JP5116619B2 - Optical communication system - Google Patents

Description

  The present invention relates to a coherent optical communication system having high interference resistance.

  An increase in demand for economical optical services using a PON (Passive Optical Network) system is expected. In the PON system, a single optical fiber and transmission equipment are shared by ONUs (Optical Network Units) of a plurality of user premises equipments under the control of OLT (Optical Line Terminal) in the station. With the spread of the PON system, the ONU may open the terminal according to the provisions of Article 49 of the Telecommunications Law. This release increases the possibility of erroneous connection to a PON system such as a media converter or a LAN device that does not follow the OLT control. The output light of these misconnected devices acts as interference light for communication of other users who share the optical fiber and the transmission equipment, and may cause a problem of communication interruption. In order to solve this problem, a method using an optical code division multiplexing (OCDM) technique has been proposed.

  One method to which the optical code multiplexing technique is applied is a method using a remover that selectively removes interfering light by interference between optical paths. In this method, signal light having an optical path length difference that can sufficiently remove interference light from a normal light source used in media converters, LAN devices, and the like is used, but the optical code does not depend on the optical frequency to be removed by the remover. The output of the multiplex signal light remover is constant, and the orthogonality between the signal lights is maintained.

  In the method using the remover, the pulse width is such that the coherency is low enough that interference does not occur with the optical path length difference of the remover, or the divided pulses do not overlap with each other with the optical path length difference of the remover. Is an optical code multiplexed signal light having an optical frequency width and a code that has a constant light regardless of the optical frequency to be removed and an orthogonal relationship between the codes and the optical frequency width. . Therefore, the interference light can be selectively removed without affecting the reception of the optical code multiplexed signal light.

  Another method using optical code multiplexing technology is to select an interfering light by providing an optical frequency chip where the interfering light and optical frequency overlap and an optical frequency chip necessary to maintain orthogonality between the optical code multiplexed signal light (For example, refer to Patent Documents 1 and 2).

On the other hand, a method using an optical frequency variation (chirp) of a direct modulation laser diode has been proposed (for example, Non-Patent Document 1). By applying an optical receiver that changes the optical frequency to be received in synchronization with optical frequency fluctuations and reducing the contribution of interfering light mixed in the received signal within one symbol time, the cost increase on the ONU side is suppressed. However, the interference light is removed regardless of the optical frequency of the interference light (for example, Non-Patent Document 1).
JP 2006-74557 A JP 2008-160707 A Yoshimoto, Naoto; Tsubokawa, Makoto, “Chirp Frequency Synchronized Detection for the Occupation of Light of the World.” 2008 OWB7

However, the conventional method has the following problems.
In the method using the remover to which the optical code multiplexing technique is applied and the method using the optical frequency chip, the optical frequency of the remover and the optical frequency chip to be removed must be matched with the optical frequency of the interference light. For this reason, there has been a problem that the communication is interrupted because the interference light cannot be removed after the occurrence of the interference light is detected until it matches the optical frequency of the interference light (Problem 1).
In addition, when adaptively removing interfering light, there is a problem that the control system becomes complicated because a feedback mechanism for sensing the optical frequency change of the interfering light and changing the optical frequency is necessary. (Problem 2).

In the method using the remover, the method of removing the interference light by using the time difference between the continuous emission of the interference light and the signal light has a problem that it is difficult to remove the interference light having a short pulse (problem) 3).
Further, when interference light is selectively removed by interference between optical paths, if light having a short coherence length is used, there is a problem that the signal-to-noise ratio is degraded due to noise of the signal light itself (Problem 4). ).

If the difference between the optical frequencies of the plurality of interfering lights is not an integral multiple of the interval between the optical frequencies removed by the remover, there is a possibility that all the interfering lights cannot be removed. When the optical frequency of the interfering light is an integral multiple of the period of the optical frequency removed by the remover, it is possible to simultaneously remove a plurality of interfering lights. However, it cannot be removed when the optical frequency of the interfering light is not an integral multiple of the period of the optical frequency removed by the remover. Therefore, when there are a plurality of interfering lights having different optical frequencies, a plurality of removers are required to remove all the interfering lights. As described above, in the method using the remover, the number of interference lights having different optical frequencies that can be removed is limited.
Similarly, even in a method that does not selectively receive optical frequency chips, the upper limit of the number of chips that are not selectively received is limited by the code length, and the number of interfering lights of different optical frequencies that can be removed is limited.
Therefore, when there are a plurality of interference light beams having different optical frequencies, the method using any of the optical code multiplexing techniques has a problem that the number of interference light beams that can be removed is limited (problem 5).

  In the method using the chirp of the directly modulated laser diode, the problems 1 to 5 of the method using the optical code multiplexing technique can be solved. However, the reception of interfering light is suppressed by selectively receiving the optical frequency by changing the optical frequency of the signal light with time. For this reason, there is a problem that the interference light cannot be removed when the optical frequency fluctuates with time similarly to the signal light and the optical frequency at each time coincides with the signal light by chance (problem). 6).

  In view of the above, an object of the present invention is to provide an optical communication system capable of solving the above-described problems and removing interference light regardless of the optical frequencies and pulse widths of signal light and interference light.

  In order to solve the above-described problems, an optical communication system according to the present invention uses a coherent optical communication system, and an optical phase change that is a natural number multiple of the period of the signal light within the same period within one symbol time modulated by transmission data. Transmission is performed in the same direction, and interference light is removed by making the average value of the reception output of interference light in one symbol time substantially zero at the time of reception.

Specifically, an optical communication system according to the present invention is an optical communication system that transmits signal light from an optical transmitter to an optical receiver, and the optical transmitter transmits continuous light having a predetermined optical frequency. A light source for output, data modulation means for modulating the intensity or optical phase of continuous light from the light source in accordance with transmission data, and the optical phase during one symbol time of modulation in the data modulation means, where n is a natural number the n period, and a transmission optical phase changing means for times and varied to substantially equal from the time the [pi to 2π from 0 to [pi, the modulated by the data modulation unit and the transmitting optical phase changing means A signal detector whose optical phase is changed by the optical receiver, and the optical receiver detects an optical signal obtained by combining the signal light transmitted from the optical transmitter and the local oscillation light, and outputs an electric signal; , Transmission of the optical transmitter In-phase detection means for detecting the output of light having the same period and a constant optical phase difference as the optical phase to be changed by the transmission light phase change means from the output of the transmitted signal light, and detecting by the optical detection means And receiving the light detected by the in-phase detection means.

