Optical transmitter, optical communication system and program

Abstract

To provide an optical transmitter capable of stably maintaining the characteristics of transmitted signal light, and an optical communication system equipped with this optical transmitter.SOLUTION: An optical transmitter (1) includes an optical modulator (20) that outputs signal light, a driver amplifier unit (103) that amplifies the voltage of an electric signal that is a data signal and applies it to electrodes (44, 45, 54, 55), an amplitude monitor unit (104) that monitors the amplitude of the voltage of the amplified electrical signal, a DC bias application unit (105) that applies a DC bias voltage (VB) to the electrode, and a DC bias control unit (106), and the DC bias control unit (106) performs first control to adjust a DC bias voltage (VB) according to the monitored amplitude, and after that, second control is performed to adjust the DC bias voltage (VB) such that the error rate detected by the optical receiver (70) that receives the signal light is minimized.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure provides: Optical transmitter, optical communication system and program Regarding. [Background technology]

[0002] In recent years, to realize high-capacity optical transmission, polarization multiplexing phase modulation (PM) schemes using digital coherent technology have been adopted, which can increase the data rate per wavelength to 100 Gbps or 400 Gbps. Known PM PM schemes include DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) modulation and DP-16QAM (Dual Polarization Quadrature Amplifier Modulation), which use MZ (Mach-Zehnder) optical modulators.

[0003] An optical transmitter that employs DP-QPSK modulation or 16QAM includes a laser light source and a QPSK modulator, which is a semiconductor optical modulator that performs phase modulation on laser light (see, for example, Patent Document 1). This optical transmitter performs phase adjustment by applying voltages to phase adjustment electrodes for each of Ich (In-phase channel), Qch (Quadrature-phase channel), and Pch (Phase channel), applies an RF signal obtained by amplifying a data signal with an RF (radio frequency) driver amplifier to the modulation electrode, and applies a DC bias voltage adjusted by a DC bias control unit to the modulation electrode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6251977 Summary of the Invention [Problem to be solved by the invention]

[0005] However, there is a problem in that the necessary RF drive voltage (ie, half-wave voltage) Vπ, which is determined by the DC bias voltage of the modulation electrode of the semiconductor optical modulator, decays with age due to the degradation of the RF driver amplifier.

[0006] An object of the present disclosure is to provide an optical transmitter capable of stably maintaining the characteristics of transmitted signal light, and an optical communication system including this optical transmitter. [Means for solving the problem]

[0007] The optical transmitter disclosed herein comprises an optical modulator having an optical waveguide and an electrode that applies an electric field to the optical waveguide, modulating laser light input to the optical waveguide with the electric field and outputting the modulated light as signal light, a driver amplifier that amplifies the voltage of an electrical signal that is a data signal and applies the amplified electrical signal to the electrode, an amplitude monitor that monitors the amplitude of the voltage of the amplified electrical signal, a DC bias application unit that applies a DC bias voltage to the electrode, and a DC bias control unit that controls the DC bias application unit, wherein the DC bias control unit performs first control to adjust the DC bias voltage in accordance with the amplitude monitored by the amplitude monitor unit, and after the first control, performs second control to receive an error rate detected by an optical receiver that receives the signal light and adjust the DC bias voltage so that the error rate is minimized. [Effects of the Invention]

[0008] of the present disclosure Optical transmitter, optical communication system and program According to this, there is an effect that the characteristics of the transmitted signal light can be stably maintained. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a block diagram schematically illustrating the configuration of an optical transmitter and an optical communication system according to a first embodiment. [Figure 2] 5 is a flowchart showing an operation related to DC bias voltage control of the optical transmitter according to the first embodiment. [Figure 3] FIG. 10 is a block diagram schematically illustrating the configuration of an optical transmitter and an optical communication system according to a second embodiment. [Figure 4] 10 is a flowchart showing an operation related to DC bias voltage control of the optical transmitter according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] The following describes the embodiments. Optical transmitter, optical communication system and program The following description will be given with reference to the drawings. The following embodiments are merely examples, and the embodiments can be appropriately combined and modified.

