JP5874896B2 - Coherent light receiving apparatus and coherent light receiving method - Google Patents

Description

  The present invention relates to a coherent light receiving apparatus and a coherent light receiving method, and more particularly, to a coherent light receiving apparatus and a coherent light receiving method for receiving multiplexed signal light by coherent detection.

  In a long-distance optical transmission system, a large-capacity information transmission is realized by using wavelength division multiplexing (WDM) transmission technology that multiplexes and transmits optical signals of a plurality of wavelengths in one optical fiber. Yes. One of the technologies for increasing the capacity is a coherent optical reception system.

  Since the coherent light receiving method can achieve higher receiving sensitivity than the direct detection method, research and development has been actively conducted in the past. However, it has been technically difficult to develop an optical PLL (Phase Locked Loop) and a narrow spectrum light source necessary for realizing a coherent light receiving system. On the other hand, since the optical amplifier has been developed, it has become possible to extend the optical communication distance even in the direct detection method, and therefore, the practical development of the coherent light receiving method has been stagnant.

  However, in recent years, the polarization multiplexed quadrature phase shift keying (DP-QPSK) method has been considered promising for realizing a transmission rate of 100 Gbps class. For this reason, research and development of coherent optical reception systems has become active again for the following reasons. The first reason is that the advancement of CMOS-LSI technology has improved digital signal processing technology, and it has become possible to compensate for the deviation of the light source frequency, so that a highly accurate optical PLL is no longer required. . Second, the digital signal processing technique can compensate for chromatic dispersion (CD) and polarization mode dispersion (PMD). Third, since the bit rate is high, a narrow spectrum light source is not necessary. Fourthly, the high sensitivity and high optical signal-to-noise ratio (OSNR) tolerance characteristic of the coherent optical reception system improves the margin shortage at a high bit rate.

  On the other hand, a non-blocking ROADM configuration using the wavelength selectivity of a coherent optical reception method has been proposed for ROADM (Reconfigurable Optical Add-Drop Multiplexer), which is an optical add / drop technique (see Non-Patent Document 1, for example). With this configuration, expensive optical components such as an arrayed waveguide grating (AWG) and an optical switch for wavelength branching are not required. In this configuration, wavelength splitting is not performed and signal light of all channels is input to the receiver. However, it is possible to extract only the interference component of the signal light of the detection channel and the local oscillation light, that is, wavelength selection.

L. Nelson et al., “Real-Time Detection of a 40 Gbps Intradyne Channel in the Presence of Multiple Received WDM Channels,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper OMJ1.

  In a wavelength-selective coherent light receiving system that extracts only a signal having a desired wavelength from a plurality of wavelengths, coherent light in which signal light of multiple channels including unnecessary channel signal light that is not used as a channel signal is collectively related. Input to the receiving device. For this reason, the average input power of the signal light in the related coherent light receiving device increases, and the current flowing through the photodetector (Photo Detector: PD) constituting the related coherent light receiving device may exceed the absolute maximum rating of the PD. Can occur. As a result, there is a problem that the related coherent optical receiver may be deteriorated or destroyed.

  As described above, the related coherent light receiving apparatus has a problem that deterioration and destruction may occur when the multiplexed signal light is selectively received by the wavelength of the local oscillation light.

  The object of the present invention is the above-described problem. In the related coherent light receiving device, if the multiplexed signal light is selectively received by the wavelength of the local oscillation light, there is a possibility that deterioration or destruction occurs. It is an object of the present invention to provide a coherent light receiving apparatus and a coherent light receiving method that solve the above problem.

  The coherent optical receiver of the present invention is connected to a coherent optical receiver that collectively receives multiplexed signal light in which signal lights having different wavelengths are multiplexed by the number of wavelengths, and the coherent optical receiver. A local oscillator that outputs local oscillation light that interferes with at least one of the signal light, a coherent optical receiver, and a control unit connected to the local oscillator, wherein the coherent optical receiver is a part of the multiplexed signal light. A 90-degree hybrid circuit and a photoelectric converter, and the 90-degree hybrid circuit receives the multiplexed signal light and the local oscillation light and interferes the multiplexed signal light with the local oscillation light. Is output to the photoelectric converter, and the control unit controls the output of the local oscillator based on the current conversion efficiency of the coherent optical receiver derived from the output of the light receiving unit and the output of the photoelectric converter.

