JP4975385B2 - Detector, polarization control optical component, and phase modulation optical communication system - Google Patents

Description

  The present invention relates to a detector for detecting an optically modulated wave of linearly polarized light that is phase-modulated in accordance with digital data, transmitted in coherent optical communication or the like, a phase-modulated optical communication system using the same, and the above-described detection The present invention relates to a polarization control optical component that can be suitably used in a container.

  In intra-station coherent optical communication using polarization-maintaining optical fiber, light that uses optical phase modulation with a low spectral occupancy ratio, which is the ratio of the occupied frequency bandwidth of each signal used in optical multiplex communication, to the entire communication bandwidth at multiple wavelengths Multiplexing communication has been studied (see Non-Patent Documents 1 to 3).

  As a detection method using the polarization-maintaining optical fiber in the intra-station coherent optical communication, there is known an operation synchronous detection method such as an absolute synchronous detection method using a local oscillator (LO) or a 1-bit delay detection method using an optical modulation wave as it is. It has been. When one 1-bit delay detection system is applied to a two-phase signal, the output of the detector becomes an OOK (On-Off Keying) type signal (binary signal).

  The OOK system is also called an ASK system (Amplitude Shift Keying), and is a modulation system in which bit information 1 and 0 of a digital signal correspond to binary amplitude (on / off; presence / absence of optical carrier) of an optical carrier. It is. In this modulation system, a discrete intensity change of light is used for communication, and the amount of information that can be transferred as the intensity change of light increases, so that the bit length of the signal can be increased accordingly. Thus, the OOK method has an advantage that signal deterioration can be suppressed.

  When detecting an optical modulation wave that has undergone multiphase phase modulation such as 4-phase modulation or 8-phase modulation exceeding 2-phase modulation, a plurality of 1-bit delay detection systems are used in combination. In each of the delay detection systems, two signals whose phase modulation amounts are shifted by π can be detected.

  An example of such a conventional detector is disclosed in Patent Document 1. Here, an example of the conventional detector will be described with reference to FIG. FIG. 15 is a block diagram showing a configuration of a conventional detector 100.

  As shown in FIG. 15, the detector 100 includes two delay elements 101 (1-bit delay elements), two phase shifters 102, and two balanced photodetectors 103. In the conventional detector 100, the optical modulation wave is divided into two, one optical modulation wave is delayed by one bit by the delay device 101, and the other optical modulation wave is phase shifted by 45 degrees by the phase shifter 102. Then, the optical modulation wave (delayed wave) that has passed through the delay device 101 and the optical modulation wave that has passed through the phase shifter 102 are superposed, and the intensity of the superposed optical modulation wave (superposition wave) is balanced. Detect at 103.

  The detector 100 detects a four-phase modulated optical wave, and therefore there are two elements. Here, a system composed of a set of the delay device 101, the phase shifter 102, and the balanced photodetector 103 is defined as a delay detection system. Each of the delayed detection systems in the conventional detector 100 can detect two signals whose phase modulation amounts are shifted by π.

In the technical fields other than the detection of the phase-modulated light modulation wave, techniques using surface plasmons excited on the surface of the metal film are known (Patent Documents 2 and 3 and Non-Patent Document 1). To 3).
JP 2004-170954 A (published on June 17, 2004) JP 11-72607 A (published March 16, 1999) Japanese Patent Laying-Open No. 2005-5338166 (published December 08, 2005) David M.M. Pozar, "Microwave Engineering", WILEY H. Shin, P.A. B. CatrYsse and S.M. Fan, PhYs. Rev. B72, 085436, 2005 Yasuo Kokubun, "Advanced Electronics Series 6 Lightwave Engineering", Kyoritsu Publishing

  However, in the conventional detector 100, in order to detect the intensity of the superimposed wave itself, when detecting a light-modulated wave that has undergone multiphase phase modulation such as four-phase modulation or eight-phase modulation exceeding two-phase modulation, Lowers the resolution.

  In the above-described conventional detector 100, the output signal of the balanced photodetector 103, which represents the intensity of the superimposed wave, is obtained by adding the amplitude of the electric field in one balanced photodetector 103 by adding the modulated wave and the delayed wave. The level is the square of the amplitude of the electric field. The intensity I of the superimposed wave of the optical modulation wave in the conventional detector 100 and the delayed wave having the phase difference φ via the delay device 101 is expressed by the following equation (1).

  FIG. 14 shows the intensity I of the superimposed wave as a function of the phase difference φ. Here, ω is the angular frequency of the modulated optical wave and the delayed wave, t is time, and c is the initial phase. It should be noted that light intensity loss inside the detector 100 was ignored.

  In the system in which the intensity of the superimposed wave is observed by the balanced photodetector 103, when applied to two-phase modulation, the balanced type when the light passing through one delay device 101 is observed by the balanced photodetector 103 is used. The output of the photodetector 103 becomes an OOK signal (binary signal) as shown in FIG. When the conventional detector 100 is applied to four phases, a high threshold value and a low threshold value are provided above and below the half wave intensity (intensity of discrete light), and the modulation wave and the delay wave Two paths are provided: a path that passes through one delayer 101 where the phase difference is 0, and a path that passes through the other delayer 101 where the phase difference between the modulated wave and the delayed wave is 0.5π. When the conventional detector 100 is applied to eight phases, two paths that pass through the two delay devices 101 where the phase difference between the modulated wave and the delayed wave is 0.25π and 0.75π are added.

  In such a conventional detector 100, as can be seen from FIG. 14, the rate of change of the output signal (superposed wave intensity) of the balanced photodetector 103 with respect to the change of the phase difference φ is relatively small. Therefore, as the number of phase modulation phases increases, the difference in superposed wave intensity (output signal level difference of the balanced photodetector 103) δI corresponding to adjacent phase modulation waves becomes very small. . For example, in the case of detecting a phase modulated wave of eight phases in the conventional detector 100, as can be seen from FIG. 14, the difference δI in the intensity of the superimposed wave corresponding to the phase modulated wave of the adjacent phase is 0.15. It becomes. Therefore, the intermediate value of the intensity of the superimposed wave corresponding to the phase-modulated wave of the adjacent phase is used as a threshold value, and the phase (bit value of the signal) of the phase-modulated wave depends on whether the intensity of the superimposed wave is larger than the threshold value. If it is determined, if the error in the intensity of the superimposed wave is greater than 0.075, a bit error will occur. Therefore, even if the conventional detector 100 includes a plurality of delay detection systems, if the phase difference between adjacent phases decreases as the number of phase modulation phases increases, a bit error occurs due to the influence of adjacent phases. Probability increases, and the resolution of the signal (the ability to separate and detect each phase of the phase-modulated wave; inversely proportional to the intensity difference δI of the superimposed wave corresponding to the phase-modulated wave of the adjacent phase) is reduced There is a point.

  Furthermore, when the intensity fluctuation or the phase fluctuation in the transmission line is large, there is a problem that the probability that a bit error will occur further increases and the resolution of the signal further decreases.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide a detector for detecting an optically modulated wave of linearly polarized light that is phase-modulated in accordance with digital data, by making it multi-phased. To provide a detector capable of suppressing a decrease in signal resolution even when the phase difference between adjacent phases is reduced, a phase modulation optical communication system using the detector, and a polarization control optical component suitably used for the detector. It is in.

  In order to solve the above problems, the detector of the present invention is a detector that detects a linearly polarized light modulated wave that has been phase-modulated in accordance with digital data, and that receives the linearly polarized light modulated wave. Branching into the first and second paths, giving a delay of 1 bit of the digital data to the optical modulation wave of the first path, and the optical modulation wave of the first path and the optical modulation wave of the second path A 90 ° polarization axis angle difference is provided between the light modulation waves, and the light modulation waves that have passed through the first and second paths are overlapped to obtain a phase difference between the light modulation waves that have passed through the first and second paths. Phase polarization conversion means for converting into a superposition wave having a polarization state that changes in response, and polarization detection for receiving the superposition wave from the phase polarization conversion means and converting the polarization state into an electrical signal Means.

  According to the above configuration, the phase polarization conversion unit receives the linearly polarized light modulation wave and branches the light into a first path and a second path. A delay corresponding to a bit is given, and a polarization axis angle difference of 90 degrees is given between the light modulation wave of the first path and the light modulation wave of the second path, and the light passes through the first and second paths. The modulated waves are superposed and converted into a superposed wave having a polarization state that changes in accordance with the phase difference between the light modulated waves that have passed through the first and second paths. The polarization detecting unit receives the superimposed wave from the phase polarization converting unit and converts the polarization state into an electric signal.

  The change rate of the polarization state with respect to the change in the phase difference between the light modulation waves via the first and second paths is the detection signal (the superposition wave) of the change in the phase difference between the two light modulation waves in the conventional detector. It becomes larger compared to the rate of change in strength. The polarization detecting means converts the change in the polarization state into an electric signal. Therefore, even if the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small, the signal resolution Can be suppressed. Therefore, multiphase formation can be facilitated.

  In the present specification, the “polarization state” refers to the type and composition ratio of polarized light (ratio of circularly polarized light component and linearly polarized light component).

  In the detector of the present invention, in addition to the above-described configuration, the phase polarization conversion unit includes an optical branching unit for branching the linearly polarized light modulation wave into first and second paths, and the light. Delay means for delaying the phase of the optical modulated wave of the first path branched by the branching means by one bit of the digital data, and optical modulated wave of the first or second path branched by the branch means The polarization rotation means for rotating the polarization axis in 90 degrees and the light modulation wave passing through the first and second paths are overlapped, and the position between the light modulation waves passing through the first and second paths is superimposed. It is preferable to have superimposing means for converting into a superposed wave having a polarization state that changes in accordance with the phase difference.

  According to the above configuration, the optical branching unit, the delay unit, the polarization rotating unit, and the superimposing unit have a polarization state that changes according to the phase difference between the light modulation waves that have passed through the first and second paths. Conversion to a superimposed wave can be performed more reliably.

  In the detector according to the present invention, in addition to the above configuration, the superimposed wave includes a circularly polarized light component and a linearly polarized light component according to the phase difference between the light modulated waves that have passed through the first and second paths. The polarization detecting means includes a polarization control means for converting the superimposed wave into scattered light by changing the chirality of the polarization component, and the scattered light emitted from the polarization control means. It is preferable to have a light detection means for detecting the intensity of the light and converting it into an electric signal.

  According to the above configuration, the polarization detection unit includes a superposed wave including at least one of a circularly polarized light component and a linearly polarized light component according to a phase difference between the light modulation waves that have passed through the first and second paths. Is converted into scattered light by changing the chirality of the polarization component. The light detection means detects the intensity of the scattered light emitted from the polarization control means and converts it into an electrical signal.

  A mode having higher-order chirality derived from the circularly polarized light component has a significantly larger propagation loss at the aperture or the like than a mode having lower-order chirality. Therefore, the mode having higher-order chirality derived from the circularly polarized light component is removed, and only the linearly polarized light component, or the linearly polarized light component and a part of the circularly polarized light component are selectively detected by the light detection means. be able to. The change rate of the intensity of only the linearly polarized light component or the change of the intensity of the combined linearly polarized light component and a part of the circularly polarized light component with respect to the change of the phase difference between the light modulation waves passing through the first and second paths. The rate is larger than the rate of change of the detection signal (superposition wave intensity) with respect to the change in phase difference between the two light modulation waves in the conventional detector. The light detection means converts the intensity change into an electric signal, so that even if the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases is reduced, the signal resolution is reduced. Can be suppressed.

