Measurement device and measurement method for optical receivers

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

[Technical Field]

[0001] The present invention relates to a measuring device and a measuring method for an optical receiver. [Background technology]

[0002] Modern society's social media, video streaming, and car navigation systems are all supported by high-speed communication networks. The core optical communication networks utilize wavelength division multiplexing and digital coherent communication technologies to achieve transmission capacities of tens of Tbps over a single optical fiber. Unlike intensity-modulated direct detection (IMDD) technology used in short-distance communication, digital coherent communication technology achieves high-capacity transmission by encoding information onto the phase of light, and also enables colorless reception, eliminating the need for wavelength-specific filters.

[0003] In wavelength division multiplexing communication, where signals of various wavelengths are multiplexed, a coherent optical receiver, instead of extracting the desired channel using an optical filter and decoding the optical signal with a photodiode (PD), superimposes a local oscillator light (local emission) and signal light of the same frequency and extracts the received signal from the resulting beat signal. This method is colorless, regardless of the wavelength of the target received light, and allows extraction of only the optical signal of the channel matching the wavelength of the local emission without using bulky and costly optical filters. Light from channels other than the desired channel generates noise through detection by the PD. This noise from non-desired channels is, in principle, canceled out by the ideal balanced reception operation of the coherent optical receiver and removed from the output signal obtained from the PD current.

[0004] However, if the overall complementary balance of a coherent optical receiver, from the optical hybrid to the photodiode and electrical amplifier, is disrupted, the aforementioned noise will not be completely canceled in the complementary signal path, degrading the receiving characteristics. The Single Port Rejection Ratio (SPRR) is an indicator of the quality of the balance in a coherent optical receiver. SPRR is a name that conforms to the definition based on the optical input port, but it is conventionally also called the Common Mode Rejection Ratio (CMRR). Correctly evaluating this SPRR (CMRR) is necessary for characterizing a coherent optical receiver. In the following explanation, we will use the conventional term CMRR.

[0005] In coherent optical receivers, noise entering from only one of the two optical fiber input terminals, specifically intensity modulation noise contained in the signal light and RIN noise (Relative Intensity Noise) contained in the station light emission, becomes a problem. The CMRR of a coherent optical receiver is an indicator of how well these noises can be removed from the beat signal between the station light emission and the signal light, which is what should be detected. Typically, a CMRR is defined for each of the four channels contained in a coherent optical receiver (XI, XQ, YI, and YQ for the two polarizations).

[0006] Figure 10 illustrates the definition of CMRR in a coherent optical receiver. Figure 10 shows one channel of a coherent optical receiver, which is a device under measurement 110 (DUT) having a signal optical input terminal (sig) and a local light emission input terminal (LO). The coherent optical receiver includes an optical hybrid 113 and a balanced PD 114 including a positive PD and a negative 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 from the numerator measurement using the measurement system shown on the upper side of Figure 10 by the value obtained from the denominator measurement using the measurement system shown on the lower side of Figure 10.

[0007] In the upper numerator measurement of Figure 10, the test light signal E1 is input from one optical input terminal, and as described above, the current output from the balance PD is expressed as Ip-In. In the lower denominator measurement of Figure 10, two optical input terminals are used, each with a phase difference φ. of Test light 141 , 142 (Electric fields E2 and E2e) jΦ The input is given, 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 value of the positive PD current can be obtained. output Furthermore, the maximum output of the negative PD current can be obtained, thereby allowing the PD current value corresponding to |Ip|+|In| to be obtained from the two maximum PD current values. CMRR is defined as the ratio of these two current values ​​(|Ip-In| and |Ip|+|In|), and can be expressed in dB as follows:

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

[0009] In molecular measurements, light input from a single optical input terminal (sig) 140 The signal is branched by the optical hybrid 113 and directly input to the two PDs. In this case, if the balance between positive and complementary signals is good in the two paths from the branching by the optical hybrid 113 to the two PDs of the balanced PD 114, then Ip ≈ In, and the numerator in the fraction of the CMRR definition formula is almost 0. The defined CMRR is a value close to 0 and is very small compared to 1, so when the CMRR value is expressed in dB, it will be a large negative value.

[0010] A coherent optical receiver has two optical input terminals, and therefore has two characteristic quantities: CMRR_sig for the signal optical input terminal and CMRR_LO for the local emission input terminal. In colorless applications, CMRR_sig for the signal optical input port is important, while CMRR_LO for the local emission input terminal is related to the removal of RIN generated by the local emission laser. As a conventional technique for measuring CMRR, the method described in Non-Patent Document 1 is widely known.

[0011] Figure 11 shows a CMRR measurement system using a conventional optical component analyzer. Figure 11(a) shows the numerator measurement system for CMRR, and (b) shows the denominator measurement system for CMRR. In the denominator measurement system 500-d in Figure 11(b), the high-frequency modulated light 123 from LCA501 is split into two by coupler 502, and the split test lights 125 and 126 are input to the signal light input terminal (sig) and local light emission input terminal (LO) of the coherent optical receiver DUT110, respectively. Each test light is connected to a variable optical attenuator (VO A The light is then input to each terminal via a variable delay line (VDL) and a polarization controller (PC) or phase modulator (PM). The intensities of the test light 125 and 126 input to the two input terminals are set to approximately the same level in the PD inside the coherent optical receiver.

[0012] Specifically, it utilizes the fact that the loss in a typical optical hybrid is approximately the same at each branch. The light input from the local light-emitting input terminal (optical input intensity P) LO ) is evenly distributed to polarization X and polarization Y, and the light intensity Pin input to the X side X_LO is P LO This results in / 2. In contrast, since the test light from the Light Component Analyzer (LCA) is polarized, the polarization state is adjusted by the polarization controller (PC) 505, so that only X-polarized light is sent to the signal light input terminal at level pin 505. x_sigIt can be adjusted to enter in psig. At this time, the optical input from the signal light input terminal is concentrated only on the X polarization wave, while the optical input from the local light emission input terminal is evenly distributed to the X polarization wave and the Y polarization wave as described above.

[0013] Therefore, the optical input intensity P LO from the local light emission input terminal is set to be twice the optical input intensity P sig from the signal light input terminal. As a result, the optical intensities incident on the two input terminals of the X polarization wave channel have the following relationship. Pin x_sig =Psig=P LO / 2=Pin x_LO

[0014] As described above, it can be made consistent between the case of signal light input (Pin x_sig ) and the case of local light emission input (Pin x_LO ). When measuring the Y polarization wave channel, the polarization state can be adjusted by PC505 so that light enters only the Y polarization wave.

