JP2013174761A - Optical transmitter, optical communication system and optical transmission method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a highly accurate bias control of an optical modulator by correcting a deviation from an optimum bias in optical transmitters using optical modulators, and thereby suppress degradation of an optical signal quality even when treating a modulation optical signal required for the highly accurate bias control such as, for example, a pre-equalized optical signal, a multi-value modulation optical signal or the like.SOLUTION: The optical transmitter comprises: an optical modulation part that modulates light based on an applied bias voltage and input modulation signal and outputs the modulated optical signal; a bias control circuit that controls the bias voltage to be applied to the optical modulation part; and an operation point off-set addition part that adds an off-set to an operation point of the bias voltage by the bias control circuit.

Description

  The present invention relates to an optical communication technique, and more particularly, to an optical transmitter using an optical modulator that modulates light, an optical communication system, and an optical transmission method.

  Conventionally, an optical waveguide formed on a substrate having an electro-optic effect, two MZ (Mach-Zehnder) type optical modulators for modulating light waves propagating through the optical waveguide, and two MZ type optical modulators ABC (automatic bias control circuit of the optical modulator for controlling the DC bias in the two MZ type optical modulators with respect to the optical modulator configured to multiplex the modulated light wave at the multiplexing part of the optical waveguide. In an Automatic Bias Control circuit, a low frequency signal is superimposed on a direct current bias applied to two MZ type optical modulators, and a low frequency signal is generated from a light wave (stray light) radiated into the substrate from the combined portion of the optical waveguide. There is known an ABC circuit that detects a change in the amount of light corresponding to 1 by a photodetector and controls the DC bias of each MZ optical modulator based on the detected change in the amount of light (example) For example, see Patent Document 1).

Japanese Patent Laying-Open No. 2004-318052 (paragraph 0048, FIG. 7 (a))

  However, in the optical transmitter using the ABC circuit of the conventional optical modulator disclosed in Patent Document 1, stray light from a multiplexing unit detected by a photodetector built in the optical modulator, as will be described later. There is an offset (hereinafter referred to as bias shift) between the extinction characteristic of the optical modulator and the extinction characteristic of the optical modulator output, and the offset amount has wavelength dependence and temperature dependence. The ABC circuit deviates from the optimum bias. In particular, when a modulated optical signal for which high-precision bias control is desired, such as a pre-equalized optical signal and a multilevel modulated optical signal, the optical signal quality is deteriorated. There was a problem.

  The present invention has been made to solve the above-described problems. In an optical transmitter using an optical modulator, high-precision bias control of the optical modulator is realized by correcting a deviation from the optimum bias. In addition, the present invention aims to suppress degradation of optical signal quality even when handling a modulated optical signal for which high-precision bias control is desired, such as a pre-equalized optical signal and a multilevel modulated optical signal.

  An optical transmitter according to the present invention modulates light based on an applied bias voltage and an input modulation signal and outputs the modulated optical signal, and the optical modulator applied to the optical modulation unit A bias control circuit that controls a bias voltage; and an operating point offset adding unit that adds an offset to an operating point of the bias voltage by the bias control circuit.

  According to the present invention, in an optical transmitter, it is possible to realize highly accurate bias control of an optical modulator by correcting a deviation from an optimum bias by adding an offset to an operating point.

1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. Explanatory drawing for demonstrating the optical transmitter by Embodiment 1 of this invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 1 of this invention 1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. Configuration diagram showing an optical transmitter according to a second embodiment of the present invention

Embodiment 1 FIG.
1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. In each figure, the same numerals indicate the same or corresponding parts. 1, the optical transmitter according to the first embodiment of the present invention includes a transmission data generation 100, a light source 101, a driver 102, an MZ (Mach-Zehnder) type light modulation unit 103, and an MZ type light modulation unit 103. Built-in monitor PD (Photo-Diode) 104, a bias control circuit 105, and an operating point offset adding unit 106.

