JP2007181036A - Optical phase multivalue modulation system, optical communication equipment, optical transmitter, and optical receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain optical phase multivalue modulation transmission by small size, low power consumption and simple structure. <P>SOLUTION: The optical phase multivalue modulation system transmits m values of phase shift modulation light with which 2m-1 ways of phase shift including negative phase shift to be separate points are associated on a signal space using an inphase signal component and an orthogonal signal component as orthogonal axes, respectively to data of positive integer m value and separate a signal value by a value of an amplitude component when the signal space is rotated on the receiving side. In the case of m=4, a unit of phase shift by a phase modulator 13 considered as equal to or less than π/4 or π/6, codes are assigned so as to be in adjacent relation in a differential encoding circuit in order to eliminate an effect of negative phase shift generated thereby and code inversion is performed by providing a fixed amount of phase shift to a delay interferometer in an optical receiver. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical phase multilevel modulation system used in an optical fiber transmission system.

  In order to respond to the explosive increase in traffic due to the widespread use of broadband connections and the diversification of access methods, time multiplexing or wavelength multiplexing transmission technology has been developed to multiplex optical signals in the time domain or wavelength domain in optical fiber transmission systems. Has been put to practical use.

  On the other hand, in order to further increase the transmission capacity, a quaternary differential phase shift keying (DQPSK) transmission system has been developed. For example, the result of an experiment in which 64 wavelength channels are multiplexed at a data transmission rate of 12.5 Gb / s and transmitted through a 6500 km fiber. It has been reported (Non-Patent Document 1). This quaternary DQPSK transmission system uses a fiber transmission of a code obtained by converting a binary data signal of two channels into a quaternary value. Unlike time multiplexing or wavelength multiplexing, in principle there is no spread of spectrum. In addition, the transmission capacity of the fiber can be increased about twice.

  FIG. 11 shows a configuration of a conventional optical transmitter of the DQPSK transmission system. The binary data signal of the two channels is code-converted into four phase shifts by the differential encoding circuit 111, and then the two phase modulators 113 and 114 are driven to phase modulate the output light of the laser light source 112. To do. The phase of the output of one phase modulator 114 is shifted by π / 2 by the phase shifter 115 and combined with the output of the phase modulator 113. The phase modulators 113 and 114 respectively perform phase modulation from 0 to π and combine them to obtain a DQPSK signal composed of four phase shifts of 0, π / 2, π, and 3π / 2. The increment of the phase shift is 1/4 of 2π, that is, π / 2. The fiber-transmitted DQPSK signal is homodyne detected using a 1-bit previous signal by a Mach-Zehnder interferometer (delay interferometer) having a 1-bit delay line, and demodulated into a 2-channel binary data signal.

  Although this DQPSK transmission system is an excellent system that can increase the transmission capacity by about twice as much as described above without accompanying spectrum broadening in principle, there is a drawback that the optical transmitter becomes a large-scale configuration. That is, as the phase modulators 113 and 114, a modulator made of a lithium niobate substrate is usually used. However, this modulator has an element length exceeding 5 cm, and the power of the drive circuit needs about 1 W. is there. On the other hand, the semiconductor laser used in combination with the phase modulator has a length of about 0.3 mm and power consumption of several tens of mW or less, which is an order of magnitude smaller.

  Therefore, conventionally, when attempting to increase the transmission capacity by wavelength multiplexing or fiber multi-core using the DQPSK transmission system, the optical modulator occupies most of the device capacity of the optical transmitter, and a large amount of power is required. Part was to be spent driving the modulator. Furthermore, even if connection and compatibility with other devices and parts are considered, there is a problem in the consistency, and these have been a major obstacle to the practical use of the DQPSK transmission apparatus.

