Optical communication device and optical communication method

Abstract

One embodiment of the present invention is an optical communication device comprising: a polarization-diversity optical heterodyne detector that receives an optical signal obtained by amplitude-modulating an optical electric field and detects the received optical signal; a converter that converts the signal detected by the optical heterodyne detector into a digital signal; and a signal processing unit that processes the digital signal. The optical heterodyne detector generates, in an intermediate frequency band of optical heterodyne detection by using separated orthogonal polarization components of the received optical signal and orthogonal polarization components of local oscillation light, a beat component between an X component of the orthogonal polarization components of the local oscillation light and an X component of the orthogonal polarization components of the received optical signal, and a beat component between a Y component of the orthogonal polarization components of the local oscillation light and a Y component of the orthogonal polarization components of the received optical signal. The converter samples each beat component and converts the sampled beat component into a digital signal. The signal processing unit performs polarization combining by converting each sampled digital signal into a baseband signal and further calculating a square root of a sum of products of components and complex conjugates of the baseband signal.

Description

Optical Communication Device and Optical Communication Method

[0001] The present invention relates to the technology of an optical communication device and an optical communication method.

[0002] As an optical communication device, a coherent optical transceiver is used. FIG. 18 is a diagram showing the configuration of an optical coherent receiver according to the prior art optical heterodyne detection method (see, for example, Non-Patent Document 2). The optical receiver includes a light source 901, an optical heterodyne detector 910, an ADC (Analog-to-Digital Convertor) 921, an ADC 922, and a signal processor 930. The optical heterodyne detector 910 has a polarization diversity configuration for receiving a single-polarization optical signal in an arbitrary polarization state, and includes two polarization splitters (first polarization splitter 911, second polarization splitter 912) and two balanced receivers (first balanced receiver 913, second balanced receiver 914).

[0003] The continuous light output from the light source 901 is used as the local light in optical heterodyne detection. The output continuous light (optical frequency f Ｌ ) is separated by the first polarization splitter 911 into polarization components (L Ｘ and L Ｙ ) that are orthogonal to each other. At this time, since the polarization of the continuous light is input at an angle of 45 degrees with respect to the orthogonal polarization axis of the first polarization splitter 911, the optical powers of L Ｘ and L Ｙ are output evenly. On the other hand, the received optical signal (optical frequency f Ｓ ) is input to the second polarization splitter 912 in an arbitrary polarization state and is separated into polarization components (S Ｘ and S Ｙ ) that are orthogonal to each other.

[0004] By detecting the beat components (S Ｙ - L Ｙ beat component, and S Ｘ - L Ｘ beat component) of each orthogonal polarization component of the received optical signal by photoelectric conversion, at least one beat component is output regardless of the polarization state of the received optical signal, and an optical signal in an arbitrary polarization state can be received. The detected beat component (S Ｘ - LＸ Beet component (g901), S Ｙ -L Ｙ The beat components (g902) are each sampled by the ADCs (921, 922), converted into digital signals, and sent to the signal processor 930.

[0005] The signal processor 930 has a baseband conversion unit 931 and a polarization combining unit 932, and generates received data by digital signal processing (DSP). The baseband conversion unit 931 and the polarization combining unit 932 use the intermediate frequency band (f ＩＦ = | f Ｓ -f Ｌ The beat component present in the baseband is converted to the baseband band by a DSP. The polarization combining unit 932 then polarizes the beat component converted to the baseband band by a DSP to obtain the received data. As a polarization combining method using a DSP, methods using the maximum ratio combining method (see, for example, Non-Patent Document 1) and MIMO (Multi-Input Multi-Output) technology (see, for example, Non-Patent Document 2) have been proposed. In the maximum ratio combining method, the beat component present in the baseband band is converted to the baseband band by a DSP. Ｙ -L Ｙ Beet component, S Ｘ -L Ｘ S was calculated based on the average of several samples of the beet component ratio. Ｘ and S Ｙ The optical power ratio is used to synthesize orthogonal polarization components and generate received data. On the other hand, in the case of MIMO, the signal processor 930 has a baseband conversion unit 931 and an adaptive equalization unit 933, and generates received data by performing polarization synthesis by sequentially updating the tap coefficient of the adaptive equalization filter according to the polarization state of the received optical signal. Note that Non-Patent Literature 2 assumes the case of receiving a polarization-multiplexed optical signal and therefore uses a 2-input 2-output MIMO, but when receiving a single-polarization optical signal, a 2-input 1-output MISO (Multi-Input Single-Output) is used.

[0006] K. Kikuchi, and S. Tsukamoto, “Evaluation of Sensitivity of the Digital Coherent Receiver,” J. Lightw. Technol., Vol.26, No.13, pp.1817-pp.1822, 2008J. Zhang, J. Yu, X. Li, K. Wang, W. Zhou, J. Xiao, L. Zhao, X. Pan, B. Liu, and X. Xin, “200 Gbit / s / λ PDM-PAM-4 PON system based on intensity modulation and coherent detection,” J. Opt. Commn. Netw., Vol. 12, No. 1, pp.A1-A8, 2020

[0007] However, when using the maximum ratio combining method for polarization combining, if the received optical signal is one in which transmitted data is superimposed on the optical field amplitude by pulse amplitude modulation (PAM), the number of samples required for averaging must be set to be longer than the continuity time of the same code. When receiving an optical signal with a long continuity of the same code, the calculation time required for averaging becomes long, increasing the processing delay of the DSP, and the polarization combining process cannot keep up if the polarization fluctuations are large. On the other hand, if MISO is used for polarization combining, the tap coefficients of the adaptive equalization filter must be sequentially updated by feedback control according to the polarization state of the received optical signal. Therefore, when using MISO for polarization combining, similar to the maximum ratio combining method, the calculation time required for the convergence of the tap coefficients becomes long, increasing the processing delay of the DSP, and the polarization combining process cannot keep up if the polarization fluctuations are large.

[0008] Thus, conventional technologies have the problem that the processing delay required for polarization synthesis is large, and the polarization synthesis process cannot keep up when polarization fluctuations are large. In view of the above circumstances, the present invention aims to provide a technology that can significantly reduce the processing delay required for polarization synthesis and can keep up with large polarization fluctuations.

[0009] One aspect of the present invention is an optical communication device comprising: a polarization diversity type optical heterodyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and performs detection of the received optical signal; a converter that converts the signal detected by the optical heterodyne detector into a digital signal; and a signal processing unit that processes the digital signal, wherein the optical heterodyne detector uses the separated orthogonal polarization components of the received optical signal and the orthogonal polarization components of the local emission to generate a beat component of the X component of the orthogonal polarization component of the local emission and the X component of the orthogonal polarization component of the received optical signal, and a beat component of the Y component of the orthogonal polarization component of the local emission and the Y component of the orthogonal polarization component of the received optical signal in the intermediate frequency band of the optical heterodyne detection; the converter samples each of the beat components and converts them into the digital signal; the signal processing unit converts each of the sampled digital signals into baseband signals, and further calculates the square root of the sum of the products of the baseband signal components and their complex conjugates to perform polarization synthesis.

[0010] One aspect of the present invention is an optical communication device comprising: a polarization diversity type optical heterodyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and detects the received optical signal; a converter that converts the signal detected by the optical heterodyne detector into a digital signal; and a signal processing unit that processes the digital signal, wherein the optical heterodyne detector uses the separated orthogonal polarization components of the received optical signal and the orthogonal polarization components of the local emission to generate beat components for each combination of the orthogonal polarization components of the received optical signal and the orthogonal polarization components of the local emission in the Y component of the orthogonal polarization component of the received optical signal, the X component of the orthogonal polarization component of the received optical signal, the Y component of the orthogonal polarization component of the local emission, and the X component of the orthogonal polarization component of the local emission; the converter samples each of the beat components and converts them into the digital signal; the signal processing unit converts each of the sampled digital signals into baseband signals; and further calculates the square root of the sum of the products of the baseband signal components and their complex conjugates to perform polarization synthesis.

[0011] One aspect of the present invention comprises an optical intradyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and performs detection of the received optical signal, a converter that converts the signal detected by the optical intradyne detector into a digital signal, and a signal processing unit that processes the digital signal, wherein the optical intradyne detector uses the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the local emission to extract the I component and the Q component from the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the local emission, respectively, and the I component of the Y component of the orthogonal polarization component of the received optical signal, the Q component of the Y component of the orthogonal polarization component of the received optical signal, the I component of the X component of the orthogonal polarization component of the received optical signal, the Q component of the Y component of the orthogonal polarization component of the local emission, and The optical communication device generates the following components in the Q component of the X component of the orthogonal polarization component of the light emitted by the station: the Q component of the Y component of the orthogonal polarization component of the received optical signal and the Q component of the Y component of the orthogonal polarization component of the light emitted by the station; the I component of the Y component of the orthogonal polarization component of the received optical signal and the I component of the Y component of the orthogonal polarization component of the light emitted by the station; the Q component of the X component of the orthogonal polarization component of the received optical signal and the Q component of the Y component of the orthogonal polarization component of the light emitted by the station; the converter samples each of the beat components and converts them into digital signals; the signal processing unit converts each of the sampled digital signals into baseband signals; and further calculates the square root of the sum of the products of each of the baseband signals with their complex conjugates to perform polarization synthesis.

[0012] One aspect of the present invention comprises an optical intradyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and performs detection of the received optical signal, a converter that converts the signal detected by the optical intradyne detector into a digital signal, and a signal processing unit that processes the digital signal, wherein the optical intradyne detector uses the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the local emission to extract the I component and the Q component from the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the local emission, respectively, and the I component of the Y component of the orthogonal polarization component of the received optical signal, the Q component of the Y component of the orthogonal polarization component of the received optical signal, the I component of the X component of the orthogonal polarization component of the received optical signal, and the X component of the orthogonal polarization component of the received optical signal This optical communication device generates beat components for each combination of the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the station's emitted light, the I component of the Y component of the orthogonal polarization component of the station's emitted light, the Q component of the Y component of the orthogonal polarization component of the station's emitted light, the I component of the X component of the orthogonal polarization component of the station's emitted light, and the beat components of each of the I and Q components of the X component of the orthogonal polarization component of the station's emitted light. The converter samples each of the beat components and converts them into digital signals. The signal processing unit converts each of the sampled digital signals into baseband signals, and further calculates the square root of the sum of the products of each of the baseband signals with their complex conjugates to perform polarization synthesis.

[0013] One aspect of the present invention is an optical communication method in which a polarization diversity type optical heterodyne detector receives an optical signal obtained by amplitude modulating an optical electric field, detects the received optical signal, a converter converts the signal detected by the optical heterodyne detector into a digital signal, a signal processing unit processes the digital signal, the optical heterodyne detector uses the separated orthogonal polarization components of the received optical signal and the orthogonal polarization components of the local emission to generate a beat component of the X component of the orthogonal polarization component of the local emission and the X component of the orthogonal polarization component of the received optical signal, and a beat component of the Y component of the orthogonal polarization component of the local emission and the Y component of the orthogonal polarization component of the received optical signal in the intermediate frequency band of the optical heterodyne detection, the converter samples each of the beat components and converts them into digital signals, the signal processing unit converts each of the sampled digital signals into baseband signals, and further calculates the square root of the sum of the products of the baseband signal components and their complex conjugates to perform polarization synthesis.

[0014] This invention significantly reduces the processing delay required for polarization synthesis, making it possible to keep up with large polarization fluctuations during the polarization synthesis process.

