JP5075739B2 - Optical PSK demodulator and optical filter phase control method using optical PSK demodulator - Google Patents

Description

  The present invention relates to an optical phase monitoring device and an optical filter with an optical phase monitor for monitoring the phase of an intensity-modulated or phase-modulated optical signal in the field of optical communication, as well as light for controlling the phase of an optical filter using the monitoring result The present invention relates to a filter phase control method.

  The information and communication network has moved away from the conventional telephone line network, and is a full IP (Internet Protocol) by light that combines the convenience and economy of the Internet network and the reliability and stability of the telephone network. It is heading to the next generation network (NGN). When talking about information communication technology in NGN, digitalization of information, IP of network, ubiquitous, and broadband are key. The broadbandization of optical communication has progressed dramatically due to wavelength multiplexing, but recently the bit rate of time division multiplexing for multiplexing information on the time axis has been remarkable.

  In conventional wavelength division multiplexing, binary amplitude shift keying, which is a binary intensity modulation method using the NRZ (Nonreturn-To-Zero) or RZ (Return-To-Zero) format ( OOF (On-Off Keying) was the mainstream. In recent years, multilevel modulation / demodulation technologies such as duobinary, CSRZ (Carrier-Suppressed Return-To Zero), and optical DPSK (Differential Phase Shift Keying) have begun to be applied to optical communications. Yes. In particular, the RZ-DPSK high-reception sensitivity modulation method that modulates the intensity of optical DPSK with RZ is superior to conventional OOF in terms of degradation resistance against signal quality degradation due to nonlinear optical effects that occur in the transmission optical fiber. This modulation / demodulation technique can also improve S / N (signal-to-noise ratio) by moving reception.

  More recently, a modulation / demodulation format of DQPSK (Differential Quadrature Phase Shift Keying) that can allocate 2 bits of data to each of the four modulated phases, which is advantageous for improving the bit rate, has been studied. At first, it is regarded as a promising candidate for future optical communications. DPSK code is a binary code that assigns a digital electrical signal of 0 or 1 to a phase change of 0 or π light, whereas DQPSK code has four phase changes (0, π / 2, π, 3π / 2) Is a quaternary code that assigns (0, 0), (0, 1), (1, 0), (1, 1) of the digital electrical signal. Therefore, if the same symbol rate is used, DQPSK can allocate twice as much information as DPSK, and its spectral efficiency is doubled. The polarization mode dispersion is alleviated.

As shown in FIG. 1, for example, a conventional optical DQPSK receiver includes a coupler (optical multiplexer / demultiplexer) 2 that splits an optical signal into two branches, and an I phase that is separately connected to two outputs of the coupler 2. And a Q-phase demodulator 4 (Non-Patent Document 1). The I-phase demodulator 3 and the Q-phase demodulator 4 are Mach-Zehnder interferometers each having an optical delay element τ corresponding to a symbol time in the transmission system. That is, in general, B (Hz) which is a free spectral range (FSR) of the Mach-Zehnder interferometers (demodulators) 3 and 4 is C (Hz) of the baud rate of the optical DQPSK signal.
B = C (1)
Is set to the relationship.

  The phase adjustment mechanism (for example, the optical path length difference is controlled so that the phase difference with respect to the optical signal provided by the arms of the interferometers 3 and 4 is + π / 4 for the I phase and −π / 4 for the Q phase. The thermo-optic phase shifters using thin film heaters 5 and 6 are set. The two optical output ports 7, 8, 9 and 10 of each interferometer 3 and 4 are light receiving devices (two photodiodes (PD)) that convert optical signals (optical pulses) into electric signals (electric pulses). (Not shown).

  FIG. 2 shows transmission spectra of the two interferometers 3 and 4 described above. To demodulate the I-phase and Q-phase data signals, the spectra of the four output ports (corresponding to positions 7, 8, 9, 10 in FIG. 1) are arranged at equal intervals on the optical frequency axis. The center frequencies of the phase and Q phase optical signals need to be set to +1/4 (11 positions in FIG. 2) and -1/4 (12 positions in FIG. 2). This means that the phase difference between the arms of the two interferometers is accurately set to + π / 4 and −π / 4, respectively, and 2 for the optical frequency of the DQPSK optical signal (1 in FIG. 1). This means that the phase characteristics (relative phase positions) of the two interferometers 3 and 4 need to be set to +1/4 and -1/4. Here, when an error occurs in the phase of the interferometers 3 and 4 as the demodulator, the signal quality Q value is significantly deteriorated. Therefore, a phase monitoring technique and a phase control technique for obtaining an accurate phase relationship between the phase of the interferometers 3 and 4 and the optical frequency of the input signal light 1 are important.

  Although FIG. 1 shows an example in which the optical DPSK code is assigned to four values, recently, studies on multi-level phase modulation codes are also being made. FIG. 3 shows a configuration of a demodulator in the case of 8 values as an example (Non-Patent Document 2). In this demodulator, four Mach-Zehnder interferometers (17, 18, 19, 20), which are optical DPSK demodulators, are arranged in parallel, and optical signals are transmitted to the demodulators 17, 18, 19, and 20 by couplers 14, 15, and 16, respectively. The signal is configured to be equally distributed. The FSR B (Hz) of each demodulator is set so as to satisfy the above-described equation (1), as in the demodulation of the optical DPSK signal.

  The respective output signals 21, 22, 23, 24, 25, 26, 27, 28 of the respective demodulators 17, 18, 19, 20 are led to a light receiver (photodiode) and converted into an electric signal. The original data is restored by performing a logical operation on the data restoration logic circuit 201. This modulation / demodulation method is optical D8PSK (Differential 8-Phase Shift Keying), and the DPSK code has eight phase changes (0, π / 4, π / 2, 3 / 4π, π, 5 / 4π, 3π / 2, 7 / 4π), and (0, 0, 0), (0, 0, 1), (0, 1, 1), (0 0, 1, 0), (1, 1, 0), (1, 1, 1), (1, 0, 1), and (1, 0, 0). Therefore, when the same symbol rate is used, the D8PSK code can allocate four times the amount of information as compared to the DPSK code, and the spectral efficiency is four times that of the optical DQPSK code. Requirements, adjustment of optical fiber dispersion, and polarization mode dispersion are alleviated.

As described above, in the optical DNPSK (differential N-value phase shift keying) coding method in which N-value (N = 2 ⁿ , where n is a positive integer) data is generally assigned to one symbol, 2 ^{(n -1)} N-value data is demodulated by one delay interferometer (Mach-Zehnder interferometer, demodulator) (see Table 1 below), and the baud rate decreases to 1/2 as the N value increases. ^Since it becomes ^(n-1) , the spectral efficiency is 2 ^(n-1) times, and the demand for the speed of the electric device, the adjustment of the dispersion of the optical fiber, and the effect of relaxing the polarization mode dispersion are expected.

