JPH0375615A - Phase shift keying system using zehnder type optical modulator - Google Patents

Abstract

PURPOSE:To suppress phase imperfect modulating components by setting an offset voltage at the time of non-modulation with driving electrodes so as to approximately minimize the light output level of the Mach-Zehnder type optical modulator and changing the polarity of the driving voltage to be applied to the offset voltage according to the modulation signal, thereby imparting a change of pi to the phases of light outputs. CONSTITUTION:The Mach-Zehnder type optical modulator 10 has an input side optical waveguide structure 2, a pair of optical waveguide structures 4 for phase shifting to impart the phase change to the guided light according to the impressed electric field branched from the input side optical waveguide structure 2, an output side optical waveguide structure 6 where the optical waveguide structures 4 for phase shifting join and the driving electrodes 8 mounted to the optical waveguide structures for phase shifting. A suitable potential difference is applied to the optical waveguide structures 4 for phase shifting and the reverse modulating electric fields are impressed to the respective optical waveguide structures 4 for phase shifting to rotate the phases of the light passing the respective optical waveguide structures 4 for phase shifting in opposite directions respectively on the phase so that the light outputs equal in any of the 0 layer and the pi layer are generated. The phase imperfect modulating components are suppressed in this way and the spectral spread of the light outputs is lessened.

Description

[Detailed Description of the Invention] Table of Contents Overview Industrial Field of Application Prior Art (Figs. 13 and 14) Problems to be Solved by the Invention (Fig. 15) Means and Effects for Solving the Problems (Fig. 15) (Figures 1 to 4) Examples (Figures 5 to 12)
Figure) Outline of the Effects of the Invention Regarding the phase shift keying method applied to coherent optical communication systems, for the purpose of suppressing phase incomplete modulation components, the input side optical waveguide structure and the applied electric field branched from the input side optical waveguide structure are a pair of phase-shifting optical waveguide structures that change the phase of the guided light accordingly; an output-side optical waveguide structure where the phase-shifting optical waveguide structures merge;
A Matsuhatsu Enda type optical modulator is provided with a drive electrode mounted on the phase shifting optical waveguide structure, and when the drive electrode is not modulated, the drive electrode is set such that the optical output level of the Matsuhatsu Enda type optical modulator is approximately minimum. The configuration is such that an offset voltage is set and the polarity of a drive voltage applied to the offset voltage is changed in accordance with a modulation signal to give a change of π to the phase of the optical output.

INDUSTRIAL APPLICATION FIELD The present invention relates to a phase shift keying method applied to a coherent optical communication method.

In the field of optical communications, an intensity modulation/direct detection method (IM/'DD method) in which intensity-modulated light is directly received by a light receiving element and converted into an electrical signal is common. On the other hand, in recent years, research into coherent optical communication systems has become active due to demands for increased communication capacity, longer transmission distances, and the like. In the coherent optical communication system, light from a laser light source with particularly high spectral purity is used as a carrier light for transmission, and its frequency, phase, etc. are modulated, and the received light and local light are mixed on the receiving side to perform, for example, heterodyne detection. Therefore, the receiving sensitivity is greatly improved compared to the IM/DD method. Among the modulation methods for digital signals, the phase shift keying method (PSK method) has the highest reception sensitivity, and fundamental improvements to the spectral linewidth of transmitted light are being studied.

BACKGROUND OF THE INVENTION FIG. 13 shows the basic configuration of a phase modulator commonly used in a conventional PSK system. This phase modulator consists of an electro-optic crystal 102 made of Z-propagating LiNbO3, etc.
electrodes 104, 106 to apply an electric field in two directions.
are provided, and a driving power source 108 is connected to these electrodes 104 and 106.

Et cos(ωt) for Z cut X propagation LiNb○3
When a plane wave of linearly polarized light is incident, the electric field of the light wave at any point in the crystal is ε(t,
−(1). Here, φ is a voltage V in two directions. is the phase shift experienced by a light wave propagating in a crystal to which is applied. If the thickness of the crystal is d, then the ordinary ray (T
The phase changes for the E light) and the extraordinary light (7M light) are given by the following equations.

