JP2011188132A - Coherent optical communication receiver and optical axis adjusting method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the accuracy of optical axis adjustment in a coherent optical communication receiver.
A present coherent optical communication receiver 100 includes an input portion IN of the reception signal light E _S by the polarization-maintaining optical fiber PMF is input, the optical coupling of the reception signal light E _S and the local oscillator light E _LO The hybrid 20 includes a light detection unit 30 that detects output light from the hybrid 20, and a polarizing plate 60 provided on an optical path from the input unit IN to the light detection unit 30. While rotating the signal light polarization maintaining optical fiber PMF _S, by the light receiving amount of the reception signal light E _S in the photodetector unit 30 is adjusted to be larger than a predetermined amount, to adjust the optical axis.
[Selection] Figure 1

Description

  The present invention relates to a receiver used for coherent optical communication and an optical axis adjustment method thereof.

  Coherent optical communication is an optical communication method in which weak received signal light is mixed with high-intensity local oscillation light for light reception detection and has properties such as high reception sensitivity and high frequency selection characteristics (for example, (Refer nonpatent literature 1). Received signal light and local oscillation light are input from a polarization maintaining optical fiber (PMF) to a hybrid circuit in the receiver, and after optical coupling is performed, are detected by a light detection unit.

  Coherent optical communication requires optical axis adjustment that matches the polarization direction of light input from the PMF to the receiver with the polarization direction of the hybrid. As a method for adjusting the optical axis, for example, there is a method of marking the PMF itself or a member for fixing the PMF, and fixing the PMF at a predetermined position using the marking as a mark.

ECOC 07 September 16-20, 2007 Berlin, Germany 33rd European Conference and Exhibition on Optical Communication P833

  Since the core diameter of the PMF is as small as, for example, several tens of μm, the optical axis adjustment method using the above-described marking requires very small marking to improve accuracy. However, since the processing and recognition of the marking becomes difficult as the size is reduced, the optical axis adjustment may not be correctly performed by the above method.

  The present invention has been made in view of the above problems, and an object of the present invention is to improve the accuracy of optical axis adjustment in a coherent optical communication receiver.

  The receiver for coherent optical communication detects an input unit to which received signal light is input by a polarization maintaining optical fiber, a hybrid that optically couples the received signal light and local oscillation light, and output light from the hybrid A light detection unit; and a polarizing plate provided on an optical path from the input unit to the light detection unit.

  According to this configuration, since the polarization direction of the received signal light input to the receiver can be detected based on the intensity of the received signal light that has passed through the polarizing plate, the accuracy of optical axis adjustment can be improved.

  The said structure WHEREIN: The polarization direction of the said polarizing plate can be set as the structure equal to the polarization direction set to the said hybrid.

  The said structure WHEREIN: It can be set as the structure which has a lens which optically couples the said polarization-maintaining optical fiber and the said hybrid.

  The said structure WHEREIN: The said polarizing plate can be set as the structure provided between the said lens and the said hybrid.

  The said structure WHEREIN: The said polarizing plate can be set as the structure provided between the said hybrid and the said photon detection part.

  In the above configuration, the polarizing plate may be provided between the polarization maintaining optical fiber and the lens.

  In the above configuration, the input unit receives the first signal light input unit to which the first reception signal light is input, the second signal light input unit to which the second reception signal light is input, and the local oscillation light. And an oscillation light input unit.

  Further, the optical axis adjustment method of the coherent optical communication receiver includes a signal light input unit to which received signal light is input by a polarization maintaining optical fiber for signal light, and a hybrid that mixes the received signal light and local oscillation light. Optical axis adjustment of a coherent optical communication receiver, comprising: a light detection unit that detects output light from the hybrid; and a polarizing plate provided on an optical path from the signal light input unit to the light detection unit The method includes a signal light optical axis adjustment step of adjusting the received light amount of the reception signal light in the light detection unit to be larger than a predetermined amount while rotating the polarization maintaining optical fiber for signal light.

