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 the light propagating the arm waveguides; based on the relationship, acquiring a voltage at which the range of the amount of change in the phase of light in the Mach-Zehnder modulator becomes a predetermined magnitude; 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 formed of a semiconductor layer and capable of modulating light has been developed (Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-164243 A [Patent Document 2] JP 2016-111398 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. In order to adjust the phase to a desired magnitude, it is important to set the range of the phase change caused by the application of voltage (phase adjustment range) to a predetermined size.

[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 a plurality of Mach-Zehnder modulators, the amount of phase change in one Mach-Zehnder modulator is large, and the amount of phase change in another Mach-Zehnder modulator is small. Even in a Mach-Zehnder modulator with low phase adjustment efficiency, the voltage can be increased to set the phase adjustment range to a predetermined size. However, there is a positive correlation between the phase adjustment efficiency and the optical absorption loss. Increasing the voltage also increases the optical absorption loss. Therefore, the object is to provide a manufacturing method, a test method, and a test program for an optical modulator capable of suppressing the increase in the 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 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; acquiring a voltage that provides a predetermined range of an amount of change in the phase of light in the Mach-Zehnder modulator based on the relationship; and storing the voltage in a memory 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 a range of an amount of change in the phase of light in the Mach-Zehnder modulator becomes a predetermined size.

[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, the 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 a range of an amount of change in the phase of light in the Mach-Zehnder modulator becomes a predetermined size based on the relationship.

[0009] An optical transmission device according to the present disclosure includes a plurality of Mach-Zehnder modulators and a memory unit, the plurality of Mach-Zehnder modulators each having 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 range of change in the phase of light becomes a predetermined magnitude. 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 a diagram illustrating the relationship between the differential voltage and the amount of phase change in a child Mach-Zehnder modulator. [Figure 4A] FIG. 4A is a diagram illustrating an example of the relationship between the voltage and the amount of phase change. [Figure 4B] FIG. 4B is a diagram illustrating the relationship between the voltage and the amount of phase change. [Figure 5A] FIG. 5A is a diagram illustrating the relationship between the differential voltage and the amount of phase change. [Figure 5B] FIG. 5B is a diagram illustrating the relationship between the differential voltage and the amount of phase change. [Figure 6A] FIG. 6A is a diagram illustrating an example of the relationship between voltage and the amount of change in optical absorption loss. [Figure 6B] FIG. 6B is a diagram illustrating the relationship between voltage and the amount of change in optical absorption loss. [Figure 7A] FIG. 7A is a diagram illustrating the relationship between the differential voltage and the amount of phase change. [Figure 7B] FIG. 7B is a diagram illustrating the relationship between the differential voltage and the amount of phase change. [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 center voltage and the phase adjustment range. [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 center voltage and the phase adjustment range. [Figure 17] FIG. 17 is a diagram illustrating the measured transmittance and the transmittance after optimization. [Figure 18A] FIG. 18A is a diagram illustrating the amount of phase change after optimization. [Figure 18B] FIG. 18B is a diagram illustrating the amount of change in absorption loss after optimization. [Figure 19] FIG. 19 is a diagram illustrating the relationship between the center voltage and the phase adjustment range. [Figure 20A] FIG. 20A is a diagram illustrating the amount of absorption loss. [Figure 20B] FIG. 20B is a diagram illustrating the extinction ratio. [Figure 21A] FIG. 21A is a diagram illustrating the amount of absorption loss. [Figure 21B] FIG. 21B is a diagram illustrating the extinction ratio. [Figure 22A] FIG. 22A is a diagram illustrating the amount of absorption loss. [Figure 22B] FIG. 22B is a diagram illustrating the extinction ratio. [Figure 23A] FIG. 23A is a diagram illustrating the amount of absorption loss. [Figure 23B] FIG. 23B is a diagram illustrating the extinction ratio. [Figure 24A] FIG. 24A is a diagram illustrating the amount of absorption loss. [Figure 24B] FIG. 24B is a diagram illustrating the extinction ratio. [Figure 25A] FIG. 25A is a diagram illustrating the amount of absorption loss. [Figure 25B] FIG. 25B is a diagram illustrating the extinction ratio. [Figure 26] FIG. 26 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 is (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 method including the steps of: preparing the Mach-Zehnder modulator; acquiring a relationship between a voltage applied to the electrode and a phase change amount of light propagating through the arm waveguide based on a transmittance of light in the arm waveguide; acquiring a voltage that makes a range of the phase change amount of light in the Mach-Zehnder modulator a predetermined size based on the relationship; and storing the voltage in a storage unit. By applying the acquired voltage to the Mach-Zehnder modulator, the range of the phase change amount can be made the predetermined size and an increase in light absorption loss can be suppressed. (2) The step of preparing the Mach-Zehnder modulator may be a step of preparing a plurality of the Mach-Zehnder modulators, and the step of acquiring the relationship between the voltage and the amount of phase change and the step of acquiring the voltage may be performed for each of the plurality of Mach-Zehnder modulators. A voltage is optimized for each of the plurality of Mach-Zehnder modulators. By applying the optimized voltage to the Mach-Zehnder modulator, the range of the amount of phase change can be set to a predetermined size and an increase in optical absorption loss can be suppressed. (3) The step of preparing the Mach-Zehnder modulator may include a step of preparing a parent Mach-Zehnder modulator and a step of preparing a child Mach-Zehnder modulator, and the step of acquiring the relationship between the voltage and the amount of phase change and the step of acquiring the voltage may be performed for each of the parent Mach-Zehnder modulator and the child Mach-Zehnder modulator. It is possible to set the range of the amount of phase change of the parent Mach-Zehnder modulator and the child Mach-Zehnder modulator to a predetermined size and to suppress an increase in optical absorption loss. (4) The step of preparing the Mach-Zehnder modulator may include the step of preparing the Mach-Zehnder modulator having a first arm waveguide, a second arm waveguide, a first electrode, and a second electrode, the first electrode being provided in the first arm waveguide and the second electrode being provided in the second arm waveguide, the step of acquiring the relationship between the voltage and the amount of phase change may include the step of acquiring the relationship between a voltage applied to the first electrode and an amount of phase change of light propagating through the first arm waveguide, and the step of acquiring the relationship between a voltage applied to the second electrode and an amount of phase change of light propagating through the second arm waveguide, a range of the amount of phase change of light in the Mach-Zehnder modulator is a range of a difference between the amount of phase change in the first arm waveguide and the amount of phase change in the second arm waveguide, and the step of acquiring the voltage may be the step of acquiring a voltage applied to the first electrode and a voltage applied to the second electrode such that the range of the amount of phase change is the predetermined magnitude. It is possible to set the range of the amount of phase change to the predetermined magnitude and to suppress an increase in optical absorption loss. (5) The voltage applied to the first electrode may be a sum of a first voltage and a second voltage, and the voltage applied to the second electrode may be a difference between the first voltage and the second voltage, and the step of acquiring the voltage may be a step of acquiring the first voltage such that the range of the amount of phase change is the predetermined magnitude. The first voltage is set as a center voltage, and the second voltage is set as a differential voltage to differentially drive a Mach-Zehnder modulator. The range of the amount of phase change can be set to a predetermined magnitude, and an increase in optical absorption loss can be suppressed. (6) The method further includes a step of measuring a first transmittance, which is the transmittance of light in the arm waveguide, and a step of calculating a second transmittance, which is the transmittance of light in the arm waveguide, and in the step of calculating the second transmittance, the second transmittance is expressed as a function of a change in the phase of light propagating through the arm waveguide, and the change in the phase of light propagating through the arm waveguide is expressed as a function of a voltage applied to the electrode, thereby calculating the second transmittance, and in the step of acquiring a relationship between the voltage and the change in the phase, the relationship between the voltage and the change in the phase may be acquired by adjusting the second transmittance to approach the first transmittance. By bringing the second transmittance close to the first transmittance, a highly accurate relationship between the voltage and the change in the phase can be acquired. The range of the change in the phase can be set to a predetermined size, and an increase in the absorption loss of light can be suppressed. (7) The step of preparing the Mach-Zehnder modulator includes a step of forming the Mach-Zehnder modulator, and the step of forming the Mach-Zehnder modulator includes a step of forming the arm waveguide having a first semiconductor layer, a core layer, and a second semiconductor layer, and the first semiconductor layer, the core layer, and the second semiconductor layer are stacked in order, and the first semiconductor layer may have a first conductivity type, and the second semiconductor layer may have a second conductivity type. A dopant is added to the first semiconductor layer and the second semiconductor layer. Variations in the amount of thermal diffusion of the dopant cause variations in the phase adjustment efficiency of the Mach-Zehnder modulator. By applying the acquired voltage to the Mach-Zehnder modulator, the range of the amount of phase change can be set to a predetermined size, and an increase in the absorption loss of light can be suppressed. (8) A method for testing 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 method comprising the steps of: acquiring a relationship between a voltage applied to the electrode and a phase change amount of light propagating through the arm waveguide based on a transmittance of light in the arm waveguide; and acquiring a voltage that makes a range of the phase change amount of light in the Mach-Zehnder modulator a predetermined size based on the relationship. By applying the acquired voltage to the Mach-Zehnder modulator, it is possible to make the range of the phase change amount a predetermined size and to suppress an increase in light absorption loss. (9) A test program 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 test program causing 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 transmittance of light in the arm waveguide, and a process of acquiring a voltage that makes a range of the amount of change in the phase of light in the Mach-Zehnder modulator a predetermined size based on the relationship. By applying the acquired voltage to the Mach-Zehnder modulator, it is possible to make the range of the amount of change in phase a predetermined size and to suppress an increase in light absorption loss. (10) An optical transmission device including a plurality of Mach-Zehnder modulators and a memory unit, the plurality of Mach-Zehnder modulators each having an electrode and an arm waveguide, the electrode being provided in the arm waveguide, and the memory unit storing, for each of the plurality of Mach-Zehnder modulators, a voltage that causes a range of a phase change amount of light to become a predetermined amount. By applying the stored voltage to the Mach-Zehnder modulator, it is possible to make the range of the phase change amount the predetermined amount and to suppress an increase in the absorption loss of light.