[0015] This figure shows an example configuration of the optical communication system of the first embodiment. This figure shows an example configuration of the optical communication device of the first embodiment. This figure illustrates the first signal processing method by the signal processing unit of the first embodiment. This figure illustrates the second signal processing method by the signal processing unit of the first embodiment. This figure illustrates the third signal processing method by the signal processing unit of the first embodiment. This is a flowchart of the processing of the optical communication device of the first embodiment. This figure shows an example configuration of a modified version of the optical communication system of the first embodiment. This figure shows an example configuration of the optical communication device of the second embodiment. This figure illustrates the fourth signal processing method by the signal processing unit of the second embodiment. This figure illustrates the fifth signal processing method by the signal processing unit of the second embodiment. This figure illustrates the sixth signal processing method by the signal processing unit of the second embodiment. This is a flowchart of the processing of the optical communication device of the second embodiment. This figure shows an example configuration of the optical communication device of the third embodiment. This is a flowchart of the processing of the optical communication device of the third embodiment. This figure shows an example configuration of the optical communication device of the fourth embodiment. This is a flowchart of the processing of the optical communication device of the fourth embodiment. This figure shows a schematic of the hardware configuration example of the information processing device applied to the embodiment. This figure shows the configuration of an optical coherent receiver using the conventional optical heterodyne detection method.

[0016] Embodiments of the present invention will be described in detail with reference to the drawings.

[0017] <First Embodiment> Figure 1 shows an example of the configuration of the optical communication system according to this embodiment. The optical communication system 1 includes, for example, an optical transmitter 2 and an optical communication device 3. The optical transmitter 2 and the optical communication device 3 are connected via an optical fiber transmission line 9. The optical transmitter 2 includes, for example, a Tx signal processing unit 21, a light source 22, and an amplitude modulator 23. The optical communication device 3 includes, for example, a light source 31, a detector 32 (optical heterodyne detector), an ADC 33 (33-1, 33-2) (converter), and a signal processing unit 34.

[0018] In this embodiment, we assume a system that receives a high-speed optical PAM-N signal with uniform amplitude level spacing of the optical field by optical heterodyne detection. A PAM-N signal is a multi-level signal that is pulse amplitude modulated (PAM). N (≧2) is the number of amplitude levels of the PAM signal. In the following embodiments, we describe examples in which the optical communication device handles digital signals (PAM), but it is not limited to this. The signal handled by the optical communication device may be, for example, an analog signal.

[0019] Optical transmitter 2 receives transmission data for optical PAM-N signals from an external device. Optical transmitter 2 performs signal processing and amplitude modulation on the input transmission data to obtain a transmission optical signal, and outputs the optical transmission signal to optical communication device 3 via optical fiber transmission line 9.

[0020] The Tx signal processing unit 21 is, for example, a DSP. The Tx signal processing unit 21 is a transmitting-side digital signal processing unit and performs digital signal processing as needed, such as Nyquist pulse shaping and transmitting device bandwidth compensation.

[0021] The light source 22 has an optical frequency f Ｓ It emits continuous light, and the emitted light frequency f Ｓ The continuous light is output to the amplitude modulator 23.

[0022] The amplitude modulator 23 receives the transmission data from the Tx signal processing unit 21 and the optical frequency f input from the light source 22. Ｓ By amplitude modulation using continuous light, the transmitted data is superimposed on the optical field, generating a transmitted optical signal. The transmitted optical signal output from the amplitude modulator 23 is transmitted to the optical communication device 3 via the optical fiber transmission line 9.

[0023] The optical communication device 3 is, for example, an optical receiver. The transmitted optical signal output by the optical transmitter 2 is input to the optical communication device 3 as a received optical signal via the optical fiber transmission line 9. The optical communication device 3 performs optical heterodyne detection, conversion to a digital signal, and other signal processing on the input received optical signal to obtain received data, and outputs the received data to an external device.

[0024] The light source 31 has an optical frequency fＳ Different optical frequencies f Ｌ It emits continuous light, and the emitted light frequency f Ｌ The continuous light is output to the detector 32. Note that the optical frequency f Ｌ The continuous light may be generated inside the optical communication device 3, or it may be supplied from an optical transmitter or the like, as will be described later using Figure 7.

[0025] The detector 32 is, for example, an optical heterodyne detector. Continuous light f output from the light source 31 Ｌ This is used as a local light source in optical heterodyne detection. The detector 32 is used for polarization separation and as a balanced receiver, as will be described later. Ｙ -L Ｙ Beet components and S Ｘ -L Ｘ The beet component will be detected.

[0026] The ADC33 (33-1, 33-2) converts the analog signal output by the detector 32 into a digital signal and outputs it to the signal processing unit 34.

[0027] The signal processing unit 34 performs signal processing on the digital signal, such as baseband conversion and polarization synthesis, to obtain received data.

[0028] In a system that receives optical PAM-N signals by optical heterodyne detection, if the optical PAM-N signal is superimposed at uniform intervals on the amplitude level of the optical field and received by optical heterodyne, the waveform distortion caused by the chromatic dispersion of the optical fiber transmission line is linear, and therefore can be fully compensated in principle using a DSP. It is also conceivable to receive high-speed optical PAM-N signals superimposed on optical power instead of optical field using a simpler direct detection method, but the waveform distortion caused by the chromatic dispersion becomes nonlinear and cannot be fully compensated by a DSP. This embodiment targets high-speed optical PAM-N signals with large waveform distortion due to the chromatic dispersion of the optical fiber transmission line. For this reason, this embodiment assumes a system configuration in which the PAM-N signal is superimposed on an optical field and received by optical heterodyne detection.

[0029] (Processing of the optical communication device) The transmitted optical signal is generated by the optical transmitter 2 by amplitude modulating the optical electric field with a PAM-N signal. Note that the modulation is assumed to be single-polarization modulation, not dual-polarization modulation. Figure 2 is a diagram showing an example of the configuration of the optical communication device 3 in this embodiment. The detector 32 includes, for example, a first polarization separator 321-1, a second polarization separator 321-2, a first balanced receiver 325-1, and a second balanced receiver 325-2. The signal processing unit 34 includes, for example, a baseband conversion unit 341 and a polarization combining unit 342.

[0030] Since the polarization of the continuous light is input at an angle of 45 degrees with respect to the orthogonal polarization axis of the first polarization separator 321-1, L Ｘ and L Ｙ The optical power is output uniformly. On the other hand, the second polarization separator 321-2 receives the received optical signal in an arbitrary polarization state and outputs mutually orthogonal polarization components (S Ｘ and S Ｙ ) are separated into these. Alternatively, an optical splitter that evenly distributes the optical power can be used instead of the first polarization separator 321-1.

[0031] Equation (1) is the complex representation of the optical field (Y polarization) representing the received optical signal input to the first balanced receiver 325-1. Equation (2) is the complex representation of the optical field (X polarization) representing the received optical signal input to the second balanced receiver 325-2. Equation (3) is the complex representation of the optical field (Y polarization) representing the station emission input to the first balanced receiver 325-1. Equation (4) is the complex representation of the optical field (X polarization) representing the station emission input to the second balanced receiver 325-2.

[0032]

[0033]

[0034]

[0035]

[0036] In equations (1) to (4), E Ｓ (t) is the optical signal input to the optical communication device 3. E Ｌθ is the optical electric field amplitude of the light emitted by the station, which is input to the optical communication device 3. t is time. α is the ratio of the optical intensity of the orthogonal polarization components of the received optical signal separated by the second polarization separator 321-2. Ｓ＿Ｙ (t) and θ Ｌ＿Ｙ (t) (Note that in the following explanation, "_" means that what follows "_" is a subscript, for example S_Y is S Ｙ δ(t) represents the optical phase of the Y-polarized component after polarization separation. Ｓ＿Ｙ This is the optical phase shift with respect to (t). ε(t) is the θ of the X-polarization component after polarization separation. Ｌ＿Ｙ This is the optical phase shift with respect to (t). In other words, the relationship with the optical phase of the X-polarized component after polarization separation is θ Ｓ＿Ｘ (t) = θ Ｓ＿Ｙ (t) - δ(t), θ Ｌ＿Ｘ (t) = θ Ｌ＿Ｙ It is expressed as (t) - ε(t). Any polarization state of a received optical signal can be described by setting the parameters α and δ(t).

[0037] Equation (5) below represents the photocurrent I output by the first balance receiver 325-1. ＩＦ＿Ｘ (g1) is given by the following equation (6), which is the photocurrent I output by the second balance receiver 325-2. ＩＦ＿Ｙ (g2)

[0038]

[0039]

[0040] Note that in equations (5) and (6), f ＩＦ This is the intermediate frequency, and f ＩＦ = | f Ｓ -f Ｌ | is the case. Also, R is the light receiving sensitivity of the balanced receiver 325 (first balanced receiver 325-1, second balanced receiver 325-2). The photocurrent of equation (5)-(6) is sampled using the ADC 33 and converted into digital data. Subsequent processing is performed in the digital domain by the signal processing unit 34.

[0041] In the signal processing unit 34, first, the I in the intermediate frequency band of optical heterodyne detection is processed. ＩＦ＿Ｙ (t) and I ＩＦ＿ＸThe baseband conversion unit 341 downconverts (t) to the baseband band by synchronous detection. If synchronous detection is performed by the first signal processing method or the third signal processing method described later, the baseband signal I shown in the following equations (7) and (8) Ｂ＿Ｙ (t) and I Ｂ＿Ｘ (t) is obtained. Furthermore, by performing polarization synthesis on the polarization synthesis unit 342 according to the following equation (9), the transmitted optical field amplitude, i.e., the transmitted data, can be detected as received data.

[0042]

[0043]

[0044]

[0045] On the other hand, when synchronous detection is performed using the second signal processing method described later, the baseband signal I shown in equations (10) and (11) below Ｂ＿Ｙ (t) and ＩＢ＿Ｘ (t) is obtained. Furthermore, by performing polarization synthesis on the polarization synthesis unit 342 according to the following equation (12), the transmitted optical field amplitude, i.e., the transmitted data, can be detected as received data.

[0046]

[0047]

[0048]

[0049] (Baseband Conversion Unit) Figure 3 is a diagram illustrating the first signal processing method by the signal processing unit of this embodiment. The signal processing unit 34 includes, for example, a baseband conversion unit 341a and a polarization combining unit 342a. The signal processing unit 34 also includes an input unit and an output unit. The baseband conversion unit 341a includes, for example, two BPFs 3411a (3411a-1, 3411a-2), two mixers 3412a (3412a-1, 3412a-2), and two LPFs 3413a (3413a-1, 3413a-2).

[0050] The BPF3411a-1 is a bandpass filter that receives the intermediate frequency band fＩＦ＿Ｘ From the PAM-N (N≧2) signal spectrum, the intermediate frequency component (f ＩＦ＿Ｘ ) extracts only the beat component (I) of the received intermediate frequency band. Mixer 3412a-1 extracts only the beat component (I) of the intermediate frequency band. ＩＦ＿Ｘ ) and the extracted intermediate frequency component f ＩＦ＿Ｘ Multiply by . LPF3413a-1 is a low-pass filter. The output of mixer 3412a-1 contains the baseband component and the harmonic component (2f ＩＦ ) contains the desired baseband component (I Ｂ＿Ｘ Extract the following:

[0051] BPF3411a-2 receives the intermediate frequency band f ＩＦ＿Ｙ From the PAM-N signal spectrum, the intermediate frequency component (f ＩＦ＿Ｙ ) extracts only the beat component (I) of the received intermediate frequency band. Mixer 3412a-2 extracts only the beat component (I) of the intermediate frequency band. ＩＦ＿Ｙ ) and the extracted intermediate frequency component f ＩＦ＿Ｙ Multiply by . LPF3413a-2 is the harmonic component (2f ＩＦ ) removes the desired baseband component (I Ｂ＿Ｙ Extract the following:

[0052] Polarization synthesis unit 342a is √(I Ｂ＿Ｘ ２ +I Ｂ＿Ｙ ２ ), or √(I Ｂ＿Ｘ ＊ I Ｂ＿Ｘ +I Ｂ＿Ｙ ＊ I Ｂ＿Ｙ The received data is obtained by calculating the complex conjugate. Note that the superscript "*" is the complex conjugate.

