Measurement device for optical receiver and measurement method

Abstract

A CMRR measurement device according to the present disclosure uses, as a test light, a broad-band light source and not a modulated light requiring sweeping by a light component analyzer (LCA). A CMRR can be acquired using low-cost general-purpose measuring equipment such as a spectrum analyzer or A / D converter as the electric signal measuring equipment. The broad-band light source can be combined with a continuous-wave (CW) light source to be used as the test light. A CMRR can be obtained very quickly because expensive measuring equipment such as an LCA is not necessary and there is no need for repeated sweeping with a modulated light.

Description

Optical receiver measuring device and measuring method

[0001] The present invention relates to an apparatus and method for measuring an optical receiver.

[0002] Modern society relies on high-speed communication networks to support social media, video streaming, car navigation, and other services. Optical communication networks, which form the backbone of modern society, use wavelength division multiplexing and digital coherent communication technologies to achieve transmission capacities of several tens of terabits per second (Tbps) over a single optical fiber. Unlike the intensity-modulated direct detection (IMDD) technology used in short-distance communications, digital coherent communication technology achieves high-capacity transmission by encoding information in the phase of light, and also enables colorless reception, eliminating the need for wavelength-specific filters.

[0003] In wavelength division multiplexing communications, in which signals of various wavelengths are multiplexed, a coherent optical receiver, instead of extracting the desired channel using an optical filter or the like and decoding the optical signal using a photodiode (PD), superimposes the signal light with a local oscillator (LOC) of the same frequency and extracts the received signal from the resulting beat signal. This technology is colorless, regardless of the wavelength of the target received light, and it is possible to extract only the optical signal of the channel that matches the wavelength of the LOC without using bulky and costly optical filters. Light other than the desired channel generates noise when detected by the PD. The noise from light other than the desired channel is theoretically canceled out by the ideal balanced receiving operation of the coherent optical receiver and is removed from the output signal obtained from the PD current.

[0004] However, if the overall positive / negative balance of a coherent optical receiver, from the optical hybrid, PD, and electrical amplifier, is disrupted, the above-mentioned noise is not completely canceled in the positive / negative signal path, degrading the reception characteristics. The single port rejection ratio (SPRR) is an indicator of the quality of the balance in a coherent optical receiver. Although the name SPRR is based on the definition of the optical input port, it is also commonly referred to as the common mode rejection ratio (CMRR). Correct evaluation of this SPRR (CMRR) is necessary for evaluating the characteristics of a coherent optical receiver. The following explanation uses the commonly used term CMRR.

[0005] In a coherent optical receiver, noise coming from only one of the two optical fiber input terminals, specifically intensity modulation noise contained in the signal light and RIN (Relative Intensity Noise) noise contained in the local oscillator light, becomes a problem. The CMRR of a coherent optical receiver is an index that indicates how much of this noise can be removed from the beat signal between the local oscillator light and the signal light that should be detected. Typically, a CMRR is defined for each of the four channels (XI, XQ, YI, and YQ for two polarizations) contained in a coherent optical receiver.

[0006] FIG. 10 illustrates the definition of CMRR in a coherent optical receiver. FIG. 10 shows one channel of a coherent optical receiver, which is a device under test (DUT) 110 having a signal light input terminal (sig) and a local optical light input terminal (LO). The coherent optical receiver includes an optical hybrid 113 and a balanced PD 114 including a positive-side PD and a negative-side PD. If the currents flowing through the two PDs are Ip and In, respectively, the current output from the midpoint of the balanced PD can be written as Ip-In. CMRR is obtained by dividing the value obtained by numerator measurement using the measurement system shown in the upper part of FIG. 10 by the value obtained by denominator measurement using the measurement system shown in the lower part of FIG. 10.

[0007] In the numerator measurement shown in the upper part of Fig. 10, a signal light E1, which is a test light, is input from one optical input terminal, and as described above, the current output from the balance PD is represented as Ip-In. In the denominator measurement shown in the lower part of Fig. 10, two test lights 121 and 122 (electric fields E2 and E2e) which are different in phase by φ are output from two optical input terminals. jΦ ) is input, and the phase difference of the test light at this time is denoted as φ. By changing the phase φ of the test light 122, the maximum positive and negative PD current outputs can be obtained, and the PD current value equivalent to |Ip| + |In| can be obtained from the two maximum PD current values. The CMRR is defined as the ratio of these two current values ​​(|Ip - In| and |Ip| + |In|) and can be expressed in dB as follows:

[0008] The above definition of CMRR uses the PD current, but it is also possible to connect an amplifier such as a transimpedance amplifier (TIA) 115 (shown by a dotted line) downstream of the balanced PD to convert the current output into a voltage and calculate the CMRR. If the same coefficient is multiplied in the numerator and denominator, the current ratio can be calculated by taking the voltage ratio. When a TIA or similar is connected, a voltage output greater than the minute current from the PD can be obtained. When using voltage output, the PD and amplifier configurations can take various forms, as will be described later.

[0009] In numerator measurement, light 120 input from one optical input terminal (sig) is split by the optical hybrid 113 and directly input to two PDs. At this time, if the balance between positive and complementary light is good in the two paths from the split by the optical hybrid 113 to the two PDs of the balance PD 114, Ip≈In, and the numerator in the fraction of the CMRR definition equation is approximately 0. Because the definition equation CMRR is a value close to 0 and much smaller than 1, if the CMRR value is expressed in dB, it will be a large negative value.

[0010] Since a coherent optical receiver has two optical input terminals, there are two types of characteristic quantities: CMRR_sig related to the signal light input terminal and CMRR_LO related to the local light input terminal. CMRR_sig related to the signal light input port is important for colorless applications, while CMRR_LO related to the local light input terminal is relevant to removing RIN generated by the local light laser. The method described in Non-Patent Document 1 is widely known as a conventional technique for measuring CMRR.

[0011] FIG. 11 shows a conventional CMRR measurement system using an optical component analyzer. (a) of FIG. 11 shows a CMRR numerator measurement system, and (b) shows a CMRR denominator measurement system. In the denominator measurement system 500-d in (b), high-frequency modulated light 123 from an LCA 501 is split into two by a coupler 502, and the split test lights 125 and 126 are input to the signal light input terminal (sig) and local light input terminal (LO) of a DUT 110, which is a coherent optical receiver. Each test light passes through a variable optical attenuator (VOL), a variable delay line (VDL), and a polarization controller (PC) or a phase modulator (PM) before being input to each terminal. The intensities of the test lights 125 and 126 input to the two input terminals are set to be approximately the same level at the PD inside the coherent optical receiver.

[0012] Specifically, this utilizes the fact that the loss of a general optical hybrid is almost the same for each branch. LO ) is equally distributed to the polarized waves X and Y, and the optical intensity Pin X_LO Is P LO On the other hand, since the test light from the light component analyzer (LCA) is polarized, by adjusting the polarization state using the polarization controller (PC) 505, only the X-polarized light is input to the signal light input terminal at the level Pin x_sig At this time, the optical input from the signal light input terminal is concentrated only on the X polarized wave, whereas the optical input from the local light input terminal is equally distributed between the X polarized wave and the Y polarized wave, as described above.

[0013] Therefore, the optical input intensity P from the local light input terminal LO The optical input intensity P from the signal light input terminal sig By doubling the input power of the X-polarized wave, the optical intensities incident on the two input terminals of the X-polarized wave channel are expressed by the following equation: x_sig =Psig=P LO / 2=Pin x_LO

[0014] As mentioned above, in the case of signal light input (Pin x_sig ) and local light input (Pin x_LO When measuring the Y-polarized channel, the polarization state can be adjusted by the PC 505 so that only Y-polarized light enters.

