Optical transmission device, control method thereof, and computer-readable storage medium

Abstract

Provided is an optical transmitter capable of suppressing an increase in voltage for controlling the phase of light, a control method therefor, and a computer-readable storage medium. The optical transmitter includes a Mach-Zehnder modulator having a branch waveguide, and a phase control section that controls the phase of light propagating in the branch waveguide by applying a voltage to the Mach-Zehnder modulator, the phase control section shifting the voltage by an amount corresponding to a change of 2π in the phase if the voltage exceeds a prescribed range.

Description

Technical Field

[0001] This disclosure relates to an optical transmitting apparatus, a control method thereof, and a computer-readable storage medium. Background Technology

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

[0003] Existing technical documents

[0004] Patent documents

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

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

[0007] The problem that the invention aims to solve

[0008] Light propagates in the branch waveguide of a Mach-Zehnder modulator. The phase of the light can be controlled by applying a voltage to the Mach-Zehnder modulator. The phase of the light in the Mach-Zehnder modulator sometimes changes depending on the stress, temperature, etc., applied to the modulator. By using automatic bias control (ABC) to control the voltage, the phase of the light is optimized. To expand the adjustment range of the phase based on ABC, the voltage applied to the Mach-Zehnder modulator can be increased. However, increasing the voltage may lead to problems such as increased power consumption, increased light loss, and larger size of the optical modulator. Therefore, the object of the present invention is to provide an optical transmitting device, its control method, and control program capable of suppressing the increase of the voltage used to control the phase of light.

[0009] Technical solutions for solving the problem

[0010] The optical transmitting apparatus disclosed herein includes: a Mach-Zehnder modulator having a branch waveguide; and a phase control unit that controls the phase of light propagating in the branch waveguide by applying a voltage to the Mach-Zehnder modulator, wherein if the voltage exceeds a predetermined range, the phase control unit shifts the voltage to the side opposite to the side exceeding the predetermined range by an amount corresponding to the phase change of 2π.

[0011] The control method for the optical transmitting device disclosed herein includes the following steps: controlling the phase of light propagating in the branch waveguide by inputting a voltage to a Mach-Zehnder modulator having a branch waveguide; and, if the voltage exceeds a predetermined range, shifting the voltage to the side opposite to the side exceeding the predetermined range by an amount corresponding to the phase change of 2π.

[0012] The control program of the optical transmitting device disclosed herein, stored in a computer-readable storage medium, causes the computer to perform the following processing: controlling the phase of light propagating in a branch waveguide by inputting a voltage to a Mach-Zehnder modulator having a branch waveguide; and, if the voltage exceeds a predetermined range, shifting the voltage to the side opposite to the side exceeding the predetermined range by an amount corresponding to the phase change of 2π.

[0013] Invention Effects

[0014] According to this disclosure, it is possible to suppress the increase of voltage used to control the phase of light. Attached Figure Description

[0015] Figure 1A This is a block diagram illustrating the optical modulator according to the first embodiment.

[0016] Figure 1B It is a block diagram representing the hardware structure of the control unit.

[0017] Figure 2A This is a top view illustrating a modulator.

[0018] Figure 2B This is a top view illustrating a modulator.

[0019] Figure 3 This is a graph illustrating the relationship between the change in voltage and phase (phase adjustment).

[0020] Figure 4 This is a diagram representing the operating point of the main MZ modulator.

[0021] Figure 5 This is a diagram showing the operating point of the secondary MZ modulator.

[0022] Figure 6 This is a flowchart illustrating the processes performed by the control unit.

[0023] Figure 7 This is a block diagram illustrating the optical transmitting apparatus according to the second embodiment.

[0024] Figure 8 This is a flowchart illustrating the processes performed by the control unit. Detailed Implementation

[0025] [Description of embodiments of this disclosure]

[0026] First, the contents of the embodiments disclosed herein will be listed for explanation.

[0027] One aspect of this disclosure is (1) an optical transmitting apparatus comprising: a Mach-Zehnder modulator having a branch waveguide; and a phase control unit that controls the phase of light propagating in the branch waveguide by applying a voltage to the Mach-Zehnder modulator, wherein, if the voltage exceeds a predetermined range, the phase control unit shifts the voltage to the side opposite to the side exceeding the predetermined range by an amount corresponding to the phase change of 2π. By shifting the voltage, the voltage converges within the predetermined range. As a result, an increase in the voltage used to control the phase of the light can be suppressed.

[0028] (2) Alternatively, the Mach-Zehnder modulator can have two branch waveguides, each with a first electrode. The phase control unit controls the phase of the light by applying a third voltage (the sum of a first voltage and a second voltage) to one of the first electrodes and a fourth voltage (the difference between the first voltage and the second voltage) to the other. If the second voltage exceeds a predetermined range, the phase control unit shifts the second voltage by an amount corresponding to the phase change of 2π. Since only the second voltage needs to be changed, the control is simple.

[0029] (3) Alternatively, the phase control unit can adjust the phase difference between the two branch waveguides to π or an equivalent value of π, or 0.5π or an equivalent value of 0.5π. By setting the phase difference of the light to an equivalent value of π, the Mach-Zehnder modulator can be adjusted to the extinction point. By setting the phase difference of the light to an equivalent value of 0.5π, the phases of the modulated light can be made orthogonal.

[0030] (4) Alternatively, the width of the specified voltage range can be greater than or equal to the range corresponding to the phase range of -π to π, and less than or equal to the phase range of -3π to 3π. The offset voltage approaches the center value of the specified range and converges within that range. Therefore, voltage increases can be suppressed.

