Method for manufacturing optical modulator, test method, storage medium, and optical transmission device

Abstract

Provided are a manufacturing method, a test method, a storage medium, and an optical transmission device for an optical modulator capable of suppressing an increase in absorption loss of light. A manufacturing method for an optical modulator having a Mach-Zehnder modulator with an electrode provided to a branch waveguide, the manufacturing method including: preparing the Mach-Zehnder modulator; acquiring a relationship between a voltage applied to the electrode and a change amount of a phase of light propagating in the branch waveguide, based on a transmittance of light in the branch waveguide; acquiring a voltage that makes a range of the change amount of the phase of the light in the Mach-Zehnder modulator a predetermined size, based on the relationship; and storing the voltage in a storage section.

Description

Technical Field

[0001] This disclosure relates to methods for manufacturing optical modulators, testing methods, storage media, and optical transmitting devices. Background Technology

[0002] A Mach-Zehnder modulator formed by a semiconductor layer and modulating light was developed (Patent Document 1 and Patent Document 2).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2014-164243

[0006] Patent Document 2: Japanese Patent Application Publication No. 2016-111398 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] Light propagates in the branch waveguide of a Mach-Zehnder modulator. By applying a voltage to the Mach-Zehnder modulator, the phase of the light can be adjusted. In order to adjust the phase to the desired magnitude, it is important to set the range of phase change caused by the application of the voltage (the phase adjustment range) to a predetermined size.

[0009] The ratio of phase change to voltage (phase adjustment efficiency) varies in each Mach-Zehnder modulator. When the same voltage is applied to multiple Mach-Zehnder modulators, some exhibit larger phase changes than others. Even in Mach-Zehnder modulators with lower phase adjustment efficiency, the voltage can be increased to achieve a predetermined phase adjustment range. However, there is a positive correlation between phase adjustment efficiency and light absorption loss. Increasing the voltage increases light absorption loss. Therefore, the object of this invention is to provide a method for manufacturing an optical modulator, a testing method, a storage medium, and an optical transmitting device capable of suppressing the increase in light absorption loss.

[0010] means for solving problems

[0011] The manufacturing method disclosed herein relates to a method for manufacturing an optical modulator having a Mach-Zehnder modulator having electrodes and branch waveguides, the electrodes being disposed in the branch waveguides. The manufacturing method includes: a preparation step in which the Mach-Zehnder modulator is prepared; a relationship acquisition step in which a relationship is acquired between a voltage applied to the electrodes and a change in the phase of light propagating in the branch waveguides, based on the transmittance of light in the branch waveguides; a voltage acquisition step in which a voltage is acquired, based on the relationship, such that the range of the change in the phase of light in the Mach-Zehnder modulator is a predetermined value; and a storage step in which the voltage is stored in a storage unit.

[0012] The testing method disclosed herein is a testing method for an optical modulator having a Mach-Zehnder modulator having electrodes and branch waveguides, the electrodes being disposed in the branch waveguides. The testing method comprises: a relationship acquisition step, in which a relationship is acquired based on the transmittance of light in the branch waveguides, between a voltage applied to the electrodes and the amount of phase change of light propagating in the branch waveguides; and a voltage acquisition step, in which a voltage is acquired based on the relationship, such that the range of the amount of phase change of light in the Mach-Zehnder modulator is a predetermined value.

[0013] The storage medium disclosed herein is a storage medium for storing test programs of an optical modulator having a Mach-Zehnder modulator having electrodes and branch waveguides, the electrodes being disposed in the branch waveguides, the test program causing a computer to perform the following processing: based on the transmittance of light in the branch waveguides, obtaining a relationship between a voltage applied to the electrodes and the amount of phase change of light propagating in the branch waveguides; and based on the relationship, obtaining a voltage that makes the range of the amount of phase change of light in the Mach-Zehnder modulator a predetermined size.

[0014] The optical transmitting apparatus disclosed herein includes a storage unit and a plurality of Mach-Zehnder modulators, the plurality of Mach-Zehnder modulators having electrodes and branch waveguides, the electrodes being disposed on the branch waveguides, and the storage unit storing, for each of the plurality of Mach-Zehnder modulators, a voltage that causes the range of changes in the phase of light to be of a predetermined magnitude.

[0015] Invention Effects

[0016] According to this disclosure, it is possible to suppress the increase in light absorption loss. Attached Figure Description

[0017] Figure 1A This is a block diagram illustrating an optical transmitting apparatus according to the first embodiment.

[0018] Figure 1B It is a block diagram representing the hardware structure of the control unit.

[0019] Figure 2A This is a top view illustrating an optical modulator.

[0020] Figure 2B It is along Figure 2A A sectional view of line AA.

[0021] Figure 3 This is a diagram illustrating the relationship between differential voltage and phase change in a sub-Mach-Zehnder modulator.

[0022] Figure 4A This is a graph illustrating the relationship between voltage and phase change.

[0023] Figure 4B This is a graph illustrating the relationship between voltage and phase change.

[0024] Figure 5A This is a graph illustrating the relationship between differential voltage and phase change.

[0025] Figure 5B This is a graph illustrating the relationship between differential voltage and phase change.

[0026] Figure 6A This is a graph illustrating the relationship between voltage and the change in light absorption loss.

[0027] Figure 6B This is a graph illustrating the relationship between voltage and the change in light absorption loss.

[0028] Figure 7A This is a graph illustrating the relationship between differential voltage and phase change.

[0029] Figure 7B This is a graph illustrating the relationship between differential voltage and phase change.

[0030] Figure 8 This is a flowchart illustrating an example of a method for manufacturing an optical modulator.

[0031] Figure 9 This is a flowchart illustrating an experiment.

[0032] Figure 10A This is a diagram illustrating the calculated phase change.

[0033] Figure 10BThis is a graph illustrating the calculated change in absorption loss.

[0034] Figure 11A This is a graph illustrating the calculated transmittance.

[0035] Figure 11B The graph shows an example of the measured transmittance and the optimized transmittance.

[0036] Figure 12A This is a diagram illustrating the optimized phase change.

[0037] Figure 12B This is a graph illustrating the change in absorption loss after optimization.

[0038] Figure 13 This is a diagram illustrating the relationship between center voltage and phase adjustment range.

[0039] Figure 14 The graph shows an example of the measured transmittance and the optimized transmittance.

[0040] Figure 15A This is a diagram illustrating the optimized phase change.

[0041] Figure 15B This is a graph illustrating the change in absorption loss after optimization.

[0042] Figure 16 This is a diagram illustrating the relationship between center voltage and phase adjustment range.

[0043] Figure 17 The graph shows an example of the measured transmittance and the optimized transmittance.

[0044] Figure 18A This is a diagram illustrating the optimized phase change.

[0045] Figure 18B This is a graph illustrating the change in absorption loss after optimization.

[0046] Figure 19 This is a diagram illustrating the relationship between center voltage and phase adjustment range.

[0047] Figure 20A This is a diagram illustrating the amount of absorption loss.

[0048] Figure 20B This is a graph illustrating the extinction ratio.

[0049] Figure 21A This is a diagram illustrating the amount of absorption loss.

[0050] Figure 21BThis is a graph illustrating the extinction ratio.

[0051] Figure 22A This is a diagram illustrating the amount of absorption loss.

[0052] Figure 22B This is a graph illustrating the extinction ratio.

[0053] Figure 23A This is a diagram illustrating the amount of absorption loss.

[0054] Figure 23B This is a graph illustrating the extinction ratio.

[0055] Figure 24A This is a diagram illustrating the amount of absorption loss.

[0056] Figure 24B This is a graph illustrating the extinction ratio.

[0057] Figure 25A This is a diagram illustrating the amount of absorption loss.

[0058] Figure 25B This is a graph illustrating the extinction ratio.

[0059] Figure 26 This is a top view illustrating an optical modulator. Detailed Implementation

[0060] [Description of embodiments of this disclosure]

[0061] First, the contents of the embodiments of this disclosure will be described.

[0062] One aspect of this disclosure is (1) a method for manufacturing an optical modulator having a Mach-Zehnder modulator having electrodes and branch waveguides, the electrodes being disposed in the branch waveguides, the manufacturing method comprising: a preparation step in which the Mach-Zehnder modulator is prepared; a relationship acquisition step in which a relationship between a voltage applied to the electrodes and a change in the phase of light propagating in the branch waveguide is acquired based on the transmittance of light in the branch waveguide; a voltage acquisition step in which a voltage is acquired based on the relationship such that the range of the change in the phase of light in the Mach-Zehnder modulator is a predetermined size; and a storage step in which the voltage is stored in a storage unit. By applying the acquired voltage to the Mach-Zehnder modulator, the range of the change in phase can be set to a predetermined size, and the increase in light absorption loss can be suppressed.

[0063] (2) Alternatively, the process of preparing the Mach-Zehnder modulator may involve preparing multiple Mach-Zehnder modulators, and for each of the multiple Mach-Zehnder modulators, performing a process of obtaining the relationship between the voltage and the change in phase, and a process of obtaining the voltage. For each of the multiple Mach-Zehnder modulators, the voltage is optimized. By applying the optimized voltage to the Mach-Zehnder modulator, the range of the phase change can be set to a predetermined size, and the increase in light absorption loss can be suppressed.

[0064] (3) Alternatively, the process of preparing the Mach-Zehnder modulator may include a process of preparing a mother Mach-Zehnder modulator and a process of preparing a daughter Mach-Zehnder modulator, wherein the mother Mach-Zehnder modulator and the daughter Mach-Zehnder modulator are respectively subjected to a process of obtaining the relationship between the voltage and the change in phase and a process of obtaining the voltage. This allows the range of the phase change of the mother Mach-Zehnder modulator and the daughter Mach-Zehnder modulator to be set to a predetermined size, and suppresses the increase in light absorption loss.

[0065] (4) Alternatively, the process of preparing the Mach-Zehnder modulator may include preparing the Mach-Zehnder modulator having a first branch waveguide, a second branch waveguide, a first electrode, and a second electrode, wherein the first electrode is disposed in the first branch waveguide, the second electrode is disposed in the second branch waveguide, and the process of obtaining the relationship between the voltage and the change in phase includes obtaining the relationship between the voltage applied to the first electrode and the change in phase of light propagating in the first branch waveguide, and obtaining the relationship between the voltage applied to the second electrode and the change in phase of light propagating in the second branch waveguide. The range of the change in phase of light in the Mach-Zehnder modulator is the range of the difference between the change in phase in the first branch waveguide and the change in phase in the second branch waveguide. The process of obtaining the voltage is to obtain the voltage applied to the first electrode and the voltage applied to the second electrode such that the range of the change in phase is the predetermined size. This allows the range of the change in phase to be set to a predetermined size and suppresses the increase in light absorption loss.

[0066] (5) Alternatively, the voltage applied to the first electrode is the sum of a first voltage and a second voltage, and the voltage applied to the second electrode is the difference between the first voltage and the second voltage. The step of obtaining the voltage is to obtain a first voltage such that the range of the phase change is the predetermined size. The first voltage is used as the center voltage, and the second voltage is used as the differential voltage to differentially drive the Mach-Zehnder modulator. This allows the range of phase change to be set to a predetermined size and suppresses the increase in light absorption loss.

[0067] (6) Alternatively, the method for manufacturing the optical modulator may further include: a step of measuring a first transmittance, which is the transmittance of light in the branch waveguide; and a step of calculating a second transmittance, which is the transmittance of light in the branch waveguide. In the step of calculating the second transmittance, the second transmittance is calculated by expressing the second transmittance as a function of the phase change of light propagating in the branch waveguide, and expressing the phase change of light propagating in the branch waveguide as a function of the voltage applied to the electrode. In the step of obtaining the relationship between the voltage and the phase change, the relationship between the voltage and the phase change is obtained by adjusting the second transmittance in a manner close to the first transmittance. By making the second transmittance close to the first transmittance, a more accurate relationship between the voltage and the phase change can be obtained. The range of the phase change can be set to a predetermined size, and the increase in light absorption loss can be suppressed.