However, the control accuracy of the demodulator becomes more severe as the multi-level is increased. FIG. 2 shows the transmission characteristics when the code level is four values. When the code level is eight values, the spectrum of the interferometer must be divided into eight equal parts in a 2π section. In general, in the case of N values, 2 ^(n-1) delay interferometers must be accurately set to phase positions that are equally divided into N in a section of 2π with respect to the phase of the optical signal. Considering the fabrication accuracy of the delay interferometer, the optical path length difference of the interferometer is 1 / N of the wavelength (≈1.5 μm). Since the error with respect to this setting value significantly affects the Q value degradation, the optical path length difference of the interferometer is an accuracy of 0.1% or less of the optical path length difference, That is, it is required to be manufactured with an accuracy of nm (nanometer). In general, with the current optical device technology, it is difficult to realize the optical path length difference with the accuracy of nm, and therefore the manufactured demodulator is finely adjusted using a control circuit.

  As a typical method for controlling the phase of the optical filter, there is dithering control. FIG. 4 shows a configuration using a multistage Mach-Zehnder interferometer as an optical filter (Patent Document 1). In this configuration, a signal having a relatively low frequency is generated from the oscillator 30, and this signal is applied to a phase control target (phase adjustment mechanism) 36, 37, 38, 39 with a minute amplitude (referred to as dithering). ), The output optical signal of the optical filter (in this case, the tunable optical demultiplexer) 31 is converted into an electrical signal by the light receiver 32, and the dithering frequency component is extracted from the main signal of the electrical signal by the low-pass filter 33 (Ve1), the dithering frequency component thus extracted and the original signal of the oscillator 30 are multiplied by the multiplier 34 to generate an error signal 35 (Ve2). 37, 38 and 39 are feedback-controlled. Such control makes it possible to accurately control the optical frequency of the input optical signal 29 and the phase relationship between the optical filter (optical Mach-Zehnder interferometer) 31 and the phase of the optical filter 31.

Japanese Unexamined Patent Publication No. 5-122261 R. A. Griffin and A. C. Carter, “Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission”, OFC2002, pp367-368, 2002. H. Yoon, D. Lee and N. Park, “Receiver Structure of Generalized M-ary Optical DPSK System And Its Semi-analytical Performance Evaluation,” OECC2004, 14C3-4, pp. 466-477, 2004. CR Doerr, S. Chandrasekhar, LL Buhl, MA Cappuzzo, EY Chen, A. Wong-Foy, and LT Gomez, “Optical Dispersion Compensator Suitable for Use With Non-Wavelength-Locked Transmitters,” J. Lightwave Technol., Vol. 24, pp. 166-170, 2006. B. Mikkelsen, C. Rasmussen, P. Mamyshev, and F. Liu, "Partial DPSK with excellent filter tolerance and OSNR sensitivity," Electron. Lett., Vol. 42, no. 23, pp. 1363-1365, Nov. 2006.

  However, the conventional dithering control as described above has a problem that a phase error due to minute amplitude modulation due to dithering essentially occurs even if accurate phase control is applied.

  On the other hand, a method has been proposed in which the operating wavelength (= operating frequency, synonymous with operating relative phase) of an optical filter is monitored and controlled with a separate optical filter (Non-Patent Document 3). FIG. 5 shows such a wavelength control circuit configuration of an optical filter using an optical Mach-Zehnder interferometer. In this configuration, the input optical signal 40 is guided to an optical filter 41 (in this case, an optical dispersion compensation filter) and an optical phase monitor 42 prepared separately. The output of the optical phase monitor 42 is power-monitored by the low-speed photo detectors 43 and 44, and the difference (difference in light intensity value) is calculated by an analog circuit so that the calculation result becomes a certain value. As a result of feedback control of the result to the temperature adjustment circuit unit (in this case, the electronic cooler), the operating wavelength of the optical filter 41 can be adjusted to the optical signal 40. This wavelength control method can avoid the occurrence of a phase error due to a dithering signal applied to the phase control mechanism that occurs in the case of the dithering control described above, and can be applied to a more accurate optical filter phase control method.

  However, this wavelength control method can be applied only when the optical filter 41 has a simple one-input one-output, as shown in FIG. For example, when it is necessary to control a plurality of phase control mechanisms including a plurality of Mach-Zehnder interferometers as shown in FIGS. 1 and 3, the conventional method using an external optical phase monitor as shown in FIG. It was impossible to realize that control.

  Therefore, conventionally, in order to individually control each of the phases of a plurality of Mach-Zehnder interferometers, such as in the case of a demodulator in a multilevel phase modulation / demodulation code, it is necessary to rely on a dithering method as shown in FIG. The phase error component added to the phase control mechanism generated by the dithering signal is essentially generated. Including the phase error in the dithering method in controlling the phase of the optical filter, as already described in the last stage of the background art section, is fatal to the Q value deterioration as the multi-value level increases. affect.

  The present invention has been made in view of the above points, and an object of the present invention is to realize a phase control that essentially does not cause a phase error regardless of the form of an optical filter or a demodulator. An object of the present invention is to provide a phase monitor device, an optical filter with an optical phase monitor, and an optical filter phase control method.

In order to achieve the above object, an optical PSK demodulator according to claim 1 comprises 2 ^(n-1) (n is a positive integer) demodulators having a phase adjustment mechanism , and An optical PSK demodulator comprising an optical branching means for branching a part of the output and an optical phase monitoring device connected to the optical output port of the optical branching means, wherein the optical phase monitoring device is in the optical frequency domain. Mach-Zehnder interference in which the phase of the optical signal from the optical modulation light source having the occupied band A (Hz) is monitored, and the free spectral range FSR (Free Spectral Range) where the optical signal from the optical modulation light source is incident is B (Hz) The phase of the optical signal from the light modulation light source is monitored by the light intensity ratio of the two light output ports of the Mach-Zehnder interferometer. It is characterized by doing.

  Here, the B (Hz) is set to a relationship of B = mA (m is an integer of 2 or more).

Further, the transmittance T at one of the two optical output ports of the Mach-Zehnder interferometer has a phase θ (radian), and the line width of the optical modulator light source with respect to the FSR of the Mach-Zehnder interferometer is d ( %)

It can be characterized by being given by.

  Here, the optical branching unit may be an optical waveguide that branches a part of the output of the optical filter and connects to the optical phase monitoring device.

  The optical phase monitoring apparatus may further include control means for controlling the phase adjustment mechanism in accordance with the monitoring result so that the monitoring result becomes a target constant value.

In order to achieve the above object, an optical filter phase control method according to a sixth aspect of the present invention is the optical PSK demodulator according to any one of the first to fifth aspects, wherein the monitoring result by the optical phase monitoring device is used. The phase adjustment mechanism is controlled in accordance with the monitoring result so that becomes a target constant value.

  With the above configuration, according to the present invention, it is possible to eliminate occurrence of a phase error due to application of a dithering signal, which is an essential problem in phase control by dithering, and to realize an accurate phase control method. An optical phase monitoring device, an optical filter with an optical phase monitor, and an optical filter phase control method can be realized.

  More specifically, according to the present invention, the optical phase monitoring device is connected to the branch output of the Mach-Zehnder interferometer (optical filter) set so that the main signal band and the given FSR have an appropriate phase relationship. Therefore, the phase relationship between the optical filter and the optical signal can be accurately obtained from a simple light intensity ratio at the output port of the optical filter.

