JP2009118471A - Controller for optical transmission device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide techniques for preventing variation (ringing) caused by changing an output waveform because of a bias voltage of an external optical modulator, therefore, changing an average refractive index in steps to control to an optimal point such as in initiation and chirp switching. <P>SOLUTION: An optical transmitter having an external modulator for modulating a driving signal on which a low-frequency signal is superposed performs: supplying the driving signal to the external modulator; monitoring the modulated light to bias the external modulator for maintaining the driving signal at a predetermined level; and controlling the intensity of the light so as to compensate for refractive index variation of the external modulator which is caused by variation of the bias. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for controlling an optical transmission device including an optical modulator.

  In recent years, with a rapid increase in the amount of information, it is desired to increase the capacity and distance of optical communication systems.

<Direct modulation method>
As an electro-optical conversion circuit in an optical communication system, an intensity modulation-direct detection method (direct modulation method) is the simplest method. In this method, the current flowing through the semiconductor laser is turned on / off in accordance with data signals “0” and “1” to control light emission / quenching. However, when the laser itself is directly turned on / off, wavelength fluctuation (chirping) occurs in the signal light due to the nature of the semiconductor. Wavelength fluctuations have an adverse effect as the data transmission speed (bit rate) increases. This is because the fiber has the property of chromatic dispersion that the propagation speed changes when the wavelength is different. If the wavelength variation is caused by direct modulation, a delay in the propagation speed occurs, and the waveform collapses during propagation through the fiber. Distance transmission and high-speed transmission become difficult.

<External modulation method>
In order to suppress the influence of the above wavelength fluctuation, in 2.5G, 10Gbps high-speed transmission, the laser diode is continuously emitted, and the light generated from this laser diode is converted to “1” and “0” of the data signal by the external modulator. Accordingly, an external modulation system that turns on (translucent) / off (shields) is used.

  As an external modulator, for example, there is a LiNbO3 external modulator (hereinafter also referred to as an LN modulator). The LN modulator causes a bias voltage drift due to DC voltage, temperature, and deterioration with time. Therefore, as shown in FIG. 2, a reference signal amplitude-modulated with a low-frequency signal is applied to the optical modulator via the drive circuit, and the low-frequency component is detected from the modulated optical signal and compared with the reference signal. In order to maintain the operating point of the external modulator appropriately, bias voltage control (ABC: Auto Bias Control) is performed. When the bias voltage is operating at the optimum point, the low frequency signal is modulated in reverse phase, and the output signal does not contain the frequency component, and thus becomes “zero”.

  It is also generally known that chirping (α parameter sign) can be reversed because the phase change between the rising and falling parts is reversed by changing the bias operating point (voltage) of the modulator. Yes.

  On the other hand, in order to keep the optical output of the transmitter constant, drive current control by auto power control (APC) using the back power of the semiconductor laser is realized.

Moreover, as a prior art relevant to this invention, there exists a technique disclosed by the following patent documents 1, 2, for example.
JP-A-2-50189 JP-A-10-164018

  However, in the process in which the bias voltage is controlled to the optimum operating point from the start of control and when the α parameter is switched, the drive is performed at a position other than the optimum operating point, and accordingly, the optical output from the LN modulator ( The average transmittance) changes greatly, and discontinuous fluctuations (ringing) occur in the light output.

As shown in FIGS. 3A to 3D, when the α parameter is switched, the input changes from the signals 41A to 41D and the output changes from the waveforms 42A to 42D until the bias voltage changes to the optimum point. Therefore, even if the laser output is controlled to be constant by APC, output fluctuation (ringing) occurs while the operating point of the LN modulator is changed.

  Although it seems that there is no problem once the driving of the light source is stopped, since the bias voltage control (ABC) cannot be realized without light emission from the light source unit, light emission of the light source is started and after bias voltage control is started. The output fluctuation occurs in any of the areas.

  For example, as shown in FIG. 9, when the output (light quantity) of the light source is linearly increased to the target value L0, a variation 91 occurs in the transmittance (FIG. 4) of the LN modulator, so that the light output (FIG. 9) is increased. An output fluctuation 92 appears.

