Transmission system and optical communication method

Abstract

This transmission system comprises a plurality of communication devices, a splitting unit, and a splitting unit. The plurality of communication devices output optical subcarrier signals of different subcarriers superimposed on light having a common carrier frequency. The splitting unit splits reference light and transmits the split reference light to the plurality of communication devices. A coupling unit generates an optical subcarrier multiplexed signal by coupling the optical subcarrier signals respectively output from the plurality of communication devices. A first communication device includes a light source, a reception unit, a wavelength control unit, and a transmission unit. The reception unit receives the split reference light. The wavelength control unit performs control so that the light source outputs local light of the carrier frequency on the basis of the difference between the optical frequency of the local light output from the light source and the optical frequency of the reference light. The transmission unit outputs an optical subcarrier signal generated by superimposing a signal of a subcarrier different from that of another communication device on the local light output by the light source.

Description

Transmission system and optical communication method

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

[0002] 18 is a diagram showing an example of the configuration of an optical access system using optical subcarrier multiplexing (SCM) technology. A station device 910 is connected to N (N is an integer greater than or equal to 2) subscriber devices 920 via an optical coupler / splitter 930 via a P2MP (Point-to-Multipoint) connection. The station device 910 includes a light source 911, a wavelength controller 912, a transmitting digital signal processor (DSP Tx) 913, an optical modulator 914, an optical circulator 915, a coherent receiver 916, and a receiving digital signal processor (DSP Rx) 917. The subscriber device 920 includes a light source 921, a wavelength controller 922, a DSP Tx 923, an optical modulator 924, an optical circulator 925, a coherent receiver 926, and a DSP Rx 927. The N subscriber devices 920 are referred to as subscriber devices 920-1 to 920-N.

[0003] Downstream data transmitted from station device 910 to each of subscriber devices 920-1 to 920-N is mapped to SCs (subcarriers) of different optical frequencies. Similarly, upstream data transmitted from each of subscriber devices 920-1 to 920-N to station device 910 is also mapped to SCs of different optical frequencies.

[0004] The optical transmission and reception configurations of the station device 910 and the subscriber device 920 are similar. That is, light sources 911 and 921 output continuous wave light. Wavelength controllers 912 and 922 control the optical frequencies of the continuous wave light output by the light sources 911 and 921, respectively. DSP Txs 913 and 923 output SC signals, in which transmission data is mapped to SCs, to optical modulators 914 and 924, respectively. Although the original transmission data is a binary digital signal, the DSP Txs 913 and 923 may convert the transmission data into multilevel symbols. The optical modulators 914 and 924 modulate the continuous wave light from the light sources 911 and 921 with the SC signals, in which the transmission data is mapped, to obtain optical SC signals. Optical circulators 915 and 925 input the optical SC signals output by the optical modulators 914 and 924 into the transmission path, respectively.

[0005] Each of the optical subscriber units 920-1 to 920-N receives an optical SC signal from the station unit 910, which has been branched by the optical coupler / branch 930. The station unit 910 receives the optical SC signals from each of the optical subscriber units 920-1 to 920-N, which have been multiplexed by the optical coupler / branch 930. The optical circulators 925 and 915 input the optical SC signals after transmission through the transmission path to the coherent receivers 926 and 916, respectively. The coherent receivers 926 and 916 receive the optical SC signals by optical intradyne detection using a portion of the continuous light output from the light sources 921 and 911, respectively. The DSP Rxs 927 and 917 demodulate the received signals for each SC using digital signal processing. In upstream transmission, the DSP Rx 917 of the station unit 910 demodulates all of the SC signals. On the other hand, in downstream transmission, the DSP Rx 927 of each subscriber device 920 only needs to demodulate the SC signal addressed to that device.

[0006] In downstream transmission, SC multiplexing is performed at the output of the DSP Tx 913 of the station device 910, i.e., in the electrical stage. On the other hand, in upstream transmission, SC multiplexing is performed at the point when each optical SC signal is combined in the optical combiner / splitter 930, i.e., in the optical stage. When each subscriber device 920 communicates with the station device 910 using multiple SCs, SC multiplexing is performed in the electrical stage as well as the optical stage. As shown in FIG. 19, upstream transmission and downstream transmission are performed at the optical frequency of the light source (approximately f ０ ) into upper and lower sidebands. In Fig. 19 and Fig. 20 described later, #n (n is an integer between 1 and N) indicates the SC corresponding to the nth subscriber unit 920-n. In Fig. 19, optical frequency control is performed, so there is no overlap between optical SC signals.

[0007] However, if optical frequency control is not performed, variations in the optical frequency of the light source 921 disposed in each subscriber unit 920 may occur. If such variations occur, when optical SC signals transmitted from each subscriber unit 920 are multiplexed in the optical stage during upstream transmission, the optical SC signals overlap in the optical frequency domain, as shown in FIG. 20, resulting in degradation of signal performance. In conventional technology, when the DSP Rx 927 of the subscriber unit 920 demodulates the downstream optical SCM signal, it detects the optical frequency offset between the light source 911 and the light source 921 (see, for example, Non-Patent Documents 1 and 2). The wavelength controller 922 uses this value as a compensation value and controls the optical frequency of the light source 921 to approach the optical frequency (reference optical frequency) of the light source 911. This prevents degradation of signal performance. Furthermore, the wavelength controller 912 of the station unit 910 adjusts the optical frequency of the light source 911 to match the optical frequency grid (f ０ ) is controlled to approach this.

[0008] D. Welch, A. Napoli, J. Back, W. Sande, J. Pedro, F. Masoud, C. Fludger, T. Duthel, H. Sun, SJ Hand, T. Chiang, A. Chase, A. Mathur, TA Eriksson, M. Plantare, M. Olson, S. Voll, and K. Wu, "Point-to-Multipoint Optical Networks Using Coherent Digital Subcarriers", Journal of Lightwave Technology, vol. 39, no. 16, pp. 5232-5247, 2021.S. Ranzini, CRS Fludger, T. Duthel, B. Liu, A. Napoli, A. Kumpera, A. Rashidinejad, A. Kakkar, M. Missey, V. Dominic,P. Samra, H. Sun, R. Maher, A. Somani, D. Welch, “Local and Remote Laser Frequency Control” in Point-to-Multipoint Networks Using Digital Subcarriers", 49th European Conference on Optical Communications (ECOC 2023), p. 807-810, 2023.

[0009] When bidirectional optical SCM transmission is performed using multiple independent light sources, it is not possible to set a reference optical frequency like the downstream signal in P2MP transmission. As a result, variations in the optical frequencies of the light sources occur, and optical SC signals overlap in the optical frequency domain, resulting in degradation of signal performance.

[0010] In view of the above circumstances, an object of the present invention is to provide a transmission system and an optical communication method that can reduce degradation of signal performance even when optical SCM transmission is performed using multiple communication devices, each having an independent light source.

