Optical transmitter, optical access system, and optical transmission method

Abstract

Provided is an optical transmission device comprising: a plurality of optical transmission units that output optical signals with different wavelengths; a multiplexer that wavelength-division multiplexes the plurality of optical signals output from the plurality of optical transmission units; and an amplification unit that amplifies a wavelength-division multiplexed optical signal. The plurality of optical transmission units each comprise: a monitoring control signal application unit that adds a monitoring control signal to a constant current for driving a laser to generate a drive signal and applies the drive signal to the laser; the laser that outputs an optical signal in accordance with the drive signal applied by the monitoring control signal application unit; and a modulator that modulates the optical signal output from the laser with a main signal.　

Description

[Technical field]

[0001] The present invention relates to an optical transmitting device, an optical access system, and an optical transmitting method. [Background technology]

[0002] 2. Description of the Related Art In optical access systems, optical access systems that utilize wavelength division multiplexing (WDM) technology have been proposed (see, for example, Non-Patent Documents 1 to 3). 7 is a diagram showing a configuration example (part 1) of a conventional optical access system 1000. In the optical access system 1000, WDM signals on which a monitor and control signal defined by AMCC (Auxiliary Management and Control Channel) is superimposed for each wavelength are transmitted and received in the upstream and downstream directions.

[0003] The optical access system 1000 includes an OLT (Optical Line Terminal) 100 and a plurality of ONUs (Optical Network Units) 200-1 to 200-N (N is an integer equal to or greater than 2). The OLT 100 and the ONUs 200-1 to 200-N are communicatively connected via an optical fiber 250 and an optical splitter 260. The OLT 100 includes N optical transmitters 110-1 to 110-N, a wavelength multiplexer 120, a WDM filter 130, a wavelength demultiplexer 140, and N optical receivers 150-1 to 150-N. The system configuration of the optical access system 1000 is, for example, an example of a TWDM-PON (Time wavelength division multiplexing - Passive Optical Network) system defined in the ITU-T G.989.2 series.

[0004] The optical transmitting units 110-1 to 110-N generate optical signals of mutually different wavelengths. Each wavelength is amplitude modulated by an electrical signal generated by adding a weak monitor and control signal to transmission data (binary on / off signal). The optical transmitting units 110-1 to 110-N each include an adding unit 111-1 to 111-N and an optical transmitter 112-1 to 112-N. The adding units 111-1 to 111-N add the input transmission data and the monitor and control signal superimposed on the subcarrier. The monitor and control signal superimposed on the subcarrier is used here in order to support an AC-coupled optical receiver.

[0005] The optical transmitters 112-1 to 112-N generate optical signals of different wavelengths by amplitude modulating the electrical signals added by the adders 111-1 to 111-N. The optical signals generated in this manner are main signals superimposed with a monitor control signal. The optical signals generated by the optical transmitters 112-1 to 112-N are multiplexed by the wavelength multiplexer 120 to become downstream WDM signals. The downstream WDM signals are transmitted to the ONUs 200-1 to 200-N via the WDM filter 130, the optical fiber 250, and the optical splitter 260.

[0006] The WDM filter 130 multiplexes light of different wavelengths and demultiplexes the input optical signal. The optical signal demultiplexed by the WDM filter 130 is input to the wavelength demultiplexer 140. The wavelength demultiplexer 140 separates the input optical signal into light of each wavelength and outputs the separated light. The wavelength demultiplexer 140 has a plurality of ports for outputting light of different wavelengths, and optical receiving units 150-1 to 150-N are connected to the respective ports.

[0007] The optical receiving units 150-1 to 150-N extract transmission data and a monitor and control signal superimposed on a subcarrier from the optical signal input to the OLT 100. The optical receiving units 150-1 to 150-N include optical receivers 151-1 to 151-N and splitters 152-1 to 152-N, respectively. The optical receivers 151-1 to 151-N optically detect the input optical signal and convert it into an electrical signal. The splitters 152-1 to 152-N split the electrical signal. Then, in a processing unit at a subsequent stage, the transmission data is obtained from one of the split signals, and the monitor and control signal superimposed on the subcarrier is obtained from the other.

[0008] The ONUs 200-1 to 200-N include WDM filters 210-1 to 210-N, optical transmitters 220-1 to 220-N, and optical receivers 230-1 to 230-N. Since the ONUs 200-1 to 200-N have the same configuration, a specific configuration will be described using the ONU 200-1 as an example. The WDM filter 210-1 multiplexes light of different wavelengths and demultiplexes an input optical signal. The optical signal demultiplexed by the WDM filter 210-1 is input to the optical receiver 230-1.

[0009] The optical transmitting unit 220-1 generates an optical signal with a wavelength set in the ONU 200-1. The optical transmitting unit 220-1 includes an adding unit 221-1 and a wavelength-tunable optical transmitter 222-1. The adding unit 221-1 adds the transmission data input to the ONU 200-1 and the monitor and control signal superimposed on the subcarrier.

