JP2013007987A - Mach-zehnder modulator and optical communication device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a bias voltage.SOLUTION: A Mach-Zehnder modulator includes: a branching optical waveguide which branches input light; a first arm optical waveguide which has a PN junction structure or a PIN junction structure and propagates light inputted from the branching optical waveguide; a second arm optical waveguide which has a quantum well structure and propagates light inputted from the branching optical waveguide; a first phase electrode which adjusts a phase of light propagated in the first arm optical waveguide; and a second phase electrode which adjusts a phase of light propagated in the second arm optical waveguide. A forward voltage is applied to the first phase electrode, and 0 V or a reverse voltage is applied to the second phase electrode.

Description

  The present invention relates to a Mach-Zehnder type modulator using a semiconductor.

  In recent years, in the field of optical communications, optical transmission schemes have increased in speed and capacity, and Mach-Zehnder type external modulators capable of high speed operation with low chirping have been put into practical use. In a general Mach-Zehnder type modulator, a ferroelectric such as lithium niobate which is a material having a large electro-optic effect is used.

  Further, as a device that can be expected to be reduced in size, cost, and power consumption as compared with a Mach-Zehnder type modulator using lithium niobate, there is a Mach-Zehnder type modulator using a semiconductor such as InP or GaAs. Among such Mach-Zehnder type modulators, attention is focused on those utilizing the quantum confined Stark effect in which the refractive index changes when an electric field is applied to the quantum well structure.

  In the method using the quantum confined Stark effect, it is common to reduce the waveguide loss by increasing the detuning between the operating wavelength and the transition wavelength of the quantum well. However, since the phase change amount is simultaneously reduced, a high bias voltage is required to obtain a desired phase change. Therefore, there is a demand for a Mach-Zehnder type modulator that can operate with a low bias voltage.

  In the conventional technique, when the electric field is applied, the quantum well structure is designed so that the electric field where the electric field absorption of light occurs is smaller than the electric field where the refractive index change blocks the light, and the refractive index change and electric field of the quantum confined Stark effect A technique for reducing a bias voltage required for light modulation by modulating light using both absorption effects is disclosed (for example, Patent Document 1 below).

JP 2001-83473 A

  However, in the prior art described in the above-mentioned patent document, in order to lower the bias voltage, the quantum well transition wavelength and the operating wavelength are operated close to each other, so that the waveguide loss due to the electroabsorption effect increases. Therefore, there is a problem that the reduction of the bias voltage is limited.

  The present invention has been made to solve the above-described problems, and an object thereof is to reduce a bias voltage in a Mach-Zehnder type modulator using a semiconductor.

  A Mach-Zehnder type modulator according to the present invention has an optical branching waveguide that branches input light and a PN junction structure or a PIN junction structure, and propagates light input from the optical branching waveguide. An arm optical waveguide; a second arm optical waveguide having a quantum well structure that propagates light input from the optical branching waveguide; and a second arm that adjusts a phase of light propagating through the first arm optical waveguide. 1 phase electrode, and a second phase electrode for adjusting the phase of light propagating through the second arm optical waveguide, applying a forward voltage to the first phase electrode, It is characterized in that 0 V or a reverse voltage is applied to the phase electrode.

  According to the present invention, the Mach-Zehnder type modulator using a semiconductor has an effect that the bias voltage can be reduced.

It is a figure which shows the structural example of the Mach-Zehnder type modulator concerning Embodiment 1 of this invention. It is a figure which shows an example about the relationship between the bias voltage applied to the phase electrode of the Mach-Zehnder type modulator concerning Embodiment 1 of this invention, and the phase difference given to the light which propagates an arm optical waveguide. It is a figure which shows the structural example of the Mach-Zehnder type modulator concerning Embodiment 1, 2 of this invention. It is a figure which shows the structural example of the optical communication apparatus concerning Embodiment 3 of this invention. It is a figure which shows the structural example of the modulator control part concerning Embodiment 3 of this invention. It is a figure which shows the structural example of the modulator control part concerning Embodiment 4 of this invention. It is a figure which shows an example of the flowchart of the modulator control operation | movement concerning Embodiment 5 of this invention.

  Embodiments of the present invention will be described below in detail with reference to the drawings. Note that each of the embodiments described below is an embodiment when the present invention is embodied, and the present invention is not limited to the scope.

