Manufacturing method, test method, and test program for optical modulator, and optical transmitter

Abstract

To provide a method for manufacturing an optical modulator, a test method, and a test program, and an optical transmitter that can prevent an increase in optical absorption loss.SOLUTION: There is provided a method for manufacturing an optical modulator. The optical modulator has Mach-Zehnder modulators. The Mach-Zehnder modulators each have electrodes and arm waveguides. The electrodes are provided on the arm waveguides. The manufacturing method includes the steps of: preparing the Mach-Zehnder modulators; based on light transmissivity in the arm waveguides, acquiring the relationship between the voltage applied to the electrodes and the amount of change in the phase of light propagating the arm waveguides; based on the relationship, acquiring a voltage at which the amount of change in the phase of light propagating the arm waveguides becomes a predetermined magnitude during modulation of the light; and storing the voltage in a storage unit.SELECTED DRAWING: Figure 9

Description

[Technical field]

[0001] The present disclosure relates to a manufacturing method, a testing method, and a testing program for an optical modulator, and an optical transmitting device. [Background technology]

[0002] A Mach-Zehnder modulator that is formed of a semiconductor layer and modulates light has been developed (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-164243 A Summary of the Invention [Problem to be solved by the invention]

[0004] Light propagates through the arm waveguide of the Mach-Zehnder modulator. The phase of the light can be adjusted by applying a voltage to the Mach-Zehnder modulator. During modulation, the amount of change in the phase is set to a predetermined amount, for example to increase the intensity of the output light.

[0005] The ratio of phase change to voltage (phase adjustment efficiency) varies from Mach-Zehnder modulator to Mach-Zehnder modulator. When the same voltage is applied to multiple Mach-Zehnder modulators, the amount of phase change in one Mach-Zehnder modulator is large, while the amount of phase change in another Mach-Zehnder modulator is small. In order to make the amount of phase change a predetermined amount even in a Mach-Zehnder modulator with low phase adjustment efficiency, the voltage can be increased. There is a positive correlation between phase adjustment efficiency and optical absorption loss. Increasing the voltage also increases the optical absorption loss. Therefore, an object of the present invention is to provide a manufacturing method, a test method, and a test program for an optical modulator that can suppress the increase in optical absorption loss, as well as an optical transmitter. [Means for solving the problem]

[0006] A manufacturing method according to the present disclosure is a method for manufacturing an optical modulator, the optical modulator having a Mach-Zehnder modulator, the Mach-Zehnder modulator having an electrode and an arm waveguide, the electrode being provided in the arm waveguide, the manufacturing method including the steps of: preparing the Mach-Zehnder modulator; acquiring, based on a light transmittance in the arm waveguide, a relationship between a voltage applied to the electrode and an amount of change in phase of light propagating through the arm waveguide; acquiring, based on the relationship, a voltage at which an amount of change in the phase of the light becomes a predetermined magnitude when the light propagating through the arm waveguide is modulated; Obtained in the process of obtaining and storing the voltage in a storage unit.

[0007] A testing method according to the present disclosure is a testing method for an optical modulator, the optical modulator having a Mach-Zehnder modulator, the Mach-Zehnder modulator having an electrode and an arm waveguide, the electrode being provided in the arm waveguide, the testing method comprising: acquiring, based on a light transmittance in the arm waveguide, a relationship between a voltage applied to the electrode and an amount of change in phase of light propagating through the arm waveguide; and acquiring, based on the relationship, a voltage at which an amount of change in the phase of the light becomes a predetermined magnitude when the light propagating through the arm waveguide is modulated.

[0008] A test program according to the present disclosure is a test program for an optical modulator, the optical modulator having a Mach-Zehnder modulator having an electrode and an arm waveguide, the electrode being provided in the arm waveguide, and the test program causes a computer to execute a process of acquiring a relationship between a voltage applied to the electrode and an amount of change in phase of light propagating through the arm waveguide based on a light transmittance in the arm waveguide, and a process of acquiring a voltage at which an amount of change in the phase of the light propagating through the arm waveguide becomes a predetermined magnitude when the light propagating through the arm waveguide is modulated, based on the relationship between the voltage applied to the electrode and the amount of change in phase of the light propagating through the arm waveguide.

[0009] An optical transmission device according to the present disclosure includes a plurality of Mach-Zehnder modulators and a memory unit, wherein the plurality of Mach-Zehnder modulators have an electrode and an arm waveguide, the electrode being provided in the arm waveguide, and the memory unit stores, for each of the plurality of Mach-Zehnder modulators, a voltage at which a change in phase of the light propagating through the arm waveguide becomes a predetermined magnitude when the light is modulated. Effect of the Invention

[0010] According to the present disclosure, it is possible to suppress an increase in light absorption loss. [Brief description of the drawings]

[0011] [Figure 1A] FIG. 1A is a block diagram illustrating an optical transmitting device according to the first embodiment. [Figure 1B] FIG. 1B is a block diagram showing the hardware configuration of the control unit. [Figure 2A] FIG. 2A is a plan view illustrating an optical modulator. [Figure 2B] FIG. 2B is a cross-sectional view taken along line AA in FIG. 2A. [Diagram 3] FIG. 3 is an example of a constellation diagram of output light. [Figure 4A] FIG. 4A is an example of a constellation diagram of output light. [Figure 4B] FIG. 4B is an example of a constellation diagram of the output light. [Figure 5A] FIG. 5A is an example of a constellation diagram of output light. [Figure 5B] FIG. 5B is an example of a constellation diagram of the output light. [Figure 6A] FIG. 6A is a diagram illustrating an example of the relationship between voltage and the amount of phase change. [Figure 6B] FIG. 6B is a diagram illustrating the relationship between the voltage and the amount of phase change. [Figure 7A] FIG. 7A is a diagram illustrating an example of the relationship between voltage and the amount of change in optical absorption loss. [Figure 7B]FIG. 7B is a diagram illustrating the relationship between voltage and the amount of change in optical absorption loss. [Figure 8] FIG. 8 is a flowchart illustrating a method for manufacturing an optical modulator. [Figure 9] FIG. 9 is a flow chart illustrating the test. [Figure 10A] FIG. 10A is a diagram illustrating the calculated amount of phase change. [Figure 10B] FIG. 10B is a diagram illustrating the calculated amount of change in absorption loss. [Figure 11A] FIG. 11A is a diagram illustrating the calculated transmittance. [Figure 11B] FIG. 11B is a diagram illustrating the measured transmittance and the transmittance after optimization. [Figure 12A] FIG. 12A is a diagram illustrating the amount of phase change after optimization. [Figure 12B] FIG. 12B is a diagram illustrating the amount of change in absorption loss after optimization. [Figure 13] FIG. 13 is a diagram illustrating the relationship between the bias voltage and the difference in the amount of phase change. [Figure 14] FIG. 14 is a diagram illustrating the measured transmittance and the transmittance after optimization. [Figure 15A] FIG. 15A is a diagram illustrating the amount of phase change after optimization. [Figure 15B] FIG. 15B is a diagram illustrating the amount of change in absorption loss after optimization. [Figure 16] FIG. 16 is a diagram illustrating the relationship between the bias voltage and the difference in the amount of phase change. [Figure 17] FIG. 17 is a plan view illustrating an optical modulator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0013] One aspect of the present disclosure relates to (1) a method for manufacturing an optical modulator, the optical modulator having a Mach-Zehnder modulator, the Mach-Zehnder modulator having an electrode and an arm waveguide, the electrode being provided in the arm waveguide, the manufacturing method including the steps of: preparing the Mach-Zehnder modulator; acquiring, based on a transmittance of light in the arm waveguide, a relationship between a voltage applied to the electrode and an amount of change in phase of light propagating through the arm waveguide; acquiring, based on the relationship, a voltage at which an amount of change in the phase of the light becomes a predetermined magnitude when the light propagating through the arm waveguide is modulated; Obtained in the process of obtaining and storing the voltage in a storage unit. By applying the acquired voltage to a Mach-Zehnder modulator to modulate light, it is possible to set the amount of phase change during modulation to a predetermined amount and to suppress an increase in light absorption loss.

[0014] [Details of the embodiment of the present disclosure] Specific examples of a manufacturing method, a test method, and a test program for an optical modulator, and an optical transmitter according to embodiments of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0015] First Embodiment (Optical transmitter) 1A is a block diagram illustrating an optical transmission device 100 according to a first embodiment. As shown in FIG. 1A, the optical transmission device 100 includes a control unit 10, a wavelength tunable laser element 22, an automatic bias control (ABC) circuit 24, a driver IC (Integrated Circuit) 26, and an optical modulator 40.

[0016] The tunable laser element 22 is a light-emitting element including, for example, a semiconductor laser element. The ABC circuit 24 applies a voltage for phase adjustment to the optical modulator 40 and performs automatic bias control. The driver IC 26 inputs a modulation signal to the optical modulator 40. The optical modulator 40 modulates the light incident from the tunable laser element 22 and emits the modulated light. The control unit 10 includes a computer such as, for example, a personal computer (PC).

[0017] FIG. 1B is a block diagram showing a hardware configuration of the control unit 10. As shown in FIG. 1B, the control unit 10 includes a CPU (Central Processing Unit) 30, a RAM (Random Access Memory) 32, a storage device 34 (storage unit), and an interface 36. The CPU 30, the RAM 32, the storage device 34, and the interface 36 are connected to each other via a bus or the like. The RAM 32 is a volatile memory that temporarily stores programs, data, and the like. The storage device 34 is, for example, a ROM (Read Only Memory), a solid state drive (SSD) such as a flash memory, a hard disk drive (HHD), or the like. The storage device 34 stores programs for executing the processes described below, and voltages obtained by the processes, and the like.

[0018] The CPU 30 executes the program stored in the RAM 32, whereby the control unit 10 realizes the phase control unit 12, the laser control unit 14, the calculation unit 15, the modulation control unit 16, and the storage control unit 18 shown in FIG. 1A. The phase control unit 12 controls the ABC circuit 24, and adjusts the voltage that the ABC circuit 24 applies to the optical modulator 40. The laser control unit 14 controls the wavelength tunable laser element 22. The calculation unit 15 calculates the transmittance, the amount of phase change, and the like, as described below. The modulation control unit 16 controls the driver IC 26. The storage control unit 18 controls the RAM 32 and the storage device 34 shown in FIG. 1B, and stores data therein. Each unit of the control unit 10 may be hardware such as a circuit.