[0053] FIG. 4 is a diagram for explaining a second signal processing method by the signal processing unit of the present embodiment. The signal processing unit 34 includes, for example, a baseband conversion unit 341b and a polarization synthesis unit 342b. The signal processing unit 34 also includes an input unit and an output unit. The baseband conversion unit 341b includes, for example, three oscillators 3414b (3414b-1, 3414b-2), two mixers 3412b (3412b-1, 3412b-2), and two LPFs 3413b (3413b-1, 3413b-2).

[0054] The oscillator 3414b-1 outputs a complex signal exp(2πjf ＩＦ t) composed of a sine wave signal and a cosine wave signal orthogonal to each other. The mixer 3412a-1 multiplies the received intermediate frequency band beat component (I ＩＦ＿Ｘ ) and the complex signal exp(2πjf ＩＦ t). Since the mixer output includes a baseband component and a harmonic signal component (2f ＩＦ ), it is passed through the LPF 3413b-1 to extract the desired baseband component (I Ｂ＿Ｘ ).

[0055] The oscillator 3414b-2 outputs a complex signal exp(2πjf ＩＦ t) composed of a sine wave signal and a cosine wave signal orthogonal to each other. The mixer 3412a-2 multiplies the received intermediate frequency band beat component (I ＩＦ＿Ｙ ) and the complex signal exp(2πjf ＩＦ t). The LPFs 3413b-2 removes the harmonic signal component (2f ＩＦ ) and extracts the desired baseband component (I Ｂ＿Ｙ ).

[0056] The polarization synthesis unit 342b calculates √(I Ｂ＿Ｘ ＊ I Ｂ＿Ｘ +I Ｂ＿Ｙ ＊ I Ｂ＿Ｙ ) to obtain the received data.

[0057] Figure 5 is a diagram illustrating a third signal processing method by the signal processing unit of this embodiment. The signal processing unit 34 includes, for example, a baseband conversion unit 341c and a polarization combining unit 342c. The signal processing unit 34 also includes an input unit and an output unit. The baseband conversion unit 341c includes, for example, two mixers 3412c (3412c-1, 3412c-2) and two LPFs 3413c (3413c-1, 3413c-2).

[0058] Mixer 3412c-1 processes the beat component (I) of the received intermediate frequency band. ＩＦ＿Ｘ The signal is split into two and multiplied. The mixer output contains the baseband component and the harmonic component (2f). ＩＦ ) is included, so the LPF3413c-1 is passed through to obtain the desired baseband component (I Ｂ＿Ｘ Extract the following:

[0059] Mixer 3415c-2 processes the beat component (I) of the received intermediate frequency band. ＩＦ＿Ｙ ) is split into two and multiplied. The LPF3416c-2 is used to filter out the harmonic components (2f ＩＦ ) removes the desired baseband component (I Ｂ＿Ｙ Extract the following:

[0060] Polarization synthesis unit 342c is √(I Ｂ＿Ｘ ２ +I Ｂ＿Ｙ ２ ), or √(I Ｂ＿Ｘ ＊ I Ｂ＿Ｘ +I Ｂ＿Ｙ ＊ I Ｂ＿Ｙ ) is calculated to obtain the received data.

[0061] The received data obtained by the configuration and processing described using Figures 3 to 5 is a PAM-N signal proportional to the transmitted data. Note that the configuration and method described using Figures 3 to 5 are examples and are not limited to the configuration or method used to acquire received data.

[0062] (Processing Procedure) Figure 6 is a flowchart of the processing of the optical communication device in this embodiment.

[0063] (Step S1) The first polarization separator 321-1 of the detector 32 separates the continuous light into orthogonal polarization components (LＸ and L Ｙ ) is separated into.

[0064] (Step S2) The second polarization separator 321-2 of the detector 32 separates the received optical signal into orthogonal polarization components (S Ｘ and S Ｙ ) is separated into.

[0065] (Step S3) The first balanced receiver 325-1 of the detector 32 separates the two polarization components obtained by the first polarization separator 321-1 into S Ｘ and L Ｘ Convert the combination into an electrical signal and the beat component is the photocurrent I ＩＦ＿Ｘ Generates.

[0066] (Step S4) The second balanced receiver 325-2 of the detector 32 separates the two polarization components obtained by the second polarization separator 321-2 into S Ｙ and L Ｙ Convert the combination into an electrical signal and the beat component is the photocurrent I ＩＦ＿Ｙ Generates.

[0067] (Step S5) ADC33-1 receives photocurrent I ＩＦ＿Ｘ It converts the analog signal to a digital signal. The ADC33-2 uses photocurrent I ＩＦ＿Ｙ Converts an analog signal to a digital signal.

[0068] (Step S6) The baseband conversion unit 341 of the signal processing unit 34 is ＩＦ＿Ｘ From the desired baseband component I Ｂ＿Ｘ Extract I ＩＦ＿Ｙ From the desired baseband component I Ｂ＿Ｙ Extract it.

[0069] (Step S7) The polarization combining unit 342 of the signal processing unit 34 calculates √(I Ｂ＿Ｘ ２ +I Ｂ＿Ｙ ２ ), or √(I Ｂ＿Ｘ ＊ I Ｂ＿Ｘ +I Ｂ＿Ｙ ＊ I Ｂ＿Ｙ ) is calculated to obtain the received data.

[0070] As described above, in this embodiment, when a PAM-N signal (where N is an integer of 2 or more) is received by heterodyne detection, the baseband component output from the baseband conversion unit is polarized and combined using a simple calculation formula (equation (13) or equation (14)). Note that equation (13) is the desired baseband component (I Ｂ＿Ｘ , I Ｂ＿Ｙ It is the square root of the sum of ). Also, equation (14) is the desired baseband component (I Ｂ＿Ｘ , I Ｂ＿Ｙ It is the square root of the sum of the products of () and their complex conjugates.

[0071]

[0072]

[0073] In other words, in this embodiment, the detector 32 detects the orthogonal polarization component (S) of the separated received optical signal. Ｘ and S Ｙ ), the orthogonal polarization component of the local emission (L Ｘ and L Ｙ Using ), L is used in the intermediate frequency band of optical heterodyne detection. Ｘ and S Ｘ The beat component (I ＩＦ＿Ｘ ), L Ｙ and S Ｙ The beat component (I ＩＦ＿Ｙ In this embodiment, the ADC 33 is configured to generate a baseband signal (I Ｂ＿Ｘ and I Ｂ＿Ｙ The waveforms are converted to ( ) and then polarization synthesis is performed by calculating equation (13) or equation (14).

[0074] As a result, according to this embodiment, when receiving an optical signal using an optical heterodyne detector with a polarization diversity configuration, polarization synthesis can be performed using the simple equation (13) or equation (14), thereby significantly reducing the processing time required for polarization synthesis in digital signal processing compared to when polarization synthesis is performed by the maximum ratio synthesis method or MISO (Multi Input Single Output). As a result, according to this embodiment, the processing delay required for polarization synthesis can be significantly reduced when performing digital signal processing, and the effect of enabling the polarization synthesis process to follow large polarization fluctuations can be expected. In the example described above, the example of N (≧2) in a PAM-N signal was explained, but in the case of multi-level modulation (N≧3), the baseband component (I Ｂ＿Ｘ , I Ｂ＿Ｙ A greater effect can be obtained by calculating the square root of the sum of the products of (14) and the complex conjugate and performing polarization synthesis. When N=2, comparing the case where the PAM signal is superimposed on the optical power and the case where the PAM signal is superimposed on the optical field, there is no difference in the binary optical signal levels of the two, so even the former optical signal can be received using the same polarization synthesis method as the latter. In contrast, when N≧3, the optical signal levels of the N values ​​of the two are different, so the former optical signal cannot be received using the same polarization synthesis method as the latter.

[0075] (Modified Configuration) Figure 7 shows an example of a modified configuration of the optical communication system of this embodiment. The optical communication system 1A includes, for example, an optical transmitter 2A and an optical communication device 3A. The optical transmitter 2A and the optical communication device 3A are connected via an optical fiber transmission line 9. The optical transmitter 2A includes, for example, a Tx signal processing unit 21, an amplitude modulator 23, a two-wavelength light source 24, and a demultiplexer 25. The optical communication device 3A includes, for example, a detector 32A (optical heterodyne detector), an ADC 33A (33A-1, 33A-2) (converter), and a signal processing unit 34A.

[0076] The configuration in Figure 7 is an example of a system configuration when the light source used as the local emitter is located in the optical transmitter. In this configuration, the optical transmitter 2A emits two wavelengths (f Ｓ and f ＬA dual-wavelength light source 24 is arranged to output a set of optical frequencies (f), and the output of the dual-wavelength light source 24 is split by a demultiplexer 25 to produce an optical frequency f Ｓ and f Ｌ It is separated into continuous light. Optical frequency f Ｓ The continuous light is sent to the amplitude modulator 23 in the optical transmitter 2A and used to generate the transmitted optical signal. Meanwhile, the optical frequency f Ｌ The continuous light is supplied to the optical communication device 3A via a transmission path separate from the optical fiber transmission path 9, and is used as local light emission in the detector 32A. Ｓ and f Ｌ The optical frequency set is a modulated optical sideband generated by modulating a single-wavelength continuous light output from a light source with a sine wave signal output from an electrical oscillator.

[0077] This modified example differs from the configuration in Figure 1, f Ｓ and f Ｌ The optical frequency difference is determined by the frequency of the sinusoidal signal output from the electrical oscillator, which has higher frequency stability than the light source, and therefore the intermediate frequency (f) during optical heterodyne detection by detector 32A ＩＦ = | f Ｓ -f Ｌ |)) remains constant. In the configuration of Figure 1, separate light sources (22, 31) are placed in the optical transmitter 2A and the optical communication device 3A, so f Ｓ and f Ｌ The optical frequency difference, i.e., f in optical heterodyne detection. ＩＦ This results in variations. Therefore, in the configuration shown in Figure 1, it was necessary to provide a margin in the bandwidth of the detector 32 and ADC 33 to account for these variations. In contrast, in the modified configuration, f ＩＦ Since this value is always constant, there is no need to provide a margin in the bandwidth of the detector 32A and ADC 33A, and the bandwidth requirements of the devices can be significantly reduced.

[0078] <Second Embodiment> Figure 8 shows an example of the configuration of the optical communication device according to this embodiment. The optical communication device 3B includes, for example, a detector 32B (optical heterodyne detector), an ADC 33B (33B-1 to 33B-4) (converter), and a signal processing unit 34B. The optical communication device 3B is, for example, an optical receiver. The detector 32B includes, for example, a first polarization separator 321B-1, a second polarization separator 321B-2, an optical splitter 323B (323B-1 to 323B-4), and a balanced receiver 325B (first balanced receiver 325B-1 to fourth balanced receiver 325B-4). The signal processing unit 34B includes, for example, a baseband converter 341B and a polarization combiner 342B. The signal processing unit 34B also includes an input unit and an output unit.