  In the coherent light receiving method of the present invention, a first light current is obtained by inputting signal light and photoelectrically converting the signal light, and a first current conversion efficiency is obtained from the intensity of the signal light and the first photocurrent. , Input local oscillation light that interferes with the signal light, photoelectrically converts the local oscillation light to obtain a second photocurrent, and obtains the second from the intensity of the local oscillation light and the second photocurrent. And the local oscillation light intensity, which is the intensity of the local oscillation light, is controlled based on at least one of the first current conversion efficiency and the second current conversion efficiency.

  According to the coherent light receiving apparatus and the coherent light receiving method of the present invention, it is possible to prevent deterioration and destruction of the coherent light receiving apparatus even when the multiplexed signal light is selectively received by the wavelength of the local oscillation light. it can.

It is a block diagram which shows the structure of the coherent optical receiver which concerns on the 1st Embodiment of this invention. It is a block diagram which shows the structure of the non-blocking ROADM system using the coherent optical receiver which concerns on embodiment of this invention. It is a block diagram which shows the structure of a related ROADM system. It is a block diagram which shows the structure of the coherent optical receiver which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the 90 degree hybrid circuit with which the coherent optical receiver which concerns on the 2nd Embodiment of this invention is provided. It is a flowchart for demonstrating the coherent light reception method which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows another structure of the coherent optical receiver which concerns on the 2nd Embodiment of this invention. It is a flowchart for demonstrating the derivation | leading-out method of the current conversion efficiency in the coherent light reception method which concerns on the 2nd Embodiment of this invention. It is a figure which shows the relationship between the local oscillation light intensity | strength and the number of wavelengths which are accept | permitted in the coherent optical receiver which concerns on the 2nd Embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

[First Embodiment]
FIG. 1 is a block diagram showing a configuration of a coherent light receiving apparatus 100 according to the first embodiment of the present invention. The coherent optical receiver 100 includes a coherent optical receiver 110, a local oscillator 120 connected to the coherent optical receiver 110, and a control unit 130 connected to the coherent optical receiver 110 and the local oscillator 120.

  The coherent optical receiver 110 collectively receives multiplexed signal light in which signal lights having different wavelengths are multiplexed by the number of wavelengths. A light receiving unit 111 that receives a part of the multiplexed signal light, a 90-degree hybrid circuit (90 ° hybrid) 112, and a photoelectric converter 113 are provided. Here, the photoelectric converter 113 is composed of a photo detector (PD) or the like.

  The local oscillator 120 outputs, to the coherent optical receiver 110, local oscillation light that interferes with at least one signal light in the multiplexed signal light. A 90-degree hybrid circuit (90 ° hybrid) 112 receives the multiplexed signal light and the local oscillation light, and outputs an interference signal light obtained by interfering the two to the photoelectric converter 113. At this time, the interfering signal light that interferes with the local oscillation light, the other multiplexed signal light, and the local oscillation light are input to the photoelectric converter 113. Then, the coherent optical receiver 110 selectively coherently detects the signal light that interferes with the local oscillation light from the multiplexed signal light, and outputs a signal after detection.

  The control unit 130 controls the output of the local oscillator based on the current conversion efficiency of the coherent optical receiver 110 derived from the output of the light receiving unit 111 and the output of the photoelectric converter 113. At this time, when the maximum intensity of each signal constituting the multiplexed signal light is determined in advance, the output of the local oscillator can be controlled using the current conversion efficiency and the number of wavelengths. Not limited to this, the output of the local oscillator may be controlled using the current conversion efficiency and the multiplexed signal light intensity derived from the output of the light receiving unit 111.

  According to the coherent optical receiver 100 of this embodiment, since the output of the local oscillator is controlled based on the current conversion efficiency of the coherent optical receiver 110, the total light intensity input to the photoelectric converter 113 is suppressed. can do. As a result, even when the multiplexed signal light is selectively received by the wavelength of the local oscillation light, the coherent light receiving device 100 can be prevented from being deteriorated or destroyed.

  Specifically, for example, the control unit 130 first obtains the multiplex signal light and the local oscillation light by photoelectric conversion from the current conversion efficiency, the number of wavelengths, and the local oscillation light intensity that is the light intensity of the local oscillation light. The maximum photocurrent which is the maximum value of the photocurrent to be obtained is derived. Thereafter, the control unit 130 can control the output of the local oscillator so that the maximum photocurrent at this time does not exceed the maximum rated value of the photoelectric converter.