  In addition, in the detector of the present invention, in addition to the above configuration, the polarization control unit removes at least one of the counterclockwise and clockwise circularly polarized components included in the superimposed wave and guides it to the light detection unit. It is preferable to include a polarization component removing unit.

According to the above configuration, since the circularly polarized light component removing unit removes at least one of the counterclockwise and clockwise circularly polarized components included in the superimposed wave and guides it to the light detecting unit, the light detecting unit is surely In addition, it is possible to selectively detect only the linearly polarized light component, or the linearly polarized light component and the low-order circularly polarized light component. Therefore, even when the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small, it is possible to reliably suppress a decrease in signal resolution.
The polarization control means includes a polarization control optical component,
In the detector of the present invention, in addition to the above configuration, the polarization control optical component includes a metal film, an opening penetrating the metal film, an end of the opening or a position near the opening, and the opening. It is preferable that a spiral groove formed in the metal film with the center of the spiral groove being formed, and the spiral groove is configured to change the chirality of the polarization component included in the superimposed wave.

  According to the above configuration, the superimposed wave is subjected to a change in the polarization component chirality by the spiral groove formed in the metal film with the end point of the opening or a position near the opening as a base point and the center of the opening as the center. . A mode having higher-order chirality derived from the circularly polarized light component has a significantly larger propagation loss at the aperture than a mode having lower-order chirality. Therefore, the mode having higher-order chirality light derived from the circularly polarized light component can be removed by the aperture. Therefore, even when the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small, it is possible to reliably suppress a decrease in signal resolution.

  In the detector of the present invention, in addition to the above configuration, it is preferable that the light detection means is disposed on a side opposite to the side on which the superimposed wave is incident in the polarization control optical component.

  According to the above configuration, the light detection unit can detect light from which the mode having higher-order chirality derived from the circularly polarized component is removed by the opening. Therefore, even when the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small, it is possible to reliably suppress a decrease in signal resolution.

  In the detector according to the present invention, in addition to the above configuration, the superimposed wave includes a circularly polarized light component and a linearly polarized light component according to the phase difference between the light modulated waves that have passed through the first and second paths. At least one of the spiral grooves, and the spiral groove increases the chirality of at least one of the counterclockwise and clockwise circularly polarized light components included in the incident light, and the opening has a high chirality due to the spiral groove. It is set to be larger than the cut-off diameter of the linearly-polarized light component and smaller than the cut-off diameter of the circularly-polarized light component whose chirality is increased by the spiral groove so as to remove the next-order circularly polarized light component. It is preferable to have a diameter.

  According to the above configuration, the spiral groove enhances the chirality of at least one of the left-handed and right-handed circularly polarized light components included in the incident light. Further, the opening has a diameter set to be larger than the cutoff diameter of the linearly polarized light component and smaller than the cutoff diameter of the circularly polarized light component whose degree of chirality is increased by the spiral groove, The circularly polarized light component having higher order of chirality can be removed by the spiral groove. Therefore, only the linearly polarized light component, or the linearly polarized light component and the low-order (not higher-order) circularly polarized light component can be selectively detected by the light detection means. The change rate of the intensity of only the linearly polarized light component or the change of the intensity of the combined linearly polarized light component and the lower-order circularly polarized light component with respect to the change of the phase difference between the light modulation waves passing through the first and second paths. The rate is larger than the rate of change of the detection signal (superposition wave intensity) with respect to the change in phase difference between the two light modulation waves in the conventional detector. The light detection means converts the intensity change into an electric signal, so that even if the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases is reduced, the signal resolution is reduced. Can be suppressed.

  In the detector according to the present invention, in addition to the above-described configuration, the light detecting means is arranged on the polarization axes of two linearly polarized light components orthogonal to each other included in the superimposed wave, respectively. It is preferable to include a photodetector.

  According to the above configuration, it is possible to separate the linearly polarized light component of the scattered light by the superposed wave and receive the light with the first and second photodetectors. Therefore, based on the change in the intensity of each of the two linearly polarized light components orthogonal to each other with respect to the change in the phase difference between the light modulation waves via the first and second paths, the first and second paths The phase difference between the modulated optical waves passing through can be determined more accurately. That is, the signal can be detected accurately.

  In the detector according to the present invention, in addition to the above configuration, the light detecting means separates two linearly polarized light components orthogonal to each other included in the superposed wave, and the polarized light separating means separates the polarized light separating means. It is preferable that the first and second photodetectors are arranged at positions where the two linearly polarized light components thus incident respectively enter.

  According to the above configuration, since the polarization separation means for separating two linearly polarized light components orthogonal to each other included in the superimposed wave is provided, the linearly polarized light component of the scattered light by the superimposed wave is sufficiently separated. Thus, the light can be received by the first and second photodetectors. Therefore, the signal can be detected more accurately.

  Further, in the detector according to the present invention, in addition to the above configuration, restoring means for restoring the digital data by determining the phase difference between the light modulation waves passing through the first and second paths based on the electrical signal. It is preferable to further have.

  According to the above configuration, the restoration unit restores the digital data by determining the phase difference between the light modulation waves that have passed through the first and second paths based on the electrical signal obtained by converting the polarization state. . Therefore, digital data can be restored while suppressing the occurrence of bit errors even when the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small.

  In order to solve the above problems, a phase-modulated optical communication system according to the present invention includes a light-modulated wave generator for phase-modulating linearly polarized light based on digital data to generate a linearly-polarized light modulated wave, and the light A polarization-maintaining optical transmission medium for transmitting the linearly-polarized light modulated wave generated by the modulation wave generator while maintaining the state of the linearly-polarized light, and the linearly-polarized light transmitted by the polarization-maintaining optical transmission medium. It has a detector according to the present invention that receives a light-modulated wave and converts it into an electric signal.

  According to the above configuration, even if the phase difference between adjacent phases is reduced due to multi-phase, an optical modulation wave corresponding to digital data is generated by the optical modulation wave generator and detected via the polarization maintaining optical transmission medium. The digital data can be restored from the modulated optical wave while suppressing a decrease in the resolution of the signal by the detector according to the present invention. Therefore, it is possible to realize a phase-modulated optical communication system capable of transmitting digital data via a polarization maintaining optical transmission medium while suppressing the occurrence of bit errors.

  In order to solve the above problems, the polarization control optical component of the present invention is based on a metal film, an opening penetrating the metal film, an end of the opening or a position near the opening, and the center of the opening. A polarization control optical component having a spiral groove formed in a metal film, wherein the spiral groove changes the chirality of a polarization component included in incident light.

  According to the said structure, the chirality of the polarization component contained in incident light can be changed with the said spiral groove. Thereby, for example, part or all of the circularly polarized light component can be made higher order, and the propagation loss at the aperture can be remarkably increased. Therefore, a part or all of the light of the mode having higher-order chirality derived from the circularly polarized light component can be removed by the opening. By removing a part or all of the circularly polarized light component in this way, a part of the polarized light component from the incident light, for example, (1) only the linearly polarized light component, (2) only a part of the circularly polarized light component, (3 ) A part of linearly polarized light component and circularly polarized light component can be taken out. As a result, processing based on some polarization components included in the incident light, for example, detection based on the intensity of some polarization components included in the incident light is possible. As described above, this contributes to an improvement in signal resolution.

  In the present specification, a “spiral groove” is a groove formed with an opening as a base point, and the azimuth angle with respect to the center of the opening monotonously increases or monotonously decreases as the distance from the opening increases. The distance from the opening side end to the opening end (groove side end) on the same azimuth is monotonously increased as the distance from the opening increases.

  Moreover, in the polarization control optical component of the present invention, in addition to the above configuration, the spiral groove is preferably a plurality of spiral grooves respectively formed from a plurality of different positions.

  According to the above configuration, by adding the chirality of the circularly polarized light component included in the previous superimposed wave and the chirality of the spiral groove, at least one of the left-handed circularly polarized light component and the right-handed circularly polarized light component is higher than the third order. Can be in mode. Therefore, if the polarization component of the third or higher mode having a remarkably large propagation loss at the aperture or the like is removed by the aperture or the like, the intensity of the linearly polarized component and one of the counterclockwise circularly polarized component and the clockwise circularly polarized component is combined Can be detected by the light detection means. Therefore, even when the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small, it is possible to reliably suppress a decrease in signal resolution.

  Moreover, in the polarization control optical component of the present invention, in addition to the above configuration, the spiral groove is preferably four or more spiral grooves respectively formed from four or more positions different from each other.

  According to the above configuration, by adding the chirality of the circularly polarized light component included in the previous superimposed wave and the chirality of the spiral groove, both the left-handed circularly polarized light component and the right-handed circularly polarized light component are in the third or higher mode. Can be. For this reason, if the polarization component of the third or higher order mode having a significantly large propagation loss at the aperture or the like is removed by the aperture or the like, the intensity of only the linearly polarized component can be detected by the light detection means. The rate of change in the intensity of only the linearly polarized light component with respect to the change in the phase difference between the light modulation waves that has passed through the first and second paths becomes extremely large near the peak of the intensity. Therefore, the resolution of the signal can be further improved by utilizing the vicinity of the intensity peak.

  Further, in the polarization control optical component of the present invention, in addition to the above configuration, the distance between the base point and the opening in the spiral groove is shorter than the wavelength of the surface plasmon excited near the opening by the incident light. preferable.

  According to the above configuration, it is possible to avoid the attenuation of the surface plasmon that changes the chirality of the polarization component included in the incident light, and to reliably change the chirality of the polarization component included in the incident light.

  As described above, the detector and the phase modulation optical communication system of the present invention can convert the change in the polarization state with respect to the change in the phase difference between the optical modulation waves via the first and second paths into an electrical signal. Even when the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small, a reduction in signal resolution can be suppressed.

  In addition, as described above, the polarization control optical component of the present invention, for example, increases the order of part or all of the circularly polarized light component, so that the light of the mode having higher-order chirality derived from the circularly polarized light component can be obtained. Part or all can be removed at the opening. Therefore, when the polarization control optical component of the present invention is used in the detector of the present invention, it contributes to an improvement in signal resolution.

[Embodiment 1]
An embodiment of the present invention will be described with reference to FIGS. 1 and 3 as follows.

  First, the structure of the detector which is one Embodiment of this invention is demonstrated based on FIG. FIG. 1 is a block diagram showing a configuration of a detector 10 in the present embodiment. The detector 10 according to the present embodiment detects an optical modulation wave that has been phase-modulated in accordance with digital data. As shown in FIG. 1, a phase polarization converter (phase polarization converter) 1 and , A condenser lens 2, a polarization detection unit (polarization detection means) 3, and a decoder (restoration means) 4. A light modulated wave phase-modulated in accordance with digital data is incident on the detector 10 as linearly polarized light in a predetermined direction. The wavelength of light that can be used is not particularly limited, and may be visible light in addition to infrared and near infrared light that are generally used for optical communication.

  The phase polarization conversion unit 1 is a part that converts a linearly polarized light modulated wave that is phase-modulated according to digital data into light including polarization information. The condensing lens 2 is a lens for condensing the light including the polarization information output from the phase polarization conversion unit 1 and inputting the light to the polarization detection unit 3. The polarization detection unit 3 is a part that detects the intensity of light including polarization information input from the condenser lens 2 and outputs it as a light detection signal. The decoder 4 restores the digital data based on the photodetection signal output from the polarization detector 3. Next, each component will be described with reference to FIG.

  First, as shown in FIG. 1, the phase polarization converter 1 includes a beam splitter (optical branching means) 11, a 1-bit delay device (delaying means) 12, a phase difference plate (polarization rotating means) 13, an optical And a coupler (superimposing means) 14.