[0011] Embodiment 1 1 is a block diagram showing a schematic configuration of an optical transmitter 1 and an optical communication system 3 according to a first embodiment. The optical transmitter 1 includes an optical modulation unit 20, an RF driver amplifier 103 as a driver amplifier unit, an amplitude monitor unit 104 as a voltage measuring device, a DC bias application unit 105 as a DC voltage application circuit, and a DC bias control unit 106 as a bias control circuit. In the first embodiment, the optical transmitter 1 also includes a laser light source 10, a phase control unit 101 as a phase control circuit, an optical intensity detector 80 that monitors the intensity of an output signal, and a data generation unit 102.

[0012] The optical modulation unit 20 has an optical waveguide and electrodes 41-45, 51-55 that apply an electric field to the optical waveguide, modulates unmodulated laser light input from the laser light source 10 to the optical waveguide using the electric field, and outputs the modulated light as signal light (also referred to as "output light"). The optical modulation unit 20 has an MZ (Mach-Zender) type optical modulator. For example, the optical modulation unit 20 has an optical branching unit 30 that is an optical branching element that branches the light input from the laser light source 10 into two, an MZ type I-channel optical modulator (also referred to as a "first optical modulator") 40 and an MZ type Q-channel optical modulator (also referred to as a "second optical modulator") 50 that modulate the lights branched by the optical branching unit 30, respectively, and an optical combining unit 60 that is an optical combining element that combines the output lights of the optical modulator 40 and the optical modulator 50. Furthermore, the signal light output from the optical transmitter 1 passes through an optical transmission line and is received by an optical receiver 70.

[0013] The RF driver amplifier 103 amplifies the voltage of the electrical signal, which is the data signal output from the data generation unit 102, and applies the amplified electrical signal to electrodes (e.g., electrodes 44, 45, 54, and 55). The amplitude monitor unit 104 monitors the amplitude of the voltage of the electrical signal amplified by the RF driver amplifier 103. The DC bias application unit 105 applies a DC bias voltage VB to the electrodes (e.g., electrodes 45 and 55). The DC bias control unit 106 controls the DC bias application unit 105 to set the DC bias voltage VB.

[0014] The optical modulation unit 20, for example, DP-QPSK modulates or 16QAM modulates the input light. In the optical modulation unit 20, the optical branching unit 30 branches the input laser light into two laser lights, and inputs the branched laser lights to the I-ch optical modulator 40 and the Q-ch optical modulator 50, respectively. The two laser lights optically modulated by the optical modulator 40 and the optical modulator 50 are polarization-combined by the optical combining unit 60, and the polarization-combined laser light is output as output light. The output light output from the optical transmitter 1 is received by the optical receiver 70.

[0015] The data signal output from the data generation unit 102 is amplified by the RF driver amplifier 103 and input to the optical modulation unit 20. The electric signal output from the DC bias application unit 105 and the electric signal output from the phase control unit 101 are input to the optical modulation unit 20, whereby the laser light is modulated. The modulated laser light is transmitted as signal light and received by the optical receiver 70. The optical receiver 70 calculates a bit error rate (BER), which is an error rate, and transmits the BER to the optical transmitter 1.

[0016] The DC bias control unit 106 performs a first control to adjust the DC bias voltage VB in accordance with the amplitude monitored by the amplitude monitoring unit 104 (steps S102 to S104 in FIG. 2 described later), and after the first control, receives the error rate detected by the optical receiver 70 that receives the signal light, and performs a second control to adjust the DC bias voltage VB so that the error rate is minimized (steps S105 to S110 in FIG. 2 described later).

[0017] 2 is a flowchart showing an operation related to DC bias voltage control of the optical transmitter 1 according to the first embodiment. First, the RF driver amplifier 103 amplifies the RF signal, which is the data signal output from the data generator 102 (step S101). Next, the amplitude monitor 104 detects the RF amplitude output from the RF driver amplifier 103 (step S102).

[0018] Next, the DC bias control unit 106 determines whether the amplitude of the voltage of the RF signal detected by the amplitude monitor unit 104 is equal to a half-wave phase modulation voltage (also referred to as a "half-wave voltage") Vπ, which is a drive voltage required to shift the phase by half a wavelength in the modulator (step S103). If the amplitude of the voltage of the RF signal is not equal to Vπ (NO in step S103), the DC bias control unit 106 adjusts the DC bias voltage VB. Specifically, if the amplitude of the voltage of the RF signal is smaller than Vπ, the DC bias control unit 106 adjusts the DC bias voltage VB to reduce Vπ, and if the amplitude of the voltage of the RF signal is larger than Vπ, the DC bias control unit 106 adjusts the DC bias voltage VB to increase Vπ (step S104).

