JP2009058910A - Adjusting device for optical modulator driver and adjusting method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an adjusting device for optical duobinary modulation type optical modulator driver and an adjusting method of the optical duobinary modulation type optical modulator driver capable of performing the adjustment of cross-points of output amplitude and driving waveform of a driver circuit of an optical duobinary modulation type optical modulator on the basis of an optical modulation waveform observed while operating an ABC circuit. <P>SOLUTION: The adjusting device for optical duobinary modulation type optical modulator driver is configured as follow: modulated light output from the optical modulator 103 is transduced to an electric signal by an optical/electric transducer (O/E) 108 and is input to a sampling scope 109; an ABC circuit is operated, at the same time, a pseudo random signal is produced by a driving signal producing circuit 104 to drive the optical modulator and an optical modulation waveform is observed by a sampling scope 109; a driver control circuit 101 performs the adjustment of the cross-points of the driver circuit 105 so that the distortion of the optical modulation waveform observed by the sampling scope may be reduced; and, further, the driver control circuit 101 performs the adjustment of the output amplitude of the driver circuit 105 so that modulated light power may become the largest. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical modulator driver adjustment apparatus and adjustment method in an optical duobinary transmitter, and more specifically, an optical modulator driver adjustment apparatus capable of adjusting an optical modulator driver circuit in a state where the optical transmitter is assembled. And an adjustment method.

  An optical transmitter used in optical communication modulates light in order to place a signal on the light. The light modulation includes direct modulation for modulating the output of the semiconductor laser and external modulation for modulating light output from the semiconductor laser by means other than the light source. Modulators used in external modulation are generally called optical modulators, and these optical modulators cause physical changes in accordance with drive signals and modulate light intensity, phase, and the like.

  Mach-Zehnder (MZ) type optical modulators are used for optical intensity modulation (NRZ, RZ, etc.), phase control type optical intensity modulation (optical duobinary, CS-RZ, etc.), optical phase modulation (DPSK, DQPSK, etc.), etc. Widely used. Since the MZ type optical modulator uses interference of light propagating through two optical waveguides, it is necessary to accurately control the phase of light propagating through both waveguides. In particular, in an MZ type optical modulator using a widely used lithium niobate (LN) substrate, since an optimum bias voltage drifts in time, a bias applied to the optical modulator by an automatic bias control (ABC) circuit. Adjust the voltage.

  Furthermore, in order to optimize the optical transmission characteristics of the optical transmitter, it is necessary to optimize the output amplitude of the optical modulator driver circuit and the cross point of the drive waveform.

  The adjustment of the output amplitude of the optical modulator driver circuit and the cross point of the drive waveform has been possible in the case of the NRZ modulation method. 4A and 4B show the driver output waveform of the NRZ modulation type optical modulator, the operation curve of the optical modulator, and the optical modulation waveform. As shown in the figure, in the NRZ modulation method, since the driver output waveform and the optical modulation waveform of the optical modulator are both binary waveforms and correspond one-to-one, the optical modulation waveform and the driver output waveform are similar. Become. Therefore, in the NRZ modulation method, the driver amplitude and crosspoint can be adjusted while observing the optical modulation waveform. As shown in FIG. 4A, the driver output waveform and the optical modulation waveform have the same cross point deviation direction, and the modulation curve can be approximated almost linearly under the condition that the cross point does not deviate greatly from 50%. It is possible to estimate the cross point of the electric waveform from the waveform. Similarly, as shown in FIG. 4B, if the cross point of the driver output waveform is optimized, the cross point of the optical modulation waveform is also optimized.

  However, in the case of an optical duobinary modulation type optical transmitter, it is necessary to drive the optical modulator with an amplitude of 2Vπ, and thus the cross point of the driver of the optical modulator is particularly adjusted while observing the optical modulation waveform. There was a problem that it was difficult to do. FIGS. 5A and 5B show a driver output waveform, a ternary electric waveform, an operation curve of the optical modulator, and an optical modulation waveform of the optical duobinary modulation optical modulator. In the case of optical duobinary modulation, a driver output having a binary waveform is converted into a ternary electric waveform by a low-pass filter (LPF) and then applied to the optical modulator. Therefore, under the condition that the automatic bias voltage control circuit (ABC circuit) is operating, the optical modulation waveform is a binary waveform obtained by folding the ternary electrical waveform with respect to the extinction voltage of the optical modulator. Waveforms and light modulation waveforms are not similar. As shown in FIG. 5A, when the drive amplitude of the optical modulator is optimum, but the cross point of the binary electric signal is deviated from 50%, the extinction ratio of the optical modulation output deteriorates. This produces the waveform distortion. In addition, deterioration of the jitter component of the waveform is also observed. For this reason, it is difficult to read the distortion of the driver output waveform from the optical modulation waveform. On the other hand, when the driver cross point is set to 50% as shown in FIG. 5B, a good light modulation waveform is obtained.

