JP5394992B2 - Optical receiver for differential phase modulation signal - Google Patents

Description

  The present invention relates to an optical receiver that receives an optical signal subjected to differential phase modulation. More specifically, the present invention relates to a circuit configuration capable of realizing high demodulation performance even for a deteriorated signal.

  With the progress of optical communication technology, it has become increasingly important to develop optical components that directly process optical signals. In particular, a waveguide-type optical interference circuit using planar lightwave circuit (PLC: Planar Light Circuit) integrated on a planar substrate and utilizing optical interference is superior in terms of mass productivity, low cost, and high reliability. Many research and developments have been made. Examples of the waveguide type optical interference circuit include an arrayed waveguide diffraction grating, a Mach-Zehnder interferometer, and a lattice circuit.

  Such a waveguide type optical interference circuit is manufactured by a standard photography method and an etching technique and a glass deposition technique such as FHD (Flame Hydrolysis Deposition). Specifically, the process is first overviewed by depositing an undercladding layer and a core layer having a higher refractive index than the periphery on the substrate. Thereafter, a waveguide pattern is formed in the core layer, and finally the waveguide is formed by a process of embedding the waveguide formed in the core layer by the over clad layer. The signal light is confined and propagated in the waveguide manufactured through the process as described above.

  Turning to modulation / demodulation processing technology in an optical transmission system, signal transmission using a phase modulation method has been widely put into practical use. Differential phase shift keying (DPSK) is particularly attracting attention because of its high resistance to signal degradation due to chromatic dispersion and polarization mode dispersion of the transmission line. In addition, multilevel modulation is also performed to increase the signal phase point in phase modulation. In addition to binary DBPSK (Differential Binary Phase Shift Keying) having two phase points, quaternary DQPSK (Differential Quadrature Phase Shift Keying) having four phase points has been studied.

  In demodulating such a DBPSK signal or DQPSK signal, an optical delay interferometer (optical delay interference circuit) that detects an optical signal corresponding to the modulation symbols that follow each other is required. That is, by branching the optical signal, delaying one optical signal by one symbol, and interfering with the other optical signal, it is possible to detect the phase difference between the preceding and succeeding modulation symbol signals. It is expected that the above-described PLC technology is applied to construct and manufacture an optical delay interference circuit, thereby stabilizing the circuit performance for a long period of time and reducing the circuit size.

“Partial DPSK with excellent filter tolerance and OSNR sensitivity” Mikkelsen, B .; Rasmussen, C .; Mamyshev, P .; Liu, F .; Electronics Letters Volume: 42, Issue: 23, Publication Year: 2006, Page (s) : 1363-1364

  However, the DPSK signal has a problem that the transmission signal is deteriorated due to the chromatic dispersion of the optical fiber or the signal band narrowing by the wavelength filter in the transmission path. In addition, the actual optical transmission network in which the transmission signal propagates has a problem that the amount of chromatic dispersion and the amount of narrowing of the signal by the filter change every moment due to changes in the state of the optical fiber due to environmental fluctuations, changes in the transmission path, etc. There is.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical receiver that can satisfactorily receive a differential phase modulation signal deteriorated due to wavelength dispersion or narrowing of a filter. is there.

  In order to achieve the above object, the present invention provides a receiver for demodulating a multilevel differential phase modulated optical signal, wherein the optical signal branches the optical signal. A splitter, a first delay unit configured by a waveguide having a different length, which gives a delay of a first delay amount between the optical signals branched by the optical splitter, and a first delay unit configured to A first coupler having a variable branching ratio for coupling an optical signal to which a delay of one delay amount is given, and a second different from the first delay amount between the optical signals output from the first coupler. A second delay unit configured by waveguides having different lengths, and a second delay unit configured to interfere with an optical signal provided with a second delay amount by the second delay unit. 2 couplers.

  The invention according to claim 2 is the receiver according to claim 1, wherein a sum of the first delay amount and the second delay amount is substantially equal to a signal interval of the optical signal. It is set as follows.

  The invention according to claim 3 is the receiver according to claim 1 or 2, wherein each of the first and second delay units has a phase shifter disposed in one waveguide. It is characterized by.

