Abstract:
An optical apparatus comprising: a branching unit branching an input light modulated by DQSPK format and thereby outputting a first branched light and a second branched light; a first branch and a second branch inputting the first branched light and the second branched light, respectively, the first branch and the second branch having an interferometer, a photo detector, and discriminator and demodulating I-signal and Q-signal, respectively; and an abnormality detection unit detecting an abnormality of the input light based on a synchronized detection of a first demodulated signal output from the photo detector in the first branch and a first recovered signal output from the discriminator in the first branch, and a synchronized detection of a second demodulated signal output from the photo detector in the second branch and a second recovered signal output from the discriminator in the second branch.

Description:
[0001]    The present invention claims foreign priority to Japanese application 2007-088805, filed on Mar. 29, 2007, which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an optical DQPSK (Differential Quadrature Phase Shift Keying) receiver and a control method of abnormality detection in the optical DQPSK receiver. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    In optical communication systems where the transmission capacity thereof has been rapidly increasing, a binary amplitude shift keying (also called as OOK: On-Off Keying) is mainly used as a way of a modulation format, which includes NRZ (Non-Return-to-Zero) modulation format or RZ (Return-to-Zero) modulation format. 
         [0004]    Recently, some other modulation/demodulation schemes have been utilized in optical communication systems, such as a Duo-Binary modulation, a CSRZ (Carrier-Suppressed Return-to-Zero) modulation, a DPSK (Differential Phase Shift Keying) modulation. 
         [0005]    In the DPSK modulation, data is modulated to a phase shift between two symbols adjacent to each other. In a binary DPSK modulation which utilizes two phase shifts, the phase shifts are 0 or π. A DPSK modulation utilizing four phase shifts of 0, π/2, π, and 3π/2 is called as DQPSK (Differential Quadrature Phase Shift Keying) modulation. 
         [0006]    Comparing with the conventional OOK modulation, DPSK modulation allows to improve an optical S/N ratio (OSNR: Optical Signal-to-Noise Ratio) by about 3 dB and to enhance an optical signal&#39;s resistance against nonlinear effects. 
         [0007]    Since optical DQPSK utilizes a quaternary symbol, a spectral efficiency of the transmission is doubled to the OOK. That eases demands for speeds of electric devices, requirements for chromatic dispersion compensation, or requirements for polarization mode dispersion. Thus, optical DQPSK is a promising candidate of next generation optical communication systems. 
         [0008]    A typical optical DQPSK receiver includes a set of Mach-Zehnder interferometers corresponding to an I-branch and Q-branch of DQPSK demodulation, as shown in “Optical Differential Quardrature Phase-Shift Key (ODQPSK) for High capacity Optical Transmission” by R. A. Griffin et al., Optical Fiber communication Conference and Exhibit, 2002. OFP2002 17-22 Mar. 2002 Pages 67-368. Each Mach-Zehnder interferometer includes an optical delay element τ which corresponds to a symbol time period in a transmission system. 
         [0009]    The optical phase difference between the branches of an interferometer is set to be π/4 in the I-branch and −π/4 in the Q-branch. The two output terminals of each interferometer are connected to a balanced photo detector for regenerating transmitted data. 
         [0010]    The configuration and operation of an optical DQPSK transmitter and receiver are also described in JP2004-516743 or WO2002/051041, for example. 
         [0011]      FIG. 1  shows a configuration of a network of optical DQPSK using RZ format. 
         [0012]    In  FIG. 1 , a narrow-band optical transmitter  1  inputs 21.5 Gbps of data  1  and data  2  from a DQPSK precoder  100  and thereby inputs the data into phase-shift modulators  120 A,  120 B respectively. 
         [0013]    The phase-shift modulator  120 A modulates a light from a light source  110  into a signal light with optical phases of 0 (rad) or π (rad), according to the data  1 . The phase-shift modulator  120 B delays the light from the light source  110  by π/2 (rad) and modulates it into a signal light with optical phases of π/2 (rad) or 3π/2 (rad), according to the data  2 . 
         [0014]    Signal lights output from the phase-shift modulators  120 A and  120 B are multiplexed to a DQPSK signal of 43 Gbps and the DQPSK signal is modulated with RZ intensity-modulation at an intensity modulator  130  into a RZ-DQPSK modulated signal and transmitted onto a network (optical transmission line)  3 , as an optical phase modulated signal. 
         [0015]    The optical signal output from the optical transmitter  1  propagates the optical transmission line (network)  3 , which comprises a WDM (Wavelength Division Multiplexing) network. The network  3  includes a WDM multiplexing (WDM MUX) circuit  4 , an optical amplifier AMP, and a demultiplexer  5  for WDM light, at some middle points. 
         [0016]    The signal/noise ratio of the optical signal is degraded during amplification in the optical amplifier (AMP). Also, amount of chromatic dispersion of the optical signal increases in a long distance transmission by optical fiber. In order to compensate the chromatic dispersion, a dispersion compensator  6  is disposed in a fore-stage of the narrow-band optical receiver  2 . 
         [0017]    The narrow-band optical receiver  2  includes delay interferometers  200 A and  200 B, photodetectors (TWIN-PD)  210 A and  210 B, equivalent amplifiers  211 A and  211 B, discrimination circuits (clock and data recovery circuits; CDR)  220 A and  220 B, decoder  230 , and an optical phase control circuit  240 . Input optical signal are branched to delay interferometer  200 A and  200 B. In the delay interferometers  200 A and  200 B, input signals are further branched to two and one of the branched optical signal is shifted by 1-bit earlier and interferes with the other branched optical signal, of which optical phase is delayed by π/4 (rad), or −π/4 (rad), respectively. In the balanced photo detectors (TWIN-PD)  210 A and  210 B, the optical phase modulated signals output from the delay interferometer  200 A and  200 B, respectively, are subjected to differential optical/electrical conversion and converted electrical signals are subjected to an equivalent amplification in the equivalent amplifier  211 A and  211 B, respectively. 
