JP2011165866A - Laser diode drive circuit and laser diode drive method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To keep an average optical output power constant, and to maintain a desired extinction ratio. <P>SOLUTION: The laser diode drive circuit 100 has a photodiode 12 for monitoring average optical output power, an APC circuit for controlling a pulse current Ip so that the average optical output power is constant, and an extinction ratio control part 22 for controlling the extinction ratio of an optical signal. The extinction ratio control part 22 has an interruption/resumption control part 28 for shutting off the feedback loop of the APC circuit to interrupt APC control, a threshold current detector 24 for detecting a threshold current for the laser diode, based on how the average optical output power changes, when individual values are changed, while keeping constant the sum of a bias current Ib and a pulse current Ip during APC control interruption; and a bias current setting part 26 for setting the bias current Ib to somewhere around the threshold current. After the bias current Ib has been set, the interruption/resumption control part 28 cancels the feedback loop shut-off and resumes the APC control. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a laser diode driving circuit and a laser diode driving method for driving a laser diode by supplying a driving current obtained by superimposing a pulse current on a bias current.

  In an optical transmission system, an optical transmission device is used that outputs an optical signal by controlling a drive current of a laser diode in accordance with transmission data. In the optical transmitter, in order to control the average optical output power of the laser diode to be constant, APC control (Auto Power Control) is performed in which the backward light of the laser diode is monitored and fed back to the drive current.

  Further, in an optical transmission apparatus, in order to ensure transmission quality, it is necessary to stably maintain an extinction ratio that is a ratio of the maximum value of the optical output power to the minimum value. The optical receiver converts the signal level into an electric signal level corresponding to the received optical signal power, and identifies “1” and “0” of the data by signal level determination. Therefore, the larger the extinction ratio, the more easily data can be identified.

  As is well known, a laser diode used in an optical transmitter has a large characteristic fluctuation due to temperature fluctuation. FIG. 1 shows an example of the relationship between the laser diode drive current and the optical output power. In FIG. 1, a curve A is a characteristic curve at a temperature of 0 ° C., a curve B is a temperature at 25 ° C., and a curve C is a characteristic curve at a temperature of 70 ° C.

  FIG. 1 shows a threshold current Ith at a temperature of 25 ° C. The laser diode normally starts laser oscillation at a threshold current Ith, and has a characteristic that the optical output power increases linearly as the current increases in a current region larger than the threshold current Ith. Further, in the current region smaller than the threshold current Ith, the laser diode hardly emits light regardless of the current value, and thus has a nonlinear characteristic in the vicinity of the threshold current Ith.

  The laser diode drive circuit sets the bias current Ib in the vicinity of the threshold current Ith of the laser diode and prevents the laser diode from superimposing the pulse current Ip on the bias current Ib in order to prevent the laser diode from delaying light emission. The structure to supply is common. FIG. 1 shows a bias current Ib and a pulse current Ip set at a temperature of 25 ° C. The optical output power when the bias current Ib is supplied to the laser diode is the minimum optical output power Pmin, the optical output power when the pulse current Ip is supplied superimposed on the bias current Ib is the maximum optical output power Pmax, and the minimum optical output power. The average of Pmin and maximum optical output power Pmax is the average optical output power Pave. The extinction ratio of the optical signal is defined by Pmax / Pmin.

  As shown in FIG. 1, the threshold current Ith of the laser diode rises as the temperature rises. Therefore, if the values of the bias current Ib and the pulse current Ip are not changed even when the temperature rises, the extinction ratio is remarkably increased. Deteriorated and stable optical signal transmission is not possible.

  Therefore, the temperature characteristics of the threshold current of the laser diode are measured in advance and stored in a memory or the like, the threshold current corresponding to the detected temperature of the laser diode is read from the memory, and the bias current or pulse current is changed. The technology to do is known.

  In addition, the pulsed current of the laser diode according to the transmission data is superposed and supplied with the current according to the low-frequency pilot tone signal compared to this pulse signal, and the optical output of the laser diode is detected by the photodiode. The pilot tone signal component is extracted and fed back to the laser diode bias current and pulse current control unit to suppress temperature fluctuations and aging. A technique for maintaining the ratio constant has also been proposed (see, for example, Patent Document 1).

