JP5423338B2 - Semiconductor optical amplifier driver - Google Patents

Description

  The present invention relates to a semiconductor optical amplifier driving apparatus, and more particularly to a technique for superimposing a dither signal on an optical output.

  In the development of high-speed and large-capacity optical fiber communication systems, a method called wavelength division multiplex communication (WDM: Wavelength Division Multiplex) that simultaneously transmits a plurality of signals having different wavelengths in one optical fiber cable has been put into practical use. Has been. Further, a method is conceivable in which the optical output is subjected to low frequency and small amplitude modulation (AM modulation) that does not affect the main signal, and a sub signal (dither signal) such as a monitoring signal is placed on the low frequency component. .

  As a method of putting such a sub signal, a method of superimposing AM modulation on an optical output by adding a dither signal to a driving power source of a semiconductor optical amplifier (SOA) can be considered.

  The SOA can amplify the light by setting the injection current in the forward direction, and can attenuate the light by setting it in the reverse direction. Since the SOA originally has a function of increasing and decreasing the optical output of the laser, a push-pull circuit is often used for driving the SOA.

  However, due to the characteristics of the push-pull circuit, when the drive power level of the SOA is located just near the middle between amplification and absorption, that is, when the push-pull circuit is controlled to produce an output close to zero, the dither signal There is a problem that distortion of the image becomes large. In other words, when the output of the push-pull circuit is controlled near zero, when the dither signal is superimposed, the push-pull circuit performs a modulation operation between the push operation and the pull operation. There has been a problem that the distortion of the signal becomes large.

  An object of the present invention is to solve this problem and to provide a driving device for a semiconductor optical amplifier capable of suppressing distortion of a dither signal.

  A semiconductor optical amplifier driving apparatus according to the present invention includes a driving signal generating unit that generates a driving signal for defining a driving current level of the semiconductor optical amplifier, a semiconductor optical signal connected to one input, and a semiconductor optical signal connected to the other input. A feedback unit that is connected to a signal indicating the drive current level input to the amplifier and outputs according to the difference, and two complementary transistors having different polarities from each other, and outputs to the semiconductor optical amplifier according to the output of the feedback unit A push-pull unit that generates a drive current; a dither control circuit that generates a dither signal superimposed on the drive signal; an offset control unit; an offset unit that is controlled by the offset control unit and offsets the drive current to the semiconductor optical amplifier; Are provided. According to the semiconductor optical amplifier driving device of the present invention, it is possible to suppress distortion of the waveform output from the push-pull unit when a dither signal is input to the push-pull unit.

  The current monitor unit that detects the drive current input to the semiconductor optical amplifier may be provided, and the offset control unit may control the offset addition by the offset unit according to the detection result of the current monitor unit. The offset control unit preferably performs offset addition by the offset unit when the absolute value of the current value detected by the current monitoring unit is a predetermined value or less.

  The offset unit preferably has a predetermined time constant with respect to offset addition control by the offset control unit. The dither control circuit preferably includes a high-pass filter having a relatively high transmission intensity on the high band side in the band of the input dither signal. After performing the offset addition control, after the offset control unit transitions to a state that does not satisfy the condition determined in performing the offset addition control, until the other conditions that define the offset release are satisfied, It is preferable to continue the offset addition control.

  According to the semiconductor optical amplifier driving device of the present invention, it is possible to suppress waveform distortion when a dither signal is input to the push-pull circuit.

1 is a circuit diagram showing an overall configuration of a semiconductor optical amplifier driving device according to a first embodiment. It is a figure for demonstrating the characteristic of an NPN transistor and a PNP transistor. When the dither signal from the dither control circuitry is outputted, is a view showing the relationship between the voltage V _base and V _SOA. In the case where the frequency of the dither signal is higher is a diagram showing the relationship between the voltage _{V base} and _{V SOA.} It is a figure for _{demonstrating} offset addition to the drive current _ISOA . It is a figure which shows an example of the flowchart performed when adding an offset to drive current _ISOA . It is a figure for demonstrating the effect | action of a high pass filter.

  Hereinafter, the best mode for carrying out the present invention will be described.

(First embodiment)
FIG. 1 is a circuit diagram showing an overall configuration of a semiconductor optical amplifier driving device 100 according to the first embodiment. As shown in FIG. 1, the semiconductor optical amplifier driving device 100 includes a controller 10, a dither control circuit 20, an SOA control circuit 30, an offset unit 40, a drive current monitor unit 50, and a PD monitor 60. In FIG. 1, a semiconductor laser 2 including a semiconductor optical amplifier 1 is also drawn.