[0015] In the denominator measurement system 500-d described above, the split test light beams 125 and 126 are multiplexed and interfered in the optical hybrid within the DUT 110, resulting in a current output from the PD. When a single test light beam 123 is split into two and input, and then re-multiplexed and interfered within the DUT, fluctuations in the phase of the optical fiber in the measurement system outside the DUT cause large fluctuations in the optical power values ​​received by each PD. The test system paths from the coupler 502 to the two input terminals in Figure 11(b) are connected by optical fiber, and the phase of the optical fiber, i.e., the optical path length, fluctuates constantly due to temperature fluctuations and vibrations.

[0016] In the denominator measurement using the conventional technique shown in Figure 11, the LCA generates an amplitude-modulated signal in the range of, for example, 0 to 50 GHz, and this test light 123 is input from the LCA 501. The aforementioned optical path length fluctuations occur continuously, even over the course of, for example, 10 seconds, during which the modulated signal is frequency-swept, making the received light intensity values ​​of each PD highly unstable. To address this fluctuation in the received light power of each PD, the LCA repeats frequency sweep measurements dozens of times and uses only the optimal results. That is, after repeated frequency sweeps, the current value at which the balanced PD current output is maximized is selected for each frequency point from the dozens of measurements, and this is used as the measurement result. From the results of repeated sweep operations, the maximum output when light is concentrated on only one of the two PDs can be selected and obtained. This maximum current value is then combined for each frequency point at appropriate frequency intervals for the two PDs to obtain the results. Note that in conventional measurements, natural phase fluctuations in the optical fiber alone may not be sufficient to reach the maximum value. For this reason, in the denominator measurement system of FIG. 11(b), a phase modulator (PM) 506 is used to vary the wide range of phase conditions until the maximum value is reached.

[0017] As explained in Figure 10 and the definition of the CMRR, the maximum values ​​of Ip and In must be acquired in the denominator measurement. The PD current values ​​obtained as a result of repeating the above-mentioned sweep operation of the modulation signal correspond to |Ip| and |In| in equation (1), and the denominator measurement of the CMRR is realized.

[0018] In the numerator measurement shown in Figure 11(a), the test light 123, which is obtained by amplitude-modulating the carrier light with the modulation signal swept by the LCA 501, is input directly to one of the optical input terminals and measured. Depending on whether the CMRR to be measured is the local light input CMRR_LO or the signal light input CMRR_sig, the test signal is input to the corresponding optical input terminal of the DUT 110 (the Sig terminal in Figure 11(a)). In the numerator measurement, there are no branch paths in the measurement system, so interference does not occur in the test light. Therefore, a stable numerator measurement result can be obtained by sweeping the frequency of the modulation signal only once. The CMRR in equation (1) can be obtained by dividing the numerator measurement result obtained using the above method by the denominator measurement result.

[0019] V. Painchaud et al., “Performance of balanced detection in a coherent receiver” OPTICS EXPRESS Vol. 17, No. 5 / pp. 3659 (2009)

[0020] However, the actual CMRR measurement method in the prior art had the following problems. First, it required expensive measurement equipment such as an optical component analyzer. Second, it required multiple frequency sweeps when measuring the denominator, which resulted in the measurement taking more than several minutes. Furthermore, optical receivers that integrate coherent optical receivers and DSPs, which have been developed in recent years, do not have electrical output terminals and therefore cannot be connected to an optical component analyzer. Therefore, the measurement itself was not possible using the prior art CMRR measurement method.

[0021] The present invention has been made in view of the above-mentioned problems, and provides a measurement device and a measurement method that can perform CMRR of a coherent optical receiver at high speed with a simpler measurement configuration.

[0022] One aspect of the present invention is an apparatus for measuring the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal light input terminal and a local optical light input terminal, an optical hybrid connected to the signal light input terminal and the local optical light input terminal, and a photodiode connected to the output of the optical hybrid, the apparatus comprising: a broadband light source that outputs test light having a coherence length longer than the optical path length of one interferometer inside the optical hybrid and having a spectral wavelength width corresponding to at least twice the frequency band of the CMRR to be measured; and an electrical measuring device for measuring an electrical signal from the coherent optical receiver, wherein the test light from the broadband light source is input to at least one of the signal light input terminal or the local optical light input terminal in each of the numerator and denominator measurements of the CMRR.

[0023] Another aspect of the present invention is a method for measuring the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal light input terminal and a local light input terminal, an optical hybrid connected to the signal light input terminal and the local light input terminal, and a photodiode connected to an output of the optical hybrid, the method comprising the steps of: in molecular measurement of the CMRR, inputting test light from a broadband light source to at least one of the signal light input terminal or the local light input terminal and acquiring a first electrical signal by an electrical measuring instrument; in molecular measurement of the CMRR, inputting the test light from the broadband light source to at least one of the signal light input terminal or the local light input terminal and acquiring a second electrical signal by the electrical measuring instrument; and calculating the CMRR based on the first electrical signal and the second electrical signal, wherein the test light has a coherence length longer than the optical path length of one interferometer inside the optical hybrid and a spectral wavelength width equivalent to at least twice the frequency band of the CMRR to be measured.

[0024] The CMRR measurement device disclosed herein can quickly and accurately obtain the CMRR of a coherent optical receiver simply by acquiring data using a spectrum analyzer or ADC and performing calculations.

[0025] FIG. 1 is a diagram showing the configuration of a CMRR measurement device for a signal light terminal of embodiment 1. FIG. 2 is a diagram showing the configuration of a CMRR measurement device for a local light terminal of embodiment 1. FIG. 3 is a diagram explaining the internal configuration of a coherent optical receiver and the propagation state of test light. FIG. 4 is a diagram explaining variations in combinations of PDs and electric amplifiers. FIG. 5 is a flow chart of the measurement procedure in the CMRR measurement device of embodiment 1. FIG. 6 is a diagram showing measurement results of the frequency dependence of CMRR in comparison with the prior art. FIG. 7 is a diagram showing the configuration of a CMRR measurement device for a signal light terminal of embodiment 2. FIG. 8 is a flow chart of the measurement procedure in the CMRR measurement device of embodiment 2. FIG. 9 is a diagram showing measurement results of the frequency dependence of CMRR in comparison with the prior art. FIG. 10 is a diagram explaining the definition of CMRR in a coherent optical receiver. FIG. 11 is a diagram showing a CMRR measurement system using an LCA according to the prior art.

[0026] The CMRR measurement device and measurement apparatus disclosed herein use a broadband light source as test light. A spectrum analyzer or an A / D converter is used as an electrical signal measurement device, allowing CMRR to be obtained using a general-purpose measurement device that is less expensive than an LCA. A broadband light source and a continuous wave (CW) light source can also be combined as test light. This means that expensive measurement devices are not required, and there is no need to repeatedly sweep modulated light, allowing CMRR to be obtained in a very short time.

[0027] The CMRR measurement device and measurement method disclosed herein do not use test light intensity-modulated by a swept modulation signal, but use a broadband light source, thereby enabling numerator and denominator measurements of CMRR to be performed in a short time.

[0028] Furthermore, even if the coherent optical receiver and the DSP are integrated and there is no analog electrical signal output terminal, if the DSP has a built-in A / D converter, the CMRR can be calculated by using the digital value output from this converter. The specific configuration and measurement procedure of the CMRR measurement device of the present disclosure will be described below.

[0029] [Embodiment 1] Fig. 1 is a diagram showing the configuration of a CMRR measurement device for a signal light input terminal (sig) of embodiment 1. Fig. 1(a) shows a measurement system 100-n for measuring the numerator of CMRR, and (b) shows a measurement system 100-d for measuring the denominator of CMRR. DUT 110 is a coherent optical receiver to be measured for CMRR, and is equipped with two optical input terminals to which optical fibers are connected and through which test light is input, and an electrical signal output terminal connected to electrical measuring instrument 120. Typically, a coherent optical receiver has four output channels, so DUT 110 includes electrical signal output terminals for four channels.