[0015] In the above-mentioned denominator measurement system 500-d, the branched test lights 125 and 126 are combined and interfered by the optical hybrid in the DUT110, and a current output is obtained from the PD. In this way, when one test light 123 is branched and input and then recombined and interfered again in the DUT, fluctuations in the phase of the optical fiber in the path in the measurement system outside the DUT will cause large fluctuations in the optical power values received by each PD. The paths of the test system from the coupler 502 to the two input terminals in (b) of FIG. 11 are each connected by an optical fiber, and the phase of the optical fiber, that is, the optical path length, fluctuates moment by moment due to temperature fluctuations and vibrations.

[0016] In the conventional denominator measurement shown in Figure 11, a signal with amplitude modulation in the LCA is generated in the range of, for example, 0 to 50 GHz, and this test light 123 is input from the LCA 501. As described above, fluctuations in optical path length occur moment by moment even during the frequency sweep time of, for example, about 10 seconds, the received light intensity value of each PD becomes very unstable. To deal with these fluctuations in the received power of each PD, the LCA repeats frequency sweep measurements dozens of times, and only the optimal result is used. That is, after repeated frequency sweeps, for each frequency point, the current value at which the balanced PD current output is maximum among dozens of measurements is selected as the measurement result. From the repeated sweep operation results, the maximum output when light is concentrated on only one of the two PDs can be selected and obtained. For the two PDs, the result is obtained by connecting these maximum current values ​​at appropriate frequency intervals for each frequency point. Note that in the conventional measurement, the fluctuation may not be sufficient to reach the maximum value due to natural phase fluctuations in the optical fiber alone. For this reason, the denominator in Figure 11(b) Mother measurement In the stable system, a phase modulator (PM) 506 is used to vary the phase over a wide range of conditions until it reaches the maximum value.

[0017] As explained in Figure 10 and the definition formula for CMRR, the denominator measurement requires obtaining the maximum values ​​of Ip and In. The PD current values ​​obtained from the repeated sweep operation of the modulated signal described above correspond to |Ip| and |In| in equation (1), thus enabling the measurement of the denominator of CMRR.

[0018] In the molecular measurement shown in Figure 11(a), the test light 123, whose carrier light has been amplitude-modulated by the modulated signal swept by the LCA501, can be input directly to one of the optical input terminals for measurement. Depending on whether the CMRR to be measured is the local emission input CMRR_LO or the signal light input CMRR_sig, the test signal is input to the corresponding optical input terminal of the DUT110 (the Sig terminal in Figure 11(a)). In molecular measurements, there are no branched paths in the measurement system and no interference occurs with the test light, so a stable molecular measurement result can be obtained by performing a frequency sweep of the modulated signal only once. The CMRR in equation (1) can be obtained by dividing the molecular measurement result obtained by the above method by the denominator measurement result. [Prior art documents] [Non-patent literature]

[0019] [Non-Patent Document 1] V. Painchaud et al., “Performance of balanced detection in a coherent receiver” OPTICS EXPRESS Vol. 17, No. 5 / pp. 3659 (2009) [Overview of the project] [Problems that the invention aims to solve]

[0020] However, conventional CMRR measurement methods had the following problems: Firstly, they required expensive measurement equipment such as optical component analyzers; secondly, they required multiple frequency sweeps during denominator measurement, resulting in measurements taking several minutes or more. Furthermore, optical receivers that integrate coherent optical receivers and DSPs, which have been developed in recent years, lack electrical output terminals and therefore cannot be connected to optical component analyzers. Conventional CMRR measurement methods were not even able to perform measurements.

[0021] This invention has been made in view of the above problems, and presents a measuring device and a measuring method that enable high-speed CMRR of a coherent optical receiver with a simpler measurement configuration. [Means for solving the problem]

[0022] One aspect of the present invention is a measuring device for the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal optical input terminal and a local light emission input terminal, an optical hybrid connected to the signal optical input terminal and the local light emission input terminal, and a photodiode connected to the output of the optical hybrid, the measuring device 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 a spectral wavelength width corresponding to at least twice the frequency band of the CMRR to be measured, and an electrical measuring instrument for the electrical signal from the coherent optical receiver, wherein in both the numerator measurement and the denominator measurement of the CMRR, the test light from the broadband light source is input to at least one of the signal optical input terminal or the local light emission input terminal.

[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 optical input terminal and a local light emission input terminal, an optical hybrid connected to the signal optical input terminal and the local light emission input terminal, and a photodiode connected to the output of the optical hybrid, the method comprising: in molecular measurement of the CMRR, inputting test light from a broadband light source to at least one of the signal optical input terminal or the local light emission input terminal, and acquiring a first electrical signal with an electrical measuring instrument; and denominator The measurement involves inputting the test light from the broadband light source to at least one of the signal light input terminal or the local light emission input terminal, and acquiring a second electrical signal using the electrical measuring instrument. ,beforeA method for measuring CMRR, comprising the steps of calculating CMRR based on a first electrical signal and a 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 corresponding to at least twice the frequency bandwidth of the CMRR to be measured. [Effects of the Invention]

[0024] The CMRR measurement device of this disclosure enables high-speed and high-precision acquisition of the CMRR of a coherent optical receiver simply by acquiring data using a spectrum analyzer or ADC and performing calculations. [Brief explanation of the drawing]

[0025] [Figure 1] This figure shows the configuration of the CMRR measuring device for the signal optical terminal of Embodiment 1. [Figure 2] This figure shows the configuration of the CMRR measuring device for the local light-emitting terminal of Embodiment 1. [Figure 3] This diagram illustrates the internal configuration of a coherent optical receiver and the propagation state of test light. [Figure 4] This diagram illustrates variations in the combination of photodiodes (PDs) and electric amplifiers. [Figure 5] This is a flowchart of the measurement procedure in the CMRR measuring device of Embodiment 1. [Figure 6] This figure shows the measurement results of the frequency dependence of CMRR in comparison with conventional techniques. [Figure 7] This figure shows the configuration of the CMRR measuring device for the signal optical terminal in Embodiment 2. [Figure 8] This is a flowchart of the measurement procedure in the CMRR measuring device of Embodiment 2. [Figure 9] This figure shows the measurement results of the frequency dependence of CMRR in comparison with conventional techniques. [Figure 10] This diagram illustrates the definition of CMRR in a coherent optical receiver. [Figure 11]This figure shows a CMRR measurement system using LCA according to conventional technology. [Modes for carrying out the invention]

[0026] CMRR measuring device and measurement method This method uses a broadband light source as the test light. A spectrum analyzer or A / D converter is used as the electrical signal measuring instrument, allowing CMRR to be acquired with inexpensive, general-purpose measuring instruments compared to LCA. A broadband light source and a continuous wave (CW) light source can also be combined as the test light. In other words, expensive measuring instruments are unnecessary, there is no need to repeatedly sweep the modulated light, and CMRR can be acquired in a very short time.

