JP2012019406A - Optical transmitter, optical receiver, optical transceiver, and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical transmitter capable of eliminating the need for polarization tracking for polarization separation on the receiving side, reducing interference between two polarized waves during polarization separation, and generating a polarization multiple signal with high yield strength against intersymbol interference without necessity of a plurality of light sources on the transmission side or giving offset to a carrier frequency of the two orthogonal polarized waves.SOLUTION: An optical transmitter includes: a light source 11; a light branch section 12 that separates the light emitted from the light source 11 into first light and second light of polarized waves orthogonal to each other; a DPSK transmission part 13 that generates a DPSK signal subjected to DPSK modulation for the first light; a zero chirp π/2 transmission part 14 that generates a zero chirp π/2 shift DPSK signal subjected to zero chirp π/2 shift DPSK modulation for the second light; and a polarization multiplex part 15 that polarization multiplexes a DPSK signal and a zero chirp π/2 shift DPSK signal.

Description

  The present invention relates to an optical transmitter, an optical receiver, an optical transceiver, and an optical transmission system.

  In order to perform large-capacity communication by optical fiber communication, the bit rate per wavelength has been increased. Since optical fibers have dispersion characteristics and the transmission distance is limited in proportion to the square of the symbol rate, for example, in high-speed transmission exceeding 40 Gb / s, it is important to reduce the symbol rate. The reduction of the symbol rate contributes to the narrowing of the occupied band and also contributes to the increase of the total transmission capacity in the wavelength division multiplex transmission. For this reason, a transmission scheme that reduces the symbol rate by using a polarization multiplexing asynchronous detection scheme as disclosed in Non-Patent Documents 1 and 2 below has been studied.

  In the method described in Non-Patent Document 1 below, the modulated signals respectively assigned to two orthogonally polarized waves (X polarized wave and Y polarized wave) are multiplexed while being shifted by a half symbol on the time axis. At this time, each modulated signal is a signal subjected to differential phase-shift keying (DPSK), and the pulse is converted to RZ (Return-to-Zero). At the identification point of one polarization, the light level of the other polarization is zero, and no interference occurs under ideal conditions. For example, in the case of 40 Gb / s transmission, by assigning such allocation to the X polarization and the Y polarization respectively, transmission of 20 Gb / s is expected for each polarization, and improvement of dispersion tolerance and narrowing of the occupied band are expected. it can. The receiving side may use a DPSK optical receiver for 40 Gb / s.

  The above-described method is called DP-DPSK (Dual-polarization DPSK). Actually, the undesired polarization component does not completely become zero at the identification point of the desired polarization due to electric band limitation and jitter, and interference occurs at the time of reception by the DPSK optical receiver. Therefore, a method of optically performing polarization multiplexing / demultiplexing is employed on the optical receiver side. In this method, the polarization controller is arranged in front of the polarization separator, and the polarization separator separates the received light into two independent signal components (X polarization component and Y polarization component). This is a system that always controls the wave controller, that is, a system that optically performs polarization tracking.

  In the method described in Non-Patent Document 2 below, on the transmitting side, in addition to the method of Non-Patent Document 1 below, a bit of each polarization is provided between the carrier frequency of the X polarization component and the carrier frequency of the Y polarization component. A frequency offset corresponding to 1/4 of the rate (repetition frequency) is given. For example, in the case of 40 Gb / s transmission, the frequency offset is 5 GHz. On the receiving side, the light is split into two, and the branched light is input to a 20 Gb / s DPSK optical receiver. One of the 20 Gb / s DPSK optical receivers is used for receiving the X polarization component, and matches the center frequency of the MZ (Mach-Zehnder) type delay interferometer with the carrier frequency of the X polarization component. Control is performed. The other 20 Gb / s DPSK optical receiver is used to receive the Y polarization component, and is controlled so that the center frequency of the MZ type delay interferometer matches the carrier frequency of the Y polarization component.

  When such a frequency offset is given, a quarter of the repetition frequency corresponds to a phase π / 2 on the complex plane. Therefore, in each DPSK optical receiver for 20 Gb / s, the undesired polarization component is a complex electric field. Even in a plane, it becomes an orthogonal component, and only a desired polarization component appears. However, as in the case of Non-Patent Document 1 described above, an undesired polarization component may remain, and this causes beat noise between signal light and ASE (Amplified Spontaneous Emission) noise. The beat noise caused by the remaining undesired polarization component is difficult to remove, and the light of the undesired polarization component is not left at the identification point of the desired polarization component by the RZ method or the electrical band design. is important. In addition, the system described in Non-Patent Document 2 below also shows that stable reception characteristics can be obtained without performing optical polarization tracking performed in the system described in Non-Patent Document 1 below.

  In Patent Document 3 below, one of two DPSK signals to be polarization multiplexed is given a periodic phase modulation with a phase shift of π / 2 at a frequency half the symbol rate. Therefore, it is possible to perform polarization multiplexing without giving an offset to the carrier frequency of two orthogonally polarized waves and to separate polarization multiplexed signals without performing optical polarization tracking on the receiving side. It is shown.

