JP4413903B2 - Optical receiver for multilevel phase modulation signal - Google Patents

Description

The present invention relates to an optical receiver that receives and decodes an optical differential phase shift keying signal obtained by multiplexing M (M = 2 ⁿ : n is a natural number) codes, and in particular, an interferometer provided in the optical receiver. The present invention relates to an optical receiver that can best control the relationship between the optical characteristics of the optical signal and the center frequency of signal light.

  Conventionally, as a transmission method used in an optical transmission system, a DPSK (Differential Phase Shift Keying) method with high nonlinear tolerance has been studied, and when receiving an optical signal modulated with DPSK (hereinafter referred to as a DPSK signal), An optical interferometer is used to convert phase modulation into intensity modulation for direct detection. A system that differentially receives light with a double balanced receiver using a Mach-Zehnder interferometer (MZI) as an optical interferometer is mainly used.

  By the way, in order to accurately demodulate the DPSK signal, it is necessary to control the relationship between the wavelength of the interference peak in MZI and the center wavelength of the transmission signal (DPSK signal) to be constant. When the wavelength drifts or the environmental temperature of the MZI fluctuates and a shift occurs between the two, it is necessary to detect the shift quickly and perform correction control.

  On the other hand, in order to control the wavelength at the interference peak of MZI, there is one in which heating is performed by a heater provided in the arm (see Patent Document 1), and the center wavelength of the DPSK signal and the interference peak of MZI are There is one that is to be tracked (see Patent Document 2).

  Here, with reference to FIG. 6, the optical receiver described in Patent Document 2 will be outlined. Here, in the DPSK signal, the optical phase between adjacent bits is 0 or π, and in FIG. 6, the vertical axis indicates the signal intensity after differential light reception in each case (that is, when the optical phase is 0 or π). The horizontal axis indicates the phase difference between the two waveguides provided in the MZI. Actually, a delay time of 1 bit is added to the illustrated phase difference, but it is omitted in the following description for convenience.

  When ΔφM = 0, as shown in the figure, the eye opening of the received signal is the best, and the absolute value of the signal intensity after the differential light reception is a maximum value of 1. Therefore, when the absolute value of the signal intensity decreases from 1, it can be determined that ΔφM has deviated from the optimum value, but it cannot be determined whether ΔφM is insufficient or excessive. For this reason, in the optical receiver described in Patent Document 2, ΔφM is slightly increased or decreased by a dither signal. As a result, in a region where ΔφM is insufficient, if ΔφM is slightly increased, the absolute value of the signal intensity after differential light reception is slightly increased, and if ΔφM is slightly decreased, the absolute value of the signal intensity is slightly decreased. .

On the other hand, in the region where ΔφM is excessive, this relationship is reversed. When ΔφM is slightly increased, the absolute value of the signal strength is slightly decreased, and when ΔφM is slightly decreased, the absolute value of the signal strength is slightly increased. . Therefore, it is possible to determine whether ΔφM should be increased or decreased by referring to the phase of the dither signal and the change in the absolute value of the signal intensity. If the heater for heating the MZI optical waveguide is controlled according to the determination result, the best reception state can be adjusted.
JP-A-63-52530 International Publication Number WO2005 / 088886

  However, if the above-described optical receiver is used for a multi-level differential phase shift keying (DMPSK) signal that multiplexes four or more values, it becomes difficult to receive the DMPSK signal. For example, a DQPSK signal of M = 4 is obtained by multiplexing an I component binary signal and a Q component binary signal, and (I component, Q component) = (1, 1), (0, 1). , (0,0), (1,0), the optical phases between four adjacent bits of 0, π / 2, π, 3π / 2 correspond (I component, Q The combination of the component and the optical phase may be other combinations). Here, the signal intensity after differential light reception in this DQPSK signal is shown in FIG.

  In FIG. 7, the eye opening is the best when ΔφM = π / 4 and −π / 4. However, when the above-described optical receiver is used, the operating point at which the signal amplitude is maximum is optimal. Thus, when ΔφM = 0 or π (π / 2 or −π / 2), the phase difference between adjacent bits is 0 and π (π / 2 and 3π / 2) and the amplitude of the signal is maximized. Become. However, the signal intensity when ΔφM = π / 2 or 3π / 2 (0 or π) falls to 0, and the eye does not open normally. That is, the conventional optical receiver has a problem that it is difficult to receive a DQPSK signal having four or more values.