[0011] One aspect of the present invention is a transmission system comprising a plurality of first communication devices that output optical subcarrier signals of different subcarriers superimposed on light of a common carrier frequency, a first branching unit that branches reference light and transmits the branched reference light to the plurality of first communication devices, and a first combining unit that generates a first optical subcarrier multiplexed signal by combining the optical subcarrier signals output from each of the plurality of first communication devices, wherein the first communication devices comprise a first light source that outputs a first local light, a first receiving unit that receives the reference light branched by the first branching unit, a first wavelength control unit that controls the first light source to output a first local light of the carrier frequency based on the difference between the optical frequency of the first local light and the optical frequency of the reference light received by the first receiving unit, and a first transmitting unit that generates an optical subcarrier signal by superimposing a signal of a subcarrier different from that of the other first communication devices on the first local light output by the first light source, and outputs the generated optical subcarrier signal.

[0012] One aspect of the present invention is an optical communication method comprising: a branching step in which a branching unit branches reference light and transmits the branched reference light to a plurality of communication devices; a receiving step in which the communication devices receive the reference light branched in the branching step; a control step in which the communication devices control the light source of the own device to output local light of a carrier frequency common to light sources of other communication devices based on the difference between the optical frequency of local light output from the light source of the own device and the optical frequency of the reference light received in the receiving step; an output step in which the communication device generates an optical subcarrier signal by superimposing a subcarrier signal different from that of the other communication devices on the local light output from the light source of the own device, and outputs the generated optical subcarrier signal; and a combining step in which a combining unit generates an optical subcarrier multiplexed signal by combining the optical subcarrier signals output from each of the plurality of communication devices.

[0013] According to the present invention, it is possible to reduce degradation of signal performance even when optical SCM transmission is performed using a plurality of communication devices each having an independent light source.

[0014] 1 is a diagram showing a schematic configuration of a transmission system according to an embodiment of the present invention; FIG. 2 is a diagram showing an optical spectrum of a coherent receiver input according to an embodiment; FIG. 3 is a diagram showing an electrical spectrum after optical intradyne detection according to an embodiment; FIG. 4 is a functional block diagram of a DSP Rx according to an embodiment; FIG. 5 is a diagram showing an example configuration of a transmission system according to an embodiment; FIG. 6 is a diagram showing an example configuration of a transmission system according to an embodiment; FIG. 7 is a diagram for explaining the operation of a transmission system according to an embodiment; FIG. 8 is a diagram showing a receiving configuration of a subscriber device according to an embodiment; FIG. 9 is a diagram showing a receiving configuration of a subscriber device according to an embodiment; FIG. 10 is a diagram showing an optical spectrum of a coherent receiver input according to an embodiment; FIG. 11 is a diagram showing an electrical spectrum after optical intradyne detection according to an embodiment; FIG. 12 is a diagram showing an electrical spectrum after optical intradyne detection according to an embodiment; FIG. 13 is a diagram showing an example configuration of a DSP Rx according to an embodiment; FIG. 14 is a diagram showing an example frame of a supervisory control signal according to an embodiment; FIG. 15 is a diagram showing an example configuration of an optical access system according to the prior art; FIG. 16 is a diagram showing the optical frequency of each SC when optical frequency control is present; FIG. 17 is a diagram showing the optical frequency of each SC when optical frequency control is not present.

[0015] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0016] FIG. 1 is a diagram showing the general configuration of a transmission system 1 according to an embodiment of the present invention. FIG. 1 shows only functional blocks relevant to the description of the overview. The transmission system 1 includes N (N is an integer equal to or greater than 2) subscriber devices 2, an optical splitter 3, and a reference light source 4. In FIG. 1, the N subscriber devices 2 are designated as subscriber devices 2-1 to 2-N. The subscriber devices 2-1 to 2-N are connected to the optical splitter 3 via a transmission line 5. Each of the subscriber devices 2-1 to 2-N transmits an optical SC signal addressed to a corresponding communication device, and receives an optical SCM signal from the corresponding communication device that includes an optical SC signal addressed to the subscriber device itself. For example, the corresponding communication device is a subscriber device 2 other than the subscriber devices 2-1 to 2-N. The reference light source 4 is located outside the subscriber device 2. The reference light source 4 is used as an optical frequency reference. The reference light source 4 transmits an optical frequency f to each subscriber device 2 via the optical splitter 3. ｃ Continuous light (f ｃ ) to supply.

[0017] The subscriber device 2 includes a light source 21, a wavelength controller 22, a coherent receiver 26, and a DSP Rx 27. The subscriber device 2 also includes components related to the transmission of optical SC signals, such as a DSP Tx and an optical modulator, but these components are not shown in FIG. 1. The light source 21 of each subscriber device 2-n (n is an integer between 1 and N) emits an optical frequency f ｎ Continuous light (f ｎ The wavelength controller 22 of the subscriber unit 2-n outputs the optical frequency f ｎ , the optical frequency grid f ０ The optical frequency grid f ０ is the carrier frequency of the optical subcarrier signals transmitted by the subscriber devices 2-1 to 2-N, as determined by the transmission system 1. The coherent receiver 26 receives the optical SCM signal transmitted through the transmission path 5 by optical intradyne detection using a portion of the continuous light output from the light source 21, and obtains a received signal for each SC signal. The DSP Rx 27 demodulates the received signal for each SC signal using digital signal processing (DSP). The DSP Rx 27 may demodulate only the SC signal addressed to its own device.

[0018] 2 is a diagram showing the optical spectrum of the input to the coherent receiver. In FIG. 2, the optical spectrum of the light input to the coherent receiver 26 of the subscriber unit 2-1 is shown in the case of n=1. The continuous light (f ｃ ) optical frequency f ｃ is the optical frequency grid f of the transmission system 1 ０ For f ｍ The value shifted by f ０ ±f ｍ ) is set. In addition, continuous light (f ｃ ) optical frequency f ｃ is controlled by a wavelength controller (not shown) of the reference light source 4. ０ ±f ｍ The optical frequency f is controlled within the range of ±Δf. ｃ ga f ０ -f ｍ That is, when the optical frequency f ｃ the optical frequency grid f ０ 4 shows the case where the frequency is set to the low frequency side.

[0019] Optical frequency f as the optical frequency reference ｃ Continuous light (hereinafter referred to as reference light or reference light (f ｃ ) is branched by the optical branching device 3 and input to the coherent receiver 26 of each of the subscriber devices 2-1 to 2-N. The coherent receiver 26 of the subscriber device 2-n receives the optical frequency f ｎ Continuous light (f ｎ ) is used as a local light, and the received reference light is optically intradyne detected. ｎ The optical frequency of the reference light is f ｃ Similarly, the wavelength controller 22 controls the optical frequency grid f ０ is controlled within the range of ±Δf.

