Optical transmitter and optical transmission method

Abstract

It is difficult to perform automatic bias control for the optical modulators with a simple configuration without using a dither signal, and therefore the optical transmitter includes a control unit configured to adjust an adjusted bias voltage, which is one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator, and a third bias voltage applied to the third optical modulator, based on the combined optical signal, and to maintain the value of the bias voltage other than the adjusted voltage among the first bias voltage, the second bias voltage, and the third voltage.

Description

[Technical field]

[0001] The present invention relates to an optical transmitter and an optical signal transmission method, and more particularly to an optical transmitter and an optical signal transmission method that perform automatic bias control for an optical modulator with a simple configuration without using a dither signal. [Background technology]

[0002] In an optical communication network, an optical transmitter outputs an optical signal that is modulated by an optical modulator based on a driving voltage. The quality of the optical signal is degraded by operating point fluctuations caused by temperature fluctuations and aging of the optical modulator. In order to suppress the operating point fluctuations, an automatic bias control using a low-frequency dither signal is adopted in the optical modulator. However, when a low-frequency dither signal is used, the amplitude of the voltage applied to the optical modulator is changed by the low-frequency dither signal, which degrades the quality of the optical signal.

[0003] Patent Document 1 discloses a method for controlling a plurality of bias voltages applied to a plurality of modulation regions in a modulator having a plurality of modulation regions for modulating an optical signal, without using a dither signal. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2018-505449 Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, since it is necessary to place a photodetector in every modulation region, the configuration of the modulator disclosed in Patent Document 1 is complicated.

[0006] A representative object of the present invention is to provide an optical transmitter and an optical signal transmission method, and more particularly, an optical transmitter and an optical signal transmission method that perform automatic bias control for an optical modulator with a simple configuration without using a dither signal. [Means for solving the problem]

[0007] An optical transmitter according to a representative embodiment of the present invention comprises: a light source configured to output light; A branching unit configured to branch light into a first branched light and a second branched light; a first optical modulator configured to modulate the first branched light to output a first modulated optical signal; a second optical modulator configured to modulate the second branched light to output a second modulated optical signal; a third optical modulator configured to adjust a phase difference between the first modulated optical signal and the second modulated optical signal; a multiplexing unit configured to multiplex the first modulated optical signal and the second modulated optical signal and output a combined optical signal; a control unit configured to adjust an adjusted bias voltage, which is one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator, and a third bias voltage applied to the third optical modulator, based on the combined optical signal; The control unit maintains the values ​​of the bias voltages other than the adjustment voltage, among the first bias voltage, the second bias voltage, and the third voltage.

[0008] An optical transmission method according to a representative aspect of the present invention includes: Outputting light; and splitting the light into a first split light and a second split light; modulating the first branched light by a first optical modulator to output a first modulated optical signal; modulating the second branched light by a second optical modulator to output a second modulated optical signal; adjusting a phase difference between the first modulated optical signal and the second modulated optical signal by a third optical modulator; combining the first modulated optical signal and the second modulated optical signal to output a combined optical signal; adjusting an adjusted bias voltage, which is one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator, and a third bias voltage applied to the third optical modulator, based on the combined optical signal; and maintaining a value of a bias voltage other than the adjustment voltage among the first bias voltage, the second bias voltage, and the third voltage. Effect of the Invention

[0009] A representative advantage of the present invention is that it provides automatic bias control for an optical modulator with a simple configuration without using a dither signal. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram showing an example of the configuration of an optical transmitter according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram for explaining the optical transmitter according to the first embodiment of the present invention. [Diagram 3] FIG. 3 is a flowchart illustrating the operation of the optical transmitter according to the first embodiment of the present invention. [Figure 4] FIG. 4 is a flowchart illustrating the operation of the optical transmitter according to the first embodiment of the present invention. [Diagram 5] FIG. 5 is a flowchart illustrating the operation of the optical transmitter according to the first embodiment of the present invention. [Figure 6] FIG. 6 is a flowchart illustrating the operation of the optical transmitter according to the first embodiment of the present invention. [Figure 7] FIG. 7 is a flowchart illustrating the operation of the optical transmitter according to the first embodiment of the present invention. [Figure 8] FIG. 8 is a flowchart for explaining the operation of the modified optical transmitter according to the first embodiment of the present invention. [Figure 9]FIG. 9 is a block diagram showing an example of the configuration of an optical transmitter according to the second embodiment of the present invention. [Figure 10] FIG. 10 is a block diagram showing an example of the configuration of an optical transmitter according to the third embodiment of the present invention. [Figure 11] FIG. 11 is a flowchart illustrating the operation of the optical transmitter according to the third embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] First embodiment The optical transmitter 1 will be described with reference to Fig. 1. Fig. 1 is a block diagram showing an example of the configuration of the optical transmitter 1. The optical transmitter 1 outputs a modulated optical signal. For example, the optical transmitter 1 outputs a quadrature amplitude modulation (QAM) optical signal.