  The invention according to claim 4 is the receiver according to any one of claims 1 to 3, wherein the second coupler has two outputs, and a light receiving element is connected to each output. It is characterized by that.

  The invention according to claim 5 is the receiver according to any one of claims 1 to 4, wherein the first coupler is an equal-length Mach-Zehnder interference circuit having a phase shifter in each arm waveguide. Each arm waveguide is provided with a phase shifter.

  The invention according to claim 6 is the receiver according to any one of claims 1 to 5, wherein at least one of the first and second delay units has different lengths. A 45-degree half-wave plate traversing the waveguide is provided.

  The invention according to claim 7 is the receiver according to any one of claims 1 to 6, wherein the optical signal is a DBPSK or DQPSK signal.

It is a figure which shows the structural example of the circuit which receives a DBPSK signal. It is a figure which shows the structural example of the circuit which receives a DQPSK signal. It is a schematic diagram for demonstrating PDPSK technique. FIG. 4 is a diagram illustrating a required OSNR with respect to chromatic dispersion in the configuration of FIG. 3. It is a figure which shows the contrast of the optical delay interference circuit by PDPSK technique, and the optical delay interference circuit by this invention. It is a figure which shows the structural example of the optical delay interference circuit by this invention. It is a figure which shows required OSNR with respect to the wavelength dispersion in the structure of FIG. 2A and 2B are diagrams illustrating a configuration example of a receiving circuit according to the present invention, in which FIG. 1A illustrates a configuration example of a circuit that receives a DBPSK signal, and FIG. 2B illustrates a configuration example of a circuit that receives a DQPSK signal. It is a figure which shows the structure of the optical delay interference circuit by Example 1 of this invention. FIG. 10 is a diagram illustrating a required OSNR with respect to chromatic dispersion in the configuration of FIG. 9. FIG. 10 is a diagram illustrating a required OSNR with respect to a transmission band of a filter in the configuration of FIG. 9. It is a figure which shows the structure of the optical delay interference circuit by Example 2 of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the configuration of a conventional DPSK receiver will be described, and then the configuration of the DPSK receiver according to the present invention will be described.

  FIG. 1 is a diagram showing a basic configuration of an optical delay interference circuit constituting the DBPSK receiving circuit. First, the operation of the optical delay line interferometer will be described using DBPSK having binary signal points as an example. The optical delay interferometer 1 is composed of an asymmetric Mach-Zehnder interferometer. That is, the optical delay interference circuit 1 includes an input waveguide 2, an optical splitter 3 connected to the input waveguide 2, output waveguides 6 and 7, and an optical coupler 10 connected to each output waveguide. Prepare. The optical splitter 3 and the optical coupler 10 are connected by two waveguides having different lengths, that is, a long arm waveguide 4 and a short arm waveguide 5. At the output ends of the output waveguides 6 and 7, photodiode (hereinafter referred to as PD) pairs 8a and 8b constituting a differential light receiving unit are arranged.

  The DBPSK signal is input to the input waveguide 2 in the optical delay interference circuit 1. The DBPSK signal is branched by the optical splitter 3 into a long arm waveguide 4 and a short arm waveguide 5. The long arm waveguide 4 and the short arm waveguide 5 constitute an optical delay unit 9 by the difference between the waveguide lengths. The amount of delay time by the optical delay unit 9 is a time corresponding to one symbol of the DBPSK signal. For example, when the symbol rate is 20 Gbit / s, a delay of 50 ps, which is the reciprocal thereof, is a delay time amount for one symbol.

  By giving this delay in the optical delay unit 9, interference occurs between adjacent symbols of the DBPSK signal. The DBPSK signal given the interference is received by the PD pair 8a, 8b of the differential light receiving unit, and differential detection is performed on the light intensity of the optical signal from the two output waveguides 6, 7. As a result, a difference output change corresponding to the phase difference of the signal occurs between the symbols that follow each other. In other words, a demodulated output is obtained from the differential light receiving unit comprising the PD pair 8a, 8b. For example, when the phase difference is 0, the differential output signal is positive, and when the phase difference is π, the differential output signal is negative.

