Optical communication device, optical communication system, and optical communication method

Abstract

One embodiment of the present invention is an optical communication instrument that uses heterodyne detection and comprises: a first polarization separator that separates a received optical signal into orthogonal polarization components; a second polarization separator that separates continuous light of a single wavelength into orthogonal polarization components; an optical splitter that splits each of the orthogonal polarization component separated by the first polarization separator and the orthogonal polarization component separated by the second polarization separator into two branches; and an optical receiver that converts eight polarization components, which are obtained by the optical splitter splitting into two branches, into an electric signal for each combination of the two branches and generates four beat components in total.

Description

Optical communication device, optical communication system, and optical communication method

[0001] The present invention relates to techniques for an optical communication device, an optical communication system, and an optical communication method.

[0002] Coherent optical transceivers are used as optical communication devices. FIG. 18 is a diagram showing the configuration of a conventional optical transceiver (see, for example, Non-Patent Document 1). The optical transceiver 900 includes a polarization controller 901, a modulator 902, a Tx DSP (Digital Signal Processor; transmitting-side digital signal processing unit) 903, a coherent receiver 904, and an Rx DSP (receiving-side digital signal processing unit) 905. In addition, in the optical transceiver 900, continuous light output from a light source 911 is sent to the polarization controller 901 via an optical fiber cable 912. Note that if the polarization controller 901 is not included, the continuous light output from the light source is split, with one branch being sent to the modulator 902 and the other being sent to the coherent receiver 904.

[0003] The continuous light sent to the polarization controller 901 is polarization-controlled and split, with one branch being sent to a modulator 902 and the other branch being sent to a coherent receiver 904. The continuous light sent to the modulator 902 is modulated with transmission data to generate a transmission optical signal. For example, in ultra-high-speed optical transmission at 100 Gb / s or higher, an IQ modulator capable of bipolarization modulation is used as the modulator. The Tx DSP 903 generates IQ signals (XI, XQ, YI, YQ) for each orthogonal polarization from the transmission data, and drives the IQ modulator with these four IQ signals to generate a transmission optical signal. Meanwhile, the received optical signal is input to the coherent receiver 904, where the split continuous light is received as local light by optical intradyne detection. The coherent receiver 904 is a polarization diversity 90-degree optical hybrid that outputs IQ components (XI, XQ, YI, YQ) for each orthogonal polarization. The Rx DSP 905 demodulates these four components by digital signal processing to obtain received data.

[0004] The coherent optical transceiver 904 is equipped with a high-power tunable laser as the light source 911. High-power characteristics are necessary to ensure sufficient transmission optical signal power and local light power. Furthermore, wavelength tunability is a function for setting an arbitrary grid wavelength during wavelength division multiplexing (WDM) transmission. However, high-density WDM transmission requires high wavelength stability in addition to wavelength tunability. Since it is difficult to ensure reliability when the coherent optical transceiver 904 is operated in a high-temperature environment, such as near an electrical switch or inside a server, the light source 911 is installed externally, as shown in Figure 18. Since the light source 911 is not installed in a high-temperature environment, reliability issues associated with high-power tunable lasers can be avoided. In this configuration, because the modulator 902 and coherent receiver 904 are polarization-dependent, a polarization controller 901 is installed to automatically track the polarization of the continuous light so that it aligns with the incident polarization axis of the modulator 902 and coherent receiver 904. A compact polarization controller has been proposed for self-coherent optical transmission, which simultaneously transmits signal light and local light, and which automatically tracks the polarization of the local light on the receiving side (see, for example, Non-Patent Document 2).

[0005] Real-Time Demonstration of Homodyne Coherent Bidirectional Transmission for Next-Generation Data Center Interconnects,” J. Light. Technol., Vol. 39, No. 4, pp.1231-1238, 2021

[0006] However, when a light source is placed outside the optical transceiver to allow for use in high-temperature environments, as in the prior art, there is a problem that the automatic tracking function of the polarization controller may not operate properly under conditions where severe polarization fluctuations occur in the transmission path. In view of the above circumstances, the present invention aims to provide a technology that enables coherent reception regardless of the input polarization state of the local light, even when a light source is remotely supplied to a coherent receiver. Note that while a polarization-independent device such as an electroabsorption modulator (EAM) can be used as a modulator, a polarization diversity receiver is essential for coherent reception, and therefore the present invention aims to solve the problems of coherent receivers.

[0007] One aspect of the present invention is an optical communication device using heterodyne detection, comprising: a first polarization separator that separates a received optical signal into orthogonal polarization components; a second polarization separator that separates continuous light of a single wavelength into orthogonal polarization components; an optical branching device that branches each of the orthogonal polarization components separated by the first polarization separator and the orthogonal polarization components separated by the second polarization separator into two; and an optical receiver that converts the eight polarization components obtained by the branching into electrical signals for each combination of the two branches, generating a total of four beat components.

[0008] One aspect of the present invention is an optical communication device using intradyne detection, comprising: a first polarization separator that separates a received optical signal into orthogonal polarization components; a second polarization separator that separates continuous light of a single wavelength into orthogonal polarization components; a first optical splitter and a second optical splitter that each split the orthogonal polarization components separated by the first polarization separator into four; a third optical splitter and a fourth optical splitter that each split the orthogonal polarization components separated by the second polarization separator into four and output the remaining two optical phases by π / 2 delay with respect to two of the split components; and an optical receiver that converts the 16 polarization components obtained by the four splits by the first optical splitter to the fourth optical splitter into electrical signals for each combination of one of the orthogonal polarization components separated by the first polarization separator and one of the orthogonal polarization components separated by the second polarization separator, with or without a π / 2 delay, to generate eight beat components.

[0009] One aspect of the present invention is an optical communication system comprising: N (N is an integer equal to or greater than 2) light sources that emit continuous light of two different optical frequencies; a first wavelength multiplexer that multiplexes the continuous light of the two optical frequencies input from the N light sources; a first wavelength demultiplexer that receives the continuous light of the two optical frequencies combined from the first wavelength multiplexer and demultiplexes the input continuous light of the two optical frequencies to N first optical communication devices; N first optical communication devices that receive each of the continuous light containing components of two optical frequencies from the first wavelength demultiplexer and output one of the input continuous light; a second wavelength multiplexer that multiplexes one of the continuous light outputs from each of the N first optical communication devices; and a second wavelength demultiplexer that receives the combined optical signal output from the second wavelength multiplexer and demultiplexes the input combined optical signal to N second optical communication devices.

[0010] One aspect of the present invention is an optical communication method for an optical communication device using heterodyne detection, in which a first polarization separator separates a received optical signal into orthogonal polarization components, a second polarization separator separates continuous light of a single wavelength into orthogonal polarization components, an optical splitter splits each of the orthogonal polarization components separated by the first polarization separator and the orthogonal polarization components separated by the second polarization separator into two, and an optical receiver converts the eight polarization components obtained by the splitting by the optical splitter into electrical signals for each combination of the two splits, thereby generating a total of four beat components.

[0011] One aspect of the present invention is an optical communication method for an optical communication device using intradyne detection, in which a first polarization splitter splits a received optical signal into orthogonal polarization components, a second polarization splitter splits continuous light of a single wavelength into orthogonal polarization components, a first optical splitter and a second optical splitter each split the orthogonal polarization components separated by the first polarization splitter into four, a third optical splitter and a fourth optical splitter each split the orthogonal polarization components separated by the second polarization splitter into four and output the remaining two optical phases by delaying two of the split optical phases by π / 2, and an optical receiver converts the 16 polarization components obtained by the first to fourth optical splitters into four into electrical signals for each combination of one of the orthogonal polarization components separated by the first polarization splitter and one of the orthogonal polarization components separated by the second polarization splitter with and without a π / 2 delay, thereby generating eight beat components.

[0012] According to the present invention, even when a light source is remotely supplied to a coherent receiver, coherent reception can be performed regardless of the input polarization state of the local light.