  An object of the present invention is to provide an optical receiver capable of always receiving a DMPSK signal satisfactorily.

In order to solve the above-described problems, the present invention is directed to use an optical interferometer to convert an optical differential phase shift keying signal having M values (M = 2 ⁿ , n is an integer of 1 or more) into an intensity-modulated signal. In an optical receiving apparatus that receives the signal after conversion into an average level detecting means for monitoring an absolute value of a high level or a low level of the intensity modulation signal over a time Tm longer than a symbol period and obtaining an average value of the signal level, the signal Delay time adjusting means for adjusting a delay time difference between the optical paths of the optical interferometer so that the average value of the levels is maximized.

  For example, the average level detecting means arbitrarily selects whether to monitor the absolute value of only the high level of the intensity modulation signal or only the absolute value of the low level of the intensity modulation signal by an external switch operation. Is possible.

  Further, for example, the delay time adjusting means selectively selects dithering means for dithering the wavelength of the interference peak of the optical interferometer with a dither signal having a period Td, and a fluctuation component having a period Td that the average value of the signal levels has. Extraction means for extracting, phase comparison means for comparing the phase of the output of the extraction means and the dither signal, and increasing or decreasing the delay time difference between the optical paths of the optical interferometer based on the result of the phase comparison Thus, the average value of the signal levels is adjusted to be maximum.

Further, the present invention converts light received by converting an optical differential phase shift keying signal multiplexed with M values (M = 2 ⁿ , n is an integer of 1 or more) into an intensity modulation signal using an optical interferometer. In the receiving apparatus, maximum level detecting means for monitoring the absolute value of the high level or low level of the intensity modulation signal over a time Tm longer than the symbol period to obtain the maximum value of the signal level, and the maximum value of the signal level being minimum And a delay time adjusting means for adjusting a delay time difference between the optical paths of the optical interferometer.

  For example, the maximum level detecting means arbitrarily selects whether to monitor the absolute value of only the high level of the intensity modulation signal or to monitor the absolute value of only the low level of the intensity modulation signal by an external switch operation. Is possible.

  Further, for example, the delay time adjusting means selectively selects dithering means for dithering the wavelength of the interference peak of the optical interferometer with a dither signal having a period Td, and a fluctuation component having a period Td that the maximum value of the signal level has. Extraction means for extracting, phase comparison means for comparing the phase of the output of the extraction means and the dither signal, and increasing or decreasing the delay time difference between the optical paths of the optical interferometer based on the result of the phase comparison Thus, the maximum value of the signal level is adjusted to be minimum.

Further, the present invention converts light received by converting an optical differential phase shift keying signal multiplexed with M values (M = 2 ⁿ , n is an integer of 1 or more) into an intensity modulation signal using an optical interferometer. In the receiving apparatus, minimum level detecting means for monitoring the absolute value of the high level or low level of the intensity modulation signal over a time Tm longer than a symbol period to obtain a minimum value of the signal level, and the minimum value of the signal level being maximum And a delay time adjusting means for adjusting a delay time difference between the optical paths of the optical interferometer.

  For example, the minimum level detecting means arbitrarily selects whether to monitor only the absolute value of only the high level of the intensity modulation signal or to monitor the absolute value of only the low level of the intensity modulation signal by an external switch operation. Is possible.

  Further, for example, the delay time adjusting means selectively selects dithering means for dithering the wavelength of the interference peak of the optical interferometer with a dither signal having a period Td, and a fluctuation component having a period Td that the minimum value of the signal level has. Extraction means for extracting, phase comparison means for comparing the phase of the output of the extraction means and the dither signal, and increasing or decreasing the delay time difference between the optical paths of the optical interferometer based on the result of the phase comparison Thus, the minimum value of the signal level is adjusted to be the maximum.

  When the symbol period of the optical differential phase shift keying signal in which the M codes are multiplexed is Ts, it is preferable that the period Td and the time Tm satisfy Ts × M <Tm <Td / 2. .