[0020] 3 is a diagram showing an electrical spectrum after optical intradyne detection. FIG. 3 also shows the case where n=1, that is, the electrical spectrum after optical intradyne detection in the coherent receiver 26 of the subscriber unit 2-1. As shown in FIG. 3, after optical intradyne detection, the local light (f ｎ) and reference light (f ｃ ) beat component is f ｎ -f ｃ The optical frequency of this beat component f ｍ The deviation from the frequency f corresponds to the desired optical frequency offset. ｎ -f ｃ -f ｍ and f ｍ The range is within ±2Δf.

[0021] In the subscriber unit 2-n, the DSP Rx 27 calculates the optical frequency offset (f ｎ -f ｃ -f ｍ ) is detected by digital signal processing, and the detected value is fed back to the light source 21 by the wavelength controller 22. As a result, the wavelength controller 22 of the subscriber device 2-n controls the optical frequency f ｎ After the control, the optical frequency of the light source 21 is f ｃ +f ｍ is.

[0022] Although not shown, the optical frequency of the reference light output from the reference light source 4 is set to an optical frequency grid f ０ f set on the high frequency side ０ +f ｍ In this case, the optical frequency offset amount in the subscriber unit 2-n is f ｎ -f ｃ +f ｍ , the optical frequency of the light source 21 after control is f ｃ -f ｍ is.

[0023] 4 is a diagram showing the functional blocks of the DSP Rx 27 that demodulates the optical SCM signal. For this configuration, for example, the technology described in Non-Patent Document 1 can be used. The diagram shows the configuration of the DSP Rx 27 in the subscriber device 2-1. The DSP Rx 27 includes analog-to-digital converters (ADCs) 271-1 to 271-4, a Fourier transformer 272, frequency shifters 273-1 to 273-N, equalizers 274-1 to 274-N, and compensation units 275-1 to 275-N.

[0024] The coherent receiver 26 of the subscriber device 2-n receives the input optical frequency f ｎ The local light (f ｎ ) and optical frequency f ｃ Reference light (f ｃ ) and separates them into four components whose optical phases are orthogonal to the polarization. The four components are the X-polarized I component (XI), the X-polarized Q component (XQ), the Y-polarized I component (YI), and the Y-polarized Q component (YQ). The coherent receiver 26 outputs these four components to the DSP Rx 27. The ADCs 271-1 to 271-4 of the DSP Rx 27 convert each of these four components from an analog signal to a digital signal.

[0025] The Fourier transformer 272 converts the digital signals output by the ADCs 271-1 to 271-4 from time domain signals to frequency domain signals. ｎ ) and reference light (f ｃ ) frequency position f of the beat component ｎ -f ｃ is identified by peak detection, and the detected value is compared with the known f ｍ and the optical frequency offset f ｎ -f ｃ -f ｍ For example, the Fourier transformer 272 of the subscriber unit 2-1 can easily calculate the optical frequency offset f １ -f ｃ -f ｍ When the optical SCM signal is input to the coherent receiver 26 together with the reference light, the Fourier transformer 272 separates each SC signal in the frequency domain. The Fourier transformer 272 outputs the j-th (j is an integer between 1 and N) SC signal to the frequency shifter 273-j.

[0026] The frequency shifters 273-1 to 273-N convert each separated SC signal into a time-domain baseband signal. Each equalizer 274-j performs polarization separation, band compensation, etc. on the baseband signal converted by the frequency shifter 273-j. Each of the compensation units 275-1 to 275-N has a frequency offset compensator (FOC) and an optical phase compensator (CPR). The compensation unit 275-j performs optical frequency offset compensation using FOC on the baseband signal output by the equalizer 274-j, performs optical phase compensation using CPR, and then demodulates the signal.

[0027] According to this embodiment, even when bidirectional optical SCM transmission is performed using multiple independent light sources, the optical frequency variation of each light source can be suppressed, and the problem of overlap of each optical SC signal in the optical frequency domain, resulting in degradation of signal performance, can be avoided.

[0028] Next, an example of the overall configuration of a transmission system using two-core subscriber devices will be described. Fig. 5 is a diagram showing the overall configuration of a transmission system 11 according to the first embodiment. The transmission system 11 includes 2N subscriber devices 201, two optical couplers 31, two optical splitters 32, and one or two reference light sources 4. The subscriber device 201 is an example of the subscriber device 2 in Fig. 1, and the optical splitter 32 is an example of the optical splitter 3 in Fig. 1.

[0029] The transmission system 11 is configured such that N pairs of opposing subscriber devices 201 are connected via an optical coupler 31 and an optical branching device 32. The 2N subscriber devices 201 are designated as subscriber devices 201-1a to 201-Na and 201-1b to 201-Nb. Fig. 5 illustrates a case where subscriber device 201-na faces subscriber device 201-nb (n is an integer between 1 and N).

[0030] The transmission path 51 to which each subscriber unit 201 is connected is a two-core optical fiber. An optical coupler 31 and an optical branching device 32 are arranged on the transmission path 51 connected to each subscriber unit 201. In Fig. 5, the optical coupler 31 and optical branching device 32 arranged on the transmission path 51 connected to each of the subscriber units 201-1a to 201-Na are referred to as the optical coupler 31a and the optical branching device 32a, respectively, and the optical coupler 31 and optical branching device 32 arranged on the transmission path 51 connected to each of the subscriber units 201-1b to 201-Nb are referred to as the optical coupler 31b and the optical branching device 32b, respectively. The reference light source 4 connected to the optical branching device 32a and supplying reference light to the subscriber units 201-1a to 201-Na is referred to as the reference light source 4a, and the reference light source 4 connected to the optical branching device 32b and supplying reference light to the subscriber units 201-1b to 201-Nb is referred to as the reference light source 4b.

[0031] The subscriber unit 201 includes a light source 21, a wavelength controller 22, a DSP Tx 23, an optical modulator 24, a coherent receiver 26, and a DSP Rx 27. The subscriber unit 201 can be the conventional subscriber unit 920 shown in FIG. 18, but an optical circulator is not used because optical signal transmission is performed over a two-core optical fiber transmission line for transmission and reception. The light source 21, wavelength controller 22, coherent receiver 26, and DSP Rx 27 have the same functions as the light source 21, wavelength controller 22, coherent receiver 26, and DSP Rx 27 of the subscriber unit 2 shown in FIG. 1. The DSP Tx 23 and optical modulator 24 have the same functions as the DSP Tx 923 and optical modulator 924 of the subscriber unit 920 shown in FIG. 18. The light source 21, wavelength controller 22, DSP Tx 23, optical modulator 24, coherent receiver 26, and DSP Rx 27 of the subscriber device 201-nx (n is an integer between 1 and N, and x is a or b) will be referred to as the light source 21-nx, wavelength controller 22-nx, DSP Tx 23-nx, coherent receiver 26-nx, optical modulator 24-nx, and DSP Rx 27-nx, respectively.