  Therefore, ringing always occurs somewhere when the α parameter is switched.

  Such ringing is not desirable because it may affect adjacent frequency channels in a transmission apparatus using the wavelength division multiplexing method.

  Therefore, the present invention provides a technique for preventing output fluctuation caused by changing the bias of the external optical modulator.

  In order to solve the above problems, the present invention employs the following configuration.

That is, the optical transmission device and the control device of the present invention are:
A signal supply unit that sends the drive signal to an external modulator that modulates the light from the light source according to the drive signal on which the low-frequency signal is superimposed;
A bias driver that controls a low-frequency component of the modulated light and controls the bias of the external modulator;
A compensation unit that controls the amount of light so as to compensate for a change in average transmittance due to a change in the input waveform of the external modulator due to a change in the bias voltage.

  The compensation unit may compensate for a change in average transmittance of the external modulator by controlling driving power of the light source.

  The compensation unit may compensate for a change in average transmittance of the external modulator by controlling an attenuation amount of an optical attenuator that attenuates the modulated light.

  The compensation unit may calculate a change amount of the transmittance of the external modulator with respect to a change amount of the bias, and compensate the change of the average transmittance based on the calculated change amount.

Moreover, the control method of the present invention includes:
A control device for an optical transmitter including an external modulator that modulates light from a light source according to a drive signal on which a low-frequency signal is superimposed,
Sending the drive signal to the external modulator;
Monitoring the low frequency component of the modulated light and controlling the bias of the external modulator to maintain the drive signal at a predetermined level;
Controlling the amount of the light so as to compensate for a change in average transmittance of the external modulator due to the fluctuation of the bias.

  ADVANTAGE OF THE INVENTION According to this invention, the technique which prevents the transmission output fluctuation | variation (ringing) which generate | occur | produces at the time of the change of the bias of an external optical modulator can be provided.

  The best mode for carrying out the present invention will be described below with reference to the drawings. The configuration of the following embodiment is an exemplification, and the present invention is not limited to the configuration of the embodiment.

<Embodiment 1>
§1. Device Configuration FIG. 1 is a schematic diagram of an optical transmitter (optical transmitter) according to the present invention.

  The light source 11 is a light source such as a semiconductor laser that outputs laser light having a predetermined frequency.

  As shown in FIG. 1, the external modulator 13 has an optical waveguide 13B and a control electrode 13C formed on a substrate 13A having an electro-optic effect. The external modulator 13 modulates the light from the light source 11 by changing the light transmittance through the optical waveguide 13B according to the voltage applied to the control electrode 13C, and generates an optical signal. The external modulator 13 of this example is an LN modulator using lithium niobate (LN: LiNbO3) for the substrate 13A. The external modulator 13 has an optical sensor (PD: Photodiode in this example) 13D that detects the optical output.

  The control device 12 includes a light source drive unit 21 that drives the light source 11, a modulation control unit 22 that controls the LN modulator 13, an attenuation control unit 23 that controls the VOA 14, and a ringing correction unit that compensates for bias fluctuations by the modulation control unit 22 ( Compensation unit, correction amount calculation unit) 24 is provided.

  In the light source drive unit 21, the APC control unit 21A determines the drive current to the light source 11 of the light source drive unit 21B based on the back power of the light source (semiconductor laser) 11 detected by the optical sensor (PD: Photodiode) 11B in this example. Control.

  The modulation control unit 22 includes an LN drive unit (signal supply unit) 22A, a bias unit (ABC control unit) 22B, and an oscillation unit 22C.

  The LN drive unit (signal supply unit) 22A uses the input signal as a drive signal of a predetermined level and supplies the drive signal to the LN modulator 13.

  The bias unit 22B adjusts the bias voltage applied to the LN modulator 13 so as to correct the shift of the drive signal with respect to the operating point of the LN modulator 13 due to the DC voltage, temperature, and deterioration with time. Therefore, in this example, the oscillator 22C generates a pilot signal having a frequency lower than that of the drive signal, and superimposes it on the drive signal and applies it to the LN modulator 13. Then, the bias unit 22B detects the shift of the drive signal with respect to the operating point of the LN modulator 13 by extracting the frequency component from the modulated optical signal detected by the PD 13D and comparing it with the pilot signal. The bias voltage to the LN modulator 13 is adjusted so as to correct.