[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 phase change amount, and the phase adjustment range, as described later. 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] (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 and the phase adjustment electrode 68a are 54 The modulation electrode 66a and the phase adjustment electrode 68a (first electrode) are spaced apart from each other and are arranged in order from the coupler 60a side to the coupler 62a side. The modulation electrode 66b and the phase adjustment electrode 68b are arranged on the arm waveguide 54 The modulation electrode 66b and the phase adjustment electrode 68b (second electrode) 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] The modulation electrode 66d and the phase adjustment electrode 68d (first electrode) are provided on the arm waveguide 54c. The modulation electrode 66e and the phase adjustment electrode 68e (second electrode) are provided on the arm waveguide 54d. The ground electrode 66f is provided between the modulation electrode 66d and the modulation electrode 66e. The ground electrode 68f is provided between the phase adjustment electrode 68d and the phase adjustment electrode 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 modulation electrodes 66a, 66b, 66d, and 66e and the lengths of the ground electrodes 66c and 66f are equal to each other. The lengths of the phase adjustment electrodes 68a, 68b, 68d, and 68e are equal to each other and shorter than the lengths of the modulation electrodes. The lengths of the ground electrodes 68c and 68f are equal to each other and shorter than the lengths 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 (first arm waveguide) is connected to the output end of the coupler 62a. A first end of the arm waveguide 55b (second arm waveguide) 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] A phase adjustment electrode 70a (first electrode) is provided on the arm waveguide 55a. A phase adjustment electrode 70b (second electrode) is provided on the arm waveguide 55b. A 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 an end face 80c.

[0038] The wiring 76a is electrically connected to an end of the phase adjustment electrode 70a and extends to an end face 80c. The wiring 76b is electrically connected to an end of the phase adjustment electrode 70b and extends to an end face 80d. The wiring 76c is electrically connected to an end of the ground electrode 70c and extends to an end face 80d. 80c 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 phase difference φ between two paired arm waveguides such as the arm waveguides 52a and 52b is expressed as the sum of an initial phase difference φ0 and a phase change amount Δφ, as shown in the following equation.

number

[0053] The initial phase difference φ0 is determined by the difference in optical path length between the arm waveguides of the optical modulator 40a, etc. The wavelength λ of light in the arm waveguide is, for example, 484 nm (1550 nm in a vacuum). The length of each of the arm waveguides 54a, 54b, 54c, and 54d of the Mach-Zehnder modulator is, for example, 6 mm, which is more than 10,000 times the wavelength λ. Variations occur in the optical path lengths of the arm waveguides due to manufacturing errors, etc. The relationship between the difference ΔP in optical path length between the two arm waveguides and the initial phase difference φ0 of light between the two arm waveguides is expressed by the following equation using an integer m.

number

[0054] The difference ΔP in optical path length between two paired arm waveguides such as the arm waveguides 52a and 52b may be 1 / 10,000 or more of the designed dimension. In this case, the difference ΔP in optical path length is equal to or greater than the wavelength λ of light. The initial phase difference φ0 is distributed in the range of 0 (rad) or more and 2π (rad) or less.

[0055] The initial phase difference φ0 may change even during operation of the optical transmitter 100. This is because the optical path length of the arm waveguide changes due to stress applied to the optical modulator 40a, temperature changes, etc. The amount of change in the initial phase difference φ0 during operation is, for example, in the range from −2π to 2π.

[0056] The phase change amount Δφ is the amount of change in the phase of light propagating through the arm waveguide. The phase change amount Δφ is adjusted by applying a voltage from the ABC circuit 24 to the phase adjustment electrode to change the optical path length of the arm waveguide. In response to the initial phase difference φ0, the phase control unit 12 changes the voltage applied from the ABC circuit 24 to the phase adjustment electrode (automatic bias control). The phase change amount in the automatic bias control is determined taking into account the initial phase difference and the phase change during operation.

[0057] The phase adjustment range is the range of the phase change amount when the voltage applied to the phase adjustment electrode is swept. In order to adjust the operating points of the child Mach-Zehnder modulators 42a and 42b to the extinction points, the phase adjustment range of each of the child Mach-Zehnder modulators 42a and 42b is preferably, for example, a range of 6π from -3π to 3π. In order to orthogonalize the phases of the two modulated lights in the parent Mach-Zehnder modulator 44a, the phase adjustment range of the parent Mach-Zehnder modulator 44a is preferably, for example, a range of 5π from -2.5π to 2.5π.