[0068] (7) Alternatively, the process of preparing the Mach-Zehnder modulator may include the process of forming the Mach-Zehnder modulator, which in turn includes the process of forming the branch waveguide having a first semiconductor layer, a core layer, and a second semiconductor layer, wherein the first semiconductor layer, the core layer, and the second semiconductor layer are stacked sequentially, the first semiconductor layer having a first conductivity type, and the second semiconductor layer having a second conductivity type. Dopants are added to the first and second semiconductor layers. Due to deviations in the thermal diffusion of the dopants, the phase adjustment efficiency of the Mach-Zehnder modulator also deviates. By applying the acquired voltage to the Mach-Zehnder modulator, the range of phase change can be set to a predetermined size, and the increase in light absorption loss can be suppressed.

[0069] (8) A method for testing an optical modulator, wherein the optical modulator has a Mach-Zehnder modulator having electrodes and branch waveguides, the electrodes being disposed in the branch waveguides, the testing method comprising: a relationship acquisition step, wherein, based on the transmittance of light in the branch waveguide, a relationship is acquired between a voltage applied to the electrodes and a change in the phase of light propagating in the branch waveguide; and a voltage acquisition step, wherein, based on the relationship, a voltage is acquired such that the range of the change in the phase of light in the Mach-Zehnder modulator is a predetermined value. By applying the acquired voltage to the Mach-Zehnder modulator, the range of the change in phase can be set to a predetermined value, and an increase in light absorption loss can be suppressed.

[0070] (9) A storage medium storing a test program for an optical modulator, wherein the optical modulator has a Mach-Zehnder modulator having electrodes and a branch waveguide, the electrodes being disposed in the branch waveguide, and the test program causing a computer to perform the following processing: obtaining a relationship between a voltage applied to the electrodes and a change in the phase of light propagating in the branch waveguide, based on the transmittance of light in the branch waveguide; and obtaining, based on the relationship, a voltage such that the range of the change in the phase of light in the Mach-Zehnder modulator is a predetermined size. By applying the obtained voltage to the Mach-Zehnder modulator, the range of the change in phase can be set to a predetermined size, and an increase in light absorption loss can be suppressed.

[0071] (10) An optical transmitting device, wherein the optical transmitting device includes a storage unit and a plurality of Mach-Zehnder modulators, the plurality of Mach-Zehnder modulators having electrodes and branch waveguides, the electrodes being disposed in the branch waveguides, and the storage unit storing, for each of the plurality of Mach-Zehnder modulators, a voltage that causes a range of phase change of light to be of a predetermined size. By applying the stored voltage to the Mach-Zehnder modulators, the range of phase change can be set to a predetermined size, and an increase in light absorption loss can be suppressed.

[0072] [Details of the embodiments disclosed herein]

[0073] Hereinafter, specific examples of the manufacturing method, testing method, testing procedure, and optical transmitting device of the optical modulator involved in the embodiments of this disclosure will be described with reference to the accompanying drawings. It should be noted that this disclosure is not limited to these examples, but is shown by way of technical solutions and is intended to include all modifications within the scope and meaning equivalent to the technical solutions.

[0074] <First Implementation Method>

[0075] (Optical transmission device)

[0076] Figure 1A This is a block diagram illustrating the optical transmitting apparatus 100 according to the first embodiment. Figure 1A As shown, the optical transmitting device 100 includes a control unit 10, a wavelength-variable laser element 22, an automatic bias control (ABC) circuit 24, a driver IC (Integrated Circuit) 26, and an optical modulator 40.

[0077] The wavelength-variable laser element 22 is, for example, a light-emitting element including a semiconductor laser element. The ABC circuit 24 applies a voltage for phase adjustment to the optical modulator 40, performing 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 wavelength-variable laser element 22 and emits modulated light. The control unit 10 includes, for example, a computer such as a personal computer (PC).

[0078] Figure 1B This is a block diagram showing the hardware structure of the control unit 10. For example... Figure 1B As shown, the control unit 10 includes a CPU (Central Processing Unit) 30, RAM (Random Access Memory) 32, a storage device 34 (storage unit), and an interface 36. The CPU 30, RAM 32, storage device 34, and interface 36 are interconnected via a bus or the like. RAM 32 is volatile memory that temporarily stores programs and data. Storage device 34 is, for example, ROM (Read Only Memory), a solid-state drive (SSD) such as flash memory, or a hard disk drive (HHD). Storage device 34 stores programs used to execute the processes described later, as well as voltages obtained through the processes.

[0079] The control unit 10 implements the program stored in RAM 32 by executing the program by CPU 30. Figure 1A The diagram shows a phase control unit 12, a laser control unit 14, a calculation unit 15, a modulation control unit 16, and a storage control unit 18. The phase control unit 12 controls the ABC circuit 24 and adjusts the voltage applied to the optical modulator 40 by the ABC circuit 24. The laser control unit 14 controls the wavelength-variable laser element 22. The calculation unit 15 calculates transmittance, phase change amount, and phase adjustment range, as described later. The modulation control unit 16 controls the driver IC 26. The storage control unit 18... Figure 1B The RAM 32 and storage device 34 shown are controlled to store data. The various parts of the control unit 10 can also be hardware such as circuits.

[0080] (Modulator)

[0081] Figure 2A This is a top view illustrating an optical modulator 40a. In the first embodiment, the optical modulator 40a is used as... Figure 1AThe optical modulator 40a is an IQ (In-phase Quadrature modulator) modulator, comprising a substrate 41, sub-Mach-Zehnder modulators 42a and 42b, and a master Mach-Zehnder modulator 44a. The substrate 41 is, for example, an insulating substrate formed of ceramic or the like. Alternatively, the substrate 41 can be provided with... Figure 1A The ABC circuit 24, driver IC 26, and lens (not shown) are used to form a module containing an optical modulator 40a.

[0082] A semiconductor substrate 80, terminating element 78a, and terminating element 78b are mounted on the upper surface of substrate 41. Terminating elements 78a and 78b include, for example, terminating resistors and capacitors. Sub-Mach-Zehnder modulators 42a and 42b, a mother Mach-Zehnder modulator 44a, an input waveguide 50, and an output waveguide 56 are formed on the semiconductor substrate 80. The semiconductor substrate 80 has four end faces: end face 80a, end face 80b, end face 80c, and end face 80d. End faces 80a and 80b are opposite each other. End faces 80c and 80d are opposite each other.

[0083] The first end of the input waveguide 50 is located at end face 80a, one of the four end faces of the semiconductor substrate 80. The second end of the input waveguide 50 is connected to coupler 58. The first end of the output waveguide 56 is connected to coupler 64. The second end of the output waveguide 56 is located at end face 80b, one of the four end faces of the semiconductor substrate 80. Coupler 58 is a 1-input, 2-output (1×2) multimode interference (MMI) coupler. Coupler 64 is a 2-input, 1-output (2×1) MMI coupler. Sub-Mach-Zehnder modulators 42a and 42b are connected in parallel between coupler 58 and coupler 64. A mother Mach-Zehnder modulator 44a is connected between sub-Mach-Zehnder modulators 42a and 42b and coupler 64.

[0084] (Sub-Mach-Zehnder modulator)

[0085] Sub-Mach-Zehnder modulator 42a is, for example, a modulator on the Ich side. Sub-Mach-Zehnder modulator 42b is, for example, a modulator on the Qch side. Sub-Mach-Zehnder modulator 42a has branch waveguides 52a, 54a, and 54b, modulation electrodes 66a and 66b, phase adjustment electrodes 68a and 68b, and ground electrodes 66c and 68c. Branch waveguide 54a is, for example, a p-side waveguide. Branch waveguide 54b is, for example, an n-side waveguide.

[0086] The first end of branch waveguide 52a is connected to the first output terminal of coupler 58. The second end of branch waveguide 52a is connected to the input terminal of coupler 60a. The first end of branch waveguide 54a (first branch waveguide) is connected to the first output terminal of coupler 60a. The second end of branch waveguide 54a is connected to the first input terminal of coupler 62a. The first end of branch waveguide 54b (second branch waveguide) is connected to the second output terminal of coupler 60a. The second end of branch waveguide 54b is connected to the second input terminal of coupler 62a.

[0087] Branch waveguide 52a is bent on the coupler 58 side. Branch waveguides 54a and 54b are bent on the coupler 60a side and on the coupler 62a side. Outside of these bent portions, branch waveguides 52a, 54a, and 54b are parallel to each other and parallel to the end face 80c of the semiconductor substrate 80.

[0088] Modulation electrode 66a and phase adjustment electrode 68a are disposed on branch waveguide 54a. Modulation electrode 66a and phase adjustment electrode 68a (first electrode) are separate from each other and are arranged sequentially from coupler 60a side to coupler 62a side. Modulation electrode 66b and phase adjustment electrode 68b are disposed on branch waveguide 54b. Modulation electrode 66b and phase adjustment electrode 68b (second electrode) are separate from each other and are arranged sequentially from coupler 60a side to coupler 62a side.

[0089] In a direction intersecting the extension directions of branch waveguides 54a and 54b, modulation electrode 66a is opposite to modulation electrode 66b. Ground electrode 66c is located between modulation electrode 66a and modulation electrode 66b. Phase adjustment electrode 68a is opposite to phase adjustment electrode 68b. Ground electrode 68c is located between phase adjustment electrode 68a and phase adjustment electrode 68b. Modulation electrode 66a and modulation electrode 66b, phase adjustment electrode 68a and phase adjustment electrode 68b, and ground electrode 66c and ground electrode 68c extend in the same direction as branch waveguides 54a and 54b and are parallel to the end face 80c of semiconductor substrate 80.

[0090] Wiring 72a and wiring 74a are electrically connected to modulation electrode 66a. Wiring 72a extends from a first end of modulation electrode 66a to end face 80a of semiconductor substrate 80. Wiring 74a extends from a second end of modulation electrode 66a to end face 80c of semiconductor substrate 80. Wiring 72b and wiring 74b are electrically connected to modulation electrode 66b. Wiring 72b extends from a first end of modulation electrode 66b to end face 80a. Wiring 74b extends from a second end of modulation electrode 66b to end face 80c. Wiring 72c and wiring 74c are electrically connected to ground electrode 66c. Wiring 72c extends from a first end of ground electrode 66c to end face 80a. Wiring 74c extends from a second end of ground electrode 66c to end face 80c.

[0091] Modulation electrode 66a is connected to wiring 72a and Figure 1A The driver IC26 is electrically connected as shown. Modulation electrode 66b is electrically connected to driver IC26 via wiring 72b. Ground electrode 66c is electrically connected to driver IC26 via wiring 72c. Wirings 74a, 74b, and 74c are electrically connected to terminal element 78a via bonding wires.

[0092] Wiring 75a is electrically connected to phase adjustment electrode 68a. Wiring 75b is electrically connected to phase adjustment electrode 68b. Wiring 75c is electrically connected to ground electrode 68c. Wiring 75a, wiring 75b, and wiring 75c extend to end face 80c. Phase adjustment electrode 68a is electrically connected to ABC circuit 24 via wiring 75a. Phase adjustment electrode 68b is electrically connected to ABC circuit 24 via wiring 75b. Ground electrode 68c is electrically connected to ABC circuit 24 via wiring 75c.

[0093] The sub-Mach-Zehnder modulator 42b has branch waveguides 52b, 54c, and 54d, modulation electrodes 66d and 66e, phase adjustment electrodes 68d and 68e, and ground electrodes 66f and 68f. Branch waveguide 54c (the first branch waveguide) is, for example, a waveguide on the p-side. Branch waveguide 54d (the second branch waveguide) is, for example, a waveguide on the n-side.

[0094] The first end of branch waveguide 52b is connected to the second output terminal of coupler 58. The second end of branch waveguide 52b is connected to the input terminal of coupler 60b. Branch waveguides 54c and 54d are connected to couplers 60b and 62b, respectively. The length of the branch waveguide of sub-Mach-Zehnder modulator 42b is equal to the length of the corresponding branch waveguide of sub-Mach-Zehnder modulator 42a. The shape of the branch waveguide of sub-Mach-Zehnder modulator 42b is the same as the shape of the corresponding branch waveguide of sub-Mach-Zehnder modulator 42a.

[0095] Modulation electrode 66d and phase adjustment electrode 68d (first electrode) are disposed on branch waveguide 54c. Modulation electrode 66e and phase adjustment electrode 68e (second electrode) are disposed on branch waveguide 54d. Ground electrode 66f is disposed between modulation electrode 66d and modulation electrode 66e. Ground electrode 68f is disposed between phase adjustment electrode 68d and phase adjustment electrode 68e.