  In addition, according to the present invention, the phase relationship between the optical filter and the main signal and the phase of the optical filter itself are accurately controlled at an arbitrary phase position by feedback-controlling the result of the phase monitoring to the phase adjustment mechanism of the optical filter. can do.

  Further, according to the present invention, the FSR (= B (Hz)) of the optical phase monitoring device is set to the FSR (= C (Hz)) of the demodulator by setting the FSR of the optical phase monitoring device to B = 2C in advance. If it is an integer multiple, a certain value can be generated as a lock point having the integer period, and phase control has the advantage that it can be realized with a simple circuit configuration that feedback-controls that constant value, and When the integer is 2 (ie, B = 2C), one lock point can be generated for each of the two FSRs of the original demodulator, thereby maximizing the number of lock points on the demodulator phase axis. In addition, the efficiency with respect to the phase control amount in the phase control is maximized.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 6 shows a specific configuration example of the optical phase monitoring device according to the first embodiment of the present invention. This optical phase monitoring device has a single wavelength CW (continuous wave) light source 45, a modulator 46, and an optical phase monitoring device 47. As the single wavelength CW light source 45, in a wavelength multiplexing system, a DFB (distributed feedback type) laser or a wavelength tunable laser provided with an external resonator by MEMS is generally used.

  The laser light from the single wavelength CW light source 45 is modulated by the modulator 46 by intensity modulation, phase modulation, or a combination thereof. In order to monitor the phase of the modulated optical signal, the optical signal is input to the optical phase monitoring device 47. The optical phase monitoring device 47 has a configuration of a Mach-Zehnder interferometer provided with two optical branches and an optical path for providing a phase therebetween.

  In the present embodiment, an example in which the optical phase monitoring device 47 is configured by a silica-based optical waveguide is shown, and an optical coupler such as a directional coupler or an MMI (Multi Mode Interference) type optical coupler can be used for optical branching. The optical path for providing is an arm waveguide in which the phase difference is set by the difference in length. When the phase monitor device 47 is composed of an optical fiber, the optical branching is an optical coupler based on melt stretching or an optical coupler using a multilayer filter, and the optical path for providing a phase is set with a phase difference based on the difference in length. Arm fiber. Further, when the phase monitor device 47 is constituted by a spatial optical system, the optical branching is a dichroic mirror or prism, and the optical path for providing a phase is a spatial optical path in which a phase difference is set by a difference in length.

  FIG. 7 shows a simulation for explaining the operating principle of the optical phase monitoring device 47 of FIG. The horizontal axis indicates the phase θ (radian) of the optical phase monitor device 47 or the relative phase relationship between the optical phase monitor device 47 and the optical signal, and the vertical axis indicates the standard of the optical signal output (48 or 49 in FIG. 6). Each curve shows a change due to the relationship between the line width of the light source 45 and the relationship d (%) of the free spectral range FSR (Free Spectral Range) of the optical phase monitoring device 47.

  In this simulation, the transmittance T at one of the two ports of the Mach-Zehnder interferometer has a phase θ (radian), and the line width of the optical modulator light source with respect to the FSR of the Mach-Zehnder interferometer is d (% )

  Given in.

  When the light source 45 has an ideal line spectrum (the line width is zero), that is, in a state where the line width of the light source 45 is zero and the modulator 46 is not operated, the transmittance of the optical phase monitor device 47 is as shown in FIG. As shown, a sine curve having a period of 2π is drawn according to the phase of the Mach-Zehnder interferometer, the minimum and maximum values are 0 and 1, and the amplitude is 1.

  Here, consider the normalized transmittance of the optical signal output in the case of a light source having a rectangular spectrum with line widths of 10%, 50%, and 80% with respect to the FSR of the Mach-Zehnder interferometer. This corresponds to the fact that the modulator 46 performs some intensity modulation, phase modulation, or a combination thereof, and the line width of the light source 45 becomes thicker according to the modulation speed. In this case, the transmittance waveform has a period of 2π as shown in FIG. 7, but its amplitude decreases as the line width increases. For example, when the line width is 33%, 50%, and 80%, the amplitudes of the transmittance waveforms are 0.83, 0.64, and 0.23, respectively.

  Furthermore, in the case of a light source having a rectangular spectrum with a line width of 100% with respect to the FSR of the Mach-Zehnder interferometer, the amplitude of the transmittance waveform is zero. This is because the FSR of the Mach-Zehnder interferometer 47 and the line width of the light source 45 match. In the ideal sine waveform indicated by the transmittance of the line spectrum light source 45, when the sine waveform is integrated in the interval of 2π, This is because the result becomes 1 regardless of the start point and end point of the integration.

  When the line width of the light source 45 is smaller than the FSR of the Mach-Zehnder interferometer 47, the above phenomenon (transmittance characteristics) is obtained by monitoring the transmittance of the Mach-Zehnder interferometer 47 so that the phase of the light source 45 is changed to Mach-Zehnder interference. This means that it can be determined by the relative relationship with the total 47 phases.

Here, considering actual optical communication, as shown in FIG. 6, the optical signal from the light source 45 is modulated by intensity modulation, phase modulation, or a combination thereof in accordance with data information, etc. The spectrum of the modulated optical signal has an occupied band (corresponding to the line width) corresponding to the modulation format and bit rate (or baud rate). Therefore, the result of FIG. 7 shows that the optical modulation signal having the occupied band A (Hz) in the optical frequency domain and the Mach-Zehnder interferometer 47 whose free spectral range FSR (Free Spectral Range) is B (Hz).
B> A (3)
This suggests that the phase of the light source 45 can be monitored by the light output intensity of the optical phase monitoring device 47. In particular, when the modulation format of the modulator 46 is an ideal spectral shape of a PSK signal such as DPSK or DQPSK in which phase modulation is used, the occupied bandwidth A (Hz) is the phase modulation speed (= baud rate) of the spectral shape. It is known that the result is an equal rectangular spectrum (see equation (1) above). From this, in the case of an optical PSK phase modulation signal whose signal light is a baud rate C (Hz),
B> C (4)
By monitoring the transmittance (light intensity ratio) of the Mach-Zehnder interferometer, the phase of the signal light can be accurately monitored.

(Second Embodiment)
Up to this point, a method for calculating the optical phase has been derived from the relationship between the FSR of the optical phase monitoring device 47 and the band occupancy rate or baud rate of the main signal, but this Mach-Zehnder interferometer 47 uses the optical phase data of the main signal. The point that it can also be used as a demodulator for sorting will be described as a second embodiment of the present invention.

That is, this embodiment is a case where the modulator 46 is an ideal DPSK signal phase modulator that modulates the phase of the signal light to binary, and the Mach-Zehnder interferometer 47 is a delay interferometer for optical DPSK demodulation. . In this case, in the ideal optical DPSK signal, the signal light becomes a rectangular spectrum light source of the occupied band A (Hz), so the occupied band A (Hz) is the modulation / demodulation baud rate (= bit rate) C (Hz). Equal, ie
A = C (5)
The relationship is established.