Since the signal light and the local oscillation light are coherently received with a constant phase difference, the integrated value of the received signal within one symbol time is also a value corresponding to the symbol value. On the other hand, the phase difference between the interfering light and the local oscillation light changes by a natural number multiple of the period within one symbol time. For this reason, the integral value of the signal due to the interference light within one symbol time becomes zero. Thereby, the interference light by the output light of a misconnection apparatus can be removed. For this reason, even if the optical frequencies of the interference light and the signal light are the same, the output of coherent detection of the interference light within the symbol time becomes zero. Therefore, the problem 6 can be solved.
Since the interference light can be removed regardless of the optical frequency of the interference light, the problems 1, 2 and 5 can be solved. In addition, since the interference light can be removed regardless of the coherence length of the signal light or the short pulse width, the problems 3 and 4 can be solved.
Therefore, it is possible to provide an optical communication system capable of removing interference light regardless of the optical frequency and pulse width of signal light and interference light.

In the optical communication system according to the present invention, the in-phase detection means is a phase obtained by inverting the optical phase of the signal light from the optical transmitter and the optical phase changed by the transmission optical phase change means and the phase change with respect to time. a receiving optical phase change means in varying shall be the structure comprising an integrating envelope detection means for envelope detection in after the intermediate frequency component obtained by integrating one symbol time of the output electrical signal of said optical detection means . (First invention)

Alternatively, in the optical communication system according to the present invention, the in-phase detection means generates a detection signal generating means for generating an electric signal whose phase changes with the same period and a constant phase difference as the optical phase changed by the transmission light phase change means. When the intermediate frequency component of the electrical signal output from said light detecting means, it shall be the structure and a synchronous detection means for synchronously detecting an electrical signal from the detection signal generating means. (Second invention)

In the first invention, the received light phase changing means receives the signal light from the optical transmitter, and changes the optical phase of the input signal light to change the optical phase of the transmitted light phase changing means. An external optical phase modulator that changes the phase change with respect to time at an inverted phase can be provided. (Third invention)

In the first to third aspects of the invention, the transmission light phase changing unit receives the light from the light source or the data modulating unit, and changes the optical phase of the input light by a natural number of periods. It can be set as the structure which is a modulator. (Fourth invention)

In the first to third aspects of the invention, the data modulation means and the transmission light phase change means are integrated, and the integrated means intensity-modulates continuous light from the light source, and one symbol of intensity modulation. It is possible to adopt a configuration in which the optical phase is changed by a natural number of periods using the optical phase change that occurs during intensity modulation over time. (Fifth invention)

In the first to third aspects of the invention, the light source, the data modulation means, and the transmission light phase change means are integrated, and the integrated means generates light that is intensity-modulated by direct intensity modulation, A direct modulation laser that changes the optical phase by a natural number of periods by using the optical phase change generated along with the direct intensity modulation during one symbol time of the direct intensity modulation can be used. (Sixth invention)

In the sixth invention, the optical transmitter further includes transmission side dispersion means for giving a delay time different for each optical frequency to the light from the direct modulation laser, and the optical receiver includes the transmission side The apparatus may further comprise inverse dispersion means for providing the signal light from the optical transmitter with a delay time opposite to the delay time for each optical frequency provided by the dispersion means. (Seventh invention)

Alternatively, in the optical communication system according to the present invention, the light source, the data modulation unit, and the transmission light phase change unit are integrated, and the integrated unit generates light that is intensity-modulated by direct intensity modulation. , A direct modulation laser that changes an optical phase by a natural number of periods using an optical phase change generated in association with direct intensity modulation during one symbol time of direct intensity modulation, and the optical transmitter includes the direct transmitter Transmitting side dispersion means for giving different delay times for each optical frequency to the light from the modulated laser, and the optical receiver has the same delay time as the delay time for each optical frequency given by the transmission side dispersion means. It further comprises constituting a local oscillator light dispersion means for imparting to the local oscillator light. (Eighth invention)

Alternatively, in the optical communication system according to the present invention, the light source outputs a plurality of continuous lights having different optical frequencies, and the data modulation unit modulates the intensity or optical phase of the continuous lights for each optical frequency, The transmission optical phase changing means changes the optical phase for each optical frequency, and the optical transmitter is a signal having a plurality of optical frequencies modulated by the data modulating means and changed in optical phase by the transmission optical phase changing means. The light detecting means uses continuous light of each optical frequency output from the light source and light having a different optical frequency by a predetermined intermediate frequency as the local oscillation light, and transmits a plurality of light transmitted by the optical transmitter. The in-phase detection means detects the light combined with the optical frequency signal light and the local oscillation light, and the in-phase detection means changes the transmission light phase change means from a plurality of optical frequency signal lights transmitted by the optical transmitter. Let The light having the same period as the optical phase and having a constant optical phase difference is detected for each optical frequency, and the optical receiver detects the light detected by the optical detection means and detected by the in-phase detection means for each optical frequency. It is configured to receive . (9th invention)
Since the signal light is composed of a plurality of optical frequency chips, the contribution of interfering light having a single optical frequency can be reduced to a fraction of the number of optical frequency chips. That is, the interference light removal capability can be further improved by applying the optical code multiplexing technique.

  ADVANTAGE OF THE INVENTION According to this invention, the optical communication system which can remove interference light irrespective of the optical frequency and pulse width of signal light and interference light can be provided.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.

(Embodiment 1)
FIG. 1 is a schematic configuration diagram of an optical communication system according to the present embodiment. The optical communication system according to the present embodiment transmits signal light from the optical transmitter 10 to the optical receiver 20. In FIG. 1, only the optical transmitter 10 and the optical receiver 20 are shown, and an optical network composed of an optical fiber, an optical multiplexer, or the like that connects the optical transmitter 10 and the optical receiver 20 is omitted.

  The optical transmitter 10 includes a light source 11, a data modulation unit 12, and a transmission light phase change unit 13. The signal light modulated by the data modulation unit 12 and changed in optical phase by the transmission light phase change unit 13. Send.

  The light source 11 outputs continuous light having a predetermined optical frequency. The data modulation unit 12 modulates the intensity or optical phase of continuous light from the light source according to transmission data. The transmission optical phase changing unit 13 changes the optical phase by a natural number of the period during one symbol time of the modulation in the data modulating unit 12. Here, one symbol time is a time for modulation with one transmission data of a data series modulated by the data modulation means 12. One transmission data is, for example, 1-bit transmission data. The transmission light phase changing unit 13 includes, for example, an external optical phase modulator 13a that receives light from the data modulation unit 12 and changes the optical phase of the input light by a natural number of periods. The external optical phase modulator 13a is, for example, an LN external modulator.