[0012] As shown in FIG. 1, the optical transmitter 1 includes a light source 10, a quadrature modulator 20, a DSP (Digital signal processor) 30, an AMP (Amplifier) ​​35, a PD (Photodiode) 40, an ADC (Analog to digital converter) 45, an error evaluation unit 50, a bias controller 55, a bias generator 60, and a DAC (Digital to analog converter) 65.

[0013] The light source 10 outputs continuous light. For example, the light source 10 is a diode laser. The light source 10 is connected to a quadrature modulator 20.

[0014] Fig. 2 is a block diagram showing an example of the configuration of the quadrature modulator 20. As shown in Fig. 2, the quadrature modulator 20 includes a branching section 21, a first optical modulator 22, a second optical modulator 23, a third optical modulator 24, and a multiplexing section 25.

[0015] 1, the quadrature modulator 20 is connected to the light source 10 and the DAC 65. The quadrature modulator 20 modulates the light from the light source 10 and outputs a QAM optical signal to the outside of the optical transmitter 1.

[0016] The branching unit 21 branches the light from the light source 10 into a first branched light and a second branched light. The branching unit 21 outputs the first branched light to the first optical modulator 22. The branching unit 21 outputs the second branched light to the second optical modulator 23.

[0017] The first optical modulator 22 modulates the first branched light and outputs a first modulated optical signal. The first optical modulator 22 is connected to the branching unit 21, the third optical modulator 24, and the DAC 65. For example, the first optical modulator 22 is a MZM (Mach-Zehnder Modulator). The first optical modulator 22 uses an amplitude modulation method. The first optical modulator 22 modulates the first branched light based on a bias voltage (Vi, described later) applied by the DAC 65 ​​and a drive signal from the AMP 35.

[0018] The second optical modulator 23 modulates the second branched light and outputs a second modulated optical signal. The second optical modulator 23 is connected to the branching unit 21, the third optical modulator 24, and the DAC 65. For example, the second optical modulator 23 is an MZM. The second optical modulator 23 uses an amplitude modulation method. The second optical modulator 23 modulates the second branched light based on a bias voltage (Vq, described later) applied by the DAC 65 ​​and a drive signal from the AMP 35.

[0019] The third optical modulator 24 adjusts the phase difference between the first modulated optical signal and the second modulated optical signal. The third optical modulator 24 is connected to the first optical modulator 22, the second optical modulator 23, the multiplexing unit 25, and the DAC 65. For example, the third optical modulator 24 is a MZI (Mach-Zehnder interferometer). The third optical modulator 24 adjusts the phase difference between the first modulated optical signal and the second modulated optical signal based on a bias voltage (Vp, described later) applied by the DAC 65. Specifically, the third optical modulator 24 adjusts the phase difference so that the first modulated optical signal and the second modulated optical signal are orthogonal to each other.

[0020] The multiplexer 25 multiplexes the first modulated optical signal and the second modulated optical signal, and outputs a combined optical signal.

[0021] The DSP 30 outputs an I signal and a Q signal to the AMP 35 based on a data stream from a client device (not shown). The AMP 35 amplifies the I signal and the Q signal. The AMP 35 further outputs the I signal and the Q signal as a drive signal to the first optical modulator 22 and the second optical modulator 23, respectively.

[0022] The PD 40 converts the combined optical signal from the quadrature modulator 20 into an analog signal, and outputs the analog signal to the ADC 45. The ADC 45 converts the analog signal into a digital signal, and outputs the digital signal to the error evaluation section 50.

[0023] The error evaluation unit 50 measures the MER (modulation error ratio) of the digital signal from the ADC 45. The error evaluation unit 50 further judges whether the MER is less than a predetermined threshold. Specifically, the error evaluation unit 50 stores an ideal constellation for the QAM modulation method in advance. The error evaluation unit 50 measures the MER of the digital signal by comparing the ideal constellation with a constellation generated from the digital signal. The error evaluation unit 50 judges whether the MER is less than a predetermined threshold and outputs the judgment result to the bias controller 55.