Therefore, if a Mach-Zehnder interferometer 47 that satisfies the above equation (3) is prepared, the Mach-Zehnder interferometer 47 operates as an optical phase monitoring device based on the discussion so far. On the other hand, when the Mach-Zehnder interferometer 47 is operated as a demodulator, due to the nature of the optical DPSK signal, it is essential that the demodulator interferes by giving a delay of one symbol to the symbol rate. Looking at this condition in detail, the condition for delaying by one symbol does not require that the delay part of the Mach-Zehnder interferometer is exactly equal to one symbol length. Interference can be generated, and the condition for demodulation can be satisfied (Non-Patent Document 4). That is,
2C> B (6)
Then, the Mach-Zehnder interferometer 47 can operate as an optical DPSK demodulator.

From the above formulas (3) to (6),
2C>B> A (or C) (However, A = C) (7)
Therefore, the Mach-Zehnder interferometer 47 has both an optical DPSK demodulation function and an optical phase monitoring function.

  From another viewpoint, the conventional demodulator that handles the optical DPSK code is used under the above equation (1). As a result, the transmission characteristic of the output signal becomes a constant value independent of the phase. As a result, it becomes impossible in principle to calculate the phase from the result of the power monitor of the demodulator. On the other hand, as in the present invention, when a relationship that satisfies the above equation (7) is set as a demodulator, the demodulator can also use the optical phase monitoring function. In this case, there is no need to control the optical filter (demodulator) by using two Mach-Zehnder interferometers, such as the conventional optical phase monitoring device 42 and optical filter (in this case, demodulator) 41 as shown in FIG. The phase control of the demodulator can be achieved with one interferometer.

  As the processing of the optical signal outputs 48 and 49 when the demodulation function and the optical phase monitoring function are combined, for example, the optical signal outputs 48 and 49 are detected by a high-speed light receiver (not shown) using a photodetector (PD). By converting the optical pulses of the optical signal outputs 48 and 49 into electrical pulses, a demodulation function is realized, and the optical output power (optical intensity) is detected by detecting the photocurrent value of each photodetector. ) To realize the optical phase monitoring function.

  As described above, in the first embodiment of the present invention, the FSR of the Mach-Zehnder interferometer is appropriately selected (see the above equation (4)) regardless of what modulation is applied to the optical signal. The basic principle that the phase of the optical signal can be monitored by the above-mentioned method is clarified, and in the second embodiment of the present invention, a special condition is set (see the above equation (7)), and one Mach-Zehnder interferometer Clarified that it operates as a demodulator and optical phase monitor. The absolute value of the photocurrent of the two output ports is affected by the incident light intensity, not the transmittance of the Mach-Zehnder interferometer, but actually takes the ratio of the two port outputs (light intensity ratio). Thus, the phase can be monitored with respect to an arbitrary signal light intensity. Specific examples are shown in the following embodiments.

(Third embodiment)
FIG. 8 shows a specific configuration of the optical filter according to the third embodiment of the present invention that implements the optical filter phase control method of the present invention. This optical filter to be controlled is an optical DQPSK demodulator composed of an optical branch 51 and two Mach-Zehnder interferometers 52 and 53. In this embodiment, this optical DQPSK demodulator is a case of a general demodulator in which the FSRs of the delay interferometers (Mach-Zehnder interferometers) 52 and 53 are equal to the baud rate C (Hz) (the above formula (1)). .

  The phases of the demodulator Mach-Zehnder interferometers 52 and 53 are monitored as follows. That is, a part of the outputs of the Mach-Zehnder interferometers 52 and 53 of the demodulator to be phase-controlled is branched and taken out, the branched output light is guided to the branched optical phase monitor devices 58 and 59, and the branched optical phase monitor The power monitors 68, 69 and 70, 71 detect the signal strength of the outputs of the devices 58, 59. Furthermore, an optical phase monitor result is generated by calibrating the detected signal through the arithmetic processing circuits 72 and 73. The composite interferometers 62 and 63 include demodulators (Mach-Zehnder interferometers) 52 and 53 and branching optical phase monitoring devices (Mach-Zehnder interferometers) 58 and 59, and the optical phase reflecting the transmission characteristics of the branching optical phase monitoring device. Functions as a monitor.

Since the optical phase monitor of the present embodiment reflects the transmission characteristics of the composite interferometers 62 and 63, the FSR of the optical phase monitor devices 58 and 59 is set to be twice the FSR of the demodulator (delay interferometer) 52 and 53. By setting, the FSR of the composite interferometers 62 and 63 was set to be twice the FSR of the Mach-Zehnder interferometers 52 and 53 of the demodulator. Therefore, the composite interferometers 62 and 63 operate as an optical phase monitoring device similar to the optical phase monitoring device 47 of FIG. 6, and B (Hz) as its FSR is
B = 2C (8)
It will satisfy the relationship. Ideally, as shown in the above equation (5), A = C.
B = 2A (9)
Is the best mode. If the above equation (9) holds, as will be described in detail later, one lock point can be generated for each of the two FSRs of the original demodulator, thereby locking on the phase axis of the demodulator. The number of points becomes the maximum, and the efficiency with respect to the phase control amount in the phase control is maximized.

  Controlling the demodulators 52 and 53 to the optimum phase point by the outputs of the complex interferometers 62 and 63 generates an error signal by the calculation of the arithmetic processing circuits 72 and 73, and uses the generated error signal as a control signal as a drive circuit. This is achieved by feedback control of the phase adjustment mechanisms 54, 55, 60, 61 through 75, 76, 79, 80. The phase adjustment mechanisms 54, 55, 60, 61 are arranged in the vicinity of the optical path of the demodulator Mach-Zehnder interferometers 52, 53. For example, a thermo-optic phase shifter using a thin film heater for controlling the optical path length difference is used. Can do.

  Next, more detailed transmission characteristics in the Mach-Zehnder of the optical DQPSK demodulator will be described. FIG. 9 shows the transmission characteristics of the optical DQPSK demodulating Mach-Zehnder interferometer 52 and the branched optical phase monitoring device 58. However, in order to simplify the drawing, the transmission characteristic of the optical phase monitor indicates the transmission characteristic only on the branched optical phase monitor device 58 side. In addition, in the description of the curves noted on the right side of FIG. 9, two curves of “2x phase bit rate” indicate the transmission characteristics of the branched optical phase monitor device 58, and the four “ Curves “I port” and “Q port” represent transmission characteristics of the Mach-Zehnder interferometer 52 for optical DQPSK demodulation.

  The transmission characteristics of the demodulator shown in FIG. 9 are the same as those shown in FIG. 2, but the spectra of the four output ports (64, 65, 66, 67 in FIG. 8) of the optical DQPSK demodulator are arranged at regular intervals on the standard optical frequency axis. The center frequencies of the optical signals of the I phase and the Q phase are set to +1/4 (position 81 in FIG. 9) and -1/4 (position 82 in FIG. 9). It shows that the main signal can be demodulated. On the other hand, the transmittance of the branched optical phase monitor device 58 when the branched optical phase monitor device 58 has a phase difference of zero with respect to the demodulator 52 is set to the position shown in FIG. However, zero phase difference means that no phase shift is generated, that is, the arm length is set to an integral multiple of the signal wavelength.