Embodiment 1 FIG.
FIG. 1 is a diagram illustrating a configuration example of a Mach-Zehnder modulator according to the present embodiment. The Mach-Zehnder modulator according to the present embodiment is formed on the semiconductor substrate 1, and includes an optical input waveguide 11, an optical branching waveguide 12, arm optical waveguides 131 and 132, arm electrodes 141 and 142, The phase electrodes 151 and 152, the optical multiplexing waveguide 16, and the optical output waveguide 17 are configured.

  Light that passes through the waveguide of the Mach-Zehnder modulator is input from the optical input waveguide 11. The input light is branched by the optical branching waveguide 12, and the branched light propagates through the arm optical waveguide 131 and the arm optical waveguide 132, respectively. The arm electrode 141 is formed on or near the arm optical waveguide 131. The phase electrode 151 is insulated from the arm electrode 141 and is formed on or in the vicinity of the arm optical waveguide 131. Similarly, the arm electrode 142 is formed on or near the arm optical waveguide 132. The phase electrode 152 is insulated from the arm electrode 142 and is formed on or near the arm optical waveguide 132. An RF voltage corresponding to the modulation signal is applied to the arm electrodes 141 and 142, and light propagating through the arm optical waveguides 131 and 132 is modulated. A bias voltage for phase adjustment is applied to the phase electrodes 151 and 152, and a phase difference depending on the modulation method is given to the light propagating through the arm optical waveguides 131 and 132. Light propagating through the arm optical waveguides 131 and 132 is multiplexed by the optical multiplexing waveguide 16. The light combined by the optical multiplexing waveguide 16 is output from the optical output waveguide 17. Note that the Mach-Zehnder type modulator is disposed in the optical communication apparatus, and the bias voltage applied to the phase electrodes 151 and 152 is introduced from one or a plurality of control circuits (not shown). The same applies to the following embodiments.

  A quantum well structure is formed in the arm optical waveguides 131 and 132 under or near the arm electrodes 141 and 142 and the phase electrodes 151 and 152. The quantum well structure is designed such that the absorption coefficient due to the quantum confined Stark effect is sufficiently small with respect to the wavelength of light input to the Mach-Zehnder type modulator, and a change in refractive index occurs.

  Next, an example of the structure of the Mach-Zehnder type modulator according to this embodiment will be described. The arm optical waveguide of this example has a PIN junction structure in which a non-doped quantum well structure is sandwiched between p-type and n-type cladding layers. An n-InP layer is formed as a cladding layer on the n-InP substrate, and a quantum well structure in which i-InGaAsP / i-InP is stacked in multiple layers is formed thereon. A p-InP layer is deposited as a cladding layer on the quantum well structure. On the p-InP layer, an InGaAs layer in which P-type impurities are diffused at a high concentration is formed as a contact layer. Thereafter, the quantum well structure and the upper layer are processed into a mesa shape by etching to form the waveguide shape of the Mach-Zehnder modulator shown in FIG. Next, an N electrode is formed of AuGeNi / Au on the back surface of the semiconductor substrate 1, Ti / Pt / Au is deposited on the InGaAs layer of the contact layer, and then an Au plating layer is formed to form a P electrode. To do. The P electrode corresponds to an example of the arm electrodes 141 and 142 and the phase electrodes 151 and 152.

  The above-described structure and formation method are merely examples, and the Mach-Zehnder type modulator realizing the present embodiment is not limited to this example. For example, in addition to the above film types, conventionally known InGaAs, InAlAs, InAlAsP, InGaAlAs, or the like may be used for the quantum well structure. Moreover, although the example formed on the InP substrate was described, even when other materials such as GaAs, Si, and Ge are used, conventionally known quantum well structures, PN junction structures, By using a PIN junction structure or the like, a Mach-Zehnder modulator that realizes the present embodiment can be obtained.

  FIG. 2 is a diagram showing an example of the relationship between the bias voltage applied to the phase electrode of the Mach-Zehnder modulator according to the present embodiment and the phase difference given to the light propagating through the arm optical waveguide. FIG. 2 shows an example of a Mach-Zehnder type modulator formed on the above-described InP substrate. The horizontal axis indicates the bias voltage applied to the phase electrode 151, and the vertical axis indicates the light transmitted through the arm optical waveguides 131 and 132. The phase difference produced between them is shown.