  Further, in Patent Document 1 below, in order to make the transmission peak of the MZ type delay interferometer on the receiving side coincide with the optical carrier frequency while having narrow spectral characteristics, the signal shifted by π / 4 is branched into two branches. An optical phase modulation circuit is disclosed in which each of the subsequent signals is phase-modulated and combined by shifting the phase of one optical signal by π / 2.

JP 2007-318483 A

J. -X. Cai, O. V. Sinkin, C.D. R. Davidson, D.D. G. Foursa, A. J. Lucero, M.M. Nissov, A. N. Pilipetskii, W. W. Patterson, and Neal S. Bergano, “40 Gb / s Transmission Using Polarization Division Multiplexing (PDM) RZ-DBPSK with Automatic Polarization Tracking,” OFC / NFOEC2008, PDP4, 2008. H. Zhang, J .; -X. Cai, C .; R. Davidson, B. Anderson, O. Sinkin, M.M. Nissov, and A. N. Pilipetskii, “Offset PDM RZ-DPSK for 40 Gb / s Long-Haul Transmission,” OFC / NFOEC2009, OThR2, 2009. J. -X. Cai, C .; R. Davidson, Y. Cai, A. N. Pilipetskii, and M. Nissov, “40 Gb / s Transmission Using a Hybrid PDM Technique” OFC / NFOEC2010, OTUD5, 2010.

  However, according to the technique described in Non-Patent Document 1, the undesired polarization component does not become completely zero but remains at the identification point of the desired polarization due to the electric band limitation and the influence of jitter. Therefore, there has been a problem that interference occurs during reception.

  In the technique described in Non-Patent Document 2, an offset is given between the carrier frequency of the X polarization component and the carrier frequency of the Y polarization component. Therefore, there is a problem that a light source is required for each of the X polarization and the Y polarization, and a total of two types of light sources are required. Further, since it is necessary to give a frequency offset between the two light sources, there is a problem that the occupied bandwidth is widened. Further, when the frequency offset fluctuates from ¼, the undesired polarization component remains large, so that there is a problem that the frequency offset needs to be controlled to be constant.

  In the technique described in Non-Patent Document 3, in order to generate a signal (π / 2 shift DPSK signal) in which a DPSK signal is given a periodic phase shift of 0 to π / 2 at a frequency half the symbol rate. In addition, an MZ type modulator and a linear waveguide type phase modulator are connected in cascade. Therefore, chirping peculiar to the linear waveguide type phase modulator, that is, the transition between signal points on the complex plane is not linear but curved, and the frequency spectrum is widened. There is a problem that it is vulnerable to intersymbol interference due to band limitation or the like.

  In order to generate a signal (π / 2 shift DPSK signal) having a cyclic phase shift of 0 to π / 2, an MZ type modulator and a linear waveguide type phase modulator are connected in cascade. Therefore, chirping peculiar to the linear waveguide type phase modulator, that is, transition between signal points on the complex plane is not linear but curved, and the frequency spectrum is widened. There is a problem that it is vulnerable to intersymbol interference due to band limitation or the like.

  Further, in the technique described in Patent Document 1, polarization multiplexing / demultiplexing is not performed optically. For this reason, there has been a problem that reduction of interference on the receiving side when optical polarization demultiplexing is performed is not described.

  The present invention has been made in view of the above, and does not require polarization tracking for optical polarization separation on the receiving side, and reduces interference remaining in a signal after polarization multiplexed signal separation. A light that does not require a plurality of light sources on the transmission side, and can generate a polarization multiplexed signal that is excellent in resistance to intersymbol interference due to band limitation or the like without giving an offset to a carrier frequency of two orthogonally polarized waves It is an object to obtain a transmitter, an optical receiver, an optical transceiver, and an optical transmission system.

  In order to solve the above-described problems and achieve the object, the present invention separates a light source and light emitted from the light source into first light and second light that are polarized waves orthogonal to each other. A polarization separation unit; a DPSK modulation unit that generates a DPSK signal obtained by subjecting the first light to DPSK modulation; and a zero chirp π / 2 obtained by subjecting the second light to π / 2 shift DPSK modulation. A zero chirp π / 2 shift DPSK modulation unit that generates a shifted DPSK signal, a polarization multiplexing unit that polarization-multiplexes the DPSK signal, and the zero chirp π / 2 shift DPSK signal.

  According to the present invention, it is possible to reduce interference on the reception side, do not need a plurality of light sources on the transmission side, and tolerate intersymbol interference such as band limitation without giving an offset to two orthogonally polarized carrier frequencies. It is possible to generate a polarization multiplexed signal that is superior to the above.