  Here, in order to simplify the drawing, the transmission characteristics of the branched optical phase monitor device 59 are omitted. However, in the case where the branched optical phase monitor device 59 is arranged so that the phase difference is zero with respect to the demodulator 53. The transmission characteristic is arranged with a shift of + π / 2 with respect to the transmission characteristic of the branched optical phase monitoring device 58 (so as to overlap with the +0.5 peak of the transmission characteristic of the curve of the Q port in the figure). Next, the transmission characteristics of the branched optical phase monitoring device 58 will be considered.

  The transmission characteristic of the branched optical phase monitor device (also referred to as an optical phase filter) 58 appears as a combined interferometer of two Mach-Zehnder interferometers 52 and 58. As described above, as a result of setting the FSR of the branch optical phase monitor device 58 to be half (B = 2A) of the Mach-Zehnder interferometer 52 of the demodulator, the cycle of the composite interferometer 62 is set to be twice the cycle of the demodulator 52. Is done. As a result, the output transmission characteristic of the demodulator 52 and the output transmission characteristic of the optical phase monitoring device 58 (that is, the transmission characteristic of the composite interferometer 62) are calculated by the FSR in the order of Equations (1) and (3), respectively. Will be set.

  Next, a phase monitoring method and an optical filter phase control method in this embodiment will be described. 10A and 10B show the ratio of the power monitor values of the two ports of the split optical phase monitoring device 58 (the output ratio between the two ports). However, in this simulation, as in FIG. 7, it is assumed that ideal phase modulation is applied to the signal light, that is, a complete rectangular spectrum having an occupied band of C (Hz) is input. In this simulation, the transmittance with a line width of 50% in FIG. 7 is converted into the power ratio between the two output ports, and only the effect of the branched optical phase monitoring device 58 included in the optical phase monitoring function unit 62 is obtained. I'm calculating. Actually, the transmission characteristic of the optical phase monitor function unit 62 is a combined effect of the two interferometers 52 and 58. Since some distortion of the light source spectrum occurs depending on the interferometer (demodulator) 52, the spectrum of the optical signal to the two input ports to the branched optical phase monitor device 58 is a rectangular spectrum as assumed in FIG. Rather, it receives some distortion. However, here, the effect of the spectral distortion is small and does not affect the explanation of the principle of operation, so it is omitted from the calculation.

  According to the simulation results of FIGS. 10A and 10B, the phase for demodulating the I-phase signal or the Q-phase signal + ¼ (position 81 in FIG. 9) and −1/4 (position 82 in FIG. 9). It can be seen that the output ratio (monitoring result) of the split optical phase monitoring device 58 in FIG. Here, it is assumed that the phase of the demodulator 52 is feedback-controlled using the monitoring result. Assuming that the initial phase of the optical signal 50 and the demodulator 52 is between 0 and 2.0, when the monitor result is smaller than 3.856, it is determined that the phase is 0.25 or more and 2.0 or less, and the phase is reduced in the direction of reducing the phase by feedback control. The control mechanism 54 is adjusted and locked to the I-phase lock point 83 (or 85).

  If the monitor result is greater than 3.85, it is determined that the phase is 0 or more and 0.25 or less, and the phase control mechanism 54 is adjusted in a direction to increase the phase by feedback control and locked to the lock point 83 (or 85) of the I phase. To do.

  Here, if the initial phase of the optical signal 50 and the demodulator 52 is between 2.0 and 4.0, if the monitoring result is smaller than 3.85, it is determined that the phase is 3.0 or more and 3.75 or less, and the phase is adjusted by feedback control. The phase control mechanism 54 is adjusted in the decreasing direction and locked to the I-phase lock point 83 (or 85). When the monitoring result is larger than 3.856, it is determined that the phase is 3.75 or more and 4.25 or less, and the phase control mechanism 54 is adjusted to increase the phase by feedback control and locked to the lock point 86 for the I phase.

  Thus, the phase inversion mechanism 74 feeds back the phase control mechanism 54 of the demodulator 52 so that the phase is decreased when the monitoring result is larger than the target value and the phase is increased when the monitoring result is smaller than the target value. By controlling, the phase of the demodulator 52 is always locked to 83, 85, 86 which are the lock points of the I phase. On the other hand, when the demodulator 52 is locked to the Q phase, the phase adjustment mechanism 54 of the demodulator 52 is feedback-controlled by positive logic (the logic inversion at the logic inverting element 74 is omitted) with 3.856 as a target value. The phase of the demodulator 52 is always locked to 84, 87, and 88 which are Q phase lock points.

  Here, when the phase adjustment mechanism 54 of the demodulator 52 is changed, in order for the spectrum of the demodulator 52 of FIG. 9 and the spectrum of the branched light phase monitor device 58 to move in parallel by the same phase amount, the branched light The change in the phase control mechanism 60 of the phase monitor device 58 needs to act in a direction to cancel the change in the phase control mechanism 54 of the demodulator 52. Considering that the FSR of the branching optical phase monitoring device 58 is twice that of the Mach-Zehnder interferometer 52 of the demodulator, the driving signal that is half of the driving signal to the demodulator (interferometer) 52, 53 is This is added to the phase adjusting mechanisms 60 and 61 provided at the arm positions on the opposite side of the phase monitoring devices 58 and 59. Therefore, in this embodiment, the amplifiers (amplifiers) 77 and 78 are set to a magnification of 1/2. Although the phase monitoring method and the phase control method of the demodulator 52 have been described above, the same phase monitoring method and phase control method as the demodulator 52 may be applied to the other demodulator 53. However, in order to avoid that both demodulators 52 and 53 are simultaneously locked to the I phase, the ethical inversion element 74 is inserted only into one demodulator (for example, the demodulator 52), and feedback control is performed. The logic is reversed.

  From the above description, the branch optical phase monitor devices configured as the composite interferometers (optical phase monitor function units) 62 and 63 by appropriately setting the FSR of the branch optical phase monitor devices 58 and 59 (B = 2C). It was shown that the phases of the demodulators 52 and 53 can be monitored using the output power ratios of the branched optical phase monitoring devices 58 and 59 by appropriately setting the FSRs of 58 and 59 (B = 2C). Further, the phase of the phase control mechanisms 54 and 55 of the demodulators 52 and 53 and the phase of the branch optical phase monitoring devices 58 and 59 are determined by simple feedback control based on the monitoring results (output intensity ratio) of the branching optical phase monitoring devices 58 and 59. By controlling the control mechanisms 60 and 61, it was clarified that the phase of the demodulator 52 and 53 can be controlled to an optimum phase value for demodulation of the I phase and the Q phase.