  The controller 10 includes an APC execution unit 11, a dither amplitude control unit 12, and an offset control unit 13. The APC execution unit 11, the dither amplitude control unit 12, and the offset control unit 13 are realized, for example, when the controller 10 executes a predetermined program. Further, the APC execution unit 11, the dither amplitude control unit 12, and the offset control unit 13 can be realized by a control circuit without using software.

  The APC execution unit 11 controls the SOA control circuit 30 so that the output light intensity of the semiconductor laser 2 becomes a target value according to the detection result of the PD monitor 60. The dither amplitude control unit 12 controls the dither amplitude and the on / off state of the dither input to the semiconductor optical amplifier 1 by controlling the dither control circuit 20. The offset control unit 13 controls on / off of offset addition to the drive current of the semiconductor optical amplifier 1 by controlling the offset unit 40.

  The dither control circuit 20 includes a high pass filter 21 and an amplifier 22. The high pass filter 21 suppresses transmission of low frequency components. The amplifier 22 amplifies the dither signal that has passed through the high-pass filter 21 with a predetermined gain, and inputs the amplified signal to the SOA control circuit 30 via the capacitor 71. The amplifier 22 adjusts the amplitude width of the dither signal in accordance with the electrical signal from the controller 10 and switches on / off the dither input to the semiconductor optical amplifier 1.

The SOA control circuit 30 includes a D / A (digital / analog) converter 31, an operational amplifier 32, and a push-pull circuit 33. The D / A converter 31 converts the digital signal input from the APC execution unit 11 into an analog signal and inputs the analog signal to the positive terminal of the operational amplifier 32 that is resistance-divided by the resistors 72a and 72b. A voltage input from the D / A converter 31 to the operational amplifier 32 is a set voltage V of the semiconductor optical amplifier 1. Therefore, the D / A converter 31 functions as a drive signal generation unit that generates a drive signal for the semiconductor optical amplifier 1. The negative terminal of the operational amplifier 32 is supplied with V _{SOA which} is a difference value between the power supply voltage V1, the resistor 73a, and the resistor 73b. The operational amplifier 32 adjusts the set voltage V to a predetermined voltage value based on the input value and inputs it to the push-pull circuit 33.

  The push-pull circuit 33 includes two complementary transistors having different polarities. In the present embodiment, the push-pull circuit 33 has a structure in which a push-side NPN transistor 331 and a pull-side PNP transistor 332 are arranged to face each other. Specifically, the emitter terminal of the NPN transistor 331 and the emitter terminal of the PNP transistor 332 are connected. A positive DC power supply VLD is connected to the collector terminal of the NPN transistor 331, and a negative DC power supply VEE is connected to the collector terminal of the PNP transistor 332. The output signal of the operational amplifier 32 is input to the base terminals of the NPN transistor 331 and the PNP transistor 332 via the resistors 74a and 74b. In the push-pull circuit 33, when the semiconductor optical amplifier 1 amplifies the output light of the semiconductor laser 2, the NPN transistor 331 drives the output of the semiconductor optical amplifier 1, and the semiconductor optical amplifier 1 absorbs the output light of the semiconductor laser 2. In this case, the PNP transistor 332 is set so as to drive the output of the semiconductor optical amplifier 1. The capacitor 75 increases the stability of the SOA control circuit 30.

  The offset unit 40 includes a transistor 41, a resistor 42, and a capacitor 43. A DC power supply VLD is connected to the emitter terminal of the transistor 41 via a resistor 42. This DC power supply VLD is also connected to the base terminal of the transistor 41 via the capacitor 43. When an electric signal is input from the offset control unit 13 to the base terminal of the transistor 41, the voltage at the base terminal is changed from a high state to a low state, and offset addition to the drive current of the semiconductor optical amplifier 1 is turned on.

The drive current monitor unit 50 includes a resistor 51, a differential amplifier 52, and an analog / digital (A / D) converter 53. The resistor 51 is inserted between the push-pull circuit 33 and the semiconductor optical amplifier 1. The differential amplifier 52 uses the ratio of the resistor 76 and the resistor 77 to generate an analog signal having a voltage that is equal to the voltage across the resistor 51. The A / D converter 53 converts the analog signal output from the differential amplifier 52 into a digital signal and inputs the digital signal to the controller 10. Thereby, the controller 10 can acquire the potential difference at the resistor 51 based on the digital signal input from the A / D converter 53 and can acquire the drive current _ISOA input to the semiconductor optical amplifier 1. .

