JP2016075880A - Optical transmitter and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily optimize a working point when the working point of an intensity modulator needs to be reviewed.SOLUTION: In an intensity modulator 100, a first light intensity detection unit 102 outputs a signal representing the intensity of light from a Mach-Zehnder optical modulation unit 101. A DC bias output unit 302a outputs the DC component of a bias voltage to the intensity modulator 100. A low-frequency signal output unit 303a outputs a low-frequency signal to the intensity modulator 100 as the AC component of the bias voltage. A synchronous detection unit 301A performs synchronous detection on the output from the first light intensity detection unit 102 using the low-frequency signal as a reference signal. Feedback control using the detection signal obtained by the synchronous detection unit 301A is exercised in the DC bias output unit 302a. A phase setting unit 305 sets a phase difference between the phase of the low-frequency signal inputted to the intensity modulator 100 and the low-frequency signal inputted to the synchronous detection unit 301A.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmitter and a control method thereof, and more particularly to an optical transmitter having an intensity modulator and a control method thereof.

  As a transmission code used in an optical transmission system, a DMPSK (Differential Multiple Phase Shift Keying) system with high nonlinear tolerance has been widely studied. In particular, an RZ (Return to Zero) -DMPSK modulation system that makes the light intensity between symbols zero by combining a pulse carver with the DMPSK system is considered to have high utility.

  According to Japanese Patent Laying-Open No. 2010-243953 (Patent Document 1), as an optical transmitter using the RZ-DMPSK modulation method, an RZ modulator that performs intensity modulation, and a DMPSK modulation that performs phase modulation on the output of the RZ modulator An optical transmitter having a transmitter is disclosed. When a voltage signal as a clock signal is input to the RZ modulator, a DC (direct current) bias voltage that defines an operating point of modulation of the RZ modulator is also applied. In order to suppress the deterioration of the optical output signal from the RZ modulator, it is necessary to finely adjust the optical path length of the modulator in accordance with the temporal variation of the modulator. Therefore, the optical transmitter is provided with a monitor unit that monitors the intensity of light from the modulator and a bias control circuit that finely adjusts the DC bias voltage. The optimization of the bias voltage is performed by feedback control using synchronous detection between a dither signal for dithering the bias voltage of the modulator and the light intensity detected by the monitor unit.

JP 2010-243953 A

  When the bias voltage is changed when no clock signal is input to the RZ modulator, the light intensity detected by the monitor unit periodically changes. Here, a voltage corresponding to a change in one cycle of the optical output is defined as 2Vπ. Further, the maximum value of intensity that the optical output can take by adjusting the bias voltage when no clock signal is input to the RZ modulator is defined as Vmax. In addition, the voltage amplitude of the clock signal in which the intensity decreases by 3 dB from Vmax when this bias voltage is used is defined as Vx.

  The modulation schemes of the RZ modulator are classified into two types: CS (Carrier-Suppressed) RZ modulation and 33% RZ modulation. When the amplitude Vc of the clock signal is not less than Vx and not more than 2Vπ, CSRZ modulation is performed when the DC bias voltage is feedback-controlled so that the light intensity detected by the monitor unit is maximized, and conversely, the light intensity is minimized. Thus, when the DC bias voltage is feedback controlled, 33% RZ modulation is performed. On the other hand, when the amplitude of the clock signal is less than Vx, 33% RZ modulation is performed when the DC bias voltage is controlled so that the light intensity detected by the monitor unit is maximized, and conversely, the light intensity is minimized. Thus, when the DC bias voltage is controlled, CSRZ modulation is performed.

  In some cases, it is required to inspect the phase modulation waveform in the optical transmitter that generates the RZ-DMPSK signal. In this case, it is necessary to stop modulation by the RZ modulator that performs intensity modulation. If the input of the clock signal to the RZ modulator is simply stopped for that purpose, whether or not the waveform phase-modulated by the phase modulator can be confirmed depends on the RZ modulation method and the amplitude Vc. Dependent. Specifically, in CSRZ modulation with Vx ≦ Vc ≦ 2Vπ or 33% modulation with Vc <Vx, feedback control is performed on the DC bias so that the light intensity becomes maximum. As a result, the RZ modulator Light is transmitted without modulation. Therefore, the modulation waveform by the phase modulator can be confirmed. On the other hand, in 33% RZ modulation with Vx ≦ Vc ≦ 2Vπ or CSRZ modulation with Vc <Vx, feedback control is performed on the DC bias so that the light intensity is minimized. As a result, the RZ modulator is in the extinction state. Therefore, it becomes impossible to confirm the modulation waveform by the phase modulator. In other words, as a result of the operating point of the intensity modulator being inappropriate, it becomes impossible to confirm the modulation waveform by the phase modulator. Therefore, it is necessary to review the operating point of the intensity modulator.

  The present invention has been made to solve the above-described problems, and its purpose is to easily optimize the operating point when it becomes necessary to review the operating point of the intensity modulator. An optical modulator and a control method thereof are provided.

