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 of an optical modulator capable of suppressing an increase in absorption loss of light. A manufacturing method of an optical modulator having a Mach-Zehnder modulator having an electrode and a branch waveguide, the electrode being provided to the branch waveguide, the manufacturing method having: a step of preparing the Mach-Zehnder modulator; a step of acquiring a relationship between a voltage applied to the electrode and an amount of change in a phase of light propagating in the branch waveguide, based on a transmittance of light in the branch waveguide; a step of acquiring a voltage that makes the amount of change in the phase of the light propagating in the branch waveguide a predetermined size when modulating the light, based on the relationship; and a step of storing the voltage in a storage unit.

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).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2014-164243 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] 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. During modulation, for example, to increase the intensity of the output light, the amount of phase change is set to a predetermined magnitude.

[0008] 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 change. 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 testing procedure, and an optical transmitting device capable of suppressing the increase in light absorption loss.

[0009] means for solving problems

[0010] The manufacturing method disclosed herein relates to a method for manufacturing an optical modulator having a Mach-Zehnder modulator having electrodes and a branch waveguide, the electrodes being disposed in the branch waveguide. The manufacturing method comprises: 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, based on the relationship, a voltage is acquired such that the change in the phase of light propagating in the branch waveguide becomes a predetermined value; and a storage step in which the voltage acquired in the voltage acquisition step is stored in a storage unit.

[0011] The testing method disclosed herein is a testing method for an optical modulator having a Mach-Zehnder modulator having electrodes and a branch waveguide, the electrodes being disposed in the branch waveguide. The testing method comprises: 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 waveguide, based on the transmittance of light in the branch waveguide; and a voltage acquisition step, in which, based on the relationship, a voltage is acquired that causes a change in the phase of light propagating in the branch waveguide to be of a predetermined magnitude when the light is modulated.

[0012] The test procedure disclosed herein relates to an optical modulator having a Mach-Zehnder modulator having electrodes and a branch waveguide, the electrodes being disposed in the branch waveguide. The test procedure causes a computer to perform the following processing: based on the transmittance of light in the branch waveguide, obtaining a relationship between the voltage applied to the electrodes and the amount of phase change of light propagating in the branch waveguide; and based on the relationship between the voltage applied to the electrodes and the amount of phase change of light propagating in the branch waveguide, obtaining a voltage that makes the amount of phase change of light of a predetermined magnitude when the light propagating in the branch waveguide is modulated.

[0013] The optical transmitting apparatus disclosed herein includes a storage unit and a plurality of Mach-Zehnder modulators, each having electrodes and a branch waveguide, the electrodes being disposed in the branch waveguide, and the storage unit storing, for each of the plurality of Mach-Zehnder modulators, a voltage that causes a predetermined change in the phase of light propagating in the branch waveguide to be modulated.

[0014] Invention Effects

[0015] According to this disclosure, it is possible to suppress the increase in light absorption loss. Attached Figure Description

[0016] Figure 1A This is a block diagram illustrating an optical transmitting apparatus according to the first embodiment.

[0017] Figure 1B It is a block diagram representing the hardware structure of the control unit.

[0018] Figure 2A This is a top view illustrating an optical modulator.

[0019] Figure 2B It is along Figure 2A A sectional view of line AA.

[0020] Figure 3 This is an example of a constellation diagram that outputs light.

[0021] Figure 4A This is an example of a constellation diagram that outputs light.

[0022] Figure 4B This is an example of a constellation diagram that outputs light.

[0023] Figure 5A This is an example of a constellation diagram that outputs light.

[0024] Figure 5B This is an example of a constellation diagram that outputs light.

[0025] Figure 6A This is a graph illustrating the relationship between voltage and phase change.

[0026] Figure 6B This is a graph illustrating the relationship between voltage and phase change.

[0027] Figure 7A This is a graph illustrating the relationship between voltage and the change in light absorption loss.

[0028] Figure 7B This is a graph illustrating the relationship between voltage and the change in light absorption loss.

[0029] Figure 8 This is a flowchart illustrating an example of a method for manufacturing an optical modulator.

[0030] Figure 9 This is a flowchart illustrating an experiment.

[0031] Figure 10A This is a diagram illustrating the calculated phase change.

[0032] Figure 10BThis is a graph illustrating the calculated change in absorption loss.

[0033] Figure 11A This is a graph illustrating the calculated transmittance.

[0034] Figure 11B The graph shows an example of the measured transmittance and the optimized transmittance.

[0035] Figure 12A This is a diagram illustrating the optimized phase change.

[0036] Figure 12B This is a graph illustrating the change in absorption loss after optimization.

[0037] Figure 13 This is a graph illustrating the relationship between the bias voltage and the difference in phase change.

[0038] Figure 14 The graph shows an example of the measured transmittance and the optimized transmittance.

[0039] Figure 15A This is a diagram illustrating the optimized phase change.

[0040] Figure 15B This is a graph illustrating the change in absorption loss after optimization.

[0041] Figure 16 This is a graph illustrating the relationship between the bias voltage and the difference in phase change.

[0042] Figure 17 This is a top view illustrating an optical modulator. Detailed Implementation

[0043] [Description of embodiments of this disclosure]

[0044] First, the contents of the embodiments of this disclosure will be described.

[0045] One aspect of this disclosure is (1) a method for manufacturing an optical modulator, wherein the optical modulator has a Mach-Zehnder modulator, the Mach-Zehnder modulator has electrodes and a branch waveguide, the electrodes being disposed in the branch waveguide, 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 change in the phase of light propagating in the branch waveguide becomes a predetermined value; and a storage step in which the voltage acquired in the voltage acquisition step is stored in a storage unit. By applying the acquired voltage to the Mach-Zehnder modulator to modulate light, the change in phase during modulation can be set to a predetermined value, and the increase in light absorption loss can be suppressed.

[0046] (2) Alternatively, the process of preparing the Mach-Zehnder modulator may involve preparing multiple Mach-Zehnder modulators, and performing a process of obtaining the relationship between the voltage and the change in phase for each of the multiple Mach-Zehnder modulators, as well as a voltage acquisition process. This allows the change in phase during modulation to be set to a predetermined value for each Mach-Zehnder modulator, and suppresses the increase in light absorption loss.

[0047] (3) 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 voltage acquisition process includes: acquiring a voltage applied to the first electrode such that the amount of phase change in the first branch waveguide is a predetermined value; and acquiring a voltage applied to the second electrode such that the amount of phase change in the second branch waveguide is a predetermined value. During modulation, the amount of phase change in the first branch waveguide and the amount of phase change in the second branch waveguide can be set to a predetermined value. It is also possible to suppress the increase in light absorption loss.