[0024] The bias controller 55 sets the values ​​of the bias voltages Vi, Vq, and Vp according to the determination result from the error evaluation unit 50. The bias controller 55 changes the values ​​of the bias voltages Vi, Vq, and Vp when the determination result indicates that the MER exceeds a predetermined threshold. The bias controller 55 maintains the values ​​of the bias voltages Vi, Vq, and Vp when the determination result indicates that the MER is less than a predetermined threshold. The bias controller 55 will be described in detail later.

[0025] The bias generator 60 obtains values ​​of bias voltages Vi, Vq, and Vp. The bias generator applies a voltage having an amplitude corresponding to the bias voltages Vi, Vq, and Vp to the DAC 65. The DAC 65 ​​converts the voltage applied from the bias generator into an analog signal and applies the analog signal to the quadrature modulator 20. Specifically, the DAC 65 ​​applies a voltage corresponding to the bias voltage Vi to the first optical modulator 22. The DAC 65 ​​applies a voltage corresponding to the bias voltage Vq to the second optical modulator 23. The DAC 65 ​​applies a voltage corresponding to the bias voltage Vp to the third optical modulator 24.

[0026] Next, the operation of the optical transmitter 1 will be described with reference to Figures 3, 4, 5, 6 and 7. Figures 3 to 7 are flowcharts showing the operation of the optical transmitter 1.

[0027] First, the light source 10 outputs light to the quadrature modulator 20 (S101). The branching unit 21 of the quadrature modulator 20 branches the light into a first branched light and a second branched light (S102). The first optical modulator 22 modulates the first branched light, and the second optical modulator 23 modulates the second branched light (S103). Next, the first optical modulator 22 outputs a first modulated optical signal. The second optical modulator 23 outputs a second modulated optical signal.

[0028] The third optical modulator 24 adjusts the phase difference between the first modulated optical signal and the second modulated optical signal (S104). The multiplexer 25 multiplexes the first modulated optical signal and the second modulated optical signal (S105). The quadrature modulator 20 outputs a combined optical signal including the first modulated optical signal and the second modulated optical signal (S106).

[0029] The error evaluation unit 50 evaluates the error of the combined optical signal from the quadrature modulator 20 (S107). Specifically, the error evaluation unit 50 measures the MER from the combined optical signal. The error evaluation unit 50 determines whether the MER is less than a predetermined threshold (S108). When the measured MER does not exceed the threshold (No in S108), the bias controller 55 maintains the values ​​of the bias voltages Vi, Vq, and Vp. When a specific time has elapsed after the bias controller 55 has maintained the values ​​(S109), the error evaluation unit 50 again evaluates the error of the combined optical signal (S107).

[0030] When the measured MER exceeds the threshold (Yes in S108), the bias controller 55 adds ΔVi to the value of the bias voltage Vi, and maintains the values ​​of the bias voltages Vq and Vp (S110). In other words, when the error exceeds the threshold, the bias controller 55 adjusts the voltage to be adjusted (hereinafter, referred to as the adjustment voltage) (bias voltage Vi). The value of ΔVi is determined in advance. In the process of S110, the bias controller 55 provides the value of Vi increased by ΔVi to the bias generator 60. The error evaluation unit 50 measures the MER of the coupled optical signal while the bias voltage Vi increased by ΔVi is being applied to the first optical modulator 22 (S111).

[0031] The bias controller 55 subtracts ΔVi from the value of the bias voltage Vi to maintain the values ​​of the bias voltages Vq and Vp (S112). In the process of S112, the bias controller 55 provides the value of Vi decreased by ΔVi to the bias generator 60. The error evaluation unit 50 measures the MER of the combined optical signal while the bias voltage Vi corresponding to the increased value is being applied to the first optical modulator 22 (S113).

