Optical transmission system and optical transmission method

Abstract

One aspect of the present invention is an optical transmitter, comprising: a first modulation unit that generates an optical modulation signal by intensity modulation of an optical signal; a second modulation unit that generates an optical modulation signal by phase / frequency modulation of an optical signal; and a modulation scheme switching unit that switches a modulation means for the optical signal to either the first modulation unit or the second modulation unit. The modulation scheme switching unit switches the modulation means to the modulation unit corresponding to a device to be connected to, among the first modulation unit and the second modulation unit.

Description

Technical Field

[0001] The present invention relates to technologies for an optical transmitter, an optical transmission system, and an optical transmission method.

Background Art

[0002] Conventionally, in order to achieve high-capacity optical communication, research and development of adaptive modulation technologies that apply optimal modulation methods and signal processing algorithms according to the transmission distance between transmission and reception nodes and the required signal speed have been underway. For example, FIG. 14 shows a configuration example of communication in the APN (APN: All Photonics Network) of IOWN (Innovative Optical and Wireless Network). FIG. 14(A) shows a configuration example (short-distance communication) in which return communication is performed by an optical gateway device between user devices connected to the same optical gateway device (Ph-GW: Photonic Gateway). On the other hand, FIG. 14(B) shows a configuration example (long-distance communication) in which data is transferred by pass-through communication between user devices connected to different optical gateway devices. In such an APN of IOWN, since it is desirable to minimize electrical processing such as optoelectronic conversion and routing processing of electrical signals, it is necessary to adaptively control transmission parameters according to the form of the transmission path.

[0003] FIG. 15 is a diagram showing an outline of the configuration of an optical transmission system according to the prior art. FIG. 15(A) shows a configuration example of an optical transmission system using an intensity modulation - direct detection method, and FIG. 15(B) shows a configuration example of an optical transmission system using a phase modulation - digital coherent detection method. Conventional optical transmitters using adaptive modulation mainly use methods such as changing the multiplicity of the signal to be transmitted using a digital signal processing circuit, or changing the redundancy of FEC (Forward Error Correction) using Reed - Solomon codes or LDPC (Low - Density Parity - Check Code) codes to obtain gain. For example, as shown in FIG. 15(A), in the intensity modulation - direct detection method often used in access networks and the like, the multiplicity of amplitude modulation is adaptively changed by switching the modulation signal to an NRZ (Non - return - to - zero) signal, a PAM (Pulse - Amplitude Modulation) 4 signal, a PAM8 signal, a PAM16 signal, etc. (see, for example, Non - Patent Document 1). Also, for example, as shown in FIG. 15(B), in the phase modulation - digital coherent reception method often used in core networks, the multiplicity of phase - amplitude modulation is adaptively changed by switching the modulation signal to a 4QAM (Quadrature amplitude modulation) signal, a 16QAM signal, a 64QAM signal, etc. (see, for example, Non - Patent Document 2).

Prior Art Documents

Non - Patent Documents

[0004]

Non - Patent Document 1

Non - Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, conventionally, in each of the intensity modulation - direct detection method and the phase modulation - digital coherent detection method, separate optical transmitters are required, and the parameters that can be adaptively changed in each case are only the parameters of the upper layer that can be changed by the digital signal processing circuit. Thus, conventionally, the optical transmitters that can be applied with modulation have not necessarily had a high degree of freedom in changing the method or parameters.

[0006] In view of the above circumstances, an object of the present invention is to provide a technology capable of improving the degree of freedom in changing the transmission method or transmission parameters in adaptive modulation of optical transmission.

Means for Solving the Problems

[0007] One aspect of the present invention includes a first modulation unit that generates an optical modulation signal by intensity modulation of an optical signal, a second modulation unit that generates an optical modulation signal by phase / frequency modulation of the optical signal, and a modulation method switching unit that switches the modulation means of the optical signal to either the first modulation unit or the second modulation unit. The modulation method switching unit switches the modulation means to the modulation unit corresponding to the destination device among the first modulation unit and the second modulation unit, and is an optical transmitter.