[0079] In the following explanation, the four components generated by the first polarization separator 321B-1 and the second polarization separator 321B-2—the X-polarization component of the local emission, the Y-polarization component of the local emission, the X-polarization component of the signal light, and the Y-polarization component of the signal light—are referred to as L, respectively. Ｘ , L Ｙ S Ｘ S Ｙ The first polarization separator 321B-1 receives continuous light f from the light source 31. Ｌ The following is input. The first polarization separator 321B-1 receives the received optical signal f Ｌ The orthogonal polarization component (L Ｘ and L Ｙ ) is separated into orthogonal polarization component L Ｙ The signal is input to the optical splitter 323B-1, and the orthogonal polarization component L Ｘ The signal is input to the optical splitter 323B-2. The received optical signal f is input to the second polarization separator 321B-2. Ｓ The following is input. The second polarization separator 321B-2 receives the received optical signal f Ｓ orthogonal polarization component (S Ｘ and S Ｙ ) is separated into orthogonal polarization component S Ｙ The signal is input to the optical splitter 323B-3, and the orthogonal polarization component S Ｘ This signal is input to the optical splitter 323B-4.

[0080] The optical splitter 323B-1 receives the orthogonal polarization component L. ＹThe signal is split into two, one branch is input to the first balanced receiver 325B-1, and the other branch is input to the third balanced receiver 325B-3. The optical splitter 323B-2 receives the input orthogonal polarization component L Ｘ The signal is split into two, one branch is input to the second balanced receiver 325B-2, and the other branch is input to the fourth balanced receiver 325B-4. The optical splitter 323B-3 receives the input orthogonal polarization component S Ｙ The signal is split into two, one branch is input to the first balanced receiver 325B-1, and the other branch is input to the second balanced receiver 325B-2. The optical splitter 323B-4 receives the input orthogonal polarization component S Ｘ The signal is split into two, one branch is input to the third balanced receiver 325B-3, and the other branch is input to the fourth balanced receiver 325B-4.

[0081] The first balance receiver 325B-1 receives the S Ｙ and L Ｙ Convert the combination into an electrical signal S Ｙ -L Ｙ The beat component is output to ADC33B-1. The second balanced receiver 325B-2 receives the S Ｙ and L Ｘ Convert the combination into an electrical signal S Ｙ -L Ｘ The beat component is output to ADC33B-2. The third balanced receiver 325B-3 receives the S Ｘ and L Ｙ Convert the combination into an electrical signal S Ｘ -L Ｙ The beat component is output to ADC33B-3. The fourth balanced receiver 325B-4 receives the S Ｘ and L Ｘ Convert the combination into an electrical signal S Ｘ -L Ｘ The beat component is output to ADC33B-4. That is, detector 32B generates beat components for the Y component of the orthogonal polarization component of the received optical signal, the X component of the orthogonal polarization component of the received optical signal, the Y component of the orthogonal polarization component of the station light emission, and the X component of the orthogonal polarization component of the station light emission, by combining the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the station light emission.

[0082] The ADC33B samples the analog output signals from each of the balanced receivers 325B (the first balanced receiver 325B-1 to the fourth balanced receiver 325B-4) and converts them into digital signals.

[0083] Alternatively, a non-balanced photoelectric converter can be used instead of a balanced receiver.

[0084] Here, assuming that in the optical communication system 1A of Figure 7, the light source is placed at the position of the optical transmitter and the local light emission is remotely supplied to the optical communication device, in a configuration where the light source and the first polarization separator are remotely located, the local light emission in an arbitrary polarization state is incident on the first polarization separator, L Ｙ Only light power is output L Ｘ It is also possible that the optical power of S is zero, or vice versa. In the former case, the received optical signal has only an X-polarization component; in the latter case, the received optical signal has only a Y-polarization component. Ｙ -L Ｙ Beet components and S Ｘ -L Ｘ None of the beat components are output, making it impossible to receive the signal.

[0085] In contrast, in the configuration of this embodiment, S Ｙ -L Ｙ Beet components and S Ｘ -L Ｘ In addition to beet components, S Ｙ -L Ｘ Beet component, S Ｘ -L Ｙ A beat component is generated. Thus, according to this embodiment, L Ｙ Only light power is output L Ｘ Even if the optical power of is zero and the received optical signal only has an X-polarization component, S Ｘ -L Ｙ Because a beat component is generated, signal reception becomes possible. Furthermore, according to this embodiment, L Ｘ Only light power is output L Ｙ Even if the optical power of is zero and the received optical signal has only a Y-polarization component, S Ｙ -L ＸBecause a beat component is generated, signal reception becomes possible.

[0086] (Baseband Conversion Unit) Figure 9 is a diagram illustrating a fourth signal processing method by the signal processing unit of this embodiment. The signal processing unit 34B includes, for example, a baseband conversion unit 341Ba and a polarization combining unit 342Ba. The baseband conversion unit 341Ba includes, for example, four BPFs 3411Ba (3411Ba-1, 3411Ba-2, 3411Ba-3, 3411Ba-4), four mixers 3412Ba (3412Ba-1, 3412Ba-2, 3412Ba-3, 3412Ba-4), and four LPFs 3413Ba (3413Ba-1, 3413Ba-2, 3413Ba-3, 3413Ba-4).

[0087] BPF3411Ba-m (where m is one of 1 to 4) extracts the intermediate frequency component (f) from the PAM-N signal spectrum of the received intermediate frequency band. ＩＦ Only the extracted intermediate frequency component (f ＩＦ The beat component (m=1; I) of the received intermediate frequency band is output to mixer 3412Ba-m. ＩＦ＿ＹＹ m=2; I ＩＦ＿ＹＸ , m=3; I ＩＦ＿ＸＹ , m=4; I ＩＦ＿ＸＸ ) is multiplied with the extracted intermediate frequency component. The LPF3413Ba-m is used to multiply the harmonic component (2f ＩＦ ) removes the desired baseband component (m=1; I Ｂ＿ＹＹ m=2; I Ｂ＿ＹＸ , m=3; I Ｂ＿ＸＹ , m=4; I Ｂ＿ＸＸ Extract ). Polarization synthesis unit 342Ba is √(I Ｂ＿ＸＸ ２ +I Ｂ＿ＸＹ ２ +I Ｂ＿ＹＸ ２ +I Ｂ＿ＹＹ ２ ), or √(I Ｂ＿ＸＸ ＊ I Ｂ＿ＸＸ +I Ｂ＿ＸＹ ＊ I Ｂ＿ＸＹ +I Ｂ＿ＹＸ ＊ IＢ＿ＹＸ +I Ｂ＿Ｙ ＊ I Ｂ＿ＹＹ ) is calculated to obtain the received data.

[0088] Figure 10 is a diagram illustrating a fifth signal processing method by the signal processing unit of this embodiment. The signal processing unit 34B includes, for example, a baseband conversion unit 341Bb and a polarization combining unit 342Bb. The baseband conversion unit 341Bb includes, for example, four oscillators 3414Bb (3414Bb-1, 3414Bb-2, 3414Bb-3, 3414Bb-4), four mixers 3412Bb (3412Bb-1, 3412Bb-2, 3412Bb-3, 3412Bb-4), and four LPFs 3413Bb (3413Bb-1, 3413Bb-2, 3413Bb-3, 3413Bb-4).

[0089] The oscillator 3414Bb-m generates a complex signal exp(2πjf) consisting of mutually orthogonal sine wave signals and cosine wave signals. ＩＦ The complex signal exp(2πjf) input from oscillator 3414Bb-m is output to mixer 3412Bb-m. ＩＦ t) and the beat component of the received intermediate frequency band (m = 1; I ＩＦ＿ＹＹ m=2; I ＩＦ＿ＹＸ , m=3; I ＩＦ＿ＸＹ , m=4; I ＩＦ＿ＸＸ Multiply by ). LPF3413Bb-m has harmonic signal components (2f ＩＦ ) removes the desired baseband component (m=1; I Ｂ＿ＹＹ m=2; I Ｂ＿ＹＸ , m=3; I Ｂ＿ＸＹ , m=4; I Ｂ＿ＸＸ ) is extracted. Polarization synthesis unit 342Bb is √(I Ｂ＿ＸＸ ＊ I Ｂ＿ＸＸ +I Ｂ＿ＸＹ ＊ I Ｂ＿ＸＹ +I Ｂ＿ＹＸ ＊ I Ｂ＿ＹＸ +I Ｂ＿Ｙ ＊ I Ｂ＿ＹＹ ) is calculated to obtain the received data.

[0090] Figure 11 is a diagram illustrating a sixth signal processing method by the signal processing unit of this embodiment. The signal processing unit 34B includes, for example, a baseband conversion unit 341Bc and a polarization combining unit 342Bc. The baseband conversion unit 341Bc includes, for example, four mixers 3412Bc (3412Bc-1, 3412Bc-2, 3412Bc-3, 3412Bc-4) and four LPFs 3413Bc (3413Bc-1, 3413Bc-2, 3413Bc-3, 3413Bc-4).

[0091] Mixer 3412Bc-m receives the beat component of the intermediate frequency band (m = 1; I ＩＦ＿ＹＹ m=2; I ＩＦ＿ＹＸ , m=3; I ＩＦ＿ＸＹ , m=4; I ＩＦ＿ＸＸ ) is split into two and multiplied. LPF3413Bc-m has harmonic components (2f ＩＦ ) removes the desired baseband component (m=1; I Ｂ＿ＹＹ m=2; I Ｂ＿ＹＸ , m=3; I Ｂ＿ＸＹ , m=4; I Ｂ＿ＸＸ Extract ). Polarization synthesis unit 342Bc is √(I Ｂ＿ＸＸ ２ +I Ｂ＿ＸＹ ２ +I Ｂ＿ＹＸ ２ +I Ｂ＿ＹＹ ２ ), or √(I Ｂ＿ＸＸ ＊ I Ｂ＿ＸＸ +I Ｂ＿ＸＹ ＊ I Ｂ＿ＸＹ +I Ｂ＿ＹＸ ＊ I Ｂ＿ＹＸ +I Ｂ＿Ｙ ＊ I Ｂ＿ＹＹ ) is calculated to obtain the received data.

[0092] The received data obtained by the configuration and processing described using Figures 9 to 11 is a PAM-N signal proportional to the transmitted data. Note that the configuration and method described using Figures 9 to 11 are examples and are not limited to the configuration and method used to acquire received data.

[0093] The configuration of the optical transmitter in this embodiment is the same as that of optical transmitter 2A in Figure 7. The transmitted optical signal is generated by amplitude modulation of the optical electric field. In this embodiment, single-polarization modulation is assumed rather than bipolarization modulation.

[0094] The complex representation of the optical field (Y polarization) representing the received optical signal input to the first balanced receiver 325B-1 and the second balanced receiver 325B-2 is the same as equation (1) above. The complex representation of the optical field (X polarization) representing the received optical signal input to the third balanced receiver 325B-3 and the fourth balanced receiver 325B-4 is the same as equation (2) above. Equation (15) below is the complex representation of the optical field (Y polarization) representing the station emission input to the first balanced receiver 325B-1 and the third balanced receiver 325B-3. Equation (16) below is the complex representation of the optical field (X polarization) representing the station emission input to the second balanced receiver 325B-2 and the fourth balanced receiver 325B-4.

[0095]

[0096]

[0097] In equations (15) to (16), E Ｓ (t) is the optical signal input to the optical communication device. E Ｌ θ is the optical electric field amplitude of the light emitted by the station, which is input to the optical communication device 3B. t is time. α is the ratio of the optical intensity of the orthogonal polarization components of the received optical signal separated by the second polarization separator 321B-2. β is the ratio of the optical intensity of the orthogonal polarization components of the received optical signal separated by the first polarization separator 321B-1. Ｓ＿Ｙ (t) and θ Ｌ＿Ｙ (t) is the optical phase of the Y-polarized component after polarization separation. δ(t) is the θ of the X-polarized component after polarization separation. Ｓ＿Ｙ This is the optical phase shift with respect to (t). ε(t) is the θ of the X-polarization component after polarization separation. Ｌ＿Ｙ This is the optical phase shift with respect to (t).