  In the present invention, the delay time adjusting means is configured to change the optical path length of the optical waveguide by controlling the heating of the optical waveguide of the optical interferometer. At this time, the optical path length is defined in advance. Change at different intervals. For example, the dithering means repeatedly heats and cools at least one optical waveguide of the optical interferometer at a period Td.

  As described above, in the present invention, the average value, the maximum value, or the minimum value of the amplitude is obtained according to the intensity modulation signal, and the average value is maximized, the maximum value is minimized, or the minimum value is obtained. Since the delay time of the optical interferometer is adjusted so as to maximize the optical interferometer, the optical differential phase shift keying signal in which M codes are multiplexed is expressed by the optical characteristics of the optical interferometer and the center frequency of the optical signal. Thus, there is an effect that the optical differential phase shift keying signal can always be satisfactorily received.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an optical receiver 10 according to an embodiment of the present invention. In FIG. 1, only an I component demodulator is shown for the sake of simplicity. The DMPSK signal is received as a received light signal, and the received light signal is branched into two by a branching means such as an optical coupler, and supplied to an I component demodulator 11 and a Q component demodulator (not shown). Since the operations of the I component demodulator 11 and the Q component demodulator are the same, the I component demodulator 11 will be described below.

  In FIG. 1, an I component demodulator 11 includes an MZI 21, a balanced light receiver (double balanced receiver) 22, an eye opening monitor unit 23, a statistical processing unit 24, a band pass filter (BPF) 25, a synchronous detection unit 26, and an oscillator. (Oscillation period = Td) 27 and a controller 28. In the illustrated example, the MZI 21 has first and second optical waveguides 21a and 21b. The first and second waveguides 21a and 21b are respectively provided with first and second heaters 29a and 29b, and the first waveguide 21a is further provided with a delay element 30 (delay time = τ). It has been.

  The received light signal applied to the I component demodulator 11 is converted from a phase modulation signal to an intensity modulation signal in the MZI 21. The operating point of the delay difference (optical path length) in the MZI 21 is performed at ΔφM = π / 4 shown in FIG. 7 (that is, the detection of the I component is performed at ΔφM = π / 4). The Q component is detected at ΔφM = −π / 4 shown in FIG. Therefore, at the operating point of ΔφM = π / 4, when the I component is 1 (phase difference between adjacent bits = 0 or 3π / 2), the signal strength is +0.7 and the I component is 0 (between adjacent bits). When the phase difference = π / 2 or π), the signal intensity becomes −0.7, and the I component is detected. On the other hand, at the operating point of ΔφM = −π / 4, when the Q component is 1 (phase difference between adjacent bits = 0 or π / 2), the signal strength is +0.7 and the Q component is 0 (between adjacent bits). When the phase difference = 3π / 2 or π), the signal intensity becomes −0.7, and the Q component is detected.

  The optical path lengths of the first and second optical waveguides 21a and 21b are controlled by the first and second heaters 29a and 29b, respectively. In the illustrated example, an oscillation signal (dither signal) having a period Td is given from the oscillator 27 to the second heater 29b, and heating and cooling are repeated at the period Td. As a result, the received light signal passing through the second optical waveguide 21b is modulated with a period Td (that is, dithered). On the other hand, a correction signal is given to the first heater 29a from the controller 28, and the optical path length of the first optical waveguide 21a is corrected as will be described later.

  The intensity modulation signal output from the MZI 21 is given to a balanced light receiver (double balanced receiver) 22 that photoelectrically converts the intensity modulation signal into an electric signal and uses the difference between the electric signals as a demodulated signal. Output. This demodulated signal is given to the eye opening monitor unit 23. The eye opening monitor unit 23 measures the amplitude of the eye opening according to the demodulated signal (demodulated waveform), and outputs the measurement result to the statistical processing unit 24 as an amplitude measurement signal. The statistical processing unit 24 statistically processes the amplitude of the eye opening based on the amplitude measurement signal. As will be described later, the statistical processing result is a voltage signal proportional to the maximum value, minimum value, or average value of the amplitude of the eye pattern.