FIG. 1 is a diagram showing a configuration example of an optical transmission system according to the present invention. FIG. 2 is a diagram illustrating a detailed configuration example of the optical transmission system. FIG. 3 is a diagram showing the signal point arrangement of the DPSK signal. FIG. 4 is a diagram illustrating a signal point arrangement of a zero chirp π / 2 shift DPSK signal (two-phase DPSK signal). FIG. 5 is a schematic diagram of transition between signal points when the conventional technique is used. FIG. 6 is a schematic diagram of transition between signal points when the polarization multiplexing system of the present embodiment is used. FIG. 7 is a diagram illustrating ΔI (t) and ΔQ (t) of DPSK signal components. FIG. 8 is a diagram illustrating ΔI (t) and ΔQ (t) of the zero chirp π / 2 shift DPSK signal component. FIG. 9 is a diagram illustrating an example of a signal component obtained by superimposing a DPSK signal component and a zero chirp π / 2 shift DPSK signal component.

  Embodiments of an optical transmitter, an optical receiver, an optical transceiver, and an optical transmission system according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment.
FIG. 1 is a diagram showing a configuration example of an optical transmission system according to the present invention. The optical transmission system according to the present embodiment includes an optical transmitter 1, a transmission unit (transmission means) 2, and an optical receiver 3. The optical transmitter 1 includes a light source (LD) 11, an optical branching unit 12, a DPSK transmission unit (DPSK-TX (Transmitter)): a DPSK modulation unit 13, a zero chirp π / 2 shift DPSK transmission unit (Zero chirp π / 2 shifted). DPSK-TX: zero chirped π / 2 shift DPSK modulation unit) 14 and polarization multiplexing unit 15. The optical receiver 3 includes an optical branching unit 31, a DPSK receiving unit (DPSK-RX (Receiver)) 32, and a zero chirped π / 2 shifted DPSK receiving unit (π / 2 shifted DPSK-RX) 33.

  In the present embodiment, the optical transmitter 1 performs DPSK modulation on one of the two orthogonally polarized signals to obtain a DPSK signal, and performs zero chirp π / 2 DPSK modulation on the other to obtain a zero chirp π / 2 DPSK signal. Then, polarization multiplexing of the DPSK signal and the zero chirp π / 2 shift DPSK signal is performed and transmitted. As a result, it is possible to generate an orthogonal two-polarized signal with reduced influence of undesired polarization components by using one light source 11. The optical receiver 3 branches the received optical signal into two, then performs DPSK demodulation on one optical signal and performs zero chirp π / 2 DPSK demodulation on the other.

  FIG. 2 is a diagram illustrating a detailed configuration example of the optical transmission system according to the present embodiment. In the configuration example shown in FIG. 2, the light source (LD) 101 of the optical transmitter 1 corresponds to the light source 11 of FIG. 1, and the polarization separator (PBS) 103 corresponds to the optical branching unit 12 of FIG. A polarization multiplexer (PBC) 108 corresponds to the polarization multiplexer 15 in FIG. Also, MZ type modulator (MZM) 104-1, delay unit (Delay) 105, attenuator (ATT) 106-1, parallel-serial converter (MUX) 112-1, DPSK encoder (ENC) 113 and data The driver (DRV) 111-1 and the oscillator (CLK) 109 constitute the DPSK transmission unit 13 in FIG. Also, a 2-parallel MZ modulator (DP-MZM) 104-2, an attenuator 106-2, a parallel-serial converter 112-2, a zero chirped π / 2 DPSK encoder 114, a data driver (two systems) 111-2, and The oscillator (CLK) 109 constitutes the zero chirp π / 2 shift DPSK transmission unit 14 of FIG. In FIG. 1, the RZ pulse carver (RZ) 102, the clock driver (DRV) 110-1, and the delay / loss control unit (Controller) 115 are not shown.

  Further, the DPSK transmission unit 13 and the zero chirp π / 2 shift DPSK transmission unit 14 include an optical unit and an electrical unit, respectively. The optical unit of the DPSK transmission unit 13 includes an MZM 104-1, a delay unit (Delay) 105, and an attenuator (ATT) 106-1, and the electrical unit of the DPSK transmission unit 13 includes a parallel-serial converter (MUX) 112-. 1, a DPSK encoder (ENC) 113, a data driver (DRV) 111-1, and a transmitter (CLK) 109. The optical unit of the zero chirp π / 2 shift DPSK transmission unit 14 includes a DP-MZM 104-2 and an attenuator 106-2, and the electric unit of the zero chirp π / 2 shift DPSK transmission unit 14 includes a parallel-serial converter 112. -2, zero chirp π / 2 shift DPSK encoder 114, data driver 111-2, and oscillator (CLK) 109.

  The transmission unit 2 includes an optical multiplexing unit 21, optical amplifiers 22-1 to 22- (n + 1), transmission path optical fibers 23-1 to 23-n, and an optical separation unit 24. The configuration of the transmission unit 2 is not limited to this and may be any configuration.