  The optical transmitter according to the present invention includes a data signal driver, a phase modulator, a clock signal driver, an intensity modulator, and a controller. The data signal driver outputs a voltage signal as a data signal. The phase modulator modulates the phase of light according to the data signal output from the data signal driver. The clock signal driver outputs a voltage signal as a clock signal. The intensity modulator is optically connected to the phase modulator and modulates the intensity according to the clock signal output from the clock signal driver. The intensity modulator has a Mach-Zehnder light modulator and a first light intensity detector. The Mach-Zehnder optical modulator operates with a clock signal with reference to an operating point defined by a bias voltage applied from the outside of the intensity modulator. The first light intensity detector outputs a signal representing the intensity of light from the Mach-Zehnder light modulator. The control unit outputs a bias voltage to the intensity modulator. The control unit includes a DC bias output unit, a low frequency signal output unit, a synchronous detection unit, and a phase setting unit. The direct current bias output unit outputs the direct current component of the bias voltage to the intensity modulator. The low frequency signal output unit outputs a low frequency signal having a frequency lower than the frequency of the clock signal to the intensity modulator as an AC component of the bias voltage. The synchronous detector performs synchronous detection on the output from the first light intensity detector using the low frequency signal as a reference signal. In the DC bias output unit, feedback control is performed using the detection signal obtained by the synchronous detection unit. The phase setting unit sets a phase difference between the phase of the low frequency signal input to the intensity modulator and the phase of the low frequency signal input to the synchronous detection unit.

  The optical transmitter controlled in the optical transmitter control method of the present invention includes a data signal driver, a phase modulator, a clock signal driver, an intensity modulator, and a controller. The data signal driver outputs a voltage signal as a data signal. The phase modulator modulates the phase of light according to the data signal output from the data signal driver. The clock signal driver outputs a voltage signal as a clock signal. The intensity modulator is optically connected to the phase modulator and modulates the intensity according to the clock signal output from the clock signal driver. The intensity modulator has a Mach-Zehnder light modulator and a light intensity detector. The Mach-Zehnder optical modulator operates with a clock signal with reference to an operating point defined by a bias voltage applied from the outside of the intensity modulator. The light intensity detector outputs a signal indicating the intensity of light from the Mach-Zehnder light modulator. The control unit outputs a bias voltage to the intensity modulator. The control unit includes a DC bias output unit, a low frequency signal output unit, and a synchronous detection unit. The direct current bias output unit outputs the direct current component of the bias voltage to the intensity modulator. The low frequency signal output unit outputs a low frequency signal having a frequency lower than the frequency of the clock signal to the intensity modulator as an AC component of the bias voltage. The synchronous detector performs synchronous detection on the output from the light intensity detector using the low frequency signal as a reference signal. In the DC bias output unit, feedback control is performed using the detection signal obtained by the synchronous detection unit. The optical transmitter control method includes the following steps. Modulation scheme information that is information about the type of modulation scheme used by the optical transmitter is acquired. Amplitude information that is information about the magnitude of the amplitude of the clock signal is acquired. Based on the modulation method information and the amplitude information, a phase difference between the phase of the low frequency signal input to the intensity modulator and the phase of the low frequency signal input to the synchronous detection unit is set. After the phase difference is set, the output of the phase modulator is observed with the input of the clock signal to the intensity modulator stopped.

  According to the optical transmitter of the present invention, the operation of the feedback control for the DC bias output unit is changed by changing the phase difference set by the phase setting unit. This changes the operating point of the intensity modulator. Therefore, reviewing the operating point of the intensity modulator, such as when observing the output of the phase modulator while the input of the clock signal to the intensity modulator is stopped, or when changing the modulation method of the optical transmitter Therefore, the operating point of the intensity modulator can be easily optimized by setting the phase difference by the phase setting unit.

  According to the control method of the optical transmitter of the present invention, by changing the phase difference between the phase of the low frequency signal input to the intensity modulator and the phase of the low frequency signal input to the synchronous detection unit. The operation of feedback control for the DC bias output unit is changed. This changes the operating point of the intensity modulator. Therefore, when the input of the clock signal to the intensity modulator is stopped to observe the output of the phase modulator, the operating point of the intensity modulator is easy so that the light intensity necessary for observation is secured. Can be optimized.

1 is a block diagram schematically showing a configuration of an optical transmitter according to Embodiment 1 of the present invention. FIG. 3 is a graph illustrating the operation of the RZ modulator as an intensity modulator in the optical transmitter of FIG. 1 in each of the CSRZ modulation method and the 33% modulation method when Vx ≦ Vc ≦ 2Vπ. In Embodiment 1 of the present invention, there is provided an optical transmitter control method for observing the output of a DMPSK modulator as a phase modulator while stopping the input of a clock signal to an RZ modulator as an intensity modulator. It is a flow figure shown roughly. It is a flowchart which shows more specifically the setting method of the phase difference in FIG. In Embodiment 1 of the present invention, a schematic method of controlling an optical transmitter, in which the optical transmitter is operated by an arbitrary modulation scheme by restarting the input of the clock signal from the state where the input of the clock signal is stopped, It is a flowchart shown in FIG. It is a block diagram which shows roughly the structure of the optical transmitter in Embodiment 2 of this invention. It is a flowchart which shows schematically the control method of the optical transmitter in Embodiment 2 of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

(Embodiment 1)
Referring to FIG. 1, an optical transmitter 901 of the present embodiment includes a light source 1, a clock signal driver 2, a data signal driver 3, a current-voltage converter 4a, and an RZ modulator 100 (intensity modulation). A DMPSK modulator 200 (phase modulator), and a control unit 300A.