  Although an example in which the light source 11, the data modulation unit 12, and the transmission light phase change unit 13 are arranged in this order has been described, the present invention is not limited to this. For example, the light source 11, the transmission light phase changing unit 13, and the data modulating unit 12 may be arranged in this order. Moreover, the predetermined optical frequency of the light source 11 may be single or plural. Since the light source 11 outputs light having a plurality of optical frequencies, removal of interfering light using an optical code multiplexing technique can be used in combination.

  The optical transmitter 10 modulates the light output from the light source 11 in accordance with the data series to be transmitted by the data modulation unit 12, and then phase-modulates the signal by the external optical phase modulator 13 a of the transmission optical phase change unit 13. Output as light. Here, the external optical phase modulator 13a performs optical phase modulation in accordance with a control signal from the signal source 13b. The control signal causes the data modulation means 12 to change the phase of the signal light approximately by a natural number during one symbol time.

  The signal light output from the optical transmitter 10 is input to the optical receiver 20 via the optical network. The optical receiver 20 generates a transmission light phase change from an output of the signal light transmitted from the local light source 21, optical multiplexing means 22, optical detection means 23, bandpass filter (BPF) 24, and optical transmitter 10. In-phase detection means for detecting the output of light having the same period and a constant optical phase difference as the optical phase to be changed by the means 13 is provided. The optical receiver 20 receives the light detected by the optical detection means 23 and detected by the in-phase detection means. Thereby, the optical receiver 20 can receive the signal light whose optical phase changes by the heterodyne envelope detection.

  In the present embodiment, the in-phase detection means includes a local oscillation light phase change means 31 and an integral envelope detection means 33. The local oscillation light phase changing means 31 receives the local oscillation light and changes the optical phase of the input local oscillation light with the same period and a constant optical phase difference as the optical phase changed by the transmission light phase changing means 13. An external optical phase modulator 31a is provided. In this case, the local oscillation light phase changing means 31 includes a signal source 31b, and the external optical phase modulator 31a changes the optical phase of the local oscillation light according to the signal from the signal source 31b.

  The local light source 21 outputs local oscillation light that interferes with signal light. The local oscillation light phase changing means 31 changes the optical phase of the local oscillation light with the same period and a constant optical phase difference as the optical phase changed by the transmission light phase changing means 13. The optical multiplexing unit 22 combines the local oscillation light and the signal light from the optical transmitter 10 via the local oscillation light phase changing unit 31 to cause the local oscillation light and the signal light to interfere with each other. The light detection means 23 is, for example, a photodiode, detects interference light of local oscillation light and signal light, photoelectrically converts it, and outputs an electric signal. The photoelectrically converted electrical signal is input to the BPF 24. The BPF 24 extracts a beat signal, which is an intermediate frequency between the local oscillation light and the signal light, from the electrical signal output from the optical detection unit 23 and inputs the beat signal to the integral envelope detection unit 33. The integral envelope detection means 33 performs envelope detection after integrating the intermediate frequency component of the electrical signal output from the optical detection means for one symbol time. The integral envelope detection means 33 includes an integral means and an envelope detection means. As the integration means, for example, there is an integrator combined with a delay line such as an optical or electrical stage transversal filter. When this type of integrator is used, the number of stages of the delay line is set to be equal to or more than the number of stages that change the optical phase by a natural number times one period in one symbol time of the phase changing means. In addition, when the signal light and the local oscillation light are combined by the optical multiplexing means 22, the polarization of both lights is combined, or the configuration of polarization diversity in which an intermediate frequency signal is output without depending on the polarization Yes.

  The optical receiver 20 of the present embodiment will be described on the assumption that signal light is received by heterodyne envelope detection. However, the optical receiver 20 may be a synchronous detection optical receiver having an optical phase locked loop, and the intermediate frequency may be zero. It may be an optical receiver for homodyne detection similar to In the case of homodyne detection, a phase diversity type optical receiver may be used, or synchronous detection may be used.

  Next, an example of the effect of this embodiment will be described with reference to FIG. FIG. 2 shows the light intensity and optical phase of the local oscillation light, signal light and interference light, (a) shows the light intensity of the local oscillation light, (b) shows the optical phase of the local oscillation light, (c ) Is the optical intensity of the transmitted signal light, (d) is the optical phase of the transmitted signal light, (e) is the optical intensity of the interfering light, and (f) is the optical phase of the interfering light. Yes, (g) is the light intensity of the locally oscillated light and the combined signal light, and (h) is the light intensity of the locally oscillated light and the interfering light after being combined.

  As shown in FIGS. 2B and 2D, the optical phase of the signal light and the local oscillation light is switched by π in half of one symbol time. For this reason, the optical phase difference between the signal light and the local oscillation light is constant, and the intermediate frequency component is constant at any time constituting the symbol time. Therefore, the light intensity obtained by integrating the local oscillation light and the combined signal light within one symbol time corresponds to the symbol value of the transmission data shown in FIG. 2C, as shown in FIG. Value. Thereby, the optical receiver can receive the transmission data.

  On the other hand, the interfering light has a constant phase throughout one symbol time, as shown in FIG. For this reason, the optical phase difference between the interference light and the local oscillation light is different by π in half of one symbol time. At this time, as shown in FIG. 2 (h), the local oscillation light and the interfering light after multiplexing have the same light intensity of the intermediate frequency component in the first half time and the second half time constituting one symbol time. The sign is the opposite sign. For this reason, the light intensity obtained by integrating the local oscillation light and the interfering light after multiplexing within one symbol time becomes zero. Therefore, the optical receiver 20 shown in FIG. 1 can remove the influence of local oscillation light and interference light after multiplexing from the electric signal output from the integral envelope detection means 33 shown in FIG.

  As described above, in the optical communication system shown in FIG. 1, the optical receiver 20 can receive the signal light and remove the influence of the interference light. Even if the interference light and the signal light have the same optical frequency, this effect is not lost.

  The change of the optical phase in the transmission light phase changing means 13 is preferably equal to the time from the phase 0 to π and the time from the phase π to 2π. Thereby, the integrated value of the signal due to the interfering light within one symbol time in the optical receiver 20 can be made zero. Even if the time from the phase 0 to π in the transmission light phase changing means 13 is not equal to the time from π to 2π, the integrated value of the signal due to the interference light can be brought close to zero. The transmission data and the signal due to the interference light can be separated.

