JP2012160970A - Phase modulation device - Google Patents

Abstract

An object of the present invention is to provide a phase modulation device that does not require assembly of components having the same length, and that can easily perform skew adjustment even if the temperature characteristics and aging of the components occur.
A phase modulator includes a light source that outputs continuous light, two phase modulators, and an intensity modulator. The continuous light from the light source is input to each of the phase modulators. The phase shifter 13 shifts one phase of the phase modulated optical signal by π / 2 to match the other of the phase modulated optical signal. The RZ phase modulation circuit 101 that intensity-modulates the combined signal that has been waved with the clock signal CLK to which the intensity modulator 15 is input and outputs it as RZ, and RZ phase modulation so that the output of the RZ phase modulation circuit 101 is maximized. A phase control circuit 111 that adjusts the phase of each of the phase-modulated optical signals generated by the phase modulator 12 of the circuit 101.
[Selection] Figure 4

Description

  The present invention relates to a phase modulation apparatus that modulates the phase of light with a digital signal sequence.

  As a method for modulating light with a digital signal, quaternary phase shift keying (QPSK) and differential quaternary phase shift keying (DQPSK: Differential QPSK) are known (see, for example, Patent Document 1). ).

In a long-distance transmission system, RZ (Return to Zero) intensity modulation may be further performed on the QPSK or DQPSK modulated optical signal for the purpose of suppressing intersymbol interference and increasing sensitivity. FIG. 1 is a diagram illustrating a phase modulation apparatus 300 that employs the RZ-QPSK modulation method. RZ-QPSK modulation is performed in the following procedure.
(1) The continuous light LD of the light source 10 is branched by the 1: 2 coupler 11.
(2) The phase modulators (12-1, 12-2) are based on digital input signals (DATA1, DATA2), and the optical phase of the continuous light (LD1, LD2) from the 1: 2 coupler 11 is 0 or π. Conversion into binary phase-modulated optical signals (LN1, LN2).
(3) The 2: 1 coupler 14 combines the two phase-modulated optical signals (LN1 ′, LN2) that have been shifted by π / 2 from each other by the phase shifter 13, and generates a quaternary phase-modulated signal QPSK. Output.
(4) The intensity modulator 15 outputs an optical signal RZ-QPSK that is RZ-modulated by intensity-modulating the phase-modulated signal QPSK with the clock signal CLK synchronized with the digital input signal.

  The above procedure is described in more detail in FIGS. FIG. 2 is a diagram illustrating a process in which a quaternary phase modulated optical signal is generated from the binary phase modulated signals generated by the phase modulators 1 and 2. Here, an intensity modulator is used as the phase modulator. That is, the amplitude of the digital input signal is set to twice the half-wave voltage of the intensity modulator, and the '0' / '1' level of the signal is set to the maximum transmission point of the intensity modulator. '0' / '1' bits are converted into light bits having a relative optical phase 0 / π, respectively. In this way, binary phase modulated optical signals LN1 and LN2 are generated. In this configuration, the optical output once becomes 0 at the phase transition point where the optical phase changes from 0 to π or vice versa. LN1 becomes LN1 'whose optical phase is further shifted by π / 2 by the π / 2 phase shifter. By combining LN1 ′ and LN2 at the timing when each bit overlaps, QPSK phase-modulated light with four optical phases of π / 4, 3π / 4, 5π / 4, and 7π / 4 is generated. The

  FIG. 3 is an eye diagram of each signal in the phase modulation apparatus 300. FIG. 3A shows a continuous light LD output from the light source 10. There is no change in the light intensity of the continuous light LD. FIG. 3B shows a binary phase modulated optical signal LN1 (LN2) output from the phase modulator 12-1 (12-2). The phase-modulated optical signal LN1 maintains a constant light intensity when the optical phase is fixed, but is once extinguished at the phase transition point as described above, so that a notch N is generated in the eye diagram. FIG. 3C shows a quaternary phase modulated optical signal QPSK output from the 2: 1 coupler 14. As described above, QPSK takes four phase states by combining two phase-modulated optical signals (LN1 ', LN2) at the timing at which the respective bits overlap. In addition, since the timing at which the phase transition occurs also overlaps, the depth of the notch N is determined in two ways depending on whether the phase transition occurs simultaneously at LN1 'and LN2. FIG. 3D shows the RZ optical signal RZ-QPSK output from the intensity modulator 15. RZ pulse modulation is applied to the region where the optical phase is determined by the clock signal CLK synchronized with the digital input signal. As shown in the figure, it is optimal to apply modulation at the timing when the peak of CLK overlaps the optimum phase (between the notch and the notch). By using RZ, intersymbol interference with adjacent pulses is suppressed, and a high pulse head value is obtained with respect to the average light intensity, so a long-distance transmission system with high sensitivity and excellent non-linear tolerance can be realized. .