  2, the optical branching unit (CPL) 301 of the optical receiver 3 corresponds to the optical branching unit 31 of FIG. 1, and also includes a delay interferometer (MZDI) 302-1 and balanced photon detection. The unit (BPD) 303-1 and the serial-parallel converter (DEMUX) 304-1 constitute the DPSK receiving unit 32 of FIG. Further, the delay interferometer 302-2, the balanced photon detector 303-2, and the serial-parallel converter 304-2 constitute the π / 2 DPSK receiver 33 in FIG. The delay length / optical phase control unit (Controller) 305 is not shown in FIG.

  2 illustrates a case where long-distance relay transmission and point-to-point transmission are performed. For example, the optical transmitter according to the present embodiment is applied to a reconfigurable optical add-drop multiplexer (ROADM) system, for example. 1 and the optical receiver 3 can also be applied. In this case, the transmission unit 2 adds the polarization multiplexed signal generated by itself to the wavelength multiplexed signal transmitted through the ring-shaped optical fiber to the signal after polarization multiplexing output from the optical transmitter 1. In addition, an optical signal having a desired wavelength is extracted from the wavelength multiplexed signal, and the extracted optical signal is input to the optical receiver 3. There are no restrictions on the transmission path system, and any transmission path other than these may be used.

  Next, an outline of the polarization multiplexing system of the present embodiment will be described. FIG. 3 is a diagram illustrating a signal point arrangement of a DPSK signal (two-phase DPSK signal). FIG. 4 is a diagram illustrating a signal point arrangement of a zero chirp π / 2 shift DPSK signal (two-phase DPSK signal). In the DPSK signal, the signal point arrangement at the time of transmission is two points as shown in FIG. 3, but in the zero chirped π / 2 shift DPSK signal, as shown in FIG. 4, it is on the In-phase channel (I-ch). And signal points on the quadrature-phase channel (Q-ch) are alternately taken every other symbol. This signal point arrangement is the same in Non-Patent Document 2 and Non-Patent Document 3 described above, but since zero chirp phase modulation is used in the present invention, more accurate arrangement is possible and the transition between signal points is linear. And has a narrow frequency spectrum. FIG. 5 is a schematic diagram of transition between signal points when the method according to Non-Patent Document 3 is used, and FIG. 6 is a schematic diagram of transition between signal points when the method according to the present invention is used. In the method of Non-Patent Document 3 in which RZ is combined with linear waveguide type phase modulation, the transition between signal points is non-linear, resulting in a wider frequency spectrum. In the present invention, of the four QPSK signal points, Since only two are selectively taken, the transition between signal points is linear and the frequency spectrum does not spread.

  On the receiving side, signals for each polarization are extracted using the delay interferometers 302-1 and 302-2, and the optical phases set at this time may be set to 0 and π / 2, respectively. Thereby, when taking out a signal, since the other polarization component has an orthogonal relationship, the interference resulting from an undesired polarization component can be reduced.

  Next, the operation of the present embodiment will be described. The optical transmitter 1 according to the present embodiment generates an optical signal based on a data sequence input from the outside (not shown), and outputs the generated optical signal to a transmission unit. In the transmission unit 2, the optical signal output from the optical transmitter 1 and the optical signal output from another optical transmitter (not shown) are multiplexed and transmitted, and the light output from the optical transmitter 1 is transmitted. The signal is separated and extracted and output to the optical receiver 3. Each optical signal input from another transmission unit (not shown) is output to another optical receiver (not shown) serving as a destination. The optical receiver 3 restores the transmitted data series based on the optical signal input from the transmission unit 2 and outputs the restored data series to the outside (not shown).

  Hereinafter, the operation of each unit of the optical transmitter 1 will be described. The parallel / serial converter 112-1 performs parallel / serial conversion on a low-speed / parallel data sequence input from the outside (not shown), and outputs the serialized data sequence to the DPSK encoder 113. The parallel / serial converter 112-2 performs parallel / serial conversion on a low-speed / parallel data sequence input from the outside (not shown), and converts the serialized data sequence to the zero chirp π / 2 shift DPSK encoder 114. Output.

  The oscillator 109 generates a clock signal having a predetermined frequency, and outputs the generated clock signal to the data drivers 111-1, 111-2, the DPSK encoder 113, and the zero chirp π / 2 shift DPSK encoder 114, respectively. The clock driver 110-1 amplifies the clock signal input from the oscillator 109 and outputs the amplified clock signal to the RZ pulse carver 102.

  The DPSK encoder 113 DPSK-encodes the data series input from the parallel-serial converter 112-1, and synchronizes the electrical signal after DPSK encoding with the clock signal input from the oscillator 109. Output to 1. The zero chirped π / 2 shift DPSK encoder 114 receives from the oscillator 109 an electrical signal obtained by performing processing based on a predetermined encoding rule for the data series input from the parallel-serial converter 112-2. The data is output to the data driver 111-2 in synchronization with the clock signal.

