Transmission device, optical communication system, and frequency control method

Abstract

This optical communication system has a plurality of transmission devices for performing optical communication. The transmission device is provided with a light source unit, a branching unit, a plurality of optical communication units, and a frequency control unit. The light source unit outputs light. The branching unit branches the light outputted by the light source unit into a plurality of branched light beams. Each of the plurality of optical communication units is opposed to a different other transmission device, and performs processing for coherent detection of an optical signal received from the opposed other transmission device by using the branched light having been branched by the branching unit, and processing for modulating the other branched light having been branched by the branching unit and transmitting the generated optical signal to the opposed other transmission device. The frequency control unit performs control so that, when a frequency offset is detected between the optical signal received from a supplier device, which is one other transmission device among the opposed other transmission devices to which the local device has its frequency adjusted, and the light outputted by the light source unit, the frequency of the light outputted from the light source unit approaches the frequency of the optical signal transmitted from the supplier device.

Description

Transmission device, optical communication system, and frequency control method

[0001] The present invention relates to a transmission device, an optical communication system, and a frequency control method.

[0002] In recent years, the increase in datacenter traffic has led to a demand for higher capacity optical transmitters and receivers, such as optical transceivers, that connect datacenters. To meet this demand for higher capacity, digital coherent transmission methods, which have traditionally been applied to medium-distance applications such as intercity and long-distance applications such as intercontinental, are now beginning to be applied to short-distance applications between and within datacenters.

[0003] FIG. 8 is a diagram illustrating an example of an optical communication network provided by a conventional optical communication system. The optical communication system includes multiple transmission devices. As shown in FIG. 9, each transmission device includes an optical transceiver for transmitting and receiving optical signals to and from multiple other transmission devices. Conventional mainstream optical transceivers are configured with a switch ASIC (Application Specific Integrated Circuit), PHY separation, and a dedicated light source, and each optical transceiver has a different light source unit. The light source unit includes a light source and a temperature control element for controlling the frequency f of the light output from the light source. In addition, optical transceivers such as optical transceivers that employ a digital coherent transmission system output signal light and LO (Local Oscillator) light from the same light source unit.

[0004] In communication systems for data centers, a system configuration is envisioned in which a switch ASIC that performs Layer 2 Ethernet (registered trademark) switching and an ASIC (PHY chip) used for an optical transmission interface are co-packaged, and this is being discussed in the standardization organization "The Optical Internetworking Forum (OIF)" (see, for example, Non-Patent Document 1). In such systems, a configuration is being considered in which multiple optical transceivers connected to the co-packaged switch share a single laser light source.

[0005] There is also a technique for performing frequency offset estimation and frequency offset compensation for an optical signal received by an optical transceiver within digital signal processing (see, for example, Non-Patent Document 2).

[0006] “OIF Co-Packaging Interoperability Demo”, OFC 2022, [online], 2022, Internet<https: / / www.oiforum.com / wp-content / uploads / OIF_Co-Packaging_Demo_OFC2022_Presentation.pdf> I. Fatadin and SJ Savory, “Compensation of Frequency Offset for 16-QAM Optical Coherent Systems Using QPSK Partitioning,” in IEEE Photonics Technology Letters, Vol. 23, No. 17, pp. 1246-1248, Sept. 1, 2011.

[0007] When performing coherent transmission using a switch with a new co-package configuration, it is necessary to compensate for the frequency offset between the local oscillator light (LO light) shared by multiple optical transceivers connected to a single transmission device and the received signals from multiple different transmission devices with frequency offsets, in order to ensure transmission performance.

[0008] FIG. 10 shows an optical communication network in which transmission devices, in which multiple optical transceivers share a single laser light source, are connected by communication paths, and FIG. 11 shows an example of the configuration of a transmission device used in the optical communication system that makes up the optical communication network of FIG. 10. The transmission devices shown in FIGS. 10 and 11 are configured using a co-package switch. Multiple optical transceivers connected to one transmission device share a light source unit. In each transmission device, multiple optical transceivers share the light source unit, so there are multiple opposing pairs of optical transceivers related to one light source. Therefore, the frequency offset of the light source must take into account the light sources of the multiple opposing optical transceivers involved.

[0009] Furthermore, digital coherent transmission systems require the use of a light source with high frequency accuracy and a high-precision digital signal processor (DSP). However, if a light source with low frequency accuracy is used without a wavelength locker, the frequency offset may increase. An optical transceiver converts the electrical signal obtained by optically detecting the received optical signal from an analog signal to an electrical signal using an analog-to-digital converter (ADC). However, if the frequency offset increases, the signal quality deteriorates due to bandwidth limitations caused by the ADC sampling rate, making it impossible to detect the frequency offset. As a result, it is considered that frequency offset estimation and frequency offset compensation performed in conventional digital signal processing, such as those described in Non-Patent Document 2, cannot be applied.