[0019] (Optical Modulator) FIG. 2A is a plan view illustrating an optical modulator 40a. In the first embodiment, the optical modulator 40a is used as the optical modulator 40 in FIG. 1A. The optical modulator 40a is an IQ (In-phase Quadrature modulator) modulator, and includes a substrate 41, two child Mach-Zehnder modulators 42a and 42b, and a parent Mach-Zehnder modulator 44a. The substrate 41 is an insulating substrate formed of, for example, ceramic. A module including the optical modulator 40a may be formed by providing the ABC circuit 24, the driver IC 26, and a lens (not shown) in FIG. 1A on the substrate 41.

[0020] A semiconductor substrate 80 and two termination elements 78a and 78b are mounted on the upper surface of the substrate 41. The termination elements 78a and 78b include, for example, a termination resistor and a capacitor. The two child Mach-Zehnder modulators 42a and 42b, the parent Mach-Zehnder modulator 44a, the input waveguide 50, and the output waveguide 56 are formed on the semiconductor substrate 80. The semiconductor substrate 80 has four end faces 80a, 80b, 80c, and 80d. The end face 80a and the end face 80b face each other. The end face 80c and the end face 80d face each other.

[0021] A first end of the input waveguide 50 is located at an end surface 80a among four end surfaces of the semiconductor substrate 80. A second end of the input waveguide 50 is connected to a coupler 58. A first end of the output waveguide 56 is connected to a coupler 64. A second end of the output waveguide 56 is located at an end surface 80b among four end surfaces of the semiconductor substrate 80. The coupler 58 is a one-input two-output (1×2) multimode interference (MMI) coupler. The coupler 64 is a two-input one-output (2×1) MMI coupler. Two child Mach-Zehnder modulators 42a and 42b are arranged in parallel between the coupler 58 and the coupler 64. A parent Mach-Zehnder modulator 44a is arranged between the two child Mach-Zehnder modulators 42a and 42b and the coupler 64.

[0022] (Mach-Zehnder Modulator) The child Mach-Zehnder modulator 42a is, for example, an Ich-side modulator. The child Mach-Zehnder modulator 42b is, for example, a Qch-side modulator. The child Mach-Zehnder modulator 42a has arm waveguides 52a, 54a, and 54b, modulation electrodes 66a and 66b, phase adjustment electrodes 68a and 68b, and ground electrodes 66c and 68c. The arm waveguide 54a is, for example, a p-side waveguide. The arm waveguide 54b is, for example, an n-side waveguide.

[0023] A first end of the arm waveguide 52a is connected to a first output end of the coupler 58. A second end of the arm waveguide 52a is connected to an input end of the coupler 60a. A first end of the arm waveguide 54a (first arm waveguide) is connected to a first output end of the coupler 60a. A second end of the arm waveguide 54a is connected to a first input end of the coupler 62a. A first end of the arm waveguide 54b (second arm waveguide) is connected to a second output end of the coupler 60a. A second end of the arm waveguide 54b is connected to a second input end of the coupler 62a.

[0024] The arm waveguide 52a is bent on the side of the coupler 58. The arm waveguides 54a and 54b are bent on the side of the coupler 60a and on the side of the coupler 62a. Except for these bent portions, the arm waveguides 52a, 54a, and 54b are parallel to each other and to an end face 80c of the semiconductor substrate 80.

[0025] The modulation electrode 66a (first electrode) and the phase adjustment electrode 68a (third electrode) are 54 The modulation electrode 66a and the phase adjustment electrode 68a are spaced apart from each other and are arranged in order from the coupler 60a side toward the coupler 62a side. The modulation electrode 66b (second electrode) and the phase adjustment electrode 68b (fourth electrode) are arranged on the arm waveguide 54 The modulation electrode 66b and the phase adjustment electrode 68b are spaced apart from each other and are arranged in order from the coupler 60a side toward the coupler 62a side.

[0026] The modulation electrodes 66a and 66b face each other in a direction intersecting the extension direction of the arm waveguides 54a and 54b. The ground electrode 66c is located between the modulation electrodes 66a and 66b. The phase adjustment electrodes 68a and 68b face each other. The ground electrode 68c is located between the phase adjustment electrodes 68a and 68b. The modulation electrodes 66a and 66b, the phase adjustment electrodes 68a and 68b, and the ground electrodes 66c and 68c extend in the same direction as the arm waveguides 54a and 54b, and are parallel to an end face 80c of the semiconductor substrate 80.

[0027] The wirings 72a and 74a are electrically connected to the modulation electrode 66a. The wiring 72a extends from a first end of the modulation electrode 66a to an end face 80a of the semiconductor substrate 80. The wiring 74a extends from a second end of the modulation electrode 66a to an end face 80c of the semiconductor substrate 80. The wirings 72b and 74b are electrically connected to the modulation electrode 66b. The wiring 72b extends from a first end of the modulation electrode 66b to the end face 80a. The wiring 74b extends from a second end of the modulation electrode 66b to the end face 80c. The wirings 72c and 74c are electrically connected to the ground electrode 66c. The wiring 72c extends from a first end of the ground electrode 66c to the end face 80a. The wiring 74c extends from a second end of the ground electrode 66c to the end face 80c.

[0028] The modulation electrode 66a is electrically connected to the driver IC 26 shown in FIG. 1A via a wiring 72a. The modulation electrode 66b is electrically connected to the driver IC 26 via a wiring 72b. The ground electrode 66c is electrically connected to the driver IC 26 via a wiring 72c. The wirings 74a, 74b, and 74c are electrically connected to a termination element 78a by bonding wires.

[0029] The wiring 75a is electrically connected to the phase adjustment electrode 68a. The wiring 75b is electrically connected to the phase adjustment electrode 68b. The wiring 75c is electrically connected to the ground electrode 68c. The wirings 75a, 75b, and 75c extend to the end face 80c. The phase adjustment electrode 68a is electrically connected to the ABC circuit 24 via the wiring 75a. The phase adjustment electrode 68b is electrically connected to the ABC circuit 24 via the wiring 75b. The ground electrode 68c is electrically connected to the ABC circuit 24 via the wiring 75c.

[0030] The Mach-Zehnder modulator 42b includes arm waveguides 52b, 54c, and 54d, modulation electrodes 66d and 66e, phase adjustment electrodes 68d and 68e, and ground electrodes 66f and 68f. The arm waveguide 54c (first arm waveguide) is, for example, a p-side waveguide. The arm waveguide 54d (second arm waveguide) is, for example, an n-side waveguide.

[0031] A first end of the arm waveguide 52b is connected to a second output end of the coupler 58. A second end of the arm waveguide 52b is connected to an input end of the coupler 60b. The arm waveguides 54c and 54d are connected to the coupler 60b and the coupler 62b. The length of the arm waveguide of the child Mach-Zehnder modulator 42b is equal to the length of the corresponding arm waveguide of the child Mach-Zehnder modulator 42a. The shape of the arm waveguide of the child Mach-Zehnder modulator 42b is the same as the shape of the corresponding arm waveguide of the child Mach-Zehnder modulator 42a.

[0032] A modulation electrode 66d (first electrode) and a phase adjustment electrode 68d (third electrode) are provided on the arm waveguide 54c. A modulation electrode 66e (second electrode) and a phase adjustment electrode 68e (fourth electrode) are provided on the arm waveguide 54d. A ground electrode 66f is provided between the modulation electrodes 66d and 66e. A ground electrode 68f is provided between the phase adjustment electrodes 68d and 68e.

[0033] The wirings 72d and 74d are electrically connected to the modulation electrode 66d. The wirings 72e and 74e are electrically connected to the modulation electrode 66e. The wirings 72f and 74f are electrically connected to the ground electrode 66f. The wirings 72d, 72e, and 72f extend to an end surface 80a of the semiconductor substrate 80. The modulation electrode 66d is electrically connected to the driver IC 26 via the wiring 72d. The modulation electrode 66e is electrically connected to the driver IC 26 via the wiring 72e. The ground electrode 66f is electrically connected to the driver IC 26 via the wiring 72f. The wirings 74d, 74e, and 74f extend to an end surface 80d of the semiconductor substrate 80 and are electrically connected to the termination element 78b.

[0034] The wiring 75d is electrically connected to the phase adjustment electrode 68d. The wiring 75e is electrically connected to the phase adjustment electrode 68e. The wiring 75f is electrically connected to the ground electrode 68f. The wirings 75d, 75e, and 75f extend to the end face 80d. The phase adjustment electrode 68d is electrically connected to the ABC circuit 24 via the wiring 75d. The phase adjustment electrode 68e is electrically connected to the ABC circuit 24 via the wiring 75e. The ground electrode 68f is electrically connected to the ABC circuit 24 via the wiring 75f.

[0035] The lengths of the phase adjustment electrodes 68a, 68b, 68d, and 68e are equal to each other. The lengths of the modulation electrodes 66a, 66b, 66d, and 66e and the lengths of the ground electrodes 66c and 66f are equal to each other. The length of one modulation electrode is greater than the length of one phase adjustment electrode, for example, 2.5 times the length of one phase adjustment electrode. The lengths of the ground electrodes 68c and 68f are equal to each other and are smaller than the length of the phase adjustment electrodes.

[0036] (Mach-Zehnder Modulator) The parent Mach-Zehnder modulator 44a has arm waveguides 55a and 55b, phase adjustment electrodes 70a and 70b, and a ground electrode 70c. A first end of the arm waveguide 55a is connected to the output end of the coupler 62a. A first end of the arm waveguide 55b is connected to the output end of the coupler 62b. A second end of each of the arm waveguides 55a and 55b is connected to the input end of the coupler 64. The arm waveguides 55a and 55b are parallel to an end face 80c of the semiconductor substrate 80 on the side closer to the child Mach-Zehnder modulator, and are bent on the side closer to the coupler 64.