  The data driver 111-1 amplifies the electrical signal input from the DPSK encoder 113 and outputs the amplified electrical signal to the MZM 104-1. The data driver 111-2 amplifies the two electric signals input from the zero chirp π / 2 shift DPSK encoder 114, and outputs the amplified two electric signals to the DP-MZM 104-2.

  The delay / loss control unit 115 controls the delay amount in the delay unit 105 and the loss amounts in the attenuators 106-1 and 106-2. The delay amount in the delay unit 105 is controlled to be, for example, half a symbol period. Further, the amount of loss in the attenuators 106-1 and 106-2 is controlled so that, for example, the loss between the DPSK transmission unit 13 and the zero chirp π / 2 shift DPSK transmission unit 14 is uniform. That is, the delay / loss control unit 115 and the attenuators 106-1 and 106-2 adjust the difference in the level of the optical signal generated by the DPSK transmission unit 13 and the zero chirp π / 2 shift DPSK transmission unit 14, respectively. A light level adjusting unit that performs the operation.

  The light source 101 generates unmodulated light and outputs it to the RZ pulse carver 102. The RZ pulse carver 102 performs RZ modulation (shaping into an RZ pulse) on the non-modulated light input from the light source 101 based on the clock signal input from the clock driver 110-1, and the modulated optical signal is the polarization separator 103. Output to. The polarization separator 103 separates the optical signal input from the RZ pulse carver 102 into two orthogonally polarized waves, outputs one of the separated optical signals to the MZM 104-1, and outputs the other to the DP-MZM 104-2. .

  The MZM 104-1 modulates the optical signal input from the polarization separator 103 based on the electrical signal input from the data driver 111-1, and outputs the modulated optical signal to the delay unit 105. The DP-MZM 104-2 modulates the optical signal input from the polarization separator 103 based on the electrical signal input from the data driver 111-2, and outputs the modulated optical signal to the attenuator 106-2. .

  The delay unit 105 gives a delay amount controlled by the delay / loss adjustment unit 115 to the optical signal input from the MZM 104-1, and outputs the optical signal to which the delay amount is given to the attenuator 106-1.

  The attenuator 106-1 gives a loss controlled by the delay / loss adjustment unit 115 to the optical signal input from the delay unit 105, and outputs the optical signal given the loss to the polarization multiplexer. The attenuator 106-2 gives a loss controlled by the delay / loss adjustment unit 115 to the optical signal inputted from the DP-MZM 104-2, and outputs the optical signal given the loss to the polarization multiplexer 108. . The polarization multiplexer 108 multiplexes the optical signal input from the attenuator 106-1 and the optical signal input from the attenuator 106-2, and outputs the multiplexed optical signal to the transmission unit 2.

  As the light source 101, for example, a distributed feedback (DFB) array type wavelength tunable laser is used and set to oscillate at 1550 nm. The type of the light source 101 and the wavelength of light emitted from the light source 101 are not limited to this.

  As the modulation performed by the RZ pulse carver 102, for example, a carrier-suppressed RZ (CSRZ: Carrier-Suppressed RZ), an RZ with a duty ratio of 50%, an RZ with a duty ratio of 33%, or the like can be used. There are no restrictions on the method. Note that the clock signal input to the RZ pulse carver 102 requires clock division by a prescaler when the RZ pulse carver 102 performs CSRZ modulation.

  Further, it is also possible to employ a configuration in which the polarization separator 103 is an optical power branch instead of polarization separation, and a polarization controller is disposed immediately before the polarization multiplexer 108 to perform orthogonal polarization multiplexing.

  The RZ pulse carver 102 and the delay unit 105 are used to improve reception characteristics by reducing the signal light-ASE beat noise. However, one or both of the RZ pulse carver 102 and the delay unit 105 may be removed.

  For example, a LiNbO 3 modulator may be used as the type of modulator used as the MZM 104-1 and DP-MZM 104-2, but is not limited thereto.

  Further, the internal configurations of the DPSK transmission unit 13 and the zero chirp π / 2 shift DPSK transmission unit 14 are not limited to the configuration example shown in FIG. 2, and various configurations are conceivable. In the DPSK transmission unit 13, the order of the MZM 104-1, the delay unit 105, and the attenuator 106-1 is not limited to the example illustrated in FIG. 2, and these three orders may be any order. In the zero chirp π / 2 shift DPSK transmission unit 14, the order of the DP-MZM 104-2 and the attenuator 106-2 is not limited to the example shown in FIG. 2, and these three orders may be any order.

  In addition, the PBS 103, MZM 104-1, attenuator 106-1, DP-MZM 104-2, and attenuator 106-2 are substituted at once by using a polarization multiplexing modulator in which two DP-MZMs are integrated. It is also possible. By making the polarization multiplexing modulator have a bit interleave configuration, it is possible to integrate up to the delay unit 105 and substitute it once.