[0030] FIG. 2 is a diagram showing the configuration of a CMRR measurement device for a local optical signal input terminal (LO) according to the first embodiment. FIG. 2(a) shows a measurement system 101-n for measuring the numerator of the CMRR, and FIG. 2(b) shows a measurement system 100-d for measuring the denominator of the CMRR. The denominator measurement systems in FIG. 1(b) and FIG. 2(b) have the same configuration. The only difference between FIG. 1 and FIG. 2 is the optical input terminal (sig, LO) of the test light 130 to the coherent optical receiver (DUT) 110 in the numerator measurement, and therefore the following description will be based on FIG. 1.

[0031] The electrical measuring instrument 120 can acquire a power spectrum, which is frequency-dependent electrical power, as an electrical signal by connecting a spectrum analyzer to the electrical signal output terminal of each output channel. Alternatively, an A / D converter can be used to convert the electrical signal of each channel into digital data, and this data can be Fourier-transformed to obtain a power spectrum similar to that obtained by a spectrum analyzer.

[0032] The CMRR measurement apparatus of this embodiment uses two test light sources: a wideband light source (WBS) 103 and a continuous wave (CW) light source 102. The wideband light source 103 can be an amplified spontaneous emission (ASE) light source, a supercontinuum (SC) light source, or a super luminescent diode (SLD) light source. The test light from the wideband light source 103 is incoherent light with no phase alignment, and it is sufficient if it has a wide wavelength width corresponding to a frequency width at least twice the frequency bandwidth required for measuring the CMRR. Here, the wavelength width of the test light from the wideband light source 103 refers to the full width at half maximum of the test light spectrum on the wavelength axis. For example, a wavelength width of 1 nm for a 1.5 μm band signal light corresponds to a frequency-equivalent bandwidth of 125 GHz, making it possible to measure CMRR at frequencies of approximately 62.5 GHz or less. The frequency used for transmission and requiring CMRR is smaller than the baud rate, so the corresponding frequency band of the wavelength width needs to be wider than the baud rate. Also, the wavelength of CW light is included in the wavelength bandwidth of broadband light.

[0033] The CW light source can be, for example, a laser light source conforming to the ITLA (Integrated Tunable Laser Assembly) specifications for communications standardized by the Optical Fiber Forum (OIF). The test light from the CW light source is monochromatic light, outputting light with high coherence that is aligned in wavelength and phase, and having a wavelength width narrower than, for example, about 10 MHz. The wavelength of the CW light source is set to fall within the wavelength width range of the broadband light source. For example, it may be set to be at the center of the wavelength width of the broadband light source.

[0034] In the molecular measurement system 100-n for CMRR_sig shown in FIG. 1A, the broadband light and CW light from the two light sources 102 and 103 are combined by a coupler 105 and then input as test light 130 to the signal light input terminal of the DUT. Therefore, in the molecular measurement system 100-n, the combined test light 130 travels through a single optical fiber and the same path within the DUT 110 to reach the PD within the DUT 110. In the molecular measurement system 101-n for CMRR_LO shown in FIG. 2A, the broadband light and CW light are combined by a coupler 105 and then input as test light 130 to the local oscillator input terminal of the DUT via a polarizer 104. The polarizer 104 is used to limit the polarization of the test light 130 to the polarization designed for the local oscillator input terminal of the coherent optical receiver.

[0035] In the denominator measurement system 100-d for CMRR_sig shown in FIG. 1B, broadband light 131 from a broadband light source 103 is input to the signal input terminal. Furthermore, CW light from a CW light source 102 is polarized by a polarizer 104, and the polarized CW light 132 is input to the local light input terminal. The polarizer 132 and the local light input terminal are connected by a polarization-maintaining fiber 107, maintaining the polarization state. In the case of the denominator measurement system 100-d, the broadband light 131 and the CW light 132 each travel through different paths in the optical hybrid within the DUT 110 to reach the PD. If the output of the CW light source is already polarized and can be matched to the polarization designed at the local light input terminal of the coherent optical receiver, the polarizer 104 is not necessary.

[0036] In both the numerator measurement system 100-n and the denominator measurement system 100-d, different test lights from two light sources are input to the DUT, but as will be explained later, the light is incident on the two PDs of the balanced PD via the same path through the optical hybrid in the case of the numerator measurement and via different paths through the optical hybrid in the case of the denominator measurement. This allows the CMRR to be determined by taking the ratio of the output power spectra of the two measurements, as will be further explained in conjunction with FIG.

[0037] The electrical signal output from the coherent optical receiver (DUT) is input to an electrical measuring instrument 120 such as a spectrum analyzer and recorded as electrical signal data. From the ratio of the electrical signal data when the numerator is measured to the electrical signal data when the denominator is measured, the CMRR value can be calculated for each modulation frequency according to equation (1).

[0038] FIG. 3 illustrates an example of the internal configuration of a coherent optical receiver and the propagation state of test light. (a) of FIG. 3 illustrates the case of numerator measurement for CMRR_sig, corresponding to the operation in numerator measurement system 100-n in FIG. 1(a). A polarization beam splitter (PBS) 111 is disposed on the signal light input side of coherent optical receiver 110, and a beam splitter (BS) 112 is disposed on the local light input side. The outputs of the PBS and BS are further input to polarization multiplexed optical hybrid 113, whose outputs constitute a total of four receiving channels: I and Q channels of X polarization, and I and Q channels of Y polarization. The interference light output of each channel of polarization multiplexed optical hybrid 113 is detected by the two corresponding balanced PDs 114 and converted into an electrical signal as a PD current. (b) of FIG. 3 illustrates the case of denominator measurement for CMRR_sig, corresponding to the operation in denominator measurement system 100-d in FIG. 1(b). In both (a) and (b) of FIG. 3, the focus is on the balance PD of the top channel of the four channels, and the path of the test light reaching this balance PD is shown in bold.

[0039] In the molecular measurement of FIG. 3(a), the electric field Es of light reaching two PDs from a light source is determined by inputting test light 130 from one signal light input terminal. In the first embodiment, test light 130 is a mixture of broadband light and CW light. If the electric field of broadband light is E1 and the electric field of CW light is E2, both lights arrive via the same single path, so there is no difference in the relative phase of the two lights between the p-side PD and the n-side PD. Therefore, if the electric field of light reaching the p-side PD is |E1 + E2|, the electric field of light reaching the n-side PD can also be written as |E1 + E2|. Furthermore, because broadband light has a wide wavelength width, this can be expressed as the electric field E for each discrete wavelength i. 1i When expressed as the sum of the above, it is expressed as follows:

[0040] At this time, the PD currents Ip and In due to the light incident on each PD are proportional to the square of the incident electric field, and are expressed as follows, where α is the proportionality coefficient:

[0041] CW light is light having a single wavelength, and the beat signal |E2| with the CW light itself is 2 contains only low-frequency components below MHz, and therefore does not appear in the output frequency spectrum of the PD current, which normally measures frequencies above 10 MHz. As will be described later, in the measurement system of Figure 1, the optical intensity of the broadband light is set to a level 10 dB or more weaker than that of the CW light, so the beat signal |E1| 2 The value of is also smaller than the beat signal of E1 and E2 expressed by the following equation (3-1).

[0042] Therefore, equation (2-2) is simplified as follows:

[0043] Here, if P0 is defined as follows, the relationship of equation (3-3) holds for the PD currents Ip and In.

[0044] P0 is the optical power generated by the beat of broadband light from a broadband light source, representing a random optical signal with a frequency band close to the wavelength width of the broadband light. The electrical output from the two PDs of a balanced PD generates a differential signal equivalent to ΔI = |Ip-In| in the case of PD current, depending on the degree of device asymmetry between the p-PD and n-PD, i.e., the imbalance between the positive and negative paths. According to the definition of CMRR described in equation (1), the numerator in the log term can be calculated by calculating |Ip-In|. |Ip-In| can be calculated from the difference in the currents of the two PDs of a balanced PD by measuring the numerator in Figures 1(a) and 3(a). In other words, the value equivalent to the numerator in the log term of the CMRR definition can be obtained directly from the differential output ΔI. Furthermore, this current can be converted to a voltage using a 50 Ω system or a TIA, and the power spectrum An(f) corresponding to the numerator measurement can finally be calculated.