[0031] (5) Alternatively, the light transmitting device may include: a shutter capable of blocking light emitted from the Mach-Zehnder modulator; a shutter control unit for controlling the opening and closing of the shutter; and a modulation control unit for modulating light propagating in the branch waveguide of the Mach-Zehnder modulator, wherein the Mach-Zehnder modulator and the shutter form a unit. When the modulation control unit modulates the light in the Mach-Zehnder modulator of the first unit among the plurality of units, and the voltage applied by the phase control unit to the Mach-Zehnder modulator of the first unit exceeds a predetermined range, the modulation control unit... The light is modulated in the Mach-Zehnder modulator of the second unit among the plurality of units. The phase control unit applies a voltage derived in the same manner as the voltage applied to the Mach-Zehnder modulator of the first unit to the Mach-Zehnder modulator. After applying the voltage to the Mach-Zehnder modulator of the second unit, the shutter control unit closes the shutter of the first unit and opens the shutter of the second unit. After closing the shutter of the first unit, the phase control unit shifts the voltage by an amount corresponding to the phase change 2π of the light propagating in the Mach-Zehnder modulator of the first unit. The unit that emits modulated light is switched by opening and closing the shutter. Since the disturbance of the modulated light is suppressed, it is less likely to cause transmission errors of the optical signal.

[0032] (6) A control method for an optical transmitting device, comprising the steps of: controlling the phase of light propagating in a branch waveguide by inputting a voltage to a Mach-Zehnder modulator having a branch waveguide; and, if the voltage exceeds a predetermined range, shifting the voltage to the side opposite to the side exceeding the predetermined range by an amount corresponding to the phase change of 2π. By shifting the voltage, the voltage converges within the predetermined range. As a result, an increase in voltage can be suppressed.

[0033] (7) A computer-readable storage medium storing a control program for an optical transmitting device, the control program causing the computer to perform the following processing: controlling the phase of light propagating in a Mach-Zehnder modulator having a branch waveguide by inputting a voltage; and, if the voltage exceeds a predetermined range, shifting the voltage to the side opposite to the side exceeding the predetermined range by an amount corresponding to the phase change of 2π. Through this voltage shift, the voltage converges within the predetermined range. As a result, voltage increases can be suppressed.

[0034] [Details of the embodiments disclosed herein]

[0035] The following description, with reference to the accompanying drawings, illustrates specific examples of the optical transmitting apparatus, its control method, and control procedure according to embodiments of the present disclosure. Furthermore, the present disclosure is not limited to these examples, but is shown in the claims and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0036] <First Implementation>

[0037] (Optical transmission device)

[0038] Figure 1A This is a block diagram illustrating the optical transmitting apparatus 100 according to the first embodiment. Figure 1A As shown, the optical transmitting device 100 includes a control unit 10, two units 20ch1 and 20ch2, and a multiplexer 21.

[0039] Units 20ch1 and 20ch2 each have a wavelength-variable laser element 22, an automatic bias control (ABC) circuit 24, a driver IC (Integrated Circuit) 26, a shutter 28, and a modulator 40.

[0040] The wavelength-tunable 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 modulator 40, performing automatic bias control. The driver IC 26 inputs a modulation signal to the modulator 40. The modulator 40 modulates the light incident from the wavelength-tunable laser element 22 and emits modulated light. The shutter 28 is capable of opening and closing, blocking light by closing and allowing light to escape by opening. The shutter 28 can be, for example, a semiconductor optical amplifier (SOA). Light is absorbed by setting the SOA to a light-absorbing state and emitted by setting the SOA to a light-transmitting state.

[0041] The multiplexer 21 can transmit either the emitted light from unit 20ch1 or the emitted light from unit 20ch2 to the outside, or it can combine the emitted light from the two units and transmit it as wavelength-multiplexed modulated light to the outside of the optical transmitting device 100. The light emitted from the multiplexer 21 can be incident on, for example, an optical fiber (not shown).

[0042] Units 20ch1 and 20ch2 operate independently of each other. For example, one of units 20ch1 and 20ch2 may modulate light while the other does not. Alternatively, both units 20ch1 and 20ch2 may modulate light. In units 20ch1 and 20ch2, the wavelength, modulation signal, and phase of the light may be the same or different. In the following example, unit 20ch1 is the unit that operates normally, while unit 20ch2 is a spare.

[0043] The control unit 10 includes a computer, such as a personal computer (PC), which controls units 20ch1 and 20ch2.

[0044] 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, and an interface 36. The CPU 30, RAM 32, storage device 34, and interface 36 are connected to each other via a bus or the like. RAM 32 is volatile memory that temporarily stores programs and data. The storage device 34 is, for example, a ROM (Read Only Memory), a solid-state drive (SSD), or a hard disk drive (HHD). The storage device 34 stores programs for performing the processes described later.

[0045] The control unit 10 enables control by executing a program stored in RAM 32 via CPU 30. Figure 1A The control unit 10 includes a phase control unit 12, a laser control unit 14, a modulation control unit 16, and a shutter control unit 18. The phase control unit 12 controls the ABC circuit 24 and receives the voltage applied to the modulator 40 by the ABC circuit 24. The laser control unit 14 controls the wavelength-variable laser element 22. The modulation control unit 16 controls the driver IC 26. The shutter control unit 18 controls the shutter 28. Each part of the control unit 10 can also be hardware such as circuitry.

[0046] (Modulator)

[0047] As Figure 1A The modulator 40 of units 20ch1 and 20ch2 can be used Figure 2A The modulator 40a shown and Figure 2B The modulator 40b is shown.

[0048] Figure 2A This is a top view illustrating modulator 40a. Modulator 40a is an IQ modulator (In-phase Quadrature modulator) and a primary Mach-Zehnder modulator with two secondary Mach-Zehnder modulators 42a and 42b. Secondary Mach-Zehnder modulator 42a generates modulated light for one of the I and Q channels (e.g., the I channel). Secondary Mach-Zehnder modulator 42b generates modulated light for the other of the I and Q channels (e.g., the Q channel). The phase of the light is adjusted so that the phase of the modulated light from secondary Mach-Zehnder modulator 42a is orthogonal to the phase of the modulated light from secondary Mach-Zehnder modulator 42b.

[0049] The modulator 40a has a substrate 43, an input waveguide 50, branch waveguides 52a and 52b, 60a to 60d, 62a and 62b, an output waveguide 64, couplers 52, 54a and 54b, 56a and 56b and 58, and electrodes 66a to 66d, 68a to 68d, 70a and 70b.