  Further, instead of the DPSK transmission unit 13 including the delay unit 105, the zero chirp π / 2 shift DPSK transmission unit 14 may include the delay unit 105. At that time, the order of the MZM 104-1, the attenuator 106-2, and the delay unit 105 of the zero chirp π / 2 shift DPSK transmission unit 14 is not limited.

  The attenuators 106-1 and 106-2 are used to compensate for the level difference of the optical signal between the DPSK transmission unit 13 and the zero chirp π / 2 shift DPSK transmission unit 14, and the DPSK transmission unit 13 and the zero chirp. Depending on the level difference of the optical signal with the π / 2 shift DPSK transmission unit 14, one or both of the attenuators 106-1 and 106-2 may be removed.

  Further, the DPSK encoder 113 may have the parallel / serial conversion function of the parallel / serial converter 112-1, and the parallel / serial converter 112-1 may not be provided. That is, it is not essential to perform DPSK encoding after parallel-serial conversion, and parallel-serial conversion may be performed after DPSK encoding, and a flexible configuration is possible depending on the configuration method of the electric circuit. Any configuration may be used as long as the DPSK-encoded serial electric signal is output from the DPSK encoder 113 to the data driver 111-1. Similarly, the zero-chirp π / 2 shift DPSK encoder 114 may have the parallel / serial conversion function of the parallel-serial converter 112-2.

  Note that the inversion and non-inversion of the sign (logic) performed by the phase modulator 107 can be realized, for example, by taking an exclusive OR with a clock signal. When the clock signal is at the LOW level, the DPSK encoded electrical signal is output as it is, and when the clock signal is at the High level, the DPSK encoded electrical signal is inverted and output.

  Further, the zero chirped π / 2 shift DPSK encoder 114 performs only normal DPSK encoding without repeating the above-described logic inversion and non-inversion, and on the receiving side, Processing that repeats logic inversion and non-inversion may be performed according to the rules illustrated in FIGS. 5 and 6.

  In practice, it is necessary to use a distributor (not shown) for distributing the clock signal output from the oscillator 109 and a clock phase shifter (not shown) for timing adjustment. It does not impose restrictions on use.

  Next, the operation of the transmission unit 2 will be described. The optical multiplexing unit 21 multiplexes optical signals input from the optical transmitter 1 and other optical transmitters (not shown), and outputs the multiplexed optical signals to the optical amplifier 22-1. The optical multiplexing unit 21 can use, for example, a wavelength division multiplexing method, a time division multiplexing method, or the like as a multiplexing method for multiplexing. In the wavelength division multiplexing method, light of different wavelengths is output from the optical transmitter 1 and a number of optical transmitters (not shown), and multiplexing is performed by combining these as one light. Although the wavelength multiplexing method has been described here as an example, the multiplexing method is not limited, and any method may be used. Further, the transmission unit 2 may be such that the optical signal transmitted from the optical transmitter 1 is directly transmitted to the optical transmitter 3 without being multiplexed with the optical signal from another optical transmitter.

  The optical amplifier 22-1 amplifies the input light and outputs it to the transmission line optical fiber 23-1. The transmission line optical fiber 23-1 transmits, for example, 50 km of the input optical signal, and outputs the transmitted optical signal to the optical amplifier 22-2. Thereafter, amplification by 222-2 to 22-n and amplification and transmission by transmission line optical fibers 23-2 to 23-n are repeated, and the optical signal output from transmission line optical fiber 23-n is optical amplifier 22- ( After being amplified in (n + 1), it is input to the light separation unit 24.

  The optical separation unit 24 separates the optical signal input from the optical amplifier 22- (n + 1). The separation performed by the light separation unit 24 is the reverse process of the optical multiplexing unit 21. For example, when the optical multiplexing unit 21 multiplexes by the wavelength division multiplexing method, the optical demultiplexing unit 24 separates the light in which a large number of wavelengths are multiplexed into optical signals for each wavelength. The optical separation unit 24 outputs the separated optical signal to the corresponding receiving device (the optical receiver 3 or another optical receiver not shown).

  For example, an AWG (Arrayed Wave Guide) may be used for the optical multiplexing unit 21 and the optical demultiplexing unit 24, but is not limited thereto. Further, for example, an erbium-doped fiber amplifier (EDFA) may be used as the optical amplifiers 22-1 to 22- (n + 1), but is not limited thereto. As the transmission line optical fibers 23-1 to 23-n, for example, ITU-TG A standard single mode fiber recommended by 652 may be used, but is not limited thereto.

  Next, the operation of the optical receiver 3 will be described. The optical branching unit 301 bifurcates the optical signal input from the transmission unit 2, outputs one of the branched lights to the delay interferometer 302-1 and outputs the other to the delay interferometer (MZDI) 302-2. .

  The delay interferometer 302-1 delays and interferes the optical signal input from the optical branching unit 301 (branches the input optical signal into delayed light and non-delayed light, delays both of them), and after the interference delay Are output to a balanced photon detector (BPD) 303-1. The delay interferometer 302-2 delays the optical signal input from the optical branching unit 301, and outputs the optical signal after the interference delay to the balanced photon detector 303-2.