  As the light source 1, any light source applicable to optical communication can be used. For example, the light source 1 is a 1.5 μm wavelength continuous wave (CW) light source using a semiconductor laser having an InP-based compound semiconductor mixed crystal as a constituent material. The wavelength band of the light source 1 is not particularly limited, and other wavelength bands such as a 1.3 μm wavelength band may be used. The light source 1 is not limited to a CW light source, and may be a pulse light source. In place of the semiconductor laser, a laser using a medium other than a semiconductor, such as a solid-state laser, may be used.

  The clock signal driver 2 outputs a voltage signal as a clock signal for the RZ modulator 100. The data signal driver 3 outputs a voltage signal as a data signal. The timing of the clock signal is adjusted so that the RZ modulator 100 is in the extinction state in the intermediate transition state between the symbols of the data signal.

  The RZ modulator 100 modulates the intensity according to the clock signal output from the clock signal driving unit 2. The RZ modulator 100 includes an RZ modulator 101 (Mach-Zehnder light modulator), a light intensity detector 102 (first light intensity detector), and optical waveguides 103a and 103b. The RZ modulator 101 operates with a clock signal with reference to an operating point defined by a bias voltage applied from the outside of the RZ modulator 100. The light intensity detector 102 outputs an electrical signal representing the intensity of light from the RZ modulator 101. The light intensity detection unit 102 is, for example, a low speed photodiode. The optical waveguide 103a guides input light to the RZ modulation unit 101, and the optical waveguide 103b guides output light from the RZ modulation unit 101.

  The DMPSK modulator 200 modulates the phase of light according to the data signal output from the data signal driver 3. As the DMPSK modulator 200, for example, a DQPSK (Differential Quadrature Phase Shift Keying) modulator may be used. The DMPSK modulator 200 includes a data modulation unit 201, a light intensity detection unit 202 (second light intensity detection unit), and optical waveguides 203a and 203b. The light intensity detection unit 202 (second light intensity detection unit) outputs an electrical signal representing the intensity of light output from the data modulation unit 201 of the DMPSK modulator 200. The light intensity detection unit 202 is, for example, a low speed photodiode. The optical waveguide 203a guides input light to the data modulation unit 201, and the optical waveguide 203b guides output light from the data modulation unit 201.

  The light source 1, the DMPSK modulator 200, and the RZ modulator 100 are optically connected in this order. Specifically, the light source 1 is connected to the optical waveguide 203a. The optical waveguides 203b and 103a are optically connected to each other. The optical waveguide 103b guides the output light of the optical transmitter 901. For optical connection, for example, an optical fiber can be used.

  The control unit 300A includes a synchronous detection unit 301A, a DC bias output unit 302a, a low frequency signal output unit 303a, an addition unit 304a, a phase setting unit 305, an amplification unit 306a, and a monitor unit 307a. The DC bias output unit 302 a outputs a DC component of the bias voltage to the RZ modulator 100. The low frequency signal output unit 303a outputs a low frequency signal having a frequency sufficiently lower than the frequency of the clock signal to the RZ modulator 100 as an AC component of the bias voltage. The adder 304a outputs the bias voltage obtained by adding the outputs from the DC bias output unit 302a and the low frequency signal output unit 303a to the RZ modulator 100.

  The amplification unit 306a amplifies the voltage signal input from the light intensity detection unit 102 via the current-voltage conversion unit 4a. The monitor unit 307a monitors the intensity of the signal from the amplification unit 306a.

  The synchronous detection unit 301A performs synchronous detection on the output from the light intensity detection unit 102 amplified by the amplification unit 306a using the low frequency signal from the low frequency signal output unit 303a as a reference signal. In the DC bias output unit 302a, feedback control using the detection signal obtained by the synchronous detection unit 301A is performed. The phase setting unit 305 sets a phase difference between the phase of the low frequency signal input to the RZ modulator 100 and the phase of the low frequency signal input to the synchronous detection unit 301A. Specifically, the phase setting unit 305 outputs light from the RZ modulator 100 by preventing the RZ modulator 100 from taking the extinction state when the input of the clock signal to the RZ modulator 100 is stopped. Set the phase difference to The phase setting unit 305 sets the phase difference according to the amplitude of the clock signal when the input of the clock signal to the RZ modulator 100 is started. Details of the operation of the phase setting unit 305 will be described later.

  The phase setting unit 305 is disposed between the low frequency signal output unit 303a and the addition unit 304a in FIG. 1, but may be disposed between the low frequency signal output unit 303a and the synchronous detection unit 301A. Good.