[0096] Wiring 72d and 74d are electrically connected to modulation electrode 66d. Wiring 72e and 74e are electrically connected to modulation electrode 66e. Wiring 72f and 74f are electrically connected to ground electrode 66f. Wiring 72d, 72e, and 72f extend to end face 80a of semiconductor substrate 80. Modulation electrode 66d is electrically connected to driver IC 26 via wiring 72d. Modulation electrode 66e is electrically connected to driver IC 26 via wiring 72e. Ground electrode 66f is electrically connected to driver IC 26 via wiring 72f. Wiring 74d, 74e, and 74f extend to end face 80d of semiconductor substrate 80 and are electrically connected to terminal element 78b.

[0097] Wiring 75d is electrically connected to phase adjustment electrode 68d. Wiring 75e is electrically connected to phase adjustment electrode 68e. Wiring 75f is electrically connected to ground electrode 68f. Wiring 75d, wiring 75e, and wiring 75f extend to end face 80d. Phase adjustment electrode 68d is electrically connected to ABC circuit 24 via wiring 75d. Phase adjustment electrode 68e is electrically connected to ABC circuit 24 via wiring 75e. Ground electrode 68f is electrically connected to ABC circuit 24 via wiring 75f.

[0098] The lengths of modulation electrodes 66a, 66b, 66d, and 66e are equal to the lengths of ground electrodes 66c and 66f. The lengths of phase adjustment electrodes 68a, 68b, 68d, and 68e are equal to the lengths of the modulation electrodes and shorter than the lengths of the modulation electrodes. The lengths of ground electrodes 68c and 68f are equal to the lengths of the phase adjustment electrodes and shorter than the lengths of the phase adjustment electrodes.

[0099] (Mach-Zehnder modulator)

[0100] The mother Mach-Zehnder modulator 44a has branch waveguides 55a and 55b, phase adjustment electrodes 70a and 70b, and a ground electrode 70c. The first end of branch waveguide 55a (the first branch waveguide) is connected to the output of coupler 62a. The first end of branch waveguide 55b (the second branch waveguide) is connected to the output of coupler 62b. The second ends of each of branch waveguides 55a and 55b are connected to the input of coupler 64. Branch waveguides 55a and 55b are parallel to the end face 80c of the semiconductor substrate 80 on the side near the daughter Mach-Zehnder modulator and are bent on the side near coupler 64.

[0101] A phase adjustment electrode 70a (first electrode) is disposed on branch waveguide 55a. A phase adjustment electrode 70b (second electrode) is disposed on branch waveguide 55b. A ground electrode 70c is disposed between branch waveguide 55a and branch waveguide 55b. The phase adjustment electrode 70a, phase adjustment electrode 70b, and ground electrode 70c extend in the same direction as the branch waveguides and are parallel to the end face 80c.

[0102] Wiring 76a is electrically connected to the end of phase adjustment electrode 70a and extends to end face 80c. Wiring 76b is electrically connected to the end of phase adjustment electrode 70b and extends to end face 80d. Wiring 76c is electrically connected to the end of ground electrode 70c and extends to end face 80c. Phase adjustment electrode 70a is electrically connected to ABC circuit 24 via wiring 76a. Phase adjustment electrode 70b is electrically connected to ABC circuit 24 via wiring 76b. Ground electrode 70c is electrically connected to ABC circuit 24 via wiring 76c.

[0103] Figure 2B It is along Figure 2A The figure shows a cross-sectional view along line AA, illustrating the cross-section of the sub-Mach-Zehnder modulator 42a. The sub-Mach-Zehnder modulator 42b and the mother Mach-Zehnder modulator 44a also have the same structure as the sub-Mach-Zehnder modulator 42a.

[0104] like Figure 2B As shown, a cladding layer 82 (first semiconductor layer) is disposed on the upper surface of the semiconductor substrate 80. The cladding layer 82 protrudes at two locations to the side opposite to the semiconductor substrate 80 (upper in the figure). A core layer 84, a cladding layer 86, and a contact layer 88 are sequentially stacked on this protruding portion. The cladding layer 82, the core layer 84, the cladding layer 86, and the contact layer 88 form mesa-shaped branch waveguides 54a and 54b. The cladding layer 86 and the contact layer 88 correspond to the second semiconductor layer.

[0105] The semiconductor substrate 80 is formed, for example, of semi-insulating indium phosphide (InP). The cladding layer 82 is formed, for example, of n-type InP (n-In P) with a thickness of 800 nm. The cladding layer 86 is formed, for example, of p-InP with a thickness of 1300 nm. The contact layer 88 is formed, for example, of p-InGaAs with a thickness of 200 nm. Silicon (Si) is doped in the n-type cladding layer 82, for example. Zinc (Zn) is doped in the p-type cladding layer 86 and the contact layer 88, for example.

[0106] The core layer 84, for example, has a multiple quantum well (MQW) structure. The core layer 84 comprises multiple alternating well layers and barrier layers. The well layers are, for example, formed of aluminum gallium indium arsenide (AlGaInAs). The barrier layers are, for example, formed of aluminum indium arsenide (AlInAs). The thickness of the core layer 84 is, for example, 500 nm.

[0107] The upper surface of the semiconductor substrate 80, the surface of the cladding layer 82, the sides of branch waveguides 54a and 54b, and the upper surface are covered by an insulating film 81. The insulating film 81 is formed of an insulator such as silicon oxide (SiO2). A resin layer 85 is formed of a resin such as benzocyclobutene (BCB) and covers the surface of the insulating film 81. The insulating film 81 and the resin layer 85 have openings in the portions between the branch waveguides on the upper surface of the cladding layer 82, and openings are also present above the branch waveguides 54a and 54b.

[0108] Modulation electrode 66a is disposed on branch waveguide 54a. Modulation electrode 66b is disposed on branch waveguide 54b. Modulation electrodes 66a and 66b are electrically connected to contact layer 88 exposed from openings in insulating film 81 and resin layer 85. Ground electrode 66c is disposed on cladding layer 82 and is electrically connected to cladding layer 82 exposed from insulating film 81 and resin layer 85. Figure 2A The phase adjustment electrodes 68a and 68b shown are also disposed on the upper surface of the contact layer 88. The ground electrode 68c is also disposed on the upper surface of the covering layer 82.

[0109] The modulation electrode and the phase adjustment electrode each have an ohmic electrode layer and a wiring layer. The ohmic electrode layer may include, for example, a platinum (Pt) layer, a titanium (Ti) layer, another platinum (Pt) layer, and a gold (Au) layer. These layers are stacked sequentially from the contact layer 88 side. The wiring layer, for example, is formed of Au and is in contact with the upper surface of the ohmic electrode layer. The ground electrode may have, for example, an alloy layer and an Au layer. The alloy layer may be formed, for example, an alloy of Au, germanium (Ge), and nickel (Ni). The Au layer is in contact with the upper surface of the alloy layer. Figure 2A The wiring shown is set up Figure 2B The resin layer 85 is formed, for example, from a metal such as Au.

[0110] (Operation of the optical transmitting device)

[0111] Next, the operation of the optical transmitting device 100 will be explained. Figure 1A The laser control unit 14 of the control unit 10 shown directs light towards the wavelength-variable laser element 22. Figure 2A The light from the input waveguide 50 of the optical modulator 40a shown is branched in coupler 58 and propagates in branch waveguides 52a and 52b. The light propagating in branch waveguide 52a is branched in coupler 60a and propagates in branch waveguides 54a and 54b. The light propagating in branch waveguide 52b is branched in coupler 60b and propagates in branch waveguides 54c and 54d.

[0112] Figure 1A The modulation control unit 16 of the control unit 10 generates a modulation signal based on the transmitted 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 sub-Mach-Zehnder modulator 42a. The modulation signal is also input from the driver IC 26 to the modulation electrodes 66d and 66e of the sub-Mach-Zehnder modulator 42b. By inputting the modulation signal, the refractive index of the branch waveguide is changed, thereby modulating the light.

[0113] The modulated light propagating in branch waveguide 54a and branch waveguide 54b are combined in coupler 62a. The combined modulated light propagates in branch waveguide 55a of the mother Mach-Zehnder modulator 44a. The modulated light propagating in branch waveguide 54c and branch waveguide 54d are combined in coupler 62b. The combined modulated light propagates in branch waveguide 55b of the mother Mach-Zehnder modulator 44a. The light propagating in branch waveguide 55a and branch waveguide 55b are combined in coupler 64 and propagates in output waveguide 56. The modulated light exits from output waveguide 56 beyond optical modulator 40a.

[0114] The phase control unit 12 of the control unit 10 uses the ABC circuit 24 for automatic bias control to adjust the phase of the light. The ABC circuit 24 changes the refractive index of the branch waveguide by applying a voltage to the phase adjustment electrode, thereby changing the optical path length. By changing the optical path length, the phase of the light propagating in the branch waveguide is changed. The phase control unit 12 can independently control the phase of the light in the mother Mach-Zehnder modulator 44a and the phase of the light in each of the daughter Mach-Zehnder modulators 42a and 42b.

[0115] When no modulation signal is input to the sub-Mach-Zehnder modulator 42a, the phase difference between the light propagating in branch waveguide 54a and the light propagating in branch waveguide 54b is π (rad) or π ± 2π × n (n is a negative or positive integer). That is, the sub-Mach-Zehnder modulator 42a is adjusted to the extinction point. The sub-Mach-Zehnder modulator 42b is also adjusted to the extinction point. The state of being adjusted to the extinction point is the operating point of the sub-Mach-Zehnder modulator.

[0116] The phase difference between the modulated light propagating in branch waveguide 55a of the mother Mach-Zehnder modulator 44a and the modulated light propagating in branch waveguide 55b of the mother Mach-Zehnder modulator 44a is 0.5π (rad) or a value equivalent to 0.5π. Values ​​equivalent to 0.5π are 0.5π ± 2π × n and 1.5π ± 2π × n (where n is a negative or positive integer). The modulated light propagating in branch waveguide 55a is orthogonal to the modulated light propagating in branch waveguide 55b.

[0117] The phase difference between two paired branch waveguides, such as branch waveguide 52a and branch waveguide 52b. As shown in the following formula, it is expressed as the initial phase difference. With phase change sum.

[0118]

Number 1

[0119] φ=φ0+Δφ

[0120] initial phase difference The optical path length is determined by factors such as the difference in optical path length between the branch waveguides of the optical modulator 40a. The wavelength λ of the light within the branch waveguide is, for example, 484 nm (1550 nm in vacuum). The lengths of the branch waveguides 54a, 54b, 54c, and 54d of the sub-Mach-Zehnder modulator are each, for example, 6 mm, which is more than 10,000 times the wavelength λ. Due to manufacturing errors, the optical path lengths of the branch waveguides deviate. The difference in optical path length ΔP between two branch waveguides is related to the initial phase difference of the light between the two branch waveguides. The relationship between them is expressed using an integer m by the following formula.

[0121]

Number 2

[0122] φ0+2mπ=2π×ΔP / λ

[0123] The difference in optical path length ΔP between paired branch waveguides, such as branch waveguides 52a and 52b, can sometimes be greater than one ten-thousandth of the designed dimensions. In this case, the difference in optical path length ΔP is greater than or equal to the wavelength λ of the light. Initial phase difference It is distributed in the range of 0 (rad) and below 2π (rad).

[0124] During the operation of the optical transmitting device 100, the initial phase difference Sometimes variations occur. This is because the optical path length of the branch waveguide changes due to stress applied to the optical modulator 40a and temperature variations, among other factors. The initial phase difference during operation... The change is, for example, in the range of -2π to 2π.

[0125] Phase change It is the change in phase of light propagating in the branch waveguide. Phase change The phase adjustment is achieved by applying a voltage from the ABC circuit 24 to the phase adjustment electrode, thereby changing the optical path length of the branch waveguide. The phase control unit 12 adjusts the phase based on the initial phase difference. The voltage applied from the ABC circuit 24 to the phase adjustment electrode is changed (automatic bias control). The phase change in automatic bias control is determined taking into account the initial phase difference and the phase change during operation.

[0126] The possible range of phase change values ​​when scanning the voltage applied to the phase adjustment electrode is defined as the phase adjustment range. To adjust the operating points of the sub-Mach-Zehnder modulators 42a and 42b to their extinction points, the phase adjustment ranges of each sub-Mach-Zehnder modulator 42a and 42b are preferably, for example, a range of -3π to 3π / 6π. To ensure that the phases of the two modulated lights in the mother Mach-Zehnder modulator 44a are orthogonal, the phase adjustment range of the mother Mach-Zehnder modulator 44a is preferably, for example, a range of -2.5π to 2.5π / 5π.