  FIG. 2 is a diagram illustrating waveforms of an input signal and an output optical signal on which a low-frequency component is superimposed in the external modulator 13. As shown in the figure, by the bias control of the bias unit 22B, the low frequency component of the input signal is smoothed and the low frequency component is removed from the output signal when there is no deviation of the drive signal (input signal) with respect to the operating point. It is burned.

  FIG. 4 is a diagram showing the relationship of the average transmittance with respect to the bias voltage.

  The ringing correction unit 24 controls the light amount of the light source so as to correct the change in transmittance of the external modulator 13 due to the variation in the bias. In this example, ringing is suppressed by changing the light amount of the light source at the start of light emission of the light source (at the start of ABC) or when switching the chirp.

§2. Control Method A control method in the optical transmitter 1 having these configurations will be described.

  FIG. 8 shows a control method at the time of start-up, that is, at the start of light emission of the light source.

  First, when a power-on or activation instruction is input, a pilot signal having a low frequency component is superimposed on the drive signal. (Step 1, hereinafter also referred to as S1) The light source drive unit 21 of the control device 12 supplies a drive current to the light source 11 (S2). That is, the output power of the light source is increased.

  After the light from the light source is input to the LN modulator 13 and the low frequency component of the transmitted light power is detected by the PD 13D, the bias unit 22B starts automatic bias control (ABC) (S3).

  When the ringing correction unit 24 detects the start of control of the bias unit 22B, the ringing correction unit 24 monitors the control amount of the bias voltage and the low frequency component of the optical output from the external modulator, and calculates the transmittance change amount of the LN modulator 13 ( S4).

  The detection of the start of control may be determined by the ringing correction unit 24 detecting a detection signal from the PD 13D or a change in the bias voltage from the bias unit 22B, or a signal that the bias unit 22B starts control. May be detected by the ringing correction unit 24.

  At this time, a change in the transmittance of the LN modulator 13 that changes due to the bias fluctuation, that is, in this example, that the transmittance sharply decreases as shown in FIG. FIG. 9 is a diagram showing ringing generated in the transmitter output at the start-up due to the change in transmittance.

  Then, the ringing correction unit 24 corrects the output of the light source 11 based on the transmittance change amount obtained in Step 4 (S5). Accordingly, the ringing correction unit 24 increases the light amount of the light source 11 in accordance with the change in the transmittance, thereby canceling the influence of the change in the transmittance of the LN modulator 13, and the optical transmitter 1 Is increased linearly. That is, ringing is removed.

  Then, it is determined whether or not the light quantity of the light source 11 has converged within a predetermined range. If it has not converged within the predetermined range, the process returns to step 2 (S6). If the light amount of the light source 11 converges within a predetermined range, it is determined whether or not the bias control has converged to the optimum control point (S7). Here, if it has not converged to the optimum control point, the process returns to step 3, and if it has converged to the optimum control point, the compensation control is terminated.

  FIG. 5 shows a control method during chirp switching.

  When a bias switching instruction is input to the modulation unit 1 by an operation of the operator and the control device 12 receives this instruction (S21), the bias unit 22B switches the bias voltage to invert the polarity of the pilot signal and the drive signal. . For example, the drive signal 41A shown in FIG. 3 is changed to the drive signal 41D (S22).

  The low frequency component of the light transmitted through the LN modulator 13 is monitored by the PD 13D, and the bias unit 22B performs automatic bias control (ABC) (S23).

  Further, the ringing correction unit 24 monitors the control amount of the bias voltage and the low frequency component from the LN modulator 13, and calculates the transmittance change amount of the LN modulator 13 (S24).

  At this time, a change in the transmittance of the LN modulator 13 that changes due to the bias fluctuation, that is, in this example, that the transmittance sharply decreases as shown in FIG. FIG. 6 is a diagram showing ringing generated in the transmitter output at the start-up due to the change in transmittance.