[0098] In other words, the relationship between the optical phase of the X-polarized component after polarization separation is θ Ｓ＿Ｘ (t) = θ Ｓ＿Ｙ (t) - δ(t), θ Ｌ＿Ｘ (t) = θ Ｌ＿ＹIt is expressed as (t) - ε(t). Any polarization state of the received optical signal can be described by setting the parameters α and δ(t). Similarly, any polarization state of the station light emitted into the optical communication device 3B can also be described by setting the parameters β and δ(t).

[0099] Equation (17) below represents the photocurrent I at the output of the first balanced receiver 325B-1. ＩＦ＿ＹＹ (t) is the result. Equation (18) below gives the photocurrent I of the output of the second balanced receiver 325B-2. ＩＦ＿ＹＸ (t) is the result. Equation (19) below gives the photocurrent I of the output of the third balanced receiver 325-3. ＩＦ＿ＸＹ (t) is the output of the fourth balanced receiver 325-4. Equation (20) is the photocurrent I ＩＦ＿ＹＹ (t)

[0100]

[0101]

[0102]

[0103]

[0104] Each ADC33 samples and converts the data into digital data using the respective photocurrents from equations (17) to (20). Subsequent processing is performed in the digital domain by the signal processing unit 34B. In the signal processing unit 34B, first, the I in the intermediate frequency band of the optical heterodyne detection is processed. ＩＦ＿ＹＹ (t), I ＩＦ＿ＹＸ (t), I ＩＦ＿ＸＹ (t), I ＩＦ＿ＸＸ (t) is down-converted to the baseband band by synchronous detection. When synchronous detection is performed using the fourth signal processing method described with Figure 9 or the sixth signal processing method described with Figure 11, the baseband signal I shown in the following equations (21) to (24) Ｂ＿ＹＹ (t), I Ｂ＿ＹＸ (t), I Ｂ＿ＸＹ (t), and I Ｂ＿ＸＸ (t) is obtained. Furthermore, by performing polarization synthesis by the polarization synthesis unit 342B according to the following equation (25), the transmitted optical field amplitude, i.e., the transmitted data, can be detected as received data.

[0105]

[0106]

[0107]

[0108]

[0109]

[0110] On the other hand, when synchronous detection is performed by the fifth signal processing method described using FIG. 10, the baseband signals I shown in the following equations (26) to (29) Ｂ＿ＹＹ (t), I Ｂ＿ＹＸ (t), I Ｂ＿ＸＹ (t), and I Ｂ＿ＸＸ (t) are obtained. Further, by the following equation (30), the polarization synthesis unit 342B performs polarization synthesis, so that the transmitted optical field amplitude, that is, the transmitted data can be detected as received data.

[0111]

[0112]

[0113]

[0114]

[0115]

[0116] As described above, in the present embodiment, when receiving a PAM-N signal (N is an integer of 2 or more) by heterodyne detection, the baseband components output from the baseband conversion unit are polarization-synthesized using a simple calculation formula (the following formula (31) or formula (32)) to obtain received data. Note that formula (31) is the square root of the sum of the desired baseband components (I Ｂ＿YY , I Ｂ＿ＹＸ , I Ｂ＿ＸＹ , I Ｂ＿ＸＸ ). Further, formula (32) is the square root of the sum of the product of the desired baseband components (I Ｂ＿ＸＸ , I Ｂ＿ＹＸ , I Ｂ＿ＸＹ , I Ｂ＿ＸＸ ) and the complex conjugate.

[0117]

[0118]

[0119] (Processing Procedure) Figure 12 is a flowchart of the processing of the optical communication device in this embodiment.

[0120] (Step S11) The first polarization separator 321B-1 of the detector 32B separates the continuous light into orthogonal polarization components (L Ｘ and L Ｙ ) is separated into.

[0121] (Step S12) The second polarization separator 321B-2 of the detector 32B separates the received optical signal into orthogonal polarization components (S Ｘ and S Ｙ ) is separated into.

[0122] (Step S13) The optical splitter 323B-1 of the detector 32B receives the input quadrature polarization component L Ｙ The signal is split into two. The optical splitter 323B-2 receives the input orthogonal polarization component L Ｘ The signal is split into two. The optical splitter 323B-3 receives the input orthogonal polarization component S Ｙ The signal is split into two. The optical splitter 323B-4 receives the input orthogonal polarization component S Ｘ It branches into two paths.

[0123] (Step S14) The first balanced receiver 325B-1 of the detector 32B receives the input S Ｙ and L Ｙ Convert the combination into an electrical signal S Ｙ -L Ｙ It generates a beat component. The second balanced receiver 325B-2 receives the S Ｙ and L Ｘ Convert the combination into an electrical signal S Ｙ -L Ｘ It generates a beat component. The third balanced receiver 325B-3 receives the S Ｘ and L Ｙ Convert the combination into an electrical signal S Ｘ -L Ｙ It generates a beat component. The fourth balanced receiver 325B-4 receives the S Ｘ and L Ｘ Convert the combination into an electrical signal S Ｘ -LＸ It contains beet components.

[0124] (Step S15) The ADC 33B of the detector 32B samples the analog signal output of each of the balanced receivers 325B (first balanced receiver 325B-1 to fourth balanced receiver 325B-4) and converts it into a digital signal.

[0125] (Step S16) The baseband conversion unit 341B of the signal processing unit 34B converts the signal input from the ADC 33B to a desired baseband component I ＩＦ＿ＸＸ , I ＩＦ＿ＸＹ , I ＩＦ＿ＹＹ and I ＩＦ＿ＹＸ Extract it.

[0126] (Step S17) The polarization combining unit 342B of the signal processing unit 34B calculates equation (31) or equation (32) to obtain the received data.

[0127] As a result, according to this embodiment, when receiving an optical signal using an optical heterodyne detector with a polarization diversity configuration, polarization synthesis can be performed using the simple equation (31) or equation (32), thereby significantly reducing the processing time required for polarization synthesis in digital signal processing compared to when polarization synthesis is performed by the maximum ratio synthesis method or MISO. As a result, according to this embodiment, when performing digital signal processing, the processing delay required for polarization synthesis can be significantly reduced, and the effect of enabling the polarization synthesis process to follow large polarization fluctuations can be expected. In the example described above, the example of N (≧2) in a PAM-N signal was explained, but in the case of multi-level modulation (N≧3), the baseband component (I Ｂ＿ＸＸ , I Ｂ＿ＹＸ , I Ｂ＿ＸＹ , I Ｂ＿ＸＸ A greater effect can be obtained by calculating the square root of the sum of the products of () and the complex conjugate (Equation (32)) and performing polarization synthesis. When N=2, comparing the case where the PAM signal is superimposed on the optical power and the case where the PAM signal is superimposed on the optical field, there is no difference in the binary optical signal levels of the two, so the former optical signal can be received using the same polarization synthesis method as the latter. In contrast, when N≧3, the optical signal levels of the N values ​​of the two are different, so the former optical signal cannot be received using the same polarization synthesis method as the latter.

[0128] <Third Embodiment> Figure 13 shows an example of the configuration of the optical communication device according to this embodiment. The optical communication device 3C includes, for example, a detector 32C (optical intradyne detector), an ADC 33C (33C-1 to 33C-4) (converter), and a signal processing unit 34C. The optical communication device 3C is, for example, an optical receiver. The detector 32C includes, for example, a first polarization separator 321C-1, a second polarization separator 321C-2, an optical splitter 323C (323C-1 to 323C-4), a π / 2 delay unit 324C (324C-1, 324C-2), and a balanced receiver 325C (first balanced receiver 325C-1 to fourth balanced receiver 325C-4). The signal processing unit 34C includes, for example, a polarization combining unit 342C. Furthermore, the signal processing unit 34C includes an input unit and an output unit.

[0129] In the following explanation, the four components generated by the first polarization separator 321C-1 and the second polarization separator 321C-2—the X-polarization component of the local emission, the Y-polarization component of the local emission, the X-polarization component of the signal light, and the Y-polarization component of the signal light—are referred to as L, respectively. Ｘ , L Ｙ S Ｘ S Ｙ The first polarization separator 321C-1 receives continuous light f from the light source 31. Ｌ The following is input. The first polarization separator 321C-1 receives the optical signal f Ｌ The orthogonal polarization component (L Ｘ and L Ｙ ) is separated into orthogonal polarization component L Ｙ The signal is input to the optical splitter 323C-1, and the orthogonal polarization component L Ｘ The signal is input to the optical splitter 323C-2. The received optical signal f is input to the second polarization separator 321C-2. Ｓ The following is input. The second polarization separator 321C-2 receives the received optical signal f Ｓ orthogonal polarization component (S Ｘ and S Ｙ ) is separated into orthogonal polarization component S Ｙ The signal is input to the optical splitter 323C-3, and the orthogonal polarization component S Ｘ This signal is input to the optical splitter 323C-4.

[0130] The π / 2 delay unit 324C delays the optical phase of the input signal by π / 2. Each of the optical splitters 323C is positioned to extract the I and Q components of the received optical signal through optical phase diversity reception. The solid arrow between the optical splitter 323C and the balanced receiver 325C represents the I component, and the dashed arrow represents the Q component.

[0131] The optical splitter 323C-1 receives the input orthogonal polarization component L Ｙ The I and Q components are extracted and split into two branches. The extracted Q component is input to the first balanced receiver 325C-1 via a π / 2 delay unit 324C-1, and the extracted I component is input to the second balanced receiver 325C-2. The optical splitter 323C-2 receives the orthogonal polarization component L. Ｘ The I and Q components are extracted and split into two. The extracted Q component is input to the third balanced receiver 325C-3 via the π / 2 delay unit 324C-2, and the extracted I component is input to the fourth balanced receiver 325C-4. The optical splitter 323C-3 receives the orthogonal polarization component S Ｙ The I and Q components are extracted and split into two branches. The extracted Q component is input to the first balanced receiver 325C-1, and the extracted I component is input to the second balanced receiver 325C-2. The optical splitter 323C-4 receives the orthogonal polarization component S. Ｘ The I and Q components are extracted and split into two branches. The extracted Q component is input to the third balanced receiver 325C-3, and the extracted I component is input to the fourth balanced receiver 325C-4.

[0132] The first balance receiver 325C-1 receives the S Ｙ The Q component and the π / 2 delayed L Ｙ The combination with the Q component is converted into an electrical signal S Ｙ -L Ｙ The beat component (Q) is output to ADC33C-1. The second balanced receiver 325C-2 receives the input S Ｙ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｙ -L Ｘ The beat component (I) is output to ADC33C-2. The third balanced receiver 325C-3 receives the input S ＸThe Q component and the π / 2 delayed L Ｙ The combination with the Q component is converted into an electrical signal S Ｘ -L Ｙ The beat component (Q) is output to ADC33C-3. The fourth balanced receiver 325C-4 receives the input S Ｘ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｘ -L Ｘ The beat component (I) is output to the ADC33C-4.

[0133] The ADC33C samples the analog output signals from each of the balanced receivers 325C (the first balanced receiver 325C-1 to the fourth balanced receiver 325C-4) and converts them into digital signals.

[0134] Alternatively, instead of the balanced receiver 325C, a photoelectric converter that does not perform balanced reception can be used.