  Each of RZ modulator 100 and DMPSK modulator 200 is formed on a substrate using, for example, lithium niobate. In this case, the RZ modulation unit 101 and the data modulation unit 201 can be configured as a Mach-Zehnder type optical modulator using a lithium niobate crystal as a constituent material. The Mach-Zehnder optical modulator is a modulator that utilizes a change in refractive index (so-called electro-optic effect) caused by application of an electric field. The Mach-Zehnder optical modulator is configured as a so-called Mach-Zehnder interferometer, and two optical waveguides each having an electrode are provided in parallel between two Y-branch optical waveguides. The Mach-Zehnder type optical modulator applies a light intensity change to the light passing through the Mach-Zehnder interferometer. This change in light intensity occurs according to the phase difference between the two optical waveguides. This phase difference is controlled by a refractive index change caused by a modulation signal input to the modulation electrode and a bias voltage applied to the bias electrode. The Mach-Zehnder type optical modulator is an optical modulator that can achieve both high signal quality such as low chirp and high speed. In addition, the data modulation unit 201 may include one or more Mach-Zehnder light modulation units therein. Each Mach-Zehnder modulation unit modulates light based on a signal input from the data signal driving unit 3.

  Next, the operation of the RZ modulator 100 will be described below.

  In the case of Vx ≦ Vc ≦ 2Vπ, in order to perform CSRZ modulation, feedback control is performed on the DC bias output unit 302a so that the light intensity output from the RZ modulator 100 receiving the clock signal is maximized. The bias voltage that defines the operating point of the RZ modulator 101 by feedback control that maximizes the light intensity output from the RZ modulator 100 is a voltage that minimizes the optical output of the RZ modulator 101 (see FIG. 2). In the left graph, the voltage that gives the minimum light output). In the left graph of FIG. 2, an arrow CSRZ represents the amplitude range of the clock signal with the bias as the center. The upper right graph in FIG. 2 shows how the maximum intensity of the optical output can be obtained by CSRZ modulation under the condition that a clock signal satisfying Vx ≦ Vc ≦ 2Vπ is applied.

  As described above, when feedback control is performed on the DC bias output unit 302a so that the intensity of the output light from the RZ modulator 100 is maximized, the input of the clock signal to the RZ modulation unit 101 is stopped. Thus, the intensity of the light output is determined only by the operating point of the RZ modulator 101. Therefore, the operating point of the RZ modulation unit 101 is that the voltage at which the optical output is maximum (in the left graph in FIG. 2) from the voltage at which the optical output is minimum (the voltage that provides the minimum optical output in the left graph in FIG. 2). Shift to the voltage that gives the maximum light output). As a result, the light from the DMPSK modulator 200 passes through the RZ modulator 100 without further modulation. That is, light whose power is lost by the insertion loss of the RZ modulator 100 is output from the RZ modulator 100. Therefore, the data modulation waveform by the DMPSK modulator 200 can be confirmed. The same applies to the 33% RZ modulation with Vc <Vx.

  On the other hand, in order to perform 33% RZ modulation when Vx ≦ Vc ≦ 2Vπ, feedback control is performed on the DC bias output unit 302a so that the light intensity output from the RZ modulator 100 receiving the clock signal is minimized. Done. The bias voltage itself that defines the operating point of the RZ modulator 101 by feedback control that minimizes the light intensity output from the RZ modulator 100 is a voltage that maximizes the optical output of the RZ modulator 101 (see FIG. 2). In the left graph, the voltage that gives the maximum light output). In the left graph of FIG. 2, the arrow 33% RZ represents the amplitude range of the clock signal centered on the bias. The upper right graph in FIG. 2 shows how the minimum intensity of the optical output can be obtained by 33% RZ modulation under the condition that a clock signal satisfying Vx ≦ Vc ≦ 2Vπ is applied.

  As described above, when feedback control is performed on the DC bias output unit 302a so that the intensity of the output light from the RZ modulator 100 is minimized, the input of the clock signal to the RZ modulation unit 101 is stopped. Thus, the intensity of the light output is determined only by the operating point of the RZ modulator 101. Therefore, if no change occurs in the feedback control, the operating point of the RZ modulation unit 101 is that the light output is from the voltage at which the light output is maximum (the voltage that gives the maximum light output in the left graph of FIG. 2). Shifts to a voltage that minimizes (the voltage that gives the minimum light output in the left graph of FIG. 2). As a result, the input light from the preceding DMPSK modulator 200 is output from the RZ modulator 100 in a state of being quenched. In other words, no light is substantially output from the RZ modulator 100. The same applies to CSRZ modulation with Vc <Vx.

  Since the data modulation waveform by the DMPSK modulator 200 cannot be confirmed in the extinction state, it is necessary to change the bias voltage that defines the operating point of the RZ modulator 100. In the present embodiment, the bias voltage is changed by changing the phase of the low frequency signal by the phase setting unit 305. Specifically, the sign of the detection signal obtained by the synchronous detection unit 301A is inverted by inverting the phase of the low-frequency signal. As a result, the bias voltage reached by feedback control to the DC bias output unit 302a is changed. Thereby, the extinction state of the RZ modulator 100 is canceled. Therefore, the data modulation waveform by the DMPSK modulator 200 can be confirmed.