[0032] The error evaluation unit 50 further judges whether the measured MER is less than a predetermined threshold value (S114). Specifically, the error evaluation unit 50 compares the smaller MER of the MER measured in S111 and the MER measured in S113 with a threshold value. When the MER does not exceed the threshold value (No in S114), the bias controller 55 changes the value of the bias voltage Vi (S115). The bias controller 55 changes the value of the bias voltage Vi to the value of the bias voltage Vi that was applied when the MER compared with the threshold value was detected (No in S114). For example, when the error evaluation unit 50 compared the MER measured in S111 with the threshold value, the bias voltage Vi is increased by ΔVi in S115. After S115, the bias controller 55 maintains the bias voltages Vi, Vq, and Vp (S109). In other words, when adjusting the bias voltage Vi in S110 and S112 does not cause the error in the combined optical signal to exceed the threshold, the bias controller 55 maintains the bias voltage Vi after the bias voltage Vi is adjusted.

[0033] When the measured MER exceeds the threshold (Yes in S114), the bias controller 55 adds ΔVq to the value of the bias voltage Vq, and maintains the values ​​of the bias voltages Vi and Vp (S116). In other words, the bias controller 55 adjusts the adjustment voltage (bias voltage Vq) when the error exceeds the threshold. The value of ΔVq is determined in advance. In the process of S116, the bias controller 55 provides the value of Vq increased by ΔVq to the bias generator 60. The error evaluation unit 50 measures the MER of the coupled optical signal while the bias voltage Vq increased by ΔVq is being applied to the second optical modulator 23 (S117).

[0034] The bias controller 55 subtracts ΔVq from the value of the bias voltage Vq to maintain the values ​​of the bias voltages Vi and Vp (S118). In the process of S118, the bias controller 55 provides the value of Vq decreased by ΔVq to the bias generator 60. The error evaluation unit 50 measures the MER of the combined optical signal while the bias voltage Vq corresponding to the increased value is being applied to the second optical modulator 23 (S119).

[0035] The error evaluation unit 50 further determines whether the measured MER is less than a predetermined threshold (S120). Specifically, the error evaluation unit 50 compares the smaller MER of the MER measured in S117 and the MER measured in S119 with the threshold. When the MER does not exceed the threshold (No in S120), the bias controller 55 changes the value of the bias voltage Vq (S121). The bias controller 55 changes the value of the bias voltage Vq to the value of the bias voltage Vq applied when the MER compared with the threshold is detected. For example, when the MER measured in S117 is compared with the threshold, the bias voltage Vq is increased by ΔVq in S121. After S121, the bias controller 55 maintains the bias voltages Vi, Vq, and Vp (S109). In other words, when the error of the coupled optical signal does not exceed the threshold by adjusting the bias voltage Vq in S116 and S118, the bias controller 55 maintains the bias voltage Vq after the bias voltage Vq is adjusted.

[0036] When the measured MER exceeds the threshold (Yes in S120), the bias controller 55 adds ΔVp to the value of the bias voltage Vp, and maintains the values ​​of the bias voltages Vi and Vq (S122). In other words, the bias controller 55 adjusts the adjustment voltage (bias voltage Vp) when the error exceeds the threshold. The value of ΔVp is predetermined. In the process of S122, the bias controller 55 provides the value of Vp increased by ΔVp to the bias generator 60. The error evaluation unit 50 measures the MER of the combined optical signal while the bias voltage Vp increased by ΔVp is being applied to the third optical modulator 24 (S123).

[0037] The bias controller 55 subtracts ΔVp from the value of the bias voltage Vp, and maintains the values ​​of the bias voltages Vi and Vq (S124). In the process of S124, the bias controller 55 provides the value of Vp decreased by ΔVp to the bias generator 60. The error evaluation unit 50 measures the MER of the combined optical signal while the bias voltage Vp corresponding to the increased value is being applied to the third optical modulator 24 (S125).

[0038] The error evaluation unit 50 further determines whether the measured MER is less than a predetermined threshold (S126). Specifically, the error evaluation unit 50 compares the smaller MER of the MER measured in S123 and the MER measured in S125 with the threshold. When the MER does not exceed the threshold (No in S126), the bias controller 55 changes the value of the bias voltage Vp (S127). The bias controller 55 changes the value of the bias voltage Vp to the value of the bias voltage Vp applied when the MER compared with the threshold is detected. For example, when the threshold is compared with the MER measured in S123, the bias voltage Vp is increased by ΔVp in S121. After S127, the bias controller 55 maintains the bias voltages Vi, Vq, and Vp (S109). In other words, when the error of the coupled optical signal does not exceed the threshold by adjusting the bias voltage Vp in S122 and S124, the bias controller 55 maintains the bias voltage Vp after the bias voltage Vp is adjusted.