[0013] 1 is a diagram illustrating an example of the configuration of an optical communication device according to an embodiment. FIG. 1 is a diagram illustrating an example of the configuration of a receiver according to a first embodiment. FIG. 2 is a diagram illustrating the configuration of a receiver using heterodyne detection according to a first comparative example. FIG. 3 is a flowchart illustrating processing by the receiver according to the first embodiment. FIG. 4 is a diagram illustrating an example of the configuration of an optical communication device in the case of intradyne detection. FIG. 5 is a diagram illustrating an example of the configuration of a receiver according to a second embodiment. FIG. 6 is a flowchart illustrating processing by the receiver according to the second embodiment. FIG. 7 is a diagram illustrating a first example of the configuration of an optical communication system. FIG. 8 is a diagram illustrating a second example of the configuration of an Rx DSP of an optical communication device. FIG. 9 is a diagram illustrating a first example of the configuration of an Rx DSP of an optical communication device. FIG. 10 is a diagram illustrating a fourth example of the configuration of an Rx DSP of an optical communication device. FIG. 11 is a diagram illustrating an example of the configuration of an optical communication system in the case of communicating with a plurality of opposing optical communication devices using supplied light. FIG. 12 is a diagram illustrating the transmission characteristics of a wavelength multiplexer (S+L) and wavelength demultiplexer (S+L) that multiplex and demultiplex WDM light, and the wavelength multiplexer (S) and wavelength demultiplexer (S). 1 is a diagram showing the transmission characteristics of a wavelength multiplexer (S+L) and a wavelength demultiplexer (S+L) that multiplex and demultiplex WDM light when m = 2, and a wavelength multiplexer (S) and a wavelength demultiplexer (S).

[0014] An embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing an example of the configuration of an optical communication device according to this embodiment. As shown in Fig. 1, the optical communication device 1 includes, for example, a wavelength demultiplexer 4, a Tx DSP 5, a modulator 6, a receiver 7 (optical communication device), and an Rx DSP 8.

[0015] A two-wavelength light source 2 is connected to the optical communication device 1, which receives transmission data and a reception optical signal and outputs the transmission optical signal and the reception data.

[0016] The two-wavelength light source 2 emits light at a frequency f Ｓ and optical frequency f Ｌ The continuous light is emitted at a frequency of f Ｓ and optical frequency f Ｌ The continuous light is input to the optical communication device 1.

[0017] The wavelength demultiplexer 4 demultiplexes the optical frequency fＳ and optical frequency f Ｌ Continuous light with optical frequency f Ｓ Continuous light and optical frequency f Ｌ and the optical frequency f Ｓ The continuous light of the optical frequency f Ｌ A continuous beam of light is output to the receiver.

[0018] The Tx DSP 5 is a transmitting-side digital signal processor that generates IQ signals (XI, XQ, YI, YQ) for each orthogonal polarization from the transmission data and outputs the four generated IQ signals to the modulator 6.

[0019] The modulator 6 converts the IQ signal input from the Tx DSP 5 into an optical frequency f Ｓ The modulated optical signal is output to, for example, another optical communication device.

[0020] The receiver 7 is a coherent receiver. The receiver 7 receives an optical signal from another optical communication device, for example, and converts the received optical signal into an optical frequency f Ｌ The optical signal is coherently detected using the optical signal, and IQ components (XI, XQ, YI, YQ) are output to the Rx DSP 8 for each orthogonal polarization.

[0021] The Rx DSP 8 is a receiving-side digital signal processing unit that demodulates the IQ components output by the receiver 7 through digital signal processing to obtain received data.

[0022] 1 is an example, and the optical communication device 1 may include other components. An example of the configuration of the receiver 7 of the optical communication device 1 shown in FIG. 1 will be described below.

[0023] In the first embodiment, optical heterodyne detection is used when performing coherent reception using a remotely supplied continuous light as a local light. Optical heterodyne detection is a detection technique in which continuous light of an optical frequency different from that of the optical signal is used as the local light, and the optical signal is read from the beat component generated in the intermediate frequency band by the optical interference between the two.

[0024] Fig. 2 is a diagram showing an example of the configuration of a receiver according to the first embodiment. As shown in Fig. 2, the receiver 7A (optical communication device) includes, for example, a first polarization separator 11-1, a second polarization separator 11-2, optical branching units 12 (12-1 to 12-4), optical coupling branching units 13-1 to 13-4, a first balanced receiver 14-1 (receiver), a second balanced receiver 14-2 (receiver), a third balanced receiver 14-3 (receiver), a fourth balanced receiver 14-4 (receiver), and ADCs (Analog to Digital Converters) 15-1 to 15-4.

[0025] The receiver 7A does not necessarily have to include ADC 15-1, ADC 15-2, ADC 15-3, and ADC 15-4. In this case, ADC 15-1, ADC 15-2, ADC 15-3, and ADC 15-4 may be connected to the receiver 7A'. That is, the optical communication device 1 may include ADC 15-1 to 15-4, or ADC 15-1 to 15-4 may be connected to the outside.

[0026] The first polarization separator 11-1 receives the received optical signal (f Ｓ The first polarization separator 11-1 receives the received optical signal (f Ｓ ) into orthogonal polarization components (X and Y).

[0027] The second polarization separator 11-2 receives continuous light (f Ｌ ) is input via an optical transmission line 3 (for example, an optical fiber cable). The second polarization separator 11-2 divides the input continuous wave light (f Ｌ ) into orthogonal polarization components (X and Y).

[0028] Here, the four components generated by the polarization separator, that is, the X polarization component of the received optical signal (signal light), the Y polarization component of the signal light, the X polarization component of the continuous light (local light), and the Y polarization component of the local light, are denoted as Sx, Sy, Lx, and Ly, respectively.

[0029] Optical splitter 12-1 splits the X-polarized component output by first polarization separator 11-1 into two, inputs one of the split X-polarized components Sx to optical coupling splitter 13-1, and inputs the other split X-polarized component Sx to optical coupling splitter 13-2.

[0030] The optical splitter 12-2 splits the Y-polarized component output by the first polarization separator 11-1 into two, inputs one of the split Y-polarized components Sy to the optical coupling splitter 13-3, and inputs the other split Y-polarized component Sy to the optical coupling splitter 13-4.

[0031] The optical splitter 12-3 splits the X-polarized component output by the second polarization separator 11-2 into two, and inputs one of the split X-polarized components Lx to the optical coupling splitter 13-1, and inputs the other split X-polarized component Lx to the optical coupling splitter 13-3.

[0032] The optical splitter 12-4 splits the Y-polarized component output by the second polarization separator 11-2 into two, and inputs one of the split Y-polarized components Ly to the optical coupling splitter 13-2, and inputs the other split Y-polarized component Ly to the optical coupling splitter 13-4.

[0033] The optical coupler / splitter 13-1 couples the input X-polarized component Sx and X-polarized component Lx, then branches the combined signal, and inputs the branched output to the first balanced receiver 14-1. The optical coupler / splitter is, for example, a 2x2 3 dB coupler, and is a module that splits an optical signal propagating through an optical transmission line into two or couples two optical signals into one fiber.

[0034] The optical coupler / splitter 13-2 couples the input X-polarized component Sx and Y-polarized component Ly, then branches them, and inputs the branched output to the second balanced receiver 14-2.

[0035] The optical coupler / splitter 13-3 couples the input Y-polarized component Sy and X-polarized component Lx, then branches them, and inputs the branched output to a third balanced receiver 14-3.

[0036] The optical coupler / splitter 13-4 couples the input Y-polarized wave components Sy and Ly, then branches them, and inputs the branched output to a fourth balanced receiver 14-4.

[0037] The first balanced receiver 14-1 receives the output branched by the optical coupling brancher 13-1 in a balanced manner, converts it into an analog electrical signal, and outputs it. Note that the balanced receiver is a module that differentially receives two input signals, and is used, for example, to receive optical phase-modulated optical signals in digital coherent communications (see, for example, Reference 1).

[0038] Reference 1: Satoshi Furuta, Kaoru Yoshino, et al., "Balanced Photodiode Module Technology," NTT Technical Journal, November 2007, pp. 60-61, 2007

[0039] The second balanced receiver 14-2 receives the output branched by the optical coupling brancher 13-2 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0040] The third balanced receiver 14-3 receives the output branched by the optical coupling brancher 13-3 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0041] The fourth balanced receiver 14-4 receives the output branched by the optical coupling brancher 13-4 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0042] The ADC 15-1 converts the analog electrical signal output by the first balanced receiver 14-1 into a digital signal and outputs the Sx-Lx beat component.