  In order to reduce the size of the phase modulator or reduce the drive amplitude and power consumption, it seems to be effective to make the phase shift step smaller than π / 2. However, simply making the phase shift step smaller than π / 2 causes the phase shift amount to deviate from an integral multiple of π / 2, and the mark (“1”) and space (“0”) signs are close. Cannot be demodulated on the optical receiver side. Therefore, in all the conventional DQPSK transmission systems, the increment of phase shift is π / 2 (for example, Non-Patent Document 2).

JP 2004-170954 A Electronics Letters, 40, 444-445 (Electronics Letters, Vol.40, p.444-445) I Triple E Journal of Lightwave Technology, Vol. 23, pp. 108-114 (IEEE Journal of Lightwave Technology, Vol.23, p.108-114)

  As described above, in the conventional DQPSK transmission system, in order to reduce the size of the phase modulator or to reduce the drive amplitude and reduce the power consumption, if the step of phase shift is made smaller than π / 2, it becomes negative. Due to the effect of the phase shift, the codes of the mark (“1”) and the space (“0”) are close to each other and overlap each other, so that it cannot be demodulated on the optical receiver side. An object of the present invention is to solve the above problems and to realize optical phase multilevel modulation transmission with a small size, low power consumption and a simple configuration.

  According to the first aspect of the present invention, negative phase shifts that become separate points on a signal space having an in-phase signal component and a quadrature signal component as orthogonal axes for data of an integer m value greater than 2, respectively. 2m-1 phase shift modulated light corresponding to 2m-1 kinds of phase shifts including the signal is transmitted, and on the receiving side, the signal value is separated by the value of the amplitude component when the signal space is rotated. A featured optical phase multilevel modulation scheme is provided.

  The integer m is 4, and quaternary data is associated with phase shifts of 0, ± π / 4, ± 2π / 4, and ± 3π / 4. On the receiving side, the signal space is ± π / 8 or ± The signal value can be separated from the amplitude component when rotated by 3π / 8.

  The integer m is 4, and quaternary data can be assigned to seven points in the range of −π / 2 to π / 2 on the signal space. In this case, quaternary data is associated with phase shifts of 0, ± π / 6, ± 2π / 6, and ± 3π / 6, and on the receiving side, amplitude components when the signal space is rotated by ± π / 4 The signal value can be separated from

  According to a second aspect of the present invention, negative phase shifts that are separate points on a signal space having an in-phase signal component and a quadrature signal component as orthogonal axes, for integer m-value data greater than 2, respectively. An optical transmitter that transmits m-value phase-shift keyed light that corresponds to 2m−1 kinds of phase shifts, and a value of an amplitude component when the signal space of the phase-shift keyed light is rotated. An optical communication device comprising an optical receiver that separates and demodulates signal values is provided.

  The integer m is 4, and the optical transmitter includes a differential encoding circuit that differentially encodes a binary data signal of two channels, and two optical phase shift key modulations that perform optical phase shift keying according to the output of the differential encoding circuit. And a phase modulator.

One of the two phase modulators gives three phase shifts Δφ ₁ of −π / 2, 0, and π / 2, and the other has three ways of −π / 4, 0, and π / 4. The phase shift Δφ ₂ can be provided. Separately, one gives three phase shifts Δφ ₁ in the range of −π / 3 ≦ Δφ ₁ ≦ π / 3, and the other in the range of −π / 6 ≦ Δφ ₂ ≦ π / 6. A configuration in which three phase shifts Δφ ₂ are provided can also be adopted. In the latter case, the phase shifts Δφ ₂ and Δφ ₁ may be any value as long as they are within the above range, but in practice, it is desirable that Δφ ₁ = 2 × Δφ ₂ . The two phase modulators are preferably connected in series.

  The differential encoding circuit preferably assigns the binary data signals of two channels so as to be adjacent to the seven phase shifts arranged in order.

  The optical receiver receives a Mach-Zehnder type delay interferometer in which one arm is provided with a 1-bit delay line and the other arm is provided with a phase shift unit for rotating a signal space, and light output from the delay interferometer. It is desirable to provide a balance type light receiver that determines the signal value from the amplitude value of the light reception output of the balance type light receiver.

