JP6566361B2 - Coherent receiver test method - Google Patents

Description

  The present invention relates to a method for testing a coherent receiver.

  As a high-speed and large-capacity optical communication system, for example, a coherent optical communication system is known. In a coherent receiver used in a coherent optical communication system, for example, signal light and local oscillation light (LO light) are processed by a polarization separation element and a 90-degree hybrid, and then converted from an optical signal to an electrical signal by a light receiving element. Do. For example, a method using a parameter called SPRR (Single Port Rejection Ratio) has been proposed for evaluating the characteristics of a coherent receiver (see Non-Patent Document 1, for example).

  In an optical device that receives a plurality of signal lights such as a coherent receiver, an optical path length of an optical path through which each signal light propagates is important. Therefore, a method for adjusting the optical path length has been proposed.

Yves Painchaud, three others, “Performance of balanced detection in a coherent receiver”, OPTICS EXPRESS, March 2, 2009, Vol. 17, No. 5, p. 3659-3672

  However, there is a case where the light intensity changes in the test result even though the input light intensity is kept constant in the test system having the adjusted optical path length. In this case, accurate testing is difficult.

  Therefore, an object is to provide a test method for a coherent receiver capable of correcting a test result.

  The coherent receiver test method according to the present invention includes a polarization filter that discriminates signal light, a light calculation unit that receives local oscillation light and signal light that is discriminated by the polarization filter, and performs light calculation by interference. A test method for a coherent receiver comprising a photoelectric conversion unit for converting the result of the optical calculation into an electric signal, wherein only the signal light or the local oscillation light is input, and in that case, from the optical calculation unit A first step of acquiring an output light intensity; a second step of inputting the signal light and the local oscillation light to perform a test of the coherent receiver; and a light intensity acquired in the first step. Based on the test result obtained in the second step, a third step of correcting an error due to a difference in light intensity input to the light calculation unit between the signal light and the local oscillation light; Including the.

  According to the above invention, the test result of the coherent receiver can be corrected.

(A) is a figure for demonstrating CMRR, (b) is a figure for demonstrating SPRR. 1 is a block diagram illustrating a test system to which a correction method according to Embodiment 1 is applied. It is a block diagram which illustrates an optical receiver. It is a figure for demonstrating the correction method. (A) And (b) is a figure which shows the measurement result of a frequency characteristic. It is an example of a flowchart.

[Description of Embodiment of Present Invention]
First, the contents of the embodiments of the present invention will be listed and described.

The present invention includes (1) a polarization filter that discriminates signal light, a light calculation unit that receives local oscillation light and signal light that is discriminated by the polarization filter, and performs light calculation by interference, and the light calculation A coherent receiver test method comprising: a photoelectric conversion unit that converts the result of the step 1 into an electrical signal, wherein only the signal light or the local oscillation light is input, and in this case, the light output from the optical calculation unit Based on the first step of acquiring the intensity, the second step of inputting the signal light and the local oscillation light and performing the test of the coherent receiver, and the light intensity acquired in the first step, And a third step of correcting an error caused by a difference in light intensity input to the light calculation unit between the signal light and the local oscillation light with respect to the test result obtained in the second step.
(2) The first step may be performed before or after the second step, and the first step and the second step may be repeated a plurality of times. Measurement accuracy is improved by multiple executions.
(3) The first step is executed by stopping the input of the local oscillation light and measuring the output light intensity of the light calculation unit obtained when the signal light is input to the coherent receiver. May be. In this case, the intensity of the light discriminated by the polarization filter can be measured using a coherent receiver.
(4) Prior to the first step to the third step, the polarization of the signal light or local oscillation light input to the coherent receiver is controlled, and the polarization of the polarization between the signal light and the local oscillation light is controlled. A polarization control step for adjusting the relative relationship may be included. In this case, accurate test results can be obtained by rough polarization control without optimizing the polarization.