  Further, although it is possible to perform adjustment by directly observing the driver output waveform before entering the optical modulator from the driver circuit, there is a problem in that adjustment takes time. In ordinary optical duobinary transmitters, the driver and the LPF are often directly connected, and it is difficult to directly observe the driver output. Even when the driver waveform can be separated and observed, the procedure of removing the connection between the driver and the LPF, adjusting the cross point, and then restoring it is complicated. In this case, considering the LPF insertion loss and the variation in Vπ of the optical modulator to be used, it is difficult to determine the optimum value of the driver amplitude only by electrical measurement.

  The present invention has been made in view of such a problem, and an object of the present invention is to adjust the output amplitude of the driver circuit of the optical duobinary modulation type optical modulator and the cross point of the driving waveform by using the ABC circuit. An object of the present invention is to provide an optical duobinary modulation type optical modulator driver adjustment apparatus and adjustment method that can be implemented based on an optical modulation waveform observed while operating.

  In order to achieve such an object, the invention described in claim 1 is an optical modulator driver adjustment device for adjusting a driver circuit for driving an optical modulator of an optical duobinary transmitter, wherein the optical modulator driver adjustment device A waveform information acquisition device that acquires optical modulation waveform information of modulated light modulated by a modulator; and a driver control circuit that adjusts the driver circuit based on the optical modulation waveform information. The point is adjusted to a value that maximizes the extinction ratio of the light modulation waveform, and the output amplitude of the driver circuit is adjusted to a value that maximizes the modulated light power.

  According to a second aspect of the present invention, in the optical modulator driver adjustment device according to the first aspect, the waveform information acquisition device detects the presence or absence of distortion of the optical modulation waveform by using an eye mask. And

  According to a third aspect of the present invention, in the optical modulator driver adjusting device according to the first aspect, the waveform information acquiring device takes distortion of the optical modulation waveform by taking a histogram in the amplitude direction of the optical modulation waveform. It is characterized by detecting.

  According to a fourth aspect of the present invention, in the optical modulator driver adjusting device according to the first aspect, the waveform information acquisition device takes a histogram of the optical modulation waveform in the time axis direction to obtain jitter of the optical modulation waveform. Is detected.

  According to a fifth aspect of the present invention, there is provided an optical modulator driver adjustment method for adjusting a driver circuit that drives an optical modulator of an optical duobinary modulation method, wherein the modulated light modulated by the optical modulator A step of observing a modulation waveform, a step of adjusting a cross point of the driver circuit to a value at which waveform distortion of the light modulation waveform is minimized based on the observed light modulation waveform, and the observed light modulation waveform And the step of adjusting the output amplitude of the driver circuit to a value at which the modulated optical power is maximized.

  According to the present invention, it is possible to adjust the output amplitude of the driver circuit of the optical modulator and the cross point of the driving waveform in the optical duobinary transmitter by observing the optical modulation waveform in a state where the optical transmitter is assembled. .

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a configuration of an adjustment apparatus for an optical duobinary modulation type optical modulator driver according to an embodiment of the present invention. The light output from the semiconductor laser (LD) 102 is input to the Mach-Zehnder optical modulator 103 and output as modulated light from the optical modulator 103 driven by the bias voltage applied from the LPF 106 and the ABC circuit 107. The The output of the LPF 106 is obtained by converting the output of the binary waveform output from the driver circuit 105 into the output of the ternary waveform. The driver circuit 105 operates based on the drive signal output from the drive signal generation circuit 104. The ABC circuit 107 applies a DC bias voltage on which an electrical signal that changes in a rectangular wave is superimposed to the optical modulator 103, detects a low-frequency component contained in the optical signal output from the optical modulator 103, and It operates so as to control the DC bias voltage applied to the modulator 103 to an optimum value.