  According to this method, the optical axis is adjusted while the polarization direction of the received signal light and the local oscillation light input to the receiver is detected based on the intensity of the received signal light that has passed through the polarizing plate. Accuracy can be improved.

  In the above configuration, the signal light input unit includes a first signal light input unit to which the first received signal light is input and a second signal light input unit to which the second received signal light is input. be able to.

  In the above configuration, a local oscillation light input unit to which the local oscillation light is input by a polarization maintaining optical fiber, and a polarizing plate on an optical path from the local oscillation light input unit to the light detection unit are provided. be able to.

  In the above configuration, the coherent optical communication receiver includes an oscillating light input unit to which a polarization maintaining optical fiber for oscillating light for introducing the local oscillating light is connected. A step of adjusting the received light amount of the local oscillation light in the light detection unit to be larger than a predetermined amount while rotating the polarization-maintaining optical fiber for transmission, on the optical path reaching the unit Then, the signal light optical axis adjustment step can be implemented.

  According to this coherent optical communication receiver and its optical axis adjustment method, the accuracy of optical axis adjustment can be improved.

FIG. 1 is a diagram illustrating a configuration of a reception system in a coherent optical communication system. 2A to 2B are diagrams illustrating the configuration of the coherent optical communication receiver according to the first embodiment. FIG. 3 is a principle diagram showing a method of adjusting the optical axis. FIG. 4 is a flowchart showing an optical axis adjustment method. FIGS. 5A and 5B are diagrams illustrating the configuration of the coherent optical communication receiver according to the second embodiment. FIGS. 6A and 6B are diagrams illustrating the configuration of the coherent optical communication receiver according to the third embodiment. FIG. 7 is a diagram illustrating a configuration near the input unit of the receiver.

FIG. 1 is a diagram illustrating a configuration of a reception system in a coherent optical communication system. In the coherent optical communication, received signal light (hereinafter referred to as signal light) ES and local oscillation light (hereinafter referred to as oscillation light) _ELO are used. The signal light ES is split into an X-direction polarization (E _SX ) and a Y-direction polarization (E _SY ) that are orthogonal to each other by an external polarization beam splitter PBS, and then the first signal of the receiver 100. The light is input to the optical input unit IN _X and the second signal light input unit IN _Y. The oscillation light E _LO is emitted from the external LO light source 10 and input to the oscillation light input unit IN _LO of the receiver 100.

The signal light E _{SX, the} signal light E _SY , and the oscillation light E _LO are input to the hybrid 20 provided inside the receiver. The hybrid 20 is an optical circuit that delays, separates, and combines input light. In this embodiment, the signal light E _SX , the signal light E _SY , and the oscillation light E _LO correspond to this input light. The hybrid 20 is designed on the assumption that the polarization plane of the input light is limited to a predetermined direction in order to delay, split, and synthesize the input light with high accuracy. Therefore, the polarization directions of the signal light E _SX , the signal light E _SY , and the oscillation light E _LO are set to be the polarization directions determined for the hybrid 20.

The hybrid 20 is composed of, for example, a quartz-based planar lightwave circuit (PLC). The signal light E _SX is combined with the oscillation light E _LO by the hybrid 20 and separated into an in-phase (In Phase; I) component and a quadrature (Q) component, respectively, and output signal light X-Ip, X-In , X-Qp, and X-Qn. The subscripts p and n shown in FIG. 1 indicate positive and negative components, respectively. For example, X-Ip indicates that the output signal light is a positive component of the I component of the signal light _ESX . Similarly, the signal light E _SY is combined with the oscillation light E _LO and output as output signal light Y-Ip, Y-In, Y-Qp, and Y-Qn, respectively. As a result, the phase and intensity information of each signal light and oscillation light is calculated, and a predetermined light output can be obtained.