  As shown in the figure, when a reverse bias voltage is applied to the phase electrode 151, the refractive index of the quantum well structure changes due to the quantum confined Stark effect, and the phase difference increases linearly according to the bias voltage. On the other hand, when a forward bias voltage is applied, a current flows through the PIN junction in the arm optical waveguide 131, and the refractive index change due to the carrier plasma effect becomes dominant. In this case, a characteristic change such as a so-called current-voltage characteristic of the diode occurs, and a refractive index change more abruptly than the quantum confined Stark effect occurs.

  In the conventional technique, since the phase difference changes linearly with respect to the bias voltage, a reverse bias voltage is applied to the phase electrode 151 and the phase electrode 152. In the present embodiment, a bias voltage is applied to the phase electrodes 151 and 152 so as to extend in both the forward direction and the reverse direction. Thereby, the adjustment range of the phase difference given to the light propagating through the arm optical waveguides 131 and 132 is expanded twice as compared with the conventional case where only the reverse bias voltage region is used. Although it is assumed that the voltage is applied across both the forward direction and the reverse direction, a forward bias can be applied to the phase electrode 151 and the phase electrode 152 can be set to 0V. Even in this case, the bias voltage can be reduced by a sudden change in refractive index due to the carrier plasma effect.

  In the above-described example of the Mach-Zehnder type modulator, both the arm optical waveguides 131 and 132 have a quantum well structure. However, the arm optical waveguide on the side utilizing the quantum confined Stark effect requires a quantum well structure, but the arm optical waveguide on the side utilizing the carrier plasma effect does not necessarily require a quantum well structure. The arm optical waveguide on the side using the quantum Stark effect is not limited to the above-described PIN junction structure, and may have a conventionally known NPIN junction structure or NIN junction structure. Further, the arm optical waveguide on the side using the carrier plasma effect may have a PN junction structure or a PIN junction structure (the non-doped layer may not be a quantum well structure).

  Next, as an application example of the present embodiment, in the Mach-Zehnder type modulator of FIG. 1, intensity modulation is performed by giving a phase difference of 0 (0 deg) or π (180 deg) to the light propagating through the arm optical waveguides 131 and 132. Will be described. When applying to the phase electrodes 151 and 152 from the reverse bias voltage region, a bias voltage of 5 V or more is required to obtain a phase difference of π. On the other hand, when a forward bias voltage is applied to the phase electrode 151 and a reverse bias voltage is applied to the phase electrode 152, the reverse bias voltage applied to the phase electrode 152 is 2 V depending on the value of the forward bias voltage. It can be lowered to the extent. In addition, unlike the prior art, it is possible to operate the quantum well so that the resonance wavelength and the operating wavelength are greatly detuned, so that waveguide loss due to the quantum confined Stark effect can be avoided.

  Although an example of intensity modulation has been described above, the same effect can be obtained by phase modulation. For example, in binary phase shift keying (BPSK), if a bias voltage for changing the phase of each light propagating through the arm optical waveguides 131 and 132 by π is Vπ, a bias application in the reverse direction is usually applied. In the region, a modulation signal of phase π is generated by applying a bias voltage of Vπ to the phase electrode 151 and −Vπ to the phase electrode 152. According to the present embodiment, since the forward bias voltage is applied to the phase electrode on one side, the bias voltage applied in the reverse direction can be reduced, and an increase in the control range of the bias voltage can be suppressed.

  The increase of the bias voltage imposes a restriction on the control circuit that controls the bias voltage applied to the Mach-Zehnder type modulator. Therefore, if the bias voltage can be reduced according to the present invention, the design of the control circuit board can be simplified, and the cost of the control circuit board can be reduced.

  As described above, in this embodiment, in the Mach-Zehnder type modulator, a forward bias is applied to the phase electrode of the arm optical waveguide having the PN junction structure or the PIN junction structure, and the arm optical waveguide having the quantum well structure is applied. The phase electrode was configured to apply a 0 V or reverse bias. Thereby, there is an effect that the bias voltage can be reduced.

Embodiment 2. FIG.
In the present embodiment, in addition to the configuration of the first embodiment, a region where a phase difference does not occur even when a bias voltage is applied (hereinafter referred to as a dead zone), which is seen near a bias voltage of 0 V, is selected. It is set as the structure excluded from.