Ordinary ray (TE light): φ, = kon, x = kox (no
-no'r l 3VO/2d) -(2) Extraordinary ray (
7M light): φz=konsx=kox (ne-n%r
33Vo/2d) ...(3) Here, is the wave number in two directions, n I (i = x, y, z) is the refractive index in each direction, no + ne is the refractive index for the ordinary ray and extraordinary ray represents. Further, r13+r33 represents a tensor component of the electro-optical constant.

Now, the incident light beam is polarized in two directions, and the signal frequency ω
Assuming that a modulation voltage Vo=V of 1 and 5 inches (ω=1) is applied, the electric field of the light wave at the output end of x=1 is as follows.

Snake, (t, A) = [Etcos (ω is one φoz + δ
zsinω, t) (4) Here, φ. 2 is a constant phase shift amount φ. 2=kon, 1. Also,
δ, sinω, t is the phase shift of the light wave due to the applied modulation voltage, and δ, −(7r/λ)ns3r33 (j'/d) V
--(5). δ2 is called the phase modulation index. Therefore, for example, δ
, sinω, t is 0. The PSK method is realized by modulating and driving the signal so as to achieve π.

By the way, when performing high-frequency modulation by effectively utilizing the broadband nature of light waves whose frequency reaches several tens of THz, a shift in phase shift speed between the light waves and the modulated microwave occurs, resulting in a decrease in modulation efficiency. In order to avoid this, a traveling wave phase modulator has been proposed in which the propagation directions of light waves and microwaves are matched and the speeds of the two are matched as much as possible.

FIG. 14 shows the configuration of a traveling wave phase modulator.

110 is a waveguide substrate, 112 is a LiNbO2 which constitutes the waveguide substrate 110, and T1. 114 is a traveling wave type electrode formed on the surface of the waveguide substrate 110 in order to apply an electric field due to a traveling wave to the waveguide 112, and 116 is the light propagation direction of the electrode 114. A driving power supply connected to the upstream side, 118 a terminating resistor connected to the downstream side of the electrode 114 in the light propagation direction, 120 a polarization-maintaining optical fiber connected to the input end of the waveguide 112, and 122 a waveguide. This is an optical fiber connected to the output side of 112. According to this configuration, the propagation directions of the light wave and the modulated microwave are matched and their speeds are matched, so that high frequency modulation is possible depending on the degree of satisfaction of speed matching.

Problems to be Solved by the Invention FIG. 15 shows a waveform diagram of the applied voltage, phase shift, and optical output in the phase modulator shown in FIG. 13 or 14. The phase shift is based on the time when no voltage is applied * (0),
It is set to π when a voltage is applied. In this case, since the rise time T1 and fall time T2 of the applied voltage are finite and the optical output P is constant, phase incomplete modulation components will occur at the rise and fall times. When an incomplete phase modulation component occurs,
The spectral spread of the optical output increases, resulting in deterioration of reception sensitivity, and when optical frequency division multiplexing transmission is performed, the band required for optical frequency multiplexing becomes wider or the number of multiplexes becomes smaller.

The present invention was created in view of such technical problems, and its purpose is to suppress phase incomplete modulation components in the PSK system.

Means and Effects for Solving the Problems FIG. 1 shows a Matsuhatsu Enda type optical modulator used for carrying out the invention.

This Matsuhatsu Enda type optical modulator 10 includes an input end optical waveguide structure 2, a pair of phase shifting optical waveguide structures 4 that change the phase of guided light according to an applied electric field branched from the input end optical waveguide structure 2, and It includes an output side optical waveguide structure 6 where the phase optical waveguide structure 4 joins, and a drive electrode 8 mounted on the phase shift optical waveguide structure. A ground electrode (not shown) may be provided opposite the drive electrode 8, or a traveling wave type may be used as shown in FIG.

This type of Matsuhatsu Enda type optical modulator is usually used as an intensity modulator, but it applies an opposite modulating electric field to each phase-shifting optical waveguide structure by applying an appropriate potential difference to the phase-shifting optical waveguide structure. , by rotating the phase of the light passing through each phase-shifting optical waveguide structure in opposite directions on the phase plane, the 0 layer,
Equal light output can be produced in any of the π layers. Below, after explaining the operating principle of the intensity modulator,
The phase shift keying method of the present invention as an application thereof will be explained.