The optical phase change in the transmission light phase changing means 13 may be sinusoidal or stepped as long as the optical phase difference between the signal light and the local oscillation light is kept substantially constant.
The speed at which the transmission light phase changing means 13 changes the optical phase is preferably constant. Since the optical phase difference between the signal light and the local oscillation light is always constant, the signal intensity in the intermediate frequency component of the signal light and the local oscillation light can be made constant at any timing within one symbol time. If the optical phase change is a sine wave with the same period or changes in a constant direction at a constant speed, the phase difference between the signal light and the local oscillation light becomes constant. Therefore, the optical transmitter 10 and the optical receiver 20 Strict synchronization can be eliminated.

Further, the principle of this embodiment will be described.
When coherent light modulated by an intensity modulator with a constant chirp amount is input to the optical receiver 20 as interference light, and the optical frequency difference from the local oscillation light enters the reception band passing through the BPF 24 shown in FIG. Is assumed. At this time, if the optical phase of the signal light with respect to the local oscillation light changes by n times (n is a natural number) within one symbol time, the interference light intensity L _j , the signal light intensity L _s , The following equation holds for the light intensity L ₁ of the local oscillation light.

Here, E _j , E _s , and E ₁ indicate the light intensities of the interference light, the signal light, and the local oscillation light, respectively. f _j , f _s , and f _l indicate the optical frequencies of the interference light, the signal light, and the local oscillation light, respectively. θ _j , θ _s , and θ _l indicate the initial phases of interference light, signal light, and local oscillation light, respectively. n is a natural number, and B is the reciprocal of one symbol time. At this time, the intermediate frequency components M _j and M _{s at} the time t of the interference light and the signal light are as follows.

Thus, when averaging the intermediate frequency component M _j and M _s within one symbol time by envelope detection, intermediate frequency component M _j of the interference light becomes zero, only the intermediate frequency components M _s of the signal light. As a result, even if the optical frequency of the interfering light is the same as that of the signal light, the output of the interfering light can be made zero and only the signal light can be obtained.

  The natural number n which is the number of times the optical phase is changed by one period during one symbol time is preferably 2 or more. This is from the viewpoint of preventing the danger that the interfering light is light from the direct modulation laser and the phase change happens to coincide with the phase change in the transmission light phase change means 13. In this case, the interfering light is phase-scrambled for one period in 1/2 symbol time, and therefore the interfering light can be removed when the natural number n is 2 or more.

  In the present embodiment, even when the optical frequency of the interfering light is a reception target, the integration of the optical frequency chip corresponding to the optical frequency of the interfering light is reduced in order to reduce the influence by integration in one symbol time. There is no communication disruption. Further, a feedback control system corresponding to the optical frequency of the interference light is not necessary. Even when interference light having a plurality of different optical frequencies enters the reception band, it is not necessary to change the optical frequency in accordance with each optical frequency. In addition, since the interference light removal is not dependent on the coherence length or pulse width of the signal light and the interfering light, the interference when the coherence length or the pulse width of the signal light and the interfering light is approximately the same, There is also an effect of avoiding deterioration of the signal-to-noise ratio when using short light.

In the present embodiment, the configuration in which the independent external optical phase modulator 13a is used as the transmission light phase changing means 13 is shown. However, from the viewpoint of cost reduction, it is also used as the data modulating means 12 and the light source 11 for modulating with data. desirable.
For example, the data modulation unit 12 and the transmission light phase change unit 13 are preferably integrated. Specifically, data modulation means that involves phase modulation in intensity modulation is used, and the intensity modulation depth is selected such that the phase modulation associated therewith is a natural number times the period. Based on the relationship between frequency modulation and phase modulation, the differential operation of the data signal is performed, and the light source is frequency-modulated to achieve such phase modulation. It integrates by using such a data modulation means. FIG. 3 shows an example of the optical transmitter in this case. The integrated means 14a shown in FIG. 3 modulates the intensity of the continuous light from the light source 11. Then, during one symbol time of the intensity modulation, the optical phase is changed by a natural number times the period by using the optical phase change generated when the intensity modulation is performed.

  For example, the light source 11, the data modulation unit 12, and the transmission light phase change unit 13 are preferably integrated. FIG. 4 shows an example of the optical transmitter in this case. The direct modulation laser 14b shown in FIG. 4 has a configuration in which the light source 11, the data modulation means 12, and the transmission light phase change means 13 shown in FIG. 1 are integrated. The direct modulation laser 14b generates light intensity-modulated by direct intensity modulation, and changes the optical phase to a natural period using the optical phase change generated in association with direct intensity modulation during one symbol time of direct intensity modulation. Change several times.

  In the direct modulation laser, chirp in which the optical phase and optical frequency of the output light fluctuate occurs mainly due to the refractive index change corresponding to the carrier fluctuation in the laser caused by the applied current and the optical output. In laser chirping, if the degree of modulation is increased even in sinusoidal modulation, the phase change of the output light with respect to time can be approximated linearly. Under this condition, the signal for changing the phase may be a sine wave signal.

  In the configuration using chirp, since the optical frequency also changes, removal by the optical frequency difference can also be applied. In the case of application, it is necessary not only to have the same period but also to modulate the signal light and the local oscillation light in synchronization. Therefore, the effect of not synchronizing the modulation of the signal light and the local oscillation light is lost, but the effect that can be removed even if the optical frequencies of the signal light and the local oscillation light overlap with each other, and there is an advantage that the interference removal capability is improved. .

  If removal by optical frequency difference is not applied in the configuration using chirp, it is not expected to improve the interference light removal capability by selecting the optical frequency range, but the effect that the optical frequency of signal light and interference light may overlap, An effect is obtained in which the modulation of the light and the local oscillation light need not be synchronized. Specifically, by widening the transmission frequency width of the BPF 24 shown in FIG. 1, the modulation of the signal light and the local oscillation light is performed particularly when the direct modulation laser 14b is sinusoidal modulation and the direct modulation laser 14b and the external light. The phase modulator 31a may be modulated with the same period. In order to obtain the removal effect by selecting the frequency domain without synchronizing the modulation of the signal light and the local oscillation light, for example, the intermediate frequency extracted by the BPF 24 is set in accordance with the optical frequency difference between the signal light and the local oscillation light at each moment. It is also possible to cope with this by making changes.