[0039] When the MER exceeds the threshold, the bias controller 55 changes the values ​​of ΔVi, ΔVq, and ΔVp (S128). For example, the bias controller 55 changes the values ​​of ΔVi, ΔVq, and ΔVp only by a predetermined value or randomly. After the process of S128, the bias controller 55 adds ΔVi to the value of the bias voltage Vi, and maintains the values ​​of the bias voltages Vq and Vp (S110). The operation of the transmitter 1 is described above. In the above operation description, the transmitter 1 adjusts the bias voltages in the order of the bias voltages Vi, Vp, and Vq, but the order is not limited to this order.

[0040] As described above, the optical transmitter 1 includes the light source 10, the first optical modulator 22, the second optical modulator 23, the third optical modulator 24, the multiplexing unit 25, and the control unit. The control unit is a combination of the PD 40, the ADC 45, the error evaluation unit 50, the bias controller 55, the bias generator 60, and the DAC 65.

[0041] The light source 10 outputs light. The branching unit 21 branches the light from the light source into a first branched light and a second branched light. The first optical modulator 22 modulates the first branched light to output a first modulated optical signal. The second optical modulator 23 modulates the second branched light to output a second modulated optical signal. The third optical modulator 24 adjusts the phase difference between the first modulated optical signal and the second modulated optical signal. The multiplexing unit 25 multiplexes the first modulated optical signal and the second modulated optical signal to output a combined optical signal. The control unit adjusts an adjustment bias voltage, which is one of a first bias voltage (bias voltage Vi) applied to the first optical modulator 22, a second bias voltage (bias voltage Vq) applied to the second optical modulator 23, and a third bias voltage (bias voltage Vp) applied to the third optical modulator 24, based on the combined optical signal. The control unit further maintains the values ​​of the bias voltages other than the adjustment voltage, among the first bias voltage, the second bias voltage, and the third voltage.

[0042] The optical transmitter 1 maintains the value of the bias voltage other than the adjustment voltage and outputs the combined optical signal. The optical transmitter 1 further adjusts the adjustment voltage based on the combined optical signal. Since the combined optical signal is affected only by the adjustment of the adjustment voltage, the optical transmitter 1 can individually adjust the bias voltages applied to the optical modulators based on the combined optical signal. As a result, it is not necessary to place a photodetector for every optical modulator, and therefore the optical transmitter 1 is simple.

[0043] For example, in the process from S110 to S113, the optical transmitter 1 adjusts the bias voltage Vi to maintain the bias voltages Vq and Vp, and outputs the combined optical signal. The optical transmitter 1 further adjusts the voltage Vi based on the combined optical signal. Since the combined optical signal is affected only by the adjustment of the bias voltage Vi, the optical transmitter 1 can adjust the bias voltage Vi individually based on the combined optical signal. The transmitter 1 does not need photodetectors for the first optical modulator 22, the second optical modulator 23, and the third optical modulator 24, because the transmitter 1 adjusts the bias voltages Vq and Vp similarly based on the combined optical signal. As a result, the optical transmitter 1 is simple.

[0044] Next, the optical transmitter 1A will be described. The optical transmitter 1A is a modified example of the optical transmitter 1. The configuration and operation of the optical transmitter 1A are the same as those of the optical transmitter 1. The optical transmitter 1A differs from the optical transmitter 1 in that it executes additional processing.

[0045] Fig. 8 is a flowchart showing an operation including an additional process for the optical transmitter 1. As shown in Fig. 8, when the error exceeds the threshold of S126 of the optical transmitter 1A, the bias controller 55 calculates the values ​​of ΔVi, ΔVq, and ΔVp (Yes in S126).

[0046] Specifically, the bias controller 55 updates the values ​​of ΔVi, ΔVq, and ΔVp based on the characteristics of the optical transmitter 1A. The bias controller 55 pre-stores a coefficient set for each characteristic of the optical transmitter 1A. For example, the characteristics of the optical transmitter 1A refer to the usage time of the optical transmitter 1A, the temperature of the optical transmitter 1A, the RMS value of the drive signal applied from the AMP 35 to the quadrature modulator 20, the values ​​of ΔVi, ΔVq, and ΔVp before S150, and the values ​​of the bias voltages Vi, Vq, and Vp. The bias controller 55 calculates the sum of the values ​​obtained by multiplying the coefficients and the characteristic values ​​as a new ΔVi. The characteristics of the optical transmitter 1A are not limited to those described above.