[0019] If the amplitude of the voltage of the RF signal is equal to Vπ (YES in step S103), the DC bias control unit 106 receives the BER measured by the optical receiver 70 (step S105).

[0020] The DC bias control unit 106 changes the DC bias voltage VB by a predetermined value (for example, +A [V]) (step S106), and determines whether the BER measured by the optical receiver 70 has improved (step S107). If the BER measured by the optical receiver 70 has improved (YES in step S107), the DC bias control unit 106 again changes the DC bias voltage VB by a predetermined value (for example, +A [V]) (step S106), and determines whether the BER measured by the optical receiver 70 has improved (step S107).

[0021] If the BER measured by the optical receiver 70 has not improved (NO in step S107), the DC bias control unit 106 changes the DC bias voltage VB by a predetermined value (for example, −A [V]) (step S108) and determines whether the BER measured by the optical receiver 70 has improved (step S109). If the BER measured by the optical receiver 70 has improved (YES in step S109), the DC bias control unit 106 again changes the DC bias voltage VB by a predetermined value (for example, −A [V]) (step S108) and determines whether the BER measured by the optical receiver 70 has improved (step S109).

[0022] If the BER measured by the optical receiver 70 has not improved (NO in step S109), the DC bias control unit 106 changes the DC bias voltage VB by a predetermined value (for example, +A [V]) (step S110). Note that the processes in steps S106 to S110 may be performed by switching the plus and minus signs in "+A" and "-A".

[0023] As described above, the optical transmitter 1 and the optical communication system 3 according to the first embodiment are configured to be able to monitor the amplitude of the output of the RF driver amplifier 103, and therefore it is possible to compensate for gain attenuation due to deterioration of the RF driver amplifier 103 over time, thereby achieving the effect of ensuring signal characteristics.

[0024] Furthermore, according to the optical transmitter 1 and the optical communication system 3 of the first embodiment, even when the amplitude or gain of the output of the RF driver amplifier 103 increases due to component replacement or the like, it is possible to make the amplitude of the RF signal equal to Vπ.

[0025] Embodiment 2 Fig. 3 is a block diagram schematically illustrating the configuration of an optical transmitter 2 and an optical communication system 4 according to the second embodiment. In Fig. 3, components that are the same as or correspond to those shown in Fig. 1 are assigned the same reference numerals as those shown in Fig. 1. The optical transmitter 2 and the optical communication system 4 according to the second embodiment differ from the optical transmitter 1 and the optical communication system 3 according to the first embodiment in that a semiconductor optical amplifier 90, which is an optical amplifier for amplifying optical output power, and an optical amplification control unit 107 are added after the optical modulation unit 20.

[0026] The second embodiment is similar to the first embodiment in that the amplitude of the voltage output from the RF driver amplifier 103 is monitored by an amplitude monitor unit 104 and fed back to a DC bias control unit 106. In the second embodiment, the gain of a semiconductor optical amplifier 90 provided in the subsequent stage of an optical modulation unit 20 is controlled by an optical amplification control unit 107 based on the power detected by an optical intensity detector 80. By increasing the DC bias voltage VB by the DC bias control unit 106, the modulation loss in the optical modulation unit 20 increases. The decrease is detected by the optical intensity detector 80, and the optical amplification control unit 107 controls the semiconductor optical amplifier 90 to compensate for the decrease in gain.

[0027] 4 is a flowchart showing the operation of the DC bias voltage control of the optical transmitter 2 according to the embodiment 2. In FIG. 4, steps S101 to S110 are the same as steps S101 to S110 in FIG.

[0028] After step S110, in the optical transmitter 2 according to the second embodiment, the optical intensity detector 80 detects the output optical power of the optical modulation unit 20 (step S201), and the optical amplification control unit 107 determines whether the output optical power is lower than a preset value (step S202). If the output optical power is lower than the preset value (YES in step S202), the optical amplification control unit 107 increases the gain of the semiconductor optical amplifier 90 by, for example, a predetermined value (step S203). The optical amplification control unit 107 repeats the processes of steps S202 and S203 until the output optical power becomes higher than the preset value.