JP 2001-94202 A

  However, in the case of the conventional technique in which the temperature characteristics of the laser diode are measured in advance and the bias current or the pulse current is changed in response to the temperature fluctuation, it is necessary to measure the temperature characteristics of the laser diode in advance and store them in the memory. However, measurement of temperature characteristics for all laser diodes is cumbersome and may increase the cost of the optical transmitter. In order to reduce this man-hour, it is considered that the same lot or the same model number in the manufacturing process of the laser diode is regarded as the same characteristic, and the characteristic of the representative laser diode is measured and stored in the memory. Actually, since the temperature characteristics of the laser diode have many variations, there is a possibility that a desired extinction ratio cannot be maintained. In addition, this conventional technique cannot cope with aging of the laser diode.

  In the case of the technique for superimposing the pilot tone signal as described in Patent Document 1, not only a complicated control circuit is required but also the noise of the main signal may increase due to the pilot tone signal. .

  The present invention has been made in view of such circumstances, and the object thereof is to keep the average optical output power of the laser diode constant and maintain a desired extinction ratio while avoiding the above-described problems. A laser diode drive circuit and a laser diode drive method are provided.

  In order to solve the above problems, a laser diode driving circuit according to an aspect of the present invention is a laser diode driving circuit that drives a laser diode by supplying a driving current in which a pulse current is superimposed on a bias current. Monitor unit for monitoring the average optical output power, APC circuit for feedback control of the pulse current so that the monitored average optical output power is constant, and extinction ratio control for controlling the extinction ratio of the optical signal output from the laser diode A part. The extinction ratio control unit interrupts the feedback loop of the APC circuit to interrupt the APC control, and changes the respective values while keeping the sum of the bias current and the pulse current constant during the interruption of the APC control. A threshold current detection unit for detecting a threshold current of the laser diode based on how the average optical output power changes at the time, a bias current setting unit for setting the bias current in the vicinity of the threshold current, A resumption unit for releasing the interruption of the feedback loop and resuming the APC control after the bias current is set.

  In the above laser diode drive circuit, APC control by an APC circuit is normally performed, and extinction ratio control by an extinction ratio control unit may be performed at predetermined time intervals.

  The threshold current detector changes each value while keeping the sum of the bias current and the pulse current constant, and searches for the bias current when the change in the average optical output power is nonlinear, thereby A threshold current may be detected.

  Another aspect of the present invention is a laser diode driving method. This method is a laser diode driving method for driving a laser diode by supplying a driving current obtained by superimposing a pulse current on a bias current, a monitoring step for monitoring the average optical output power of the laser diode, and a monitored average light An APC control step that feedback-controls the pulse current so that the output power becomes constant, and an extinction ratio control step that controls the extinction ratio of the optical signal output from the laser diode. The extinction ratio control step includes an interruption step of interrupting the APC control by interrupting the feedback loop of the APC control, and changing each value while keeping the sum of the bias current and the pulse current constant during the interruption of the APC control. A threshold current detecting step for detecting a threshold current of the laser diode based on a change in the average optical output power at the time, a bias current setting step for setting the bias current in the vicinity of the threshold current, After the bias current is set, the method includes a restarting step for releasing the interruption of the feedback loop and restarting the APC control.

  In the above laser diode driving method, the APC control step may be normally performed, and the extinction ratio control step may be performed every predetermined time interval.

  The threshold current detection step changes each value while keeping the sum of the bias current and the pulse current constant, and searches for the bias current when the change in the average optical output power becomes nonlinear, thereby detecting the laser diode. You may have the step which detects a threshold current.

  It should be noted that any combination of the above-described constituent elements and a representation of the present invention converted between an apparatus, method, system, program, recording medium storing the program, etc. are also effective as an aspect of the present invention.

  According to the present invention, it is possible to provide a laser diode driving circuit and a laser diode driving method capable of keeping the average optical output power of a laser diode constant and maintaining a desired extinction ratio while avoiding the above problems. .

It is a figure which shows an example of the relationship between the drive current of a laser diode, and optical output power. It is a figure which shows the structure of the laser-diode drive circuit which concerns on embodiment of this invention. It is a figure for demonstrating the change of the extinction ratio by a temperature change. It is a figure for demonstrating extinction ratio control in the case of a temperature fall. It is a figure for demonstrating extinction ratio control in the case of a temperature fall. It is a figure for demonstrating extinction ratio control in the case of a temperature rise. It is a figure for demonstrating extinction ratio control in the case of a temperature rise. It is a flowchart of bias Ib setting control according to the present embodiment.