  When the phase modulation is multi-valued to two or more values, the phase change amount cannot be detected with only one delay interferometer. For example, a four-valued DQPSK receiving circuit uses two delay interference circuit elements to demodulate a modulated signal.

  FIG. 2 is a block diagram showing an optical delay interference circuit for DQPSK. In the basic configuration, the DQPSK signal is branched by an optical splitter, and each is input to two different optical delay interference circuits. Specifically, one of the DQPSK signals branched by the optical splitter 23 is the first optical delay interference from the optical splitter 3a through the arm waveguides 4a and 5a and the output waveguides 6a and 7a to the PD pair 8a and 8b. Input to the circuit. The other of the DQPSK signals is input from the optical splitter 3b to the second optical delay interference circuit that reaches the PD pairs 8c and 8d via the arm waveguides 4b and 5b and the output waveguides 6b and 7b. The two delay interference circuits include optical delay units 9a and 9b, respectively, each having a delay of one symbol. Furthermore, one optical delay interference circuit is provided with a π / 2 phase transition unit 12 (in the case of FIG. 2, an interference circuit having an optical delay unit 9a).

  The optical delay units 9a and 9b and the phase transition unit 12 include a fine adjustment function unit (not shown) and can finely adjust the delay time or the phase transition amount. For example, when the fine adjustment function unit is configured by a heater, a change in the optical path length due to a change in refractive index can be generated by heating the heater disposed on the upper portion of the waveguide. Therefore, the delay time or phase shift amount to be given can be finely adjusted according to the heater heating amount. According to the above-described configuration, separate binary signals (I signal and Q signal) can be received in each optical delay interference circuit, and the quaternary DQPSK signal can be demodulated as a whole.

  The basic receiving circuit that receives the DPSK signal has been described above. Once again, it ’s easy to organize here. The DPSK receiving circuit includes an optical delay interference circuit and a light receiving PD pair. In the case of a DBPSK receiving circuit, it is composed of one optical delay interference circuit and one light-receiving PD pair (FIG. 1). In the case of the DQPSK receiving circuit, it is composed of two optical delay interference circuits and two light receiving PD pairs (FIG. 2).

  On the other hand, such a differential phase modulation signal is effective because it can transmit a large-capacity signal, but the transmission signal is deteriorated due to the chromatic dispersion of the fiber and the signal band narrowing by the wavelength filter in the transmission line. There's a problem. These are described in detail below.

  The chromatic dispersion of the fiber indicates the amount that the propagation speed of light propagating through the fiber varies depending on the wavelength, and is expressed in units such as ps / nm. For example, 1 ps / nm means that the propagation speed differs between lights having different wavelengths of 1 nm, and a delay difference of 1 ps occurs. The transmitted optical signal is composed of a plurality of wavelength components. Accordingly, since light of different wavelengths constituting the transmission signal propagates at different speeds, the waveform degradation of the transmission signal occurs after propagation.

  Next, transmission signal degradation due to narrowing of the signal band by the wavelength filter will be described. In an optical fiber transmission system, as represented by a reconfigurable optical add-drop multiplexing (ROADM) system, many nodes pass during transmission. Each node is provided with a wavelength filter, and light is added to or extracted from the transmission path. When passing a signal, the wavelength filter is ideally a rectangular band-pass filter having a wide band so that the entire wavelength band of the signal can be transmitted. However, the filter actually used does not have perfect rectangularity and a band that can transmit all the optical signals. Therefore, if the number of filters that pass through increases, the wavelength band of the propagating optical signal is narrowed, and the transmission signal is degraded.

  As described above, the band narrowing by the filter and the chromatic dispersion of the fiber deteriorate the transmission signal. When the transmission signal is deteriorated, the demodulated signal obtained by demodulating the transmission signal by the demodulation circuit is similarly deteriorated. If the demodulated signal is severely degraded, communication becomes impossible. There has been proposed a method capable of demodulating even when such transmission signal degradation occurs. (Here, “transmission signal” and “demodulation signal” are particularly distinguished. “Transmission signal” is an optical signal transmitted through an optical fiber, and is a signal input to a DPSK receiving circuit. "Signal" is an electrical signal obtained by receiving the optical signal demodulated by the demodulation circuit in the receiving circuit by the PD pair.)