[0058] (Voltage) The following describes the voltage applied to the child Mach-Zehnder modulator 42a by the ABC circuit 24. The voltage Vp applied to the phase adjustment electrode 68a on the arm waveguide 54a is expressed as follows using a center voltage Vcc (first voltage) and a differential voltage Vdc (second voltage):

number

number

[0059] The difference between voltages Vp and Vn is 2 Vdc. Phase control unit 12 fixes center voltage Vcc to a constant value and changes differential voltage Vdc to change voltages Vp and Vn and adjust the operating point of child Mach-Zehnder modulator 42a. Voltage Vp is applied to phase adjustment electrode 68d of child Mach-Zehnder modulator 42b, and voltage Vn is applied to phase adjustment electrode 68e.

[0060] The voltage applied to the parent Mach-Zehnder modulator 44a by the ABC circuit 24 will be described below. The voltage VI applied to the phase adjustment electrode 70a on the arm waveguide 55a is expressed as follows using a center voltage Vcp (first voltage) and a differential voltage Vdp (second voltage):

number

number

[0061] The difference between voltage VI and voltage VQ is 2Vdp. Phase control section 12 fixes center voltage Vcp to a constant value and changes differential voltage Vdp to change voltages VI and VQ and adjust the operating point of parent Mach-Zehnder modulator 44a.

[0062] The magnitude of the voltages will be explained using the voltages Vp and Vn of the child Mach-Zehnder modulator as an example. The minimum value of the voltages Vp and Vn is Vmin, and the maximum value is Vmax. The wider the adjustment range of the differential voltage Vdc, the wider the phase adjustment range. To widen the adjustment range of the differential voltage Vdc, the center voltage Vcc and the differential voltage Vdc are determined, for example, as follows: Vcc=(Vmin+Vmax) / 2 Vdc adjustment range: Vmax-Vmin range from -(Vmax-Vmin) / 2 to (Vmax-Vmin) / 2 The minimum value Vmin and the maximum value Vmax are determined according to, for example, the power consumption and the withstand voltage of the optical modulator 40. When Vmin=0V and Vmax=20V, Vcc=10V. The range of the differential voltage Vdc is from -10V to 10V (-Vcc≦Vdc≦Vcc). The voltages applied to the child Mach-Zehnder modulator 42b and the parent Mach-Zehnder modulator 44a can be similar to the above.

[0063] 3 is a diagram illustrating the relationship between the differential voltage and the phase change amount Δφ in the child Mach-Zehnder modulator 42a. The horizontal axis represents the differential voltage Vdc, and the vertical axis represents the phase change amount Δφ. The dashed line represents the phase change amount in the arm waveguide 54a. The dotted line represents the phase change amount in the arm waveguide 54b. The solid line represents the phase change amount of the child Mach-Zehnder modulator 42a. The phase change amount of the child Mach-Zehnder modulator 42a is the phase difference between the arm waveguides (phase change amount in the arm waveguide 54a-phase change amount in the arm waveguide 54b). The center voltage Vcc=10V, and the range of the differential voltage Vdc is from -10V to 10V.

[0064] As shown in FIG. 3, the more the differential voltage Vdc is made positive, the more the phase change in the arm waveguide 54a becomes positive. The phase change in the arm waveguide 54b approaches 0. The phase change (phase difference) indicated by the solid line increases to the positive side. The more the differential voltage Vdc is made negative, the more the phase change in the arm waveguide 54b becomes positive. The phase change in the arm waveguide 54a approaches 0. The phase difference increases to the negative side. With the differential voltage Vdc in the range of -10V to 10V, the phase difference is in the range of approximately -9π to 9π.

[0065] 3, the range of the phase change amount Δφ (phase adjustment range) of the child Mach-Zehnder modulator 42a is approximately from -9π to 9π, which exceeds the phase adjustment range of 6π from -3π to 3π required for the child Mach-Zehnder modulator. In order to reduce power consumption and set the phase adjustment range to a predetermined size, the center voltage Vc is set to a value lower than 10V, for example, 7V. The differential voltage Vdc is set to a range of -7V to 7V, for example.

[0066] The rate of phase change relative to voltage (phase adjustment efficiency) may vary from one Mach-Zehnder modulator to another. The difference in phase adjustment efficiency is believed to be due to the difference in the amount of thermal diffusion of the dopant into the cladding layers 82 and 86, the contact layer 88, and the like. Differences in the amount of thermal diffusion of the dopant cause differences in the strength of the electric field generated in the core layer 84 when a voltage is applied. Differences in the electric field strength also cause differences in the refractive index, resulting in different amounts of phase change. Differences in the amount of thermal diffusion of the dopant cause variations in the band gap energy, which in turn changes the amount of phase change.

[0067] The variation in phase adjustment efficiency may occur between the child Mach-Zehnder modulator 42a and the child Mach-Zehnder modulator 42b in one optical modulator 40a. Also, the variation in phase adjustment efficiency may occur among a plurality of optical modulators 40a.

[0068] Take two optical modulators 40a-1 and 40a-2 as an example. The optical modulators 40a-1 and 40a-2 are respectively Figure 2A First, the phase adjustment efficiency between the child Mach-Zehnder modulators will be described. Fig. 4A and Fig. 4B are diagrams illustrating the relationship between voltage and phase change amount. The horizontal axis is the voltage (Vp and Vn) applied to the phase adjustment electrodes of the child Mach-Zehnder modulators. The vertical axis is the phase change amount in the arm waveguide.

[0069] FIG. 4A shows the arm waveguide of the Mach-Zehnder modulator 42a of the optical modulator 40a-1. Voltage and Phase change amount Relationship with 4B shows the phase change amount of the arm waveguide (arm waveguide 54a) on the p-side, and the dotted line shows the phase change amount of the arm waveguide (arm waveguide 54b) on the n-side. Voltage and Phase change amount Relationship with4A and 4B. 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). Due to the difference in phase adjustment efficiency, the amount of phase change in the p-side arm waveguide is slightly different from the amount of phase change in the n-side arm waveguide in each of FIGS. 4A and 4B. The difference in the amount of phase change between the child Mach-Zehnder modulators is larger than the difference in the amount of phase change between the arm waveguides.

[0070] Comparing the cases where the same voltage is applied in FIG. 4A and FIG. 4B, the phase change amount Δφ in FIG. 4A is small, and the phase change amount Δφ in FIG. 4B is large. For example, when the voltage is 10V, the phase change amount Δφ of the two arm waveguides 54a and 54b in FIG. 4A is about 1.5π. The phase change amount Δφ of the two arm waveguides 54c and 54d in FIG. 4B is about 2.5π. The phase adjustment efficiency of the child Mach-Zehnder modulator 42b shown in FIG. 4B is higher than that of the child Mach-Zehnder modulator 42a shown in FIG. 4A. In this way, the phase adjustment efficiency varies due to the variation in the thermal diffusion amount of the dopant within the same optical modulator 40a-1.

[0071] 5A and 5B are diagrams illustrating the relationship between the differential voltage and the amount of phase change. The horizontal axis represents the differential voltage Vdc. The vertical axis represents the amount of phase change Δφ. The center voltage Vcc is 7V.

[0072] 5A shows the amount of phase change in the child Mach-Zehnder modulator 42a of the optical modulator 40a-1. The dashed 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). The solid line represents the amount of phase change (phase difference between the arm waveguides) in the child Mach-Zehnder modulator 42a. Since the child Mach-Zehnder modulator 42a is differentially driven, the amount of phase change is symmetrical with Vdc=0 as the reference. When the differential voltage Vdc is swept from -7V to 7V, the amount of phase change is -3π or more and 3π or less.