[0050] Branch waveguides 52a, 60a, 60b, and 62a, couplers 54a and 56a, and electrodes 66a, 66b, 68a, and 68b constitute a secondary Mach-Zehnder modulator 42a. Branch waveguides 52b, 60c, 60d, and 62b, couplers 54b and 56b, and electrodes 66c, 66d, 68c, and 68d constitute a secondary Mach-Zehnder modulator 42b. Secondary Mach-Zehnder modulators 42a and 42b are connected in parallel between couplers 52 and 58. Branch waveguides 60a and 60c are p-side waveguides. Branch waveguides 60b and 60d are n-side waveguides.

[0051] The substrate 43 is, for example, a semiconductor substrate formed of indium phosphide (InP) or other semiconductors. An input waveguide 50, an output waveguide 64, branch waveguides, and couplers are formed on the substrate 43. Couplers 52, 54a and 54b, 56a and 56b, and 58 are 1×2 couplers. The electrodes are formed of metal. Electrodes 66a to 66c are modulation electrodes for modulating light. Electrodes 68a to 68d, and electrodes 70a and 70b are phase adjustment electrodes for adjusting the phase of light.

[0052] One end of the input waveguide 50 is located on one end face of the substrate 43, and the other end is connected to the input terminal of the coupler 52. One of the two output terminals of the coupler 52 is connected to a branch waveguide 52a, and the other is connected to one end of a branch waveguide 52b.

[0053] The other end of branch waveguide 52a is connected to the input terminal of coupler 54a. One end of branch waveguides 60a and 60b is connected to the two output terminals of coupler 54a. The other ends of branch waveguides 60a and 60b are connected to the two input terminals of coupler 56a. Branch waveguides 60a and 60b extend side-by-side between couplers 54a and 56a. Electrodes 66a and 68a are disposed on branch waveguide 60a and arranged in this order between couplers 54a and 56a. Electrodes 66b and 68b are disposed on branch waveguide 60b and arranged in this order between couplers 54a and 56a.

[0054] The other end of branch waveguide 52b is connected to the input terminal of coupler 54b. One end of branch waveguides 60c and 60d is connected to the two output terminals of coupler 54b. The other ends of branch waveguides 60c and 60d are connected to the two input terminals of coupler 56b. Branch waveguides 60c and 60d extend side-by-side between coupler 54b and coupler 56b. Electrodes 66c and 68c are disposed on branch waveguide 60c and arranged sequentially from the coupler 54b side to the coupler 56b side. Electrodes 66d and 68d are disposed on branch waveguide 60d and arranged sequentially from the coupler 54b side to the coupler 56b side.

[0055] One end of branch waveguide 62a is connected to the output terminal of coupler 56a. One end of branch waveguide 62b is connected to the output terminal of coupler 56b. The other ends of branch waveguides 62a and 62b are connected to the two input terminals of coupler 58. Branch waveguides 62a and 62b extend side-by-side between couplers 56a and 56b and coupler 58. Electrode 70a is disposed on branch waveguide 62a. Electrode 70b is disposed on branch waveguide 62b. One end of output waveguide 64 is connected to the output terminal of coupler 58, and the other end is located on the side of the end face of substrate 43 opposite to the side where one end of input waveguide 50 is located.

[0056] Figure 1A The wavelength-variable laser element 22 shown is... Figure 2A The input terminal of the modulator 40a (the end of the input waveguide 50) is coupled. Figure 1A The shutter 28 shown is located at the same position as... Figure 2A The relative positions of the output terminals (ends of output waveguide 64) of modulator 40a.

[0057] Figure 1A The laser control unit 14 of the control unit 10 shown causes the wavelength-variable laser element 22 to emit light. The light incident on... Figure 2AThe light from the input waveguide 50 of the modulator 40a shown branches into branch waveguides 52a and 52b in coupler 52. The light propagating in branch waveguide 52a branches into branch waveguides 60a and 60b in coupler 54a. The light propagating in branch waveguide 52b branches into branch waveguides 60c and 60d in coupler 54b.

[0058] When to Figure 1A When transmission data is input to the control unit 10, the modulation control unit 16 of the control unit 10 generates a modulation signal based on the transmission data and inputs it to the driver IC 26. By inputting the modulation signal from the driver IC 26 to the electrodes 66a to 66d, the refractive index of the branch waveguides 60a to 60d changes, and light modulation is performed.

[0059] The modulated light propagating in branch waveguide 60a and branch waveguide 60b merges in coupler 56a and propagates in branch waveguide 62a. The modulated light propagating in branch waveguide 60c and branch waveguide 60d merges in coupler 56b and propagates in branch waveguide 62b. The modulated light propagating in branch waveguide 62a and branch waveguide 62b merges in coupler 58 and propagates in output waveguide 64. The modulated light exits from the end of output waveguide 64. Figure 1A When the shutter 28 is open, the modulated light is emitted from the light transmitting device 100 via the multiplexer 21. When the shutter 28 is closed, the modulated light is blocked by the shutter 28 and does not escape from the light transmitting device 100.

[0060] Figure 2B This is a top view illustrating modulator 40b. Modulator 40b is a DP (Dual Polarization)-IQ modulator with two modulators 40a1 and 40a2. Modulators 40a1 and 40a2 are... Figure 2A The modulator 40a1 is the same IQ modulator as modulator 40a1. Modulator 40a2 has sub-Mach-Zehnder modulators 42a and 42b. Modulator 40a2 has sub-Mach-Zehnder modulators 42c and 42d. The sub-Mach-Zehnder modulators 42c and 42d have the same structure as sub-Mach-Zehnder modulators 42a and 42b.

[0061] One end of the input waveguide 71 is located on the end face of the substrate 43, and the other end is connected to the input terminal of the coupler 72. One of the two output terminals of the coupler 72 is connected to the input waveguide 50a of the modulator 40a1, and the other is connected to the input waveguide 50b of the modulator 40a2. The output waveguide 64a of the modulator 40a1 and the output waveguide 64b of the modulator 40a2 are located on the end face of the substrate 43 on the side opposite to the input waveguide 71.