[0037] The phase adjustment electrode 70a is provided on the arm waveguide 55a. The phase adjustment electrode 70b is provided on the arm waveguide 55b. The ground electrode 70c is provided between the arm waveguide 55a and the arm waveguide 55b. The phase adjustment electrodes 70a and 70b and the ground electrode 70c extend in the same direction as the arm waveguides and are parallel to the end face 80c.

[0038] The wiring 76a is electrically connected to an end of the phase adjustment electrode 70a and extends to an end surface 80c. The wiring 76b is electrically connected to an end of the phase adjustment electrode 70b and extends to an end surface 80d. The wiring 76c is electrically connected to an end of the ground electrode 70c and extends to an end surface 80d. c The phase adjustment electrode 70a is electrically connected to the ABC circuit 24 via a wiring 76a. The phase adjustment electrode 70b is electrically connected to the ABC circuit 24 via a wiring 76b. The ground electrode 70c is electrically connected to the ABC circuit 24 via a wiring 76c.

[0039] 2B is a cross-sectional view taken along line AA in FIG. 2A, illustrating a cross section of the child Mach-Zehnder modulator 42a. The child Mach-Zehnder modulator 42b and the parent Mach-Zehnder modulator 44a also have a similar configuration to the child Mach-Zehnder modulator 42a.

[0040] As shown in Fig. 2B, a cladding layer 82 (first semiconductor layer) is provided on the upper surface of a semiconductor substrate 80. The cladding layer 82 protrudes at two positions toward the opposite side to the semiconductor substrate 80 (upward in the figure). A core layer 84, a cladding layer 86, and a contact layer 88 are laminated in this order on the protruding portions. The cladding layer 82, the core layer 84, the cladding layer 86, and the contact layer 88 form mesa-shaped arm waveguides 54a and 54b. The cladding layer 86 and the contact layer 88 correspond to a second semiconductor layer.

[0041] The semiconductor substrate 80 is made of, for example, semi-insulating indium phosphide (InP). The cladding layer 82 is made of, for example, n-type InP (n-InP) having a thickness of 800 nm. The cladding layer 86 is made of, for example, p-InP having a thickness of 1300 nm. The contact layer 88 is made of, for example, p-InGaAs having a thickness of 200 nm. The n-type cladding layer 82 is doped with, for example, silicon (Si). The p-type cladding layer 86 and the contact layer 88 are doped with, for example, zinc (Zn).

[0042] The core layer 84 has, for example, a multiple quantum well structure (MQW: Multiple Quantum Well). The core layer 84 includes a plurality of well layers and barrier layers that are alternately stacked. The well layers are made of, for example, aluminum gallium indium arsenide (AlGaInAs). The barrier layers are made of, for example, aluminum indium arsenide (AlInAs). The thickness of the core layer 84 is, for example, 500 nm.

[0043] The upper surface of the semiconductor substrate 80, the surface of the cladding layer 82, and the side and upper surfaces of the arm waveguides 54a and 54b are covered with an insulating film 81. The insulating film 81 is made of an insulator such as silicon oxide (SiO2). The resin layer 85 is made of benzocyclobutene (BCB) or the like, and covers the surface of the insulating film 81. The insulating film 81 and the resin layer 85 have openings in the portion between the arm waveguides on the upper surface of the cladding layer 82, and have openings above the arm waveguides 54a and 54b.

[0044] The modulation electrode 66a is provided on the arm waveguide 54a. The modulation electrode 66b is provided on the arm waveguide 54b. The modulation electrodes 66a and 66b are electrically connected to the contact layer 88 exposed from the opening of the insulating film 81 and the resin layer 85. The ground electrode 66c is provided on the cladding layer 82 and is electrically connected to the cladding layer 82 exposed from the insulating film 81 and the resin layer 85. The phase adjustment electrodes 68a and 68b shown in FIG. 2A are also provided on the upper surface of the contact layer 88. The ground electrode 68c is also provided on the upper surface of the cladding layer 82.

[0045] The modulation electrode and the phase adjustment electrode each have an ohmic electrode layer and a wiring layer. The ohmic electrode layer includes, for example, a platinum (Pt) layer, a titanium (Ti) layer, a platinum (Pt) layer, and a gold (Au) layer. These layers are stacked in order from the contact layer 88 side. The wiring layer is made of, for example, Au, and contacts the upper surface of the ohmic electrode layer. The ground electrode has, for example, an alloy layer and an Au layer. The alloy layer is made of, for example, an alloy of Au, germanium (Ge), and nickel (Ni). The Au layer contacts the upper surface of the alloy layer. The wiring shown in FIG. 2A is provided on the resin layer 85 of FIG. 2B and is made of a metal such as Au.

[0046] (Operation of optical transmitter) Next, the operation of the optical transmitter 100 will be described. The laser control unit 14 of the control unit 10 shown in Fig. 1A causes the wavelength tunable laser element 22 to emit light. The light incident on the input waveguide 50 of the optical modulator 40a shown in Fig. 2A branches at the coupler 58 and propagates through the arm waveguides 52a and 52b. The light propagating through the arm waveguide 52a branches at the coupler 60a and propagates through the arm waveguides 54a and 54b. The light propagating through the arm waveguide 52b branches at the coupler 60b and propagates through the arm waveguides 54c and 54d.

[0047] The modulation control unit 16 of the control unit 10 in Fig. 1A generates a modulation signal based on transmission data and inputs it to the driver IC 26. The modulation signal is input from the driver IC 26 to the modulation electrodes 66a and 66b of the child Mach-Zehnder modulator 42a. The modulation signal is input from the driver IC 26 to the modulation electrodes 66d and 66e of the child Mach-Zehnder modulator 42b. The input of the modulation signal changes the refractive index of the arm waveguide, modulating the light.

[0048] The modulated light propagating through the arm waveguide 54a and the modulated light propagating through the arm waveguide 54b are multiplexed by the coupler 62a. The modulated light after the multiplexing propagates through the arm waveguide 55a of the parent Mach-Zehnder modulator 44a. The modulated light propagating through the arm waveguide 54c and the modulated light propagating through the arm waveguide 54d are multiplexed by the coupler 62b. The modulated light after the multiplexing propagates through the arm waveguide 55b of the parent Mach-Zehnder modulator 44a. The light propagating through the arm waveguide 55a and the light propagating through the arm waveguide 55b are multiplexed by the coupler 64 and propagate through the output waveguide 56. The modulated light is emitted from the output waveguide 56 to the outside of the optical modulator 40a.

[0049] The phase control section 12 of the control section 10 performs automatic bias control using the ABC circuit 24 to adjust the phase of the light. The ABC circuit 24 applies a voltage to the phase adjustment electrode, which changes the refractive index of the arm waveguide and changes the optical path length. The change in the optical path length changes the phase of the light propagating through the arm waveguide. The phase control section 12 can independently control the phase of the light in the parent Mach-Zehnder modulator 44a and the phase of the light in each of the child Mach-Zehnder modulators 42a and 42b.

[0050] When no modulation signal is input to the child Mach-Zehnder modulator 42a, the phase difference between the light propagating through the arm waveguide 54a and the light propagating through the arm waveguide 54b is π (rad) or π±2π×n (n is a negative or positive integer). That is, the child Mach-Zehnder modulator 42a is adjusted to its extinction point. The child Mach-Zehnder modulator 42b is also adjusted to its extinction point. The state adjusted to the extinction point is the operating point of the child Mach-Zehnder modulator.

[0051] The phase difference between the modulated light propagating through the arm waveguide 55a of the parent Mach-Zehnder modulator 44a and the modulated light propagating through the arm waveguide 55b is 0.5π (rad) or a value equivalent to 0.5π. The value equivalent to 0.5π is 0.5π±2π×n or 1.5π±2π×n (n is a negative or positive integer). The modulated light propagating through the arm waveguide 55a and the modulated light propagating through the arm waveguide 55b are orthogonal to each other.

[0052] The optical modulator 40a may use, for example, a QPSK modulation method. In the QPSK modulation method, modulated light corresponding to each of the four symbol codes 00, 01, 10, and 11 of the modulated signal is generated. To generate the modulated light, the driver IC 26 applies a voltage to the modulation electrode.

[0053] The voltage applied to the modulation electrode 66a is VIp, and the voltage applied to the modulation electrode 66b is VIn. The voltage applied to the modulation electrode 66d is VQp, and the voltage applied to the modulation electrode 66e is VQn. Each of the voltages VIp, VIn, VQp, and VQn is calculated from the bias voltage Vb and the amplitude voltage Vpp.

[0054] Number 1 indicates voltages VIp, VIn, VQp and VQn corresponding to the symbol code 00.

number

number

number

number

[0055] 3 to 5B are examples of constellation diagrams of output light. In each diagram, Ip is output light from the p-side arm waveguide 54a of the Ich child Mach-Zehnder modulator 42a. In is output light from the n-side arm waveguide 54b of the Ich child Mach-Zehnder modulator 42a. Qp is output light from the p-side arm waveguide 54c of the Qch child Mach-Zehnder modulator 42b. Qn is output light from the n-side arm waveguide 54d of the Qch child Mach-Zehnder modulator 42b.

[0056] FIG. 3 is a constellation diagram in an unmodulated state. The amplitude voltage Vpp is 0V. A bias voltage Vb is applied to each of the multiple modulation electrodes. The child Mach-Zehnder modulator is adjusted to the extinction point. The phase of the output light Ip from the p-side arm waveguide 54a of the child Mach-Zehnder modulator 42a and the phase of the output light In from the n-side arm waveguide 54b are inverted to each other. The output lights of the two arm waveguides cancel each other. The phase of the output light Qp from the arm waveguide 54c of the child Mach-Zehnder modulator 42b and the phase of the output light Qn from the arm waveguide 54d are inverted to each other. The output lights of the two arm waveguides cancel each other.