  The beam splitter 11 is for branching an optical modulation wave phase-modulated according to digital data into two paths, a path (second path) 15 and a path (first path) 16.

  The 1-bit delay unit 12 delays the phase of the optical modulation wave of one path 16 branched by the beam splitter 11 by one bit of the digital data (a time equal to the period for transmitting each bit of the digital data). It is something to be made. The 1-bit delay unit 12 is configured so that the optical path length of the path 16 is longer than the optical path length of the path 15 by one bit, that is, by the distance that the light travels in the time corresponding to one bit of the digital data. The light is induced. The 1-bit delay device 12 is not particularly limited. For example, an optical fiber having a length equal to the distance that light travels in a time corresponding to 1 bit of the digital data can be used.

  The phase difference plate 13 is disposed in the path 16 and transmits the optical modulation wave of the path 16 branched by the beam splitter 11 to give a polarization rotation (rotation of the polarization axis) to the optical modulation wave of the path 16. Is. In the present embodiment, a half-wave plate that is disposed in the path 16 and applies a 90 ° (π / 2) polarization rotation to the optical modulation wave is used as the phase difference plate 13.

  The optical coupler 14 superimposes the light modulation wave given the 90 degree polarization rotation and the light modulation wave not given the 90 degree polarization rotation, and converts them into a superposition wave. It is. The superposed wave obtained by the optical coupler 14 has a polarization state that changes in accordance with the phase difference between the optical modulation waves via the paths 15 and 16. More specifically, the superposed wave includes at least one of a circularly polarized light component and a linearly polarized light component, as will be described later, and the circularly polarized light component according to the phase difference between the light modulation waves that have passed through the paths 15 and 16. And the ratio of the linearly polarized light component change. As the optical coupler 14, a fiber coupler, an optical waveguide type optical coupler, or the like can be used.

  Next, as shown in FIG. 1, the polarization detection unit 3 includes a polarization control optical component (polarization control means) 31 and a photodetector (light detection means) 32.

  The polarization control optical component 31 converts the superimposed wave obtained by the optical coupler 14 into scattered light by changing the chirality of the polarization component. More specifically, as will be described later, the polarization control optical component 31 is an element that adds the chirality of the polarization control optical component 31 described later to the chirality of the polarization component of the light modulation wave superimposed by the optical coupler 14. is there.

  The photodetector 32 detects the intensity of the scattered light emitted from the polarization control optical component 31, converts it to an electrical signal representing the intensity, and outputs the electrical signal. As the photodetector 32, a known photodetector such as a balanced photodetector is preferably used. However, the photodetector 32 is not limited to this, and another photodetector having a similar function may be used. The form of the electrical signal output from the photodetector 32 is not particularly limited, but in the present embodiment, the electrical signal changes in voltage according to the detected light intensity.

  In the polarization detection unit 3, the polarization control optical component 31 has circular polarization component removal means that removes at least one of the counterclockwise and clockwise circular polarization components included in the superimposed wave and guides them to the photodetector 32. It is preferable. As a result, the light detector 32 detects the intensity of the polarization component that has not been removed by the circularly polarized light component removing means among the polarization components included in the superimposed wave, that is, (1) the intensity of the linearly polarized light component, or (2) The intensity of the combination of the linearly polarized light component and one of the counterclockwise and clockwise circularly polarized light components can be detected. When the optical modulation wave received by the detector 10 is multiphased to a phase larger than two phases, the change in the phase difference between the optical modulation waves via the phase modulation wave paths 15 and 16 (1) or (2) The rate of change in intensity is greater than the rate of change in the intensity of the superimposed wave with respect to the change in phase difference between the modulated optical waves via the two paths in the conventional detector 100. Therefore, when the optical modulation wave received by the detector 10 is multiphased to a phase larger than two phases, the detector 10 can suppress a decrease in resolution as compared with the conventional detector 100, and a bit error can be prevented. Generation can be suppressed. The polarization control optical component 31 having the circular polarization component removing means is not particularly limited, but in the polarization control optical component 31 according to the second embodiment to be described later, the aperture diameter is included in the superimposed wave. It is preferable to set it so that it is larger than the cutoff diameter of the linearly polarized light component and smaller than the cutoff diameter of the circularly polarized light component whose chirality is increased by the polarization control optical component 31. The polarization control optical component 31 may have the function of removing at least part of the linearly polarized light component from the superimposed wave and guiding it to the photodetector 32 instead of the circularly polarized light component removing unit. (Effect of suppressing a decrease in resolution) can be achieved.

  Finally, the decoder 4 determines the phase difference between the optical modulation waves via the paths 15 and 16 based on the electrical signal output from the polarization detector 3, and restores and outputs the digital data. Is.

  Next, the operation of the detector 10 of the present embodiment will be described based on FIG.

  A linearly polarized light modulation wave (the direction of the linear polarization axis is hereinafter referred to as “x-axis direction”) input to the detector 10 is branched into two paths 15 and 16 by the beam splitter 11.

  The path 16 is provided with a 1-bit delay unit 12, and the 1-bit delay unit 12 gives a delay of 1 bit of the digital data to the optical modulation wave of the path 15.

  The phase difference plate 13 is a delayed optical modulation wave output from the 1-bit delay device 12 (hereinafter, the delayed optical modulation wave is referred to as a “delay wave”, and an undelayed optical modulation wave is referred to as a “non-delay wave”. ”) Is rotated 90 degrees with respect to the non-delayed wave (this direction is hereinafter referred to as“ y-axis direction ”).

  The delayed wave and the non-delayed wave are superimposed again by the optical coupler 14. Since the delayed wave and the non-delayed wave are optical modulation waves corresponding to adjacent bits of the digital data, depending on the phase difference between the optical modulation waves corresponding to adjacent bits of the digital data due to the superposition described above. , A superposed wave whose polarization state changes, such as linearly polarized light, elliptically polarized light, and circularly polarized light.

  Thereafter, the superimposed wave is incident on the polarization control optical component 31. Scattered light corresponding to the polarization state is emitted from the polarization control optical component 31. Then, the detector 32 detects the intensity of the scattered light, and outputs an electrical signal according to the detected light intensity. The electrical signal output from the detector 32 is input to the decoder 4. The decoder 4 restores and outputs the digital data based on the electrical signal.

  Here, an example of the digital data and the corresponding optical modulation wave will be described with reference to FIG. FIG. 3A shows an example of a waveform of digital data (digital signal), a value of each bit of the digital data, and a phase difference with respect to an initial phase (reference phase) of a two-phase signal (two-phase optical modulation wave). It is a figure which shows the example of these correspondence. A two-phase signal is obtained by associating values 0 and 1 of each bit of digital data with a signal (optical modulation wave) having a phase difference of 0 and π (radians) with respect to the initial phase. FIG. 3B is a diagram illustrating an example of a waveform of digital data and an example of a correspondence relationship between the value of each bit of the digital data and the phase difference with respect to the initial phase of the four-phase signal (four-phase optical modulation wave). It is. The four-phase signal is obtained by converting the values 00, 01, 10, and 11 of two bits of digital data into signals (optical modulation waves) having phase differences of 0, 0.5π, π, and 1.5π with respect to the initial phase, respectively. It is a thing made to correspond. FIG. 3C illustrates an example of a waveform of digital data and an example of a correspondence relationship between the value of each bit of the digital data and the phase difference with respect to the initial phase of the 8-phase signal (eight-phase optical modulation wave). It is. The 8-phase signal has values 000, 001, 010, 011, 100, 101, 110, and 111 corresponding to 3 bits of digital data, respectively, with phase differences of 0, 0.25π, 0.5π, 0,. This corresponds to signals (optical modulation waves) of 75π, π, 1.25π, 1.5π, and 1.75π.

  Next, the polarization of the superimposed wave will be described.

  The amplitude of the superposed wave of the non-delayed wave having the linear polarization axis in the X-axis direction and the delayed wave having the linear polarization axis in the Y-axis direction is expressed by the following equation (2).

  Here, ω is the angular frequency of the superimposed wave, t is time, c is the initial phase, and φ is the phase difference between the non-delayed wave and the delayed wave corresponding to the adjacent bits of the digital data. . In addition, the amplitude of the superimposed wave is normalized so that the intensity becomes 1 on the coordinate axis AB described below.

  The coordinate axis A-B is constructed by setting the coordinate axis overlapping with the X axis as the A axis and the coordinate axis overlapping with the Y axis as the B axis when the XY coordinate axis consisting of the X axis and the Y axis is rotated by -45 degrees. Is done.

  As can be seen from the above equation (2), the polarization of the superposition wave is generally elliptical polarization. However, when the phase difference φ is a specific value, special polarization (complete linear polarization or complete polarization) is obtained. Circularly polarized). First, the case of special polarized light will be described, and then general elliptically polarized light will be described. According to the above equation (2), when the phase difference φ is 90 degrees, the polarization of the superimposed wave is counterclockwise (counterclockwise with respect to the traveling direction of the superimposed wave, and the same description is omitted hereinafter). When the phase difference φ is 270 degrees, the polarization of the superimposed wave is clockwise (rightward with respect to the traveling direction of the superimposed wave; hereinafter, the same description is omitted) and circularly polarized. That is, in these cases, the polarization of the superposition wave is completely circular polarization. Further, according to the above equation (2), when the phase difference φ = 0 degrees, the superimposed wave polarization is linearly polarized in the B-axis direction, and when the phase difference φ = 180 degrees, the superimposed wave polarization. Becomes linearly polarized light in the A-axis direction. That is, in these cases, the polarization of the superposed wave is completely linearly polarized light.

  Here, general elliptically polarized light will be described. When the phase difference is 0 <φ <90 °, the polarization of the superposed wave is counterclockwise elliptically polarized light having the long side of the A axis. Further, when the phase difference is 90 ° <φ <180 °, the polarization of the superposed wave is counterclockwise elliptically polarized light having the B axis as its long side. Further, when the phase difference is 180 ° <φ <270 °, the polarization of the superposed wave is a clockwise elliptically polarized light with the B axis as the long side, and when the phase difference is 270 ° <φ <360 °, The polarization of the superimposed wave is clockwise elliptical polarization with the long side of the A axis.

However, as is generally known, elliptically polarized light is given by superposition (sum) of circularly polarized light and linearly polarized light. For example, when the phase difference is 0 <φ <90 °, as described above, the polarization of the superposition wave is a left-handed elliptical polarization with the long side of the A axis, but this elliptical polarization is
Amplitude is

Left-handed circularly polarized light,
Amplitude is

Is superimposed with linearly polarized light having an axis in the A-axis direction. The ratio between the circularly polarized light component and the linearly polarized light component constituting the elliptically polarized light changes according to the phase difference φ. For example, when the phase difference is 0 <φ <90, the ratio of the circularly polarized light component and the linearly polarized light component constituting the elliptically polarized light (superimposed wave) is the linearly polarized light component (A-axis direction as the phase difference φ approaches 0). The ratio of circularly polarized light components (counterclockwise circularly polarized light) increases as the phase difference φ approaches 90 degrees.

  Therefore, the superimposed wave has a polarization type (linearly polarized light, elliptically polarized light, or circularly polarized light) corresponding to the phase difference φ between the non-delayed wave and the delayed wave, and the phase difference φ between the non-delayed wave and the delayed wave. It becomes the ratio of the corresponding circularly polarized light component and the linearly polarized light component. That is, the superimposed wave is in a polarization state corresponding to the phase difference φ between the non-delayed wave and the delayed wave.