  The phase shift amount of the phase shift unit is adjusted so that the signal strength-to-noise ratio of the two-channel binary data signal output from the discrimination circuit is maximized, or is output from the discrimination circuit It is desirable to adjust so that the code error rate of the two-channel binary data signal is minimized.

  The delay interferometer and the balanced photoreceiver are provided separately for each of the in-phase signal component and the quadrature signal component, and the phase shift amount of each phase shift unit of the two delay interferometers is an absolute value. It is desirable that the signs be adjusted equally and in opposite relations.

  The determination circuit may include a threshold value identification circuit in which a threshold value is set in the vicinity of the balanced output value of the balanced light receiver. The discrimination circuit can include means for demodulating a binary data signal from a multi-value amplitude signal by a combination of a threshold value identification circuit, an AND circuit, and an OR circuit.

  According to another aspect of the invention, for positive integer m-valued data, including negative phase shifts that are separate points on the signal space with the in-phase signal component and the quadrature signal component as orthogonal axes, respectively. There is provided an optical transmitter comprising phase modulation means for generating m-value phase shift modulated light corresponding to 2m−1 kinds of phase shifts.

  According to still another aspect of the present invention, there is provided an optical receiver comprising means for separating and demodulating a signal value based on an amplitude component value when a signal space of phase shift keying light is rotated A vessel is provided.

  When the phase shift increment is π / 2 as in the prior art, there is no need to consider negative phase shift. This is because, for example, when considering a negative phase shift of −π / 2, 3π / 2 obtained by adding 2π has the same value in the phase modulation method. On the other hand, if the phase shift step is smaller than π / 2, the negative phase shift and the value obtained by adding four times the phase shift step are not the same value. The effect of this must be taken into account.

  Conventionally, in the example disclosed regarding the m-value phase modulation method, the increment of the phase shift is 2π / m, and the negative phase shift is not considered (for example, Patent Document 1). That is, conventionally, no consideration has been given to configuring a DQPSK optical transmission / reception apparatus in the case where a negative phase shift occurs when the phase shift increment given by the phase modulator is smaller than π / 2.

  On the other hand, in the present invention, 2m−1 kinds of phase shifts are made to correspond to m-value data, and it can be demodulated even on the receiving side. Since the step of the phase shift becomes small, the phase modulator can be miniaturized, and the driving amplitude can be reduced to reduce the power consumption.

  Embodiments of an optical transceiver according to the present invention will be described below in detail with reference to the drawings. Hereinafter, in order to simplify the description, a case will be described as an example in which a binary data signal of two channels is differentially encoded and transmitted by four-level differential phase shift keyed light.

  1 and 2 are block diagrams showing a first embodiment of the present invention. FIG. 1 shows the configuration of an optical transmitter, and FIG. 2 shows the configuration of an optical receiver.

The optical transmitter includes a differential encoding circuit 11 that differentially encodes a binary data signal of two channels, a laser light source 12, and an optical phase shift according to the output of the laser light source 12 according to the output of the differential encoding circuit 11. Two phase modulators 13 and 14 for modulation are provided. The laser light source 12 and the phase modulators 13 and 14 are connected in series on the optical path, and quaternary differential phase shift keyed light (DQPSK signal light) is output. The phase modulator 13 gives three phase shifts Δφ ₁ of −π / 2, 0, and π / 2, and the phase modulator 14 sets three phase shifts of −π / 4, 0, and π / 4. The shift Δφ ₂ is given. Either of the phase modulator 13 and the phase modulator 14 may be connected first.