[0057] FIG. 4A is a constellation diagram for symbol code 00. A voltage shown in Equation 1 is applied to each modulation electrode. The voltage Vpp / 2 in Equation 1 rotates the output light Ip of the arm waveguide 54a counterclockwise by about π / 2 from FIG. 3. The voltage -Vpp / 2 rotates the output light In of the arm waveguide 54b clockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42a is obtained by multiplexing the output light Ip and the output light In, and is represented by a vector on the I axis. The output light Qp of the arm waveguide 54c rotates counterclockwise by about π / 2 from FIG. 3. The output light Qn of the arm waveguide 54d rotates clockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42b is obtained by multiplexing the output light Qp and the output light Qn, and is represented by a vector on the Q axis. By multiplexing the output light from the child Mach-Zehnder modulator 42a and the output light from the child Mach-Zehnder modulator 42b, output light from the optical modulator 40a corresponding to the symbol code 00 is obtained. The output light is represented by a vector located in the first quadrant.

[0058] FIG. 4B is a constellation diagram for symbol code 01. A voltage shown in equation 2 is applied to each modulation electrode. The output light Ip of the arm waveguide 54a rotates clockwise by about π / 2 from FIG. 3. The output light In of the arm waveguide 54b rotates counterclockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42a is represented by a vector on the I axis. The output light Qp of the arm waveguide 54c rotates counterclockwise by about π / 2 from FIG. 3. The output light Qn of the arm waveguide 54d rotates clockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42b is represented by a vector on the Q axis. The output light of the optical modulator 40a corresponding to symbol code 01 is represented by a vector located in the second quadrant.

[0059] FIG. 5A is a constellation diagram for the symbol code 10. A voltage shown in the formula 3 is applied to each modulation electrode. The output light Ip of the arm waveguide 54a rotates counterclockwise by about π / 2 from FIG. 3. The output light In of the arm waveguide 54b rotates clockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42a is represented by a vector on the I axis. The output light Qp of the arm waveguide 54c rotates clockwise by about π / 2 from FIG. 3. The output light Qn of the arm waveguide 54d rotates counterclockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42b is represented by a vector on the Q axis. The output light of the optical modulator 40a is represented by a vector located in the fourth quadrant.

[0060] FIG. 5B is a constellation diagram for symbol code 11. 4 A voltage indicated by is applied to each modulation electrode. The output light Ip of the arm waveguide 54a rotates clockwise by about π / 2 from FIG. 3. The output light In of the arm waveguide 54b rotates counterclockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42a is represented by a vector on the I axis. The output light Qp of the arm waveguide 54c rotates clockwise by about π / 2 from FIG. 3. The output light Qn of the arm waveguide 54d rotates counterclockwise by about π / 2 from FIG. 3. The output light of the child Mach-Zehnder modulator 42b is represented by a vector on the Q axis. The output light of the optical modulator 40a is represented by a vector located in the third quadrant.

[0061] As shown in Figures 4A to 5B, the intensity of the modulated light is maximized by rotating the phase of the light in each arm waveguide by approximately π / 2 clockwise or counterclockwise from the unmodulated state. The amplitude voltage Vpp should be equal to the voltage Vπ that rotates the signal phase by π. The voltage Vπ / 2, which is 1 / 2 the voltage Vπ, is the voltage that rotates the phase by π / 2. Vpp= Vπ Then, Vpp / 2=Vπ / 2. ±Vpp / 2 in equations 1 to 4 is equivalent to a voltage of ±Vπ / 2, and rotates the phase of the light by ±π / 2.

[0062] In order to suppress an increase in the power consumption of the driver IC 26, it is effective to reduce the amplitude voltage Vpp. The upper limit of the amplitude voltage Vpp is set to, for example, 1.7 V. On the other hand, as described in Figures 6A and 6B, by determining the bias voltage Vb, the amplitude voltage Vpp corresponding to the phase rotation of π / 2 is also determined.

[0063] 6A is a diagram illustrating the relationship between voltage and phase change amount. The horizontal axis represents the voltage applied to the modulation electrode of the child Mach-Zehnder modulator 42a. The vertical axis represents the amount of change Δφ in the phase of light in the arm waveguide of the child Mach-Zehnder modulator 42a. As the voltage increases, the amount of phase change Δφ also increases.

[0064] The dashed line in FIG. 6A indicates 11.5V. When the bias voltage Vb is 11.5V, at the position indicated by the dotted line, the phase change amount is ±π / 2 with respect to the position of 11.5V. That is, the difference in the phase change amount between the two arm waveguides is π. Specifically, the voltage applied to the modulation electrode 66a is Vb+Vpp / 2=12.35V. The output light of the arm waveguide 54a rotates by π / 2. The voltage applied to the modulation electrode 66b is Vb-Vpp / 2=10.65V. The output light of the arm waveguide 54b rotates by -π / 2. The modulation shown in FIG. 4A is possible. At this time, Vπ / 2=Vpp / 2=0.85V. The amplitude voltage Vpp is 1.7V, which is equal to the upper limit value.

[0065] As shown in FIG. 6A, the phase change amount Δφ has a nonlinear relationship with the voltage. The higher the voltage, the greater the rate of change (slope) of the phase change amount Δφ. The slope of the phase change amount when Vb=11.5V is greater than the slope when Vb<11.5V. This makes Vπ / 2 smaller. When the bias voltage Vb is less than 11.5V, the slope of the phase change amount becomes smaller, so the amplitude voltage Vpp exceeds 1.7V. To make the amplitude voltage Vpp 1.7V or less, the bias voltage Vb is set to 11.5V or more.

[0066] The rate of phase change with respect to voltage (phase adjustment efficiency) may vary for each Mach-Zehnder modulator. The difference in phase adjustment efficiency is considered to be due to the difference in the amount of thermal diffusion of the dopant in the cladding layers 82 and 86, the contact layer 88, etc. The difference in the amount of thermal diffusion of the dopant causes a difference in the strength of the electric field generated in the core layer 84 when a voltage is applied. If there is a difference in the electric field strength, the refractive index also differs, and the amount of phase change also differs. The difference in the amount of thermal diffusion of the dopant causes a difference in the band gap energy, and therefore the amount of phase change also changes. Here, it is assumed that the phase adjustment efficiency of the child Mach-Zehnder modulator 42b is higher than that of the child Mach-Zehnder modulator 42a.

[0067] FIG. 6B is a diagram illustrating the relationship between voltage and phase change amount. The horizontal axis represents the voltage applied to the modulation electrode of the child Mach-Zehnder modulator 42b. The vertical axis represents the change amount Δφ of the phase of light in the arm waveguide of the child Mach-Zehnder modulator 42b. When compared at the same voltage, the phase change amount Δφ in FIG. 6B is larger than that in FIG. 6A. As with the child Mach-Zehnder modulator 42a, if Vb=11.5V, the voltage Vπ / 2 corresponding to a phase rotation of π / 2 is 0.7V. The amplitude voltage Vpp of the child Mach-Zehnder modulator 42b is 1.4V, which is smaller than that of the child Mach-Zehnder modulator 42a. As shown by the dotted line in FIG. 6B, Vb+Vpp / 2 =12.2V. Vb-Vpp / 2 =10.8V.

[0068] 6A and 6B, by setting the bias voltage Vb to, for example, 11.5 V, the amplitude voltage Vpp can be set to 1.7 V or less in both the child Mach-Zehnder modulator 42b with high phase adjustment efficiency and the child Mach-Zehnder modulator 42a with low phase adjustment efficiency. By reducing the amplitude voltage Vpp, the power consumption of the driver IC 26 can be reduced.

[0069] There is a positive correlation between the phase adjustment efficiency and the optical absorption loss in the arm waveguide. This is because the Kramers-Kronig relationship holds between the change in the refractive index of the arm waveguide and the amount of optical absorption. The smaller the phase adjustment efficiency, the smaller the absorption loss. The larger the phase adjustment efficiency, the larger the absorption loss.

[0070] 7A and 7B are diagrams illustrating the relationship between voltage and the amount of change in optical absorption loss. The horizontal axis represents the voltage applied to the modulation electrode of the Mach-Zehnder modulator. The vertical axis represents the amount of change in optical absorption loss in the arm waveguide.

[0071] 7A shows an example of the child Mach-Zehnder modulator 42a. FIG. 7B shows an example of the child Mach-Zehnder modulator 42b. The change in absorption loss in the child Mach-Zehnder modulator 42b is larger than the change in absorption loss in the child Mach-Zehnder modulator 42a. For example, at 11.5V, the change in absorption loss in the child Mach-Zehnder modulator 42a is 0.18 dB. The change in absorption loss in the child Mach-Zehnder modulator 42b is 1.1 dB.

[0072] In order to reduce the power consumption of the driver IC 26, the amplitude voltage Vpp is reduced in both the child Mach-Zehnder modulator 42b with high phase adjustment efficiency and the child Mach-Zehnder modulator 42a with low phase adjustment efficiency. As shown in FIG. 6A and FIG. 6B, the bias voltage Vb may be set to, for example, 11.5V for both the child Mach-Zehnder modulators 42a and 42b. However, as shown in FIG. 7B, the change in absorption loss of the child Mach-Zehnder modulator 42b is larger than that of the child Mach-Zehnder modulator 42a. In order to suppress the increase in the absorption loss of light, it is important to optimize the voltage applied to the modulation electrode for each Mach-Zehnder modulator.

[0073] (Manufacturing method) 8 is a flow chart illustrating a method for manufacturing the optical modulator 40a, including a step of optimizing the voltage. As shown in FIG. 8, first, a Mach-Zehnder modulator is formed (steps S1 to S3). A cladding layer 82 is formed on the upper surface of a wafer (semiconductor substrate 80) by metal organic chemical vapor deposition (MOCVD) or the like. , a core layer 84, a cladding layer 86, and a contact layer 88. Epitaxial growth do. By adding dopants to the source gas, an n-type cladding layer 82, a p-type cladding layer 86, and a contact layer 88 are formed. (Step S1) If there is variation in the amount of thermal diffusion of the dopant, then the phase adjustment efficiency will also vary, as shown in Figures 4A and 4B and Figures 7A and 7B.

