JP2012151752A - Coherent light receiving device, and optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coherent light receiving device demanding a low setting accuracy of optical components, and being configured by simple optical components.SOLUTION: A coherent light receiving device uses a plurality of local oscillation lights having initial phases and phase fluctuations in accordance with each other. The coherent light receiving device multiplies respective intermediate frequency signals of the plurality of local oscillation lights and signal light by sinusoidal signals having the same frequencies as the respective intermediate frequency signals and having phase fluctuations in accordance with each other and having a predetermined relation of the initial phases at multiplication, or alternatively, samples the intermediate frequency signals in a cycle at a fraction of the natural number of a frequency difference of the intermediate frequency signals. In the case of a diversity system equivalent to an optical multi-terminal coupling circuit having the number of terminals of K, by adjusting an output of an intermediate frequency signal corresponding to each terminal so as to have a phase of 2πq/K (q is 0 to K-1), coherent light detection is realized without using an optical 90-degree phase hybrid circuit and without causing problems by phase modulation in a cycle equal to or less than half of one symbol time.

Description

  The present invention relates to a coherent optical receiver that receives an optical signal by a phase diversity reception method, and an optical communication system including the same.

  Synchronous detection of coherent optical communication performs optical detection by mixing signal light and local light whose phase is tracked with signal light. Here, when the phases of the signal light and the local light are not matched, the detection efficiency is deteriorated and the signal is not output. There is a phase diversity reception method as a method for realizing coherent optical detection without tracking a phase using an optical phase locked loop.

  The phase diversity reception system is based on the idea of simultaneous measurement of the phase and amplitude of an optical signal using an optical multi-terminal coupling circuit. Taking a two-phase (in-phase and quadrature) diversity reception system as an example, an optical 90-degree phase hybrid circuit with two inputs and four outputs is provided as an optical multi-terminal coupling circuit. This optical 90-degree phase hybrid circuit corresponds to a four-terminal optical multi-terminal coupling circuit. When local light from the local light source and received signal light are input to different input terminals, a π / 2 phase difference is obtained. Outputs four series of mixed lights. Here, four series of mixed lights are used, but mixed lights having different π / 2 phase differences are detected without differential detection of mixed lights having different π phase differences from each other. In this case, a two-input two-output optical 90 degree phase hybrid circuit that treats as a four-terminal optical multi-terminal coupling circuit but does not use two of the four outputs, and has two series with different π / 2 phase differences. Can be output.

  Each of the two series of differential optical detectors 2 performs differential optical detection on each of the two sets of mixed light having different π phase differences from the optical 90-degree phase hybrid circuit and converts them into electrical signals. Output intermediate frequency signal in relation. Each of these two intermediate frequency signals is demodulated by a demodulation circuit. The adder appropriately adds the demodulated signals to output a signal component. This makes it possible to obtain a signal output of a certain level or more in the receiving device regardless of the phase state of the signal light (see, for example, Patent Document 1).

JP 2009-192746 A

Dany-Sebastian Ly-Gagnon et al. , “Unrepeated 210-km transmission with coherent detection and digital signal processing of 20-Gb / s QPSK Signal”, OFC 2005, OTuL4.

  However, the configuration of a coherent optical receiver using a phase diversity system using an optical multi-terminal coupling circuit such as an optical 90-degree phase hybrid circuit is expensive and complicated. For example, when an optical 90-degree phase hybrid circuit is composed of optical components such as a spatial optical system and PLC, it is robust so that the relative optical path difference generated by these optical circuits is not affected by environmental changes such as temperature and vibration. It is necessary to maintain the optical path length difference or to correct the optical path difference. For example, in order to adjust the phase difference of signal light in the 1550 nm band by about 1/100 of the wavelength, it is necessary to maintain or correct the optical path length difference on the order of 15 nm. For this reason, even if affected by disturbances such as environmental temperature fluctuations and vibrations, a highly accurate and highly resistant coherent receiver that can maintain a sufficient accuracy by maintaining the optical path length difference and phase errors due to the effects of disturbances. There is a coherent optical receiver that performs correction by feeding back to an optical circuit so as to reduce this. Each of the coherent optical receivers has a problem that the configuration of the optical 90-degree phase hybrid circuit becomes expensive or processing such as feedback becomes complicated.

  Accordingly, the present invention has been made to solve the above-described problems, and provides a coherent optical receiver that can be configured with simple optical components with low required accuracy for setting optical components, and an optical communication system including the same. With the goal.

  In order to solve the above-described problem, the coherent optical receiver according to the present invention uses a plurality of local lights having the same initial phase and phase fluctuation, and each of the intermediate frequencies generated by optically detecting the signal light and the local light. A sine wave signal in which the intermediate frequency signal is sampled at a period that is a natural fraction of the frequency difference of the signal, or the same phase as each of the intermediate frequency signals and the initial phase when the phase fluctuations are aligned together has a predetermined relationship Is multiplied by the intermediate frequency signal and demodulated, and the output of the intermediate frequency signal corresponding to each terminal is 2πq / K in the case of a diversity system that is equivalent to an optical multi-terminal coupling circuit having K terminals (q: 0 to 0). By adjusting the output to be K-1), coherent optical detection is realized without using the conventional optical 90-degree phase hybrid circuit.

  Specifically, the coherent light receiving device according to the present invention includes a local light source unit that outputs a plurality of local light sources having the same initial phase and phase fluctuation, and the local light source that is output from the local light source unit. A mixing unit that mixes signal light and outputs mixed signal light; and an optical detection unit that optically detects the mixed signal light and outputs intermediate frequency signals between the local light and the signal light; A processing unit that separates the intermediate frequency signals output from the optical detection unit to demodulate signal components of the signal light, and a phase relationship between the intermediate frequency signals output from the optical detection unit is a predetermined relationship. And an adjusting unit that adjusts to become.

  This coherent light receiving apparatus uses a plurality of local lights having the same initial phase and phase fluctuation, and adjusts the phases of the intermediate frequency signals output from the optical detection unit to have a predetermined relationship. This coherent light receiving apparatus uses a plurality of local lights having the same initial phase and phase fluctuation, and a period of a natural fraction of the frequency difference between the intermediate frequency signals generated by optically detecting the signal light and the local light. In the case of a diversity system in which the intermediate frequency signal is sampled at the same time and the number of terminals is equivalent to that of an optical multi-terminal coupling circuit with K, the phase at the time of sampling the intermediate frequency signal corresponding to each terminal is 2πq / K, To K-1). In the case of the two-phase diversity system corresponding to the optical four-terminal coupling circuit, 2 corresponding to each of the orthogonal phase components of the signal light is adjusted by adjusting the phase difference in the intermediate frequency signal to be a quarter cycle. Two intermediate frequency signals can be simultaneously output by frequency multiplexing. In the adjustment by the adjustment unit, the frequency difference between the local lights is set to a natural number multiple of the sampling period, and the phase of the sampling is π / 2 if the phase of the intermediate frequency signal at the time of sampling is a predetermined phase or a two-phase diversity system. Adjust the sampling cycle so that For this reason, this coherent optical receiver can implement | achieve a phase diversity structure with one optical detector, without using an optical 90 degree phase hybrid circuit.

  By eliminating the need for the optical 90-degree phase hybrid circuit, the phase error due to the influence of disturbance can be ignored as compared with the conventional configuration, and adjustment of optical components in the wavelength order becomes unnecessary. Therefore, the present invention can provide a coherent optical receiver that has a low required accuracy for setting optical components and can be configured with simple optical components.