[0073] 5B shows the amount of phase change in the child Mach-Zehnder modulator 42b of the optical modulator 40a-1. The dashed 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). The solid line represents the amount of phase change in the child Mach-Zehnder modulator 42b. The phase adjustment efficiency of the child Mach-Zehnder modulator 42b is higher than that of the child Mach-Zehnder modulator 42a. Therefore, the amount of phase change in FIG. 5B is larger than that in FIG. 5A, and is not less than −4π and not more than 4π.

[0074] In order to set the range of the phase change amount (phase adjustment range) to a predetermined value, such as -3π or more and 3π or less, even in the child Mach-Zehnder modulator 42a with low phase adjustment efficiency, the center voltage Vcc may be set to 7V and the differential voltage Vdc may be set to the range from -7V to 7V as shown in Fig. 5A. However, as shown in Fig. 5B, the phase adjustment range of the child Mach-Zehnder modulator 42b with high phase adjustment efficiency is 8π, from -4π to 4π, which exceeds the predetermined range of 6π. This increases the optical absorption loss.

[0075] 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.

[0076] 6A and 6B are diagrams illustrating the relationship between voltage and the amount of change in optical absorption loss. The horizontal axis represents the voltages (Vp and Vn) applied to the phase adjustment electrodes of the Mach-Zehnder modulator. The vertical axis represents the amount of change in optical absorption loss.

[0077] 6A shows the amount of change in absorption loss in the arm waveguide of the child Mach-Zehnder modulator 42a of the optical modulator 40a-1. 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).

[0078] 6B shows the amount of change in absorption loss in the arm waveguide of the child Mach-Zehnder modulator 42b of the optical modulator 40a-1. 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).

[0079] Comparing the cases where the same voltage is applied in Figures 6A and 6B, the change in absorption loss in Figure 6A is small, while the change in absorption loss in Figure 6B is large. For example, when the voltage is 15V, the change in absorption loss in Figure 6A is less than 1dB. The change in absorption loss in Figure 6B is more than 3dB. The absorption loss increases nonlinearly with voltage, and increases more as the voltage increases.

[0080] 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, the phase adjustment range is set to a predetermined size, such as 6π from −3π to 3π. To achieve this, the center voltage Vcc is set to 7V and the differential voltage Vdc is set to a range from −7V to 7V for both the child Mach-Zehnder modulators 42a and 42b. However, as shown in FIG. 5B, the phase adjustment range of the child Mach-Zehnder modulator 42b exceeds the predetermined range of 6π. The change in absorption loss of the child Mach-Zehnder modulator 42b shown in FIG. 6B is larger than that of the child Mach-Zehnder modulator 42a. That is, if the voltage is determined according to the child Mach-Zehnder modulator 42a with low phase adjustment efficiency, the phase adjustment range becomes excessive and the absorption loss increases in the child Mach-Zehnder modulator 42b with high phase adjustment efficiency. The insertion loss of light increases, and the extinction ratio decreases as described below.

[0081] 4A to 6B, the two child Mach-Zehnder modulators 42a and 42b in one optical modulator 40a-1 have been described. Phase adjustment efficiency also differs between a plurality of optical modulators.

[0082] The optical modulator 40a-2 is a modulator different from the optical modulator 40a-1. The child Mach-Zehnder modulator 42a of the optical modulator 40a-2 has the same phase adjustment efficiency and absorption loss as the child Mach-Zehnder modulator 42a of the optical modulator 40a-1 (see Figs. 4A, 5A, and 6A). The child Mach-Zehnder modulator 42b of the optical modulator 40a-2 has the same phase adjustment efficiency and absorption loss as the child Mach-Zehnder modulator 42b of the optical modulator 40a-1 (see Figs. 4B, 5B, and 6B). The parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 has a higher phase adjustment efficiency and a larger absorption loss than the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1.

[0083] 7A and 7B are diagrams illustrating the relationship between the differential voltage and the amount of phase change. The horizontal axis represents the differential voltage Vdp. The vertical axis represents the amount of phase change Δφ. The center voltage Vcp is 7.4V.

[0084] 7A shows the amount of phase change in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1. The dashed line represents the amount of phase change in the arm waveguide (arm waveguide 55a) on the Ich side. The dotted line represents the amount of phase change in the arm waveguide (arm waveguide 55b) on the Qch side. The solid line represents the amount of phase change in the parent Mach-Zehnder modulator 44a (amount of phase change in arm waveguide 55a-amount of phase change in arm waveguide 55b). When the differential voltage Vdc is in the range of -7V to 7.4V, the amount of phase change is in the range of -2.5π or more and 2.5π or less.

[0085] FIG. 7B shows the parent Mach-Zehnder modulator 44 of the optical modulator 40a-2. a 7B shows the phase change amount in FIG. 7B. The phase adjustment efficiency of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 is higher than that of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1. When the differential voltage Vdc is in the range of −5.5V to 5.1V, the phase change amount is in the range of −2.5π or more and 2.5π or less. The phase change amount in FIG. 7B is larger than that in FIG. 7A. For example, when Vdp=4V, the phase change amount in FIG. 7A is approximately π. The phase change amount in FIG. 7B is approximately 2π.

[0086] As described above, there is a positive correlation between the phase adjustment efficiency and the optical absorption loss in the arm waveguide. Compared to the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1, the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 has a high phase adjustment efficiency and a large absorption loss. When the voltage is determined according to the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1 with a low phase adjustment efficiency, the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 with a high phase adjustment efficiency a In this case, the phase adjustment range becomes larger than the predetermined range (5π), and the absorption loss increases.

[0087] There is a difference in phase adjustment efficiency between the child Mach-Zehnder modulators in the same optical modulator 40a-1. There is also a difference in phase adjustment efficiency between the optical modulator 40a-1 and the optical modulator 40a-2. In order to set the phase change amount to a predetermined magnitude and suppress the increase in the optical absorption loss, it is important to optimize the voltage applied to the phase adjustment electrode for each Mach-Zehnder modulator.

[0088] (Manufacturing method) FIG. 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, a Mach-Zehnder modulator is formed (steps S1 to S3). A cladding layer 82 is epitaxially grown on the upper surface of a wafer (semiconductor substrate 80) by metal organic chemical vapor deposition (MOCVD) or the like. A protruding portion corresponding to an arm waveguide is formed by etching the cladding layer 82. A core layer 84, a cladding layer 86, and a contact layer 88 are epitaxially grown in this order on the protruding portion (step S1). An n-type cladding layer 82, a p-type cladding layer 86, and a contact layer 88 are formed by adding a dopant to the raw material gas. If there is a variation in the amount of thermal diffusion of the dopant, the phase adjustment efficiency also varies as shown in FIG. 4A, FIG. 4B, FIG. 7A, and FIG. 7B.

[0089] 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.

[0090] 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 phase adjustment electrodes 68a and 68b (step S4). The child Mach-Zehnder modulator 42b is tested to optimize the voltages applied to the phase adjustment electrodes 68d and 68e (step S5). The parent Mach-Zehnder modulator 44a is tested to optimize the voltages applied to the phase adjustment electrodes 70a and 70b (step S6). Through the above steps, the optical modulator 40a is formed.

[0091] (test) Fig. 9 is a flow chart illustrating the test. Each of steps S4, S5 and S6 in Fig. 8 is a process for performing the test shown in Fig. 9.

[0092] First, a test of the optical modulator 40a-1 out of the multiple optical modulators 40a will be described. Tests of the child Mach-Zehnder modulator 42a of the optical modulator 40a-1, the child Mach-Zehnder modulator 42b, and the parent Mach-Zehnder modulator 44a are performed in this order.