[0032] The optical SC signals transmitted from the subscriber units 201-1a to 201-Na are combined in the optical coupler 31a to generate an optical SCM signal. The upper part of Fig. 5 shows the optical frequencies of the optical SC signals contained in the optical SCM signal generated by the optical coupler 31a. #na represents the optical SC signal transmitted from the subscriber unit 201-na. The optical branching unit 32b branches the optical SCM signal generated by the optical coupler 31a and transmits it to the subscriber units 201-1b to 201-Nb.

[0033] Meanwhile, the optical SC signals transmitted from subscriber units 201-1b to 201-Nb are combined in optical coupler 31b to form an optical SCM signal. The optical frequencies of the optical SC signals contained in the optical SCM signal generated by optical coupler 31b are shown at the bottom of Figure 5. #nb represents the optical SC signal transmitted from subscriber unit 201-nb. Optical splitter 32a splits the optical SCM signal generated by optical coupler 31b and transmits it to subscriber units 201-1a to 201-Na. Of the received optical SCM signals, each subscriber unit 201 only needs to receive the optical SC signal addressed to it.

[0034] Furthermore, the optical splitter 32a splits the reference light output by the reference light source 4a and transmits it to the subscriber units 201-1a to 201-Na. The optical splitter 32b distributes the reference light output by the reference light source 4b and transmits it to the subscriber units 201-1b to 201-Nb. The transmission system 11 does not necessarily have to have either the reference light source 4a or the reference light source 4b.

[0035] Next, the overall configuration when single-core subscriber devices are used will be described. Fig. 6 is a diagram showing the overall configuration of a transmission system 12. In Fig. 6, the same components as those in the transmission system 11 shown in Fig. 5 are designated by the same reference numerals, and their description will be omitted. The transmission system 12 has 2N subscriber devices 202, two optical couplers 33, and one or two reference light sources 4. The subscriber device 202 is an example of the subscriber device 2 in Fig. 1, and the optical coupler 33 is an example of the optical coupler 3 in Fig. 1.

[0036] The transmission system 12 is configured such that N pairs of opposing subscriber devices 202 are connected via two optical couplers 33. The 2N subscriber devices 202 are designated as subscriber devices 202-1a to 202-Na and 202-1b to 202-Nb. Fig. 6 illustrates a case where subscriber device 202-na faces subscriber device 202-nb (n is an integer between 1 and N).

[0037] The transmission path 52 to which each subscriber unit 202 is connected is a single optical fiber. The optical coupler / branch 33 arranged on the transmission path 52 connected to each of the subscriber units 202-1a to 202-Na is referred to as optical coupler / branch 33a, and the optical coupler / branch 33 arranged on the transmission path 52 connected to each of the subscriber units 202-1b to 202-Nb is referred to as optical coupler / branch 33b. The reference light source 4 connected to the optical coupler / branch 33a and supplying reference light to the subscriber units 202-1a to 202-Na is referred to as reference light source 4a, and the reference light source 4 connected to the optical coupler / branch 33b and supplying reference light to the subscriber units 202-1b to 202-Nb is referred to as reference light source 4b.

[0038] The subscriber unit 202 is configured to include a light source 21, a wavelength controller 22, a DSP Tx 23, an optical modulator 24, an optical circulator 25, a coherent receiver 26, and a DSP Rx 27. The subscriber unit 202 has the same configuration as the subscriber unit 201 shown in Fig. 5 except for the inclusion of the optical circulator 25. The optical circulator 25 outputs the optical SC signal output by the optical modulator 24 to the transmission line 52, and outputs the optical SCM signal input from the transmission line 52 to the coherent receiver 26. The subscriber unit 202 can be the conventional subscriber unit 920 shown in Fig. 18.

[0039] The light source 21, wavelength controller 22, DSP Tx 23, optical modulator 24, optical circulator 25, coherent receiver 26, and DSP Rx 27 of the subscriber device 202-nx (n is an integer between 1 and N, and x is a or b) will be referred to as the light source 21-nx, wavelength controller 22-nx, DSP Tx 23-nx, optical modulator 24-nx, optical circulator 25-nx, coherent receiver 26-nx, and DSP Rx 27-nx, respectively.

[0040] The optical SC signals transmitted from the subscriber units 202-1a to 202-Na are combined by the optical coupler / splitter 33a to generate an optical SCM signal. The upper part of Fig. 6 shows the optical frequencies of the optical SC signals contained in the optical SCM signal generated by the optical coupler / splitter 33a. #na represents the optical SC signal transmitted from the subscriber unit 202-na. The optical coupler / splitter 33b branches the optical SCM signal generated by the optical coupler / splitter 33a and transmits it to the subscriber units 202-1b to 202-Nb.

[0041] On the other hand, the optical SC signals transmitted from the subscriber units 202-1b to 202-Nb are combined by the optical coupler / splitter 33b to become an optical SCM signal. The optical frequencies of the optical SC signals contained in the optical SCM signal generated by the optical coupler / splitter 33b are shown at the bottom of Fig. 6. #nb represents the optical SC signal transmitted from the subscriber unit 202-nb. The optical coupler / splitter 33a branches the optical SCM signal generated by the optical coupler / splitter 33b and transmits it to the subscriber units 202-1a to 202-Na. Each subscriber unit 202 only needs to receive the optical SC signal addressed to it among the received optical SCM signals. In Fig. 6, the frequencies of the optical SC signals transmitted from the subscriber units 202-1a to 202-Na are determined by the optical frequency grid f ０ The frequency of the optical SC signal transmitted from the subscriber units 202-1b to 202-Nb is the upper sideband of the optical frequency grid f ０ , but these may be reversed.

[0042] Furthermore, the optical coupler / splitter 33a branches the reference light output by the reference light source 4a and transmits it to the subscriber units 202-1a to 202-Na. The optical coupler / splitter 33b branches the reference light output by the reference light source 4b and transmits it to the subscriber units 202-1b to 202-Nb. Note that the transmission system 12 does not necessarily have to have either the reference light source 4a or the reference light source 4b.

[0043] In the following, the operation of the transmission system 11 will be explained using the case of two-core bidirectional transmission as an example, but the same applies to the case of one-core bidirectional transmission, that is, the operation of the transmission system 12.

[0044] The transmission system 11 performs a series of processes in time series, such as (Process 1) controlling the optical frequency of the local light of the subscriber unit 201-na using the reference light of the reference light source 4, (Process 2) controlling the optical frequency of the local light of the subscriber unit 201-nb using the transmission signal of the subscriber unit 201-na, and (Process 3) the subscriber unit 201-na receives the transmission signal of the opposing subscriber unit 201-nb in addition to the reference light, and controls the optical frequency of the local light. As a result, with the passage of a small amount of time from (Process 1) to (Process 3), the optical frequency f of the reference light source 4 changes. ｃ ga f ｃ Even if the optical frequency of the reference light changes to ', the optical frequency of the local light in the subscriber unit 201 can be controlled using the optical frequency of the reference light as an absolute reference. Thereafter, the transmission system 11 repeats the operations of (Process 1) to (Process 3). Each of the operations of (Process 1) to (Process 3) will be explained using subscriber unit 201-1a and subscriber unit 201-1b as an example.