JP 2007-208472 A

  So far, RZ-QPSK is generated by synthesizing two binary phase-modulated lights at the timing when each bit overlaps to generate quaternary phase-modulated light. The procedure of applying RZ modulation is described. As is obvious from this, it is an issue in RZ-QPSK generation that the timing of three kinds of signals, that is, two data strings (LN1 'and LN2) and clock CLK are aligned. The effect when the timing between these signals is not aligned will be described below.

  FIG. 4 shows the influence of the timing shift (data-clock skew) between the quaternary QPSK phase-modulated light and the CLK. When there is no skew, the output waveform is a single-width repetitive pulse as shown in FIG. However, when a skew occurs, the notch and the RZ modulation clock interfere with each other and the pulse is deleted. As a result, the output waveform becomes a distorted pulse as shown in FIG. Such distortion causes loss of information and causes significant deterioration in transmission characteristics.

  FIG. 5 is a diagram for explaining the influence of the data-to-data skew between the binary phase modulated lights LN1 'and LN2 input to the 2: 1 coupler 14. As described above, the quaternary QPSK phase-modulated light has a phase determination region R1 in which four phase states are determined and a phase transition region R2 in which a phase transition occurs and a notch is generated. When there is no skew, the phase transition timings between the two signals are aligned, so that notches having the same width with different depths are generated in the eye diagram as shown in FIG. However, when skew occurs, the phase transition timing between the two signals is shifted and combined, so that the notches seen in the eye diagram are separated. For this reason, as shown in FIG. 5B, the phase determination region is narrowed, and even if the clock phase is adjusted to the optimum timing, interference with the phase transition region is unavoidable and transmission characteristics are significantly deteriorated.

  Factors that cause skew between these three signals include variations in delay times of individual components (such as the modulator 12 and the driver) and wiring, delay fluctuations depending on temperature and power supply fluctuations, and secular fluctuations. If the delay times of individual parts vary, it is possible to cope with the problem by adjusting the delay time at the time of manufacture and adjusting the delay time individually by using a phase shifter. However, delay fluctuations due to temperature and power supply fluctuations and aging fluctuations cannot be dealt with only by initial adjustment, and correction is required to always operate at the optimum operating point.

  Accordingly, the present invention provides a phase modulation apparatus that does not require the assembly of each component part to be equal in length and solves the above-described problem, and can easily adjust skew even if the temperature characteristics of the part and changes with time occur. For the purpose.

  In order to achieve the above object, the phase modulation apparatus according to the present invention monitors the average intensity of the optical output signal and feedback-controls the phase shift amount of the phase shifter provided in the electric signal path so that the average intensity is maximized. The configuration.

  Specifically, the phase modulation apparatus according to the present invention includes a light source that outputs continuous light having a constant optical phase, and two optical phases obtained by branching the output of the light source. Two phase modulators that respectively modulate based on one electrical data signal, a π / 2 phase shifter that shifts the optical phase of the output of one of the phase modulators by π / 2, and an output of the π / 2 phase shifter And a combiner for combining the output of the other phase modulator, an intensity modulator for intensity-modulating the output of the combiner with an electrical clock signal synchronized with the electrical data signal, and the electrical data signal Are arranged in two of the two electrical paths for propagating to the phase modulator and the electrical path for propagating the electrical clock signal to the intensity modulator, and the phase of the electrical data signal or the electrical clock signal is determined. A shifting phase shifter, and Based on the phase of the electrical data signal or electrical clock signal in the electrical path where the shifter is not disposed as a reference, the phase of the electrical data signal or electrical clock signal in the electrical path where the phase shifter is disposed is modulated with the intensity. And a phase control circuit for controlling each so that the light output intensity from the device is maximized.