In the design of the present embodiment, the monitoring result of the branched optical phase monitoring devices 58 and 59 is 3.856, indicating that it is a lock point suitable for the demodulation of the I phase and the Q phase. It is important to prepare an assumed optical signal and determine the monitoring result of the branched optical phase monitoring devices 58 and 59 by a procedure of mapping while changing the phase of the branched optical phase monitoring devices 58 and 59. For example, in the simulation of FIG. 10, it is assumed that the signal light is given as a perfect rectangular spectrum with ideal phase modulation and an occupied band of C (Hz). A phase modulator using ordinary lithium niobate (LiNbO ₃ ) cannot achieve ideal phase modulation of 0 and π, and a region where the phase value transitions between 0 and π occurs between symbols. Because it does. Furthermore, the intensity of the optical signal decreases at the phase transition (from 0 to π, or from π to 0). In addition, depending on the modulation / demodulation method, as described in the prior art, a method of converting the phase modulation signal into an RZ pulse is also conceivable. As a result of the above, the spectrum of the phase modulation signal has an almost occupied band of C (Hz) (C is a baud rate), but does not have an ideal rectangular spectrum shape. Further, the branching accuracy of the outputs of the branching optical phase monitoring devices 58 and 59 may deviate from the ideal value due to manufacturing errors, and there may be a slight difference in the photoelectric conversion efficiency of the photodetector used in the power monitor. .

  However, even if any event occurs, as long as the configuration shown in FIG. 8 of the present embodiment is used, the power monitor of the output of the demodulator itself cannot in principle monitor the phase information, and the branch output of the demodulator The fact that the phase relationship between the optical signal and the demodulator can be monitored based on the monitoring result of the connected branching optical phase monitoring device is based on the essential operation principle of the present invention. Therefore, if the target value is set using the calibration procedure based on the mapping described above, the essential optical phase monitor and demodulator phase control shown in the present embodiment can be achieved.

  Next, an example of a specific configuration on the control side in the configuration of the demodulator in FIG. 8 is shown. Arithmetic processing circuits 72 and 73 convert the output power of the optical phase monitoring function units 62 and 63 detected by the photodetector into a digital signal by an analog-to-digital converter, and perform a digital signal processor (DSP) for arithmetic processing of the digital signal. It can be constituted by a digital circuit such as a CPU (Central Processing Unit) or FPGA (Field Programmable Gate Array). The arithmetic processing circuits 72 and 73 can constitute a feedback loop by a combination of analog circuits as another means. The amplifiers (amplifiers) 77 and 78 may be incorporated into the digital circuits of the arithmetic processing circuits 72 and 73 for arithmetic processing, or may be mounted with analog amplifier components.

  Further, when the optical DQPSK demodulator with an optical phase monitoring function of the present embodiment is configured by a silica-based optical waveguide, the phase adjustment mechanisms 54, 55, 60, and 61 are metal thin film heaters loaded on the waveguide. A method of controlling the phase using the thermo-optic effect by conducting energization overheating is suitable. Furthermore, when the optical DQPSK demodulator with an optical phase monitoring function of the present embodiment is configured by an optical fiber, the phase adjustment mechanisms 54, 55, 60, 61 are provided with a heater on the surface of the optical fiber in the same manner as the silica-based optical waveguide. There are a method of controlling the phase using a thermo-optic effect by loading and overheating the current, and a method of controlling the phase by winding an optical fiber around the piezo element and expanding and contracting the piezo element. Furthermore, when the optical DQPSK demodulator with an optical phase monitoring function of this embodiment is configured by a spatial optical system, any method can be used as long as it directly controls the optical path length of the space. There is a method of finely adjusting the optical path length by the position control.

(Fourth embodiment)
The optical DQPSK demodulator with an optical phase monitoring function has been described above with reference to FIG. 8. However, only the configuration of the upper half of FIG. 8 including one demodulator 52, the branched optical phase monitoring device 58, and the control system is used. Thus, an optical DPSK demodulator with an optical phase monitoring function is realized. This optical DPSK demodulator with an optical phase monitoring function is a fourth embodiment of the present invention. In this case, the optical branch 51 in FIG. 8 is not necessary. Also, the parameters, optical phase monitoring operation, optical phase monitor control operation, calibration procedure by mapping, and the like of this embodiment are redundantly described in accordance with the description of the optical DQPSK demodulator with an optical phase monitoring function in FIG. Detailed description thereof will be omitted.

(Fifth embodiment)
In the fifth embodiment of the present invention, the configurations of the optical DQPSK demodulator and the optical phase monitor function unit are the same as those in FIG. 8, and the relative phases of the demodulators 52 and 53 and the branched optical phase monitor devices 58 and 59 are π. An example in some cases is shown. This embodiment shows that the I phase and the Q phase are locked even in an extreme case where the phase shift is π.

  FIG. 11 shows the transmission characteristics when the phase of the demodulator and the branched optical phase monitor apparatus are shifted by π with the configuration of FIG. Here, the phase difference π means that the arm length is set to be shifted by a half wavelength in addition to an integral multiple of the signal wavelength. The other important parameters, ie, the FSR of the demodulators 52 and 53 are equal to the baud rate, and the FSR of the branch optical phase monitoring devices 58 and 59 are set to be twice the FSR of the demodulators 52 and 53. The FSRs 62 and 63 are twice the FSRs of the demodulators 52 and 53, which is the same as the third embodiment.

  Similar to FIG. 9, the transmission characteristics are obtained by arranging the spectrums of the four output ports (64, 65, 66, and 67 in FIG. 8) of the optical DQPSK demodulator at equal intervals on the optical frequency axis. The center frequency of the optical signal of the phase is set to +1/4 (position 89 in FIG. 11) and -1/4 (position 90 in FIG. 11), so that the main signals of the I phase and Q phase Can be demodulated. On the other hand, since the phases of the branched optical phase monitoring devices 58 and 59 are shifted by the phase π with respect to the demodulators 52 and 53, the transmittances of the branched optical phase monitoring devices 58 and 59 are the positions shown in FIG. Set to This is the position of the optical phase monitor shifted by π with respect to FIG.

  The optical phase monitoring results in this case are shown in FIGS. 12 (a) and 12 (b). However, in this simulation, the ideal phase modulation is applied to the signal light as in FIGS. 10A and 10B, that is, a complete rectangular spectrum with an occupied band of C (Hz) is input. And the effect of some light source spectral distortion by the Mach-Zehnder interferometer 52 of the demodulator is ignored. As a result of the simulations shown in FIGS. 12A and 12B, it can be seen that the optical phase monitoring result (output ratio between two ports) is 1 at the lock point (positions 91 to 97 in FIG. 12). That is, the optical DQPSK signal is demodulated by the demodulator 52 by adjusting the phase adjustment mechanism 54 of the demodulator 52 so that the output power ratio of the optical phase monitor device (corresponding to the light intensity ratio between the two ports) becomes 1. Will be.

  As described above, the optical phase monitoring results in FIGS. 9 and 11 show the relative phase difference between the demodulator and the optical phase monitoring device (that is, the relative positional relationship between the demodulator spectrum and the optical phase monitor spectrum). Is calculated in advance, the monitor result indicating the position of the target phase is known by calculating the ratio of the two powers of the monitor output of the optical phase monitor device, and the feedback loop is set so that the target value is obtained. , It means that the phase of the demodulator can be locked stably. As already explained in the last paragraph of the background art section, this is generally difficult to realize with the accuracy of nm as the optical path length difference in the current optical device technology as the production accuracy of the delay interferometer. This means that the present invention is still effective even in a situation where it is extremely difficult to set the phase to 0 or π accurately. In other words, by calibrating the relative phase difference between the demodulator and the optical phase monitor device in advance, the phase modulation signal can be converted into a phase modulation signal by imperfection of phase modulation generated by the phase modulator, power fluctuation between symbols, and modulation / demodulation method. RZ intensity modulation is combined and calibration is performed including the branch error of the output of the optical phase monitor, the error of the photoelectric conversion efficiency of the photodetector, and the phase error between the demodulator and the optical phase monitor. A demodulator phase monitor and a demodulator phase control are provided.