[0074] A mesa-shaped arm waveguide as shown in FIG. 2B is formed by dry etching or the like (step S2). An insulating film 81 and a resin layer 85 are formed. Openings are formed in the insulating film 81 and the resin layer 85 by dry etching or the like. Electrodes (modulation electrodes, phase adjustment electrodes, and ground electrodes) are formed by vacuum deposition or the like (step S3). Child Mach-Zehnder modulators 42a and 42b and parent Mach-Zehnder modulator 44a are formed on the semiconductor substrate 80. The wafer is diced to form a plurality of optical modulators 40a.

[0075] Each of the optical modulators 40a is disposed on the substrate 41 and electrically connected to the ABC circuit 24 and the driver IC 26. A test is performed for each optical modulator 40a. Specifically, the child Mach-Zehnder modulator 42a is tested to optimize the voltages applied to the modulation electrodes 66a and 66b (step S4). The child Mach-Zehnder modulator 42b is tested to optimize the voltages applied to the modulation electrodes 66d and 66e (step S5). Through the above steps, the optical modulator 40a is formed.

[0076] (test) Fig. 9 is a flow chart illustrating the test. Each of steps S4 and S5 in Fig. 8 is a process for performing the test shown in Fig. 9.

[0077] When testing the child Mach-Zehnder modulator 42a (step S4 in FIG. 8), the phase control section 12 of the control section 10 adjusts the phase of the child Mach-Zehnder modulator 42b. electrode A voltage is applied to the child Mach-Zehnder modulator 42b to adjust the child Mach-Zehnder modulator 42b to the extinction point. The laser control unit 14 of the control unit 10 drives the wavelength-tunable laser element 22 to cause light to be incident from the wavelength-tunable laser element 22 to the optical modulator 40a. A light-receiving element (not shown) or the like receives the light emitted from the child Mach-Zehnder modulator 42a. The control unit 10 measures the light transmittance in the arm waveguide by comparing the intensity of the incident light with the intensity of the emitted light.

[0078] The control unit 10 measures the optical transmittance (first transmittance) in the arm waveguide 54a of the child Mach-Zehnder modulator 42a while sweeping the voltage applied from the ABC circuit 24 to the phase adjustment electrode 68a of the child Mach-Zehnder modulator 42a. The control unit 10 measures the optical transmittance (first transmittance) in the arm waveguide 54b of the child Mach-Zehnder modulator 42a while sweeping the voltage applied from the ABC circuit 24 to the phase adjustment electrode 68b (step S10 in FIG. 9). The calculation unit 15 of the control unit 10 calculates the optical transmittance (second transmittance) in the arm waveguide 54a and the optical transmittance (second transmittance) in the arm waveguide 54b (step S12).

[0079] The calculation unit 15 optimizes the transmittance so that the transmittance calculated in step S12 approaches the transmittance measured in step S10 (step S14). The optimization of the transmittance results in a parameter (coefficient). The parameter is corrected based on the ratio between the length of the phase adjustment electrode and the length of the modulation electrode (step S15). The calculation unit 15 obtains a relationship between the voltage applied to the modulation electrode and the amount of phase change in the arm waveguide (step S16). The memory control unit 18 obtains a voltage that provides a predetermined amount of phase change based on the relationship between the voltage and the amount of phase change, and stores the voltage in, for example, the memory device 34 (step S18).

[0080] The test will be described in detail. The calculation unit 15 calculates the transmittance T as a function of the change in absorption loss ΔL1, the initial phase difference φ0, and the phase change amount Δφ. The calculation unit 15 calculates the phase change amount Δφ in one arm waveguide as a function of the voltage V applied to the phase adjustment electrode as shown in the following equation.

number

[0081] An example of the initial values ​​of the coefficients is shown below. k1=1×10 -1 (π / V), k2=1×10 -2 (π / V 2 ), k3=1×10 -5 (π / V 3 ), k4=1×10 -6 (π / V 4 ), k5=1×10 -7 (π / V 5 ), k6=1×10 -8 (π / V 6 )

[0082] 10A is a diagram illustrating the calculated phase change amount. The horizontal axis represents the voltage applied to the phase adjustment electrodes 68a and 68b of the child Mach-Zehnder modulator 42a. The vertical axis represents the phase change amount Δφ. The solid lines represent the phase change amount of the p-side arm waveguide (arm waveguide 54a) and the phase change amount of the n-side arm waveguide (arm waveguide 54b). The calculation unit 15 performs calculations using the same function (Equation 5) and the same coefficients (initial values) for the arm waveguides 54a and 54b, so the phase change amount Δφ is also equal between the arm waveguides.

[0083] The calculation section 15 calculates the amount of change ΔL1 in the absorption loss of light in the arm waveguide as a function of the voltage V applied to the phase adjustment electrode, as shown in the following equation.

number

[0084] The initial values ​​of the coefficients a1 and a2 are shown below. a1=1×10 -4 (dB), a2=1.5(V)

[0085] 10B is a diagram illustrating the calculated change in absorption loss. The horizontal axis represents the voltage applied to the phase adjustment electrodes 68a and 68b of the child Mach-Zehnder modulator 42a. The vertical axis represents the change in absorption loss ΔL1. The solid lines represent the change in absorption loss ΔL1 of the arm waveguide 54a and the change in absorption loss ΔL1 of the arm waveguide 54b. Since the calculation is performed using the same function (Equation 6) and the same coefficients for the arm waveguides 54a and 54b, the change in absorption loss ΔL1 is also the same.

[0086] The calculation unit 15 calculates the transmittance T (step S12). As shown in the following equation, the transmittance T in each arm waveguide is expressed as a function of the amount of change in absorption loss ΔL1, the initial phase difference φ0, and the amount of phase change Δφ.

number

[0087] The phase change amount Δφ is expressed by equation 5. The change amount ΔL1 of the absorption loss is expressed by equation 6. The sign in the cosine function (cos) in equation 7 is positive for the arm waveguide on the p side and negative for the arm waveguide on the n side. The initial phase difference φ0 is expressed by the following equation. acos in equation 8 is reverse This is the cosine function.

number

[0088] T0 is the transmittance when the applied voltage is 0 V, and is measured in step S10. When the voltage applied to the phase adjustment electrode 68a is swept, if the first pole that appears in the transmittance is a minimum, the sign of the initial phase difference φ0 is set to plus, and if it is a maximum, the sign is set to minus. In the example of the child Mach-Zehnder modulator 42a, φ0=0.4π.

[0089] Fig. 11A is a diagram illustrating the calculated transmittance. Fig. 11B is a diagram illustrating the measured transmittance and the transmittance after optimization. The horizontal axis of Fig. 11A and Fig. 11B represents the voltage applied to the phase adjustment electrode of the child Mach-Zehnder modulator 42a. The vertical axis represents the light transmittance.

[0090] The solid line in FIG. 11A represents the transmittance of the p-side arm waveguide (arm waveguide 54a). The dotted line represents the transmittance of the n-side arm waveguide (arm waveguide 54b). The transmittance shown in FIG. 11A is calculated by the calculation unit 15 using Equation 7 and initial values ​​in step S12 of FIG. 9. The solid line in FIG. 11B represents the transmittance after optimization of the arm waveguide 54a. The dotted line represents the transmittance after optimization of the arm waveguide 54b. The circles represent the measurement results of the transmittance of the arm waveguide 54a. The triangles represent the measurement results of the transmittance of the arm waveguide 54b.

[0091] The optimization in step S14 in Fig. 9 refers to bringing the transmittance calculated in step S12 closer to the transmittance measured in step S10 and reducing the error between the two. The transmittance shown by the solid line in Fig. 11B changes from the transmittance shown by the solid line in Fig. 11A and approaches the measured transmittance shown by the circle in Fig. 11B. The transmittance shown by the dashed line in Fig. 11B changes from the transmittance shown by the dashed line in Fig. 11A and approaches the measured transmittance shown by the triangle in Fig. 11B.

[0092] By optimizing the transmittance, the initial phase difference φ0, the phase change amount Δφ, and the change amount of absorption loss ΔL1 included in the transmittance equation (Equation 7) change. The phase change amount Δφ and the change amount of absorption loss ΔL1 become functions that more accurately indicate the relationship with voltage.

[0093] More specifically, the coefficients k1 to k6 in the equation for the phase change amount Δφ (Equation 5) and the coefficients a1 and a2 in the equation for the change amount ΔL1 (Equation 6) are changed from their initial values. The coefficients after optimization are shown below. Coefficient for arm waveguide 54a k1=8.36×10 -2 (π / V), k2=2.14×10 -3 (π / V 2), k3=1.33×10 -6 (π / V 3 ), k4=1.37×10 -5 (π / V 4 ), k5=3.15×10 -7 (π / V 5 ), k6=9.54×10 -9 (π / V 6 ), a1=4.18×10 -4 (dB), a2=2.24(V) Coefficient for arm waveguide 54b k1 = 1.09 × 10 -1 (π / V), k2=1.28×10 -3 (π / V 2 ), k3=9.04×10 -7 (π / V 3 ), k4=6.66×10 -6 (π / V 4 ), k5=9.04×10 -8 (π / V 5 ), k6=1.98×10 -8 (π / V 6 ), a1=8.96×10 -4 (dB), a2=2.60(V) The initial phase difference φ0 after optimizing the transmittance is 0.38π.

[0094] These coefficients represent the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change. In order to obtain the relationship between the voltage applied to the modulation electrode and the amount of phase change, the above coefficients are corrected.

[0095] As shown in FIG. 2A, the modulation electrode and the phase adjustment electrode are provided on the arm waveguide. It is assumed that the amount of phase change per unit length of the phase adjustment electrode when a voltage is applied to the phase adjustment electrode is equal to the amount of phase change per unit length of the modulation electrode when a voltage is applied to the modulation electrode. The length of the modulation electrode is longer than the length of the phase adjustment electrode, for example, 2.5 times the length of the phase adjustment electrode. It is assumed that the amount of phase change when a voltage is applied to the modulation electrode changes at a rate approximately equal to the ratio of the lengths compared to the amount of phase change when a voltage is applied to the phase adjustment electrode.