  The optical receiver includes Mach-Zehnder type delay interferometers 21-1 and 21-2 in which one arm is provided with a 1-bit delay line and the other arm is provided with a phase shift unit for rotating a signal space. Signals are obtained from the balance type light receivers 22-1 and 22-2 that receive the output light of the delay interferometers 21-1 and 21-2 and the amplitude values of the light reception outputs of these balance type light receivers 22-1 and 22-2 And a determination circuit 23 for determining the value. The delay interferometers 21-1 and 21-2 and the balanced light receivers 22-1 and 22-2 detect the DQPSK signal light using the signal one bit before, and perform homodyne detection. The discriminating circuit 23 discriminates a signal by a combination of the threshold discriminating circuit 24 and the AND circuit 25, and demodulates a 2-channel binary data signal.

  Table 1 shows an example of assignment of binary data signals of two channels in the differential encoding circuit 11 to the phase shift of the DQPSK signal light. In this table, the first bit of the 2-bit code corresponds to the binary data signal of the first channel, and the second bit from the head corresponds to the binary data signal of the second channel. However, either one of the channels may be in the order. 2 channel binary for 7 phase shifts arranged in the order of 3π / 4, 2π / 4, π / 4, 0, -π / 4, -2π / 4, -3π / 4. The data signals are allocated so as to be adjacent to each other, that is, so that only one bit is different between neighbors. Here, for the negative phase shift, the same data signal as the phase shift obtained by adding four times the increment of the phase shift to the negative phase shift is assigned. In addition, four data signals 00, 01, 11, and 10 can be assigned to the phase shift 0, and two adjacency assignments are also possible. That is, a total of eight ways of assigning the binary data signals of the two channels to the phase shift are possible. Table 1 shows an example in which the data signal assigned to the phase shift 0 is 11 and the data signal assigned to the phase shift π / 4 is 10.

  FIG. 3 shows a configuration example of a delay interferometer and a balance type light receiver used in the optical receiver shown in FIG. The delay interferometer 21 includes a Mach-Zehnder interferometer having a 1-bit delay line 31 in one arm and a phase shift unit 32 having a phase shift amount θ in the other arm. Two outputs of the meter are respectively received by two light receiving elements connected in series, and one electric signal is output.

3, the k-th bit of the DQPSK signal light phase phi _k, the phase of phi _k-1 of the DQPSK signal light of k-1 th bit of the 1-bit ago, and when, the balanced photodetector 22 The output V is
V = cos (Δφ + θ). . . (1)
Δφ = φ _k −φ _k−1 . . . (2)
It is expressed. From equation (1), the sign of the binary data signal demodulated by the optical receiver is arranged on the circumference of the I (in-phase) -Q (orthogonal) plane, and its position is the phase shift Δφ and the phase shift amount θ. It can be seen that the magnitude of the output V of the balanced light receiver 22 is given by the magnitude of its I component.

In the case of DQPSK, assuming that the phase shift increment is φ ₀ , there are generally seven possible phase shifts of 0, ± φ ₀ , ± 2φ ₀ , and ± 3φ ₀ . Since φ ₀ = π / 2 in the conventional DQPSK, as can be seen from the equation (1), the phase shift of −π / 2, −π, −3π / 2 and 3π / 2, π, π / 2 The phase shifts have the same value. Therefore, the phase shift is degenerated in four ways: 0, π / 2, π, and 3π / 2. On the other hand, if the phase shift step φ ₀ is made smaller than π / 2, the possible phase shift values can be degenerated from 4 types to 7 types, so that the mark (“1”) and space (“0” )) Overlaps in code arrangement on the IQ plane. Accordingly, the maximum phase shift step φ ₀ where no code overlap occurs in the range of φ ₀ <π / 2 is φ ₀ ≦ 2π / 7, which divides the range of 2π into seven equal parts. Further, since the magnitude of the output V of the balanced light receiver 22 is given by an I component, the inter-symbol distance between the mark (“1”) and the space (“0”) on the I axis, that is, the signal strength to noise ratio. It can be seen that the maximum is when φ ₀ = π / 4 when the phase shift step φ ₀ is within the range of π / 6 <φ ₀ ≦ 2π / 7.