[0008] One aspect of the present invention includes the above optical transmitter, one or more user devices including an optical receiver that detects an optical signal by a direct detection method or a digital coherent detection method, one or more network node devices that house the one or more user devices, and a path control device that controls a communication path by the one or more network node devices. The optical transmitter exchanges information related to the negotiation to be carried out with the user device of the communication partner via the path control device, and is an optical transmission system.

[0009] One aspect of the present invention is a modulation step of modulating an optical signal by any means of a first modulation unit that generates an optical modulation signal by intensity modulation of the optical signal and a second modulation unit that generates an optical modulation signal by phase / frequency modulation of the optical signal, and a modulation method switching step of switching the modulation means of the optical signal to either the first modulation unit or the second modulation unit. In the modulation method switching step, the modulation means is switched to the modulation unit corresponding to the device that is the communication partner among the first modulation unit and the second modulation unit. This is an optical transmission method.

Advantages of the Invention

[0010] According to the present invention, in adaptive modulation of optical transmission, it becomes possible to improve the degree of freedom in changing the transmission method or transmission parameters.

Brief Description of the Drawings

[0011]

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DETAILED DESCRIPTION OF THE INVENTION

[0012] Embodiments of the present invention will be described in detail with reference to the drawings. <First Embodiment> FIG. 1 is a diagram showing a configuration example of an optical transmission system 1A according to the first embodiment. The optical transmission system 1A includes an optical transmitter 10, an optical receiver 20, and an optical transmission line 30. The optical transmitter 10 modulates data to be transmitted (hereinafter referred to as "target data") into an optical signal and outputs it to the optical transmission line 30. The optical transmission line 30 includes an optical fiber and connects the optical transmitter 10 and the optical receiver 20. The optical receiver 20 restores the target data transmitted by the optical transmitter 10 by inputting and demodulating the optical signal from the optical transmission line 30. Hereinafter, the configurations of the optical transmitter 10 and the optical receiver 20 will be described in more detail.

[0013] The optical transmitter 10 includes, for example, a light source 110, a digital signal processing circuit 120, a DA converter 130, and an intensity modulator 140. The light source 110 sends continuous light to the DA converter 130 and the intensity modulator 140. The light source 110 outputs, for example, a DFB laser (Distributed Feedback Laser) as continuous light.

[0014] The digital signal processing circuit 120 is a circuit that performs digital signal processing related to the transmission of target data. Specifically, the digital signal processing circuit 120 includes a modulation signal output unit 121 and a modulation method switching unit 122. The modulation signal output unit 121 generates a modulation signal for modulating the output light of the light source 110 and outputs it to the DA converter 130. For example, the digital signal processing circuit 120 is configured using a processor such as a CPU (Central Processing Unit) and a memory. The digital signal processing circuit 120 functions as the modulation signal output unit 121, the modulation method switching unit 122, and a digital signal processing unit for optical signals when the processor executes a program. Note that all or part of these functions may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above program may be recorded on a computer-readable recording medium. A computer-readable recording medium is, for example, a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, a semiconductor storage device (e.g., SSD: Solid State Drive), or a storage device such as a hard disk or a semiconductor storage device built into a computer system. The above program may also be transmitted via a telecommunication line.

[0015] The DA converter 130 receives the modulation signal from the digital signal processing circuit 120, converts the input modulation signal into an analog signal, and outputs it. Hereinafter, the modulation signal converted into an analog signal is referred to as an "analog modulation signal". The analog modulation signal output from the DA converter 130 is input to the light source 110 or the intensity modulator 140 based on the control of the modulation method switching unit 122 described later.

[0016] The modulation method switching unit 122 has a function of switching the modulation method of the light source 110 between intensity modulation and phase / frequency modulation. Specifically, the modulation method switching unit 122 switches the modulation method of the light source 110 by changing the amplitude and bias point of the modulation signal, the device to which the modulation signal is applied, etc. to those corresponding to the changed modulation method. The modulation method switching unit 122 can be configured as a part of the functions of the digital signal processing circuit 120. For example, the modulation method switching unit 122 is configured to switch the modulation method according to the type of optical receiver of the pre-registered communication partner. Also, for example, the modulation method switching unit 122 may be configured to switch the modulation method of the light source 110 in response to the input of a switching operation. In this case, the digital signal processing circuit 120 may include an input device such as a button or a switch that receives the input of the switching operation of the modulation method.