  Non-Patent Document 1 proposes a technique called partial DPSK (PDPSK: Partial DPSK). When demodulating a transmission signal that has deteriorated due to band narrowing by a filter or chromatic dispersion of a fiber, the deterioration of the demodulated signal can be reduced by reducing the delay amount of the optical delay interference circuit that constitutes the demodulation circuit.

  FIG. 3 is a schematic diagram for explaining the PDPSK technique. The DPSK signal has information on the phase of light. Therefore, the intensity is always a constant signal. Although the intensity changes once when the phase is switched, it is always 1 at the signal point (time for reading the signal). On the other hand, since information is transmitted by phase, the phase of the light of the transmission signal changes every moment. FIG. 3 shows a case where a DBPSK signal is transmitted. The phase changes between 0 and π, and a transmission signal is sent from the transmitter.

  However, the transmission signal reaching the receiver deteriorates due to signal band narrowing or chromatic dispersion caused by a filter in the transmission path. This signal degradation appears as intensity fluctuations as shown in the transmission signal immediately before the receiver in FIG. When a transmission signal having no intensity fluctuation is demodulated by a conventional DPSK receiving circuit having a 1-bit delay amount, it is demodulated into a 0 or 1 signal according to the phase difference between the preceding and succeeding signals. However, when the intensity of the transmission signal varies, the demodulated signal changes according to the intensity variation. For this reason, the demodulated signal changes with respect to both the phase difference of the transmission signal and the intensity fluctuation, and the demodulated signal is deteriorated.

  Therefore, as shown in FIG. 3, when the delay amount of the optical delay interferometer in the demodulation circuit is appropriately adjusted, the degradation of the demodulated signal can be prevented. By reducing the delay amount, a demodulated signal that does not fluctuate due to fluctuations in the intensity of the transmission signal can be obtained.

  FIG. 4 shows the required OSNR with respect to chromatic dispersion in the transmission path. The OSNR is an optical signal-noise ratio and is a parameter representing the quality of a transmission signal. A larger OSNR indicates a higher quality signal. The required OSNR is an OSNR required to achieve a certain error rate in the transmission system. “Required OSNR is high” indicates that a high-quality transmission signal must be transmitted in order to achieve a certain error rate. Therefore, the lower the required OSNR, the better the transmission state.

  As shown in FIG. 4, when the amount of chromatic dispersion in the transmission path increases, the demodulated signal deteriorates and the required OSNR increases. However, it can be seen that the amount of deterioration differs depending on the delay amount of the optical delay interference circuit of the receiving circuit. In the demodulator A having a 1-bit delay, which is a conventional demodulation method, when the amount of dispersion is small, the required OSNR is low and good transmission is possible. However, as the amount of dispersion increases, the required OSNR rapidly deteriorates.

  On the other hand, when the PDPSK technique, that is, the demodulator C in which the delay amount is reduced to about 0.6 bits is used, the deterioration amount of the required OSNR amount is small even in a transmission line with a large dispersion amount. Therefore, when the amount of dispersion is small, A is the optimal demodulator, and C is the demodulator that enables the best transmission in a transmission line with a large amount of dispersion. Similarly, good signal transmission can be achieved by reducing the amount of delay even when the signal is narrowed by a filter arranged in the transmission path.

  As described above, satisfactory transmission can be realized by optimizing the delay amount of the optical delay interference circuit in accordance with the state of the transmission line, particularly the amount of chromatic dispersion and the amount of signal band narrowing by the filter.

  However, in an actual optical transmission network in which a transmission signal propagates, the amount of chromatic dispersion and the amount of signal narrowing due to a filter change every moment due to changes in the state of the optical fiber due to environmental changes, changes in the transmission path, and the like. Therefore, since the amount of deterioration of the transmission signal due to these changes, a demodulator capable of adjusting the delay amount is necessary for good demodulation. Therefore, the present invention proposes a configuration of a waveguide variable band interference circuit that can realize a good demodulation state according to the state of the transmission line (the amount of chromatic dispersion and the amount of filter narrowing).