Signal light and an oscillation light output from the hybrid 20 are inputted to the light detecting section _{30 X-I ~30 Y-Q} . Each of the light detection units 30 includes two photodiodes (PD: Photodiode) and one transimpedance amplifier (TIA) connected thereto. The photodetector composed of a set of two photodiodes PD is optically coupled with output signal light of positive and negative components, respectively. For example, the photodetector _{30 X-I,} the signal light _{E SX} of the I component of the positive and negative components in the form of the output signal light X-Ip, is optically coupled with X-an In. The transimpedance amplifier TIA converts the combined current from the two photodiodes PD into a voltage signal and outputs it to a subsequent circuit. As described above, the light detecting section _{30 X-I, 30 X-} Q, 30 Y-I, and ₃₀ in the _Y-Q, is detected the output light from the hybrid 20, the electrical signals respectively X-I, X-Q , Y-I, and Y-Q and output.

The output signal X-I~Y-Q from the optical detection unit _{30 X-I ~30 Y-Q} is respectively inputted to the subsequent ADC (Analog Digital Converter) circuit and DSP (Digital Signal Processor) circuit, including a demodulation Predetermined signal processing is performed.

FIG. 2A is a schematic top view illustrating the configuration of the coherent optical communication receiver 100 according to the first embodiment, and FIG. 2B is a schematic cross-sectional view along the optical waveguide. FIG. 2B shows only one of the optical waveguides shown in FIG. 1, but all the waveguides branched from the signal light ES and the oscillation light _ELO have the same configuration. Yes.

As shown in FIG. 2A, the signal lights E _SX and E _SY are respectively input to the receiver 100 by the polarization maintaining optical fibers PMF _X and PMF _Y for signal light, and the oscillation light E _LO is for oscillation light. The signal is input to the receiver 100 by the polarization maintaining optical fiber PMF _LO . The polarization maintaining optical fibers PMF _X , PMF _Y , and PMF _LO are fixed to the receiver 100 by fixing means 40 _X , 40 _Y , and 40 _LO , respectively. In the receiver 100, an oscillation light input section IN _LO is provided between the first signal light input section IN _X and the second signal light input section IN _Y.

As shown in FIG. 2B, a window 92 is provided on the inner wall surface of the package accommodating the receiver 100 in the input unit IN. A lens 50 for optically coupling the polarization maintaining optical fiber PMF and the hybrid 20 is provided between the window 92 (input unit IN) and the hybrid 20. A polarizing plate 60 for adjusting the optical axis is provided between the lens 50 and the hybrid 20. Both surfaces of the polarizing plate 60 are subjected to AR coating (anti-reflective coating) for suppressing the reflectance (the reflectance is preferably about 2% or less). Polarization direction of the polarizing plate 60 Are positioned so as to coincide with the polarization direction determined for the hybrid 20. Therefore, when the polarization directions of the signal light E _SX , E _SY and the oscillation light E _LO introduced from the polarization maintaining optical fibers PMF _X , PMF _Y and PMF _LO coincide with the polarization direction determined for the hybrid 20, The light intensity transmitted through the polarizing plate 60 is maximized.

  A photodiode PD and a transimpedance amplifier TIA are provided on the opposite side across the hybrid 20. In addition, an RF board 70 and a wiring board 80 for mounting the ADC and the DSP are provided around the hybrid 20 (the ADC and the DSP may be provided outside the receiver 100).

  A first carrier 110, a second carrier 120, and a third carrier 130 are provided on the base 105 of the package constituting the receiver 100. A lens 50 and a polarizing plate 60 are mounted on the second carrier 120. Here, the polarization direction of the polarizing plate 60 is positioned so that a predetermined direction is realized with respect to the first carrier 110. The hybrid 20 is mounted on the second carrier 120. On the third carrier 130, the light detection unit 30 (PD and TIA) is mounted.