[0043] The ADC 15-2 converts the analog electrical signal output by the second balanced receiver 14-2 into a digital signal and outputs the Sx-Ly beat component.

[0044] The ADC 15-3 converts the analog electrical signal output by the third balanced receiver 14-3 into a digital signal and outputs the beat component of Sy-Lx.

[0045] The ADC 15-4 converts the analog electrical signal output by the fourth balanced receiver 14-4 into a digital signal and outputs the Sy-Ly beat component.

[0046] The configuration shown in Figure 2 allows optical heterodyne detection. It is also possible to use a lower-cost optical-to-electrical converter instead of the balanced receiver. When using an optical-to-electrical converter, the optical splitter may also be replaced with an optical coupler (e.g., a 2x1 type 3 dB coupler), and the output obtained by combining the two input beat components may be input to the optical-to-electrical converter.

[0047] 3 is a diagram showing the configuration of a receiver using heterodyne detection according to Comparative Example 1. The receiver 910 using heterodyne detection according to Comparative Example 1 includes a first polarization separator 911-1, a second polarization separator 911-2, an optical coupling splitter 912-1, an optical coupling splitter 912-2, a first balanced receiver 913-1, a second balanced receiver 913-2, an ADC 914-1, and an ADC 914-2.

[0048] The first polarization separator 911-1 receives the received optical signal (f Ｓ The second polarization separator 911-2 receives continuous light (f Ｌ) is input. In the first comparative example, in order to perform polarization diversity reception, the second polarization separator 912-2 is used to separate the local light into two orthogonal polarization components. At this time, since the light source 915 and the second polarization separator 911-2 are close to each other, they are connected with a polarization-maintaining optical fiber cable to maintain the polarization state of the local light input to the second polarization separator 911-2, and the optical power of the orthogonal polarization components output from the second polarization separator 911-2 is always kept equal. As a result, regardless of the polarization state of the received optical signal that reaches the first polarization separator 911-1, a beat component (Sx-Lx beat component or Sy-Ly beat component) with at least one of the orthogonal polarization components of the local light is generated, making signal reception possible. However, if the light source 915 and the second polarization separator 911-2 are located remotely, it may happen that only the optical power of Lx is output and the optical power of Ly is zero, or vice versa. In the first comparative example, if only Lx optical power is output, Ly optical power is zero, and the received optical signal has only a Y polarization component, or if only Ly optical power is output, Lx optical power is zero, and the received optical signal has only an X polarization component, neither the Sx-Lx beat component nor the Sy-Ly beat component is output, and signal reception is not possible.

[0049] In contrast, in this embodiment, the configuration of Figure 2 generates an Sx-Lx beat component and a Sy-Ly beat component in addition to an Sx-Lx beat component and a Sy-Ly beat component. As a result, according to this embodiment, even if only Lx optical power is output and Ly optical power is zero, and the received optical signal has only a Y polarization component, the Sx-Ly beat component is generated, making it possible to receive a signal. Furthermore, according to this embodiment, even if only Ly optical power is output and Lx optical power is zero, and the received optical signal has only an X polarization component, the Sy-Lx beat component is generated, making it possible to receive a signal.

[0050] FIG. 4 is a flowchart showing the processing of the receiver of this embodiment.

[0051] (Step S1) The first polarization separator 11-1 separates the received optical signal into orthogonal polarization components (Sx and Sy).

[0052] (Step S2) The second polarization separator 11-2 separates the continuous light of a single wavelength into orthogonal polarization components (Lx and Ly).

[0053] (Step S3) The first optical branching device 12-1 to the fourth optical branching device 12-4 branch each of Sx, Sy, Lx, and Ly into two.

[0054] (Step S4) The first optical receiver (first balanced receiver 14-1) to the fourth optical receiver (fourth balanced receiver 14-4) convert the eight polarization components obtained by splitting into two by the first optical splitter 12-1 to the fourth optical splitter 12-4 into electrical signals for each combination of Sx and Lx, Sx and Ly, Sy and Lx, and Sy and Ly, thereby generating a total of four beat components.

[0055] As shown in FIG. 2, this embodiment includes a first polarization separator 11-1 that separates a received optical signal into orthogonal polarization components (Sx and Sy), a second polarization separator 11-2 that separates continuous light of a single wavelength into orthogonal polarization components (Lx and Ly), a first optical branching device 12-1 to a fourth optical branching device 12-4 that branch Sx, Sy, Lx, and Ly into two components, respectively, and a first optical receiver (first balanced receiver 14-1) to a fourth optical receiver (fourth balanced receiver 14-4) that convert the eight polarization components obtained by branching into two components by the first optical branching device 12-1 to the fourth optical branching device 12-4 into electrical signals for each combination of Sx and Lx, Sx and Ly, Sy and Lx, and Sy and Ly, and generate a total of four beat components.

[0056] As a result, according to this embodiment, in polarization diversity optical heterodyne detection, eight polarization components obtained by splitting each of the four outputs of two polarization separators into two are combined with each other to generate Sx-Lx beat components and Sy-Ly beat components, as well as Sx-Ly beat components and Sy-Lx beat components.As a result, according to this embodiment, optical coherent reception that is not dependent on the input polarization state of the local light becomes possible.

[0057] Second Embodiment In a second embodiment, optical intradyne detection is used. Note that optical intradyne detection is detection that does not use an optical PLL (Phase Locked Loop), and the frequency f Ｓ and local oscillator light f ＬThe frequency of the frequency condition f Ｓ ≒f Ｌ In this state, two signals (I signal and Q signal) are extracted in the electrical domain from the received optical signal using local light that is 90 degrees out of phase with each other.

[0058] 5 is a diagram showing an example of the configuration of an optical communication system in the case of intradyne detection. As shown in FIG. 5, the optical communication system 10 includes, for example, optical communication device 1B-1, optical communication device 1B-2, light source 2A-1, light source 2A-2, and an optical transmission path 3 (e.g., an optical fiber cable). Furthermore, the optical communication device 1B (1B-1, 1B-2) includes, for example, an optical branching device 9, a Tx DSP 5, a modulator 6, a receiver 7B (optical communication device), and an Rx DSP 8. In the optical communication system 10, the light source 2A (2A-1, 2A-2) may be shared between the optical communication devices 1B-1 and 1B-2.

[0059] The optical splitter 9 splits the optical frequency f ｃ The continuous light is split into two, and the split outputs are input to the modulator 6 and the receiver 7B. In this way, the optical frequency in the optical intradyne detection is set to f ｃ Let's say.

[0060] The optical communication device 1B-1 receives an optical frequency f ｃ The optical communication device 1B-2 receives a continuous light beam of optical frequency f from a light source 2A-2. The optical communication device 1B-2 receives a continuous light beam of optical frequency f from a light source 2A-2. The optical communication device 1B-2 receives an optical transmission signal of optical frequency f from a light source 2A-2. ｃ The continuous light is input via the optical transmission path 3, transmission data is input from an external device, the optical transmission signal output by the optical communication device 1B-1 is input as a received optical signal via the optical transmission path 3, and the transmitted optical signal is output to the optical communication device 1B-1.

[0061] 6 is a diagram illustrating an example of the configuration of a receiver according to the present embodiment. As shown in FIG. 6, a receiver 7B (optical communication device) includes, for example, a first polarization separator 21-1, a second polarization separator 21-2, optical branching units 22-1 to 22-4, optical branching units 23-1 to 23-8, π / 2 delay units 24-1 to 24-4, optical coupling branching units 25-1 to 25-8, a first balanced receiver 26-1 (receiver), a second balanced receiver 26-2 (receiver), a third balanced receiver 26-3 (receiver), a fourth balanced receiver 26-4 (receiver), a fifth balanced receiver 26-5 (receiver), a sixth balanced receiver 26-6 (receiver), a seventh balanced receiver 26-7 (receiver), an eighth balanced receiver 26-8 (receiver), and ADCs 27-1 to 27-8.

[0062] In the configuration of FIG. 6, optical intradyne detection is used, and therefore, unlike optical heterodyne detection, Ｓ ≒f Ｌ The first polarization separator 21-1 receives the received optical signal (f ｃ The first polarization separator 21-1 receives the received optical signal (f ｃ ) into orthogonal polarization components (X and Y).