         [0018]    In the CDR  220 A and  220 B, the electrical signals output from the equivalent amplifier are converted to an I-channel signal and a Q-channel signal, respectively, and the CDR  220 A and  220 B work as the data recovery circuit. In decoder  230 , the I-channel signal and the Q-channel signal are subjected to a bit swap logic inversion processing, which corresponds to the processing of the DQPSK precoder  100  in the optical transmitter  100 . 
         [0019]    In the receiver  2  of  FIG. 1 , it is important to keep the optical phase difference accurately at π/4 (rad) and −π/4 (rad) between the branches of the delay interferometer  200 A and  200 B, respectively. If the optical phase difference of the delay interferometer  200 A or  200 B becomes otherwise, waveform distortion of the output signal exceeds an allowable range. To keep the accurate optical phase difference, a feedback control is performed by an optical phase control circuit  240 . 
         [0020]    The optical phase control circuit  240  monitors a phase error detected in the receiver  2  and generates a phase adjustment signal that adjusts the phase of the interferometers so that the phase differences are maintained at a target value. 
         [0021]    A typical feedback control method is known as a dither-peak-detection method. In this method, the phase shift added to the optical signal is slightly fluctuated at a frequency f and signal component with frequency  2   f  is monitored as an error signal. The  2   f  component of the error signal become minimized when the phase at the interferometers are maintained at a target value. 
         [0022]    When the dither-peak-detection method is used in the optical phase control circuit  240  for controlling the delay of optical phase to be π/4 (rad) or −π/4 (rad) at the delay interferometers  200 A and  200 B, respectively, following problems arise. 
         [0023]    First, fluctuating the phase causes a degradation of waveform distortion of the optical end electrical signal. 
         [0024]    Second, the peak detection (detection of the above described minimum value) only indicates whether or not the phase is adjusted to the target value and does not indicate whether the target phase is larger or smaller than the target value. 
         [0025]    Third, since a relation between the peak level of the detection signal and the phase error generally varies in a quadratic curve, the sensitivity of the peak detection signal against the phase to be adjusted is reduced as the phase error approaches to zero. 
         [0026]    Fourth, the speed of phase control is restricted by the fluctuation frequency (frequency f in the description above). 
         [0027]      FIG. 2  shows a configuration of the optical DQPSK receiver of the embodiment, also described in a Japanese patent application of JP2005-305052, now published as JP2007-20138 (prior application). In  FIG. 2 , one of the two branches, I-branch and Q-branch, is referred to as an A-branch, and the other is referred to as a B-branch. 
         [0028]    In  FIG. 2 , an input DQPSK signal (or RZ-DQPSK signal) is branched and directed to a delay interferometer  11   a  in the A-branch and a delay interferometer  11   b  in the B-branch. 
         [0029]    In the delay interferometers  11 A and  11 B, input signals are further branched to two. The delay interferometers  11   a  and  11   b  include an optical delay element and a phase-shift element, respectively, and one of the branched optical signal is shifted by 1-bit earlier by the phase shift element and interferes with the other branched optical signal, of which optical phase is delayed by π/4 (rad), or −π/4 (rad) by the delay element, respectively. 
         [0030]    In  FIG. 2 , the phase-shift amount of the phase-shift element is adjusted by its temperature. For example, as the temperature of the phase-shift element rises, its phase-shift amount increases. 
         [0031]    The photo detection circuits (Twin-PD)  12   a  and  12   b  generate current signals corresponding to the optical phase modulated signals output from the delay interferometer  11   a  and  11   b , respectively. Trans-impedance amplifiers (TIA)  13   a  and  13   b  convert the electric signal currents generated by the photo detection circuits  12   a  and  12   b , respectively, into electric signals with a corresponding voltage level and limiter amplifier (LIA)  16   a  and  16   b  amplify the electric signals, respectively. 
         [0032]    In the discrimination circuits (clock and data recovery circuits; CDR)  17   a  and  17   b , the electrical signals output from the LIA  16   a  and  16   b , respectively, are converted to an I-channel signal and a Q-channel signal, respectively, which work as clock and data recovery circuits. 
         [0033]    Optical phase error detection unit IA includes low-pass filters (LPF)  14   a ,  20   a , and  21 A, a mixer  15   a , and AD converter  22   a . An electric signal output from the TIA  13   a  is provided to a mixer  15   a  through the low-pass filter  14   a . Also, an electric signal output from the CDR  17   b  is provided to the mixer  15   a  through the low-pass filter  20   a.    
         [0034]    Similarly, optical phase error detection unit IB includes low-pass filters  14   b ,  20   b , and  21   b , a mixer  15   b , and AD converter  22   b . An electric signal output from the TIA  13   b  is provided to the mixer  15   b  through the low-pass filter  14   b . Also, an electric signal output from the CDR  17   a  is provided to the mixer  15   b  through the low-pass filter  20   b.    
         [0035]    The cut-off frequencies of the low-pass filters  14   a ,  14   b ,  20   a , and  20   b  are for example about 100 MHz. 
         [0036]    In the optical phase error detection unit IA, the mixer  15   a  multiplies output signals of the low-pass filter  14   a  and the low-pass filter  20   a . Similarly, in the optical phase error detection unit IB, the mixer  15   b  multiplies the output signals of low-pass filter  14   b  and the low-pass filter  20   b.    
         [0037]    High frequency components of electrical signals output from the mixers  15   a  and  15   b  are eliminated by the low-pass filters  21   a  and  21   b , respectively. An A-branch monitor signal and a B-branch monitor signal output from the low-pass filters  21   a  and  21   b , respectively, are converted into a digital data by A/D converters (ADCs)  22   a  and  22   b , respectively. 
         [0038]    Thus, in the optical phase error detection unit IA, the mixer  15   a  multiplies the electric signal not processed by CDR  17   a  in the A-branch and the electric signal processed by CDR  17   b  in the B-branch. Similarly, in the optical phase error detection unit IB, the mixer  15   b  multiplies the electric signal not processed by CDR  17   b  in the B-branch and the electric signal processed by CDR  17   a  in the A-branch. 