[0127] (Voltage)

[0128] The voltage applied to the sub-Mach-Zehnder modulator 42a by the ABC circuit 24 will be explained. The voltage Vp applied to the phase adjustment electrode 68a on the branch waveguide 54a is expressed as follows using the center voltage Vcc (first voltage) and the differential voltage Vdc (second voltage).

[0129]

Number 3

[0130] Vp = Vcc + Vdc

[0131] The voltage Vn applied to the phase adjustment electrode 68b on the branch waveguide 54b is expressed by the following formula.

[0132]

Number 4

[0133] Vn = Vcc - Vdc

[0134] The difference between voltage Vp and voltage Vn is 2Vdc. The phase control unit 12 adjusts the operating point of the sub-Mach-Zehnder modulator 42a by fixing the center voltage Vcc to a constant value and changing the differential voltage Vdc, thereby changing voltages Vp and Vn. Voltage Vp is applied to the phase adjustment electrode 68d of the sub-Mach-Zehnder modulator 42b, and voltage Vn is applied to the phase adjustment electrode 68e.

[0135] The voltage applied to the main Mach-Zehnder modulator 44a by the ABC circuit 24 will be explained. The voltage VI applied to the phase adjustment electrode 70a on the branch waveguide 55a is expressed as follows using the center voltage Vcp (first voltage) and the differential voltage Vdp (second voltage).

[0136]

Number 5

[0137] VI = Vcp + Vdp

[0138] The voltage VQ applied to the phase adjustment electrode 70b on the branch waveguide 55b is expressed by the following formula.

[0139]

Number 6

[0140] VQ = Vcp - Vdp

[0141] The difference between voltage VI and voltage VQ is 2Vdp. The phase control unit 12 adjusts the operating point of the main Mach-Zehnder modulator 44a by fixing the center voltage Vcp to a constant value and changing the differential voltage Vdp, thereby changing voltage VI and VQ.

[0142] Taking the voltages Vp and Vn of a sub-Mach-Zehnder modulator as examples, the magnitudes of the voltages are explained. The minimum value of voltages Vp and Vn is set as Vmin, and the maximum value as Vmax. The wider the adjustment range of the differential voltage Vdc, the wider the phase adjustment range. To expand the adjustment range of the differential voltage Vdc, for example, the center voltage Vcc and the differential voltage Vdc can be determined as follows.

[0143] Vcc = (Vmin + Vmax) / 2

[0144] The adjustment range of Vdc is: -(Vmax-Vmin) / 2 to the range of (Vmax-Vmin) / 2 from Vmax to Vmin.

[0145] The minimum and maximum values ​​Vmin and Vmax are determined, for example, based on power consumption and the withstand voltage of the optical modulator 40. With Vmin = 0V and Vmax = 20V, Vcc = 10V. The differential voltage Vdc ranges from -10V to 10V (-Vcc ≤ Vdc ≤ Vcc). The voltages applied to the sub-Mach-Zehnder modulator 42b and the mother Mach-Zehnder modulator 44a can also be the same as described above.

[0146] Figure 3 This example illustrates the differential voltage and phase change in a sub-Mach-Zehnder modulator 42a. A graph showing the relationship between the two. The horizontal axis represents the differential voltage Vdc, and the vertical axis represents the phase change. The dashed lines represent the phase change in branch waveguide 54a. The dashed lines represent the phase change in branch waveguide 54b. The solid lines represent the phase change in sub-Mach-Zehnder modulator 42a. The phase change of sub-Mach-Zehnder modulator 42a is the phase difference between the branch waveguides (phase change in branch waveguide 54a - phase change in branch waveguide 54b). The center voltage Vcc = 10V, and the differential voltage Vdc ranges from -10V to 10V.

[0147] like Figure 3 As shown, the higher the differential voltage Vdc becomes on the positive side, the greater the phase change in branch waveguide 54a becomes on the positive side. The phase change in branch waveguide 54b gets closer to 0. The phase change (phase difference) shown by the solid line increases more on the positive side. The higher the differential voltage Vdc becomes on the negative side, the greater the phase change in branch waveguide 54b becomes on the positive side. The phase change in branch waveguide 54a gets closer to 0. The phase difference increases more on the negative side. By setting the differential voltage Vdc to the range of -10V to 10V, the phase difference is approximately in the range of -9π to 9π.

[0148] exist Figure 3 In this case, the phase change of the sub-Mach-Zehnder modulator 42a The phase adjustment range is approximately -9π to 9π, exceeding the required phase adjustment range of -3π to 3π (6π) for sub-Mach-Zehnder modulators. To reduce power consumption and to 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, etc.

[0149] In each Mach-Zehnder modulator, the ratio of phase change to voltage (phase adjustment efficiency) sometimes deviates. It is believed that the difference in phase adjustment efficiency stems from variations in the thermal diffusion of the dopant in cladding layers 82, 86, and contact layers 88. Because of these variations in dopant thermal diffusion, the intensity of the electric field generated in the core layer 84 when a voltage is applied also varies. When the electric field intensity differs, the refractive index also differs, resulting in different magnitudes of phase change. Due to the deviation in dopant thermal diffusion, the bandgap energy also deviates, thus altering the phase change.

[0150] Deviations in phase adjustment efficiency sometimes occur between sub-Mach-Zehnder modulators 42a and 42b within an optical modulator 40a. Additionally, deviations in phase adjustment efficiency can sometimes occur between multiple optical modulators 40a.

[0151] Taking optical modulators 40a-1 and 40a-2 as examples, optical modulators 40a-1 and 40a-2 respectively have... Figure 2A The structure is described first. The phase adjustment efficiency between the sub-Mach-Zehnder modulators is explained. Figure 4A as well as Figure 4B This is a graph illustrating the relationship between voltage and phase change. The horizontal axis represents the voltages (Vp and Vn) applied to the phase adjustment electrodes of the sub-Mach-Zehnder modulator. The vertical axis represents the phase change in the branch waveguide.

[0152] Figure 4A This represents the phase change in the branch waveguide of the sub-Mach-Zehnder modulator 42a of the optical modulator 40a-1. The solid line represents the phase change in the branch waveguide on the p side (branch waveguide 54a). The dashed line represents the phase change in the branch waveguide on the n side (branch waveguide 54b). Figure 4B This represents the phase change in the branch waveguide of the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1. The solid line represents the phase change in the branch waveguide on the p-side (branch waveguide 54c). The dashed line represents the phase change in the branch waveguide on the n-side (branch waveguide 54d). Due to the different phase adjustment efficiencies, in Figure 4A as well as Figure 4B Within each branch waveguide, the phase change is slightly different from that of the n-side branch waveguide. The difference in phase change between sub-Mach-Zehnder modulators is greater than the difference in phase change between branch waveguides.

[0153] When in Figure 4A as well as Figure 4B When comparing cases where the same voltage is applied, Figure 4A Phase change Smaller Figure 4B Phase change It is relatively large. For example, at a voltage of 10V, Figure 4A Phase change of branch waveguide 54a and branch waveguide 54b Approximately 1.5π. At a voltage of 10V, Figure 4B Phase change of branch waveguide 54c and branch waveguide 54d It is approximately 2.5π. Figure 4B The phase adjustment efficiency of the sub-Mach-Zehnder modulator 42b shown is compared to Figure 4A The sub-Mach-Zehnder modulator 42a shown has high phase adjustment efficiency. However, within the same optical modulator 40a-1, the thermal diffusion of the dopant causes a deviation, which in turn causes a deviation in phase adjustment efficiency.

[0154] Figure 5A as well as Figure 5B This is a graph illustrating the relationship between differential voltage and phase change. The horizontal axis represents the differential voltage Vdc, and the vertical axis represents the phase change. The center voltage Vcc is 7V.

[0155] Figure 5A The values ​​represent the phase change in the sub-Mach-Zehnder modulator 42a of the optical modulator 40a-1. The dashed lines represent the phase change in the branch waveguide on the p-side (branch waveguide 54a). The dashed lines represent the phase change in the branch waveguide on the n-side (branch waveguide 54b). The solid lines represent the phase change in the sub-Mach-Zehnder modulator 42a (the phase difference between the branch waveguides). Since the sub-Mach-Zehnder modulator 42a is differentially driven, the phase change is symmetrical with Vdc = 0 as a reference. When scanning the differential voltage Vdc from -7V to 7V, the phase change is greater than -3π and less than 3π.

[0156] Figure 5B The dashed lines represent the phase change in the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1. The dashed lines represent the phase change in the p-side branch waveguide (branch waveguide 54c). The dashed lines represent the phase change in the n-side branch waveguide (branch waveguide 54d). The solid lines represent the phase change in the sub-Mach-Zehnder modulator 42b. The phase adjustment efficiency of the sub-Mach-Zehnder modulator 42b is higher than that of the sub-Mach-Zehnder modulator 42a. Therefore, Figure 5B Phase change ratio Figure 5A Large, meaning above -4π and below 4π.

[0157] In the sub-Mach-Zehnder modulator 42a, which has relatively low phase adjustment efficiency, in order to set the range of phase change (phase adjustment range) to a predetermined size such as above -3π and below 3π, for example... Figure 5AThat would be sufficient, setting the center voltage Vcc = 7V and the differential voltage Vdc to a range of -7V to 7V. However, if... Figure 5B As shown, the phase adjustment range of the sub-Mach-Zehnder modulator 42b, which has a high phase adjustment efficiency, is from -4π to 4π / 8π, exceeding the predetermined range of 6π. This leads to increased light absorption loss.

[0158] There is a positive correlation between phase adjustment efficiency and light absorption loss in the branch waveguide. This is because the Cramer-Kroni relationship holds between the change in refractive index of the branch waveguide and the amount of light absorption. Lower phase adjustment efficiency results in lower absorption loss, while higher phase adjustment efficiency results in higher absorption loss.

[0159] Figure 6A as well as Figure 6B This is a graph illustrating the relationship between voltage and the change in light absorption loss. The horizontal axis represents the voltage (Vp and Vn) applied to the phase adjustment electrodes of the sub-Mach-Zehnder modulator. The vertical axis represents the change in light absorption loss.

[0160] Figure 6A This represents the variation in absorption loss in the branch waveguide of the sub-Mach-Zehnder modulator 42a of the optical modulator 40a-1. The solid line represents the variation in absorption loss in the branch waveguide on the p side (branch waveguide 54a). The dashed line represents the variation in absorption loss in the branch waveguide on the n side (branch waveguide 54b).

[0161] Figure 6B This represents the variation in absorption loss in the branch waveguide of the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1. The solid line represents the variation in absorption loss in the branch waveguide on the p side (branch waveguide 54c). The dashed line represents the variation in absorption loss in the branch waveguide on the n side (branch waveguide 54d).

[0162] When in Figure 6A as well as Figure 6B When comparing cases where the same voltage is applied, Figure 6A The change in absorption loss is relatively small. Figure 6B The variation in absorption loss is relatively large. For example, at a voltage of 15V, Figure 6A The change in absorption loss is less than 1 dB. Figure 6B The variation in absorption loss exceeds 3dB. The absorption loss increases non-linearly with respect to voltage; the higher the voltage, the greater the absorption loss.

[0163] In both the sub-Mach-Zehnder modulator 42b with higher phase adjustment efficiency and the sub-Mach-Zehnder modulator 42a with lower phase adjustment efficiency, the phase adjustment range is set to a predetermined size, for example, from -3π to 6π. Therefore, for both sub-Mach-Zehnder modulators 42a and 42b, the center voltage Vcc is set to 7V, and the differential voltage Vdc is set to a range of -7V to 7V. However, if... Figure 5B As shown, the phase adjustment range of the sub-Mach-Zehnder modulator 42b exceeds the predetermined range of 6π. Figure 6B The variation in absorption loss of the sub-Mach-Zehnder modulator 42b shown is greater than that of the sub-Mach-Zehnder modulator 42a. That is, when the voltage is determined based on the sub-Mach-Zehnder modulator 42a with lower phase adjustment efficiency, the phase adjustment range becomes excessively large and the absorption loss increases in the sub-Mach-Zehnder modulator 42b with higher phase adjustment efficiency. The insertion loss of light increases, and the extinction ratio decreases as described later.