  The output (voltage signal) of the statistical processing unit 24 fluctuates in the period Td due to the influence of the dither signal, but when the statistical processing result takes an extreme value, the component of the period Td disappears and the 2 × Td period Ingredients appear. From this voltage signal, only the frequency 1 / Td component is extracted by the BPF 25 and given to the synchronous detector 26. Then, the synchronous detection unit 26 synchronously detects the output signal of the BPF 25 by the dither signal supplied from the oscillator 27 and supplies the detection signal to the controller 28. The controller 28 determines the correction amount and the correction direction of the delay time in the first optical waveguide 21a based on the detection signal, and gives the correction signal to the first heater 29a so that the eye opening is maximized. The delay time in one optical waveguide 21a is corrected.

  By the way, when the DQPSK signal is differentially received by the MZI and the double balanced receiver and the I component is demodulated, the optimum value of the delay time ΔφM of the MZI is 0.25π as shown in FIG. However, the eye pattern after differential light reception is 0.7 at the high level and -0.7 at the low level. Therefore, the amplitude of the eye pattern after differential light reception is 0.7 × 2 = 1.4.

  Now, assuming that ΔφM increases by 0.15π to 0.4π due to fluctuations in the wavelength of the received light signal or fluctuations in the environmental temperature of the MZI, the eye pattern after differential light reception is at a high level and a low level. Each of the two splits into four values of +0.95 (P1) and +0.3 (P2) at the high level and -0.95 (P4) and -0.3 (P3) at the low level. The eye pattern amplitude is 0.95 × 2 = 1.9 when it is large, and 0.3 × 2 = 0.6 when it is small. Whether the amplitude of the eye pattern takes a large or small value is determined by whether the optical phase difference between adjacent bits is 0, π / 2, π, or 3π / 2. The eye pattern amplitude is randomly selected. Moreover, it fluctuates at high speed.

  The waveform obtained by the experiment is shown in FIG. 2 (a) and 2 (b) are diagrams showing the amplitude of the eye pattern when ΔφM = 0.25π and 0.25π <ΔφM <0.5π, respectively. In FIGS. 2 (a) and 2 (b), The horizontal axis represents the phase difference (Δφbit) between the bits, and the vertical axis represents the amplitude. As can be understood from FIGS. 2 (a) and 2 (b) and FIG. 7, when the eye opening is the best, the larger amplitude This is when the maximum value (difference when the optical phase difference between adjacent bits is 0 and π) becomes maximum.

  Subsequently, considering the average value of the amplitude of the eye pattern, from FIG. 7, when ΔφM = 0.25π, the average value of the amplitude of the eye pattern is 0.7 × 2 = 1.4, and ΔφM = 0.4π. Then, the average value of the high level is (0.95 + 0.3) /2=0.63, and the average value of the amplitude is 0.63 × 2 = 1.26 since it is also symmetric at the low level. Here, the result of expressing the average value of the amplitude as a function of ΔφM is shown in FIG.

As shown in FIG. 3, ΔφM at which the average value of the amplitude is maximum is 0.25π, which corresponds to ΔφM at which the eye opening is maximum. From the above description, one of the following conditions is necessary to maximize the eye opening.
(1) ΔφM is controlled so that the minimum value of the amplitude of the eye pattern is maximized.
(2) ΔφM is controlled so as to minimize the maximum value of the eye pattern amplitude.
(3) ΔφM is controlled so as to maximize the average value of the eye pattern amplitude.
The condition (3) can also be used in common with DPSK.

  In the eye pattern, whether the high level and the low level are split in the high direction or the low direction is determined by the sign of the transmission signal. Become. Therefore, in order to obtain the maximum value, the minimum value, or the average value, it is necessary to sample at least M bits (in order to ensure accuracy, it is desirable to increase the number of sampling bits).

  By the way, when ΔφM is not the optimum value, it is necessary to determine whether ΔφM should be decreased or increased. For example, as shown in FIG. 7, when ΔφM is excessive and ΔφM = 0.4π, the eye pattern is +0.95 (P1) and +0.3 (P2) at the high level, and −0.95 (P4) at the low level. ) And −0.3 (P3), but when ΔφM is insufficient and ΔφM = 0.1π, the eye pattern is at high level +0.95 (P1) and +0.3 (P2). At the low level, there are four values of -0.95 (P4) and -0.3 (P3). Although both can be determined to be optimal values, it cannot be determined whether ΔφM is excessive or insufficient. Therefore, in order to clarify the direction of control, dithering is also used as described above.