[Details of the embodiment of the present invention]
A specific example of a coherent receiver test method according to an embodiment of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to a claim are included.

First, CMRR (Common-Mode Rejection Ratio) and SPRR (Single Port Rejection Ratio) will be described. CMRR is defined by the equation of FIG. FIG. 1A is a diagram for explaining CMRR. As illustrated in FIG. 1A, it is assumed that a pair of differential signal lights is received by the light receiving elements 12a and 12b. The light receiving elements 12a and 12b are connected to an amplifier (TIA) 36 and have a dual type connection configuration. Each signal light is intensity-modulated at frequency f. i ₁ (f) is a photocurrent output from the light receiving element 12a. i ₂ (f) is a photocurrent output from the light receiving element 12b.

| I ₁ (f) | is the absolute value of the current output from the light receiving element 12a when the intensity of the optical signal received by the light receiving element 12a is maximized. When the intensity of the optical signal received by the light receiving element 12a is maximized, the current output from the light receiving element 12b is minimized. | I ₂ (f) | is the absolute value of the current output from the light receiving element 12b when the intensity of the optical signal received by the light receiving element 12b is maximized. When the intensity of the optical signal received by the light receiving element 12b is maximized, the current output from the light receiving element 12a is minimized. | I ₁ (f) −i ₂ (f) | is the photocurrent i ₁ (f) output from the light receiving elements 12a and 12b when the light receiving element 12a and the light receiving element 12b receive optical signals having the same phase. And i ₂ (f).

CMRR is effective when i ₁ (f) and i ₂ (f) can be measured individually. However, it is difficult to measure i ₁ (f) and i ₂ (f) individually in a coherent receiver including an optical calculation unit. Therefore, when a test is performed on the coherent receiver, SPRR (Single Port Rejection Ratio) is used as the removal ratio. Here, the optical calculation unit is a multi-mode interference waveguide (MMI). The optical calculation is to spectrally and optically combine input signal light and reference light (local oscillation light: LO light) by optical interference. For example, the optical transmission signal is equally divided and output as output light. A transmission signal can be obtained by photoelectrically converting the output light with a light receiving element and demodulating the obtained electrical signal.

SPRR is defined by the equation in FIG. FIG.1 (b) is a figure for demonstrating SPRR. As illustrated in FIG. 1B, the light receiving elements 12a and 12b and the amplifier 36 similar to those in FIG. When the LO light is input without inputting the signal light to the coherent receiver, the light receiving element 12a and the light receiving element 12b receive the optical signals having the same phase. In this case, the voltage output from the amplifier 36 is Vout _LO . When both the signal light and the LO light are input to the coherent receiver, the absolute value when the voltage output from the amplifier 36 is maximum is | Vout _S21 |. By measuring this SPRR, it is possible to test a coherent receiver including an optical calculation unit.

  In the coherent receiver, the input light is discriminated inside by the polarization. For this reason, a polarization filter is built in the coherent receiver. In the SPRR test system, the polarization control unit optimizes the polarization. However, the amount of polarization may vary depending on the pressure and temperature applied to the optical fiber of the SPRR test system or the stability of the polarization controller. As a result, a change in light intensity may appear in the test result even though the input light intensity is kept constant in the test system having the adjusted optical path length. In this case, accurate testing is difficult. Therefore, in the following embodiment, a test method capable of correcting the test result of the coherent receiver will be described.

  FIG. 2 is a block diagram illustrating a test system to which the correction method according to the first embodiment is applied. In the test system of FIG. 2, a coherent receiver is used as the optical receiver 30. As illustrated in FIG. 2, intensity-modulated signal light emitted from an optical communication analyzer 14 (LCA: Lightwave Communication Analyzer) is branched into two by a splitter 16. One branched light is incident on the optical receiver 30 via the attenuator 18, the optical path length correcting means 20, and the polarization controller 22. The optical path length correcting means 20 may be an apparatus / method capable of correcting the optical path length, such as a manually operated delay line or an electric delay line. The other branched light is incident on the optical receiver 30 via the attenuator 18, the phase modulator 24, and the polarization controller 22. The other branched light is subjected to low-frequency phase modulation that is not synchronized with intensity modulation by the phase modulator 24 controlled by the function generator 26.