[0029] As described above, the optical transmitter 2 and optical communication system 4 according to the second embodiment are configured to include the semiconductor optical amplifier 90 in the subsequent stage of the optical modulation unit 20, which makes it possible to compensate for increased optical loss by controlling the DC bias voltage VB, thereby achieving the effect of maintaining the optical output power of the transmitter. This makes it possible to continue outputting the optical power required by the transmission line or the opposing optical receiver, and simultaneously ensures signal characteristics as in the first embodiment.

[0030] Furthermore, according to the optical transmitter 2 and the optical communication system 4 of the second embodiment, even if the optical loss compensation due to other factors or the optical transmitter output power required for the system changes, it is possible to adjust the optical output power to the desired level.

[0031] Moreover, instead of the semiconductor optical amplifier 90 that amplifies the optical output, an erbium-doped optical fiber amplifier may be used.

[0032] In addition, the data generation unit 102, the DC bias control unit 106, and the phase control unit 101 can be formed by a processing circuit, and the processing circuit may have a memory for storing a program and a processor such as a CPU (central processing unit) for executing the program.

[0033] In all other respects, the second embodiment is the same as the first embodiment.

[0034] Embodiment 3 According to the third embodiment Optical transmitter, optical communication system and program The difference between the first and second embodiments is that the relationship between the wavelength of the laser light and the DC bias voltage VB is determined in advance and stored as a table. Optical transmitter, optical communication system and programIn the third embodiment, the DC bias voltage VB is set by referring to a table of DC bias voltages VB corresponding to wavelengths obtained in advance. In other words, in the third embodiment, the DC bias control unit 106 obtains and stores in advance a table of DC bias voltages VB corresponding to each wavelength of the laser light to be used, and adjusts the DC bias voltage VB using the table so that the amplitude monitored by the amplitude monitoring unit 104 becomes equal to the half-wave voltage Vπ for the wavelength of the laser light to be used.

[0035] As described above, the third embodiment Optical transmitter, optical communication system and program According to this, by determining the value of the DC bias voltage VB for the wavelength to be used in advance, it is possible to obtain the effect that the transmission characteristics and optical output power can be maintained even when the wavelength of the laser light source is changed.

[0036] In all other respects, the third embodiment is the same as the first or second embodiment.

[0037] Embodiment 4 According to the fourth embodiment Optical transmitter, optical communication system and program The point that Vπ is adjusted for each wavelength of the laser light is the same as that of the third embodiment. Optical transmitter, optical communication system and programThe optical transmitter according to the fourth embodiment varies the wavelength of the laser light emitted from the laser light source 10 from the shortest wavelength to the longest wavelength within the applicable range of the device, and calculates the DC bias voltage VB so that the desired Vπ is obtained for each wavelength. The optical transmitter according to the fourth embodiment then checks the BER of the received signal at the optical receiver 70, varies the DC bias voltage VB, and determines the DC bias voltage VB that minimizes the BER. By pre-storing a table of the laser light wavelength vs. DC bias voltage VB obtained by the above method, it becomes possible to set the DC bias voltage VB required when the wavelength of the laser light source 10 is changed. In other words, in the fourth embodiment, the DC bias control unit 106 pre-obtains the DC bias voltage VB that minimizes the error rate of the received signal at the optical receiver 70 for each wavelength of the laser light used, and stores the DC bias voltage VB as a table. The DC bias control unit 106 then uses the table to adjust the DC bias voltage VB so that the amplitude monitored by the amplitude monitor unit 104 for the wavelength of the laser light used becomes equal to the half-wave voltage Vπ.

[0038] As described above, the fourth embodiment Optical transmitter, optical communication system and program According to this, by determining the value of the DC bias voltage VB for the wavelength to be used in advance, it is possible to maintain transmission performance with high precision.

[0039] In all other respects, the fourth embodiment is the same as the third embodiment. [Explanation of symbols]

[0040] 1, 2 Optical transmitter, 3, 4 Optical communication system, 10 Laser light source (laser light emitting element), 20 Optical modulation unit, 30 Optical branching unit (optical branching element), 40 Optical modulator (first optical modulator), 41 to 45 Electrodes, 50 Optical modulator (second optical modulator), 51 to 55 Electrodes, 60 Optical combining unit (optical combining element), 70 Optical receiver, 80 Optical intensity detector, 90 Semiconductor optical amplifier, 101 Phase control unit, 102 Data generation unit, 103 RF driver amplifier (driver amplifier unit), 104 Amplitude monitor unit, 105 DC bias application unit, 106 DC bias control unit, 107 Optical amplification control unit.