[0093] When testing the child Mach-Zehnder modulator 42a (step S4 in FIG. 8), the phase control unit 12 of the control unit 10 applies a voltage to the phase adjustment electrode 68b of 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 enter the optical modulator 40a-1 from the wavelength-tunable laser element 22. 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.

[0094] 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).

[0095] The calculation unit 15 optimizes the transmittance so that the transmittance calculated in step S12 approaches the transmittance measured in step S10 (step S14). Based on the optimization of the transmittance, the calculation unit 15 acquires a relationship between the voltage applied to the phase adjustment electrode and the amount of phase change in the arm waveguide (step S16). The memory control unit 18 acquires a voltage that provides a predetermined phase adjustment range 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).

[0096] 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

[0097] An example of the initial values ​​of the coefficients is shown below. k1=3×10 -1 (π / V), k2=3×10 -2 (π / V 2 ), k3=3×10 -3 (π / V 3 ), k4=1×10 -4 (π / V 4 ), k5=1×10 -6 (π / V 5 ), k6=1×10 -8 (π / V 6 )

[0098] 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 7) 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.

[0099] 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

[0100] The initial values ​​of the coefficients a1 and a2 are shown below. a1=1×10 -3 (dB), a2=2(V)

[0101] 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. Since the calculation is performed using the same function (Equation 8) and the same coefficients for the arm waveguides 54a and 54b, the change in absorption loss ΔL1 is also the same, as shown by the solid line.

[0102] 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

[0103] The amount of phase change Δφ is expressed by equation 7. The amount of change in absorption loss ΔL1 is expressed by equation 8. The sign in the cosine function (cos) in equation 9 is positive for the p-side arm waveguide and negative for the n-side arm waveguide. The initial phase difference φ0 is expressed by the following equation. acos in equation 10 is an inverse cosine function.

number

[0104] 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=0.28π.

[0105] 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.

[0106] 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 9 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.

[0107] 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.

[0108] By optimizing the transmittance, the initial phase difference φ0, the phase change amount Δφ, and the absorption loss change amount ΔL1 included in the transmittance equation (Equation 9) are also optimized. The phase change amount Δφ and the absorption loss change amount ΔL1 become functions that more accurately indicate the relationship with the voltage (Step S16 in FIG. 9).

[0109] More specifically, the initial values ​​of the coefficients k1 to k6 in the equation for the phase change amount Δφ (equation 7) and the coefficients a1 and a2 in the equation for the change amount ΔL1 (equation 8) are changed. The coefficients after optimization are shown below. Coefficient for arm waveguide 54a k1=1.32×10 -1 (π / V), k2=1.90×10 -2 (π / V2 ), k3=3.33×10 -3 (π / V 3 ), k4=1.43×10 -4 (π / V 4 ), k5=9.50×10 -7 (π / V 5 ), k6=9.50×10 -8 (π / V 6 ), a1=1×10 -3 (dB), a2=2.5(V) Coefficient for arm waveguide 54b k1=1.40×10 -1 (π / V), k2=2.00×10 -2 (π / V 2 ), k3=3.50×10 -3 (π / V 3 ), k4=1.50×10 -4 (π / V 4 ), k5=1.00×10 -6 (π / V 5 ), k6=1.00×10 -7 (π / V 6 ), a1=1.2×10 -3 (dB), a2=2.4(V) The initial phase difference φ0 after the transmittance is optimized is 0.25π.

[0110] FIG. 12A is a diagram illustrating the amount of phase change after optimization. The horizontal axis, vertical axis, solid line, and dashed line are the same as those corresponding to FIG. 4A. As shown in FIG. 12A, the coefficient obtained by optimizing the transmittance is substituted into Equation 7 for calculation, and the amount of phase change close to that of FIG. 4A is obtained. FIG. 12B is a diagram illustrating the amount of change in absorption loss after optimization. The horizontal axis, vertical axis, solid line, and dashed line are the same as those corresponding to FIG. 6A. As shown in FIG. 12B, the coefficient obtained by optimizing the transmittance is substituted into Equation 8 for calculation, and the amount of change in absorption loss close to that of FIG. 6A is obtained.

[0111] FIG. 13 is a diagram illustrating the relationship between the center voltage and the phase adjustment range. The horizontal axis represents the center voltage Vcc, which is swept from 0 V to 10 V in this example. For each value of the center voltage Vcc, the differential voltage Vdc ranges from -Vcc to Vcc The range is set to 1 / 2. The vertical axis represents the range of the phase change amount (phase adjustment range). The calculation unit 15 applies the optimized coefficients k1 to k6 to Equation 7 to calculate the phase change amount of the arm waveguide 54a and the phase change amount of the arm waveguide 54b for each voltage. The calculation unit 15 calculates the difference between the phase change amount of the arm waveguide 54a and the phase change amount of the arm waveguide 54b to obtain the phase adjustment range in the child Mach-Zehnder modulator 42a. In the child Mach-Zehnder modulator 42a, the phase adjustment range may be 6π (from -3π to 3π). The minimum value of the center voltage Vcc that makes the phase adjustment range 6π is obtained. As shown in FIG. 13, in order to make the phase adjustment range 6π, the center voltage Vcc may be 7V. The storage device 34 shown in FIG. 1B stores the center voltage Vc of the child Mach-Zehnder modulator 42a of the optical modulator 40a-1 as 7V.

[0112] 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).

[0113] 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 acquires a relationship between the voltage applied to the phase adjustment electrode and the amount of phase change (step S16). The memory control unit 18 acquires a voltage that provides a predetermined range of the amount of phase change based on the relationship between the voltage and the amount of phase change, and stores the voltage in the memory device 34 (step S18).

[0114] 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.

[0115] FIG. 15A is a diagram illustrating the amount of phase change after optimization. The horizontal axis, vertical axis, solid line, and dotted line are the same as those corresponding to FIG. 4B. As shown in FIG. 15A, the amount of phase change close to that of FIG. 4B is obtained by optimization. FIG. 15B is a diagram illustrating the amount of change in absorption loss after optimization. The horizontal axis, vertical axis, solid line, and dotted line are the same as those corresponding to FIG. 6B. As shown in FIG. 15B, the amount of change in absorption loss close to that of FIG. 6B is obtained by optimization.

[0116] FIG. 16 is a diagram illustrating the relationship between the center voltage and the phase adjustment range. The horizontal axis represents the center voltage Vcc. The vertical axis represents the range of the phase change amount (phase adjustment range). The calculation unit 15 applies the optimized coefficients k1 to k6 to Equation 7 to calculate the phase adjustment range for each voltage. As shown in FIG. 16, in order to set the phase adjustment range to 6π in the child Mach-Zehnder modulator 42b, the center voltage Vcc needs to be 5.7V. The storage device 34 stores the center voltage Vcc of the child Mach-Zehnder modulator 42b of the optical modulator 40a-1. Vcc Remember this as 5.7V.

[0117] Next, the parent Mach-Zehnder modulator 44a is tested (step S6 in FIG. 8). The phase control unit 12 of the control unit 10 sets the child Mach-Zehnder modulators 42a and 42b to the maximum transmission point. The control unit 10 measures the light transmittance (first transmittance) in the arm waveguide 55a of the parent Mach-Zehnder modulator 44a while sweeping the voltage applied from the ABC circuit 24 to the phase adjustment electrode 70a of the parent Mach-Zehnder modulator 44a. The control unit 10 measures the light transmittance (first transmittance) in the arm waveguide 55b while sweeping the voltage applied to the phase adjustment electrode 70b (step S10 in FIG. 9). The calculation unit 15 of the control unit 10 calculates the light transmittance (second transmittance) in the arm waveguide 55a and the light transmittance (second transmittance) in the arm waveguide 55b (step S12).