Now, the light Po input to the input-side optical waveguide structure 2 is divided into three parts at the branch point to the phase-shifting optical waveguide structure 4, and one of the phase-shifting optical waveguide structures 4 (hereinafter referred to as "first arm") is divided into three parts. If the guided light Pl propagating in It undergoes a phase change of 1 Δφ depending on the voltage. When these two guided lights are combined and interfered in the output side optical waveguide structure 6, the optical output intensity changes corresponding to the phase difference 2 Δφ between them. Therefore, for example, Z Ka) LiNb○.

When the TM mode is excited using the TM mode, the guided light undergoes phase changes in opposite directions in the first and second arms, that is, push-pull operation becomes possible, so optical modulation of the mode is performed efficiently. be able to.

For example, when the electrode length is 11 and the electrode spacing is d, the phase difference 2Δφ is as follows: 2Δφ=π(V/Lea), V+eo=λd/
It is given by (2r na3rs31 ). V here
lso is the half-wave voltage and r is the applied electric field reduction factor. Now, the input power P0 is divided at the branch point, and the first arm and the second arm are each given a complex electric field amplitude Bl+ +E2.
It is assumed that guided light beams with a wavelength of

For simplicity, if we ignore the scattering loss at the branching point and the confluence point, ε, B+1 B, l ''=1...(
6), and the power branch ratio rP on the input side is r= = (
IEll/ lε, l)2 ・ (7) If the phase difference is 2Δφ, the output P is: ■ P=-(IE, l-1巳21)'+21 [i, cos'Δφ and Become. The above equation is illustrated in Figure 2, and the extinction ratio is given by: When the power branching ratio is 1, the extinction ratio becomes infinite.

When using a Matsuhatsu Enda type optical modulator as an intensity modulator, in Fig. 2, the driving voltage is changed between 0 and +1 or between -1 and 0 on the horizontal axis to create a light on/off signal. However, in the phase shift keying method of the present invention, the offset voltage is set during non-modulation so that, for example, the +1 point on the horizontal axis in FIG. By varying the voltage between the two, a digital phase modulation signal of (0, π) is created. That is, the offset voltage for the drive electrode 8 during non-modulation is set to the voltage vary that gives a substantially minimum value of the optical output level of the Matsuha Tsuender optical modulator 10, and the drive voltage V is added to this offset voltage. By changing the polarity of the modulation signal according to the modulation signal, the phase of the optical output is changed by π. By doing this,
When the phase changes from 0 phase to π phase or from π phase to 0 phase, the optical output always passes through a point (corresponding to +l on the horizontal axis in the graph shown in Figure 2) where the optical output becomes 0, and the pulse It is possible to suppress incomplete components of the phase modulation at the rise and fall of the signal, and as a result, the spectral spread of the optical output becomes smaller.

This principle will be explained with reference to FIGS. 3 and 4.

FIG. 3 is a waveform diagram of drive voltage, phase shift, and optical output, and FIG. 4 is a waveform diagram of the drive voltage, phase shift, and optical output, and FIG. , 12°t, is a diagram schematically showing the relationship between the electric field amplitude near the confluence point and the distance in the light propagation direction. Time tl corresponds to the time when the horizontal axis of the graph shown in Fig. 2 becomes +2, time t2 corresponds to the time when the horizontal axis becomes +1, and time ts corresponds to the time when the horizontal axis becomes 0. ing. In this example, the optical output is set to be maximum when the drive voltage is added to the offset voltage, so the maximum S/N ratio can be obtained. At time t, the phase of the light propagating through the first arm (indicated by the solid line in FIG. 4) and the phase of the light propagating through the second arm (indicated by the broken line in FIG. 4) are , coincide when they are combined.At time t2, the lights that have propagated through the first and second arms undergo a phase change of π/2 in different directions when they are combined. , these cancel each other out and the optical output becomes 0.Intensity modulation is a method in which the state at time tl+t2 is switched according to a modulation signal.

In the present invention, at time t3, the light that has propagated through the first and second arms undergoes a phase change of π in opposite directions when combined, so that the phase of the combined light is different from the phase of the combined light at time t. Phase shift keying is performed by focusing on the point that is shifted by π.

In this case, when the drive voltage falls from Vo to -Vn or vice versa, it always passes through a point where the optical output becomes 0 in accordance with the phase shift, so the phase incomplete modulation component is suppressed. .