  The light source 101 and the light input unit of the MZ type light modulation unit 103 are optically connected by, for example, an optical fiber. The transmission data generator 100, which is means for generating transmission data as a modulation signal, and the modulation electrode of the MZ type optical modulation unit 103 are electrically connected via the driver 102, and the bias control circuit 105 and the MZ type optical modulation unit 103 are connected to each other. The bias electrode is electrically connected, and the bias control circuit 105 and the operating point offset adding unit 106 are electrically connected.

  In FIG. 1, a light source 101 includes an LD (Laser Diode) which is a semiconductor laser made of, for example, an InP compound semiconductor mixed crystal, and emits and outputs laser light as a continuous wave in a 1.5 μm wavelength band. . The configuration of the light source 101 is not limited to this. For example, a 1.3 μm wavelength band, a pulse wave, a solid-state laser, or the like may be used. In short, light for realizing desired optical communication is used. Can be applied as an embodiment of the present invention.

  In FIG. 1, an MZ type optical modulation unit 103 converts two MZ type optical modulators optically connected in parallel by an optical waveguide and the optical phase of light propagating through these two MZ type optical modulators. An optical phase adjustment unit for adjustment and a built-in monitor PD 104 are provided, and are formed on an LN (Lithium Niobate) substrate made of, for example, lithium niobate.

  The two MZ type optical modulators of the MZ type optical modulation unit 103 are optical modulators using, for example, an LN crystal as a constituent material and utilizing a so-called electro-optical effect, which is a change in refractive index by applying an electric field. The MZ type optical modulator is configured as a so-called MZ interferometer by connecting two optical waveguides having electrodes formed in parallel between two Y branch optical waveguides. The MZ type optical modulator is configured such that, for light passing through the MZ interferometer, a modulation signal input to the modulation electrode and a gap between two optical waveguides caused by a refractive index change caused by a DC bias voltage applied to the bias electrode. A light intensity change corresponding to the phase difference is given and output. The MZ type optical modulator is an optical modulator capable of achieving both high optical signal quality such as low chirp and high speed.

  The MZ type optical modulation unit 103 optically connects these two MZ type optical modulators in parallel and is configured as a main MZ interferometer, and is an I-ch (In-phase channel) phase modulation that is a real part. An optical signal and a Q-ch (Quadrature-phase channel) phase-modulated optical signal, which is an imaginary part, are combined with a carrier phase difference of π / 2 to multiplex a complex optical field of DQPSK (Differential Quadrature Shift Shifting). Is an optical modulator. The MZ type optical modulation unit 103 includes a modulation electrode and a bias electrode in each MZ type optical modulator, and a bias electrode as an optical phase adjustment unit in an optical waveguide connecting the MZ type optical modulators in parallel. ing.

  The configuration of the MZ type light modulation unit 103 is not limited to this. For example, an InP-based compound semiconductor mixed crystal or the like may be used as a constituent material. In short, it is similar to a bias shift described later. If the optical modulator has an operating point offset, the same effects as those of the embodiment of the present invention can be obtained.

  In FIG. 1, the bias control circuit 105 includes an ABC (Automatic Bias Control) circuit similar to the configuration disclosed in, for example, Patent Document 1. That is, the amplitude of the low frequency signal component superimposed on the monitor signal decreases near the operation stable point of the ABC circuit and becomes zero at the operation stable point. Therefore, the ABC circuit adds a low frequency signal having an appropriate amplitude to the monitor signal, so that the amplitude of the low frequency signal component of the monitor signal becomes zero, that is, the position of the operation stable point of the ABC circuit. It is possible to adjust the bias voltage applied to each bias electrode.

  However, the bias control circuit 105 compensates for a later-described bias shift included in the monitor signal from the built-in monitor PD 104, so that an offset is added to the position of the operation stable point of the ABC circuit by the compensation amount from the operating point offset adding unit 106. A configuration for determining the added bias voltage is provided.

