JP4975775B2 - Transmitter and transmission method - Google Patents

Description

  The present invention relates to an automatic bias control of an optical modulator, and in particular, a multi-valued optical differential phase shift keying signal (DMPSK) with a pulse carver (RZ: Return to Zero). The present invention relates to a transmitter for transmission and a transmission method.

  As a transmission code used in an optical transmission system, a DMPSK system having a high nonlinear resistance has been widely studied. In particular, a modulation method in which a pulse carver is combined with the DMPSK method and the light intensity between symbols is “0” is effective. Hereinafter, in order to simplify the explanation, RZ-DQPSK (Return to Zero-DQPSK) in which a pulse carver is combined with differential quadrature phase shift keying (DQPSK), which is quaternary multi-level phase modulation. The description will be limited to the modulation method.

  In order to generate an RZ-DQPSK signal, a configuration in which a DQPSK modulator and an RZ modulator are combined in series is common. A typical example is shown in FIG. In FIG. 9, the CW light input from the CW (Continuous Wave) light source 1 to the DQPSK modulator 2 is divided into two by the first coupler 3, and the first phase modulation means 4 and the second phase modulation means 4 Input to the phase modulation means 5. Each of these two phase modulators 4 and 5 is modulated by the binary digital signals DATA1 and DATA2 to generate two phase-modulated lights whose optical phase varies by π.

  These two phase-modulated lights are combined by the second coupler 7. At this time, the two phase-modulated lights are combined after adding a delay corresponding to a quarter of the wavelength by the π / 2 phase adjusting means 6. . The output of the second coupler 7 is output to the outside of the DQPSK modulator 2 as NRZ-DQPSK signal light. The NRZ-DQPSK signal light is intensity-modulated by the RZ modulator 8. The period of this intensity modulation is synchronized with the clock of each data, and is adjusted so that the intermediate transition state between symbols of the NRZ-DQPSK signal light is quenched. The output of the RZ modulator 8 is an RZ-DQPSK signal.

  As is well known, when an optical modulator is used, it is necessary to apply a bias voltage in order to finely adjust the optical path length. In general, the optimum value of the bias voltage is not constant, and drift occurs due to the temporal variation of the optical modulator. In order to suppress signal quality degradation, a bias control circuit that keeps the bias voltage at an optimum value is extremely important. As shown in FIG. 9, in order to generate one RZ-DQPSK signal, four types of bias voltages Bias1 to Bias4 are required, and a bias control circuit (not shown) is required for all of them. A configuration using dithering is widely known as means for detecting drift of the bias voltages Bias1 to Bias4.

  FIG. 10 is a block diagram showing a configuration of an RZ-DQPSK transmitter that realizes the above-described bias control and drift detection in the prior art. Note that portions corresponding to those in FIG. 9 are denoted by the same reference numerals and description thereof is omitted. In FIG. 10, bias voltages Bias1 to Bias3 are generated by first to third DC power supplies 10-1, 10-2, and 10-3, respectively. The first low-frequency signal generator 11 generates a sine wave having a frequency f1 sufficiently lower than the bit rate, and the first low-frequency superposition circuit 13-is time-shared through the first changeover switch 12. The first low frequency superposition circuit 13-2 and the third low frequency superposition circuit 13-3 are supplied.

  In the first low frequency superposition circuit 13-1, the second low frequency superposition circuit 13-2, and the third low frequency superposition circuit 13-3, the first to third DC power supplies 10-1 to 10- are respectively provided. By superimposing a sine wave of frequency f1 on each output of 3, dithering is added to each bias voltage Bias1 to Bias3 by time sharing. The NRZ-DQPSK signal output from the second coupler 7 is subject to fluctuations in the frequency f1 due to the three types of dithering. A part of the output of the second coupler 7 is tapped by the first monitor coupler 14 and supplied to the first monitor means 15. Here, the first monitor means 15 is a low-speed photo detector.

  The fluctuation of the optical power obtained by the first monitoring means 15 is synchronously detected by the first synchronous detection circuit 16 together with the output signal of the reference first low frequency generator 11. In the first synchronous detection circuit 16, a voltage control signal is generated based on the obtained synchronous detection result, and the first to third DC power supplies 10-1 to 10-10 are time-shared by the second changeover switch 17. -3. The first changeover switch 12 and the second changeover switch 17 are synchronized, and a voltage control signal is fed back to the first to third DC power supplies 10-1 to 10-3 to which a dither signal is applied. The As a result, the bias voltages Bias1 to Bias3 are controlled to appropriate bias values.

  On the other hand, the same applies to the bias voltage Bias4 of the RZ modulator 8. That is, the bias voltage Bias4 is generated by the fourth DC power supply 10-4. The second low frequency signal generator 21 generates a sine wave having a frequency f2 sufficiently lower than the bit rate, and supplies the sine wave to the fourth low frequency superposition circuit 13-4. In the fourth low-frequency superimposing circuit 13-4, the sine wave having the frequency f2 is superimposed on the output of the fourth DC power supply 20, whereby dithering is added to the bias voltage Bias4.

  On the other hand, a part of the output of the RZ modulator 8 is tapped by the second monitor coupler 23 and supplied to the second monitor 24. The fluctuation of the optical power obtained by the second monitoring means 24 is synchronously detected by the second synchronous detection circuit 25 together with the output signal of the second low frequency generator 21 for reference, and the obtained synchronous detection result is obtained. A voltage control signal is originally generated and fed back to the fourth DC power supply 10-4. As a result, the bias voltage Bias4 is controlled to an appropriate bias value.