[0135] Thus, in this embodiment, the signal light and the local light emission are each divided into orthogonal polarization components (S Ｘ S Ｙ , L Ｘ , L Ｙ ) are separated and S is detected using optical intradyne detection. Ｘ -L Ｘ Beet component (I), S Ｘ -L Ｘ Beat component (Q), S Ｙ -L Ｙ Beat component (I), and S Ｙ -L Ｙ By generating a beat component (Q), it is possible to perform coherent reception using the baseband band instead of the intermediate frequency band, regardless of the input polarization state of the station's light emission. Furthermore, according to this embodiment, this configuration makes it possible to perform coherent reception using the baseband band instead of the intermediate frequency band, regardless of the input polarization state of the station's light emission.

[0136] (Processing of optical communication equipment) In the following explanation, f Ｓ and f Ｌ If they match (= f Ｃ ) will be explained, f Ｓ ≠f ＬEven in such a case, the following holds.

[0137] The transmitted optical signal is generated by amplitude-modulating the optical electric field. In this embodiment, single polarization modulation is assumed instead of dual polarization modulation. The following equation (33) is the complex representation of the optical electric field (Y polarization) of the received optical signal input to the first balanced receiver 325C-1 and the second balanced receiver 325C-2. The following equation (34) is the complex representation of the optical electric field (X polarization) of the received optical signal input to the third balanced receiver 325C-3 and the fourth balanced receiver 325C-4. The following equation (35) is the complex representation of the optical electric field (Y polarization / I component) of the local light emission input to the second balanced receiver 325C-2. The following equation (36) is the complex representation of the optical electric field (Y polarization / Q component) of the local light emission input to the first balanced receiver 325C-1. The following equation (37) is the complex representation of the optical electric field (X polarization / I component) of the local light emission input to the fourth balanced receiver 325C-4. The following equation (38) is the complex representation of the optical electric field (X polarization / Q component) of the local light emission input to the third balanced receiver 325C-3.

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144] In equations (33) to (38), E Ｓ (t) is the optical signal input to the optical communication device 3C. E Ｌ is the optical electric field amplitude of the local light emission input to the optical communication device 3C. t is time. α is the light intensity ratio of the orthogonal polarization component of the received optical signal separated by the second polarization separator 321C-2. θ Ｓ＿Ｙ (t) and θ Ｌ＿Ｙ (t) are the optical phases of the Y polarization component after polarization separation. δ (t) is the θ of the X polarization component after polarization separation Ｓ＿ＹThis is the optical phase shift with respect to (t). ε(t) is the θ of the X-polarization component after polarization separation. Ｌ＿Ｙ This is the optical phase shift with respect to (t). In other words, the relationship with the optical phase of the X-polarized component after polarization separation is θ Ｓ＿Ｘ (t) = θ Ｓ＿Ｙ (t) - δ(t), θ Ｌ＿Ｘ (t) = θ Ｌ＿Ｙ It is expressed as (t) - ε(t). Any polarization state of a received optical signal can be described by setting the parameters α and δ(t).

[0145] Equation (39) below shows the baseband photocurrent I of the output of the first balanced receiver 325C-1. ＩＹ (t) is given by the following equation (40), which is the baseband photocurrent I of the output of the second balanced receiver 325C-2. ＱＹ (t) is given by the following equation (41), which is the baseband photocurrent I of the output of the third balanced receiver 325C-3. ＩＸ (t) is given by the following equation (42), which is the baseband photocurrent I of the output of the fourth balanced receiver 325C-4. ＱＸ (t) is the result. In equations (39) to (42), R is the light receiving sensitivity of the balanced receiver.

[0146]

[0147]

[0148]

[0149]

[0150] The photocurrents in equations (39) to (40) are sampled using the ADC 33C and converted into digital data. Subsequent processing is performed in the digital domain by the signal processing unit 34C. The polarization combining unit 342C can detect the transmitted optical field amplitude, i.e., transmitted data, as received data by performing polarization combining using the following equation (43). Equation (43) is the square root of the sum of the squares of each baseband photocurrent.

[0151]

[0152] (Processing Procedure) Figure 14 is a flowchart of the processing of the optical communication device in this embodiment.

[0153] (Step S21) The first polarization separator 321C-1 of the detector 32C separates the continuous light into orthogonal polarization components (L Ｘ and L Ｙ ) Separates into:

[0154] (Step S22) The second polarization separator 321C-2 of the detector 32C separates the received optical signal into orthogonal polarization components (S Ｘ and S Ｙ ) Separates into:

[0155] (Step S23) The optical splitter 323-1 of the detector 32C receives the input quadrature polarization component L Ｙ The I and Q components are extracted and split into two. The optical splitter 323C-2 receives the orthogonal polarization component L Ｘ The optical splitter 323C-3 extracts the I and Q components from the input and splits it into two, and the input orthogonal polarization component S Ｙ The I and Q components are extracted and split into two. The optical splitter 323C-4 receives the orthogonal polarization component S Ｘ The I and Q components are extracted and the signal is split into two branches.

[0156] (Step S24) The π / 2 delay circuit 324C-1 of the detector 32C receives the quadrature polarization component L input from the optical splitter 323C-1. Ｙ The optical phase of the Q component is delayed by π / 2. The π / 2 delay unit 324C-2 delays the orthogonal polarization component L input from the optical splitter 323C-2. Ｘ The Q component of the optical phase is delayed by π / 2.

[0157] (Step S25) The first balanced receiver 325C-1 of the detector 32C receives the input S Ｙ The Q component and the π / 2 delayed L Ｙ The combination with the Q component is converted into an electrical signal S Ｙ -L Ｙ It generates a beat component (Q). The second balanced receiver 325C-2 receives the input S Ｙ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｙ -L Ｘ It generates a beat component (I). The third balanced receiver 325C-3 receives the input S Ｘ The Q component and the π / 2 delayed LＹ The combination with the Q component is converted into an electrical signal S Ｘ -L Ｙ It generates a beat component (Q). The fourth balanced receiver 325C-4 receives the input S Ｘ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｘ -L Ｘ Generates the beat component (I).

[0158] (Step S26) The ADC 33C of the detector 32C samples the analog signal output of each of the balanced receivers 325C (first balanced receiver 325C-1 to fourth balanced receiver 325C-4) and converts it into a digital signal.

[0159] (Step S27) The polarization combining unit 342C of the signal processing unit 34C calculates equation (39) to obtain the received data.

[0160] As described above, in this embodiment, when receiving a PAM-N signal (where N is an integer of 2 or more) by intradyne detection, the detector 32C is configured to separate it into an I component and a Q component. In this embodiment, the signal processing unit 34C obtains received data by polarization combining the components output from the detector 32C using a simple calculation formula (equation (39)). Note that equation (31) is the desired beat component of the I component and the Q component (I ＩＸ , I ＩＱ , I ＩＸ , I ＱＸ It is the square root of the sum of ).

[0161] <Fourth Embodiment> Figure 15 shows an example of the configuration of the optical communication device according to this embodiment. The optical communication device 3D includes, for example, a detector 32D (optical intradyne detector), an ADC 33D (33D-1 to 33D-8) (converter), and a signal processing unit 34D. The optical communication device 3D is, for example, an optical receiver. The detector 32D includes, for example, a first polarization separator 321D-1, a second polarization separator 321D-2, an optical splitter 322D (322D-1 to 322D-4), an optical splitter 323D (323D-1 to 323D-8), a π / 2 delay unit 324D (324D-1 to 324D-4), and a balanced receiver 325D (first balanced receiver 325D-1 to eighth balanced receiver 325D-8). The signal processing unit 34D includes, for example, a polarization combining unit 342D. The signal processing unit 34D also includes an input unit and an output unit.

[0162] In the following explanation, the four components generated by the first polarization separator 321D-1 and the second polarization separator 321D-2—the X-polarization component of the local emission, the Y-polarization component of the local emission, the X-polarization component of the signal light, and the Y-polarization component of the signal light—are referred to as L, respectively. Ｘ , L Ｙ S Ｘ S Ｙ The first polarization separator 321D-1 receives continuous light f from the light source 31. Ｌ The following is input. The first polarization separator 321D-1 receives the received optical signal f Ｌ The orthogonal polarization component (L Ｘ and L Ｙ ) is separated into orthogonal polarization component L Ｙ The signal is input to the optical splitter 322D-1, and the orthogonal polarization component L Ｘ The signal is input to the optical splitter 322D-2. The received optical signal f is input to the second polarization separator 321D-2. Ｓ The following is input. The second polarization separator 321D-2 receives the received optical signal f Ｓ orthogonal polarization component (S Ｘ and S Ｙ ) is separated into orthogonal polarization component S Ｙ The signal is input to the optical splitter 322D-3, and the orthogonal polarization component S Ｘ This signal is input to the optical splitter 322D-4.

[0163] The π / 2 delay unit 324D delays the optical phase of the input signal by π / 2. Each of the optical splitters 323D is positioned to extract the I and Q components of the received optical signal through optical phase diversity reception. The solid arrow between the optical splitter 323D and the balanced receiver 325D represents the I component, and the dashed arrow represents the Q component.

[0164] The optical splitter 323D-1 receives the orthogonal polarization component L. Ｙ The I and Q components are extracted and split into two branches. The extracted Q component is input to the first balanced receiver 325D-1 via a π / 2 delay unit 324D-1, and the extracted I component is input to the second balanced receiver 325D-2. The optical splitter 323D-2 receives the orthogonal polarization component L. Ｙ The I and Q components are extracted and split into two branches. The extracted Q component is input to the fifth balanced receiver 325D-5 via the π / 2 delay unit 324D-2, and the extracted I component is input to the sixth balanced receiver 325D-6. The optical splitter 323D-3 receives the orthogonal polarization component L. Ｘ The I and Q components are extracted and split into two. The extracted Q component is input to the third balanced receiver 325D-3 via the π / 2 delay unit 324D-3, and the extracted I component is input to the fourth balanced receiver 325D-4. The optical splitter 323D-4 receives the orthogonal polarization component L. Ｘ The I and Q components are extracted and split into two branches. The extracted Q component is input to the seventh balanced receiver 325D-7 via a π / 2 delay unit 324D-4, and the extracted I component is input to the eighth balanced receiver 325D-8.

[0165] The optical splitter 323D-5 receives the input orthogonal polarization component S Ｙ The I and Q components are extracted and split into two branches. The extracted Q component is input to the first balanced receiver 325D-1, and the extracted I component is input to the second balanced receiver 325D-2. The optical splitter 323D-6 receives the orthogonal polarization component S. Ｙ The I and Q components are extracted and split into two branches. The extracted Q component is input to the third balanced receiver 325D-3, and the extracted I component is input to the fourth balanced receiver 325D-4. The optical splitter 323D-7 receives the orthogonal polarization component S. ＸThe I and Q components are extracted and split into two branches. The extracted Q component is input to the fifth balanced receiver 325D-5, and the extracted I component is input to the sixth balanced receiver 325D-6. The optical splitter 323D-8 receives the orthogonal polarization component S Ｘ The I and Q components are extracted and split into two branches. The extracted Q component is input to the seventh balanced receiver 325D-7, and the extracted I component is input to the eighth balanced receiver 325D-8.

[0166] The first balance receiver 325D-1 receives the S Ｙ The Q component and the π / 2 delayed L Ｙ The combination with the Q component is converted into an electrical signal S Ｙ -L Ｙ The beat component (Q) is output to ADC33D-1. The second balanced receiver 325D-2 receives the input S Ｙ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｙ -L Ｘ The beat component (I) is output to ADC33D-2. The third balanced receiver 325D-3 receives the input S Ｙ The Q component and the π / 2 delayed L Ｘ The combination with the Q component is converted into an electrical signal S Ｙ -L Ｘ The beat component (Q) is output to ADC33D-3. The fourth balanced receiver 325D-4 receives the input S Ｙ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｙ -L Ｘ The beat component (I) is output to the ADC33D-4.