  The PD monitor 60 includes a photodiode 61 and an analog / digital (A / D) converter 62. The photodiode 61 receives the output light of the semiconductor laser 2 and outputs an analog current proportional to the output light intensity of the semiconductor laser 2. This current is converted into a voltage by the resistor 78 and input to the A / D converter 62. The A / D converter 62 converts the input value into a digital signal and inputs the digital signal to the APC execution unit 11.

  The semiconductor optical amplifier 1 and the semiconductor laser 2 of this embodiment are integrally formed. Specifically, in the semiconductor optical amplifier 1 and the semiconductor laser 2, the optical waveguide of the semiconductor laser 2 and the optical waveguide of the semiconductor optical amplifier 1 are optically coupled to each other. The semiconductor optical amplifier 1 and the semiconductor laser 2 may be independent from each other. The semiconductor optical amplifier 1 amplifies the output light of the semiconductor laser 2 when a positive voltage is applied from the SOA control circuit 30, and the output light of the semiconductor laser 2 when a negative voltage is applied from the SOA control circuit 30. To absorb. In other words, the SOA control circuit 30 outputs a current to the semiconductor optical amplifier 1 when optically amplifying the semiconductor optical amplifier 1, and outputs a current from the semiconductor optical amplifier 1 when optically absorbing the semiconductor optical amplifier 1. Absorb.

Next, characteristics of the NPN transistor 331 and the PNP transistor 332 in the push-pull circuit 33 will be described. FIG. 2A shows a part of the SOA control circuit 30 extracted. As shown in FIG. 2A, the voltage V _base is a base drive voltage of the transistor of the push-pull circuit 33 of the SOA control circuit 30. The voltage V _SOA is a voltage value on the downstream side of the resistor 51 of the drive current monitor unit 50. Therefore, this voltage V _SOA is equal to the drive voltage applied to the semiconductor optical amplifier 1. FIG. 2B is a diagram illustrating the relationship between the set voltage V, the voltage V _base, and the voltage V _SOA . In FIG. 2B, the horizontal axis indicates the set voltage V, and the vertical axis indicates the base drive voltage V _base and the drive voltage V _SOA of the transistor of the push-pull circuit 33.

The SOA control circuit 30 controls the drive voltage V _SOA to a target voltage value according to the input set voltage V. For this purpose, the SOA control circuit 30 controls the drive voltage V _SOA by controlling the base voltages of the NPN transistor 331 and the PNP transistor 332. However, the output voltage of the push-pull circuit 33 greatly fluctuates near the voltage at which the operations of the NPN transistor 331 and the PNP transistor 332 are switched. Specifically, V _SOA is higher than V _base at a voltage lower than the switching voltage, and V _SOA is lower than V _base at a voltage higher than the switching voltage. Therefore, the base voltages of the NPN transistor 331 and the PNP transistor 332 need to be largely changed near the switching between the NPN transistor 331 and the PNP transistor 332. As an example, in the example of FIG. 2B, it is necessary to satisfy V _base ≧ V _SOA + 0.6V when the SOA control circuit 30 outputs current, and V _base ≦ V _SOA −0.6V when current is absorbed.

The output light of the semiconductor laser 2 to AM modulation, it is necessary to superimpose a dither signal to the driving voltage V _SOA having target amplitude. However, since it is the set voltage V that contributes to the modulation in the SOA control circuit 30, the waveform of the dither signal superimposed on the set voltage V needs to be transmitted to the drive voltage V _SOA without being corrupted.

FIG. 3A shows that when the drive voltage V _SOA is 1.7 V that exceeds the switching voltage of the NPN transistor 331 and the PNP transistor 332, and the dither signal is output from the dither control circuit 20, the voltages V _base and V _SOA The relationship is shown.

If voltage above the switching voltage in FIG. 3 (a) NPN transistor 331 driving voltage _{V SOA} as shown in and PNP transistor 332, because it can use the linear characteristic in FIG. 2 _(b), it is superimposed on the _{V SOA} The reproducibility of the dither signal is high. Therefore, as shown in FIG. 3B, the reproducibility is kept high even when the frequency of the dither signal is increased.

On the other hand, when the drive voltage V _SOA is 1.0 V near the switching between the NPN transistor 331 and the PNP transistor 332, the amplitude by the dither signal is implemented in the nonlinear region of FIG. Thus, the dither signal superimposed on V _SOA will be distorted.