In the optical transmitter shown in FIG. 1, the element length or drive amplitude of the phase modulators 13 and 14 is ½ of the conventional, that is, the phase shift step φ ₀ is π / 4. That is, the phase modulator 13 gives three phase shifts Δφ ₁ of −π / 2, 0, and π / 2, and the phase modulator 14 gives three types of phase shifts of −π / 4, 0, and π / 4. By giving the phase shift Δφ ₂ , a DQPSK signal having four phase values of 0, π / 4, π / 2, and 3π / 4 is generated.

  4 and 5 are diagrams showing the code arrangement in the signal space, and the DQPSK signal having the four phase values of 0, π / 4, 2π / 4, and 3π / 4 described above is input to the delay interferometer 21. FIG. In this case, the code arrangement of the binary data signal of one channel is expressed on the IQ plane. In these figures, the symbol ● represents the mark “1” and the symbol ○ represents the space “0”. In FIG. 4, the phase shift amount θ of the phase shift unit 32 is θ = 0, and in FIG. 5, θ = + 3π / 8.

  When the phase shift amount θ = 0, as can be seen from FIG. 4, since the I component of the mark arranged at Δφ = π / 4 and the I component of the space arranged at Δφ = −π / 4 are equal, In this case, the value of the mark and the space is the same as the output V of the balanced light receiver 22, and separation is impossible.

  On the other hand, as can be seen from the equation (1), when the phase shift amount θ is given to the phase shift unit 32, the entire code arrangement of the mark and the space can be rotated counterclockwise on the circumference while maintaining the interval. It is possible to avoid the overlap of the I component that is the output V of the balanced light receiver 22. If the phase shift amount θ is θ = + 3π / 8 as shown in FIG. 5, the distance between the marks on the I-axis of the mark and the space is maximized and demodulated as the output V of the balanced light receiver 22 2 The signal-to-noise ratio of the value data is maximized.

  In the encoding shown in Table 1, by assigning binary data of two channels so as to be adjacent to each other, in FIG. 5, the I component of the mark is all arranged inside the range surrounded by the broken line. Therefore, if the threshold value identifying circuit 24 for judging the threshold value of the I component at the position of the threshold value 51 which is the intersection of the broken line and the I axis is used, the mark and the space are separated and the binary data signal of one channel is demodulated. You can see that you can.

  Also, the binary data signal of the other channel can be demodulated in the same manner by rotating the code arrangement in the signal space in the reverse direction. In this case, the phase shift amount of the phase shift unit 32 has the same absolute value and the opposite sign from the case of demodulation of the binary data signal of one channel described above from the coding relationship of Table 1. . Further, the phase shift amount of the phase shift unit 32 is adjusted so that the signal strength-to-noise ratio of the output binary data signal is maximized or the code error rate of the binary data signal is minimized. .

In the optical transmitter shown in FIG. 1, the element lengths or drive amplitudes of the phase modulators 13 and 14 are set to 1/3 of the prior art, that is, the phase shift step φ _{0 is set} to π / 6. That is, the phase modulator 13 gives three phase shifts Δφ ₁ of −π / 3, 0, and π / 3, and the phase modulator 14 has three ways of −π / 6, 0, and π / 6. by providing a phase shift _{Δφ 2, 0, π / 6,2π} / 6,3π / 6, to generate a DQPSK signal having a phase value of the four values.

  Table 2 shows an example of assignment of binary data signals of two channels to the phase shift of the DQPSK signal light. 2 channel binary for 7 phase shifts arranged in the order of 3π / 6, 2π / 6, π / 6, 0, -π / 6, -2π / 6, -3π / 6 Data signals are assigned so as to be adjacent.