  A configuration in which the first monitor coupler 14 and the first monitor means 15 are incorporated in the DQPSK modulator is widely used. The configuration shown in FIG. 10 is simple because the feedback loop of the bias voltage Bias1 to 3 and the feedback loop of the bias voltage Bias4 are independent.

  The feedback loop described above is somewhat complicated, but a configuration as shown in FIG. 11 has already been proposed. Note that portions corresponding to those in FIG. 10 are denoted by the same reference numerals and description thereof is omitted. In the configuration shown in FIG. 11, the first monitor coupler 14 and the first power monitoring means 15 in FIG. 10 are eliminated, and only the RZ-DQPSK signal that is the output of the RZ modulator 8 is monitored. The output of the first low-frequency generator 11 is superimposed on all of the bias voltages Bias1 to 4 by time sharing, and the voltage control signal obtained by synchronous detection is also used for the first to fourth DC power supplies 10 by time sharing. -1 to 10-4. Note that the first synchronous detection circuit 16 and the second synchronous detection circuit 25 are also integrated into one of the synchronous detection circuits 28.

Japanese Patent No. 2642499

  However, the above-described prior art has the following problems.

  Even if dithering is performed on the bias voltage Bias3 applied to the π / 2 phase adjusting means 6 in the prior art, the optical power at the output of the DQPSK modulator 2 varies only slightly. This is because when the bias voltage Bias3 deviates from an appropriate value, there are both a sign that the optical power increases and a sign that the light power decreases, and cancel the influences. The total optical power fluctuates only slightly, and synchronous detection is difficult. Increasing the dithering amplitude also increases the amount of fluctuation in the optical power of the output of the DQPSK modulator 2, but this is undesirable because it causes quality degradation of the transmission signal.

  Another problem is that a random intensity change always occurs in the NRZ-DQPSK signal light that is the output of the DQPSK modulator 2. The first and second phase modulators 4 and 5 are accompanied by intensity modulation when the phase is switched. The signal light is subjected to intensity modulation that differs between symbols depending on whether both of them change phase, only one of them changes phase, or both hold the previous phase. For this reason, the efficiency of synchronous detection falls.

  In the configuration shown in FIG. 11, since the RZ-DQPSK signal is monitored and the intensity modulation between symbols is suppressed, this problem is solved. However, the feedback with respect to the bias voltage Bias1 to 3 and the feedback with respect to the bias voltage Bias4 form a double loop, and the control tends to become unstable. Further, there is a problem that the monitor coupler 14 and the monitor means 15 built in the DQPSK modulator 2 cannot be used.

  Further, in the double loop configuration, when dithering is time sharing, no dither is added to the RZ modulator 8 in the time zone in which the π / 2 phase adjustment unit 6 is dithered. . However, the generation of circuit noise due to the fluctuation of the fourth DC power supply 10-4 and the external noise picked up by the conducting wire connected to the bias voltage Bias4 is unavoidable to some extent. In particular, the latter is disadvantageous as the circuit scale increases.

  Incidentally, as described above, detection of π / 2 bias dithering is generally very low sensitivity. In the configuration shown in FIG. 11, since the RZ modulator 8 is interposed between the second monitor coupler 23 and the bias voltage Bias3 (π / 2) bias, the π / 2 received by the second monitor coupler 23 is obtained. There is a problem that the circuit noise coming from the bias voltage Bias4 described above is superimposed on the dithering signal.

  The above description is for the case where the dithering is time sharing, but when all the bias voltages Bias1 to Bias4 are dithered simultaneously (this is possible with different dither frequencies and is a known technique). The dithering to the bias voltage Bias4 in FIG. 11 becomes noise for the dithering signal to the bias voltage Bias3 (π / 2) bias. If the dither frequencies of the bias voltage Bias4 and the bias voltage Bias3 are different, they can be separated by synchronous detection, but detection of π / 2 bias dithering is generally disadvantageous because it is very low in sensitivity.

  The present invention has been made in consideration of such circumstances, and its purpose is to accurately control the bias voltage of the differential multiphase phase shift modulator while minimizing signal quality degradation. It is an object of the present invention to provide a transmitter and a transmission method that can be used.

In order to solve the above-described problem, the present invention provides a signal transmitter that outputs an RZ-DMPSK signal obtained by pulse-curving differential multiphase phase shift keying, and a pulse generator that inputs a light source and the light source output. and use the modulator, before Symbol pulse generating modulator dithering means for dithering the bias pulse generating modulator, a pulse light power monitor for monitoring the optical power of the optical pulse train output from the pulse generating modulator And a pulse generator modulator bias for optimizing the bias of the pulse generator modulator by synchronous detection of the output of the pulse light power monitor means and the dithering signal output from the pulse generator modulator dithering means differential multi-phase to the control unit, and the pulse light bias controller provided with the, an input optical pulse train output from the pulse generating modulator A phase shift modulator, and a plurality of differential multi-phase phase-shift modulator dithering means for performing dithering on all of the plurality of bias having the said differential multiphase phase shift modulator, said differential polyphase phase Modulated light power monitoring means for monitoring the optical power of the output of the shift modulator, and the dithering signal output from the output of the modulated light power monitor means and the differential polyphase phase shift modulator dithering means Differential multiphase phase shift modulator bias control means for optimizing each of a plurality of biases of the differential polyphase phase shift modulator by synchronous detection, the light source and the differential polyphase phase shift between the modulator, the pulse generating modulator and the pulsed light bias controller is arranged, a feedback loop with the said pulse generator for the modulator bias control means, said differential multiphase phase shift modulator bias And be made independent of a feedback loop with the control means, an optical signal to be monitored by the modulated light power monitoring means that a RZ-DMPSK signals, a transmitter according to claim.