  The local light source unit of the coherent optical receiver according to the present invention includes a light source that outputs one light, an oscillator that oscillates a sine wave signal, and modulates light from the light source with a sine wave signal from the oscillator. And a plurality of modulators for generating the local light.

  The local light source unit can output a plurality of local lights having the same initial phase and phase fluctuation.

  The local light source unit of the coherent optical receiver according to the present invention outputs the local light in which an absolute value of a frequency difference between the plurality of intermediate frequency signals is larger than half of a modulation band of the signal light, and the processing unit Is multiplied by a plurality of intermediate frequency signals and a sine wave signal having the same frequency as each of the intermediate frequency signals and the initial phase when the phase fluctuations are multiplied together, and the adjustment unit has a number of terminals. In the case of a diversity system that is equivalent to an optical multi-terminal coupling circuit of K, it is sine so that the output of the intermediate frequency signal corresponding to each terminal is an output of 2πq / K, (q: 0 to K−1) phase. It adjusts so that it may become the phase of a wave signal.

  By taking the frequency difference of the intermediate frequency signal, that is, the local light, to be larger than half of the modulation band, there is at least one point in the signal mark where the intermediate frequency signal is maximum, so that the optical signal is most efficiently transmitted. An electric sine wave signal that can be detected and phase fluctuations are aligned and the initial phase of each other is in a predetermined relationship with the local frequency signal generated from the local light and the signal light. By adjusting the phase of the electrical sine wave signal when multiplying to a predetermined relationship, the required accuracy of optical component setting is low, adjustment of the optical component in the wavelength order is unnecessary, and it can be configured with a simple optical component A coherent optical receiver can be provided.

  Further, when demodulating, in the case of a diversity system that is equivalent to an optical multi-terminal coupling circuit having K terminals, the output of the intermediate frequency signal corresponding to each terminal is 2πq / K, (q: 0 to K−1). The phase difference between the intermediate frequencies is set by setting the phase of the electrical sine wave signal with the phase fluctuations aligned so that the phase of the sine wave signal becomes the output of The phase difference between the sine wave signals can be coherently detected by managing only the relationship between the phase differences in a non-correlated manner with the phase of the signal light.

The local light source unit of the coherent optical receiver according to the present invention has an optical frequency difference smaller than twice the bandwidth of the signal light, and the light source spectral line width of the signal light and the local light Outputs local light larger than the light source spectral line width,
The processing unit includes an AD converter that converts the intermediate frequency signal having orthogonal phases into a digital signal;
A digital arithmetic unit that calculates a digital signal output from the AD converter using equation (C1) and outputs a signal vector s (t) for estimating data information included in the signal light; and the digital arithmetic unit Discriminating the signal vector s (t) output from the discriminator, and outputting the signal component of the signal light as received data,
The adjusting unit adjusts the phase difference of the plurality of intermediate frequency signals so that the phase difference differs from the phase difference of the plurality of sine wave signals multiplied by the intermediate frequency signal by a quarter cycle by the digital calculator. It is characterized by that.
(Formula C1)
s (t) = [I ′ + jQ ′] / [X + Y exp (−jw0t)]
Here, the amplitude of the intermediate frequency signal is X and Y, the angular frequency difference of the local light is w0, and the imaginary unit is j.

  The bandwidth required for differential optical detection can be reduced by AD conversion of the output of the optical detection unit, digital signal processing, and optimization of the phase difference and the angular frequency difference between the local light emissions.

  The coherent optical receiver according to the present invention further includes an intensity ratio control circuit that changes a ratio of the intensity of the local light. When the intensity ratio control circuit adjusts the light intensity between the local lights, it is possible to prevent the digital calculator from being unable to calculate the signal vector.

  An optical communication system according to the present invention branches an optical fiber that transmits a wavelength-multiplexed signal, a plurality of the coherent optical receivers, and a wavelength-multiplexed signal that transmits the optical fiber, and couples them to the coherent optical receivers, respectively. An optical branching unit, and each of the coherent optical receivers shares one local light source unit.

  Since a plurality of coherent optical receivers share one local light source unit, it is possible to greatly reduce the number of parts, and it is possible to greatly reduce device manufacturing costs and device management costs.

  The present invention can provide a coherent optical receiver that can be configured with simple optical components with low required accuracy for setting optical components, and an optical communication system including the same.

It is a figure explaining the optical communication system containing the coherent optical receiver which concerns on this invention. It is a figure explaining local light emission of the coherent optical receiver concerning the present invention. It is a figure explaining local light emission of the coherent optical receiver concerning the present invention. It is a figure explaining the optical communication system containing the coherent optical receiver which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In addition, in this specification and drawing, the component with the same code | symbol shall show the mutually same thing.

(Embodiment 1)
FIG. 1 is a diagram illustrating an optical communication system 401 including a coherent optical receiver 301 according to the first embodiment. The optical communication system 401 includes an optical transmission device 351 and a coherent optical reception device 301 connected via an optical network 352. The optical transmission device 351 includes, for example, intensity modulation, BPSK (Binary Phase Shift Keying), M-phase PSK (M-ary Phase Shift Keying) such as QPSK, and M-phase DPSK (M-ary Differential Phase Shifting, APSK). Optical modulation is performed by a modulation scheme such as M-ary Amplitude and Phase Shift Keying, M-phase QAM (M-ary Quadrature Amplitude Modulation), and the optical signal is output.

  The coherent optical receiver 301 performs data extraction processing (demodulation processing) by performing signal processing substantially equivalent to phase diversity without adopting the phase diversity configuration. In other words, the optical 90-degree phase hybrid circuit is not required, and the optical detector is one line. Specifically, a plurality of local lights with the same initial phase and phase fluctuation are used, the local light and the input signal light are mixed, the mixed signal light is detected, and the intermediate between the local light and the signal light is detected. The frequency signal is separated and adjusted so that the phases of the intermediate frequency signals have a predetermined relationship, so that the phase with a period of half or less of one symbol time is not used without using the optical 90-degree phase hybrid circuit. We decided to realize coherent optical detection without causing problems due to modulation.

  The coherent light receiving device 301 uses a plurality of local lights having the same initial phase and phase fluctuation, and each of the intermediate frequency signals generated by optical detection of the signal light and the local light and the same frequency as each of the intermediate frequency signals. In addition, each of the adjustment units is multiplied by a sine wave signal whose initial phase has a predetermined relationship when the phase fluctuations are multiplied together. The phase of the sine wave signal is adjusted so that the output of the intermediate frequency signal corresponding to the terminal becomes an output of 2πq / K, (q: 0 to K−1). Alternatively, the coherent light receiving apparatus 301 uses a plurality of local lights with the same initial phase and phase fluctuation, and optical detection of the signal light and the local light is a natural number of a frequency difference between the intermediate frequency signals respectively generated. In the case of a diversity method in which an intermediate frequency signal is sampled at a period of and the number of terminals is equivalent to that of an optical multi-terminal coupling circuit with K, the phase at the time of sampling of the intermediate frequency signal corresponding to each terminal is 2πq / K, (q : 0 to K-1).

  Hereinafter, details of the coherent optical receiver 301 will be described in accordance with the two-phase diversity reception. The coherent light receiving apparatus 301 includes a local light source section 10, an adjustment section 11, a mixing section 12 of an input / two-output multiplexer, a differential optical detector 13, and a processing section 14.