[0096] The calculation unit 15 calculates the following coefficients by multiplying each of k1 to k6 and a1 among the above coefficients by 2.5 based on the ratio of the electrode lengths (step S15 in FIG. 9). Coefficient for arm waveguide 54a k1 = 2.09 × 10 -1 (π / V), k2=5.34×10 -3 (π / V 2 ), k3=3.32×10 -6 (π / V 3 ), k4=3.42×10 -5 (π / V 4 ), k5=7.88×10 -7 (π / V 5 ), k6=2.38×10 -8 (π / V 6 ), a1=1.05×10 -3 (dB) Coefficient for arm waveguide 54b k1=2.73×10 -1 (π / V), k2=3.21×10 -3 (π / V 2 ), k3=2.26×10 -6 (π / V 3 ), k4=1.66×10 -5 (π / V 4 ), k5=2.26×10 -7 (π / V 5 ), k6=4.96×10 -8 (π / V 6 ), a1=2.24×10 -3 (dB) a2 is the same as before the correction of k1 etc. The calculation unit 15 applies these coefficients to Equations 5 and 6 to calculate the amount of change in phase and the amount of change in absorption loss when a voltage is applied to the modulation electrode.

[0097] FIG. 12A is a diagram illustrating the amount of phase change after optimization. The horizontal axis represents the voltage applied to the modulation electrodes 66a and 66b of the Mach-Zehnder modulator 42a. The vertical axis represents the amount of phase change Δφ. The solid line represents the amount of phase change in the p-side arm waveguide (arm waveguide 54a). The dotted line represents the amount of phase change in the n-side arm waveguide (arm waveguide 54b). As shown in FIG. 12A, by substituting the coefficients obtained by optimizing and correcting the transmittance into Equation 5 for calculation, an amount of phase change close to that in FIG. 6A can be obtained.

[0098] Fig. 12B is a diagram illustrating the amount of change in absorption loss after optimization. The horizontal axis represents the voltage applied to the modulation electrodes 66a and 66b of the child Mach-Zehnder modulator 42a. The vertical axis represents the amount of change in absorption loss. The solid line represents the amount of change in absorption loss in the p-side arm waveguide (arm waveguide 54a). The dotted line represents the amount of change in absorption loss in the n-side arm waveguide (arm waveguide 54b). As shown in Fig. 12B, by substituting the coefficients obtained by optimizing and correcting the transmittance into Equation 6 for calculation, an amount of change close to that in Fig. 7A can be obtained.

[0099] FIG. 13 is a diagram illustrating the relationship between the bias voltage and the difference in the phase change amount. The horizontal axis represents the bias voltage Vb. The vertical axis represents the difference between the phase change amount in the arm waveguide 54a and the phase change amount in the arm waveguide 54b when the voltage Vb+Vpp / 2 is applied to the modulation electrode 66a and the voltage Vb-Vpp / 2 is applied to the modulation electrode 66b. In order to suppress an increase in the power consumption of the driver IC 26, Vpp=1.7V is set. A predetermined amplitude voltage Vpp=1.7V is applied for each bias voltage Vb on the horizontal axis of FIG. 13, and the difference in the phase change amount is obtained. It is sufficient that the difference in the phase change amount between the arm waveguide 54a and the arm waveguide 54b is π. As shown in FIG. 13, when Vb=11.5V, the difference in the phase change amount is π. The storage device 34 shown in FIG. 1B stores the bias voltage Vb of the child Mach-Zehnder modulator 42a as 11.5V.

[0100] Next, the child Mach-Zehnder modulator 42b is tested (step S5 in FIG. 8). The phase control unit 12 of the control unit 10 applies a voltage to the phase adjustment electrode of the child Mach-Zehnder modulator 42a to adjust the child Mach-Zehnder modulator 42a to the extinction point. The control unit 10 measures the optical transmittance in the arm waveguides 54c and 54d of the child Mach-Zehnder modulator 42b while sweeping the voltage applied from the ABC circuit 24 to the phase adjustment electrode of the child Mach-Zehnder modulator 42b (step S10 in FIG. 9). The calculation unit 15 of the control unit 10 calculates the optical transmittance in the arm waveguide 54c and the optical transmittance in the arm waveguide 54d (step S12).

[0101] The calculation unit 15 performs optimization so that the transmittance calculated in step S12 approaches the transmittance measured in step S10 (step S14). The calculation unit 15 corrects the parameters based on the ratio between the length of the phase adjustment electrode and the length of the modulation electrode (step S15). The calculation unit 15 obtains the relationship between the voltage applied to the modulation electrode and the amount of phase change (step S16). The memory control unit 18 obtains the voltage at which the amount of phase change becomes a predetermined magnitude based on the relationship between the voltage and the amount of phase change, and stores it in the memory device 34 (step S18).

[0102] FIG. 14 is a diagram illustrating the measured transmittance and the transmittance after optimization. The horizontal axis represents the voltage applied to the phase adjustment electrodes 68d and 68e of the child Mach-Zehnder modulator 42b. The vertical axis represents the optical transmittance. The solid line represents the transmittance after optimization of the p-side arm waveguide (arm waveguide 54c). The dotted line represents the transmittance after optimization of the n-side arm waveguide (arm waveguide 54d). The circles represent the measurement results of the transmittance of the arm waveguide 54c. The triangles represent the measurement results of the transmittance of the arm waveguide 54d. By optimizing the transmittance, the amount of change in phase and the amount of change in absorption loss can be obtained.

[0103] FIG. 15A is a diagram illustrating the amount of phase change after optimization. The horizontal axis represents the voltage applied to the modulation electrodes 66d and 66e of the child Mach-Zehnder modulator 42b. The vertical axis represents the amount of phase change Δφ. The solid line represents the amount of phase change in the p-side arm waveguide (arm waveguide 54c). The dotted line represents the amount of phase change in the n-side arm waveguide (arm waveguide 54d). As shown in FIG. 15A, by substituting the coefficients obtained by optimizing and correcting the transmittance into Equation 5 for calculation, an amount of phase change close to that in FIG. 6B can be obtained.

[0104] Fig. 15B is a diagram illustrating the amount of change in absorption loss after optimization. The horizontal axis represents the voltage applied to the modulation electrodes 66d and 66e of the child Mach-Zehnder modulator 42b. The vertical axis represents the amount of change in absorption loss. The solid line represents the amount of change in absorption loss in the p-side arm waveguide (arm waveguide 54c). The dotted line represents the amount of change in absorption loss in the n-side arm waveguide (arm waveguide 54d). As shown in Fig. 15B, by substituting the coefficients obtained by optimizing and correcting the transmittance into Equation 6 for calculation, an amount of change close to that in Fig. 7B can be obtained.

[0105] FIG. 16 is a diagram illustrating the relationship between the bias voltage and the difference in the phase change amount. The horizontal axis represents the bias voltage Vb. The vertical axis represents the difference between the phase change amount in the arm waveguide 54c and the phase change amount in the arm waveguide 54d when a voltage Vb+Vpp / 2 is applied to the modulation electrode 66d and a voltage Vb-Vpp / 2 is applied to the modulation electrode 66e. A predetermined amplitude voltage Vpp=1.7V is applied for each bias voltage Vb on the horizontal axis, and the difference in the phase change amount is calculated. The difference in the phase change amount may be π. As shown by the dotted line in FIG. 16, when Vb=8.9V, the difference in the phase change amount is π. The storage device 34 shown in FIG. 1B stores the bias voltage Vb of the child Mach-Zehnder modulator 42b as 8.9V.

[0106] Table 1 is an example of a data table stored in storage device 34. [Table 1]

[0107] The bias voltage Vb on the Ich side (child Mach-Zehnder modulator 42a) is 11.5 V The bias voltage Vb on the Qch side (child Mach-Zehnder modulator 42b) is 8.9V.

[0108] According to the first embodiment, the control unit 10 obtains the relationship between the voltage applied to the modulation electrode and the amount of phase change, and obtains the voltage at which the amount of phase change during modulation is a predetermined magnitude. The optimized voltage is applied to the Mach-Zehnder modulator to modulate the light. The amount of phase change during modulation can be made to a predetermined magnitude, and an increase in the absorption loss of light can be suppressed.

[0109] The optical modulator 40a has two child Mach-Zehnder modulators 42a and 42b. The phase adjustment efficiency of the child Mach-Zehnder modulator 42b is higher than that of the child Mach-Zehnder modulator 42a. When the same bias voltage as that of the child Mach-Zehnder modulator 42a is applied to the child Mach-Zehnder modulator 42b, In the Mach-Zehnder modulator 42b The light absorption loss is , which is larger than the optical absorption loss in the Mach-Zehnder modulator 42a. 15B, when the bias voltage Vb of the child Mach-Zehnder modulator 42b is set to 11.5 V, which is equal to the bias voltage of the child Mach-Zehnder modulator 42a, the amount of change in absorption loss is 1.1 dB.

[0110] Figure 9 of The voltage is optimized for each child Mach-Zehnder modulator through testing. As shown in Table 1, the bias voltage Vb of the child Mach-Zehnder modulator 42a is set to 11.5V, and the bias voltage Vb of the child Mach-Zehnder modulator 42b is set to 8.9V. This allows the phase change amount of the child Mach-Zehnder modulators 42a and 42b to be a predetermined amount, and also allows the increase in the absorption loss of light to be suppressed. By setting the bias voltage Vb of the child Mach-Zehnder modulator 42b to 8.9V, the change amount of the absorption loss can be reduced to 0.26dB.

[0111] As shown in FIG. 13, the control unit 10 Mach-Zehnder Modulator 42aDuring modulation, a voltage is obtained that makes the difference between the amount of phase change in the arm waveguide 54a and the amount of phase change in the arm waveguide 54b equal to π. As shown in FIG. 16, the control unit 10 also obtains a voltage in the child Mach-Zehnder modulator 42b that makes the difference between the amount of phase change equal to π. As shown in FIG. 4A to FIG. 5B, the output light of each arm waveguide rotates ±π / 2. For example, in FIG. 4A, the phase of the output light Ip of the arm waveguide 54a and the phase of the output light Qp of the arm waveguide 54c rotate +π / 2 compared to FIG. 3. The phase of the output light In of the arm waveguide 54b and the phase of the output light Qn of the arm waveguide 54d rotate -π / 2. In QPSK modulation, the modulated light is maximized.