  Since the first carrier 110 and the second carrier 120 are mounted on the base 105, the relative positional accuracy between the polarizing plate 60 and the hybrid 20 is high. For this reason, the polarization direction of the polarizing plate 60 with respect to the light input part of the hybrid 20 can be determined with high accuracy. In other words, since the carrier of the polarizing plate 50 (first carrier 110) and the carrier of the hybrid 20 (second carrier 120) are mounted on a common base member (base 105), the polarization direction of the polarizing plate. And the positional accuracy of the hybrid optical input unit can be improved.

  Next, a method for adjusting the optical axis of the coherent optical communication receiver 100 according to the first embodiment will be described.

FIG. 3 is a principle diagram showing a method of adjusting the optical axis. The signal light E _SX (E _SY ) or the oscillation light E _LO passes through the polarizing plate 60 having a predetermined polarization direction (arrow in the figure) and is input to the hybrid 20. Here, the polarization direction of the polarizing plate 60 is formed to be equal to the polarization direction of the optical waveguide defined in the hybrid 20. Therefore, if the polarization direction of the light incident on the polarizing plate matches the polarization direction of the hybrid 20, the amount of light transmitted through the polarizing plate increases. Conversely, if the polarizing direction is different, the polarizing plate The amount of transmitted light is reduced. By utilizing this, the light detection unit 30 monitors the amount of light transmitted through the polarizing plate (actually, the amount of light output from the hybrid 20) while rotating the polarization maintaining optical fiber PMF. The optical axis can be adjusted to adjust the polarization direction.

FIG. 4 is a flowchart of the optical axis adjustment method. The optical axis adjustment operation may be performed automatically using a dedicated device, or some processes may be performed manually. First, while rotating the polarization maintaining optical fiber PMF _LO for oscillation light little by little (step S1), the output of the light detection unit 30 is monitored to determine whether the output light is maximum (step S2). When the output light detected by the light detection unit 30 becomes maximum, the polarization maintaining optical fiber PMF _LO for oscillation light is fixed at that position (step S3). If the output light is not the maximum, the process returns to step S1 and the adjustment is continued until the output light reaches the maximum.

Next, while rotating the polarization maintaining optical fiber PMF _X for signal light little by little (step S4), the output of the light detection unit 30 is monitored to determine whether the amount of received light is maximum (step S5). If the received light amount is not the maximum, the process returns to step S4, and the adjustment is continued until the received light amount reaches the maximum. When the amount of received light detected by the light detection unit 30 is maximized, the polarization maintaining optical fiber PMF _X for signal light is fixed at that position (step S6). Similarly, while the signal light polarization maintaining fiber PMF _Y is rotated little by little (step S7), and monitors the output of the optical detection unit 30, the amount of received light to determine whether the maximum (Step S8). If the received light amount is not the maximum, the process returns to step S4, and the adjustment is continued until the received light amount reaches the maximum. Amount of received light is detected If the maximum, to fix the polarization maintaining optical fiber PMF _Y signal light at that position in the photodetector section 30 (step S9). In the first embodiment, there are two polarization-maintaining optical fibers for signal light, PMF _X and PMF _Y , but either one of the optical fibers may be optically adjusted first. Through the above steps, the optical axis adjustment of the receiver 100 is completed.

Oscillation light _{E LO} is to be input to the optical detecting section _{30 X-I ~30 Y-Q} shown in FIG. 1, in step S2, among the four light detecting section _{30 X-I ~30 Y-Q,} any The output of the light detector may be monitored. On the other hand, in step S5, the two light detection units to which each signal light is input (the light detection units _{30X -I} and _30X-Q if the signal light E _SX , and the light detection unit 30 if the signal light E _{SY. Y-I} and 30 _Y-Q ), the output of one of the light detection units is monitored.

  In steps S2, S5, and S8, it is theoretically preferable to proceed to the next step when the amount of received light reaches a maximum, but in actuality, when the amount of received light exceeds a predetermined threshold value. If the above condition is satisfied, the process may proceed to the next step.