  The local oscillation light phase changing means 31 changes the optical phase of the local oscillation light continuously at a constant period regardless of the presence or absence of signal light. On the other hand, the transmission light phase changing unit 13 may change the optical phase continuously, or may apply the optical phase change only when the data modulating unit 12 outputs light. For this reason, in the configuration in which the optical phase change is applied by the direct modulation laser in which the light source 11, the data modulation unit 12, and the transmission light phase change unit 13 are integrated, there is an intensity change such that the output frequency fluctuates sufficiently at each time. Since data modulation is in progress, if the output intensity of the direct modulation laser is changed so that the optical phase rotates by a natural number of cycles due to the change, the data signal and optical phase that control the intensity of the light oscillated by the direct modulation laser It is not necessary to include a mixer that mixes the signal for phase modulation for controlling the signal. For example, in the case of a data signal on an RZ (Return to Zero) pulse that has passed through a low-pass filter or the like having a pass band of about 75% below the symbol rate, the signal intensity with respect to time varies.

  In the local oscillation light phase changing means 31, the local light source 21 and the external optical phase modulator 31a are preferably integrated. For example, the local oscillation light phase changing means 31 replaces the local light source 21 and the external optical phase modulator 31a with a direct modulation laser that generates local oscillation light by direct intensity modulation in accordance with a signal input from the signal source 31b. Prepare. In this case, the local oscillation light phase changing means 31 changes the optical phase of the local oscillation light by using the optical phase change generated along with the direct intensity modulation of the direct modulation laser.

  In this embodiment, the example in which the data modulation unit 12 performs the light intensity modulation has been described. However, the data modulation unit 12 may perform the optical phase modulation. FIG. 5 shows a schematic configuration diagram of an optical communication system when data modulation is performed by optical phase modulation. In the case of PSK (Phase Shift Keying) modulation, the signal light is subjected to phase modulation by the data modulation means 12 in addition to phase change by the transmission light phase change changing means 13. In this case, unlike the signal light, the local oscillation light is not phase-modulated by the data modulation means 12, so that the local oscillation light phase change means 31 only changes the phase in the same cycle direction as the phase change by the transmission light phase change means 13. Given.

  A synchronous detection will be described as an example. The optical multiplexing means 22 and the optical detection means 23 shown as 1 × 2 optical multiplexers and 1 optical input optical detection means in FIG. 1 are respectively 2 × 2 optical multiplexing means 27 and 2 optical inputs in FIG. This corresponds to the balanced optical detection means 28. The BPF 24 and the integral envelope detection means 33 shown in FIG. The signal light and the local oscillation light are combined by the 2 × 2 optical combining means 27. The combined light is output from the two outputs of the optical combining means 27 and input to the two inputs of the balanced optical detection means 28, and the respective outputs after photoelectric conversion are added with opposite polarities and detected. The local oscillation light is subjected to automatic frequency control by extracting the control signal of the local light source 21 from the optical detection output by the external optical phase modulator 31a. In the case of homodyne in which the intermediate frequency is approximated to zero, phase synchronization control is performed in addition to automatic frequency control.

  As described above, the optical communication system according to the present embodiment changes the phase of the signal light by approximately a natural number multiple of one period during one symbol time of transmission data, so that the signal light and the interfering light are transmitted. Even if the optical frequency is the same, interference light can be removed. Furthermore, by using signal light in which the phase of the signal light changes at a natural number multiple of the period at the same speed in the same direction within one symbol time, it is simple without synchronizing the phase modulation period of the signal light and the local oscillation light. Interfering light can be removed.

(Embodiment 2)
FIG. 6 is a schematic configuration diagram of the optical communication system according to the present embodiment. The difference between the present embodiment and the first embodiment is that the optical receiver 20 includes a received light phase changing means 32 instead of the local oscillation light phase changing means 31 shown in FIG. The optical receiver 20 performs coherent detection on the signal light after the opposite phase is changed by the received light phase changing means 32.

  The local light source 21 outputs local oscillation light that interferes with signal light. The received light phase changing unit 32 changes the optical phase of the signal light from the optical transmitter 10 with a phase obtained by inverting the optical phase changed by the transmitting light phase changing unit 13 and the phase change with respect to time. The optical multiplexing means 22 combines the signal light via the received light phase changing means 32 and the local oscillation light from the local light source 21 to cause the local oscillation light and the signal light to interfere with each other. The subsequent steps are the same as in the first embodiment. As a result, after integrating the intermediate frequency component of the electrical signal output from the optical detection means 23 for one symbol time, the integral envelope detection means 33 detects the envelope, thereby reducing the output of the interference light to zero. Can only receive.

このように、受信光位相変化手段３２が送信光位相変化手段１３の変化させる光位相と時間に対する位相変化の極性を反転させた位相で、信号光の光位相を変化させる。これにより、１つの光に対して、光位相θ_１（ｔ）及び光位相θ_３（ｔ）の２度の位相変化を加えて、両方の位相変化の和を一定に保つことができる。 Here, the light phase changed by the transmission light phase changing means 13 is θ ₁ (t), the light phase changed by the local oscillation light phase changing means 31 is θ ₂ (t), and the light changed by the received light phase changing means 32 is changed. Let the phase be θ ₃ (t). At this time, the optical phase θ ₁ (t), the optical phase θ ₂ (t), and the optical phase θ ₃ (t) have the following relationship.

In this way, the received light phase changing unit 32 changes the optical phase of the signal light with the phase obtained by inverting the optical phase changed by the transmitting light phase changing unit 13 and the polarity of the phase change with respect to time. As a result, two phase changes of the optical phase θ ₁ (t) and the optical phase θ ₃ (t) are added to one light, and the sum of both phase changes can be kept constant.

  In the configuration shown in FIG. 7, instead of the 1 × 2 light combining means 22 and the one-light input light detection means 23 shown in FIG. 6, a 2 × 2 light combining means 27 and a two-light input balanced light detection means 28 are used. It is. The BPF 24 and the integral envelope detection means 33 shown in FIG. Further, the local light source 21 shown in FIG. 6 is unnecessary. Transmission data for driving the data modulation means 12 by the optical transmitter 10 is differentially encoded in advance. The signal light from the optical transmitter 10 passes through the received light phase changing means 32 and is branched into two by the optical branching means 25, one being input to the optical multiplexing means 27 and the other being input to the optical delaying means 26. The received light phase changing means 32 has a phase obtained by inverting the optical phase changed by the transmitting light phase changing means 13 and the phase change with respect to time for one of the signal lights branched by the optical branching means 25 and the transmitted light phase changing means 13. The phase is changed at the same period and at the same speed in the same direction with the same optical phase difference. In the case of changing with a constant optical phase difference, the data signal is output in a form in which “0” and “1” are inverted. The optical delay means 26 adds an optical delay of one symbol time to the other signal light. One is multiplexed by 2 × 2 optical multiplexing means 27 with an optical delay of one symbol time compared to the other. The combined light is output from the two outputs of the optical combining means 27 and is input to the two inputs of the balanced optical detection means 28, respectively. The balanced optical detection means 28 detects each of the outputs after adding the opposite polarities after photoelectric conversion. Since the two branched signal lights are shifted by 1 bit, a signal corresponding to the phase difference is obtained. That is, differential decoding is automatically performed, and a detection result of an optical DPSK (Differential Phase-Shift-Keying) signal is obtained.