  That is, whether the variation in the dither signal and the variation in the minimum value of the amplitude of the eye pattern are in phase or in phase corresponds to whether ΔφM is insufficient or excessive. The same applies to the maximum value or average value of the amplitude of the eye pattern. Therefore, it is possible to determine whether ΔφM is insufficient or excessive based on the phase comparison result between the two.

  Note that the time required for the sampling needs to be shorter than the cycle of the dither signal, and for the phase comparison between them, the sampling and averaging process may be completed in at most half of the cycle of the dither signal. desirable. That is, when the period of the dither signal is Td, the time for obtaining the maximum value, the minimum value, or the average value is Tm, the symbol period is Ts, and the multiplexing number is M, Ts × M <Tm <Td / 2 is satisfied. desirable.

  In the eye pattern monitoring, it may be possible to arbitrarily select whether to monitor only the high level amplitude or only the low level amplitude. That is, when obtaining the average value, maximum value, or minimum value of the amplitude of the intensity modulation signal, the high level or the low level is selectively switched by operating an external switch. Also good. As described in the next paragraph, by adopting this configuration, the interferometer can be correctly controlled even when the waveform is degraded due to the group velocity dispersion.

  Next, a case where there is group velocity dispersion in the transmission path and the received signal has undergone waveform deterioration will be described. In such a case, the demodulated waveform becomes asymmetric between the high level and the low level due to the combined effect of the waveform degradation on the transmission line and the nonlinearity of MZI. FIG. 8 shows a demodulation waveform when the group velocity dispersion of the transmission path is +75 ps / nm. The ΔZM of MZI is adjusted so that the eye opening is maximized, but on the low level side, the pulse expands in the time axis direction and the amplitude decreases. Due to the symmetry of MZI, the eye opening is maximized even when ΔφM is increased by π. In this case, the high-level pulse expands in the time axis direction and the peak value decreases. For this reason, the transition of the amplitude of the eye pattern is more complicated than when the group velocity dispersion is 0, which causes an error in the control signal.

In order to avoid this problem, only one of the high level and the low level needs to be monitored. As described above, the high level and the low level of the demodulated waveform are split, but the high level most deviating from the GND level is H1, the high level closest to the GND level is H2, and the low level most deviating from the GND level is L1, GND. When the low level closest to the level is defined as L2, the MZI may be controlled to satisfy one of the following conditions.
(1 ′) ΔφM is controlled so that the difference between H2 and the GND level is maximized.
(2 ′) ΔφM is controlled so that the difference between H1 and the GND level is minimized.
(3 ′) ΔφM is controlled so that the difference between the average value of the high level and the GND level is maximized.
(4 ′) ΔφM is controlled so that the difference between L2 and the GND level is maximized.
(5 ′) ΔφM is controlled so that the difference between L1 and the GND level is minimized.
(6 ′) ΔφM is controlled so that the difference between the average value of the low level and the GND level is maximized.

  By the way, when it controls so that one of the conditions of said (1 ')-(6') may be satisfy | filled, some of several eye opening points may not be detected. For example, in the case of a DQPSK signal, there are four types of eye opening points: I component, Q component, inversion of I component, and inversion of Q component, but two of them may not be detected. However, due to the periodicity of MZI, the two detectable eye opening points always include two multiplexed signal components.

  FIG. 4 shows an example in which a sampling oscilloscope 31 is used as the eye opening monitor unit 23 and the statistical processing unit 24. The amplitude of the eye pattern according to the demodulated signal (demodulated waveform) for a predetermined time using the sampling oscilloscope 31. The maximum value and the minimum value of are detected. The demodulated waveform is supplied to the clock extraction system 32, and the clock extraction system 32 extracts the clock signal from the demodulated waveform and provides it as a trigger signal for the sampling oscilloscope 31.