The coherent optical communication receiver 100 according to the first embodiment includes a polarizing plate 60 for adjusting the optical axis between the lens 50 and the hybrid 20. Since the polarization direction of the light input to the receiver 100 can be detected by the intensity of the signal light E _SX and E _SY that have passed through the polarizing plate 60 and the intensity of the oscillation light E _LO , the accuracy of optical axis adjustment is improved. Can do.

Optical axis adjusting method for coherent optical communication receiver 100 according to the first embodiment, a first step of adjusting the oscillation light _{E LO} (step S1 to S3), a second step of adjusting the signal light ES (step S3~ S9). By adjusting the received light amount in the light detection unit to be larger than a predetermined amount while rotating the polarization maintaining optical fiber PMF _LO for oscillation light and the polarization maintaining optical fibers PMF _X and PMF _Y for signal light, The optical axis can be adjusted with high accuracy.

  The optical axis adjustment of the coherent optical communication receiver 100 according to the first embodiment can be performed using a CW light source (continuous light) and detection means (linear detection means) for detecting the CW light source (continuous light). The CW light source and the linear detection means for detecting it are inexpensive, and the cost of the optical axis adjusting device is low. In addition, according to the present embodiment, since the polarizing plate 60 is provided in front of the hybrid 20, input light having a polarization direction different from the polarization direction defined for the hybrid 20 can be suppressed. This leads to an improvement in the accuracy of optical calculation in the optical circuit of the hybrid 20.

  In Example 1, the polarizing plate 60 is disposed between the lens 50 and the hybrid 20 as illustrated in FIG. 2, but the polarizing plate 60 may be directly attached to the end surface of the hybrid 20.

  The following examples are examples in which the position where the polarizing plate 60 is disposed is variously changed.

FIGS. 5A and 5B are diagrams illustrating the configuration of the coherent optical communication receiver according to the second embodiment. Unlike FIGS. 2A to 2B, the polarizing plates 60 _X-I , 60 _X-Q , 60 _Y-I , and 60 _Y-Q are respectively detected by the light detection units 30 _X-I , 30 _X-Q , Corresponding to 30 _Y-I and 30 _Y-Q , it is provided between the hybrid 20 and the photodiode PD. Other configurations are the same as those of the first embodiment, and detailed description thereof is omitted.

  Also in the receiver 100A having the configuration shown in FIGS. 5A to 5B, the optical axis can be adjusted with high accuracy by the same method as in the first embodiment. The polarizing plate 60 may be directly attached to the end face of the photodiode PD.

  Note that a first carrier 110, a second carrier 120, and a third carrier 130 are provided on the base 105 of the package constituting the receiver 100. A lens 50 is mounted on the first carrier 110. The hybrid 20 is mounted on the second carrier 120. On the third carrier 130, the polarizing plate 60 and the light detection unit 30 (PD and TIA) are mounted. Here, the polarization direction of the polarizing plate 60 is positioned so that a predetermined direction is realized with respect to the third carrier 130.

  Since the second carrier 120 and the third carrier 130 are mounted on the base 105, the relative positional accuracy between the hybrid 20 and the polarizing plate 60 is high. For this reason, the polarization direction of the polarizing plate 60 with respect to the light input part of the hybrid 20 can be determined with high accuracy. In other words, since the carrier of the hybrid 20 (second carrier 120) and the carrier of the polarizing plate 50 (third carrier 130) are mounted on the common base member (base 105), the polarization direction of the polarizing plate. And the positional accuracy of the hybrid optical input unit can be improved.

  FIGS. 6A and 6B are diagrams illustrating the configuration of the coherent optical communication receiver according to the third embodiment. Unlike FIGS. 2A and 2B, a window pipe 90 is provided in the base of the receiver 100 (between the input unit IN and the lens 50), and a polarizing plate 60 is provided in the window pipe 90. . Other configurations are the same as those of the first embodiment, and detailed description thereof is omitted.