  The modulated light output from the optical modulator 103 is converted into an electrical signal by an optical / electrical converter (O / E) 108 and input to the sampling scope 109. While operating the ABC circuit, the drive signal generation circuit 104 generates a pseudo random signal to drive the optical modulator, and the sampling scope 109 observes the optical modulation waveform. The sampling scope 109 transmits the observed light modulation waveform information to the driver control circuit 101. The driver control circuit 101 analyzes the received waveform information according to a reference to be described later, and changes the control voltage applied to the amplitude adjustment terminal 105a and the crosspoint adjustment terminal 105b of the driver circuit 105 based on the analysis result, thereby operating parameters of the driver. Make adjustments.

  FIGS. 2A and 2B show cross point shifts in the driver output and the corresponding light modulation waveforms. In the case of optical duobinary modulation, the shift of the cross point of the driver output waveform is indirectly observed as the extinction ratio deterioration of the optical modulation waveform. When the ABC circuit is in operation, the potentials at the “1” and “−1” levels of the ternarized electric signal are substantially controlled to the maximum value of the light transmittance of the modulator. As shown in FIG. 2A, when the cross point of the driver output waveform is deviated, the voltage value of the “0” level of the ternary electric signal is shifted from the extinction potential of the optical modulator. Accordingly, the level of the light modulation waveform corresponding to the “0” level increases, and as a result, the extinction ratio defined by the ratio between the light modulated “1” level and the “0” level deteriorates. When the deviation of the cross point of the driver output waveform is large, the jitter of the trace corresponding to the transition from “0” → “1” or “1” → “0” of the symbol after optical modulation increases. Therefore, the driver control circuit 101 adjusts the cross point of the driver circuit 105 so that the extinction ratio of the light modulation waveform observed by the sampling scope is increased.

  The driver control circuit 101 monitors the extinction ratio observed by the sampling scope while changing the position of the crosspoint by changing the voltage applied to the crosspoint adjustment terminal 105b of the driver circuit 105, and maximizes the extinction ratio. Detect cross points to be performed. When the cross point is changed in one direction, the extinction ratio increases as the cross point approaches the optimum value, and is maximized when the cross point takes the optimum value. Further, when the cross point is changed in the same direction, the cross point is deviated from the optimum value, and the extinction ratio becomes small again. Therefore, the cross point corresponding to the maximum value of the extinction ratio is the optimal cross point. The driver control circuit 101 adjusts the cross point of the driver circuit 105 to a cross point that maximizes the extinction ratio. The shape change of the light modulation waveform is evaluated by, for example, taking a histogram in the amplitude direction at the eye opening timing in the eye waveform acquired by the sampling scope.

  As shown in FIG. 2, when the cross point of the modulator driver is deviated, the “0” level of the modulated light is double-traced, whereas when the cross point is controlled to approximately 50%. Single trace. When this is used, the width in the amplitude direction of the “0” level of the optical modulation waveform is used as an evaluation parameter, and the applied voltage to the crosspoint adjustment terminal 105b of the modulator driver 105 is optimized so as to minimize it. Is also possible.

  In addition, when the cross point of the modulator driver is shifted, the jitter of the transition from “0” to “1” or “1” to “0” of the optical modulation waveform increases. The crosspoint control voltage of the modulator driver 105 may be controlled so as to minimize the jitter in the time axis direction.

  FIGS. 3A to 3C show the correspondence between the amplitude of the drive voltage of the ternary optical modulator and the distortion of the optical modulation waveform. Since the modulation characteristic of the optical modulator changes at a cycle of 2Vπ, when the amplitude of the drive voltage is smaller than 2Vπ, as shown in FIG. 3A, the amplitude is optimized as compared with FIG. The modulated light power is observed to be low. When the amplitude of the driver output is larger than 2Vπ, the optical modulation waveform is folded back at the “1” and “−1” levels of the ternary output, so that waveform degradation is observed as shown in FIG. When going back and forth between the two values, the maximum amplitude is temporarily reached, but the amplitude of the “1” continuous portion decreases. As a result, the average power of the modulated light is reduced from the optimum amplitude condition. Therefore, the driver control circuit 101 adjusts the output amplitude of the driver circuit 105 so that the modulated light power is maximized by changing the voltage applied to the amplitude adjustment terminal 105a of the driver circuit 105.