  The superimposed wave is incident on the polarization control optical component 31 and converted into scattered light corresponding to the polarization state by the polarization control optical component 31. The intensity of the scattered light is detected by the photodetector 32 and converted into an electric signal. Thereby, the polarization state is converted into an electric signal.

  As described above, the detector 10 in the present embodiment superimposes the light modulation waves that have passed through the paths 15 and 16 and changes the polarization state according to the phase difference between the light modulation waves that have passed through the paths 15 and 16. The polarization state is converted into an electrical signal by the polarization detector 3. The change rate of the polarization state with respect to the change in the phase difference between the optical modulation waves via the paths 15 and 16 is the change in the detection signal (intensity of the superimposed wave) with respect to the change in the phase difference between the two optical modulation waves in the conventional detector. Greater than rate. Since the polarization detection unit 3 converts the change in the polarization state into an electric signal, the signal resolution can be obtained even when the optical modulation wave is multiphased into three or more phases and the phase difference between adjacent phases becomes small. Can be suppressed. Therefore, multiphase formation can be facilitated.

  The detector of the present invention is not limited to the above-described embodiment, and various modifications can be made within the scope indicated in the claims. For example, instead of the beam splitter 11, other optical branching means such as a half mirror, an optical waveguide type optical branching device, a fiber coupler, or the like may be used.

  In addition, the phase difference plate 13 may be disposed on the path 15 to give a 90-degree polarization rotation to the optical modulation wave of the path 15 branched by the beam splitter 11. Further, instead of the phase difference plate 13, other polarization rotation means capable of rotating the polarization by 90 degrees, such as a TN (twisted nematic) liquid crystal to which a voltage is applied, may be used. Further, in the present invention, it is sufficient that a 90-degree polarization axis angle difference can be given between the light modulation wave of the path 15 and the light modulation wave of the path 16, and the polarization rotation of 90 degrees. It is not always necessary to provide a polarization rotation means for providing For example, instead of the phase difference plate 13, a polarization rotation unit that applies polarization rotation of 270 degrees may be used.

  Alternatively, the decoder 4 may be omitted from the detector 10 and the electric signal output from the photodetector 32 of the detector 10 may be processed by an external decoder.

[Embodiment 2]
The following will describe another embodiment of the present invention with reference to FIGS. Configurations other than those described in the present embodiment are the same as those in the first embodiment. For convenience of explanation, members having the same functions as those shown in the drawings of the first embodiment are given the same reference numerals, and explanation thereof is omitted.

  First, the configuration of the polarization control optical component 31 according to another embodiment of the present invention will be described with reference to FIG. FIG. 4 is a perspective view showing the configuration of the polarization control optical component 31 in the present embodiment.

  FIG. 4 is a perspective view in which a part of the polarization control optical component 31 is cut so that the cross section can be seen. As shown in FIG. 4, the polarization control optical component 31 is formed by forming a spiral groove 42 on one surface of the metal film 41 and forming an opening 43 penetrating the metal film 41.

  The metal film 41 is preferably made of at least one metal of gold (Au), silver (Ag), and aluminum (Al) that can excite surface plasmons. The film thickness of the metal film 41 is preferably less than or equal to the wavelength λ of incident light (incident wave) so that surface plasmons can be effectively excited.

  The spiral groove 42 is formed in the metal film 41 with the end of the opening 43 or the vicinity thereof as a base point (end point) and the center of the opening 43 as the center. The spiral groove 42 has a function of increasing the degree of chirality (degree of freedom) of at least one of the left-handed and right-handed circularly polarized light components included in the incident light. The spiral groove 42 in the present embodiment has an Archimedean spiral (the spiral whose distance from the center of the opening 43 changes in proportion to the azimuth angle with respect to the center of the opening 43) having the opening 43 as a base point. Therefore, the spiral groove 42 has a constant pitch (the interval between the spiral grooves 42 on a straight line drawn outward from the center of the opening 43). The number of base points 44 near the opening 43 of the spiral groove 42 in the polarization control optical component 31 shown in FIG. 4, that is, the number of spiral curves constituting the spiral groove 42 is 2 as shown in FIG. It is a piece. Note that even if the number of base points 44 (the number of spiral curves) is a plurality of other cases, a near field with controlled chirality is generated. For example, the number of base points 44 may be four as shown in FIG.

  The width of the spiral groove 42 is preferably λ / 4 or less (the wavelength λ of incident light). The pitch of the spiral grooves 42 (the distance between the spiral groove centers with respect to the radial direction between the nearest spiral grooves) is preferably λ / 2 or less. The depth of the spiral groove 42 is preferably λ or less. The inclination angle of the side surface of the spiral groove 42 is preferably substantially vertical. The number of turns of the spiral groove 42 is preferably in the range of 0.5 to 5.

  The opening 43 is circular. The diameter (diameter) of the opening 43 is larger than the cut-off diameter of the linearly polarized light component so that the circularly polarized component whose chirality is enhanced by the spiral groove 42 is removed, and the chirality is enhanced by the spiral groove. It is preferable to have a diameter set to be smaller than the cut-off diameter of the circularly polarized light component. The opening 43 may have another shape, for example, an ellipse or a rectangle. When the opening 43 is oval or rectangular, the length of the short axis or short side preferably satisfies the above dimensional condition.

  Further, the spiral groove 43 and the opening 43 can be formed by an existing patterning process. In the formation process of the spiral groove 43 and the opening 43, there are various processes such as light exposure with a stepper, electron beam (EB) exposure, sweep type electron beam (SEM) exposure, processing with a focused ion beam (FIB), and nanoimprint. Methods and means can be utilized.

  The spiral groove 42 is not limited to the one having the Archimedean spiral shape, and is a groove formed with the opening 43 as a base point, and monotonously as the azimuth angle with respect to the center of the opening 43 increases from the opening 43. Any groove that increases or monotonously decreases and monotonously increases as the distance from the opening 43 increases from the opening may be used. Therefore, the shape of the spiral groove 42 is a logarithmic spiral (Bernoulli spiral), a parabolic spiral, a hyperbolic spiral, a Lithus spiral, other geometrical spirals; an Archimedean spiral or other geometrical spirals, For example, it may be a shape approximated by a set of straight lines forming 90 degrees with each other.

  Next, the function of the polarization control optical component 31 will be described with reference to FIG.

  First, incident light is irradiated from the upper side of FIG. 4 onto the surface (one surface) of the metal film 41 which is the formation surface of the spiral groove 42. At this time, an electric dipole is excited around the opening 43 located at the center of the spiral groove 42. The excited electric dipole propagates through the opening 43 as surface plasmon, reaches the back surface of the metal film 41 (surface opposite to the surface irradiated with incident light), and emits electric dipole (the electric dipole vibrates). Is emitted as scattered light due to a phenomenon in which light is emitted anisotropically.

  The incident light may be irradiated near the center of the spiral groove 42 of the polarization control optical component 31, but is preferably incident perpendicularly to the surface of the metal film 41 on which the spiral groove 42 is formed.

  The incident light is converted into surface plasmons having higher-order rotational symmetry by the spiral groove 42 of the polarization control optical component 31. Here, surface plasmon is a kind of electron density wave generated at the metal-dielectric interface, and the wave number depends on the thickness and optical characteristics (dielectric constant, refractive index) of the sample up to several hundreds of nanometers in contact with the metal thin film surface. Change.

  The surface plasmon forms an electric field distribution by an electric dipole distribution near the entrance of the opening 43 of the polarization control optical component 31. FIG. 7 shows an electric field distribution 70 in the radial direction in the vicinity of the entrance of the opening 43 of the polarization control optical component 31 excited by such surface plasmons. The surface plasmon is a TM wave (transverse magnetic wave) propagating on the metal surface of the polarization control optical component 31, and the radial electric field is dominant.

  (A) and (b) of FIG. 7 are respectively secondary clockwise (clockwise with respect to the incident direction of light with respect to the polarization control optical component 31; hereinafter, the same description is omitted). This shows a case in which circularly polarized light and counterclockwise circularly polarized light are incident (in the right direction with respect to the incident direction of light with respect to the polarization control optical component 31; the same description is omitted hereinafter). The electric field distribution modes excited by the surface plasmons in this case are clockwise third-order and first-order modes, respectively.

  Here, the chirality of each mode of the electric field distribution 70 will be described with reference to FIG. As shown in FIG. 7, the electric field distribution 70 includes an electric field directed toward the center of the opening 43 and an electric field directed away from the center. Therefore, the chirality of the mode is defined by the number of sets of the two types of electric fields. For example, in FIG. 7A, the electric field directed toward the center of the opening 43 is separated into three parts, and the electric field directed away from the center is also separated into three parts. Therefore, it is considered that there are three sets of the above two types of electric fields. Therefore, the chirality of the electric field distribution mode in FIG. Further, since each has a rotating direction, for example, the electric field distribution mode in FIG. 7A is referred to as a clockwise third-order mode.

  Similarly, although not shown, when clockwise circularly polarized light and counterclockwise circularly polarized light are incident on the fourth-order clockwise spiral groove 42, clockwise fifth-order and third-order modes are formed. When the incident light is linearly polarized light, the even-order spiral groove 42 generates a non-rotating first-order mode as shown in FIG.

  Next, the chirality of the spiral groove 42 will be described with reference to FIGS. In the structure shown in FIG. 4, the spiral groove 42 is clockwise when viewed from the incident direction of light with respect to the polarization control optical component 31. A state consisting of clockwise and counterclockwise is called chirality. Here, clockwise is defined by + and counterclockwise is defined by-. Further, the degree of chirality of the spiral groove 42 is defined as the number of base points 44 near the opening 43 of the spiral groove 42 in the polarization control optical component 31, that is, the number of spiral curves constituting the spiral groove 42. For example, the chirality of the spiral groove 42 shown in (a) of FIG. 5 is 2, and the chirality of the spiral groove 42 shown in (b) of FIG.

FIG. 6 shows a primary clockwise spiral groove 42. The distance r (θ) from the center of the opening 43 of the spiral groove 42 is as follows when the radius of the opening 43 is r ₀ :
r (θ) = r ₀ + c + 2aθ / 2π
It shall be represented by

Here, c is the distance between the base point 44 of the spiral groove 42 and the circumference of the opening 43, and 2 a represents the pitch of the spiral groove 42. Further, θ is an azimuth angle starting from an arbitrary angle, and increases clockwise.
However, in reality, it is necessary to consider the thickness of the spiral groove 42, but there is no problem if the pitch 2a is sufficiently larger than the thickness of the spiral groove 42. Therefore, the ratio of the pitch 2a to the thickness of the spiral groove 42 (pitch 2a / thickness of the spiral groove 42) is desirably 1.5 or more.

Further, in order to avoid attenuation of the surface plasmon, the distance c between the opening 43 and the base 44 in the helical groove 42, it is smaller than the wavelength 2 [pi / k _sp of the surface plasmon is desirable.

Here, how to determine the pitch 2a of the spiral groove 42 will be described. In the pitch 2a, δ is the incident angle of light (δ = 0 in the case of vertical incidence), k ₀ is the wave number of light in vacuum k ₀ = 2π / λ ₀ (where λ ₀ is the wavelength of light in vacuum), and When k _sp is a wave number of a surface plasmon described later, the following equation is obtained.

2a = 4π / (Re (k _sp ) −k ₀ sin (δ)) (3)
Here, k _sp is a complex parameter, and Re represents the real part of the complex number.

Therefore, if the wave number k _{sp of the} surface plasmon, the wave number k ₀ of the light in vacuum, and the incident angle δ of the incident light are known from the above equation (3), the pitch 2a can be determined.