[0063] The second polarization separator 21-2 receives continuous light (f ｃ ) is input via an optical transmission line 3 (for example, an optical fiber cable). The second polarization separator 21-2 divides the input continuous wave light (f ｃ ) into orthogonal polarization components (X and Y).

[0064] Optical splitter 22-1 (first optical splitter) splits the X-polarized component output by first polarization separator 21-1 into two, inputs one of the split X-polarized components Sx to optical splitter 23-1, and inputs the other split X-polarized component Sx to optical splitter 23-2.

[0065] Optical splitter 22-2 (second optical splitter) splits the Y-polarized component output by first polarization separator 21-1 into two, inputs one of the split Y-polarized components Sy to optical splitter 23-3, and inputs the other split Y-polarized component Sy to optical splitter 23-4.

[0066] Optical splitter 22-3 (third optical splitter) splits the X-polarized component output by second polarization separator 21-2 into two, inputs one of the split X-polarized components Lx to optical coupling splitter 23-5, and inputs the other split X-polarized component Lx to optical splitter 23-6.

[0067] The optical splitter 22-4 (fourth optical splitter) splits the Y-polarized component output by the second polarization separator 21-2 into two, inputs one of the split Y-polarized components Ly to the optical coupling splitter 23-7, and inputs the other split Y-polarized component Ly to the optical splitter 23-8.

[0068] The π / 2 delay devices 24 (24-1 to 24-4) are used to extract I and Q signals from the received optical signal using local light that is out of phase with each other by 90 degrees (π / 2).

[0069] The optical branching device 23-1 (first optical branching device) branches the X-polarized component Sx input from the optical branching device 22-1 into two, inputs one of the branched Sx to the optical coupling branching device 25-1, and inputs the other branched Sx to the optical coupling branching device 25-2.

[0070] The optical splitter 23-2 (first optical splitter) splits the X-polarized component Sx input from the optical splitter 22-1 into two, and inputs one of the splits to the optical coupling splitter 25-3 and the other to the optical coupling splitter 25-4.

[0071] The optical branching device 23-3 (second optical branching device) branches the Y polarization component Sy input from the optical branching device 22-2 into two, inputs one of the branches to the optical coupling branching device 25-5, and inputs the other branch to the optical coupling branching device 25-6.

[0072] The optical branching device 23-4 (second optical branching device) branches the Y polarization component Sy input from the optical branching device 22-2 into two, inputs one of the branches to the optical coupling branching device 25-7, and inputs the other branch to the optical coupling branching device 25-8.

[0073] The optical branching device 23-5 (third optical branching device) branches the X-polarized component Lx input from the optical branching device 22-3 into two, inputs one of the branches to an optical coupling branching device 25-1, and inputs the other branch to an optical coupling branching device 25-2 via an n / 2 delay device 24-1. The π / 2 delay device 24-1 delays the optical phase of the input signal by π / 2.

[0074] The optical branching device 23-6 (third optical branching device) branches the X-polarized component Lx input from the optical branching device 22-3 into two, inputs one of the branches to an optical coupling branching device 25-5, and inputs the other branch to an optical coupling branching device 25-6 via an n / 2 delay device 24-3. The π / 2 delay device 24-3 delays the optical phase of the input signal by π / 2.

[0075] The optical branching device 23-7 (fourth optical branching device) branches the Y polarization component Ly input from the optical branching device 22-4 into two, inputs one of the branches to an optical coupling branching device 25-3, and inputs the other branch to an optical coupling branching device 25-4 via a π / 2 delay device 24-2. The n / 2 delay device 24-2 delays the optical phase of the input signal by π / 2.

[0076] The optical branching device 23-8 (fourth optical branching device) branches the Y polarization component Ly input from the optical branching device 22-4 into two, inputs one of the branches to the optical coupling branching device 25-7, and inputs the other branch to the optical coupling branching device 25-8 via the n / 2 delay device 24-4. The n / 2 delay device 24-8 delays the optical phase of the input signal by π / 2.

[0077] The optical coupler / splitter 25-1 couples the input X-polarized wave component Sx and X-polarized wave component Lx, then branches them, and inputs the branched output to the first balanced receiver 26-1.

[0078] The optical coupler / splitter 25-2 couples the input X-polarized wave component Sx and X-polarized wave component Lx, then branches the combined wave, and inputs the branched output to the second balanced receiver 26-2.

[0079] The optical coupler / splitter 25-3 couples the input X-polarized component Sx and Y-polarized component Ly, then branches them, and inputs the branched output to a third balanced receiver 26-3.

[0080] The optical coupler / splitter 25-4 couples the input X-polarized component Sx and Y-polarized component Ly, then branches them, and inputs the branched output to a fourth balanced receiver 26-4.

[0081] The optical coupler / splitter 25-5 couples the input Y-polarized component Sy and X-polarized component Lx, then branches them, and inputs the branched output to a fifth balanced receiver 26-5.

[0082] The optical coupler / splitter 25-6 couples the input X-polarized wave component Sy and the X-polarized wave component Lx, then branches them, and inputs the branched output to a sixth balanced receiver 26-6.

[0083] The optical coupler / splitter 25-7 couples the input Y-polarized component Sy and X-polarized component Lx together, then branches them, and inputs the branched output to a seventh balanced receiver 26-7.

[0084] The optical coupler / splitter 25-8 couples the input Y-polarized wave components Sy and Ly, then branches them, and inputs the branched output to an eighth balanced receiver 26-8.

[0085] The first balanced receiver 26-1 receives the output branched by the optical coupling brancher 25-1 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0086] The second balanced receiver 26-2 receives the output branched by the optical coupling brancher 25-2 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0087] The third balanced receiver 26-3 receives the output branched by the optical coupling brancher 25-3 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0088] The fourth balanced receiver 26-4 receives the output branched by the optical coupling brancher 25-4 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0089] The fifth balanced receiver 26-5 receives the output branched by the optical coupling brancher 25-5 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0090] The sixth balanced receiver 26-6 receives the output branched by the optical coupling brancher 25-6 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0091] The seventh balanced receiver 26-7 receives the output branched by the optical coupling brancher 25-7 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0092] The eighth balanced receiver 26-8 receives the output branched by the optically coupled branching device 25-8 in a balanced manner, converts it into an analog electrical signal, and outputs it.

[0093] The ADC 27-1 converts the analog electrical signal output by the first balanced receiver 26-1 into a digital signal, and outputs the beat component (I component) of Sx-Lx (without π / 2 delay).

[0094] The ADC 27-2 converts the analog electrical signal output by the second balanced receiver 26-2 into a digital signal, and outputs the beat component (Q component) of Sx-Lx (with a π / 2 delay).

[0095] The ADC 27-3 converts the analog electrical signal output by the third balanced receiver 26-3 into a digital signal, and outputs the beat component (I component) of Sx-Ly (without π / 2 delay).

[0096] The ADC 27-4 converts the analog electrical signal output by the fourth balanced receiver 26-4 into a digital signal, and outputs the beat component (Q component) of Sx-Ly (with a π / 2 delay).

[0097] The ADC 27-5 converts the analog electrical signal output by the fifth balanced receiver 26-5 into a digital signal, and outputs the beat component (I component) of Sy-Lx (without π / 2 delay).

[0098] The ADC 27-6 converts the analog electrical signal output by the sixth balanced receiver 26-6 into a digital signal, and outputs the beat component (Q component) of Sy-Lx (with a π / 2 delay).

[0099] The ADC 27-7 converts the analog electrical signal output by the seventh balanced receiver 26-7 into a digital signal, and outputs the beat component (I component) of Sy-Ly (without π / 2 delay).

[0100] The ADC 27-8 converts the analog electrical signal output by the eighth balanced receiver 26-8 into a digital signal, and outputs the beat component (Q component) of Sy-Ly (with a π / 2 delay).

[0101] In the configuration of FIG. 4, the optical splitters 22 (22-1 to 22-4) and the optical splitters 23 (23-1 to 23-8) arranged for optical phase diversity reception may be integrated into a 1x4 type optical splitter.

[0102] FIG. 7 is a flowchart showing the processing of the receiver of this embodiment.