  Signal light and local oscillation light (LO light) are incident on the optical receiver 30. Here, the branched light not subjected to phase modulation is used as signal light, and the branched light subjected to phase modulation is used as local oscillation light. As illustrated in FIG. 2, the branched light used as the signal light and the branched light used as the local oscillation light propagate through different optical paths from the subsequent stage of the splitter 16. That is, the optical path 28a through which the branched light used as the signal light propagates is different from the optical path 28b through which the branched light used as the local oscillation light propagates.

  FIG. 3 is a block diagram illustrating the optical receiver 30. As illustrated in FIG. 3, the optical receiver 30 includes a polarization beam splitter (PBS) 32, a beam splitter (BS), 90-degree hybrids 10x and 10y, light receiving elements 12a and 12b, and an amplifier. 36 and a polarization rotation element 38.

  The signal light incident on the optical receiver 30 is separated into X-polarized light and Y-polarized light orthogonal to each other by the polarization separation element 32. The X-polarized light is incident on the 90-degree hybrid 10x on the X side. The Y-polarized light is rotated by 90 ° by the polarization rotation element 38 to become X-polarized light, and then enters the 90-degree hybrid 10y on the Y side. For example, TE light can be used as the X-polarized light, and TM light can be used as the Y-polarized light. However, the X-polarized light may be TM light and the Y-polarized light may be TE light.

  The LO light incident on the optical receiver 30 is branched into two by the beam splitter 34. For the LO light, light set in advance to X polarization is introduced by a polarization maintaining fiber. For this reason, the branched light branched by the beam splitter 34 is also X-polarized light. The branched lights branched by the beam splitter 34 are incident on the X-side 90-degree hybrid 10x and the Y-side 90-degree hybrid 10y, respectively.

  The 90-degree hybrids 10x and 10y are composed of an MMI that separates, synthesizes, and delays incident signal light and LO light in an internal optical waveguide and emits interference light from four ports. The 90-degree hybrids 10x and 10y can be configured by, for example, a planar lightwave circuit (PLC). The signal light incident on the 90-degree hybrid 10x on the X side is combined with the LO light, and then the positive component (Positive) and negative component (negative) of the in-phase component (In-Phase) and the quadrature component (Quadrature-Phase) ) And output as four interference signal lights (X-Ip, X-In, X-Qp, X-Qn). Of the four interference signal lights, X-Ip and X-In are a pair of differential optical signals, and X-Qp and X-Qn are also a pair of differential optical signals.

  Similarly, the signal light incident on the 90-degree hybrid 10y on the Y side is combined with the LO light, and then separated into a positive component and a negative component of an in-phase component and a quadrature component, and four interference signal beams (Y− Ip, Y-In, Y-Qp, Y-Qn). Of the four interference signal lights, Y-Ip and Y-In are a pair of differential optical signals, and Y-Qp and Y-Qn are also a pair of differential optical signals.

  The light receiving elements 12a and 12b are provided corresponding to the interference signal light emitted from the 90-degree hybrids 10x and 10y. Each of the light receiving elements 12a and 12b receives the interference signal light emitted from the 90-degree hybrids 10x and 10y, and generates a photocurrent by photoelectric conversion. The light receiving elements 12a and 12b are, for example, photodiodes (PD). The light receiving element 12a receives a positive component, and the light receiving element 12b receives a negative component. The pair of light receiving elements 12a and 12b are arranged in a dual type. The amplifier 36 converts a pair of photocurrents output from the pair of light receiving elements 12a and 12b into a voltage and amplifies the voltage. The amplifier 36 is a transimpedance amplifier (TIA), for example. The pair of electrical signals amplified by the amplifier 36 is output to the outside of the optical receiver 30.