[0027] The CMRR measurement apparatus and measurement method of this disclosure do not use test light whose intensity is modulated by a swept modulated signal, but instead use a broadband light source, thereby enabling numerator and denominator measurements of CMRR in a short time.

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

[0029] [Embodiment 1] Figure 1 shows the configuration of the CMRR measuring device for the signal optical input terminal (sig) of Embodiment 1. Figure 1(a) shows the measurement system 100-n for measuring the numerator of CMRR, and (b) shows the measurement system 100-d for measuring the denominator of CMRR. DUT110 is a coherent optical receiver that is the target of CMRR measurement, and is equipped with two optical input terminals to which optical fibers are connected and test light is input, and an electrical signal output terminal connected to an electrical measuring instrument 120. Typically, a coherent optical receiver has four output channels, so DUT110 includes electrical signal output terminals for four channels.

[0030] Figure 2 shows the configuration of the CMRR measuring device for the local light emission input terminal (LO) of Embodiment 1. Figure 2(a) shows the measurement system 101-n for measuring the numerator of CMRR, and (b) shows the measurement system 10 for measuring the denominator of CMRR. 1 -d is shown. The denominator measurement systems in Figure 1(b) and Figure 2(b) have the same configuration. The only difference between Figure 1 and Figure 2 is the optical input terminals (sig, LO) of the test light 130 to the coherent optical receiver (DUT) 110 in the numerator measurement, so the explanation below will be based on Figure 1.

[0031] As an electrical measuring instrument 120, a spectrum analyzer can be connected to the electrical signal output terminal of each output channel to acquire a power spectrum, which is an electrical power with frequency dependence, as an electrical signal. Alternatively, an A / D converter can be used to convert the electrical signals of each channel into digital data, and a power spectrum similar to that obtained by a spectrum analyzer can be obtained by performing a Fourier transform on this data.

[0032] In this embodiment of the CMRR measurement apparatus, two test light sources are used: a wideband light source (WBS) 103 and a continuous wave (CW) light source 102. The wideband light source 103 can be an ASE (Amplified Spontaneous Emission) light source, a supercontinuum (SC) light source, or an SLD (Super Luminescent Diode) light source. The test light from the wideband light source 103 is incoherent light with disjointed phases and a wide wavelength range corresponding to a frequency range of at least twice the frequency bandwidth required for CMRR measurement. Here, the wavelength range 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, in the case of a 1.5 μm band signal light, a wavelength range of 1 nm corresponds to a frequency-equivalent bandwidth of 125 GHz, making it possible to measure CMRR of approximately 62.5 GHz or less. Since the frequency at which CMRR is required for transmission is smaller than the baud rate, the corresponding frequency bandwidth of the wavelength must be wider than the baud rate. Also, the wavelength of CW light is included within the wavelength bandwidth of broadband light.

[0033] As a CW light source, for example, a laser light source specified in the ITLA (Integrated Tunable Laser Assembly) specification for communications, as standardized by the OIF, can be used. The test light from the CW light source is monochromatic, emitting highly coherent light with synchronized 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 broadband light source's wavelength width.

[0034] In molecular measurement system 100-n for CMRR_sig in Figure 1(a), broadband and CW light from the two light sources 102 and 103 are coupled by coupler 105 and then input as test light 130 to the signal light input terminal of the DUT. Therefore, in molecular measurement system 100-n, the coupled test light 130 reaches the PD in the DUT 110 via a single optical fiber and the same path within the DUT 110. In molecular measurement system 101-n for CMRR_LO in Figure 2(a), broadband and CW light are coupled by coupler 105 and then input as test light 130 to the local emission input terminal of the DUT via polarizer 104. Polarizer 104 directs the polarization of the test light 130 to the local emission input terminal of the coherent optical receiver's local oscillator. light Used to limit the input polarization to the polarization designed at the input terminal.

[0035] In the denominator measurement system 100-d for CMRR_sig in Figure 1(b), broadband light 131 from broadband light source 103 is used as the signal. light It is input to the input terminal. In addition, the CW light from the CW light source 102 is polarized by the polarizer 104, and the polarized CW light 132 is input to the local emission input terminal. 104 The connection between the local oscillator and the local light-emitting input terminal is made by polarization-maintaining fiber 107, which maintains the polarization state. In the case of denominator measurement system 100-d, the broadband light 131 and CW light 132 reach the PD via different paths of the optical hybrid within the DUT 110, respectively. The output of the CW light source is already polarized and the local oscillator of the coherent optical receiver light If the polarization can be matched to the design at the input terminal, the polarizer 104 is unnecessary.

[0036] In both the molecular measurement system 100-n and the denominator measurement system 100-d, different test light from two light sources is input to the DUT. However, as will be explained later, in the case of molecular measurement, the light is incident on the two PDs of the balance PD via the same path of the optical hybrid, while in the case of denominator measurement, it is incident on the two PDs of the optical hybrid via different paths. 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 with reference to Figure 3.

[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 at the numerator measurement to the electrical signal data at the denominator measurement, the CMRR value can be calculated for each modulation frequency according to equation (1).

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

[0039] In the molecular measurement shown in Figure 3(a), the electric field Es of the light reaching the two PDs from the light source is input from a single signal light input terminal as test light 130. In Embodiment 1, test light 130 is a mixture of broadband light and CW light. If the electric field of the broadband light is E1 and the electric field of the CW light is E2, then since both lights reach the PDs via the same single path, there is no difference in the relative phase between the p-side PD and the n-side PD. Therefore, if the electric field of the light reaching the p-side PD is |E1+E2|, then the electric field of the light reaching the n-side PD can also be written as |E1+E2|. Furthermore, since broadband light has a wide wavelength range, this can be expressed as a discrete electric field E for each wavelength i.1i Expressed as a sum, it can be written as follows:

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[0040] In this case, 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 if the proportionality constant is α, they can be expressed as follows.

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[0041] CW light is light having a single wavelength, and the beat signal |E2| with itself is a beat signal with CW light. 2 Since it contains only low-frequency components below MHz, it does not appear in the output frequency spectrum of PD currents that are normally measured above 10 MHz. Also, as will be described later, in the measurement system of Figure 1, the light intensity of the broadband light is set to a level that is 10 dB or more weaker than that of the CW light, so the beat signal |E1| 2 The value of is also small compared to the beat signals E1 and E2, which are represented by the following equation (3-1).

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[0042] Therefore, equation (2-2) is simplified as shown below.

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[0043] If we define P0 as shown in the following equation, then the relationship (3-3) holds for the PD currents Ip and In.