[0010] The wavelength-tunable optical transmitter 222-1 generates an optical signal with a wavelength set for the ONU 200-1 by amplitude-modulating the electrical signal added by the adder 221-1. The optical signals generated by the ONUs 200-1 to 200-N are multiplexed by the optical splitter 260 to become an upstream WDM signal. The upstream WDM signal is transmitted to the OLT 100 via the optical splitter 260 and the optical fiber 250.

[0011] The optical receiving unit 230-1 extracts the transmission data and the monitor / control signal superimposed on the subcarrier from the input optical signal. The optical receiving unit 230-1 includes a tunable optical receiver 231-1 and a splitter 232-1. The tunable optical receiver 231-1 is equipped with a tunable optical filter, and when the ONU 200-1 is connected to the system, its transmission wavelength is swept. When the ONU 200-1 detects the downstream wavelength, it reads the free wavelength information of the pair of upstream and downstream directions embedded in the monitor / control signal, and sets the transmission wavelength of the tunable optical filter and the output wavelength of the tunable optical transmitter 222-1 to the free wavelength (for example, see Non-Patent Document 1). Note that in FIG. 7, the ONU 200-1 transmits λ d1 +λ u1 In this example, the wavelength pair λ is selected, but if ONU200-1 has an available wavelength, d1 +λ u1 From λ dN +λ uN Any wavelength pair up to

[0012] The tunable optical receiver 231-1 converts an optical signal with a set wavelength into an electrical signal. The splitter 232-1 splits the electrical signal. Then, in a downstream processing unit, transmission data is obtained from one of the split signals, and a monitor / control signal superimposed on a subcarrier is obtained from the other. The monitor / control signal is also superimposed on the upstream optical signal, and an Ack is returned to the OLT 100.

[0013] 8 is a diagram showing a configuration example (part 2) of a conventional optical access system 2000. In the optical access system 1000, a monitor control signal is collectively superimposed on a downstream WDM signal. The optical access system 2000 includes an OLT 300 and a plurality of ONUs 400-1 to 400-N. The OLT 300 and the ONUs 400-1 to 400-N are connected via an optical fiber 450 and an optical splitter 460.

[0014] The OLT 100 includes N coherent optical transmitters 310-1 to 310-N, a wavelength multiplexer 320, and a semiconductor optical amplifier 330. The N coherent optical transmitters 310-1 to 310-N, the wavelength multiplexer 320, and the semiconductor optical amplifier 330 function as a wavelength division multiplexing transmitter. N coherent optical transmitters 310-1 to 310-N generate optical signals with different wavelengths on which a main signal is superimposed. These optical signals are multiplexed by a wavelength multiplexer 320 to generate a downstream WDM signal. A semiconductor optical amplifier 330 is used to amplify the intensity of the WDM signal incident on the transmission line, and modulates the drive current with a weak monitor and control signal to collectively superimpose the monitor and control signal on the intensity of the WDM signal. The baseband monitor and control signal may be subcarrier modulated, or may remain a baseband signal as in Non-Patent Document 3.

[0015] The ONUs 400-1 to 400-N include optical splitters 410-1 to 410-N, coherent optical receivers 420-1 to 420-N, photoelectric converters 430-1 to 430-N, and AMCC demodulators 440-1 to 440-N. Since the ONUs 400-1 to 400-N have the same configuration, a specific configuration will be described using the ONU 400-1 as an example.

[0016] The optical splitter 410-1 splits the downstream WDM signal input to the ONU 400-1. A part of the downstream WDM signal split by the optical splitter 410-1 is input to the coherent optical receiver 420-1. The coherent optical receiver 420-1 coherently receives the desired optical signal by matching the wavelength of the local light with the wavelength of the WDM signal to be received. Intradyne detection or heterodyne detection can be used as the coherent detection method.

[0017] The other downstream WDM signal branched by the optical brancher 410-1 is input to the photoelectric converter 430-1. The photoelectric converter 430-1 converts the input downstream WDM signal into an electrical signal. The AMCC demodulator 440-1 receives the monitor and control signal by demodulating the electrical signal. Note that all downstream WDM signals are input to the photoelectric converter 430-1, but the monitor and control signal can be received as long as the transmission bit rate of the monitor and control signal is sufficiently slower than the transmission bit rate of the main signal. [Prior art documents] [Non-patent literature]

[0018] [Non-Patent Document 1] Kyosuke Sone, Goji Nakagawa, Yoshio Hirose, and Takeshi Hoshida, "Demonstration of simultaneous activation of multiple ONUs in a WDM-PON system for 5G mobile fronthaul," IEICE Technical Report, CS2018-7, 2018. [Non-Patent Document 2] Goji Nakagawa, Kyosuke Sone, Seto Yoshida, Shoichiro Oda, Motoyuki Takizawa, Tomoo Takahara, Yoshio Hirose, and Takeshi Hoshida, "Proposal and Characteristic Evaluation of a Compact and Low-cost AMCC Superposition Method Using Magneto-Optical Effect-Based Optical Attenuator," IEICE Technical Report, CS2018-94, 2019. [Non-Patent Document 3] Satoshi Yoshima, Takaaki Katsumata, Hiroshi Miura, Yuita Noguchi, Akiko Nagasawa, Naoki Suzuki, and Masaki Noda, “Experimental Investigation of an Optically-superimposed AMCC in 100 Gb / s Coherent WDM-PON for 5G Mobile Fronthaul”, ECOC2016, Th.1.D.1, 2016. Summary of the Invention [Problem to be solved by the invention]