  Next, the operation will be described. In FIG. 1, the modulation electrical signal output from the transmission data generator 100 is amplified by the driver 102 and input to the MZ type optical modulation unit 103 as an I-ch and Q-ch modulation electrical signal. The MZ type optical modulation unit 103 modulates the continuous light from the light source 101 by driving each modulation electrode, which is a traveling wave electrode, with an I-ch and Q-ch modulation electrical signal as a modulation signal from the driver 102. And output as an optical signal.

  In addition to the modulation electrical signal, the bias electrode of the MZ type optical modulation unit 103 includes I-ch, Q-ch, and P-ch that determine the operating point of the MZ type optical modulation unit 103 from the bias control circuit 105. (Phase channel) A bias voltage is input. The built-in monitor PD 104 of the MZ type light modulation unit 103 is used as a means for detecting the deviation of the operating point.

  Note that, as described above, a known technique as disclosed in Patent Document 1 can be applied to the bias control for the MZ type light modulation unit 103. For example, the ABC circuit of the bias control circuit 105 is based on an error signal from the built-in monitor PD 104 of the MZ type optical modulation unit 103 which is a detection means for superimposing a low frequency dither signal on the DC bias and detecting a response to the superimposed signal. However, a configuration that controls an optimum DC bias for the MZ type light modulator 103 may be employed.

  As a DC bias voltage finally applied to the MZ type optical modulation unit 103, a voltage obtained by adding a compensation amount from the operating point offset adding unit 106 to the DC bias voltage value controlled by the bias control circuit 105 is input. Is done.

  FIG. 2 is an explanatory diagram for explaining the optical transmitter according to the first embodiment of the present invention, and shows an extinction characteristic curve of the MZ type optical modulation unit 103. In each figure, the same numerals indicate the same or corresponding parts. In FIGS. 1 and 2, when light of an opposite phase is coupled at the coupling portion of the Y-branch optical waveguide of the main MZ interferometer of the MZ type optical modulation unit 103, high-order mode light is formed and cannot propagate through the optical waveguide. In this case, the light emitted into the LN substrate, that is, the stray light described above, becomes the extinction state of the optical modulator output.

  At this time, when monitoring is performed using the built-in monitor PD 104, since this radiated light is received, the extinction characteristic curve (curve indicated by a broken line in FIG. 2) and the modulation with respect to the bias voltage of the monitor PD current flowing through the built-in monitor PD 104 are modulated. The phase difference from the extinction characteristic curve (curve indicated by the solid line in FIG. 2) with respect to the bias voltage of the output of the detector is θ = π + α. Ideally, the extinction characteristic curve is completely opposite in phase, and it is desirable that α = 0. The coupling efficiency at the monitor PD 104 changes and does not necessarily satisfy α = 0, and appears as an offset of the phase difference of the extinction characteristic curve. This offset α is called a bias shift.

  In ordinary OOK (On Off Keying), DPSK (Differential Phase Shift Keying), and DQPSK modulation methods, the extinction curve is used in full swing, so the effect of bias shift on optical transmission quality is relatively minor. In the case of an optical signal or a multi-level modulated optical signal, the transient region of the extinction characteristic curve is used, and the distance between symbols is narrowed, so the deterioration of optical signal quality caused by bias shift cannot be ignored.

  The pre-equalized optical signal is used to electrically equalize and transmit waveform distortion due to chromatic dispersion and / or polarization dispersion of the optical transmission line in advance by digital signal processing on the transmission side. This pre-equalization technique is superior in that the system configuration can be simplified compared to optical dispersion compensation and the amount of equalization can be increased compared to dispersion compensation on the reception side using a conventional optical dispersion compensation device. This is an effective compensation means even in a high-speed optical transmission technology in which the influence of waveform distortion in the optical transmission path becomes large, such as 40 Gbps or 100 Gbps or higher.