  FIG. 3 schematically shows a control method of the optical transmitter 901 for observing the output of the DMPSK modulator 200 while stopping the input of the clock signal to the RZ modulator 100.

  In step S10, information regarding the control conditions of the optical transmitter 901 is acquired. Specifically, in step S11, modulation scheme information that is information about the type of modulation scheme used by the optical transmitter 901 is acquired. In step S12, amplitude information that is information about the amplitude of the clock signal is acquired.

  In step S20, based on the modulation scheme information and amplitude information described above, the phase between the phase of the low-frequency signal input to the RZ modulator 100 and the phase of the low-frequency signal input to the synchronous detector 301A. Phase difference is set. Details of the phase difference setting method will be described later.

  In step S30, after the phase difference is set as described above, the output of the DMPSK modulator 200 is observed in a state where the input of the clock signal to the RZ modulator 100 is stopped. The timing for stopping the input of the clock signal is arbitrary.

  With reference to FIG. 4, a specific method for setting the phase difference (FIG. 3: step S20) will be described below.

  In step S21, the modulation scheme information is determined. Specifically, if CSRZ modulation is used, the process proceeds to step S22a, and if 33% modulation is used, the process proceeds to step S22b.

  In step S22a, a determination is made regarding the amplitude of the clock signal. Specifically, if Vc <Vx is satisfied, the process proceeds to step S23a. Conversely, if Vc <Vx is not satisfied, in other words, if Vc ≧ Vx is satisfied, the process proceeds to step S23b.

  In step S22b, a determination is made regarding the amplitude of the clock signal. Specifically, if Vc <Vx is satisfied, the process proceeds to step S23c. Conversely, when Vc <Vx is not satisfied, in other words, when Vc ≧ Vx is satisfied, the process proceeds to step S23d.

  In step S23a, the phase setting unit 305 performs a phase inversion operation. That is, the low frequency signal input from the low frequency signal output unit 303a is output to the addition unit 304a after the phase is inverted. As a result, the phase of the low frequency signal input from the adder 304a to the RZ modulator 100 and the phase of the low frequency signal input from the low frequency signal output unit 303a to the synchronous detector 301A are inverted. Thus, a phase difference between the two is set. Similar processing is performed in step S23d.

  In step S23b, the phase setting unit 305 performs a phase holding operation. That is, the low frequency signal input from the low frequency signal output unit 303a is output to the addition unit 304a while maintaining the phase. Thereby, the phase difference between the phase of the low frequency signal input from the adding unit 304a to the RZ modulator 100 and the phase of the low frequency signal input from the low frequency signal output unit 303a to the synchronous detection unit 301A is increased. Zero. Similar processing is performed in step S23c.

  When the input of the clock signal to the RZ modulation unit 101 is stopped under the setting of the phase difference as described above, the operating point of the RZ modulation unit 101 is such that the optical output is maximized by the feedback control of the DC bias output unit 302a. It becomes the voltage which becomes. As a result, the input light from the preceding DMPSK modulator 200 passes through the RZ modulator 100 without being further modulated. That is, light whose power is lost by the insertion loss of the RZ modulator 100 is output from the RZ modulator 100. Therefore, the data modulation waveform by the DMPSK modulator 200 can be confirmed.

  Referring to FIG. 5, next, a method of controlling optical transmitter 901 in which optical transmitter 901 is operated by an arbitrary modulation method from the state where the input of the clock signal is stopped (for example, step S30 in FIG. 3). Is described below.

  In step S40, information regarding the control conditions that the optical transmitter 901 will use is acquired. Specifically, in step S41, modulation scheme information that is information about the type of modulation scheme to be used by the optical transmitter 901 is acquired. In step S42, phase information that is information about the phase difference to be used by the optical transmitter 901 is acquired. The phase information is “inverted” or “held” in the present embodiment. “Inverted” means that the phase of the low frequency signal input from the adder 304a to the RZ modulator 100 and the phase of the low frequency signal input from the low frequency signal output unit 303a to the synchronous detector 301A are inverted. It means that the phase difference between the two is set so as to be. That is, it means that the low frequency signal input from the low frequency signal output unit 303a is output to the adding unit 304a after the phase is inverted. On the other hand, “hold” is between the phase of the low-frequency signal input from the adding unit 304a to the RZ modulator 100 and the phase of the low-frequency signal input from the low-frequency signal output unit 303a to the synchronous detection unit 301A. This means that the phase difference of is zero. That is, it means that the low frequency signal input from the low frequency signal output unit 303a is output to the adding unit 304a while maintaining the phase.

  In step S51, a determination is made regarding the amplitude of the clock signal. Specifically, if Vc <Vx is satisfied, the process proceeds to step S52a. Conversely, if Vc <Vx is not satisfied, in other words, if Vc ≧ Vx is satisfied, the process proceeds to step S52b.

  In step S52a, the phase information is determined. Specifically, if the phase information is “inverted”, the process proceeds to step S53a. Similar processing is performed in step S53c. On the other hand, if the phase information is not “inverted”, in other words, if the phase information is “hold”, the process proceeds to step S53b. Similar processing is performed in step S53d.