[0047] The usage time is obtained by a timer (not shown) that monitors the time from the input power to the optical transmitter 1. The temperature is obtained by a temperature sensor (not shown). The RMS value is obtained by the bias controller 55 that monitors the voltage applied from the AMP 35 to the quadrature modulator 20.

[0048] By using the ridge regression analysis, the bias controller 55 may further calculate a new ΔVi using an L2 norm based on the coefficients for the calculated values. By using the lasso regression analysis, the bias controller 55 may further calculate a new ΔVi using an L1 norm based on the coefficients for the calculated values.

[0049] The bias controller 55 further calculates a new ΔVq based on the temperature, the time of use, the RMS value of the drive signal applied to the second optical modulator 23, the value of ΔVq before S150, and the value of the bias voltage Vq. The bias controller 55 further calculates a new ΔVp based on the temperature, the time of use, the value of ΔVp before S150, and the value of the bias voltage Vp.

[0050] The bias controller 55 changes the values ​​of ΔVi, ΔVq, and ΔVp to the new values ​​of ΔVi, ΔVq, and ΔVp calculated in S150 (S128). As described above, the operation of the optical transmitter 1A is explained.

[0051] As described above, the bias controller 55 updates the change amounts (ΔVi, ΔVp, ΔVq) based on the characteristics of the optical transmitter 1 A. This allows the transmitter 1 A to adjust the bias voltage with an optimal optical change amount, so that the optical transmitter 1 A reduces the number of processes to reduce errors. Second embodiment

[0052] An optical transmission system 2 in the second embodiment will be described with reference to Fig. 9. Fig. 9 is a block diagram showing a configuration example of the optical transmission system 2. The optical transmission system 2 includes two optical transmitters 1, a PD 40, an ADC 45, an error evaluation unit 50, a bias controller 55, a bias generator 60, and a DAC 65. The optical transmitter 1 includes a light source 10, a quadrature modulator 20, a DSP 30, and an AMP 35. The PD 40 and the DAC 65 ​​are connected to the two optical transmitters 1. The optical transmission system 2 differs from the optical transmitter 1 in that it includes an additional optical transmitter 1.

[0053] The optical transmission system 2 executes the operation described in the first embodiment for the two optical transmitters 1. Specifically, the optical transmission system 2 executes the processes from S101 to S128 by using one of the optical transmitters 1. Then, when the error of the combined optical signal from one of the optical transmitters 1 does not exceed the threshold, the optical transmission system 2 starts to evaluate the error of the combined optical signal from the other optical transmitter 1. Specifically, after executing the process in S109 using one of the optical transmitters 1, the optical transmission system 2 evaluates the error of the combined optical signal from the other optical transmitter 1 in S107.

[0054] The optical transmission system 2 is described above. In the above description of the operation, the transmitter 1A adjusts the bias voltages in the order of the bias voltages Vi, Vp, and Vq, but the order is not limited to this order. The optical transmission system 2 has a similar configuration to the optical transmitter 1. As a result, like the optical transmitter 1, it is not necessary to arrange a photodetector for every optical modulator, so the optical transmission system 2 is simple. Third embodiment

[0055] The optical transmitter 3 in the third embodiment will be described with reference to Fig. 10. Fig. 10 is a block diagram showing a configuration example of the optical transmitter 3. The optical transmitter 3 includes a light source 10, a first optical modulator 22, a second optical modulator 23, a third optical modulator 24, a multiplexing unit 25, and a control unit 70.

[0056] The light source 10 outputs light. The branching unit 21 branches the light from the light source into a first branched light and a second branched light. The first optical modulator 22 modulates the first branched light to output a first modulated optical signal. The second optical modulator 23 modulates the second branched light to output a second modulated optical signal. The third optical modulator 24 adjusts the phase difference between the first modulated optical signal and the second modulated optical signal. The multiplexing unit 25 multiplexes the first modulated optical signal and the second modulated optical signal to output a combined optical signal. The control unit 70 adjusts an adjustment bias voltage, which is one of a first bias voltage (bias voltage Vi) applied to the first optical modulator 22, a second bias voltage (bias voltage Vq) applied to the second optical modulator 23, and a third bias voltage (bias voltage Vp) applied to the third optical modulator 24, based on the combined optical signal. The control unit 70 further maintains the values ​​of the bias voltages other than the adjustment voltage, among the first bias voltage, the second bias voltage, and the third voltage.