  The demodulation method described in Non-Patent Document 1 is to vary the delay amount in accordance with the chromatic dispersion amount of the transmission line and the filter narrowing amount. In this case, if a waveguide type variable delay interference circuit can be realized, a delay interferometer as shown in FIG. As shown in FIG. 5A-2, the transmission spectrum of this delay interferometer becomes a sin function-like spectrum, and the period changes when the delay amount is changed. The delay amount is determined by the variable delay unit 52. For example, in the case of transmission of a DBPSK signal of 40 Gbit / s, since 1 bit is approximately 25 ps, the 1-bit delay is 25 ps. Therefore, the period of the sin function is 40 GHz.

FIG. 5A-2 shows a spectrum when the delay amount is changed in three stages of 1 bit, (1-x) bit, and (1-2x) bit by the variable delay unit 52, and the center f _{0 is shown.} Is the operating frequency of the delay interferometer. This transmission spectrum is the transmission spectrum at the output end on one side in FIG. 5 (a-1), but the transmission spectrum at the other output end is an inversion of this. The inversion is G (f) = 1−F (f) when the transmission spectrum of FIG. 5 (a-1) is a function F (f) of the frequency f. That is, the transmission spectrum at the other output end is G (f). Which of the two output terminals becomes F (f) is equivalent as a demodulator function, and either of them may be used.

  As described above, it is difficult to realize the variable delay interference circuit as described above. This is because it is not easy to change the length of the waveguide freely. Therefore, the present invention proposes a circuit configuration that can easily realize the same function. Specifically, as illustrated in FIG. 5B-1, the circuit configuration includes two types of delay units having different delay amounts and these are connected in series by a branching ratio variable coupler 54. Since the branching ratio variable coupler 54 can be easily realized by a waveguide, such a demodulation circuit can be easily realized. The two delay units have different delay amounts. A method of determining these delay amounts will be described next.

As shown in FIG. 5A-1, consider a case where the delay amount is desired to be changed by X bits, (X-dx) bits, and (X-2 * dx) bits. X is the maximum delay amount, and dx is the step of changing. However, X-2 * dx is a value larger than 0. In this case, the delay amounts Y1 and Y2 of the delay unit in FIG.
Y1 = (X-2 * dx) / 2 (Formula 1)
Y2 = (X + 2 * dx) / 2 (Formula 2)
Given in.

FIG. 5B-2 shows a transmission spectrum of the demodulation circuit when delay amounts are set in Y1 and Y2 given by Expressions 1 and 2. This is the transmission spectrum of one of the two output ends, and the other output end is an inversion thereof. Compared with the spectrum of FIG. 5A-2, a different transmission spectrum is obtained. Since the demodulator configuration is different, the transmission spectrum is also different. However, paying attention only to the frequency near f ₀ , FIGS. 5A-1 and 5B-1 can realize the same spectrum.

What should be noted here is the optical spectrum band of the transmission signal. The optical spectrum of the transmission signal exists only in a limited band centered on f ₀ . Specifically, 80% or more of the signal spectrum intensity exists in a band defined by the period of the delay interference circuit at the time of 1-bit delay, and the region is defined as a transmission signal spectrum band as shown in FIG. As shown in FIG. What is important as a demodulation circuit is the transmission spectrum of the demodulation circuit within this transmission signal spectrum band. Therefore, when compared within the transmission signal spectrum band, the ideal variable delay interference circuit of FIG. 5A-1 and the variable band interference circuit of FIG. Performance can be realized.

As described above, according to the present invention, it is possible to obtain a good demodulated state by adjusting the branch ratio of the branch ratio variable coupler for the transmission signal deteriorated due to the dispersion of the transmission path or the signal narrowing by the filter. Furthermore, by using the configuration of the present invention, it is possible to realize a simple demodulation circuit that can be realized. More specifically, as shown in FIG. 6, phase shifters 62 and 64 are mounted on two delay units to adjust the center frequency f ₀ of the demodulation circuit in accordance with the center frequency of the transmission signal. be able to.