As shown in FIG. 4A, it is conceivable that the reproducibility of the dither signal can be maintained even when driving in the nonlinear region of FIG. 2B by increasing the amplitude of the voltage V _base . However, as shown in FIG. 4B, when the frequency of the dither signal is increased, the modulation drive of the signals of the NPN transistor 331 and the PNP transistor 332 is not in time, and as a result, the waveform of the drive voltage V _SOA is destroyed.

Therefore, in the present embodiment, the offset unit 40 adds a predetermined positive current as an offset to the drive current _ISOA to the semiconductor optical amplifier 1. If the offset current is added to the voltage value of V _SOA is temporarily increased, is input to the operational amplifier 32. The operational amplifier 32 changes the value of the control input (V _base ) of the push-pull circuit 33 so as to eliminate the difference between this value and the output of the D / A converter 31. In the present embodiment, since a positive current is added as an offset, the output of the operational amplifier 32 changes so as to decrease the output of the push-pull circuit 33. When the offset is added in this way, the driving current _ISOA is driven near the switching voltage of the NPN transistor 331 and the PNP transistor 332, and then deep driving is performed with only the PNP transistor 332 operating. .

  As shown in FIG. 5A, this is because the switching of the NPN transistor 331 and the PNP transistor 332 with respect to the set voltage (output of the D / A converter 31) due to the addition of the offset is positive as shown by the dotted line. Indicates a shift.

The addition of the offset, since it becomes possible to dither drive in the linear region, thereby improving the reproducibility of the dither signal superimposed on I _SOA. This eliminates the need to increase the output amplitude of the operational amplifier 32 in order to perform dither driving in the non-linear region of the push-pull circuit 33. Also, as shown in FIG. This leads to an effect of sufficiently following the output drive of the push-pull circuit 33.

As for the offset to the drive current _ISOA , a negative current may be added in addition to a positive current. In this case, the switching of the NPN transistor 331 and the PNP transistor 332 with respect to the set voltage (output of the D / A converter 31) is shifted to the minus side. Even in this case, since the linear region of the push-pull circuit 33 can be used, distortion of the dither signal can be suppressed.

Note that if the offset current is added from the offset unit 40 to the drive current _ISOA in a short time, the drive voltage V _SOA will rapidly increase. However, the resistor 42 and the capacitor 3 set a predetermined time constant for offset addition. As a result, the sudden addition of the offset current is suppressed, so that the rapid increase of the drive voltage _VSOA is suppressed.

Next, details of offset control of the drive current _ISOA will be described. FIG. 6 is a diagram illustrating an example of a flowchart executed when an offset is added to the drive current _ISOA . As shown in FIG. 6, first, when the optical output of the target wavelength is started from the semiconductor laser 2, the APC execution unit 11 determines that the output light intensity of the semiconductor laser 2 is the target value according to the detection result of the PD monitor 60. The SOA control circuit 30 is controlled so as to become (step S1).

Next, the controller 10 acquires the detection value of the drive current monitor unit 50 and acquires the drive current _ISOA (step S2). Then, the controller 10 determines whether the drive current _{I SOA} is within a range of 0 ± αmA (step S3). Note that “α” in step S3 is set in the offset control unit 13 at the time of a shipping test, for example, and can be set to about 0.5 mA. If the driving current I _SOA in Step S3 is determined to be within the scope of 0 ± αmA, offset control unit 13, so that the offset is started for the drive current ISOA of the SOA 1, controls the offset section 40 (Step S4).

Subsequently, the dither amplitude controller 12 starts dither input to the semiconductor optical amplifier 1 by controlling the dither control circuit 20 (step S5). If the driving current I _SOA is not determined to be within the scope of 0 ± αmA in step S3 step S5 is executed, the dither input is started. Incidentally, before the addition control of the offset in step S4, performs an input of the dither signal in step S5, a while even after adding the offset, dither signal superimposed on the drive current I _SOA remains distorted and Become. In order to obtain the effect of offset addition at an early stage, as described above, it is preferable to input a dither signal after the offset addition control.

According to the flowchart of FIG. 6, when driving in the vicinity of switching operation of the NPN transistor 331 and PNP transistor 332, the dither signal in a state where the offset in the drive current I _SOA only transistors of one is added is shifted to the state of operation Is entered. Thereby, deformation of the waveform of the dither signal in the driving voltage V _SOA is suppressed.

  When the offset is added and the optical output of the semiconductor laser 2 is stable, it is preferable not to change the offset setting unnecessarily. This is because the characteristics of the semiconductor optical amplifier 1 do not change abruptly. For example, the flowchart of FIG. 6 may be executed again only when conditions such as when the target light output is changed and the set voltage V is changed, or when the channel of the semiconductor laser 2 is changed, are satisfied.

