JP2009296623A - Coherent optical transceiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized and polarization-independent optical transceiver capable of receiving high-speed signal light in accordance with a coherent reception system. <P>SOLUTION: In a coherent optical transceiver, a coherent light beam output from a light source 21 is split into two by a beam splitter 41, one coherent light beam is modulated by an optical modulator 42 and transmitted to an optical transmission line 50, and the other coherent light beam is sent to an acousto-optic polarization mode converter 31 of a local oscillation light generating unit 11. The local oscillation light generating unit 11 generates local oscillation light by polarizing and multiplexing orthogonal polarization components in optical frequencies different from each other, and the local oscillation light beam and a reception signal light beam are combined by a 2×4 hybrid circuit 12 and then photo-electric-converted by two differential photodetectors 13, 14, further converted into digital signals by A/D conversion circuits 15, 16 and then subjected to signal processing by a digital arithmetic circuit 17, thereby estimating reception data. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical transceiver used in an optical transmission system, and more particularly to a coherent receiving optical transceiver that does not depend on the polarization state of signal light.

  In order to realize an ultra high-speed optical transmission system of 40 gigabits per second (Gbit / s) or more, a transmitter / receiver of an RZ-DQPSK (Return-to-Zero Differential Quadrature Phase Shift Keying) modulation system has been developed. In the future, it is desired to further improve the optical noise immunity of the RZ-DQPSK transceiver and to reduce the size of the optical tunable dispersion compensator, which occupies a large size, by replacing it with powerful electric signal processing. Yes. As means for realizing them, for example, a coherent reception system such as a homodyne system, an intradyne system, or a heterodyne system is promising and has been studied (for example, non-patented). Reference 1). According to the coherent reception method, the optical noise resistance is improved by 3 dB, and it is considered that the compensation capability of chromatic dispersion distortion by electrical signal processing after photoelectric conversion is significantly increased as compared with delayed direct detection.

  However, in the above coherent reception method, the principle that the polarization state of the local oscillation light output from the local oscillation light source provided in the optical receiver is orthogonal to the polarization state of the received signal light cannot be received. There are some problems. Since the polarization state of the signal light received through the optical transmission line always varies depending on the state of the optical transmission line, a mechanism for solving the above-described problems is important.

As conventional techniques for eliminating the polarization dependency of the coherent optical receiver, for example, the following methods are known (for example, see Non-Patent Documents 2 and 3).
(I) A system comprising an infinite-tracking automatic polarization controller and always controlling the polarization state of the received signal light to coincide with the polarization state of the local oscillation light. (II) Phase hybrid circuit and photoelectric conversion unit (III) Using locally polarized light as polarization multiplexed light, and shifting one optical frequency of polarization components orthogonal to each other by about twice or more compared to the signal bandwidth To detect each polarization component by coherent reception using the prepared and performing frequency separation after photoelectric conversion

F. Derr, "Coherent optical QPSK intradyne system: Concept and digital receiver realization", Journal of Lightwave Technology.Vol. 10, No. 9, p. 1290-1296, September 1992. L. G. Kazovsky, "Phase- and polarization-diversity coherent optical techniques", Journal of Lightwave Technology, Vol. 7, No. 2, p. 279-292, February 1989. A. D. Kersey et al., "New polarisation-insensitive detection technique for coherent optical fiber heterodyne communications", Electronics Letters, Vol. 23, p. 924-926, Aug. 27, 1987.

  However, the conventional technology as described above is a problem that it is difficult to realize a small-size and polarization-independent coherent optical receiver that can receive high-speed signal light such as 40 Gbit / s. That is, in order to realize the method (I), an infinite tracking automatic polarization controller is required, and thus it is difficult to reduce the size. In addition, the realization of the above-described method (II) requires a large-scale light receiving front-end circuit that is about twice or more, so that it is difficult to reduce the size. Furthermore, in order to realize the method (III), an electronic circuit having a light receiving band that is about three times as large as the signal bandwidth is required. Therefore, it is difficult to deal with high bit rate signal light. is there.

  Here, the problem of the method (III) will be specifically described.