(Sixth embodiment)
In the sixth embodiment of the present invention, the configuration is the same as in FIG. 8, the FSR of the branch optical phase monitoring devices 58 and 59 is B = 3C, and the relative relationship between the demodulators 52 and 53 and the branch optical phase monitoring devices 58 and 59 is relative. The example in case a phase is zero is shown. The phase difference of zero means that the arm length is set to an integral multiple of the signal wavelength.

  FIG. 13 shows the transmission characteristics. Here, the spectrum of the four output ports (64, 65, 66, 67 in FIG. 8) of the demodulator is arranged at equal intervals on the optical frequency axis as in FIGS. Is set to +1/4 (position 98 in FIG. 13) and -1/4 (position 99 in FIG. 13), so that the main signals of I phase and Q phase can be demodulated. it can. On the other hand, the outputs (3x phase bit rate curves) of the split optical phase monitoring devices 58 and 59 are broader than those in FIG. 9, and are three times the spectrum of the demodulators 52 and 53.

  FIGS. 14A and 14B show the optical phase monitoring results of the present embodiment. However, in this simulation, as in FIGS. 10A, 10B, 12A, and 12B, the ideal phase modulation is applied to the signal light, that is, the occupied band is C (Hz). And the effect of the light source spectrum distortion by the Mach-Zehnder interferometer (demodulator) 52 is ignored.

  As a result of the simulations of FIGS. 14A and 14B, it is found that a lock point for demodulating the I phase and the Q phase can be obtained at the power ratio (output ratio between two ports) 8.933. That is, as in the third embodiment and the fifth embodiment described above, the power ratio of the optical phase monitor device indicates a certain value (8.933 in this case) that is determined by the phase between the filters. It can be seen that the optimal point can be obtained. However, as described in the third embodiment, control is performed so as to cancel the phase of the branch optical phase monitor device with respect to the phase control of the demodulator, and the spectrum of the demodulator and the branch optical phase monitor device of FIG. Therefore, in this embodiment, the gains of the amplifiers (amplifiers) 77 and 78 are set to 1/3.

  Here, the important point is that the number of phase lock points in FIGS. 14A and 14B is smaller (sparser) than in FIGS. 12A and 12B. This is because the FSR of the optical phase monitor is large (three times that of the demodulator) in this embodiment, and the monitor result does not have a phase resolution equal to or less than FSR. However, conversely, when the FSR of the branching optical phase monitoring device is reduced and a parameter satisfying B> A is selected, the phase is changed at any adjacent period of the demodulator based on the result of the branching optical phase monitoring device. Although it is possible to monitor, the monitoring result is not a constant value (3.856 in the third embodiment, 1 in the fifth embodiment, 8.933 in the sixth embodiment), and the value is a phase value. Since it becomes a function, the phase mapping must be previously provided on the control circuit side and the target value must be set stepwise according to the initial phase. Therefore, in addition to the configuration of FIG. Therefore, it is necessary to learn the frequency information and determine the target value of the lock point, which makes it difficult to apply realistic control.

  In the third embodiment and the fifth embodiment, the FSR of the branch optical phase monitor devices 58 and 59 is set to B = 2C in advance. The FSR of the branch optical phase monitor devices 58 and 59 is demodulator 52, 53. If it is an integer multiple of FSR, a certain value can be generated as a lock point with that integer period, so phase control has the advantage of a simple circuit configuration that feedback-controls that constant value, When the integer is 2, one lock point can be generated for two of the FSRs of the original demodulator, and the phase axis of the demodulator (the horizontal axis of FIGS. 2, 9, 11, and 13). This is because the maximum number of lock points is obtained on the normalized optical frequency), and the efficiency with respect to the phase control amount in the phase control is maximized.

(Seventh embodiment)
As described above, in the third, fifth, and sixth embodiments, the example in which the optical phase monitoring device and the optical filter phase control method of the present invention are applied to the demodulation of the optical DQPSK signal has been described, but the seventh embodiment of the present invention is described. In the embodiment, an example in which the optical phase monitoring device and the optical filter phase control method of the present invention are extended to further phase multilevel modulation is shown. FIG. 15 shows the configuration.

  As shown in FIG. 15, the optical filter to be phase-controlled is an optical D8PSK demodulator composed of a plurality of interferometers 109, 110, 111, and 112. Here, each demodulator 109, 110, 111, 112 is assumed to be a general delay interferometer (Mach-Zehnder interferometer), and B (Hz) which is FSR is an optical D8PSK signal as shown in the above equation (1). It shall be equal to the baud rate.

  Compared with the configuration of the DQPSK demodulator according to each of the third, fifth, and sixth embodiments of the present invention described above, the present embodiment is merely an increase in the number of delay interferometers of the demodulator. The basic principles of the phase monitoring method and the optical filter phase control method are the same. That is, a part of the optical signal of the demodulators 109, 110, 111, and 112 is taken out and guided to the branched optical phase monitor devices 117, 118, 119, and 120, and the output of the branched optical phase monitor device is output to the power monitor 129. , 130, 131, 132, 133, 134, 135, 136, and the detected values are calibrated through the arithmetic processing circuits 137, 138, 139, 140, thereby generating an optical phase monitor result.

  The optical phase monitor function units 125, 126, 127, and 128 are configured by demodulators 109, 110, 111, and 112 and branched optical phase monitor devices 117, 118, 119, and 120.

  In order to control the demodulators 109, 110, 111, and 112 to the optimum phase points by the branched optical phase monitor devices 117, 118, 119, and 120, error signals are generated by the arithmetic processing circuits 137, 138, 139, and 140, The phase adjustment mechanisms 145, 146, 147, 148, 121, 122, 123, and 124 are feedback-controlled by the drive circuits 141, 142, 143, 144, 153, 154, 155, and 156 using the error signal as a control signal.

  The relationship B> C between B (Hz), which is the FSR of the branched optical phase monitoring devices 117, 118, 119, and 120, and the baud rate C (Hz) of the optical D8PSK signal is the minimum requirement for optical phase monitoring. Furthermore, in the case of B = 2C, the target value can be set to a constant value from the result of the optical phase monitor, phase control can be realized by simple feedback control, and there are the most lock points on the phase axis, The point which can achieve the phase control which can obtain the maximum efficiency is the same as in the third to sixth embodiments. In the case of DMPSK with M-value phase multiplexing, the target value is π / M. For example, in the case of DQPSK, the target value is π / 4. Further, even if an error generated in the phase modulator, a combination with intensity modulation, an interferometer manufacturing error, and an error in photoelectric conversion efficiency of the photodetector (power monitor) are generated, calibration is performed including them, so that FIG. The configuration is similar to the third to sixth embodiments in that the phase monitor of the demodulator and the phase control of the demodulator can be realized.