[0010] Therefore, in the transmission devices shown in Figures 10 and 11, there is a method of estimating the frequency offset value of the received signal from the power difference between the positive-side frequency component and the negative-side frequency component of the received signal, and controlling the frequency of the LO light output by the light source unit to compensate for the frequency offset. However, for example, if the transmission device #1 shown in Figures 10 and 11 controls the frequency of the light source unit #1 based on the received signal of the optical transceiver #1-1, the frequency of the LO light used in the optical transceiver #1-2 will also change. As a result, frequency offset compensation for the optical transceiver #1-2 will not be possible. As such, frequency offset compensation cannot be performed simultaneously for multiple optical transceivers connected to one transmission device.

[0011] In view of the above circumstances, an object of the present invention is to provide a transmission device, an optical communication system, and a frequency control method that can perform frequency offset compensation even when a light source is shared by multiple optical transceivers with different communication destinations.

[0012] One aspect of the present invention is a transmission device in an optical communication system having a plurality of transmission devices that perform optical communication, the transmission device comprising: a light source unit that outputs light; a branching unit that branches the light output by the light source unit into a plurality of branched lights; a plurality of optical communication units that each face a different other transmission device and perform a process of coherently detecting an optical signal received from the other facing transmission device using the branched light branched by the branching unit, and a process of modulating the other branched light branched by the branching unit to generate an optical signal and transmitting it to the other facing transmission device; and a frequency control unit that, when a frequency offset is detected between the optical signal received from a supplier device, which is another transmission device with which the transmission device itself is frequency-tuned, and the light output by the light source unit, controls the frequency of the light output from the light source unit to approach the frequency of the optical signal transmitted from the supplier device.

[0013] One aspect of the present invention is an optical communication system in which a plurality of the above-described transmission devices are connected by optical transmission paths.

[0014] One aspect of the present invention is a frequency control method performed by a transmission device in an optical communication system having a plurality of transmission devices that perform optical communication, the method comprising: a branching step of branching light output by a light source unit into a plurality of branched lights; an optical communication step in which a plurality of optical communication units, each facing a different other transmission device, perform a process of coherently detecting an optical signal received from the other facing transmission device using the branched light branched by the branching step, and a process of modulating the other branched light branched by the branching step and transmitting the generated optical signal to the other facing transmission device; and a frequency control step of, when a frequency offset is detected between the optical signal received from a supplier device, which is another transmission device with which the transmission device itself is frequency-tuned, and the light output by the light source unit, controlling the frequency of the light output from the light source unit to approach the frequency of the optical signal transmitted from the supplier device.

[0015] According to the present invention, it is possible to perform frequency offset compensation even when a light source is shared by a plurality of optical transceivers with different communication destinations.

[0016] FIG. 1 is a configuration diagram of an optical communication system according to a first embodiment. FIG. 1 is a configuration diagram of an optical communication system according to a first embodiment. FIG. 2 is a block diagram showing the configuration of a transmission device according to the first embodiment. FIG. 3 is a diagram showing an example of a tree graph of an optical communication network according to the first embodiment. FIG. 4 is a flow diagram showing processing of the optical communication system according to the first embodiment. FIG. 4 is a configuration diagram of an optical communication system according to a second embodiment. FIG. 5 is a block diagram showing the configuration of a transmission device according to the second embodiment. FIG. 5 is a diagram showing an optical communication network using an optical communication system of the prior art. FIG. 6 is a block diagram showing the configuration of a transmission device of the prior art. FIG. 6 is a diagram showing an optical communication network using an optical communication system of the prior art. FIG. 7 is a block diagram showing the configuration of a transmission device of the prior art.

[0017] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0018] (First Embodiment) Fig. 1 is a configuration diagram of an optical communication system 1 according to a first embodiment of the present invention. Fig. 1 shows only the configuration related to this embodiment. The optical communication system 1 has J (J is an integer of 2 or more) transmission devices 10. The J transmission devices 10 are referred to as transmission devices 10-1 to 10-J, respectively. Fig. 1 illustrates an example where J=3.

[0019] The transmission device 10 includes a light source unit 11, a plurality of optical communication units 12, and one or more communication units 13. The light source unit 11 outputs signal light and LO (Local Oscillator) light to the plurality of optical communication units 12. The frequency of the light output from the light source unit 11 of the transmission device 10-j (j is an integer between 1 and J) is denoted by f ｊ Each optical communication unit 12 is connected to the optical communication unit 12 of a different other transmission device 10 by two optical transmission paths 20. Note that multiple optical communication units 12 of a transmission device 10 may be connected to multiple optical communication units 12 of the same other transmission device 10, respectively. The optical transmission paths 20 are, for example, SMF (single mode fiber) or MCF (multicore fiber). The optical communication unit 12 of the transmission device 10-j transmits a frequency f ｊ and transmits an optical signal of frequency f ｉThe communication unit 13 is connected to the communication unit 13 of another opposing transmission device 10 via a communication path 21. The communication path 21 is an optical fiber, a LAN (Local Area Network) cable, or the like.

[0020] The communication units 13 of the transmission devices 10 may be connected via a network. Fig. 2 is a configuration diagram of an optical communication system 1a. In Fig. 2, the same components as those in the optical communication system 1 shown in Fig. 1 are denoted by the same reference numerals, and their description will be omitted. The optical communication system 1a shown in Fig. 2 differs from the optical communication system 1 shown in Fig. 1 in that the communication units 13 of the transmission devices 10 are connected via a network.