FIG. 8 is a diagram showing a configuration of a coherent optical receiver to which the above-described method (III) is applied. In this conventional coherent optical receiver, the local oscillator light generating section 101, the light of optical angular frequency omega _L outputted from the light source 111, through an optical isolator 112 a polarization beam splitter (Polarization Beam Splitter: PBS) 113 And is separated into orthogonal polarization components. Then, one polarization component is an acousto-optic modulator (Acousto-Optic Modulator: AOM) 114 is input to the optical angular frequency is shifted by omega _O, in the polarization component and PBS113 the optical angular frequency ω _{L +} ω _O The other separated polarization component is synthesized by a polarization beam combiner (PBC) 115. Thus, for example, as shown in the conceptual diagram of FIG. 9, the polarization component of the optical angular frequency ω _L (E _x (t) component in the figure) and the polarization component of the optical angular frequency ω _L + ω _O orthogonal thereto ( The local oscillation light E _LO is generated by polarization multiplexing the E _y (t) component in the figure.

The local oscillation light E _LO output from the local oscillation light generation unit 101 is combined with the reception signal light E _S having the optical angular frequency ω _S by the multiplexer 102 and then received by the photodetector 103 to be electrically supplied. Converted to a signal. The electrical signal includes a signal component A ₁ of an intermediate frequency ωi due to the beat of the polarization component and the reception signal light E _S of the optical angular frequency omega _L included in the local oscillator light E _LO, included in the local oscillator light E _LO it means to include the signal a ₂ of the intermediate frequency .omega.i + omega _O by beat optical angular frequency ω _{L +} ω _O polarization component and the reception signal light E _S, band output signal of the light detector 103 pass filter (band Pass filter (BPF) 104 and 105 are respectively provided to separate the intermediate frequency signals A ₁ and A ₂ according to the frequency. Then, the intermediate data A ₁ and A ₂ are input to the reception electronic circuit 106 and necessary signal processing is executed to reproduce the reception data DATA.

At this time, the intermediate frequency signals A ₁ and A ₂ input to the receiving electronic circuit 106 have an electric spectrum as shown in the schematic diagram of FIG. 10, for example. Specifically, the intermediate-frequency signal A ₁ has a approximately twice the spectral width of the signal bandwidth centered on the frequency .omega.i, intermediate-frequency signal A ₂ is approximately twice the spectrum of the signal band width around the frequency .omega.i + .omega.o It has a width. The power difference ΔP between the intermediate frequency signals A ₁ and A ₂ varies depending on the polarization state of the received signal light. For this reason, the bandwidth of the reception electronic circuit 106 needs to be at least four times the signal bandwidth in the example of FIG. In the case where an intermediate frequency ωi sets the optical angular frequency omega _L of the local oscillator light so that 0Hz, the bandwidth of the receiving circuit 106 is close to three times the signal bandwidth.

  Therefore, the conventional coherent optical receiver to which the method (III) is applied is an electron having a bandwidth of about 3 times or more, that is, about 120 GHz or more, for a signal light of 40 Gbit / s, for example. A circuit is required, and it is very difficult to realize high-speed signal light of about 40 Gbit / s or more.

  The present invention has been made paying attention to the above-described problems, and an object thereof is to provide a compact and polarization-independent optical transceiver capable of receiving high-speed signal light by a coherent reception method.

  In order to achieve the above object, an aspect of a coherent optical transceiver according to the present invention includes a coherent light source and an optical branching unit that branches light output from the coherent light source into a first coherent light and a second coherent light. An optical modulation unit that modulates the first coherent light branched by the optical branching unit according to transmission data and transmits the modulated light to the outside, and the second coherent light branched by the optical branching unit or the second coherent light A local oscillation light generating unit that outputs light generated based on light modulation as local oscillation light, and a multiplexing unit that combines the received signal light and the local oscillation light output from the local oscillation light generation unit and outputs the combined signal And a photoelectric conversion unit that converts light output from the multiplexing unit into an electrical signal, and a data identification unit that performs identification processing of received data based on the output of the photoelectric conversion unit. Is shall.

  According to the coherent optical transmitter / receiver of the present invention as described above, the configuration can be simplified and miniaturized by sharing the light sources of the local oscillation light and the transmission signal light.