         [0039]    A microcontroller  23   a  calculates a digital signal output from the A/D converter  22   a  and generates a phase adjustment signal for the A-branch. Similarly, a microcontroller  23   b  calculates from a digital signal output from the A/D converter  22   b  and generates a phase adjustment signal for the B-branch. Details of the calculations are explained later. The microcontrollers  23   a  and  23   b  are not necessarily separated ones, and may be a common controller. 
         [0040]    The phase adjustment signals generated by the microcontrollers  23   a  and  23   b  are converted into analog signals and provided to heaters  24   a  and  24   b , respectively. 
         [0041]    In the A-branch, temperature of a phase-shift element in the delay interferometer  11   a  is adjusted by the heater  24   a  controlled by the microcontroller  23   a . In the B-branch, temperature of a phase-shift element in the delay interferometer  11   b  is adjusted by the heater  24   b  controlled by the microcontroller  23   b . The temperatures of the phase shift elements in the interferometer  11   a  and the interferometer  11   b  are adjusted separately. 
         [0042]    As phase-shift amounts of the phase-shift elements of the delay interferometers  11   a  and  11   b  depend on temperature, the phase-shift amounts of the phase-shift elements of the delay interferometers  11   a  and  11   b  are adjusted by the phase adjustment signals generated by the microcontrollers  23   a  and  23   b , respectively. 
         [0043]    A temperature detector  25  detects a temperature around the delay interferometers  11   a  and  11   b . A temperature control circuit  26  generates a temperature control signal based on a detection result of the temperature detector  25 . A Peltier device  27  changes the temperature around the delay interferometers  11   a  and  11   b , based on the temperature control signal. As the temperature control circuit  26  generates the temperature control signal for maintaining the temperature around the delay interferometers  11   a  and  11   b  to a predetermined value, the Peltier device  27  maintains the temperature around the delay interferometers  11   a ,  11   b  at a predetermined temperature according to the temperature control signal. 
         [0044]    The Peltier device  27  is used for supplemental temperature control device to control the temperatures of the phase-shift elements in the delay interferometers  11   a  and  11   b . Therefore, if temperature control for the phase-shift amounts of the phase-shift elements in the delay interferometer  11   a  and  11   b  can be done only by the heaters  24   a  and  24   b , respectively, temperature control is done without the temperature detector  25 , the temperature control circuit  26 , or the Peltier device  27 . 
         [0045]    In the optical DQPSK receiver shown in  FIG. 2 , when the phase error of the phase-shift element of the delay interferometer  11   a  is δA, the A-branch monitor signal is proportional to −sin(δA). Also, when the phase error of the phase-shift element of the delay interferometer  11   b  is δB, the B-branch monitor signal is proportional to −sin(δB). 
         [0046]    Therefore, the microcontroller  23   a  controls the heater  24   a  such that the A-branch monitor signal output from the low-pass filter  21   a  becomes zero. Similarly, the microcontroller  23   b  controls the heater  24   b  such that the B-branch monitor signal output from the low-pass filter  21   b  becomes zero. 
         [0047]    As described above, optical phase error detection unit IA and IB, including the mixers  15   a  and  15   b , and the low-pass filters  21   a  and  21   b , and the microprocessors  23   a  and  23   b  operate as a phase monitoring apparatus and a phase control apparatus. 
         [0048]    In  FIG. 2 , the phases of light of the optical phase modulated signals in the A-branch (I-branch) and the B-branch (Q-branch) are orthogonal. In performing signal extraction separately from the A-branch and the B-branch, when the optical phase control of the delay interferometer  11   a  ( 11   b ) is maintained in a target state, a discrimination output component of the CDR  17   a  (or  17   b ) is not mixed into the output signal of the TIA  13   b  (or  13   a ) of the B (or A)-branch, and therefore the output of the optical phase error detection unit (output of the A/D converter  22   b  (or  22   a )) will become 0 [V]. 
         [0049]    On the other hand, when the optical phase control of the delay interferometer  11   a  ( 11   b ) is deviated from the target state, a discrimination output component of the CDR  17   a  (or  17   b ) of the A (or B)-branch is mixed into the output signal of the TIA  13   b  (or  13   a ) of the B (or A)-branch, therefore, a +/− voltage will be generated at the output of the optical phase error detection unit IA (or IB) (output of the A/D converter  22   b ). This makes it possible to determine the phase-shift amount and its direction. 
         [0050]    When there is no correlation between the A-branch and B-branch, the time integration value of the multiplication output of both the signals will converge to zero; however, when the output signal of the discrimination circuit  17   a  (or  17   b ) of the B (or A)-branch is mixed into the output signal of the trans-impedance amplifier  13   b  (or  13   a ) of the B (or A)-branch, the relation represented by the following equation holds. 
         [0051]    That is, letting the output wp of the TIA  13   b  (or  13   a ), i.e. the signal not processed by CDR  17   b  ( 17   a ) of the B (or A)-branch be [B(A)-arm TIA OUT], and the signal output from the discrimination circuit  17   b  ( 17   a ), i.e. the signal processed by the CDR  17   b  ( 17   a ) be [A(B)-arm MUX OUT], the relation is represented as follows: 
         [0000]      [ B ( A )-arm TIA OUT]=( n*B ( A )-arm TIA OUT)+( m*A ( B )-arm  MUX  OUT) 
         [0052]    Here, [B(A)-arm TIA OUT] is the signal to be primarily extracted, and [A(B)-arm MUX OUT] is the signal component which has been mixed. Further, n and m are coefficients generated by optical phase shifting. 
         [0053]    Then, the output of the mixer  15   a ,  15   b , [B(A)-arm TIA OUT×A(B)-arm MUX OUT] will be represented as follows. 
         [0000]      [ B ( A )-arm TIA OUT×A-arm  MUX  OUT]=( n*B ( A )-arm TIA OUT+ m*A ( B )-arm TIA OUT)× A ( B )-arm  MUX  OUT 
         [0000]      =( n*B ( A )-arm TIA OUT)×( A ( B )-arm  MUX  OUT)+( m*A ( B )-arm TIA OUT)×( A ( B )-arm  MUX  OUT) 
         [0054]    Where, (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT), which is the output of synchronized detection, is a term which is supposed to be zero by nature; and when control is deviated from a target state, the term (m*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT) will not become zero (i.e. m will not be 0), but will generate a +/− voltage depending on the direction of the optical phase shift. 