[0045] FIG. 7 is a diagram for explaining the operation of the transmission system 11. While FIG. 7 illustrates a case in which reference light is supplied from the reference light source 4a to the subscriber units 201-1a to 201-Na, reference light may also be supplied from the reference light source 4b to the subscriber units 201-1b to 201-Nb. Alternatively, both the reference light source 4a and the reference light source 4b may supply reference light. The operations of (Process 1) and (Process 2) above will be explained using FIG. 7. That is, by the above-mentioned (Process 1) process, when no optical SC signal is received, the subscriber unit 201-1a controls the optical frequency of the light source 21-1a using the reference light from the reference light source 4a. Thereafter, by the above-mentioned (Process 2) process, an optical SC signal (SC#1a) is transmitted from the subscriber unit 201-1a to the subscriber unit 201-1b, and the optical SC signal is received by the subscriber unit 201-1b, which then controls the optical frequency of the light source 21-1b.

[0046] (Process 1) When no optical SC signal is received, the optical frequency of the light source 21-1a of the subscriber unit 201-1a is f １ａ From f ｃ +f ｍ 1. That is, the optical splitter 32a splits the reference light (f ｃ) and outputs it to the subscriber units 201-1a to 201-Na. The coherent receiver 26-1a of the subscriber unit 201-1a receives the optical frequency f １ａ The local light (f １ａ ) and the optical frequency f branched by the optical branching device 32a ｃ Reference light (f ｃ ) and separates it into four components whose polarization and optical phase are orthogonal. The ADCs 271-1 to 271-4 of the DSP Rx 27-1a convert each of these four components from analog to digital signals, and the Fourier transformer 272 converts these digital signals from time domain signals to frequency domain signals. The Fourier transformer 272 of the subscriber unit 201-1a converts the local oscillator light (f １ａ ) and reference light (f ｃ ) frequency f of the beat component １ａ -f ｃ When the optical frequency offset f １ａ -f ｃ -f ｍ The wavelength controller 22-1a calculates the optical frequency f １ａ f ｃ +f ｍ Control to.

[0047] (Process 2) The optical modulator 24-1a of the subscriber unit 201-1a modulates the continuous light (f １ａ ) by the SC signal generated by the DSP Tx 23-1a. The optical modulator 24-1a inputs the optical SC signal (SC#1a) obtained by the modulation into the transmission line 51. The optical SC signals (SC#1a to SC#Na) transmitted from the subscriber units 201-1a to 201-Na are combined by the optical coupler 31a to become an optical SCM signal. This optical SCM signal is branched by the optical brancher 32b and sent to the subscriber units 201-1b to 201-Nb.

[0048] The coherent receiver 26-1b of the subscriber unit 201-1b receives the continuous light (f １ｂ) as local light, and receives the optical SCM signal transmitted through the transmission path 51 by optical intradyne detection. The DSP Rx 27-1b demodulates the received signal using a DSP. Note that the DSP Rx 27-1b only needs to demodulate the SC signal (SC#1a) transmitted by the remote subscriber unit 201-1a. The DSP Rx 27-1b detects an optical frequency offset, which is the difference between the optical frequency of the light source 21-1b and the optical frequency of the light source 21-1a of the remote subscriber unit 201-1a, using a Fourier transformer or the like. The wavelength controller 22-1b controls the optical frequency of the light source 21-1b using the optical frequency offset detected by the DSP Rx 27-1b. The wavelength controller 22-1b adjusts the optical frequency of the light source 21-1b to the optical frequency f of the light source 21-1a of the remote subscriber unit 201-1a. ｃ +f ｍ Control it to approach .

[0049] Figure 8 shows the receiving configuration of subscriber unit 201-1b. The receiving configuration of subscriber unit 201-1b is similar to that of subscriber unit 2 shown in Figure 4. Like DSP Rx 27 shown in Figure 4, DSP Rx 27-1b includes ADCs 271-1 to 271-4, a Fourier transformer 272, frequency shifters 273-1 to 273-N, equalizers 274-1 to 274-N, and compensation units 275-1 to 275-N. However, the Fourier transformer 272 of DSP Rx 27-1b outputs SC signal #ja of subscriber unit 201-ja (j is an integer between 1 and N) to frequency shifter 273-j.

[0050] The ADCs 271-1 to 271-4 convert the XI, XQ, YI, and YQ components of the optical SCM signal optically intradyne detected by the coherent receiver 26-1b from analog signals to digital signals. The Fourier transformer 272 converts the digital signals output by the ADCs 271-1 to 271-4 from the time domain to the frequency domain, separates them into subcarriers, and calculates the amount of optical frequency offset. The optical frequency offset (f １ｂ -f ｃ -f ｍ ) can be easily calculated, for example, by detecting the center of gravity on the frequency axis of the electrical spectrum of the SC signal.

[0051] The Fourier transformer 272 separates SC#1 from the remote subscriber unit 201-1a. The frequency shifter 273-1 converts the separated SC#1a into a time-domain baseband signal. At this time, the offset detected by the Fourier transformer 272 may be used to compensate for the optical frequency offset. The equalizer 274-1 performs polarization separation and bandwidth compensation for the baseband signal, and the compensator 275-1 performs optical frequency offset compensation for SC#1a using FOC and optical phase compensation using CPR. When optical frequency offset compensation is performed by the frequency shifter 273-1, the compensation performed by the frequency shifter 273-1 is coarse adjustment, and the compensation performed by the compensator 275-1 using FOC is fine adjustment. The compensator 275-1 demodulates SC#1a. The wavelength controller 22-1b uses the optical frequency offset detected by one or both of the Fourier transformer 272 and the FOC to adjust the optical frequency f １ｂ is the optical frequency f of the light source 21-1a １ａ (=f ｃ +f ｍ The reference light (f ｃ ) optical frequency f ｃ f ０ +f ｍ When set to , the optical frequency f １ａ , optical frequency f １ｂ Both f ０ -f ｍ is controlled to approach

[0052] (Process 3) As described above, the optical frequency f １ｂ After carrying out the above control, the subscriber unit 201-1a simultaneously receives the reference light from the reference light source 4a and the optical SC signal (SC#1b) transmitted from the subscriber unit 201-1b. This operation will be described.

[0053] The optical modulator 24-1b of the subscriber unit 201-1b modulates the continuous light (f １ｂ) by the SC signal generated by the DSP Tx 23-1b. The optical modulator 24-1b inputs the optical SC signal (SC#1b) obtained by the modulation into the transmission path 51. The optical SC signals (SC#1b to SC#Nb) transmitted from each of the subscriber units 201-1b to 201-Nb are combined in the optical coupler 31b to become an optical SCM signal. The optical branching unit 32a branches the optical SCM signal output from the optical coupler 31b and the reference light output from the reference light source 4a, and outputs them to the subscriber units 201-1a to 201-Na.