[0103] (Step S11) The first polarization separator 21-1 separates the received optical signal into orthogonal polarization components (Sx and Sy).

[0104] (Step S12) The second polarization separator 21-2 separates the continuous light of a single wavelength into orthogonal polarization components (Lx and Ly).

[0105] (Step S13) The first optical branching devices (22-1, 23-1, 23-2) to the second optical branching devices (22-2, 23-3, 23-4) each branch Sx and Sy into four.

[0106] (Step S14) The third optical branching devices (22-3, 23-5, 23-6) to the fourth optical branching devices (22-4, 23-7, 23-8) each branch Lx and Ly into four. Subsequently, the π / 2 delay devices 24-1 to 24-4 delay the remaining two optical phases of the four branched optical signals by π / 2 relative to the optical phases of two of the four branched optical signals.

[0107] (Step S15) The first balanced receiver 26-1 to the eighth balanced receiver 26-8 convert the 16 polarization components obtained by splitting into four by the first to fourth optical splitters into electrical signals for each combination of Sx and Lx (without π / 2 delay), Sx and Lx (with π / 2 delay), Sx and Ly (without π / 2 delay), Sx and Ly (with π / 2 delay), Sy and Lx (without π / 2 delay), Sy and Lx (with π / 2 delay), Sy and Ly (without π / 2 delay), and Sy and Ly (with π / 2 delay), and generate eight beat components.

[0108] As shown in FIG. 6, this embodiment includes a first polarization splitter 21-1 that splits a received optical signal into orthogonal polarization components (Sx and Sy), a second polarization splitter 21-2 that splits a single-wavelength continuous light into orthogonal polarization components (Lx and Ly), a first optical splitter (22-1, 23-1, 23-2) to a second optical splitter (22-2, 23-3, 23-4) that split each of Sx and Sy into four, and a third optical splitter (22-3, 23-5, 23-6) to a fourth optical splitter (22-4, 23-5) that split each of Lx and Ly into four and delay two of the optical phases by π / 2 before outputting the remaining two optical phases. The optical receiver is provided with first to eighth optical receivers (first balanced receiver 26-1) to eighth optical receivers (eighth balanced receiver 26-8) that convert the 16 polarization components obtained by branching into four by the first to fourth optical branchers into electrical signals for each combination of Sx and Lx (without π / 2 delay), Sx and Lx (with π / 2 delay), Sx and Ly (without π / 2 delay), Sx and Ly (with π / 2 delay), Sy and Lx (without π / 2 delay), Sy and Lx (with π / 2 delay), Sy and Ly (without π / 2 delay), and Sy and Ly (with π / 2 delay), and generate eight beat components.

[0109] As described above, in this embodiment, the signal light and the local light are each separated into orthogonal polarization components (Sx, Sy, Lx, Ly), and optical intradyne detection is used to generate the Sx-Lx beat component (I component), Sx-Lx beat component (Q component), Sy-Ly beat component (I component), and Sy-Ly beat component (Q component), in addition to the Sx-Ly beat component (I component), Sx-Ly beat component (Q component), Sy-Lx beat component (I component), and Sy-Lx beat component (Q component). As a result, this embodiment enables coherent reception regardless of the input polarization state of the local light.

[0110] <Configuration Example of Optical Communication System> Next, a configuration example of an optical communication system using an optical communication device 1 equipped with the above-described receiver 7A or 7B will be described. Fig. 8 is a diagram showing a first configuration example of an optical communication system. As shown in Fig. 8, an optical communication system 10A includes, for example, an optical communication device 1-1, an optical communication device 1-2, a dual-wavelength light source 2-1, a dual-wavelength light source 2-2, and an optical transmission path 3 (for example, an optical fiber cable).

[0111] The optical communication device 1 (1-1, 1-2) includes the above-mentioned receiver 7A or 7B. The optical communication device 1-1 receives an optical frequency f Ｓ and f Ｌ The optical communication device 1-2 receives a continuous light of optical frequency f from a two-wavelength light source 2-2. Ｓ and f Ｌ The continuous light is input via the optical transmission line 3, transmission data is input from an external device, an optical transmission signal output by the optical communication device 1-1 is input as a received optical signal via the optical transmission line 3, and the transmitted optical signal is output to the optical communication device 1-1. When the receiver 7A performs heterodyne detection, the intermediate frequency (f ＩＦ ) is f Ｓ -f Ｌ is.

[0112] In the configuration shown in Figure 8, for example, a dual-wavelength light source 2 (2-1, 2-2) that outputs two wavelengths is located remotely from an optical communication device 1 (1-1, 1-2) and supplies the light to the optical communication device via an optical transmission path 3. Note that a dual-wavelength light source can also be configured by arranging two normal light sources that output one wavelength. Alternatively, the output of a normal light source can be modulated with a sinusoidal signal to generate two modulated optical sidebands.

[0113] Fig. 9 is a diagram showing a second configuration example of an optical communication system. As shown in Fig. 9, an optical communication system 10A includes, for example, an optical communication device 1-1, an optical communication device 1-2, a dual-wavelength light source 2, and an optical transmission path 3 (for example, an optical fiber cable). In the configuration example of Fig. 9, the dual-wavelength light source 2 is shared by the optical communication devices 1-1 and 1-2.

[0114] <Examples of Formulas Used in Receiver> Below, examples of formulas used in the receiver 7 (7A, 7B) described above will be described. Here, as an example, a case will be illustrated in which an optical field is amplitude-modulated to generate an optical transmission signal. Furthermore, single-polarization modulation will be used instead of dual-polarization modulation. The following formula (1) is a complex representation of the optical field (X polarization) representing the received optical signal input to the first balanced receiver and the second balanced receiver. The following formula (2) is a complex representation of the optical field (Y polarization) representing the received optical signal input to the third balanced receiver and the fourth balanced receiver. The following formula (3) is a complex representation of the optical field (X polarization) representing the local light input to the first balanced receiver and the third balanced receiver. The following formula (4) is a complex representation of the optical field (Y polarization) representing the local light input to the second balanced receiver and the fourth balanced receiver.

[0115]

[0116]

[0117]

[0118]

[0119] In the formulas (1) to (4), α (the first optical intensity ratio or the second optical intensity ratio) and β (the first optical intensity ratio or the second optical intensity ratio) are optical intensity ratios of the orthogonal polarization components separated by the first polarization separator and the second polarization separator. Ｓ＿Ｙ In ＿ " (underscore) represents a subscript. θ Ｓ＿Ｙ (t) and θ Ｌ＿Ｙ (t) is the optical phase of the Y polarization component after polarization separation. Ｓ (t) and P Ｌ are the optical intensities of the optical signal and local light input to the receiver. t is time. δ(t) and ε(t) are the θ Ｓ＿Ｙ (t) and θ Ｌ＿Ｙ The optical phase shift with respect to (t) is expressed as θ Ｓ＿Ｘ (t) = θ Ｓ＿Ｙ (t)-δ(t), θ Ｌ＿Ｘ (t) = θ Ｌ＿Ｙ(t) - ε(t). By setting the parameters α and δ(t), it is possible to describe any polarization state of the received optical signal. Similarly, by setting the parameters β and ε(t), it is possible to describe any polarization state of the local oscillator light input to the receiver.

[0120] The following equation (5) expresses the photocurrent I at the output of the first balanced receiver. ＩＦ＿１ (t). The following equation (6) expresses the photocurrent I at the output of the second balanced receiver. ＩＦ＿２ (t). The following equation (7) expresses the photocurrent I ＩＦ＿３ (t). The following equation (8) expresses the photocurrent I ＩＦ＿４ (t).

[0121]

[0122]

[0123]

[0124]

[0125] In formulas (5) to (8), f ＩＦ is the intermediate frequency during heterodyne detection, and f ＩＦ = f ｓ -f Ｌ R is the light receiving sensitivity of the balanced receiver. In each embodiment, the photocurrents of equations (5) to (8) are sampled using an ADC and converted into digital data. The subsequent processing is performed in the digital domain by the Rx DSP. Also, as shown in equations (5) to (8), I ＩＦ＿１ (t) to I ＩＦ＿４ The phases of (t) are different from each other. If the received signal converted to the intermediate frequency band by optical heterodyne detection is further down-converted by synchronous detection, the amplitude components of equations (5) to (8) can be extracted as baseband signals.