       SUMMARY 
       [0055]    An optical apparatus comprising: a branching unit branching an input light modulated by DQSPK format and thereby outputting a first branched light and a second branched light; a first branch and a second branch inputting the first branched light and the second branched light, respectively, the first branch and the second branch having an interferometer, a photo detector, and discriminator and demodulating I-signal and Q-signal, respectively; and an abnormality detection unit detecting an abnormality of the input light based on a synchronized detection of a first demodulated signal output from the photo detector in the first branch and a first recovered signal output from the discriminator in the first branch, and a synchronized detection of a second demodulated signal output from the photo detector in the second branch and a second recovered signal output from the discriminator in the second branch. 
         [0056]    The above summary describes only an example embodiment. All embodiments are not limited to including all the features in this example. 
         [0057]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0058]      FIG. 1  shows a configuration of a network of optical DQPSK using RZ format in a related art; 
           [0059]      FIG. 2  shows a configuration of an optical DQPSK receiver in a related art; 
           [0060]      FIG. 3A  and  FIG. 3B  are block diagrams showing a configuration of an embodiment; 
           [0061]      FIG. 4  is a diagram showing a DC offset elimination processing of a phase detection unit; 
           [0062]      FIG. 5A  and  FIG. 5B  are block diagrams showing a configuration of an embodiment; 
           [0063]      FIG. 6  is a flow chart of detecting and notifying an abnormality corresponding to Case  1 ; 
           [0064]      FIGS. 7A and 7B  are flow chart of detecting and notifying abnormalities corresponding to the Cases  2  and  3 ; 
           [0065]      FIG. 8  is a diagram showing a threshold for the Case  1 ; and 
           [0066]      FIG. 9  is a diagram showing a threshold for the Cases  2  and  3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0067]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0068]    In the configuration shown in  FIG. 2 , when the term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT), which generates a +/− voltage depending on the direction of optical phase shift, can not be detected, the feedback loop may not be able to function properly causing a runaway in the configuration of the invention of the prior application. 
         [0069]    That is, in a normal sate in which the outputs of the TIA  13   a ,  13   b  and the outputs of the CDR  17   a ,  17   b  are at generally expected levels, the feedback control functions properly. On the other hand, when optical input state is abnormal, or when any one of the TIA  13   a ,  13   b  or the CDR  17   a ,  17   b  is in failure, the above described term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT), which generates a +/− voltage depending on the direction of optical phase shift, can not be detected so that the optical phase error detection value will become abnormal. 
         [0070]    However, in the configuration shown in  FIG. 2 , when the term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT) cannot be detected, it cannot detect that the optical phase error detection value becomes abnormal. 
         [0071]    Shown below are examples of the state in which the optical phase error detection value becomes abnormal. 
         [0072]    Case  1 : cases when the received optical input includes no optical phase modulated signal, such as an amplified spontaneous emission (ASE) light alone, or cases when the ratio of the signal light S and ASE light (A/ASE) is abnormally small to such an extant as not to be able to detect the above described term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT) by a predetermined amount, the detection of optical input disconnection becomes impossible; that is, in which optical input disconnection detection, thereby the detection of abnormalities, is disabled because of a large amount of ASE. 
         [0073]    Case  2 : cases when the distortion of the optical input signal is so large that the optical signal cannot be discriminated, and therefore MUX OUT of the above described term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT) is indeterminate. 
         [0074]    As a bandwidth of the linear amplified signal (TIA OUT) is limited by the low-pass filter  14   a ,  14   b  in the optical phase error detection circuit unit IA or IB, the linear amplified signal has a large tolerance against a residual dispersion, relating to the distortion of the optical input signal. 
         [0075]    Chromatic dispersion generated in the optical transmission line  3  is compensated by the dispersion compensator  6 . As the control of the dispersion compensator  6  is performed in such a way to decrease the error correction rate in a signal communication state, when the delay interferometer optical phase control is abnormal and the optical signal cannot be discriminated, it is impossible to make a dispersion compensation and impossible to ensure a signal communication state. 
         [0076]    Case  3 : cases where the data discrimination circuit (CDR)  17   a  or  17   b  is abnormal. An abnormality of the data discrimination circuit  17   a ,  17   b  will cause errors due to the discrimination errors of phase shift of the data and the clock. In such a case, (MUX OUT), which is the discrimination circuit output of the above described term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT), becomes indeterminate and thus optical phase error detection values cannot be obtained, as occurred in Case  2 . 
         [0077]    Case  4 : cases when any one of the TIA  13   a ,  13   b  and CDR  17   a ,  17   b  is abnormal, the above described term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT) cannot be detected regardless of the optical phase control value. 
         [0078]    Case  5 : cases when optical input is disconnected and the output signals of TIA  13   a , TIA  13   b , CDR  17   a , and CDR  17   b  become indeterminate. That makes it impossible to obtain the optical phase error detection value. And it is also the case in which optical input disconnection detection, thereby the detection of abnormalities is disabled due to circuit abnormalities and the like. 
         [0079]      FIG. 3A  and  FIG. 3B  are block diagrams showing a configuration of an embodiment.  FIG. 3A  shows an optical receiver of the embodiment and  FIG. 3B  shows a detailed configuration of the optical phase error detection units IA and IB, surrounded by a dotted line in  FIG. 3A , and an abnormal state detection unit  300 . 
         [0080]    In  FIG. 3A , an input DQPSK signal (or RZ-DQPSK signal) is branched and directed to a delay interferometer  11   a  in the A-branch and a delay interferometer  11   b  in the B-branch. 
         [0081]    In the delay interferometers  11 A and  11 B, input signals are further branched to two. The delay interferometers  11   a  and  11   b  include an optical delay element and a phase-shift element, respectively, and one of the branched optical signal is shifted by 1-bit earlier by the phase shift element and interferes with the other branched optical signal, of which optical phase is delayed by π/4 (rad), or −π/4 (rad) by the delay element, respectively. 