It is a block diagram which shows the structural example of the coherent optical receiver relevant to this invention. FIG. 2 is a block diagram illustrating a specific configuration example of a local oscillation light generation unit in the coherent optical receiver of FIG. 1. It is a figure which shows typically the electrical spectrum of the intermediate frequency signal in the coherent optical receiver of FIG. It is a block diagram which shows the other structural example of the local oscillation light generation part relevant to the coherent optical receiver of FIG. It is a block diagram which shows another structural example of the local oscillation light generation part relevant to the coherent optical receiver of FIG. It is a block diagram which shows one Embodiment of the coherent optical transmitter-receiver by this invention. It is a perspective view which shows an example of the PLC circuit applied to the coherent optical transmitter-receiver of FIG. It is a block diagram which shows the structural example of the conventional coherent optical receiver. It is a conceptual diagram which shows the orthogonal polarization component of the local oscillation light in the conventional coherent optical receiver. It is a figure which shows typically the electric spectrum of the intermediate frequency signal in the conventional coherent optical receiver.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings. Note that the same reference numerals denote the same or corresponding parts throughout the drawings.

  FIG. 1 is a block diagram showing a configuration example of a coherent optical receiver related to the present invention.

  In FIG. 1, the coherent optical receiver includes, for example, a local oscillation light generation unit 11, a 2 × 4 optical hybrid circuit 12 as a multiplexing unit, differential optical detectors 13 and 14 as photoelectric conversion units, AD conversion circuits 15 and 16 as AD conversion units, a digital calculation circuit 17 as a digital calculation unit, and an identification circuit 18 as a data identification unit are configured.

The local oscillation light generator 11 generates local oscillation light E _LO that is polarization-multiplexed with the polarization component of the optical angular frequency ω _{L and} the polarization component of the optical angular frequency ω _L + ω _O orthogonal thereto. Optical angular frequency difference omega _O between the orthogonal polarization components of the local oscillator light E _LO is 2 times smaller than the bandwidth of the signal light E _S received by the optical receiver, and an optical transmitter (not shown) It is preset to be larger than the signal light spectrum line width of the E light source for generating _S (FWHM) and spectral line width of the light source for generating the local oscillator light E _LO (FWHM) in the machine.

  FIG. 2 is a block diagram showing a specific configuration example of the local oscillation light generator 11. The local oscillation light generation unit 11 includes, for example, a light source 21, a polarization beam splitter (PBS) 22, a frequency shifter (FS) 23, an oscillator 24, a variable optical attenuator (VOA) 25, a polarization A wave beam combiner (PBC) 26, an optical splitter 27, a monitor circuit 28, and an intensity ratio control circuit 29 are included.

Light source 21 generates a constant polarization state that has an optical angular frequency omega _L, for example a linearly polarized wave. The spectral line width (full width at half maximum) of the light source 21 is, for example, about 100 kHz to 10 MHz.

  The PBS 22 separates the output light from the light source 21 into two polarization components orthogonal to each other. When the output light from the light source 21 is linearly polarized light, the output light is input to the PBS 22 so that the polarization direction is inclined by 45 degrees with respect to the optical axis of the PBS 22. Further, although not shown, an optical isolator may be disposed between the light source 21 and the PBS 22.

The frequency shifter 23 receives one polarization component output from the PBS 22 and shifts the optical angular frequency of the input light by ω _O according to the output signal from the oscillator 24. As this frequency shifter 23, a general FM modulator, an acousto-optic modulator (AOM), a single side-band (SSB) modulator, or the like can be used.

As described above, the oscillator 24 has an oscillation frequency Δf (corresponding to an optical angular frequency ω _O that is smaller than twice the signal bandwidth and larger than the spectral line width of the signal light source and the spectral line width of the local oscillation light source. = operate at ω _{O /} 2π), and outputs the oscillation signal to the control terminal of the frequency shifter 23. If you leave by way of specific example of the frequency Delta] f, if the received signal light _{E S} is DQPSK signal light of 40 Gbit / s, the signal band width is 20 GHz, less than twice the 40 GHz, and, above Since the spectral width of the light source 21 in the local oscillation light generation unit 11 may be larger than 100 kHz to 10 MHz (the spectral width of the signal light source is basically the same), the frequency Δf in this case is, for example, 100 MHz. It can be set in the range of ˜1 GHz. However, the frequency Δf is not limited to the above specific example.

  The VOA 25 receives the other polarization component output from the PBS 22 and attenuates the intensity of the input light. The attenuation amount of the VOA 25 is variably controlled in accordance with an output signal from an intensity ratio control circuit 29 described later.

The PBC 26 receives the polarization component of the optical angular frequency ω _L + ω _O output from the frequency shifter 23 and the polarization component of the optical angular frequency ω _L output from the VOA 25, and polarization-multiplexes each polarization component. The generated local oscillation light E _LO is generated.