  Even when the number of wavelengths is “1”, that is, when single wavelength signal light is received, if the current conversion efficiency of the coherent optical receiver is high, the current flowing through the photoelectric converter becomes excessive, causing deterioration and destruction. A similar problem can occur. However, even in this case, according to the coherent light receiving apparatus 100 of the present embodiment, since the total light intensity input to the photoelectric converter 113 can be suppressed, the occurrence of such a problem can be avoided.

  FIG. 2 shows an example of a non-blocking ROADM system 1000 using a coherent optical receiver according to an embodiment of the present invention. The optical splitter 1-1 power-divides the wavelength-multiplexed multiplexed signal light into the wavelength blocker 2 and the coherent optical receivers RX4-1 to 4-N. Among the multiplexed signal light branched to the wavelength blocker 2, the signal light having a specific wavelength is blocked. Signal light of other wavelengths is transmitted, amplified by the optical amplifier 7, and then sent to the optical splitter 1-2. The multiplexed signal lights branched by the optical splitter 1-1 are respectively combined with the local oscillation lights output from the local oscillators LO3-1 to 3-N and received by the coherent optical receivers RX4-1 to 4-N. . Then, after optical-electrical conversion, it is sent to each of the clients 6-1 to 6-N. On the other hand, the signals from the clients 6-1 to 6-N are subjected to electro-optical conversion by the transmitters TX5-1 to 5-N, respectively, and then sent to the optical splitter 1-2. The control unit 8 controls the wavelength blocker 2, the local oscillators LO3-1 to 3-N, the coherent optical receivers RX4-1 to 4-N, and the transmitters TX5-1 to 5-N. Here, the coherent optical receiver RX, the local oscillator LO, and a part of the control unit 8 constitute the coherent optical receiver 100 according to the present embodiment.

  With such a configuration, according to the non-blocking ROADM system 1000 using the coherent optical receiver 100 of this embodiment, an ROADM system having wavelength selectivity can be obtained.

  For comparison, FIG. 3 shows an example of the configuration of a related ROADM system 2000. The AWG 21-1 that is the arrayed waveguide diffraction grating branches the multiplexed signal light into each wavelength and sends it to the Add-Drop-SW 22 that is an optical switch. The Add-Drop-SW 22 sends the signal light having the wavelength to be dropped (dropped) to the associated coherent optical receivers RX24-1 to 24-N, and sends the signal light having the wavelength to be passed (through) to the AWG 21-2. The branched (dropped) signal light is combined with the local oscillation light output from the local oscillators LO23-1 to 23-N and received by the associated coherent optical receivers RX24-1 to 24-N. The Then, after optical-electrical conversion, each is sent to the clients 26-1... 26-N. On the other hand, the signals from the clients 26-1 to 26-N of the channels corresponding to the wavelength of the branched (dropped) signal light are subjected to electro-optical conversion by the transmitters TX25-1 to 25-N, respectively. And then sent to Add-Drop-SW22. The Add-Drop-SW 22 transmits the inserted signal light to the AWG 21-2.

  As apparent from FIGS. 2 and 3, according to the non-blocking ROADM system 1000 according to the present embodiment shown in FIG. 2, the arrayed waveguide grating AWG and the optical switch Add-Drop- which are indispensable for the configuration shown in FIG. SW is unnecessary. Since these optical devices are expensive, there is a problem that the cost increases, and the degree of design freedom in the WDM optical transmission system is limited. On the other hand, in the non-blocking ROADM system 1000 according to the present embodiment, it is possible to selectively receive multiplexed signal light based on the wavelength of the local oscillation light without causing such a problem.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 4 is a block diagram showing a configuration of a coherent light receiving apparatus 200 according to the second embodiment of the present invention. This embodiment demonstrates the case where it uses for a polarization multiplexing four phase phase modulation (Dual Polarization-Quadrature Phase Shift Keying: DP-QPSK) system.