[0164] exist Figures 4A to 6B The text describes two sub-Mach-Zehnder modulators 42a and 42b in an optical modulator 40a-1. The phase adjustment efficiency also differs among multiple optical modulators.

[0165] Optical modulator 40a-2 is a different modulator from optical modulator 40a-1. The sub-Mach-Zehnder modulator 42a of optical modulator 40a-2 has the same phase adjustment efficiency and absorption loss as the sub-Mach-Zehnder modulator 42a of optical modulator 40a-1 (see reference). Figure 4A , Figure 5A as well as Figure 6A The sub-Mach-Zehnder modulator 42b of optical modulator 40a-2 has the same phase adjustment efficiency and absorption loss as the sub-Mach-Zehnder modulator 42b of optical modulator 40a-1 (see reference). Figure 4B , Figure 5B as well as Figure 6B Compared with the mother Mach-Zehnder modulator 44a of optical modulator 40a-1, the mother Mach-Zehnder modulator 44a of optical modulator 40a-2 has higher phase adjustment efficiency and larger absorption loss.

[0166] Figure 7A as well as Figure 7B This is a graph illustrating the relationship between differential voltage and phase change. The horizontal axis represents the differential voltage Vdp, and the vertical axis represents the phase change. The center voltage Vcp is 7.4V.

[0167] Figure 7AThis represents the phase change in the mother Mach-Zehnder modulator 44a of the optical modulator 40a-1. The dashed line represents the phase change in the branch waveguide (branch waveguide 55a) on the Ich side. The dashed line represents the phase change in the branch waveguide (branch waveguide 55b) on the Qch side. The solid line represents the phase change in the mother Mach-Zehnder modulator 44a (phase change of branch waveguide 55a - phase change of branch waveguide 55b). By setting the differential voltage Vdc to the range of -7V to 7.4V, the phase change is made to be in the range of -2.5π to 2.5π.

[0168] Figure 7B This indicates the phase change in the mother Mach-Zehnder modulator 44a of optical modulator 40a-2. The phase adjustment efficiency of the mother Mach-Zehnder modulator 44a of optical modulator 40a-2 is higher than that of the mother Mach-Zehnder modulator 44a of optical modulator 40a-1. By setting the differential voltage Vdc to the range of -5.5V to 5.1V, the phase change is made to be in the range of -2.5π or higher and 2.5π or lower. Figure 7B Phase change ratio Figure 7A The phase change is large. For example, when Vdp = 4V, Figure 7A The phase change in the equation is approximately π. Figure 7B The phase change is approximately 2π.

[0169] As stated above, there is a positive correlation between phase adjustment efficiency and light absorption loss in the branch waveguide. Compared to the mother Mach-Zehnder modulator 44a of optical modulator 40a-1, the mother Mach-Zehnder modulator 44a of optical modulator 40a-2 has a higher phase adjustment efficiency and a larger absorption loss. When the voltage is determined based on the mother Mach-Zehnder modulator 44a of optical modulator 40a-1 with lower phase adjustment efficiency, in the mother Mach-Zehnder modulator 44a of optical modulator 40a-2 with higher phase adjustment efficiency, the phase adjustment range becomes larger than the predetermined range (5π), and the absorption loss increases.

[0170] Within the same optical modulator 40a-1, the phase adjustment efficiency varies between the sub-Mach-Zehnder modulators. The phase adjustment efficiency also varies between optical modulator 40a-1 and optical modulator 40a-2. To set the phase change to a predetermined value and suppress the increase in light absorption loss, it is important to optimize the voltage applied to the phase adjustment electrode for each Mach-Zehnder modulator.

[0171] (Manufacturing method)

[0172] Figure 8 This is a flowchart illustrating an example of a manufacturing method for an optical modulator 40a, including steps for optimizing the voltage. For example... Figure 8As shown, a Mach-Zehnder modulator is formed (steps S1 to S3). A cladding layer 82, a core layer 84, a cladding layer 86, and a contact layer 88 are epitaxially grown on the upper surface of a wafer (semiconductor substrate 80) using methods such as Metal Organic Chemical Vapor Deposition (MOCVD). 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 (step S1). When there is a deviation in the thermal diffusion of the dopant, such as... Figure 4A as well as Figure 4B , Figure 7A as well as Figure 7B As shown, the phase adjustment efficiency also deviates.

[0173] Forming such as through dry etching, etc. Figure 2B A mesa-shaped branch waveguide as shown is formed (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 electrode, phase adjustment electrode, and ground electrode) are formed by vacuum evaporation or the like (step S3). Sub-Mach-Zehnder modulators 42a and 42b, and a mother Mach-Zehnder modulator 44a are formed on the semiconductor substrate 80. The wafer is diced to form a plurality of optical modulators 40a.

[0174] Multiple optical modulators 40a are respectively disposed on substrate 41 and electrically connected to ABC circuit 24 and driver IC 26. Each optical modulator 40a is tested. Specifically, the sub-Mach-Zehnder modulator 42a is tested to optimize the voltage applied to phase adjustment electrodes 68a and 68b (step S4). The sub-Mach-Zehnder modulator 42b is tested to optimize the voltage applied to phase adjustment electrodes 68d and 68e (step S5). The mother Mach-Zehnder modulator 44a is tested to optimize the voltage applied to phase adjustment electrodes 70a and 70b (step S6). The optical modulator 40a is formed through the above steps.

[0175] (test)

[0176] Figure 9 This is a flowchart illustrating an experiment. Figure 8 Steps S4, S5, and S6 are respectively for performing Figure 9 The procedure for the experiment is shown.

[0177] First, the experiment of optical modulator 40a-1 among multiple optical modulators 40a will be described. The experiments of sub-Mach-Zehnder modulator 42a, sub-Mach-Zehnder modulator 42b, and mother Mach-Zehnder modulator 44a of optical modulator 40a-1 will be carried out in sequence.

[0178] During the testing of the sub-Mach-Zehnder modulator 42a ( Figure 8 In step S4), the phase control unit 12 of the control unit 10 applies a voltage to the phase adjustment electrode 68b of the sub-Mach-Zehnder modulator 42b, adjusting the sub-Mach-Zehnder modulator 42b to the extinction point. The laser control unit 14 of the control unit 10 drives the wavelength-variable laser element 22, and light is incident from the wavelength-variable laser element 22 onto the optical modulator 40a-1. A light-receiving element (not shown) receives the emitted light from the sub-Mach-Zehnder modulator 42a. The control unit 10 measures the transmittance of light in the branch waveguide by comparing the intensity of the incident light and the intensity of the emitted light.

[0179] The control unit 10 scans the voltage applied from the ABC circuit 24 to the phase adjustment electrode 68a of the sub-Mach-Zehnder modulator 42a while measuring the light transmittance (first transmittance) in the branch waveguide 54a of the sub-Mach-Zehnder modulator 42a. The control unit 10 also scans the voltage applied from the ABC circuit 24 to the phase adjustment electrode 68b while measuring the light transmittance (first transmittance) in the branch waveguide 54b of the sub-Mach-Zehnder modulator 42a. Figure 9 (Step S10). The calculation unit 15 of the control unit 10 calculates the transmittance (second transmittance) of light in the branch waveguide 54a and the transmittance (second transmittance) of light in the branch waveguide 54b (step S12).

[0180] The calculation unit 15 optimizes the transmittance so that the transmittance calculated in step S12 is close to the transmittance measured in step S10 (step S14). Based on the transmittance optimization, the calculation unit 15 obtains the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change in the branch waveguide (step S16). Based on the relationship between the voltage and the amount of phase change, the storage control unit 18 obtains a voltage that makes the phase adjustment range of the sub-Mach-Zehnder modulator 42a a predetermined size, and stores this voltage, for example, in the storage device 34 (step S18).

[0181] The experiment is described in detail. The calculation unit 15 uses the change in absorption loss ΔL1 and the initial phase difference... and phase change The calculation unit 15 calculates the transmittance T as a function of the applied voltage V for the phase adjustment electrode, as shown in the following formula. Perform the calculation.

[0182]

Number 7

[0183] Δφ=k1×V+k2×V 2 +k3×V 3 +k4×V 4 +k5×V 5 +k6×V 6

[0184] The following shows an example of the initial values ​​for the coefficients.

[0185] 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 )

[0186] Figure 10A This is a graph illustrating the calculated phase change. The horizontal axis represents the voltage applied to the phase adjustment electrodes 68a and 68b of the sub-Mach-Zehnder modulator 42a. The vertical axis represents the phase change. The solid lines represent the phase change of the branch waveguide on the p-side (branch waveguide 54a) and the phase change of the branch waveguide on the n-side (branch waveguide 54b). The calculation unit 15 uses the same function (number 7) and the same coefficients (initial values) for both branch waveguides 54a and 54b, thus calculating the phase change between the branch waveguides. They are also equal.

[0187] The calculation unit 15 calculates the change in light absorption loss ΔL1 in the branch waveguide as a function of the applied voltage V for the phase adjustment electrode, as shown in the following formula.

[0188]

Number 8

[0189] ΔL1=a1×(1-exp(-V / a2))

[0190] The initial values ​​of coefficients a1 and a2 are shown below.

[0191] a1 = 1 × 10 -3 (dB), a2=2(V)

[0192] Figure 10B This is a graph illustrating the calculated variation in absorption loss. The horizontal axis represents the voltage applied to the phase adjustment electrodes 68a and 68b of the sub-Mach-Zehnder modulator 42a. The vertical axis represents the variation in absorption loss ΔL1. Since the same function (number 8) and the same coefficients are used for the calculations of branch waveguides 54a and 54b, the variation in absorption loss ΔL1 is also the same as shown by the solid line.

[0193] The calculation unit 15 calculates the transmittance T (step S12). As shown in the following formula, the transmittance T in each branch waveguide is expressed as the change in absorption loss ΔL1 and the initial phase difference. and phase change The function.

[0194]

Number 9

[0195] T=(1+10^(ΔL1 / 10)+2×10^(ΔL1 / 20)×cos(φ0±Δφ)) / (1+10^(ΔL1 / 10)) 2

[0196] Phase change The change in absorption loss, ΔL1, is represented by number 7. The sign of the cosine function (cos) in number 9 is positive for the branch waveguide on the p-side and negative for the branch waveguide on the n-side. Initial phase difference. It is expressed by the following formula. In number 10, acos is the inverse cosine function.

[0197]

Number 10

[0198] φ0=±acos(T0 0.5 )

[0199] T0 is the transmittance when the applied voltage is 0V, and it is measured in step S10. When scanning the applied voltage against the phase adjustment electrode 68a, if the initial pole in the transmittance is a minimum, the initial phase difference... The sign is positive, and negative when it is a maximum. In the example of the sub-Mach-Zehnder modulator 42a, it is set to...

[0200] Figure 11A This is a graph illustrating the calculated transmittance. Figure 11B The graph shows an example of the measured transmittance and the optimized transmittance. Figure 11A as well as Figure 11BThe horizontal axis represents the voltage applied to the phase adjustment electrode of the sub-Mach-Zehnder modulator 42a. The vertical axis represents the transmittance of light.

[0201] Figure 11A The solid line represents the transmittance of the branch waveguide on the p side (branch waveguide 54a). The dashed line represents the transmittance of the branch waveguide on the n side (branch waveguide 54b). Figure 11A The transmittance shown is calculated by the calculation unit 15. Figure 9 The transmittance is calculated using the number 9 and the initial value in step S12. Figure 11B The solid line represents the optimized transmittance of branch waveguide 54a. The dashed line represents the optimized transmittance of branch waveguide 54b. The circle represents the measured transmittance of branch waveguide 54a. The triangle represents the measured transmittance of branch waveguide 54b.

[0202] Figure 9 The optimization in step S14 refers to making the transmittance calculated in step S12 close to the transmittance measured in step S10, thereby reducing the error between the two. Figure 11B The transmittance shown by the solid line is from Figure 11A The transmittance change shown by the solid line is close to Figure 11B The circle in the diagram represents the measured transmittance. Figure 11B The transmittance shown by the dashed line is from Figure 11A The transmittance change shown by the dashed line is close to Figure 11B The triangle in the figure represents the measured transmittance.