[0019] In the conventional optical access system 1000 shown in FIG. 7, downstream optical signals are generated by modulating each wavelength with an electrical signal generated by adding a monitor control signal to transmission data. When the transmission data is a high-speed signal exceeding 10 Gbps (bit per second), it is necessary to perform modulation using an external modulator (an EA (Electro-Absorption) modulator or an LN (Lithium Niobate) modulator). However, if a part of the amplitude range of the electrical signal that drives the modulator is assigned to the monitor control signal and the driving amplitude of the transmission data signal is reduced, the performance of the main signal deteriorates. Furthermore, Non-Patent Document 2 describes a configuration in which a high-speed variable optical attenuator is placed at the output of the modulator, and each is individually driven by the transmission data signal and the monitor control signal, but N variable optical attenuators are required, which increases the cost.

[0020] In the optical access system 2000 shown in FIG. 8, a semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier) ​​is used to superimpose the monitor and control signal on the intensity of the downstream WDM signal all at once. However, when the semiconductor optical amplifier is used in the gain saturation region to increase the output of the WDM signal, the main signal superimposed on the amplitude and phase is significantly distorted, and the receiving sensitivity is deteriorated. The lower limit of the input optical signal intensity at which gain saturation occurs does not depend on the number of wavelengths of the WDM signal. Therefore, when the number of wavelengths used increases, the optical intensity per wavelength cannot be increased in the configuration of FIG. 8. An erbium doped fiber amplifier (EDFA) is an optical amplifier that can increase the output without causing distortion in the WDM main signal. However, the gain response speed of the EDFA is slower than that of the SOA, and it is difficult to superimpose the monitor and control signal on the WDM signal by changing the intensity of the pump light of the EDFA at the transmission bit rate of the monitor and control signal. As described in Non-Patent Document 2, it is possible to superimpose the monitor and control signal using a variable optical attenuator that can change the amount of attenuation at high speed, but there is a problem that the number of optical components increases when used in combination with an EDFA.

[0021] As described above, the conventional technology has a problem in that it is not possible to suppress degradation of downstream WDM signals with a simple configuration.

[0022] In view of the above circumstances, an object of the present invention is to provide a technique capable of suppressing degradation of a downstream WDM signal with a simple configuration. [Means for solving the problem]

[0023] One aspect of the present invention is an optical transmitting device comprising a plurality of optical transmitting units that output optical signals of different wavelengths, a multiplexer that wavelength-multiplexes the plurality of optical signals output from the plurality of optical transmitting units, and an amplifier that amplifies the wavelength-multiplexed optical signal, wherein the plurality of optical transmitting units comprise a monitor and control signal application unit that adds a monitor and control signal to a constant current for driving a laser to generate a drive signal and applies the drive signal to the laser, a laser that outputs an optical signal in response to the drive signal applied to the monitor and control signal application unit, and a modulator that modulates the optical signal output from the laser with a main signal.

[0024] One aspect of the present invention is an optical access system comprising an optical transmitting device and an optical receiving device, wherein the optical transmitting device comprises a plurality of optical transmitting units that output optical signals of different wavelengths, a multiplexer that wavelength-multiplexes the plurality of optical signals output from the plurality of optical transmitting units, and an amplifier that amplifies the wavelength-multiplexed optical signal, wherein the plurality of optical transmitting units comprise a monitor / control signal application unit that adds a monitor / control signal to a constant current for driving a laser to generate a drive signal and applies the drive signal to the laser, a laser that outputs an optical signal in response to the drive signal applied to the monitor / control signal application unit, and a modulator that modulates the optical signal output from the laser with a main signal, and the optical receiving device comprises an optoelectric conversion unit that converts the optical signal into an electrical signal, and a demodulation unit that demodulates the monitor / control signal based on the electrical signal.

[0025] One aspect of the present invention is a method for transmitting a signal from a plurality of optical transmitters, the plurality of optical transmitters outputting optical signals of different wavelengths; In the plurality of optical transmission steps, a multiplexing step of wavelength-multiplexing a plurality of the optical signals output from a plurality of optical transmitters; and an amplifying step of amplifying the wavelength-multiplexed optical signal. a synchronizing step of synchronizing the plurality of optical transmitters; and in the plurality of optical transmission steps, a monitor and control signal application step of generating a drive signal by adding a monitor and control signal to a constant current for driving a laser, and applying the drive signal to the laser, and a modulation step of modulating, with a main signal, the optical signal output from a laser that outputs an optical signal in response to the drive signal applied in the monitor and control signal application step. and in the monitor and control signal applying step, the drive signal is applied to the laser at the same timing in response to a synchronization signal for synchronizing application timings of the drive signal, which is supplied in the synchronization step. It is an optical transmission method. One aspect of the present invention is an optical transmission method comprising a plurality of optical transmission steps of outputting optical signals of different wavelengths, a multiplexing step of wavelength-multiplexing the plurality of optical signals output from a plurality of optical transmitters in the plurality of optical transmission steps, and an amplifying step of amplifying the wavelength-multiplexed optical signal, wherein the plurality of optical transmission steps comprise a monitor and control signal application step of adding a monitor and control signal to a constant current for driving a laser to generate a drive signal and applying the drive signal to the laser, and a modulation step of modulating, with a main signal, the optical signal output from a laser that outputs an optical signal in response to the drive signal applied in the monitor and control signal application step, wherein the drive signals are applied to the laser at the same timing in the plurality of monitor and control signal application steps, and the amplifying unit used in the amplifying step is an Erbium Doped Fiber Amplifier (EDFA) and is operated in a gain saturation region.