  Part of the multiplexed signal light input to the coherent light receiving apparatus 200 is branched to the light receiving unit (PD) 32 and used as a power monitor. Other multiplexed signal lights are separated into X-polarized light and Y-polarized light by PBS 34 which is a polarizing beam splitter and input to 90-degree hybrid circuits (90 ° hybrid) 36-1 and 36-2, respectively. . On the other hand, the local oscillation light output from the local oscillator LO33 is branched into two by the optical coupler 35 and input to the 90-degree hybrid circuits (90 ° hybrid) 36-1 and 36-2, respectively. Interference light between signal light and local oscillation light is acquired inside 90-degree hybrid circuits (90 ° hybrid) 36-1 and 36-2, and input to balanced PDs 37-1 to 37-4 that are balanced photodetectors. . The amplitude of the signal light photoelectrically converted by the balance PD is adjusted by TIA / AGCs 38-1 to 38-4 including an impedance conversion amplifier (TIA) and an automatic gain control (Automatic Gain Control: AGC) circuit. Thereafter, the signals are input to ADCs 39-1 to 39-4 which are analog-to-digital converters (ADCs) through AC (Alternating Current) coupling, and are digital signal processors (Digital Signal Processors: DSPs). Digital signal processing is performed by the DSP 30-1. The operation of each unit is monitored or controlled by the monitoring control unit 30-2. In the coherent light receiving apparatus 200, a portion other than the local oscillator LO33 and the monitoring control unit 30-2 constitutes the coherent light receiver 210.

  FIG. 5 shows an example of a specific configuration of the 90-degree hybrid circuits (90 ° hybrid) 36-1 and 36-2. The signal light is branched into four by the optical couplers 41-1, 41-2, and 41-4. On the other hand, the local oscillation light is branched into four by the optical couplers 41-3, 41-5, and 41-6, and then the phase is set to 0 by the π / 2 phase shifter 43 and the π phase shifters 42-1 and 42-2, respectively. Shifted by π, π / 2, 3π / 2. Thereafter, interference light between the local oscillation light and the signal light is generated by the optical mixers 44-1 to 44-4 and output to the photodetectors PD45-1 to 45-4 as photoelectric converters. Therefore, in the signal light and the local oscillation light, an optical loss of 6 dB corresponding to four branches occurs in the 90-degree hybrid circuit (90 ° Hybrid).

Next, the output of the photodetectors PD45-1 to 45-4 will be described. Assuming that the multiplexed signal light is S (t) and the local oscillation light is L (t), each is expressed as follows.

Here, ω _k and ω indicate the frequencies of the signal light and the local oscillation light (= light speed / wavelength), respectively. Φ _k is the phase of the signal light, and transmission information is put on this phase in the phase modulation method. For example, in the QPSK system is phi _k 0, [pi, is any value π / 2,3π / 2.

Assuming that the wavelength of the signal light ω ₁ to be extracted and the wavelength of the local oscillation light ω coincide (ω ₁ = ω), the outputs of the photodetectors PD45-1 to PD 45-1 are expressed as follows.

In Expressions (1) to (4), a, b, c, and d are current conversion efficiencies of the coherent optical receiver, and are assumed to be equal in the signal light port and the local oscillation light port. At this time, the signal light other than k = 1 is normally separated from the local oscillation light by 50 GHz or more, so that the interference component between the signal light and the local oscillation light is outside the band of each photodetector PD. Therefore, these interference components do not appear in the equations (1) to (4).

Next, the current conversion efficiency will be specifically described. The current conversion efficiency of the photo detector PD is a value indicating how much current flows when light of 1 W intensity is input to the photo detector PD. The relationship between input light intensity and photocurrent in the photodetector PD is expressed by the following equation.

Here, I [A] is the photocurrent, e is the charge density, h is the Planck constant, λ is the wavelength, c is the speed of light, η is the quantum efficiency, and P [W] is the input light intensity. From the above equation, the current conversion efficiency of the photodetector PD can be obtained as follows.

On the other hand, the current conversion efficiency of the coherent optical receiver indicates how much current flows through the photodetector PD when light of 1 W intensity is input to the input port of the optical receiver. The signal light input to the signal light port of the coherent optical receiver 210 is branched into two branches by the PBS 34 and four branches by the 90-degree hybrid circuit (90 ° hybrid) 36-1 (36-2). Therefore, a 9 dB optical loss corresponding to 8 branches occurs. Therefore, when the input power to the signal light port of the coherent optical receiver 210 is P _sig [dBm], the input intensity P [dBm] to the photodetector PD is expressed by the following equation.

Here, L _sig indicates an excessive loss of the signal optical port. The excess loss is a loss of the waveguide or a coupling loss between the waveguide and each photodetector PD. From the above two equations, the current conversion efficiency at the signal light port of the coherent optical receiver 210 is expressed by the following equation.