Claims

1. an optical modulation unit having an optical waveguide and an electrode for applying an electric field to the optical waveguide, modulating laser light input to the optical waveguide with the electric field and outputting the modulated light as signal light; a driver amplifier unit that amplifies the voltage of an electrical signal that is a data signal and applies the amplified electrical signal to the electrode; an amplitude monitor that monitors the amplitude of the voltage of the amplified electrical signal; a DC bias applying unit that applies a DC bias voltage to the electrode; a DC bias control unit that controls the DC bias application unit; and The DC bias control unit performing a first control of adjusting the DC bias voltage in accordance with the amplitude monitored by the amplitude monitor; After the first control, an error rate detected by an optical receiver that receives the signal light is received, and a second control is performed to adjust the DC bias voltage so that the error rate is minimized.

1. An optical transmitter comprising:

2. The DC bias control unit adjusts the DC bias voltage in the first control so that the amplitude monitored by the amplitude monitor unit becomes equal to a half-wave voltage.

2. The optical transmitter according to claim 1.

3. the optical modulation unit has an MZ type optical modulator, The MZ type optical modulator has the optical waveguide and the electrodes.

3. The optical transmitter according to claim 1 or 2.

4. The optical modulation unit an optical branching unit that branches light incident from a laser light source into two; a first MZ-type optical modulator and a second MZ-type optical modulator that modulate the light beams branched by the optical branching unit, respectively; an optical combining unit that combines output lights of the first optical modulator and the second optical modulator; and The first optical modulator and the second optical modulator each have the optical waveguide and the electrode.

3. The optical transmitter according to claim 1 or 2.

5. further comprising a laser light source that outputs the laser light; The DC bias control unit adjusts the DC bias voltage in the first control so that the amplitude monitored by the amplitude monitor unit becomes equal to a half-wavelength voltage of the laser light.

5. The optical transmitter according to claim 1, wherein the optical transmitter is a semiconductor laser.

6. a light intensity detector that detects the intensity of the signal light output from the optical modulation unit; an optical amplifier that amplifies the signal light output from the optical modulation unit; an optical amplification control unit that adjusts a gain of the optical amplifier based on the intensity detected by the optical intensity detector; 6. The optical transmitter according to claim 1, further comprising:

7. The DC bias control unit acquires and stores in advance a table of the DC bias voltage for each wavelength of the laser light to be used, and adjusts the DC bias voltage using the table so that the amplitude monitored by the amplitude monitor unit becomes equal to a half-wave voltage for the wavelength of the laser light to be used.

7. The optical transmitter according to claim 1, wherein the optical transmitter is a semiconductor laser.

8. The DC bias control unit determines the DC bias voltage such that the error rate received from the optical receiver is minimized.

8. The optical transmitter according to claim 1, wherein the optical transmitter is a semiconductor laser.

9. The DC bias control unit acquires in advance, for each wavelength of the laser light used, a DC bias voltage that minimizes the error rate of the received signal at the optical receiver and stores the acquired voltage as a table, and adjusts the DC bias voltage using the table so that the amplitude monitored by the amplitude monitor unit becomes equal to a half-wave voltage for the wavelength of the laser light used.

8. The optical transmitter according to claim 1, wherein the optical transmitter is a semiconductor laser.

10. An optical transmitter according to any one of claims 1 to 9; the optical receiver; An optical communication system having:

11. A first step of adjusting a DC bias voltage applied to an electrode that applies an electric field to an optical waveguide in accordance with the amplitude of the amplified voltage of an electrical signal that is a data signal; a second step of receiving an error rate detected by an optical receiver that receives signal light, which is light obtained by modulating the laser light input to the optical waveguide by the electric field, after the first step, and adjusting the DC bias voltage so that the error rate is minimized; A program that is executed by a processor.

12. The program described in Claim 11, characterized in that the first step adjusts the DC bias voltage so that the amplitude is equal to a half-wave voltage.

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