  According to the present invention, in the above invention, the differential polyphase phase shift modulator dithering means increases the dithering amplitude when the apparatus is started up, and a predetermined time has elapsed since the apparatus start-up. After that, the dithering amplitude is reduced.

  According to the present invention, in the above-described invention, a 1 to N branch coupler that branches an optical pulse train output from the pulse generating modulator into N branches, and an optical pulse that is N branched by the 1 to N branch coupler. , The first to Nth differential multiphase phase shift modulators that perform different modulations of the baud rate B, and the optical pulses mutually output from the first to Nth differential multiphase phase shift modulators. And a multiplexing means for performing time multiplexing by adding a delay difference so as not to interfere, and the pulse generating modulator generates an optical pulse train having a period of 1 / B and a duty ratio of 1 / N or less, The multiplexing means outputs an optical differential multiphase shift signal having a baud rate of B × N.

  According to the present invention, in the above invention, the one-to-two branch coupler for branching the optical pulse train output from the pulse generating modulator and the two outputs from the one-to-two branch coupler are respectively modulated. 1 and a second differential polyphase phase shift modulator, and a multiplexing means for polarization multiplexing the outputs from the first and second differential polyphase phase shift modulators. It is characterized by that.

  According to the present invention, in the above invention, in place of the modulated light power monitoring means, modulated light waveform monitoring means for monitoring the waveform of the modulated light output from the differential multiphase phase shift modulator, and the modulated light A peak hold circuit that detects the peak value from the waveform of the modulated light monitored by the waveform monitoring means and holds the peak value for a duration longer than the same sign continuous time of the transmitter. Features.

  The present invention is the above invention, wherein the pulse generating modulator dithering means and the differential polyphase phase shift modulator dithering means have a dithering frequency equal to or lower than a low cutoff frequency of a receiving system. It is characterized by that.

  According to the present invention, in the above invention, the differential polyphase phase shift modulator bias control means applies feedback so that the synchronous detection output takes a positive or negative extreme value, and controls a π / 2 bias. / 2 bias control means, and the π / 2 bias control means determines whether or not the result of synchronous detection at the current bias value is an extreme value, and if not, increases the current bias value. An extremum determination function that determines whether the current value is to be held or not, and if it is an extremum, has an extremum determination function that keeps the dithering amplitude constant. Compare and refer to the synchronous detection result while slightly changing the average voltage of the ring, and determine whether the synchronous detection result at the current bias value is an extreme value based on the comparison of the comparison reference result, Also, Based on the comparison reference result if the result of the synchronous detection in the bias value of the resident is not extreme, the result of the synchronous detection defines the change amount of the bias value to approach the extremum, characterized in that.

  According to the present invention, in the above invention, at the time of starting up the apparatus, before operating the extreme value determination function, synchronous detection is performed while sweeping a bias voltage in advance, and a rate of change of the synchronous detection result with respect to a change in the bias voltage. Is measured in advance.

In order to solve the above-described problem, the present invention is a signal transmission method for outputting an RZ-DMPSK signal obtained by pulse-curving differential multiphase phase shift keying, and includes a light source in a pulse generation modulator. a pulse generating modulation step of inputting the output, pulse generating modulator dithering means comprises a bias dithering pulse generation modulator dithering step of the pulse generator for the modulator, the pulse light power monitor means A pulse light power monitoring step for monitoring the optical power of the optical pulse train output from the pulse generation modulator, and a pulse generation modulator bias control means comprising: an output of the pulse light power monitor means; and the pulse generation modulation. A filter that optimizes the bias of the modulator for pulse generation by synchronous detection with the dithering signal output from the dithering means. Differential multi-phase to scan the generation modulator bias control step, a pulsed light bias control step including a differential multiphase phase shift modulator, and inputs the optical pulse train output from the pulse generating modulator A phase shift keying step and a plurality of differential polyphase phase shift modulator dithering means dithering all of the plurality of biases of the differential polyphase phase shift modulator; A phase shift modulator dithering step, a modulated optical power monitor means for monitoring the optical power of the output of the differential multiphase phase shift modulator, and a differential multiphase phase shift keying The bias bias control means comprises the differential multi-phase phase shift keying by synchronous detection of the output of the modulated optical power monitor means and the dithering signal output from the differential multi-phase phase shift modulator dithering means. Of including a differential multiphase phase shift modulator bias control step of each optimize the plurality of bias, and the prior differential multiphase phase shift keying step, the pulse generating modulation step and the pulse light Performing a bias control step, making the feedback loop of the pulse generator modulator bias control means independent of the feedback loop of the differential polyphase phase shift modulator bias control means , and modulating light power The transmission method is characterized in that the optical signal monitored by the monitoring means is an RZ-DMPSK signal.