[0021] FIG. 3 is a block diagram showing the configuration of a transmission device 10. The transmission device 10 optically communicates with K other transmission devices 10 (K is an integer equal to or greater than 1). The transmission device 10 includes a light source unit 11, N (N≧K, N is an integer equal to or greater than 2) optical communication units 12, M (M is an integer equal to or greater than 1) communication units 13, a topology generation unit 14, a frequency offset estimation unit 15, a frequency control unit 16, a data signal generation unit 17, a data signal demodulation unit 18, and a control unit 19. The N optical communication units 12 are referred to as optical communication units 12-1 to 12-N, respectively, and the M communication units 13 are referred to as communication units 13-1 to 13-M, respectively. FIG. 3 shows an example where K=N=M.

[0022] The light source unit 11 outputs light. The light source unit 11 includes a light source, such as a DFB (Distributed Feedback) laser or a VCSEL (Vertical Cavity Surface Emitting Laser), and a temperature control element for frequency control. The light output from the light source unit 11 is branched into N beams, which are input to the optical communication units 12-1 to 12-N, respectively.

[0023] The optical communication unit 12 includes an electric signal generation unit 121, an optical modulation unit 122, an optical detection unit 123, an ADC (analog-to-digital converter) 124, and an electric signal reception unit 125. The optical communication unit 12 is an optical transceiver that transmits and receives optical signals to and from the optical communication unit 12 of another opposing transmission device 10. The electric signal generation unit 121, the optical modulation unit 122, the optical detection unit 123, the ADC 124, and the electric signal reception unit 125 included in an optical communication unit 12-n (n is an integer between 1 and N) will be referred to as the electric signal generation unit 121-n, the optical modulation unit 122-n, the optical detection unit 123-n, the ADC 124-n, and the electric signal reception unit 125-n, respectively. The light output from the light source unit 11 to the optical communication unit 12-n is branched, and one of the branched lights is input to the optical modulation unit 122-n as signal light, and the other branched light is input to the optical detection unit 123-n as local oscillator (LO) light. In this way, the transmission device 10 branches the light output from the light source unit 11 to the optical communication units 12-1 to 12-N, and each of the optical communication units 12-1 to 12-N includes a branching unit that further branches the branched light into signal light and local oscillator light.

[0024] The electrical signal generation unit 121 is realized by, for example, an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). The electrical signal generation unit 121 maps a transmission signal to symbols and samples the symbol-mapped transmission signal. The electrical signal generation unit 121 performs band limiting and pre-equalization on the sampled transmission signal, and then converts the digital signal into an analog signal. The electrical signal generation unit 121 outputs the I (in-phase) component and Q (quadrature) component of each of the X polarization and the Y polarization, which have been converted into analog signals, to the optical modulation unit 122.

[0025] The optical modulation unit 122 is an optical modulator that modulates the signal light output from the light source unit 11 with the electrical signal generated by the electrical signal generation unit 121 to generate an optical signal to be transmitted. The optical modulation unit 122 includes a modulator driver and a Mach-Zehnder modulator. The modulator driver amplifies the I component of the X polarization, the Q component of the X polarization, the I component of the Y polarization, and the Q component of the Y polarization of the transmission signal output from the electrical signal generation unit 121, and drives the Mach-Zehnder modulator with these amplified signal components. As a result, the optical modulation unit 122 generates an optical signal in which the X polarization optical signal and the Y polarization optical signal are polarization-multiplexed, and outputs the generated optical signal to the optical transmission path 20.

[0026] The optical detection unit 123 is composed of an optical 90-degree hybrid and a BPD (balanced photodiode). The optical detection unit 123 receives an optical signal output from the opposing transmission device 10 and transmitted through the optical transmission path 20. The optical detection unit 123 coherently detects the optical signal using the local light output from the light source unit 11. The optical detection unit 123 outputs a received electrical signal obtained by coherent detection.

[0027] The ADC 124 is a digital-to-analog converter that converts the electrical signal output from the optical detection unit 123 from an analog signal to a digital signal. The ADC 124 outputs the received signal, which has been converted into a digital signal, to the electrical signal receiving unit 125.

[0028] The electrical signal receiving unit 125 is a DSP (Digital Signal Processor). The electrical signal receiving unit 125 is realized by, for example, an FPGA or an ASIC. The electrical signal receiving unit 125 performs compensation for the frequency characteristics in the optical detection unit 123, compensation for chromatic dispersion in the optical transmission line 20, compensation for phase noise, and frequency offset compensation on the received signal output by the ADC 124, and then outputs the signal to the data signal demodulation unit 18.

[0029] The communication unit 13 transmits and receives electrical signals to and from the communication unit 13 of the other transmission device 10 via the communication path 21 .

[0030] The topology generation unit 14 transmits and receives control signals of a tree generation protocol to and from the topology generation unit 14 of the opposing transmission device 10, and generates a tree graph of a logical network including all of the transmission devices 10 in the optical communication network of the optical communication system 1, 1a. The tree generation protocol is an autonomous distributed protocol that configures a tree-type logical network. In the first embodiment, the control signals of the tree generation protocol are transmitted and received by the communication unit 13. As the tree generation protocol, protocols for Ethernet (registered trademark) such as STP (Spanning Tree Protocol) and SPB (Short Path Bridging), and link-state routing protocols for IP (Internet Protocol) routers such as OSPF (Open Shortest Path First) and ISIS (Intermediate System to Intermediate System) can be used.