  As illustrated in FIG. 2, the pair of electrical signals output from the optical receiver 30 is converted into a digital signal by an analog-digital conversion circuit 40 (ADC: Analog Digital Converter). One of the digital signals output from the analog-digital conversion circuit 40 is input to the optical communication analyzer 14. Thereby, the optical communication analyzer 14 can measure the frequency characteristic indicating the relationship between the modulation frequency of the intensity modulation and the intensity of the output signal from the optical receiver 30. Moreover, in the test system illustrated in FIGS. 2 and 3, the optical receiver 30 can be tested by measuring SPRR.

  However, as described above, a change in light intensity may appear in the test result even though the input light intensity is kept constant. Therefore, the test results are corrected as follows. First, the optical power is measured using the optical receiver 30 when only one of the signal light and the LO light is input to the optical receiver 30. Since only one of the signal light and the LO light is input, the optical calculation using the signal light and the LO light is not performed inside the optical receiver 30. In this case, if the polarization of the input light deviates from a predetermined value, the measured optical power changes accordingly. That is, the optical power after being filtered by the polarization separation element 32 (polarization filter) inside the optical receiver 30 can be obtained. If the test result is corrected using this, the accuracy of the test result can be increased.

FIG. 4 is a diagram for explaining the correction method. As illustrated in FIG. 4, “Single Port LO” is a current value I _LO output from each of the light receiving elements 12 a and 12 b when LO light is input to the optical receiver 30 without inputting signal light. It is. “Single Port Signal” is a current value I _Signal output from each of the light receiving elements 12 a and 12 b when signal light is input without inputting LO light to the optical receiver 30. The “S21 test” is a current value I _S21 output from each of the light receiving elements 12a and 12b when both signal light and LO light are input to the optical receiver 30. IphLO is a current value caused by LO light among the currents output from the light receiving elements 12a and 12b. IphSignal is a current value caused by the signal light among the currents output from the light receiving elements 12a and 12b. In the example of FIG. 4, there is a difference between I _LO and I _Signal . As a result, there are also differences between I _LO and I _Signal and I _S12 . This is because an error occurs due to a difference in light intensity between the signal light and the LO light input to the 90-degree hybrids 10x and 10y.

In this embodiment, the correction is performed so that I _LO and I _Signal approach I _S21 . Preferably, correction is performed so that I _LO and I _Signal coincide with I _S21 . Therefore, the SPRR (LO) of the LO light is corrected as shown in the following formula (1), and the SPRR (Signal) of the signal light is corrected as shown in the following formula (2). Thereby, I _LO and I _Signal are corrected so as to coincide with I _S21 .
SPRR (LO) = 20 × log (Vout _LO × I _S21 / I _LO / Vout _S21 ) (1)
SPRR (Signal) = 20 × log (Vout _Signal × I _S21 / I _Signal / Vout _S21 ) (2)
With these corrections, the test result of the optical receiver 30 can be corrected even if a change in the light intensity appears. As described above, it is possible to correct an error caused by a difference in light intensity between the signal light and the LO light input to the 90-degree hybrid 10x, 10y.

  In the above example, the intensity of the light discriminated by the polarization separation element 32 is measured by inputting only one of LO light and signal light to the optical receiver 30. In this case, the intensity of light discriminated by the polarization separation element 32 can be measured using the optical receiver 30. However, other methods may be used. For example, the amount of variation in the intensity of the light discriminated by the polarization separation element 32 may be measured by measuring the transmitted light or reflected light of the polarization separation element 32 with a light receiving element or the like.