  The balanced photon detector 303-1 detects the optical signal input from the delay interferometer 302-1 and converts it into an electrical signal, and outputs this electrical signal to a serial-parallel converter (DEMUX) 304-1. The balanced photon detector 303-2 detects the optical signal input from the delay interferometer 302-2, converts it into an electrical signal, and outputs this electrical signal to the series-parallel converter 304-2.

  The serial / parallel converter 304-1 performs serial / parallel conversion on the (serial) electric signal input from the delay interferometer 302-1 to develop it into a low-speed parallel signal, and outputs it to the outside (not shown). In the serial-parallel converter 304-2, the (serial) electric signal input from the delay interferometer 302-2 is converted from serial to parallel to develop into a low-speed parallel signal and output to the outside (not shown). The delay length / optical phase control unit 305 (Controller) controls the delay length and optical phase (phase difference between delayed light and non-delayed light) of the delay interferometers 302-1 and 302-2.

  The delay interferometers 302-1 and 302-2 may be Mach-Zehnder type, but are not limited to this.

  For example, the delay length / optical phase control unit 305 controls the delay length as one symbol for both the delay interferometer 302-1 and the delay interferometer 302-2, and sets the optical phase to 0 in the delay interferometer 302-1. The delay interferometer 302-2 controls to be π / 2. By setting in this way, the delay interferometer 302-1 can extract the DPSK signal, and the delay interferometer 302-2 can extract the zero chirp π / 2 shift DPSK signal.

At this time, the input light to the optical receiver 3 is r (t), the electric signal output from the balanced photon detector 303-1 is ΔI (t), and is output from the balanced photon detector 303-2. ΔI (t) ｔRe {r (t) r ^* (t−T)} and ΔQ (t) 電 気 Im {r (t) r ^* (t−T) }. However, T represents one symbol period, * represents a complex conjugate, Re {} represents a real part in {}, and Im {} represents an imaginary part in {}.

  An example of the above ΔI (t) and ΔQ (t) is shown in FIGS. FIG. 7 is a diagram illustrating DPSK signal components, and FIG. 8 is a diagram illustrating zero chirped π / 2 shift DPSK signal components. ΔI (t) and ΔQ (t) are formed by superposing the points shown in FIG. 7 and the points shown in FIG. FIG. 9 is a diagram illustrating an example of a signal component obtained by superimposing a DPSK signal component and a zero chirp π / 2 shift DPSK signal component. Since the polarization used for the DPSK signal and the polarization used for the zero chirped π / 2-shift DPSK signal are orthogonal to each other, the signal components basically do not interfere even if delayed detection is performed. Therefore, it is possible to perform sign determination by performing binary determination on ΔI (t) and ΔQ (t). Further, since the undesired polarization component at the identification point of the desired polarization is basically zero, the influence of the residual error of the undesired polarization component can be reduced.

  It is also possible to use a reception module in which the optical branching unit 301 and the delay interferometers 302-1 and 302-2 are integrated. Further, instead of the balanced photon detectors 303-1 and 303-2, single photon detectors that are not balanced may be used.

  The serial-parallel converters 304-1 and 304-2 also perform clock recovery. At this time, the clock timing may be shifted between the serial-parallel converter 304-1 and the serial-parallel converter 304-2. This timing shift depends on the configuration of the optical transmitter 1. That is, it depends on the presence / absence of the delay device 105 and the setting in the optical transmitter 1. In this case, the serial / parallel converters 304-1 and 304-2 need to output a parallel signal to the outside after performing the skew adjustment.

  The optical transmitter / receiver having both functions as the optical transmitter 1 and the optical receiver 3 described in the present embodiment is configured, and an optical signal is transmitted / received between the optical transmitter / receiver via the transmission unit 2. You may make it perform.

  As described above, in the present embodiment, the optical transmitter 1 includes the DPSK transmission unit 13, the zero chirped π / 2 shift DPSK transmission unit 14, and the polarization multiplexing unit 15, and the light emitted from the light source 11. The DPSK transmission unit 13 and the zero chirped π / 2 shift DPSK transmission unit 14 modulate two orthogonally polarized waves, respectively, and multiplex and transmit the modulated optical signals. This eliminates the need for polarization tracking for polarization separation on the reception side, reduces interference between the two polarizations during polarization separation, and does not require a plurality of light sources on the transmission side, and also two orthogonal polarizations It is possible to obtain an optical transmitter capable of generating a polarization multiplexed signal having a high tolerance to intersymbol interference without giving an offset to the carrier frequency.

  As described above, the optical transmitter, the optical receiver, the optical transceiver, and the optical transmission system according to the present invention are useful for an optical transmission system that employs a polarization multiplexing asynchronous detection method, and in particular, an undesired polarization component. It is suitable for an optical transmission system that reduces the occurrence of intersymbol interference due to residual of the interference and interference between polarized waves on the receiving side.