  According to the present invention, it is possible to obtain an advantage that the bias voltage of the differential multiphase phase shift modulator can be controlled with high accuracy while minimizing signal quality degradation.

It is a block diagram which shows the structure of the RZ-DQPSK transmitter by 1st Embodiment of this invention. In order to compare the first embodiment with the prior art shown in FIG. 10, it is an explanatory diagram showing system fluctuations when the bias voltage Bias3 is intentionally changed. It is a block diagram which shows the structure of the RZ-DQPSK transmitter by this 2nd Embodiment. It is a block diagram which shows the structure of the RZ-DQPSK transmitter by this 3rd Embodiment. It is a conceptual diagram which shows the operation principle of this 3rd Embodiment. FIG. 4 is a block diagram showing a configuration of an RZ-DQPSK transmitter, in which an output of an RZ modulator is branched into two, and each output is DQPSK modulated and then polarization multiplexed. It is a block diagram which shows the structure which concerns on a RZ-DQPSK transmitter and carries out time multiplexing. It is explanatory drawing for demonstrating the control method of bias voltage Bias3 by this 4th Embodiment. It is a block diagram which shows the structure of the typical RZ-DQPSK transmitter by a prior art. It is a block diagram which shows the structure of the RZ-DQPSK transmitter which implement | achieves bias control and drift detection in a prior art. It is a block diagram which shows the other structural example of the RZ-DQPSK transmitter by a prior art.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

A. First Embodiment FIG. 1 is a block diagram illustrating a configuration of an RZ-DQPSK transmitter according to a first embodiment of the present invention. The first embodiment corresponds to the prior art shown in FIG. 10 described above, and the same reference numerals are given to the portions corresponding to FIG. In the figure, the first monitoring means 15 is a low-speed photo detector (PD) and monitors the optical power. As the first monitoring means 15, a low-speed PD built in a commercially available DQPSK modulator may be used. The first embodiment is obtained by replacing the order of the RZ modulator 8 and the accompanying bias control circuit 30 with respect to the DQPSK modulator 2 in the prior art shown in FIG. The modulator 8 and the accompanying bias control circuit 30 are interposed between the CW light source 1 and the DQPSK modulator 2.

  As described above, the bias that is most difficult to control is the bias voltage Bias3 that performs the π / 2 phase adjustment. Hereinafter, bias control of the π / 2 phase adjusting unit 6 will be described.

The DQPSK modulator 2 converts the output light of one of the two optical phase modulators, that is, the first and second optical phase modulators 4 and 5, by the π / 2 phase adjusting means 6 into an optical phase of π / 2. In addition, it interferes with the other output light. Specifically, the π / 2 phase adjusting means 6 gives a delay corresponding to ¼ of the carrier wavelength. Here, the electric fields of the output light of the optical phase modulator 4 and the output light of the π / 2 phase adjusting means 6 are expressed as D _I × exp (iωt) and D _Q × exp (iωt + π / 2), respectively. To do. Here, ω is a carrier frequency, t is a time, D _I and D _Q are data signs, and can be normalized to a value of ± 1 at the center of the time slot.

The light intensity I ₀ after D _I × exp (iωt) and D _Q × exp (iωt + π / 2) interfere with each other at the second coupler 7 is
I ₀ = | D _I × exp (iωt) + D _Q × exp (iωt + π / 2) | ²
= | D _I | ² + | D _Q | ²

It is expressed. At the center of the time slot, D _I and D _Q are ± 1, so I ₀ is constant.
Here, it is assumed that the phase difference created by the π / 2 phase adjusting means 6 changes from the optimal value π / 2 to π / 2 + α due to the change in optical characteristics of the modulator over time. The output light I ₁ of the DQPSK modulator 2 at this time is

I ₁ = | D _I × exp (iωt) + D _Q × exp (iωt + π / 2 + α) | ²
= | D _I | ² + | D _Q | ² + 2 × D _I × D _Q × cos (π / 2 + α)
= | D _I | ² + | D _Q | ² −2 × D _I × D _Q × sin (α)

  It becomes.

As described above, since π / 2 is an optimum value for the phase difference created by the π / 2 phase adjusting means 6, when the phase difference fluctuates due to the change of the modulator over time, the bias voltage Bias3 is corrected to obtain α Must be kept at 0. That is, it must be reduced when α> 0 and increased when α <0. However, sin (α) is an odd function, and D _I × D _Q can take both +1 and −1 at random in the center of the time slot. The size of ₁ is constant regardless of α. In reality, modulator and collapse symmetry is slightly by the band limiting of the modulator driver, since I ₁ also varies slightly with changes in alpha, detecting variations in the alpha by monitoring the variation of I ₁ It is possible. However, as described in the above-mentioned problem of the prior art, the fluctuation of I ₁ is extremely small, so that it is generally difficult to detect this. D _I and D _Q take a value of ± 1 at the center of the time slot, but irregularly change at the boundary of the time slot. As described in the problem of the prior art described above, this is due to the intensity change accompanying the phase modulation. The magnitude of this intensity change depends on the change of the sign and the characteristics of the modulator. It becomes difficult.