  FIG. 6 shows a block diagram of an optical receiver used in this embodiment. This optical receiver includes delay interferometers 61-1 and 61-2 and balanced light receivers 62-1 and 62-2, and each of the delay interferometers 61-1 and 61-2 has the same absolute value and a sign. Includes phase shift units having opposite phase shift amounts, and each phase shift unit has a maximum signal strength-to-noise ratio of the output binary data signal, and the code error rate of the binary data signal is It has been adjusted to be minimal. A two-channel binary data signal is demodulated by a discrimination circuit 63 that is a combination of a threshold identification circuit 64, an AND circuit 65, and an OR circuit 66.

FIG. 7 shows a code arrangement on the signal space when the phase shift amount of the phase shifter section of the delay interferometers 61-1 and 61-2 is θ = 0. When the binary data signal is demodulated, codes are arranged on the IQ plane as shown in FIG. The phase shift Δφ ₁ provided by the phase modulator 13 and the phase shift Δφ ₂ provided by the phase modulator 14 are −π / 3 ≦ Δφ ₁ ≦ π / 3 and −π / 6 ≦ Δφ ₂ ≦ π /. In the case of 6, unlike the case of the first embodiment, the code arrangement of the mark and the space is always within a semicircumference, that is, within the range of π on the IQ plane. Therefore, if an appropriate amount of phase shift is given by the phase shift unit, the mark and space I components do not overlap in any case. That is, if the conditions of −π / 3 ≦ Δφ ₁ ≦ π / 3 and −π / 6 ≦ Δφ ₂ ≦ π / 6 are satisfied, the value of the phase shift given by the phase modulators 13 and 14 is an arbitrary value. You can choose.

  As can be seen from the equation (1), in order to obtain a high signal strength-to-noise ratio, the threshold value of the threshold value identification circuit 64 that separates the mark and the space is in the vicinity of the balanced output value of the balanced light receivers 62-1 and 62-2. Must be set to

  8A and 8B show the binary data signal of one channel when the phase shift amount θ of the delay interferometer is θ = + π / 4 and the opposite sign θ = −π / 4. Is expressed on the IQ plane. In these figures, if the mark and space are identified by the threshold value 81 that is the intersection of the broken line and the I axis, the binary data of the two channels are assigned so as to be adjacent by the coding of Table 2, and the mark Can be identified in groups of two.

  FIGS. 9A and 9B show the binary data signal of the other channel when the phase shift amount θ of the delay interferometer is θ = + π / 4 and θ = −π / 4 of the opposite sign. Is expressed on the IQ plane. If the phase shift amount of one set of delay interferometers has the same absolute value and the opposite sign, a delay interferometer common to one of the channels described above can be used. Specifically, a binary data signal of two channels can be demodulated by the configuration of the optical receiver shown in FIG.

  Here, an example in which the element length or drive amplitude of the phase modulators 13 and 14 is set to 1/3 of the conventional one is shown, but it is not necessary to limit to this value. It is possible to choose. For example, it is possible to realize an ultra-compact and low power consumption DQPSK optical transmission / reception apparatus in which the element length is 1/10 of the conventional one or the drive amplitude is 1/10.

  FIG. 10 is a block configuration diagram showing a usage form of the present invention and FIG. 10, and shows an example in which the present invention is used in a high-density wavelength division multiplexing transmission apparatus.

  The DQPSK signal lights output from the optical transmitters 101-1 to 101-100 are combined by the wavelength multiplexers 102-1 and 102-2 and input to the polarization beam combiner 103. Here, the odd-numbered wavelength channels from the wavelength 1 to the wavelength 99 are multiplexed by the wavelength multiplexer 102-1, and the even-numbered wavelength channels from the wavelength 2 to the wavelength 100 are combined with the other wavelength multiplexer 102. -2 is input to the polarization multiplexer 103 so that the polarizations are orthogonal to each other. The 100-channel DQPSK signal light in which the wavelength and the polarization are multiplexed is input to the polarization separator 105 on the reception side through the optical fiber transmission line 104. The DQPSK signal light is separated into wavelength channels by the polarization separator 105 and the wavelength separators 106-1 and 106-2, and demodulated using the optical receivers 107-1 to 107-100.