[0048] (4) Alternatively, the voltage applied to the first electrode may be the sum of a first voltage and a second voltage, and the voltage applied to the second electrode may be the difference between the first voltage and the second voltage. The voltage acquisition process includes acquiring a first voltage such that the second voltage is below a predetermined value. During modulation, the amount of phase change in the first branch waveguide and the amount of phase change in the second branch waveguide can be set to a predetermined value. By setting the second voltage below a predetermined value, an increase in power consumption can be suppressed.

[0049] (5) Alternatively, the process of preparing the Mach-Zehnder modulator may include preparing a Mach-Zehnder modulator having a first branch waveguide, a second branch waveguide, a first electrode, a second electrode, a third electrode, and a fourth electrode, wherein the third electrode is disposed in the first branch waveguide, the fourth electrode is disposed in the second branch waveguide, and the process of obtaining the relationship between the voltage applied to the electrodes and the change in phase of light propagating in the branch waveguides includes the following steps: obtaining the relationship between the voltage applied to the first electrode and the change in phase of light propagating in the first branch waveguide based on the relationship between the voltage applied to the third 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 based on the relationship between the voltage applied to the fourth electrode and the change in phase of light propagating in the second branch waveguide. This allows for a highly accurate relationship between voltage and phase changes. It also allows for setting the phase change to a predetermined value and suppressing an increase in light absorption loss.

[0050] (6) Alternatively, the method for manufacturing the optical modulator may further include: a step of measuring a first transmittance, the first transmittance being the transmittance of light in the branch waveguide; and a step of calculating a second transmittance, the second transmittance being the transmittance of light in the branch waveguide. In the step 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 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 third 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. The process of calculating the second transmittance in the second branch waveguide and expressing the change in phase of light propagating in the second branch waveguide as a function of the voltage applied to the fourth electrode, thereby calculating the second transmittance in the second branch waveguide and obtaining the relationship between the voltage applied to the electrode and the change in phase, includes the following steps: adjusting the second transmittance in the first branch waveguide to be close to the first transmittance in the first branch waveguide, thereby obtaining the relationship between the voltage applied to the third electrode and the change in phase of light propagating in the first branch waveguide; and adjusting the second transmittance in the second branch waveguide to be close to the first transmittance in the second branch waveguide, thereby obtaining the relationship between the voltage applied to the fourth electrode and the change in phase of light propagating in the second branch waveguide. By making the second transmittance close to the first transmittance, a more accurate relationship between voltage and phase change can be obtained. The change in phase can be set to a predetermined value, and the increase in light absorption loss can be suppressed.

[0051] (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 semiconductor layer and the second semiconductor layer. Due to the deviation 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 amount of phase change can be set to a predetermined magnitude, and the increase in light absorption loss can be suppressed.

[0052] (8) A method for testing 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, 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 that causes the change in the phase of light propagating in the branch waveguide to be of a predetermined magnitude when the light is modulated. By applying the acquired voltage to the Mach-Zehnder modulator to modulate the light, the change in phase during modulation can be set to a predetermined magnitude, and an increase in light absorption loss can be suppressed.

[0053] (9) A storage medium that stores 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 causes a computer to perform the following processing: based on the transmittance of light in the branch waveguide, obtaining a relationship between a voltage applied to the electrodes and the amount of phase change of light propagating in the branch waveguide; and based on the relationship between the voltage applied to the electrodes and the amount of phase change of light propagating in the branch waveguide, obtaining a voltage that, when modulating light propagating in the branch waveguide, causes the amount of phase change of the light to be of a predetermined magnitude. By applying the obtained voltage to the Mach-Zehnder modulator to modulate light, the amount of phase change during modulation can be set to a predetermined magnitude, and the increase in light absorption loss can be suppressed.

[0054] (10) An optical transmitting apparatus, wherein the optical transmitting apparatus 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 such that the amount of phase change of the light propagating in the branch waveguide is of a predetermined magnitude when the light is modulated. By applying the stored voltage to the Mach-Zehnder modulators to modulate the light, the amount of phase change during modulation can be set to a predetermined magnitude, and the increase in light absorption loss can be suppressed.

[0055] [Details of the embodiments disclosed herein]

[0056] 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 meaning and scope equivalent to the technical solutions.

[0057] <First Implementation Method>

[0058] (Optical transmission device)

[0059] Figure 1A This is a block diagram illustrating, by way of example, 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.

[0060] 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).

[0061] 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.

[0062] The control unit 10 implements the program stored in RAM 32 by executing the program by CPU 30. Figure 1AThe 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, etc., 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.

[0063] (Optical modulator)

[0064] Figure 2A This is a top view illustrating an optical modulator 40a. In the first embodiment, the optical modulator 40a is used as... Figure 1A The 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.

[0065] 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.

[0066] 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 both sub-Mach-Zehnder modulators 42a and 42b and coupler 64.

[0067] (Sub-Mach-Zehnder modulator)

[0068] 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.

[0069] 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.

[0070] 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.

[0071] A modulation electrode 66a (first electrode) and a phase adjustment electrode 68a (third electrode) are disposed on branch waveguide 54a. The modulation electrode 66a and the phase adjustment electrode 68a are separate from each other and are arranged sequentially from the coupler 60a side towards the coupler 62a side. A modulation electrode 66b (second electrode) and a phase adjustment electrode 68b (fourth electrode) are disposed on branch waveguide 54b. The modulation electrode 66b and the phase adjustment electrode 68b are separate from each other and are arranged sequentially from the coupler 60a side towards the coupler 62a side.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] Modulation electrode 66d (first electrode) and phase adjustment electrode 68d (third electrode) are disposed on branch waveguide 54c. Modulation electrode 66e (second electrode) and phase adjustment electrode 68e (fourth 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.

[0079] 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.

[0080] 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.

[0081] Phase adjustment electrodes 68a, 68b, 68d, and 68e are of equal length. Modulation electrodes 66a, 66b, 66d, and 66e are of equal length, as are ground electrodes 66c and 66f. The length of one modulation electrode is greater than the length of one phase adjustment electrode, for example, 2.5 times the length of the phase adjustment electrode. The lengths of ground electrodes 68c and 68f are equal and less than the lengths of the phase adjustment electrodes.

[0082] (Mach-Zehnder modulator)

[0083] The mother Mach-Zehnder modulator 44a has branch waveguides 55a and 55b, phase adjustment electrodes 70a and 70b, and a ground electrode 70c. A first end of branch waveguide 55a is connected to the output of coupler 62a. A first end of branch waveguide 55b is connected to the output of coupler 62b. The second ends of both 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 sub-Mach-Zehnder modulator and are bent on the side near coupler 64.

[0084] Phase adjustment electrode 70a is disposed on branch waveguide 55a. Phase adjustment electrode 70b is disposed on branch waveguide 55b. Ground electrode 70c is disposed between branch waveguide 55a and branch waveguide 55b. Phase adjustment electrode 70a, phase adjustment electrode 70b, and ground electrode 70c extend in the same direction as the branch waveguide and are parallel to end face 80c.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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-InP) 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] (Operation of the optical transmitting device)

[0094] 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.