[0118] 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 obtains the relationship between the voltage applied to the phase adjustment electrode 70a and the phase change amount in the arm waveguide 55a (step S16). The memory control unit 18 obtains a voltage that provides a predetermined range of phase change amount based on the relationship between the voltage and the phase change amount, and stores the voltage in the memory device 34 (step S18).

[0119] The phase adjustment range of the parent Mach-Zehnder modulator 44a may be, for example, 5π (−2.5π to 2.5π). The storage device 34 stores 7V as the center voltage Vcp at which the phase adjustment range of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1 is 5π.

[0120] Next, a test is performed on the optical modulator 40a-2, which is different from the optical modulator 40a-1. The test process for each Mach-Zehnder modulator is the same as the corresponding test process for the optical modulator 40a-1. It is assumed that the center voltage of the child Mach-Zehnder modulator 42a of the optical modulator 40a-2 is 7V, which is the same as the child Mach-Zehnder modulator 42a of the optical modulator 40a-1. It is assumed that the center voltage of the child Mach-Zehnder modulator 42b of the optical modulator 40a-2 is 5.7V, which is the same as the child Mach-Zehnder modulator 42b of the optical modulator 40a-1.

[0121] A test is also performed on the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2. FIG. 17 is a diagram illustrating the measured transmittance and the transmittance after optimization. The horizontal axis represents the voltage applied to the phase adjustment electrodes 70a and 70b of the parent Mach-Zehnder modulator 44a. The vertical axis represents the transmittance of light. The solid line represents the transmittance after optimization of the arm waveguide (arm waveguide 55a) on the Ich side. The dotted line represents the transmittance after optimization of the arm waveguide (arm waveguide 55b) on the Qch side. The circles represent the measurement results of the transmittance of the arm waveguide 55a. The triangles represent the measurement results of the transmittance of the arm waveguide 55b. The coefficients in Equation 7 and Equation 8 change due to the optimization of the transmittance.

[0122] FIG. 18A is a diagram illustrating the amount of phase change after optimization. The horizontal axis represents the voltage applied to the phase adjustment electrodes 70a and 70b. The vertical axis represents the amount of phase change. The solid line represents the amount of phase change in the arm waveguide 55a. The dashed line represents the amount of phase change in the arm waveguide 55b. FIG. 18B is a diagram illustrating the amount of change in absorption loss after optimization. The horizontal axis represents the voltage applied to the phase adjustment electrodes 70a and 70b. The vertical axis represents the amount of change in absorption loss. The solid line represents the amount of change in the arm waveguide 55a. The dashed line represents the amount of change in the arm waveguide 55b.

[0123] FIG. 19 is a diagram illustrating the relationship between the center voltage and the phase adjustment range. The horizontal axis represents the center voltage Vcp. The vertical axis represents the range of the phase change amount (phase adjustment range). The calculation unit 15 acquires the phase adjustment range in the parent Mach-Zehnder modulator 44a for each voltage. As shown in FIG. 19, in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2, the center voltage Vcp may be 5.7V to set the phase adjustment range to 5π. The storage device 34 stores the center voltage Vc of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 as 5.7V.

[0124] Table 1 is an example of a data table stored in storage device 34. [Table 1]

[0125] In Table 1, Vcc of Ich is the center voltage of the child Mach-Zehnder modulator 42a. Vcc of Qch is the center voltage of the child Mach-Zehnder modulator 42b. Vcp is the center voltage of the parent Mach-Zehnder modulator 44a. In the optical modulators 40a-1 and 40a-2, Vcc of Ich is 7.0V, and Vcc of Qch is 5.7V. Vcp in the optical modulator 40a-1 is 7.4V. Vcp in the optical modulator 40a-2 is 5.7V.

[0126] The storage device 34 stores the center voltage Vcc of the child Mach-Zehnder modulator 42a, the center voltage Vcc of the child Mach-Zehnder modulator 42b, and the center voltage Vcp of the parent Mach-Zehnder modulator 44a in each of the optical modulators 40a-1 and 40a-2. The differential voltage Vcd of the child Mach-Zehnder modulator is set to be -Vcc or more and Vcc or less. The differential voltage Vdp of the parent Mach-Zehnder modulator is set to be -Vcp or more and Vcp or less. The voltages are optimized by the test in FIG. 9. When the optical modulators 40a-1 and 40a-2 are used, the phase adjustment range can be set to a predetermined size and an increase in optical absorption loss can be suppressed by applying the voltage.

[0127] (Absorption loss and extinction ratio of Mach-Zehnder modulator) The amount of absorption loss and the extinction ratio of the child Mach-Zehnder modulator will be described with reference to Fig. 20A to Fig. 22B. Fig. 20A to Fig. 21B show an example in which the test shown in Fig. 9 was performed and the voltage was optimized as shown in Table 1, and correspond to the first embodiment. Fig. 22A and Fig. 22B show an example in which the voltage is not optimized and the same voltage is applied to multiple child Mach-Zehnder modulators.

[0128] 20A, 21A, and 22A are diagrams illustrating the amount of change in absorption loss. The horizontal axis indicates the differential voltage Vdc. The vertical axis indicates the amount of optical absorption loss. The dashed line indicates the amount of absorption loss in the p-side arm waveguide (arm waveguide 54a or 54c). The dotted line indicates the amount of absorption loss in the n-side arm waveguide (arm waveguide 54b or 54d). The solid line indicates the difference in absorption loss between the arm waveguides, ΔL2 (the amount of absorption loss in the p-side arm waveguide−the amount of absorption loss in the n-side arm waveguide).

[0129] 20B, 21B, and 22B are diagrams illustrating the extinction ratio. The horizontal axis indicates the differential voltage Vdc. The vertical axis indicates the extinction ratio. The extinction ratio (ER) is calculated by the following formula.

number

[0130] The smaller the difference ΔL2 in the amount of absorption loss, the larger the extinction ratio ER. The larger the difference ΔL2, the smaller the extinction ratio ER. The larger the difference ΔL2, the less the light can be canceled out even when the light from the two arm waveguides is multiplexed in antiphase. As a result, the extinction ratio ER becomes smaller.

[0131] FIG. 20A shows the amount of absorption loss in the child Mach-Zehnder modulator 42a of the optical modulator 40a-1 when the center voltage Vcc=7V. The differential voltage Vdc has a value in the range of −6.8V to 7V. As the differential voltage Vdc becomes more negative, the amount of absorption loss in the arm waveguide 54b increases, and the amount of absorption loss in the arm waveguide 54a approaches 0. The difference in absorption loss ΔL2 becomes more negative. As the differential voltage Vdc becomes more positive, the amount of absorption loss in the arm waveguide 54a increases, and the amount of absorption loss in the arm waveguide 54b approaches 0. The difference in absorption loss ΔL2 becomes more positive. The maximum absolute value of the difference in absorption loss ΔL2 is 0.38 dB.

[0132] FIG. 20B shows the extinction ratio of the child Mach-Zehnder modulator 42a of the optical modulator 40a-1. The extinction ratio ER in FIG. 20B is calculated from the difference ΔL2 in FIG. 20A. As the differential voltage Vdc becomes larger on the positive side and the negative side, the extinction ratio ER decreases. The minimum value of the extinction ratio ER is 33.3 dB.