  The reception light phase changing unit 32 can have the same configuration as the transmission light phase changing unit 13. For example, the reception light phase changing unit 32 receives the signal light from the optical transmitter 10 and inverts the optical phase of the input signal light and the phase change with respect to time that the transmission light phase changing unit 13 changes. The configuration includes an external optical phase modulator that changes with the phase.

The principle of this embodiment will be described. In this embodiment, when the optical phase of the signal light with the local oscillation light changes by n times (n is a natural number) in one symbol time, the interference light intensity L _j and the signal light intensity L _{s 1} , the following expression holds for the light intensity L ₁ of the local oscillation light.

Thus, when averaging the intermediate frequency component M _j and M _s within one symbol time by integration, the intermediate frequency component M _j of the interference light becomes zero, only the intermediate frequency components M _s of the signal light. As a result, even if the optical frequency of the interfering light is the same as that of the signal light, the output of the interfering light can be made zero and only the signal light can be obtained.

(Embodiment 3)
FIG. 8 is a schematic configuration diagram of the optical communication system according to the present embodiment. In the first and second embodiments, the optical phase of the local oscillation light or signal light is changed in the optical receiver 20, but in this embodiment, the optical phase of the local oscillation light or signal light is not changed in the optical receiver 20. Instead, a detection signal generating means 34a and a synchronous detection means for performing synchronous detection by multiplying an output signal from the BPF 24 with an electric signal whose phase changes in the same cycle as the phase change of the beat signal obtained by extracting the intermediate frequency component of the signal light. 34b is provided. In this embodiment, since it can process with an electrical signal, there exists an effect which reduces the component which makes the phase change of the light in an optical receiver.

The local light source 21 outputs local oscillation light that interferes with signal light. The optical multiplexing means 22 combines the local oscillation light from the local light source 21 and the signal light from the optical transmitter 10 to cause the local oscillation light and the signal light to interfere with each other. The optical detection means 23 detects the interference light of the local oscillation light and the signal light, performs photoelectric conversion, and outputs an electrical signal. The photoelectrically converted electrical signal is input to the BPF 24. The BPF 24 extracts a beat signal, which is an intermediate frequency between the local oscillation light and the signal light, from the electrical signal output from the optical detection means 23 and inputs the beat signal to the synchronous detection means 34b. The detection signal generation unit 34 a generates an electrical signal whose phase changes with the same period and a constant phase difference as the optical phase changed by the transmission light phase change unit 13. The synchronous detection means 34b synchronously detects the intermediate frequency component of the electrical signal output from the optical detection means 23 with the electrical signal from the detection signal generation means 34a.
Others are the same as in the first and second embodiments.

(Embodiment 4)
FIG. 9 is a schematic configuration diagram of the optical communication system according to the present embodiment. The difference between this embodiment and Embodiment 1 is that the optical transmitter 10 further includes a transmission-side dispersion unit 41, and the optical receiver 20 further includes an inverse dispersion unit 42.

  The transmission side dispersion means 41 gives different delay times for each optical frequency. For example, a different delay time is given to the signal light output from the transmission light phase changing means 13 for each optical frequency. The inverse dispersion means 42 gives the signal light from the optical transmitter 10 a delay time opposite to the delay time for each optical frequency given by the transmission side dispersion means 41.

  In the present embodiment, it is premised that the data modulation means 12 that generates an optical frequency change accompanying intensity modulation, such as a direct modulation laser, is used. For example, a direct modulation laser in which the light source 11, the data modulation unit 12, and the transmission light phase change unit 13 are integrated is used. It is desirable that the error of the sum of the delay times due to the optical frequency be within the transmission frequency of the BPF 24 that transmits the beat signal.

  This embodiment is particularly effective in order to prevent the risk that the interference light is light from a directly modulated laser and the phase modulation of the interference light coincides with the optical receiver 20 of this embodiment. In the case of signal light in which the optical frequency of the signal light changes from the low frequency side to the long frequency side in one symbol time, the signal of one symbol is expanded in the time direction. Even if the time-phase characteristics of the interference light coincide with each other by chance, the transmission side dispersion means 41 gives dispersion that expands or shortens to a symbol time that does not match the symbol period of the interference light. Since the inverse dispersion means 42 changes the phase change of the interfering light with respect to time, the coincidence of the phase change can be reduced. Further, when the change in the optical frequency is large, the optical frequency difference between the interfering light and the local oscillation light at a certain time is different from the optical frequency width to be received by the optical receiver 20, and there is an effect that the optical frequency difference is not a reception target.

  Here, the delay time with respect to the optical frequency provided by the transmission side dispersion unit 41 and the inverse dispersion unit 42 may be monotonously changed or may be randomly changed. By changing randomly, separation from interfering light becomes easy.

  When the delay time with respect to the optical frequency provided by the transmission side dispersion unit 41 and the inverse dispersion unit 42 is monotonously changed, there is a possibility that the signal light and the interference light by the inverse dispersion unit 42 are synchronized. For this reason, the ratio of the delay time width to the optical frequency applied by the inverse dispersion means 42 with respect to the symbol time T is greatly different from the ratio of the difference between the symbol time of the data modulation means 12 and the symbol time of other symbol rates. It is desirable. Thereby, it can suppress that the signal light and disturbance light by the reverse dispersion means 42 synchronize.

  From the viewpoint of reducing the possibility that the symbol rates of the signal light and the interference light coincide with each other, it is desirable that the expanded or shortened symbol time does not match the symbol time used in the existing equipment. For example, in the case of the symbol rates of 155 Mbit / s and 622 Mbit / s, if the dispersion of the signal rate of the signal light having a transmission rate of 155 Mbit / s is increased by four times, the symbol rate of the interference light of 622 Mbit / s There is a possibility of coincidence. On the other hand, if the signal light symbol having a transmission rate of 622 Mbit / s is reduced to ¼, it may coincide with the symbol rate of 155 Mbit / s interference light. For this reason, it is desirable to give such dispersion that the symbol time that is not used in existing equipment is expanded or shortened.

  When the delay time is given in the direction in which the real time waveform of the interference light is expanded by the inverse dispersion means 42, the symbols gradually overlap each other, and interference light having a plurality of optical frequencies is present at the same time. In this case, since the contribution of the disturbing light may approach a certain value as the limit, it is desirable to provide a delay time in the direction in which the real-time waveform of the disturbing light is reduced.