[0203] By optimizing the transmittance, the initial phase difference contained in the transmittance equation (Chapter 9) is obtained. Phase change The change in absorption loss, ΔL1, was also optimized. And the change in absorption loss ΔL1 becomes a function that more accurately represents the relationship with voltage. Figure 9 Step S16).

[0204] More specifically, phase change The coefficients k1 to k6 in equation (7), and the coefficients a1 and a2 in equation (8) representing the change ΔL1, change relative to their initial values. The optimized coefficients are shown below.

[0205] For the coefficients of branch waveguide 54a

[0206] k1 = 1.32 × 10 -1 (π / V), k2 = 1.90 × 10 -2 (π / V 2 k3 = 3.33 × 10 -3 (π / V3 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)

[0207] For the coefficients of branch waveguide 54b

[0208] 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)

[0209] Initial phase difference after transmittance optimization It is 0.25π.

[0210] Figure 12A This is a graph illustrating the optimized phase change. The horizontal axis, vertical axis, solid line, and dashed line are respectively... Figure 4A The corresponding content is the same. For example... Figure 12A As shown, by substituting the coefficients obtained through transmittance optimization into the number 7 for calculation, a result close to... Figure 4A The amount of phase change. Figure 12B This is a graph illustrating the change in absorption loss after optimization. The horizontal axis, vertical axis, solid line, and dashed line are respectively... Figure 6A The corresponding content is the same. For example... Figure 12B As shown, by substituting the coefficients obtained through transmittance optimization into the number 8 for calculation, a result close to... Figure 6A The change in absorption loss.

[0211] Figure 13This is a graph illustrating the relationship between the center voltage and the phase adjustment range. The horizontal axis represents the center voltage Vcc, which in this example is scanned from 0V to 10V. For each value of the center voltage Vcc, the differential voltage Vdc is set to the range of -Vcc to Vcc. The vertical axis represents the range of phase change (phase adjustment range). The calculation unit 15 applies the optimized coefficients k1 to k6 to number 7, and for each voltage, calculates the phase change of branch waveguide 54a and the phase change of branch waveguide 54b. The calculation unit 15 calculates the difference between the phase change of branch waveguide 54a and the phase change of branch waveguide 54b, and obtains the phase adjustment range in the sub-Mach-Zehnder modulator 42a. In the sub-Mach-Zehnder modulator 42a, the phase adjustment range is 6π (from -3π to 3π). The minimum value of the center voltage Vcc that makes the phase adjustment range 6π is found. Figure 13 As shown, in order to set the phase adjustment range to 6π, the center voltage Vcc can be 7V. Figure 1B The storage device 34 shown stores the center voltage Vc of the sub-Mach-Zehnder modulator 42a of the optical modulator 40a-1 as 7V.

[0212] Next, experiments were conducted on the sub-Mach-Zehnder modulator 42b. Figure 8 Step S5). The phase control unit 12 of the control unit 10 applies a voltage to the phase adjustment electrode of the sub-Mach-Zehnder modulator 42a, adjusting the sub-Mach-Zehnder modulator 42a to the extinction point. While scanning the voltage applied from the ABC circuit 24 to the phase adjustment electrode of the sub-Mach-Zehnder modulator 42b, the control unit 10 measures the transmittance of light in the branch waveguide 54c and branch waveguide 54d of the sub-Mach-Zehnder modulator 42b. Figure 9 (Step S10). The calculation unit 15 of the control unit 10 calculates the transmittance of light in the branch waveguide 54c and the transmittance of light in the branch waveguide 54d (step S12).

[0213] The calculation unit 15 performs optimization to make the transmittance calculated in step S12 close to the transmittance measured in step S10 (step S14). The calculation unit 15 obtains the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change (step S16). Based on the relationship between voltage and phase change, the storage control unit 18 obtains a voltage that makes the range of phase change of the sub-Mach-Zehnder modulator 42b a predetermined size, and stores the voltage in the storage device 34 (step S18).

[0214] Figure 14This is a graph illustrating the measured transmittance and the optimized transmittance. The horizontal axis represents the voltage applied to the phase adjustment electrodes 68d and 68e of the sub-Mach-Zehnder modulator 42b. The vertical axis represents the transmittance of light. The solid line represents the optimized transmittance of the p-side branch waveguide (branch waveguide 54c). The dashed line represents the optimized transmittance of the n-side branch waveguide (branch waveguide 54d). The circle represents the measured transmittance of branch waveguide 54c. The triangle represents the measured transmittance of branch waveguide 54d. By optimizing the transmittance, the changes in phase change and absorption loss can be obtained.

[0215] Figure 15A This is a graph illustrating the optimized phase change. The horizontal axis, vertical axis, solid line, and dashed line are respectively... Figure 4B The corresponding content is the same. For example, Figure 15A As shown, an approximation is obtained through optimization. Figure 4B The amount of phase change. Figure 15B This is a graph illustrating the change in absorption loss after optimization. The horizontal axis, vertical axis, solid line, and dashed line are respectively... Figure 6B The corresponding content is the same. For example, Figure 15B As shown, an approximation is obtained through optimization. Figure 6B The change in absorption loss.

[0216] Figure 16 This is a graph 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 phase change (phase adjustment range). The calculation unit 15 applies the optimized coefficients k1 to k6 to number 7 to calculate the phase adjustment range for each voltage. Figure 16 As shown, in the sub-Mach-Zehnder modulator 42b, in order to set the phase adjustment range to 6π, the center voltage Vcc can be 5.7V. The storage device 34 stores the center voltage Vcc of the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1 as 5.7V.

[0217] Next, the test of the master Mach-Zehnder modulator 44a was carried out. Figure 8 Step S6). The phase control unit 12 of the control unit 10 sets the sub-Mach-Zehnder modulator 42a and sub-Mach-Zehnder modulator 42b to the maximum transmission point. The control unit 10 scans the voltage applied from the ABC circuit 24 to the phase adjustment electrode 70a of the mother Mach-Zehnder modulator 44a while measuring the transmittance (first transmittance) of light in the branch waveguide 55a of the mother Mach-Zehnder modulator 44a. The control unit 10 scans the voltage applied to the phase adjustment electrode 70b while measuring the transmittance (first transmittance) of light in the branch waveguide 55b. Figure 9(Step S10). The calculation unit 15 of the control unit 10 calculates the transmittance (second transmittance) of light in the branch waveguide 55a and the transmittance (second transmittance) of light in the branch waveguide 55b (step S12).

[0218] The calculation unit 15 performs optimization to make the transmittance calculated in step S12 close to 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 amount of phase change in the branch waveguide 55a (step S16). Based on the relationship between the voltage and the amount of phase change, the storage control unit 18 obtains a voltage that makes the range of the amount of phase change a predetermined size, and stores the voltage in the storage device 34 (step S18).

[0219] The phase adjustment range of the mother Mach-Zehnder modulator 44a can be, for example, 5π (-2.5π to 2.5π). 7V is stored in the storage device 34 as the center voltage Vcp that makes the phase adjustment range of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-1 5π.

[0220] Next, tests were conducted on optical modulator 40a-2, which is different from optical modulator 40a-1. The test procedures for each Mach-Zehnder modulator were the same as those for the corresponding tests on optical modulator 40a-1. The center voltage of sub-Mach-Zehnder modulator 42a of optical modulator 40a-2 was 7V, the same as that of sub-Mach-Zehnder modulator 42a of optical modulator 40a-1. The center voltage of sub-Mach-Zehnder modulator 42b of optical modulator 40a-2 was 5.7V, the same as that of sub-Mach-Zehnder modulator 42b of optical modulator 40a-1.

[0221] Experiments were also conducted on the mother Mach-Zehnder modulator 44a for the optical modulator 40a-2. Figure 17 This is a graph illustrating the measured transmittance and the optimized transmittance. The horizontal axis represents the voltage applied to the phase adjustment electrodes 70a and 70b of the mother Mach-Zehnder modulator 44a. The vertical axis represents the transmittance of light. The solid line represents the optimized transmittance of the branch waveguide (branch waveguide 55a) on the Ich side. The dashed line represents the optimized transmittance of the branch waveguide (branch waveguide 55b) on the Qch side. The circle represents the measured transmittance of branch waveguide 55a. The triangle represents the measured transmittance of branch waveguide 55b. By optimizing the transmittance, the coefficients in number 7 and number 8 are changed.

[0222] Figure 18AThis is a graph illustrating the optimized phase change. The horizontal axis represents the voltage applied to phase adjustment electrodes 70a and 70b. The vertical axis represents the phase change. The solid line represents the phase change in branch waveguide 55a. The dashed line represents the phase change in branch waveguide 55b. Figure 18B This is a graph illustrating the change in absorption loss after optimization. The horizontal axis represents the voltage applied to phase adjustment electrodes 70a and 70b. The vertical axis represents the change in absorption loss. The solid line represents the change in branch waveguide 55a. The dashed line represents the change in branch waveguide 55b.

[0223] Figure 19 This is a graph 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 phase change (phase adjustment range). The calculation unit 15 obtains the phase adjustment range in the master Mach-Zehnder modulator 44a for each voltage. Figure 19 As shown, in the mother Mach-Zehnder modulator 44a of the optical modulator 40a-2, in order to set the phase adjustment range to 5π, the center voltage Vcp can be 5.7V. The storage device 34 stores the center voltage Vc of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-2 as 5.7V.

[0224] Table 1 is an example of a data table stored in storage device 34.

[0225] Table 1

[0226]

[0227] Table 1 shows that Vcc for Ich is the center voltage of sub-Mach-Zehnder modulator 42a. Vcc for Qch is the center voltage of sub-Mach-Zehnder modulator 42b. Vcp is the center voltage of mother Mach-Zehnder modulator 44a. In optical modulators 40a-1 and 40a-2, Vcc for Ich is 7.0V, and Vcc for Qch is 5.7V. Vcp for optical modulator 40a-1 is 7.4V. Vcp for optical modulator 40a-2 is 5.7V.

[0228] Storage device 34 stores the center voltage Vcc of sub-Mach-Zehnder modulator 42a, the center voltage Vcc of sub-Mach-Zehnder modulator 42b, and the center voltage Vcp of master Mach-Zehnder modulator 44a in optical modulators 40a-1 and 40a-2 respectively. The differential voltage Vcd of the sub-Mach-Zehnder modulator is above -Vcc and below Vcc. The differential voltage Vdp of the master Mach-Zehnder modulator is above -Vcp and below Vcp. Figure 9The experiment optimized the voltage. When using optical modulators 40a-1 and 40a-2, by applying this voltage, the phase adjustment range can be set to a predetermined size, and the increase in light absorption loss can be suppressed.

[0229] (Absorption loss and extinction ratio of the sub-Mach-Zehnder modulator)

[0230] Reference Figures 20A to 22B The absorption loss and extinction ratio of the sub-Mach-Zehnder modulator are explained. Figures 20A to 21B Is to carry out Figure 9 The experiments shown, and the examples of voltage optimization as shown in Table 1, correspond to the first embodiment. Figure 22A as well as Figure 22B This is an example of applying the same voltage to multiple sub-Mach-Zehnder modulators without optimizing the voltage.

[0231] Figure 20A , Figure 21A as well as Figure 22A This is a graph illustrating the variation in absorption loss. The horizontal axis represents the differential voltage Vdc. The vertical axis represents the amount of light absorption loss. The dashed line represents the absorption loss of the p-side branch waveguide (branch waveguide 54a or branch waveguide 54c). The dashed line represents the absorption loss of the n-side branch waveguide (branch waveguide 54b or branch waveguide 54d). The solid line represents the difference in absorption loss between the branch waveguides, ΔL2 (absorption loss of the p-side branch waveguide - absorption loss of the n-side branch waveguide).

[0232] Figure 20B , Figure 21B as well as Figure 22B This is a graph illustrating the extinction ratio. The horizontal axis represents the differential voltage Vdc. The vertical axis represents the extinction ratio. The extinction ratio (ER) is calculated using the following formula.

[0233]

Number 11

[0234] ER = 20 × log 10 ((10^(ΔL2 / 20)+1) / (10^(ΔL2 / 20)-1))

[0235] The smaller the difference in absorption loss ΔL2, the larger the extinction ratio ER. Conversely, the larger the difference ΔL2, the smaller the extinction ratio ER. When the difference ΔL2 becomes very large, even if the light from the two branch waveguides is combined with the light in opposite phases, the light cannot completely cancel each other out. As a result, the extinction ratio ER decreases.