         [0082]    In  FIG. 3A , the phase-shift amount of the phase-shift element is adjusted by its temperature. For example, as the temperature of the phase-shift element rises, its phase-shift amount increases. 
         [0083]    The photo detection circuits (Twin-PD)  12   a  and  12   b  generate current signals corresponding to the optical phase modulated signals output from the delay interferometer  11   a  and  11   b , respectively. Trans-impedance amplifiers (TIA)  13   a  and  13   b  convert the electric signal currents generated by the Twin-PD  12   a  and  12   b , respectively, into electric signals with a corresponding voltage level and limiter amplifier (LIA)  16   a  and  16   b  amplify the electric signals, respectively. 
         [0084]    In the discrimination circuits (clock and data recovery circuits; CDR)  17   a  and  17   b , the electrical signals output from the LIA  16   a  and  16   b , respectively, are converted to an I-channel signal and a Q-channel signal, respectively, which work as clock and data recovery circuits. 
         [0085]    Optical phase error detection unit IA includes low-pass filters (LPF)  14   a ,  20   a , and  21 A, a mixer  15   a , and AD converter (ADC)  22   a . An electric signal output from the TIA  13   a  (first demodulated signal) is provided to a mixer  15   a  through the LPF  14   a . Also, an electric signal output from the CDR  17   b  (second recovered signal) is provided to the mixer  15   a  through the LPF  20   a.    
         [0086]    Similarly, optical phase error detection unit IB includes LPF  14   b ,  20   b , and  21   b , a mixer  15   b , and ADC  22   b . An electric signal output from the TIA  13   b  (second demodulated signal) is provided to the mixer  15   b  through the low-pass filter  14   b . Also, an electric signal output from the CDR  17   a  (first recovered signal) is provided to the mixer  15   b  through the LPF  20   b.    
         [0087]    The cut-off frequencies of the LPF  14   a ,  14   b ,  20   a , and  20   b  are about 100 MHz, for example. 
         [0088]    In the optical phase error detection unit IA, the mixer  15   a  multiplies output signals of the LPF  14   a  and the LPF  20   a . Similarly, in the optical phase error detection unit IB, the mixer  15   b  multiplies the output signals of LPF  14   b  and the LPF  20   b.    
         [0089]    High frequency components of electrical signals output from the mixers  15   a  and  15   b  are eliminated by the LPFs  21   a  and  21   b , respectively. An A-branch monitor signal and a B-branch monitor signal output from the LPFs  21   a  and  21   b , respectively, are converted into a digital data by A/D converters (ADCs)  22   a  and  22   b , respectively. 
         [0090]    Thus, in the optical phase error detection unit IA, the mixer  15   a  multiplies the electric signal not processed by CDR  17   a  in the A-branch (the first demodulated signal) and the electric signal processed by CDR  17   b  in the B-branch (the second recovered signal). Similarly, in the optical phase error detection unit IB, the mixer  15   b  multiplies the electric signal not processed by CDR  17   b  in the B-branch (the second demodulated signal) and the electric signal processed by CDR  17   a  in the A-branch (the first recovered signal). 
         [0091]    A microcontroller  23   a  calculates a digital signal output from the A/D converter  22   a  and generates a phase adjustment signal for the A-branch. Similarly, a microcontroller  23   b  calculates from a digital signal output from the A/D converter  22   b  and generates a phase adjustment signal for the B-branch. The microcontrollers  23   a  and  23   b  are not necessarily separated ones, and may be a common controller. 
         [0092]    The abnormal state detection unit  300  has the same configuration as that of the optical phase error detection units IA, IB. 
         [0093]    That is, the optical phase error detection unit IA is a functional unit for obtaining the multiplication result A from the output signal of the TIA  13   a  of the A-branch (the first demodulated signal) and the output signal of the CDR  17   b  of the B-branch (the second recovered signal). On the other hand, the phase error detection unit IB is a functional unit for obtaining the multiplication result B of the output signal from the TIA  13   b  of the B-branch (the second demodulated signal) and the output signal of the CDR  17   a  of the A-branch (the first recovered signal). 
         [0094]    In contrast to that, the abnormal state detection unit  300  is a functional unit for obtaining a multiplication result C from the output signal of the TIA  13   a  of the A-branch (the first demodulated signal) and the output signal of the CDR  17   a  of the A-branch (the first recovered signal), and a functional unit for obtaining a multiplication result D from the output signal of the TIA  13   b  of the B-branch (the second demodulated signal) and the output signal of the CDR  17   b  of the B-branch (the first recovered signal). 
         [0095]    In such a configuration, the multiplication results C and D in the abnormal state detection unit  300  are as follows. 
         [0096]    The output C, which is the multiplication result from the output signal of the TIA  13   a  of the A-branch (the first demodulated signal) and the output signal of the CDR  17   a  of the A-branch (the first recovered signal), is at a positive voltage level during a normal state and 0 volt or a lowered voltage level during an abnormal state. 
         [0097]    Similarly, the output D, which is the multiplication result from the output signal of the TIA  13   b  of the B-branch (the second demodulated signal) and the output signal of the CDR  17   b  of the B-branch (the second recovered signal), is at a positive voltage level during a normal state and 0 volt or under voltage level during an abnormal state. 
         [0098]    In  FIGS. 3A and 3B , a microcontroller  301  inputs the multiplication results A, B, C, and D of each functional unit described above and performs abnormality detection. The microcontroller  301  may be common to the microcontrollers  23   a ,  23   b  in the B-branch and the A-branch in  FIG. 3A . 
         [0099]    A microcontroller  23   a  calculates a digital signal output from the A/D converter  22   a  and generates a phase adjustment signal for the A-branch. Similarly, a microcontroller  23   b  calculates from a digital signal output from the A/D converter  22   b  and generates a phase adjustment signal for the B-branch. The microcontrollers  23   a  and  23   b  are not necessarily separated ones, and may be a common controller. 
         [0100]    The phase adjustment signals generated by the microcontrollers  23   a  and  23   b  are converted into analog signals and provided to heaters  24   a  and  24   b , respectively. 