[0167] The fifth balanced receiver 325D-5 receives the S Ｘ The Q component and the π / 2 delayed L Ｙ The combination with the Q component is converted into an electrical signal S Ｘ -L Ｙ The beat component (Q) is output to ADC33D-5. The sixth balanced receiver 325D-6 receives the input S Ｘ Component I and L Ｙ The combination with the I component is converted into an electrical signal SＸ -L Ｙ The beat component (I) is output to ADC33D-6. The seventh balanced receiver 325D-7 receives the input S Ｘ The Q component and the π / 2 delayed L Ｘ The combination with the Q component is converted into an electrical signal S Ｘ -L Ｘ The beat component (Q) is output to ADC33D-7. The eighth balanced receiver 325D-8 receives the input S Ｘ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｘ -L Ｘ The beat component (I) is output to the ADC33D-8. Although the output of each balanced receiver 325D (1st balanced receiver 325D-1 to 8th balanced receiver 325D-8) is an analog signal, it is sampled by the ADC33D and converted into a digital signal.

[0168] Alternatively, a non-balanced optical converter can be used instead of the balanced receiver 325D.

[0169] Thus, in this embodiment, the signal light and the local light emission are each divided into orthogonal polarization components (S Ｘ S Ｙ , L Ｘ , L Ｙ ) are separated and S is detected using optical intradyne detection. Ｘ -L Ｘ Beet component (I), S Ｘ -L Ｙ Beat component (Q), S Ｙ -L Ｙ Beet component (I), S Ｙ -L Ｙ In addition to the beet component (Q), S Ｘ -L Ｙ Beet component (I), S Ｘ -L Ｙ Beat component (Q), S Ｙ -L Ｘ Beat component (I), and S Ｙ -L ＸBy generating a beat component (Q), coherent reception becomes possible regardless of the input polarization state of the signal light and the local emission light. With this configuration, according to this embodiment, coherent reception is possible using the baseband band instead of the intermediate frequency band, regardless of the input polarization state of the local emission light. The optical splitters 328D and 322D, which are arranged for optical phase diversity reception, can also be combined into a 1x4 type optical splitter.

[0170] (Processing of optical communication equipment) In the following explanation, f Ｓ and f Ｌ If they match (= f Ｃ ) will be explained, f Ｓ ≠f Ｌ Even in that case, the following still holds true.

[0171] The transmitted optical signal is generated by amplitude modulation of the optical electric field. In this embodiment, single-polarization modulation is assumed, rather than bipolarization modulation. The complex representation of the optical electric field (Y polarization) representing the received optical signal input to the first balanced receiver 325D-1 to the fourth balanced receiver 325D-4 is the same as equation (33). The complex representation of the optical electric field (X polarization) representing the received optical signal input to the fifth balanced receiver 325D-5 to the eighth balanced receiver 325D-8 is the same as equation (34).

[0172] Equation (44) is the complex representation of the optical field (Y polarization / I component) representing the station emission input to the second balanced receiver 325D-2 and the sixth balanced receiver 325D-6. Equation (45) is the complex representation of the optical field (Y polarization / Q component) representing the station emission input to the first balanced receiver 325D-1 and the fifth balanced receiver 325D-5. Equation (46) is the complex representation of the optical field (X polarization / I component) representing the station emission input to the fourth balanced receiver 325D-4 and the eighth balanced receiver 325D-8. Equation (47) is the complex representation of the optical field (X polarization / I component) representing the station emission input to the third balanced receiver 325D-3 and the seventh balanced receiver 325D-7.

[0173]

[0174]

[0175]

[0176]

[0177] In equations (44) to (47), E Ｓ (t) is the optical signal input to the optical communication device 3D. E Ｌ θ is the optical electric field amplitude of the light emitted by the station, which is input to the optical communication device 3D. t is time. α is the ratio of the optical intensity of the orthogonal polarization components of the received optical signal separated by the second polarization separator 321D-2. β is the ratio of the optical intensity of the orthogonal polarization components of the received optical signal separated by the first polarization separator 321D-1. Ｓ＿Ｙ (t) and θ Ｌ＿Ｙ (t) is the optical phase of the Y-polarized component after polarization separation. δ(t) is the θ of the X-polarized component after polarization separation. Ｓ＿Ｙ This is the optical phase shift with respect to (t). ε(t) is the θ of the X-polarization component after polarization separation. Ｌ＿Ｙ This is the optical phase shift with respect to (t). In other words, the relationship with the optical phase of the X-polarized component after polarization separation is θ Ｓ＿Ｘ (t) = θ Ｓ＿Ｙ (t) - δ(t), θ Ｌ＿Ｘ (t) = θ Ｌ＿Ｙ It is expressed as (t) - ε(t). Any polarization state of the received optical signal can be described by setting the parameters α and δ(t). Similarly, any polarization state of the station light emitted into the optical communication device 3D can be described by setting the parameters β and ε(t).

[0178] Equation (48) below shows the baseband photocurrent I of the output of the first balanced receiver 325D-1. ＩＹＹ (t) is given by the following equation (49), which is the baseband photocurrent I of the output of the second balanced receiver 325D-2. ＱＹＹ (t) is given by the following equation (50), which is the baseband photocurrent I of the output of the third balanced receiver 325D-3. ＩＹＸ (t) is given by the following equation (51), which is the baseband photocurrent I of the output of the fourth balanced receiver 325D-4. ＱＹＸ (t) is given by the following equation (52), which is the baseband photocurrent I of the output of the first balanced receiver 325D-1. ＩＸＹ(t) is given by the following equation (53), which is the baseband photocurrent I of the output of the second balanced receiver 325D-2. ＱＸＹ (t) is given by the following equation (54), which is the baseband photocurrent I of the output of the third balanced receiver 325D-3. ＩＸＸ (t) is given by the following equation (55), which is the baseband photocurrent I of the output of the fourth balanced receiver 325D-4. ＱＸＸ (t)

[0179]

[0180]

[0181]

[0182]

[0183]

[0184]

[0185]

[0186]

[0187] The photocurrents in equations (48) to (55) are sampled using the ADC 33D and converted into digital data. Subsequent processing is performed in the digital domain by the signal processing unit 34D. The polarization combining unit 342D can detect the transmitted optical field amplitude, i.e., the transmitted data, as received data by performing polarization combining according to the following equation (56).

[0188]

[0189] (Processing Procedure) Figure 15 is a flowchart of the processing of the optical communication device in this embodiment.

[0190] (Step S31) The first polarization separator 321D-1 of the detector 32D separates the continuous light into orthogonal polarization components (L Ｘ and L Ｙ ) Separates into:

[0191] (Step S32) The second polarization separator 321D-2 of the detector 32D separates the received optical signal into orthogonal polarization components (S Ｘ and S Ｙ ) Separates into:

[0192] (Step S33) The optical splitter 322D-1 of the detector 32D receives the input quadrature polarization component L Ｙ The signal is split into two. The optical splitter 322D-2 receives the input orthogonal polarization component L. Ｘ The signal is split into two. The optical splitter 322D-3 receives the input orthogonal polarization component S Ｙ The signal is split into two. The optical splitter 322D-4 receives the input orthogonal polarization component S Ｘ It branches into two paths.

[0193] (Step S34) The optical splitter 323D-1 of the detector 32D receives the input orthogonal polarization component L Ｙ The I and Q components are extracted and split into two. The optical splitter 323D-2 receives the orthogonal polarization component L. Ｙ The I and Q components are extracted and split into two. The optical splitter 323D-3 receives the orthogonal polarization component L Ｘ The I and Q components are extracted and split into two. The optical splitter 323D-4 receives the orthogonal polarization component L Ｘ The I and Q components are extracted and split into two. The optical splitter 323D-5 receives the orthogonal polarization component S Ｙ The I and Q components are extracted and split into two. The optical splitter 323D-6 receives the orthogonal polarization component S Ｙ The I and Q components are extracted and split into two. The optical splitter 323D-7 receives the orthogonal polarization component S Ｘ The I and Q components are extracted and split into two. The optical splitter 323D-8 receives the orthogonal polarization component S Ｘ The I and Q components are extracted and the signal is split into two branches.

[0194] (Step S35) The π / 2 delay circuit 324D-1 of the detector 32D receives the quadrature polarization component L input from the optical splitter 323D-1. Ｙ The optical phase of the Q component is delayed by π / 2. The π / 2 delay unit 324D-2 delays the orthogonal polarization component L input from the optical splitter 323D-1. Ｙ The optical phase of the Q component is delayed by π / 2. The π / 2 delay unit 324D-3 delays the quadrature polarization component L input from the optical splitter 323D-3. ＸThe optical phase of the Q component is delayed by π / 2. The π / 2 delay unit 324D-4 delays the orthogonal polarization component L input from the optical splitter 323D-4. Ｘ The Q component of the optical phase is delayed by π / 2.

[0195] (Step S36) The first balanced receiver 325D-1 of the detector 32D receives the input S Ｙ The Q component and the π / 2 delayed L Ｙ The combination with the Q component is converted into an electrical signal S Ｙ -L Ｙ It generates a beat component (Q). The second balanced receiver 325D-2 receives the input S Ｙ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｙ -L Ｘ It generates a beat component (I). The third balanced receiver 325D-3 receives the input S Ｙ The Q component and the π / 2 delayed L Ｘ The combination with the Q component is converted into an electrical signal S Ｙ -L Ｘ It generates a beat component (Q). The fourth balanced receiver 325D-4 receives the input S Ｙ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｙ -L Ｘ It generates a beat component (I). The fifth balanced receiver 325D-5 receives the input S Ｘ The Q component and the π / 2 delayed L Ｙ The combination with the Q component is converted into an electrical signal S Ｘ -L Ｙ It generates a beat component (Q). The sixth balanced receiver 325D-6 receives the input S Ｘ Component I and L Ｙ The combination with the I component is converted into an electrical signal S Ｘ -L Ｙ It generates a beat component (I). The seventh balanced receiver 325D-7 receives the input S Ｘ The Q component and the π / 2 delayed L Ｘ The combination with the Q component is converted into an electrical signal S Ｘ -L ＸIt generates a beat component (Q). The eighth balanced receiver 325D-8 receives the input S Ｘ Component I and L Ｘ The combination with the I component is converted into an electrical signal S Ｘ -L Ｘ Generates the beat component (I).

[0196] (Step S37) The ADC 33D of the detector 32D samples the analog signal output of each of the balanced receivers 325D (first balanced receiver 325D-1 to eighth balanced receiver 325D-8) and converts it into a digital signal.

[0197] (Step S38) The polarization combining unit 342D of the signal processing unit 34D calculates equation (56) to obtain the received data.

[0198] As described above, in this embodiment, when receiving a PAM-N signal (where N is an integer of 2 or more) by intradyne detection, the detector 32D is configured to separate it into the I component and the Q component. In this embodiment, the signal light and the local light emission are each divided into orthogonal polarization components (S Ｘ S Ｙ , L Ｘ , L Ｙ ) are separated and S is detected using optical intradyne detection. Ｘ -L Ｘ Beet component (I), S Ｘ -L Ｙ Beat component (Q), S Ｙ -L Ｙ Beet component (I), S Ｙ -L Ｙ In addition to the beet component (Q), S Ｘ -L Ｙ Beet component (I), S Ｘ -L Ｙ Beat component (Q), S Ｙ -L Ｘ Beat component (I), and S Ｙ -L Ｘ A beat component (Q) is generated. In this embodiment, the signal processing unit 34D obtains received data by polarization combining the components output from the detector 32D using a simple calculation formula (equation (56)). Equation (56) is a desired beat component (I) of the I component and the Q component. ＩＹＹ , IＱＹＹ , I ＩＹＸ , I ＱＹＸ , I ＩＸＹ、 I ＱＸＹ、 I ＩＸＸ、 I ＱＸＸ ) This is the square root of the sum of the products of each complex conjugate.

