JP6531318B2 - Optical member evaluation device - Google Patents

Description

  The present invention relates to an evaluation apparatus for an optical member such as an optical fiber.

  Mode-multiplexed optical communication is a communication technology in which light of a plurality of propagation modes, each of which carries different information, is transmitted by one optical fiber. Each propagation mode is orthogonal to one another, and the orthogonality is defined by spatial intensity and phase. The lowest order propagation mode is called the fundamental mode and has a flat spatial phase. On the other hand, higher order propagation modes have spatially non-uniform phase, and the intensity also shows a specific distribution accordingly. In the mode multiplexing optical communication system, each propagation mode is treated as an individual channel, but when crosstalk occurs between the propagation modes, the transmission quality is degraded.

  Therefore, it is required to measure and evaluate how much crosstalk occurs in optical members such as an optical fiber and a mode converter used in a mode multiplexing optical communication system. As mentioned above, the propagation mode is determined by the spatial intensity and phase of the light, ie the distribution of the complex amplitude. In order to measure the crosstalk, it is necessary to measure the spatial intensity and phase of light (hereinafter, referred to as object light) propagated through the optical member to be measured. Usually, the photodetector can only observe the spatial intensity distribution of light. Therefore, spatial intensity and phase distribution are known, and the spatial distribution of intensity and phase of object light is determined by causing object light to interfere with coherent reference light and object light.

  For this reason, usually, the light generated by the light source is branched into two, one of which is made to pass through the optical member to be measured to be an object light, and the other is used as a reference light. At this time, the reference light is caused to interfere with the object light after propagating the dedicated light path different from the optical member to be measured through which the object light passes, and the optical path length of the dedicated light path of the reference light is the optical member to be measured It must be the same optical path length as Therefore, when evaluating a long optical member such as an optical fiber, it is necessary to prepare an optical fiber having the same length as the optical fiber to be measured as a dedicated optical path for reference light.

  Patent Document 1 discloses a configuration in which a dedicated light path of reference light is made unnecessary by generating reference light from object light. FIG. 3 shows an evaluation device described in Patent Document 1. In FIG. 3, the light generated by the light source 1 is converted into a predetermined propagation mode in the mode converter 2 and is input to the measured fiber 3 which is an optical member to be measured. The splitter 4 splits the object light propagated through the fiber to be measured 3, outputs one of the object lights to the interference unit 6, and generates the other object light from the object light as a spatial filter. Output to 5 The spatial filter 5 suppresses high frequency components of the object light and passes direct current and low frequency components. That is, the spatial filter 5 extracts a plane wave component having a uniform amplitude and phase distribution from the object light and uses it as a reference light. This is because the spatial distribution of the intensity and phase of the reference light must be known. The interference unit 6 causes the object light from the branching filter 4 to interfere with the reference light that can be regarded as a substantially plane wave output from the spatial filter 5, and the imaging unit 7 photographs interference fringes and processes the photographed data. Output to section 8. The processing unit 8 obtains, for example, a spatial distribution of the intensity and phase of the object light based on the interference fringes.

WO 2014/050141 pamphlet

  However, when the object light input to the spatial filter 5 contains almost no plane wave component, the configuration described in Patent Document 1 degrades the measurement accuracy. That is, in the configuration described in Patent Document 1, if the object light does not contain a plane wave component sufficiently, it is not possible to accurately evaluate the optical member.

  The present invention provides an evaluation apparatus capable of generating reference light from object light and evaluating optical members with high accuracy regardless of the phase distribution of object light.

  According to one aspect of the present invention, an evaluation apparatus for evaluating the optical member by the object light having passed through the optical member to be evaluated includes: a branching unit that branches the object light; and one object light branched by the branching unit Phase adjustment means for giving a phase delay according to the spatial position passing through to the one object light, and performing low-pass filter processing based on the spatial frequency on the object light passing through the phase adjustment means Interference means for generating interference fringes by causing generation means for outputting as light, the reference light, and the other object light branched by the branching means, imaging means for imaging the interference fringes, and the imaging means A spatial distribution of the phase of the object light is determined based on the captured interference fringes, and control means is provided for controlling the phase adjustment unit based on the spatial distribution of the determined phase.

  According to the present invention, regardless of the phase distribution of the object light, the reference light can be generated from the object light and the optical member can be evaluated with high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS The block diagram of the evaluation apparatus by one Embodiment. FIG. 2 is a block diagram of a spatial filter according to one embodiment. The block diagram of the evaluation apparatus by background art.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. The following embodiment is an exemplification, and the present invention is not limited to the contents of the embodiment. Further, in each of the following drawings, components that are not necessary for the description of the embodiment will be omitted from the drawings.

  FIG. 1 is a block diagram of an evaluation device 100 according to the present embodiment. In the configuration of FIG. 1, the optical member to be evaluated is an optical fiber, but in the present embodiment, any optical member that needs to measure the spatial distribution of phase and intensity is to be evaluated. can do. In FIG. 1, the light generated by the light source 1 is converted into a predetermined propagation mode in the mode converter 2 and is input to the measured fiber 3 to be measured. Then, the light propagated through the measured fiber 3 is input to the evaluation device 100 as object light. The light source 1 and the mode converter 2 may be included in the evaluation device.