  In order to detect such small fluctuations as described above, the dither method is generally used. When dithering is performed by superimposing a sine wave having a minute frequency f0 on the bias voltage Bias3 of the π / 2 phase adjusting means 6, the bias voltage Bias3 is expressed by V0 + ΔV × cos (2π · f0 × t). The relationship between the bias voltage Bias3 and the phase difference is k · V0 + k · ΔV × cos (2π · f0 × t) using the proportional coefficient k that can be configured by the modulator, so that the light intensity Id when dither is applied is ,

I _d = | D _I × exp (iωt) + D _Q × exp (iωt + k · V0 + k · ΔV × cos (2π · f0 × t)) | ²
= | D _I | ² + | D _Q | ²
+ 2 × D _I × D _Q × cos (k · V0 + k · ΔV × cos (2π · f0 × t))

It becomes. The target state is a state in which the argument of COS slightly vibrates in the vicinity of π / 2. For this purpose, the dither amplitude ΔV is sufficiently small, and the fluctuation component of the period f0 appearing in Id is maximized. V0 may be adjusted as follows. As described above, the fluctuation of Id is extremely small, but it is possible to increase the detection efficiency by selectively amplifying the fluctuation component of frequency f0 from Id and performing synchronous detection with the dithering signal. It has been known.
In the present invention, the light is already pulsed when the light enters the DQPSK modulator. Therefore, there is no irregular intensity change in the vicinity of the time slot boundary described above, the detection efficiency of the fluctuation component of the frequency f0 is improved, and the bias 3 can be controlled with high accuracy.

  In the configuration shown in FIG. 1, unlike the conventional configuration shown in FIG. 11, the low-speed PD built in the commercially available DQPSK modulator 2 can be used as the first monitoring means 15.

  FIGS. 2A to 2C are explanatory diagrams showing system fluctuations when the bias voltage Bias3 is intentionally changed in order to compare the first embodiment with the prior art shown in FIG. is there. First, FIG. 2A is an explanatory diagram when the change in the Q value of the output signal with respect to the change in the bias voltage Bias3 is measured. It can be seen that the best output waveform is obtained when the bias voltage Bias3 is 1.7V.

  FIG. 2B is an explanatory diagram when measuring the output of the first synchronous detection circuit 16 when the bias voltage Bias3 is intentionally varied in the configuration of the first embodiment shown in FIG. . Here, since the purpose is to examine the influence of the drift of the bias voltage Bias3, the feedback to the third DC power supply 10-3 is cut off. When the bias voltage Bias3 is the optimum value 1.7V, the output of the first synchronous detection circuit 16 is maximized.

FIG. 2C is an explanatory diagram showing the result of the same measurement as in FIG. 2B performed with the configuration of the conventional technique shown in FIG. As shown in FIG. 2C, in the case of the prior art configuration shown in FIG. 10, the first synchronization is compared with FIG. 2A even though the dithering amplitude and frequency are set equal. The peak of the output of the detection circuit 16 is ambiguous. This indicates that the configuration according to the first embodiment can control the bias voltage Bias3 with higher accuracy than the configuration of the prior art shown in FIG.

  In the configuration shown in FIG. 1, the feedback loop is not doubled, and the feedback circuit can be reduced in size, so that circuit noise from the bias voltage Bias4 can be reduced. Furthermore, since the DQPSK modulator 2 itself has an optical loss, the circuit noise derived from the bias voltage Bias4 is also attenuated, and the dither signal of the bias voltage Bias3 (π / 2) received by the first monitor coupler 3 of FIG. The sensitivity of the component can be improved.

B. Second Embodiment Next, a second embodiment of the present invention will be described.
FIG. 3 is a block diagram showing a configuration of the RZ-DQPSK transmitter according to the second embodiment. It should be noted that portions corresponding to those in FIG. In the second embodiment, the output of the first monitoring means 15 in the first embodiment shown in FIG. 1 is branched into two, and one of them is input to the amplitude control device 40. The amplitude of the first low frequency generator 11 is changed based on the change in the magnitude of the output of the monitor means 15. The first low-frequency generator 11 can be controlled with high accuracy if the output amplitude is large, but on the other hand, there is a trade-off that the signal quality deteriorates. In the second embodiment, this problem is solved.

  In the configuration shown in FIG. 3, in the unstable initial state immediately after starting up the transmitter, the amplitude of the first low frequency generator 11 is increased to improve the accuracy of bias control. Then, after a while after the transmitter is started up and the transmitter starts to stabilize to some extent, the amplitude of the first low-frequency generator 11 is reduced to suppress signal quality deterioration. When the CW light source 1 outputs light, the first monitor means 15 receives some optical power, so the output of the first monitor means 15 is not zero. The amplitude control device 40 detects the time when the output of the first monitor means 15 is not 0, and regards it as the start-up time of the transmitter. The elapsed time T from the startup time is measured by a timer in the amplitude control device 40. If the elapsed time T is smaller than a predetermined time t, the amplitude control device 40 increases the amplitude of the first low-frequency generator 11, and the first time when the elapsed time T exceeds the time t. The amplitude of the low frequency generator 11 is reduced.

  According to the second embodiment described above, in an unstable initial state immediately after startup of the transmitter, the amplitude control device 40 increases the amplitude of the first low-frequency generator 11 to increase the bias control. After the transmitter is stabilized, the amplitude control device 40 increases the amplitude of the first low-frequency generator 11 and decreases the amplitude of the first low-frequency generator 11. As a result, it is possible to suppress degradation of signal quality.