[0095] Figure 1AThe 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] As for the modulation method used in the optical modulator 40a, for example, there is the QPSK (Quadrature Phase Shift Keying) modulation method. In the QPSK modulation method, modulated light is generated, which corresponds to the four symbol codes 00, 01, 10, and 11 of the modulation signal, respectively. In order to generate the modulated light, the driver IC26 applies a voltage to the modulation electrode.

[0101] The voltage applied to modulation electrode 66a is denoted as VIp, and the voltage applied to modulation electrode 66b is denoted as VIn. The voltage applied to modulation electrode 66d is denoted as VQp, and the voltage applied to modulation electrode 66e is denoted as VQn. Voltages VIp, VIn, VQp, and VQn are calculated based on the bias voltage Vb and the amplitude voltage Vpp, respectively.

[0102] The number 1 represents the voltages VIp, VIn, VQp, and VQn corresponding to the symbol code 00.

[0103]

Number 1

[0104] VIp=Vb+Vpp / 2, VIn=Vb-Vpp / 2, VQp=Vb+Vpp / 2, VQn=Vb-Vpp / 2

[0105] Number 2 represents the voltages VIp, VIn, VQp, and VQn corresponding to symbol code 01.

[0106]

Number 2

[0107] VIp=Vb-Vpp / 2, VIn=Vb+Vpp / 2, VQp=Vb+Vpp / 2, VQn=Vb-Vpp / 2

[0108] Number 3 represents the voltages VIp, VIn, VQp, and VQn corresponding to symbol code 10.

[0109]

Number 3

[0110] VIp=Vb+Vpp / 2, VIn=Vb-Vpp / 2, VQp=Vb-Vpp / 2, VQn=Vb+Vpp / 2

[0111] Number 4 represents the voltages VIp, VIn, VQp, and VQn corresponding to symbol code 11.

[0112]

Number 4

[0113] VIp=Vb-Vpp / 2, VIn=Vb+Vpp / 2, VQp=Vb-Vpp / 2, VQn=Vb+Vpp / 2

[0114] Figures 3 to 5B These are examples of constellation diagrams for the output light. In each diagram, Ip is the output light of the p-side branch waveguide 54a of the sub-Mach-Zehnder modulator 42a of Ich. In is the output light of the n-side branch waveguide 54b of the sub-Mach-Zehnder modulator 42a of Ich. Qp is the output light of the p-side branch waveguide 54c of the sub-Mach-Zehnder modulator 42b of Qch. Qn is the output light of the n-side branch waveguide 54d of the sub-Mach-Zehnder modulator 42b of Qch.

[0115] Figure 3 This is the constellation diagram in the unmodulated state. The amplitude voltage Vpp is 0V. Bias voltage Vb is applied to each of the multiple modulation electrodes. The sub-Mach-Zehnder modulator is adjusted to the extinction point. The phase of the output light Ip of the p-side branch waveguide 54a of the sub-Mach-Zehnder modulator 42a is reversed with the phase of the output light In of the n-side branch waveguide 54b. The output lights of the two branch waveguides cancel each other out. The phase of the output light Qp of the branch waveguide 54c of the sub-Mach-Zehnder modulator 42b is reversed with the phase of the output light Qn of the branch waveguide 54d of the sub-Mach-Zehnder modulator 42b. The output lights of the two branch waveguides cancel each other out.

[0116] Figure 4A This is the constellation diagram with symbol code 00. A voltage as shown in number 1 is applied to each modulation electrode. By applying voltage Vpp / 2 as shown in number 1 to the electrodes, the output light Ip of branch waveguide 54a is synchronized with... Figure 3 Compared to a counterclockwise rotation of approximately π / 2, by applying a voltage of -Vpp / 2 to the electrodes, the output light In of the branch waveguide 54b is made to... Figure 3 Compared to a clockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42a is obtained by combining the output light ID and the output light In, represented by a vector on the I-axis. The output light Qp of the branch waveguide 54c is... Figure 3 Compared to a counterclockwise rotation of approximately π / 2, the output light Qn of the 54d branch waveguide is... Figure 3 Compared to a clockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42b is obtained by combining the output lights Qp and Qn, and is represented by a vector on the Q-axis. By combining the output lights of the sub-Mach-Zehnder modulator 42a and the sub-Mach-Zehnder modulator 42b, the output light of the optical modulator 40a corresponding to the symbol code 00 is obtained. The output light is represented by a vector located in the first quadrant.

[0117] Figure 4B This is a constellation diagram with symbol code 01. The voltage shown in number 2 is applied to each modulation electrode. The output light Ip of branch waveguide 54a is... Figure 3Compared to a clockwise rotation of approximately π / 2, the output light In of the 54b branch waveguide is... Figure 3 Compared to a counterclockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42a is represented by a vector on the I-axis. The output light Qp of the branch waveguide 54c is... Figure 3 Compared to a counterclockwise rotation of approximately π / 2, the output light Qn of the 54d branch waveguide is... Figure 3 Compared to a clockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42b is represented by a vector on the Q-axis. The output light of the optical modulator 40a, corresponding to symbol code 01, is represented by a vector located in the second quadrant.

[0118] Figure 5A This is the constellation diagram of symbol code 10. The voltage shown in number 3 is applied to each modulation electrode. The output light Ip of branch waveguide 54a is... Figure 3 Compared to a counter-clockwise rotation of approximately π / 2, the output light In of the 54b branch waveguide is... Figure 3 Compared to a clockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42a is represented by a vector on the I-axis. The output light Qp of the branch waveguide 54c is... Figure 3 Compared to a clockwise rotation of approximately π / 2, the output light Qn of branch waveguide 54d is... Figure 3 Compared to a counterclockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42b is represented by a vector on the Q-axis. The output light of the optical modulator 40a, corresponding to symbol code 10, is represented by a vector located in the fourth quadrant.

[0119] Figure 5B This is the constellation diagram of symbol code 11. The voltage shown in number 4 is applied to each modulation electrode. The output light Ip of branch waveguide 54a is... Figure 3 Compared to a clockwise rotation of approximately π / 2, the output light In of the 54b branch waveguide is... Figure 3 Compared to a counterclockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42a is represented by a vector on the I-axis. The output light Qp of the branch waveguide 54c is... Figure 3 Compared to a clockwise rotation of approximately π / 2, the output light Qn of branch waveguide 54d is... Figure 3 Compared to a counterclockwise rotation of approximately π / 2, the output light of the sub-Mach-Zehnder modulator 42b is represented by a vector on the Q-axis. The output light of the optical modulator 40a, corresponding to symbol code 11, is represented by a vector located in the third quadrant.