  Further, the driver control circuit 101 monitors the change of the modulated light power while changing the output amplitude, and detects the peak of the modulated light power. When the output amplitude is changed in one direction, the modulated light power increases as the output amplitude approaches the optimum value, and is maximized when the output amplitude takes the optimum value. Further, when the output amplitude is changed in the same direction, the output amplitude deviates from the optimum value, and the modulated light power decreases again. For this reason, the output amplitude corresponding to the maximum value of the modulated light power is the optimum output amplitude. The driver control circuit 101 adjusts the output amplitude of the driver circuit 105 to an output amplitude that maximizes the modulated light power.

  Method for determining whether or not the waveform acquired by the sampling scope protrudes from a preset eye mask in order to check whether the amplitude and crosspoint adjustment values of the modulator driver 105 are within a desired range There is also. The eye mask setting parameters include required conditions such as the distribution width of the eye in the amplitude direction, the allowable jitter amount, and the extinction ratio. By examining the match with the eye mask while adjusting the amplitude and crosspoint parameters of the modulator driver, the operation of the driver can be optimized.

It is a block diagram of the adjustment apparatus of the optical duobinary modulation type | mold optical modulator driver which concerns on one Embodiment of this invention. (A) is a figure which shows the response | compatibility of the shift | offset | difference of the cross point in a driver output, distortion of a ternary electric waveform, and distortion of an optical modulation waveform, (b) is the optimized driver output waveform, ternary electricity It is a figure which shows the distortion of a waveform, and the optical modulation waveform corresponding to them. (A) is a figure which shows an optical modulation waveform when the amplitude of the ternary modulator drive voltage is smaller than 2Vπ, and (b) is a figure which shows an optical modulation waveform when the amplitude is 2Vπ. (C) is a figure which shows an optical modulation waveform when an amplitude is larger than 2V (pi). (A) is a figure which shows the driver output waveform of the NRZ modulation system when the cross point is passing, the optical modulation characteristic, and the optical modulation waveform, and (b) is the NRZ when the cross point is optimized. It is a figure which shows the driver output waveform of a modulation system, an optical modulation characteristic, and an optical modulation waveform. (A) is a figure which shows the driver output waveform in the optical duobinary modulation system when the cross point has shifted | deviated, a ternary electric waveform, an optical modulation characteristic, and an optical modulation waveform, (b) is an optimal cross point. It is a figure which shows the driver output waveform, the ternary electric waveform, and the optical modulation waveform in the optical duobinary modulation system when it is made into.

Explanation of symbols

101 Driver Control Circuit 102 Semiconductor Laser (LD)
103 Mach-Zehnder optical modulator 104 Drive signal generation circuit 105 Driver circuit 105a Amplitude adjustment terminal 105b Crosspoint adjustment terminal 106 Low-pass filter (LPF)
107 Automatic Bias Control (ABC) Circuit 108 Optical / Electric Converter (O / E)
109 Sampling scope

Claims (5)

An optical modulator driver adjusting device for adjusting a driver circuit for driving an optical modulator of an optical duobinary transmitter,
A waveform information acquisition device for acquiring optical modulation waveform information of modulated light modulated by the optical modulator;
A driver control circuit for adjusting the driver circuit based on the light modulation waveform information,
The driver circuit adjusts the cross point to a value that maximizes the extinction ratio of the light modulation waveform, and adjusts the output amplitude of the driver circuit to a value that maximizes the modulated light power. Optical modulator driver adjustment device.   2. The optical modulator driver adjustment device according to claim 1, wherein the waveform information acquisition device detects the presence or absence of distortion of the optical modulation waveform by using an eye mask.   2. The optical modulator driver adjustment device according to claim 1, wherein the waveform information generation device detects distortion of the optical modulation waveform by taking a histogram in an amplitude direction of the optical modulation waveform. 3.   2. The optical modulator driver adjustment device according to claim 1, wherein the waveform information generation device detects jitter of the optical modulation waveform by taking a histogram of the optical modulation waveform in a time axis direction. An optical modulator driver adjustment method for adjusting a driver circuit for driving an optical modulator of an optical duobinary modulation method,
Observing an optical modulation waveform of modulated light modulated by the optical modulator;
Adjusting the cross point of the driver circuit based on the observed light modulation waveform to a value that minimizes the waveform distortion of the light modulation waveform; and the driver circuit based on the observed light modulation waveform, Adjusting the output amplitude of the driver circuit to a value at which the modulated optical power is maximized.

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