C. Third Embodiment Next, a third embodiment of the present invention will be described.
FIG. 4 is a block diagram showing the configuration of the RZ-DQPSK transmitter according to the third embodiment. The parts corresponding to those in FIG. The third embodiment uses a waveform measuring means 50 and a peak hold circuit 60 instead of the output of the first monitoring means 15 in the second embodiment shown in FIG. The waveform measuring means 50 has a band comparable to the data bit rate and can monitor the intensity of each optical pulse of the RZ-DQPSK signal. The peak hold circuit 60 holds the peak intensity of the waveform obtained by the waveform measuring means 50. The holding time is assumed to be longer than the maximum same-code continuous time that the transmission signal has.

  FIGS. 5A and 5B are conceptual diagrams showing the operation principle of the third embodiment. When all of the bias voltages Bias1 to Bias4 are appropriate, the RZ-DQPSK signal becomes an optical pulse train. FIG. 5A shows this state in the form of an eye pattern. Next, it is assumed that the bias voltage Bias3 deviates from an appropriate value. At this time, as shown in FIG. 5B, the waveform trace is split, and an optical pulse with a higher peak value and an optical pulse with a lower peak value appear. Note that the peak interval in FIGS. 5A and 5B is determined by the transmission rate. In this example, since the DQPSK signal is 21.5 Gbit / s, the peak interval is 93 psec (the horizontal scale interval is 23.2 ps / div).

As described in the above-described problem of the conventional technology and the first embodiment, whether the peak value becomes higher or lower is random because it is determined by the sign of D _I × D _Q × sin (α). Here, it is assumed that the peak value is monitored and the maximum peak value is held for a time sufficiently longer than the time when the same code continuation is predicted. The peak hold value is minimum when the bias voltage Bias3 is optimum, and is maximum when the bias voltage Bias3 is maximum. By feeding back the result to the bias voltage Bias3, the bias voltage Bias3 can be maintained at an appropriate value.

  In the configuration described above, the output of the RZ modulator 8 is connected to only one DQPSK modulator 2. However, the output of the RZ modulator 8 may be divided into N parts, connected in parallel to N DQPSK modulators to create modulated light, and then multiplexed by optical multiplexing.

  FIG. 6 is a block diagram showing a configuration of the RZ-DQPSK transmitter, in which the output of the RZ modulator is branched into two, and each output is DQPSK modulated and then polarization multiplexed. In the figure, the output from the RZ modulator 70 is divided into two by an OC (Optical Coupler) 71 and input to each of two DQPSK modulators 2-1 and 2-2 arranged in parallel. The outputs of the two DQPSK modulators 2-1 and 2-2 are combined and output by a PBC (Polarization Beam Combiner) 72.

  As described above, the RZ conversion performed by the RZ conversion modulator 70 is performed before the DQPSK modulation performed by the DQPSK modulators 2-1 and 2-2, so that the RZ conversion modulator 70 and its drive amplifier (not shown) are provided. Since only one unit needs to be prepared and the number of necessary parts can be reduced, there is a further effect that downsizing and power consumption can be reduced.

Next, a configuration in the case of time multiplexing will be described.
FIG. 7 is a block diagram showing a time-multiplexed configuration for the RZ-DQPSK transmitter. In the figure, the output of the RZ modulator 80 is branched by N (= 4) by the OC 81 and then time-multiplexed by N (= 4) DQPSK modulators 2-1 to 2-4. Are combined and output by the combining means 82. When the baud rate of the DQPSK signal generated by each DQPSK modulator 2-1 to 2-4 is B, the period of the output pulse of the RZ modulator 80 is 1 / B. So that the RZ modulator 80 has a duty ratio of 1 / N or less. That is, the width of the light pulse is 1 / (NB). In addition, it is necessary to give an appropriate optical delay to each of the N optical signals before being input to the multiplexing means 82 and to time-multiplex so that the N types of optical pulses do not overlap. The FSR (Free Spectral Range) of the delay interferometer for demodulation is set to B / N, and demodulation is performed by detecting the phase difference of optical pulses separated by N bits.

  As N increases, the optical pulse width, which is the output of the RZ modulator 80, must be narrowed, so the bandwidth required for the RZ modulator 80 increases, but the drive signal for the RZ modulator 80 is increased. Is not a random signal but a periodic clock, and therefore, no bandwidth is required up to the low frequency side. Further, since the baud rate of the combined optical signal is B × N, it is possible to generate a signal that is faster than the operation speed of each DQPSK modulator 2-1 to 2-4. Note that the polarization multiplexing shown in FIG. 6 and the time multiplexing shown in FIG. 7 can be used in combination.

  In the configuration described above, the frequencies f1 and f2 of the first low-frequency signal generator 11 and the second low-frequency signal generator 21 are not mentioned. These frequencies f1 and f2 must be slower than the response speed of the bias voltage change in the DQPSK modulator 2 so that the synchronous detection of the dithering means output can be performed. Desirably, these frequencies f1 and f2 may be configured to fall below the low-frequency cutoff frequency of the reception system in the receiver. In general, a demodulating photoelectric conversion element or an amplifying amplifier (not shown) is designed to pass no signal in the vicinity of DC in order to operate at high speed. If dithering is performed at a frequency equal to or lower than the low-frequency cutoff frequency, it is possible to prevent fluctuation due to dithering from leaking into the main signal at the time of reception, so that deterioration of the quality of the main signal can be suppressed.