[0120] like Figures 4A to 5BAs shown, the intensity of the modulated light is maximized by rotating the phase of the light in each branch waveguide by approximately π / 2 clockwise or counterclockwise compared to the unmodulated state. The amplitude voltage Vpp only needs to be equal to the voltage Vπ, which is defined as the voltage that rotates the phase of the signal by π. A voltage Vπ / 2, which is half the voltage Vπ, is the voltage that rotates the phase by π / 2. When Vpp = Vπ, Vpp / 2 = Vπ / 2. Voltages ±Vpp / 2 (numbers 1 to 4) are equal to voltage ±Vπ / 2, rotating the phase of the light by ±π / 2.

[0121] To suppress the increase in power consumption of driver IC26, reducing the amplitude voltage Vpp is effective. The upper limit of the amplitude voltage Vpp can be set to, for example, 1.7V. On the other hand, as in... Figure 6A as well as Figure 6B As explained in the text, by determining the bias voltage Vb, the amplitude voltage Vpp corresponding to the phase rotation of π / 2 is also determined.

[0122] Figure 6A This is a graph illustrating the relationship between voltage and phase change. The horizontal axis represents the voltage applied to the modulation electrode of the sub-Mach-Zehnder modulator 42a. The vertical axis represents the phase change of the light in the branch waveguide of the sub-Mach-Zehnder modulator 42a. As voltage increases, the phase change... It also gets bigger.

[0123] Figure 6A The dashed line represents 11.5V. With a bias voltage Vb of 11.5V, the phase change in the voltages shown by the dashed line, using 11.5V as a reference, is ±π / 2. That is, the difference in phase change between the two branch waveguides is π. Specifically, the voltage applied to the modulation electrode 66a is set to Vb + Vpp / 2 = 12.35V. The output light of branch waveguide 54a rotates by π / 2. The voltage applied to the modulation electrode 66b is set to Vb - Vpp / 2 = 10.65V. The output light of branch waveguide 54b rotates by -π / 2. This allows for... Figure 4A The modulation is shown. At this time, Vπ / 2 = Vpp / 2 = 0.85V. The amplitude voltage Vpp is 1.7V, which is equal to the upper limit.

[0124] like Figure 6A As shown, the phase change amount The relationship with voltage is non-linear. The larger the voltage, the greater the phase change. The greater the rate of change (slope), the greater the slope of the phase change. The slope of the phase change when the bias voltage Vb = 11.5V is greater than the slope of the phase change when the bias voltage Vb < 11.5V. Therefore, at a bias voltage Vb of 11.5V, Vπ / 2 decreases. When the bias voltage Vb is less than 11.5V, the slope of the phase change decreases, and therefore the amplitude voltage Vpp exceeds 1.7V. To keep the amplitude voltage Vpp below 1.7V, the bias voltage Vb is set to above 11.5V.

[0125] 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 deviations in the thermal diffusion of the dopant in the cladding layer 82, cladding layer 86, contact layer 88, etc. Due to the difference in the thermal diffusion of the dopant, the strength of the electric field generated in the core layer 84 when a voltage is applied also varies. If the electric field strength differs, the refractive index also differs, resulting in different magnitudes of phase change. Due to the deviation in the thermal diffusion of the dopant, the bandgap energy also deviates, thus changing the phase change. Here, the sub-Mach-Zehnder modulator 42b has a higher phase adjustment efficiency compared to the sub-Mach-Zehnder modulator 42a.

[0126] Figure 6B This is a graph illustrating the relationship between voltage and phase change. The horizontal axis represents the voltage applied to the modulation electrode of the sub-Mach-Zehnder modulator 42b. The vertical axis represents the phase change of the light in the branch waveguide of the sub-Mach-Zehnder modulator 42b. When comparing at the same voltage Figure 6B Phase change Compare Figure 6A The voltage Vpp of the sub-Mach-Zehnder modulator 42b is larger than that of the sub-Mach-Zehnder modulator 42a. Similarly, if Vb = 11.5V, then the voltage Vπ / 2 corresponding to a phase rotation of π / 2 is 0.7V. The amplitude voltage Vpp of the sub-Mach-Zehnder modulator 42b is smaller than that of the sub-Mach-Zehnder modulator 42a, at 1.4V. Figure 6B As shown by the dashed line, Vb + Vpp / 2 = 12.2V. Vb - Vpp / 2 = 10.8V.

[0127] like Figure 6A as well as Figure 6B As shown, by setting the bias voltage Vb, for example, to 11.5V, the amplitude voltage Vpp can be set to below 1.7V in both the sub-Mach-Zehnder modulator 42b with high phase adjustment efficiency and the sub-Mach-Zehnder modulator 42a with low phase adjustment efficiency. By reducing the amplitude voltage Vpp, the power consumption of the driver IC26 can be reduced.

[0128] 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. The lower the phase adjustment efficiency, the lower the absorption loss; the higher the phase adjustment efficiency, the greater the absorption loss.

[0129] Figure 7A as well as Figure 7B This is a graph illustrating the relationship between voltage and the change in light absorption loss. The horizontal axis represents the voltage applied to the modulation electrode of the sub-Mach-Zehnder modulator. The vertical axis represents the change in light absorption loss in the branch waveguide.

[0130] Figure 7A This represents an example of a sub-Mach-Zehnder modulator 42a. Figure 7B This illustrates an example of a sub-Mach-Zehnder modulator 42b. The variation in absorption loss in sub-Mach-Zehnder modulator 42b is greater than that in sub-Mach-Zehnder modulator 42a. For example, at 11.5V, the variation in absorption loss of sub-Mach-Zehnder modulator 42a is 0.18dB, while the variation in absorption loss of sub-Mach-Zehnder modulator 42b is 1.1dB.

[0131] To reduce the power consumption of driver IC26, the amplitude voltage Vpp is reduced 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. For example... Figure 6A as well as Figure 6B As shown, for both sub-Mach-Zehnder modulators 42a and 42b, the bias voltage Vb can be set to, for example, 11.5V. However, as... Figure 7B As shown, the variation in absorption loss of sub-Mach-Zehnder modulator 42b is greater than that of sub-Mach-Zehnder modulator 42a. To suppress the increase in light absorption loss, it is important to optimize the voltage applied to the modulation electrode for each Mach-Zehnder modulator.

[0132] (Manufacturing method)

[0133] 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 first 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 the 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). If there is a deviation in the thermal diffusion of the dopant, then... Figure 4A as well as Figure 4B , Figure 7A as well as Figure 7B As shown, the phase adjustment efficiency also deviates.

[0134] 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.

[0135] 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, a sub-Mach-Zehnder modulator 42a is tested to optimize the voltage applied to modulation electrodes 66a and 66b (step S4). A sub-Mach-Zehnder modulator 42b is tested to optimize the voltage applied to modulation electrodes 66d and 66e (step S5). The optical modulator 40a is formed through the above steps.

[0136] (test)

[0137] Figure 9 This is a flowchart illustrating an experiment. Figure 8 Steps S4 and S5 are respectively for performing Figure 9 The procedure for the experiment is shown.