[0026] One aspect of the present invention is an optical transmission method performed by an optical access system including an optical transmitter and an optical receiver, the method comprising: a plurality of optical transmission steps in which the optical transmitter outputs optical signals of different wavelengths; In the plurality of optical transmission steps, a multiplexing step of wavelength-multiplexing a plurality of the optical signals output from a plurality of optical transmitters; and an amplifying step of amplifying the wavelength-multiplexed optical signal. a synchronizing step of synchronizing the plurality of optical transmitters; and in the plurality of optical transmission steps, a monitor / control signal application step of generating a drive signal by adding a monitor / control signal to a constant current for driving a laser to generate a drive signal and applying the drive signal to the laser, and a modulation step of modulating, with a main signal, an optical signal output from a laser that outputs an optical signal in response to the drive signal applied in the monitor / control signal application step, and the optical receiving device includes a photoelectric conversion step of converting the optical signal into an electrical signal, and a demodulation step of demodulating the monitor / control signal based on the electrical signal. and applying the drive signal to the laser at the same timing in the monitor and control signal applying step in response to a synchronization signal for synchronizing application timings of the drive signal, the synchronization step being provided. It is an optical transmission method. One aspect of the present invention is an optical transmission method performed by an optical access system including an optical transmitting device and an optical receiving device, the optical transmitting device having a plurality of optical transmission steps of outputting optical signals of different wavelengths, a multiplexing step of wavelength-multiplexing the plurality of optical signals output from a plurality of optical transmitting units in the plurality of optical transmitting steps, and an amplification step of amplifying the wavelength-multiplexed optical signal, the plurality of optical transmitting steps having a monitor and control signal application step of adding a monitor and control signal to a constant current for driving a laser to generate a drive signal and applying the drive signal to the laser, and a modulation step of modulating, with a main signal, the optical signal output from a laser that outputs an optical signal in response to the drive signal applied in the monitor and control signal application step, the optical receiving device having a photoelectric conversion step of converting the optical signal into an electrical signal, and a demodulation step of demodulating the monitor and control signal based on the electrical signal, the drive signals being applied to the laser at the same timing in a plurality of the monitor and control signal application steps, and the amplification unit used in the amplification step being an Erbium Doped Fiber Amplifier (EDFA) and operated in a gain saturation region. Effect of the Invention

[0027] According to the present invention, it is possible to suppress degradation of downstream WDM signals with a simple configuration. [Brief description of the drawings]

[0028] [Figure 1] 1 is a diagram illustrating an example of the configuration of an optical access system according to a first embodiment. [Diagram 2] FIG. 4 is a sequence diagram showing a processing flow of the optical access system in the first embodiment. [Diagram 3] 1 is an explanatory diagram regarding degradation of signal performance of a monitor / control signal when the superimposition ratio of the monitor / control signal to the main signal is small; [Figure 4] 11 is an explanatory diagram regarding degradation of the signal performance of a monitor / control signal when the superimposition ratio of the monitor / control signal is large. FIG. [Diagram 5] FIG. 4 is an explanatory diagram regarding a drive current applied to a laser. [Figure 6] FIG. 11 is a diagram illustrating an example of the configuration of an optical access system according to a second embodiment. [Figure 7] FIG. 1 is a diagram showing a configuration example (part 1) of a conventional optical access system. [Figure 8] FIG. 1 is a diagram showing a second example of a configuration of a conventional optical access system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (First embodiment) 1 is a diagram showing a configuration example of an optical access system 1 in the first embodiment. The optical access system 1 in the present invention is applicable to any system that shares at least a part of a transmission line by WDM. In the following explanation, for the sake of simplicity, the optical access system 1 will be explained by taking a PON, which is a 1:N network, as an example.

[0030] The optical access system 1 includes an OLT 10 and a plurality of ONUs 20-1 to 20-N. The OLT 10 and the ONUs 20-1 to 20-N are connected via an optical fiber 30 and an optical splitter 40. The OLT 10 and the ONUs 20-1 to 20-N communicate with each other via the optical fiber 30 and the optical splitter 40. In the following description, the ONUs 20-1 to 20-N will be referred to as ONUs 20 when there is no particular need to distinguish them. The direction from the ONUs 20 to the OLT 10 will be referred to as upstream, and the direction from the OLT 10 to the ONUs 20 will be referred to as downstream.