[0062] Modulator 40a1 generates modulated light in the X-channel (X-polarized wave). Modulator 40a2 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. Figure 1A The multiplexer 21 shown combines the modulated light from the X channel and the modulated light from the Y channel and outputs them. Hereinafter, it will be assumed that it is used... Figure 2A The modulator 40a as Figure 1A Modulator 40.

[0063] The phase control unit 12 of the control unit 10 uses the ABC circuit 24 for automatic bias control. Specifically, the phase control unit 12 causes the ABC circuit 24 to apply voltages to electrodes 68a-68d, 70a, and 70b of the modulator 40a. By applying voltages, the refractive index of the branch waveguides changes, thereby changing the phase of the light. The phase control unit 12 can independently control the phase difference in the main Mach-Zehnder modulator (modulator 40a) and the phase difference in the secondary Mach-Zehnder modulators 42a and 42b.

[0064] The phase control unit 12 adjusts the phase difference between the phase of the light propagating in branch waveguide 62a and the phase of the light propagating in branch waveguide 62b to, for example, 0.5π (rad) or a value equivalent to 0.5π. That is, the phases of the light are orthogonalized in the main Mach-Zehnder modulator. The phase difference equivalent to 0.5π is the value at which the light phases are orthogonalized, which is 0.5π ± 2π × n or 1.5π ± 2π × n (where n is a negative or positive integer). The state where the phases of the light are adjusted to be orthogonal is the operating point of the main Mach-Zehnder modulator.

[0065] The phase control unit 12 adjusts the phase difference between the phase of the light propagating in branch waveguide 60a and the phase of the light propagating in branch waveguide 60b to, for example, π (rad) or a value equivalent to π. That is, without a modulation signal applied, the sub-Mach-Zehnder modulators 42a and 42b are adjusted to their extinction points. The phase difference equivalent to π is the value at which the extinction point is reached, which is π ± 2π × n (where n is a negative or positive integer). The state of being adjusted to the extinction point is the operating point of the sub-Mach-Zehnder modulator.

[0066] The voltages applied from the ABC circuit 24 to the modulator 40a will be explained. The voltage VI (third voltage) applied to the electrode 70a on the branch waveguide 62a and the voltage VQ (fourth voltage) applied to the electrode 70b on the branch waveguide 62b are represented using the center voltage Vcp (first voltage) and the differential voltage Vdp (second voltage) as follows.

[0067] VI = Vcp + Vdp (1)

[0068] VQ = Vcp - Vdp (2)

[0069] The difference between voltage VI and voltage VQ is 2Vdp. The phase control unit 12 fixes the center voltage Vcp to a constant value and changes voltage VI and VQ by changing the differential voltage Vdp, thereby adjusting the operating point of the modulator 40a.

[0070] The voltage applied from the ABC circuit 24 to the sub-Mach-Zehnder modulator 42a will be explained. The voltage Vp (third voltage) applied to the electrode 68a on the branch waveguide 60a and the voltage Vn (fourth voltage) applied to the electrode 68b on the branch waveguide 60b are represented using the center voltage Vcc (first voltage) and the differential voltage Vdc (second voltage) as follows.

[0071] Vp=Vcc+Vdc (3)

[0072] Vn=Vcc-Vdc (4)

[0073] The difference between voltage Vp and voltage Vn is 2Vdc. The phase control unit 12 fixes the center voltage Vcc to a constant value and adjusts the operating point of the sub-Mach-Zehnder modulator 42a by changing the differential voltage Vdc, thereby changing voltages Vp and Vn. Voltages Vp and Vn are also applied to the sub-Mach-Zehnder modulator 42b.

[0074] Figure 3 This is a graph illustrating the relationship between the change in voltage and phase (phase adjustment). The horizontal axis represents the differential voltage Vdp or Vdc, and the vertical axis represents the phase adjustment. like Figure 3 As shown, the differential voltage and phase adjustment amount The relationship between them is non-linear and point-symmetric with respect to the origin. If the differential voltage is 0, then the phase adjustment amount... It is also 0. If the differential voltage increases towards the positive side, the phase adjustment amount will be 0. It also increases towards the positive side. If the differential voltage increases towards the negative side, then the phase adjustment amount... It also increases towards the negative side. With differential voltages V1, V2, and V3 respectively, the phase adjustment amount... The phase adjustment amounts are π, 2π, and 3π, respectively. These are the differential voltages -V1, -V2, and -V3. The values ​​are -π, -2π, and -3π.

[0075] Phase difference between the two branch waveguides The initial phase difference is expressed as follows: With phase adjustment amount sum.

[0076]

[0077] initial phase difference The phase adjustment is determined by the difference in optical path length between the branch waveguides of modulator 40a. The phase adjustment amount is based on the change in phase caused by the applied voltage.

[0078] The wavelength λ of the light emitted from the wavelength-variable laser element 22 within the branch waveguide is, for example, 484 nm (1550 nm in vacuum). On the other hand, the lengths of the branch waveguides 60a and 60b of the secondary Mach-Zehnder modulator 42a, and the branch waveguides 60c and 60d of the secondary Mach-Zehnder modulator 42b, are, for example, 6 mm, which is more than 10,000 times the wavelength λ. Due to manufacturing errors, the optical path lengths of the branch waveguides deviate. The difference ΔL between the optical path lengths of the two branch waveguides is related to the initial phase difference of the light between the two branch waveguides. The relationship between them is expressed using the integer m as follows.

[0079]

[0080] For example, as with branch waveguides 60a and 60b, the difference ΔL in the optical path length between a pair of branch waveguides can sometimes be greater than one ten-thousandth of the designed dimensions. In this case, since the difference in optical path length ΔL is greater than or equal to the wavelength λ of light, the initial phase difference... It is distributed in the range of 0 (rad) and below 2π (rad).