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

  FIG. 2 is a diagram showing a configuration of the laser diode drive circuit 100 according to the embodiment of the present invention. As shown in FIG. 2, the laser diode driving circuit 100 includes a laser diode 10, a photodiode 12, a current-voltage conversion resistor 14, an operational amplifier 16, a reference voltage source 18, a feedback loop control circuit 20, and a quenching. A ratio control unit 22, a pulse current constant current source 30, a bias current constant current source 32, and a modulation circuit 34 are provided.

  In the laser diode 10, a power supply voltage Vcc is applied to the anode, and a constant current source 32 for bias current for supplying a bias current Ib is connected to the cathode. Further, a constant current source 30 for pulse current for supplying a pulse current Ip is also connected to the cathode of the laser diode 10, and is input between the laser diode 10 and the constant current source 30 for pulse current. A modulation circuit 34 that modulates the pulse current Ip according to the data signal is provided. As described above, the drive current obtained by superimposing the pulse current Ip on the bias current Ib is supplied to the laser diode 10 using the constant current source for bias current 32 and the constant current source for pulse current 30.

  The photodiode 12 monitors the average optical output power of the laser diode 10. When the laser diode 10 emits light, a monitor current proportional to the light output flows through the photodiode 12. This monitor current is converted into a monitor voltage Vm by the current-voltage conversion resistor 14 and input to the operational amplifier 16. The operational amplifier 16 also receives the reference voltage Vref from the reference voltage source 18. The operational amplifier 16 compares the monitor voltage Vm and the reference voltage Vref, and outputs a feedback signal to the pulse current constant current source 30 so that the monitor voltage Vm = the reference voltage Vref. Based on the input feedback signal, the pulse current constant current source 30 performs feedback control (APC control) on the pulse current Ip so that the average optical output power monitored by the photodiode 12 becomes constant. When the pulse current Ip is feedback-controlled, the bias current constant current source 32 holds the bias current Ib at a fixed value. The operational amplifier 16, the reference voltage source 18 and the like constitute an APC circuit.

  A feedback loop control circuit 20 is provided between the feedback loop from the operational amplifier 16 to the constant current source 30 for pulse current. The feedback loop control circuit 20 has a function of interrupting a feedback loop from the operational amplifier 16 to the pulse current constant current source 30 in accordance with an instruction from an interruption / resumption control unit 28 of an extinction ratio control unit 22 described later. When the cutoff of the feedback loop is released, the feedback loop control circuit 20 outputs the feedback signal from the operational amplifier 16 to the pulse current constant current source 30 as it is. When the feedback loop is interrupted by the feedback loop control circuit 20, the pulse current constant current source 30 continues to hold the pulse current Ip immediately before the interruption.

  The extinction ratio control unit 22 has a function of controlling the extinction ratio of the optical signal output from the laser diode 10. The extinction ratio control unit 22 includes an interruption / resumption control unit 28, a threshold current detection unit 24, and a bias current setting unit 26.

  The interruption / resumption control unit 28 instructs the feedback loop control circuit 20 to shut off and release the feedback loop, and controls the timing of interruption and resumption of APC control. The laser diode driving circuit 100 is normally subjected to APC control by the APC circuit described above, and the APC control is interrupted by the interruption / resumption control unit 28 at predetermined time intervals, and the extinction ratio control is performed. This predetermined time interval can be appropriately set according to the use environment of the laser diode driving circuit 100 or the like. For example, in a use environment where temperature fluctuation is severe, this time interval is preferably set to a relatively short time, for example, about 1 minute. On the contrary, in the case of a usage environment in which a temperature change hardly occurs, this time interval may be a relatively long time of about 1 hour, for example.

  A threshold voltage detector 24 receives a monitor voltage Vm representing the average optical output power monitored by the photodiode 12. While the APC control is interrupted, the threshold current detection unit 24 changes each value while keeping the sum of the bias current Ib and the pulse current Ip constant while monitoring the input monitor voltage Vm. Then, the threshold current of the laser diode 10 is detected based on how the average optical output power changes at that time.

  More specifically, the threshold current detector 24 changes each value while keeping the sum of the bias current Ib and the pulse current Ip constant, and the monitored change in the average optical output power becomes nonlinear. The threshold current of the laser diode 10 is detected by searching for the bias current Ib.