Specifically, for example, when the current conversion efficiency of the photodetector PD is 0.8 [A / W] and the excess loss L _sig [dB] of the signal light port is 2 dB, the current conversion efficiency of the signal light port of the coherent optical receiver Is

It is obtained.

  Next, a case where the coherent optical receiver is used in the ROADM system will be described. In the related ROADM system 2000 shown in FIG. 3, as described above, wavelength branching is performed using the AWG 21-1. Then, the add-drop-SW 22 switches according to whether it is branched (Drop) or passed (Through), and extracts and receives signal light of a specific wavelength. Further, a signal corresponding to the wavelength of the branched signal light is inserted (Add) into the Add-Drop-SW 22, multiplexed by the AWG 21-2, and then transmitted to the transmission line.

  On the other hand, in the non-blocking ROADM system 1000 using the coherent optical receiver according to the present embodiment shown in FIG. 2, as described above, the wavelength-multiplexed multiplexed signal light is first split in power by the optical splitter 1-1. . Thereafter, the signals are received by the respective coherent optical receivers RX4-1 to 4-N. The local oscillation light from the local oscillators LO3-1 to 3-N is also input to the coherent optical receivers RX4-1 to 4-N, respectively, and is caused to interfere with the signal light. By interfering with the local oscillation light having the same wavelength as the signal light having the wavelength to be extracted, it is possible to extract only the signal light having a specific wavelength despite receiving all the channels without splitting the wavelength. It is assumed that the channel through which the signal light passes (Through) does not install the transmitter TX5-k and the coherent optical receiver RX5-k, or does not operate even if they are installed. At this time, the wavelength blocker 2 passes only the signal of the channel to be passed (Through). These controls are performed by the control unit 8.

  However, in the wavelength division multiplexing (WDM) system, signal light of a plurality of wavelengths is input simultaneously, so that high intensity light is input to the coherent optical receivers RX4-1 to 4-N. For example, if 32 channels of signal light having a maximum light intensity of 0 dBm per channel are input, light having a total intensity of +15.1 dBm is input to the coherent optical receiver. Inside the coherent optical receiver, as described above, there are two branches for separation of X polarization and Y polarization, and four branches in a 90-degree hybrid circuit (90 ° Hybrid), for a total of eight branches. 9 dB is produced. There are other excess losses. If these losses are 2 dB, light with an intensity of +4.1 dBm is input per photo detector PD. In addition to this, if the intensity of the local oscillation light is +15 dBm, light with an intensity of +4 dBm is further input, and a light with an intensity of +7.1 dBm is also input. If the current conversion efficiency of the photodetector PD is 0.8 A / W, an average current of about 4 mA flows through the photodetector PD, but this exceeds the absolute maximum rating of the photodetector PD. For this reason, the coherent optical receiver may be deteriorated or destroyed.

  However, according to the coherent light receiving method according to the present embodiment, as described below, even when the multiplexed signal light is selectively received by the wavelength of the local oscillation light, the coherent light receiving device is deteriorated or destroyed. Can be prevented.

  FIG. 6 is a flowchart for explaining the coherent light receiving method according to the present embodiment. First, the coherent optical receiver receives signal light (step S11), and acquires the first photocurrent by photoelectrically converting the signal light (step S12). Then, a first current conversion efficiency that is a current conversion efficiency for the signal light is acquired from the intensity of the signal light and the first photocurrent (step S13).

  Next, the coherent optical receiver inputs the local oscillation light that interferes with the signal light (step S14), and acquires the second photocurrent by photoelectrically converting the local oscillation light (step S15). Then, a second current conversion efficiency that is a current conversion efficiency for the local oscillation light is acquired from the intensity of the local oscillation light and the second photocurrent (step S16).

  Finally, the coherent optical receiver controls the local oscillation light intensity, which is the intensity of the local oscillation light, based on at least one of the first current conversion efficiency and the second current conversion efficiency (step S17). In the above description, the second current conversion efficiency is acquired after acquiring the first current conversion efficiency. However, the present invention is not limited to this, and the second current conversion efficiency is acquired after acquiring the second current conversion efficiency. It is good as well.