  The phase control circuit superimposes a pilot signal of a predetermined frequency on a control signal indicating a phase shift amount by which the phase shifter shifts the phase of the electrical data signal or the electrical clock signal, and sets the phase shift amount at the predetermined frequency. A pilot signal supply unit that superimposes a pilot signal to be oscillated, a synchronous detection unit that performs synchronous detection of an electrical signal obtained by photoelectrically converting the output of the intensity modulator at the same frequency as the pilot signal, and an output of the synchronous detection unit And an adjustment unit that performs an adjustment operation to output the control signal of the previous phase shift amount adjusted so that the error signal becomes zero to the phase shifter.

  Here, the phase control circuit of the phase modulation apparatus according to the present invention uses two pilot signals having different frequencies prepared for each phase shifter, and the pilot signal supply unit includes two pilot signals. Is superimposed on the control signal, the synchronous detection unit performs synchronous detection for each pilot signal, and the adjustment unit performs the adjustment operation for each pilot signal. Good. In this phase modulation apparatus, two pilot signals are assigned to two phase shifters, and skew adjustment can be performed simultaneously.

  On the other hand, the phase control circuit of the phase modulation apparatus according to the invention uses one pilot signal and the pilot signal superposition performed by the pilot signal supply unit, the synchronous detection performed by the synchronous detection unit, and the adjustment unit The adjustment work may be performed in a time division manner. In this phase modulation apparatus, skew adjustment by two phase shifters is alternately performed by one pilot signal. Since the phase control circuit can be simplified, the phase modulation device can be reduced in size.

  The present invention can provide a phase modulation device that does not require assembling of each component part and that is easy to adjust even if the temperature characteristics of the parts and changes with time occur.

It is a figure explaining the phase modulation apparatus which employ | adopts RZ-QPSK modulation system. It is a figure explaining the production | generation of a QPSK signal. It is the figure which showed the eye diagram of the signal in a phase modulation apparatus. (A) is a signal output from the light source. (B) is a signal output from the phase modulator. (C) is a signal output by the 2: 1 coupler. (D) is a signal output from the intensity modulator. It is a figure explaining the influence of the skew between data-clocks. (A) is an output waveform when there is no skew. (B) is an output waveform when skew occurs. (C) is a diagram for explaining data-clock skew. It is a figure explaining the influence of the skew between data. (A) is QPSK phase-modulated light when there is no skew. (B) is QPSK phase-modulated light when skew occurs. (C) is a diagram for explaining the inter-data skew. It is a figure explaining the phase modulation apparatus which concerns on this invention. It is a figure explaining the principle of operation concerning the present invention. (A) is a figure explaining a mode that the average optical intensity of a RZ optical signal changes with the skew between LN1'-CLK. (B) is a result of observing an RZ optical signal with a skew of 0.5 UI with an oscilloscope. (C) is the result of observing an RZ optical signal with a skew of 0.25 UI with an oscilloscope. (D) is the result of observing an RZ optical signal when there is no skew with an oscilloscope. It is a figure explaining the principle of operation concerning the present invention. It is a figure explaining the phase modulation apparatus which concerns on this invention. It is a figure explaining the superimposition of the pilot signal on a phase modulation signal. It is a figure explaining the determination method of the control direction of a phase shifter. (A) is a figure explaining the state in which the phase of data is ahead of the phase of a clock. (B) is a diagram for explaining a state in which the phase of the data and the phase of the clock coincide with each other. (C) is a diagram illustrating a state in which the data phase is delayed from the clock phase. It is a figure explaining the phase modulation apparatus which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(First embodiment)
FIG. 6 is a diagram illustrating a phase modulation device 301 which is an example of the present embodiment. The phase modulation device 301 includes an RZ phase modulation circuit 101 and a phase control circuit 111. The RZ phase modulation circuit 101 includes a light source 10 that outputs continuous light, two phase modulators (12-1, 12-2), and an intensity modulator 15, and these phase modulators (12-1, 12-). 2) generates two phase-modulated optical signals by phase-modulating continuous light from the light source 10 based on the data signals (DATA1, DATA2) input thereto, and the phase shifter 13 generates one of the phase-modulated optical signals. Is shifted by π / 2 to generate a quaternary phase modulated signal that is combined with the other of the phase modulated optical signal, and the intensity modulator 15 intensifies the quaternary phase modulated signal with the clock signal CLK synchronized with the data signal. Modulate, convert to RZ and output. The phase control circuit 111 outputs the phase shifters (21-1, 21-2) arranged in the electrical paths of the data signals (DATA1, DATA2) and the intensity modulator 15 based on the phase of the clock signal (CLK). An adjustment unit that determines the phase shift amount of the data signals (DATA1, DATA2) so that the optical intensity of the RZ optical signal RZ-QPSK to be maximized and outputs a control signal to the phase shifters (21-1, 21-2) 125.