In addition, the multi-level modulated optical signal has a background in which it is difficult to increase the speed of the component parts, and it is possible to increase the speed of 40 Gbps and 100 Gbps by increasing the amount of information that can be transmitted with one symbol. As such a modulation method, OFDM (Orthogonal Frequency Division Multiplexing) which is orthogonal frequency division multiplexing that reduces the symbol rate by dividing the carrier into a plurality of subcarriers, and the amplitudes of two carriers whose phases are different by 90 degrees are independent. QAM (Quadrature Amplitude Modulation), which is quadrature amplitude modulation in which each carrier wave is modulated with a plurality of amplitude values, is promising. In QAM taking 2 ^N values, N bits can be transmitted in one symbol, and the frequency utilization efficiency can be improved N times compared to OOK.

  Next, FIG. 3 is an explanatory diagram for explaining the optical transmitter according to the first embodiment of the present invention, and shows the wavelength dependence and temperature dependence of the bias shift of the MZ type optical modulation unit 103. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 3, since the distribution of the radiated light and the optical path length of the transmission path change depending on the wavelength of the input light from the light source 101, the bias shift has wavelength dependence. Therefore, performing optimal bias shift compensation for each wavelength leads to realization of highly accurate bias control of the optical modulator.

  In FIG. 3, since the distribution of the radiated light and the optical path length of the transmission path change depending on the temperature of the MZ type light modulation unit 103, the bias shift has temperature dependence. Therefore, performing optimal bias shift compensation for each temperature leads to realization of highly accurate bias control of the optical modulator. However, for example, in the case where the ambient temperature is kept constant or when there is a temperature control means for keeping the temperature of the MZ type light modulation unit 103 constant, only compensation of the wavelength dependence of the bias shift described above is performed. Next, an embodiment for that purpose will be described.

  FIG. 4 is a block diagram showing the optical transmitter according to the first embodiment of the present invention, and shows an example for compensating the wavelength dependence of the bias shift described above. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 4, the optical transmitter includes a light emission wavelength Ch. (Channel) information 107 is held, and a bias shift wavelength dependence compensation table in which the wavelength and the compensation amount correspond to each other is held in the operating point offset addition unit 106. The light source 101 and the operating point offset adding unit 106 are electrically connected.

  Next, the operation will be described. In FIG. 4, the LD of the light source 101 has an emission wavelength Ch. In response to a light emission command including information 107, light is emitted, and the light emission wavelength Ch. Continuous light having the wavelength indicated by the information 107 is input to the MZ type light modulation unit 103. On the other hand, the modulation electrical signal output from the transmission data generation 100 is amplified by the driver 102 and input to the MZ type optical modulation unit 103 as an I-ch and Q-ch modulation electrical signal. The MZ type optical modulation unit 103 modulates the continuous light from the light source 101 by driving each modulation electrode, which is a traveling wave electrode, with an I-ch and Q-ch modulation electrical signal as a modulation signal from the driver 102. And output as an optical signal.

  In addition to the modulation electrical signal, the bias electrode of the MZ type optical modulation unit 103 includes I-ch, Q-ch, and P-ch that determine the operating point of the MZ type optical modulation unit 103 from the bias control circuit 105. A bias voltage is input. The built-in monitor PD 104 of the MZ type light modulation unit 103 is used as a means for detecting the deviation of the operating point.

  As a DC bias voltage finally applied to the MZ type optical modulation unit 103, a voltage obtained by adding a compensation amount from the operating point offset adding unit 106 to the DC bias voltage value controlled by the bias control circuit 105 is input. Is done. As the compensation amount, the current emission wavelength Ch. A bias shift compensation amount corresponding to the information 107 is input to the bias control circuit 105, and the bias voltage is optimized.

  That is, the operating point offset adding unit 106 receives the emission wavelength Ch. In response to the emission command including the information 107, the current emission wavelength Ch. Is obtained from the bias shift wavelength dependency compensation table, which is a table storing the compensation amount of the bias shift for each wavelength. The compensation value corresponding to the wavelength indicated by the information 107 is referred to feed forward to the bias voltage of the bias control circuit 105. The bias shift wavelength dependence compensation table may have compensation amounts for all wavelengths, or may be interpolated from several representative points to generate compensation amounts.