  The ringing correction unit 24 corrects the output of the light source 11 based on the transmittance change amount obtained in step 24 (S25). Accordingly, the ringing correction unit 24 cancels the influence of the transmittance change of the LN modulator 13 by increasing the light quantity of the light source 11 as shown in FIG. Remove ringing from the light output.

  Then, it is determined whether or not the bias control has converged to the optimum control point (S7). Here, if it has not converged to the optimum control point, the process returns to step 23, and if it has converged to the optimum control point, the compensation control is terminated.

<Embodiment 2>
FIG. 11 shows a schematic configuration of the optical transmission apparatus according to the second embodiment of the present invention.

  The optical transmission apparatus according to the present embodiment includes a variable optical attenuator (attenuating unit: VOA: Variable Optical Attenuator), and differs from the first embodiment in that the ringing correction unit controls the VOA instead of the light source unit. Other configurations are the same. For this reason, the same elements are denoted by the same reference numerals and the description thereof is omitted.

  The ringing correction unit 24A of the present embodiment controls the amount of light after being modulated by the LN modulator 13 so as to compensate for the change in transmittance of the external modulator due to the variation of the bias. In this example, ringing is suppressed by controlling the transmittance of the VOA 14, that is, the amount of attenuation, at the start of light emission of the light source (at the start of ABC) or at the time of chirp switching.

  A control method in the optical transmission apparatus of this embodiment will be described.

  FIG. 15 shows a control method at the time of start-up, that is, at the start of light emission of the light source.

  First, when a power-on or activation instruction is input, a pilot signal having a low frequency component is superimposed on the drive signal (S1), and the light source drive unit 21 of the control device 12 supplies a drive current to the light source 11. Supply (S2). That is, the output power of the light source is increased.

  After the light from the light source is input to the LN modulator 13 and the low frequency component of the transmitted light power is detected by the PD 13D, the bias unit 22B starts automatic bias control (ABC) (S3).

  When the ringing correction unit 24 detects the start of control of the bias unit 22B, the ringing correction unit 24 monitors the control amount of the bias voltage and the low frequency component of the output light from the external modulator 13, and calculates the transmittance change amount of the LN modulator 13. (S4).

  FIG. 9 is a diagram showing ringing generated in the transmitter output at the start-up due to the change in transmittance.

  Then, the ringing correction unit 24A corrects the transmittance of the VOA 14 based on the transmittance change amount obtained in Step 4 (S5A). Therefore, the ringing correction unit 24A cancels the influence of the change in the transmittance of the LN modulator 13 by increasing the transmittance of the VOA 14 as shown in FIG. 10 in accordance with the decrease in the transmittance of the LN modulator 13. The optical output of the optical transmitter 1 is increased linearly. That is, ringing is removed.

  Then, it is determined whether or not the light quantity of the light source 11 has converged within a predetermined range. If it has not converged within the predetermined range, the process returns to step 2 (S6). If the light amount of the light source 11 converges within a predetermined range, it is determined whether or not the bias control has converged to the optimum control point (S7). Here, if it has not converged to the optimum control point, the process returns to step 3, and if it has converged to the optimum control point, the compensation control is terminated.

  FIG. 16 shows a control method at the time of chirp switching.

  When a bias switching instruction is input to the modulation unit 1 by an operation of the operator and the control device 12 receives this instruction (S21), the bias unit 22B switches the bias voltage to invert the polarity of the pilot signal and the drive signal. . For example, the drive signal 41A shown in FIG. 3 is changed to the drive signal 41D (S22).

  The low frequency component of the light transmitted through the LN modulator 13 is monitored by the PD 13D, and the bias unit 22B performs automatic bias control (ABC) (S23).

  Further, the ringing correction unit 24 monitors the control amount of the bias voltage and the low frequency component from the LN modulator 13, and calculates the transmittance change amount of the LN modulator 13 (S24).

  At this time, a change in the transmittance of the LN modulator 13 that changes due to the bias fluctuation, that is, in this example, that the transmittance sharply decreases as shown in FIG. FIG. 12 is a diagram showing ringing generated in the transmitter output at the start-up due to the change in transmittance.