  FIG. 5 is a block diagram showing another example of the I component demodulator in the optical receiver according to the present invention. In FIG. 5, the same components as those in FIG. 1 are given the same reference numerals. The I component demodulator 40 shown in FIG. 5 has a limiter amplifier 41, a smoothing circuit 42, a resistor 43, and a power supply 44. An output signal from the balanced light receiver 22 is given to the limiter amplifier 41, After being amplified by the limiter amplifier 41, it is output as a demodulated signal.

  In the illustrated I component demodulator 40, the amplitude of the eye pattern is monitored by the magnitude of the drive current of the limiter amplifier 41. As described above, in the DMPSK, the high level and low level of the eye pattern are respectively set. When split, there are two types of eye pattern amplitude. Since whether to take a large amplitude or a small amplitude changes randomly at the same speed as the symbol rate of the signal, here, as shown in the figure, the smoothing circuit 42 smoothes high-speed fluctuation components and averages the amplitudes. The value is given to the synchronous detector 26 via the BPF 25.

  Since it is necessary to monitor fluctuations due to the dither signal, the response speed of the smoothing circuit 42 needs to be sufficiently faster than the cycle Td of the dither signal.

  Thus, in this embodiment, the average value, maximum value, or minimum value of the amplitude of the intensity modulation signal (demodulation waveform) is obtained, and the average value is maximized or the maximum value is minimized, Alternatively, since the delay time of the MZI is adjusted so as to maximize the minimum value, an optical differential phase shift keying signal in which M codes are multiplexed can be obtained by combining the optical characteristics of the MZI and the center frequency of the optical signal. The effect is that the relationship can always be kept in the best condition.

  As is clear from the above description, in this embodiment, the eye opening monitor unit 23, the statistical processing unit 24, and the oscillator 27 function as an average value acquisition unit, a maximum value acquisition unit, or a minimum value acquisition unit. , BPF 25, synchronous detector 26, controller 28, and heaters 29a and 29b function as delay time adjusting means.

  An optical interferometer that obtains an average value, maximum value, or minimum value of the amplitude according to the intensity modulation signal, and maximizes the average value, minimizes the maximum value, or maximizes the minimum value. Since the delay time is adjusted, the optical differential phase shift keying signal in which M codes are multiplexed always maintains the best relationship between the optical characteristics of the optical interferometer and the center frequency of the optical signal. As a result, it can be applied to an optical receiver that receives an optical differential phase shift keying signal.

It is a block diagram which shows the structure of the I component demodulator used with the optical receiver by embodiment of this invention. It is a figure which shows the amplitude change of the eye pattern after differential light reception, (a) is a figure which shows the case of delay time (phase difference) (DELTA) (phi) M = 0.25 (pi), (b) is 0.25 (pi) <(DELTA) phiM <0.5 (pi). It is a figure which shows the case of. It is a figure which shows the result which represented the average value of the amplitude of an eye pattern as a function of delay time (DELTA) phiM. It is a block diagram which shows an example of I component demodulator used with the optical receiver by embodiment of this invention. It is a block diagram which shows the other example of I component demodulator used with the optical receiver by embodiment of this invention. It is a figure which shows the relationship between the signal strength in a DPSK signal, and the phase difference between waveguides. It is a figure which shows the relationship between the signal strength in the optical differential phase shift keying signal with which the M code | symbol was multiplexed, and the phase difference between waveguides. It is a figure which shows a demodulation waveform in case group velocity dispersion | distribution of a transmission line is + 75ps / nm.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Optical receiver 11, 40 I component demodulator 21 MZI 22 Balance type light receiver 23 Eye opening monitor part 24 Statistical processing part 25 Band pass filter 26 Synchronous detection part 27 Oscillator 28 Controller 21a, 21b Optical waveguide 29a, 29b Heater 30 Delay Element 31 Sampling oscilloscope 32 Clock extraction system 41 Limiter amplifier 42 Smoothing circuit 43 Resistance 44 Power supply

Claims (12)