[0045] Next, although the definition of CMRR (1) uses the PD current, the CMRR can also be calculated by converting the current output to voltage. If the same coefficient is multiplied in the numerator and denominator, the current ratio can be calculated by taking the voltage ratio. If a TIA or similar is connected, a voltage output greater than the minute current from the PD can be obtained.

[0046] In the denominator measurement of Fig. 3(b), broadband light 131 is input from the signal light input terminal, and CW light 132 is input from the local light input terminal. The polarization multiplexed optical hybrid 113 detects the light (electric field E S ) and the light from the local light input terminal (electric field E L The p-PD and n-PD are designed so that the phases of the light reaching them are shifted by 180° with respect to the relative phase of the broadband light 131 entering from the signal light input terminal and reaching the p-PD on the p side, and the electric field of the CW light 132 entering from the local light input terminal and reaching the n-PD on the n side is defined as E1 and E2, respectively. If the absolute value of the electric field entering the p-PD is |E1 + E2|, then the absolute value of the electric field entering the n-PD can be written as |E1 - E2|.

[0047] In CMRR measurement, in order for the light waves input and output to the optical hybrid to maintain the above-mentioned phase relationship as designed, the coherence length of the test light from the light source must be longer than the optical path length of the interferometer inside the optical hybrid. If the wavelength of the light from the light source is λ and the wavelength width is Δλ, the coherence length is λ 2 For example, when the optical path length L of the interference system inside the optical hybrid is 0.5 mm for a wavelength of 1.55 μm, the condition for the optical path length L is Δλ<λ 2 / L= (1.55×10 -6 ) 2 / 0.5 x 10 -3 = 5 nm. Therefore, the wavelength width of the spectral band of the broadband light needs to be narrower than 5 nm. Here, the optical path length of the interferometer inside the optical hybrid refers to the optical path length of the optical coupling and interference path configured so that the signal light and the local light are coupled inside the optical hybrid and a phase difference of 180 degrees is obtained when they enter the two PDs of one channel.

[0048] At this time, the output currents Ip and In of the PD are proportional to the square of the electric field incident on the PD, and are expressed as follows, where α is the proportionality coefficient:

[0049] In equations (4) and (5), the first two terms have the same sign, so they cancel out and almost disappear in the differential current ΔI of the p-PD and n-PD. On the other hand, the third terms in equations (4) and (5) have opposite signs, so they are added together in the differential current and become dominant, and the output current Iout obtained as the differential output current of the p-PD and n-PD is expressed by the following equation:

[0050] According to the CMRR definition equation described in equation (1), the denominator in the log term can be calculated by adding |Ip| and |In|. Referring to equation (3-3) for the numerator measurement in the CMRR measurement apparatus of this embodiment, |Ip| and |In| each approximately equal the value αP0. Furthermore, referring to equation (6), 2αP0 is calculated by the denominator measurement in Figures 1(b) and 3(b), and this value corresponds to the sum (|Ip| + |In|) of |Ip| (approximately P0) and |In| (approximately P0). Therefore, the value corresponding to the denominator in the log term of the CMRR definition equation can be directly obtained from the output of the denominator measurement system in Figures 1(b) and 3(b). Furthermore, this current can be converted to a voltage using a 50 Ω system or a TIA, ultimately yielding the power spectrum Ad(f) corresponding to the denominator measurement.

[0051] If the CMRR corresponding to equation (1) is found using An(f) obtained by measuring the numerator and Ad(f) obtained by measuring the denominator, it can be calculated using the following equation, since power is proportional to the square of the current and log is used when expressed in dB.

[0052] In the measurement system of Figure 1, when the output of the DUT 110 is connected to a 50Ω measurement system, 50 × Iout is obtained as a voltage output. Also, when a TIA (not shown) is connected to the output side of each PD of the coherent optical receiver 110 of Figure 3, the transimpedance is set to Z t As Z tIn both the numerator and denominator measurements, the power spectrum of the voltage output can be obtained by the electrical measuring device 120.

[0053] As described above, the numerator measurement system 100-n in FIG. 1A obtains ΔI = |Ip - In| or the value of the power spectrum An(f) corresponding to ΔI, while the denominator measurement system 100-d in FIG. 1B obtains |Ip| + |In| or the value of the power spectrum Ad(f) corresponding to this. The CMRR can be calculated for each frequency band by taking the ratio of these two values. However, the numerator and denominator measurement systems have different configurations of the paths from the two light sources 102 and 103 to the optical input terminals (sig, LO) of the DUT, and the optical loss from the light source to the PD for each test light is different. To obtain the correct CMRR value, the absolute value of the electric field of the light reaching the PD must be consistent.

[0054] Specifically, the output power spectrum must be acquired under the condition that the absolute value of the electric field of the light reaching the PD during the numerator measurement is the same as the absolute value of the electric field of the light reaching the PD during the denominator measurement. To achieve this, for example, the output level of the test light from the light source may be adjusted so that the power supply currents from the two PDs are the same.

[0055] Therefore, the present invention can be implemented as an apparatus for measuring the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal light input terminal and a local optical light input terminal, an optical hybrid connected to the signal light input terminal and the local optical light input terminal, and a photodiode connected to the output of the optical hybrid, the apparatus comprising: a broadband light source that outputs test light having a coherence length longer than the optical path length of one interferometer inside the optical hybrid and having a spectral wavelength width corresponding to at least twice the frequency band of the CMRR to be measured; and an electrical measuring device for measuring an electrical signal from the coherent optical receiver, wherein in each of the numerator and denominator measurements of the CMRR, the test light from the broadband light source is input to at least one of the signal light input terminal or the local optical light input terminal.

[0056] For the numerator measurement of CMRR, light obtained by combining the broadband light from the broadband light source and CW light from a continuous wave (CW) light source is input to either the signal light input terminal or the local light input terminal, and the wavelength of the CW light is included in the wavelength bandwidth of the broadband light. For the denominator measurement of CMRR, the broadband light from the broadband light source is input to the signal light input terminal, and CW light from a continuous wave (CW) light source is input to the local light input terminal.

[0057] 4 is a diagram illustrating variations in the combination of a PD and a TIA. In the above description of FIGS. 1 to 3, the coherent optical receiver outputs a differential current from the balanced PD 114 shown in FIG. 4(a). As a modification, a single-input, single-ended output TIA 115-1 may be connected to the balanced PD 114. Alternatively, a single-input, differential output TIA 115-2 having differential electrical outputs 116p and 116n may be connected to the balanced PD 114.

[0058] 4(b) shows a case where two PD pairs (dual PDs) 116 are used in a non-balanced configuration. Even when combined with a differential input / single output TIA 115-3 or a differential input / differential output TIA 115-4 having differential electrical outputs 116p and 116n, an output proportional to the current difference between the two PDs can be obtained. In either case, a larger electrical output can be obtained when an amplifier such as a TIA is connected. (c) and (d) of FIG. 4 will be described later.

[0059] Next, a more specific measurement procedure for the CMRR measurement apparatus of Figure 1 will be described. After the DUT 110 is set in the numerator measurement system (a) or denominator measurement system (b) of Figure 1, the broadband light source 103 and CW light source 102 are set either manually or under the control of the processor 121, and an electrical signal is acquired from the electrical measuring instrument 120. The acquired electrical signal is converted into digital data by an A / D converter or the like built into the electrical measuring instrument, and then processed by the processor 121. The processor 121 can also temporarily store data in the associated memory 122 for further processing. The processor does not need to be constantly connected to the measuring instrument; it can retrieve data from the measuring instrument after the measurement is completed.

[0060] 5 is a flow diagram of the measurement procedure in the CMRR measurement apparatus of embodiment 1. This is the procedure for measuring CMRR_sig related to the signal light input terminal using the measurement systems 100-n and 100-d shown in FIG. 1. Flow 300 includes the steps of numerator measurement, denominator measurement, and CMRR calculation, and also includes a calibration step indicated as Cal.