[0138] During the testing of the sub-Mach-Zehnder modulator 42a ( Figure 8In step S4), the phase control unit 12 of the control unit 10 applies a voltage to the phase adjustment electrode 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. 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.

[0139] 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).

[0140] 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). Through transmittance optimization, parameters (coefficients) are obtained. The parameters are corrected based on the ratio of the length of the phase adjustment electrode to the length of the modulation electrode (step S15). The calculation unit 15 obtains the relationship between the voltage applied to the modulation electrode and the amount of phase change 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 amount of phase change a predetermined value, and stores this voltage, for example, in the storage device 34 (step S18).

[0141] 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.

[0142]

Number 5

[0143] Δφ=k1×V+k2×V 2 +k3×V 3 +k4×V4 +k5×V 5 +k6×V 6

[0144] The following shows an example of the initial values ​​for the coefficients.

[0145] k1 = 1 × 10 -1 (π / V), k2=1×10 -2 (π / V 2 k3 = 1 × 10 -5 (π / V 3 k4 = 1 × 10 -6 (π / V 4 k5 = 1 × 10 -7 (π / V 5 k6 = 1 × 10 -8 (π / V 6 )

[0146] 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 5) 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.

[0147] 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.

[0148]

Number 6

[0149] ΔL1=a1×(1-exp(-V / a2))

[0150] The initial values ​​of coefficients a1 and a2 are shown below.

[0151] a1 = 1 × 10 -4 (dB), a2 = 1.5 (V)

[0152] Figure 10BThis 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. The solid lines represent the variations ΔL1 in branch waveguide 54a and branch waveguide 54b. Since the same function (number 6) and the same coefficients are used for the calculations of branch waveguide 54a and branch waveguide 54b, the variation in absorption loss ΔL1 is also the same.

[0153] 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.

[0154]

Number 7

[0155] T=(1+10^(ΔL1 / 10)+2×10^(ΔL1 / 20)×cos(φ0±Δφ)) / (1+10^(ΔL1 / 10)) 2

[0156] Phase change The change in absorption loss, ΔL1, is represented by number 5. The sign of the cosine function (cos) in number 7 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 8, acos is the inverse cosine function.

[0157]

Number 8

[0158] φ0=±acos(T0 0.5 )

[0159] 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...

[0160] 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 11B The 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.

[0161] 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 7 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.

[0162] 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.

[0163] By optimizing the transmittance, the initial phase difference contained in the formula for transmittance (Chapter 7) is changed. Phase change And the change in absorption loss ΔL1. Phase change. And the change in absorption loss ΔL1 becomes a function that more accurately represents the relationship with voltage.

[0164] More specifically, the amount of phase change The coefficients k1 to k6 in equation (5), and the coefficients a1 and a2 in equation (6) representing the change ΔL1, change relative to their initial values. The optimized coefficients are shown below.

[0165] For the coefficients of branch waveguide 54a

[0166] k1 = 8.36 × 10 -2 (π / V), k2 = 2.14 × 10 -3 (π / V 2 k3 = 1.33 × 10 -6 (π / V 3 k4 = 1.37 × 10 -5 (π / V 4 k5 = 3.15 × 10 -7 (π / V5 k6 = 9.54 × 10 -9 (π / V 6 a1 = 4.18 × 10 -4 (dB), a2 = 2.24 (V)

[0167] For the coefficients of branch waveguide 54b

[0168] k1 = 1.09 × 10 -1 (π / V), k2 = 1.28 × 10 -3 (π / V 2 k3 = 9.04 × 10 -7 (π / V 3 k4 = 6.66 × 10 -6 (π / V 4 k5 = 9.04 × 10 -8 (π / V 5 k6 = 1.98 × 10 -8 (π / V 6 a1 = 8.96 × 10 -4 (dB), a2 = 2.60 (V)

[0169] Initial phase difference after transmittance optimization It is 0.38π.

[0170] These coefficients represent the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change. To obtain the relationship between the voltage applied to the modulation electrode and the amount of phase change, these coefficients are corrected.

[0171] like Figure 2A As shown, the modulation electrode and the phase adjustment electrode are disposed on the branch waveguide. It is presumed that the phase change per unit length of the phase adjustment electrode when a voltage is applied is equal to the phase change per unit length of the modulation electrode when a voltage is applied. The length of the modulation electrode is greater than the length of the phase adjustment electrode, for example, 2.5 times the length of the phase adjustment electrode. It is presumed that the phase change when a voltage is applied to the modulation electrode changes proportionally to the phase change when a voltage is applied to the phase adjustment electrode, in a manner similar to the ratio of their lengths.

[0172] Based on the ratio of electrode lengths, the calculation unit 15 sets k1 to k6 and a1 in the above coefficients to 2.5 times, thereby calculating the following coefficients ( Figure 9 Step S15).

[0173] For the coefficients of branch waveguide 54a

[0174] k1 = 2.09 × 10 -1(π / V), k2 = 5.34 × 10 -3 (π / V 2 k3 = 3.32 × 10 -6 (π / V 3 k4 = 3.42 × 10 -5 (π / V 4 k5 = 7.88 × 10 -7 (π / V 5 k6 = 2.38 × 10 -8 (π / V 6 a1 = 1.05 × 10 -3 (dB)

[0175] For the coefficients of branch waveguide 54b

[0176] k1 = 2.73 × 10 -1 (π / V), k2 = 3.21 × 10 -3 (π / V 2 k3 = 2.26 × 10 -6 (π / V 3 k4 = 1.66 × 10 -5 (π / V 4 k5 = 2.26 × 10 -7 (π / V 5 k6 = 4.96 × 10 -8 (π / V 6 a1 = 2.24 × 10 -3 (dB)

[0177] a2 and k1 are the same as before the correction. The calculation unit 15 applies these coefficients to numbers 5 and 6 to calculate the amount of phase change and the amount of change of absorption loss when a voltage is applied to the modulation electrode.

[0178] Figure 12A This is a graph illustrating the optimized phase change. The horizontal axis represents the applied voltage to the modulation electrodes 66a and 66b of the sub-Mach-Zehnder modulator 42a. The vertical axis represents the phase change. The solid line represents the phase change of the branch waveguide on the p-side (branch waveguide 54a). The dashed line represents the phase change of the branch waveguide on the n-side (branch waveguide 54b). For example... Figure 12A As shown, by substituting the coefficients obtained through transmittance optimization and correction into number 5 for calculation, a result close to... Figure 6A The amount of phase change.

[0179] Figure 12BThis is a graph illustrating the variation in absorption loss after optimization. The horizontal axis represents the applied voltage for the modulation electrodes 66a and 66b of the sub-Mach-Zehnder modulator 42a. The vertical axis represents the variation in absorption loss. The solid line represents the variation in absorption loss of the p-side branch waveguide (branch waveguide 54a). The dashed line represents the variation in absorption loss of the n-side branch waveguide (branch waveguide 54b). (Example...) Figure 12B As shown, by substituting the coefficients obtained through transmittance optimization and correction into the number 6 for calculation, a result close to... Figure 7A The change in quantity.