In an optical receiving apparatus that receives and converts an optical differential phase shift keying signal multiplexed with M values (M = 2n, n is an integer of 2 or more) into an intensity modulation signal using an optical interferometer,
Average level detection means for monitoring an absolute value of only one of the high level and low level of the intensity modulation signal over a time Tm longer than a symbol period to obtain an average value of the signal level;
An optical receiver for multi-level phase modulation signal, comprising delay time adjusting means for adjusting a delay time difference between optical paths of the optical interferometer so that an average value of the signal level is maximized.   The average level detection means can arbitrarily select whether to monitor the absolute value of only the high level of the intensity modulation signal or only the absolute value of the low level of the intensity modulation signal by an external switch operation. The multi-level phase modulation signal optical receiver according to claim 1, wherein:   The delay time adjusting means is a dithering means for dithering the wavelength of the interference peak of the optical interferometer with a dither signal having a period Td, and an extracting means for selectively extracting a fluctuation component having a period Td that the average value of the signal levels has. And phase comparison means for performing phase comparison between the output of the extraction means and the dither signal, and increasing or decreasing a delay time difference between optical paths of the optical interferometer based on the result of the phase comparison 3. The optical receiver for multi-level phase modulation signals according to claim 1, wherein the average value of the signal level is adjusted to be maximum. In an optical receiving apparatus that receives and converts an optical differential phase shift keying signal multiplexed with M values (M = 2n, n is an integer of 2 or more) into an intensity modulation signal using an optical interferometer,
Maximum level detecting means for monitoring the absolute value of only one of the high level and low level of the intensity modulation signal over a time Tm longer than the symbol period to obtain the maximum value of the signal level;
An optical receiver for multilevel phase modulation signals, comprising delay time adjusting means for adjusting a delay time difference between optical paths of the optical interferometer so that the maximum value of the signal level is minimized.   The maximum level detection means can arbitrarily select whether to monitor the absolute value of only the high level of the intensity modulation signal or only the absolute value of the low level of the intensity modulation signal by an external switch operation. 5. The multi-level phase modulation signal optical receiver according to claim 4, wherein   The delay time adjusting means is a dithering means for dithering the wavelength of the interference peak of the optical interferometer with a dither signal having a period Td, and an extracting means for selectively extracting a fluctuation component of the period Td that the maximum value of the signal level has. And phase comparison means for performing phase comparison between the output of the extraction means and the dither signal, and increasing or decreasing a delay time difference between optical paths of the optical interferometer based on the result of the phase comparison 6. The multi-level phase modulation signal optical receiver according to claim 4, wherein adjustment is performed so that the maximum value of the signal level is minimized. In an optical receiving apparatus that receives and converts an optical differential phase shift keying signal multiplexed with M values (M = 2n, n is an integer of 2 or more) into an intensity modulation signal using an optical interferometer,
Minimum level detecting means for monitoring the absolute value of only one of the high level and low level of the intensity modulation signal over a time Tm longer than the symbol period to obtain a minimum value of the signal level;
An optical receiver for multi-level phase modulation signal, comprising delay time adjusting means for adjusting a delay time difference between optical paths of the optical interferometer so that the minimum value of the signal level becomes maximum.   The minimum level detection means can arbitrarily select whether to monitor the absolute value of only the high level of the intensity modulation signal or only the absolute value of the low level of the intensity modulation signal by an external switch operation. The optical receiver for multi-level phase modulation signal according to claim 7, wherein   The delay time adjusting means is a dithering means for dithering the wavelength of the interference peak of the optical interferometer with a dither signal having a period Td, and an extracting means for selectively extracting a fluctuation component of the period Td that the minimum value of the signal level has. And phase comparison means for performing phase comparison between the output of the extraction means and the dither signal, and increasing or decreasing a delay time difference between optical paths of the optical interferometer based on the result of the phase comparison 9. The optical receiver for multi-level phase modulation signals according to claim 7 or 8, wherein the minimum value of the signal level is adjusted to be maximum.   The period Td and time Tm satisfy Ts × M <Tm <Td / 2, where Ts is a symbol period of the optical differential phase shift keying signal in which the M codes are multiplexed. Item 10. The multi-level phase modulation signal optical receiver according to any one of Items 3, 6, and 9.   11. The multiple delay unit according to claim 1, wherein the delay time adjusting unit changes the optical path length of the optical waveguide by heating and controlling the optical waveguide of the optical interferometer. Optical receiver for value phase modulation signal.   12. The optical receiver for multi-level phase modulation signals according to claim 11, wherein the dithering means repeats heating and cooling at least one optical waveguide of the optical interferometer with a period Td.

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