  Further, in the test system, the intensity of the light filtered by the polarization separation element 32 in the optical receiver 30 by the correction method according to the present embodiment without optimizing the polarization of the signal light and the LO light by the polarization controller 22. If the correction based on is performed, accurate test results can be obtained only by roughly controlling the polarization of the signal light and the LO light. In this case as well, it is only necessary to give a correction similar to that described above to the test result. However, it is preferable to perform approximate alignment in the polarization direction. For example, it is preferable to perform matching (polarization matching) so that the polarization directions of LO light and signal light are the same combination of polarizations. According to this method, the number of man-hours required for adjusting the polarization direction during the test can be greatly reduced.

  A specific example of a test method for performing the above correction method after adjusting the optical path length will be described below. First, experimental results will be described. In the test system of FIG. 2, the optical path lengths of the optical paths 28a and 28b through which the signal light and the LO light propagate are deliberately shifted from the optimum values, and the delay time difference between the signal lights propagating through the optical paths 28a and 28b, respectively. Was 53 ps. In this state, the frequency characteristic was measured with the optical communication analyzer 14. The frequency characteristic was measured only once (1 sweep). FIG. 5A and FIG. 5B are diagrams showing measurement results of frequency characteristics.

  A thin line in FIG. 5A indicates measurement data, and a thick line indicates an envelope connecting the maximum values of the measurement data. For reference, envelopes in the case where the optical path lengths of the optical paths 28a and 28b are optimum are indicated by broken lines. As shown in FIG. 5A, the envelope (thick line) in the case where the optical path length is greatly deviated from the optimum value is output from the optical receiver 30 as compared with the envelope (dashed line) in the optimum case. It can be seen that the intensity of the signal is reduced.

  FIG. 5B shows a line B (= A × cos (πfτ)) obtained by multiplying an envelope (hereinafter referred to as “A”) with cos (πfτ) when the optical path length is optimal as a one-dot chain line. FIG. 6 is a diagram in which a line C (= A × sin (πfτ) / √2) multiplied by sin (πfτ) / √2 is added by a two-dot chain line. As shown in FIG. 5B, the measurement data (thin line) when the optical path length deviates greatly from the optimum value is B (= A × cos (πfτ)) line (dashed line) and C (= A × sin). It can be seen that the intensity changes in accordance with the phase modulation within the region surrounded by the line (two-dot chain line) of (πfτ) / √2). As a result, the envelope (thick line) connecting the maximum values of the measurement data is almost equal to the line connecting the maximum values of A × cos (πfτ) and A × sin (πfτ) / √2. I understand.

Furthermore, the measurement data (thin line) indicates that vibration due to phase modulation occurs at a frequency f at which the line B (= A × cos (πfτ)) and the line C (= A × sin (πfτ) / √2) intersect. It turns out that it has not occurred. Therefore, the following formula (3) is established.
cos (πfτ) = sin (πfτ) / √2 (3)
From the above equation (3), it can be seen that the delay time difference τ can be obtained by specifying the frequency f. Since the value obtained by multiplying the delay time difference τ by the speed of light is the optical path length difference between the optical paths 28a and 28b, the optical path length is corrected by the optical path length correcting means 20 in FIG. 2 based on the delay time difference τ. By doing so, the optical path length difference between the optical paths 28a and 28b can be made appropriate.

  Hereinafter, a flow of a correction method capable of correcting the test result of the coherent receiver after adjusting the optical path length will be described. FIG. 6 is an example of a flowchart. Hereinafter, the correction method will be described with reference to FIGS. 3 and 6. First, a test system is attached to a DUT (device under test) (step S1). Next, measurement system members (for example, attenuator 18, polarization controller 22, and phase modulator 24 in FIG. 2) used in the two optical paths 28a and 28b of the test system for measuring the device under test (for example, an optical device). The physical length such as the size is measured (step S2). The measurement of the physical length may be an approximation, for example. After measuring the physical length, the test system member and the optical receiver are connected by an optical fiber or the like, and a test system as shown in FIG. 2 is assembled (step S3). At this time, the optical path lengths of the two optical paths 28a and 28b are made different by making full use of the optical fiber, the adapter, the optical path length correcting means 20 and the like as necessary. For example, the optical path length difference between the two optical paths 28a and 28b is set to 4 mm or more. Thereafter, the optical receiver 30 connected to the test system is operated to adjust the relative relationship of polarization for each of the optical paths 28a and 28b (step S4). Thereby, the relative relationship of the polarization between the signal light and the LO light can be adjusted.