[0180] Figure 13 This is a graph illustrating the relationship between the bias voltage and the phase change. The horizontal axis represents the bias voltage Vb. The vertical axis represents the difference between the phase change in branch waveguide 54a and the phase change in branch waveguide 54b when a voltage Vb + Vpp / 2 is applied to modulation electrode 66a and a voltage Vb - Vpp / 2 is applied to modulation electrode 66b. To suppress the increase in power consumption of driver IC26, Vpp is set to 1.7V. Figure 13 For each bias voltage Vb on the horizontal axis, apply a predetermined amplitude voltage Vpp = 1.7V to calculate the difference in phase change. The phase change difference between branch waveguide 54a and branch waveguide 54b only needs to be π. For example... Figure 13 As shown, when Vb = 11.5V, the difference in phase change is π. Figure 1B The storage device 34 shown stores the bias voltage Vb of the sub-Mach-Zehnder modulator 42a as 11.5V.

[0181] 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).

[0182] 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 corrects the parameters based on the ratio of the length of the phase adjustment electrode to the length of the modulation electrode (step S15). The calculation unit 15 obtains the relationship between the voltage applied to the modulation electrode and the amount of phase change (step S16). Based on the relationship between the voltage and the amount of phase change, the storage control unit 18 obtains the voltage that makes the amount of phase change a predetermined value and stores the voltage in the storage device 34 (step S18).

[0183] Figure 14 This 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.

[0184] Figure 15A This is a graph illustrating the optimized phase change. The horizontal axis represents the applied voltage to the modulation electrodes 66d and 66e of the sub-Mach-Zehnder modulator 42b. The vertical axis represents the phase change. The solid line represents the phase change of the branch waveguide on the p-side (branch waveguide 54c). The dashed line represents the phase change of the branch waveguide on the n-side (branch waveguide 54d). For example... Figure 15A As shown, by substituting the coefficients obtained through transmittance optimization and correction into number 5 for calculation, a result close to... Figure 6B The amount of phase change.

[0185] Figure 15B This is a graph illustrating the variation in absorption loss after optimization. The horizontal axis represents the applied voltage for the modulation electrodes 66d and 66e of the sub-Mach-Zehnder modulator 42b. The vertical axis represents the variation in absorption loss. The solid line represents the variation in absorption loss of the p-side branch waveguide (branch waveguide 54c). The dashed line represents the variation in absorption loss of the n-side branch waveguide (branch waveguide 54d). (Example...) Figure 15B As shown, by substituting the coefficients obtained through transmittance optimization and correction into the number 6 for calculation, a result close to... Figure 7B The change in quantity.

[0186] Figure 16This is a graph illustrating the relationship between the bias voltage and the phase change. The horizontal axis represents the bias voltage Vb. The vertical axis represents the difference between the phase change in branch waveguide 54c and branch waveguide 54d when a voltage Vb + Vpp / 2 is applied to modulation electrode 66d and a voltage Vb - Vpp / 2 is applied to modulation electrode 66e. The difference in phase change is calculated by applying a predetermined amplitude voltage Vpp = 1.7V to each bias voltage Vb on the horizontal axis. The difference in phase change only needs to be π. Figure 16 As shown by the dashed line, when Vb = 8.9V, the difference in phase change is π. Figure 1B The storage device 34 shown stores the bias voltage Vb of the sub-Mach-Zehnder modulator 42b as 8.9V.

[0187] Table 1 is an example of a data table stored in storage device 34.

[0188] Table 1

[0189]

[0190] The bias voltage Vb on the Ich side (sub-Mach-Zehnder modulator 42a) is 11.5V. The bias voltage Vb on the Qch side (sub-Mach-Zehnder modulator 42b) is 8.9V.

[0191] According to the first embodiment, the control unit 10 acquires the relationship between the voltage applied to the modulation electrode and the phase change, and acquires a voltage that makes the phase change during modulation a predetermined value. Light is modulated by applying the optimized voltage to the Mach-Zehnder modulator. The phase change during modulation can be set to a predetermined value, and the increase in light absorption loss can be suppressed.

[0192] Optical modulator 40a has a sub-Mach-Zehnder modulator 42a and a sub-Mach-Zehnder modulator 42b. The phase adjustment efficiency of sub-Mach-Zehnder modulator 42b is higher than that of sub-Mach-Zehnder modulator 42a. If the same bias voltage is applied to sub-Mach-Zehnder modulator 42b as to sub-Mach-Zehnder modulator 42a, the light absorption loss in sub-Mach-Zehnder modulator 42b is greater than that in sub-Mach-Zehnder modulator 42a. Figure 15B As shown, when the bias voltage Vb of the sub-Mach-Zehnder modulator 42b is set to 11.5V, which is equal to the bias voltage of the sub-Mach-Zehnder modulator 42a, the change in absorption loss is 1.1dB.

[0193] pass Figure 9The experiment optimized the voltage for each sub-Mach-Zehnder modulator. As shown in Table 1, the bias voltage Vb of sub-Mach-Zehnder modulator 42a was set to 11.5V, and the bias voltage Vb of sub-Mach-Zehnder modulator 42b was set to 8.9V. This allowed the phase change of sub-Mach-Zehnder modulators 42a and 42b to be set to a predetermined value, suppressing the increase in light absorption loss. By setting the bias voltage Vb of sub-Mach-Zehnder modulator 42b to 8.9V, the change in absorption loss was reduced to 0.26dB.

[0194] like Figure 13 As shown, the control unit 10 acquires a voltage such that the difference between the phase change in branch waveguide 54a and the phase change in branch waveguide 54b is π when the sub-Mach-Zehnder modulator 42a is modulated. Figure 16 As shown, the control unit 10 also acquires a voltage in the sub-Mach-Zehnder modulator 42b that makes the difference in phase change π. For example... Figures 4A to 5B As shown, the output light of each branch waveguide rotates by ±π / 2. For example, in... Figure 4A In, with Figure 3 In comparison, the phase of the output light Ip from branch waveguide 54a and the phase of the output light Qp from branch waveguide 54c are rotated by +π / 2. The phase of the output light In from branch waveguide 54b and the phase of the output light Qn from branch waveguide 54d are rotated by -π / 2. In QPSK modulation, the modulated light becomes the largest.

[0195] The sub-Mach-Zehnder modulator is differentially driven. In the absence of modulation, Vpp = 0, and a bias voltage Vb is applied to the modulation electrode. For example... Figure 3 As shown, the phase difference between the two paired branch waveguides is π. For example, when the modulation signal is symbol code 00, the voltage applied to the modulation electrode 66a of the sub-Mach-Zehnder modulator 42a and the modulation electrode 66d of the sub-Mach-Zehnder modulator 42b is Vb + Vpp / 2. The voltage Vpp / 2 rotates the phase of the light by π / 2. The voltage applied to the modulation electrode 66b and the modulation electrode 66e is Vb - Vpp / 2. The voltage -Vpp / 2 rotates the phase of the light by -π / 2. Figure 4A By synthesizing the above light, the output light during modulation is maximized.