  The local light source unit 10 generates a local light EL composed of a plurality of local lights having the same initial phase and phase fluctuation for receiving coherent light. The local light source unit 10 modulates light from a single light source 101 using a modulator 103 that modulates the intensity of the light with a sine wave signal having a frequency fm of the oscillator 102. As a result, a sideband having a difference in frequency fm with respect to the light output from the light source 101 is generated. Three lights can be generated from one light source 101 by the light transmitted from the light source 101 and the sideband generated by the modulator 103. Of these three lights, any two lights may be selected using an optical demultiplexer (not shown), and both sidebands having a frequency interval of 2fm are suppressed by suppressing the output light of the light source 101. It is also possible to select only.

  For example, a Mach-Zehnder type optical modulator is used as the modulator 103, and an sine wave signal of fm is applied and modulated by the oscillator 102. At this time, if an appropriate bias voltage is selected, the optical frequency component of the input light is suppressed, and a sideband having a difference of the modulation frequency fm with respect to the optical frequency of the input light is generated (DSB-SC system). In the DSB-SC system, it is not necessary to select two lights as local lights from the three lights, so that the output light power of the light source 101 can be used effectively.

  Further, a part of the light with the angular frequency wL output from the light source 101 is shifted by an angular frequency w0 (= 2πf0) by using an acousto-optic modulator (AOM), and the light with the angular frequency wL + w0. And local light EL in which light having an angular frequency wL is mixed may be output. Further, the light of the angular frequency wL + w0 and the light of the angular frequency wL are mixed using a frequency shifter that shifts the angular frequency by w0, an FM modulator, a single sideband (SSB) modulator, or the like. Local light emission EL may be output. In the case of the K-phase diversity system, K local light-emitting ELs each having a frequency that is a natural number multiple of w0 may be output.

  In either case, the optical frequency interval of the local light is controlled with the frequency accuracy of the oscillator 102 by the electric signal, so that the optical frequency control of the signal light is facilitated.

  Further, a mode-locked light source that operates with an external clock may be used as the light source 101, and other light sources may be used as long as the above conditions are satisfied.

  Since a plurality of local lights from the local light source unit 10 that uses a modulated sideband or outputs a light whose frequency is shifted from a part of the light is generated from a single light, the same initial phase is generated at each moment. Have the same phase fluctuation. When the propagation path for each local light is different, the difference in the path length for propagating the local light is negligible compared to the coherent length in order to maintain the correlation of the phase relationship between the local lights, for example, 1/10 of that. The following.

  Here, it is assumed that the local light EL is light whose optical frequencies are separated from the received signal light Es by fIF1 and fIF2. As shown in FIG. 2, the local light source unit 10 outputs the signal light and the light whose optical frequencies are separated by fIF1 and + fIF2, respectively, and the optical frequency between the local lights is f0 = fIF1 + fIF2. Alternatively, as shown in FIG. 3, the local light source unit 10 is arranged so that the optical frequency difference between the two local lights is f0 = fIF2−fIF1, and the local light for the signal light is light by fIF1 and fIF2. Similarly, it is possible to obtain two intermediate frequency signals by using light with different frequencies. In either case, fIF1 ≠ fIF2.

  In order to detect the output signal most efficiently, it is sufficient that at least one point where the intermediate frequency signal is maximized exists in the mark of the signal. This is possible by setting | fIF1-fIF2 |, which is the frequency difference of the intermediate frequency signal, to be larger than half of the modulation band. Here, it is assumed that the light intensity of the local light is equal, and the transmission characteristics in consideration of the reception sensitivity of the optical detector corresponding to each of the optical receivers 301 described later and the gain and attenuation of the intermediate frequency signal are equal. Even if they are different, the optical signal intensity or the intermediate frequency signal intensity may be adjusted so that the light intensity, reception sensitivity, and transmission characteristics are equal.

  The 2 × 2 multiplexer of the mixing unit 12 mixes the local light EL and the received signal light Es. That is, in the 2 × 2 multiplexer, the local light EL from the adjustment unit 11 and the received signal light Es are mixed and output to the differential optical detector 13.

  The differential optical detector 13 optically detects the mixed signal light mixed in the mixing unit 12. That is, the differential optical detector 13 performs differential optical detection on the mixed light from the mixing unit 12 and converts it into an electrical signal.

  The processing unit 14 includes an intermediate signal using, as a carrier wave, the light of the angular frequency wL included in the local light EL and the wi (= 2πfIF1) due to the beat of the received signal light Es in the electric signal output from the differential optical detector 13. Optical modulation method in transmission-side apparatus 351 using frequency signal X and intermediate frequency signal Y using wi + w0 (= 2πfIF2) based on the light of angular frequency wL + w0 included in local light EL and the beat of received signal light Es as a carrier wave Process according to. The intermediate frequency signal X is assumed to have a spectrum width about twice the signal bandwidth centered on the frequency wi, and the intermediate frequency signal Y is assumed to have a spectrum width about twice the signal bandwidth centered on the frequency wi + w0. . Further, the difference in power between the intermediate frequency signals X and Y varies depending on the phase state of the received signal light. At this time, the bandwidth of the receiving electronic circuit needs to be four times or more the signal bandwidth in the example of FIG. When the local light angular frequency wL is set so that the intermediate frequency wi becomes 0 Hz, the bandwidth of the electric circuit used for reception approaches three times the signal bandwidth.

In the processing unit 14, the intermediate frequency signals X and Y are demodulated by, for example, multiplying them by a sine wave signal having the same frequency. The sine wave signal multiplied by each intermediate frequency signal may be frequency shifted so that the sine wave signal from the oscillator 102 of the local light source unit becomes the respective intermediate frequency signal. Further, as shown in FIG. 1, an oscillator different from the oscillator 102 of the local light source unit is provided, one intermediate frequency signal is output, and a part of the output is frequency-shifted to become the other intermediate frequency signal. Also good. Moreover, you may provide individually the oscillator which outputs each intermediate frequency signal. However, the tuning operation is performed so as to align the initial phase and the phase fluctuation between the oscillators to be used. The adjustment unit (not shown) phase shifts the phase of the sine wave signal when multiplying the intermediate frequency signal by a sine wave signal having the same frequency as each intermediate frequency signal from the oscillator and having the same initial phase and phase fluctuation. By adjusting with the adjuster, the output of the intermediate frequency signal corresponding to each terminal is 2πq / K, (q: 0 to K−1) in the case of the diversity system equivalent to the optical multi-terminal coupling circuit having K terminals. ) To adjust the phase of the sine wave signal so that the phase is output. In the case of the two-phase diversity method, the output is set so that the π / 2 phase is different. In the case of the two-phase diversity method, if the phase fluctuations of the intermediate frequency signals X and Y are Θx (t) and Θy (t), and the phase fluctuations of the corresponding sine wave signals are φx (t) and φy (t), It becomes relation of expression.
(Formula 1)
Θx (t) −Θy (t) = φx (t) −φy (t) ± kπ / 2
(K = 1, 3, 5,...) (1)
At this time, the demodulated intermediate frequency signal is expressed by the following equation when the constant term independent of the phase is 1.
(Formula 2)
cos (Θx (t) −φx (t))
cos (Θy (t) −φy (t) ± kπ / 2) = sin (Θx (t) −φx (t))
(2)
As shown in the equation (2), the phase difference between the intermediate frequencies and the phase difference between the sine wave signals only need to be constant only in the relationship between the phase differences of the signal light and the local light. .