  Next, the operation of the high pass filter 21 will be described. When the frequency of the dither signal is increased, the operation of the operational amplifier 32 may not be in time, and the frequency characteristics of the SOA control circuit 30 may deteriorate on the high band side. FIG. 7A is a diagram illustrating frequency characteristics of the SOA control circuit 30. In FIG. 7A, the horizontal axis indicates the frequency of the dither signal, and the vertical axis indicates the transfer characteristic of the SOA control circuit 30. As shown in FIG. 7A, the modulation amplitude is not degraded until the frequency of the dither signal is about 10 kHz to 100 kHz, but the modulation amplitude is degraded when the frequency of the dither signal exceeds 100 kHz. For example, when the frequency of the dither signal is 1 MHz, the modulation amplitude is reduced by about −8 dB. Therefore, by providing the high-pass filter 21 in the dither control circuit 20, the difference between the modulation frequency on the low frequency side and the modulation amplitude on the high frequency side can be suppressed.

  FIG. 7B shows the frequency characteristics of the high-pass filter 21. FIG. 7C is a graph in which the frequency characteristic of the high-pass filter 21 is superimposed on the frequency characteristic of the SOA control circuit 30. In FIG. 7B, the horizontal axis indicates the frequency of the dither signal, and the vertical axis indicates the transfer characteristic of the high-pass filter 21. In FIG. 7C, the horizontal axis indicates the frequency of the dither signal, and the vertical axis indicates the transfer characteristic when passing through the SOA control circuit 30 and the high-pass filter 21.

  As shown in FIG. 7B, the high-pass filter 21 has a high transmission intensity on the high band side of the frequency band of the dither signal. In this case, as shown in FIG. 7C, the synthesized frequency characteristics of the SOA control circuit 30 and the high-pass filter 21 are substantially constant regardless of the frequency. Thus, by providing the high-pass filter 21, it is possible to suppress the deterioration of the amplitude of the dither signal in the high band.

In the above embodiment, whether or not to add an offset is determined according to the drive current _ISOA , but is not limited thereto. For example, it may be determined that an offset is added when the intensity of light output from the semiconductor laser 2 is smaller than a predetermined value. However, the light intensity output from the semiconductor laser 2 since not to be proportional to the drive current I _SOA, it is preferable to directly detect the driving current I _SOA as in the above embodiment.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

DESCRIPTION OF SYMBOLS 1 Semiconductor optical amplifier 2 Semiconductor laser 10 Controller 11 APC execution part 12 Dither amplitude control part 13 Offset control part 20 Dither control circuit 21 High pass filter 30 SOA control circuit 31 D / A converter 32 Operational amplifier 33 Push pull circuit 40 Offset part 50 Drive current Monitor unit 51 Resistance 60 PD monitor 61 Photodiode 100 Semiconductor optical amplifier driver

Claims (6)

A drive signal generator for generating a drive signal for defining a drive current level of the semiconductor optical amplifier;
The drive signal is connected to one input, the signal indicating the drive current level input to the semiconductor optical amplifier is connected to the other input, and a feedback unit that makes an output according to the difference;
A push-pull unit that includes two complementary transistors having different polarities, and generates a drive current to the semiconductor optical amplifier according to the output of the feedback unit;
A dither control circuit for generating a dither signal superimposed on the drive signal;
An offset control unit;
An apparatus for driving a semiconductor optical amplifier, comprising: an offset unit that is controlled by the offset control unit and offsets a drive current to the semiconductor optical amplifier. A current monitor unit for detecting a drive current input to the semiconductor optical amplifier;
2. The semiconductor optical amplifier drive device according to claim 1, wherein the offset control unit controls addition of an offset by the offset unit in accordance with a detection result of the current monitoring unit.   3. The drive of a semiconductor optical amplifier according to claim 2, wherein the offset control unit performs offset addition by the offset unit when the absolute value of the current value detected by the current monitoring unit is equal to or less than a predetermined value. apparatus.   2. The semiconductor optical amplifier drive device according to claim 1, wherein the offset unit has a predetermined time constant with respect to offset addition control by the offset control unit.   2. The semiconductor optical amplifier drive device according to claim 1, wherein the dither control circuit includes a high-pass filter having a relatively high transmission intensity on a high band side in a band of an input dither signal.   The offset control unit, after performing the offset addition control, after transitioning to a state that does not satisfy the condition determined in performing the offset addition control, until another condition that defines the offset cancellation is satisfied 2. The semiconductor optical amplifier drive device according to claim 1, wherein the offset addition control is continued.

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