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[0044] P0 is the optical power generated by the beat between broadband light sources from a broadband light source, and represents 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 corresponding 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 unbalance of the complementary path. According to the definition of CMRR explained in equation (1), the numerator in the log term can be found by determining |Ip-In|. |Ip-In| can be determined from the difference in current between the two PDs of a balanced PD by molecular measurements in Figure 1(a) and Figure 3(a). In other words, the value corresponding to the numerator in the log term of the definition of CMRR can be obtained directly from the differential output ΔI. Furthermore, this current can be converted to a voltage using a 50Ω system or TIA, and finally the power spectrum An(f) corresponding to the molecular measurement can be obtained.

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

[0046] In the denominator measurement in Figure 3(b), broadband light 131 is input from the signal light input terminal, and CW light 132 is input from the station light emission input terminal. The polarization multiplexed optical hybrid 113 uses light (electric field E) from the signal light input terminal in the wavelength band used. S ) and light from the local light-emitting input terminal (electric field E L Regarding the relative phase of the two, the design ensures that the phases of the light reaching the p-PD and n-PD are shifted by 180°. Let E1 be the electric field of the broadband light 131 entering from the signal light input terminal and reaching the p-side p-PD, and E2 be the electric field of the CW light 132 entering from the station light emission input terminal and reaching the n-side n-PD. If the absolute value of the electric field entering the p-PD is |E1+E2|, then the absolute value of the electric field incident on the n-PD can be written as |E1-E2|.

[0047] In CMRR measurements, for the light waves input and output from the optical hybrid to maintain the phase relationship described above 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, then the coherence length is λ 2 It is expressed as / Δλ. For example, if the optical path length L of the interferometric 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 / 0.5×10 -3 = 5 nm. Therefore, the wavelength width of the spectral bandwidth of broadband light must 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 inside the optical hybrid such that a phase difference of 180 degrees is obtained when the signal light and local emission are coupled and incident on the two PDs of one channel.

[0048] The output currents Ip and In of the PD at this time are proportional to the square of the electric field incident on the PD, and if the proportionality constant is α, they can be expressed as follows:

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[0049] In equations (4) and (5), the first two terms have the same sign and therefore cancel each other out and almost disappear in the differential current ΔI of the p-PD and n-PD. On the other hand, the third term in equations (4) and (5) has the opposite sign and is added in the differential current, becoming dominant, and the output current Iout obtained as the differential output current of the p-PD and n-PD is given by the following equation.

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[0050] According to the definition of CMRR explained in equation (1), the denominator in the log term should be calculated by adding |Ip| and |In|. Referring to equation (3-3) for numerator measurement in the CMRR measuring device of this embodiment, |Ip| and |In| are approximately equal to the value αP0. Referring to equation (6), 2αP0 can be obtained by the denominator measurement in Figure 1(b) and Figure 3(b), and this value corresponds to the sum of |Ip| (approximately P0) and |In| (approximately P0) (|Ip| + |In|). Therefore, the value corresponding to the denominator in the log term of the CMRR definition can be obtained immediately from the output of the denominator measurement system in Figure 1(b) and Figure 3(b). Furthermore, this current can be converted to a voltage using a 50Ω system or TIA, and finally the power spectrum Ad(f) corresponding to the denominator measurement can be obtained.

[0051] Using An(f) obtained from the numerator measurement and Ad(f) obtained from the denominator measurement, the CMRR corresponding to equation (1) can be calculated. In the case of dB expression, log is used, and since power is proportional to the square of the current, it can be calculated using the following equation.

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[0052] In the measurement system shown in Figure 1, if the output of DUT110 is connected to a 50Ω measurement system, 50 × Iout is obtained as the voltage output. Also, if a TIA (not shown) is connected to the output side of each PD of the coherent optical receiver 110 in Figure 3, the transimpedance is Z t Z t ×Iout is obtained as the voltage output. In both numerator and denominator measurements, the power spectrum of the voltage output can be obtained by the electrical measuring instrument 120.

[0053] As described above, the molecular measurement system 100-n in Figure 1(a) yields a value of ΔI = |Ip - In| or the power spectrum An(f) corresponding to ΔI, and the denominator measurement system 100-d in Figure 1(b) yields a value of |Ip| + |In| or the power spectrum Ad(f) corresponding to this. By taking the ratio of these two values, the CMRR can be calculated for each frequency band. However, the configuration of the paths from the two light sources 102 and 103 to the optical input terminals (sig, LO) of the DUT differs between the molecular and denominator measurement systems, and the optical loss from the light source to the PD differs for each test light. To obtain the correct CMRR value, it is necessary to equalize the absolute value of the electric field of the light reaching the PD.

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

[0055] Accordingly, the present invention relates to a common-mode rejection ratio (CMRR) measuring device for a coherent optical receiver having a signal optical input terminal and a local light emission input terminal, an optical hybrid connected to the signal optical input terminal and the local light emission input terminal, and a photodiode connected to the output of the optical hybrid, 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 a spectral wavelength width corresponding to at least twice the frequency band of the CMRR to be measured, and an electrical measuring instrument for the electrical signal from the coherent optical receiver, wherein in both the numerator measurement and the denominator measurement of the CMRR, the test light from the broadband light source is input to at least one of the signal optical input terminal or the local light emission input terminal.

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

[0057] Figure 4 illustrates variations in the combination of PD and TIA. In the above explanation of Figures 1 to 3, the coherent optical receiver was assumed to output differential current from the balanced PD114 shown in Figure 4(a). As a variation, a single-input single-ended output TIA115-1 may be connected to the balanced PD114. Alternatively, the balanced PD114 may be connected to a differential electrical output 11 7 p, 11 7 A TIA115-2 single-input differential output with n may also be connected.

[0058] Figure 4(b) shows the case using two juxtaposed PD pairs (dual PDs) 116 that are not balanced connected, with a differential input single output TIA115-3 or a differential electrical output 11 7 p, 11 7 Even when combined with the TIA115-4, which has a differential input and differential output with n, an output proportional to the current difference between the two PDs can be obtained. In any case where an amplifier such as a TIA is connected, a larger electrical output can be obtained. Figures 4(c) and 4(d) will be discussed later.

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

[0060] Figure 5 is a flowchart of the measurement procedure in the CMRR measuring device of Embodiment 1. The measurement systems 100-n and 100-d shown in Figure 1 are used to measure the signal optical input terminal. ru This is the procedure for measuring CMRR_sig. Flow 300 includes the steps of numerator measurement, denominator measurement, and CMRR calculation, as well as the calibration step labeled "Cal".

[0061] In S301, in the molecular measurement system shown in Figure 1(a), only broadband light is input to the sig terminal from the broadband light source 103. For one channel of the DUT, the total value of the photocurrent supplied from the power supply to the PD, as shown in Figures 4(c) and (d) and described later, is (I p0 +I n0 ) That is, the current value I 1wbs Measure (calibration step 1). This current value I 1wbs This is used for calibration of broadband light in the denominator measurement described later.