[0031] The transmission device 10 at the apex of the tree graph is defined as the root node. The root node is a transmission device 10 having a light source unit 11 that outputs a reference frequency within the optical communication network of the optical communication system 1, 1a. Of two transmission devices 10 connected in the tree graph, the transmission device 10 closer to the root node is called the supplier or supplier device 10, and the transmission device 10 farther from the root node is called the subscriber or subscriber device 10. The subscriber device 10 adjusts the frequency of its light source unit 11 to the frequency used by the supplier device 10 among the other transmission devices 10 connected to it. In this way, the supplier is a device that provides communication frequency information to the subscriber, and the subscriber is a device that adjusts the communication frequency of its own device based on the communication frequency information provided by the supplier. The communication frequency information provided by the supplier to the subscriber is the communication frequency of its own device. The storage unit 141 included in the topology generation unit 14 stores the tree graph and the supplier / subscriber relationship in the tree graph.

[0032] The frequency offset estimation unit 15 estimates the frequency offset between the frequency of the supplier device 10 and the frequency of the own device. The frequency offset estimation unit 15 of the subscriber device 10-j estimates the frequency offset between the frequency f used by the supplier device 10-i and the frequency of the own device. ｉ In the first embodiment, the frequency offset estimation unit 15 of the subscriber device 10-j receives information on the frequency f used by the supplier device 10-i through communication via the communication unit 13. ｉ The frequency offset estimation unit 15 of the subscriber device 10-j acquires the frequency f of the local light output from the light source unit 11 of the device itself. ｊ and the frequency f used by the supplier device 10-i. ｉ The frequency offset estimation unit 15 estimates the frequency offset amount based on the difference between the frequency f used by the subscriber device 10-i' (i' ≠ i, j, and i' is an integer between 1 and J). The frequency offset estimation unit 15 outputs the estimated frequency offset amount to the frequency control unit 16. ｉ’ is not used to estimate the frequency offset.

[0033] The frequency control unit 16 controls the light source unit 11 to change the frequency of the light output. The frequency control unit 16 calculates a control amount for compensating for the frequency offset amount estimated by the frequency offset estimation unit 15. The control amount is the temperature of the light source unit 11, the amount of current injected into the laser used as the light source unit 11, etc. The frequency control unit 16 controls the frequency of the local light output from the light source unit 11 using the calculated control amount, thereby compensating for the frequency offset. As a result, the frequency control unit 16 of the subscriber device 10-j controls the frequency f of the light source unit 11 of its own device. ｊ is the frequency f of the optical signal transmitted from the supplier device 10-i. ｉ The control is made to match the

[0034] The data signal generation unit 17 outputs a data signal to be transmitted to the opposing transmission device 10 to the electrical signal generation unit 121 of the optical communication unit 12 corresponding to that opposing transmission device 10. The data signal demodulation unit 18 demodulates and decodes a received signal output from the electrical signal receiving unit 125 of the optical communication unit 12. The control unit 19 controls each unit within the transmission device 10. The control unit 19 relays signals between each unit within the transmission device 10.

[0035] If the light source unit 11 does not have a wavelength locker, the frequency accuracy may be low and the frequency offset may increase. This may cause the electrical signal receiving unit 125 to be unable to compensate for the frequency offset, making it difficult to demodulate the optical signal received from the other transmission device 10. In this case, information cannot be obtained from the ADC 124 at a later stage, and the topology generation unit 14 and frequency offset estimation unit 15 cannot acquire information from the optical signal received from the other opposing transmission device 10. The topology generation unit 14 and frequency offset estimation unit 15 of this embodiment can transmit and receive tree generation protocol control signals and supplier communication frequency information to and from the topology generation unit 14 and frequency offset estimation unit 15 of the opposing transmission device 10 via the communication unit 13, even if the frequency offset of the optical signal cannot be compensated for and demodulation is difficult.

[0036] FIG. 4 shows a tree graph of the optical communication network of the optical communication system 1 shown in FIG. 1 and the optical communication network of the optical communication system 1a shown in FIG. 2. The optical communication network is configured by connecting multiple transmission devices 10 via optical communication units 12. In FIG. 4, the transmission device 10-1 is the root node. The root node is a transmission device 10 in the optical communication network that has a light source unit 11 that outputs a reference frequency. Transmission devices 10-2 and 10-3 are connected downstream of the transmission device 10-1. The transmission device 10-1 is the supplier of the transmission devices 10-2 and 10-3, and the transmission devices 10-2 and 10-3 are subscribers of the transmission device 10-1.

[0037] Next, the operation of the optical communication systems 1 and 1a will be described with reference to a flow chart of FIG.