[0133] 21A shows the amount of absorption loss in the child Mach-Zehnder modulator 42b of the optical modulator 40a-1 when the center voltage Vcc is 5.7 V. The differential voltage Vdc has a value in the range of −5.8 V to 5.5 V. The maximum absolute value of the difference ΔL2 in the amount of absorption loss is 0.50 dB.

[0134] Fig. 21B shows the extinction ratio in the child Mach-Zehnder modulator 42b of the optical modulator 40a-1. The extinction ratio ER in Fig. 21B is calculated from the difference ΔL2 in Fig. 21A. The minimum value of the extinction ratio ER is 30.8 dB.

[0135] 22A shows the amount of absorption loss in the child Mach-Zehnder modulator 42b of the optical modulator 40a-1 when the center voltage Vcc is 7 V. The maximum absolute value of the difference ΔL2 in the amount of absorption loss is 0.88 dB.

[0136] Fig. 22B shows the extinction ratio in the child Mach-Zehnder modulator 42b of the optical modulator 40a-1. The extinction ratio ER in Fig. 22B is calculated from the difference ΔL2 in Fig. 22A. The minimum value of the extinction ratio ER is 26.0 dB.

[0137] As shown in FIGS. 22A and 22B, when the center voltage of the child Mach-Zehnder modulator 42b is made equal to the center voltage of the child Mach-Zehnder modulator 42a, the amount of absorption loss increases and the extinction ratio decreases.

[0138] 21A and 21B, according to the first embodiment, the amount of optical absorption loss can be reduced by optimizing the center voltage of the child Mach-Zehnder modulator 42b, and the decrease in the extinction ratio can be suppressed. In both the child Mach-Zehnder modulators 42a and 42b, an extinction ratio of 30 dB or more can be obtained.

[0139] (Absorption loss and extinction ratio of parent Mach-Zehnder modulator) The amount of absorption loss and extinction ratio of the parent Mach-Zehnder modulator will be described with reference to Figures 23A to 25B. Figures 23A to 24B show an example in which the test shown in Figure 9 was performed and the voltage was optimized as shown in Table 1. Figures 25A and 25B show an example in which the voltage is not optimized and the same voltage is applied to multiple parent Mach-Zehnder modulators.

[0140] 23A, 24A, and 25A are diagrams illustrating the amount of absorption loss. The horizontal axis indicates the differential voltage Vdp. The vertical axis indicates the amount of light absorption loss. The dashed line indicates the amount of absorption loss of the Ich-side arm waveguide (arm waveguide 55a). The dotted line indicates the amount of absorption loss of the Qch-side arm waveguide (arm waveguide 55b). The solid line indicates the difference ΔL2 in the amount of absorption loss between the arm waveguides (absorption loss amount of the Ich-side arm waveguide-absorption loss amount of the Qch-side arm waveguide). FIG. 23B, 24B, and 25B are diagrams illustrating the extinction ratio. The horizontal axis indicates the differential voltage Vdp. The vertical axis indicates the extinction ratio.

[0141] 23A shows the amount of absorption loss in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1 when the center voltage Vcp is 7.4 V. The differential voltage Vdp takes a value in the range of −7 V to 7.4 V. The maximum absolute value of the difference ΔL2 in the amount of absorption loss is 0.53 dB.

[0142] Fig. 23B shows the extinction ratio in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1. The extinction ratio ER in Fig. 23B is calculated from the difference ΔL2 in Fig. 23A. The minimum value of the extinction ratio ER is 30.3 dB.

[0143] 24A shows the amount of absorption loss in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 when the center voltage Vcp is 5.7 V. The maximum absolute value of the difference ΔL2 in the amount of absorption loss is 0.44 dB.

[0144] Fig. 24B shows the extinction ratio in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2. The extinction ratio ER in Fig. 24B is calculated from the difference ΔL2 in Fig. 24A. The minimum value of the extinction ratio ER is 31.9 dB.

[0145] 25A shows the amount of absorption loss in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 when the center voltage Vcp is 7.4 V. The differential voltage Vdp has a value in the range of −5.5 V to 5.1 V. The maximum absolute value of the difference ΔL2 in the amount of absorption loss is 0.89 dB.

[0146] Fig. 25B shows the extinction ratio in the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2. The extinction ratio ER in Fig. 25B is calculated from the difference ΔL2 in Fig. 25A. The minimum value of the extinction ratio ER is 25.8 dB.

[0147] As shown in FIGS. 25A and 25B, if the center voltage of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 is made equal to the center voltage of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1, the amount of absorption loss increases and the extinction ratio decreases.

[0148] 24A and 24B, according to the first embodiment, the amount of optical absorption loss can be reduced by optimizing the center voltage of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2, and the decrease in the extinction ratio can be suppressed. In the parent Mach-Zehnder modulators 44a of both the optical modulators 40a-1 and 40a-2, an extinction ratio of 30 dB or more can be obtained.

[0149] According to the first embodiment, the control unit 10 obtains the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change, and obtains the voltage that provides a predetermined phase adjustment range. The phase of light in the Mach-Zehnder modulator is adjusted with the voltage optimized for each Mach-Zehnder modulator. The phase adjustment range can be set to a predetermined size, and an increase in light absorption loss can be suppressed.

[0150] The voltage is optimized for each of the Mach-Zehnder modulators in one optical modulator, such as the child Mach-Zehnder modulators 42a and 42b in the optical modulator 40a-1. In each Mach-Zehnder modulator, the phase adjustment range is set to a predetermined size, and the increase in the optical absorption loss can be suppressed. For example, the center voltage Vcc of the child Mach-Zehnder modulator 42a is set to 7V, and the center voltage Vcc of the child Mach-Zehnder modulator 42b is set to 5.7V. As shown in FIG. 13 and FIG. 16, the phase adjustment range can be set to 6π in both the child Mach-Zehnder modulators 42a and 42b. As shown in FIG. 20A to FIG. 21B, the extinction ratio can be set to 30 dB or more by suppressing the increase in the absorption loss.

[0151] Voltages are optimized in a plurality of optical modulators, such as the optical modulator 40a-1 and the optical modulator 40a-2. The central voltage Vcp of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-1 is set to 7V, and the central voltage Vcp of the parent Mach-Zehnder modulator 44a of the optical modulator 40a-2 is set to 5.7V. In the two parent Mach-Zehnder modulators 44a, the phase adjustment range can be set to 5π. As shown in Figs. 23A to 24B, the extinction ratio can be set to 30 dB or more by suppressing an increase in absorption loss.

[0152] The phase adjustment range of the child Mach-Zehnder modulator is, for example, a range of 6π from -3π to 3π, but may be 6π or more or 6π or less. The phase adjustment range of the parent Mach-Zehnder modulator is, for example, 5π from -2.5π to 2.5π, but may be 5π or more or 5π or less. The phase adjustment range may be an appropriate size depending on, for example, the initial phase difference φ0. The phase adjustment range of the child Mach-Zehnder modulator may be, for example, 5π or 7π. The phase adjustment range of the parent Mach-Zehnder modulator may be, for example, 4π or 6π.