[0031] The OLT 10 transmits downstream WDM signals to each of the ONUs 20-1 to 20-N by using WDM technology. The OLT 10 includes N optical transmitters 11-1 to 11-N, a wavelength multiplexer 12, an optical amplifier 13, and a synchronization unit 18.

[0032] The optical transmitters 11-1 to 11-N generate optical signals with different wavelengths. The optical transmitters 11-1 to 11-N each include a monitor and control signal application unit 14-1 to 14-N, a main signal generation unit 15-1 to 15-N, a laser 16-1 to 16-N, and a modulator 17-1 to 17-N. Since the optical transmitters 11-1 to 11-N each have the same configuration, the specific configuration will be described using the optical transmitter 11-1 as an example.

[0033] The monitor and control signal application unit 14-1 generates a monitor and control signal (AMCC) including monitor and control information. The monitor and control signal application unit 14-1 generates a drive signal by adding the generated monitor and control signal to a constant current for driving the laser 16-1. The monitor and control information includes information about available wavelengths (hereinafter referred to as "available wavelength information"). The monitor and control signal application units 14-1-14-N may use the monitor and control signal by superimposing it on a subcarrier, or may use it as a baseband signal. The monitor and control signal application unit 14-1 applies a drive signal to the laser 16-1 at the same timing as the other monitor and control signal application units 14 in response to a synchronization signal supplied from the synchronization unit 18. The main signal generator 15-1 generates a main signal using data to be transmitted that is supplied from the outside.

[0034] Laser 16-1 outputs an optical signal according to the drive signal applied from monitor / control signal application unit 14-1. By using the drive signal applied from monitor / control signal application unit 14-1, the monitor / control signal is superimposed on the amplitude of the optical signal output from laser 16-1. Lasers 16-1 to 16-N output optical signals of different wavelengths.

[0035] The modulator 17-1 amplitude-modulates the optical signal output from the laser 16-1 with the input transmission data.

[0036] The optical signals modulated by the optical transmitters 11-1 to 11-N are multiplexed by a wavelength multiplexer 12 to become a downstream WDM signal. The monitor and control signal superimposed on the current applied to the multiple lasers 16-1 to 16-N outputting different wavelengths is the same signal. The multiple monitor and control signal application units 14-1 to 14-N are synchronized with each other to drive the lasers 16-1 to 16-N at the same timing. Therefore, the multiple optical signals output from the lasers 16-1 to 16-N and modulated are input to the multiplexer / demultiplexer 12 at approximately the same timing.

[0037] The optical amplifier 13 amplifies the downstream WDM signal. The optical amplifier 13 is, for example, an EDFA. The downstream WDM signal amplified by the optical amplifier 13 is transmitted to the ONUs 20-1 to 20-N via the optical fiber 30 and the optical splitter 40. The synchronization unit 18 synchronizes the monitor and control signal application units 14-1 to 14-N to make the application timing of the drive signals the same.

[0038] The ONUs 20-1 to 20-N include optical splitters 21-1 to 21-N, tunable optical receivers 22-1 to 22-N, photoelectric converters 23-1 to 23-N, AMCC demodulators 24-1 to 24-N, wavelength setting units 25-1 to 25-N, and tunable optical transmitters 26-1 to 26-N. Since the ONUs 20-1 to 20-N have the same configuration, a specific configuration will be described using the ONU 20-1 as an example.

[0039] The optical splitter 21-1 splits the downstream WDM signal input to the ONU 20-1 into two paths. The downstream WDM signal split into the first path by the optical splitter 21-1 is input to the tunable optical receiver 22-1. The tunable optical receiver 22-1 is equipped with a tunable optical filter, and when the ONU 20-1 is connected to the optical access system 1, its transmission wavelength is swept. When the ONU 20-1 detects the downstream wavelength, it acquires free wavelength information for a pair of upstream and downstream directions embedded in the monitor and control signal. The wavelength setting unit 25-1 sets the transmission wavelength of the tunable optical filter and the output wavelength of the tunable optical transmitter 26-1 to free wavelengths based on the acquired free wavelength information. The wavelength setting unit 25-1 refers to free wavelength information for the downstream direction, for example, and sets an unused wavelength as the transmission wavelength of the tunable optical filter. The wavelength setting unit 25-1 refers to, for example, available wavelength information in the upstream direction and sets an unused wavelength as the output wavelength of the wavelength-tunable optical transmitter 26-1. The wavelength-tunable optical transmitter 26-1 transmits an optical signal having the wavelength set in the setting unit 25-1 to the OLT 10.

[0040] The downstream WDM signal branched to the second path by the optical brancher 21-1 is input to the photoelectric converter 23-1. The photoelectric converter 23-1 converts the input downstream WDM signal into an electrical signal. The AMCC demodulator 24-1 receives the monitor and control signal by demodulating the electrical signal. Note that all downstream WDM signals are input to the photoelectric converter 23-1, but the monitor and control signal can be received as long as the transmission bit rate of the monitor and control signal is sufficiently slower than the transmission bit rate of the main signal.