         [0101]    In the A-branch, temperature of a phase-shift element in the delay interferometer  11   a  is adjusted by the heater  24   a  controlled by the microcontroller  23   a . In the B-branch, temperature of a phase-shift element in the delay interferometer  11   b  is adjusted by the heater  24   b  controlled by the microcontroller  23   b . The temperatures of the phase shift elements in the interferometer  11   a  and the interferometer  11   b  are adjusted separately. 
         [0102]    As phase-shift amounts of the phase-shift elements of the delay interferometers  11   a  and  11   b  depend on temperature, the phase-shift amounts of the phase-shift elements of the delay interferometers  11   a  and  11   b  are adjusted by the phase adjustment signals generated by the microcontrollers  23   a  and  23   b , respectively. 
         [0103]    A temperature detector  25  detects a temperature around the delay interferometers  11   a  and  11   b . A temperature control circuit  26  generates a temperature control signal based on a detection result of the temperature detector  25 . A Peltier device  27  changes the temperature around the delay interferometers  11   a  and  11   b , based on the temperature control signal. As the temperature control circuit  26  generates the temperature control signal for maintaining the temperature around the delay interferometers  11   a  and  11   b  to a predetermined value, the Peltier device  27  maintains the temperature around the delay interferometers  11   a ,  11   b  at a predetermined temperature according to the temperature control signal. 
         [0104]    The Peltier device  27  is used for supplemental temperature control device to control the temperatures of the phase-shift elements in the delay interferometers  11   a  and  11   b . Therefore, if temperature control for the phase-shift amounts of the phase-shift elements in the delay interferometer  11   a  and  11   b  can be done only by the heaters  24   a  and  24   b , respectively, temperature control is done without the temperature detector  25 , the temperature control circuit  26 , or the Peltier device  27 . 
         [0105]    In the optical DQPSK receiver shown in  FIG. 3A , when the phase error of the phase-shift element of the delay interferometer  11   a  is δA, the A-branch monitor signal is proportional to −sin(δA). Also, when the phase error of the phase-shift element of the delay interferometer  11   b  is δB, the B-branch monitor signal is proportional to −sin(δB). 
         [0106]    Therefore, the microcontroller  23   a  controls the heater  24   a  such that the A-branch monitor signal output from the low-pass filter  21   a  becomes zero. Similarly, the microcontroller  23   b  controls the heater  24   b  such that the B-branch monitor signal output from the low-pass filter  21   b  becomes zero. 
         [0107]    As described above, optical phase error detection unit IA and IB, including the mixers  15   a  and  15   b , and the low-pass filters  21   a  and  21   b , and the microprocessors  23   a  and  23   b  operate as a phase monitoring apparatus and a phase control apparatus. 
         [0108]    In  FIG. 3A , the phases of light of the optical phase modulated signals in the A-branch (I-branch) and the B-branch (Q-branch) are orthogonal. In performing signal extraction separately from the A-branch and the B-branch, when the optical phase control of the delay interferometer  11   a  ( 11   b ) is maintained in a target state, a discrimination output component of the output signal of CDR  17   a  (the first recovered signal) (or  17   b /the second recovered signal) is not mixed into the output signal of the TIA  13   b  (the second demodulated signal) (or  13   a /the first demodulate signal) of the B (or A)-branch, and therefore the output of the optical phase error detection unit (output of the A/D converter  22   b  (or  22   a )) will become 0 [V]. 
         [0109]    Moreover, logic inversion circuits  210   a ,  211   a  ( 210   b ,  211   b ) are provided between the LPFs  14   a ,  20   a  ( 14   b ,  20   b ) and the mixer  15   a  ( 15   b ), which have an integration function in the phase error detection units IA, IB. Similarly, a logic inversion circuit is provided in the functional unit for detecting and notifying an abnormal state  300  as well. 
         [0110]    This is because an optical phase error detection value is feeble, and the DC offset voltage generated in the circuit components used for the phase error detection unit is not negligible. Therefore, DC offset elimination processing is performed through computation by the microcontroller  301  of a total of four kinds of outputs A, B, C, and D which are obtained by subjecting each of the TIA outputs and the CDR outputs to logic conversion/inversion and multiplication. 
         [0111]      FIG. 4  is a detailed drawing to illustrate the above described matter; there is shown as the representative only the A-branch which includes Bessel LPFs  14   a ,  20   a , analog switches  210   a ,  211   a  as the logic inversion circuit, a mixer  15   a , a LPF  20   a , and an A/D converter  22   a  of  FIG. 3 . 
         [0112]    In  FIG. 4 , the output of the TIA  13   a  is subjected to the LPF  14   a  to provide a differential output A 1 −A 2 , and the output of the CDR  17   b  is subjected to the LPF  29   a  to provide a differential output B 1 −B 2 . 
         [0113]    Let the DC offset voltages at the input/output units of the mixer  15   a  be respectively DC A , DC B , and DC out  as shown in  FIG. 4 . Further letting the optical phase error detection outputs, which are obtained through the logic conversion/inversion processing (polarity inversion processing), performed by controlling the analog switches  210   a ,  211   a  by microcontroller  301 , be W 1  to W 4  respectively, the outputs of the mixer  15   a  will be given as follows. 
         [0114]    The output will take the following four values: 
         [0000]        W 1= DC   out +( A 1− A 2+ DC   A )( B 1− B 2+ DC   B ) 
         [0000]        W 2= DC   out +( A 2− A 1+ DC   A )( B 1− B 2+ DC   B ) 
         [0000]        W 3= DC   out +( A 2− A 1+ DC   A )( B 2− B 1+ DC   B ) 
         [0000]        W 4= DC   out +( A 1− A 2+ DC   A )( B 2− B 1+ DC   B ) 
         [0000]    By using these four values, the following computation is performed by the microcontroller  301 . 
         [0115]    Thus, the result is given as W 1 +W 2 +W 3 +W 4 =4(A 1 −A 2 )(B 1 −B 2 ) showing that providing inversion switches  210   a ,  211   a  as shown in  FIGS. 3 and 4  makes it possible to eliminate DC offset generated in the circuit. 