[0126] Coherent detection is performed using the intermediate frequency component extracted from the received signal. The baseband signal I obtained by synchronous detection is Ｂ＿１ (t) to I Ｂ＿４(t) is as shown in the following equations (9) to (12): Even if envelope detection is performed instead of synchronous detection, results proportional to equations (9) to (12) can be obtained.

[0127]

[0128]

[0129]

[0130]

[0131] <Configuration Example of Rx DSP> Figure 10 is a diagram showing a first configuration example of an Rx DSP of an optical communication device. As shown in Figure 10, the Rx DSP 8A includes, for example, ADCs 81-1 to 81-4 (converters), a synchronous detection unit 82 (82-1 to 82-4), a first maximum ratio polarization combiner 83A, a second maximum ratio polarization combiner 84A, a comparator 85A, and a third maximum ratio polarization combiner 86A. The synchronous detection unit 82 includes, for example, a band pass filter (BPF) 821, a mixer 822, and a low pass filter (LPF) 823.

[0132] Each of the ADCs 81-1 to 81-4 converts the current I of the analog electrical signal output by the receiver 7A (or 7B). ＩＦ＿１ ~I ＩＦ＿４ Converts the signal from analog to digital.

[0133] The BPF 821 extracts the intermediate frequency component (f ＩＦ ) is extracted.

[0134] The mixer 822 multiplies the received signal by the intermediate frequency component extracted by the BPF 821. The output of the mixer 822 contains a baseband signal component and a harmonic signal component (2f ＩＦ ) are included.

[0135] LPF 823 cuts harmonic signal components from the signal output by mixer 822 and passes a predetermined frequency to obtain a desired baseband signal.

[0136] The first maximum ratio polarization combiner 83A and the second maximum ratio polarization combiner 84A calculate β by the maximum ratio polarization combining method.

[0137] The comparator 85A compares and determines which of the values ​​calculated by the first maximum ratio polarization combiner 83A and the second maximum ratio polarization combiner 84A should be used. Ｂ＿１ (k))￣|≧|(I Ｂ＿３ (t) − |, and the first maximum ratio polarization combiner 83A calculates β １＿２ This shows the case where is adopted. The superscript "￣" indicates the average. １＿２ is the light intensity ratio between the synchronous detection unit 82-1 and the synchronous detection unit 82-2, and β ３＿４ is the light intensity ratio between the synchronous detector 82-3 and the synchronous detector 82-4.

[0138] The third maximum ratio polarization combiner 86A calculates α using the same method as described above.

[0139] FIG. 11 illustrates a second exemplary configuration of an Rx DSP of an optical communication device. The Rx DSP 8B includes, for example, ADCs 81-1 to 81-4, synchronous detectors 82 (82-1 to 82-4), a first maximum ratio polarization combiner 83B, a second maximum ratio polarization combiner 84B, a comparator 85B, and a third maximum ratio polarization combiner 86B. In the configuration illustrated in FIG. 11, the first maximum ratio polarization combiner 83B and the second maximum ratio polarization combiner 84B calculate α using the maximum ratio polarization combiner method. The comparator 85B compares the values ​​calculated by the first maximum ratio polarization combiner 83B and the second maximum ratio polarization combiner 84B to determine which value to use. The third maximum ratio polarization combiner 86B calculates β.

[0140] Next, an example of a procedure for detecting a received current by applying the maximum ratio polarization combining method to the embodiment will be described. Fig. 12 is a flowchart of the procedure for detecting a received current by the Rx DSP.

[0141] (Step S21) The Rx DSP 8A (or 8B) calculates β using the maximum ratio polarization combining method and detects the X and Y components of the received current. Ｂ＿１ (t), I Ｂ＿２ (t), I Ｂ＿３ (t), I Ｂ＿４ The k-th sampling point of (t) is I Ｂ＿１ (k), I Ｂ＿２ (k), I Ｂ＿３(k), I Ｂ＿４ (k), and r in the following equations (13) and (14) Ｘ (k), r Ｙ As shown in equations (15) and (16), the Rx DSP 8 defines r Ｘ (k), r Ｙ For (k), the average of l points before and after the kth sampling point (total of 2l + 1 points) is taken. This average value is equal to √β / (1-β), and the Rx DSPA (or 8B) calculates β using this formula. If α≒0, then I Ｂ＿１ (k) and I Ｂ＿２ The value of (k) is small and Ｘ Similarly, when α≒1, I Ｂ＿３ (t), I Ｂ＿４ The value of (t) is small and r Ｙ Therefore, the Rx DSPA (or 8B) cannot accurately calculate I Ｂ＿１ (k) and I Ｂ＿３ (t), or I Ｂ＿２ (k) and I Ｂ＿４ (t) and take the time average of I Ｂ＿１ (k) ≧ I Ｂ＿３ In the case of (t), using equation (15), (β = β １＿２ ) is calculated, and I Ｂ＿２ (k) ≧ I Ｂ＿４ In the case of (t), β (β = β) is calculated using equation (16). ３＿４ ) can avoid the above problem. Note that the Rx DSPA (or 8B) may compare the root mean squares to determine the magnitude relationship of the received current. Furthermore, the Rx DSPA (or 8B) substitutes the calculated β into the following equations (17) and (18) to obtain the X component (IX(t)) and Y component (IY(t)) of the received current.

[0142] (Step S22) The Rx DSPA (or 8B) calculates α using the maximum ratio polarization combining method and detects the received current. Note that α is calculated using the same method as in step S21, and the Rx DSPA (or 8B) detects the received current (I(t)) of the following equation (19) using the same method as equations (17) and (18). Note that the time fluctuation amount √Ps(t) in I(t) corresponds to the optical field of the amplitude-modulated received optical signal.

[0143] (Step S23) The Rx DSPA (or 8B) executes the processes of steps S21 and S22 performed for the k-th sampling point for subsequent sampling points as needed.

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150]

[0151] <Other synchronous detection methods> When formulas (9) to (12) are generated from formulas (5) to (8), the intermediate frequency component (f ＩＦ In the above description, synchronous detection is performed using the synchronous detection method 100. However, the present invention is not limited to this. Synchronous detection can also be performed using other methods.

[0152] 13 is a diagram showing a third example of the configuration of an Rx DSP of an optical communication device. As shown in FIG. 13, the Rx DSP 8C includes, for example, ADCs 81-1 to 81-4, a synchronous detection unit 82C (82C-1 to 82C-4), a first maximum ratio polarization combiner 83A, a second maximum ratio polarization combiner 84A, a comparator 85A, and a third maximum ratio polarization combiner 86A. The synchronous detection unit 82C includes, for example, an oscillator 824, a mixer 822, and an LPF 823.

[0153] The oscillator 824 generates a complex signal exp(2πjf ＩＦ The mixer 822 multiplies the complex signal output by the oscillator 824 by the received signal. The mixer output contains a baseband signal component and a harmonic signal component (2f ＩＦ ) are included.

[0154] LPF 823 cuts harmonic signal components from the signal output by mixer 822 and passes a predetermined frequency to obtain a desired baseband signal.

[0155] In the example of FIG. 13, the comparator 85A calculates |(I Ｂ＿１ (k))￣|≧|(I Ｂ＿３ (t) − |, and the first maximum ratio polarization combiner 83A calculates β １＿２ This shows the case where is adopted.

[0156] FIG. 14 illustrates a fourth exemplary configuration of an Rx DSP of an optical communication device. The Rx DSP 8D includes, for example, ADCs 81-1 to 81-4, synchronous detectors 82C (82C-1 to 82C-4), a first maximum ratio polarization combiner 83B, a second maximum ratio polarization combiner 84B, a comparator 85B, and a third maximum ratio polarization combiner 86B. In the configuration of FIG. 14, the first maximum ratio polarization combiner 83B and the second maximum ratio polarization combiner 84B calculate α using the maximum ratio polarization combiner method. The comparator 85B compares the values ​​calculated by the first maximum ratio polarization combiner 83B and the second maximum ratio polarization combiner 84B to determine which value to use. The third maximum ratio polarization combiner 86B calculates β.