  This eliminates the need for a bias shift monitoring function outside the optical modulator, thereby enabling a compact and low-cost optical transmitter configuration. Another advantage is that no extra light output loss due to a monitoring coupler or the like is caused outside the optical modulator.

  As a method of creating the bias shift wavelength dependence compensation table, ABC is performed by the bias control circuit 105 in a state where there is no offset amount at a certain emission wavelength, and the transmitted light waveform in that state is monitored with an oscilloscope or the like. The offset amount when the bias voltage offset is adjusted so as to maximize the eye opening is measured in advance as a compensation amount at the wavelength and stored in the bias shift wavelength dependence compensation table.

  Alternatively, ABC is performed by the bias control circuit 105 in a state where there is no offset amount at a certain emission wavelength, and the transmission light in that state is input to the reference receiver, and the bit error rate becomes the best, or optical OSNR (Optical Signal to (Noise Ratio) The offset amount when the offset of the bias voltage is adjusted so that the proof stress is maximized is measured in advance as a compensation amount at the wavelength, and stored in the bias shift wavelength dependence compensation table.

  As described above, in the optical transmitter according to the first embodiment of the present invention, the operating point offset adding unit 106 adds an offset to the operating point of the bias control circuit 105 that is an ABC circuit so as to compensate for the bias shift. I am doing so. This realizes highly accurate bias control of the optical modulator by correcting the deviation from the optimum bias. Consequently, highly accurate bias control such as a pre-equalized optical signal and a multilevel modulated optical signal is desired. Even when a modulated optical signal is handled, there is an effect that deterioration of the optical signal quality can be suppressed.

  In the optical transmitter according to the example of the first embodiment of the present invention, the operating point offset adding unit 106 refers to the bias shift wavelength dependence compensation table, determines the compensation amount based on the emission wavelength information, An offset is added to the operating point of the bias control circuit 105 which is an ABC circuit so as to compensate for the bias shift. Thereby, in addition to the above-described effects, there is an effect that the configuration can be reduced in size and cost without causing an extra loss of light output.

  In the optical transmitter according to the example of the first embodiment of the present invention, the table in the operating point offset adding unit 106 includes data on both wavelength dependency and temperature dependency of the bias shift as shown in FIG. 3, for example. May be held, the compensation amount may be determined based on the emission wavelength information and the temperature information, and an offset may be added to the operating point of the bias control circuit 105 which is an ABC circuit so as to compensate for the bias shift. Thereby, in addition to the above-described operation and effect, there is an operation and effect that deterioration of the optical signal quality can be suppressed even when the MZ type optical modulation unit 103 has a temperature variation. It goes without saying that it is possible to compensate only for the temperature dependence of the bias shift.

Embodiment 2. FIG.
As described above, the optical transmitter according to the first embodiment of the present invention refers to, for example, a table in which wavelengths and compensation amounts are associated with each other, and determines a bias shift compensation amount based on emission wavelength information. However, the optical transmitter according to the second embodiment of the present invention does not use a table and performs bias shift based on the output monitor of the MZ type optical modulation unit 103. The compensation amount is determined.

  FIG. 5 is a block diagram showing an optical transmitter according to Embodiment 2 of the present invention. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 5, instead of the operating point offset adding unit 106, the configuration shown in FIG. The configuration is the same as that of the optical transmitter according to the example of the first embodiment, and description of the same configuration and the similar operation may be omitted. The optical branching unit 108, the power monitoring unit 109, and the bias shift amount detecting unit 110 constitute an operating point offset adding unit.