         [0116]      FIG. 5A  and  FIG. 5B  are block diagrams showing a configuration of another embodiment. In contrast to the configuration of  FIG. 3A  and  FIG. 3B , in this embodiment, a node-switching circuit  302  is provided and abnormal state detection unit  300  is not provided. Thus, by controlling the node-switching circuit  302  through the microcontroller  301  so as to function the optical phase error detection unit for optical phase error detection or abnormal state detection by time division, it simplifies the circuit configuration. 
         [0117]      FIGS. 6 and 7  show the operational processing flow of a functional unit for detecting and notifying an abnormal state  300  corresponding to the Cases  1  to  5  described above, which cause problems in the related art configurations, described in  FIG. 2  for example. 
         [0118]    Hereinafter, referring to such operational flow, the operation of embodiments according to the present invention in the Cases  1  to  5  will be described. 
         [0119]    First, temperature of the phase-shift elements in the delay interferometers  11   a  and  11   b  are stabilized by utilizing the heaters  24   a ,  24   b  (step S 0 ). Next, it is judged whether or not there is a signal communication (step S 1 ) in the processing circuit of data A and data B, which is positioned aft-stage of the CDR  17   a ,  17   b  and which is not shown in  FIG. 5A . 
         [0120]    The absence of signal communication indicates a state in which there is no incoming signal, and the presence of signal communication is a state in which a signal with an error rate Pe of at least not less than about 10 −2  is obtained regardless of the signal quality. 
         [0121]    (Case  1 ) 
         [0122]    When it is judged that there is signal communication (Yes at step S 1 ), if the abnormality detection operation output value determined by the microcontroller  301  from the signal determined by the abnormal state detection unit in  FIG. 3A ,  3 B,  5 A, or  5 B is larger than a first threshold TH 1  (No at step S 2 ), it is judged that there is no abnormality detection (step S 3 ). 
         [0123]      FIG. 8  illustrates the threshold for the Case  1 . That is, Case  1  is a case in which the received optical input is the ASE light alone and does not include an optical phase modulated signal, or a case in which S/ASE ratio is extremely small to an extent that the term (n*B(A)-arm TIA OUT)×(A(B)-arm MUX OUT) described above (hereinafter, simply referred to as * term) cannot be detected. 
         [0124]    In  FIG. 8 , with an abscissa being S/ASE [dB], there are shown on the ordinate the multiplication results C and D; that is, the values represented by the following expressions respectively: 
         [0000]      (A-arm TIA OUT×A-arm)/Rx_POW_MON_A 
         [0000]      (B-arm TIA OUT×B-arm MUX OUT)/Rx_POW_MON_B 
         [0125]    The numerator of the above equation is a synchronized detection output of the output signal of the TIA  13   a  (the first demodulation signal) ( 13   b /the second demodulation signal) of A (B) arm and the output signal (MUX OUT) of the CDR  17   a  (the first recovered signal), ( 17   b /the second recovered signal); the denominator of the above equation is an optical input power monitor value; and the entire term of numerator/denominator indicates the value processed by the microcontroller  301 . 
         [0126]    Then, considering that the optical input power [mW] is proportional to the TIA output, the synchronized detection output of the same branch (Arm) is divided by the optical input power monitor value so that normalized output is indicated on the ordinate. 
         [0127]    Further, since the numerator acts on the input signal optical power S [mW], and the denominator acts on the total optical power (S+ASE) [mW], an S/ASE abnormality judgment threshold VTH 1  is set from allowable S/ASE using the characteristics that the output will vary in the ratio of S[mW]/(S+ASE)[mW]. 
         [0128]    However, this requires a premise that the judgment threshold VTH 1  is a threshold by which S/ASE ratio abnormality can be judged when there is no other abnormality. 
         [0129]    Referring back to  FIG. 6 , if the input signal optical power S is larger than the S/ASE abnormality judgment threshold VTH 1  (No at step S 2 ), no abnormality is detected (step S 3 ). 
         [0130]    On the other hand, if the input signal optical power S is not larger than the S/ASE abnormality judgment threshold value VTH 1  during abnormality detection operation (Yes at step S 2 ), it is judged to be abnormal and the control of the delay interferometer  11   a  ( 11   b ) by the microcontroller  23   a  ( 23   b ) is temporally halted, that is, the status quo is maintained, thereby causing the optical amplifier AMP on the optical transmission line  3  to perform control to decrease the input light and update the Psig/Pase ratio (step S 3 ). 
         [0131]    Thus, control is performed such that a control signal is inserted into a predetermined channel from the receiver and sent towards the transmitter, and the ratio of the signal light level to the ASE light level (Psig/Pase) is updated at each optical amplifier AMP. 
         [0132]    Based on such control, control of the delay interferometer is restarted after the control and updating at the optical amplifier AMP (step S 4 ). 
         [0133]    After restarting the control of delay interferometer, abnormality detection operation is further performed as with the step S 2  described above, to make judgment with reference to the threshold VTH 1  (step S 5 ). 
         [0134]    Then, when the number of updates of the optical amplifier control as described above becomes larger than N 3  for example (Yes at step S 6 ), it is judged that discrimination improvement by the discrimination circuit (CDR)  17   a ,  17   b  becomes impossible or there is a hardware fault, and an alarm ALM is output from the microcontroller  301  (step S 7 ). 
         [0135]    (Case  2 ) 
         [0136]    Case  2  is a case in which because of large waveform distortion, the optical input becomes a signal with an error rate Pe of not larger than 10 −2 . 
         [0137]    Therefore, in such a case, it is judged that there is no signal communication at the signal communication state judgment in  FIG. 6 , (No at step S 1 ). Then, processing advances by moving to the flow of  FIGS. 7A and 7B . 
         [0138]    That is, the case in which the output MUX OUT of the discrimination circuit  17   a  ( 17   b ) in the above described * term becomes indeterminate. 