[0061] In step S301, in the molecular measurement system shown in FIG. 1A, only broadband light is input from the broadband light source 103 to the sig terminal. For one channel of the DUT, the total value of the photocurrent (I p0 +I n0 ) i.e., the current value I 1wbs (calibration step 1). This current value I 1wbs is used for calibration for broadband light in the denominator measurement described below.

[0062] I p0 and I n0 In the case of a configuration using a differential output IC among the variations of the combination of PD and TIA in Figure 4, it can be measured using a power supply with an ammeter as part of the electrical measuring instrument. Specifically, as shown in Figure 4(c) and (d), it can be obtained by measuring the current supplied to the PD from the power supply with an ammeter 118. In the case of Figure 4(d), the sum of the currents I p0 +I n0can be obtained.

[0063] In step S302, in the molecular measurement system shown in FIG. 1A, only CW light is input from the CW light source 102 to the sig terminal. The total value of the PD photocurrent (I p0 +I n0 ) i.e., the current value I 1CW (calibration step 2). This current value I 1CW is used for calibration of CW light in the denominator measurement described below.

[0064] In S303, in the molecular measurement system 100-n in FIG. 1A, test light is input from the broadband light source 103 and the CW light source, and the mixed test light 130 is input, and the output power spectrum An(f) is measured by the electrical measuring instrument 120. As described above, the frequency characteristic An(f) can be obtained directly using a spectrum analyzer. An(f) can also be obtained by converting a fixed-time electrical signal into digital output data using an A / D converter and then Fourier transforming that data. Control, arithmetic processing, etc. for spectrum acquisition are performed by the processor 121, and memory 122 can also be used.

[0065] In step S304, in the denominator measurement system 200-d in FIG. 1B, only the broadband light from the broadband light source 103 is input to the sig terminal. The total value of the PD photocurrent (I p0 +I n0 ) i.e., the current value I 2wbs (calibration step 3).

[0066] In S305, the measured current value I 2wbs is the current value I obtained in S301. 1wbs So that (I 1wbs =I 2wbs ), and adjust the output intensity of the broadband light source 103 (calibration step 4).

[0067] In S306, in the denominator measurement system 200-d in FIG. 1B, only the CW light from the CW light source 102 is input to the LO terminal. The total value of the PD photocurrent (I p0 +In0 ) i.e., the current value I 2CW (calibration step 5).

[0068] In S307, the measured current value I 2CW is the current value I obtained in S302. 1CW So that (I 2CW =I 1CW ), and adjust the output intensity of the CW light source 102 (calibration step 6).

[0069] In S308, test light is input from the broadband light source 103 and the CW light source to the denominator measurement system 100-d in FIG. 1B, and the output power spectrum Ad(f) is measured by the electrical measuring instrument 120. As with the output power spectrum An(f), Ad(f) can also be acquired by a spectrum analyzer or an A / D converter. Control, arithmetic processing, and the like for acquiring the spectrum are performed by the processor 121, and the memory 122 can also be used.

[0070] In S309, the output power spectrum Ad(f) is divided by the output power spectrum Ad(f) for each frequency band to obtain a ratio, and the CMRR value is calculated.

[0071] As is clear from the above procedure, the method also has an aspect of being a method for measuring CMRR, including each of the steps S301 to S309.

[0072] In the above-mentioned S302, the total value of the PD photocurrent (I p0 +I n0 ) Current value I 1CW In the measurement of the CMRR, the I when only broadband light is input to either the signal light input terminal or the local light input terminal is measured. p0 and I n0 In addition, in the above-mentioned step S304, the sum of the PD photocurrents (I p0 +I n0 ), current value I 2CW In the measurement of the CMRR denominator, the I when only broadband light is input to either the signal light input terminal or the local light input terminal is p0 and I n0 The sum of the above is calculated as the second total current.

[0073] Therefore, for each light source used in the measurement, the sum of the power supply currents of the two photodiodes when test light is input to either the signal light input terminal or the local light input terminal in the denominator measurement of CMRR (I p0 +I n0 ) is the sum of the power supply currents of the two photodiodes during molecular measurement of the CMRR (I p0 +I n0 The light intensity of the test light from the light source is adjusted during the numerator and denominator measurements so that it is the same as

[0074] In S305, I 1wbs =I 2wbs The intensity adjustment amount used to adjust the output intensity of the broadband light source 103 so that the CMRR is satisfied can also be used as a correction value (correction coefficient) for calculating the CMRR. For example, instead of aligning the currents, spectrum Ad(f) and Ad(f) data can be first acquired at a test light intensity that places the electrical signal from the DUT within a satisfactory operating range of the electrical measuring instrument, and then the acquired data can be corrected using the intensity adjustment amount. For example, if the measurement was performed at a light intensity twice that of the originally set current, the output result can be halved for correction. The calibration step in flow 300 is an example of acquiring spectrum Ad(f) after aligning the PD detection levels; a correction coefficient obtained by performing a separate calibration procedure can also be applied to the acquired data later.

[0075] Figure 6 shows the measurement results of the frequency dependence of CMRR compared with the conventional technology. The figure shows CMRR_sig for the sig terminal up to a frequency band of 50 GHz, using the measurement system of embodiment 1 shown in Figure 1. The measurement results using LCA of the conventional technology shown in Figure 11 are also shown to compare the two. Compared to the conventional technology, the difference in the measured values ​​is only a few dB at most in any band, and the frequency dependence is generally accurately reproduced, confirming that the results match with a certain degree of accuracy.

[0076] In the CMRR measurement device disclosed herein, broadband light is used as one of the test light sources, eliminating the need for test light whose modulation frequency is swept by an LCA or the like. This simplifies the CMRR measurement device and enables measurements to be performed in a short time. Even in the case of measuring CMRR_LO for the LO terminal shown in Figure 2, the sig terminal in the molecular measurement is simply replaced with the LO terminal, so the measurement flow procedure shown in Figure 5 can be applied as is with only slight adaptations to some of the molecular measurement.

[0077] In the CMRR measurement device disclosed herein, broadband light having a wide bandwidth is used as the test light, so that the beat signal generated by the PD in the coherent optical receiver has a frequency band sufficient for measuring CMRR, and replaces the test light intensity-modulated with a frequency-swept modulation signal. Although the CMRR measurement device of this embodiment uses two types of light sources, including a broadband light source, equivalent CMRR measurements can be achieved with an even simpler configuration that uses only a broadband light source.

[0078] [Embodiment 2] The CMRR measurement device of the above-described embodiment 1 uses two types of test light: broadband light and CW light. In this embodiment, a CMRR measurement method with a simpler configuration that uses only broadband light as test light is presented.

[0079] 7 is a diagram showing the configuration of a CMRR measurement device for a signal light terminal (sig) of embodiment 2. Figure 7(a) shows a measurement system 200-n for measuring the numerator of CMRR, and (b) shows a measurement system 200-d for measuring the denominator of CMRR. The DUT 110 is a coherent optical receiver to be measured for CMRR, and is equipped with two optical input terminals to which optical fibers are connected and through which test light is input, and an electrical signal output terminal connected to an electrical measuring instrument 120.

[0080] In the molecular measurement system 200-n, broadband light 133 from the broadband light source 103 is input as test light to the signal light input terminal of the DUT. Therefore, in the molecular measurement system 200-n, the test light 133 reaches the PD via a single optical fiber and one path within the DUT 110.

[0081] In the CMRR denominator measurement system 200-d of FIG. 7(b), broadband light 131 from the broadband light source 103 is split into two paths by a coupler 105. One path is input to the signal light input terminal as test light 134, and the other path is input to the local light input terminal as test light 135. The test light 134 is level-adjusted by an optical attenuator 106 and then input to the signal light input terminal. The test light 135 input to the local light input terminal is polarized by a polarizer 104, and a polarization-maintaining fiber is used between the polarizer 104 and the local light input terminal to maintain the polarization state. In the case of the denominator measurement system 200-d, the two split broadband light beams 134 and 135 each travel through different paths in the optical hybrid within the DUT 110 to reach the PD. Note that if polarized broadband light can be input with polarization appropriate for the local light input of the optical receiver, the polarizer 104 is not necessary.