[0038] The topology generation unit 14 of each transmission device 10 generates a logical topology based on a tree graph including all transmission devices 10 in the optical communication network using a tree generation protocol such as STP (step S1). When generating a logical topology using a tree generation protocol, the topology generation unit 14 of each transmission device 10 first determines a root node. For example, the topology generation unit 14 of each transmission device 10 determines the root node using one of the following methods (1) to (5) based on information set in a control message of the tree generation protocol received from another transmission device 10 or a manual setting.

[0039] (1) A node ID for uniquely identifying the transmission device 10 is assigned to each transmission device 10. Among the transmission devices 10-1 to 10-J, the transmission device 10 with the smallest node ID is determined to be the root node.

[0040] (2) Among the transmission devices 10-1 to 10-J, the transmission device 10 with the longest sum of transmission distances is determined as the root node. The sum of the transmission distances of the transmission device 10-j is calculated by summing the transmission distances of the optical transmission paths 20 connected to the optical communication units 12-1 to 12-N of the transmission device 10-j.

[0041] (3) Among the transmission devices 10-1 to 10-J, the transmission device 10 with the largest sum of bandwidths is determined as the root node. The sum of bandwidths of the transmission device 10-j is calculated by summing the bandwidths of the optical communication units 12-1 to 12-N that the transmission device 10-j has.

[0042] (4) Among the transmission devices 10-1 to 10-J, the transmission device 10 with the largest sum of "transmission distance x bandwidth" is determined as the root node. The sum of "transmission distance x bandwidth" for the transmission device 10-j is obtained by calculating the product of the transmission distance and bandwidth for each optical communication unit 12-n, n = 1 to N, that the transmission device 10-j has, and then summing up the products calculated for each of the optical communication units 12-1 to 12-N.

[0043] (5) An operator manually sets the root node. The operator may input the root node setting to each transmission device 10, or the operator may set the root node to each transmission device 10 based on an instruction input by the operator from a management device (not shown) connected to the transmission device 10.

[0044] In this embodiment, the control message of the above-mentioned tree generation protocol is extended, and the topology generation unit 14 stores the root node determination indicators (2) to (4) above in the control message exchanged between the transmission devices 10. In this way, the root node determination indicators are exchanged between the transmission devices 10.

[0045] The topology generation unit 14 of each transmission device 10 generates a control message of the tree generation protocol and transmits it to the opposing transmission device 10 from each communication unit 13. The topology generation unit 14 sets in the control message at least one of the following: the node ID of the own device; the sum of the transmission distances of each optical communication unit 12-1 to 12-N of the own device; the sum of the bandwidths of each optical communication unit 12-1 to 12-N of the own device; and the sum of the transmission distances x bandwidths of each optical communication unit 12-1 to 12-N of the own device. The communication unit 13 of each transmission device 10 outputs the control message received from the opposing transmission device 10 to the topology generation unit 14. The topology generation unit 14 of each transmission device 10 determines a root node using any of (1) to (5) above based on the information in the received control message or manual settings, and generates a tree graph such as that shown in FIG. 4, for example.

[0046] After constructing the tree in step S1, the topology generation unit 14 of each transmission device 10 determines, among the other transmission devices 10 facing the own device, those transmission devices that are closer to the root node than the own device as suppliers of the own device, and determines the own device as a subscriber of the suppliers. Furthermore, the topology generation unit 14 of each transmission device 10 determines, among the other transmission devices 10 facing the own device, those transmission devices that are farther from the root node than the own device as subscribers of the own device, and determines the own device as a supplier of those subscribers. In this way, a tree graph is generated with the root node determined based on the above (1) to (5) as its apex, and suppliers and subscribers are determined based on the generated tree graph. In other words, which of the multiple transmission devices 10 facing each other in the optical communication systems 1 and 1a is the supplier and which is the subscriber is determined based on the above (1) to (5). The topology generation unit 14 stores the generated tree graph and information about other transmission devices 10 that have supplier / subscriber relationships with the own device in the storage unit 141.

[0047] Each transmission device 10 changes the communication frequency of the light source unit 11 of the own device to match the communication frequency of the supplier of the own device (step S2). Specifically, the frequency offset estimation unit 15 of the supplier device 10-i changes the frequency f ｉ The frequency offset estimator 15 of the subscriber device 10-j receives the frequency information that the communication unit 13 has received from the supplier device 10-i, and estimates the frequency f of the light source unit 11 of the subscriber device 10-j. ｊ and the frequency f indicated by the received frequency information ｉ The frequency offset estimation unit 15 calculates the frequency offset amount based on the frequency offset estimation result and instructs the frequency control unit 16 to compensate for the calculated frequency offset amount. The frequency control unit 16 controls the frequency of the light source unit 11 of the transmission device 10-j so as to compensate for the frequency offset amount instructed by the frequency offset estimation unit 15. As a result, the frequency f ｊ The frequency f of the light source unit 11 of the supplier device 10 ｉ The above operation is performed by all transmission devices 10 on the tree, thereby synchronizing the light source frequencies throughout the entire optical communication network. Note that, although the communication path 21 is used to transmit and receive frequency information in the same way as the topology signal as described above, the optical transmission path 20 may also be used. Transmission and reception of frequency information using the optical transmission path 20 will be described in the second embodiment.