  The Mach-Zehnder type modulator according to this embodiment has the same configuration as that shown in FIG. 1, and the change in phase difference when a bias voltage is applied to the phase electrodes 151 and 152 is also as shown in FIG. In FIG. 2, between the inflection point 1 near the bias voltage 0V and the inflection point 2 corresponding to the voltage at which the phase change increases abruptly as in the diode characteristics when the bias voltage is applied in the forward direction. Make the area a dead zone. The bias voltage at the inflection points 1 and 2 is determined by a voltage value at which a change in phase difference with respect to the bias voltage is equal to or greater than a predetermined threshold, and should be appropriately adjusted according to the configuration of the Mach-Zehnder modulator and how to determine the threshold. Can do.

  A bias voltage higher than a threshold value (a bias voltage higher than the inflection points 1 and 2) is applied to the phase electrodes 151 and 152. In the example of FIG. 2, the width of the dead zone is about 0.6V. The dead zone is examined in advance, and when the Mach-Zehnder type modulator is actually operated in the optical communication apparatus, the dead zone is excluded from the range in which the optimum bias voltage is selected. It should be noted that the above configuration may be applied to only one side, such as not using the dead band only for the forward bias voltage.

  As described above, in this embodiment, in addition to the configuration of the first embodiment, the bias control is performed so as not to use the dead zone. Thereby, in addition to the effect obtained by the configuration of the first embodiment, an efficient region having a large phase change with respect to the bias voltage can be used, and the control of the Mach-Zehnder type modulator can be simplified.

  In the example of FIG. 1, the arm optical waveguide has two structures, but the configuration of the Mach-Zehnder modulator is not limited to this. For example, a configuration in which the Mach-Zehnder type modulators according to the first and second embodiments as shown in FIG. 3 are nested is also possible. The light input from the optical input waveguide 21 is branched by the optical branching waveguide 22. A phase difference π / 2 is given between the branched lights by the phase electrodes 231 and 232. Thereafter, one of the branched lights propagates through the nested structure 241, and the other of the branched lights propagates through the nested structure 242. In the optical modulation of the nested structures 241 and 242, the same Mach-Zehnder modulator control as in the first and second embodiments is applied. The configuration for providing the phase difference π / 2 may be in the subsequent stage of the nested structures 241 and 242.

  The Mach-Zehnder type optical modulator shown in FIG. 3 includes four-phase phase shift keying (QPSK), differential quadrature phase shift keying (DQPSK), quadrature phase shift keying (DQPSK), quadrature phase shift keying (AM), and quadrature phase shift keying (DQPSK). : Quadrature amplitude modulation), which corresponds to a modulation scheme that realizes a higher transmission rate. The first and second embodiments may also be applied to a Mach-Zehnder type optical modulator having an increased nesting structure as compared with the configuration of FIG.

Embodiment 3 FIG.
On the side using the quantum confined Stark effect, the refractive index changes in proportion to the applied voltage. On the other hand, the refractive index changes in proportion to the applied current on the side using the carrier plasma effect. When the bias voltage is controlled using a microcomputer or the like because the change in refractive index with respect to the bias voltage differs between when the forward bias voltage is applied and when the reverse bias voltage is applied, the control accuracy of the forward and reverse bias voltages Is different. The transmission characteristics of the optical communication apparatus using the Mach-Zehnder type modulator as described above vary depending on the bias voltage applied to the external modulator. The bias voltage condition that achieves the best transmission characteristics changes depending on the wavelength of the optical signal to be transmitted, the temperature of the modulator, etc., so the forward and reverse bias voltages must be controlled with different control accuracy. is there.

  FIG. 4 is a diagram illustrating a configuration example of the optical communication apparatus according to the present embodiment. As illustrated, the optical communication apparatus 1000 includes an optical modulation unit 3, an optical output monitor unit 4, and a modulator control unit 5.

  The optical modulation unit 3 includes the Mach-Zehnder type modulator described in the first and second embodiments, and transmits an optical signal. The optical output monitor unit 4 monitors the optical output of the optical signal output from the optical modulation unit 3, receives a part of the transmitted optical signal, and outputs an optical output monitor value. An example of the optical output monitor unit 4 includes a photodetector and a transimpedance amplifier, detects the intensity of the received optical signal as a monitor voltage, and outputs this as an optical output monitor value. For example, the light output monitor unit 4 may detect the monitor current as a monitor current without converting it into a voltage, and output this as a light output monitor value. The modulator control unit 5 calculates a bias voltage control amount based on the output from the optical output monitor unit 4. The modulator control unit 5 controls the bias voltage of the Mach-Zehnder type modulator included in the optical modulation unit 3 based on the calculated bias voltage control amount.