[0082] In the measurement apparatus of the first embodiment, similarly to the explanation given with reference to the internal configuration of the coherent optical receiver in Fig. 3, it will be explained that the numerator and denominator of CMRR can be measured using the measurement systems in Fig. 7. The internal configuration and operation of the DUT in Fig. 3 are the same in the CMRR measurement apparatus of this embodiment, except for the test light used in the first embodiment.

[0083] In the molecular measurement of FIG. 7A, the electric field E of the light reaching the two PDs from the broadband light source 103 can be expressed as the sum for each discrete wavelength i, as shown in the following equation, because the broadband light has a wide wavelength width.

[0084] At this time, the PD currents Ip and In due to the light incident on each PD are proportional to the square of the electric field E1 of the light, and are expressed as follows, where α is a proportionality coefficient:

[0085] Here, if P1 is defined as follows, I p and I n is expressed as the following equation (7-4).

[0086] In equation (7-3), P1 is the optical power generated by the beat of broadband light from the broadband light source, and equation (7-3) indicates that a random optical signal having a frequency band close to the wavelength width of the spectrum of the broadband light reaches the PD. That is, when the same random optical signal having optical power P1 is input to the two PDs, the electrical output from the two PDs of the balanced PD is expressed as ΔI = |I in the case of PD current, depending on the degree of device asymmetry between the p-PD and n-PD, i.e., the imbalance of the positive and negative paths. p -I n According to the definition of CMRR explained in equation (1), the numerator in the log term is |I p -I n All you need to do is find |. p -I n | can be calculated from the difference in current between the two balanced PDs by the numerator measurement in Figure 7(a). In other words, the value equivalent to the numerator in the log term of the CMRR definition equation can be obtained directly from the differential output ΔI. This current can be converted into a voltage using a 50Ω system or a TIA, and finally the power spectrum An(f) corresponding to the numerator measurement can be calculated.

[0087] In the denominator measurement of FIG. 7(b), the electric field caused by the beat between the test light 134 incident from the signal light input terminal and the test light 135 incident from the local light input terminal has a phase shift of 180 degrees between the p-PD and the n-PD, similar to the case of the denominator measurement system of the first embodiment shown in FIG. 1(b).

[0088] 3B of the denominator measurement system of the first embodiment, the electric field of the broadband light 134 that enters from the signal light input terminal and reaches the p-PD is defined as E1, and the electric field of the broadband light 135 that enters from the local light input terminal and reaches the n-PD is defined as E2. In this case, the absolute value of the electric field that enters the p-PD can be written as |E1 + E2|, and the absolute value of the electric field that enters the n-PD can be written as |E1 - E2|.

[0089] In CMRR measurement, in order for the light waves input and output to the optical hybrid to maintain this phase relationship as designed, the coherence length of the test light from the light source must be longer than the optical path length of the interferometer inside the optical hybrid. If the wavelength of the light from the light source is λ and the wavelength width is Δλ, the coherence length is λ 2 For example, when the optical path length L of the interferometer inside the optical hybrid is 0.5 mm for a wavelength of 1.55 μm, the condition for the optical path length L is Δλ<λ 2 / L = (1.55 x 10 -6 ) 2 / 0.5 x 10 -3 = 5 nm. Therefore, the wavelength width of the spectral band of the broadband light needs to be narrower than 5 nm. Here, the optical path length of the interferometer inside the optical hybrid refers to the optical path length of the optical coupling and interference path configured so that the signal light and the local light are coupled inside the optical hybrid and a phase difference of 180 degrees is obtained when they enter the two PDs of one channel.

[0090] In this case, each output current of the PD is determined by the electric field incident on the PD. The electric field when the wideband signal reaches the PD via the signal light input terminal is defined as E1, and the electric field when the wideband signal reaches the PD via the local light input terminal is defined as E2. As in the case of the numerator, E1 can be expressed as the sum for each discrete wavelength i, as shown in the following equation:

[0091] As a result, the PD currents Ip and In due to the light incident on each PD are proportional to the square of the incident electric field, and are expressed as follows, where α is the proportionality coefficient:

[0092] In equations (8a) and (8b), the first two terms have the same sign, but the third term has an inverted sign, so when calculating the difference between Ip and In, which are the currents generated by the electric fields of E1 and E2, the last term makes a dominant contribution. In other words, the output current Iout obtained as a differential output current is expressed by the following equation:

[0093] E1 and E2 are the electric fields of the test light that reaches the PD from the same broadband light source 103 via different paths and have the same spectrum. Furthermore, if the intensity of the test light from the light source and the optical attenuator are adjusted so that the two PD currents are the same, |E1| = |E2| can be satisfied by aligning the intensity of the test light incident from the signal light input terminal and the intensity of the test light incident from the local light input terminal. Specifically, in FIG. 7B, |E1| = |E2| can be satisfied by adjusting the output intensity of the test light from the broadband light source 103 and the attenuation amount of the variable attenuator 106. In this case, equation (8c) for the differential output current Iout can be written as follows:

[0094] Here, if P2 is defined as in the following equation (9-2) for the right-hand side of equation (9-1), the relationship in equation (9-3) is obtained.

[0095] P2 represents the optical signal power consisting of the beat between the test light 134, 135 split into two from the broadband light source 103. Therefore, just like P1 in molecular measurement, since it is an optical signal generated by the beat between broadband lights from the broadband light source, a random optical signal having a frequency band close to the wavelength width of the broadband light is input.

[0096] According to the definition of CMRR explained in equation (1), the denominator in the log term can be calculated by adding |Ip| and |In|. P2 in the above equation (9-3) is an electrical signal generated by the beat of test light from a broadband light source. Furthermore, the optical intensity input to the PD from the signal light input terminal and the optical intensity input to the PD from the local light input terminal are set equal to the optical intensity input from the signal light input terminal during numerator measurement by the calibration process described below. Therefore, even if the test light 134 and 135 are optical signals with optical powers P1 and P2 and the same spectrum, the optical intensity detected by each of the two PDs will be greater by the amount of test light input from the two input terminals. This coefficient is called a. The coefficient a can be calculated from theoretical calculations and actual measurements. For example, in an ideal example, this value is 21 / 2 However, it is possible to use a different calculated value by making adjustments depending on conditions such as the spectral shape. Taking into account the possibility of adjusting the coefficient a, equation (9-3) can be written as follows:

[0097] Referring to equation (9-4), the value a × 2αP1 finally obtained by the denominator measurement system in FIG. 7(b) corresponds to a times the sum of |Ip| (approximately P1) and |In| (approximately P1). Therefore, a value equivalent to a times the denominator in the log term of the CMRR definition equation can be immediately obtained from the output of the denominator measurement system in FIG. 7(b). When calculating the CMRR, the above-mentioned coefficient a can be corrected by applying a correction to the output of the denominator measurement system. The PD current is converted to a voltage using a 50 Ω system or a TIA, and the power spectrum Ad(f) corresponding to the denominator measurement can finally be obtained.

[0098] As described above, the value of ΔI = |Ip - In| is obtained by the numerator measurement system 200-n in Figure 7(a), and a value equivalent to |Ip| + |In| is obtained by the denominator measurement system 200-d in Figure 7(b). The currents obtained during these numerator and denominator measurements are converted to voltages using a 50 Ω system or TIA, and the power spectra An(f) and Ad(f) corresponding to the numerator and denominator measurements, respectively, can be finally determined. Using these values, the CMRR can be expressed as follows:

[0099] 8 is a flow diagram of the measurement procedure in the CMRR measurement apparatus of embodiment 2. This is the procedure when measuring CMRR_sig related to the signal light input terminal using the measurement systems 200-n and 200-d shown in Fig. 7. Flow 400 includes the steps of numerator measurement, denominator measurement, and CMRR calculation, and also includes a calibration step indicated as Cal.