Next, the wave number k _sp of the surface plasmon will be described. Here, the thickness of the metal film 41 constituting the polarization control optical component 31 is thicker than the wavelength λ ₀ of light in vacuum, and the surface of the metal film 41 of the polarization control optical component 31 including the spiral groove 42 has a complex dielectric constant. ε when in contact with the dielectric consisting of _d is the wave number k _sp of the surface plasmon, complex permittivity ε with _m obtained is known as the next metal film 41 constituting the polarization control optics 31 It has been.
k _sp = (ε _m ε _d / (ε _m + ε _d )) ^0.5
On the other hand, the film thickness of the metal film 41 of the polarization control optical component 31 is thinner than the wavelength λ ₀ of light in vacuum, and the surface on one side of the metal film 41 of the polarization control optical component 31 including the spiral groove 42 has a complex dielectric constant. When the surface on the opposite side of the metal film 41 is in contact with the first medium having ε _d1 and the second medium having the complex dielectric constant ε _d2 , the wave number k _{sp of the} surface plasmon is the polarization It is known that the film thickness t of the metal film 41 of the control optical component 31 and the complex dielectric constant ε _m of the metal film 41 of the polarization control optical component 31 are converted as shown in the following equation (4).

However, k _X1 , k _X2 , and k _X3 are wave numbers of light propagating through the metal film 41 constituting the first medium, the second medium, and the polarization control optical component 31, respectively.

In addition, when the light condensed by the condensing lens 2 is incident on the polarization control optical component 31, a plurality of wave numbers k ₀ sin (δ) in the in-surface direction of the metal film 41 of the polarization control optical component 31 are provided. It is known to be a superposition of plane waves. Therefore, the incident angle δ treated by the above equation (3) is 0 <δ ≦ arcsin (NA).
It is in the range. Here, NA is the numerical aperture of the condenser lens 2.

The phase γ (θ) at the innermost circumference of the surface plasmon excited on the surface of the polarization control optical component 31 by the spiral groove 42 is γ (θ) = − 2aθk ₀ sin (δ) / 2π.
Is described. Note that θ is an azimuth angle starting from an arbitrary angle, and increases clockwise.

Here, the electric field in the radial direction is E _ir eXp (−iωT) × eXp (iσ _i θ)
Consider the circularly polarized incident light described in. Here, σ _i = + 1 is clockwise circularly polarized light, and σ _i = −1 is chirality of counterclockwise circularly polarized light. E _ir is the electric field amplitude in the radial direction of the incident light, ω is the angular frequency of the electric field, and T is time. Note that θ is an azimuth angle starting from an arbitrary angle, and increases clockwise.

The radial electric field around the circumference of the opening 43 of the surface plasmon excited by the circularly polarized light is E _r = E _pr eXp (−iωT) × eXp (iσ _i θ) × eXp (i (2aθk _sp / 2π
+ Γ (θ)))
It is expressed. Note that E _pr is the amplitude of the radial electric field around the circumference of the opening 43 due to the surface plasmons excited by the circularly polarized light.

Here, when the electric field _Er in the radial direction when E _pr = 1 and T = 0 is obtained,
E _r = eXp (iθ (σ _i + 2aθ / 2π × (k _sp −k ₀ sin (φ))))
It becomes.

Further, ignoring the attenuation term of the amplitude _Er due to the imaginary term of the wave number _ksp , and considering only the phase term of the amplitude _Er ,
E _r = eXp (iθ (σ _i +2))
It becomes.

As a result, the chirality of the mode derived from the circularly polarized light (incident light) formed in the opening 43 of the polarization control optical component 31 is the sum of the chirality 2 of the spiral groove 42 and the chirality σ _i of the circularly polarized light (incident light). It shows that it becomes. Similarly, for any n, in the n-th order spiral groove 42, the chirality of the mode derived from the circularly polarized light (incident light) formed in the opening 43 is the chirality n of the spiral groove 42 and the chirality σ _{i of the} circularly polarized light. And the sum.

  When the incident light is linearly polarized light, there are only two degrees of freedom that are symmetrical and asymmetrical along the polarization axis. Therefore, the non-rotating first-order mode (n is an even number) or zero-order mode (n is an odd number) is excited in the opening 43 by the spiral groove 42 of the polarization control optical component 31. Therefore, when the chirality of the spiral groove 42 of the polarization control optical component 31 is second order or fourth order, a non-rotating first order mode is excited in the opening 43.

  When the chirality of the spiral groove 42 of the polarization control optical component 31 is second-order and fourth-order, the relationship between the above-described chiralities is summarized as shown in Table 1 and Table 2.

  As can be seen from Table 1 and the like, when the chirality of the spiral groove 42 is quadratic (when the spiral groove 42 is two spiral grooves formed with two different positions as base points, respectively), it is excited by the opening 43. The mode is a first-order mode derived from linearly polarized light and a third-order mode derived from circularly polarized light.

  As can be seen from Table 2 and the like, when the chirality of the spiral groove 42 is quadratic (when the spiral groove 42 is two spiral grooves formed with two different positions as base points, respectively), the opening 43 is excited. The modes are a first-order mode derived from linearly polarized light, and a third-order mode and fifth-order mode derived from circularly polarized light.

By the way, light (surface plasmon) having a wavelength smaller than the radius r ₀ of the circular opening 43 in the polarization control optical component 31 cannot be propagated through the opening 43 and is filtered. At this time, the frequency (light speed C / wavelength λ) corresponding to the wavelength is referred to as a cutoff frequency. The wavelength, that is, the diameter of the smallest opening 43 that can propagate light (surface plasmon) is called a cutoff diameter. Accordingly, there is a respective cutoff frequency and cutoff diameter for each mode of the electric field distribution 70.

By the way, the cut-off frequency for the 0th to 5th mode light (surface plasmon) is as follows: primary mode TE ₀₁ , 0th mode TM ₀₀ , secondary mode TE ₀₂ , tertiary mode TE ₀₁ , fourth mode TE ₀₁ , it is known that high cut-off frequency in the order of the following modes TE _01. That is, in the above example, it is known that the cutoff frequency for each mode of the electric field distribution is high in the order of the first-order mode, the third-order mode, and the fifth-order mode. Therefore, each mode of light (surface plasmon) is first mode, 0th mode, second mode, third mode, fourth mode, fifth mode,... is there. The above order is the same even in a system in which surface plasmons can exist on the surface of the metal film 41.

That is, as the order of the mode of the electric field distribution 70 excited by the aperture 43 of the polarization control optical component 31 increases from the first order to the third order, the electric dipole distribution itself that causes the electric field distribution 70 increases. Go. Therefore, if the radius r _{0 of the} opening 43 is set in the middle of the size of the electric dipole distribution corresponding to the first-order mode and the third-order mode, it is possible to filter a signal that generates a mode of one order.

  Therefore, when the chirality of the spiral groove 42 is second order, the diameter of the opening 43 is larger than the diameter (cutoff diameter) corresponding to the cut-off frequency of the first-order mode light (surface plasmon) and the third-order mode light. Is set to be smaller than the diameter corresponding to the cut-off frequency (cut-off diameter), the third-order mode light originating from one of the left-handed and right-handed circularly polarized light is removed by the opening 43, and the remaining circles Propagating the third-order mode light derived from the polarized light and the first-order mode light derived from the linearly polarized light to the back side of the metal film 41 (the side opposite to the side where the incident light is incident on the metal film 41). it can.

  When the chirality of the spiral groove 42 is fourth order, the diameter of the opening 43 is larger than the diameter corresponding to the cut-off frequency of light in the first-order mode and larger than the diameter corresponding to the cut-off frequency of light in the third-order mode. When set to be small, the aperture 43 removes the light of the third-order mode and the fifth-order mode derived from the counterclockwise and right-handed circularly polarized light, and only the light of the first-order mode derived from the linearly polarized light is the metal film 41. It can be propagated to the back side.

  Similarly, when the chirality of the spiral groove 42 is third order or when the chirality of the spiral groove 42 is fifth order or more, the diameter of the opening 43 corresponds to the cutoff frequency of light of the mode derived from linearly polarized light. If it is set to be smaller than the diameter corresponding to the cutoff frequency of the light of the mode light derived from circularly polarized light, the aperture 43 removes the light of the mode derived from the circularly polarized light and derives from the linearly polarized light. Only light of the mode to be transmitted can be propagated to the back side of the metal film 41.

  As described above, the chirality of the spiral groove 42 is second order or higher, the diameter of the opening 43 is larger than the diameter corresponding to the cut-off frequency of light in the first-order mode, and the cut-off frequency of light in the third-order mode. If the diameter is set to be smaller than the diameter corresponding to, the aperture 43 removes light of a mode derived from circularly polarized light (right-handed circularly polarized light or left-handed circularly polarized light) higher-ordered by the spiral groove 42 (1 The light of the mode derived from the remaining circularly polarized light and the light of the mode derived from linearly polarized light, or (2) the light of the mode derived from linearly polarized light can be guided to the back surface side of the metal film 41.

  Although the polarization control optical component 31 according to the present embodiment can be used for various applications, it is suitable as the polarization control optical component 31 constituting the detector 10 according to the first embodiment.

  When the polarization control optical component 31 according to the present embodiment is used in the detector 10 according to the first embodiment, the photodetector 32 is disposed on the opposite side of the metal film 41 from the side on which incident light is incident. It is preferable. Thereby, the chirality of the spiral groove 42 is set to the second order or more, the diameter of the opening 43 is larger than the diameter corresponding to the cutoff frequency of the light in the first order mode, and corresponds to the cutoff frequency of the light in the third order mode. If the diameter is set to be smaller than the diameter, the opening 43 removes light in a mode derived from circularly polarized light (right-handed circularly polarized light or left-handed circularly polarized light) higher-ordered by the spiral groove 42, and (1) remaining The light intensity of the mode derived from the circularly polarized light and the light of the mode derived from the linearly polarized light or (2) the intensity of the light derived from the mode derived from the linearly polarized light can be detected by the photodetector 32.

  (1) The intensity of the combined light of the mode derived from the remaining circularly polarized light and the light of the mode derived from linearly polarized light is in contrast to the change in the phase difference between the light modulation waves via the paths 15 and 16. As shown in FIG. Therefore, when the optical modulation wave received by the detector 10 is multiphased to a phase larger than two phases, the detected light intensity (photodetector) with respect to the change in the phase difference between the optical modulation waves via the paths 15 and 16 The rate of change of the light intensity detected at 32 is larger than the rate of change of the intensity of the superimposed wave with respect to the change in the phase difference between the light modulated waves via the two paths in the conventional detector 100. Therefore, when the optical modulation wave received by the detector 10 is multiphased to a phase larger than two phases, the detector 10 can suppress a decrease in resolution as compared with the conventional detector 100, and a bit error can be prevented. Generation can be suppressed.

  In addition, when the intensity of the light of the mode derived from the above (2) linearly polarized light is detected after separating the two linearly polarized light components orthogonal to each other, the intensity of each linearly polarized light component passes through the paths 15 and 16. It changes as shown in FIG. 12 with respect to the change of the phase difference between the modulated optical waves. Therefore, when the optical modulation wave received by the detector 10 is multiphased to a phase larger than two phases, the detected light intensity (photodetector) with respect to the change in the phase difference between the optical modulation waves via the paths 15 and 16 The rate of change of the light intensity detected at 32 is larger than the rate of change of the intensity of the superimposed wave with respect to the change in the phase difference between the light modulated waves via the two paths in the conventional detector 100. Therefore, when the optical modulation wave received by the detector 10 is multiphased to a phase larger than two phases, the detector 10 can suppress a decrease in resolution as compared with the conventional detector 100, and a bit error can be prevented. Generation can be suppressed.