[0081] During the operation of the optical transmitting device 100, the optical path length of the branch waveguide may sometimes change due to stress and temperature variations applied to the modulator 40. This change in optical path length affects the initial phase difference. Changes occur, phase difference It also changes. The phase control unit 12 performs automatic bias control based on the phase difference. The change in voltage applied to electrodes 68a-68d, 70a, and 70b adjusts the operating point.

[0082] Figure 4 This is a graph showing the operating point of the main MZ modulator. The horizontal axis represents the differential voltage Vdp, and the vertical axis represents the phase difference. Multiple solid lines, starting from the top, represent the initial phase difference. Examples include 2π, 1.25π, π, 0.75π, and 0. V1p is, for example, 0.8V, and V2p is, for example, 1.5V.

[0083] In the initial phase difference In order to make the phase difference As long as the phase adjustment amount is adjusted 0.5π is sufficient. The phase control unit 12 sets the differential voltage Vdp to V2p and adjusts the phase adjustment amount. Set the phase difference to 0.5π. Adjust to 0.5π. At the initial phase difference... In order to make the phase difference As long as the phase adjustment amount is adjusted -0.25π is sufficient. The phase control unit 12 sets the differential voltage Vdp to -V1p. At the initial phase difference... In order to make the phase difference As long as the phase adjustment amount is adjusted A value of 0.5π is sufficient. The phase control unit 12 sets the differential voltage Vdp to V2p. At the initial phase difference... In order to make the phase difference As long as the phase adjustment amount is adjusted A value of 0.25π is sufficient. The phase control unit 12 sets the differential voltage Vdp to V1p. At the initial phase difference... In order to make the phase difference As long as the phase adjustment amount is adjusted -0.5π is sufficient. The phase control unit 12 sets the differential voltage Vdp to -V2p.

[0084] Figure 5 This is a graph showing the operating point of the secondary MZ modulator. The horizontal axis represents the differential voltage Vdc, and the vertical axis represents the phase difference. Multiple solid lines, starting from the top, represent the initial phase difference. Examples are 2π, 0.75π, 0, -0.5π, -2π, and -2.5π. Figure 5 Examples of voltages are shown below. V1c is 0.8V, V2c is 2.9V, V3c is 4.3V, V4c is 7.4V, and V5c is -1.5V.

[0085] like Figure 5 As shown, at the initial phase difference In order to make the phase difference As long as the phase adjustment amount is adjusted π is sufficient. The phase control unit 12 adjusts the phase by setting the differential voltage Vdc to V2c. Let the phase difference be π. Let the initial phase difference be π. In order to make the phase difference As long as the phase adjustment amount is adjusted A value of 0.25π is sufficient. The phase control unit 12 sets the differential voltage Vdc to V1c. At the initial phase difference... In order to make the phase difference As long as the phase adjustment amount is adjusted -π is sufficient. The phase control unit 12 sets the differential voltage Vdc to -V2c.

[0086] As described above, during the operation of the optical transmitting device 100, the initial phase difference Sometimes this can change. The phase control unit 12 follows the initial phase difference. The differential voltage Vdc changes due to the change in voltage, thereby adjusting the operating point.

[0087] The initial phase difference between the two branch waveguides of the secondary Mach-Zehnder modulator (e.g., branch waveguides 60a and 60b of the secondary Mach-Zehnder modulator 42a) Let it be 0.75π. In this case, such as Figure 5 of As shown by the solid line, the phase control unit 12 sets the differential voltage Vdc to V1c. During operation, the initial phase difference... When the voltage changes from 0.75π to 0, the phase control unit 12 adjusts the phase by setting the differential voltage Vdc to V2c. Let the phase difference be π. Adjusted to π. At the initial phase difference When the voltage changes to -0.5π, the phase control unit 12 sets the differential voltage Vdc to V3c.

[0088] In the initial phase difference When the value changes from -0.5π to, for example, -2π, such as Figure 5 As shown by the middle arrow A1, by setting the differential voltage Vdc to V4c, which is larger than V3c, it is possible to... The point is taken as the operating point. However, because the differential voltage Vdc increases in the positive direction, the voltage Vp increases. Furthermore, because the differential voltage Vdc increases in the negative direction, the voltage Vn increases. That is, due to the increase in the absolute value of the differential voltage Vdc, the voltages Vp and Vn used for phase adjustment increase.

[0089] As voltages Vp and Vn increase, power consumption and optical loss in the branch waveguides increase. The difference between Vp and Vn, 2Vdc, increases, leading to a greater difference in optical loss between branch waveguides and a decrease in the extinction ratio. Stress can be suppressed by fixing the modulator 40 to a robust package made of ceramic or similar material. Temperature variations can be suppressed by using Peltier elements or similar components. However, the increased number of components increases the size and cost of the optical transmitting device 100.

[0090] Therefore, in the first embodiment, the differential voltage Vdc is set to a range, for example, between -V3c and V3c (-V3c ≤ Vdc ≤ V3c). If this range is exceeded, the differential voltage Vdc is shifted by an amount equivalent to a 2π phase change (2π equivalent). By shifting the differential voltage, the increase in voltages Vp and Vn is suppressed.

[0091] exist Figure 5 In the initial phase difference When the phase changes from -0.5π to the negative direction, the phase control unit 12 does not change the differential voltage Vdc to a voltage higher than V3c, but instead shifts the differential voltage Vdc to the side opposite to the high voltage side by an amount corresponding to the phase change of 2π. Figure 5 initial phase difference On the line, with the phase difference A change of 2π from π to -π corresponds to a change of 2π in the differential voltage Vdc from V3c to -V5c. That is, as... Figure 5 As shown by arrow A2, the action point moves from π to -π. This is achieved by adjusting the phase difference. Becoming -π, the sub-Mach-Zehnder modulator is adjusted to the extinction point without a modulating signal. The voltage -V5c is greater than -V3c and less than 0, falling within the range of -V3c to V3c. In other words, the absolute value of the differential voltage Vdc after the offset is smaller than its absolute value before the offset. Therefore, the increase in voltages Vp and Vn is suppressed.