[0054] 9 is a diagram showing the receiving configuration of subscriber unit 201-1a. The receiving configuration of subscriber unit 201-1a is the same as the receiving configuration of subscriber unit 2 shown in FIG. 4. Like DSP Rx 27 shown in FIG. 4, DSP Rx 27-1a includes ADCs 271-1 to 271-4, a Fourier transformer 272, frequency shifters 273-1 to 273-N, equalizers 274-1 to 274-N, and compensation units 275-1 to 275-N. However, the Fourier transformer 272 of DSP Rx 27-1a outputs the SC signal of subscriber unit 201-jb (j is an integer between 1 and N) to frequency shifter 273-j. In addition to the reference light from the reference light source 4a, the optical SC signal (SC#1b) transmitted from the subscriber device 201-1b is simultaneously input to the coherent receiver 26-1a of the subscriber device 201-1a. The optical frequency of the reference light from the reference light source 4a is f within the round trip time between the transmission of the optical signal (SC#1a) from the subscriber device 201-1a and the arrival of the optical signal (SC#1b) from the subscriber device 201-1b by the operation of (Process 2) above. ｃ From f ｃ ', which becomes the new optical frequency standard.

[0055] The optical frequency (f ｃ +f ｍ The offset amount between the local oscillator light (f １ａ ), SC#1b, and reference light (f ｃ ') is entered.

[0056] 10 is a diagram showing the optical spectrum input to the coherent receiver 26-1a. １ａ ), SC#1b, and reference light (f ｃ 10, the optical frequency of the local light (P1) is f ０ f within ±Δf ｃ +f ｍ and the optical frequency of the reference light (P3) is f ０ -f ｍ f within ±Δf ｃ ' is.

[0057] 11 is a diagram showing an electrical spectrum after optical intradyne detection by the coherent receiver 26-1a. As shown in FIG. 11, optical intradyne detection generates a beat component (B1) between the local light (P1) and SC#1b (P2), a beat component (B2) between the local light (P1) and the reference light (P3), and a beat component (B3) between SC#1b (P2) and the reference light (P3). The beat component (B1) between the local light (P1) and SC#1b (P2) is the received signal of SC#1b, and is demodulated via the frequency shifter 273-1, equalizer 274-1, and compensation unit 275-1 in the functional blocks subsequent to the Fourier transformer 272. The beat component (B2) between the local light (P1) and the reference light (P3) is calculated based on the optical frequency offset (f ｃ -f ｃ '), but the amount that can be directly detected is f ｃ +f ｍ -f ｃ Therefore, the optical frequency f of the detected beat component (B2) ｃ +f ｍ -f ｃ ' to the offset f provided in the reference light source 4 ｍ It is necessary to subtract this beat component (B2). ｍ Since there may be a variation in the range of ±2Δf with respect to the frequency of the local light (P1) and the SC#1b (P2), if the beat component (B1) of the local light (P1) and the SC#1b (P2) is in this range, it becomes impossible to detect the optical frequency offset. In order to avoid this, the electrical bandwidth in which the SC signal exists (including the portion widened by modulation) is set to f ｍAlthough not shown, the electrical band limit of this SC signal is set to the reference light (f ｃ ) optical frequency is f ０ +f ｍ The same applies when the setting is made as follows. The beat component (B3) of SC#1b (P2) and the reference light (P3) is an unnecessary component.

[0058] 12 is a diagram showing the electrical spectrum after optical intradyne detection by the coherent receiver 26-1a when an optical SC signal from another subscriber unit 201 is also received. As shown in FIG. 12, the unwanted beat component (B3) of SC#1b is superimposed in the frequency domain when receiving an optical SC signal (SC#kb) transmitted by another subscriber unit 201-kb (k is an integer between 2 and N), degrading the signal performance of SC#kb. As the set intensity of the reference light increases, the intensity of the unwanted beat component (B3) also increases. Therefore, to relatively reduce the influence of the unwanted beat component (B3), it is desirable to set the intensity of the reference light received from the reference light source 4a as low as possible.

[0059] FIG. 13 shows the electrical spectrum after optical intradyne detection by the coherent receiver 26-1a when the intensity of the reference light is reduced. As shown in FIG. 13, setting the intensity of the reference light lower than that shown in FIG. 12 can improve the signal performance of SC#kb. However, at the same time, the intensity of the beat component (B2) used for optical frequency offset detection is also reduced, resulting in a decrease in the accuracy of optical frequency offset detection. Furthermore, the beat component (B2) appears on the high-frequency side of the received signal, i.e., in the region where the frequency response characteristics of the coherent receiver 26 and ADCs 271-1 to 271-4 are reduced, and therefore its intensity is weaker than that of the received signal component. However, unlike the received signal, the beat component (B2) used for optical frequency offset detection does not have any information superimposed on it. Therefore, when detecting the offset, the DSP Rx 27 can average the results of multiple Fourier transforms. Averaging the beat component (B2) improves the signal-to-noise ratio of the detected component, thereby improving the accuracy of offset detection even when the reference light intensity is reduced. The DSP configuration for performing the averaging process will be described later with reference to FIG.

[0060] Reference light (f ｃ ) optical frequency f ｃ f ０ -f ｍ When the frequency is set to 1 / f, this unnecessary beat component (B2) is superimposed in the frequency domain when receiving other optical SC signals only when the optical SC signal (SC#1b) is placed in the lower sideband with respect to the light source frequency of the subscriber unit 201-1b. Although not shown, the beat component of the optical SC signal and the reference light in the upper sideband is superimposed in the reception band (DC to f) of the SC signal. ｍ -2Δf), it does not affect the reception performance of the SC signal. ｃ ) optical frequency f ｃ f ０ +f ｍ When the setting is made as above, this unwanted beat component (B2) will be superimposed in the frequency domain when receiving other optical SC signals only when the optical SC signal (SC#1b) is placed in the upper sideband with respect to the light source frequency of the subscriber device 201-1b.

[0061] In the above process, the reference light can be amplitude-modulated and superimposed by the monitor control signal. Fig. 14 shows the optical spectrum input to the coherent receiver 26-1a, and Fig. 15 shows the electrical spectrum after optical intradyne detection by the coherent receiver 26-1a. The reference light (f ｃ Even if the monitor and control signal is superimposed on the local light (P1) and the reference light (P3), the Fourier transformer 272 can detect the peak frequency position of the beat component (B2) of the local light (P1) and the reference light (P3). The beat component (B2) can be detected by peak detection in the frequency domain or by finding the center of gravity of the monitor and control signal spectrum. As in the example shown in FIG. 13, the accuracy of offset detection can be improved by averaging processing.

[0062] The supervisory control signal is written with information required to open the subscriber device 201. For example, information to be written may include permission to transmit an upstream optical signal, the wavelength setting of the light source when transmitting an optical SCM signal by wavelength division multiplexing (WDM), and the like.