D. Fourth Embodiment Next, a fourth embodiment of the present invention will be described.
In the above description, the difference in behavior of each of the bias voltages Bias1 to Bias4 has not been described. As shown in FIG. 2A, the bias voltage Bias3 (π / 2 bias) has an appropriate bias value when the synchronous detection output is maximum. However, the bias voltage Bias1 and the bias voltage Bias2 (data bias group) have appropriate bias values when the synchronous detection result is “0”. The latter is easy to control, and the control direction can be determined by the sign of synchronous detection. For example, when the synchronous detection result is positive, it is possible to determine that the bias voltage is increased, and when the synchronous detection result is negative, the bias voltages Bias1 and Bias2 are decreased.

  On the other hand, in the case of the bias voltage Bias3 (π / 2 bias), it is difficult to determine whether or not the extreme value is taken. Further, when it is determined that the value is not an extreme value, it is difficult to determine whether the bias voltage Bias3 should be increased or decreased. Hereinafter, a method for solving this problem will be described.

  FIGS. 8A and 8B are explanatory diagrams for explaining a method of controlling the bias voltage Bias3 according to the fourth embodiment. 8A and 8B, dithering is performed at three types of bias voltages V-, V0, and V +, and the respective synchronous detection results are compared. These three types of bias voltages are equally spaced and V− <V0 <V +.

  FIG. 8A is a diagram illustrating an example in which V0 is lower than the optimum bias value. In this case, the difference ΔV between the synchronous detection result at V + and the synchronous detection result at V− is positive. On the other hand, FIG. 8B is a diagram showing an example where V0 is higher than the optimum bias value. In this case, the aforementioned ΔV becomes negative. That is, V0 can be converged to the optimum bias value by applying feedback that increases V0 when ΔV is positive and decreases V0 when ΔV is negative. At this time, in order to minimize the influence on the signal quality, it is necessary to sufficiently reduce the ternary width of V−, V0, and V + and sufficiently reduce the dithering amplitude.

  Here, since the change in the optical output with respect to the dithering of the bias voltage Bias3 (π / 2 bias) is extremely small, the synchronous detection result is very noisy. For this reason, there is a possibility of locking to a false peak due to noise, and the operation becomes unstable. However, in the fourth embodiment, synchronous detection is performed by a sufficiently large dithering amplitude when the transmitter is started up. This can be dealt with by measuring the change in the synchronous detection result in advance while improving the accuracy and sweeping the average value of the bias voltage. When the transmitter is started up, since the dithering amplitude is large, the signal quality is greatly deteriorated, but there is no problem because the processing is limited to the startup of the transmitter. When the optimum bias voltage is predicted, V0 is set in the vicinity of the voltage, the dithering amplitude is sufficiently reduced, and feedback is applied to keep V0 at the optimum value in the same manner as in the prior art. . Even in this configuration, there is a possibility of locking to the false peak due to the noise described above, but since the processing is performed in the vicinity of the optimum bias value, the possibility of deviating greatly from the optimum value is extremely small.

  In the fourth embodiment, since the change of the synchronous detection result with respect to the bias voltage is measured in advance before normal operation, the deviation amount of V0 from the optimum bias value is estimated from the sign and absolute value of ΔV described above. It is also possible. By determining the magnitude of the feedback signal based on this estimation result, it is possible to converge the bias voltage with higher speed and accuracy.

  In the fourth embodiment, the synchronous detection results are compared at three points of V0, V +, and V−. However, the comparison point may be two or more, and V0 may be omitted and only V− and V + may be used. . In this case, control is performed so that the average value of V− and V + converges to the optimum bias value.

DESCRIPTION OF SYMBOLS 1 CW light source 2 DQPSK modulator 3 1st coupler 4 1st phase adjustment means 5 2nd phase adjustment means 6 (pi) / 2 phase adjustment means 7 2nd coupler 8 RZ modulator 10-1 to 10-4 1st, 2nd, 3rd, 4th DC power supply 11 1st low frequency generator 12 1st change-over switch 13-1 to 13-4 1st, 2nd, 3rd, 4th low frequency superposition Circuit 14 First monitor coupler 15 First monitor means 16 First synchronous detection circuit 17 Second changeover switch 21 Second low-frequency signal generator 24 Second monitor means 25 Second synchronous detection circuit 30 Bias Control circuit

Claims (9)