  In step 53a, the low frequency signal input from the low frequency signal output unit 303a is output to the addition unit 304a after the phase is inverted. Accordingly, when a clock signal is input to the RZ modulator 100, the RZ modulator 100 performs CSRZ modulation as a result of feedback control of the DC bias output unit 302a. Note that the timing of starting the input of the clock signal is arbitrary.

  In step 53b, the low frequency signal input from the low frequency signal output unit 303a is output to the addition unit 304a while maintaining the phase. Accordingly, when a clock signal is input to the RZ modulator 100, the RZ modulator 100 performs 33% RZ modulation as a result of feedback control of the DC bias output unit 302a. Note that the timing of starting the input of the clock signal is arbitrary.

  In step 53c, the low frequency signal input from the low frequency signal output unit 303a is output to the addition unit 304a after the phase is inverted. Accordingly, when a clock signal is input to the RZ modulator 100, the RZ modulator 100 performs 33% RZ modulation as a result of feedback control of the DC bias output unit 302a. Note that the timing of starting the input of the clock signal is arbitrary.

  In step 53d, the low frequency signal input from the low frequency signal output unit 303a is output to the addition unit 304a while maintaining the phase. Accordingly, when a clock signal is input to the RZ modulator 100, the RZ modulator 100 performs CSRZ modulation as a result of feedback control of the DC bias output unit 302a. Note that the timing of starting the input of the clock signal is arbitrary.

  As described above, 33% RZ modulation or CSRZ modulation can be selected by acquiring amplitude information and phase information (step S40).

  According to the optical transmitter 901 of the present embodiment, the feedback control operation for the DC bias output unit 302a is changed by changing the phase difference set by the phase setting unit 305. As a result, the operating point of the RZ modulator 100 is changed. Therefore, when the output of the DMPSK modulator 200 is observed in a state where the input of the clock signal to the RZ modulator 100 is stopped, or when the modulation scheme of the optical transmitter is changed, the operation of the RZ modulator 100 is performed. When it becomes necessary to review the points, the operating point of the RZ modulator 100 can be easily optimized by setting the phase difference by the phase setting unit 305.

  The phase setting unit 305 sets a phase difference so that light is output from the RZ modulator 100 when the input of the clock signal to the RZ modulator 100 is stopped. As a result, even if the RZ modulator 100 is in the extinction state as it is by stopping the input of the clock signal, the extinction state is canceled by changing the phase difference. Therefore, the output of the DMPSK modulator 200 can be observed in a state where the input of the clock signal to the RZ modulator 100 is stopped.

  When the input of the clock signal to the RZ modulator 100 is started, the phase setting unit 305 sets the phase difference according to the magnitude of the clock signal amplitude Vc, as shown in FIG. Thus, a desired modulation method can be selected when the optical transmitter shifts to a normal operation when the input of the clock signal starts.

  According to the control method of optical transmitter 901 of the present embodiment, the phase difference between the phase of the low-frequency signal input to RZ modulator 100 and the phase of the low-frequency signal input to synchronous detector 301A. Is changed, the feedback control operation for the DC bias output unit 302a is changed. As a result, the operating point of the RZ modulator 100 is changed. Therefore, when the input of the clock signal to the RZ modulator 100 is stopped in order to observe the output of the DMPSK modulator 200, the operation of the RZ modulator 100 is ensured so that the light intensity necessary for observation is ensured. The points can be easily optimized.

(Embodiment 2)
Referring to FIG. 6, in optical transmitter 902 of the present embodiment, light source 1, RZ modulator 100, and DMPSK modulator 200 are optically connected in this order. Therefore, the DMPSK modulator 200 modulates the phase of the light output from the RZ modulator 100. Specifically, the light source 1 is connected to the optical waveguide 103a. The optical waveguides 103b and 203a are optically connected to each other. The optical waveguide 203b guides the output light of the optical transmitter 902. The optical transmitter 902 includes a current-voltage converter 4b that converts the current signal from the light intensity detector 202 into a voltage signal.

  The optical transmitter 902 includes a control unit 300B instead of the control unit 300A (FIG. 1). In addition to the configuration of the control unit 300A, the control unit 300B includes a DC bias output unit 302b, a low frequency signal output unit 303b, an addition unit 304b, an amplification unit 306b, and a monitor unit 307b. The DC bias output unit 302 b outputs a DC component of the bias voltage to the DMPSK modulator 200. The low frequency signal output unit 303b outputs a low frequency signal having a frequency sufficiently lower than the frequency of the clock signal to the DMPSK modulator 200 as an AC component of the bias voltage. Adder 304b outputs the bias voltage obtained by adding the outputs from DC bias output unit 302b and low-frequency signal output unit 303b to DMPSK modulator 200.

  The amplification unit 306b amplifies the voltage signal input from the light intensity detection unit 202 via the current-voltage conversion unit 4b. The monitor unit 307b monitors the intensity of the signal from the amplification unit 306b. The monitor unit 307b stores a reference value of the intensity of the signal from the amplification unit 306b based on the monitoring result. Which monitor result is selected as the reference value will be described later. The amplification unit 306b controls the amplification amount according to the reference value. The output amplified by the amplification unit 306b is input to the synchronous detection unit 301B.