DESCRIPTION OF SYMBOLS 1 Optical transmitter 2 Transmission part 3 Optical receiver 11,101 Light source (LD)
12 Optical branching unit 13 DPSK transmission unit (DPSK-TX)
14 Zero Chirp π / 2 Shift DPSK Transmitter (Zero Chirp π / 2 sifted DPSK-TX)
DESCRIPTION OF SYMBOLS 15 Polarization multiplexing part 21 Optical multiplexing part 22-1-22- (n + 1) Optical amplifier 23-1-23-n Transmission path optical fiber 24 Optical separation part 31,301 Optical branch part 32 DPSK receiving part (DPSK-RX)
33 π / 2 shifted DPSK receiver (π / 2 shifted DPSK-RX)
102 RZ pulse carver (RZ)
103 Polarization separator (PBS)
104-1 Mach-Zehnder modulator (MZM)
104-2 Two-parallel Mach-Zehnder modulator (DP-MZM)
105 Delayer (Delay)
106-1, 106-2 Attenuator (ATT)
108 Polarization Multiplexer (PBC)
109 Transmitter (CLK)
110-1 Clock driver (DRV)
111-1, 111-2 Data Driver (DRV)
112-1, 112-2 Parallel to serial converter (MUX)
113 DPSK encoder (ENC)
114 Zero Chirp π / 2 Shift DPSK Encoder (ENC)
115 Delay / Loss Control Unit (Controller)
302-1 and 302-2 Delay Interferometer (MZDI)
303-1, 303-2 Balanced Photon Detector (BPD)
304-1, 304-2 Series-parallel converter (DEMUX)
305 Delay Length / Optical Phase Control Unit (Controller)

Claims (8)

A light source;
A polarization separation unit that separates light emitted from the light source into first light and second light that are polarized waves orthogonal to each other;
A DPSK modulator that generates a DPSK signal obtained by subjecting the first light to DPSK modulation;
A zero chirp π / 2 shift DPSK modulator that generates a zero chirp π / 2 shift DPSK signal that has undergone π / 2 shift DPSK modulation so that the transition between signal points is linear with respect to the second light;
A polarization multiplexing unit that polarization-multiplexes the DPSK signal and the zero chirped π / 2 shift DPSK signal;
An optical transmitter comprising: An RZ pulse carver for shaping the light emitted from the light source into an RZ pulse;
Further comprising
The polarization separation unit separates the light shaped into the RZ pulse by the RZ pulse carver into the first light and the second light.
The optical transmitter according to claim 1. A delay unit providing a predetermined time difference between the DPSK signal and the zero chirped π / 2 shift DPSK signal;
The optical transmitter according to claim 1, further comprising: A light level adjustment unit for adjusting a difference in light level between the DPSK signal and the zero chirp π / 2 shift DPSK signal;
The optical transmitter according to claim 1, further comprising: The zero chirp π / 2 shift DPSK modulator is
DPSK encoding based on the transmission data sequence, and further encoding two sets of points (ODD, EVEN) having a phase difference of π among the four QPSK signal points, and shifting to zero chirp π / 2 A zero chirped π / 2 shift DPSK encoder that generates a DPSK encoded electrical signal;
A zero chirped π / 2 shift DPSK modulator that modulates the second light based on the DPSK encoded electrical signal;
The optical transmitter according to claim 1, further comprising: An optical receiver that receives a polarization multiplexed signal transmitted from the optical transmitter according to any one of claims 1 to 5,
An optical branching unit that bifurcates the polarization multiplexed signal into first received light and second received light;
A first delay interferometer that branches the first received light into delayed light and non-delayed light, and causes delayed interference between the delayed light and the non-delayed light;
A second delay interferometer for branching the second received light into delayed light and non-delayed light, and causing delayed interference between the delayed light and the non-delayed light;
A first photon detector for detecting delayed interference light by the first delayed interferometer;
A second photon detector for detecting delayed interfering light by the second delayed interferometer;
With
The delay amount given to the delayed light by the first delay interferometer and the second delay interferometer is a delay amount for one symbol, and the optical levels of the delayed light and the non-delayed light in the first delayed interferometer The phase difference is 0, and the optical phase difference between the delayed light and the non-delayed light in the second delay interferometer is π / 2.
An optical receiver. An optical transmitter according to any one of claims 1 to 5,
The optical receiver according to claim 6 for receiving a polarization multiplexed signal transmitted from the optical transmitter according to any one of claims 1 to 5 other than the optical transmitter;
An optical transceiver characterized by comprising: An optical transmitter according to any one of claims 1 to 5,
An optical receiver according to claim 6;
Multiplexed polarization multiplexed signals transmitted from the plurality of optical transmitters are multiplexed to generate a multiplexed signal, the multiplexed signal is transmitted, the multiplexed signal after transmission is separated, and the separated optical signal is A transmission unit for input to the optical receiver;
An optical transmission system comprising:

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