Furthermore, in the conventional phase shift keying method explained in FIGS. 13 to 15, the phase changes continuously from 0 to π in response to changes in the voltage applied to the phase modulator. According to the invention method, the phase between times tl and t2 is constant based on the principle of wave synthesis, and the phase between times tl and t2 is constant.
The phase between 2 and t3 is constant, and therefore, at time t2 when the optical output becomes 0, the phase changes discontinuously as shown in Figure 3. Therefore, an incomplete phase modulation component is generated. do not have.

In the example of this explanation, drive voltages with equal absolute values and different polarities are applied so that the optical output is maximized to the offset voltage set so that the optical output is minimized, but the S/N ratio This drive voltage does not necessarily need to be set so that the optical output is maximized, as long as a slight decrease in is allowed. This is because when phase shift keying is performed using arbitrary drive voltages (those with the same absolute value and different polarities), the amount of phase shift is always π, and furthermore,
This is because the 0-phase light and the π-phase light have the same intensity.

In this way, in the method of the present invention, while continuous phase changes between the 0 phase and the π phase are important in the analog phase modulation method, the above continuous phase changes are not required at all and rather have a detrimental effect. This method actively utilizes this feature to eliminate phase-incomplete modulation components.

In addition, since the method of the present invention uses a Matsuhatsu Enda type optical modulator, when this is made into a traveling wave type, the 14th
Compared to the case where the traveling wave phase modulator shown in the figure is used, the electrode length can be halved for the same driving power source, and the modulation band is expanded. As a result, application to faster systems becomes possible.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 5 is a block diagram of a coherent optical transmission system using the DPSK method (differential phase shift keying method) showing an embodiment of the present invention.

First, on the transmitting side, a DFB with a fiber external resonator
- Light from a light source 12 consisting of an LD module (distributed feedback semiconductor laser module) or the like is DPSK-modulated by a Matsuha Tsuenda type optical modulator 10 and sent to an optical transmission line 20. 14 is an offset of the Matsuhatsu Enda type optical modulator 10 An offset circuit 16 is a drive circuit that adjusts the magnitude of the drive voltage added to the offset voltage and changes its polarity based on the modulation signal from the precoder 18.The precoder 18 is used to perform differential encoding. This is because the receiving side performs demodulation using 1-bit delay detection.

The light transmitted to the receiving side via the optical transmission line 20 is combined with local light (local oscillation light) from a local light source 24 configured in the same manner as the transmitting light source 12 in an optical coupler 22 consisting of an optical coupler or the like. The multiplexed light is input to the optical detector 26. The photodetector 26 is configured such that PIN photodiodes are connected in series in a double-balanced manner, and a HEMT amplifier amplifies a signal component from which intensity noise components have been removed from the connection point.

When the received light and the local light sonic light are input to the optical detector 26, an intermediate frequency signal containing transmission information in the phase shift is generated due to, for example, its square law detection characteristic.

Therefore, demodulation can be performed after passing this intermediate frequency signal through the amplifier 28, bandpass filter 30, and amplifier 32. This demodulation is performed by mixing the signal from the amplifier 32 with a signal delayed by a time T (one time slot, that is, the reciprocal of the bit rate) corresponding to one bit by a delay circuit 34 in a mixer 36. The signal from mixer 36 is passed through a low pass filter 38.
After passing through , it is identified by the identifier 40 . The intermediate frequency signal that has passed through the band pass filter 30 is demodulated, and after being amplified by an amplifier 42, a carrier signal is regenerated by a frequency doubler 44, and an AFC circuit is used to keep the frequency of this carrier signal constant. 46, the local light source 24
The driving conditions are feedback-controlled.

The Matsuhatsu Enda type optical modulator 10 used in the example has a Tr:
It is of LiNbO2 type, and the half-wave voltage is 10.2 (
V), the cutoff frequency is approximately 7Gllz. And its drive is 1. 2Gb/s differentially encoded NRZ
(Depends on the 215-1> signal.

FIG. 6 shows how the optical output of the Matsuhatsu Enda type optical modulator 10 changes with respect to the applied voltage. The maximum extinction ratio is 33.3
dB, the insertion loss between single mode fibers is 1.9 (I
It was B. Then, the applied voltage V a r r which gives the minimum value of the optical output level in the vicinity of the applied voltage 0 (V)
was -2.3 (V), so an offset voltage is set to this voltage.

The driving voltage added to the offset voltage is 1/2 of the half-wave voltage.
5.1 (V).