[0196] The Mach-Zehnder modulator can also be driven using methods other than differential drive. Regardless of the driving method used, the phase change of the Mach-Zehnder modulator can be controlled at the optimal voltage, and the increase in absorption loss can be suppressed.

[0197] Depending on the modulation method, the phase change of each branch waveguide during modulation can be set to a value other than ±π / 2. The control unit 10 acquires a voltage that makes the phase change a predetermined value. Besides QPSK, the modulation method can be, for example, Quadrature Amplitude Modulation (QAM). In QAM modulation, phase modulation and amplitude modulation are combined.

[0198] To suppress the increase in power consumption of the driver IC26, the amplitude voltage Vpp is preferably, for example, below 1.7V. Figure 13 As shown, with the amplitude voltage Vpp set to a predetermined value such as 1.7V, a bias voltage Vb is obtained that results in a phase change difference of π between the two branch waveguides. By setting the bias voltage Vb to an optimal value, the phase change can be set to a predetermined value, and the increase in light absorption loss can be suppressed. Since the amplitude voltage Vpp is 1.7V, the power consumption of the driver IC26 can be reduced. The upper limit of the amplitude voltage Vpp can also be a value other than 1.7V.

[0199] like Figure 2A As shown, modulation electrode 66a and phase adjustment electrode 68a are disposed on branch waveguide 54a. Modulation electrode 66b and phase adjustment electrode 68b are disposed on branch waveguide 54b. Modulation electrode 66a and modulation electrode 66b are connected to the same terminal element 78a. Therefore, it is difficult to make the DC voltage applied to modulation electrode 66a different from the DC voltage applied to modulation electrode 66b. It is also difficult to measure transmittance while scanning the voltage for the modulation electrode. On the other hand, it is possible to make the DC voltage applied to phase adjustment electrode 68a different from the DC voltage applied to phase adjustment electrode 68b. It is possible to measure transmittance while scanning the voltage for the phase adjustment electrode.

[0200] The control unit 10 acquires the relationship between the voltage applied to the phase adjustment electrode and the amount of phase change. Figure 9 (Steps S14 and S15). The control unit 10 corrects the coefficients based on the ratio of the length of the modulation electrode to the length of the phase adjustment electrode, and obtains the relationship between the voltage applied to the modulation electrode and the phase change (steps S15 and S16). The control unit 10 obtains a voltage such that the phase change in each branch waveguide is, for example, π / 2 (step S18). The phase change can be set to a predetermined value, and the increase in light absorption loss is suppressed.

[0201] Specifically, control unit 10 in Figure 9 In step S10, the transmittance is measured while scanning the voltage applied to the phase adjustment electrode, and in step S12, the transmittance is calculated. Figure 11B as well as Figure 14 As shown, the control unit 10 performs transmittance fitting to make the calculated transmittance close to the measured transmittance. The transmittance shown in number 7 is the phase change. The function. The phase change shown in number 5. This is a function of the voltage applied to the phase adjustment electrode. By fitting the transmittance, the coefficients within number 5 are made to a more appropriate value. The ratio of the length of the modulation electrode to the length of the phase adjustment electrode is, for example, 2.5. Based on the ratio, the coefficients are set to a constant multiple of the ratio (for example, 2.5 times). By applying the constant multiple coefficients to number 5, a more accurate relationship between the voltage applied to the modulation electrode and the phase change can be obtained. The control unit 10 acquires the applied voltage to the modulation electrode when the phase change is of a predetermined size. The phase change can be set to a predetermined size, and the increase in light absorption loss can be suppressed. The transmittance, the phase change, and the change in absorption loss can also be calculated using formulas other than those mentioned above.

[0202] 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. Due to the deviation in the thermal diffusion of the dopant, the phase adjustment efficiency of the Mach-Zehnder modulator also deviates. According to the first embodiment, a voltage is obtained for each Mach-Zehnder modulator such that the phase change is of a predetermined magnitude. By driving the Mach-Zehnder modulator with a voltage corresponding to the phase adjustment efficiency, the phase of the modulated light can be set to a predetermined magnitude, and the increase in absorption loss can be suppressed.

[0203] exist Figure 13 as well as Figure 16 In the example, the control unit 10, for instance, varies the bias voltage Vb in increments of 0.1V within a range of 6V to 13V, thereby calculating the phase change. The control unit 10 may also use algorithms such as the bisection method and Newton's method to determine the appropriate bias voltage Vb.

[0204] Depending on the wavelength of the light, the relationship between voltage and phase change, and the relationship between voltage and absorption loss change, can sometimes vary. A wavelength-variable laser element 22 receives light of multiple wavelengths presumably used during the operation of the optical modulator 40a. The control unit 10 controls each of the multiple wavelengths. Figure 9The storage device 34 stores the bias voltage for each of the multiple wavelengths. The control unit 10 can also obtain the bias voltage by conducting tests on, for example, the shortest and longest wavelengths in the wavelength range. The control unit 10 can also obtain the bias voltage by conducting tests on, for example, three or more wavelengths in the wavelength range. By performing interpolation using linear interpolation, polynomial interpolation, spline interpolation, etc., the bias voltage at wavelengths where no tests were conducted can also be obtained.

[0205] In the first embodiment, there is a deviation in the phase adjustment efficiency of the sub-Mach-Zehnder modulators 42a and 42b of an optical modulator 40a. An optimal voltage is obtained for each of the sub-Mach-Zehnder modulators 42a and 42b. There is also a situation where the phase adjustment efficiency deviates for each optical modulator 40a. The control unit 10 performs tests on multiple optical modulators 40a to obtain the optimal voltage for each optical modulator 40a. Figure 8 as well as Figure 9 In the process, Figure 1A The optical transmitting device 100 serves as a test apparatus for the optical modulator 40. An optical modulator 40a is installed in the optical transmitting device 100 and tested. The optical modulator 40a is replaced with another optical modulator, and further testing is conducted. The storage device 34 can also store voltages for multiple optical modulators 40a. When the optical transmitting device 100 is used for communication, etc., testing can be performed on only one optical modulator 40a included in the optical transmitting device 100. The storage device 34 can also store only the voltage for that single optical modulator 40a.

[0206] <Second Implementation Method>

[0207] 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.

[0208] Figure 17 This is a top view illustrating an optical modulator 40b. The optical modulator 40b is a DP-IQ modulator, which includes optical modulators 43a and 43b.

[0209] 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.

[0210] 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 further downstream of the coupler 59.

[0211] 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.

[0212] 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.

[0213] 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.