  Here, the first current conversion efficiency and the second current conversion efficiency are acquired in advance by the coherent light receiving method according to the present embodiment (steps S11 to S16), and the values are stored in the coherent light receiver ( Memory etc.). In this case, during actual operation, the coherent optical receiver refers to the values of the first current conversion efficiency and the second current conversion efficiency stored in the storage unit (memory or the like), thereby determining the local oscillation light intensity. Control (step S17).

  In addition, when the coherent light receiving device 200 according to the present embodiment is used, the light receiving unit (PD) 32 included in the coherent light receiving device 200 acquires the intensity of the multiplexed signal light that is signal light even during actual operation. Can do. Therefore, the current conversion efficiency during actual operation can be acquired, and the local oscillation light intensity can be controlled based on the current conversion efficiency.

  Next, a case where the coherent light receiving apparatus 200 is used will be described as an example for controlling the local oscillation light intensity. Accordingly, the signal light is multiplexed signal light in which signal lights having different wavelengths are multiplexed by the number of wavelengths. First, the photoelectric current obtained by photoelectrically converting both the multiple signal light and the local oscillation light from at least one of the first current conversion efficiency and the second current conversion efficiency, the number of wavelengths, and the local oscillation light intensity. The maximum photocurrent, which is the maximum value, is derived. Then, the local oscillation light intensity is controlled so that the maximum photocurrent does not exceed a predetermined value. At this time, the local oscillation light intensity may be controlled in accordance with the increase or decrease in the number of wavelengths of the multiplexed signal light.

  These local oscillation light intensities can be controlled by controlling the output of the local oscillator LO33 by the monitoring control unit 30-2 controlling the drive current of the local oscillator LO33. Not limited to this, like the coherent optical receiver 300 shown in FIG. 7, the supervisory control unit 30-2 controls the local oscillation light, which is the output of the local oscillator LO33, by controlling the variable optical attenuator VOA31. It is good. Here, the variable optical attenuator VOA 31 is disposed in the optical path of the local oscillation light between the local oscillator LO and the coherent optical receiver 310.

  Next, the coherent light receiving method according to the present embodiment will be described in more detail. FIG. 8 is a flowchart for explaining a method of deriving current conversion efficiency. First, in a state where the local oscillation light is blocked (OFF) (step S21), a first photocurrent is obtained by photoelectrically converting the multiplexed signal light (step S22). Specifically, the output of the local oscillation light is turned off, and the signal light group is input from the signal light port. At this time, the photocurrents of the light receiving unit (PD) 32 and the balance PDs 37-1 to 37-4 are measured. Since the relationship of the photocurrent with respect to the input light intensity of the light receiving unit (PD) 32 can be obtained by measuring in advance, the multiple signal light intensity that is the signal group intensity in the light receiving unit (PD) 32 is obtained. The current conversion efficiency for the multiplexed signal light at the signal light port is calculated from the multiplexed signal light intensity and the photocurrents of the balance PDs 37-1 to 37-4 that are the first photocurrents (step S23).

  Next, the local oscillation light is output (ON) with a predetermined intensity (step S24). Here, the predetermined local oscillation light intensity is within a range not exceeding the absolute maximum rating of the photodetector PD even when combined with the signal light group intensity. At this time, the third photocurrent obtained by photoelectrically converting both the multiplexed signal light and the local oscillation light is obtained by measuring the photocurrents of the balance PDs 37-1 to 37-4 (step S25). Then, the current conversion efficiency for the local oscillation light in the local oscillation optical port is derived from the difference between the first photocurrent and the third photocurrent and the local oscillation light intensity (step S26). Through the above steps, the current conversion efficiency for the multiplexed signal light and the local oscillation light can be derived.

  FIG. 9 is a diagram illustrating the relationship between the allowable local oscillation light intensity and the number of wavelengths with respect to different current conversion efficiencies. Here, the absolute maximum rating of the photodetector PD is 2 mA, the maximum reception intensity per channel is 0 dBm, and the current conversion efficiencies in the signal light port and the local oscillation light port are assumed to be equal for simplicity.

From the above equation (1), the maximum value of the photocurrent flowing through the photodetector PD45-1 is represented by the following equation.

Figure 9 is one in which the maximum value of the photocurrent was determined local oscillation light intensity B ² so as not to exceed the 2mA that the absolute maximum ratings of the photo detector PD. The photodetectors PD45-2 to 45-4 can be similarly determined using the equations (2) to (4). The current conversion efficiency can be obtained by the above-described derivation method, and the information on the number of channels is notified from the external device. Therefore, the local oscillation light intensity can be determined based on these two pieces of information.