[0062] I p0 and I n0In the case of a configuration using a differential output IC among the variations of PD and TIA combinations in Figure 4, the current can be measured using a power supply with an ammeter as part of the electrical measuring instrument. Specifically, as shown in Figures 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 currents I is directly measured by the power supply with an ammeter. p0 +I n0 You can obtain this.

[0063] In S302, in the molecular measurement system shown in Figure 1(a), only CW light from the CW light source 102 is input to the sig terminal. For one channel of the DUT, the total value of the PD photocurrent (I p0 +I n0 ) That is, the current value I 1CW Measure (calibration step 2). This current value I 1CW This is used for calibration of CW light in the denominator measurement described later.

[0064] In S303, in the molecular measurement system 100-n shown in Figure 1(a), test light is input from the broadband light source 103 and the CW light source, respectively, and the mixed test light 130 is input to measure the output power spectrum An(f) using the electrical measuring instrument 120. As mentioned above, the frequency characteristic An(f) can be directly obtained using a spectrum analyzer. Alternatively, An(f) can be obtained by converting an electrical signal for a certain period of time into digital output data using an A / D converter and then performing a Fourier transform on that data. Control and calculation processing for spectrum acquisition are performed by the processor 121, and memory 122 can also be used.

[0065] In S304, the denominator measurement system is shown in Figure 1(b). 1 In 00-d, only broadband light from the broadband light source 103 is input to the sig terminal. For one channel of the DUT, the total value of the PD photocurrent (I p0 +I n0 ) That is, the current value I 2wbs Measure (calibration process 3).

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

[0067] In S306, the denominator measurement system is shown in Figure 1(b). 1 In 00-d, only CW light from the CW light source 102 is input to the LO terminal. For one channel of the DUT, the total value of the PD photocurrent (I p0 +I n0 ) That is, the current value I 2CW Measure (calibration step 5).

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

[0069] In S308, in the denominator measurement system 100-d shown in Figure 1(b), test light is incident from the broadband light source 103 and the CW light source, respectively, and the output power spectrum Ad(f) is measured by the electrical measuring instrument 120. Similar to the output power spectrum An(f), Ad(f) can also be obtained using a spectrum analyzer or an A / D converter. Control and calculation processing for spectrum acquisition are performed by the processor 121, and memory 122 can also be used.

[0070] In S309, output power spectrum A for each frequency band. n The ratio is obtained by dividing (f) by the output power spectrum Ad(f), and the CMRR value is calculated.

[0071] As is clear from the procedure described above, it also has aspects as a method for measuring CMRR, including each step from S301 to S309.

[0072] In the above S302, the total value of the PD photocurrent (I p0 +I n0 ) 、 Current value I 1CW In the measurement, for molecular measurement of CMRR, when only broadband light is input to either the signal light input terminal or the local light emission input terminal, I p0 and I n0 The sum of these is calculated as the first total current. Also, in S304 mentioned above, the total value of the PD photocurrent (I p0 +I n0 ), current value I 2wbs In the measurement, for CMRR denominator measurement, when only broadband light is input to either the signal light input terminal or the local light emission input terminal, the I p0 and I n0 The sum of these 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 emission input terminal in the CMRR denominator measurement (I p0 +I n0 ) is the sum of the power supply currents of the two photodiodes during molecular measurement with CMRR (I p0 +I n0 The light intensity of the test light from the light source during numerator and denominator measurements is adjusted to be the same as ).

[0074] In S305, I 1wbs =I 2wbsThe amount of intensity adjustment used when adjusting the output intensity of the broadband light source 103 to achieve this can also be used as a correction value (correction coefficient) for CMRR calculation. For example, instead of equalizing the current, spectral Ad(f) and Ad(f) data may be acquired first at a test light intensity where the electrical signal from the DUT is within the good operating range of the electrical measuring instrument, and then the acquired data may be corrected using the intensity adjustment amount. For example, if the measurement was taken at twice the light intensity for the current that should have been set, the output result should be halved for correction. The calibration process in Flow 300 is just one example of acquiring spectral Ad(f) after equalizing the detection levels of the PD, and the correction coefficient obtained by separately performing the calibration procedure may also be applied to the acquired data later.

[0075] Figure 6 shows the measurement results of the frequency dependence of CMRR in comparison with the conventional technology. The measurement system of Embodiment 1 in Figure 1 ru The CMRR_sig for the sig terminal up to a frequency band of 50 GHz is shown. The measurement results using the conventional LCA technique shown in Figure 11 are also shown, and the two are compared. Compared to the conventional technique, the measured values ​​differ by only a few dB at most in all frequency bands, and the frequency dependence is reproduced fairly accurately, confirming that the results agree with a certain degree of precision.

[0076] In the CMRR measurement apparatus of this disclosure, broadband light is used as one of the test lights, eliminating the need for test light whose modulation frequency has been swept by LCA or the like. This simplifies the CMRR measurement apparatus and enables short measurement times. In the case of measuring CMRR_LO for the LO terminal shown in Figure 2, only the sig terminal in the molecular measurement is replaced with the LO terminal, so the procedure of the measurement flow shown in Figure 5 can be applied as is with only minor adjustments to a part of the molecular measurement.

[0077] In the CMRR measuring apparatus of this disclosure, by using broadband light with a wide bandwidth as the test light, the beat signal generated by the PD in the coherent light receiver has a frequency bandwidth sufficient for measuring CMRR, and the frequency-swept modulated signal replaces the intensity-modulated test light. In the CMRR measuring apparatus of this embodiment, two types of light sources, including a broadband light source, are used, but equivalent CMRR measurement can be achieved with an even simpler configuration that uses only a broadband light source.

[0078] [Embodiment 2] In the CMRR measurement apparatus of Embodiment 1 described above, two types of light, broadband light and CW light, were used as test light. In this embodiment, a simpler method for measuring CMRR is presented, which uses only broadband light as test light.

[0079] Figure 7 shows the configuration of the CMRR measuring device for the signal optical terminal (sig) of Embodiment 2. Figure 7(a) shows the measurement system 200-n for measuring the numerator of CMRR, and (b) shows the measurement system 200-d for measuring the denominator of CMRR. DUT110 is a coherent optical receiver that is the target of CMRR measurement, and is equipped with two optical input terminals to which optical fibers are connected and test light is input, and an electrical signal output terminal that is connected to an electrical measuring instrument 120.

[0080] In molecular measurement system 200-n, broadband light 133 from broadband light source 103 is input to the signal light input terminal of the DUT as test light. Therefore, in 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 in Figure 7(b), broadband light 131 from the broadband light source 103 is split into two paths by the coupler 105. One path is input to the signal light input terminal as test light 134, and the other is input to the local light emission input terminal as test light 135. The test light 134 is level-adjusted by the optical attenuator 106. Sign lightThe test light 135 to the local light emission input terminal is polarized by polarizer 104, and the polarizer 104 and the local light emission input terminal are connected by a polarization-maintaining fiber, maintaining the polarization state. In the case of the denominator measurement system 200-d, the two branched broadband lights 134 and 135 each reach the PD via different paths in the optical hybrid within the DUT 110. Note that if polarized broadband light can be input with a polarization suitable for the local light emission input of the optical receiver, polarizer 104 is unnecessary.