A signal transmitter for outputting an RZ-DMPSK signal obtained by pulse-curving differential polyphase phase shift keying,
A light source;
A pulse generating modulator for inputting the light source output;
A pulse generator for the modulator dithering means for dithering the bias before Symbol pulse generating modulator,
Pulsed light power monitoring means for monitoring the optical power of the optical pulse train output from the pulse generating modulator;
Pulse generation modulator bias control means for optimizing the bias of the pulse generation modulator by synchronous detection of the output of the pulse light power monitoring means and the dithering signal output from the pulse generation modulator dithering means; ,
A pulsed light bias controller comprising:
A differential multiphase phase shift modulator that receives an optical pulse train output from the pulse generating modulator;
A plurality of differential polyphase phase shift modulator dithering means for dithering all of the plurality of biases of the differential polyphase phase shift modulator;
Modulated optical power monitoring means for monitoring the optical power of the output of the differential polyphase phase shift modulator;
A plurality of biases of the differential multiphase phase shift modulator are respectively detected by synchronous detection of the output of the modulated optical power monitor means and the dithering signal output from the differential polyphase phase shift modulator dithering means. Differential multiphase phase shift modulator bias control means to optimize;
With
Between the light source and the differential polyphase phase shift modulator, the pulse generating modulator and the pulsed light bias controller are arranged,
Independence of a feedback loop of the pulse generator modulator bias control means and a feedback loop of the differential multiphase phase shift modulator bias control means , and light monitored by the modulated light power monitor means The signal is an RZ-DMPSK signal;
A transmitter characterized by. The differential polyphase phase shift modulator dithering means comprises:
Increase the amplitude of dithering at the time of startup of the device, and decrease the amplitude of dithering after a predetermined time has elapsed since startup of the device.
The transmitter according to claim 1. A 1-to-N branch coupler for N-branching the optical pulse train output from the pulse generating modulator;
First to Nth differential multiphase phase shift modulators for performing different modulation of baud rate B on each of the N pulses branched by the 1 to N branch coupler;
Combining means for performing time multiplexing by adding a delay difference so that optical pulses do not interfere with each other, the outputs from the first to N-th differential polyphase phase shift modulators,
The pulse generating modulator is:
An optical pulse train having a period of 1 / B and a duty ratio of 1 / N or less is generated,
The multiplexing means is
Output an optical differential polyphase phase shift signal with a baud rate of B × N.
The transmitter according to claim 1 or 2. A one-to-two branch coupler for bifurcating the optical pulse train output from the pulse generating modulator;
First and second differential polyphase phase shift modulators that respectively modulate the two outputs from the one-to-two branch coupler;
Multiplexing means for polarization multiplexing the outputs from the first and second differential polyphase phase shift modulators;
The transmitter according to claim 1, further comprising: Instead of the modulated light power monitoring means,
Modulated light waveform monitoring means for monitoring the waveform of the modulated light output from the differential multiphase phase shift modulator;
A peak hold circuit for detecting the peak value from the modulated light waveform monitored by the modulated light waveform monitoring means, and holding the peak value for a duration longer than the same sign continuous time of the transmitter;
The transmitter according to claim 1, further comprising: The pulse generating modulator dithering means and the differential polyphase phase shift modulator dithering means are:
It has a dithering frequency that is below the low cutoff frequency of the receiving system,
The transmitter according to claim 1 or 2. The differential polyphase phase shift modulator bias control means includes:
Π / 2 bias control means for controlling a π / 2 bias for applying feedback so that the synchronous detection output takes a positive or negative extreme value;
The π / 2 bias control means includes
Determines whether the result of synchronous detection at the current bias value is an extreme value, and if not, determines whether the current bias value should be increased or decreased. It has an extreme value judgment function that makes a judgment to hold the bias value of
The extreme value determination function is
Although the dithering amplitude is kept constant, the synchronous detection result is compared and referenced while slightly changing the average dithering voltage, and the result of synchronous detection at the current bias value based on the comparison of the comparison reference results If the result of synchronous detection at the current bias value is not an extreme value, the amount of change in the bias value so that the result of synchronous detection approaches the extreme value based on the comparison reference result Determine
The transmitter according to claim 1. At the time of starting up the apparatus, before operating the extreme value determination function, synchronous detection is performed while sweeping the bias voltage in advance, and the rate of change of the synchronous detection result with respect to the change in the bias voltage is measured in advance.
The transmitter according to claim 7. A method of transmitting a signal for outputting an RZ-DMPSK signal obtained by pulse-curving differential polyphase phase shift keying,
A pulse generation modulation step of inputting a light source output to the pulse generation modulator;
Pulse generating modulator dithering means comprises a bias dithering pulse generation modulator dithering step of the pulse generator for the modulator,
A pulsed light power monitoring means for monitoring a light power of an optical pulse train output from the pulse generating modulator;
The pulse generator modulator bias control means optimizes the bias of the pulse generator modulator by synchronous detection of the output of the pulse light power monitor means and the dithering signal output from the pulse generator modulator dithering means. A modulator bias control step for generating a pulse;
A pulsed light bias control step comprising:
A differential multi-phase phase shift keying modulator, the differential multi-phase phase shift key modulation step, which receives the optical pulse train output from the pulse generation modulator;
A plurality of differential polyphase phase shift modulator dithering means dithers all of a plurality of biases of the differential polyphase phase shift modulator. Steps,
A modulated light power monitoring means for monitoring the optical power of the output of the differential polyphase phase shift modulator;
The differential polyphase phase shift modulator bias control means is configured to detect the difference by synchronous detection between an output of the modulated optical power monitor means and a dithering signal output from the differential polyphase phase shift modulator dithering means. A differential polyphase phase shift modulator bias control step that each optimizes a plurality of biases of the dynamic polyphase phase shift modulator;
Including
Before the differential multiphase phase shift key modulation step, execute the pulse generation modulation step and the pulsed light bias control step,
Independence of a feedback loop of the pulse generator modulator bias control means and a feedback loop of the differential multiphase phase shift modulator bias control means , and light monitored by the modulated light power monitor means The signal is an RZ-DMPSK signal;
A transmission method characterized by the above.

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