  In addition, the upper limit of the output intensity of the tunable laser module (Integrable Tunable Laser Assembly: ITLA) used for the local oscillator LO33 is usually about +15 dBm. Therefore, even when +15 dBm or more is allowed in FIG. 9, the output of the local oscillator LO33 is set to +15 dBm.

  If the current passed through the wavelength tunable laser module ITLA is changed in order to control the output of the local oscillator LO33, the phase of the local oscillation light also changes, and reception of signal light may be blocked. In order to avoid this, it is possible to set the local oscillation light intensity in accordance with the maximum number of wavelengths received in advance. In addition, the local oscillation light intensity may be changed by controlling the attenuation rate of the variable optical attenuator VOA31 arranged at the subsequent stage of the local oscillator LO33 using the coherent optical receiver 300 shown in FIG. . In this case, since the phase of the local oscillation light does not change, reception of the signal light is not blocked.

  When controlling the drive current of the local oscillator LO33, even if the number of reception channels is small at the beginning of introducing the coherent optical receiver, the local oscillation light intensity is set low in anticipation of an increase in the number of channels thereafter. It is necessary to keep. Therefore, when the number of reception channels is small, it is disadvantageous in terms of minimum reception sensitivity. On the other hand, when the variable optical attenuator VOA31 is used, the number of parts increases, and the local oscillation light intensity decreases by the amount of loss in the variable optical attenuator VOA31. It is possible to set the intensity.

  Specifically, for example, when the current conversion efficiency is determined to be 0.03 A / W by the above-described current conversion efficiency derivation method (FIG. 8), and the number of channels finally increases to 24 channels, local oscillation from FIG. It can be seen that the light intensity may be set to +15 dBm.

  Similarly, when the current conversion efficiency is found to be 0.05 A / W and the number of wavelengths finally increases to 24 wavelengths, the local oscillation light intensity may be set to +9.8 dBm. When the variable optical attenuator VOA 31 is used, when the number of wavelengths is 1, the local oscillation light intensity is set to +14.5 dBm, and the local oscillation light intensity is reduced as the number of wavelengths increases. Then, the variable optical attenuator VOA 31 is controlled so as to be +9.8 dBm at 24 wavelengths. Thereby, the optimal local oscillation light intensity can be set for each number of wavelengths.

  When the variable optical attenuator VOA 31 is used, the local oscillation light intensity is adaptively changed using the current conversion efficiency and the multiplexed signal light intensity monitored by the light receiving part (PD) 32 without using the number of wavelengths. A configuration may be adopted.

  The present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the invention described in the claims, and it is also included within the scope of the present invention. Not too long.

100, 200, 300 Coherent optical receiver 110, 210, 310 Coherent optical receiver 111 Light receiving unit 112 90 degree hybrid circuit (90 ° Hybrid)
113 Photoelectric Converter 120 Local Oscillator 130 Control Unit 1-1, 1-2 Optical Splitter 2 Wavelength Blocker 3-1 to 3-N, 23-1 to 23-N Local Oscillator LO
4-1 to 4-N coherent optical receiver RX
5-1 to 5-N, 25-1 to 25-N transmitter TX
6-1 to 6-N, 26-1 to 26-N Client 7 Optical amplifier 8 Control unit 21-1, 21-2 AWG
22 Add-Drop-SW
24-1 to 24-N Related Coherent Optical Receiver RX
30-1 DSP
30-2 Monitoring and Control Unit 31 Variable Optical Attenuator VOA
32 Light receiver (PD)
33 Local oscillator LO
34 PBS
35 Optical coupler 36-1, 36-2 90 degree hybrid circuit (90 ° Hybrid)
37-1 to 37-4 Balance PD
38-1 to 38-4 TIA / AGC
39-1 to 39-4 ADC
41-1 to 41-6 optical couplers 42-1, 42-2 π phase shifter 43 π / 2 phase shifter 44-1 to 44-4 optical mixer 45-1 to 45-4 photo detector PD
1000 Non-blocking ROADM system 2000 Related ROADM system

Claims (10)