  The light source 21, the PBS 22, the frequency shifter 23, the VOA 25, and the PBC 26 are optically coupled using, for example, a polarization maintaining fiber, an optical waveguide, a spatial optical system, etc. It is assumed that the polarization state is maintained.

The optical branching device 27 branches a part of the local oscillation light E _LO output from the PBC 26 as monitor light and outputs it to the monitor circuit 28.

The monitor circuit 28 detects the intensity (amplitude) of each polarization component of the optical angular frequencies ω _L and ω _L + ω _O included in the local oscillation light E _LO using the monitor light from the optical branching device 27, and Monitor the ratio.

  The intensity ratio control circuit 29 generates a control signal for changing the attenuation amount of the VOA 25 according to the monitoring result of the monitor circuit 28 and the calculation result of the digital arithmetic circuit 17 described later, and outputs the control signal to the VOA 25. . Details of the control of the VOA 25 by the intensity ratio control circuit 29 will be described later.

The 2 × 4 optical hybrid circuit 12 (FIG. 1) is an optical 90-degree hybrid circuit having two input ports and four output ports. One input port is connected to an optical transmission line or the like from an optical transmitter (not shown). received signal light E _S of the optical angular frequency omega _S inputted to the optical receiver is input through to the other input port, the local oscillator light E _LO output from the local oscillator light generating section 11 is inputted The The 2 × 4 optical hybrid circuit 12 combines the reception signal light E _S and the local oscillator light E _LO is input, the optical phase outputs 90 degrees two different sets of light from each other. Here, the phases of the light output from the two output ports of one set located on the upper side in FIG. 1 are 0 degrees and 180 degrees, respectively, and are output from the two output ports of the other set located on the lower side in the figure. The phase of the emitted light is 90 degrees and 270 degrees.

  The differential photodetector 13 receives each light having an optical phase of 0 degrees and 180 degrees output from the 2 × 4 optical hybrid circuit 12 and performs differential photoelectric conversion detection (balanced detection). Further, the differential photodetector 14 receives each light whose optical phase is 90 degrees and 270 degrees output from the 2 × 4 optical hybrid circuit 12, and performs differential photoelectric conversion detection. The received signals detected by the differential photodetectors 13 and 14 are amplified (standardized) by an AGC amplifier (not shown) or the like.

  Each of the AD conversion circuits 15 and 16 converts the analog reception signal output from each of the differential photodetectors 13 and 14 into a digital signal and outputs the digital signal to the digital arithmetic circuit 17.

The digital arithmetic circuit 17 uses the digital signal from each of the AD conversion circuits 15 and 16 and executes arithmetic processing according to an algorithm described in detail later, whereby an optical angular frequency difference ω _O between orthogonal polarization components. However, signal processing is performed so that the signal light Es can be received coherently using the local oscillation light E _LO set in the range as described above.

  The identification circuit 18 performs data identification processing of the reception signal based on the calculation result in the digital calculation circuit 17, and outputs a reception data signal DATA indicating the result.

  Next, the operation of the coherent optical receiver configured as described above will be described.

First, the operation principle of this optical receiver will be described in detail. The signal light E _S received by the present optical receiver is represented by, for example, an electric field vector E _S (t) shown in the following equation (1).

In the above equation (1), s (t) is a signal vector corresponding to the received signal light data, e _x (t) is a unit vector in the x direction, e _y (t) is a unit vector in the y direction, and A _x Is the amplitude of the x polarization component of the received signal light, A _y is the amplitude of the y polarization component of the received signal light, ω is the average angular frequency (= ω _S ) of the received signal light, and φ (t) is the received signal light Optical phase fluctuation, t represents time, and j represents an imaginary unit.

Further, the local oscillation light E _LO output from the local oscillation light generation unit 11 is represented by, for example, an electric field vector E _LO (t) represented by the following equation (2).

In the above equation (2), A _{LO_x} is the amplitude of the x polarization component of the local oscillation light, A _{LO_y} is the amplitude of the y polarization component of the local oscillation light, ω _LO is the average optical angular frequency of the local oscillation light, and Δωt is The optical angular frequency difference (= ω _O ) between the orthogonal polarization components of the local oscillation light, φ _LO (t) represents the optical phase fluctuation of the local oscillation light, and φ ₀ represents the initial phase of the local oscillation light.