[0153] The child Mach-Zehnder modulator 42a has two paired arm waveguides 52a and 52b. The phase adjustment electrode 68a is provided in the arm waveguide 52a. The phase adjustment electrode 68b is provided in the arm waveguide 52b. The control unit 10 acquires the relationship between the voltage applied to the phase adjustment electrode 68a and the amount of phase change in the arm waveguide 52a, and the relationship between the voltage applied to the phase adjustment electrode 68b and the amount of phase change in the arm waveguide 52b (FIG. 12A). Based on the relationship between the voltage and the amount of phase change, the control unit 10 can acquire a voltage that provides a phase adjustment range of 6π in the child Mach-Zehnder modulator 42a, as shown in FIG. 13. The control unit 10 also acquires the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change in the arm waveguide in the child Mach-Zehnder modulator 42b and the parent Mach-Zehnder modulator 44a, and acquires a voltage that provides a desired phase adjustment range based on the relationship.

[0154] The Mach-Zehnder modulator is differentially driven. The voltage Vp applied to the child Mach-Zehnder modulator is Vcc+Vdc, and the voltage Vn is Vcc-Vdc. The voltage VI applied to the parent Mach-Zehnder modulator is Vcp+Vdp, and the voltage VQ is Vcp-Vdp. In step S18 of FIG. 9, the control unit 10 acquires the center voltages Vcc and Vcp. As shown in FIG. 13, FIG. 16, and FIG. 19, the phase adjustment range becomes a predetermined size by acquiring the optimal center voltage. As shown in Table 1, the storage device 34 stores the optimal center voltages Vcc and Vcp for each Mach-Zehnder modulator. When driving the Mach-Zehnder modulator, the control unit 10 acquires the center voltage stored in the storage device 34, and calculates the sum and difference between the center voltage and the differential voltage to acquire the voltages Vp, Vn, VI, and VQ. The ABC circuit 24 applies the voltage to the phase adjustment electrode. In each Mach-Zehnder modulator, a predetermined phase adjustment range can be obtained and an increase in absorption loss can be suppressed. In the first embodiment, the center voltage in differential driving is optimized. The Mach-Zehnder modulator may be driven by a method other than differential driving. Regardless of the driving method, the phase adjustment range of the Mach-Zehnder modulator can be controlled with an optimal voltage and an increase in absorption loss can be suppressed.

[0155] The differential voltage Vdc of the child Mach-Zehnder modulator is set to be equal to or higher than -Vcc and equal to or lower than Vcc. The differential voltage Vdp of the parent Mach-Zehnder modulator is set to be equal to or higher than -Vcp and equal to or lower than Vcp. The differential voltages may be changed from those described above.

[0156] In step S12 of FIG. 9, the transmittance is calculated as a function of the amount of phase change (Equation 9). The transmittance is fitted to bring the calculated transmittance closer to the measured transmittance. The amount of phase change is also optimized by optimizing the transmittance. The amount of phase change shown in Equation 7 is a function of voltage. The coefficient in Equation 7 changes by fitting the transmittance, and the relationship between the voltage and the amount of phase change becomes more accurate. The control unit 10 obtains a voltage that provides a predetermined size of the phase adjustment range based on the amount of phase change. It is possible to set the phase adjustment range to a predetermined size 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.

[0157] 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 amount of thermal diffusion of the dopant cause variations in the phase adjustment efficiency of the Mach-Zehnder modulator. According to the first embodiment, a voltage that provides a predetermined phase adjustment range is obtained for each Mach-Zehnder modulator. The phase adjustment range can be set to a predetermined size and an increase in absorption loss can be suppressed.

[0158] In the steps of Fig. 8 and Fig. 9, the optical transmission device 100 of Fig. 1A is used as a test device for the optical modulator 40. One optical modulator 40 (for example, the optical modulator 40a-1) is incorporated into the optical transmission device 100 and a test is performed. The optical modulator 40a-1 is replaced with the optical modulator 40a-2 and a test is performed. The storage device 34 stores the voltages for both the optical modulators 40a-1 and 40a-2 as shown in Table 1. When the optical transmission device 100 is used for communication, a test may be performed on one optical modulator 40a included in the optical transmission device 100. The storage device 34 may store only the voltage for that one optical modulator 40a.

[0159] <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.

[0160] 26 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.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] Table 2 is an example of a data table stored in the storage device 34. The storage device 34 stores the voltages of each of the multiple optical modulators 40b (optical modulators 40b-1 and 40b-2 in Table 2). [Table 2]

[0167] The Vcc of XI is the center voltage of the child Mach-Zehnder modulator 42a of the optical modulator 43a on the Xch side of the optical modulator 40b. The Vcc of XQ is the center voltage of the child Mach-Zehnder modulator 42b of the optical modulator 43a. The Vcc of YI is the center voltage of the child Mach-Zehnder modulator 42c of the optical modulator 43b on the Qch side. The Vcc of YQ is the center voltage of the child Mach-Zehnder modulator 42d of the optical modulator 43b. The Vcp of Xch is the center voltage of the parent Mach-Zehnder modulator 44a. The Vcp of Ych is the center voltage of the parent Mach-Zehnder modulator 44b. The voltages in the optical modulator 40b-1 are, for example, 5.8V, 6.2V, 6.0V, 6.1V, 5.9V, and 6.0V. The specific values ​​of the voltages in the optical modulator 40b-2 are omitted.

[0168] 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 phase adjustment range to a predetermined size and suppress an increase in optical absorption loss.

[0169] 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.

[0170] 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]

[0171] 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, 43a, 43b Optical modulator 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 and a voltage to be applied to the second electrode, based on the first relationship and the second relationship, such that a range of a difference between an amount of phase change in the first arm waveguide and an amount of phase change in the second arm waveguide becomes a predetermined size; 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, wherein the step of acquiring the first relationship and the second relationship and the step of acquiring the voltage applied to the first electrode and the voltage applied to the second electrode are performed for each of the plurality of Mach-Zehnder modulators.

3. providing the Mach-Zehnder modulator includes providing a parent Mach-Zehnder modulator and providing a child Mach-Zehnder modulator; 3. The method for manufacturing an optical modulator according to claim 1, wherein the step of acquiring the first relationship and the second relationship and the step of acquiring the voltage applied to the first electrode and the voltage applied to the second electrode are performed for each of the parent Mach-Zehnder modulator and the child Mach-Zehnder modulator.

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; 2 . The method for manufacturing an optical modulator according to claim 1 , wherein the step of acquiring the voltage is a step of acquiring the first voltage that provides the range of the amount of change in the phase with the predetermined magnitude.

5. A method for manufacturing an optical modulator as described in Claim 4, wherein, in the process of acquiring the voltage, the minimum value of the first voltage that results in a range of the phase change amount of the specified magnitude is acquired.

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, expressing the second transmittance in the first arm waveguide as a function of a change in a phase of light propagating through the first arm waveguide and expressing the change in the phase of the light propagating through the first arm waveguide as a function of a voltage applied to the first electrode, thereby calculating the second transmittance in the first arm waveguide; expressing the 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 second electrode, thereby calculating the second transmittance in the second arm waveguide; 6. The method for manufacturing an optical modulator according to claim 1, wherein, in the step of acquiring the first relationship and the second relationship, the second transmittance is adjusted so as to approach the first transmittance in each of the first arm waveguide and the second arm waveguide, thereby acquiring the first relationship and the second relationship.

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 and a voltage to be applied to the second electrode such that a range of a difference between an amount of phase change in the first arm waveguide and an amount of phase change in the second arm waveguide becomes a predetermined size based on the first relationship and the second relationship.

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 and a voltage to be applied to the second electrode such that a range of a difference between an amount of phase change in the first arm waveguide and an amount of phase change in the second arm waveguide becomes a predetermined size 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 range of a difference between an amount of phase change in the first arm waveguide and an amount of phase change in the second arm waveguide becomes a predetermined size.

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