  Note that the phase adjustment by the adjustment unit has been adjusted by the explicit phase adjuster so as to maintain the phase with the intermediate frequency signal in the relationship shown in the equation (1), but for the multiplication of the sine wave signal multiplied by the intermediate frequency signal. It is not necessary to use the propagation path length to the mixer by previously satisfying equation (1). Conversely, in addition to adjusting the phase between the intermediate frequency signals so as to maintain the relationship shown in equation (1), the phase of the output after multiplying the intermediate frequency signal and the sine wave signal is relative to each intermediate frequency signal. The output may be adjusted so as to be constant. Furthermore, although the description has been given by adjusting the phase between the sine wave signals multiplied by the intermediate frequency signal, the phase between the local lights may be adjusted by adjusting the propagation distance of the local light. When adjusting the phase between the local lights, the phase of the sine wave signal multiplied by the intermediate frequency signal need not be adjusted. Further, both the sine wave signal multiplied by the intermediate frequency signal and the local light may be adjusted so as to have a relationship represented by the equation (1). The local light is input to the differential optical detector 13 via the mixing unit 12. When adjusting the propagation distance of local light at this time, the phase between the local lights is adjusted so that the phase of the intermediate frequency signal generated by the differential optical detector 13 differs by a predetermined relationship, for example, π / 2. To do.

For example, assuming that the optical frequency difference is 2 GHz, the refractive index of the propagation path is 1.5, and the speed of light is 3 × 10 ⁸ m / s, the phase rotates once when 2.5 cm is propagated. For this reason, in the case of 1/4 phase adjustment, it may be set in units of 0.625 cm, which is 1/4 of the phase adjustment. Note that the setting of the propagation path may be constant as long as the variation in the length of the propagation path and the refractive index from the light source 101 due to changes over time such as temperature can be ignored. For example, if the phase may be shifted up to π / 40, the effective propagation path length variation considering the refractive index may be 0.6 mm or less.

  Further, in the processing unit 14, the output of the differential optical detector 13 may be processed after being separated into the intermediate frequency signals X and Y by an electric filter (Band Pass Filter: BPF) in advance. In the case of processing after separating each of the intermediate frequency signals X and Y with an electric filter (Band Pass Filter: BPF), instead of multiplying the sine wave signal of the same frequency, each is detected by envelope detection such as double detection. It may be processed.

  Further, the coherent light receiving device 301 has an intermediate frequency signal with a period of a natural number of a frequency difference between the intermediate frequency signals generated by optically detecting the signal light and the local light so as to satisfy the expression (1). In the case of the diversity system in which the number of terminals is equal to that of an optical multi-terminal coupling circuit having K terminals, the phase at the time of sampling of the intermediate frequency signal corresponding to each terminal is 2πq / K, (q: 0 to K−1). You may adjust so that it may become. That is, between all samplings between the sampling period τ for sampling so as to process the phase difference corresponding to each local light, and the frequency difference Δf (= w0 / 2π) of the intermediate frequency signal. When sampling with the same sign value, Δfτ = m (m is an arbitrary integer other than 0), but when sampling with the opposite sign value every sampling, Δfτ = m / 2 (m is other than 0) Any integer) may be established. Sampling may be performed, for example, by multiplying a sine wave signal multiplied by the intermediate frequency signal only at the sampling timing, or the intermediate frequency signal may be input to the mixer only at the sampling timing, or the output of the mixer May be sampled. In general, in the case of the diversity system in which the number of terminals is equivalent to that of an optical multi-terminal coupling circuit having K terminals, the difference in frequency of the intermediate frequency signal corresponding to each local light is the same. Note that the frequency of each intermediate frequency signal is different.

  Further, desirably, the phase when demodulating the intermediate frequency signal is one of 0 and π / 2, 0 and −π / 2, π and π / 2, and π and −π / 2 regardless of the sampling opportunity. It is desirable to perform sampling by matching the phases so that one is a fixed relationship. In this case, since the demodulated phase component of the intermediate frequency signal is constant at every sampling, regardless of the sampling opportunity, the processing of the phase modulation component is simplified.

  Further, in the processing in the processing unit, the same processing as in Patent Document 1 may be processed for each intermediate frequency signal instead of each output of the optical 90-degree phase hybrid circuit.

  Note that the coherent optical receiver 301 in FIG. 1 uses a 2 × 2 multiplexer mixing unit 12 and a differential optical detector to differentially detect outputs having different π phase differences in each of the 2 × 2 multiplexers. 13, the mixing unit 12 may be a 2 × 1 multiplexer, and may be a non-differential single-input optical detector instead of the differential optical detector 13, or a difference from the multiplexer. One of the dynamic detectors may be changed. Further, the coherent light receiving apparatus 301 uses a single local light instead of using a plurality of local lights, frequency-shifts or intensity-modulates a part of the signal light, and the multiple sidebands and a single local light. In the diversity system, the signal light is modulated so that the phase difference of the intermediate frequency signal and the phase difference of the sine wave signal differ by a quarter period, and the number of terminals is equivalent to that of an optical multi-terminal coupling circuit having K terminals. You may adjust so that the output of the intermediate frequency signal corresponding to a terminal may become the phase of a sine wave signal so that it may become the output of 2 (pi) q / K and the phase of (q: 0-K-1).

  The polarization controller 15 matches the polarization state of the signal light input to the mixing unit 15 with the polarization state of the local light. For example, the polarization controller 15 can be configured by the above-described infinite tracking automatic polarization controller, or a polarization diversity configuration including two sets of receivers of the present application that are mixed with signal light with orthogonal polarizations. Alternatively, a configuration in which polarization diversity is performed in addition to phase diversity using a group of a plurality of orthogonally polarized local light sources and a total of four local light sources may be used.

  Next, an optical communication system (not shown) provided with a plurality of coherent optical communication devices 301 will be described. The optical communication system to which the present embodiment is applied is, for example, a multi-point connection in which a plurality of optical transmission devices 351 and a plurality of coherent optical communication devices 301 are connected via an optical coupler, an optical fiber transmission line, and an optical branching unit. It is a multipoint optical communication system.

  The signal light transmitted from the optical transmission device 351 side is combined with all signal lights using an optical coupler to form a wavelength multiplexed signal, and then transmitted to the optical fiber transmission line. At this time, the wavelength of each signal light is set to be different. For example, the signal light transmitted from the optical transmission device 351 is set to fs1 and fs2 as the optical frequency, and similarly, the signal light from n is transmitted to be fsn. When time division multiplexing is also performed, a single optical frequency may be shared between the optical transmitters 351 that perform time division multiplexing. The light transmitted from the optical transmitter 351 is further multiplexed by the optical multiplexer / demultiplexer in the optical fiber transmission line and transmitted to the coherent optical communication device 301. Each coherent optical communication device 301 similarly demodulates signal light to obtain a signal output.