The reception signal light E _S and the local oscillation light E _LO as described above are combined by the 2 × 4 optical hybrid circuit 12, then photoelectrically converted by the differential photodetectors 13 and 14, and further amplified by the AGC amplifier. The normalized complex current is defined by the following equation (3). The real part I of this complex current corresponds to the output of one differential photodetector 13, and the imaginary part Q corresponds to the output of the other differential photodetector 14.

In the above formula (3), the phase difference of the x polarization component is represented by θ _x (t), and the phase difference of the y polarization component is represented by θ _y (t). A _x ′ and A _y ′ satisfy the relationship of the following equation (4), where the gain of the AGC amplifier is g (a positive number).

  Next, focusing on the term derived from the x polarization component in the above equation (3) and compensating for the frequency difference and relative phase noise between the transmission side carrier wave and the local oscillation light, the complex current I after the compensation is obtained. '+ JQ' is expressed by the following equation (5).

  Here, the above compensation will be described. The complex current signals output from the differential photodetectors 13 and 14 may include a frequency mismatch between the local oscillation light and the carrier wave of the signal light, and a declination rotation due to a phase shift. There is a need. As a technique related to this compensation, for example, in literature: DS. Ly-Gagnon et al., “Unrepeated 210-km transmission with coherent detection and digital signal processing of 20-Gb / s QPSK signal,” OFC 2005, OTuL4. Shows a method of calculating the phase difference θ (t) between the received signal light and the local oscillation light when the received signal light is a quaternary phase shift keying (PSK) system. By this extension, it is shown that in the case of the m-value PSK system, it can be approximately calculated according to the relationship of the following equation (6).

Therefore, in the present optical receiver, with reference to the relationship of the above equation (6), the phase difference θ _x (t) of the x polarization component and the phase difference θ _{y of the} y polarization component included in the above equation (3). Each approximate value of (t) is calculated according to the following equation (7).

At this time, the integration time width Δt in the above equation (7) is sufficiently larger than the reciprocal of the frequency difference between the orthogonal polarization components of the local oscillation light, that is, 2π / Δω, and the average frequency of the received signal light And the reciprocal of the maximum value of the frequency difference between the average frequency of the local oscillation light, that is, 1 / max (| ω _LO −ω _S ) / 2π |). When the received signal light is DQPSK, the value of m is 4.

When approximate values of θ _x (t) and θ _y (t) are calculated by the above equation (7), the ratio of A _x ′ and A _y ′ included in the above equation (3) is Approximately obtained from equation (8).

However, the integration time T in the above equation (8) needs to be sufficiently larger than the reciprocal 2π / Δω of the frequency difference between the orthogonal polarization components of the local oscillation light.

By using the ratio of A _x ′ and A _y ′ determined according to the relationship of the above equation (8) and the relationship of A _x ′ ² + A _y ′ ² = 1 shown in the above equation (4), A It becomes possible to calculate the values of _x ′ and _Ay ′.

Therefore, _if the values of A _x ′ and A _y ′ are known, the values of I ′ and Q ′ in the above-described equation (5) are known as the current values output from the differential photodetectors 13 and 14. , and the addition, because the value of Derutaomegati are known by a value corresponding to the frequency Δf of the oscillator _{24 (Δωt = ω O = 2πΔf} ), rationalization denominator solved for s (t) and (5) the relationship The value of the signal vector s (t) can be calculated by the following equation (9).

  Therefore, by executing the data identification process in accordance with the threshold corresponding to the modulation method of the received signal light in the identification circuit 18 based on the calculated value of the signal vector s (t), it is possible to reproduce the received data.

  However, since the relationship of the above equation (9) diverges under the condition shown in the following equation (10), measures to avoid such a state are necessary.

  The condition that the above equation (10) has a real number solution is expressed by the following equation (11).

Here, since A _x ' ² + A _y ' ² = 1 shown in the above-mentioned formula (4), there is a relationship of 0 ≦ A _x '≦ 1 and 0 ≦ A _y ' ≦ 1. The condition of the expression (11) is only in the case of the following expression (12).

Therefore, when the values of A _x ′ and A _y ′ calculated using the above-described equations (4) and (8) approach the condition of the above equation (12), for example, the local oscillation light When the ratio of the amplitude A _{LO_x} of the x polarization component and the amplitude A _{LO_y} of the y polarization component is changed, the relationship of the above equation (9) diverges and the calculation of the signal vector s (t) becomes impossible Can be avoided.