  Here, the operation principle of this embodiment will be described with reference to FIGS. FIG. 7A shows the skew between LN1 ′ and CLK using the phase shifter 21-1 from the state where the phase relationship between the phase modulated optical signals LN1 ′ and LN2 output from the phase modulator 12 and CLK is optimal. Is a diagram for explaining how the average light intensity of the RZ optical signal RZ-QPSK changes when the value is changed from −0.5 UI (Unit Interval) to +0.5 UI. 7B to 7D show the results of observing the RZ optical signal at each phase with an oscilloscope. When the skew increases as shown in FIG. 7D to FIG. 3B, interference between the notch N and CLK occurs as shown in FIG. 7D to FIG. 7B. Thus, the average light intensity of the RZ optical signal RZ-QPSK is reduced. This shows that the skew adjustment can be performed by maximizing the average light intensity of the RZ optical signal RZ-QPSK.

  FIG. 8 is a diagram for explaining how the average light intensity of the RZ optical signal RZ-QPSK changes when the skew between the phase-modulated optical signal LN1 ′ and the CLK of the LN2 is changed from −0.5 UI to +0.5 UI. It is. When the skew adjustment of LN1 'is performed, even if the phase of LN2 is not optimal, it is shown that the skew adjustment of LN1' can be performed by maximizing the light intensity of RZ-QPSK. Therefore, if the phase shifters (21-1, 21-2) control the phases of LN1 ′ and LN2 in the direction in which the average optical intensity of the RZ optical signal RZ-QPSK increases, both skew adjustments are performed. You can see that

  In the embodiment of FIG. 6, the average light intensity is obtained as a photocurrent obtained by photoelectrically converting the monitor light tapped by the 1: 2 coupler 16 by the light receiving element 23. The adjustment unit 125 varies the phase shift amount of the phase shifters (21-1, 21-2), and if the average light intensity, that is, the photocurrent is decreased, the phase shift amount is varied to the opposite side. Therefore, the skew can be adjusted at the optimum timing.

  In the present embodiment, the skew between LN1 ′ and LN2 is adjusted with reference to the phase of CLK. However, the skew between LN2 and CLK is adjusted with reference to LN1 ′, and LN1 ′ and CLK with respect to LN2. It is also possible to adjust the skew. Alternatively, the light intensity modulator 15 may be arranged immediately after the light source 10 and the optical pulse generated by the light intensity modulator 15 may be branched to apply phase modulation.

(Second Embodiment)
FIG. 9 is a diagram illustrating a phase modulation device 302 that is an example of the present embodiment. The phase modulation device 302 is mounted with an example of the configuration of the adjustment unit 125 described in the phase modulation device 301 of FIG.

  The phase control circuit 111 superimposes a pilot signal of a predetermined frequency on a control signal J that instructs the phase shift amount by which the phase shifter (21-1, 21-2) shifts the phase of the data signal, and sets the phase shift amount to the predetermined frequency. A pilot signal supply unit 22 for superimposing a pilot signal to vibrate, a synchronous detection unit 24 for performing synchronous detection on the electrical signal obtained by photoelectrically converting the output of the intensity modulator 15 at the same frequency as the pilot signal, and a synchronous detection unit 24 An adjustment unit 25 that performs an adjustment operation to output the control signal J of the phase shift amount adjusted so that the error signal becomes zero to the phase shifters (21-1, 21-2). Have.

  In particular, the phase modulation device 302 uses two pilot signals with different frequencies prepared for each phase shifter (21-1, 21-2), and the pilot signal supply unit (22-1, 22-2) The pilot signals are superimposed by superimposing the two pilot signals on the control signal J, and the synchronous detectors (24-1, 24-2) perform synchronous detection for each pilot signal, and the adjusting units (25-1, 25-2) performs adjustment work for each pilot signal.

  In the present embodiment, the control direction of the phase shifter can be known in advance by using the pilot signal. An outline of a method for determining the control direction of the phase shifter in the phase control circuit 111 will be described with reference to FIGS. FIG. 10 is a diagram schematically showing a change in the phase state of the phase modulated optical signal 33 due to the pilot signal 32. Assume that the input data string 30 is input to the phase modulator via the phase shifter 31. It is assumed that a notch N is present at the center of the input data string 30. When the pilot signal 32 having the frequency f is introduced into the phase shifter 31, the notch N of the data signal output from the phase shifter 31 oscillates at the frequency f. Therefore, the phase-modulated optical signal 33 modulated based on this data signal The notch N also vibrates. The time oscillation of the notch is equivalent to the time when the optimum phase oscillates.