  In the case of the present embodiment, in order to receive the signal light transmitted from the plurality of optical transmission devices 351 by the coherent optical communication device 301, compared to the case where the optical phase synchronization receiver is arranged in each coherent optical communication device 301, Since it is not necessary to perform phase synchronization according to different phases of the signal light from the optical transmitter 351, the local light source units 10 can be integrated into one. In this embodiment, since the local light source unit 10 is single, each coherent optical communication device 301 needs to be finely adjusted or a phase diversity receiver using a high-precision 90-degree optical phase hybrid circuit is used as a transmitter. Compared with the case where it arranges for every, it becomes a simple structure. Further, the local light source unit 10 can be shared by a plurality of multipoint-multipoint optical communication systems or a plurality of point-multipoint optical communication systems. Therefore, the number of parts can be greatly reduced compared to the optical phase-synchronized receiver, and the number of optical parts can be significantly reduced compared to the phase diversity receiver using the optical 90-degree phase hybrid circuit. It is. For this reason, it becomes possible to significantly reduce device manufacturing costs and device management costs. When the local light source unit 10 is shared by a plurality of coherent optical communication devices 301, and when an oscillator that outputs a sine wave multiplied by the intermediate frequency signal is also shared, their propagation distances are made uniform among the coherent optical communication devices 301. Or adjust individually.

  As described above, in the case of the present embodiment, unlike the configuration using the optical 90-degree phase hybrid circuit, a set of local light can be shared by a plurality of receiving apparatuses, so the optical 90-degree phase hybrid circuit is used. There is also an advantage of better scalability than configuration.

  In the above description, a description was added in accordance with the two-phase diversity method. However, in the case of the diversity method that is generally equivalent to an optical multi-terminal coupling circuit having K terminals, the phase difference is 2π / K, 2π2 / K, ... the same if 2π (K-1) / K. In addition, if the polarization of local light of this embodiment is an orthogonal polarization and the 2 × 2 demultiplexer is an optical 90-degree phase hybrid circuit, it can be applied to polarization diversity.

  In a conventional coherent optical receiver, in a 90-degree optical phase hybrid circuit, a plurality of mixed lights having a phase difference between the signal light and the local light that differ by π / 2 are used as two series of mixed light of the received signal light and the local light. It was generated. Then, each of these four output mixed lights is optically detected by two different optical detectors. On the other hand, in the coherent light receiving apparatus 301 of the present embodiment, the local light source unit 10 outputs a plurality of local lights having the same initial phase and phase fluctuation, and uses the local lights, so that a predetermined sampling period is used. Thus, two intermediate frequency signals (first sequence and second sequence) having orthogonal phase relationships are simultaneously output by frequency multiplexing. Therefore, a phase diversity configuration can be realized with one differential optical detector without using a complicated optical phase loop, an optical 90-degree phase hybrid circuit, or a high-speed phase modulator. For this reason, since the received signal light can be converted into an electrical signal by a configuration using simple optical components, it is possible to reduce the size and simplify the configuration while supporting high speed.

(Embodiment 2)
FIG. 4 is a diagram illustrating an optical communication system 402 including the coherent optical receiver 302 according to the second embodiment in the case of two-phase diversity. The coherent light receiving apparatus 302 includes a local light source unit 20 and a processing unit 24 which are different from the coherent light receiving apparatus 301 of FIG.

  The coherent light receiving apparatus 302 includes a local light source unit 20, a 2 × 2 multiplexer mixing unit 12, a differential optical detector 13, and a processing unit 24. The processing unit 24 includes an analog / digital converter 121 (A / D converter: Analog to Digital converter), a digital arithmetic unit 122, an identification unit 123, an oscillator 124 serving as a sine wave signal source, a frequency divider 125, and a pattern generation unit 126. Is provided.

  The local light source unit 20 receives two outputs of the local light having different frequencies from each other at a frequency of 1 / integer of a sine wave signal generated by the oscillator 124 of the processing unit 24 described later by the modulator 103 that has received the output from the oscillator 102. Is output. Specifically, a local light EL that multiplexes local light having an angular frequency wL and an angular frequency wL + w0 is generated. The frequency difference Δf (= w0 / 2π) between the plurality of local lights of the local light EL is smaller than twice the bandwidth of the received signal light Es and the spectral line width of the light source that generates the signal light Es. (Full width at half maximum) and a spectrum line width (full width at half maximum) of the light source that generates the local light emission EL. Here, the sine wave signal output from the oscillator 102 to the modulator 103 is in phase with the sine wave signal generated by the oscillator 124 of the processing unit 24 from the viewpoint of simplifying the processing in the processing unit 24. desirable.

  The local light source unit 20 includes, for example, a combination of a light source 101, a modulator 103, an optical frequency-dependent variable optical attenuator 105 (VOA), an optical splitter 106, a monitor circuit 107, and an intensity ratio control circuit 108. Composed.

  The light source 101 outputs light having an angular frequency wL. The spectral line width (full width at half maximum) of the light source 101 can be, for example, about 100 kHz to 10 MHz.

  The modulator 103 is a frequency shifter that shifts the angular frequency of a part of the light from the light source 101 by the angular frequency w 0 of the frequency signal supplied from the frequency divider 104. The modulator 103 is, for example, a general FM modulator, acousto-optic modulator, intensity modulator, or SSB modulator. When the modulator 103 is an intensity modulator, if the carrier is suppressed and the upper and lower sidebands are used, the angular frequency of the light output from the light source 101 is wL0 + w0 / 2 instead of wL0 and is supplied from the frequency divider 104. The angular frequency of the frequency signal is w0 / 2. Further, when the optical frequency of a part of the light from the light source 101 cannot be changed, the optical frequency of one of the lights branched and branched from the light source 101 is changed, and the other optical frequency is left as it is. It is good also as a structure which aligns both polarization states and multiplexes.

  For example, when the signal light is 40 Gbit / s QPSK or DQPSK signal light, the frequency w0 (= 2πf0) is about 20 GHz, so that the frequency w0 is smaller than about 40 GHz, and the local light source 20 Since the spectral width of the light source 101 and the signal light source may be larger than 100 kHz to 10 MHz, the frequency f0 in this case is set in the range of 100 MHz to 1 GHz, for example.

  The VOA 105 is an attenuator whose attenuation is different depending on the optical frequency, and receives local light and changes the intensity ratio between the multiple local lights. The intensity ratio to be changed is variably controlled according to an output signal from an intensity ratio control circuit 108 described later. Further, the VOA 105 may be an amplifier whose amplification amount differs at the optical frequency instead of being an attenuator. The VOA 105 may be replaced with adjusting the intensity ratio of each local light generated by the modulator 103. Alternatively, a configuration may be adopted in which a plurality of local lights are branched, and one or both of them are combined with the polarization states of the two again through an attenuator. A plurality of local light branching recombinations may be shared by the modulator 103 and the VOA 105 described above.

  The light source 101, the modulator 103, and the VOA 105 are optically coupled using, for example, a polarization maintaining fiber, an optical waveguide, or a spatial optical system, and the polarization state of light propagating therebetween is determined. Is retained.

  The optical branching device 106 branches a part of the local light EL as monitor light and outputs it to the monitor circuit 107. The monitor circuit 107 detects the intensity (amplitude) of local light at each of the angular frequencies wL and wL + w0 included in the local light EL using the monitor light from the optical branching device 106, and monitors the ratio. The intensity ratio control circuit 108 generates a control signal for changing the attenuation amount of the VOA 105 according to the monitoring result of the monitor circuit 107 and the calculation result of the digital arithmetic unit 122 described later, and outputs the control signal to the VOA 105. .

  The mixing unit 12 is a 2 × 2 multiplexer that mixes the local light from the local light source 10 and the received signal light. The signal light Es having the angular frequency ws input to the coherent light receiving device 302 via an optical transmission line or the like is input to one input port of the 2 × 2 multiplexer in a state where local light and polarization are aligned. The local light EL output from the local light source 10 is input to the other input port. The 2 × 2 multiplexer combines the input signal light and the local light, and outputs two sets of light whose optical phases are substantially different from each other by π.