  After the threshold current is detected by the threshold current detector 24, the bias current setting unit 26 controls the bias current constant current source 32 so that the bias current Ib is in the vicinity of the detected threshold current. Set. After the bias current Ib is controlled by the bias current constant current source 32, the suspension / resumption control unit 28 issues a feedback loop cutoff cancellation instruction to the feedback loop control circuit 20 to resume APC control.

  Next, the extinction ratio control according to the present embodiment will be described in more detail. FIG. 3 is a diagram for explaining a change in the extinction ratio due to a temperature change. FIG. 3 shows a characteristic curve 40 (broken line) and a characteristic curve 42 (solid line) of the laser diode 10. Here, a case will be described in which the characteristic curve of the laser diode 10 changes from 40 to 20 due to a decrease in the temperature of the laser diode 10. In the example shown in FIG. 3, since the extinction ratio control is not started, the APC control feedback loop has not been cut off yet.

  It is assumed that the bias current Ib (old) set to a value equal to the threshold current Ith (old) and the pulse current Ip (old) are supplied to the laser diode 10 before the temperature is lowered. In this case, the time when only the bias current Ib is supplied is the minimum optical output power Pmin (old) of the laser diode 10, and the time when the current obtained by superimposing the pulse current Ip (old) on the bias current Ib (old) is supplied. The maximum optical output power Pmax (old) of the laser diode 10 is obtained. Further, the average light output power Pave (old) detected by the photodiode 12 is an average value of Pmax (old) and Pmin (old).

  When the characteristic curve changes from 40 to 42 and the threshold current Ith changes from Ith (old) to Ith (new) due to the temperature drop of the laser diode 10, the bias current Ib (old) is fixed in the APC control. Therefore, the minimum optical output power increases to Pmin (new), and the average optical output power also increases. Therefore, the pulse current is reduced to Ip (new) by APC control, and the maximum optical output power is also reduced to Pmax (new). Since the maximum optical output power is reduced by the increase in the minimum optical output power, the average optical output power Pave (new) after the temperature change is not changed from the average optical output power Pave (old) before the temperature decrease and is kept constant. Be drunk. However, the extinction ratio was Pmax (old) / Pmin (old) before the temperature decrease, but decreased to Pmax (new) / Pmin (new) after the temperature decrease.

  FIG. 4 is a diagram for explaining the extinction ratio control in the case of a temperature drop. When the APC control is interrupted by the interruption / resumption control unit 28, the threshold current detection unit 24 starts searching for a new threshold current Ith (new). In the present embodiment, the threshold current detection unit 24 gradually decreases the bias current Ib (old) before the temperature decrease by a predetermined amount while monitoring the average optical output power Pave. At this time, the sum of the bias current Ib (old) and the pulse current Ip (new) when the extinction ratio control is started is kept constant. That is, the pulse current Ip is increased by the decrease of the bias current Ib.

  When the bias current is decreased from Ib (old) to Ib1, the average optical output power decreases linearly from Pave (old) to Pave1. When the bias current is decreased from Ib1 to Ib2 next time, the average optical output power decreases approximately linearly from Pave1 to Pave2. However, when the bias current is then decreased from Ib2 to Ib3, the average optical output power Pave3 hardly changes from Pave2. In other words, the change in average optical output power is non-linear with respect to the change in bias current. This is because the threshold current Ith (new) after temperature decrease exists between the bias currents Ib1 and Ib2. That is, since the bias currents Ib2 and Ib3 are smaller than the threshold current Ith (new) after the temperature decrease, even if the bias current is decreased from Ib2 to Ib3, the minimum optical output powers Pmin2 and Pmin3 are It is almost constant. Further, as described above, since the sum of the bias current Ib (old) and the pulse current Ip (new) is kept constant, Pmax (new) is constant. As a result, even if the bias current is decreased from Ib2 to Ib3, the average optical output powers Pave2 and Pave3 are almost unchanged.

  FIG. 5 is also a diagram for explaining the extinction ratio control in the case of a temperature drop. FIG. 5 shows the relationship between the number of times the bias current is changed and the average optical output power. The average optical output power Pave that was Pave (old) before the start of the extinction ratio control is reduced to Pave1 in the first control, and is reduced to Pave2 in the second control. However, in the third control, the average optical output power Pave3 is substantially the same value as Pave2 as described in FIG. Therefore, the threshold current detection unit 24 detects that the average optical output power Pave does not decrease even when the bias current Ib is decreased. In other words, the threshold current detection unit 24 detects the average optical output with respect to the change of the bias current Ib. By detecting a point at which the change in the power Pave becomes non-linear, a new threshold current Ith (new) after a temperature drop is generated between the bias current Ib1 in the first control and the bias current Ib2 in the second control. It can be detected that it exists.