[0041] Fig. 2 is a sequence diagram showing a flow of processing in the optical access system 1 in the first embodiment. Note that the processing in Fig. 2 will be described taking as an example a case where the optical access system 1 is provided with two ONUs 20 (ONUs 20-1 and 20-2). The monitor and control signal application units 14-1-14-N included in the optical transmission units 11-1-11-N of the OLT 10 apply drive signals to the lasers 16-1-16-N at the same timing synchronized by the synchronization unit 18 (step S101). The lasers 16-1-16-N each output an optical signal with a wavelength corresponding to the applied drive signal. The modulators 17-1-17-N amplitude-modulate the optical signals output from the lasers 16-1-16-N with the input transmission data (step S102).

[0042] The wavelength multiplexer 12 multiplexes the modulated optical signals output from the optical transmitters 11-1 to 11-N to generate a downstream WDM signal (step S103). The generated downstream WDM signal is output to the optical amplifier 13. The optical amplifier 13 amplifies the input downstream WDM signal (step S104). The optical amplifier 13 outputs the amplified downstream WDM signal to the optical fiber 30.

[0043] The downstream WDM signal output to the optical fiber 30 is split by the optical splitter 40 and input to each of the ONUs 20-1 to 20-N. The downstream WDM signal input to the ONU 20-1 is branched by the optical branching device 21-1 (step S105). The downstream WDM signal branched to the first path by the optical branching device 21-1 is input to the wavelength tunable optical receiver 22-1, and the downstream WDM signal branched to the second path by the optical branching device 21-1 is input to the photoelectric converter 23-1.

[0044] The tunable optical receiver 22-1 optically detects the input downstream WDM signal at a set wavelength (step S106). The photoelectric converter 23-1 converts the input downstream WDM signal into an electrical signal (step S107). The photoelectric converter 23-1 outputs the electrical signal to the AMCC demodulator 24-1. The AMCC demodulator 24-1 receives a monitor control signal by demodulating the electrical signal output from the photoelectric converter 23-1 (step S108).

[0045] The downstream WDM signal input to the ONU 20-2 is branched by the optical branching device 21-2 (step S109). One of the downstream WDM signals branched by the optical branching device 21-2 is input to the wavelength tunable optical receiver 22-2, and the other downstream WDM signal branched by the optical branching device 21-2 is input to the photoelectric converter 23-2.

[0046] The tunable optical receiver 22-2 optically detects the input downstream WDM signal at a set wavelength (step S110). The photoelectric converter 23-2 converts the input downstream WDM signal into an electrical signal (step S111). The photoelectric converter 23-2 outputs the electrical signal to the AMCC demodulator 24-2. The AMCC demodulator 24-2 receives a monitor control signal by demodulating the electrical signal output from the photoelectric converter 23-2 (step S112).

[0047] Here, a case where the optical amplifier 13 is an EDFA will be described. The EDFA has a slow gain response speed, and when the bit rate of the monitor and control signal is equivalent to the gain response speed and the EDFA is operated in a gain saturation region, there is a concern that the signal performance degradation of the monitor and control signal due to the transient response of the EDFA may occur. The signal performance degradation of the monitor and control signal will be described with reference to Figs. 3 and 4. Fig. 3 is an explanatory diagram of the signal performance degradation of the monitor and control signal when the superimposition ratio of the monitor and control signal to the main signal is small. Fig. 4 is an explanatory diagram of the signal performance degradation of the monitor and control signal when the superimposition ratio of the monitor and control signal is large.

[0048] As shown in Figure 3, when the superposition ratio of the monitor / control signal to the main signal is small (Figure 3(a)), even if an optical signal is input into the saturation region of the EDFA gain, the EDFA gain is fixed (gain clamped) at the gain for the main signal, which accounts for the majority of the input optical intensity, and the gain received by the AMCC, which is a weak signal, is constant within that amplitude range, so there is no degradation of the signal characteristics of the monitor / control signal due to transient response.

[0049] On the other hand, when the superposition ratio of the monitor and control signal is large (FIG. 4(a)), the gain of the EDFA changes depending on the amplitude range of the monitor and control signal, and degradation of the signal characteristics of the monitor and control signal due to a transient response may occur.

[0050] FIG. 5 is an explanatory diagram regarding the drive current applied to the laser 16. As shown in FIG. Fig. 5(a) shows an example of the drive current 52 of the laser 16. When the drive current 52 of the laser 16 is amplitude modulated (baseband modulation or subcarrier modulation) by a weak monitor control signal after setting the bias point high, the wavelength changes slightly to a short wave when the laser is on and to a long wave when the laser is off due to a change in refractive index caused by a change in carrier density in the semiconductor (Fig. 5(b)). This wavelength change causes performance degradation of the received signal when the main signal is coherently detected.