  The splitter 4 of the evaluation device 100 splits the input object light, and outputs one of the branched light to the interference unit 6 as the object light. Also, the other object light is output to the spatial light phase modulator 9, and the spatial light phase modulator 9 converts the object light into intermediate light and outputs it. The spatial light phase modulator 9 is, for example, a liquid crystal device having a plane orthogonal to the traveling direction of the passing light, and provides the passing light with an independent delay amount for each position on the passing plane. It is configured. In other words, the spatial light phase modulator 9 functions as a phase adjustment unit that gives the passing light a phase delay according to the spatial position. Here, the spatial position means a position on a plane orthogonal to the traveling direction of the passing light. In the initial state, phase delays at all spatial positions are the same. Therefore, in the initial state, the intermediate light output from the spatial light phase modulator 9 is the same as the object light. The intermediate light having passed through the spatial light phase modulator 9 is input to the spatial filter 5. FIG. 2 shows a configuration example of the spatial filter 5. The spatial filter includes, for example, a condenser lens 51, a member 52 provided with a pinhole, and a collimator lens 53. With this configuration, the spatial filter 5 suppresses high frequency components of the spatial frequency of the intermediate light, and extracts direct current and low frequency components as reference light. That is, the spatial filter 5 performs low-pass filter processing for removing the high frequency component of the intermediate light, and extracts a plane wave component having a uniform amplitude and phase distribution from the intermediate light. This is because the spatial distribution of the intensity and phase of the reference light must be known. Here, the frequency means a spatial frequency. The interference unit 6 causes the object light from the splitter 4 and the reference light from the spatial filter 5 to interfere with each other, and the imaging unit 7 captures interference fringes and outputs the captured data to the processing unit 10.

  The processing unit 10 determines the phase distribution (complex amplitude distribution) of the object light from the interference fringes, and feeds back the determined conjugate of the phase distribution to the spatial light phase modulator 9. That is, the processing unit 10 controls each of the spatial light phase modulators 9 so that the delay amount given to the light passing through the spatial light phase modulator 9 at each spatial position becomes the phase conjugate of the phase distribution of the object light. Control the amount of phase delay in position. This corresponds to adjusting the phase so as to reduce the phase difference at each position of the object light to form an intermediate light, and the intermediate light output from the spatial light phase modulator 9 by this operation is a plane wave from the object light It will be closer to That is, the plane wave component contained in the intermediate light increases more than the intermediate light in the first measurement. Furthermore, the processing unit 10 determines the phase distribution (complex amplitude distribution) of the object light from the interference fringes by the second measurement, and feeds back the determined conjugate of the phase distribution to the spatial light phase modulator 9. By repeating this process, the spatial light phase modulator 9 smoothes the phase and intensity distribution of the input object light, and the intermediate light output from the spatial light phase modulator 9 approaches a plane wave. As a result, regardless of the phase distribution of the object light that has passed through the fiber to be measured 3, it is possible to extract reference light of sufficient intensity from the object light, and therefore, it is possible to improve the measurement accuracy. The number of repetitions can be set in advance. Alternatively, the repetition may be terminated when the setting state of the spatial light phase modulator 9 converges.

  When the repetition is completed, the processing unit 10 evaluates the measured optical fiber 3 based on the spatial distribution of the phase and the intensity of the object light. The contents that can be evaluated are arbitrary contents that can be calculated based on the spatial distribution of the phase and intensity of the object light, and, for example, crosstalk between propagation modes, wavelength dependency of spatial distribution of phase and intensity, ideal Deviation of the propagation mode from the spatial distribution of phase and intensity, inter-mode delay, etc.

  4: Branching part, 9: Spatial light phase modulator, 5: Spatial filter, 6: Interference part, 7: Imaging part, 10: Processing part

Claims (3)

It is an evaluation device which evaluates the optical member concerned by object light which passed optical member of evaluation object,
A branching unit that branches the object light;
Phase adjusting means for giving a phase delay according to a spatial position where one object light branched by the branching means passes and passes to the one object light;
Generation means for performing low-pass filter processing based on spatial frequency on the object light having passed through the phase adjustment means, and outputting as reference light;
Interference means for causing interference fringes by causing the reference light and the other object light branched by the branching means to interfere with each other;
Imaging means for imaging the interference fringes;
Control means for determining the spatial distribution of the phase of the object light based on the interference fringes imaged by the imaging means, and controlling the phase adjustment means based on the spatial distribution of the determined phase;
An evaluation device characterized by comprising:   The control means sets the phase delay given by the phase adjustment means so as to reduce the phase difference at the spatial position of the object light passing through the phase adjustment means from the spatial distribution of the phase obtained. The evaluation apparatus according to claim 1.   The evaluation apparatus according to claim 1, wherein the phase adjustment unit is a spatial light phase modulator.

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