Other effects are the same as in the first embodiment.
Although the present embodiment has been described on the assumption of the configuration of the first embodiment, the same effect can be obtained even if it is applied to any configuration of the first, second, and third embodiments.

(Embodiment 5)
In the present embodiment, instead of the inverse dispersion means 42 according to the fourth embodiment shown in FIG. 9, the local oscillation light dispersion means for giving different delay times to the input local oscillation light according to the optical frequency is provided. The local oscillation light dispersion unit provides the local oscillation light with the same delay time as the delay time for each optical frequency provided by the transmission side dispersion unit 41.

Here, the transmission side dispersion means 41 and the local oscillation light dispersion means give the same delay time to optical frequencies that differ by only the intermediate frequency. For this reason, the phase relationship of the signal light and the local oscillation light with respect to time is the same as in the first embodiment. If the delay time provided by the local oscillation light dispersion means is the same as in the fourth embodiment, the same effect as in the fourth embodiment can be obtained.
In addition, although this embodiment demonstrated on the assumption of the structure of Embodiment 1, even if it provides to any structure of Embodiment 1, 2, and 3, it is the same.

(Embodiment 6)
The difference of this embodiment from Embodiment 1 is in signal light and local oscillation light. In the present embodiment, the signal light is signal light encoded with the presence or absence or phase of light of different optical frequencies, and the local oscillation light differs in optical frequency from the encoded signal light by a predetermined intermediate frequency. In addition, the local oscillation light has a plurality of optical frequencies necessary for decoding a code to be selected and canceling a code not selected. Therefore, the local light source may extract the local light source control signal from the optical detection output and perform automatic frequency control. In the case of homodyne detection in which the intermediate frequency is approximated to zero, phase synchronization control may be performed in addition to automatic frequency control.

Specifically, the optical communication system according to the present embodiment differs from the optical communication system shown in FIG.
The light source 11 outputs a plurality of continuous lights having different optical frequencies. Each optical frequency is assigned for each symbol. The data modulation means 12 modulates the intensity of continuous light for each optical frequency. The transmission light phase changing unit 13 changes the optical phase for each optical frequency. As a result, the optical transmitter 10 transmits signal light having a plurality of optical frequencies modulated by the data modulation unit 12 and whose optical phase is changed by the transmission light phase changing unit 13.

  The optical detection means 23 uses local oscillation light, and detects light obtained by combining the signal light having a plurality of optical frequencies transmitted from the optical transmitter 10 and the local oscillation light. Here, the local oscillation light differs in optical frequency from the continuous light of each optical frequency output from the light source 11 by a predetermined intermediate frequency. The predetermined intermediate frequency is an intermediate frequency between each optical frequency output from the light source 11 and its coherent light. The phases of the local oscillation light at the respective optical frequencies coincide with each other when the phase difference from the signal light of each optical frequency transmitted by the optical transmitter 10 is multiplexed by the optical multiplexing means. The in-phase detection means detects, for each optical frequency, light having the same period and a constant optical phase difference as the optical phase to be changed by the transmission optical phase change means 13 from the signal light having a plurality of optical frequencies transmitted by the optical transmitter 10. To do. As a result, the optical receiver 20 receives the light detected by the optical detection means and detected by the in-phase detection means for each optical frequency.

  In this configuration, since the signal light is composed of a plurality of optical frequency chips, the contribution of a single interfering light can be reduced to a fraction of the number of optical frequency chips. That is, the interference light removal capability is further improved by applying the optical code multiplexing technique. Although the present embodiment has been described based on the configuration of the first embodiment, the same applies to the configurations of other embodiments.

  The present invention can be used for an economical optical service using a PON system.

1 is a schematic configuration diagram of an optical communication system according to Embodiment 1. FIG. The light intensity and optical phase of the local oscillation light, signal light and interference light are shown, (a) is the light intensity of the local oscillation light, (b) is the optical phase of the local oscillation light, and (c) is transmitted. (D) is the optical phase of the transmitted signal light, (e) is the optical intensity of the interfering light, (f) is the optical phase of the interfering light, (g ) Is the light intensity of the locally oscillated light and the combined signal light, and (h) is the light intensity of the locally oscillated light and the interfering light after being combined. An example of the optical transmitter when the data modulation means and the transmission optical phase change means are integrated is shown. An example of an optical transmitter in the case of a direct modulation laser in which a light source, a data modulation means, and a transmission light phase change means are integrated is shown. 1 is a schematic configuration diagram of an optical communication system when optical phase modulation is used for data modulation. FIG. 3 is a schematic configuration diagram of an optical communication system according to Embodiment 2. FIG. 1 is a schematic configuration diagram of an optical communication system when optical phase modulation is used for data modulation. FIG. FIG. 6 is a schematic configuration diagram of an optical communication system according to a third embodiment. FIG. 6 is a schematic configuration diagram of an optical communication system according to a fourth embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical transmitter 11 Light source 12 Data modulation means 13 Transmission light phase change means 13a External optical phase modulator 13b Signal source 14a Integrated means 14b Direct modulation laser 20 Optical receiver 21 Local light source 22 Optical multiplexing means 23 Optical detection Means 24 BPF
25 Optical branching means 26 Optical delay means 27 Optical multiplexing means 28 Balanced optical detection means 31 Local oscillation optical phase change means 31a, 32a External optical phase modulators 31b, 32b Signal source 32 Received optical phase change means 33 Integral envelope detection means 34a Detection signal generation means 34b Synchronous detection means 35 Integration means 41 Transmission side dispersion means 42 Inverse dispersion means

Claims (9)