  In order to improve the detection accuracy, the photodetector 32 is more preferably disposed immediately below the end (exit) on the side opposite to the side on which the incident light is incident as shown in FIG. .

  Hereinafter, in the detector 10 which concerns on Embodiment 1, the specific example at the time of using the polarization control optical component 31 which concerns on this embodiment is demonstrated as Examples 1-3. In this case, the superimposed wave condensed by the condenser lens 2 is incident on the polarization control optical component 31 as the incident light.

Example 1
Here, based on FIG. 8, 11, 14, and Table 2, the Example regarding the digital data restoration method of the detector in one Embodiment of this invention is demonstrated. Here, a polarization control optical component 31 having secondary clockwise and counterclockwise spiral grooves (σ _s = + 2, -2) and phase shifters of 0.25π, 0.5π, and 0.75π are used. A detection method in which light detection is performed by the photodetector 32 disposed immediately below the opening 43 as shown in FIG.

  First, the relationship between the cutoff frequency related to the polarization control optical component 31 and the mode used in this embodiment will be described with reference to FIG. FIG. 8 is a back view showing an embodiment of the polarization control optical component in the present invention.

In the polarization control optical component 31 having the second-order clockwise spiral groove (σ _s = + 2), the cutoff frequency is located between the frequencies of the first-order mode and the third-order mode, that is, the aperture 43 The radius r _{0 of the} opening 43 is adjusted so that the diameter is larger than the diameter corresponding to the cutoff frequency of the first-order mode light and smaller than the diameter corresponding to the cutoff frequency of the third-order mode light. In this case, as shown in FIG. 8, when the photodetector 32 is arranged immediately below the opening 43, the clockwise third-order mode is filtered by the opening 43 and is not detected by the photodetector 32. That is, the intensity of light detected by the photodetector 32 in the clockwise third-order mode is zero.

Next, based on FIG. 11, the detected signal phase difference (phase difference between adjacent bits) when using the polarization control optical component 31 having the second-order clockwise and counterclockwise spiral grooves (σ _s = + 2). ) And the light intensity will be described. FIG. 11 is a graph showing the relationship between the light intensity related to the detector 10 and the phase difference of the digital signal.

FIG. 11A shows the relationship between the intensity of light and the phase difference of the detected signal when the polarization control optical component 31 having a secondary clockwise spiral groove (σ _s = + 2) is used. On the other hand, FIG. 11B shows the relationship between the light intensity and the phase difference of the detected signal when the polarization control optical component 31 having the second-order counterclockwise spiral groove (σ _s = −2) is used. ing.

  FIG. 11 shows the normalized light output as a function of the phase difference φ. As shown in FIG. 11, the phase difference of 0.5π (when the polarization control optical component 31 having a second-order clockwise spiral groove is used) or 1.5π (2 In the case of using the polarization control optical component 31 having the next left-handed spiral groove), the light intensity becomes zero. On the other hand, in the case of linearly polarized light, counterclockwise circularly polarized light, and elliptically polarized light, the light intensity is ideally 1.

  Here, based on Table 3, a digital data restoration method in the present embodiment will be described.

  In the present embodiment, a case where 8-phase phase modulation waves are detected will be described. In the detector 10 of this embodiment, the phase polarization converter 1 of FIG. 1 and the phase shifters of 0.25π, 0.5π, and 0.75π are added to the phase polarization converter 1 of FIG. However, there are eight types of detection systems in which a total of four types of phase polarization conversion units and two types of polarization control optical components 31 having secondary clockwise and counterclockwise spiral grooves are combined. Table 2 shows the relationship between the types of elements (polarization control optical component 31 and phase shifter) used in each detection system and the phase difference of signals detected by the photodetector 32 of each detection system. It is.

In this embodiment, in the detection system using the phase polarization conversion unit 1 of FIG. 1 and the polarization control optical component 31 having the secondary clockwise spiral groove (σ _s = + 2), the signal phase difference is 0. When .5π, clockwise circularly polarized light is not detected due to the cutoff. That is, the intensity of the detected light is zero. On the other hand, in the detection system using the phase polarization converter 1 of FIG. 1 and the polarization control optical component 31 having the secondary counterclockwise spiral groove (σ _s = + 2), the signal phase difference is 1.5π. The counterclockwise circularly polarized light is not detected by the cut-off. That is, the intensity of the detected light is zero. Here, the point where the intensity of the two lights becomes 0 is used.

  As shown in Table 3, signals having a phase difference of 0.5π and 1.5π can be detected by detecting a signal whose light intensity is 0 in a detection system that does not use a phase shifter. The digital signals corresponding to the phase differences of 0.5π and 1.5π are 010 and 110, respectively. Next, as shown in Table 3, signals having a phase difference of 0.75π and 1.75π can be detected by detecting a signal in which the light intensity becomes 0 in a detection system to which a phase shifter of 0.25π is added. . Similarly, other phase difference signals of π, 0, 1.25π, and 0.25π are detected by a detection system to which phase shifters of 0.5π and 0.75π are added as shown in Table 3. It can be detected by detecting a signal whose light intensity is zero.

  Here, based on FIGS. 11 and 14, the resolution of the signal when the detector 10 of the present embodiment and the conventional detector 100 are used will be compared. In FIG. 11 and FIG. 14, when a light intensity difference δI is compared between a detected signal and a phase signal adjacent thereto (a signal having a phase difference of 0.25π from the detected signal), a detector is obtained. 10 and the conventional detector 100 are 0.29 and 0.15, respectively. Then, it can be seen that the detector 10 has a resolution approximately twice that of the conventional detector 100. That is, the detector 10 according to the present embodiment can detect with higher resolution than the conventional detector 100.

  The method for restoring digital data in the case of two phases and four phases will be described as follows.

First, in the case of a two-phase signal, a polarization control optical component 31 having secondary clockwise and counterclockwise spiral grooves (σ _s = + 2, −2) and a phase shifter of 0.5π are shown in FIG. A detection method in which light detection is performed by the light detector 32 arranged immediately below the opening 43 will be described.
If detection is performed by adding a phase shifter of 0.5π to a detection system using a second-order clockwise spiral groove (σ _s = + 2), the light intensity of the detection system is 0 when the phase difference φ = 0. become. Therefore, if a signal in which the intensity of light detected by the photodetector 32 is detected in this detection system is detected, a signal having a phase difference φ = 0 can be detected. Since the original digital data of the signal having the phase difference φ = 0 is 0, the digital data “0” is restored when the intensity of light detected by the photodetector 32 becomes 0.

Further, if a detection system using a polarization control optical component 31 having a secondary left-handed spiral groove (σ _s = −2) is added and detected by adding a phase shifter of 0.5π, the phase difference φ = π At this time, the light intensity of the detection system becomes zero. Therefore, if a signal in which the intensity of light detected by the photodetector 32 is detected in this detection system is detected, a signal having a phase difference φ = π can be detected. Since the original digital data of the signal having the phase difference φ = π is 1, when the light intensity detected by the photodetector 32 becomes 0, the digital data “1” is restored.

Next, in the case of a four-phase signal, if a detection system using the polarization control optical component 31 having secondary clockwise and counterclockwise spiral grooves (σ _s = + 2, −2) is used, the phase difference φ = It is possible to detect a signal of 0.5π (restoring digital data “01”) and a signal of phase difference φ = 1.5π (restoring digital data “11”). Further, if a phase shifter of 0.5π is added to the detection system using the polarization control optical component 31 having the secondary clockwise and counterclockwise spiral grooves (σ _s = + 2, −2), the case of a two-phase signal Similarly to the above, it is possible to detect a signal having a phase difference φ = 0 (restoring digital data “00”) and a signal having a phase difference φ = π (restoring digital data “10”).

(Example 2)
Next, an example relating to the digital data restoration method of the detector 10 according to the embodiment of the present invention will be described with reference to FIGS. Here, a polarization control optical component 31 having a secondary clockwise spiral groove (σ _s = + 4) and a phase shifter of 0.25π, 0.5π, and 0.75π are used to separate orthogonal linear polarization components. The detection method to detect will be described.

  With the arrangement of the photodetectors as shown in FIG. 8, the linearly polarized light components included in the superimposed wave that are orthogonal to each other cannot be separated and detected. Therefore, in this embodiment, in order to separate and detect two orthogonally polarized light components included in the superimposed wave, the A axes orthogonal to each other, such as a polarizing prism 90 as shown in FIG. The polarization separation means for separating the linearly polarized light component in the direction and the B axis direction is disposed below the opening 43, and the photodetector 32A and the photodetector 32B are separated in the A axis direction and the B axis direction separated by the polarization separation means. The linearly polarized light components are arranged at the positions where the linearly polarized light components are incident, and the linearly polarized light components in the A-axis direction and the B-axis direction are observed by the photodetectors 32A and 32B.

  First, the relationship between the cutoff frequency related to the polarization control optical component 31 and the mode used in this embodiment will be described.

In the polarization control optical component 31 having the fourth-order clockwise spiral groove (σ _s = + 4), the cut-off frequency is located between the frequencies of the first-order mode and the third-order mode, that is, the aperture 43 The radius r _{0 of the} opening 43 is adjusted so that the diameter is larger than the diameter corresponding to the cutoff frequency of the first-order mode light and smaller than the diameter corresponding to the cutoff frequency of the third-order mode light. In this case, even when the photodetector 32 is arranged immediately below the opening 43 as shown in FIG. 8 or when the photodetectors 32A and 32B are arranged as shown in FIG. This mode is not detected by the photodetector 32 or the photodetectors 32A and 32B. That is, light in the clockwise third-order and fifth-order modes derived from circularly polarized light is not detected, and only the first-order mode derived from linearly polarized light is detected.

Based on FIGS. 12 and 13, the relationship between the detected phase difference and the light intensity when using the polarization control optical component 31 having the fourth-order clockwise spiral groove (σ _s = + 4) will be described. 12 and 13 are graphs showing the relationship between the light intensity and the phase difference of the digital signal related to the detector 10 and the phase-modulated optical communication system 20. 12 and 13 show the normalized light output as a function of the phase difference φ.

FIG. 12A shows the intensity of light detected by the photodetector 32A in the A-axis direction using the polarization control optical component 31 having a fourth-order clockwise spiral groove (σ _s = + 4). The relationship with the phase difference of a signal is shown. On the other hand, FIG. 12B shows the intensity and detection of light when detected by the photodetector 32B in the B-axis direction using the polarization control optical component 31 having the fourth-order left-handed spiral groove (σ _s = + 4). The relationship with the phase difference of the signal to be shown is shown.

  FIG. 13 is a sum of the light intensities shown in FIGS. 12 (a) and 12 (b). As shown in FIG. 13, the intensity becomes 0 at the phase differences of 0.5π and 1.5π in which the superimposed wave is completely circularly polarized.

  Here, based on Table 4, a digital data restoration method in the present embodiment will be described.

  In the present embodiment, as in the first embodiment, a case where 8-phase phase modulation is performed will be described. Table 2 shows the relationship between the elements used in this example and the detected phase difference.