  After the threshold current detection unit 24 detects the threshold value Ith (new) after the temperature drop, the bias current setting unit 26 controls the bias current constant current source 32 to be in the vicinity of Ith (new). Is set to a new bias current Ib (new). For example, the bias current constant current source 32 can set the new bias current Ib (new) to the bias current Ib2 when the average optical output power Pave starts to become constant. In this way, the bias current Ib (new) after the temperature drop can be set in the vicinity of Ith (new) after the temperature drop.

  Thereafter, when the drive current of the laser diode 10 is set to Ib (new) + Ip (new), the suspension / resumption control unit 28 instructs the feedback loop control circuit 20 to release the feedback loop cutoff, and the APC control is resumed. The optimum pulse current Ip is set by the APC control so that the average light output power Pave of the laser diode 10 is the same as before the execution of the extinction ratio control unit. By the extinction ratio control as described above, the new bias current Ib (new) after the temperature decrease can be set in the vicinity of the new Ith (new), so even if the temperature of the laser diode 10 decreases, the extinction ratio The fluctuation of the ratio can be suppressed.

  Next, the operation when the temperature of the laser diode 10 rises will be described. FIG. 6 is a diagram for explaining extinction ratio control in the case of a temperature rise. FIG. 6 shows the temperature characteristic 44 after the temperature rise.

  In the example of FIG. 6, since the temperature of the laser diode 10 has increased, the threshold current has increased from Ith (old) to Ith (new). Therefore, the bias current Ib (old) set to the same value as the threshold current Ith (old) before the temperature change is smaller than the threshold current Ith (new) after the temperature rise. .

  When the APC control is interrupted by the interruption / resumption control unit 28, the threshold current detection unit 24 starts searching for a new threshold current Ith (new). Here again, the threshold current detector 24 gradually decreases the bias current Ib (old) before the temperature rise by a predetermined amount while monitoring the average optical output power Pave. At this time, the sum of the bias current Ib (old) and the pulse current Ip (new) when the extinction ratio control is started is kept constant.

  When the bias current is decreased from Ib (old) to Ib1, the average optical output power hardly changes (Pmin (old) ≈Pmin1). In other words, the change in average optical output power is non-linear with respect to the change in bias current. As shown in FIG. 6, since the bias current Ib (old) before the temperature change is smaller than the threshold current Ith (new) after the temperature rise, even if the bias current is decreased to Ib1, the minimum light output This is because the power Pmin1 is almost unchanged from Pmin (old), while the sum of the bias current Ib (old) and the pulse current Ip (new) is kept constant, so that Pmax (new) is constant.

  As described above, when it is found that the average optical output power hardly changes with the change of the bias current, the threshold current detection unit 24 switches the control to gradually increase the bias current from Ib (old) by a predetermined amount. . As shown in FIG. 6, when the bias current is increased from Ib (old) to Ib2, the average optical output power increases approximately linearly from Pave (old) to Pave2. When the bias current is increased from Ib2 to Ib3 next time, the average optical output power increases linearly from Pave2 to Pave3.

  FIG. 7 is also a diagram for explaining the extinction ratio control when the temperature rises. FIG. 7 shows the relationship between the number of times the bias current is changed and the average optical output power, as in FIG. The average light output power Pave that was Pave (old) before the start of the extinction ratio control is the same average light output power Pave1 as Pave (old) in the first control. Since the threshold current detection unit 24 has found that the average optical output power hardly changes with the change of the bias current, the bias current is gradually increased from Ib (old) by a predetermined amount after the second control. . In the second control, the average light output power increases from Pave (old) to Pave2, and in the third control, the average light output power increases from Pave2 to Pave3. Therefore, the threshold current detection unit 24 changes from a state where the average optical output power Pave does not decrease even when the bias current Ib is decreased to a state where the average optical output power Pave increases by increasing the bias current Ib. In other words, by detecting a point at which the change in the average optical output power Pave becomes non-linear with respect to the change in the bias current Ib, the bias current Ib (old) before the start of the extinction ratio control and the second time are detected. It can be detected that there is a new threshold current Ith (new) after the temperature rise between the bias current Ib2 and the control.