  Here, as the wavelength multiplexers 102-1 and 102-2 and the wavelength separators 106-1 and 106-2, an AWG (Arrayed Waveguide) with optimized center wavelength and band characteristics can be used. For the polarization multiplexer 103 and the polarization separator 105, a multiplexer / demultiplexer using a polarization maintaining fiber can be used.

  By using the optical transmitter and the optical receiver shown in the second embodiment as the optical transmitters 101-1 to 101-100 and the optical receivers 107-1 to 107-100, the drive amplitude of the phase modulator can be reduced. Each can be reduced to 1/10 of the conventional one. If the clock speed is 20 GHz, the number of wavelength channels is 100, and the wavelength channel interval is 50 GHz, optical fiber transmission with a bit density of 0.8 bits / Hz and a total capacity of 4 Tb / s is possible. Therefore, it is possible to realize a Tb / s class large-capacity optical transmission / reception device that is significantly reduced in size and power consumption compared to the conventional one.

  According to the present invention, it is possible to reduce the size and power consumption of a DQPSK optical transceiver. This is particularly effective in a high-density wavelength division multiplex transmission apparatus having a large number of wavelength channels.

The block block diagram of the optical transmitter in the 1st Example of this invention. The block block diagram of an optical receiver. FIG. 3 is a diagram illustrating a configuration example of a delay interferometer and a balanced light receiver used in the optical receiver illustrated in FIG. 2. The figure which shows code arrangement | positioning on signal space. The figure which shows the state which rotated code | symbol arrangement | positioning on signal space. The block block diagram of the optical receiver in the 2nd Example of this invention. The figure which shows code arrangement | positioning on signal space in case a phase shift amount is (theta) = 0. The figure which shows signal arrangement | positioning of one channel in case a phase shift amount is (theta) = + (pi) / 4 and (theta) =-(pi) / 4. The figure which shows signal arrangement | positioning of the other channel in case a phase shift amount is (theta) = + (pi) / 4 and (theta) =-(pi) / 4. The block block diagram which shows the example which utilized this invention for the high-density wavelength multiplexing transmission apparatus. The figure which shows the structure of the optical transmitter of the conventional DQPSK transmission system.

Explanation of symbols

11 differential encoding circuit 12 laser light source 13, 14 phase modulator 21, 21-1, 21-2, 61-1, 61-2 delay interferometer 22, 22-1, 22-2, 62-1, 62- 2 Balance type optical receivers 23 and 63 Discrimination circuits 24 and 64 Threshold identification circuits 25 and 65 AND circuit 31 Bit delay line 32 Phase shift unit 51 and 81 Threshold 66 OR circuit
101-1 to 101-100 Optical transmitters 102-1 and 102-2 Wavelength multiplexer 103 Polarization multiplexer 104 Optical fiber transmission path 105 Polarization separators 106-1 and 106-2 Wavelength separator 107-1 ~ 107-100 Optical receiver

Claims (19)