[0017] Specifically, when phase / frequency modulation is specified as the modulation method, the modulation method switching unit 122 controls the circuit so that the analog modulation signal output by the DA converter 130 is input to the light source 110. The light source 110 is configured to phase / frequency modulate the bias current by the input analog modulation signal. When the bias current is phase / frequency modulated based on the analog modulation signal, frequency changes and phase changes occur in the output light of the light source 110 (generally called chirp), and the phase / frequency modulation of the light source 110 is realized by this chirp. Since such a modulation method controls the operation itself of the light source 110 for outputting continuous light, it can be said that it is a more direct modulation method than the intensity modulation described later. Also, hereinafter, for simplicity, in some cases, modulating the output light of the light source 110 may be expressed as "modulating the light source 110".

[0018] On the one hand, when intensity modulation is specified as the modulation method, the modulation method switching unit 122 controls the circuit so that the analog modulation signal output by the DA converter 130 is input to the intensity modulator 140. In this case, the intensity modulator 140 inputs the analog modulation signal from the DA converter 130 and modulates the output light of the light source 110 by intensity modulation based on the input analog modulation signal. Specifically, the intensity modulator 140 inputs continuous light from the light source 110 and generates an optical modulation signal by intensity-modulating the input continuous light based on the analog modulation signal. The intensity modulator 140 sends the generated optical modulation signal to the optical transmission line 30. The intensity modulator 140 is, for example, an EA (Electro-Absorption) modulator. The intensity modulator 140 may be a Mach-Zehnder modulator.

[0019] FIG. 2 is an image diagram showing an outline of switching the modulation method of the light source 110. FIG. 2(A) represents an image of modulating the light source 110 by phase / frequency modulation, and FIG. 2(B) represents an image of modulating the light source 110 by intensity modulation. The example of FIG. 2 shows the case where the light source 110 is modulated with an NRZ signal that can take two values of 0 or 1.

[0020] The optical transmitter 10 of the first embodiment configured as described above can realize phase / frequency modulation of the light source 110 by modulating the bias current applied to the light source 110 with an analog modulation signal, and can intensity-modulate the light source 110 by including the intensity modulator 140. Therefore, according to the optical transmitter 10 of the first embodiment, it is not necessary to prepare separate optical transmitters for the intensity modulation method and the phase / frequency modulation method, and one optical transmitter 10 can support both methods. That is, according to the optical transmitter 10 of the first embodiment, in adaptive modulation of optical transmission, it is possible to improve the degree of freedom in changing the transmission method or transmission parameters.

[0021] (Modification example) In the optical transmitter 10 of the first embodiment, when the light source 110 is modulated by phase / frequency modulation, not only the phase / frequency modulation component of the analog modulation signal but also the intensity modulation component is applied along with the modulation of the bias current of the light source 110. In this case, the optical transmitter 10 may be configured to correct the optical modulation signal so that the intensity modulation component is removed by operating the intensity modulator 140 to modulate it with an inverse pattern of the modulation component of the light source 110. FIG. 3 is an image diagram of the correction of the optical modulation signal by the optical transmitter 10 of the modified example. FIG. 3(A) is a diagram showing the intensity of the optical signal (optical modulation signal) output from the light source 110 as a result of modulating the bias current applied to the light source 110 with a modulation signal. On the other hand, FIG. 3(B) is a diagram showing the intensity of the optical modulation signal corrected by the intensity modulator 14 modulating the optical modulation signal output in FIG. 3(A) with a modulation signal having an inverse pattern to the modulation signal in FIG. 3(A). With such a configuration, the optical transmitter 10 of the modified example can improve the accuracy of phase / frequency modulation.

[0022] Also, the adaptive modulation configuration in the optical transmitter 10 of the first embodiment may be combined with a conventional adaptive modulation that changes the modulation level or FEC type.

[0023] Also, the adaptive modulation configuration in the optical transmitter 10 of the first embodiment may be combined with a configuration in which chirping is applied to improve wavelength dispersion tolerance by modulating a DFB laser in intensity modulation by an EA (Electro-Absorption) modulator.