  The differential optical detector 13 optically detects the mixed signal light mixed by the mixing unit 12. Specifically, the differential optical detector 13 receives each light output from the 2 × 2 multiplexer with a phase difference of π and performs differential optical detection. Here, in order to perform differential optical detection of outputs having different π phase differences of the 2 × 2 multiplexer, the 2 × 2 multiplexer and the differential optical detector 13 are configured. The detector may be a 2 × 1 multiplexer, and instead of the differential optical detector, a non-differential single-input optical detector may be used, or one of the multiplexer and the differential detector may be changed. .

  The AD converter 121 converts the analog electrical signal input from the differential optical detector 13 into a digital signal using the sine wave signal from the oscillator 124 that generates the sine wave signal source as a sampling timing, and converts the signal to the digital calculator 122. Output.

  The digital calculator 122 uses the digital signal from the AD converter 121 and performs digital calculation processing to obtain information consisting of a real part and an imaginary part for the amplitude information and phase information of the electric field of the received signal light. To make an estimate. Here, the digital computing unit 122 has a first series of digital signals (I) derived from the mixed light of the local light corresponding to the phase state “0” per symbol of the received signal light and the received signal light, The second series of digital signals (Q) derived from the mixed light of the local light corresponding to the phase state “π / 2” and the received signal light can be captured. Thereby, in the digital calculator 122, the information which consists of a real part and an imaginary part can be obtained about the amplitude information and phase information of the optical electric field of the received signal light by arithmetic processing.

  The identification unit 123 is a data identification unit that performs reception data identification processing based on a calculation result from the digital arithmetic unit. The identification unit 123 uses the digital signal as a result of estimating the amplitude and phase information by the digital computing unit 122 and performs identification processing of received data according to the light modulation scheme in the transmission side device 351. By the identification unit 123 identifying in this way, the coherent light receiving apparatus can reproduce the signal light data.

  The digital computing unit 122 normally operates at an operating sine wave signal frequency lower than the bit rate of received data included in the received signal light.

  The oscillator 124 is an oscillation circuit that generates a sine wave signal that defines the sampling timing to the AD converter 121. Based on the result of the arithmetic processing for extracting the received data described above, the digital calculator 122 sets the phase and frequency of the sine wave signal generated by the oscillator 124 to the phase state “0”. Control is performed so that the local light emission corresponding to “0” and the phase state “π / 2” corresponds to the phase state “π / 2”. Thereby, the digital calculator 122 performs the above-described digital calculation process on the digital signal from the AD converter 121 by parallel processing according to the frequency division number of the sine wave signal performed by the frequency divider 125, for example.

  The frequency divider 125 divides the sine wave signal from the oscillator 124 and outputs it as an operating sine wave signal to the digital calculator 122. Thereby, the digital arithmetic unit 122 can perform the above-described digital arithmetic processing on the digital signal from the AD converter 121 by parallel processing according to the frequency division number. The frequency divider 125 is configured so that the sampling frequency of the AD converter 121 is a digital arithmetic unit so that the digital arithmetic unit 122 can perform arithmetic processing in units of one set of first and second series digital signals per symbol. The frequency division ratio is set so as to be at least twice the operating sine wave signal 122.

  Further, the frequency division number of the frequency divider 125 is set to a frequency division number (2 × N frequency division) that is an integer N times 2 to calculate the first series (I) digital signal, When the calculation for the digital signal of the series (Q) is set as one set, the digital calculator 122 can execute N parallel processing.

  The AD converter 121 and the digital arithmetic unit 122 can be configured in advance as a single module. In this way, wiring for connection as the AD converter 121 and the digital arithmetic unit 122 can be designed in advance in the module, which is advantageous from the viewpoint of downsizing the apparatus and reducing wiring load. is there. The AD converter 121 and the digital arithmetic unit 122 perform digital signal processing (analog / digital conversion processing and digital arithmetic processing) based on a sine wave signal with respect to the intermediate frequency signal converted into the electric signal by the differential optical detector 13. A reception data processing unit that performs processing for extracting reception data included in the received signal light is configured.

The transmission bit rate of the signal light is Z, and the multilevel modulation number of the signal light is m [information amount per symbol] is log ₂ m (bit / symbol), the first sequence (I) and the second sequence (one symbol) If the number of times of acquisition of the digital signal of Q) is h, the oscillator 124 generates a sine wave signal having a frequency of 2hZ / log ₂ m, and the AD converter 121 performs sampling in synchronization with this sine wave signal.

Next, the operation will be described. The received signal light and local light are represented by, for example, electric field vectors Es (t) and EL (t) shown in the following equations.
(Formula 3)
Es (t) = Asexp {j (wst + Os (t))} s (t) (3)
(Formula 4)
EL (t)
= [AL1 + AL2exp {j (w0t)}] exp {j (wLt + OL (t))}
(4)
Here, s (t) is a signal vector corresponding to the received signal light data, As is the amplitude of the received signal light, ws is the average angular frequency of the received signal light, and Os (t) is the received signal light. Optical phase fluctuation, AL1 is the amplitude of one local light, AL2 is the amplitude of the other local light, wL is the average angular frequency of one local light, w0 is the angular frequency difference between the local lights, OL (t) is the local light The optical phase fluctuation of light emission, t represents time, and j represents an imaginary unit.

After the received signal light Es and the local light EL are mixed by the 2 × 2 multiplexer, the complex current optically detected by the differential optical detector 13 is defined by the following equation. The real part I of this complex current corresponds to an intermediate frequency signal related to one local light, and the imaginary part Q corresponds to an intermediate frequency signal related to the other local light.
(Formula 5)
I + jQ
= RAsAL1exp {j (wst + Os (t) -wLt-OL (t))} s (t)
+ RAsAL2exp {j (wst + Os (t) −wLt−OL (t) −w0t} s (t)
(≡ [RAsAL1exp {j (θ1 (t))} + RAsAL2exp {j (θ2 (t))}] s (t))
= Exp {j (wst + Os (t) -wLt-OL (t))} [RAsAL1 + RAsAL2exp {-j (w0t)}] s (t)
= Exp {j (θ (t))} [RAsAL1 + RAsAL2exp {−j (w0t)}] s (t) (5)
Here, it is assumed that the following expression is satisfied.
(Formula 6)
(RAsAL1) ² + (RAsAL2) ² = 1 (6)
Next, focusing on the term derived from the local light of the optical frequency wL in the equation (5) and compensating for the frequency difference between the signal light and the local light and the relative phase noise, the complex current I ′ + jQ ′ after the compensation is obtained. Is given by
(Formula 7)
I′jQ ′ = [RAsAL1 + RAsAL2exp {−j (w0t)}] s (t) (7)

Here, an example of the above compensation method will be described. The complex current signal output from the differential optical detector is a technique related to this compensation. For example, Non-Patent Document 1 discloses that the received signal light is quaternary phase shift keying (PSK). In the case of the method, a method for calculating the phase difference θ (t) between the received signal light and the local light is shown. By this extension, in the case of the m-value PSK method, the approximation is performed according to the relationship of the following equation: It can be calculated automatically.
(Formula 8)
θ (t) ≈ (1 / m) (1 / Δt) ∫arg {(I + jQ) ^m } dt (8)
Therefore, in the present invention, each of the local light emission phase difference θ1 (t) and the local light emission phase difference θ2 (t) of the optical frequency wL + w0 included in the equation (5) is the same as the equation (8). An approximate value is calculated according to the following equation:
(Formula 9)
θ1 (t) ≈ (1 / m) (1 / Δt) ∫arg {(I + jQ) ^m } dt
θ2 (t) ≈ (1 / m) (1 / Δt) ∫arg {([I + jQ] exp {−j (w0t)}) ^m } dt (9)
At this time, the integration time width in the equation (9) is sufficiently larger than the reciprocal of the frequency difference between the plurality of local lights, that is, 2π / w0, and the average frequency of the received signal light and the average frequency of the local light It is sufficiently smaller than the reciprocal of the maximum value of the frequency difference, that is, 1 / max (| wL−ws | / 2π |). When the received signal light is DQPSK, the value of m is 4. Note that the above compensation method is an example, and a compensation method having another configuration can be applied to the present invention.