(Other embodiments)
As described above, in the seventh embodiment of the present invention, the specific example adapted to the optical D8PSK signal has been shown. However, the optical phase monitoring apparatus and the optical filter phase control method of the present invention can be further multi-level phase codes (D16PSK, D64PSK, etc.). ). As described in the present embodiment, this is achieved by a configuration in which the branched optical phase monitor device is connected to the output of each demodulator. Furthermore, the present invention can be applied to a QAM code, which is an application of a multilevel phase code, with a similar configuration.

  In the above description, the preferred embodiment of the present invention has been illustrated and described. However, the embodiment of the present invention is not limited to the above-described example, and is within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease of the number of components, design change of the shape, etc. are all included in the embodiment of the present invention.

  For example, in the embodiment of the present invention, an example of application to a demodulating interferometer in the multi-level phase code method has been shown. Further, the optical phase monitoring apparatus and the optical filter phase control method of the present invention include other general optical filters, For example, adaptation to a multiplexing / demultiplexing filter, a dispersion compensation filter, and a variable dispersion compensation filter is also possible. In this case, when it is necessary to monitor and control the optical filter and the optical signal at the optical frequency level or the optical phase level, the effect is exhibited regardless of the signal format and speed of the main signal.

  In addition, the present invention may be applied to a system composed of a plurality of devices, or may be applied to an apparatus composed of one device.

  INDUSTRIAL APPLICABILITY The optical phase monitoring device, the optical filter with an optical phase monitor, and the optical filter phase control method of the present invention can be used for optical communication devices used in optical communication networks and the like, and are expected to contribute to the development of optical communication. .

It is a block diagram of a general optical DQPSK demodulator in an optical communication system. It is a figure which shows the transmission characteristic of a general optical DQPSK demodulator, and the optimal demodulation phase value (lock point). It is a block diagram of the demodulation part of the conventional optical phase multilevel modulation signal. It is a block diagram of the optical filter frequency control using the conventional dithering signal. It is a block diagram of optical filter wavelength control using the conventional optical interferometer. It is a block diagram of the optical phase monitor of the 1st Embodiment of this invention. It is a transmission characteristic figure of the optical phase monitor of the 1st Embodiment of this invention. It is a block diagram of the optical DQPSK demodulator using the optical phase monitor of the 3rd Embodiment of this invention. It is a figure which shows the transmission characteristic and optimal demodulation phase value (lock point) of the optical DQPSK demodulator and optical phase monitor of the 3rd Embodiment of this invention. However, B = 2C and relative phase 0. It is a figure which shows the result (power ratio) and lock point of the optical phase monitor of the 3rd Embodiment of this invention. However, B = 2C and relative phase 0. It is a figure which shows the transmission characteristic and optimal demodulation phase value (lock point) of the optical DQPSK demodulator and optical phase monitor of the 5th Embodiment of this invention. However, B = 2C and relative phase π. It is a figure which shows the result (power ratio) and lock point of the optical phase monitor of the 5th Embodiment of this invention. However, B = 2C and relative phase π. It is a figure which shows the transmission characteristic and optical demodulation phase value (lock point) of the optical DQPSK demodulator and optical phase monitor of the 6th Embodiment of this invention. However, B = 3C and relative phase 0. It is a figure which shows the result (power ratio) and lock point of the optical phase monitor of the 6th Embodiment of this invention. However, B = 3C and relative phase 0. It is a block diagram of the optical multilevel PSK demodulator using the optical phase monitor of the 7th Embodiment of this invention.

Explanation of symbols

1, 50 Optical DQPSK signal 2, 14, 15, 16, 51, 106, 107, 108 Coupler (Optical splitter)
3, 4, 17, 18, 19, 20, 52, 53, 109, 110, 111, 112
Demodulator (Mach-Zehnder interferometer)
5, 6, 36, 37, 38, 39, 54, 55, 60, 61, 121, 122, 123, 124, 145, 146, 147, 148 Phase adjustment mechanism 7, 8, 9, 10, 21, 22, 23, 24, 25, 26, 27, 28, 64, 65,
66, 67 Data signal (I phase, Bar I phase, Q phase, Bar Q phase)
11, 12, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91,
92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104 Lock point of any of I phase, Bar I phase, Q phase, or Bar Q phase 13, 105 Optical D8PSK signal 29 , 40 Optical signal input 30 Oscillator 31, 41 Optical filter 32 Light receiver 33 Low pass filter 34 Multiplier 35 Error signal 42, 58, 59, 117, 118, 119, 120 Branched optical phase monitor device (Mach-Zehnder interferometer)
43, 44, 68, 69, 70, 71, 129, 130, 131, 132, 133,
134, 135, 136 Power monitor 45 Single wavelength CW light source 46 Modulator 47 Demodulator / phase monitor device 48, 49, 101, 102 Optical signal output 62, 63, 125, 126, 127, 128 Optical phase monitor function unit ( Compound interferometer)
72, 73, 137, 138, 139, 140 Arithmetic processing circuit 74 Logic inversion circuit 75, 76, 79, 80, 141, 142, 143, 144, 153, 154, 155, 156 Drive circuit 77, 78, 149, 150 151, 152 Amplifier (Amplifier)
96 Optical DPSK signal 201 Data recovery logic circuit

Claims (6)

2 ^(n-1) demodulators (n is a positive integer) with phase adjustment mechanism ;
Optical branching means for branching a part of the output of the demodulator;
An optical phase monitoring device connected to the optical output port of the optical branching means;
An optical PSK demodulator comprising:
The optical-phase monitoring apparatus monitors the phase of the optical signal from the optical modulation source having an occupied band A (Hz) in the optical frequency domain,
A free spectral region FSR (Free Spectral Range) for incident light signals from the light modulation light source has a Mach-Zehnder interferometer that is B (Hz),
B (Hz) is
B> A
Of which is set in relation, and an optical PSK demodulator characterized by monitoring the phase of the optical signal from the light modulation source by the light intensity ratio of the two light output ports of the Mach-Zehnder interferometer. B (Hz) is
B = mA (m is an integer of 2 or more)
The optical PSK demodulator according to claim 1, wherein The transmittance T at one of the two ports of the Mach-Zehnder interferometer has a phase θ (radian), and the line width of the optical modulator light source with respect to the FSR of the Mach-Zehnder interferometer is d (%). If

The optical PSK demodulator according to claim 1 or 2, characterized by: 2. The optical PSK demodulator according to claim 1 , wherein the optical branching means is an optical waveguide that branches a part of the output of the optical filter and connects to the optical phase monitor device. 2. The optical PSK demodulation according to claim 1 , further comprising control means for controlling the phase adjustment mechanism in accordance with the monitoring result so that the monitoring result by the optical phase monitoring device becomes a target constant value. vessel. In 請 Motomeko 1 to the optical PSK demodulator according to 5 any one,
An optical filter phase control method, wherein the phase adjustment mechanism is controlled according to the monitoring result so that the monitoring result by the optical phase monitoring device becomes a target constant value.

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