[0024] Further, the optical transmitter 10 of the first embodiment may be configured to realize phase / frequency modulation and intensity modulation by modulating the bias current of the light source 110. FIG. 4 is a diagram showing a configuration example of the optical transmitter 10 of a modified example. The optical transmitter 10 of the modified example is different from the optical transmitter 10 of the first embodiment in that it does not include the intensity modulator 140. In this configuration, phase / frequency modulation and intensity modulation can be realized by changing the magnitude of the bias current value applied to the light source 110. FIG. 5 is an image diagram showing an outline of switching of the modulation method of the light source 110 by the optical transmitter 10 of the modified example. FIG. 5(A) represents an image of phase / frequency modulation, and FIG. 5(B) represents an image of intensity modulation. In this case, the modulation method switching unit 122 can realize phase / frequency modulation in the same manner as in the first embodiment. On the other hand, when performing intensity modulation, the modulation method switching unit 122 sets the bias current value applied to the light source 110 to a current value larger than that during phase / frequency modulation. As a result, a desired extinction ratio can be obtained, so the modulation method switching unit 122 can realize intensity modulation by modulating the bias current. That is, the optical transmitter 10 of the modified example is configured to modulate the light source 110 by controlling the output operation of the light source 110 itself for both phase / frequency modulation and intensity modulation. Thus, according to the optical transmitter 10 of the modified example, since there are more parts where the operations of the circuits can be made common in phase / frequency modulation and intensity modulation, the optical transmitter 10 can be configured with a smaller circuit scale.

[0025] Further, the optical transmitter 10 of the first embodiment may be configured to include a Mach-Zehnder modulator 150 instead of the intensity modulator 140. FIG. 6 is a diagram showing a configuration example of the optical transmitter 10 of the modification. In this configuration, the optical transmitter 10 can achieve intensity modulation and phase modulation of the light source 110 by changing the bias voltage and modulation amplitude applied to the Mach-Zehnder modulator 150. FIG. 7 is an image diagram showing an outline of switching the modulation method of the light source 110 by the optical transmitter 10 of the modification. FIG. 7(A) represents an image of intensity modulation, and FIG. 7(B) represents an image of phase modulation. In the case of intensity modulation, the Mach-Zehnder modulator 150 has an extinction characteristic that changes sinusoidally with respect to voltage. In the case of binary intensity modulation, the bias voltage and amplitude of the modulation signal are determined from the point where the light transmittance is 100% to the point where it becomes 0%. At this time, there is no fluctuation in the output phase. On the other hand, during phase modulation, the bias voltage and amplitude of the modulation signal are determined from the point where the light transmittance is 100% to the next point where it becomes 100%. As a result, the phase fluctuates by 180 degrees. The adaptive modulation configuration in the optical transmitter 10 of the modification may be combined with a conventional adaptive modulation that changes the modulation order or FEC type. According to such a configuration of the modification, a multilevel intensity modulation signal can be transmitted, and a multilevel intensity modulation component can be added to binary phase modulation for transmission. Note that the Mach-Zehnder modulator 150 may be an IQ modulator capable of modulating both the I-axis and the Q-axis. In this case, the Mach-Zehnder modulator 150 can transmit an intensity modulation signal by modulating both the I-axis and the Q-axis with the same signal, the same bias, and the same amplitude.

[0026] <Second Embodiment> FIG. 8 is a diagram showing a configuration example of the optical transmission system 1B of the second embodiment. The optical transmission system 1 is a system in which user devices each including the optical transmitter 10 of the first embodiment communicate with each other. Here, as configurations of user devices including the optical transmitter 10 of the first embodiment, two types of configurations of user devices 2A and 2B shown in FIG. 8 are assumed. The user device 2A is configured to include an optical receiver 20A that receives an optical signal by a direct detection method, while the user device 2B is configured to include an optical receiver 20B that receives an optical signal by a digital coherent detection method. The multiplexer / demultiplexer 21 multiplexes and demultiplexes optical signals of different wavelengths in the transmission and reception of optical signals and inputs / outputs them.