[0048] The optical communication systems 1 and 1a perform optical communication using a light source frequency synchronized across the entire optical communication network (step S3). That is, in the transmission device 10-i, where i=1 to J, the electrical signal generator 121 maps the data signal transmitted from the data signal generator 17 to a symbol and then samples it. The electrical signal generator 121 performs band limitation and pre-equalization on the sampled transmission signal, and converts it into an electrical signal from an analog signal. The optical modulator 122 modulates the frequency f output from the light source 11. ｉThe light from the transmission device 10-i is modulated by an electrical signal generated by the electrical signal generator 121 to generate an optical signal to be transmitted. The optical modulator 122 of the transmission device 10-i outputs the generated optical signal to the optical transmission path 20 between the transmission device 10-i and the opposing transmission device 10-j.

[0049] In the transmission device 10-j where j=1 to J, the optical detection unit 123-n of each optical communication unit 12-n receives the optical signal output from the corresponding transmission device 10-i and transmitted through the optical transmission path 20. The optical detection unit 123-n detects the frequency f ｊ The optical detector 123-n performs coherent detection of the received optical signal using the LO light. The optical detector 123-n outputs the received signal obtained by coherent detection. The ADC 124-n converts the received electrical signal output by the optical detector 123-n from an analog signal to a digital signal and outputs it to the electrical signal receiver 125-n. The electrical signal receiver 125-n performs compensation for the frequency characteristics of the optical detector 123-n, compensation for chromatic dispersion received in the optical transmission path 20, compensation for phase noise, frequency offset compensation, etc. on the received signal, and then outputs it to the data signal demodulator 18. The data signal demodulator 18 demodulates and decodes the received signal.

[0050] Each transmission device 10 periodically checks the difference in communication frequency with the supplier device 10. Specifically, the topology generation unit 14 of each supplier device 10-i checks the difference in communication frequency between the light source unit 11 of the own device and the supplier device 10-i. ｉ The communication unit 13 transmits the frequency information in which the frequency offset f indicated in the frequency information received from the supplier device 10-i to the subscriber device 10-j. ｉ and the frequency f of the light source unit 11 of the device itself. ｊ Based on this, it is determined whether a frequency offset has occurred (step S4). The frequency offset estimation unit 15 of the subscriber device 10-j periodically transmits the communication frequency f ｉ In response to the inquiry, the frequency offset estimator 15 of the supplier device 10-i may return frequency information.

[0051] If the frequency offset estimation unit 15 of the transmission device 10 determines that a frequency offset of a predetermined value or more has not occurred between the transmission device 10 and the supplier device 10 (step S4: NO), the frequency offset estimation unit 15 repeats the process of step S4 after a predetermined time has elapsed. On the other hand, if the frequency offset estimation unit 15 determines that a frequency offset of a predetermined value or more has occurred between the transmission device 10 and the supplier device 10 (step S4: YES), the frequency offset estimation unit 15 performs the same process as the transmission device 10-j in step S2 to estimate the communication frequency f ｊ , the communication frequency f of the supplier device 10 ｉ (Step S5) The optical communication systems 1 and 1a repeat the process from step S3 using the light source frequency synchronized in step S5.

[0052] According to this embodiment, in an optical communication network in which transmission devices are connected by optical transceivers, a tree generation protocol is used to generate a logical topology based on a tree graph including all transmission devices in the optical communication network. All transmission devices on the tree select a transmission device closest to the root node as a supplier from among the other connected transmission devices. Each transmission device changes the communication frequency of its own light source to match the communication frequency of the supplier. This enables frequency offset compensation in a transmission device configured to share a single laser light source among multiple optical transceivers.

[0053] Second Embodiment In a second embodiment, a control signal for generating a topology is transmitted and received by an optical signal. Also, in the second embodiment, information on the communication frequency of the supplier's transmission device is obtained from the received optical signal. The second embodiment will be described, focusing on the differences from the first embodiment.

[0054] FIG. 6 is a configuration diagram of an optical communication system 5 according to the second embodiment. FIG. 6 shows only the configuration related to this embodiment. In FIG. 6, the same components as those in the optical communication system 1 according to the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and their description will be omitted. The optical communication system 5 has J transmission devices 50. The J transmission devices 50 are respectively referred to as transmission devices 50-1 to 50-J. FIG. 5 illustrates an example in which J=3. The transmission device 50 has a light source unit 11 and multiple optical communication units 12. Hereinafter, the supplier's transmission device 50 will be referred to as the supplier device 50, and the subscriber's transmission device 50 will be referred to as the subscriber device 50.

[0055] Fig. 7 is a block diagram showing the configuration of a transmission device 50. In Fig. 7, the same components as those of the transmission device 10 according to the first embodiment shown in Fig. 3 are denoted by the same reference numerals, and their description will be omitted. The transmission device 50 optically communicates with K other transmission devices 50. The transmission device 50 shown in Fig. 7 differs from the transmission device 10 shown in Fig. 3 in that it does not have the communication unit 13, and that it includes a topology generation unit 54 and a frequency offset estimation unit 55 instead of the topology generation unit 14 and the frequency offset estimation unit 15.

[0056] The topology generation unit 54 has the same functions as the topology generation unit 14 except that it transmits and receives control signals of the tree generation protocol via the optical communication unit 12 .