When approximate values of θx (t) and θy (t) are calculated by the equation (9), the ratio of RAsAL1 and RAsAL2 included in the above equation (5) is approximately obtained by the following equation.
(Formula 10)
RAsAL1: RAsAL2
≈∫ | exp {−j (θ1 (t))} (I + jQ)} | dt
: ∫ | exp {−j (θ2 (t))} (I + jQ)} | dt (10)
However, the integration time width in the equation (10) is sufficiently larger than the reciprocal 2π / w0 (= 1 / f0) of the frequency difference between the plurality of local lights.

The values of RAsAL1 and RAsAL2 are calculated by using the ratio of RAsAL1 and RAsAL2 obtained according to the relationship of equation (10) and the relationship of equation (6). The value of RAsAL1 and RAsAL2, the current value of the intermediate frequency signal output from the differential optical detector 13, and w0 (= 2πf0) calculated from the frequency of the oscillator 124 are solved for Equation (7) for s (t). And rationalize the denominator to calculate the value of the signal vector s (t). This s (t) corresponds to the estimation result of the amplitude and phase of the received signal light electric field output from the digital computing unit 122.
(Formula 11)
s (t)
= [I '+ jQ'] / [RAsAL1 + RAsAL2exp {-j (w0t)}]
= [I '+ jQ'] [RAsAL1 + RAsAL2exp {j (w0t)] / [1 + 2RAsAL1RAsAL2cos (w0t)] (11)
Therefore, based on the calculated value of the signal vector s (t), the identification unit 123 can perform the data identification process according to the threshold value corresponding to the modulation method of the received signal light, whereby the received data can be reproduced. .

However, since the above equation (11) diverges under the condition of the following equation (12), measures are required to avoid such a state.
(Formula 12)
cos (w0t) = − 1 / [2RAsAL1RAsAL2] (12)
The expression (12) is established only in the case of the following expression in consideration of the expression (4).
(Formula 13)
RAsAL1 = RAsAL2 = √2 / 2 (13)
Therefore, when the condition of the equation (13) is approached, for example, by changing the amplitude ratio of a plurality of local lights, the relationship of the above equation (11) diverges and the signal vector s (t) is calculated. It becomes possible to avoid a situation in which it becomes impossible.

  In the local light source unit 20, a part of the light having the angular frequency wL output from the light source 101 is shifted by the modulator 103 by the angular frequency w 0. The light from the modulator 103 is input to the VOA 105, the intensity (amplitude) ratio between a plurality of local lights having the angular frequency wL and the angular frequency wL + w0 is adjusted, and the local light comprising a plurality of local lights having an angular frequency difference of w0. It becomes EL.

  The local light EL is sent to the 2 × 2 multiplexer, and a part thereof is branched by the optical splitter 106 and sent to the monitor circuit 107. The monitor circuit 107 monitors the ratio of the intensity (amplitude) of each local light included in the local light EL and transmits the monitoring result to the intensity ratio control circuit 108.

  The intensity ratio control circuit 108 changes the attenuation amount of the VOA 105 according to the monitor result of the monitor circuit 107 and the calculation result of the digital calculator 122.

  The local light EL input to the 2 × 2 multiplexer is combined with the received signal light Es having the angular frequency wS, and each light having an optical phase difference of π is output to the differential optical detector 13. In the differential optical detector 13, the output light from the 2 × 2 multiplexer is subjected to differential optical detection 13. Thereby, the signal I having the intermediate frequency wi due to the beat between the local light of the angular frequency wL included in the local light EL and the received signal light Es is output from the differential photodetector 13 and included in the local light EL. A signal Q having an intermediate frequency wi + w0 due to a beat between the local light having the angular frequency wL + w0 and the received signal light Es is output from the differential photodetector 13.

  Here, the intermediate frequency signals I and Q are set so that the frequency difference is smaller than twice the signal bandwidth and larger than the spectral line width of the signal light source and the local light source unit 20. Each spectrum overlaps on the frequency axis. As a result, the bandwidth required for the differential optical detector 13 and the electronic circuit disposed downstream thereof is, for example, about twice the signal bandwidth. When the local light angular frequency wL is set so that the intermediate frequencies fIF1 and fIF2 are in the vicinity of 0 Hz, the required bandwidth can be reduced to the same level as the signal bandwidth.

  The intermediate frequency signals I and Q output from the differential optical detector 13 are AD converted at high speed by the AD converter 121, and a digital signal sequence corresponding to the intermediate frequency signals I and Q is input to the digital calculator 122. In the digital calculator 122, digital signal processing is executed according to a series of algorithms corresponding to the equations (3) to (11), and the value of the signal vector s (t) is calculated. Further, when the values of RAsAL1 and RAsAL2 obtained in the calculation process approach the condition of the above expression (13), the information is transmitted from the digital calculator 122 to the intensity ratio control circuit 108 in the local light source unit 20, and the intensity By controlling the VOA 105 by the ratio control circuit 108, the ratio of the intensity between the local lights included in the local light EL is changed, the expression (11) diverges and the calculation of the signal vector s (t) becomes impossible. Is avoided.

  When the calculated value of the signal vector s (t) in the digital calculator 122 is transmitted to the identification unit 123, the identification unit 123 determines the signal vector s (t) according to the threshold corresponding to the modulation method of the received signal light. An identification process of which data value the calculated value corresponds to is executed, and the identification result is output as received data.

  As described above, by combining AD conversion of received signals and digital signal processing, and setting the angular frequency difference w0 between the local lights included in the local light EL, the bandwidth required for the differential optical detector or the like is improved. The width can be reduced.

  In the calculation of the signal vector s (t) in the digital calculator 122, as shown in the above equation (11), trigonometric functions sin (w0t) and cos () regarding the frequency difference w0 to be given to one local light. It is necessary to calculate w0t). Here, in the case where the frequency signal for giving the frequency difference is configured to be supplied from an independent oscillation circuit, it is conceivable that the set value of w0t takes a value within a certain range. The values of trigonometric functions sin (w0t) and cos (w0t) corresponding to the oscillation frequency of the circuit need to be stored in advance in a table configuration.

  In this case, it is desirable to synchronize the oscillator 102 and the digital calculator 122 used in the local light source unit 20. For example, a frequency difference between local lights (fIF1 + fIF2 in FIG. 2, fIF1 + fIF2 in FIG. 2, (fIF2- By making the frequency signal defining fIF1), regularity is given to the values of sin (w0t) and cos (w0t) at each sampling timing in the AD converter 121, and the signal vector s (t) corresponding to the sampling timing This calculation may be simplified.