In the present embodiment, when the polarization control optical component 31 having the fourth-order clockwise spiral groove (σ _s = + 4) is used, the above-mentioned cut-off causes the phase difference of 0.5π and 1.5π. Rotating and clockwise circularly polarized light is not detected. That is, the intensity of the detected light is zero. On the other hand, the primary mode derived from linearly polarized light is detected by the photodetector 32A in the A-axis direction when the polarization axis is A-axis, and by the photodetector 32B in the B-axis direction when the B-axis is. Of the first-order modes derived from linearly polarized light, the intensity of light whose polarization axis is the A axis is ideally 1 with a phase difference π, and the intensity of light whose polarization axis is the B axis is ideally no phase difference 0. It becomes 1. Here, the point where the intensity of the two lights becomes 1 is used.

  As shown in Table 4, a signal having a phase difference of 0 and π can be detected by detecting a signal whose light intensity is 1 in a detection system that does not use a phase shifter. The digital signals corresponding to the phase differences of 0 and π are 000 and 100, respectively. Next, as shown in Table 4, when detecting signals having a phase difference of 1.25π and 0.25π, a detection system to which a phase shifter of 0.25π is added is used in this embodiment. A signal having a phase difference of 1.25π and 0.25π can be detected by detecting a signal whose light intensity is 1 in a detection system to which a phase shifter of 0.25π is added. Similarly, to detect signals with other phase differences of 1.5π, 0.5π, 1.75π, and 0.75π, as shown in Table 4, 0.5π and 0.75π phase shifters are used. What is necessary is just to detect the signal in which the light intensity becomes 1 in the detection system to which is added.

  Here, based on FIG. 13 and FIG. 14, the resolution of the signal when the detector 10 of the present embodiment and the conventional detector 100 are used will be compared. Comparing the light intensity difference δI in FIGS. 13 and 14, the detector 10 and the conventional detector 100 are 0.71 and 0.15, respectively. Then, it can be seen that the resolution of the detector 10 is about 4.7 times that of the conventional detector 100. That is, the detector 10 according to the present embodiment can detect with higher resolution than the conventional detector 100. In addition, when the phase fluctuation in a transmission line is small, it can apply also to further multiphase.

  The method for restoring digital data in the case of two phases and four phases will be described as follows.

  First, in the case of a two-phase signal, when the phase difference φ = 0, the light is linearly polarized with the B axis as the polarization axis, and the intensity of light detected by the photodetector 32B is 1. Therefore, if a signal in which the light intensity detected by the photodetector 32B is 1 is detected, a signal having a phase difference φ = 0 can be detected. Since the original digital data of the signal having the phase difference φ = 0 is 0, when the intensity of light detected by the photodetector 32B becomes 1, the digital data “0” is restored. Further, when the phase difference φ = π, the light is linearly polarized with the A axis as the polarization axis, and the intensity of the light detected by the photodetector 32A is 1. Therefore, if a signal whose light intensity detected by the photodetector 32A is 1 is detected, a signal having a phase difference φ = π can be detected. Since the original digital data of the signal having the phase difference φ = π is 1, when the intensity of the light detected by the photodetector 32A becomes 1, the digital data “1” is restored. In this way, the digital data of the two-phase signal can be restored.

  Next, in the case of a four-phase signal, when the phase difference φ = 0, the light is linearly polarized with the B axis as the polarization axis, and the intensity of the light detected by the photodetector 32B is 1. Therefore, if a signal in which the light intensity detected by the photodetector 32B is 1 is detected, a signal having a phase difference φ = 0 can be detected. Since the original digital data of the signal having the phase difference φ = 0 is 00, the digital data “00” is restored when the intensity of light detected by the photodetector 32B becomes 1. Further, when the phase difference φ = π, the light is linearly polarized with the A axis as the polarization axis, and the intensity of the light detected by the photodetector 32A is 1. Therefore, if a signal whose light intensity detected by the photodetector 32A is 1 is detected, a signal having a phase difference φ = π can be detected. Since the original digital data of the signal having the phase difference φ = π is 10, when the intensity of light detected by the photodetector 32A becomes 1, the digital data “10” is restored.

  In the case of a four-phase signal, in order to detect light having a phase difference φ = 0.5π and a phase difference φ = 1.5π, a detection system with a 0.5π phase shifter is added to the same detection system. It ’s fine. When the phase difference φ = 0.5π, the light is linearly polarized with the B axis as the polarization axis, and the intensity of the light detected by the photodetector 32B is 1. Therefore, if a signal in which the intensity of light detected by the photodetector 32B is 1 is detected by a detection system to which a phase shifter of 0.5π is added, a signal having a phase difference φ = 0.5π can be detected. Since the original digital data of the signal having the phase difference φ = 0.5π is 01, when the intensity of light detected by the photodetector 32B becomes 1 in the detection system to which the phase shifter of 0.5π is added. Restores the digital data “01”. Further, when the phase difference φ = 1.5π, the light is linearly polarized with the A axis as the polarization axis, and the light intensity is 1. Therefore, if a signal in which the light intensity detected by the photodetector 32A is 1 is detected by a detection system to which a phase shifter of 0.5π is added, a signal having a phase difference φ = 1.5π can be detected. Since the original digital data of the signal having the phase difference φ = 1.5π is 11, when the light intensity detected by the photodetector 32A becomes 1 in the detection system to which the phase shifter of 0.5π is added. Restores the digital data “11”.

In this embodiment, the polarization control optical component 31 having a fourth-order clockwise spiral groove (σ _s = + 4) is used, but it has a fourth-order counterclockwise spiral groove (σ _s = −4). Even if the polarization control optical component 31 is used, the same effect can be obtained. In addition, the polarization control optical component 31 having a fifth or higher order n-order clockwise or counterclockwise spiral groove is used, and corresponds to the cutoff frequency of light in a mode derived from linearly polarized light (0th-order mode or first-order mode). Even if the diameter of the opening 43 is adjusted so as to be smaller than the diameter corresponding to the cutoff frequency of the light of the (n−1) -order mode derived from circularly polarized light and larger than the diameter to be obtained, the same effect can be obtained. . Furthermore, using the polarization control optical component 31 having a third-order clockwise or counterclockwise spiral groove, the diameter is larger than the diameter corresponding to the cutoff frequency of the zero-order mode light derived from the linearly polarized light, and is circularly polarized. Even if the diameter of the opening 43 is adjusted so as to be smaller than the diameter corresponding to the cutoff frequency of the light of the second-order mode that is derived, the effect of improving the resolution can be obtained. However, since the cutoff frequency of the zero-order mode light and the cutoff frequency of the second-order mode light are close, the second-order mode light derived from the optical circularly polarized light is removed by the opening 43 and the light is derived from the linearly polarized light. It is relatively difficult to extract only the 0th mode. Therefore, it is more preferable to use the polarization control optical component 31 having a fourth-order or higher spiral groove in that the mode 43 derived from the optically circularly polarized light can be easily removed by the opening 43.

(Example 3)
By the way, it is generally known that the pattern of electric dipole radiation of the wavelength of light in vacuum is not isotropic. The primary mode derived from the linearly polarized light behaves as an electric dipole at the exit of the opening 43 of the polarization control optical component 31. FIG. 10 shows a structure in which two photodetectors 32 </ b> A and 32 </ b> B are arranged on the A axis and the B axis below the opening 43 of the polarization control optical component 31. In the present embodiment, the photodetector 32A is disposed on one polarization axis (A axis) of two linearly polarized light components orthogonal to each other included in the superimposed wave, and the photodetector 32B is disposed on the superimposed wave. It is arranged on the other polarization axis (B axis) of two linearly polarized light components orthogonal to each other included in the combined wave.

  Here, the linearly polarized light formed in the opening 43 of the polarization control optical component 31 is parallel to the A axis, and the photodetectors 32A and 32B are in contact with the metal film 41 of the polarization control optical component 31, or Consider a case where the distance (interval) from the metal film 41 is less than or equal to the wavelength of light in vacuum. In this case, surface plasmons propagating on the surface (hereinafter referred to as “back”) of the metal film 41 on the radiation side of the polarization control optical component 31 are predominantly observed, and the intensity of the photodetector 32A on the A axis increases. . Similarly, the linearly polarized light formed in the opening 43 of the polarization control optical component 31 is parallel to the B axis, and the photodetectors 32A and 32B are in contact with the metal film 41 of the polarization control optical component 31, or the metal film When the distance (interval) from 41 is equal to or less than the wavelength of light in vacuum, the intensity of the photodetector 32A on the A axis increases.

  On the other hand, when the linearly polarized light formed in the opening 43 of the polarization control optical component 31 is parallel to the A axis and the photodetectors 32A and 32B are sufficiently separated from the metal film 41 and the opening 43 of the polarization control optical component 31 The intensity of the photodetector 32 on the B axis increases according to the electric dipole radiation pattern. Similarly, when the linearly polarized light formed in the opening 43 of the polarization control optical component 31 is parallel to the B axis and the photodetectors 32A and 32B are sufficiently separated from the metal film 41 and the opening 43 of the polarization control optical component 31, The intensity of the photodetector 32 on the B axis increases according to the electric dipole radiation pattern. When the photodetectors 32A and 32B are in contact with the metal film 41 of the polarization control optical component 31, or the distance (interval) from the metal film 41 is equal to or less than the wavelength of light in vacuum, the light detection on the A axis The strength of the container 32A increases.

  Therefore, in this embodiment, in the detector 10 of the second embodiment, the polarizing prism 90 is omitted, and the photodetector 32A and the photodetector 32B are (1) the position in contact with the metal film 41 or the distance from the metal film 41. Is disposed at a position that is equal to or less than the wavelength of light in vacuum, or (2) is disposed at a position sufficiently separated from the metal film 41 and the opening 43. In the case of (1), the linearly polarized light parallel to the A axis is mainly detected by the photodetector 32A on the A axis, and the linearly polarized light parallel to the B axis is mainly detected by the photodetector 32A on the B axis. . In the case of (2), the linearly polarized light parallel to the B axis is mainly detected by the photodetector 32A on the A axis, and the linearly polarized light parallel to the A axis is mainly detected by the photodetector 32A on the B axis. .

  In this way, even if the polarizing prism 90 is not used, it is possible to separate and observe orthogonal linear components using two or more photodetectors 32.

  As described above, the polarization control optical component 31 in the present embodiment can change the chirality of the polarization component included in the incident light by the spiral groove 42. Thereby, for example, part or all of the circularly polarized light component can be made higher order, and the propagation loss at the opening 43 can be remarkably increased. Therefore, a part or all of the light of the mode having higher order chirality derived from the circularly polarized light component can be removed by the opening 43. By removing a part or all of the circularly polarized light component in this way, a part of the polarized light component from the incident light, for example, (1) only the linearly polarized light component, (2) only a part of the circularly polarized light component, (3 ) A part of linearly polarized light component and circularly polarized light component can be taken out. As a result, processing based on some polarization components included in the incident light, for example, detection based on the intensity of some polarization components included in the incident light is possible. Performing detection based on the intensity of some polarization components included in the incident light contributes to improvement in signal resolution as described above.

  In the detector of the present embodiment, the polarization state of the superimposed wave is indirectly detected by detecting the light intensity after removing at least a part of the circularly polarized component from the superimposed wave. It is good also as a structure which detects using the photodetector which can detect the polarization state of a superposition wave directly.

[Embodiment 3]
The following will describe still another embodiment of the present invention with reference to FIG. Configurations other than those described in the present embodiment are the same as those in the first and second embodiments. For convenience of explanation, members having the same functions as those shown in the drawings of Embodiments 1 and 2 are given the same reference numerals, and explanation thereof is omitted.

  First, the configuration of the phase modulation optical communication system 20 according to the present embodiment will be described with reference to FIG. FIG. 2 is a block diagram showing a configuration of the phase modulation optical communication system 20.