[0082] In the measuring device of Embodiment 1, Figure 3 no Ko As explained with reference to the internal configuration of the Herent optical receiver, we will now explain how the numerator and denominator of the CMRR can be measured using each measurement system in Figure 7. The internal configuration and operation of the DUT in Figure 3 are the same as in Embodiment 1, except that the test light is different, and this remains unchanged in the CMRR measurement apparatus of this embodiment.

[0083] In the molecular measurement shown in Figure 7(a), the electric field E1 of the light from the broadband light source 103 reaching the two PDs can be expressed as a sum for each discrete wavelength i, since broadband light has a wide wavelength range. It can be expressed as follows:

number

[0084] In this case, 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 if the proportionality constant is α, it can be expressed as follows.

number

[0085] Here, if we define P1 as follows, I p and I n It can be expressed as shown in equation (7-4) below.

number

number

[0086] In equation (7-3), P1 is the optical power generated by the beat between broadband light sources, and equation (7-3) represents that a random optical signal with a frequency band close to the wavelength width of the broadband light spectrum reaches the PD. That is, with the same random optical signal having optical power P1 input to both PDs, the electrical output from the two PDs of a balanced PD is given by ΔI = |I|, depending on the degree of device asymmetry between the p-PD and n-PD, i.e., the unbalance of the complementary path, for the PD current. p -I n A difference signal corresponding to | is generated. According to the definition of CMRR explained in equation (1), for the numerator in the log term, |I p -I n We just need to find |. |I p -I n The | value is determined from the difference in current between the two PDs of the balanced PD by molecular measurement in Figure 7(a). In other words, the value corresponding to the numerator in the log term of the CMRR definition can be obtained directly from the differential output ΔI. This current can be converted to a voltage using a 50Ω system or TIA, and finally the power spectrum An(f) corresponding to the molecular measurement can be obtained.

[0087] In the denominator measurement in Figure 7(b), the electric field due to the beat between the test light 134 incident from the signal light input terminal and the test light 135 incident from the local light emission input terminal is shifted in phase by 180 degrees between the p-PD and the n-PD, similar to the denominator measurement system in Embodiment 1 of Figure 1(b).

[0088] As explained in Figure 3(b) of the denominator measurement system of Embodiment 1, the electric field of the broadband light 134 entering from the signal light input terminal and reaching the p-PD is denoted as E1, and the electric field of the broadband light 135 entering from the local light emission input terminal and reaching the n-PD is denoted as E2. In this case, the absolute value of the electric field entering the p-PD can be written as |E1+E2|, and the absolute value of the electric field incident on the n-PD can be written as |E1-E2|.

[0089] In CMRR measurements, for the light waves input and output from the optical hybrid to maintain the designed phase relationship, 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, then the coherence length is λ 2 It is expressed as / Δλ. For example, if 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=(1.55×10 -6 ) 2 / 0.5×10 -3 = 5 nm. Therefore, the wavelength width of the spectral bandwidth of broadband light must 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 inside the optical hybrid such that a phase difference of 180 degrees is obtained when the signal light and local emission are coupled and incident on the two PDs of one channel.

[0090] The output current of the PD at this time is determined by the electric field incident on the PD. Let E1 be the electric field that reaches the PD via the signal optical input terminal when a broadband signal is transmitted, and E2 be the electric field that reaches the PD via the local light emission input terminal. E1, as in the case of the numerator, can be expressed as a discrete sum for each wavelength i, as shown in the following equation.

number

[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 can be expressed as follows, where α is the proportionality constant.

number

number

[0092] In equations (8a) and (8b), the first two terms have the same sign, but the third term has the opposite sign. Therefore, when calculating the difference between Ip and In, which are currents generated by the electric fields E1 and E2, the last term is the dominant contributor. That is, the output current Iout obtained as a differential output current is expressed by the following equation.

number

[0093] E1 and E2 are the electric fields of the test light that reaches the PD via different paths from the same broadband light source 103, 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 made by matching the intensity when the test light is incident from the signal light input terminal with the intensity when the test light is incident from the local light emission input terminal. Specifically, in Figure 7(b), |E1|=|E2| can be made 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.

number

[0094] Here, if we define P2 on the right-hand side of equation (9-1) as shown in equation (9-2), we obtain the relationship shown in equation (9-3).

number

number

[0095] P2 represents the optical signal power consisting of beats between test beams 134 and 135, which are branched from the broadband light source 103. Therefore, as with P1 in molecular measurements, since it is an optical signal generated by the beats of broadband light from a broadband light source, it means that a random optical signal with 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 should be calculated by adding |Ip| and |In|. In equation (9-3) above, P2 is an electrical signal generated by the beat between test lights from broadband light sources. Furthermore, through the calibration process described later, the light intensity input to the PD from the signal light input terminal and the light intensity input to the PD from the local light emission input terminal are set to be equal to the light intensity input from the signal light input terminal during molecular measurement. Therefore, even though test lights 134 and 135 are optical signals with the same spectrum and optical powers P1 and P2, the light intensity detected by each of the two PDs will be larger by the amount that test light is input from two input terminals. Let this coefficient be a. The coefficient a can be determined from theoretical calculations and experimental measurements. For example, in an ideal case, this value is 2 1 / 2 However, by making adjustments based on conditions such as spectral shape, different values ​​can be used. Considering the adjustability of coefficient a, equation (9-3) can be written as follows.

number

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

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

number

[0099] Figure 8 is a flowchart of the measurement procedure in the CMRR measuring device of Embodiment 2. This is the procedure for measuring CMRR_sig related to the signal optical input terminal using the measurement systems 200-n and 200-d shown in Figure 7. Flowchart 400 includes the steps of numerator measurement, denominator measurement, and CMRR calculation, as well as the calibration step indicated as Cal.

[0100] In S401, in the molecular measurement system shown in Figure 7(a), broadband light is input from the broadband light source 103 to the signal light input terminal. For the channel to be measured in the DUT, the total value of the photocurrent (I) supplied from the power supply with ammeter to the PD is the same as in Embodiment 1. 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 of FIG. 7(a), test light 133 is incident from the broadband light source 103 to the signal light input terminal, and the output power spectrum An(f) is measured by the electrical measuring instrument 120. As described above, An(f) can be obtained by the spectrum analyzer. It is also possible to convert the electrical signal for a certain period of time into digital output data using an A / D converter and obtain An(f) by performing Fourier transform on the data. The control, arithmetic processing, etc. for spectrum acquisition are implemented by the processor 121, and the memory 122 can also be utilized.