  Based on the above operating principle, the specific operation of the present optical receiver will be described next.

In this optical receiver, the local oscillator light generating section 11, light of optical angular frequency omega _L outputted from the light source 21 is separated into orthogonal polarization components given in PBS 22. Then, one of the polarization components (e.g., y polarization component) is inputted with the optical angular frequency is shifted by omega _O to the frequency shifter 23, the other polarization component (e.g., x-polarization component) VOA25 And the intensity (amplitude) is adjusted.

Next, the polarization component of the optical angular frequency ω _L + ω _O output from the frequency shifter 23 and the polarization component of the optical angular frequency ω _L output from the VOA 25 are input to the PBC 26, and the optical angular frequency difference is omega _O quadrature local oscillator light E _LO of the polarization components and polarization multiplexing that of is generated, with該局unit oscillator light E _LO is sent to the 2 × 4 optical hybrid circuit 12, a portion of the optical divider 27 The data is branched and sent to the monitor circuit 28. The monitor circuit 28 monitors the intensity (amplitude) ratio of each polarization component included in the local oscillation light E _LO and transmits the monitoring result to the intensity ratio control circuit 29. The intensity ratio control circuit 29 determines that the operation values of A _x ′ and A _y ′ approach the condition of the above-described expression (12) according to the monitor result of the monitor circuit 28 and the operation result of the digital operation circuit 17. The attenuation amount of the VOA 25 is changed. As a result, the intensity ratio between the orthogonal polarization components of the local oscillation light _ELO changes, and the divergence of the above-described equation (9) is avoided.

2 × 4 optical hybrid circuit local oscillator light E _LO input to 12 is combined with the reception signal light E _S having optical angular frequency omega _S, each light of the light phase of 0 degrees and 180 degrees differential optical detector Are output to the detector 13, and each light having an optical phase of 90 degrees and 270 degrees is output to the differential photodetector 14. In each of the differential photodetectors 13 and 14, the output light from the 2 × 4 optical hybrid circuit 12 is subjected to differential photoelectric conversion detection. Thus, the signal having an intermediate frequency ωi due to the beat of the polarization component of optical angular frequency omega _L included in the local oscillator light E _LO and (x polarization component) and the x polarization component of the received signal light E _S, the local optical angular frequency ω _{L +} ω _O polarization components contained in the oscillation light E _LO (y polarization component) and the signal having the intermediate frequency .omega.i + omega _O by the beat between the y polarization component of the received signal light E _S is the differential optical Output from the detectors 13 and 14.

FIG. 3 is a diagram schematically showing the electrical spectrum of the intermediate frequency signal. Since the intermediate frequency signal is 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 oscillation light source, each spectrum has a frequency. It overlaps on the axis. As a result, the bandwidth required for each of the differential photodetectors 13 and 14 and the electronic circuit arranged at the subsequent stage is about twice as large as the signal bandwidth in the example of FIG. When not shown, the frequency ωi sets the optical angular frequency omega _L of the local oscillator light so that 0Hz vicinity, it is possible to narrow the bandwidth required to the same degree as the signal bandwidth. In the state where the spectrum of the intermediate frequency signal overlaps as described above, it is not possible to separate each intermediate frequency signal using a bandpass filter as in the case of the conventional coherent optical receiver shown in FIG. Is possible. However, in the present optical receiver, each can be separated by digital signal processing of the intermediate frequency signal in accordance with the above-described operation principle.

Specifically, the intermediate frequency signals I and Q output from the differential photodetectors 13 and 14 are AD-converted at high speed by the AD conversion circuits 15 and 16, and a digital signal sequence corresponding to the intermediate frequency signals I and Q is obtained. Is input to the digital arithmetic circuit 17. In the digital arithmetic circuit 17, digital signal processing is executed according to a series of algorithms corresponding to the above equations (1) to (9), and the value of the signal vector s (t) is calculated. When each value of A _x ′ and A _y ′ obtained in the calculation process approaches the condition of the above expression (12), the information is transferred from the digital calculation circuit 17 to the intensity ratio control circuit in the local oscillation light generation unit 11. transmitted to 29, the intensity ratio control circuit 29 that VOA25 is controlled, the ratio of intensities between the orthogonal polarization components of the local oscillator light E _LO is changed, and diverging (9) signal vector s ( It is avoided that the calculation of t) becomes impossible.