  Next, signal light subjected to intensity modulation is emitted from the optical communication analyzer 14 and is incident on the optical receiver 30. The emitted signal light is branched by the splitter 16, and one is incident on the optical receiver 30 via the attenuator 18, the optical path length correcting means 20, and the polarization controller 22. The other is passed through the attenuator 18, the phase modulator 24, and the polarization controller 22, is subjected to phase modulation at a low frequency of, for example, about 1 Hz by the phase modulator 24, and enters the optical receiver 30.

  As described with reference to FIG. 3, the optical receiver 30 includes the 90-degree hybrids 10x and 10y, the light receiving elements 12a and 12b, and the amplifier 36. The 90-degree hybrids 10x and 10y emit interference signal light by causing interference between two incident signal lights (signal light and LO light). The light receiving elements 12a and 12b receive the interference signal light and output a photocurrent. This photocurrent is amplified by the amplifier 36 and output to the outside of the optical receiver 30.

  The electrical signal output from the optical receiver 30 is converted into a digital signal by the analog-digital conversion circuit 40 and then input to the optical communication analyzer 14 to measure the frequency characteristics. For example, the frequency characteristic may be measured once (1 sweep). In step S3, since the optical path lengths of the two optical paths 28a and 28b are different, for example, measurement results of frequency characteristics as shown in FIGS. 5A and 5B are obtained.

From the measurement result of the frequency characteristic, a delay time difference between the signal lights propagating through the two optical paths 28a and 28b is obtained (step S5). For example, the frequency f _{0 at} which the influence of phase modulation is not seen in the frequency characteristics is specified, and the delay time difference τ is obtained using the above equation (3). That is, τ = {arctan (√2)} / (πf ₀ ) is calculated to obtain a delay time difference between the signal lights propagating through the two optical paths 28a and 28b. The frequency f _{0 at which} the influence of phase modulation is not observed can be in a range of ± 0.5 GHz from the frequency at which no fluctuation due to phase modulation occurs.

Next, a correction amount ΔL for equalizing the optical path lengths of the two optical paths 28a and 28b is obtained from the delay time difference obtained in step S5 (step S6). For example, when the speed of light in the optical path length correcting means 20 for correcting the optical path length is c ₀ , the correction amount ΔL is obtained by calculating ΔL = {arctan (√2)} × c ₀ / (πf ₀ ). be able to.

  Next, the optical path length of the optical path 28a is corrected by using the optical path length correcting means 20 so as to increase by the correction amount ΔL (step S7). Here, only the delay time difference obtained in step S5 does not tell which optical path length of the two optical paths 28a and 28b is longer (or shorter). Accordingly, in step S7, the optical path length may be corrected in the opposite direction, that is, the place where the optical path length of the optical path 28a should be shortened may be lengthened.

  Therefore, after correcting the optical path length in step S7, it is confirmed whether the optical path length is corrected correctly (step S8). For example, after correcting the optical path length, the output signal from the optical receiver 30 is input to the optical communication analyzer 14 to measure the frequency characteristics. Then, it is confirmed whether or not the frequency at which the influence of phase modulation is not seen in the frequency characteristics is lower than before the correction. If the frequency is low, the optical path length is not correctly corrected (No in step S8), so the process returns to step S7, and the optical path 28a is shortened by the correction amount ΔL using the optical path length correction means 20. The optical path length is corrected.