[0057] Next, the operation of the transmitter 3 will be described with reference to Fig. 11. Fig. 11 is a flowchart showing the operation of the transmitter 3. First, the light source 10 outputs light to the quadrature modulator 20 (S201). The branching section of the quadrature modulator 20 branches the light into a first branched light and a second branched light (S202). The first optical modulator 22 modulates the first branched light, and the second optical modulator 23 modulates the second branched light (S203). Next, the first optical modulator 22 outputs a first modulated optical signal. The second optical modulator 23 outputs a second modulated optical signal.

[0058] The third optical modulator 24 adjusts the phase difference between the first modulated optical signal and the second modulated optical signal (S204). The multiplexing unit 25 multiplexes the first modulated optical signal and the second modulated optical signal (S205). The quadrature modulator 20 outputs a combined optical signal including the first modulated optical signal and the second modulated optical signal (S206). The control unit 70 adjusts the adjusted bias voltage based on the combined optical signal, and maintains the values ​​of the bias voltages other than the adjusted voltage among the first bias voltage, the second bias voltage, and the third voltage (S207).

[0059] As described above, the optical transmitter 3 is described. The optical transmitter 3 has a similar configuration to the optical transmitter 1. As a result, like the optical transmitter 1, it is not necessary to arrange a photodetector for every optical modulator, so the optical transmitter 3 is simple.

[0060] Although the present invention has been shown and described in detail with reference to embodiments thereof, the present invention is not limited to these embodiments, and it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the claims.

Claims

1. a light source configured to output light; a branching unit configured to branch the light into a first branched light and a second branched light; a first optical modulator configured to modulate the first branched light to output a first modulated optical signal; a second optical modulator configured to modulate the second branched light to output a second modulated optical signal; a third optical modulator configured to adjust a phase difference between the first modulated optical signal and the second modulated optical signal; a multiplexing unit configured to multiplex the first modulated optical signal and the second modulated optical signal to output a combined optical signal; a control unit configured to adjust an adjusted bias voltage, which is one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator, and a third bias voltage applied to the third optical modulator, based on the combined optical signal; The control unit is maintaining a value of a bias voltage other than the adjusted bias voltage among the first bias voltage, the second bias voltage, and the third bias voltage; measuring the error of the combined optical signal by a modulation error ratio (MER); adjusting the regulated bias voltage when the error exceeds a threshold; maintaining the value of the adjusted bias voltage when the error in adjusting the adjusted bias voltage does not exceed the threshold; and evaluating the error again a specified time after maintaining the adjusted bias voltage. Optical transmitter.

2. The control unit increases and decreases the value of the adjusted bias voltage by a change amount.

2. The optical transmitter of claim 1.

3. the control unit calculates the amount of change based on one of the first bias voltage, the second bias voltage, and the third bias voltage.

3. The optical transmitter according to claim 2.

4. The control unit calculates the amount of change based on the temperature of the optical transmitter.

4. An optical transmitter according to claim 2 or 3.

5. the control unit calculates the amount of change based on an RMS value of a drive signal for modulating the first branched light or an RMS value of a drive signal for modulating the second branched light.

5. An optical transmitter according to claim 2.

6. The control unit calculates the amount of change based on a usage time of the optical transmitter.

6. An optical transmitter according to claim 2.

7. Outputting light; and branching the light into a first branched light and a second branched light; modulating the first branched light by a first optical modulator to output a first modulated optical signal; modulating the second branched light by a second optical modulator to output a second modulated optical signal; adjusting a phase difference between the first modulated optical signal and the second modulated optical signal by a third optical modulator; combining the first modulated optical signal and the second modulated optical signal to output a combined optical signal; adjusting an adjusted bias voltage, which is one of a first bias voltage applied to the first optical modulator, a second bias voltage applied to the second optical modulator, and a third bias voltage applied to the third optical modulator, based on the combined optical signal; maintaining a value of a bias voltage other than the adjusted bias voltage among the first bias voltage, the second bias voltage, and the third bias voltage; measuring an error in the combined optical signal by a modulation error ratio (MER); adjusting the regulated bias voltage when the error exceeds a threshold; maintaining the value of the adjusted bias voltage when the error in adjusting the adjusted bias voltage does not exceed the threshold; evaluating the error again a specified time after maintaining the adjusted bias voltage; and 2. An optical transmission method comprising:

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