  The synchronous detection unit 301B not only performs the same function as the synchronous detection unit 301A (FIG. 1) described above, but also outputs a low-frequency signal output unit with respect to the output from the light intensity detection unit 202 amplified by the amplification unit 306b. Synchronous detection is performed using the low-frequency signal from 303b as a reference signal. The DC bias output unit 302b performs feedback control using the detection signal obtained by the synchronous detection unit 301B. As a result, the voltage value output from the DC bias output unit 302b is optimized.

  The DMPSK modulator 200 can be applied with a plurality of types of bias voltages, each of which is feedback controlled. The number of types of bias voltage varies depending on the modulation method. For example, when DQPSK is used as DMPSK, three types of bias voltages are used.

  Further, with reference to FIG. 7, a method of controlling the amplification amount of the amplification unit 306b will be described below.

  In step S60, the intensity of the signal from the amplification unit 306b monitored by the monitor unit 307b when each of the RZ modulator 100 and the DMPSK modulator 200 is in a stable state is stored as a reference value in the monitor unit 307b. . The reference value is, for example, an effective value of the input voltage to the monitor unit 307b. Next, in step S70, at least one of the modulation scheme of RZ modulator 100 and the clock signal amplitude is changed. Next, in step S80, standby processing is performed until the RZ modulator 100 becomes stable. Next, in step S90, the amplification unit 306b controls the amplification amount according to the reference value. Specifically, the amplification amount of the amplifying unit 306b is controlled so that an effective value comparable to the reference value is input to the monitor unit 307b. In particular, when the amplitude of the clock signal changes greatly, the intensity of the optical output from the RZ modulator 100 input to the DMPSK modulator 200 also changes greatly, so that the amount of amplification is greatly changed.

  When the RZ modulator 100 is in the “stable state”, the bias voltage control of the RZ modulator 100 and the input from the clock signal driving unit 2 are stable. This is when the change in light output is below a certain amount of change. This amount of change is, for example, about 0.2 dB as an effective value. When the DMPSK modulator 200 is in the “stable state”, the optical output from the RZ modulator 100, the bias voltage control of the DMPSK modulator 200, and the input from the data signal driving unit 3 are stable. As a result, the change in the optical output from the DMPSK modulator 200 is less than a certain amount of change. This amount of change is, for example, about 0.2 dB as an effective value.

  Since the configuration other than the above is substantially the same as the configuration of the first embodiment described above, the same or corresponding elements are denoted by the same reference numerals, and description thereof is not repeated.

  As in the first embodiment, also in this embodiment, when the output of the DMPSK modulator 200 is observed with the clock signal input to the RZ modulator 100 stopped, or the modulation scheme of the optical transmitter is changed. When the operating point of the RZ modulator 100 needs to be reviewed as in the case of the operation, the operating point of the RZ modulator 100 can be optimized by setting the phase difference by the phase setting unit 305. Here, since the DMPSK modulator 200 is disposed downstream of the RZ modulator 100 in the present embodiment, if the state of the output light of the RZ modulator 100 changes, the light output from the DMPSK modulator 200 is changed. The intensity also varies. If the amplification amount of the amplifying unit 306b is always constant, the voltage value monitored by the monitoring unit 307b may be saturated or extremely reduced in accordance with the variation. In that case, the stability of the bias voltage control by the feedback control using the signal from the monitor unit 307b deteriorates.

  In the present embodiment, even when the intensity of light output from the DMPSK modulator 200 provided on the downstream side of the RZ modulator 100 changes due to the change in the modulation scheme of the RZ modulator 100, Since the amplification amount of the amplifying unit 306b is controlled according to the reference value, the monitor unit 307b can acquire a signal representing the intensity with an appropriate intensity. Therefore, feedback control using the signal is stabilized.

  In the present embodiment, RZ modulator 100 is arranged upstream of DMPSK modulator 200. That is, the RZ modulator 100 is arranged on the upstream side of the phase modulator. The advantage of this arrangement is particularly great when a polarization multiplexing modulator such as a DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) modulator is applied as the phase modulator. This is because the intensity modulation using the RZ modulator can be performed only for one polarization, and if the RZ modulator is arranged downstream of the polarization multiplexing modulator, the RZ for each polarization This is because it is necessary to provide a modulator. On the other hand, when the RZ modulator is arranged on the upstream side of the phase modulator, the optical transmitter can be configured with only one RZ modulator while using the polarization multiplexing modulator.

  Note that the function of the control unit of the optical transmitter in each of the above embodiments may be realized by software processing using a computer program that causes a microcomputer or the like provided in the optical transmitter to execute a control method. Further, by using the above-described optical transmitter, an optical communication system may be configured in which an optical signal transmitted from the optical transmitter is transmitted through an optical fiber and received by an optical receiver. Further, by using two or more optical transmitters as described above, the optical signals transmitted from the two or more optical transmitters are wavelength-multiplexed to transmit the optical fiber, and wavelength-separated on the receiving side, 2 for each wavelength. A WDM (Wavelength Division Multiplexing) optical communication system in which reception is performed by one or more optical receivers may be configured.