[0157] The above-described synchronous detection technique is equivalent to multiplying a received signal by a quadrature sine wave signal and a quadrature cosine wave signal, and extracting IQ components from the received signal. Therefore, in the above-described embodiments, the receivable modulation method is not limited to amplitude modulation, and can also be applied to, for example, phase modulation.

[0158] Next, an example of processing by the Rx DSP 8C (or 8D) shown in Figures 13 and 14 will be described. The Rx DSP 8C (or 8D) applies exp(2πjf ＩＦThe following equations (20) to (23) express the relationship between the generated baseband signal component and harmonic signal component (2f ＩＦ ) is obtained by extracting the desired baseband signal component from the signal using an LPF.

[0159] In addition, the Rx DSP 8C (or 8D) uses r Ｘ (k), r Ｙ Furthermore, the Rx DSP 8C (or 8D) defines r Ｘ (k), r Ｙ For (k), the average of l points (total of 2l + 1 points) before and after the k-th sampling point is taken. Ｘ (k), r Ｙ Since (k) is a complex number, the Rx DSP 8C (or 8D) calculates r Ｘ (k), r Ｙ For (k), β and ε are calculated by treating the real part and the imaginary part separately as in the following equations (24) to (25) and (26) to (27).

[0160] In order to handle the cases of α≒0 and α≒1, the Rx DSP 8C (or 8D) Ｂ＿１ (k) and I Ｂ＿３ (t), or I Ｂ＿２ (k) and I Ｂ＿４ (t) is taken as the time average |(I Ｂ＿１ (k))￣|≧|(I Ｂ＿３ (t)) ￣|, use equation (24) to obtain β (= β １＿２ ) is calculated, and |(I Ｂ＿２ (k))￣|≧|(I Ｂ＿４ (t)) - |, use equation (26) to obtain β (= β ３＿４ The Rx DSP 8C (or 8D) may compare the received currents using root mean squares in order to determine the magnitude relationship between the received currents.

[0161]

[0162]

[0163]

[0164]

[0165]

[0166]

[0167]

[0168]

[0169] Furthermore, the Rx DSP 8C (or 8D) substitutes the calculated β into the following equations (28) and (29) to obtain the X component of the received current (I Ｘ (t)) and Y component (I Ｙ (t)). Then, the Rx DSP 8C (or 8D) calculates α and δ in the same way as explained for the Rx DSP 8A (or 8B). The Rx DSP 8C (or 8D) detects the received current (I(t)) of the following equation (30) using a method similar to equations (28) to (29). Note that the time fluctuation amount √Ps(t) in I(t) corresponds to the optical field of the amplitude-modulated received optical signal. Note that, unlike equation (19), the phase component (θ Ｌ＿Ｙ (t)-θ Ｓ＿Ｙ (t)), and it is possible to extract √Ps(t) by taking the absolute value of equation (30). Furthermore, the Rx DSP 8C (or 8D) executes the above-described process performed for the k-th sampling point for subsequent sampling points as needed.

[0170]

[0171]

[0172]

[0173] <Example of communication between multiple optical transmitters and receivers> Next, a case will be described in which, for example, the receiver has the configuration shown in FIG. 5 , a different wavelength is assigned to each optical communication device using wavelength division multiplexing (WDM), and communication is performed between multiple optical communication devices facing each other on the optical transmission path of the same optical fiber cable.

[0174] 15 is a diagram showing an example of the configuration of an optical communication system for communicating with a plurality of opposing optical communication devices using supplied light. The optical communication system 10C includes, for example, a plurality of optical communication devices 1 (1-1 to 1-N, 1-1A to 1-NA), wavelength multiplexers 31 (31-1, 31-2), wavelength demultiplexers 32 (32-1, 32-2), two-wavelength light sources 2 (2-1 to 2-N), and optical transmission paths 3 (3-1, 3-2). The two-wavelength light sources 2-1 to 2-N (N is an integer of 2 or more) output f Ｓ１ and f Ｌ１ , f Ｓ２ and f Ｌ２ , ..., f Ｓｉ and f Ｌｉ (i is an integer between 2 and N)..., f ＳＮ and f ＬＮ The combinations of optical frequencies are supplied to the optical communication devices 1-1 to 1-N. The example in Fig. 15 shows a case where the supplied light is used to communicate with the opposing optical communication devices 1-1A to 1-NA. For simplicity, Fig. 15 shows only one-way transmission.

[0175] The combinations of optical frequencies are multiplexed by a wavelength multiplexer 32-1 (S+L) to become WDM light, which is transmitted over the optical transmission path 3-1 of the optical fiber cable, and then separated into the original combinations of optical frequencies by a wavelength demultiplexer 32-1 (S+L), and each is input to the optical communication devices 1-1 to 1-N.

[0176] Local light for reception (f Ｌｉ ) is used for coherent reception, while the light source for transmission (f Ｓｉ ) is modulated by transmission data to generate an optical signal. The optical signals output from each of the optical communication devices 1-1 to 1-N are multiplexed by a wavelength multiplexer 31-2(S) to become a WDM signal, which is transmitted over an optical transmission path 3-2 of an optical fiber cable, and then demultiplexed by a wavelength demultiplexer 32-2(S) to be received by the corresponding optical communication devices 1-1A to 1-NA.

[0177] In addition, for example, an arrayed waveguide grating (AWG) or a wavelength selective switch (WSS) can be used as the wavelength multiplexer 31-1 (S+L), wavelength demultiplexer 32-1 (S+L), wavelength multiplexer 31-2 (S), and wavelength demultiplexer 32-2 (S).

[0178] Next, an example of optical signals at each part in the configuration of FIG. 15 will be described. In the following example, Ｓ１ and f Ｌ１ , f Ｓ２ and f Ｌ２ , f Ｓ３ and f Ｌ３ The following description will be given of a case where the optical frequency sets of the above are supplied to the optical communication devices 1-1 to 1-3. An optical interleaver (IL) is arranged as the wavelength demultiplexer 32 in FIG. 15. The following settings are also made: (I) The transmission center optical frequencies of the two output ports of the IL are set to the same frequency as the transmission light source (f Ｓｉ ) and the receiving local light (f Ｌｉ ) optical frequency grid (f Ｓｉ and f Ｌｉ ) optical frequency interval (ΔF = f Ｌ（ｉ＋１） -f Ｌｉ = f Ｓ（ｉ＋１） -f Ｓｉ） The channel spacing (Δf) between the IL and the modulator is set to the relationship ΔF = m × Δf, where m is a natural number (m = 2 in Figure 16). With this setting and the same (single product) configuration of the optical communication device, any set of optical frequencies can be supplied to the modulator and coherent receiver.

[0179] FIG. 16 is a diagram showing the transmission characteristics of a wavelength multiplexer (S+L) and wavelength demultiplexer (S+L) that multiplex and demultiplex WDM light, and a wavelength multiplexer (S) and wavelength demultiplexer (S). The horizontal axis represents optical frequency, and the vertical axis represents optical intensity. The solid line g101 represents the transmission characteristics of the wavelength multiplexer 31-1 (S+L) and wavelength demultiplexer 32-1 (S+L). The dashed line g102 represents the transmission characteristics of the wavelength multiplexer 31-2 (S) and wavelength demultiplexer 32-2 (S). FIG. 17 is a diagram showing the transmission characteristics of a wavelength multiplexer (S+L) and wavelength demultiplexer (S+L) that multiplex and demultiplex WDM light, and a wavelength multiplexer (S) and wavelength demultiplexer (S) when m = 2. The horizontal axis represents optical frequency, and the vertical axis represents optical intensity. The solid line g111 indicates the transmission characteristics (first port), and the dashed line g112 indicates the transmission characteristics (second port). Also, the symbol g115 indicates the IL channel spacing (=Δf), and the symbol g116 indicates the WDM optical frequency spacing (=ΔF).