[0092] In the initial phase difference For example, when the voltage changes from -0.5π to -2π, the phase control unit 12 changes the differential voltage Vdc from -V5c to V2c, maintaining the operating point at -π. If the initial phase difference... For example, if the change is -2.5π, the differential voltage Vdc increases to V3c. At this time, the phase control unit 12 shifts the differential voltage Vdc to the side opposite to the high voltage side by an amount corresponding to the phase change of 2π. For example... Figure 5 As indicated by the middle arrow A3, the differential voltage Vdc changes from V3c to -V5c, falling within the range above -V3c and below V3c. Phase difference. The value is -3π. In the example shown by arrows A2 and A3, when the differential voltage Vdc is higher than V3c, the phase control unit 12 shifts the differential voltage Vdc towards the lower voltage side by an amount equivalent to 2π. When the differential voltage Vdc is lower than -V3c, the phase control unit 12 shifts the differential voltage Vdc towards the higher voltage side by an amount equivalent to 2π. Without applying a modulation signal, the sub-Mach-Zehnder modulator is adjusted to the extinction point.

[0093] Figure 6 This is a flowchart illustrating the process executed by the control unit 10, showing the resetting of the voltage of the sub-Mach-Zehnder modulator. At the start time of the process, Figure 1AUnit 20ch1 of the optical transmitting device 100 is in operation. The wavelength-variable laser element 22 of unit 20ch1 emits light. The ABC circuit 24 of unit 20ch1 applies voltage to electrodes 68a-68d, 70a, and 70b, performing automatic bias control. Under the control of the modulation control unit 16, the driver IC 26 of unit 20ch1 applies voltage to electrodes 66a-66d. The modulator 40a of unit 20ch1 generates modulated light. Unit 20ch2 is not in operation. The shutter 28 of unit 20ch1 is open, and the shutter 28 of unit 20ch2 is closed. The modulated light from unit 20ch1 is emitted after passing through the combiner 21.

[0094] like Figure 6 As shown, the phase control unit 12 of the control unit 10 acquires the differential voltage Vdc applied to the sub-Mach-Zehnder modulator as a monitoring signal and determines whether the differential voltage Vdc is V3c or higher (step S10). If not, step S10 is repeated. If yes, the control unit 10 starts unit 20ch2 and makes it operate under the same conditions as unit 20ch1 (step S12). The wavelength and intensity of the emitted light from the wavelength-variable laser element 22 in unit 20ch2, the voltages Vp and Vn, VI and VQ applied by the ABC circuit 24, and the voltage applied by the driver IC 26 are all equal to the corresponding voltages in unit 20ch1. The modulator 40 of unit 20ch2 generates modulated light equivalent to that of the modulator 40 in unit 20ch1. Since the shutter 28 of unit 20ch2 is closed, the modulated light of unit 20ch2 does not exit the light transmitting device 100.

[0095] The shutter control unit 18 of the control unit 10 closes the shutter 28 of unit 20ch1 and opens the shutter 28 of unit 20ch2 (step S14). The modulated light from unit 20ch1 is not emitted outside the light transmitting device 100. On the other hand, the same modulated light as unit 20ch1 is emitted from unit 20ch2. Since the modulated light is emitted continuously from the light transmitting device 100, it is not easy to cause turbulence in the modulated light, and transmission errors are suppressed.

[0096] Phase control unit 12 shifts the differential voltage Vdc of unit 20ch1 by an amount of 2π (equivalent to 2π) in phase (step S16). For example, as in Figure 5 As explained in the text, the initial phase difference in the sub-Mach-Zehnder modulator 42a of unit 20ch1 When the voltage changes from -0.5π to -2π, the phase control unit 12 shifts the voltage from V3c to -V5c. The shutter control unit 18 closes the shutter 28 of unit 20ch2 and opens the shutter 28 of unit 20ch1 (step S18). The modulated light from unit 20ch2 is not emitted outside the light transmitting device 100, while the modulated light from unit 20ch1 is emitted. The control unit 10 stops unit 20ch2 (step S20). The process ends here.

[0097] According to the first embodiment, when the differential voltage Vdc exceeds a range of -V3c and below V3c, the phase control unit 12 shifts the differential voltage Vdc by an amount corresponding to the phase change 2π to the side opposite to the side exceeding this range. This shift brings the differential voltage Vdc closer to the range of -V3c to V3c. The absolute value of the differential voltage Vdc decreases, and the increase in voltages Vp and Vn is suppressed. This achieves suppression of power consumption and light loss in the branch waveguides. Since the increase in the voltage difference 2Vdc between the branch waveguides is suppressed, the decrease in the extinction ratio is also suppressed. Furthermore, it eliminates the need for a robust package for fixing the modulator 40 and temperature adjustment elements. This reduces the number of components, thereby suppressing an increase in the size and cost of the optical transmitting device 100.

[0098] The offset of the differential voltage Vdc is equivalent to 2π. Therefore, after the voltage offset, the phase difference of the light is also adjusted to make the modulator the operating point. Figure 5 As shown, the phase control unit 12 controls the phase difference of the light between the two branch waveguides of the sub-Mach-Zehnder modulator to π. Through automatic bias control, the sub-Mach-Zehnder modulator is adjusted to the extinction point.

[0099] The voltage applied to the phase adjustment electrodes 68a and 68c of the sub-Mach-Zehnder modulator is Vpc, the sum of the center voltage Vcc and the differential voltage Vdc. The voltage applied to the phase adjustment electrodes 68b and 68d of the sub-Mach-Zehnder modulator is Vpn, the difference between the center voltage Vcc and the differential voltage Vdc. The phase control unit 12 can adjust the phase by changing the voltages Vpc and Vpn by changing the differential voltage Vdc. The phase control unit 12 changes the differential voltage Vdc by an amount corresponding to a 2π phase change. Since only the differential voltage Vdc needs to be adjusted, control is simple.