[0027] In order to communicate between these user devices 2, it is necessary to determine, at the start of communication (negotiation), which modulation method, intensity modulation or phase / frequency modulation, is used to exchange optical signals. For example, as a configuration for realizing negotiation, a configuration can be considered in which each user device 2 sequentially transmits a signal of intensity modulation and a signal of phase / frequency modulation, and mutually notifies which modulation method it can receive. Also, the negotiation exchange is assumed to be performed in a scheme similar to a general initial authentication phase at the time of establishing a communication connection, such as communication mode negotiation in Ethernet (registered trademark) or three-way handshake in TCP (Transmission Control Protocol).

[0028] In the optical transmission system 1 of the second embodiment configured as described above, the user devices 2 can determine, by negotiation at the start of communication, which modulation method, intensity modulation or phase / frequency modulation, is used to transmit and receive optical signals. That is, according to the optical transmission system 1 of the second embodiment, each user device 2 can determine a modulation method corresponding to the transmission path between the user device of the communication partner by transmitting and receiving main signals, and can adaptively switch the modulation method in response to changes in the transmission path and communicate. Therefore, in adaptive modulation of optical transmission, it is possible to improve the degree of freedom in changing the transmission method or transmission parameters.

[0029] <Third Embodiment> FIG. 9 is a diagram showing a configuration example of the optical transmission system 1C according to the third embodiment. In the optical transmission system 1B of the second embodiment, negotiation is performed in the initial authentication phase using the communication of the main signal. In contrast, in the optical transmission system 1C of the third embodiment, each user device 2C has a transmission path for a control signal (hereinafter referred to as the "second path") separately from the transmission path of the main signal (hereinafter referred to as the "first path"), and via the second path, information such as the loss of the optical signal (path loss) in the transmission path to the destination user device 2C and the configuration of the optical receiver in the destination user device 2C (hereinafter referred to as "control information") is exchanged. In the optical transmission system 1C of the third embodiment, the user device 2C determines the modulation method based on the control information notified from the destination user device 2C.

[0030] Specifically, in this case, each user device 2C includes a control signal communication unit 22 in addition to the optical transmitter 10, the optical receiver 20, and the multiplexer / demultiplexer 21 of the first embodiment. The control signal communication unit 22 transmits and receives a signal (control signal) indicating control information via the second path. For example, as shown in FIG. 9(A), the second path may be configured as a physically separate network from the first path, or as shown in FIG. 9(B), it may be configured as a logically different transmission path using the same physical network as the first path.

[0031] For example, in the example of FIG. 9(B), the second path may be configured as a separate channel using a wavelength independent of the first path in the optical transmission line used as the first path, or as a pseudo control channel realized by an AMCC (Auxiliary Management and Control Channel) that enables transmission of the control signal within the wavelength range of the main signal.

[0032] In the optical transmission system 1C of the third embodiment configured as described above, each user device 2C can perform modulation scheme negotiation using a second path that is physically or logically independent of the first path for transmitting the main signal. That is, according to the optical transmission system 1C of the third embodiment, each user device 2C can adaptively switch the modulation scheme for communication in response to a change in the transmission path by recognizing the configuration of the transmission path based on the control signals transmitted and received between each other, so that in adaptive modulation of optical transmission, it is possible to improve the degree of freedom in changing the transmission scheme or transmission parameters.

[0033] (Modification example) In the optical transmission system 1C of the third embodiment, information indicating the communication destination user device (hereinafter referred to as "destination information") may be registered in advance in each user device 2C. In this case, the modulation scheme switching unit 122 of the optical transmitter 10 may be configured to determine the modulation scheme of the optical transmitter 10 based on the pre-registered destination information. In this case, since it is not necessary to transmit and receive control signals for determining the modulation scheme, the second path can be omitted in the optical transmission system 1C, and the system configuration can be simplified.

[0034] <Fourth Embodiment> FIG. 10 is a diagram showing a configuration example of the optical transmission system 1D of the fourth embodiment. In the optical transmission systems 1B of the second embodiment and 1C of the third embodiment, the user devices 2 communicate directly with each other. In contrast, in the optical transmission system 1D of the fourth embodiment, each user device 2D communicates via a network node device (hereinafter referred to as "NW node device") 40. The user device 2D is different from the user device 2C of the third embodiment in that it communicates directly with the network node device 40, but the basic configuration is the same as that of the user device 2C. The NW node device 40 is a device that transfers optical communication by an optical switch. Communication between the user devices 2D is realized by one or more NW node devices 40 transferring the communication. The integrated controller 50 is a device that integrally controls one or more NW node devices 40.