  FIG. 5 is a diagram illustrating a configuration example of the modulator control unit according to the present embodiment. As illustrated, the modulator control unit 5 includes a control signal input unit 51, a drive signal output unit 52, an operational amplifier 53, resistors 54, 55, and 56, a switch 57, and a gain switching unit 58.

  The modulator control unit 5 includes an inverting amplifier circuit using an operational amplifier 53. An electric signal for driving the Mach-Zehnder type modulator is input from the control signal input unit 51. The input electric signal is amplified by the operational amplifier 53 according to the gain of the modulator control unit 5. The amplified electrical signal is output from the drive signal output unit 52. The electric signal output from the drive signal output unit 52 is applied to the phase electrode of the Mach-Zehnder type modulator in the optical modulation unit 3. The gain switching unit 58 changes the gain of the electric signal by changing the combined resistance under the control of the switch 57. The switch 57 controls on / off based on the gain control signal. The switch only needs to be able to be turned on and off, and is realized by an analog switch, for example.

  When the resistance value of the resistor 54 is R1, the resistance value of the resistor 55 is R2, and the resistance value of the resistor 56 is R3, the gain G1 of the modulator control unit 5 when the switch 57 is turned off is expressed by Equation (1). become.

G1 = R2 / R1 (1)

  The gain G2 of the modulator control unit 5 when the switch 57 is turned on is as shown in Expression (2).

G2 = ((R2 × R3) / (R2 + R3)) / R1 (2)

  The modulator control unit switches the gains G1 and G2 according to the voltage application direction (forward direction or reverse direction). For example, when applying a reverse bias voltage, control is performed in the state of G1, and when applying a forward bias voltage, control is performed in the state of G2. If G1> G2, the control width of the forward bias voltage controlled in the G2 state is narrower than the control width of the reverse bias voltage controlled in the G1 state. As a result, the forward bias voltage can be set with a finer precision than the reverse bias voltage. As described above, the forward and reverse bias voltages can be controlled with different control accuracy by switching between G1 and G2 according to the voltage application direction.

Embodiment 4 FIG.
In the third embodiment, the gain switching unit 58 is configured by the resistors 55 and 56 and the switch 57. However, in the present embodiment, the gain switching unit 58 is configured by the variable resistor 61.

  FIG. 6 is a diagram illustrating a configuration example of the modulator control unit according to the present embodiment. The same components as those in FIG. 5 are denoted by the same reference numerals, and redundant description is omitted. As shown in the figure, the gain switching unit 68 of the modulator control unit 6 of the present embodiment has a variable resistor 61. The gain switching unit 68 switches the gain of the modulator control unit 6 by changing the resistance value of the variable resistor 61 based on the gain control signal. The modulator control unit 6 controls the resistance value of the variable resistor 61 according to the forward or reverse bias condition. The variable resistor 61 only needs to be able to control the resistance value according to the gain control signal, and can be realized by, for example, DPM (Digital PotentioMeter).

  When the resistance value of the resistor 54 is R1 and the resistance value of the variable resistor 61 is R4, the gain G1 of the modulator control unit 6 is expressed by Expression (3).

  G1 = R4 / R1 (3)

  Similar to the third embodiment, the gain is controlled by adjusting the resistance value of the variable resistor 61 in accordance with the voltage application direction. When applying a forward bias voltage, the gain is reduced by lowering the value of R4 as compared to applying a reverse bias voltage. As a result, the control width of the forward bias voltage is narrower than the control width of the reverse bias voltage, and the forward bias voltage is set with a finer precision than the reverse bias voltage. As described above, the bias voltage in the forward direction and the reverse direction can be controlled with different control accuracy by adjusting the variable resistance value according to the voltage application direction and setting the gain. Also, by controlling with a variable resistor, even if the bias voltage conditions that achieve the best transmission characteristics change depending on the wavelength of the transmitted optical signal, the temperature of the modulator, etc., the bias conditions can be flexibly changed. It becomes possible to do.

  In the third and fourth embodiments, the gain is changed by controlling the resistance value in the voltage amplifier circuit of the modulator control unit, but the gain may be switched by a method other than the switching of the resistance value. For example, the gain value may be switched by storing gain information in the table in advance and extracting the gain setting value from the table in accordance with the optical output monitor value. In the third and fourth embodiments, the inverting amplifier circuit is shown as an example of the voltage amplifier circuit of the modulator control unit, but the present invention is not limited to this. For example, the same effect can be obtained by switching the gain by applying another known voltage amplifier circuit such as a non-inverting amplifier circuit to the third and fourth embodiments.