A coherent optical receiver that collectively receives multiplexed signal light in which signal lights having different wavelengths are multiplexed by the number of wavelengths;
A local oscillator that is connected to the coherent optical receiver and outputs a local oscillation light that interferes with at least one signal light in the multiplexed signal light;
A controller connected to the coherent optical receiver and the local oscillator;
And
The coherent optical receiver includes a light receiving unit that receives a first multiplexed signal light obtained by branching the multiplexed signal light , a 90-degree hybrid circuit, and a photoelectric converter.
The 90-degree hybrid circuit has a first input port and a second input port, and receives a second multiplexed signal light obtained by branching the multiplexed signal light from the first input port , the local oscillator light from the second input port is input, and outputs an interference signal light of the said second multiplexed signal light local oscillator light to the photoelectric converter,
The control unit is an interference signal between the second multiplexed signal light and the local oscillation light based on the current conversion efficiency of the coherent optical receiver derived from the output of the light receiving unit and the output of the photoelectric converter. derives the maximum light current is the maximum value of the photoelectric current obtained by photoelectric conversion of light, the so that the maximum light current does not exceed the maximum rated value of the photoelectric converter, the local to the local oscillator output Coherent optical receiver that controls the intensity of oscillation light . The control unit photoelectrically converts the interference signal light of the second multiplexed signal light and the local oscillation light from the current conversion efficiency, the number of wavelengths, and the local oscillation light intensity that is the light intensity of the local oscillation light. Deriving the maximum photocurrent that is the maximum value of the photocurrent obtained by
Coherent optical receiver according to claim 1 for controlling the strength of the local oscillator light to the maximum light current is the local oscillator output so as not to exceed the maximum rated value of the photoelectric converter. The coherent optical receiver according to claim 1, wherein the control unit derives a current conversion efficiency for at least one of the multiplexed signal light and the local oscillation light. Wherein, the coherent optical receiving apparatus according to any one of claims 1-3 for controlling the intensity of the local oscillator light, wherein the local oscillator is output by controlling the drive current of the local oscillator. A variable optical attenuator disposed in the optical path of the local oscillation light between the local oscillator and the coherent optical receiver;
Wherein, the coherent optical receiver as claimed in any one of claims 1 to 3 for controlling the intensity of the local oscillator light, wherein the local oscillator is output by controlling the variable optical attenuator. In a coherent light receiving method for obtaining a received signal by causing interference between signal light and locally transmitted light,
A first photocurrent is obtained by photoelectrically converting the signal light,
Obtaining a first current conversion efficiency from the intensity of the signal light and the first photocurrent ;
To obtain a second optical current by photoelectric conversion of pre Symbol local oscillation light,
Obtaining a second current conversion efficiency from the intensity of the local oscillation light and the second photocurrent;
Based on at least one of the first current conversion efficiency and the second current conversion efficiency, the maximum photocurrent obtained by photoelectrically converting the signal light and the interference signal light of the local oscillation light A coherent light receiving method for deriving a photocurrent and controlling a local oscillation light intensity that is an intensity of the local oscillation light so that the maximum photocurrent does not exceed a predetermined value . The signal light is a multiplexed signal light in which signal lights having different wavelengths are multiplexed by the number of wavelengths,
Obtained by photoelectrically converting both the multiple signal light and the local oscillation light from at least one of the first current conversion efficiency and the second current conversion efficiency, the number of wavelengths, and the local oscillation light intensity. The maximum photocurrent, which is the maximum value of the photocurrent
The coherent light receiving method according to claim 6, wherein the local oscillation light intensity is controlled so that the maximum photocurrent does not exceed a predetermined value. The coherent light receiving method according to claim 7, wherein the local oscillation light intensity is controlled according to an increase or decrease in the number of wavelengths. Obtaining a multiple signal light intensity by receiving a portion of the multiple signal light;
A first photocurrent is obtained by photoelectrically converting the multiplexed signal light in a state where the local oscillation light is blocked,
The coherent light receiving method according to claim 7 or 8 , wherein the first current conversion efficiency is derived from the multiplexed signal light intensity and the first photocurrent. A first photocurrent is obtained by photoelectrically converting the multiplexed signal light in a state where the local oscillation light is blocked,
A third photocurrent is obtained by photoelectrically converting the multiplexed signal light and the local oscillation light together,
The coherent light according to any one of claims 7 to 9, wherein the second current conversion efficiency is derived from a difference between the first photocurrent and the third photocurrent and the local oscillation light intensity. Reception method.

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