  Then, the ringing correction unit 24A corrects the transmittance of the VOA 14 based on the transmittance change amount obtained in step 24 (S25A). Therefore, the ringing correction unit 24A cancels the influence of the change in the transmittance of the LN modulator 13 by increasing the transmittance of the VOA 14 as shown in FIG. 13 in accordance with the decrease in the transmittance of the LN modulator 13. , Ringing is removed from the optical output of the optical transmitter 1 as shown in FIG.

  Then, it is determined whether or not the bias control has converged to the optimum control point (S7). Here, if it has not converged to the optimum control point, the process returns to step 23, and if it has converged to the optimum control point, the compensation control is terminated.

<Others>
The present invention is not limited to the illustrated examples described above, and various modifications can be made without departing from the scope of the present invention.

  For example, the same effects as those of the above-described embodiment can be obtained even with the configurations described below. These components can be combined as much as possible.

(Appendix 1)
A signal supply unit that sends the drive signal to an external modulator that modulates the light from the light source according to the drive signal on which the low-frequency signal is superimposed;
A bias unit that monitors a low-frequency component of the modulated light and controls a bias of the external modulator;
And a compensation unit that controls the amount of light so as to compensate for a change in transmittance of the external modulator due to the change in the bias.

(Appendix 2)
The control apparatus according to appendix 1, wherein the compensation unit compensates for a change in transmittance of the external modulator by controlling driving power of the light source.

(Appendix 3)
The control apparatus according to appendix 1, wherein the compensating unit compensates for a change in transmittance of the external modulator by controlling an attenuation amount of an optical attenuator that attenuates the modulated light.

(Appendix 4)
The compensation unit according to any one of appendices 1 to 3, wherein the compensation unit calculates a change amount of the transmittance of the external modulator with respect to the change amount of the bias, and compensates the change in the transmittance based on the calculated change amount. Control device.

(Appendix 5)
A control device for an optical transmitter including an external modulator that modulates light from a light source according to a drive signal on which a low-frequency signal is superimposed,
Sending the drive signal to the external modulator;
Monitoring the low frequency component of the modulated light to control the bias of the external modulator;
And a step of controlling the amount of light so as to compensate for a change in transmittance of the external modulator due to a change in the bias.

(Appendix 6)
The control method according to appendix 5, wherein the drive power of the light source is controlled to compensate for a change in transmittance of the external modulator.

(Appendix 7)
The control method according to appendix 5, wherein an attenuation amount of an optical attenuator that attenuates the modulated light is controlled to compensate for a change in transmittance of the external modulator.

(Appendix 8)
The control method according to any one of appendices 5 to 7, wherein a change amount of the transmittance of the external modulator with respect to the change amount of the bias is calculated, and the change in the transmittance is compensated based on the calculated change amount.

(Appendix 9)
An optical transmitter comprising: a light source; an external modulator that modulates light from the light source; and a control device that controls the light source and the external modulator,
The control device is
A signal supply unit that sends the drive signal to an external modulator that modulates the light from the light source according to the drive signal on which the low-frequency signal is superimposed;
A bias unit for monitoring a low frequency component of the modulated light and controlling a bias voltage of the external modulator;
And a compensation unit that controls a light amount of the light source so as to compensate for a change in transmittance of the external modulator due to a change in the bias.

(Appendix 10)
An optical transmitter comprising: an external modulator that modulates light from a light source; an optical attenuator that attenuates light modulated by the external modulator; and a controller that controls the external modulator and the optical attenuator,
The control device is
A signal supply unit that sends the drive signal to an external modulator that modulates the light from the light source according to the drive signal on which the low-frequency signal is superimposed;
A bias unit that monitors a low-frequency signal of the modulated light and controls a bias voltage of the external modulator;
An optical transmission device comprising: a compensation unit that controls an attenuation amount of the optical attenuator so as to compensate for a change in transmittance of the external modulator due to a change in the bias.