[0236] Figure 20AThis represents the absorption loss of the sub-Mach-Zehnder modulator 42a in optical modulator 40a-1 when the center voltage Vcc = 7V. The differential voltage Vdc takes values ​​in the range of -6.8V to 7V. The greater the negative side of the differential voltage Vdc, the greater the absorption loss of branch waveguide 54b, and the closer the absorption loss of branch waveguide 54a is to 0. The difference in absorption losses ΔL2 increases towards the negative side. Conversely, the greater the positive side of the differential voltage Vdc, the greater the absorption loss of branch waveguide 54a, and the closer the absorption loss of branch waveguide 54b is to 0. The difference in absorption losses ΔL2 increases towards the positive side. The maximum absolute value of the difference in absorption losses ΔL2 is 0.38dB.

[0237] Figure 20B This indicates the extinction ratio of the sub-Mach-Zehnder modulator 42a of the optical modulator 40a-1. Figure 20B The extinction ratio ER in the middle is based on Figure 20A The difference ΔL2 is used for calculation. The differential voltage Vdc increases towards the positive and negative sides, and the extinction ratio ER decreases. The minimum extinction ratio ER is 33.3 dB.

[0238] Figure 21A This represents the absorption loss of the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1 when the center voltage Vcc = 5.7V. The differential voltage Vdc takes values ​​in the range of -5.8V to 5.5V. The maximum absolute value of the difference in absorption losses ΔL2 is 0.50dB.

[0239] Figure 21B This indicates the extinction ratio of the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1. Figure 21B The extinction ratio ER in the middle is based on Figure 21A The difference ΔL2 is used for calculation. The minimum extinction ratio ER is 30.8 dB.

[0240] Figure 22A This represents the absorption loss of the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1 when the center voltage Vcc = 7V. The maximum absolute value of the difference in absorption loss ΔL2 is 0.88dB.

[0241] Figure 22B The extinction ratio of the sub-Mach-Zehnder modulator 42b of the optical modulator 40a-1 is indicated. Figure 22B The extinction ratio ER in the middle is based on Figure 22A The difference ΔL2 is used for calculation. The minimum extinction ratio ER is 26.0 dB.

[0242] like Figure 22A as well as Figure 22BAs shown, when the center voltage of the sub-Mach-Zehnder modulator 42b is equal to that of the sub-Mach-Zehnder modulator 42a, the absorption loss increases and the extinction ratio decreases.

[0243] like Figure 21A as well as Figure 21B As shown, according to the first embodiment, by optimizing the center voltage of the sub-Mach-Zehnder modulator 42b, the amount of light absorption loss can be reduced, and the decrease in extinction ratio can be suppressed. Therefore, an extinction ratio of 30 dB or more can be obtained in both the sub-Mach-Zehnder modulator 42a and the sub-Mach-Zehnder modulator 42b.

[0244] (Absorption loss and extinction ratio of the mother Mach-Zehnder modulator)

[0245] Reference Figures 23A to 25B The absorption loss and extinction ratio of the mother Mach-Zehnder modulator are explained. Figures 23A to 24B Is to carry out Figure 9 The experiments shown are examples of voltage optimization as presented in Table 1. Figure 25A as well as Figure 25B This is an example of applying the same voltage to multiple master Mach-Zehnder modulators without optimizing the voltage.

[0246] Figure 23A , Figure 24A as well as Figure 25A This is a graph illustrating absorption loss. The horizontal axis represents the differential voltage Vdp. The vertical axis represents the light absorption loss. The dashed line represents the absorption loss of the branch waveguide on the Ich side (branch waveguide 55a). The dashed line represents the absorption loss of the branch waveguide on the Qch side (branch waveguide 55b). The solid line represents the difference in absorption loss between the branch waveguides, ΔL2 (absorption loss of the branch waveguide on the Ich side - absorption loss of the branch waveguide on the Qch side). Figure 23B , Figure 24B as well as Figure 25B This is a graph illustrating the extinction ratio. The horizontal axis represents the differential voltage Vdp, and the vertical axis represents the extinction ratio.

[0247] Figure 23A This represents the absorption loss of the mother Mach-Zehnder modulator 44a in optical modulator 40a-1 when the center voltage Vcp = 7.4V. The differential voltage Vdp is taken as a value in the range of -7V to 7.4V. The maximum absolute value of the difference in absorption loss ΔL2 is 0.53dB.

[0248] Figure 23B This indicates the extinction ratio of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-1. Figure 23B The extinction ratio ER in the middle is based on Figure 23AThe difference ΔL2 is used for calculation. The minimum extinction ratio ER is 30.3 dB.

[0249] Figure 24A This represents the absorption loss of the mother Mach-Zehnder modulator 44a in optical modulator 40a-2 when the center voltage Vcp = 5.7V. The maximum absolute value of the difference in absorption loss ΔL2 is 0.44dB.

[0250] Figure 24B This indicates the extinction ratio of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-2. Figure 24B The extinction ratio ER in the middle is based on Figure 24A The difference ΔL2 is used for calculation. The minimum extinction ratio ER is 31.9 dB.

[0251] Figure 25A This represents the absorption loss of the mother Mach-Zehnder modulator 44a in optical modulator 40a-2 when the center voltage Vcp = 7.4V. The differential voltage Vdp takes values ​​in the range of -5.5V to 5.1V. The maximum absolute value of the difference in absorption loss ΔL2 is 0.89dB.

[0252] Figure 25B This indicates the extinction ratio of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-2. Figure 25B The extinction ratio ER in the middle is based on Figure 25A The difference ΔL2 is used for calculation. The minimum extinction ratio ER is 25.8 dB.

[0253] like Figure 25A as well as Figure 25B As shown, when the center voltage of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-2 is equal to the center voltage of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-1, the absorption loss increases and the extinction ratio decreases.

[0254] like Figure 24A as well as Figure 24B As shown, according to the first embodiment, by optimizing the center voltage of the mother Mach-Zehnder modulator 44a of the optical modulator 40a-2, the amount of light absorption loss can be reduced, and the decrease in extinction ratio can be suppressed. An extinction ratio of 30 dB or more can be obtained in the mother Mach-Zehnder modulators 44a of both the optical modulators 40a-1 and 40a-2.

[0255] According to the first embodiment, the control unit 10 acquires the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change, and acquires a voltage that makes the phase adjustment range a predetermined size. The phase of the light in the Mach-Zehnder modulator is adjusted using a voltage optimized for each Mach-Zehnder modulator. The phase adjustment range can be set to a predetermined size, and the increase in light absorption loss is suppressed.

[0256] Similar to sub-Mach-Zehnder modulators 42a and 42b within optical modulator 40a-1, voltage optimization is performed for each of the multiple Mach-Zehnder modulators within an optical modulator. In each Mach-Zehnder modulator, the phase adjustment range can be set to a predetermined size, and the increase in light absorption loss can be suppressed. For example, the center voltage Vcc of sub-Mach-Zehnder modulator 42a is set to 7V, and the center voltage Vcc of sub-Mach-Zehnder modulator 42b is set to 5.7V. Figure 13 as well as Figure 16 As shown, in both sub-Mach-Zehnder modulators 42a and 42b, the phase adjustment range can be set to 6π. Figures 20A to 21B As shown, by suppressing the increase in absorption loss, the extinction ratio can be set to 30dB or higher.

[0257] Similar to optical modulators 40a-1 and 40a-2, voltage optimization is achieved among multiple optical modulators. The center voltage Vcp of the mother Mach-Zehnder modulator 44a of optical modulator 40a-1 is set to 7V, and the center voltage Vcp of the mother Mach-Zehnder modulator 44a of optical modulator 40a-2 is set to 5.7V. In both mother Mach-Zehnder modulators 44a, the phase adjustment range can be set to 5π. Figures 23A to 24B As shown, by suppressing the increase in absorption loss, the extinction ratio can be made above 30 dB.

[0258] The phase adjustment range of the sub-Mach-Zehnder modulator can be set, for example, from -3π to 6π (3π), but it can also be above or below 6π. The phase adjustment range of the mother Mach-Zehnder modulator can be set, for example, from -2.5π to 5π (2.5π), but it can also be above or below 5π. The phase adjustment range is, for example, based on the initial phase difference. The phase adjustment range can be set to an appropriate value. For example, the phase adjustment range of the sub-Mach-Zehnder modulator can be 5π or 7π. For example, the phase adjustment range of the mother Mach-Zehnder modulator can be 4π or 6π.

[0259] The sub-Mach-Zehnder modulator 42a has paired branch waveguides 52a and 52b. A phase adjustment electrode 68a is disposed on branch waveguide 52a. A phase adjustment electrode 68b is disposed on branch waveguide 52b. The control unit 10 acquires the relationship between the voltage applied to the phase adjustment electrode 68a and the change in phase in branch waveguide 52a, and the relationship between the voltage applied to the phase adjustment electrode 68b and the change in phase in branch waveguide 52b. Figure 12A The control unit 10 is based on the relationship between voltage and phase change, such as... Figure 13 As shown, a voltage that makes the phase adjustment range of the sub-Mach-Zehnder modulator 42a 6π can be obtained. In the sub-Mach-Zehnder modulator 42b and the mother Mach-Zehnder modulator 44a, the control unit 10 also obtains the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change in the branch waveguide, and obtains the voltage that makes the phase adjustment range the desired size based on this relationship.

[0260] 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 mother Mach-Zehnder modulator is Vcp + Vdp, and the voltage VQ is Vcp - Vdp. Figure 9 In step S18, the control unit 10 acquires the center voltage Vcc and the center voltage Vcp. For example... Figure 13 , Figure 16 as well as Figure 19 As shown, by obtaining the optimal center voltage, the phase adjustment range is made to a predetermined size. As shown in Table 1, the storage device 34 stores the optimal center voltage Vcc and center voltage Vcp for each Mach-Zehnder modulator. When driving the Mach-Zehnder modulator, the control unit 10 obtains the center voltage stored in the storage device 34 and calculates the sum of the center voltage and the differential voltage, as well as the difference between the center voltage and the differential voltage, thereby obtaining voltages Vp, Vn, VI, and VQ. The ABC circuit 24 applies voltage to the phase adjustment electrode. In each Mach-Zehnder modulator, a predetermined phase adjustment range can be obtained, and the increase in absorption loss can be suppressed. In the first embodiment, the center voltage in differential drive is optimized. The Mach-Zehnder modulator can also be driven by methods other than differential drive. Regardless of the driving method, the phase adjustment range of the Mach-Zehnder modulator can be controlled with the optimal voltage, and the increase in absorption loss can be suppressed.

[0261] The differential voltage Vdc of the sub-Mach-Zehnder modulator is set to a value above -Vcc and below Vcc. The differential voltage Vdp of the mother Mach-Zehnder modulator is set to a value above -Vcp and below Vcp. The differential voltage can also be changed relative to the differential voltages described above.

[0262] exist Figure 9 In step S12, the transmittance is calculated as a function of the phase change (number 9). The transmittance is fitted to make the calculated transmittance close to the measured transmittance. Through transmittance optimization, the phase change is also optimized. The phase change shown in number 7 is a function of voltage. By fitting the transmittance, the coefficients in number 7 are changed to improve the accuracy of the relationship between voltage and phase change. Based on the phase change, the control unit 10 obtains the voltage that makes the phase adjustment range a predetermined size. The phase adjustment range can be set to a predetermined size, and the increase in light absorption loss can be suppressed. The transmittance, phase change, and change in absorption loss can also be calculated using formulas other than those described above.

[0263] like Figure 2B As shown, branch waveguides 54a and 54b have a cladding layer 82, a core layer 84, a cladding layer 86, and a contact layer 88. Other branch waveguides also have the same structure. The cladding layer 82 is an n-type semiconductor layer. The cladding layer 86 and the contact layer 88 are p-type semiconductor layers. Dopants are added to obtain both n-type and p-type conductivity types. Due to the deviation in the thermal diffusion of the dopants, the phase adjustment efficiency of the Mach-Zehnder modulator also deviates. According to the first embodiment, for each Mach-Zehnder modulator, a voltage is obtained that makes the phase adjustment range a predetermined size. The phase adjustment range can be set to a predetermined size, and the increase in absorption loss is suppressed.