[0180] As shown in FIG. 16, the transmission center optical frequency of the wavelength multiplexer 31-1 (S+L) and the wavelength demultiplexer 32-1 (S+L) is f Ｓｉ and f Ｌｉ On the other hand, the transmission center wavelengths of the wavelength multiplexer 31-2(S) and wavelength demultiplexer 32-2(S) are set to f Ｓｉ It is recommended to set it to match the

[0181] As shown in Figure 17, an offset is applied to the transmission center optical frequency by a wavelength multiplexer / demultiplexer (wavelength multiplexer, wavelength demultiplexer) for WDM light and a wavelength multiplexer / demultiplexer (wavelength multiplexer, wavelength demultiplexer) for WDM signals. ｓｉ Therefore, when passing through the wavelength multiplexer 31-2(S) and wavelength demultiplexer 32-2(S), if no offset is added, the optical spectrum f Ｓｉ However, this can be avoided by offsetting the transmission center optical frequencies of both the modulated light spectrums.

[0182] In the optical communication device 1 configured in this manner, a coherent optical receiver is configured using optical heterodyne detection or optical intradyne detection, so that even if a light source is placed at a location away from the optical communication device and the continuous light output by the light source is remotely supplied to the coherent receiver as local light, coherent reception can be performed regardless of the input polarization state of the local light.

[0183] Furthermore, in the optical communication system 10A (or 10B), the Rx DSP is configured with a synchronous detection unit (wavelength multiplexer, wavelength demultiplexer), first maximum ratio polarization multiplexer to third maximum ratio polarization multiplexer, and a comparator to calculate the optical intensity ratios α and β of the orthogonal polarization components separated by the polarization separator. Therefore, the received signal converted to the intermediate frequency band by optical heterodyne detection can be further downconverted by synchronous detection, thereby extracting the amplitude component as a baseband signal.

[0184] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0185] The present invention is applicable to optical communication devices, optical transmitting devices, optical receiving devices, optical communication systems, and the like.

[0186] 1, 1-1 to 1-N, 1-1A to 1-NA... Optical communication device, 2... Light source, 2A-1 to 2A-N, 2B... Two-wavelength light source, 3, 3-1, 3-2... Optical transmission line, 4... Wavelength demultiplexer, 5... Tx DSP, 6... Modulator, 7, 7A, 7B, 7C, 7D... Receiver, 8, 8A, 8B, 8C, 8D... Rx DSP, 9...optical splitter, 10, 10A, 10B, 10C...optical communication system, 11-1, 21-1...first polarization splitter, 11-2, 21-2...second polarization splitter, 12, 12-1 to 12-4, 22-1 to 22-4, 23-1 to 23-8...optical splitters, 13-1 to 13-4, 25-1 to 25-8...optical coupling splitters, 14-1, 26-1...first balanced receiver, 14-2, 26-2...second balanced receiver, 14-3, 26-3...third balanced receiver, 14-4, 26-4...fourth balanced receiver, 26-5...fifth balanced receiver, 26-6...sixth balanced receiver, 26-7...seventh balanced receiver receiver, 26-8...eighth balanced receiver, 15-1 to 15-4, 27-1 to 27-8, 81-1 to 81-4...ADC, 24-1 to 24-4...π / 2 delay device, 82-1 to 82-4, 82C-1 to 82C-4...synchronous detection unit, 83A, 83B...first maximum ratio polarization combiner, 84A, 84B...second maximum ratio polarization combiner, 85A, 85B...comparator, 86A, 86B...third maximum ratio polarization combiner, 821...BPF, 822...mixer, 823...LPF, 824...oscillator, 31-1...wavelength multiplexer (S+L), 32-1...wavelength demultiplexer (S+L), 31-2...wavelength multiplexer (S), 32-2...wavelength demultiplexer (S)

Claims

1. An optical communication device using heterodyne detection, comprising: a first polarization separator that separates a received optical signal into orthogonal polarization components; a second polarization separator that separates continuous light of a single wavelength into orthogonal polarization components; an optical splitter that splits each of the orthogonal polarization components separated by the first polarization separator and the orthogonal polarization components separated by the second polarization separator into two; and an optical receiver that converts the eight polarization components obtained by the splitting by the optical splitter into electrical signals for each combination of the two splits, generating a total of four beat components.

2. An optical communication device using intradyne detection, comprising: a first polarization separator that separates a received optical signal into orthogonal polarization components; a second polarization separator that separates continuous light of a single wavelength into orthogonal polarization components; a first optical splitter and a second optical splitter that each split the orthogonal polarization components separated by the first polarization separator into four; a third optical splitter and a fourth optical splitter that each split the orthogonal polarization components separated by the second polarization separator into four, and output the remaining two optical phases by delaying two of the split components by π / 2; and an optical receiver that converts the 16 polarization components obtained by the four splits by the first to fourth optical splitters into electrical signals for each combination of one of the orthogonal polarization components separated by the first polarization separator and one of the orthogonal polarization components separated by the second polarization separator, with or without a π / 2 delay, to generate eight beat components.

3. The optical communications device according to claim 1, further comprising a receiving-side digital signal processing unit that processes the electrical signal output by the optical receiver, the receiving-side digital signal processing unit comprising: a converter that converts the electrical signal output by the optical receiver into a digital signal; a synchronous detection unit that detects baseband signal components from the digital signal; a first maximum ratio polarization combiner that determines a first optical intensity ratio of the orthogonal polarization components separated by the polarization separator using maximum ratio polarization combining; a second maximum ratio polarization combiner that determines the first optical intensity ratio of the orthogonal polarization components separated by the polarization separator using maximum ratio polarization combining; a comparator that compares the first optical intensity ratio determined by the first maximum ratio polarization combiner with the first optical intensity ratio determined by the second maximum ratio polarization combiner and selects one of them; and a third maximum ratio polarization combiner that determines a second optical intensity ratio different from the first optical intensity ratio of the orthogonal polarization components separated by the polarization separator using maximum ratio polarization combining.

4. An optical communication system comprising: N (N is an integer of 2 or greater) light sources that emit continuous light of two different optical frequencies; a first wavelength multiplexer that multiplexes the continuous light of the two optical frequencies input from the N light sources; a first wavelength demultiplexer that receives the continuous light of the two optical frequencies combined from the first wavelength multiplexer and demultiplexes the input continuous light of the two optical frequencies to output to N first optical communication devices; N first optical communication devices that receive each continuous light containing components of two optical frequencies from the first wavelength demultiplexer and output one of the input continuous light; a second wavelength multiplexer that multiplexes one of the continuous light outputs from each of the N first optical communication devices; and a second wavelength demultiplexer that receives the combined optical signal output from the second wavelength multiplexer and demultiplexes the input combined optical signal to N second optical communication devices.

5. An optical communication system according to claim 4, wherein the transmission center optical frequencies of the first wavelength multiplexer and the first wavelength demultiplexer are set to an intermediate optical frequency between the two so that the output optical powers of the two different optical frequencies are equal, and the transmission center wavelengths of the second wavelength multiplexer and the second wavelength demultiplexer are set to match one of the continuous light beams input.

6. The optical communication system according to claim 4, wherein the first wavelength demultiplexer and the second wavelength demultiplexer have an offset in their transmission center optical frequencies, and the first wavelength multiplexer and the second wavelength multiplexer have an offset in their transmission center optical frequencies.

7. An optical communication method for an optical communication device using heterodyne detection, wherein a first polarization separator separates a received optical signal into orthogonal polarization components; a second polarization separator separates continuous light of a single wavelength into orthogonal polarization components; an optical splitter splits each of the orthogonal polarization components separated by the first polarization separator and the orthogonal polarization components separated by the second polarization separator into two; and an optical receiver converts the eight polarization components obtained by the splitting by the optical splitter into electrical signals for each combination of the two splits, generating a total of four beat components.

8. An optical communication method for an optical communication device using intradyne detection, wherein a first polarization separator separates a received optical signal into orthogonal polarization components; a second polarization separator separates continuous light of a single wavelength into orthogonal polarization components; a first optical splitter and a second optical splitter each split the orthogonal polarization components separated by the first polarization separator into four; a third optical splitter and a fourth optical splitter each split the orthogonal polarization components separated by the second polarization separator into four, and output the remaining two optical phases by delaying two of the split optical phases by π / 2; and an optical receiver converts the 16 polarization components obtained by the first to fourth optical splitters into four into electrical signals for each combination of one of the orthogonal polarization components separated by the first polarization separator and one of the orthogonal polarization components separated by the second polarization separator, with or without a π / 2 delay, to generate eight beat components.

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