[0035] In this case, each user device 2D negotiates with the NW node device 40 to which it is connected regarding the modulation method. Specifically, each NW node device 40 includes a control signal communication unit 41 similar to the control signal communication unit 22 of the user device 2D. In each user device 2, the control signal communication unit 22 communicates with the control signal communication unit 41 of the NW node device 40 to notify the NW node device 40 of its own control information and acquire the control information of the destination user device 2D from the NW node device 40. In each user device 2D, the modulation method switching unit 122 of the optical transmitter 10 determines the modulation method for communication with the destination user device 2D based on the control information acquired from the NW node device 40 and performs the switching of the modulation method.

[0036] On the other hand, in the NW node device 40, the control signal communication unit 41 supplies the control signal input from the user device 2D that is the negotiation partner to the integrated controller 50 and outputs a control signal regarding the control information of the user device 2D that is the communication destination of that user device 2D to the user device 2D that is the negotiation partner. Here, the control signal communication unit 41 of each NW node device 40 has a function of sharing the control information acquired from each user device 2D via the integrated controller 50. Due to this control information sharing function, the user device 2D can acquire the control information of the destination user device 2D from the NW node device 40 to which it is connected and can determine the modulation method for communication with the destination user device 2D by negotiating with the directly connected NW node device 40.

[0037] <Fifth Embodiment> FIG. 11 is a diagram showing a configuration example of the optical transmission system 1E according to the fifth embodiment. As shown in FIG. 11, the optical transmission system 1E according to the fifth embodiment is applied to an APN network NW including a first access area A1, a second access area A2, local networks LN1 and LN2, and a core network CN. Here, the first access area A1 and the second access area A2 represent the ranges of service areas that accommodate end-user terminals via the user device 2E. For example, the first access area A1 includes a user device 2E-1 that wirelessly connects a mobile communication terminal TM1 such as a smartphone or a mobile phone, and a user device 2E-2 that wiredly connects a device TM2 such as a broadcasting device. Further, for example, the second access area A2 includes a user device 2E-3 that wiredly connects a server device group TM3 installed in a data center or the like. Here, it is assumed that the optical receiver 20 of each user device 2E detects an optical signal by a direct detection method.

[0038] Note that the configurations of the first access area A1 and the second access area A2 illustrated in FIG. 11 are merely examples, and the access area is not limited to the illustrated devices and may connect any devices. Also, the APN network NW may be configured to include one access network, or may be configured to include three or more access networks.

[0039] The local networks LN1 and LN2 are networks that accommodate the devices in the corresponding access areas. Also, the core network CN is a network that accommodates the local networks LN1 and LN2. For simplicity, FIG. 11 shows the case where the core network CN accommodates two local networks LN1 and LN2, but the number of local networks accommodated by the core network CN may be one, or may be three or more.

[0040] For example, the local network LN1 may be configured as a full-mesh network by a plurality of optical gateway devices 60 (Ph-GW: Photonic Gateway). In this case, in the local network LN1, the optical gateway device 60-1 that accommodates the first access area A1 and the optical gateway device 60-2 that accommodates the second access area A2 can perform long-distance communication by relaying communication via an arbitrary transfer path. In such a local network LN1, in order to improve the SNR degraded in the optical transmission path 30, the optical gateway device serving as the transfer destination may be configured as a 3R repeater.

[0041] FIG. 12 is a diagram showing a configuration example of the connection between the user device 2E and the optical gateway device 60 as a 3R repeater. For example, the user device 2E shown in FIG. 12 is the user device 2E-3 in the long-distance communication of FIG. 12, and the optical gateway device 60 as the 3R repeater shown in FIG. 12 is the optical gateway device 60-2 in the long-distance communication of FIG. 12.