FIG. 7 shows the bit error rate characteristics measured in the above embodiment. The minimum receiving sensitivity at a bit error rate of 10-9 is -
40,8 dBm and 14 to the shot noise limit.
．． A difference of 3 dB was observed. It cannot be said that the bit error rate characteristic in this example is necessarily better than that in the conventional technology using a traveling wave phase modulator, but this is due to the branched phase shifting optical waveguide structure. This is thought to be due to phase modulation imperfection due to asymmetry or setting errors in the applied voltage, and by improving these, it is expected that the bit error rate characteristics will be dramatically improved.

An embodiment for dealing with setting errors in applied voltage will be described below.

An embodiment in which the optical output level of the Matsuhatsu Enda type optical modulator 10 is detected and the offset voltage and drive voltage during modulation are controlled so that the optical output level is maximized will be described with reference to FIG. In this embodiment, the output light of the Matsuhatsu Enda type optical modulator 10 is transferred to an optical splitter 48 consisting of a half mirror or the like.
The branched light intensity is detected by an optical power meter 50.

The optical power meter 50 includes a photoreceiver 52 that allows the branched light to enter and generates a photocurrent, and a current-voltage converter 54 that converts the photocurrent into a voltage.

The voltage signal from the optical power meter 50 is sent to the switch circuit 56.
is selectively sent to the offset voltage control system and drive voltage control system. When controlling the offset voltage, a comparator 58 detects the difference between the voltage signal from the optical power meter 50 and the set value of the offset voltage at the time of non-modulation, or a change from the initial value of the ratio, and cancels the difference. A control signal is sent to the offset circuit \@14 via the offset voltage control circuit 62. Further, when controlling the drive voltage, the comparator 60 performs similar detection of the drive voltage, and sends a control signal to the drive circuit 16 via the drive voltage control circuit 64.

FIG. 9 is a diagram for explaining the decrease in the optical output level due to the shift of the characteristic curve. The characteristic curve in FIG. 6 shown by the solid line shifts to the direction of lower applied voltage as shown by the broken line,
The voltage that gives the minimum value of optical output is V. fr to V. (r
′ is shown. Despite this shift in the characteristic curve, the offset voltage is VO
It is clear that if left set to ff, the light output level will decrease. Therefore, in such a case, the offset voltage control system is selected by the switch circuit 56 in FIG. 8 and the above-described control is performed to maximize the optical output. By doing so, stable (0, π) phase modulation without deterioration of reception sensitivity becomes possible. Although control in the case where the characteristic curve shifts has been described here, similar control can be performed also in the case where (the shape of) the characteristic curve itself changes.

FIG. 10 is a diagram for explaining the decrease in the optical output level due to the deformation of the characteristic curve. The voltage V that gives the minimum value of the optical output as shown by the broken line in the characteristic curve of FIG. 6 shown by the solid line
. The case is shown expanded in the direction of the applied voltage with f as the center. In this case, if the drive voltage ゎ is left unchanged, the optical output level will decrease. Therefore, in such a case, the drive voltage control system is selected by the switch circuit 56 in FIG. 8 and the above-mentioned control is performed so that the optical output level is maximized. By doing this, stable (0, π) phase modulation without deterioration of receiving sensitivity becomes possible. When V changes without deforming the characteristic curve, the state shown in Fig. 10 occurs. Similar control can be performed on

By the way, in reality, is the optical output level fluctuating due to an inappropriate offset voltage?
It is not always possible to determine whether the optical output level is changing due to an inappropriate drive voltage. Therefore, in such a case, the control of the offset voltage and the control of the drive voltage may be sequentially performed selectively by the switch circuit 56. This improves reception sensitivity and enables stable (0.π>) phase modulation at all times.

On the other hand, if the Matsuhatsu Enda type optical modulator 10 employs a LiNbO3 waveguide structure, it is desirable to input the light as TM polarized light that can use a large electro-optic constant, and the polarization plane of the input light is the same as that of TM polarized light. If it deviates from the plane of polarization, problems such as an increase in incomplete phase modulation components occur.

Therefore, in such a case, as shown in Figure 11,
A polarization controller 70 is provided upstream of the Matsuhatsu Enda type optical modulator 10 in the propagation direction of the propagating light, and the voltage signal from the optical power meter 50 is compared with the initial value of the control voltage of the polarization controller 70 in the comparator 66. However, the control voltage of the polarization controller 70 is controlled via the polarization control circuit 68 so as to correct the difference. As a result, phase modulation can always be performed as TM polarized light, resulting in stable (
0, π) phase modulation becomes possible.