1 is a configuration diagram of an optical transmitter according to a first embodiment of the present invention. Figure showing an example of the output signal waveform of the external modulator Diagram showing an example of the optical output waveform when the bias voltage changes Diagram showing an example of the optical output waveform when the bias voltage changes Diagram showing an example of the optical output waveform when the bias voltage changes Diagram showing an example of the optical output waveform when the bias voltage changes Explanatory diagram of average transmittance change when bias voltage changes Explanatory drawing of control method at chirp switching Explanatory diagram of the relationship between light source output and transmitter output during chirp switching Explanatory drawing of the relationship between the light source output and the transmitter output when the correction at the time of chirp switching of the present invention is performed Explanatory diagram of compensation control method at startup Illustration of relationship between light source output and transmitter output at startup Explanatory diagram of relationship between light source output and transmitter output when correction at start-up of the present invention is performed Explanatory diagram of bias switching Configuration diagram of optical transmitter according to embodiment 2 of the present invention The figure which shows the relationship (without correction | amendment) of the LN output and transmitter output at the time of chirp switching The figure which shows the relationship between the LN transmittance | permeability change at the time of chirp switching, and the transmittance | permeability correction | amendment of VOA The figure which shows the relationship (correction execution) of LN output and transmitter output at the time of chirp switching Explanatory diagram of compensation control method at startup Explanatory drawing of control method at chirp switching

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical transmitter 1 Modulation unit 11 Light source 12 Control apparatus 13 External modulator (modulation part: LN modulator)
13A Substrate 13B Optical waveguide 13C Control electrode 13D Optical sensor 14 Variable optical attenuator (attenuator: VOA)
2 Multiplexer 21 Light source driver 21A APC controller 21B Light source driver 22 Modulation controller 22A LN driver (signal supply unit)
22B Bias part (ABC control part)
22C Oscillator 24, 24A Ringing corrector (compensator)

Claims (7)

A signal supply unit that sends the drive signal to an external modulator that modulates the light from the light source according to the drive signal on which the low-frequency signal is superimposed;
A bias unit that monitors a low-frequency component of the modulated light and controls a bias of the external modulator;
And a compensation unit that controls the amount of light so as to compensate for a change in transmittance of the external modulator due to the change in the bias.   The control device according to claim 1, wherein the compensation unit compensates for a change in transmittance of the external modulator by controlling a light amount of the light source.   The control device according to claim 1, wherein the compensation unit compensates a change in transmittance of the external modulator by controlling an attenuation amount of an optical attenuator that attenuates the modulated light.   4. The compensation unit according to claim 1, wherein the compensation unit calculates a change amount of the transmittance of the external modulator with respect to a change amount of the bias, and compensates for the change in transmittance based on the calculated change amount. 5. Control device. A control device for an optical transmitter including an external modulator that modulates light from a light source according to a drive signal on which a low-frequency signal is superimposed,
Sending the drive signal to the external modulator;
Monitoring the low frequency component of the modulated light to control the bias of the external modulator;
And a step of controlling the amount of light so as to compensate for a change in transmittance of the external modulator due to a change in the bias. An optical transmitter comprising: a light source; an external modulator that modulates light from the light source; and a control device that controls the light source and the external modulator,
The control device is
A signal supply unit that sends the drive signal to an external modulator that modulates the light from the light source according to the drive signal on which the low-frequency signal is superimposed;
A bias unit for monitoring a low frequency component of the modulated light and controlling a bias voltage of the external modulator;
And a compensation unit that controls a light amount of the light source so as to compensate for a change in transmittance of the external modulator due to a change in the bias. An optical transmitter comprising: an external modulator that modulates light from a light source; an optical attenuator that attenuates light modulated by the external modulator; and a controller that controls the external modulator and the optical attenuator,
The control device is
A signal supply unit that sends the drive signal to an external modulator that modulates the light from the light source according to the drive signal on which the low-frequency signal is superimposed;
A bias unit that monitors a low-frequency signal of the modulated light and controls a bias voltage of the external modulator;
An optical transmission device comprising: a compensation unit that controls an attenuation amount of the optical attenuator so as to compensate for a change in transmittance of the external modulator due to a change in the bias.

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