  In step S8, if the frequency where the influence of phase modulation is not seen in the frequency characteristic is high or the point where the influence of phase modulation is not seen disappears, the optical path length has been corrected correctly. (Yes in step S8), the adjustment of the optical path length is completed. In step S7, first, the optical path length of the optical path 28a is changed so as to increase by the correction amount ΔL. However, even if the optical path length of the optical path 28a is changed so as to decrease by the correction amount ΔL. Of course.

Next, the SPRR measurement is started (step S9). Specifically, Vout _LO , Vout _Signal, and Vout _S21 are measured. Vout _LO , Vout _Signal, and Vout _S21 are preferably measured a plurality of times. This is because the measurement accuracy is improved. For example, Vout _LO , Vout _Signal, and Vout _S21 may be measured at predetermined time intervals to calculate the average value. Next, the amount of current output from each light receiving element 12a, 12b is monitored (step S10). Specifically, when only signal light is input to the optical receiver 30, the light intensity when only LO light is input to the optical receiver 30 is acquired. This light intensity can be obtained by monitoring the amount of current output from the light receiving elements 12a and 12b. Further, both the LO light and the signal light are input to the optical receiver 30, and the amount of current output from the light receiving elements 12a and 12b when the coherent receiver is tested is monitored. Thereby, I _LO , I _Signal and I _S21 are obtained. The measurement of I _LO , I _Signal and I _S21 is preferably performed a plurality of times. This is because the measurement accuracy is improved. For example, the average values may be calculated by measuring I _LO , I _Signal and I _S21 at predetermined time intervals. The order in which step S9 and step S10 are performed is not particularly limited.

  If these values are obtained, the SPRR measurement is terminated (step S11). Using this result, current amount correction is performed on the test result of each SPRR using the above formula (1) and the above formula (2) (step S12). As a result, it is possible to correct an error caused by a difference in light intensity input to the optical calculation unit for the signal light and the LO light. Next, the corrected SPRR test result is extracted (step S13). Next, the test system is removed from the device under test (DUT) (step S14).

According to this embodiment, I _LO and I _Signal are corrected so as to approach I _S21 . Preferably, correction is performed so that I _LO and I _Signal coincide with I _S21 . Thereby, even if a change appears in the light intensity, the test result of SPRR can be corrected.

    10x, 10y 90 degree hybrid, 12a, 12b light receiving element, 14 optical communication analyzer, 16 splitter, 18 attenuator, 20 optical path length correcting means, 22 polarization controller, 24 phase modulator, 26 function generator, 28a, 28b optical path, 30 Optical receiver, 32 Polarization separation element, 34 Beam splitter, 36 Amplifier, 38 Polarization rotation element, 40 Analog-digital conversion circuit

Claims (4)

A polarization filter for discriminating signal light, a local oscillation light and a signal light discriminated by the polarization filter are input, an optical calculation unit for performing optical calculation by interference, and converting the result of the optical calculation into an electric signal A test method for a coherent receiver comprising:
A first step of inputting only the signal light or the local oscillation light, and obtaining the light intensity output from the optical calculation unit in that case;
A second step of inputting the signal light and the local oscillation light and performing a test of the coherent receiver;
Based on the light intensity acquired in the first step, the test result acquired in the second step is used to correct an error caused by a difference in light intensity input to the light calculation unit between the signal light and the local oscillation light. A coherent receiver testing method comprising: a third step of:   The coherent receiver testing method according to claim 1, wherein the first step is performed before or after the second step, and the first step and the second step are repeatedly performed a plurality of times.   The first step is performed by stopping the input of the local oscillation light and measuring the output light intensity of the light calculation unit obtained when the signal light is input to the coherent receiver. Item 3. A test method for a coherent receiver according to Item 1 or 2.   Prior to the first step to the third step, the polarization of the signal light or local oscillation light input to the coherent receiver is controlled, and the relative relationship of polarization between the signal light and the local oscillation light is controlled. The coherent receiver test method according to claim 1, comprising a polarization control step of adjusting.

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