  In the receiver 100A configured as shown in FIGS. 6A to 6B, the optical axis can be adjusted with high accuracy by the same method as in the first embodiment. The polarizing plate 60 may be directly attached to the end surface of the window 92 in the input unit IN.

  FIG. 7 is a diagram illustrating a configuration near the input unit IN of the receiver 100. A window 92 into which light is input is formed in the window pipe 90. The window pipe 90 has a slit 94 and is positioned on the base. A polarizing plate 60 is attached to the surface of the window 92. When the polarizing plate 60 is attached, it is preferable to use an adhesive having a small difference in refractive index between the polarizing plate 60 and the window 92.

  Note that a first carrier 110, a second carrier 120, and a third carrier 130 are provided on the base 105 of the package constituting the receiver 100. A lens 50 is mounted on the first carrier 110. The hybrid 20 is mounted on the second carrier 120. On the third carrier 130, the light detection unit 30 (PD and TIA) is mounted. A window pipe 90 is positioned and provided by a slit 94 on the side wall of the package constituting the receiver 100. The slit 94 is fitted with a notch (not shown) provided on the side wall of the package. That is, since the window pipe 90 holding the polarizing plate 60 and the second carrier 120 on which the hybrid 20 is mounted are positioned in a common package, the relative relationship between the polarizing plate 60 and the hybrid 20 is relatively small. Position accuracy is high. For this reason, the polarization direction of the polarizing plate 60 with respect to the light input part of the hybrid 20 can be determined with high accuracy.

  As described in the first to third embodiments, the polarizing plate 60 can be provided at any position on the optical waveguide from the input unit IN to the light detection unit 30. When the polarizing plate 60 is provided in the subsequent stage of the hybrid 20 as in the second embodiment, the polarization maintaining optical fiber PMF may be directly coupled to the hybrid 20 without providing the lens 50. However, in order to improve the optical coupling degree between the polarization maintaining optical fiber PMF and the hybrid 20, it is preferable to provide the lens 50.

In the first to third embodiments, the optical axis of the oscillation light E _{LO is} first adjusted and then the optical axes of the signal light E _SX and the signal light E _SY are adjusted. However, this order may be reversed. However, as in the first to third embodiments, by first adjusting the optical axis of the oscillation light E _LO , all of the light detection units 30 _X-I , 30 _X-Q , 30 _Y-I , 30 _Y-Q There is an effect that it is possible to confirm the positioning status of the. That is, according to the present invention, in addition to confirming the accuracy of the polarization direction of the input light, when the absolute value of the detection light falls below a predetermined value, it can be determined that there is a problem in positioning of the light detection unit.

Here, since the oscillation light E _LO is connected to all the optical output terminals of the hybrid 20, when positioning the oscillation light E _LO , the light detection units 30 _X-I , 30 _X-Q , 30 _Y Optical output is detected from all of _−I and 30 _YQ . In this case, if each light detecting section _{30 X-I, 30 X-} Q, 30 Y-I, 30 one of the optical detection result of the _Y-Q is, deviates from a predetermined value, the light detection unit is normal It can be determined that the positioning has not been performed. Thus, in order to confirm the positioning status of all the light detection units at a time, it is preferable to first position the polarization maintaining optical fiber of the oscillation light _ELO according to the present invention. The manufacturing quality of the optical receiver 100 can be improved by repairing a solid that has been determined that the positional deviation of the light detection unit is not normal or not flowing to a subsequent process.

Although it is not possible to confirm the positioning status of all photodetection units at once, depending on the manufacturing process, the absolute value of the detection light of the photodetection unit can be confirmed when positioning polarization maintaining optical fibers other than the oscillation light _ELO. Thus, the positioning state of the light detection unit can be confirmed.