  When the calculated value of the signal vector s (t) in the digital arithmetic circuit 17 is transmitted to the identification circuit 18, the identification circuit 18 calculates the signal vector s (t) according to the threshold corresponding to the modulation method of the received signal light. An identification process for identifying which data value corresponds to the value is executed, and the identification result is output as received data DATA.

As described above, according to the present optical receiver, the setting of the optical angular frequency difference ω _O between the orthogonal polarization components of the local oscillation light E _LO is optimized by combining AD conversion of the received signal and digital signal processing. Compared with the above-described conventional method (III), the bandwidth required for each of the differential photodetectors 13 and 14 can be significantly reduced. For example, high-speed signal light of about 40 Gbit / s or more Can be received coherently without depending on the polarization state. Further, compared with the above-described conventional methods (I) and (II), polarization-independent coherent reception can be realized with a simple configuration, so that a small-sized optical receiver can be provided.

In the above optical receiver, as a specific configuration of the local oscillator light generating section 11, together with the shifting only omega _O the optical angular frequency of one polarization component using a frequency shifter 23, with VOA25 orthogonal Although an example (FIG. 2) for controlling the amplitude ratio between polarization components has been shown, the configuration of the local oscillation light source is not limited to this. For example, as shown in FIG. 4, instead of the VOA 25, a polarization rotator 30 is disposed between the light source 21 and the PBS 22, and the polarization rotator 30 is controlled according to the output signal from the intensity ratio control circuit 29. It is also possible to change the intensity (amplitude) ratio between the orthogonal polarization components separated by the PBS 22.

Further, for example, as shown in FIG. 5, by providing the output light from the light source 21 to the acousto-optic polarization mode converter 31, the orthogonal polarization components of the optical angular frequencies ω _L and ω _L + ω _O are included, and A configuration in which local oscillation light E _{LO in} which the intensity ratio between orthogonal polarization components is controlled may be generated. In this case, the output signal from the oscillator 24 oscillating at the frequency Δf is supplied to the drive circuit 32 that drives the acousto-optic polarization mode converter 31, and the power of the drive signal output from the drive circuit 32 is the intensity ratio. By controlling according to the control signal from the control circuit 29, the local oscillation light E _LO similar to the case of the configuration example shown in FIG. Will be output from. As for the acoustooptic polarization mode converter 31, for example, the document: David A. Smith et al., “Integrated-optic acoustically-tunable filters for WDM networks”, IEEE Journal on Selected Areas in Communications, Vol. No. 6, August 1990., etc. can be used. Since the configuration is further simplified by applying the local oscillation light generator 11 using the acoustooptic polarization mode converter 31 as described above, it is possible to provide a smaller optical receiver.

  Furthermore, in the above optical receiver, an example in which digital signal processing is executed in the digital arithmetic circuit 17 in accordance with a series of algorithms corresponding to the above equations (1) to (9) is shown. The algorithm is not limited to the above example. In this connection, if it is not necessary to consider the divergence condition of equation (9) as described above by executing digital signal processing by applying another algorithm, the orthogonal polarization of the local oscillation light is used. A configuration for changing the intensity ratio between components (for example, in the configuration of FIG. 2, the VOA 25, the optical branching device 27, the monitor circuit 28, and the intensity ratio control circuit 29) can be omitted.

  Next, as an application example of the optical receiver described above, an embodiment of a coherent optical transceiver according to the present invention in which the light source in the local oscillation light source is shared with the signal light source on the transmission side will be described.

  FIG. 6 is a block diagram showing a configuration of the coherent optical transceiver. In this coherent optical transceiver, for example, a configuration using the acousto-optic polarization mode converter 31 shown in FIG. 5 described above is applied as the local oscillation light generator 11, and the light source 21 of the local oscillation light generator 11 is used. Part of the light transmitted to the acousto-optic polarization mode converter 31 is branched by the optical branching device 41. Then, the light branched by the optical branching device 41 is sent to the optical modulator 42 driven according to the transmission data DATA, and the signal light modulated by the optical modulator 42 is transmitted to the optical transmission line 50. The configuration of the other parts than the optical splitter 41 and the optical modulator 42 is basically the same as the configuration shown in FIGS. 1 and 5 described above.