[0102] In S403, in the denominator measurement system 200-d of FIG. 7(b), the input to the input terminal of the DUT 110 is removed, and broadband light from the broadband light source 103 is input only to the local light emission input terminal. For the measurement target channel of the DUT, the total value of the PD photocurrents (I light +I p0 +I n0 ), that is, the current value I 2LO is measured (calibration step 2).

[0103] In S404, the output intensity of the broadband light source 103 is adjusted so that the measured current value I 2LO becomes the current value I1 obtained in S401 (I1 = I 2LO ) (calibration step 3).

[0104] In S405, in the denominator measurement system 200-d of FIG. 7(b), the input to the local light emission 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. For the measurement target channel of the DUT, the total value of the PD photocurrents (I light +I p0 +I n0 ), that is, the current value I 2Sig is measured (calibration step 4).

[0105] In S406, the output intensity of the broadband light source 103 is adjusted so that the measured current value I 2Sig becomes the current value I1 obtained in S401 (I 2Sig=I1), while maintaining the same intensity of test light 135 to the already adjusted local light input terminal, the intensity of test light 134 to the signal light input terminal is adjusted. In other words, in the configuration of Figure 7(b), the intensity of test light 135 is maintained at the state after the completion of S404, and the intensity of test light 134 is adjusted with the variable attenuator 106 (calibration step 5). Once this intensity adjustment is complete, the photodetection power by PD is matched between the molecular measurement system and the denominator measurement system for broadband light, and the accuracy of the CMRR measurement value is maintained.

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

[0107] In S408, output power spectrum A for each frequency. n The CMRR is calculated by dividing (f) by the output power spectrum Ad(f) corrected by the correction factor a to obtain the ratio. The correction factor a used in the CMRR calculation should be determined by theoretical calculation or computer simulation using the spectrum of the test light source and the device model. It can also be calculated from the measurement results of a device for which the CMRR value is known.

[0108] Accordingly, the present invention relates to a method for measuring the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal optical input terminal and a local light emission input terminal, an optical hybrid connected to the signal optical input terminal and the local light emission input terminal, and a photodiode connected to the output of the optical hybrid, wherein in molecular measurement of the CMRR, the steps are: (S402) input test light from a broadband light source to at least one of the signal optical input terminal or the local light emission input terminal, and a first electrical signal is acquired by an electrical measuring instrument; and denominatorThe measurement includes the steps of inputting the test light from the broadband light source to at least one of the signal light input terminal or the local light emission input terminal and acquiring a second electrical signal with the electrical measuring instrument (S407), and calculating the CMRR based on the first electrical signal and the second electrical signal (S408), wherein the test light can be configured such that its coherence length is longer than the optical path length of one interferometer inside the optical hybrid and has a spectral wavelength width corresponding to at least twice the frequency bandwidth of the CMRR to be measured.

[0109] Figure 9 shows the measurement results of the frequency dependence of CMRR in comparison with the conventional technology. The CMRR_sig for the sig terminal up to a frequency bandwidth of 50 GHz is shown using the measurement system of Embodiment 2 in Figure 7. The measurement results of the conventional technology using LCA shown in Figure 11 are also shown. , both We compared the two methods. Compared to conventional technology, the measured values ​​differed by only a few dB at most in all frequency bands, and the frequency dependence was reproduced fairly accurately, confirming that the results agreed with a certain degree of precision.

[0110] In the CMRR measuring device of Embodiment 2, the numerator and denominator measurements are completed with short-duration measurements using an electrical measuring instrument. By using only broadband light as the test light, it is unnecessary to use test light whose modulation frequency has been swept by LCA or the like. In this way, the CMRR measuring device is simplified and short-duration measurements are achieved. 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, the measurement of CMRR_LO can be performed.

[0111] Various modifications and changes are possible to any of the embodiments described above, as follows. The electrical measuring instrument can be any device capable of analyzing the frequency characteristics of the input electrical signal. For example, it can be a real-time oscilloscope, a device with a built-in A / D converter such as a signal analyzer, a receiver with a built-in A / D converter, or a spectrum analyzer that directly analyzes the frequency characteristics. For calculating CMRR using a DSP, the CMRR can be obtained in logarithmic form by accumulating data over a certain period of time, performing a Fourier transform, and taking the logarithm of the ratio of the absolute values ​​of the numerator and denominator measurements.

[0112] Furthermore, while variations in the combination of PD and TIA were described in Embodiment 1 along with Figure 4, similar variations are possible in Embodiment 2 as well.

[0113] Switching between numerator and denominator measurement paths, and removing the optical fiber during the calibration process (Cal), can be done using an optical switch. Regarding the test light from the light source, while not mandatory if the test light from the light source meets the wavelength width requirements, it may be beneficial to set the final required wavelength width of the light source by applying appropriate bandwidth limiting using an optical filter.

[0114] In the embodiments shown in Figures 1 and 2, since the CW light source is polarized, when incident on a coherent light receiver, the ratio of light entering the X-polarized and Y-polarized sides through the polarization beam splitter inside the 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 embodiments described above, the intensity of the test light is adjusted in the calibration process to match the received intensity of the PD between the numerator and denominator measurements. To adjust the intensity of the test light, an optical amplifier or a variable optical attenuator may be provided in at least one of the paths from the light source to the signal light input terminal and the path to the local light emission input terminal in the measurement systems of Figures 1, 2, and 7. As described in the measurement procedures in Figures 5 and 8, the power spectra An(f) and Ad(f) can be obtained in both the denominator and numerator measurement systems after performing the calibration process to adjust the light intensity of the light source.

[0116] Furthermore, in the descriptions of each embodiment above, the photocurrent supplied from the ammeter-equipped power supply 118 is measured for each output channel of the coherent optical receiver and used for calibration. If the currents for each output channel are considered to be approximately the same, the currents from the power supplies for multiple channels can be measured together and divided by the number of channels to obtain the current for each channel, which can then be used for calibration.

[0117] Instead of matching the light intensity during the numerator and denominator measurements, it is also possible to acquire a power spectrum with different test light intensities for the numerator and denominator measurements, taking into account the output current range of the PD and the detection sensitivity and accuracy of the electrical measuring instrument, in order to obtain an appropriate electrical signal, and then correct the acquired measurement data. In the case of a measurement using one type of light source and 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 should be corrected by multiplying it by 1 / A. Also, in the case of a measurement using two types of light sources and test light as in Embodiment 1, and using the beat output of two types of test light, when the light intensity of one test light is multiplied by A, the output amplitude of the acquired electrical signal should be 1 / √A(1 / A 1 / 2 You can simply multiply it by two and correct it.