[0100] In step S401, in the molecular measurement system shown in FIG. 7A, broadband light is input from the broadband light source 103 to the signal light input terminal. As in the first embodiment, the total value of the photocurrent (I p0 +I n0) that is, the current value I1 is measured (calibration step 1). This current value I1 is used for calibration for broadband light in the denominator measurement described later.

[0101] In S402, in the molecular measurement system 200-n in FIG. 7(a), test light 133 is input from the broadband light source 103 to the signal light input terminal, and the output power spectrum An(f) is measured by the electrical measurement device 120. As described above, An(f) can be obtained by a spectrum analyzer. An(f) can also be obtained by converting a fixed-time electrical signal into digital output data using an A / D converter and then Fourier transforming that data. Control, arithmetic processing, etc. for spectrum acquisition are performed by the processor 121, and the memory 122 can also be used.

[0102] In step S403, in the denominator measurement system 200-d in FIG. 7B, the input to the signal input terminal of the DUT 110 is removed, and broadband light from the broadband light source 103 is input only to the local light input terminal. The total value of the PD photocurrent (I p0 +I n0 ) i.e., the current value I 2LO (calibration step 2).

[0103] In S404, the measured current value I 2LO becomes the current value I1 acquired in S401 (I1 = I 2LO ), and adjust the output intensity of the broadband light source 103 (calibration step 3).

[0104] In step S405, in the denominator measurement system 200-d in FIG. 7B, the input to the local oscillator input terminal of the DUT 110 is removed, and broadband light from the broadband light source 103 is input only to the signal light input terminal. The total value of the PD photocurrent (I p0 +I n0 ) i.e., the current value I 2Sig (calibration step 4).

[0105] In S406, the measured current value I 2Sig becomes the current value I1 acquired in S401 (I 2Sig= I1), the intensity of the test light 134 to the signal light input terminal is adjusted while maintaining the already adjusted intensity of the test light 135 to the local light input terminal unchanged. That is, in the configuration of Figure 7(b), the intensity of the test light 134 is adjusted by the variable attenuator 106 while maintaining the intensity of the test light 135 in the state after completion of S404 (calibration step 5). Once this intensity adjustment is complete, the light power received by the PDs for the broadband light is aligned between the numerator measurement system and the denominator measurement system, maintaining the accuracy of the CMRR measurement value.

[0106] In S407, test light 134 and 135 are incident on two input terminals from the broadband light source 103 using the denominator measurement system 200-d in Figure 7(b), and the output power spectrum Ad(f) is measured using the electrical measuring instrument 120. As with the output power spectrum An(f), Ad(f) can be obtained using a spectrum analyzer or an A / D converter. Control, arithmetic processing, etc. for spectrum acquisition are performed by the processor 121, and the memory 122 can also be used.

[0107] In S408, the CMRR is calculated by dividing the output power spectrum Ad(f) by the output power spectrum Ad(f) corrected with the correction coefficient a for each frequency to obtain a ratio. The correction coefficient a used to calculate the CMRR is obtained in advance by theoretical calculation or computer simulation using the spectrum of the test light from the test light source and a device model. It can also be calculated in advance from the measurement results of an element whose CMRR value is known.

[0108] Therefore, the present invention provides a method for measuring the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal light input terminal and a local light input terminal, an optical hybrid connected to the signal light input terminal and the local light input terminal, and a photodiode connected to an output of the optical hybrid, the method comprising: in molecular measurement of the CMRR, a step of inputting test light from a broadband light source to at least one of the signal light input terminal or the local light input terminal and acquiring a first electrical signal by an electrical measuring instrument (S402); in molecular measurement of the CMRR, a step of inputting the test light from the broadband light source to at least one of the signal light input terminal or the local light input terminal and acquiring a second electrical signal by the electrical measuring instrument (S407); and a step of calculating the CMRR based on the first electrical signal and the second electrical signal (S408), wherein the test light has a coherence length longer than the optical path length of one interferometer inside the optical hybrid and has a spectral wavelength width equivalent to at least twice the frequency band of the CMRR to be measured.

[0109] Figure 9 shows the measurement results of the frequency dependence of CMRR compared with the conventional technology. The CMRR_sig for the sig terminal up to a frequency band of 50 GHz is shown using the measurement system of embodiment 2 in Figure 7. The measurement results using LCA of the conventional technology are also shown in Figure 11 to compare the two. Compared to the conventional technology, the difference in the measured values ​​is only a few dB at most in any band, and the frequency dependence is generally accurately reproduced, confirming that the results match with a certain degree of accuracy.

[0110] Even with the CMRR measurement device of embodiment 2, numerator and denominator measurements can be completed in a short time using an electrical measuring device. By using only broadband light as the test light, test light with a swept modulation frequency using an LCA or the like is not required. In this way, the CMRR measurement device is simplified, enabling measurements in a short time. In the measurement of CMRR_sig for the sig terminal shown in Figure 7, if the sig terminal in the numerator measurement is replaced with the LO terminal, CMRR_LO measurement can be performed.

[0111] Any of the above-described embodiments can be modified in various ways, as described below. The electrical measuring instrument may be any device capable of analyzing the frequency characteristics of an input electrical signal. For example, it may be a device with a built-in A / D converter, such as a real-time oscilloscope or a signal analyzer, a receiver with a built-in A / D converter, or a spectrum analyzer that directly analyzes frequency characteristics. The CMRR is calculated using a DSP by accumulating data over a certain period of time, performing a Fourier transform, and converting the ratio of the absolute values ​​of the numerator and denominator measurements into a logarithm, thereby obtaining a logarithmic CMRR.

[0112] Furthermore, in the first embodiment, variations in the combination of PD and TIA have been described with reference to FIG. 4, but similar variations are possible in the second embodiment as well.

[0113] The optical switch can be used to switch between the numerator and denominator measurement paths and to remove the optical fiber during the calibration process (Cal). Regarding the test light from the light source, although it is not essential if the test light from the light source satisfies the wavelength width conditions, it is also possible to set the wavelength width of the light source to the final required value by applying an appropriate band limit using an optical filter.

[0114] 1 and 2, the CW light source is polarized, so when it enters the coherent optical receiver, the ratio of light that enters the X-polarized side and the Y-polarized side through the polarization beam splitter inside the optical receiver depends on the polarization state. To keep this ratio constant during measurement, a polarization controller may be used between the light source and the DUT.

[0115] In each of the above-described embodiments, the intensity of the test light is adjusted in the calibration process to equalize the received power of the PD between the numerator and denominator measurements. To adjust the intensity of the test light, the measurement systems shown in Figures 1, 2, and 7 may include an optical amplifier or a variable optical attenuator on at least one of the paths from the light source to the signal light input terminal and the local light input terminal. As described in the measurement procedures shown in Figures 5 and 8, the power spectra An(f) and Ad(f) can be acquired in both the denominator and numerator measurement systems after performing the calibration process to adjust the light intensity of the light source.

[0116] In the above-described embodiments, the photocurrent supplied from the power supply with an ammeter 118 is measured for each output channel of the coherent optical receiver and used for calibration. If the current for each output channel is considered to be approximately the same, the current for each channel can be determined by measuring the currents from the power supplies for multiple channels together and dividing by the number of channels to determine the current for each channel and use it for calibration.

[0117] Instead of making the light intensity uniform during denominator measurement and numerator measurement, it is also possible to acquire power spectra at different test light intensities for each of the numerator and denominator measurements so as to acquire an appropriate electrical signal, taking into consideration the output current range of the PD and the detection sensitivity and detection accuracy of the electrical measuring instrument, and then correct the acquired measurement data. In the case of a measurement using one type of light source and one type of test light as in embodiment 2, when data is acquired with the light intensity increased by A times, the output intensity of the electrical signal can be corrected by multiplying it by 1 / A. Furthermore, in the case of a measurement using two types of light source and two types of test light and using beat outputs of the two types of test light as in embodiment 1, when the light intensity of one test light is increased by A times, the output amplitude of the acquired electrical signal can be corrected by multiplying it by 1 / √A (1 / A 1 / 2 ) to correct it.