  It should be noted that the present invention can be freely combined with each other within the scope of the invention, and each embodiment can be appropriately modified or omitted.

  DESCRIPTION OF SYMBOLS 1 Light source, 2 Clock signal drive part, 3 Data signal drive part, 4a, 4b Current voltage conversion part, 100 RZ modulator (intensity modulator), 101 RZ modulation part (Mach-Zehnder type light modulation part), 102 1st light Intensity detection unit, 202 Second light intensity detection unit, 103a, 103b, 203a, 203b Optical waveguide, 200 DMPSK modulator (phase modulator), 201 Data modulation unit, 300A, 300B control unit, 301A, 301B Synchronous detection unit , 302a, 302b DC bias output unit, 303a, 303b low frequency signal output unit, 304a, 304b addition unit, 305 phase setting unit, 306a, 306b amplification unit, 307a, 307b monitor unit, 901, 902 optical transmitter.

Claims (5)

An optical transmitter,
A data signal driver that outputs a voltage signal as a data signal;
A phase modulator that modulates the phase of light according to the data signal output from the data signal driver;
A clock signal driver that outputs a voltage signal as a clock signal;
An intensity modulator that is optically connected to the phase modulator and modulates the intensity according to the clock signal output from the clock signal driver, the intensity modulator comprising:
A Mach-Zehnder optical modulator that operates with the clock signal based on an operating point defined by a bias voltage applied from the outside of the intensity modulator;
A first light intensity detector that outputs a signal representing the intensity of light from the Mach-Zehnder light modulator, and the optical transmitter further includes a controller that outputs the bias voltage to the intensity modulator, The controller is
A DC bias output unit that outputs a DC component of the bias voltage to the intensity modulator;
A low-frequency signal output unit that outputs a low-frequency signal having a frequency lower than the frequency of the clock signal to the intensity modulator as an AC component of the bias voltage;
A synchronous detection unit that performs synchronous detection on the output from the first light intensity detection unit using the low frequency signal as a reference signal, and obtained by the synchronous detection unit in the DC bias output unit Feedback control using a detection signal is performed, and the control unit further includes a phase between the phase of the low frequency signal input to the intensity modulator and the phase of the low frequency signal input to the synchronous detection unit. Including a phase setting unit for setting a phase difference,
Optical transmitter.   2. The light according to claim 1, wherein the phase setting unit sets the phase difference so that light is output from the intensity modulator when input of the clock signal to the intensity modulator is stopped. 3. Transmitter.   3. The phase setting unit according to claim 1, wherein when the input of the clock signal to the intensity modulator is started, the phase setting unit sets the phase difference according to the amplitude of the clock signal. Optical transmitter. The phase modulator modulates the phase of light output from the intensity modulator, and the phase modulator outputs a second signal indicating the intensity of light output from the phase modulator. Having a light intensity detector,
The control unit includes an amplification unit that amplifies a signal from the second light intensity detection unit, and a monitor unit that monitors the intensity of the signal from the amplification unit, and the monitor unit receives a signal from the amplification unit. A reference value of the intensity of, and the amplification unit controls the amplification amount according to the reference value,
The optical transmitter according to claim 1. An optical transmitter control method comprising:
The optical transmitter includes a data signal driver that outputs a voltage signal as a data signal;
A phase modulator that modulates the phase of light according to the data signal output from the data signal driver;
A clock signal driver that outputs a voltage signal as a clock signal;
An intensity modulator that is optically connected to the phase modulator and modulates intensity according to the clock signal output from the clock signal driver, the intensity modulator comprising:
A Mach-Zehnder optical modulator that operates with the clock signal based on an operating point defined by a bias voltage applied from the outside of the intensity modulator;
A light intensity detector that outputs a signal representing the intensity of light from the Mach-Zehnder light modulator, and the optical transmitter further includes a controller that outputs the bias voltage to the intensity modulator, the controller Is
A DC bias output unit that outputs a DC component of the bias voltage to the intensity modulator;
A low-frequency signal output unit that outputs a low-frequency signal having a frequency lower than the frequency of the clock signal to the intensity modulator as an AC component of the bias voltage;
A synchronous detection unit that performs synchronous detection on the output from the light intensity detection unit using the low-frequency signal as a reference signal, and in the DC bias output unit, the detection signal obtained by the synchronous detection unit Used feedback control,
The method of controlling the optical transmitter is as follows:
Obtaining modulation scheme information that is information about the type of modulation scheme used by the optical transmitter;
Obtaining amplitude information which is information about the amplitude of the clock signal;
Based on the modulation method information and the amplitude information, a phase difference between the phase of the low frequency signal input to the intensity modulator and the phase of the low frequency signal input to the synchronous detection unit is set. And a process of
And a step of observing an output of the phase modulator in a state where input of the clock signal to the intensity modulator is stopped after the step of setting the phase difference.

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