[0042] In this case, the optical gateway device 60 includes, for example, a multiplexer / demultiplexer 61 that inputs and outputs an optical signal to and from the optical transmission path 30, an optical receiver 62 that demodulates the optical signal of the upstream communication by phase / frequency modulation, an optical transmitter 63 that modulates the optical signal of the downstream communication by intensity modulation, and a digital signal processing circuit 64 that performs digital signal processing of the optical signal. The multiplexer / demultiplexer 61 is assumed to be a device with low wavelength dependence such as a splitter or a circulator, and a device that multiplexes and demultiplexes according to the communication wavelength band such as a wavelength demultiplexing filter.

[0043] The optical receiver 62 includes, for example, an optical amplifier 621, a local light source 622, a polarization and phase diversity optical receiver 623, an AD converter 624, and a digital coherent detection unit 625. The digital coherent detection unit 625 is realized by a digital signal processing circuit 120 and detects an optical signal by a digital coherent detection method. The optical transmitter 63 includes, for example, a DA converter 631, a light source 632, an intensity modulator 633, and an optical amplifier 634. Hereinafter, the optical gateway device 60 configured as a 3R repeater may be described as the "3R repeater 60".

[0044] For the 3R repeater 60 configured as described above, the optical transmitter 10 of the user device 2E is configured to communicate with the optical receiver 62 of the 3R repeater 60 by a phase / frequency modulation method. Thereby, in the upstream communication, high-sensitivity reception is possible in the 3R repeater 60. On the other hand, as described above, here it is assumed that the user device 2E directly detects an optical signal by a direct detection method, so it is assumed that the optical receiver 20 is configured to communicate with the optical transmitter 63 of the 3R repeater 60 by an intensity modulation method.

[0045] In this case, it is assumed that the reception sensitivity of the optical receiver 20 of the user device 2E in the downstream communication is lower than the reception sensitivity of the optical receiver 62 of the 3R repeater 60 in the upstream communication. In order to reduce such a difference in reception sensitivity and make the allowable transmission path loss equal in the upstream communication and the downstream communication, the 3R repeater 60 may be configured to improve the intensity of the output light by the optical amplifier 634. For example, when the difference in reception sensitivity between the upstream communication and the downstream communication is A [dB], the gain of the optical amplifier 634 may be adjusted so that the intensity of the output light of the optical transmitter 63 is A [dB] higher than the intensity of the output light in the upstream communication.

[0046] Note that currently, it is assumed that a pre-amplifier is applied to a digital coherent optical receiver used in a core network or the like for high-sensitivity reception. Therefore, a pre-amplifier may be arranged in front of the digital signal processing circuit 64 of the 3R repeater 60 also in the optical transmission system 1E of the fifth embodiment.

[0047] Also, in the configuration of long-distance communication, the modulation method can be determined by the same negotiation method as in the fourth embodiment. Also, in the user device 2E or the 3R repeater 60, when the information of the destination device is registered in advance, the modulation method may be determined based on the registered information.

[0048] On the other hand, when user devices 2E belonging to the same access area network communicate with each other, the optical gateway device 60 that accommodates the access area network functions as an optical switch, and short-distance communication is possible by folding back the communication of the user device 2E. For example, in the example of FIG. 11, the communication of the user device 2E-1 with the user device 2E-2 as the communication destination is folded back to the user device 2E-2 by the optical gateway device 60-1.

[0049] FIG. 13 is a diagram showing a configuration example of short-distance communication between user devices 2E via the optical gateway device 60. As described above, in short-distance communication within the same access network, since the communication of the user device 2E is folded back by the optical switch function of the optical gateway device 60, the connection configuration between the user devices 2E becomes a configuration directly connected via the optical transmission path 30 as shown in FIG. 13. Note that the digital signal processing circuit 120 in FIG. 13 includes a DA converter and an AD converter for transmission and reception. That is, in short-distance communication, each user device 2E can determine the modulation method by the same negotiation method as in the third embodiment. Also, in this case, in each user device 2E, when the information of the destination device is registered in advance, the modulation method may be determined based on the registered information.

[0050] According to the optical transmission system 1E of the fifth embodiment configured as described above, in the APN, when the transmission path loss is small, for example, during the return communication at the optical gateway device 60, the user devices 2E can communicate directly with each other. When the transmission path loss is large, the communication is transferred via the network of the optical gateway device 60, and at the same time, the optical signal is amplified by the 3R repeater 60 to relay the communication to the user device 2E of the communication partner.