         [0139]    In  FIG. 9 , with the abscissa being the chromatic dispersion, there are shown on the ordinate the multiplication results C and D as with  FIG. 8 ; that is, the values represented by the following equation: 
         [0000]      (A-arm TIA OUT×A-arm MUX OUT)/Rx_POW_MON_A 
         [0000]      (B-arm TIA OUT×B-arm MUX OUT)/Rx_POW_MON_B 
         [0140]    When the data discrimination in the discrimination circuit (CDR)  17   a  ( 17   b ) has been extremely degraded (when the error rate becomes extremely large), due to the waveform distortion including residual dispersion, the above described term (MUX OUT) becomes indeterminate and the numerator converges to zero. 
         [0141]    The effect of error rate of the MUX OUT term, which is the output of the discrimination circuit  17   a  ( 17   b ), on the ordinate of  FIG. 9  will be the error rate itself. Thus, when the error rate is given as Pe=10 −2 , the likelihood of the MUX OUT term will be reduced at the rate of 10 −2  thereby affecting the ordinate. 
         [0142]    Further, the trans-impedance amplifier output (TIA OUT) has a high residual dispersion resistance since the bandwidth of the linear amplified signal of input signal is limited by the LPF (Bessel Fil)  14   a  ( 14   b ) 
         [0143]    In  FIG. 9 , considering the loop gain of the feedback control of the delay interferometer optical phase, the lower limit value due to waveform distortion for normal functioning is set to be a threshold VTH 2 . 
         [0144]    However, this threshold VTH 2  requires a premise that it is a threshold by which waveform distortion abnormality can be judged when there is no other abnormality. 
         [0145]    Referring to  FIGS. 7A and 7B , during the abnormality detection computation by the microcontroller  301 , if the above described multiplication result C or D is larger than judgment threshold VTH 2  with respect to the residual dispersion of the input signal optical power (No at step S 10 ), the process returns to the judgment of Case  1  of  FIG. 6  described above. 
         [0146]    On the contrary, when the multiplication result C or D is smaller than the judgment threshold VTH 2  (Yes at step S 10 ), the control of the delay interferometer  11   a  ( 11   b ) by the microcontroller  23   a  ( 23   b ) is temporally halted, that is, the status quo is preserved, and the dispersion compensator  6  on the network  3  is updated (step S 11 ). 
         [0147]    After the dispersion compensation value is updated by the dispersion compensator  6 , control on the delay interferometer  11   a  ( 11   b ) is restarted (step S 12 ). 
         [0148]    After restarting control of the delay interferometer, further abnormality detection computation is performed as with the above described step S 2 , and judgment is made with reference to the threshold VTH 2  (step S 13 ). 
         [0149]    When the computation result output C or D described above is larger than the threshold VTH 2 , the process returns to the judgment of Case  1  previously described in  FIG. 6  (No at step S 13 ). 
         [0150]    Then, when the number of judgments that the computation result output C or D is smaller than the threshold VTH 2  becomes larger than N 1  (Yes at step S 14 ), it is judged that improvement of the discrimination circuit (CDR)  17   a  ( 17   b ) only by the dispersion compensator  6  is not possible (NG) (step S 15 ). 
         [0151]    (Case  3 ) 
         [0152]    In Case  3 , in a similar fashion with Case  2 , when the output of the discrimination circuit (CDR)  17   a ,  17   b  becomes indeterminate due to the threshold abnormality thereof in a similar fashion with Case  2 , the processing is as follows. 
         [0153]    That is, in  FIGS. 7A and 7B , the dispersion compensation control by the dispersion compensator  6  and the phase control on the delay interferometer  11   a  ( 11   b ) are temporally halted, and thereafter the threshold of the discriminator (CDR)  17   a  ( 17   b ) and the phase control are updated (step S 16 ). 
         [0154]    Then, after the update of control value, control on the dispersion compensator  6  and phase control on the delay interferometer  11   a  ( 11   b ) are restarted (step S 17 ). 
         [0155]    Then, when the number of judgments that computation result output C or D is smaller than the threshold VTH 2  becomes larger than N 2  (Yes at step S 19 ), the discrimination point of the discrimination circuit (CDR)  17   a  ( 17   b ) is returned to the original state (step S 20 ), and it is judged that improvement of the dispersion compensation by the dispersion compensator  6  and the discrimination output by the update of the threshold of the discrimination circuit (CDR)  17   a  ( 17   b ) is not possible (step S 21 ). 
         [0156]    In such a case, since ASE light is large, or the hardware is abnormal, a countermeasure is taken by improving the signal to ASE ratio of the optical amplifier AMP on the network  3  (step S 22 ). 
         [0157]    (Case  4  and Case  5 ) 
         [0158]    Case  4  is a case in which either of the output signal of the trans-impedance amplifier (TIA) (the first/second demodulated signal) or the output signal of the discrimination circuit (CDR)  17   a  ( 17   b ) (the first/second recovered signal) is abnormal, and in which the control value updating processing in the above described Cases  1  to  3  will not allow recovery. That is, it is judged that the optical input power monitor value is normal and any of input light loss detection unit or fore-stage units before the interferometers is likely to be in fault. 
         [0159]    Further, in Case  5 , when output of the abnormal state detection unit is abnormal and shows no change during a series of abnormality detection control flow of the above described Cases  1  to  3 , it is judged that input light loss detection unit is likely to be in failure regardless of the failure judgment of other units in Case  4 . 
         [0160]    As described above, utilizing a circuit for detecting and notifying an abnormal state, a runaway of the feedback control of the delay interferometer optical phase is prevented. Also, utilizing an abnormality detection computation and control unit, the dispersion compensator, the optical amplifier, and others are halted during abnormality and recovered therefrom upon detection of abnormality. 
         [0161]    In the control procedure for the above described abnormality recovery, as the abnormalities can happen at the same time, it is preferable to recover the abnormalities and avoid the abnormalities affecting the dispersion compensation amount feedback control of the dispersion compensator in the optical receiver unit or the delay interferometer optical phase feedback control embedded in the optical receiver in order to avoid runaway. 
         [0162]    Thereafter, abnormality recovery control is performed in which abnormality recovery is performed by time division so that abnormality judgment is recovered on the abnormality occurrence causes. 
         [0163]    Then, from a series of the results of abnormality detection control which is performed by such time division, abnormality occurrence locations are identified. 
         [0164]    Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claim and their equivalents.