  In addition, as shown in FIG. 7, when the dispersion amount of the transmission line is large or when the narrowing due to the filter is large, by setting the branching ratio of the branching ratio variable coupler to 100%, even a deteriorated transmission signal can be obtained. Therefore, it is possible to convert to a good demodulated signal. Further, when the state of the transmission path is good, good demodulation is possible by setting the branching ratio to 0%. Good demodulation can be realized by changing the branching ratio in accordance with the amount of dispersion or narrowing of the transmission path.

  Here, the maximum delay amount X will be described in more detail. The delay amount may be approximately equal to the signal interval of the multi-level differential phase modulation optical signal, and is not necessarily exactly equal. This is because the multi-level differential phase modulation optical signal is a signal with a finite time width, so even if the delay amount does not exactly match the signal interval, the preceding and following signals can interfere with each other in the demodulation circuit, so that the demodulation operation Is possible. Therefore, in principle, there may be a deviation of the signal width Tw with respect to the signal interval Td. Here, the signal interval Td is a time width corresponding to a signal repetition period, and the signal width Tw is a time width in which the signal strength is not zero. Therefore, the maximum delay amount X may be set as Td−Tw / 2 <X <Td + Tw / 2.

  Up to this point, the variable band interference circuit has been described. FIG. 8 shows a configuration example when this is actually used as a demodulation circuit. FIG. 8A shows a DBPSK demodulation circuit, and FIG. 8B shows a DQPSK demodulation circuit.

  FIG. 9 shows a schematic diagram of an actually manufactured circuit. As the planar lightwave circuit, a quartz glass waveguide deposited on a Si substrate was used. A lower cladding layer and a core layer were deposited on the Si substrate by a flame deposition method, the core layer was formed by photolithography and etching, and an upper cladding layer was again produced by the flame deposition method. Unlike the cladding layer, the core layer is doped with germanium to increase the refractive index and form a waveguide.

  The transmission signal used was a 21.9 Gbit / s DBPSK signal. Assuming a delay amount of three stages of 45 ps corresponding to a 1-bit delay, 37 ps, and 29 ps as a delay amount, in order to realize a filter characteristic equivalent to this delay amount, Equations (1) and (2) are used. The amount of delay was determined. In this case, since X = 45 ps and dx = 8 ps, Y1 and Y2 were 8 ps and 37 ps, respectively.

  In the experiment, an equal-length Mach-Zehnder interference circuit in which the phase shifters 91a and 91b are arranged is used as the branching ratio variable coupler. The branching ratio can be varied from 0 to 100% by driving the phase shifters 91a and 91b. As the couplers 92, 94, 96 and 98 having a fixed branch ratio of 50%, multimode interference couplers (MMI couplers) were used.

Using this variable band interference circuit, a DBPSK signal demodulation experiment was performed. The required OSNR was measured while degrading the transmission signal by chromatic dispersion. The required OSNR is an OSNR when the bit error rate (BER) is 10 ⁻³ . For comparison, the demodulation characteristics of a conventional delay interference circuit (1-bit delay) were also measured. The variable band interference circuit fabricated in this example was adjusted so that the best demodulation characteristics were obtained by adjusting the branching ratio of the branching ratio variable coupler by the phase shifters 91a and 91b.

  The results are shown in FIG. When the chromatic dispersion is increased, the demodulated signal is deteriorated as the transmission signal is deteriorated. Therefore, the required OSNR amount increases. However, it can be seen that the degradation amount is remarkably suppressed in the circuit of the present invention as compared with the conventional delay interference circuit.

  Next, the resistance against the narrowing of the band of the transmission signal was examined by the filter characteristic of the transmission line. In order to narrow the band of the DBPSK signal input to the demodulator, a band pass filter was used. The demodulation characteristics were examined by changing the transmission bandwidth (3 dB band) of the bandpass filter.

  The results are shown in FIG. When the band of the band-pass filter is narrowed to increase the signal constriction, the demodulated signal also deteriorates as the transmission signal deteriorates. Therefore, the required OSNR amount is increased. However, comparing the amount of deterioration with a conventional demodulator, it can be seen that the degree of deterioration is suppressed.

  Here, the isometric Mach-Zehnder interference circuit is used as the branching ratio variable coupler, but the invention is not limited to this, and any coupler having a variable branching ratio may be used. Further, although the MMI coupler is used as the fixed branch ratio coupler, the present invention is not limited to this, and a directional coupler, an X coupler, or the like may be used.