By the way, since the offset voltage and drive voltage control and the polarization plane control described above are all performed based on the voltage signal from the optical power meter 50, these controls cannot be performed simultaneously. Therefore, in order to prevent imperfections in phase modulation due to variations in offset voltage, drive voltage, and polarization plane all at once, the arrangement shown in FIG. 12 may be used. That is, the switch circuit 74 selectively switches the control of the offset voltage or drive voltage, and the switch circuit 72 selectively switches the control of the offset voltage or drive voltage or the polarization plane. . By sequentially performing this switching at appropriate timing, stable (0, π
) Phase modulation becomes possible.

As described in detail, according to the present invention, phase incomplete modulation components can be suppressed, and the spectral spread of output light can be reduced.

As a result, the design of optical heterogain receivers becomes easier;
It becomes easier to improve reception sensitivity, and when performing frequency division multiplex transmission, it becomes possible to increase the number of multiplexes if the transmission band is the same.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the Matsuhatsu Enda type optical modulator used in carrying out the invention, and Fig. 2 is an explanatory diagram of the Matsuhatsu Enda type optical modulator used in the implementation of the invention.
Graph showing the relationship between /P and V/V+eo, Figure 3 is a diagram explaining the principle of the invention, Figure 4 is a diagram to help explain the principle of the invention, Figure 5 is a block diagram of an embodiment of a coherent optical transmission system using the DPSK method. , Fig. 6 is a characteristic diagram of the Matsuhatsu Enda type optical modulator shown in Fig. 5, Fig. 7 is a graph showing the bit error rate characteristics in the embodiment shown in Fig. 5, and Fig. 8 is a graph showing the offset voltage and drive voltage. Figure 9 is a diagram illustrating a decrease in the optical output level due to a shift of the characteristic curve; Figure 1 is a diagram illustrating a decrease in the optical output level due to a deformation of the characteristic curve. Figure 11 is a diagram showing an example of controlling the polarization state, Figure 12 is a diagram showing an example of controlling the offset voltage, drive voltage, and polarization state, and Figure 13 is the basic configuration of the phase modulator. FIG. 14 is a diagram showing the configuration of a traveling wave phase modulator, and FIG. 15 is an explanatory diagram of phase incomplete modulation components. 1 2... Input side optical waveguide structure, 4... Optical waveguide structure for phase shifting, 6... Output side optical waveguide structure, 8... Drive electrode, 0... Matsuhatsu Enda type optical modulator.

Claims (1)

[Claims] (1) An input-side optical waveguide structure (2) and a pair of phase-shifting optical waveguides that change the phase of guided light according to the applied electric field branched from the input-side optical waveguide structure (2). structure (4), and an output side optical waveguide structure (6) where the phase shifting optical waveguide structure (4) joins;
The drive electrode (8) mounted on the phase shifting optical waveguide structure (4)
), and an offset voltage is set for the drive electrode (8) during non-modulation so that the optical output level of the Mach-Zehnder optical modulator (10) is approximately minimum. A phase shift keying method characterized in that the polarity of the drive voltage applied to the offset voltage is changed in accordance with the modulation signal to give a change of π to the phase of the optical output. (2) The phase according to claim 1, characterized in that the optical output level of the Mach-Zehnder optical modulator (10) is detected and the offset voltage during modulation is controlled so that the optical output level is maximized. Shift keying method. (3) The phase shift keying method according to claim 1, wherein the optical output level of the Mach-Zehnder optical modulator (10) is detected, and the drive voltage is controlled so that the level is maximized. . (4) The phase shift keying method according to claim 1, wherein the offset voltage control according to claim 2 and the drive voltage control according to claim 3 are performed selectively and sequentially. (5) The optical output level of the Mach-Zehnder optical modulator (10) is detected, and the polarization state of the light input to the Mach-Zehnder optical modulator (10) is controlled so that the level is maximized. The phase shift keying method according to claim 1, characterized in that: (6) The offset voltage control according to claim 2, the drive voltage control according to claim 3, and the polarization state control according to claim 5 are performed selectively and sequentially. The phase shift keying method according to claim 1.