In the first to third embodiments, the input unit IN _{X corresponds} to the first signal light input unit, the input unit IN _{Y corresponds} to the second signal light input unit, and the input unit IN _LO corresponds to the oscillation light input unit. The signal light E _{SX corresponds} to the first received signal light, and the signal light E _SY corresponds to the second received signal light.

In the first to third embodiments, the number of optical fibers for signal light input is two. However, when the polarizing beam splitter PBS in FIG. 1 is formed inside the hybrid 20, the number of optical fibers for signal light input. May be one. In that case, it is preferable to provide the polarizing plate 60 at the rear stage of the hybrid 20 as in the second embodiment. In the first to third embodiments, the number of optical fibers for oscillating light input is one. However, when the oscillating light _ELO is dispersed outside the receiver 100, the number of optical fibers for oscillating light input. May be two or more.

  The optical communication receivers according to Embodiments 1 to 3 use various communication methods (for example, PSK (Phase Shift Keying), DPSK (Differential Phase Shift Keying), and DQPSK (Differential Quadrature Phase Shift Keying). Etc.).

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

DESCRIPTION OF SYMBOLS 10 LO light source 20 Hybrid 30 Optical detection part 50 Lens 60 Polarizing plate 100 Receiver IN input part PMF Polarization-maintaining optical fiber

Claims (11)

An input unit to which received signal light is input by a polarization maintaining optical fiber;
A hybrid that optically couples the received signal light and the local oscillation light;
A light detection unit for detecting output light from the hybrid;
A polarizing plate provided on an optical path from the input unit to the light detection unit;
A coherent optical communication receiver.   The coherent optical communication receiver according to claim 1, wherein a polarization direction of the polarizing plate is equal to a polarization direction set in the hybrid.   The coherent optical communication receiver according to claim 1, further comprising a lens that optically couples the polarization maintaining optical fiber and the hybrid.   The coherent optical communication receiver according to claim 3, wherein the polarizing plate is provided between the lens and the hybrid.   The coherent optical communication receiver according to claim 3, wherein the polarizing plate is provided between the hybrid and the light detection unit.   The coherent optical communication receiver according to claim 3, wherein the polarizing plate is provided between the polarization maintaining optical fiber and the lens. The input unit is
A first signal light input unit to which the first received signal light is input;
A second signal light input unit to which the second received signal light is input;
The coherent optical communication receiver according to claim 1, further comprising: an oscillation light input unit to which local oscillation light is input. A signal light input unit to which received signal light is input by a polarization maintaining optical fiber for signal light;
A hybrid that mixes the received signal light and the local oscillation light;
A light detection unit for detecting output light from the hybrid;
A polarizing plate provided on an optical path from the signal light input unit to the light detection unit;
A method for adjusting the optical axis of a coherent optical communication receiver comprising:
A coherent optical communication receiver having a signal light optical axis adjustment step of adjusting the received light amount of the received signal light in the light detection unit to be larger than a predetermined amount while rotating the polarization maintaining optical fiber for signal light Optical axis adjustment method.   8. The signal light input unit includes a first signal light input unit to which a first reception signal light is input and a second signal light input unit to which a second reception signal light is input. The optical axis adjustment method of the receiver for coherent optical communication described in 1. A local oscillation light input unit to which the local oscillation light is input by a polarization maintaining optical fiber;
The coherent optical communication receiver according to claim 1, further comprising: a polarizing plate on an optical path from the local oscillation light input unit to the light detection unit. The coherent optical communication receiver includes an oscillation light input unit to which a polarization maintaining optical fiber for oscillation light for introducing the local oscillation light is connected,
A polarizing plate is provided on the optical path from the oscillation input unit to the light detection unit,
The step of adjusting the signal light optical axis is performed after the step of adjusting the received light amount of the local oscillation light in the light detection unit to be larger than a predetermined amount while rotating the polarization maintaining optical fiber for transmission. The method of adjusting an optical axis of a coherent optical communication receiver according to claim 7.

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