[0057] The frequency offset estimator 55 has the same function as the frequency offset estimator 15, except that when the own device is a subscriber, the frequency offset estimator 55 acquires the frequency of the received signal obtained by coherently detecting an optical signal received from the transmission device 50 of the supplier of the own device as the supplier's communication frequency. Alternatively, the frequency offset estimator 55 of the supplier device 50 may notify the frequency offset estimator 55 of the subscriber device 50 of information about the frequency used by the own device via an optical control signal from the optical communication unit 12. The frequency offset estimator 55 of the subscriber device 50 acquires the frequency information via the control signal received by the optical communication unit 12 from the supplier device 50.

[0058] Next, a description will be given of the operation of the optical communication system 5. The optical communication system 5 operates in the same manner as the processing of the optical communication systems 1 and 1a of the first embodiment shown in FIG.

[0059] First, in step S1, the topology generation unit 54 of each transmission device 50 generates a control message of the tree generation protocol and transmits it as an optical signal from each optical communication unit 12 to the opposing transmission device 50. The optical communication unit 12 of each transmission device 50 converts the control message received from the opposing transmission device 50 from an optical signal to an electrical signal and outputs it to the topology generation unit 54. The topology generation unit 54 of each transmission device 50 determines a root node based on the information in the received control message or manual settings, and generates a tree graph of the optical communication network.

[0060] In step S2, each transmission device 50 changes the communication frequency of the light source unit 11 of its own device to match the communication frequency of its own device's supplier. Specifically, the transmission devices 50-i, i = 1 to J, transmit optical signals to the opposing transmission device 50-j, similar to step S3 of the first embodiment. The optical detection units 123-n and ADCs 124-n of the transmission devices 50-j, j = 1 to J, operate similarly to step S3 of the first embodiment. The electrical signal receiving unit 125-n of the transmission device 50-j branches the digital signal output by the ADC 124-n into two, and performs compensation for the frequency characteristics of one of the branched received signals in the optical detection unit 123-n, compensation for chromatic dispersion received in the optical transmission path 20, phase noise compensation, frequency offset compensation, etc., before outputting the signal to the data signal demodulation unit 18. The data signal demodulation unit 18 demodulates and decodes the received signal. The electrical signal receiving unit 125 - n outputs the other branched received signal to the frequency offset estimating unit 55 .

[0061] The frequency offset estimation unit 55 of the transmission device 50-j performs a fast Fourier transform on a received signal output by the electrical signal receiving unit 125-n of the optical communication unit 12-n connected to the supplier device 50-i via the optical transmission path 20 to obtain a received signal in the frequency domain. The frequency offset estimation unit 55 may receive the result of the fast Fourier transform on the received signal from the electrical signal receiving unit 125-n. The frequency offset estimation unit 55 of the transmission device 50-j calculates a frequency offset based on the frequency of the received signal obtained by coherently detecting the optical signal received from the supplier device 50-i, and instructs the frequency control unit 16 to compensate for the calculated frequency offset. The frequency control unit 16 controls the frequency of the light source unit 11 of the transmission device 50-j so as to compensate for the frequency offset instructed by the frequency offset estimation unit 55.

[0062] The frequency offset estimation unit 55 of the supplier device 50-i estimates the frequency f ｉ The frequency offset estimation unit 55 of the subscriber device 50-j receives the frequency information that the optical communication unit 12 has received from the supplier device 50-i.

[0063] In step S3, the optical communication system 5, like the optical communication systems 1 and 1a, performs optical communication using a light source frequency synchronized across the entire optical communication network. Then, in step S4, each transmission device 50 periodically checks the difference in communication frequency with the supplier device 50 based on the communication frequency information of the supplier device 50-i acquired by the same process as that of the transmission device 50-j in step S2. Specifically, the frequency offset estimation unit 55 of each transmission device 50-j acquires frequency information of the received signal obtained by coherently detecting the optical signal received from the supplier device 50-i by the same process as that of step S2, and determines whether a frequency offset has occurred based on the acquired frequency information. Alternatively, the frequency offset estimation unit 55 of the supplier device 50-i may transmit its own frequency information to the subscriber device 50-j from its optical communication unit 12. The frequency offset estimation unit 55 of the subscriber device 50-j receives the frequency information received from the supplier device 50-i by its optical communication unit 12 and determines whether a frequency offset has occurred based on the communication frequency indicated by the received frequency information.

[0064] If the frequency offset estimation unit 55 of the transmission device 50-j determines that a frequency offset of a predetermined value or more has not occurred between the transmission device 50-j and the supplier device 50-i (step S4: NO), the frequency offset estimation unit 55 of the transmission device 50-j repeats the process of step S4 after a predetermined time has elapsed. On the other hand, if the frequency offset estimation unit 55 of the transmission device 50-j determines that a frequency offset of a predetermined value or more has occurred between the transmission device 50-j and the supplier device 50-i (step S4: YES), the frequency offset estimation unit 55 of the transmission device 50-j performs the same process as the transmission device 50-j in step S2 to estimate the communication frequency f ｊ the communication frequency f of the supplier device 50-i ｉ (Step S5) The optical communication system 5 repeats the process from step S3 using the light source frequency synchronized in step S5.