  FIG. 11 is a diagram for explaining the relationship between the skew state of the phase-modulated optical signal LN1 'and the clock signal CLK and the average optical intensity of the RZ optical signal RZ-QPSK. As described with reference to FIG. 7, the average light intensity is maximized at the optimum timing (FIG. 11B) where the optimum phase of the phase-modulated optical signal LN1 ′ coincides with the peak of the clock signal CLK. It decreases (FIGS. 11A and 11C). Here, when a sine wave pilot signal having the frequency f1 is superimposed on the control signal J for controlling the phase shifter 21-1, the optimum phase of the phase-modulated optical signal LN1 'oscillates at the frequency f1. This vibration causes time variation in the average light intensity, but the state of time variation changes depending on the phase advance or delay state of the phase-modulated optical signal LN1 'with respect to the clock signal CLK.

  Specifically, in the case of FIG. 11A in which the phase of the phase-modulated optical signal LN1 'is advanced, the average light intensity varies with time at the frequency f1 and in the same phase as the pilot signal. In the case where the phase of the phase-modulated optical signal LN1 'is the optimum point (B), the optical average intensity takes the maximum value. In this state, the average light intensity is attenuated regardless of whether the phase of the phase-modulated optical signal LN1 'is advanced or delayed from the optimum point. For this reason, when the phase of the phase-modulated optical signal LN1 'is vibrated with the pilot signal, the temporal variation of the optical average intensity varies at twice the frequency f1. When the phase of the phase-modulated optical signal LN1 'is delayed, the average light intensity varies with time at the frequency f1 and in the opposite phase to the pilot signal.

  Accordingly, if the phase control circuit 111 can detect whether the time variation of the optical average output is the same phase as that of the pilot signal or the opposite phase, the phase control circuit 111 determines whether to advance or delay the phase-modulated optical signal LN1 ′, that is, the phase shifter. The phase shift direction of 21-1 can be determined. The description here is the same for the phase-modulated optical signal LN2.

  The above operation will be described more specifically with reference to FIG. A pilot signal having a frequency f1 is applied to the phase shifter 21-1 from the pilot signal supply unit 22-1 to oscillate the phase of the phase modulated optical signal LN1 '. The average light intensity of the RZ optical signal RZ-QPSK at this time is photoelectrically converted by the light receiving element 23 and detected as a photocurrent value, and is synchronously detected by the pilot signal by the synchronous detector 24-1. If the time variation of the photocurrent is in phase with the pilot signal (the phase of the phase modulated optical signal LN1 ′ is advanced) or opposite in phase (the phase of the phase modulated optical signal LN1 ′ is delayed), the synchronous detection output K is positive or negative, respectively. Takes the value of If the phase-modulated optical signal LN1 'is in the optimum phase, the harmonic wave of f1 does not appear in the synchronous detection output, and K becomes 0. Thus, the adjustment unit 25-1 adjusts the magnitude of the phase shift amount of the phase shifter 21-1 based on the sign of K, thereby completing the skew adjustment.

  The skew adjustment of the phase modulated optical signal LN1 has been described above. However, the phase modulated light is obtained by using the pilot signal of the frequency f2 from the pilot signal supply unit 22-2, the synchronous detection unit 24-2, and the adjustment unit 25-2. The skew adjustment of the signal LN2 can be similarly performed.

  Furthermore, since phase modulator 302 uses pilot signals of two frequencies (f1, f2), it is possible to simultaneously adjust the skew of phase modulated optical signal LN1 and phase modulated optical signal LN2.

  In this embodiment, the skew between LN1 ′ and LN2 is adjusted based on the phase of CLK. For example, the skew between LN2 (LN1 ′) and CLK can be adjusted based on LN1 ′ (LN2), for example. It is. Alternatively, the intensity modulator 15 may be disposed immediately after the light source 10 and the optical pulse generated by the intensity modulator 15 may be branched to apply phase modulation.

  Since the phase modulator 302 can perform this skew adjustment at any time, it is not necessary to assemble each component with the same length, and even if the temperature characteristics of the component and changes with time occur, the skew fluctuation due to the change can be easily converged. it can.