[0051] Signal 53 in Figure 5(b) represents the wavelength shift due to the thermal effect, and signal 54 represents the wavelength shift due to the carrier effect. Signal 55 in Figure 5(c) represents the oscillation wavelength shift. If the monitor and control signal is a slow signal similar to the speed of thermal change in the semiconductor, the temperature of the semiconductor will follow the on / off state even if it is adjusted using a Peltier element or thermistor. In the on state, the semiconductor's drive current increases and the added heat causes a change in refractive index that is opposite to the change in carrier density, shifting the wavelength to the longer wavelength side. On the other hand, when the state changes from on to off, the driving current of the semiconductor decreases, and the heat decreases, causing a change in the refractive index opposite to the change in carrier density, and the wavelength shifts to the shorter wavelength side. As a result, the change in the refractive index due to the change in carrier density and the change in the thermal change cancel each other out, which is expected to suppress the wavelength shift of the semiconductor laser due to weak signal modulation.

[0052] The above effect can be expected when using a semiconductor laser having a structure that makes it difficult for heat generated by the drive current to escape. Examples of such semiconductor laser structures include a ridge waveguide type and a high mesa waveguide type.

[0053] The optical access system 1 configured as above makes it possible to suppress degradation of downstream WDM signals with a simple configuration. Specifically, in the optical access system 1, by superimposing a monitor control signal using the lasers 16-1 to 16-N, which are essential components of the optical transmitters 11-1 to 11-N, it becomes possible to increase the output power of downstream WDM signals without degradation with a simple configuration in which only the optical amplifier 13 is disposed after the WDM signals are generated by the multiplexer / demultiplexer 12.

[0054] In the OLT 10 in the optical access system 1, the main signal (transmission data) and the monitor and control signal are not added together, but are applied separately to the modulator 15 and the laser 16. This allows the entire amplitude range of the electrical signal that drives the modulator 15 to be assigned to the main signal, making it possible to suppress performance degradation of the main signal.

[0055] Although the effect of chromatic dispersion in the optical fiber 30 causes a shift in the time position of the monitor and control signals superimposed individually for each wavelength when they are received, the speed of the monitor and control signals is sufficiently slower than that of the main signal. Therefore, by adjusting the timing at the OLT 10, it is possible to suppress deterioration of the reception performance of the monitor and control signals caused by the shift in time position.

[0056] Second embodiment 6 is a diagram showing a configuration example of an optical access system 1a in the second embodiment. The optical access system 1a in the present invention is applicable to any system that shares at least a part of a transmission line by WDM. In the following explanation, for the sake of simplicity, the optical access system 1a will be explained by taking a PON, which is a 1:N network, as an example.

[0057] The optical access system 1a includes an OLT 10a and a plurality of ONUs 20-1 to 20-N. The OLT 10a and the ONUs 20-1 to 20-N are connected via an optical fiber 30 and an optical splitter 40. The OLT 10a and the ONUs 20-1 to 20-N communicate with each other via the optical fiber 30 and the optical splitter 40.

[0058] The optical access system 1a differs from the optical access system 1 in the first embodiment in that a monitor control signal is superimposed on a subcarrier. That is, the optical access system 1 has both patterns of using the monitor control signal superimposed on a subcarrier and using the monitor control signal as a baseband signal, whereas the optical access system 1a is limited to using the monitor control signal superimposed on a subcarrier only.

[0059] The OLT 10a includes N optical transmitters 11a-1 to 11a-N, a wavelength multiplexer 12, an optical amplifier 13, and a synchronization unit 18. The optical transmitters 11a-1 to 11a-N generate optical signals with different wavelengths. The optical transmitters 11a-1 to 11a-N include monitor and control signal application units 14a-1 to 14a-N, main signal generation units 15-1 to 15-N, lasers 16a-1 to 16a-N, and modulators 17-1 to 17-N, respectively. The OLT 10a is different in configuration in that it includes monitor and control signal application units 14a-1 to 14a-N and lasers 16a-1 to 16a-N instead of the monitor and control signal application units 14-1 to 14-N and lasers 16-1 to 16-N.

[0060] The monitor / control signal application units 14a-1-14a-N in the optical transmitters 11a-1-11a-N included in the OLT 10a superimpose the monitor / control signal on a subcarrier for use. The monitor / control signal application units 14a-1-14a-N are synchronized by a synchronization unit 18 and apply the drive signal, in which the monitor / control signal is superimposed on the subcarrier, to the lasers 16a-1-16a-N at the same timing.

[0061] When optically amplified by the optical amplifier 13, degradation of the signal characteristics of the monitor and control signal may occur as shown in Fig. 4. In this regard, as in the second embodiment, by subcarrier modulating the monitor and control signal and setting the signal band to exceed the response speed of the gain of the optical amplifier 13, this can be avoided, thereby making it possible to avoid the problem shown in Fig. 4.

[0062] A part of the functions of the OLT 10, 10a and the ONU 20 in the above-mentioned embodiment may be realized by a computer. In that case, a program for realizing the functions may be recorded in a computer-readable recording medium, and the program recorded in the recording medium may be read into a computer system and executed to realize the functions. The term "computer system" as used herein includes hardware such as an OS and peripheral devices. The term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, and storage devices such as hard disks built into a computer system. The term "computer-readable recording medium" may also include a medium that dynamically holds a program for a short period of time, such as a communication line when a program is transmitted via a network such as the Internet or a communication line such as a telephone line, and a medium that holds a program for a certain period of time, such as a volatile memory inside a computer system that is a server or client in that case. The above-mentioned program may be a program for realizing a part of the above-mentioned functions, or may be a program that can realize the above-mentioned functions in combination with a program already recorded in the computer system, or may be a program that is realized using a programmable logic device such as an FPGA.