Embodiment 5 FIG.
In this embodiment, in addition to the configurations of the first to fourth embodiments, bias voltage control is not performed when the forward direction (positive) and the reverse direction (negative) of the bias voltage are reversed.

  FIG. 7 is a diagram illustrating an example of a flowchart of the modulator control operation according to the present embodiment. When the bias voltage control is started, the modulator control unit reads the optical output monitor value (step S701). The modulator control unit calculates a bias voltage control amount based on the read optical output monitor value (step S702). The case where the control is performed based on the calculated bias voltage control amount and the case where the control is not performed are compared to determine whether the positive / negative of the bias voltage after the control is reversed (step S703). Here, when the sign is reversed, the bias voltage is not controlled (step S704). On the other hand, if the sign is not reversed, control is performed with the calculated bias voltage (step S705). For example, when a forward bias voltage is applied to the phase electrode of the Mach-Zehnder type modulator and the bias voltage of the phase electrode reverses from the forward direction to the reverse direction, bias control is performed to reverse the voltage application direction. Without changing the current voltage application direction. Similarly, when a reverse bias voltage is applied to the phase modulator and the bias voltage of the phase electrode is reversed from the reverse direction to the forward direction, the current state is not performed without performing the bias control for reversing the voltage application direction. The voltage application direction is maintained. After steps S704 and S705, the process returns to step S701 again.

  By including the operation flow as described above, the voltage application direction of the bias voltage applied to the Mach-Zehnder modulator can be maintained in the first to fourth embodiments.

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 11 Optical input waveguide 12 Optical branching waveguide 131, 132 Arm optical waveguide 141, 142 Arm electrode 151, 152 Phase electrode 16 Optical multiplexing waveguide 17 Optical output waveguide 1000 Optical communication apparatus 241, 242 Nested structure 3 Optical modulation Unit 4 optical output monitor unit 5, 6 modulator control unit 51 control signal input unit 52 drive signal output unit 53 operational amplifier 54, 55, 56 resistor 57 switch 58, 68 gain switching unit 61 variable resistor

Claims (9)

An optical branching waveguide for branching input light; and
A first arm optical waveguide having a PN junction structure or a PIN junction structure and propagating light input from the optical branching waveguide;
A second arm optical waveguide having a quantum well structure and propagating light input from the optical branching waveguide;
A first phase electrode for adjusting a phase of light propagating through the first arm optical waveguide;
A second phase electrode for adjusting a phase of light propagating through the second arm optical waveguide, a forward voltage is applied to the first phase electrode, and 0 V or reverse is applied to the second phase electrode. A Mach-Zehnder type modulator characterized by applying a directional voltage.   2. The forward voltage applied to the first phase electrode is set within a range in which a change in phase of light propagating through the first arm optical waveguide is a predetermined threshold value or more. A Mach-Zehnder type modulator described in 1.   2. The reverse voltage applied to the second phase electrode is set within a range in which a change in phase of light propagating through the second arm optical waveguide is equal to or greater than a predetermined threshold value. Or a Mach-Zehnder modulator according to 2.   A Mach-Zehnder type modulator having at least one of the Mach-Zehnder type modulators according to claim 1 as a nested structure.   An optical communication apparatus comprising the Mach-Zehnder type modulator according to claim 1. An optical modulator having at least one of the Mach-Zehnder modulators according to claim 1,
An optical output monitor for monitoring an optical signal output from the optical modulator;
A modulator control unit that controls a voltage applied to the Mach-Zehnder type modulator based on an output from the optical output monitor unit;
The optical communication apparatus, wherein the modulator control unit switches a gain according to a voltage application direction to a phase electrode of the Mach-Zehnder type modulator.   The optical communication device according to claim 6, wherein the modulator control unit includes a voltage amplification circuit, and switches a gain of the voltage amplification circuit.   The optical communication apparatus according to claim 6, wherein the modulator control unit includes a voltage amplification circuit, and performs gain switching by adjusting a resistance value of a variable resistor of the voltage amplification circuit.   The modulator control unit performs control so as not to change a voltage application direction when a voltage application condition calculated based on an output from the light output monitor unit reverses the voltage application direction. The optical communication device according to any one of 6 to 8.

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