[0118] For example, in the case of numeral measurement, as is clear from equation (1), the better the CMRR characteristics, the more the p-PD and n-PD currents 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 for the numerator and denominator, such as using stronger test light in the numeral measurement than in the denominator measurement. This allows the electrical signal output from the DUT to be measured so that it falls within the optimal input range of the electrical measuring instrument 120. Since the light intensity is adjusted (by a factor of A), the measured electrical signal is corrected (by a factor of 1 / A or 1 / A) as described above. 1 / 2 By multiplying the measurement by two, it is also possible to measure the CMRR under the optimal conditions of the electrical measuring instrument 120.

[0119] When applying this method, the power spectrum is acquired while the electrical signal is within the optimal input range of the electrical measuring instrument, and then the measured value is corrected. Specifically, in the flowchart 300 of Figure 5, the light intensity is adjusted in S305 and S307 so that the electrical signal output amplitude is within the optimal input range of the electrical measuring instrument 120. Then, in the CMRR calculation formula in S309, the calculated value of CMRR is corrected using the photocurrent obtained in S301, S302, S305, and S308.

[0120] Furthermore, in any embodiment of the CMRR measuring device, a simpler calibration method similar to that described in Non-Patent Document 1 can also be used. More specifically, consider the case where the loss to each channel inside the coherent optical receiver is the same between the signal light input path and the local light emission input path. For example, in Embodiment 1, if the signal light is polarized light, such as by passing it through a polarizer, then when the polarization controller is adjusted to concentrate the signal light only to the X polarization, the input intensity of the local light emission should be set to twice the input intensity of the signal light. In this case, unlike the signal light which is incident only on the X side, the local light emission is evenly distributed to the X and Y polarizations, and in the X polarization, the signal light and the local light emission are incident at the same intensity. With this method, the light intensity reaching the PD of the measurement channel can be made equal by measuring only the light intensity of the test light input to the two terminals.

[0121] In the numerator and denominator measurement systems described above, an optical bandpass filter can be used to limit the bandwidth of the broadband light from the broadband light source 103 and reduce noise caused by unwanted light. Alternatively, as an alternative to the broadband light source, light randomly modulated at a speed exceeding the measurement bandwidth of the CMRR can be used.

[0122] When using an A / D converter as an electrical measuring instrument to measure a time signal, if the amount of memory available for calculations is limited, measurement errors can be reduced by averaging multiple measurements. For example, the acquired time signal can be Fourier transformed, and then the average of the absolute values ​​from different measurements can be taken.

[0123] By integrating the coherent optical receiver and DSP, even if the optical receiver lacks an analog electrical signal output terminal, the output signal of the coherent optical receiver can be acquired by the DSP. The electrical signal acquired by the DSP can be obtained from the output of the A / D converter typically found in the DSP for the coherent optical receiver, replacing the electrical measuring instruments shown in Figures 1, 2, and 7.

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

[0125] In the measurement apparatus of Embodiment 1, the beat signal between two test lights, broadband light and CW light, is used as the input to the PD, but as examined in equation (3), the beat signal between broadband light |E1| 2 These components, though small, also contribute to measurement errors. Accuracy can be improved by measuring the output power Bn(f) and Bd(f) respectively when only broadband light is input in both the numerator and denominator measurements, and using these values ​​for correction to calculate the CMRR (Common Measurement Rate of Reaction).

number

[0126] In the measuring apparatus of Embodiment 2, in the denominator measurement, the beat component of the test light from the signal light input force terminal and the test light from the local light emission input terminal is the main input to the PD, and the beat signal |E1| of the test lights from the signal light input force terminal 2 and the beat signal |E2| of the test lights from the local light emission input terminal 2 also become factors of measurement error. When test light is input only from the signal light input force terminal and when test light is input only from the local light emission input force terminal, the output powers Bds(f) and Bdl(f) are measured, and the accuracy can also be improved by obtaining CMRR for use in correction by the following equation. [Equation]

[0127] In addition, the present invention can also be used for measuring other quantities that include CMRR in the formula. As described in detail above, with the CMRR measuring apparatus of the present invention, the CMRR of a coherent optical receiver can be implemented at high speed with a less expensive configuration. [Industrial Applicability]

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

Claims

1. A device for measuring the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal optical input terminal and a local light emission input terminal, an optical hybrid connected to the signal optical input terminal and the local light emission input terminal, and a photodiode connected to the 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 inside the optical hybrid and a spectral wavelength width corresponding to more than twice the frequency band of the CMRR to be measured, The electrical measuring instrument for the electrical signal from the aforementioned coherent optical receiver and Equipped with, A measuring device characterized in that, in both the numerator measurement and the denominator measurement of CMRR, the test light from the broadband light source is input to at least one of the signal light input terminal or the local light emission input terminal.

2. The measuring apparatus according to claim 1, characterized in that, for the molecular measurement of CMRR, the test light from the broadband light source is input to only one of the signal light input terminal or the local light emission input terminal.

3. The measuring apparatus according to claim 1 or 2, characterized in that, for measuring the denominator of the CMRR, the test light is input from the broadband light source to both the signal light input terminal and the station light emission input terminal.

4. The measuring apparatus according to claim 1, characterized in that, for the molecular measurement of CMRR, light obtained by coupling 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 emission input terminal, and the wavelength of the CW light is included within the wavelength bandwidth of the test light.

5. The measuring apparatus according to claim 1 or 4, characterized in that, for measuring the denominator of the CMRR, 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 station light emission input terminal.

6. The light intensity of the test light from the broadband light source during the numerator and denominator measurements is adjusted such 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 emission 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 The measuring device according to any one of claims 1, 2, or 4, characterized in that the CMRR is calculated by correcting the numerator measurement result and the denominator measurement result using the sum of the respective power supply currents.

7. The measuring apparatus according to claim 1, characterized in that the broadband light source is one of an ASE (Amplified Spontaneous Emission) light source, an SC (Super-continuum) light source, or an SLD (Superluminescent diode) light source.

8. A method for measuring the common-mode rejection ratio (CMRR) of a coherent optical receiver having a signal optical input terminal and a station light emission input terminal, an optical hybrid connected to the signal optical input terminal and the station light emission input terminal, and a photodiode connected to the output of the optical hybrid, In molecular measurement of CMRR, the steps include inputting test light from a broadband light source to at least one of the signal light input terminal or the local light emission input terminal, and acquiring a first electrical signal using an electrical measuring instrument; In measuring the denominator of the CMRR, the steps include inputting the test light from the broadband light source to at least one of the signal light input terminal or the station light emission input terminal, and acquiring a second electrical signal with the electrical measuring instrument, A step of calculating CMRR based on the first electrical signal and the second electrical signal. Equipped with, A method for measuring CMRR, characterized in that 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 corresponding to more than twice the frequency band of the CMRR to be measured.

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