An optical communication system for transmitting signal light from an optical transmitter to an optical receiver,
The optical transmitter is
A light source that outputs continuous light of a predetermined optical frequency;
Data modulation means for modulating the intensity or optical phase of continuous light from the light source according to transmission data;
A transmission optical phase in which n is a natural number and the optical phase is changed so that an n period, a time from 0 to π, and a time from π to 2π are substantially equal during one symbol time of modulation in the data modulation means. A change means,
Transmitting the signal light modulated by the data modulation means and having the optical phase changed by the transmission light phase change means;
The optical receiver is:
Optical detection means for detecting the light combined with the signal light transmitted from the optical transmitter and the local oscillation light and outputting an electric signal;
In-phase detection means for detecting the output of light having the same period and a constant optical phase difference as the optical phase to be changed by the transmission light phase change means from the output of the signal light transmitted by the optical transmitter,
The in-phase detection means includes
A received light phase changing means for changing the optical phase of the signal light from the optical transmitter at a phase obtained by inverting the optical phase changed by the transmitting light phase changing means and the phase change with respect to time;
Integrating envelope detection means for detecting an envelope after integrating an intermediate frequency component of the electrical signal output from the optical detection means for one symbol time,
An optical communication system , wherein the optical receiver receives light detected by the optical detection means and detected by the in-phase detection means. An optical communication system for transmitting signal light from an optical transmitter to an optical receiver,
The optical transmitter is
A light source that outputs continuous light of a predetermined optical frequency;
Data modulation means for modulating the intensity or optical phase of continuous light from the light source according to transmission data;
A transmission optical phase in which n is a natural number and the optical phase is changed so that an n period, a time from 0 to π, and a time from π to 2π are substantially equal during one symbol time of modulation in the data modulation means. A change means,
Transmitting the signal light modulated by the data modulation means and having the optical phase changed by the transmission light phase change means;
The optical receiver is:
Optical detection means for detecting the light combined with the signal light transmitted from the optical transmitter and the local oscillation light and outputting an electric signal;
In-phase detection means for detecting the output of light having the same period and a constant optical phase difference as the optical phase to be changed by the transmission light phase change means from the output of the signal light transmitted by the optical transmitter,
The in-phase detection means includes
A detection signal generating means for generating an electric signal whose phase changes with the same period and a constant phase difference as the optical phase to be changed by the transmission light phase changing means;
Synchronous detection means for synchronously detecting an intermediate frequency component of the electrical signal output from the optical detection means with the electrical signal from the detection signal generation means ,
Light communication systems that wherein the optical receiver, for receiving the light detected by the detection by and said phase detecting means by said light detecting means. The received light phase changing means includes:
An external optical phase modulator that receives the signal light from the optical transmitter and changes the optical phase of the input signal light at a phase that is obtained by inverting the optical phase changed by the transmission optical phase changing means and the phase change with respect to time. The optical communication system according to claim 1 , further comprising: The transmission light phase change means is an external optical phase modulator that receives light from the light source or the data modulation means and changes the optical phase of the input light by a natural number of periods. Item 4. The optical communication system according to any one of Items 1 to 3 . The data modulation means and the transmission light phase change means are integrated,
The integrated means modulates the intensity of continuous light from the light source, and changes the optical phase by a natural number of the period by using the optical phase change generated during the intensity modulation during one symbol time of the intensity modulation. optical communication system according to any of claims 1 3, characterized in that to vary. The light source, the data modulation means and the transmission light phase change means are integrated,
The integrated means generates light that is intensity-modulated by direct intensity modulation, and periodically cycles the optical phase using the optical phase change that occurs with direct intensity modulation during one symbol time of direct intensity modulation. optical communication system according to any of claims 1 3, characterized in that the a directly modulated laser to natural number times change. The optical transmitter is
Further comprising transmission-side dispersion means for giving different delay times to the light from the directly modulated laser for each optical frequency,
The optical receiver is:
The light according to claim 6 , further comprising inverse dispersion means for assigning a delay time opposite to a delay time for each optical frequency provided by the transmission side dispersion means to the signal light from the optical transmitter. Communications system. An optical communication system for transmitting signal light from an optical transmitter to an optical receiver,
The optical transmitter is
A light source that outputs continuous light of a predetermined optical frequency;
Data modulation means for modulating the intensity or optical phase of continuous light from the light source according to transmission data;
A transmission optical phase in which n is a natural number and the optical phase is changed so that an n period, a time from 0 to π, and a time from π to 2π are substantially equal during one symbol time of modulation in the data modulation means. A change means,
Transmitting the signal light modulated by the data modulation means and having the optical phase changed by the transmission light phase change means;
The optical receiver is:
Optical detection means for detecting the light combined with the signal light transmitted from the optical transmitter and the local oscillation light and outputting an electric signal;
In-phase detection means for detecting the output of light having the same period and a constant optical phase difference as the optical phase to be changed by the transmission light phase change means from the output of the signal light transmitted by the optical transmitter,
Receiving light detected by the light detection means and detected by the in-phase detection means;
The light source, the data modulation means and the transmission light phase change means are integrated,
The integrated means generates light that is intensity-modulated by direct intensity modulation, and periodically cycles the optical phase using the optical phase change that occurs with direct intensity modulation during one symbol time of direct intensity modulation. Is a direct modulation laser that changes the natural number of
The optical transmitter is
Further comprising transmission-side dispersion means for giving different delay times to the light from the directly modulated laser for each optical frequency,
The optical receiver is:
Light communication system that further comprising a local oscillator light dispersion means for imparting to the local oscillator light to the same delay time as the delay time for each optical frequency to impart the sender dispersion means. An optical communication system for transmitting signal light from an optical transmitter to an optical receiver,
The optical transmitter is
A light source that outputs continuous light of a predetermined optical frequency;
Data modulation means for modulating the intensity or optical phase of continuous light from the light source according to transmission data;
A transmission optical phase in which n is a natural number and the optical phase is changed so that an n period, a time from 0 to π, and a time from π to 2π are substantially equal during one symbol time of modulation in the data modulation means. A change means,
Transmitting the signal light modulated by the data modulation means and having the optical phase changed by the transmission light phase change means;
The optical receiver is:
Optical detection means for detecting the light combined with the signal light transmitted from the optical transmitter and the local oscillation light and outputting an electric signal;
In-phase detection means for detecting the output of light having the same period and a constant optical phase difference as the optical phase to be changed by the transmission light phase change means from the output of the signal light transmitted by the optical transmitter,
The light source outputs a plurality of continuous lights having different optical frequencies,
The data modulation means modulates the intensity or optical phase of the continuous light for each optical frequency,
The transmission light phase changing means changes the optical phase for each optical frequency,
The optical transmitter transmits signal light of a plurality of optical frequencies modulated by the data modulation means and changed in optical phase by the transmission optical phase change means;
The optical detection means uses continuous light of each optical frequency output from the light source and light having a different optical frequency by a predetermined intermediate frequency as the local oscillation light, and signal light having a plurality of optical frequencies transmitted by the optical transmitter. And detecting the combined light of the local oscillation light,
The in-phase detection unit is configured to detect, for each optical frequency, light having a constant optical phase difference with the same period as the optical phase to be changed by the transmission optical phase change unit from signal light having a plurality of optical frequencies transmitted by the optical transmitter. Detect
Said optical receiver, an optical communication system that is characterized in that to receive the light detected by the detection by and said phase detecting means by said light detecting means for each optical frequency.

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