  For data of an integer m value greater than 2, 2m−1 phase shifts including negative phase shifts that are different points on the signal space having the in-phase signal component and the quadrature signal component as orthogonal axes, respectively. An optical phase multilevel modulation method characterized in that m-value phase-shift keyed light corresponding to the above is transmitted, and the signal value is separated on the receiving side by the value of the amplitude component when the signal space is rotated.   The integer m is 4, and quaternary data is associated with phase shifts of 0, ± π / 4, ± 2π / 4, and ± 3π / 4. On the receiving side, the signal space is ± π / 8 or ± 2. The optical phase multilevel modulation system according to claim 1, wherein a signal value is separated from an amplitude component when rotated by 3π / 8.   The optical phase multilevel modulation system according to claim 1, wherein the integer m is 4, and quaternary data is assigned to seven points in a range of -π / 2 to π / 2 on a signal space.   Corresponding phase shifts of 0, ± π / 6, ± 2π / 6, and ± 3π / 6 to the four-valued data, the signal value from the amplitude component when the signal space is rotated ± π / 4 on the receiving side The optical phase multilevel modulation system according to claim 3, which separates the two. For data of an integer m value greater than 2, 2m−1 phase shifts including negative phase shifts that are different points on the signal space having the in-phase signal component and the quadrature signal component as orthogonal axes, respectively. An optical transmitter for transmitting phase-shift keyed light of m value corresponding to
An optical receiver comprising: an optical receiver that separates and demodulates a signal value based on a value of an amplitude component when the signal space of the phase shift keying light is rotated.   The integer m is 4, and the optical transmitter includes a differential encoding circuit that differentially encodes a binary data signal of two channels, and two optical phase shift key modulations that perform optical phase shift keying according to the output of the differential encoding circuit. The optical communication device according to claim 5, further comprising a phase modulator. One of the two phase modulators gives three phase shifts Δφ ₁ of −π / 2, 0, and π / 2, and the other has three ways of −π / 4, 0, and π / 4. The optical communication device according to claim 6, wherein the optical communication device is configured to provide a phase shift Δφ ₂ . One of the two phase modulators gives three phase shifts Δφ ₁ in a range of −π / 3 ≦ Δφ ₁ ≦ π / 3, and the other has a range of −π / 6 ≦ Δφ ₂ ≦ π / 6. 7. The optical communication apparatus according to claim 6, wherein the optical communication apparatus is configured to provide three kinds of phase shifts Δφ ₂ . 9. The optical communication apparatus according to claim 8, wherein the phase shift Δφ ₁ and the phase shift Δφ ₂ are in a relationship of Δφ ₁ = 2 × Δφ ₂ .   The optical communication apparatus according to claim 6, wherein the two phase modulators are connected in series.   The optical communication apparatus according to claim 6, wherein the differential encoding circuit allocates two-channel binary data signals so as to be adjacent to seven phase shifts arranged in order.   The optical receiver receives a Mach-Zehnder type delay interferometer in which one arm is provided with a 1-bit delay line and the other arm is provided with a phase shift unit for rotating a signal space, and light output from the delay interferometer. The optical communication apparatus according to claim 5, further comprising: a balance-type light receiver that performs determination, and a determination circuit that determines a signal value from an amplitude value of a light-receiving output of the balance-type light receiver.   13. The optical communication apparatus according to claim 12, wherein the phase shift amount of the phase shift unit is adjusted so that the signal intensity-to-noise ratio of the two-channel binary data signal output from the determination circuit is maximized.   13. The optical communication apparatus according to claim 12, wherein the phase shift amount of the phase shift unit is adjusted so that a code error rate of a two-channel binary data signal output from the determination circuit is minimized.   The delay interferometer and the balanced photoreceiver are provided separately for each of the in-phase signal component and the quadrature signal component, and the phase shift amount of each phase shift unit of the two delay interferometers is an absolute value. 13. The optical communication apparatus according to claim 12, wherein the optical communication apparatuses are equally adjusted to have opposite signs.   13. The optical communication apparatus according to claim 12, wherein the determination circuit includes a threshold identification circuit in which a threshold is set in the vicinity of a balanced output value of the balanced light receiver.   13. The optical communication apparatus according to claim 12, wherein the determination circuit includes means for demodulating a binary data signal from a multi-value amplitude signal by a combination of a threshold identification circuit, an AND circuit, and an OR circuit.   For positive integer m-valued data, 2m−1 phase shifts including negative phase shifts that are different points on the signal space having the in-phase signal component and the quadrature signal component as orthogonal axes, respectively. An optical transmitter comprising phase modulation means for generating phase-shift keyed light having m values corresponding thereto. An optical receiver comprising means for separating and demodulating a signal value based on an amplitude component value when a signal space of phase shift keying light is rotated.

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