In the configuration shown in FIG. 6, the portion 40 surrounded by a broken line can be a planar optical integrated circuit (PLC) in which part or all of the portion 40 is integrated as an optical waveguide device. is there. FIG. 7 is a perspective view showing an example of the PLC. In this PLC, for example, an optical waveguide is formed in a required pattern on a substrate such as lithium niobate (LiNbO ₃ : LN), and an optical branching device 41 is provided in the middle of the optical waveguide into which output light from the light source 21 is incident. Is formed. Further, in the vicinity of both ends of the optical waveguide connecting between one output port of the optical splitter 41 and the input port on the local oscillation light side of the 2 × 4 optical hybrid circuit 12, a surface acoustic wave (Surface Acoustic Wave) is provided. A comb-shaped electrode (Interdigital Transducer: IDT) 31A and a SAW absorber 31B for generating SAW) are formed, and an acousto-optic polarization mode converter 31 is realized. Further, a traveling wave electrode to which a modulation signal corresponding to transmission data is applied is formed in the optical waveguide connected to the other output port of the optical branching device 41, and the optical modulator 42 is realized.

  In the coherent optical transceiver configured as described above, the received signal light that propagates through the optical transmission path 50 and is input to the signal light input port of the PLC is the same as in the optical receiver shown in FIG. Coherently received without depending on the polarization state, and part of the output light of the light source 21 that generates local oscillation light is branched by the optical branching device 41 and modulated by the optical modulator 42 according to the transmission data. Thus, signal light to be transmitted to the optical transmission line 50 is generated.

  According to the coherent optical transmitter / receiver as described above, the configuration can be simplified and miniaturized by sharing the light sources of the local oscillation light and the transmission signal light. In addition, a very small coherent optical transceiver is realized by applying a PLC that integrates the 2 × 4 optical hybrid circuit 12, the acousto-optic polarization mode converter 31, the optical splitter 41, the optical modulator 42, and the like. It becomes possible to do.

DESCRIPTION OF SYMBOLS 11 ... Local oscillation light generation part 12 ... 2 * 4 optical hybrid circuit 13, 14 ... Differential photodetector 15, 16 ... AD converter circuit 17 ... Digital arithmetic circuit 18 ... Identification circuit 21 ... Light source 22 ... Polarization beam splitter ( PBS)
23 ... Frequency shifter (FS)
24 ... Oscillator 25 ... Variable optical attenuator (VOA)
26 ... Polarized beam combiner (PBC)
27, 41 ... Optical splitter 28 ... Monitor circuit 29 ... Intensity ratio control circuit 30 ... Polarization rotator 31 ... Acousto-optic polarization mode converter 32 ... Drive circuit 40 ... PLC
42 ... optical modulator E _{LO ...} local oscillator light E S _... reception signal light

Claims (3)

A coherent light source,
An optical branching unit for branching light output from the coherent light source into first coherent light and second coherent light;
An optical modulation unit that modulates the first coherent light branched by the optical branching unit according to transmission data and transmits the modulated light to the outside;
A local oscillating light generator that outputs, as local oscillating light, second coherent light branched by the optical branching unit or light generated based on modulation of the second coherent light;
A multiplexing unit that combines the received signal light and the local oscillation light output from the local oscillation light generation unit and outputs the combined signal;
A photoelectric conversion unit that converts light output from the multiplexing unit into an electrical signal;
Based on the output of the photoelectric conversion unit, a data identification unit that performs a process of identifying received data;
A coherent optical transceiver characterized by comprising: The coherent optical transceiver according to claim 1,
The local oscillation light generation unit is a local oscillation light generated by modulating the second coherent light and having orthogonal polarization components having different optical frequencies, and having an optical frequency between the orthogonal polarization components. The difference is smaller than twice the bandwidth of the received signal light and generates a local oscillation light that is larger than the light source spectral line width of the received signal light and the light source spectral line width of the local oscillation light;
An AD converter that converts an electrical signal output from the photoelectric converter into a digital signal;
A digital arithmetic unit that performs arithmetic processing for estimating data information contained in the received signal light, using a digital signal output from the AD converter, according to a calculation algorithm corresponding to the modulation method of the received signal light And comprising
The coherent optical transceiver is characterized in that the data identification unit executes a process of identifying received data based on a calculation result of the digital calculation unit. The coherent optical transceiver according to claim 1,
A coherent optical transceiver characterized in that at least the multiplexing unit and the optical branching unit are integrated as an optical waveguide device.

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