[0063] Fig. 16 is a diagram showing an example of the configuration of the DSP Rx 28 when detecting the amount of optical frequency offset by averaging processing. The subscriber devices 2, 201, and 202 are provided with the DSP Rx 28 shown in Fig. 16 instead of the DSP Rx 27. In Fig. 16, the same components as those in the DSP Rx 27 shown in Fig. 4 and the DSP Rx 27-1a shown in Fig. 9 are designated by the same reference numerals, and their description will be omitted. Fig. 16 shows the configuration of the DSP Rx 28 provided in the subscriber device 201-1a.

[0064] The DSP Rx 28 differs from the DSP Rx 27-1a shown in FIG. 9 in that it further includes a first averaging processor 281, a frequency shifter 282, a second averaging processor 283, and a monitor and control signal demodulator 284. The first averaging processor 281 receives the received digital signal converted into the frequency domain by the Fourier transformer 272 multiple times. The first averaging processor 281 averages the received digital signals to detect with high precision the beat component (B2) of the local light (P1) and the reference light (P3), i.e., the frequency position of the frequency offset detection component. The first averaging processor 281 calculates the amount of optical frequency offset as a detected value f ｃ +f ｍ -f ｃ ' to the offset (f ｍ ) is subtracted. ｃ f ０ +f ｍ (The optical frequency of the reference light is set to f ０ When the frequency is set to the high frequency side relative to f, the first averaging processor 281 adds an offset (f ｍ ) is added to calculate the

[0065] For reception of the downstream supervisory control signal, detection accuracy can also be improved by using averaging processing. The Fourier transformer 272 converts the received digital signal into the frequency domain, separates the beat component (B2) of the local oscillator light (P1) and reference light (P3) from other received SCs, and outputs the separated beat component (B2) to the frequency shifter 282. The frequency shifter 282 converts the separated beat component (B2) into a baseband signal in the time domain, and outputs the signal to the second averaging processor 283. The supervisory control information written in the supervisory control signal is not updated frequently, and the same information is transmitted repeatedly for most of the time.

[0066] FIG. 17 is a diagram showing an example of a frame of a monitor / control signal. For example, as shown in FIG. 17, the payload of the monitor / control signal is divided into frames with a period T. A monitor / control device that superimposes a monitor / control signal on a reference light repeatedly writes the same information in the payload and transmits it. The processing time of the DSP is much shorter than the update frequency of the monitor / control signal. Therefore, the second averaging unit 283 performs averaging on a frame-by-frame basis, enabling highly sensitive reception of the repeatedly transmitted monitor / control signal. This averaging is performed in the time domain, unlike the detection of the optical frequency offset. In this case, frame synchronization is required to accurately determine the start position of the payload of each frame. Therefore, for example, a preamble is added to the start of each frame. The second averaging unit 283 determines the start position by performing autocorrelation using the preamble added to each frame. This averaging may be performed across multiple frames included in the data range processed by the Fourier transformer 272 at one time, or across multiple frames included in the data range processed by the Fourier transformer 272 multiple times. The monitor and control signal demodulator 284 demodulates the frames averaged by the second averaging processor 283 to obtain a monitor and control signal.

[0067] In (Process 2), the subscriber unit 201-nb may receive reference light from the reference light source 4b, but does not use it. The transmission system 11 may also execute (Process 1) to (Process 3) by switching the subscriber unit 201-na and the subscriber unit 201-nb. When the transmission system 12 shown in FIG. 6 executes the above-described (Process 1) to (Process 3), the subscriber units 202-na and 202-nb perform the same processing as those of the subscriber units 201-na and 201-nb, the processing of the optical coupler 31a and the processing of the optical splitter 32a are performed by the optical coupler / splitter 33a, and the processing of the optical coupler 31b and the processing of the optical splitter 32b are performed by the optical coupler / splitter 33b.

[0068] According to the above-described embodiment, even when multiple communication devices perform optical SCM transmission using multiple independent light sources, the optical SC signals transmitted from each communication device do not overlap in the optical frequency domain, so that signal performance is not degraded.

[0069] According to the embodiment described above, the transmission system includes a plurality of first communication devices, a first branching unit, and a first coupling unit. For example, the plurality of first communication devices correspond to the subscriber devices 2, 201-1a to 201-Na, and 202-1a to 202-Na in the embodiment, the first branching unit corresponds to the optical branching units 3 and 32a and the optical coupling branching unit 33a in the embodiment, and the first coupling unit corresponds to the optical coupler 31a and the optical coupling branching unit 33a in the embodiment. The plurality of first communication devices output optical subcarrier signals of different subcarriers superimposed on light of a common carrier frequency. The common carrier frequency is, for example, an optical frequency grid (f ０). The first branching unit branches the reference light and transmits the branched reference light to a plurality of first communication devices. The first coupling unit generates a first optical subcarrier multiplexed signal by combining optical subcarrier signals output from each of the plurality of first communication devices. The first communication device includes a first light source, a first receiving unit, a first wavelength control unit, and a first transmitting unit. For example, the first receiving unit corresponds to the coherent receiver 26 and the DSP Rx 27 in the embodiment, and the first transmitting unit corresponds to the DSP Tx 23 and the optical modulator 24. The first light source outputs a first local optical signal. The first receiving unit receives the reference light branched by the first branching unit. The first wavelength control unit controls the first light source to output a first local optical signal having a carrier frequency based on a difference between the optical frequency of the first local optical signal and the optical frequency of the reference light received by the first receiving unit. The first transmitting unit generates an optical subcarrier signal by superimposing a subcarrier signal different from that of the other first communication devices on the first local optical signal output by the first light source, and outputs the generated optical subcarrier signal.

[0070] The transmission system may further include a plurality of second communication devices and a second branching unit. For example, the plurality of second communication devices correspond to subscriber units 201-1b to 201-Nb and 202-1b to 202-Nb, and the second branching unit corresponds to the optical branching unit 32b and the optical coupling branching unit 33b in the embodiment. The plurality of second communication devices face each of the plurality of first communication devices. The second branching unit branches the first optical subcarrier multiplexed signal and transmits the branched first optical subcarrier multiplexed signal to the plurality of second communication devices. The second communication device includes a second light source, a second receiving unit, and a second wavelength control unit. For example, the second receiving unit corresponds to the coherent receiver 26 and the DSP Rx 27 in the embodiment. The second light source outputs a second local light. The second receiving unit performs reception processing of the first optical subcarrier multiplexed signal using the second local light, and extracts the optical subcarrier signal from the facing first communication device. The second wavelength control unit controls the second light source to output the second local light of the carrier frequency based on the difference between the optical frequency of the second local light and the optical frequency of the first local light on which the optical subcarrier signal extracted by the second receiving unit is superimposed.