  An example in which the sampling frequency of the AD converter 121 (that is, the sine wave signal frequency from the oscillator 124) is 1 / T and the pattern generator 126 divides the sine wave signal from the oscillator 124 by a quarter is illustrated. In this case, when sampling by the AD converter 121 is performed in the order of the timing numbers # 0, # 1,..., # 6, the sampling times are set to 0, T,. , 6T.

  At this time, since Δw = 1 / 4T, w0t at timing numbers # 0 to # 6 increases from 0 by π / 2. Accordingly, cosw0t at timing numbers # 0 to # 6 is sequentially 1, 0, −1, 0, 1, 0, −1, and includes four values (1,0, −1) including timing numbers # 7 and thereafter. , 0) constitutes a circulating sequence. Similarly, sinw0t at timing numbers # 0 to # 6 is also 0, 1, 0, −1, 0, 1 in order, and four values (0, 1, 0, − including timing number # 7 and thereafter). 1) constitute a cyclic sequence in which the values of 1) circulate.

  Therefore, even when the frequency division ratio in the frequency divider 125 is set to “1”, the values of sin (w0t) and cos (w0t) are stored in a table configuration when the digital arithmetic unit 122 calculates. It is not necessary to do so, and the values of sin (w0t) and cos (w0t) can be derived from the above-described simple cyclic number sequence to simplify the calculation.

  In addition, when the division ratio in the frequency divider 125 is “2” and the calculation in the digital calculator 122 is paralleled by two channels ch1 and ch2, a signal based on the digital signal input according to the sampling timing. Since the calculation of the vector s (t) can be performed alternately on the channels ch1 and ch2, the assignment of the values of sin (w0t) and cos (w0t) for each of the channels ch1 and ch2 can be further simplified.

  That is, sin (w0t) assigned in the calculation of the signal vector s (t) in the channel ch1 forming the digital calculator 122 is a fixed value 0, and the value of cos (w0t) is a binary value (1 , -1). On the other hand, in channel ch2, sin (w0t) is a binary (1, -1) value that appears alternately, and the value of cos (w0t) is a fixed value of 0.

  Further, assuming that the frequency division ratio at the frequency divider is “4”, the frequency signal generated by the pattern generator 126 and the sine wave signal obtained by frequency division at the frequency divider 125 have substantially the same frequency. In this case, the computation in the digital computing unit 122 is parallelized by four channels ch1 to ch4, but the assignment of the values of sin (w0t) and cos (w0t) in each channel ch1 to ch4 is further simplified. Can be

  In this case, w0t always takes the same point on the phase plane in each of the channels ch1 to ch4, so that the values of sine and cosine can always be fixed values. That is, sin (w0t) in each of the channels ch1 to ch4 has a fixed value of 0, 1, 0, −1, and cos (w0t) has a fixed value of 1, 0, −1, 0, respectively.

  The frequency signal generated by the pattern generator 126 is not limited to being ¼ of the sine wave signal frequency from the oscillator 124. Also in this case, in particular, the sine wave signal obtained by the frequency division by the frequency divider 125 is matched with the frequency of the frequency signal generated by the pattern generator 126 to be used for the calculation by the digital arithmetic unit 122. The values of sin (w0t) and cos (w0t) can be fixed values.

  Thereby, in the above equation (11) for obtaining the signal vector s (t), the denominator cos (w0t) term (Δφ0 is taken into account, cos (w0t−Δφ0)) and the numerator ej (w0t) ( If Δφ0 is taken into account, the term ej (w0t−Δφ0)) can be made constant for each channel, so that the calculation load can be greatly reduced. In particular, if the frequency divider 125 and the pattern generator 126 are shared, the operation of the digital arithmetic unit 122 can be simplified as described above, and the apparatus configuration can be simplified.

10, 20: Local light source 12: Mixer 13: Differential optical detector 14, 24: Processing unit 15: Polarization controller 16: Differential optical detector 101: Light source 102: Oscillator 103: Modulator 104: Frequency divider 105: VOA
106: Brancher 107: Monitor circuit 108: Intensity ratio control circuit 121: AD converter 122: Digital calculator 123: Discriminator 124: Oscillator 125: Divider 126: Pattern generator 301, 302: Coherent light receiving device 351 : Optical transmission device 352: Optical network 401, 402: Optical communication system

Claims (6)

A local light source section that outputs a plurality of local light sources with the same phase fluctuation and initial phase;
Mixing the local light output from the local light source unit and the input signal light, and outputting a mixed signal light; and
An optical detection unit that optically detects the mixed signal light and outputs an intermediate frequency signal between the local light and the signal light;
A processing unit that separates each of the intermediate frequency signals output from the optical detection unit and demodulates a signal component of the signal light;
An adjustment unit that adjusts the phase relationship between the local lights so that the phases of the intermediate frequency signals output by the optical detection unit have a predetermined relationship;
A coherent optical receiver. The local light source section is
A light source that outputs one light;
An oscillator that oscillates a sine wave signal;
A modulator that modulates light from the light source with a sine wave signal from the oscillator to generate a plurality of local lights;
The coherent optical receiver according to claim 1, comprising: The local light source unit outputs the local light in which the absolute value of the frequency difference between the plurality of intermediate frequency signals is larger than half of the modulation band of the signal light,
The processing unit multiplies each of the intermediate frequency signals and a sine wave signal having the same frequency as each of the intermediate frequency signals and the initial phase when the phase fluctuations are multiplied together, and having a predetermined relationship,
In the case of a diversity system in which the adjusting unit is equivalent to an optical multi-terminal coupling circuit having K terminals, the output of the intermediate frequency signal corresponding to each terminal is a phase of 2πq / K, (q: 0 to K−1). The coherent optical receiver according to claim 1, wherein the phase of the sine wave signal is adjusted so that the output of the coherent optical signal becomes the output of the coherent optical receiver. The local light source section is
A difference in optical frequency is smaller than twice the bandwidth of the signal light, and the local light emission greater than the light source spectral line width of the signal light and the light source spectral line width of the local light is output,
The processor is
An AD converter for converting the intermediate frequency signal having the orthogonal phase into a digital signal, and a digital signal output from the AD converter by the equation (C1) to estimate data information included in the signal light A digital computing unit that outputs a signal vector s (t);
A discriminator that discriminates the signal vector s (t) output by the digital arithmetic unit and outputs a signal component of the signal light as received data;
The adjustment unit is
The phase difference between the plurality of intermediate frequency signals is adjusted by the digital computing unit so that the phase difference differs from the phase difference between the plurality of sine wave signals multiplied by the intermediate frequency signal by a quarter period. The coherent optical receiver according to claim 1 or 2.
(Formula C1)
s (t) = [I ′ + jQ ′] / [X + Y exp (−jw0t)]
Here, the amplitude of the intermediate frequency signal is X and Y, the angular frequency difference of the local light is w0, and the imaginary unit is j.   The coherent optical receiver according to claim 4, further comprising an intensity ratio control circuit that changes an intensity ratio of the local light. An optical fiber for transmitting a wavelength multiplexed signal;
A plurality of coherent optical receivers according to any one of claims 1 to 5;
An optical branching device for branching a wavelength division multiplexed signal transmitted through the optical fiber and coupling it to the coherent optical receiving device;
And each of the coherent optical receivers shares one local light source unit.

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