[0214] Table 2 is an example of a data table stored in storage device 34. Storage device 34 stores the voltage of optical modulator 40b.

[0215] Table 2

[0216]

[0217] Xch represents optical modulator 43a. In Xch, Ich represents sub-Mach-Zehnder modulator 42a. Qch represents sub-Mach-Zehnder modulator 42b. Ych represents optical modulator 43b. In Ych, Ich represents sub-Mach-Zehnder modulator 42c. Qch represents sub-Mach-Zehnder modulator 42d. The bias voltage of sub-Mach-Zehnder modulator 42a is, for example, 8.9V. The bias voltage of sub-Mach-Zehnder modulator 42b is, for example, 9.5V. The bias voltage of sub-Mach-Zehnder modulator 42c is, for example, 9.4V. The bias voltage of sub-Mach-Zehnder modulator 42d is, for example, 9.5V.

[0218] According to the second embodiment, by driving the Mach-Zehnder modulator with a voltage optimized for each Mach-Zehnder modulator, the amount of phase change during modulation can be set to a predetermined value, and the increase in light absorption loss can be suppressed.

[0219] 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.

[0220] 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 has the following characteristics: The preparation process involves preparing the Mach-Zehnder modulator. The relationship acquisition process involves obtaining, based on the transmittance of light in the first branch waveguide and the transmittance of light in the second branch waveguide, a first relationship (i.e., the change in phase of the voltage applied to the first electrode) and a second relationship (i.e., the change in phase of the light propagating in the first branch waveguide) are acquired. The relationship acquisition process includes: measuring the first transmittance, which is the transmittance of light in each of the first and second branch waveguides; calculating the second transmittance, which is the transmittance of light in each of the first and second branch waveguides; and acquiring the first and second relationships by adjusting the second transmittance to be close to the first transmittance. A voltage acquisition process, in which, based on the first relationship and the second relationship, when modulating the light propagating in the first branch waveguide and the light propagating in the second branch waveguide, acquires a voltage applied to the first electrode such that the phase change in the first branch waveguide is of a predetermined magnitude, and acquires a voltage applied to the second electrode such that the phase change in the second branch waveguide is of a predetermined magnitude; and The storage process involves storing the voltage applied to the first electrode and the voltage applied to the second electrode, which were obtained in the voltage acquisition process, in a storage unit.

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, During the modulation, the difference between the phase change in the first branch waveguide and the phase change in the second branch waveguide is π.

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 voltage acquisition process includes the process of acquiring a first voltage such that the second voltage is below a predetermined value.

5. The method for manufacturing an optical modulator according to claim 4, characterized in that, The process of preparing the Mach-Zehnder modulator includes the process of preparing the Mach-Zehnder modulator having a first branch waveguide, a second branch waveguide, a first electrode, a second electrode, a third electrode, and a fourth electrode. The third electrode is disposed on the first branch waveguide. The fourth electrode is disposed on the second branch waveguide. The process of obtaining the first relationship and the second relationship includes the following steps: Based on the relationship between the voltage applied to the third electrode and the change in phase of the light propagating in the first branch waveguide, the relationship between the voltage applied to the first electrode and the change in phase of the light propagating in the first branch waveguide, i.e., the first relationship, is obtained. as well as Based on the relationship between the voltage applied to the fourth electrode and the change in phase of the light propagating in the second branch waveguide, the relationship between the voltage applied to the second electrode and the change in phase of the light propagating in the second branch waveguide, namely the second relationship, is obtained.

6. The method for manufacturing an optical modulator according to claim 5, characterized in that, 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 third electrode. The second transmittance in the second branch waveguide is calculated by expressing the second transmittance as a function of the phase change of the light propagating in the second branch waveguide, and by expressing the phase change of the light propagating in the second branch waveguide as a function of the voltage applied to the fourth electrode. The process of obtaining the first relationship and the second relationship includes the following steps: By adjusting the second transmittance in the first branch waveguide to be close to the first transmittance in the first branch waveguide, the relationship between the voltage applied to the third electrode and the change in phase of the light propagating in the first branch waveguide is obtained. as well as By adjusting the second transmittance in the second branch waveguide to be close to the first transmittance in the second branch waveguide, the relationship between the voltage applied to the fourth electrode and the change in phase of the light propagating in the second branch waveguide is obtained.

7. The method for manufacturing an optical modulator according to any one of claims 1, 2, 4 to 6, 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 the process of forming a first branch waveguide having a first semiconductor layer, a core layer, and a second semiconductor layer, as well as a second branch waveguide. 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.

8. 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 experimental method has the following characteristics: The relationship acquisition process involves obtaining, based on the transmittance of light in the first branch waveguide and the transmittance of light in the second branch waveguide, a first relationship (i.e., the change in phase of the voltage applied to the first electrode) and a second relationship (i.e., the change in phase of the light propagating in the first branch waveguide) are acquired. The relationship acquisition process includes: measuring the first transmittance, which is the transmittance of light in each of the first and second branch waveguides; calculating the second transmittance, which is the transmittance of light in each of the first and second branch waveguides; and acquiring the first and second relationships by adjusting the second transmittance to be close to the first transmittance. In the voltage acquisition process, based on the first relationship and the second relationship, when modulating the light propagating in the first branch waveguide and the light propagating in the second branch waveguide, a voltage applied to the first electrode is acquired such that the phase change in the first branch waveguide is of a predetermined magnitude, and a voltage applied to the second electrode is acquired such that the phase change in the second branch waveguide is of a predetermined magnitude.

9. A storage medium for storing experimental programs of 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 experimental procedure causes the computer to perform the following processing: In the first process, based on the transmittance of light in the first branch waveguide and the transmittance of light in the second branch waveguide, a first relationship (i.e., a relationship between the voltage applied to the first electrode and the phase change of light propagating in the first branch waveguide) and a second relationship (i.e., a relationship between the voltage applied to the second electrode and the phase change of light propagating in the second branch waveguide) are obtained. The first process includes: measuring the first transmittance, where the first transmittance is the transmittance of light in each of the first and second branch waveguides; calculating the second transmittance, where the second transmittance is the transmittance of light in each of the first and second branch waveguides; and obtaining the first relationship and the second relationship by adjusting the second transmittance to be close to the first transmittance. In the second process, based on the first relationship and the second relationship, when modulating the light propagating in the first branch waveguide and the light propagating in the second branch waveguide, a voltage applied to the first electrode is obtained such that the amount of phase change in the first branch waveguide is a predetermined amount, and a voltage applied to the second electrode is obtained such that the amount of phase change in the second branch waveguide is a predetermined amount.

10. 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, for each of the plurality of Mach-Zehnder modulators, stores the voltage applied to the first electrode and the voltage applied to the second electrode, such that the phase change of the light is of a predetermined magnitude when the light propagating in the first branch waveguide and the light propagating in the second branch waveguide are modulated, by executing a test program stored in the storage medium of claim 9.

Images