[0264] exist Figure 8 as well as Figure 9 In the process, Figure 1A The optical transmitting device 100 is used as a test device for the optical modulator 40. An optical modulator 40 (e.g., optical modulator 40a-1) is installed in the optical transmitting device 100 and tested. Then, optical modulator 40a-1 is replaced with optical modulator 40a-2, and tested again. The storage device 34 stores the voltages for both optical modulator 40a-1 and optical modulator 40a-2 as shown in Table 1. When the optical transmitting device 100 is used for communication, etc., testing can also be performed on only one optical modulator 40a included in the optical transmitting device 100. The storage device 34 may also store only the voltage for that one optical modulator 40a.

[0265] <Second Implementation Method>

[0266] The second embodiment uses a DP (Dual Polarization)-IQ modulator as an example of optical modulator 40. The structure of the optical transmitting device 100 is the same as that of the first embodiment.

[0267] Figure 26 This is a top view illustrating an optical modulator 40b. The optical modulator 40b is a DP-IQ modulator that includes both optical modulator 43a and optical modulator 43b.

[0268] A semiconductor substrate 80 and four terminating elements 78a, 78b, 78c, and 78d are mounted on the upper surface of substrate 41. Terminating elements 78a, 78b, 78c, and 78d include, for example, terminating resistors and capacitors. Terminating elements 78a and 78b face the end face 80c of the semiconductor substrate 80. Terminating elements 78c and 78d face the end face 80d of the semiconductor substrate 80. An input waveguide 51, an optical modulator 43a, and an optical modulator 43b are formed on the semiconductor substrate 80.

[0269] The first end of the input waveguide 51 is located at the end face 80a of the semiconductor substrate 80. The second end of the input waveguide 51 is connected to the coupler 59. Optical modulators 43a and 43b are connected in parallel at a position following the coupler 59.

[0270] Optical modulator 43a is an IQ modulator, and... Figure 2A Similarly, optical modulator 40a has sub-Mach-Zehnder modulators 42a and 42b, and a mother Mach-Zehnder modulator 44a. Optical modulator 43b is an IQ modulator, having sub-Mach-Zehnder modulators 42c and 42d, and a mother Mach-Zehnder modulator 44b. The structures of sub-Mach-Zehnder modulators 42c and 42d are the same as those of sub-Mach-Zehnder modulators 42a and 42b. The structure of mother Mach-Zehnder modulator 44b is the same as that of mother Mach-Zehnder modulator 44a.

[0271] Optical modulator 43a generates modulated light in the X channel (X-polarized wave). Optical modulator 43b generates modulated light in the Y channel (Y-polarized wave). The polarization plane of the X-polarized wave is orthogonal to the polarization plane of the Y-polarized wave. Using polarization wave rotation elements (not shown) and wave combining elements, the two modulated lights are combined in a manner with orthogonal polarization planes.

[0272] The manufacturing method of the optical modulator 40b is similar to... Figure 8 The same process. The control unit 10 performs the same operation on the Mach-Zehnder modulator within the optical modulator 40b. Figure 9The experiment involved testing the sub-Mach-Zehnder modulators 42a and 42b of optical modulator 43a, as well as the master Mach-Zehnder modulator 44a. The sub-Mach-Zehnder modulators 42c and 42d of optical modulator 43b were then adjusted to their extinction points.

[0273] Table 2 is an example of a data table stored in storage device 34. Storage device 34 stores the voltages of each of the multiple optical modulators 40b (optical modulator 40b-1 and optical modulator 40b-2 in Table 2).

[0274] Table 2

[0275]

[0276] Vcc of XI is the center voltage of the sub-Mach-Zehnder modulator 42a of optical modulator 43a on the Xch side of optical modulator 40b. Vcc of XQ is the center voltage of the sub-Mach-Zehnder modulator 42b of optical modulator 43a. Vcc of YI is the center voltage of the sub-Mach-Zehnder modulator 42c of optical modulator 43b on the Qch side. Vcc of YQ is the center voltage of the sub-Mach-Zehnder modulator 42d of optical modulator 43b. Vcp of Xch is the center voltage of the mother Mach-Zehnder modulator 44a. Vcp of Ych is the center voltage of the mother Mach-Zehnder modulator 44b. The voltages in 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 optical modulator 40b-2 are omitted.

[0277] According to the second embodiment, by driving the Mach-Zehnder modulator with an optimized voltage for each Mach-Zehnder modulator, the phase adjustment range can be set to a predetermined size, and the increase in light absorption loss can be suppressed.

[0278] As an example of optical modulator 40, it is configured as an IQ modulator in the first embodiment and as a DP-IQ modulator in the second embodiment. This disclosure can also be applied to optical modulators other than those described above.

[0279] The embodiments of this disclosure have been described in detail above, but this disclosure is not limited to this specific embodiment. Various modifications and alterations can be made within the scope of the spirit of this disclosure as described in the technical solution.

Claims

1. A method for manufacturing an optical modulator, characterized in that, The optical modulator has a Mach-Zehnder modulator. The Mach-Zehnder modulator has a first electrode, a second electrode, a first branch waveguide, and a second branch waveguide. The first electrode is disposed on the first branch waveguide. The second electrode is disposed on the second branch waveguide. The manufacturing method includes the following steps: Prepare the Mach-Zehnder modulator; Based on the light transmittance in the first branch waveguide and the light transmittance in the second branch waveguide, the relationship between the voltage applied to the first electrode and the change in the phase of the light propagating in the first branch waveguide, i.e., the first relationship, and the relationship between the voltage applied to the second electrode and the change in the phase of the light propagating in the second branch waveguide, i.e., the second relationship, are obtained. Based on the first relationship and the second relationship, obtain the voltage applied to the first electrode and the voltage applied to the second electrode such that the range of the difference between the phase change in the first branch waveguide and the phase change in the second branch waveguide is a predetermined size. The voltage applied to the first electrode and the voltage applied to the second electrode, obtained in the process of obtaining the voltage applied to the first electrode and the voltage applied to the second electrode, are stored in the storage unit; The transmittance of light, i.e., the first transmittance, of the first branch waveguide and the second branch waveguide is measured. as well as The transmittance, or second transmittance, of light in both the first and second branch waveguides is calculated. In the process of calculating the second transmittance, the second transmittance in the first branch waveguide is calculated by expressing the second transmittance in the first branch waveguide as a function of the phase change of the light propagating in the first branch waveguide, and by expressing the phase change of the light propagating in the first branch waveguide as a function of the voltage applied to the first electrode. The second transmittance in the second branch waveguide is calculated by expressing the second transmittance as a function of the phase change of light propagating in the second branch waveguide, and by expressing the phase change of light propagating in the second branch waveguide as a function of the voltage applied to the second electrode. In the process of obtaining the first relationship and the second relationship, in each of the first branch waveguide and the second branch waveguide, the second transmittance is adjusted in a manner close to the first transmittance, thereby obtaining the first relationship and the second relationship.

2. The method for manufacturing an optical modulator according to claim 1, characterized in that, The process of preparing the Mach-Zehnder modulator is the process of preparing multiple Mach-Zehnder modulators. The process of obtaining the first relationship and the second relationship, and obtaining the voltage applied to the first electrode and the voltage applied to the second electrode, are performed on the multiple Mach-Zehnder modulators respectively.

3. The method for manufacturing an optical modulator according to claim 1 or 2, characterized in that, The process of preparing the Mach-Zehnder modulator includes the process of preparing a mother Mach-Zehnder modulator and the process of preparing a daughter Mach-Zehnder modulator. The process of obtaining the first relationship and the second relationship, and obtaining the voltage applied to the first electrode and the voltage applied to the second electrode, are performed on the mother Mach-Zehnder modulator and the daughter Mach-Zehnder modulator, respectively.

4. The method for manufacturing an optical modulator according to claim 1, characterized in that, The voltage applied to the first electrode is the sum of the first voltage and the second voltage. The voltage applied to the second electrode is the difference between the first voltage and the second voltage. The process of obtaining the voltage applied to the first electrode and the voltage applied to the second electrode is a process of obtaining the first voltage such that the range of the change in phase is the predetermined size.

5. The method for manufacturing an optical modulator according to claim 1 or 2, characterized in that, The process of preparing the Mach-Zehnder modulator includes the process of forming the Mach-Zehnder modulator. The process of forming the Mach-Zehnder modulator includes forming a first branch waveguide and a second branch waveguide 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 sequentially. The first semiconductor layer has a first conductivity type. The second semiconductor layer has a second conductivity type.

6. A test method for an optical modulator, characterized in that, The optical modulator has a Mach-Zehnder modulator. The Mach-Zehnder modulator has a first electrode, a second electrode, a first branch waveguide, and a second branch waveguide. The first electrode is disposed on the first branch waveguide. The second electrode is disposed on the second branch waveguide. The test method includes the following steps: Based on the light transmittance in the first branch waveguide and the light transmittance in the second branch waveguide, the relationship between the voltage applied to the first electrode and the change in the phase of the light propagating in the first branch waveguide, i.e., the first relationship, and the relationship between the voltage applied to the second electrode and the change in the phase of the light propagating in the second branch waveguide, i.e., the second relationship, are obtained. Based on the first relationship and the second relationship, the voltage applied to the first electrode and the voltage applied to the second electrode are obtained such that the difference between the phase change in the first branch waveguide and the phase change in the second branch waveguide is within a predetermined range. The transmittance of light, i.e., the first transmittance, of the first branch waveguide and the second branch waveguide is measured. as well as The transmittance, or second transmittance, of light in both the first and second branch waveguides is calculated. In the process of calculating the second transmittance, the second transmittance in the first branch waveguide is calculated by expressing the second transmittance in the first branch waveguide as a function of the phase change of the light propagating in the first branch waveguide, and by expressing the phase change of the light propagating in the first branch waveguide as a function of the voltage applied to the first electrode. The second transmittance in the second branch waveguide is calculated by expressing the second transmittance as a function of the phase change of light propagating in the second branch waveguide, and by expressing the phase change of light propagating in the second branch waveguide as a function of the voltage applied to the second electrode. In the process of obtaining the first relationship and the second relationship, in each of the first branch waveguide and the second branch waveguide, the second transmittance is adjusted in a manner close to the first transmittance, thereby obtaining the first relationship and the second relationship.

7. A storage medium that stores a test program for an optical modulator. Its features are, The optical modulator has a Mach-Zehnder modulator. The Mach-Zehnder modulator has a first electrode, a second electrode, a first branch waveguide, and a second branch waveguide. The first electrode is disposed on the first branch waveguide. The second electrode is disposed on the second branch waveguide. The experimental procedure causes the computer to perform the following processing: Based on the light transmittance in the first branch waveguide and the light transmittance in the second branch waveguide, the relationship between the voltage applied to the first electrode and the change in the phase of the light propagating in the first branch waveguide, i.e., the first relationship, and the relationship between the voltage applied to the second electrode and the change in the phase of the light propagating in the second branch waveguide, i.e., the second relationship, are obtained. Based on the first relationship and the second relationship, obtain the voltage applied to the first electrode and the voltage applied to the second electrode such that the difference between the phase change in the first branch waveguide and the phase change in the second branch waveguide is within a predetermined range; and The transmittance of light in the first branch waveguide and the second branch waveguide, i.e., the first transmittance, is measured. The transmittance of light in the first branch waveguide and the second branch waveguide, i.e., the second transmittance, is calculated by expressing the second transmittance in the first branch waveguide as a function of the phase change of light propagating in the first branch waveguide, and by expressing the phase change of light propagating in the first branch waveguide as a function of the voltage applied to the first electrode. The second transmittance in the second branch waveguide is calculated by expressing the second transmittance in the second branch waveguide as a function of the phase change of light propagating in the second branch waveguide, and by expressing the phase change of light propagating in the second branch waveguide as a function of the voltage applied to the second electrode. In each of the first and second branch waveguides, the second transmittance is adjusted in a manner close to the first transmittance to obtain the first relationship and the second relationship.

8. An optical transmitting device, characterized in that, The optical transmitting device includes a storage unit and multiple Mach-Zehnder modulators. The plurality of Mach-Zehnder modulators have a first electrode, a second electrode, a first branch waveguide, and a second branch waveguide. The first electrode is disposed on the first branch waveguide. The second electrode is disposed on the second branch waveguide. The storage unit stores, for each of the plurality of Mach-Zehnder modulators, the voltage applied to the first electrode and the voltage applied to the second electrode such that the range of the difference between the phase change in the first branch waveguide and the phase change in the second branch waveguide is a predetermined size.

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