[0065] The optical communication systems 1 and 1a of the first embodiment perform processing similar to that of the optical communication system 5 described above, and when it is difficult to demodulate the optical signal, for example, when the frequency offset cannot be compensated for, the frequency offset estimation unit 55 may transmit and receive information using the communication unit 13 as in the first embodiment.

[0066] According to the above-described embodiment, the optical communication system has a configuration in which multiple transmission devices that perform optical communication are connected via an optical transmission path. The transmission device includes a light source unit, a branching unit, multiple optical communication units, and a frequency control unit. The light source unit outputs light. The branching unit branches the light output by the light source unit into multiple branched lights. The multiple optical communication units are respectively connected to different other transmission devices. Each optical communication unit performs a process of coherently detecting an optical signal received from the other transmission device using the branched light branched by the branching unit, and a process of modulating the other branched light branched by the branching unit to generate an optical signal and transmitting it to the other transmission device. When the frequency control unit detects a frequency offset between an optical signal received from a supplier device, which is another transmission device whose frequency the own device tunes, and the light output from the light source unit of the own device, the frequency control unit controls the frequency of the light output from the light source unit to approach the frequency of the optical signal transmitted from the supplier device.

[0067] Which of multiple opposing transmission devices in an optical communication system is the supplier device is determined based on the sum of the communication bandwidth between the transmission device and each of the other opposing transmission devices, the sum of the transmission distance between the transmission device and each of the other multiple opposing transmission devices, or the transmission distance and communication bandwidth between the transmission device and each of the other multiple opposing transmission devices.

[0068] The transmission device may further include a topology generation unit that acquires a tree-type logical network of a plurality of transmission devices included in the optical communication system using an autonomous decentralized protocol for configuring a tree-type logical network, and determines a supplier device of the transmission device from among other transmission devices connected to the transmission device in the acquired logical network.

[0069] Although the embodiments of the present invention have been described in detail above with reference to the drawings, the specific configurations are not limited to these embodiments, and include designs within the scope of the present invention that do not deviate from the gist of the present invention.

[0070] 1, 1a, 5 Optical communication system 10, 10-1 to 10-3, 50, 50-1 to 50-3 Transmission device 11 Light source unit 12, 12-1, 12-2 Optical communication unit 13-1, 13-2 Communication unit 14, 54 Topology generation unit 15, 55 Frequency offset estimation unit 16 Frequency control unit 17 Data signal generation unit 18 Data signal demodulation unit 19 Control unit 20 Optical transmission path 121-1, 121-2 Electrical signal generation unit 122-1, 122-2 Optical modulation unit 123-1, 123-2 Optical detection unit 124-1, 124-2 Analog-to-digital conversion unit 125-1, 125-2 Electrical signal receiving unit 141 Storage unit

Claims

1. A transmission device in an optical communication system having a plurality of transmission devices for performing optical communication, the transmission device comprising: a light source unit that outputs light; a branching unit that branches the light output by the light source unit into a plurality of branched lights; a plurality of optical communication units that perform, respectively, a process of coherently detecting an optical signal received from another transmission device facing each other using the branched light branched by the branching unit, and a process of transmitting an optical signal generated by modulating another branched light branched by the branching unit to the other transmission device facing each other; and a frequency control unit that controls the frequency of the light output from the light source unit to approach the frequency of the optical signal transmitted from a supplier device, which is another transmission device with which the own device synchronizes the frequency, when detecting a frequency offset between the optical signal received from the supplier device and the light output by the light source unit among the other transmission devices facing each other.

2. Among the plurality of transmission devices facing each other in the optical communication system, which one is the supplier device is determined based on the sum of the communication bands between the transmission device and each of the other transmission devices facing it, the sum of the transmission distances between the transmission device and each of the plurality of other transmission devices facing it, or the transmission distances and communication bands between the transmission device and each of the plurality of other transmission devices facing it. The transmission device according to claim 1.

3. The transmission device according to claim 2, further comprising a topology generation unit that acquires a tree-type logical network of the plurality of transmission devices included in the optical communication system by means of an autonomous distributed protocol for constructing a tree-type logical network, and determines the supplier device of the own device from among the other transmission devices connected to the own device in the acquired logical network.

4. An optical communication system in which a plurality of the transmission devices according to any one of claims 1 to 3 are connected by an optical transmission line.

5. A frequency control method executed by a transmission device in an optical communication system having a plurality of transmission devices for performing optical communication, the method comprising: a branching step of branching the light output from a light source unit into a plurality of branched lights; a processing step in which a plurality of optical communication units each facing a different other transmission device perform coherent detection of an optical signal received from the other transmission device facing them using the branched lights branched in the branching step; an optical communication step of performing a process of modulating the other branched lights branched in the branching step to generate an optical signal and transmitting the generated optical signal to the other transmission device facing them; and a frequency control step of controlling the frequency of the light output from the light source unit to approach the frequency of the optical signal transmitted from the supplier device, which is the other transmission device whose frequency is to be adjusted by the own device, when a frequency offset between the optical signal received from the supplier device and the light output from the light source unit is detected.

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