[0112] The child Mach-Zehnder modulators are differentially driven. When not modulated, Vpp=0, and a bias voltage Vb is applied to the modulation electrode. As shown in FIG. 3, the phase difference between the two paired arm waveguides is π. For example, when the modulation signal has a symbol code of 00, the voltage applied to the modulation electrode 66a of the child Mach-Zehnder modulator 42a and the modulation electrode 66d of the child Mach-Zehnder modulator 42b is Vb+Vpp / 2. The voltage Vpp / 2 rotates the phase of the light by π / 2. The voltage applied to the modulation electrodes 66b and 66e is Vb-Vpp / 2. The voltage -Vpp / 2 rotates the phase of the light by -π / 2 (FIG. 4A). These lights are combined to maximize the output light during modulation.

[0113] The Mach-Zehnder modulator may be driven by a method other than differential driving. Regardless of the driving method, the amount of phase change of the Mach-Zehnder modulator can be controlled with an optimal voltage and an increase in absorption loss can be suppressed.

[0114] The phase change amount of each arm waveguide during modulation may be a value other than ±π / 2 depending on the modulation method. The control unit 10 acquires a voltage that provides a predetermined magnitude of phase change amount. The modulation method may be, for example, quadrature amplitude modulation (QAM) method other than QPSK. The QAM modulation method combines phase modulation and amplitude modulation.

[0115] In order to suppress an increase in the power consumption of the driver IC 26, it is preferable that the amplitude voltage Vpp is, for example, 1.7V or less. As shown in FIG. 13, when the amplitude voltage Vpp is set to a predetermined value such as 1.7V, a bias voltage Vb is obtained such that the difference in the amount of phase change between the two arm waveguides is π. By setting the bias voltage Vb to an optimal value, the amount of phase change can be set to a predetermined value and an increase in the absorption loss of light can be suppressed. Since the amplitude voltage Vpp is 1.7V, the power consumption of the driver IC 26 can be reduced. The upper limit of the amplitude voltage Vpp may be a value other than 1.7V.

[0116] As shown in FIG. 2A, the modulation electrode 66a and the phase adjustment electrode 68a are provided on the arm waveguide 54a. The modulation electrode 66b and the phase adjustment electrode 68b are provided on the arm waveguide 54b. The modulation electrodes 66a and 66b are connected to the same terminating element 78a. For this reason, it is difficult to make the DC voltage applied to the modulation electrode 66a different from the DC voltage applied to the modulation electrode 66b. It is also difficult to measure the transmittance while sweeping the voltage to the modulation electrode. On the other hand, it is possible to make the DC voltage applied to the phase adjustment electrode 68a different from the DC voltage applied to the phase adjustment electrode 68b. It is possible to measure the transmittance while sweeping the voltage to the phase adjustment electrode.

[0117] The control unit 10 obtains the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change (step S14 in FIG. 9, equation 5). The control unit 10 corrects the coefficient based on the ratio between the length of the modulation electrode and the length of the phase adjustment electrode, and obtains the relationship between the voltage applied to the modulation electrode and the amount of phase change (steps S15 and S16). The control unit 10 obtains a voltage that makes the amount of phase change in each arm waveguide, for example, π / 2 (step S18). It is possible to set the amount of phase change to a predetermined magnitude and suppress an increase in optical absorption loss.

[0118] Specifically, in step S10 of Fig. 9, the control unit 10 measures the transmittance while sweeping the voltage applied to the phase adjustment electrode, and calculates the transmittance in step S12. As shown in Figs. 11B and 14, the control unit 10 performs a fitting of the transmittance to bring the calculated transmittance closer to the measured transmittance. The transmittance shown in equation 7 is a function of the phase change amount Δφ. The phase change amount Δφ shown in equation 5 is a function of the voltage applied to the phase adjustment electrode. By fitting the transmittance, the coefficient in equation 5 becomes a more appropriate value. The ratio between the length of the modulation electrode and the length of the phase adjustment electrode is, for example, 2.5. The coefficient is set according to the ratio. Ratio The multiplication is multiplied by a constant (e.g., 2.5 times). By substituting the coefficient after the multiplication into Equation 5, a highly accurate relationship between the voltage applied to the modulation electrode and the amount of phase change can be obtained. The control unit 10 obtains the voltage applied to the modulation electrode when the amount of phase change is a predetermined amount. It is possible to make the amount of phase change a predetermined amount and suppress an increase in the absorption loss of light. The transmittance, the amount of phase change, and the amount of change in absorption loss may be calculated from equations other than those above.

[0119] As shown in FIG. 2B, the arm waveguides 54a and 54b have a cladding layer 82, a core layer 84, a cladding layer 86, and a contact layer 88. The other arm waveguides have the same configuration. The cladding layer 82 is an n-type semiconductor layer. The cladding layer 86 and the contact layer 88 are p-type semiconductor layers. A dopant is added to obtain n-type and p-type conductivity. Variations in the thermal diffusion amount of the dopant also occur in the phase adjustment efficiency of the Mach-Zehnder modulator. According to the first embodiment, a voltage at which the phase change amount becomes a predetermined magnitude is obtained for each Mach-Zehnder modulator. By driving the Mach-Zehnder modulator with a voltage according to the phase adjustment efficiency, the phase of the light during modulation can be made to a predetermined magnitude and an increase in absorption loss can be suppressed.

[0120] 13 and 16, the control unit 10 changes the bias voltage Vb in increments of 0.1 V within a range of 6 V to 13 V, for example, and calculates the amount of phase change. The control unit 10 may use an algorithm such as the bisection method or Newton's method to determine an appropriate bias voltage Vb.

[0121] The relationship between the voltage and the amount of phase change, and the relationship between the voltage and the amount of change in absorption loss may change depending on the wavelength of light. The wavelength-tunable laser element 22 receives light of multiple wavelengths that are assumed to be used when the optical modulator 40a is in operation. The control unit 10 performs the test of FIG. 9 for each of the multiple wavelengths. The storage device 34 stores the bias voltage for each of the multiple wavelengths. The control unit 10 may perform the test at, for example, the shortest wavelength and the longest wavelength in the wavelength range to obtain the bias voltage. The control unit 10 may perform the test at, for example, three or more wavelengths in the wavelength range to obtain the bias voltage. By performing interpolation using linear interpolation, polynomial interpolation, spline interpolation, or the like, the bias voltage at wavelengths that are not tested can also be obtained.

[0122] In the first embodiment, there is variation in phase adjustment efficiency between the child Mach-Zehnder modulators 42a and 42b of one optical modulator 40a, and an optimal voltage is obtained for each child Mach-Zehnder modulator 42a and 42b. There may also be variation in phase adjustment efficiency between the optical modulators 40a. The control unit 10 tests a plurality of optical modulators 40a and obtains an optimal voltage for each optical modulator 40a. In the steps of Figs. 8 and 9, the optical transmission device 100 of Fig. 1A is used as a test device for the optical modulator 40. One optical modulator 40a is incorporated into the optical transmission device 100 and tested. The optical modulator 40a is Other optical modulators The optical transmitter 100 is replaced and further tested. The storage device 34 may store voltages for multiple optical modulators 40a. When the optical transmitter 100 is used for communication, a test may be performed on one optical modulator 40a included in the optical transmitter 100. The storage device 34 may store only the voltage for that one optical modulator 40a.

[0123] <Second embodiment> The second embodiment is an example in which a DP (Dual Polarization)-IQ modulator is used as the optical modulator 40. The configuration of the optical transmitter 100 is the same as that of the first embodiment.

[0124] 17 is a plan view illustrating the optical modulator 40b. The optical modulator 40b is a DP-IQ modulator and includes two optical modulators 43a and 43b.

[0125] A semiconductor substrate 80 and four termination elements 78a, 78b, 78c, and 78d are mounted on the upper surface of the substrate 41. The termination elements 78a, 78b, 78c, and 78d include, for example, a termination resistor and a capacitor. The termination elements 78a and 78b face an end surface 80c of the semiconductor substrate 80. The termination elements 78c and 78d face an end surface 80d of the semiconductor substrate 80. The input waveguide 51 and the optical modulators 43a and 43b are formed on the semiconductor substrate 80.

[0126] A first end of the input waveguide 51 is located on an end surface 80a of the semiconductor substrate 80. A second end of the input waveguide 51 is connected to a coupler 59. Two optical modulators 43a and 43b are arranged in parallel after the coupler 59.

[0127] The optical modulator 43a is an IQ modulator, and has two child Mach-Zehnder modulators 42a and 42b and a parent Mach-Zehnder modulator 44a, similar to the optical modulator 40a in FIG. 2A. The optical modulator 43b is an IQ modulator, and has two child Mach-Zehnder modulators 42c and 42d and a parent Mach-Zehnder modulator 44b. The configurations of the child Mach-Zehnder modulators 42c and 42d are the same as those of the child Mach-Zehnder modulators 42a and 42b. The configuration of the parent Mach-Zehnder modulator 44b is the same as that of the parent Mach-Zehnder modulator 44a.

[0128] The optical modulator 43a generates modulated light of the X channel (X polarization). The optical modulator 43b generates modulated light of the Y channel (Y polarization). The polarization plane of the X polarization is orthogonal to the polarization plane of the Y polarization. Using a polarization rotation element and a multiplexing element (not shown), the two modulated lights are multiplexed so that the polarization planes are orthogonal.

[0129] The manufacturing method of the optical modulator 40b is the same as the process shown in FIG. 8. The control unit 10 performs the test shown in FIG. 9 on each of the Mach-Zehnder modulators in the optical modulator 40b. When testing the child Mach-Zehnder modulators 42a and 42b of the optical modulator 43a and the parent Mach-Zehnder modulator 44a, the child Mach-Zehnder modulators 42c and 42d of the optical modulator 43b are adjusted to their extinction points. When testing the child Mach-Zehnder modulators 42c and 42d of the optical modulator 43b and the parent Mach-Zehnder modulator 44b, the child Mach-Zehnder modulators 42a and 42b of the optical modulator 43a are adjusted to their extinction points.