  After the threshold current detection unit 24 detects the threshold value Ith (new) after the temperature rise, the bias current setting unit 26 controls the constant current source 32 for bias current to be in the vicinity of Ith (new). Is set to a new bias current Ib (new). For example, the bias current constant current source 32 can set the new bias current Ib (new) to the bias current Ib2 when the average optical output power Pave starts to increase. In this way, the bias current Ib (new) after the temperature rise can be set in the vicinity of Ith (new) after the temperature rise.

  Thereafter, when the drive current of the laser diode 10 is set to Ib (new) + Ip (new), the suspension / resumption control unit 28 instructs the feedback loop control circuit 20 to release the feedback loop cutoff, and the APC control is resumed. The optimum pulse current Ip is set by the APC control so that the average light output power Pave of the laser diode 10 is the same as before the execution of the extinction ratio control unit. By the extinction ratio control as described above, the new bias current Ib (new) after the temperature rise can be set in the vicinity of the new Ith (new), so even if the temperature of the laser diode 10 rises, The fluctuation of the ratio can be suppressed.

  If the threshold current has not changed since the previous extinction ratio control, the threshold current detector 24 cannot detect a point where the change in the average optical output power is non-linear. In such a case, the bias current setting unit 26 sets the bias current Ib (old) set by the previous extinction ratio control as the bias current Ib for the current extinction ratio control.

  Note that the amount of change in the bias current Ib during the search for the threshold current Ith, the amount of change in Pave that the average optical output power Pave is assumed to be constant, and the bias current Ib that determines that the average optical output power Pave is constant The number of times of control and the like can be appropriately set by experiments or simulations according to the characteristics of the laser diode 10 to be used, the usage environment of the laser diode driving circuit 100, and the like.

  FIG. 8 is a flowchart of bias Ib setting control according to the present embodiment. The control of this flowchart is performed after the interruption / resumption control unit 28 interrupts the feedback loop of the APC control.

  First, various parameters are initialized (S10). In this initialization step, the average optical output power Pave (old) before the start of control is set as the comparison optical output power Pref, and the bias current Ib (old) before the start of control is set as Ib (0). The number of times N is set to 1.

  Next, the threshold current detector 24 changes each value while keeping the sum of the bias current Ib and the pulse current Ip constant (S12). Specifically, the bias Ib (N−1) with the previous control count N is decreased by a predetermined amount ΔIb, and this is used as the current bias current Ib (N). Further, in order to keep the sum of the bias current Ib and the pulse current Ip constant, the pulse current Ip is obtained by adding a predetermined amount ΔIb to the pulse current Ip.

  Next, the threshold current detector 24 determines whether or not the change in the average optical output power Pave from the comparative optical output power Pref is within a predetermined value ΔP (S14). When the change in the average light output power Pave from the comparison light output power Pref is not within the predetermined value ΔP (N in S14), the control number N is incremented, and the current average light output power Pave is set as the comparison light output power Pref. Set (S16), return to S12.

  On the other hand, when the change in the average optical output power Pave from the comparative optical output power Pref is within the predetermined value ΔP (Y in S14), the threshold current detection unit 24 determines whether the number of times of control N is 1 or not. Determine (S18).

  When the control count N is not 1 (N in S18), the threshold current detection unit 24 determines the current bias current Ib (N) as the bias current Ib to be set by the bias current setting unit 26 (S20). . On the other hand, when the control count N is 1 (Y in S18), the threshold current detection unit 24 proceeds to control for increasing the bias current Ib.

  First, the threshold current detection unit 24 increases the bias Ib (N−1) with the previous control count N by a predetermined amount ΔIb and sets it as the current bias current Ib (N). Further, in order to keep the sum of the bias current Ib and the pulse current Ip constant, the current pulse current Ip is obtained by subtracting a predetermined amount ΔIb from the pulse current Ip (S22).

  Next, the threshold current detector 24 determines whether or not the change in the average optical output power Pave from the comparative optical output power Pref is equal to or greater than a predetermined value ΔP (S24). ). When the change in the average light output power Pave from the comparison light output power Pref is not greater than or equal to the predetermined value ΔP (N in S24), the control number N is incremented, and the current average light output power Pave is set as the comparison light output power Pref. Set (S26) and return to S22.