  As shown in FIG. 2, the phase modulation optical communication system 20 according to the present embodiment includes an optical modulation wave generator 21, a polarization maintaining optical fiber (polarization maintaining optical transmission medium) 22, and the detector 10 according to the first embodiment. It is composed of Next, each component will be described with reference to FIG.

  First, the optical modulation wave generator 21 includes a laser light source 23 and a phase modulator 24 as shown in FIG. The laser light source 23 emits a linearly polarized optical carrier wave. The phase modulator 24 modulates the phase of the linearly polarized light based on the input digital data to generate a linearly polarized light modulation wave. As a phase modulation method in the phase modulator 24, a known phase modulation method can be used.

  Next, the polarization maintaining optical fiber 22 transmits the linearly polarized light modulation wave generated by the light modulation wave generator 21 to the detector 10 while maintaining the polarization. As the polarization maintaining optical fiber 22, a known polarization maintaining optical fiber can be used.

  Finally, the detector 10 detects the optical modulation wave transmitted by the polarization maintaining optical fiber 22 and restores the digital data. The details of the detector 10 are as described in the first embodiment. The example of the polarization control optical component 31 that is preferably used for the detector 10 and the detection method of the detector 10 are also as described in the second embodiment.

  Next, the operation of the phase modulation optical communication system 20 in the present embodiment will be described based on FIG.

  First, digital data to be transmitted is input to the phase modulator 24. Next, linearly polarized light is emitted from the laser light source 23. Then, the linearly polarized light emitted from the laser light source 23 undergoes phase modulation according to the digital data by the phase modulator 24 and is emitted as a light modulation wave.

  Thereafter, the modulated optical wave emitted from the phase modulator 24 is transmitted by the polarization maintaining optical fiber 22. Then, the modulated optical wave transmitted by the polarization maintaining optical fiber 22 is input to the detector 10.

  The optical modulation wave input to the detector 32 is restored to digital data by the detector 32 and output to the outside as described in the first embodiment. As a result, in the phase modulation optical communication system 20, the digital data input to the optical modulation wave generator 21 is transmitted to the output destination of the detector 32 by optical communication via the polarization maintaining optical fiber 22.

  As described above, the phase-modulated optical communication system 20 according to the present embodiment includes the detector 10 that converts and detects information based on the phase difference of the optically modulated wave into a polarization state corresponding to the information. Compared with a conventional detector that detects the intensity of the light, the rate of change of the detection target (polarization state) with respect to the change of the phase difference can be increased. Therefore, since the resolution of detection can be improved, it is possible to reduce the influence of adjacent phases in multiphase phase modulation coherent optical communication and facilitate multiphase.

  The phase modulation optical communication system 20 according to the present embodiment can be suitably used for intra-station transmission of digital data.

  As described above, the detector according to the present invention applies 90-degree deflection rotation to the delay detection system, and reads out phase information of the optical modulation wave based on the polarization state obtained by superimposing the modulation wave and the delay wave. It is. The superposed wave has two degrees of freedom (chirality), clockwise and counterclockwise, except for linearly polarized light. In the present invention, a high-resolution observation of a signal is provided by using a method for nonlinearly observing the polarization state of the superimposed wave.

  As described above, in the detector of the present invention, it is preferable that the deflection control optical component including the opening penetrating the metal film capable of generating surface plasmon and the secondary or quaternary spiral groove surrounding the opening, After changing the chirality of the superimposed wave, the polarization state of the superimposed wave is observed. When the chirality of the circularly polarized light component and the chirality of the spiral groove have different signs, the chirality of the output light of the spiral groove derived from the circularly polarized light component is one less than the chirality of the spiral groove. On the other hand, when the chirality of the circular polarization component and the chirality of the spiral groove have the same sign, the chirality of the output light of the spiral groove derived from the circular polarization component is one more than the chirality of the spiral groove. A mode having higher-order chirality generated from the circularly polarized light component generated at the entrance of the opening by such a spiral groove has a significantly large propagation loss in the opening. Therefore, the circularly polarized light component can be removed from the output from the aperture exit. Furthermore, since the output from the aperture has a specific radiation pattern, orthogonally polarized light can be separated and observed by a spatially separated optical detector. Accordingly, even when the phase difference from the adjacent phase is reduced, it is possible to suppress a decrease in the resolution of the signal and to facilitate multiphase.

  INDUSTRIAL APPLICABILITY The present invention uses a deflection control optical component that adds optical chirality, facilitates detection in multiphase phase modulation optical coherent communication, improves the reliability of optical communication, and reduces the size of a detector. Introduction can be facilitated.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the technical means disclosed in different embodiments can be appropriately combined. Such embodiments are also included in the technical scope of the present invention.

  The detector, polarization control optical component, and phase modulation optical communication system of the present invention can be applied to detection in coherent optical communication. Specifically, in intra-station coherent optical communication using a polarization-maintaining optical fiber, a plurality of optical phase modulations with low spectrum occupancy, which is the ratio of the occupied frequency bandwidth of each signal used in optical multiplex communication, to the entire communication bandwidth It can be used for optical multiplex communication that uses wavelengths. The polarization control optical component of the present invention can be used as a near-field generating light source with controlled chirality by combining with an appropriate light source.

It is a block diagram which shows the structure of one Embodiment of the detector in this invention. It is a block diagram which shows the structure of one Embodiment of the phase modulation optical communication system in this invention. It is a wave form diagram which shows the example of the digital signal in the said detector. It is a perspective view which shows the structure of one Embodiment of the polarization control optical component in this invention. It is a perspective view which shows other embodiment of the polarization control optical component in this invention. It is a top view which shows one Embodiment of the polarization control optical component in this invention. It is a conceptual diagram which shows the instantaneous in-plane electric field vector in the periphery of the opening in the said polarization control optical component. It is a back view which shows one Embodiment of the polarization control optical component in this invention. It is a perspective view which shows an example of the arrangement method of the said polarization control optical component and the photodetector regarding this invention. It is a perspective view which shows another example of the arrangement method of the said polarization control optical component and the photodetector regarding this invention. It is a graph which shows the relationship between the intensity | strength of light regarding the said detector, and the phase difference of a digital signal. It is a graph which shows the relationship between the intensity | strength of light regarding the said detector, and the phase difference of a digital signal. It is a graph which shows the relationship between the intensity | strength of light regarding the said detector, and the phase difference of a digital signal. It is a graph which shows the relationship between the intensity | strength of light regarding the conventional detector, and the phase difference of a digital signal. It is a block diagram which shows the structure of the conventional detector.

Explanation of symbols

1 Phase polarization converter (phase polarization converter)
3 Polarization detection unit (polarization detection means)
4 Decoder (restoration means)
10 Detector 11 Beam splitter (optical branching means)
12 1-bit delay device (delay means)
13 Retardation plate (polarization rotating means)
14 Optical coupler (superposition means)
15 route (second route)
16 route (first route)
21 optical modulation wave generator 22 polarization maintaining optical fiber (polarization maintaining optical transmission medium)
31 Polarization control optical component (polarization control means)
32 photodetector (light detection means)
32A photodetector (first photodetector)
32B photodetector (second photodetector)
41 Metal film 42 Spiral groove 43 Opening (circular polarization component removing means)
44 Base point 90 Polarizing prism (polarization separating means)

Claims (13)

A metal film,
An opening penetrating the metal film;
A polarization control optical component having a spiral groove formed in a metal film with the end of the opening or the vicinity thereof as a base point and the center of the opening as a center,
The spiral groove is a groove that monotonously increases or monotonously decreases as the azimuth angle with respect to the center of the opening increases from the opening, and monotonously increases as the distance from the opening increases from the opening, and a polarization component included in incident light. A polarization control optical component characterized by changing the chirality of the light. The spiral groove is
The polarization control optical component according to claim 1, wherein the polarization control optical component is a plurality of spiral grooves respectively formed with a plurality of different positions as base points. The spiral groove is
The polarization control optical component according to claim 1, wherein the polarization control optical component is four or more spiral grooves respectively formed with four or more different positions as base points.   The distance between the base point and the opening in the spiral groove is shorter than the wavelength of a surface plasmon excited near the opening by the incident light. Polarization control optics. A detector that detects a linearly polarized light modulated wave that is phase-modulated according to digital data,
The linearly polarized light modulation wave is received and branched into a first path and a second path, a delay of one bit of the digital data is given to the light modulation wave of the first path, and the first path A polarization axis angle difference of 90 degrees is given between the light modulation wave of the path and the light modulation wave of the second path, and the light modulation waves passing through the first and second paths are superimposed, Phase polarization conversion means for converting into a superposed wave having a polarization state that changes in accordance with the phase difference between the optically modulated waves that have passed through the first and second paths;
Receiving the superimposed wave from the phase polarization conversion means, and having a polarization detection means for converting the polarization state into an electric signal,
The superimposed wave includes at least one of a circularly polarized light component and a linearly polarized light component according to a phase difference between the light modulated waves that have passed through the first and second paths,
The incident light is the superimposed wave,
The polarization detection means is
The polarization control optical component according to any one of claims 1 to 4,
And a light detecting means for detecting the intensity of the scattered light emitted from the polarization control optical component and converting it into an electric signal. The phase polarization conversion means includes
Light branching means for branching the linearly polarized light-modulated wave into the first and second paths;
Delay means for delaying the phase of the optically modulated wave of the first path branched by the optical branching means by one bit of the digital data;
Polarization rotation means for rotating the polarization axis of the light modulated wave of the first or second path branched by the branch means by 90 degrees;
Superimposing the modulated light waves that have passed through the first and second paths into a superimposed wave having a polarization state that changes according to the phase difference between the modulated light waves that have passed through the first and second paths. The detector according to claim 5, further comprising superimposing means for performing the operation. The light detection means includes
The detector according to claim 5, wherein the detector is disposed on a side opposite to a side on which the superimposed wave is incident in the polarization control optical component.   8. The polarization control optical component according to claim 5, wherein at least one of a counterclockwise circularly polarized light component included in the superimposed wave is removed and led to the light detection means. The detector according to item 1. The spiral groove increases the chirality of at least one of the counterclockwise and clockwise circularly polarized light components included in the incident light,
The opening is larger than the cut-off diameter of the linearly polarized light component so as to remove the circularly polarized component whose degree of chirality is enhanced by the spiral groove, and the circularly polarized component whose degree of chirality is enhanced by the spiral groove The detector according to any one of claims 6 to 7, wherein the detector has a diameter set to be smaller than a cut-off diameter. The light detection means includes
10. The apparatus according to claim 5, further comprising first and second photodetectors respectively disposed on polarization axes of two linearly polarized light components orthogonal to each other included in the superimposed wave. The detector according to item. The light detection means includes
Polarization separating means for separating two linearly polarized light components orthogonal to each other included in the superimposed wave;
The first and second photodetectors respectively disposed at positions where the two linearly polarized light components separated by the polarization separating means are incident, respectively. The detector according to item.   12. A restoring means for restoring digital data by determining a phase difference between optically modulated waves passing through the first and second paths based on the electrical signal. The detector according to any one of the above. An optical modulation wave generator for phase-modulating linearly polarized light based on digital data to generate an optically modulated wave of linearly polarized light;
A polarization-maintaining optical transmission medium for transmitting the linearly polarized light modulated wave generated by the light modulated wave generator while maintaining the state of the linearly polarized light;
The detector according to any one of claims 5 to 12, which receives a linearly polarized light modulated wave transmitted by the polarization maintaining optical transmission medium and converts it into an electric signal. Phase modulation optical communication system.

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