[0071] The second communication device may further include a second transmitter. For example, the second transmitter corresponds to the DSP Tx 23 and the optical modulator 24. The second transmitter generates an optical subcarrier signal by superimposing a subcarrier signal different from that of the other second communication devices on a second local light output by the second light source, and outputs the generated optical subcarrier signal. The transmission system further includes a second coupler. For example, the second coupler corresponds to the optical coupler 31b and the optical coupler / branch 33b in the embodiment. The second coupler generates a second optical subcarrier multiplexed signal by combining optical subcarrier signals output from each of the multiple second communication devices. The first branching unit branches the reference light and the second optical subcarrier multiplexed signal and transmits them to the multiple first communication devices. The first receiving unit of the first communication device performs reception processing of the reference light and the second optical subcarrier multiplexed signal using the first local light and extracts the optical subcarrier signal from the opposing second communication device. The first wavelength control unit of the first communication device controls the first light source to output a first local light of a carrier frequency, based on the optical frequency of the beat component between the reference light and the first local light obtained by the reception processing.

[0072] The first communication device may be connected to the first branching section and the first coupling section by a two-core transmission line, and the second communication device may be connected to the second branching section and the second coupling section by a two-core transmission line.

[0073] The transmission system may include a first optical coupling / branching device that operates as a first branching unit and a first coupling unit, and a second optical coupling / branching device that operates as a second branching unit and a second coupling unit. The first communication device and the first optical coupling / branching device are connected by a single-core transmission line, and the second communication device and the second optical coupling / branching device are connected by a single-core transmission line. The multiple first communication devices output optical subcarrier signals superimposed on one sideband of the carrier frequency, and the multiple second communication devices output optical subcarrier signals superimposed on the other sideband of the carrier frequency.

[0074] 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.

[0075] 1, 11, 12 Transmission system 2-1 to 2-N Subscriber device 3 Optical splitter 4, 4a, 4b Reference light source 5, 51, 52 Transmission path 21, 21-1a, 21-1b Light source 22, 22-1a, 22-1b Wavelength controller 23, 23-1a, 23-1b Transmitting side digital signal processing unit 24, 24-1a, 24-1b Optical modulator 25-1a, 25-1b Optical circulator 26, 26-1a, 26-1b Coherent receiver 27, 27-1a, 27-1b, 28 Receiving side digital signal processing unit 31a, 31b Optical coupler 32a, 32b Optical splitter 33a, 33b Optical coupler splitter 201-1a to 201-Na, 201-1b to 201-Nb: subscriber units 202-1a to 202-Na, 202-1b to 202-Nb: subscriber units 271-1 to 271-4: analog-to-digital converters 272: Fourier transformers 273-1 to 273-N: frequency shifters 274-1 to 274-N: equalizers 275-1 to 275-N: compensation units 281: first averaging processing unit 282: frequency shifters 283: second averaging processing unit 284: supervisory control signal demodulation unit 910: station equipment 911, 921: light sources 912, 922: wavelength controllers 913, 923: transmitting-side digital signal processing units 914, 924: optical modulators 915, 925: optical circulators 916, 926 Coherent receiver 917, 927 Receiving side digital signal processing unit 920-1 to 920-N Subscriber device 930 Optical coupling splitter

Claims

a plurality of first communication devices that output optical subcarrier signals each having a different subcarrier superimposed on light of a common carrier frequency; a first branching unit that branches a reference light and transmits the branched reference light to a plurality of the first communication devices; a first combining unit that generates a first optical subcarrier multiplexed signal by combining the optical subcarrier signals output from each of the plurality of first communication devices, The first communication device a first light source that outputs a first local light; a first receiving unit that receives the reference light branched by the first branching unit; a first wavelength control unit that controls the first light source to output a first local oscillator light having the carrier frequency based on a difference between an optical frequency of the first local oscillator light and an optical frequency of the reference light received by the first receiving unit; a first transmitter that generates an optical subcarrier signal by superimposing a signal of a subcarrier different from that of other first communication devices on the first local light output from the first light source, and outputs the generated optical subcarrier signal, Transmission system. 　 The transmission system comprises: a plurality of second communication devices that face the plurality of first communication devices, respectively; a second branching unit that branches the first optical subcarrier multiplexed signal and transmits the branched first optical subcarrier multiplexed signal to a plurality of the second communication devices; The second communication device a second light source that outputs a second local light; a second receiving unit that performs a receiving process of the first optical subcarrier multiplexed signal using the second local light and extracts the optical subcarrier signal from the opposing first communication device; a second wavelength control unit that controls the second light source to output second local light of the carrier frequency based on a difference between the optical frequency of the second local light and the optical frequency of the first local light on which the optical subcarrier signal extracted by the second receiving unit is superimposed; Equipped with The transmission system according to claim 1 . 　 The second communication device a second transmitter that generates an optical subcarrier signal by superimposing a signal of a subcarrier different from that of other second communication devices on the second local light output from the second light source, and outputs the generated optical subcarrier signal; The transmission system comprises: a second combining unit that combines the optical subcarrier signals output from each of the plurality of second communication devices to generate a second optical subcarrier multiplexed signal; the first branching unit branches the reference light and the second optical subcarrier multiplexed signal and transmits the branched reference light and the second optical subcarrier multiplexed signal to a plurality of the first communication devices; the first receiving unit performs a receiving process of the reference light and the second optical subcarrier multiplexed signal using the first local light, and extracts the optical subcarrier signal from the opposing second communication device; the first wavelength control unit controls the first light source to output a first local light having the carrier frequency, based on an optical frequency of a beat component between the reference light and the first local light obtained by the receiving process.

3. The transmission system according to claim 2. 　 the first communication device is connected to the first branching unit and the first coupling unit by a two-core transmission line; The transmission system according to claim 1 . 　 The transmission system comprises: a first optical coupler / splitter that operates as the first branching unit and the first coupling unit; a second optical coupler / splitter that operates as the second branching unit and the second coupling unit, the first communication device and the first optical coupling / branching device are connected by a single-core transmission line, the second communication device and the second optical coupling / branching device are connected by a single-core transmission line, the plurality of first communication devices output the optical subcarrier signal superimposed on one sideband of the carrier frequency; the plurality of second communication devices output the optical subcarrier signal superimposed on the other sideband of the carrier frequency; 4. The transmission system according to claim 3. 　 a branching step in which a branching unit branches the reference light and transmits the branched reference light to a plurality of communication devices; a receiving step in which the communication device receives the reference light branched in the branching step; a control step in which the communication device controls the light source of the communication device to output local light having a carrier frequency common to light sources of other communication devices, based on a difference between the optical frequency of local light output by the light source of the communication device itself and the optical frequency of the reference light received in the receiving step; an output step in which the communication device generates an optical subcarrier signal by superimposing a signal of a subcarrier different from that of the other communication devices on the local light output from the light source of the communication device itself, and outputs the generated optical subcarrier signal; a combining step in which a combining unit generates an optical subcarrier multiplexed signal by combining the optical subcarrier signals output from each of the plurality of communication devices; An optical communication method comprising:

Images