[0051] <Modification example> In the optical transmission system 1E of the fifth embodiment, a Mach-Zehnder modulator may be used instead of the intensity modulator.

[0052] In the optical transmitter 10 or the optical transmission system 1E of the embodiment described above, conventionally, different optical transmitters (intensity modulator, DP-IQ modulator: Dual Polarization In-phase Quadrature modulator) were required for each modulation method. However, by changing the signal applied to the modulation element, its bias value, and amplitude according to the desired modulation method, it is possible to switch between intensity modulation and phase / frequency modulation using only the intensity modulator. Therefore, according to the optical transmitter 10 or the optical transmission system 1E of the embodiment, an economical optical transmitter capable of communicating with both optical receivers of the direct detection method and the digital coherent reception method can be realized with the same optical transmitter.

[0053] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Industrial applicability

[0054] The present invention is applicable to a transmission device, a reception device, or an optical transmission system that communicates via an optical transmission path.

Explanation of reference numerals

[0055] 1... Optical transmission system, 2... User device, 10... Optical transmitter, 110... Light source, 120... Digital signal processing circuit, 121... Modulation signal output section, 122... Modulation method switching section, 130... DA converter, 140... Intensity modulator, 150... Mach-Zehnder modulator, 20... Optical receiver, 21... Optical multiplexer / demultiplexer, 22... Control signal communication section, 30... Optical transmission line, 40... Network node device, 41... Control signal communication section, 50... Integrated controller, 60... Optical gateway device, 61... Optical multiplexer / demultiplexer, 62... Optical receiver, 621... Optical amplifier, 622... Local light source, 623... Polarization and phase diversity optical receiver, 624... AD converter, 625... Digital coherent detection section, 63... Optical transmitter, 631... DA converter, 632... Light source, 633... Intensity modulator, 634... Optical amplifier, 64... Digital signal processing circuit

Claims

1. An optical transmission system comprising a plurality of user devices having an optical transmitter for transmitting an optical signal and an optical receiver for receiving an optical signal, wherein the user device, a first modulation unit that generates an optical modulation signal by intensity modulation of the optical signal, a second modulation unit that generates an optical modulation signal by phase / frequency modulation of the optical signal, a modulation method switching unit that switches modulation means to any one of the modulation units according to the destination user device among the first modulation unit and the second modulation unit, and is provided with, the modulation method switching unit, at the time of initial communication with the destination user device, transmits a signal of intensity modulation and a signal of phase / frequency modulation to the destination user device in order, and determines by negotiation the modulation method to be used for communication with the destination user device by mutually notifying which modulation method can be received, and communicates control information regarding the negotiation via a second path that is logically independent with a different wavelength from a first path for communicating a main signal and physically the same path as the first path, Optical transmission system.

2. The second modulation unit realizes phase / frequency modulation of the optical signal by modulating a bias current applied to a light source that emits the optical signal with a modulation signal related to the modulation means, The optical transmission system according to claim 1.

3. The first modulation unit realizes intensity modulation of the optical signal by an electroabsorption modulator or a Mach-Zehnder modulator, The optical transmission system according to claim 2.

4. The first modulation unit realizes intensity modulation of the optical signal by applying a bias current with a current value larger than the bias current applied by the second modulation unit to the light source, The optical transmission system according to claim 2.

5. An optical transmission method by an optical transmission system comprising a plurality of user devices having an optical transmitter for transmitting an optical signal and an optical receiver for receiving an optical signal, wherein the user device executes a modulation method switching step of switching modulation means to any one of a first modulation unit that generates an optical modulation signal by intensity modulation of the optical signal and a second modulation unit that generates an optical modulation signal by phase / frequency modulation of the optical signal according to the destination user device, The modulation method switching step is to transmit a signal of intensity modulation and a signal of phase / frequency modulation to the user device at the connection destination in order during the initial communication with the user device at the connection destination, and to mutually notify which modulation method can be received, thereby determining the modulation method to be used for communication with the user device at the connection destination by negotiation. The control information regarding the negotiation is communicated via a second path that is logically independent by a wavelength different from that of the first path for communicating the main signal and is physically the same path as the first path. Optical transmission method.

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