  On the other hand, when the change in the average optical output power Pave from the comparative optical output power Pref is equal to or greater than the predetermined value ΔP (Y in S24), the threshold current detector 24 biases the current bias current Ib (N). The current setting unit 26 determines the bias current Ib to be set (S28). The bias current Ib can be set in the vicinity of the threshold current Ith by the flowchart as described above.

  As described above, according to the laser diode drive circuit 100 according to the present embodiment, even if the threshold current Ith of the laser diode 10 varies due to a temperature change, the bias current after the variation is changed. It can be set in the vicinity of the current Ith. Thereby, the fluctuation | variation of an extinction ratio can be suppressed and a desired extinction ratio can be maintained.

  Further, according to the laser diode driving circuit 100 according to the present embodiment, since the APC control is performed by the APC circuit in the normal time, the average optical output power of the laser diode 10 can be kept constant. Although the APC control is interrupted during the extinction ratio control, the extinction ratio control according to the present embodiment is completed in a relatively short time, so that there is no substantial influence due to the interruption of the APC control.

  Further, according to the laser diode driving circuit 100 according to the present embodiment, the extinction ratio can be controlled in accordance with the characteristics of the individual laser diodes 10. Therefore, the temperature characteristics of each individual laser diode are measured in advance. Such an operation is unnecessary, and it is possible to cope with the secular change of the laser diode 10.

  In addition, according to the laser diode driving circuit 100 according to the present embodiment, processing such as superimposing the pilot tone signal on the main signal is not necessary, so that noise of the main signal does not increase.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are also within the scope of the present invention. is there.

  DESCRIPTION OF SYMBOLS 10 Laser diode, 12 Photodiode, 14 Current-voltage conversion resistance, 16 Operational amplifier, 18 Reference voltage source, 20 Feedback loop control circuit, 22 Extinction ratio control part, 24 Threshold current detection part, 26 Bias current setting part, 28 Suspension / resumption control unit, 30 constant current source for pulse current, 32 constant current source for bias current, 100 laser diode drive circuit.

Claims (6)

A laser diode driving circuit for driving a laser diode by supplying a driving current in which a pulse current is superimposed on a bias current,
A monitor unit for monitoring the average optical output power of the laser diode;
An APC circuit that feedback-controls the pulse current so that the monitored average optical output power is constant;
An extinction ratio control unit for controlling an extinction ratio of an optical signal output from the laser diode;
The extinction ratio control unit comprises
An interruption unit for interrupting the APC control by interrupting the feedback loop of the APC circuit;
The threshold current of the laser diode is detected based on how the average optical output power changes when each value is changed while the sum of the bias current and pulse current is kept constant while the APC control is interrupted. A threshold current detector to perform,
A bias current setting unit for setting the bias current in the vicinity of the threshold current;
After the bias current is set, a resumption unit that releases the interruption of the feedback loop and resumes APC control;
A laser diode driving circuit comprising:   2. The laser diode driving circuit according to claim 1, wherein the APC circuit is normally controlled by the APC circuit, and the extinction ratio control is performed by the extinction ratio control unit at every predetermined time interval.   The threshold current detector changes each value while keeping the sum of the bias current and the pulse current constant, and searches for the bias current when the change in the average optical output power becomes nonlinear, thereby 3. The laser diode driving circuit according to claim 1, wherein a threshold current of the diode is detected. A laser diode driving method for driving a laser diode by supplying a drive current obtained by superimposing a pulse current on a bias current,
A monitoring step of monitoring an average optical output power of the laser diode;
An APC control step for feedback-controlling the pulse current so that the monitored average optical output power is constant;
An extinction ratio control step for controlling an extinction ratio of an optical signal output from the laser diode;
The extinction ratio control step comprises:
An interruption step of interrupting the APC control by interrupting the feedback loop of the APC control;
The threshold current of the laser diode is detected based on how the average optical output power changes when each value is changed while the sum of the bias current and pulse current is kept constant while the APC control is interrupted. A threshold current detection step to perform,
A bias current setting step for setting the bias current in the vicinity of the threshold current;
After the bias current is set, a restart step for releasing the interruption of the feedback loop and restarting the APC control;
A laser diode driving method comprising:   5. The laser diode driving method according to claim 4, wherein the APC control step is normally performed, and the extinction ratio control step is performed at predetermined time intervals.   The threshold current detection step changes each value while keeping the sum of the bias current and the pulse current constant, and searches for the bias current when the change in the average optical output power is nonlinear. 6. The laser diode driving method according to claim 4, further comprising a step of detecting a threshold current of the diode.

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