[0063] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included. [Industrial Applicability]

[0064] The present invention can be applied to an optical access system that uses WDM. [Explanation of symbols]

[0065] 10, 10a...OLT, 20-1 to 20-N...ONU, 11-1 to 11-N, 11a-1 to 11a-N...optical transmitter, 12...multiplexer / demultiplexer, 13...optical amplifier, 16-1 to 16-N, 14a-1 to 14a-N...laser, 17-1 to 17-N...modulator, 21-1 to 21-N...optical splitter, 22-1 to 22-N...wavelength tunable optical receiver, 23-1 to 23-N...optical-to-electrical converter, 24-1 to 24-N...AMCC demodulator, 25-1 to 25-N...wavelength setting unit, 26-1 to 26-N...wavelength tunable optical transmitter

Claims

1. A plurality of optical transmitters outputting optical signals of different wavelengths; a wavelength multiplexer that multiplexes the optical signals output from the optical transmitters; an amplifier for amplifying a wavelength-multiplexed optical signal; Equipped with The plurality of optical transmitters include a monitor / control signal application unit that adds a monitor / control signal to a constant current for driving a laser to generate a drive signal and applies the drive signal to the laser; a laser that outputs an optical signal in response to a drive signal applied to the monitor and control signal application unit; a modulator that modulates the optical signal output from the laser with a main signal; Equipped with a synchronization unit that synchronizes the plurality of monitor and control signal application units, an optical transmission device, wherein the plurality of monitor and control signal application units apply the drive signal to the laser at the same timing in response to a synchronization signal for synchronizing the application timing of the drive signal supplied from the synchronization unit;

2. An optical access system including an optical transmitting device and an optical receiving device, The optical transmitter comprises: A plurality of optical transmitters outputting optical signals of different wavelengths; a wavelength multiplexer that multiplexes the optical signals output from the optical transmitters; an amplifier for amplifying a wavelength-multiplexed optical signal; Equipped with The plurality of optical transmitters include a monitor / control signal application unit that adds a monitor / control signal to a constant current for driving a laser to generate a drive signal and applies the drive signal to the laser; a laser that outputs an optical signal in response to a drive signal applied to the monitor and control signal application unit; a modulator that modulates the optical signal output from the laser with a main signal; Equipped with The optical receiving device comprises: A photoelectric conversion unit that converts the optical signal into an electrical signal; a demodulation unit that demodulates the monitor and control signal based on the electrical signal; Equipped with The optical transmitter comprises: a synchronization unit that synchronizes the plurality of monitor and control signal application units, the plurality of monitor and control signal application units apply the drive signal to the laser at the same timing in response to a synchronization signal for synchronizing application timings of the drive signals supplied from the synchronization unit.

3. A plurality of optical transmission steps for outputting optical signals of different wavelengths; a multiplexing step of wavelength-multiplexing the plurality of optical signals output from the plurality of optical transmitters in the plurality of optical transmission steps; an amplifying step of amplifying the wavelength-multiplexed optical signal; having In the plurality of optical transmission steps, a monitor and control signal applying step of generating a drive signal by adding the monitor and control signal to a constant current for driving a laser, and applying the drive signal to the laser; a modulation step of modulating, with a main signal, an optical signal output from a laser that outputs an optical signal in response to the drive signal applied in the monitor and control signal application step; A synchronization step of synchronizing operations in the plurality of monitor and control signal application steps, In the monitor and control signal applying step, the drive signals are applied to the laser at the same timing in response to a synchronization signal for synchronizing application timings of the drive signals, which is provided in the synchronization step. Optical transmission method.

4. An optical transmission method performed by an optical access system including an optical transmitting device and an optical receiving device, comprising: A plurality of optical transmission steps in which the optical transmitter outputs optical signals of different wavelengths; a multiplexing step of wavelength-multiplexing the plurality of optical signals output from the plurality of optical transmitters in the plurality of optical transmission steps; an amplifying step of amplifying the wavelength-multiplexed optical signal; having In the plurality of optical transmission steps, a monitor and control signal applying step of generating a drive signal by adding the monitor and control signal to a constant current for driving a laser, and applying the drive signal to the laser; a modulation step of modulating, with a main signal, an optical signal output from a laser that outputs an optical signal in response to the drive signal applied in the monitor and control signal application step; having The optical receiving device, a photoelectric conversion step of converting the optical signal into an electrical signal; a demodulation step of demodulating the monitor and control signal based on the electrical signal; having The optical transmitter, A synchronization step of synchronizing operations in the plurality of monitor and control signal application steps is further included. an optical transmission method in which, in a plurality of the monitor and control signal applying steps, the drive signals are applied to the laser at the same timing in response to a synchronization signal for synchronizing application timings of the drive signals, which is supplied in the synchronization step.

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