[0100] The first embodiment can also be applied to a master Mach-Zehnder modulator. The phase control unit 12 will... Figure 2AThe phase difference of the light between branch waveguides 62a and 62b shown is controlled to be 0.5π. The phase control unit 12 can suppress the increase of Vdp by shifting the differential voltage Vdp by an amount corresponding to the phase change of 2π. Through automatic bias control, the phase difference is maintained at 0.5π, and the phases of the two modulated lights of the main Mach-Zehnder modulator are orthogonal. The phase adjustment of the light phase by the phase control unit 12 is not limited to adjusting the secondary Mach-Zehnder modulator to the extinction point or making the phases orthogonal in the main Mach-Zehnder modulator.

[0101] The differential voltage after offsetting by an equivalent amount of 2π is preferably closer to the center value of the differential voltage range, i.e., 0V, compared to before the offset. For example, assuming the voltage range is proportional to... Figure 5 of The range corresponding to the phase adjustment amount above -3π and below 3π in the curve ( Figure 5 The range is wide (from -V4c to V4c). Even if the differential voltage Vdc is offset by an amount equivalent to 2π, Vdc will not converge within the range corresponding to the phase from -π to π. Furthermore, the voltage range is wider than the range corresponding to the phase adjustment from -π to π (in...). Figure 5 When the voltage is narrow (V2c to V2c), if the differential voltage Vdc is shifted by an equivalent amount of 2π, the voltage will move away from the central value (0) and the absolute value will increase compared to before the shift.

[0102] The differential voltage range preferably has a central value of 0V, is above the range corresponding to a phase adjustment amount of -π or more and π or less, and is below the range corresponding to a phase adjustment amount of -3π or more and 3π or less. This allows the voltage after a shift of approximately 2π to converge to, for example, a range corresponding to a phase adjustment amount of -π or more and π or less. By making the absolute value of the differential voltage after the shift smaller than that before the shift, the increase in voltages Vp and Vn can be suppressed.

[0103] In the first embodiment, the voltages VI and VQ used for phase adjustment of the main Mach-Zehnder modulator are represented by the sum and difference of the center voltage Vcp and the differential voltage Vdp. The voltages Vp and Vn used for phase adjustment of the secondary Mach-Zehnder modulator are represented by the sum and difference of the center voltage Vcc and the differential voltage Vdc. The voltages used for phase adjustment may also exclude the center voltage and the differential voltage. The phase control unit 12 only needs to use a 2π offset from the voltages VI and VQ, Vp and Vn used for phase adjustment.

[0104] Units 20ch1 and 20ch2 each have a shutter 28. For example, unit 20ch1 is set to normal operation, and unit 20ch2 is set to standby. Before the phase control unit 12 resets the voltage of unit 20ch1, the same voltage as unit 20ch1 is applied to unit 20ch2. Figure 6(Step S12). The modulation control unit 16 modulates the light in unit 20ch2 in the same way as unit 20ch1. The shutter control unit 18 opens the shutter 28 of unit 20ch2 synchronously with the shutter 28 of unit 20ch1 (step S14). Modulated light is emitted from unit 20ch2, and modulated light is not emitted from unit 20ch1 which is being reset. Disorders in the modulated light are suppressed, making it less likely to cause transmission errors of the optical signal.

[0105] For example, if the transmission speed is set to 11 Gb / s, a 1 Mbit buffer period is set for every 10 Mbits of data transmitted. This results in a buffer period of approximately 100 μs every 1 ms. By switching cells during this buffer period, data transmission without transmission errors can be achieved.

[0106] Alternatively, after resetting the voltage in unit 20ch1, the shutter 28 of unit 20ch1 can be kept closed, allowing unit 20ch2 to continue operating and emitting light from unit 20ch2. Alternatively, unit 20ch1 can be set as a standby unit, while unit 20ch2 can be set for normal operation.

[0107] <Second Implementation>

[0108] Figure 7 This is a block diagram illustrating the optical transmitting apparatus 200 according to the second embodiment. Descriptions of structures identical to those in the first embodiment are omitted. The optical transmitting apparatus 200 has 101 units, namely units 20ch1 to 20ch101. Each unit has the same structure. 100 units (e.g., units 20ch1 to 20ch100) are for normal operation, and one unit (e.g., unit 20ch101) is for standby.

[0109] Figure 8 This is a flowchart illustrating the process executed by the control unit 10, showing the resetting of the voltage of the sub-Mach-Zehnder modulator. When the differential voltage Vdc in any of units 20ch1 to 20ch100 (e.g., unit 20ch1) reaches V3c or higher (step S10), the control unit 10 starts unit 20ch101 (step S22). The shutter control unit 18 closes the shutter 28 of unit 20ch1 and opens the shutter 28 of unit 20ch101 (step S24). After the phase control unit 12 shifts the differential voltage Vdc of unit 20ch1 by an amount equivalent to 2π (step S16), the shutter control unit 18 closes the shutter 28 of unit 20ch101 and opens the shutter 28 of unit 20ch1 (step S28). The control unit 10 stops unit 20ch101 (step S30).

[0110] According to the second embodiment, similarly to the first embodiment, by keeping the differential voltage Vdc within the range of -V3c to V3c, the increase in voltages Vp and Vn is suppressed. 100 units 20ch1 to 20ch100 are for normal operation, and one unit 20ch101 is for standby. Since only one standby unit is needed relative to the transmission capacity of 100 channels, the cost of the optical transmission device 200 is reduced. The number of normally operating units can be less than 100 or more. The number of standby units can also be one or more. The higher the ratio of normally operating units to standby units, the lower the cost.

[0111] The embodiments of this disclosure have been described in detail above, but this disclosure is not limited to the specific embodiments involved. Various modifications and alterations can be made within the scope of the spirit of this disclosure as set forth in the claims.