[0199] As a result, according to this embodiment, coherent reception is possible regardless of the input polarization state of the signal light and the station light emission. Furthermore, according to this embodiment, coherent reception is possible regardless of the input polarization state of the station light emission using the baseband band instead of the intermediate frequency band.

[0200] In each of the embodiments described above, the signal processing unit 34 (34A, 34B, 34C, 34D) is configured using a processor such as a CPU (Central Processing Unit) and memory. The signal processing unit 34 (34A, 34B, 34C, 34D) functions, for example, as a baseband conversion unit and a polarization synthesis unit, when the processor executes a program. All or part of the functions of the signal processing unit 34 (34A, 34B, 34C, 34D) may be implemented using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above program may be recorded on a computer-readable recording medium. Computer-readable recording media include, for example, portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, semiconductor memory devices (e.g., SSDs: Solid State Drives), and memory devices such as hard disks and semiconductor memory devices built into computer systems. The above program may be transmitted via a telecommunications line.

[0201] Figure 17 is a schematic diagram of an example hardware configuration of an information processing device applied to an embodiment. The information processing device 6 comprises a processor 601, a main memory 602, a communication interface 603, an auxiliary storage device 604, an input / output interface 605, and an internal bus 606. The processor 601, the main memory 602, the communication interface 603, the auxiliary storage device 604, and the input / output interface 605 are connected to each other via the internal bus 606 so as to be able to communicate with each other. The information processing device 6 may be applied, for example, to the signal processing unit 34 (34A, 34B, 34C, 34D) of an optical communication device 3 (3A, 3B, 3C, 3D). In this case, for example, the input and output units of the signal processing unit 34 (34A, 34B, 34C, 34D) may be configured using the communication interface 603. For example, the storage unit of the signal processing unit 34 (34A, 34B, 34C, 34D) may be configured using the main memory 602 and the auxiliary storage device 604. Furthermore, the signal processing unit 34 (34A, 34B, 34C, 34D) may be configured using a processor 601, a main memory 602, and an auxiliary memory 604.

[0202] In the embodiments described above, examples of N (≧2) in the PAM-N signal were explained. However, in the case of multi-level modulation (N≧3), a greater effect can be obtained by calculating the square root of the sum of the products of the baseband component and its complex conjugate and performing polarization synthesis. When N=2, comparing the case where the PAM signal is superimposed on optical power and the case where the PAM signal is superimposed on optical field, there is no difference in the binary optical signal levels of the two. Therefore, the former optical signal can be received using the same polarization synthesis method as the latter. In contrast, when N≧3, the optical signal levels of the N values ​​of the two are different, and the former optical signal cannot be received using the same polarization synthesis method as the latter.

[0203] In the embodiments described above, examples were given in which the optical communication device handles digital signals (PAM), but this is not the only example. The signals handled by the optical communication device can be any optical signals obtained by amplitude modulation of an optical electric field, such as OFDM (Orthogonal Frequency Division Multiplexing) signals used in terrestrial digital television broadcasting, or analog signals used before terrestrial digital television broadcasting.

[0204] While embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention.

[0205] <Addendum> An optical communication method comprising: a polarization diversity type optical heterodyne detector receiving an optical signal obtained by amplitude modulating an optical electric field, detecting the received optical signal, a converter converting the signal detected by the optical heterodyne detector into a digital signal, a signal processing unit processing the digital signal, the optical heterodyne detector using the separated orthogonal polarization components of the received optical signal and the orthogonal polarization components of the local emission to generate beat components for each combination of the orthogonal polarization components of the received optical signal and the orthogonal polarization components of the local emission in the Y component of the orthogonal polarization component of the received optical signal, the X component of the orthogonal polarization component of the received optical signal, the Y component of the orthogonal polarization component of the local emission, and the X component of the orthogonal polarization component of the local emission, the converter sampling each of the beat components and converting them into the digital signal, the signal processing unit converting each of the sampled digital signals into baseband signals, and further calculating the square root of the sum of the products of the baseband signal components and their complex conjugates to perform polarization synthesis.

[0206] The present invention is applicable to optical communication systems, optical communication devices, optical receivers, optical communication equipment, and the like.

[0207] 1, 1A, 1B, 1C, 1D... Optical communication system, 2... Optical transmitter, 3, 3A, 3B, 3C, 3D... Optical communication device, 9... Optical fiber transmission line, 21... Tx signal processing unit, 22... Light source, 23... Amplitude modulator, 31... Light source, 32... Detector, 33, 33A, 33B, 33C, 33D... ADC33, 34, 34A, 34B, 34D... Signal processing unit, 321, 321A, 321B, 321C, 321D... Polarization separator, 323, 323A, 323B, 323C, 323D... Optical splitter, 325, 325 A, 325B, 325C, 325D... Balanced receivers; 341, 341B, 341a, 341b, 341c, 341Ba, 341Bb, 341Bc... Baseband converters; 342, 342B, 342C, 342D, 342a, 342b, 342c, 342Ba, 342Bb, 342, 342a, 342b, 342Bc... Polarization combiners; 324C, 342D... π / 2 delay converters; 322D... Optical splitter

Claims

A polarization diversity type optical heterodyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and performs detection of the received optical signal, A converter that converts the signal detected by the optical heterodyne detector into a digital signal, The system comprises a signal processing unit for processing the aforementioned digital signal, The optical heterodyne detector uses the orthogonal polarization components of the separated received optical signal and the orthogonal polarization component of the local emission to generate a beat component of the X component of the orthogonal polarization component of the local emission and the X component of the orthogonal polarization component of the received optical signal, and a beat component of the Y component of the orthogonal polarization component of the local emission and the Y component of the orthogonal polarization component of the received optical signal in the intermediate frequency band of the optical heterodyne detection. The converter samples each of the beat components and converts them into digital signals. The signal processing unit converts each of the sampled digital signals into a baseband signal, and further calculates the square root of the sum of the products of the baseband signal components and their complex conjugates to perform polarization synthesis. Optical communication device. 　 A polarization diversity type optical heterodyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and performs detection of the received optical signal, A converter that converts the signal detected by the optical heterodyne detector into a digital signal, The system comprises a signal processing unit for processing the aforementioned digital signal, The aforementioned optical heterodyne detector is, Using the orthogonal polarization components of the separated received optical signal and the orthogonal polarization components of the local emission, In the Y component of the orthogonal polarization component of the received optical signal, the X component of the orthogonal polarization component of the received optical signal, the Y component of the orthogonal polarization component of the station emission, and the X component of the orthogonal polarization component of the station emission, beat components are generated for each combination of the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the station emission. The converter samples each of the beat components and converts them into digital signals. The signal processing unit converts each of the sampled digital signals into a baseband signal, and further calculates the square root of the sum of the products of the baseband signal components and their complex conjugates to perform polarization synthesis. Optical communication device. 　 An optical intradyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and performs detection of the received optical signal, A converter that converts the signal detected by the aforementioned optical intradyne detector into a digital signal, The system comprises a signal processing unit for processing the aforementioned digital signal, The aforementioned optical intradyne detector is, Using the orthogonal polarization components of the separated received optical signal and the orthogonal polarization components of the local emission, The I component and the Q component are extracted from the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the station's emitted light, respectively. In the I component of the Y component of the orthogonal polarization component of the received optical signal, the Q component of the Y component of the orthogonal polarization component of the received optical signal, the I component of the X component of the orthogonal polarization component of the received optical signal, the Q component of the X component of the orthogonal polarization component of the received optical signal, the I component of the Y component of the orthogonal polarization component of the station emission, the Q component of the Y component of the orthogonal polarization component of the station emission, and the Q component of the X component of the orthogonal polarization component of the station emission, the orthogonal polarization of the received optical signal The following are generated: the Q component of the Y component of the component and the Q component of the Y component of the orthogonal polarization component of the station emission; the I component of the Y component of the orthogonal polarization component of the received optical signal and the I component of the Y component of the orthogonal polarization component of the station emission; the Q component of the X component of the orthogonal polarization component of the received optical signal and the Q component of the Y component of the orthogonal polarization component of the station emission; and the I component of the X component of the orthogonal polarization component of the received optical signal and the I component of the X component of the orthogonal polarization component of the station emission. The converter samples each of the beat components and converts them into digital signals. The signal processing unit converts each of the sampled digital signals into a baseband signal, and then calculates the square root of the sum of the products of each baseband signal with its complex conjugate to perform polarization synthesis. Optical communication device. 　 An optical intradyne detector that receives an optical signal obtained by amplitude modulating an optical electric field and performs detection of the received optical signal, A converter that converts the signal detected by the aforementioned optical intradyne detector into a digital signal, The system comprises a signal processing unit for processing the aforementioned digital signal, The aforementioned optical intradyne detector is, Using the orthogonal polarization components of the separated received optical signal and the orthogonal polarization components of the local emission, The I component and the Q component are extracted from the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the station's emitted light, respectively. In the I component of the Y component of the orthogonal polarization component of the received optical signal, the Q component of the Y component of the orthogonal polarization component of the received optical signal, the I component of the X component of the orthogonal polarization component of the received optical signal, the Q component of the X component of the orthogonal polarization component of the received optical signal, the I component of the Y component of the orthogonal polarization component of the station emission, the Q component of the Y component of the orthogonal polarization component of the station emission, and the Q component of the X component of the orthogonal polarization component of the station emission, beat components of the I component and Q component of each combination of the orthogonal polarization component of the received optical signal and the orthogonal polarization component of the station emission are generated. The converter samples each of the beat components and converts them into digital signals. The signal processing unit converts each of the sampled digital signals into a baseband signal, and then calculates the square root of the sum of the products of each baseband signal with its complex conjugate to perform polarization synthesis. Optical communication device. 　 The optical signal obtained by amplitude modulating the aforementioned optical field is an analog signal. The optical communication device according to claim 1 or claim 2. The optical signal obtained by amplitude modulating the optical electric field is an optical PAM (Pulse Amplitude Modulation)-N signal (where N is an integer of 2 or more) with uniform spacing between optical electric field amplitude levels. The optical communication device according to claim 1 or claim 2. 　 The above N is an integer greater than or equal to 3. The optical communication device according to claim 6. 　 A polarization diversity type optical heterodyne detector receives an optical signal obtained by amplitude modulating the optical field, and performs detection of the received optical signal. The converter converts the signal detected by the optical heterodyne detector into a digital signal. The signal processing unit processes the digital signal. The optical heterodyne detector uses the orthogonal polarization components of the separated received optical signal and the orthogonal polarization component of the local emission to generate a beat component of the X component of the orthogonal polarization component of the local emission and the X component of the orthogonal polarization component of the received optical signal, and a beat component of the Y component of the orthogonal polarization component of the local emission and the Y component of the orthogonal polarization component of the received optical signal in the intermediate frequency band of the optical heterodyne detection. The converter samples each of the beat components and converts them into digital signals. The signal processing unit converts each of the sampled digital signals into a baseband signal, and further calculates the square root of the sum of the products of the baseband signal components and their complex conjugates to perform polarization synthesis. Optical communication method.

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