  Next, the operation will be described. In FIG. 5, the LD of the light source 101 has an emission wavelength Ch. In response to a light emission command including information 107, light is emitted, and the light emission wavelength Ch. Continuous light having the wavelength indicated by the information 107 is input to the MZ type light modulation unit 103. On the other hand, the modulation electrical signal output from the wavelength transmission data generation 100 is amplified by the driver 102 and input to the MZ type optical modulation unit 103 as an I-ch and Q-ch modulation electrical signal. The MZ type optical modulation unit 103 modulates the continuous light from the light source 101 by driving each modulation electrode, which is a traveling wave electrode, with an I-ch and Q-ch modulation electrical signal as a modulation signal from the driver 102. And output as an optical signal.

  In addition to the modulation electrical signal, the bias electrode of the MZ type optical modulation unit 103 includes I-ch, Q-ch, and P-ch that determine the operating point of the MZ type optical modulation unit 103 from the bias control circuit 105. A bias voltage is input. The built-in monitor PD 104 of the MZ type light modulation unit 103 is used as a means for detecting the deviation of the operating point.

  The modulated light output from the MZ-type optical modulation unit 103 passes through the optical branching unit 108 optically connected to the destination, and is transmitted to the power monitor unit 109 as transmission light transmitted from the optical transmitter as a main signal. Is branched to monitor light. Of the modulated light output from the MZ type light modulator 103, the light intensity branched as the monitor light may be sufficient as long as the power monitor 109 can detect the light. The ratio may be a branching ratio used in a general transmission light power monitor.

  The bias shift amount detection unit 110 changes the DC bias voltage of the bias control circuit 105 at the current emission wavelength, and sweeps the operating point of the MZ type optical modulation unit 103 while monitoring light detected by the power monitor unit 109. And the monitor PD current detected by the built-in monitor PD 104 of the MZ type light modulator 103 are measured. Thereby, the bias shift amount detector 110 can obtain the relationship between the modulator output and the monitor PD current as shown in FIG. 2, that is, the bias shift α at the current emission wavelength can be detected as the compensation amount.

  The DC bias voltage finally applied to the MZ type optical modulation unit 103 is a voltage obtained by adding the compensation amount detected by the bias shift amount detection unit 110 to the DC bias voltage value controlled by the bias control circuit 105. Is input, and the bias voltage is optimized.

  In the actual operation of the optical transmitter, as a startup procedure when starting the optical transmitter, after the light source 101 emits light at an arbitrary wavelength and the light output intensity is stabilized, the DC bias from the bias control circuit 105 is obtained. While changing the voltage and sweeping the operating point of the MZ light modulator 103, the light intensity of the monitor light detected by the power monitor 109 and the monitor PD current detected by the built-in monitor PD 104 of the MZ light modulator 103 Is measured by the bias shift amount detection unit 110, so that an accurate bias shift compensation amount at an arbitrary wavelength can be grasped and compensation corresponding thereto can be performed.

  In the above configuration, it is not necessary to have a compensation amount table corresponding to the emission wavelength. For this reason, it is not necessary to perform a prior measurement for creating the table, and the adjustment time in the optical transmitter inspection process can be shortened as compared with the first embodiment.

  If the optical transmitter has a power monitor function and an optical branching unit and a monitor PD or tap PD are provided outside the MZ type optical modulation unit 103 in advance, the optical branching unit 108 and the power monitoring unit 109 are newly provided. There is no need to add, and the configuration for the power monitor function can be shared.

  As described above, in the optical transmitter according to the second embodiment of the present invention, the bias shift amount detection unit 110 detects the compensation amount based on the output monitor outside the optical modulator and compensates for the bias shift. An offset is added to the operating point of the bias control circuit 105 which is an ABC circuit. This realizes highly accurate bias control of the optical modulator by correcting the deviation from the optimum bias. Consequently, highly accurate bias control such as a pre-equalized optical signal and a multilevel modulated optical signal is desired. Even when a modulated optical signal is handled, there is an effect that deterioration of the optical signal quality can be suppressed. Furthermore, there is an effect that the time in the inspection process of the optical transmitter can be shortened as compared with the case where the table is used.