(Embodiment 3)
FIG. 12 is a diagram illustrating the phase modulation device 303 according to the second embodiment. The difference between the phase modulation device 303 and the phase modulation device 302 in FIG. 9 is that a phase control circuit 112 is provided as an alternative to the phase control circuit 111. The difference between the phase control circuit 112 and the phase control circuit 111 is the number of pilot signals. The phase modulation device 302 performs skew adjustment with one pilot signal having a frequency f. Specifically, the skew adjustment of the phase-modulated optical signal LN1 and the phase-modulated optical signal LN2 is performed by shifting the time. For this reason, the phase modulation device 302 includes a changeover switch 26 that switches the output destination of the pilot signal to the phase control circuit 112. The skew adjustment method is the same as the skew adjustment method of the phase modulation device 302 described with reference to FIGS.

  Although RZ-QPSK has been described as an embodiment of the present invention, the present invention can also be applied to RZ-DQPSK, RZ-DP-QPSK (bit aligned), and RZ-DP-QPSK (symbol interleaving).

10: light source 11: 1: 2 coupler 12-1, 12-2: phase modulator 13: phase shifter 14: 2: 1 coupler 15: intensity modulator 16: 1: 2 coupler 21-1, 21-2: Phase shifters 22, 22-1, 22-2: pilot signal supply unit 23: light receiving elements 24, 24-1, 24-2: synchronous detection units 25, 25-1, 25-2: adjustment unit 26: changeover switch 30 : Input data string 31: phase shifter 32: pilot signal 33: phase modulation light signal 101: RZ phase modulation circuit 111, 112: phase control circuit 125: adjustment units 300 to 303: phase modulation devices LD, LD 1 and LD 2: continuous light LN1, LN1 ′, LN2: Phase modulated optical signal QPSK: Combined signal RZ-QPSK: RZ optical signal K: Detection signal J: Control signal

Claims (4)

A light source that outputs continuous light with a constant optical phase;
Two phase modulators for respectively modulating the optical phases of two continuous lights obtained by branching the output of the light source based on two electrical data signals;
A π / 2 phase shifter that shifts the optical phase of the output of one of the phase modulators by π / 2;
A multiplexer that multiplexes the output of the π / 2 phase shifter and the output of the other phase modulator;
An intensity modulator that modulates the output of the multiplexer with an electric clock signal synchronized with the electric data signal;
The electrical data signal or the electrical clock is disposed in two of two electrical paths through which the electrical data signal propagates to the phase modulator and the electrical path through which the electrical clock signal propagates to the intensity modulator. A phase shifter that shifts the phase of the signal;
With reference to the phase of the electrical data signal or electrical clock signal of the electrical path where the phase shifter is not disposed, the phase of the electrical data signal or electrical clock signal of the electrical path where the phase shifter is disposed, A phase control circuit for controlling the light output intensity from the intensity modulator to be maximum,
A phase modulation apparatus. The phase control circuit includes:
Pilot signal superposition is performed so that the phase shifter superimposes a pilot signal of a predetermined frequency on a control signal indicating a phase shift amount for shifting the phase of the electrical data signal or the electrical clock signal, and causes the phase shift amount to vibrate at the predetermined frequency. A pilot signal supply unit to perform;
A synchronous detection unit that performs synchronous detection on an electrical signal obtained by photoelectrically converting the output of the intensity modulator at the same frequency as the pilot signal;
An adjustment unit that performs an adjustment operation to output the control signal of the previous phase shift amount adjusted so that the error signal becomes zero to the phase shifter, using the output of the synchronous detection unit as an error signal;
The phase modulation apparatus according to claim 1, wherein: The phase control circuit includes:
Using two pilot signals having different frequencies prepared for each phase shifter,
The pilot signal supply unit performs the pilot signal superposition so that two pilot signals are superimposed on the control signal, respectively.
The synchronous detection unit performs synchronous detection for each pilot signal,
The phase modulation apparatus according to claim 2, wherein the adjustment unit performs the adjustment operation for each pilot signal. The phase control circuit includes:
Using one pilot signal,
The pilot signal superposition performed by the pilot signal supply unit;
The phase modulation apparatus according to claim 2, wherein the synchronous detection performed by the synchronous detection unit and the adjustment operation performed by the adjustment unit are performed in a time division manner.

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