[0130] Table 2 is an example of a data table stored in the storage device 34. The storage device 34 stores the voltage of the optical modulator 40b. [Table 2]

[0131] Xch represents the optical modulator 43a. In Xch, Ich represents the child Mach-Zehnder modulator 42a. Qch represents the child Mach-Zehnder modulator 42b. Y ch is optical modulator 43 b In Ych, Ich represents the child Mach-Zehnder modulator 42c. Qch represents the child Mach-Zehnder modulator 42d. The bias voltage of the child Mach-Zehnder modulator 42a is, for example, 8.9V. The bias voltage of the child Mach-Zehnder modulator 42b is, for example, 9.5V. The bias voltage of the child Mach-Zehnder modulator 42c is, for example, 9.4V. The bias voltage of the child Mach-Zehnder modulator 42d is, for example, 9.5V.

[0132] According to the second embodiment, by driving the Mach-Zehnder modulators with a voltage optimized for each Mach-Zehnder modulator, it is possible to set the amount of phase change during modulation to a predetermined magnitude and suppress an increase in optical absorption loss.

[0133] The optical modulator 40 is an IQ modulator in the first embodiment and a DP-IQ modulator in the second embodiment, but the present disclosure may be applied to optical modulators other than these.

[0134] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present disclosure described in the claims. [Explanation of symbols]

[0135] 10 Control section 12 Phase control section 14 Laser control unit 15 Calculation section 16 Modulation control section 18 Memory control unit 22 Tunable laser element 24 ABC circuit 26 Driver IC 30 CPU 32 RAM 34 Storage device 36 Interface 40, 40a, 40a-1, 40a-2, 40b, 40b-1, 40b-2 Modulators 41 Substrate 42a, 42b, 42c, 42d Mach-Zehnder Modulator 44a, 44b Parent Mach-Zehnder Modulator 52a, 52b, 54a, 54b, 55a, 55b arm waveguides 50, 51 Input waveguide 56 Output waveguide 58, 59, 60a, 60b, 62a, 62b, 64 coupler 66a, 66b, 66d, 66e, Modulation electrodes 66c, 66f, 68c, 68f, 70c Ground electrode 68a, 68b, 68d, 68e, 70a, 70b Phase adjustment electrodes 72a, 72b, 72c, 72d, 72e, 72f, 74a, 74b, 74c, 74d, 74e, 74f, 75a, 75b, 75c, 75d, 75e, 75f, 76a, 76b, 76c Wiring 78a, 78b, 78c, 78d Termination element 80 Semiconductor Substrate 80a, 80b, 80c, 80d end face 81 Insulating film 82, 86 Cladding layer 84 Core Layer 85 Resin layer 88 Contact Layer 100 Optical transmitter

Claims

1. A method for manufacturing an optical modulator, comprising the steps of: the optical modulator comprises a Mach-Zehnder modulator; the Mach-Zehnder modulator has a first electrode, a second electrode, a first arm waveguide, and a second arm waveguide; the first electrode is provided on the first arm waveguide, the second electrode is provided on the second arm waveguide, The manufacturing method includes: providing the Mach-Zehnder modulator; acquiring a first relationship, which is a relationship between a voltage applied to the first electrode and an amount of change in the phase of light propagating through the first arm waveguide, and a second relationship, which is a relationship between a voltage applied to the second electrode and an amount of change in the phase of light propagating through the second arm waveguide, based on a light transmittance in the first arm waveguide and a light transmittance in the second arm waveguide; acquiring a voltage to be applied to the first electrode such that a phase change amount in the first arm waveguide becomes a predetermined amount when light propagating through the first arm waveguide and light propagating through the second arm waveguide are modulated based on the first relationship and the second relationship, and acquiring a voltage to be applied to the second electrode such that a phase change amount in the second arm waveguide becomes a predetermined amount. and storing in a memory unit the voltage applied to the first electrode and the voltage applied to the second electrode acquired in the acquiring step.

2. the step of preparing the Mach-Zehnder modulator includes a step of preparing a plurality of the Mach-Zehnder modulators; 2. The method for manufacturing an optical modulator according to claim 1, further comprising the steps of: acquiring the first relationship and the second relationship for each of the plurality of Mach-Zehnder modulators; and acquiring a voltage applied to the first electrode and a voltage applied to the second electrode.

3. A method for manufacturing an optical modulator as described in claim 1 or claim 2, wherein during modulation, the difference between the amount of phase change in the first arm waveguide and the amount of phase change in the second arm waveguide is π.

4. the voltage applied to the first electrode is the sum of a first voltage and a second voltage; a voltage applied to the second electrode is a difference between the first voltage and the second voltage; The method for manufacturing an optical modulator according to claim 1 , wherein the step of obtaining the voltage includes the step of obtaining the first voltage such that the second voltage is equal to or lower than a predetermined value.

5. the step of preparing the Mach-Zehnder modulator includes the step of preparing the Mach-Zehnder modulator having the first arm waveguide, the second arm waveguide, the first electrode, the second electrode, a third electrode, and a fourth electrode; the third electrode is provided on the first arm waveguide, the fourth electrode is provided on the second arm waveguide, The step of acquiring the first relationship and the second relationship includes: acquiring the first relationship, which is a relationship between a voltage applied to the first electrode and an amount of change in the phase of light propagating through the first arm waveguide, based on a relationship between a voltage applied to the third electrode and an amount of change in the phase of light propagating through the first arm waveguide; and acquiring the second relationship, which is a relationship between a voltage applied to the second electrode and an amount of change in phase of light propagating through the second arm waveguide, based on a relationship between a voltage applied to the fourth electrode and an amount of change in phase of light propagating through the second arm waveguide.

6. measuring a first transmittance which is a transmittance of light in each of the first arm waveguide and the second arm waveguide; calculating a second transmittance which is a transmittance of light in each of the first arm waveguide and the second arm waveguide, in the step of calculating the second transmittance, the second transmittance in the first arm waveguide is expressed as a function of an amount of change in a phase of light propagating through the first arm waveguide, and the amount of change in the phase of the light propagating through the first arm waveguide is expressed as a function of a voltage applied to the third electrode, thereby calculating the second transmittance in the first arm waveguide; expressing a second transmittance in the second arm waveguide as a function of a change in a phase of light propagating through the second arm waveguide, and expressing the change in the phase of the light propagating through the second arm waveguide as a function of a voltage applied to the fourth electrode, thereby calculating the second transmittance in the second arm waveguide; the step of acquiring the first relationship and the second relationship includes acquiring a relationship between a voltage applied to the third electrode and an amount of change in a phase of light propagating through the first arm waveguide by adjusting a second transmittance in the first arm waveguide so as to approach a first transmittance in the first arm waveguide; and adjusting a second transmittance in the second arm waveguide to approach a first transmittance in the second arm waveguide, thereby acquiring a relationship between a voltage applied to the fourth electrode and an amount of change in a phase of light propagating through the second arm waveguide.

7. providing the Mach-Zehnder modulator includes forming the Mach-Zehnder modulator; the step of forming the Mach-Zehnder modulator includes the step of forming the first arm waveguide and the second arm waveguide each having a first semiconductor layer, a core layer, and a second semiconductor layer; the first semiconductor layer, the core layer, and the second semiconductor layer are stacked in this order; the first semiconductor layer has a first conductivity type; The method for manufacturing an optical modulator according to claim 1 , wherein the second semiconductor layer has a second conductivity type.

8. A method for testing an optical modulator, comprising the steps of: the optical modulator comprises a Mach-Zehnder modulator; the Mach-Zehnder modulator has a first electrode, a second electrode, a first arm waveguide, and a second arm waveguide; the first electrode is provided on the first arm waveguide, the second electrode is provided on the second arm waveguide, The test method comprises: acquiring a first relationship, which is a relationship between a voltage applied to the first electrode and an amount of change in the phase of light propagating through the first arm waveguide, and a second relationship, which is a relationship between a voltage applied to the second electrode and an amount of change in the phase of light propagating through the second arm waveguide, based on a light transmittance in the first arm waveguide and a light transmittance in the second arm waveguide; acquiring a voltage to be applied to the first electrode such that a phase change amount in the first arm waveguide becomes a predetermined amount when light propagating through the first arm waveguide and light propagating through the second arm waveguide are modulated based on the first relationship and the second relationship, and acquiring a voltage to be applied to the second electrode such that a phase change amount in the second arm waveguide becomes a predetermined amount.

9. A test program for an optical modulator, comprising: the optical modulator comprises a Mach-Zehnder modulator; the Mach-Zehnder modulator has a first electrode, a second electrode, a first arm waveguide, and a second arm waveguide; the first electrode is provided on the first arm waveguide, the second electrode is provided on the second arm waveguide, The test program includes: On the computer, acquiring a first relationship, which is a relationship between a voltage applied to the first electrode and an amount of change in the phase of light propagating through the first arm waveguide, and a second relationship, which is a relationship between a voltage applied to the second electrode and an amount of change in the phase of light propagating through the second arm waveguide, based on a light transmittance in the first arm waveguide and a light transmittance in the second arm waveguide; and acquiring a voltage to be applied to the first electrode such that an amount of phase change in the first arm waveguide becomes a predetermined amount, and acquiring a voltage to be applied to the second electrode such that an amount of phase change in the second arm waveguide becomes a predetermined amount, when modulating light propagating through the first arm waveguide and light propagating through the second arm waveguide, based on the first relationship and the second relationship.

10. a plurality of Mach-Zehnder modulators; A storage unit, the plurality of Mach-Zehnder modulators each have a first electrode, a second electrode, a first arm waveguide, and a second arm waveguide; the first electrode is provided on the first arm waveguide, the second electrode is provided on the second arm waveguide, the storage unit stores, for each of the plurality of Mach-Zehnder modulators, a voltage to be applied to the first electrode and a voltage to be applied to the second electrode such that a change in phase of the light propagating through the first arm waveguide and the light propagating through the second arm waveguide becomes a predetermined magnitude when the light propagating through the first arm waveguide and the light propagating through the second arm waveguide are modulated.

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