[0118] For example, in the case of numerator measurement, as is clear from equation (1), the better the CMRR characteristics are, the more the currents in the p-PD and n-PD cancel each other out, and the output power obtained from the differential current ΔI becomes very small. In such a situation, it is possible to use different test light intensities in the numerator and denominator, such as using a stronger test light in the numerator measurement than in the denominator measurement. This allows the electrical signal output from the DUT to be measured so that it falls within the optimum input range of the electrical measuring instrument 120. Since the light intensity is adjusted (by a factor of A), the electrical signal measured as described above can be corrected by a correction calculation (1 / A or 1 / A 1 / 2 By multiplying the temperature by 100° C., the CMRR can be measured under the optimum conditions of the electrical measuring instrument 120.

[0119] When this method is applied, the power spectrum is acquired while the electrical signal is within the optimal input range of the electrical measuring device, and then the measurement value is corrected. Specifically, in the flow chart 300 of FIG. 5 , the light intensity is adjusted in steps S305 and S307 so that the electrical signal output amplitude falls within the optimal input range of the electrical measuring device 120. Thereafter, in the CMRR calculation formula in step S309, the photocurrent obtained in steps S301, S302, S305, and S308 is used to correct the calculated CMRR value.

[0120] Furthermore, in any of the CMRR measurement devices of the present embodiment, a method similar to that described in Non-Patent Document 1 can also be used as a simpler calibration method. More specifically, consider the case where the loss to each channel within the coherent optical receiver is the same between the signal light input path and the local light input path. For example, in the first embodiment, if the signal light is polarized by passing it through a polarizer, for example, the polarization controller can be adjusted to concentrate the signal light only on the X polarization, and the input intensity of the local light can be input so that it is twice the input intensity of the signal light. In this case, unlike the signal light incident only on the X side, the local light is equally distributed between the X polarization and the Y polarization, and the signal light and the local light are incident with the same intensity on the X polarization. With this method, the optical intensity reaching the PD of the measurement channel can be made uniform simply by measuring the optical intensity of the test light input to the two terminals.

[0121] In the above-described numerator and denominator measurement systems, an optical bandpass filter can be used to reduce noise due to unwanted light by limiting the bandwidth of the broadband light from the broadband light source 103. Alternatively, light that is randomly modulated at a speed equal to or greater than the measurement bandwidth of the CMRR can be used instead of a broadband light source.

[0122] When measuring a time signal using an A / D converter as an electrical measuring instrument, if the memory capacity for calculation processing is small, measurement errors can be reduced by averaging multiple measurements. For example, after Fourier transforming the acquired time signal, the absolute values ​​of different measurements can be averaged.

[0123] By integrating the coherent optical receiver with a DSP, even if the optical receiver does not have an output terminal for analog electrical signals, the output signal of the coherent optical receiver can be acquired by the DSP. By acquiring the electrical signal acquired by the DSP from the output of an A / D converter that is normally included in the DSP for the coherent optical receiver, this can replace the electrical measuring instruments shown in Figures 1, 2, and 7.

[0124] Furthermore, when measuring CMRR, the acquired electrical signal contains thermal noise from the electrical amplifier (TIA) included in the DUT. Accuracy can also be improved by acquiring in advance the output data of the electrical signal from the DUT when no optical input is present and subtracting the noise intensity spectrum from An(f) and Ad(f).

[0125] In the measurement device of the first embodiment, the beat signal between two test lights, broadband light and CW light, is used as the input to the PD. As discussed in equation (3), the beat signal |E1| between the broadband lights 2 Although small, the component of also causes measurement error. In both the denominator and numerator measurements, the output powers Bn(f) and Bd(f) when only broadband light is input can be measured, and the accuracy can be improved by using these for correction in the following equation to find the CMRR.

[0126] In the measurement device of the second embodiment, in the denominator measurement, the main input to the PD is the beat component of the test light from the signal light input terminal and the test light from the local light input terminal, and the beat signal |E1| between the test lights from the signal light input terminals is 2 , the beat signal |E2| between the test lights from the local light input terminal 2 The accuracy can also be improved by measuring the output powers Bds(f) and Bdl(f) when test light is input only from the signal light input terminal and when test light is input only from the local light input terminal, and then using these values ​​for correction using the following equation to find the CMRR.

[0127] Furthermore, the present invention can also be used to measure other quantities that include the CMRR in their equations. As described above in detail, the CMRR measurement device of the present invention makes it possible to perform CMRR of a coherent optical receiver at high speed with a cheaper configuration.

[0128] The present invention can be used in the manufacture of optical receivers.

Claims

1. 1. An apparatus for measuring a common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal light input terminal and a local light input terminal, an optical hybrid connected to the signal light input terminal and the local light input terminal, and a photodiode connected to an output of the optical hybrid, a broadband light source that outputs test light having a coherence length longer than the optical path length of one interferometer in the optical hybrid and a spectral wavelength width corresponding to at least twice the frequency band of the CMRR to be measured; an electrical measurement device for the electrical signal from the coherent optical receiver; Equipped with A measuring apparatus characterized in that, in each of the numerator and denominator measurements of CMRR, the test light from the broadband light source is input to at least one of the signal light input terminal and the local light input terminal.

2. 2. The measuring device according to claim 1, wherein the test light from the broadband light source is input to only one of the signal light input terminal and the local light input terminal for the molecular measurement of CMRR.

3. 3. The measuring device according to claim 1, wherein the test light is input from the broadband light source to both the signal light input terminal and the local light input terminal in order to measure the denominator of the CMRR.

4. 2. The measuring device according to claim 1, wherein for the molecular measurement of CMRR, light obtained by combining the test light from the broadband light source and CW light from a continuous wave (CW) light source is input to either the signal light input terminal or the local light input terminal, and the wavelength of the CW light is included in the wavelength bandwidth of the test light.

5. 5. The measuring apparatus according to claim 1, wherein the test light from the broadband light source is input to the signal light input terminal, and CW light from a continuous wave (CW) light source is input to the local light input terminal, in order to measure the denominator of the CMRR.

6. The light intensity of the test light from the broadband light source is adjusted during the numerator measurement and the denominator measurement so that the sum of the power supply currents of the two photodiodes when test light is input to either the signal light input terminal or the local light input terminal during the denominator measurement of the CMRR is the same as the sum of the power supply currents of the two photodiodes during the numerator measurement of the CMRR, or 5. The measuring device according to claim 1, wherein the CMRR is calculated by correcting the results of the numerator measurement and the results of the denominator measurement using the sum of the respective power supply currents.

7. 2. The measuring device according to claim 1, wherein the broadband light source is one of an ASE (Amplified Spontaneous Emission) light source, an SC (Super-continuum) light source, and an SLD (Superluminescent diode) light source.

8. 1. A method for measuring a common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal light input terminal and a local light input terminal, an optical hybrid connected to the signal light input terminal and the local light input terminal, and a photodiode connected to an output of the optical hybrid, comprising: In a molecular measurement of CMRR, a step of inputting test light from a broadband light source to at least one of the signal light input terminal and the local light input terminal, and acquiring a first electrical signal by an electrical measuring device; In measuring a denominator of CMRR, the test light from the broadband light source is input to at least one of the signal light input terminal and the local light input terminal, and a second electrical signal is acquired by the electrical measuring instrument; calculating a CMRR based on the first electrical signal and the second electrical signal; Equipped with The test light has a coherence length longer than the optical path length of one interferometer inside the optical hybrid and a spectral wavelength width equivalent to at least twice the frequency band of the CMRR to be measured.

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