  As described above, in the optical transmitters according to the first and second embodiments of the present invention, the operating point offset adding unit 106, the bias control circuit 105, and the bias shift amount detecting unit 110 have been described as digital signal processing circuits. However, the control function may be realized by software processing using a computer program that causes a microcomputer or the like provided in the optical transmitter to execute the control method.

  Further, the optical transmitters according to the first and second embodiments of the present invention are applied to an optical communication system in which an optical signal transmitted from the optical transmitter is transmitted through an optical transmission line such as an optical fiber and received by an optical receiver. You may do it. Also, the optical transmitters according to the first and second embodiments of the present invention are provided with two or more optical transmitters, wavelength-multiplexed optical signals transmitted from the two or more optical transmitters, and optical transmission such as an optical fiber. It may be applied to a WDM (Wavelength Division Multiplexing) optical communication system in which transmission is performed on a path and wavelength is separated on the receiving side and received by two or more optical receivers for each wavelength. Also, the optical transmitters according to the first and second embodiments of the present invention are provided with two such optical transmitters, and optical signals transmitted from the two optical transmitters are polarization-multiplexed with mutually orthogonal polarizations. You may make it apply to the polarization multiplexing optical communication system which transmits with an optical transmission line called an optical fiber, carries out polarization separation on the receiving side, and receives with two optical receivers for every polarization.

100 transmission data generation, 101 light source, 102 driver, 103 MZ (Mach-Zehnder) type light modulator, 104 built-in monitor PD (Photo-Diode), 105 bias control circuit, 106 operating point offset adder, 107 emission wavelength Ch. (Channel) information, 108 optical branching unit, 109 power monitoring unit, 110 bias shift amount detection unit.

Claims (8)

An optical modulator that modulates light based on the applied bias voltage and the input modulation signal, and outputs the modulated optical signal;
A bias control circuit for controlling the bias voltage applied to the light modulator;
An operating point offset adding unit for adding an offset to the operating point of the bias voltage by the bias control circuit;
An optical transmitter comprising: The optical modulation unit includes an MZ (Mach-Zehnder) type optical modulator,
The bias control circuit controls the bias voltage based on the monitoring result of the radiated light in the MZ type optical modulator,
The operating point offset adding unit adds the offset corresponding to the bias shift between the emitted light in the MZ type optical modulator and the optical signal to the operating point of the bias voltage by the bias control circuit. The optical transmitter according to claim 1. The operating point offset adding unit refers to a table indicating a correspondence relationship between the wavelength of the light input to the light modulating unit and the offset, and adds the offset to the operating point of the bias voltage by the bias control circuit. The optical transmitter according to claim 1, wherein addition is performed. The operating point offset adding unit adds the offset to an operating point of the bias voltage by the bias control circuit based on a monitoring result of the optical signal output from the optical modulation unit. The optical transmitter according to claim 1 or 2. The optical transmitter according to any one of claims 1 to 4, wherein the optical signal is a pre-equalized optical signal or a multilevel modulated optical signal. A light source for inputting the emitted light to the light modulator;
A built-in monitor PD (Photo-Diode) for monitoring the light intensity of the emitted light,
The bias control circuit adds a low-frequency signal having a frequency lower than that of the modulation signal to the bias voltage, and the bias voltage is based on the low-frequency signal included in the emitted light monitored by the built-in monitor PD. The optical transmitter according to claim 2, wherein the optical transmitter is controlled.   An optical communication system, wherein the optical signal transmitted from the optical transmitter according to any one of claims 1 to 6 is transmitted through an optical transmission path and received by an optical receiver. An optical modulation step of modulating light by the optical modulation unit based on the applied bias voltage and the input modulation signal, and outputting the modulated optical signal;
A bias control step for controlling the bias voltage applied to the light modulator;
An operating point offset adding step of adding an offset to the operating point of the bias voltage by the bias control step;
An optical transmission method comprising:

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