[0112] Label Explanation

[0113] 10. Control Department

[0114] 12 Phase Control Unit

[0115] 14 Laser Control Department

[0116] 16 Modulation Control Unit

[0117] 18. Shutter control unit

[0118] Units 20ch1 to 20ch101

[0119] 22 Wavelength Variable Laser Element

[0120] 24 ABC circuit

[0121] 26 Driver ICs

[0122] 28 shutter speeds

[0123] 40, 40a, 40a1, 40a2, 40b modulators

[0124] 42a~42d Sub-Mach-Zehnder Modulator

[0125] 43 substrate

[0126] Branch waveguides 52a, 52b, 60a-60d, 62a, 62b

[0127] Input waveguides 50, 50a, 50b, and 71

[0128] Output waveguides 64, 64a, and 64b

[0129] Couplers 52, 54a, 54b, 56a, 56b, 58, and 72

[0130] Electrodes 66a~66d, 68a~68d, 70a, 70b

[0131] 100, 200 optical transmission devices

Claims

1. An optical transmitting apparatus comprising: a Mach-Zehnder modulator having a branch waveguide; and a phase control section that controls a phase of light propagating in the branch waveguide by applying a voltage to the Mach-Zehnder modulator, wherein in a case where the voltage exceeds a prescribed range, the phase control section shifts the voltage by an amount corresponding to a change of 2π in the phase to a side opposite to a side on which the prescribed range is exceeded, the optical transmitting apparatus comprises: a shutter capable of blocking the light emitted from the Mach-Zehnder modulator; a shutter control section that controls opening and closing of the shutter; and a modulation control section that modulates the light propagating in the branch waveguide of the Mach-Zehnder modulator, wherein the Mach-Zehnder modulator and the shutter form one unit, in a case where the modulation control section modulates the light in the Mach-Zehnder modulator of a first unit among a plurality of the units and the voltage applied by the phase control section to the Mach-Zehnder modulator of the first unit exceeds a prescribed range, the modulation control section modulates the light in the Mach-Zehnder modulator of a second unit among the plurality of the units, the phase control section applies a voltage derived in the same manner as the voltage applied to the Mach-Zehnder modulator of the first unit to the Mach-Zehnder modulator of the second unit, after the voltage is applied to the Mach-Zehnder modulator of the second unit, the shutter control section closes the shutter of the first unit and opens the shutter of the second unit, and after the shutter of the first unit is closed, the phase control section shifts the voltage by an amount corresponding to a change of 2π in the phase of the light propagating in the Mach-Zehnder modulator of the first unit.

2. The optical transmitting apparatus according to claim 1, wherein the Mach-Zehnder modulator has two of the branch waveguides, a first electrode is provided to each of the two branch waveguides, the phase control section controls the phase of the light by applying a third voltage that is a sum of a first voltage and a second voltage to one of the two first electrodes and applying a fourth voltage that is a difference between the first voltage and the second voltage to the other of the two first electrodes, and in a case where the second voltage exceeds a prescribed range, the phase control section shifts the second voltage by an amount corresponding to a change of 2π in the phase.

3. The optical transmitting apparatus according to claim 1 or 2, wherein the phase control section adjusts a phase difference of the light between the two branch waveguides to π or a value equivalent to π, or 0.5π or a value equivalent to 0.5π.

4. The optical transmitting apparatus according to claim 1 or 2, wherein a width of the prescribed range of the voltage is a range corresponding to a range of the phase of -π or more and π or less or more, and a range corresponding to a range of the phase of -3π or more and 3π or less or less.

5. A control method of an optical transmitting apparatus, comprising the steps of: controlling a phase of light propagating in a branch waveguide of a Mach-Zehnder modulator by inputting a voltage to the Mach-Zehnder modulator having the branch waveguide. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ in a case where the voltage exceeds a prescribed range, shifting the voltage to the opposite side from the side on which the prescribed range is exceeded by an amount corresponding to a change of 2π in the phase; an opening and closing shutter capable of blocking light emitted from the Mach-Zehnder modulator; and modulating light propagating in the branch waveguide of the Mach-Zehnder modulator, the Mach-Zehnder modulator and the shutter form a unit, in the modulating process, the light is modulated in the Mach-Zehnder modulator of a first unit among a plurality of the units, and in the process of controlling the phase, in a case where the voltage applied to the Mach-Zehnder modulator of the first unit exceeds a prescribed range, the light is modulated in the Mach-Zehnder modulator of a second unit among the plurality of the units, and a voltage derived in the same manner as the voltage applied to the Mach-Zehnder modulator of the first unit is applied to the Mach-Zehnder modulator of the second unit, after the voltage is applied to the Mach-Zehnder modulator of the second unit, the process of opening and closing the shutter is performed, the shutter of the first unit is closed and the shutter of the second unit is opened, after the shutter of the first unit is closed, the voltage is shifted by an amount corresponding to a change of 2π in the phase of the light propagating in the Mach-Zehnder modulator of the first unit.

6. A computer-readable storage medium storing a control program of an optical transmission apparatus, the control program causing a computer to execute the following processes: controlling a phase of light propagating in a branch waveguide of a Mach-Zehnder modulator by inputting a voltage to the Mach-Zehnder modulator having the branch waveguide; in a case where the voltage exceeds a prescribed range, shifting the voltage to the opposite side from the side on which the prescribed range is exceeded by an amount corresponding to a change of 2π in the phase; an opening and closing shutter capable of blocking light emitted from the Mach-Zehnder modulator; and modulating light propagating in the branch waveguide of the Mach-Zehnder modulator, the Mach-Zehnder modulator and the shutter form a unit, in the modulating process, the light is modulated in the Mach-Zehnder modulator of a first unit among a plurality of the units, and in the process of controlling the phase, in a case where the voltage applied to the Mach-Zehnder modulator of the first unit exceeds a prescribed range, the light is modulated in the Mach-Zehnder modulator of a second unit among the plurality of the units, and a voltage derived in the same manner as the voltage applied to the Mach-Zehnder modulator of the first unit is applied to the Mach-Zehnder modulator of the second unit, after the voltage is applied to the Mach-Zehnder modulator of the second unit, the process of opening and closing the shutter is performed, the shutter of the first unit is closed and the shutter of the second unit is opened, after the shutter of the first unit is closed, the voltage is shifted by an amount corresponding to a change of 2π in the phase of the light propagating in the Mach-Zehnder modulator of the first unit.

Images