  Thus, with the circuit configuration of the present invention, a demodulator capable of obtaining good demodulation characteristics can be easily realized by using a band variable interference circuit according to the chromatic dispersion amount of the transmission line and the signal narrowing amount by the filter. did it.

  As a demodulator of a DBPSK or DQPSK signal, a very low polarization dependency may be required. Polarization dependence means that the delay amount of the demodulator slightly changes depending on the incident polarization of the transmission signal. Since the refractive index of the silica-based glass waveguide depends on the incident polarization, the optical path length of the waveguide changes depending on the incident polarization. The dependence of the refractive index on the incident polarization occurs between light having an electric field component parallel to the Si substrate (horizontal polarization) and light having a vertical electric field component (vertical polarization). The difference in refractive index that is felt is called birefringence. In order to accurately control the delay amount, it is required to eliminate the polarization dependence caused by birefringence.

  Therefore, FIG. 12 shows a configuration example using a 45-degree half-wave plate in order to eliminate the birefringence of the waveguide. In the figure, 45-degree half-wave plates 122 and 124 operate to switch between horizontal polarization and vertical polarization. Accordingly, when the light is disposed at the center of the waveguide in a certain section, the polarization is switched at the intermediate point, so that the light feels the average refractive index in the section. Accordingly, birefringence can be eliminated.

  Actually, a variable band interference circuit was manufactured by the same method and the same design as in Example 1, and a 45-degree half-wave plate was arranged at the center of the two delay portions as shown in FIG. However, the phase shifter arranged in each delay unit was divided into two and arranged before and after the 45 degree half-wave plate. This is to eliminate unnecessary birefringence generated by arranging the phase shifter. Thereby, the polarization dependence of the delay amount in the first delay unit and the second delay unit is eliminated.

  While the present invention has been described with respect to several specific embodiments, the embodiments described herein are merely illustrative in view of the many possible embodiments to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. The configuration and details of the embodiment exemplified here can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

DESCRIPTION OF SYMBOLS 1 Optical delay interference circuit 2 Input waveguide 3 Optical splitter 4 Long arm waveguide 5 Short arm waveguide 6, 7 Output waveguide 8 Photodiode pair 9 Optical delay part 10 Optical coupler 12 pi / 2 phase transition part 23 Light Splitter 52 Variable delay unit 54 Branch ratio variable coupler 62, 64 Phase shifter 91 Phase shifter 92, 94, 96, 98 Fixed branch ratio coupler 122, 124 45 degree half-wave plate

Claims (7)

A receiver that demodulates a multilevel differential phase modulated optical signal,
An optical splitter for branching the optical signal;
A first delay unit configured by waveguides having different lengths, which gives a delay of a first delay amount between the optical signals branched by the optical splitter;
A first coupler having a variable branching ratio that couples an optical signal to which a delay of a first delay amount is given by the first delay unit;
A second delay unit configured by waveguides having different lengths, which gives a delay of a second delay amount different from the first delay amount between optical signals output from the first coupler;
A receiver comprising: a second coupler that interferes with an optical signal to which a delay of a second delay amount is given by the second delay unit. The receiver according to claim 1, wherein
The receiver according to claim 1, wherein the sum of the first delay amount and the second delay amount is set to be substantially equal to a signal interval of the optical signal. The receiver according to claim 1 or 2,
Each of the first and second delay units includes a phase shifter disposed in one waveguide. The receiver according to any one of claims 1 to 3,
The receiver, wherein the second coupler has two outputs, and a light receiving element is connected to each output. The receiver according to any one of claims 1 to 4,
The first coupler comprises a isometric Mach-Zehnder interference circuit having a phase shifter in each arm waveguide, and each arm waveguide has a phase shifter. The receiver according to any one of claims 1 to 5,
At least one of the first delay unit and the second delay unit includes a 45-degree half-wave plate that traverses the waveguides having different lengths. The receiver according to any one of claims 1 to 6,
The receiver, wherein the optical signal is a DBPSK or DQPSK signal.

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