JP2007173611A - Laser diode driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maintain a predetermined extinction ratio by compensating a fluctuation in the characteristic of a laser diode regarding a laser diode driving circuit. <P>SOLUTION: The laser diode driving circuit comprises a constant current source 11 for a bias current, first and second current sources 12, 13 for a modulation current, and a photodiode 2 for monitoring. The laser diode driving circuit also comprises a bias current control loop for controlling the source 11 so as to suppress a fluctuation in the average value of the light output power of the laser diode 1; a modulation current control loop for contacting and separating the source 13 to and from the source 12 on the basis of a control condition for varying the bias current of the bias current control loop not smaller than the predetermined value, and controlling the sources 12, 13 by detecting the variation of the average value of the light output power of the laser diode 1; and a control circuit 10 for controlling the contact and separation of the source 13 by a switch 14. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a laser diode drive circuit that compensates for characteristic fluctuations due to temperature fluctuations of a laser diode for converting an electrical signal into an optical signal for transmission.

  In the optical signal transmission system, the drive current of the laser diode is controlled according to the transmission data, converted into an optical signal, and transmitted through the optical fiber transmission line. The laser diode that constitutes the optical signal transmission circuit is already Various configurations are known, but characteristic fluctuations due to temperature fluctuations are relatively large. The relationship between the laser diode drive current [mA] and the optical output power [mW] is, for example, as shown in FIG. In the figure, 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. Also, Ib represents a bias current, Im represents a modulation current, the optical output power due to the bias current Ib at a temperature of 25 ° C. is P2, and the optical output power due to the modulation current Im supplied to be superimposed on the bias current Ib is P1. The average optical output power is Pav.

  The laser diode drive circuit generally has a configuration in which the bias current Ib of the laser diode is set in the vicinity of the threshold current and the modulation current Im is superimposed on the bias current Ib and supplied to the laser diode, and the optical signal is quenched. The ratio is equivalent to P1 / P2. On the optical signal receiving side, the signal is converted into an electric signal level corresponding to the received optical signal power, and data “1” and “0” are identified by signal level determination. Therefore, the larger the extinction ratio, the more easily data can be identified. Further, as the temperature rises, the threshold current value of the laser diode rises as shown by curves A, B and C. Therefore, if the values of the bias current Ib and the modulation current Im are not changed even if the temperature rises, the quenching The ratio is significantly deteriorated, and stable optical signal transmission cannot be performed. 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 or its vicinity is read from the memory, and the bias current or modulation current is changed. Means are known. In this case, a means is also known in which the modulation current corresponding to the detected temperature is read from the memory to drive the laser diode, the optical output of the laser diode is detected by the photodiode, and the bias current is controlled by feedback control. .

  Various means for controlling the extinction ratio of the laser diode are also known. For example, in the characteristic curve diagram shown in FIG. 6, when the modulation current Im is changed, the optical output power also changes. Therefore, the modulation current Im is increased by a test amount ΔIm at a period slower than the transmission data rate, The average optical output power Pav due to increases by ΔPav. Therefore, since the slope L1 of the characteristic curve is expressed by ΔPav / ΔIm, when this slope indicates the reference slope at the reference temperature, the bias current continues the previous state, and the reference slope and If they are different, the bias current is controlled so that the reference slope is obtained. Accordingly, there has been proposed a means for controlling the light output characteristics of the laser diode due to temperature fluctuations so as to keep the extinction ratio constant (see, for example, Patent Document 1).

The laser diode modulation current according to the transmission data is supplied by superimposing the current according to the low-frequency pilot tone signal compared to this modulation 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 modulation current control unit to suppress temperature fluctuations and aging, and with a relatively low frequency feedback control configuration, the extinction ratio is reduced. Means for keeping the value constant has also been proposed (see, for example, Patent Document 2).
JP 2001-94202 A JP-T-2001-519098

  The laser diode that converts the input transmission data into an optical signal changes its characteristics due to temperature fluctuations and deteriorates the extinction ratio. In order to maintain the desired extinction ratio, the laser diode characteristics are measured in advance. In conventional means for controlling bias current and / or modulation current in response to temperature fluctuations, it is necessary to pre-measure the characteristics of the laser diode and store it in memory, The characteristic measurement is complicated, and there is a problem that the cost of the optical signal transmission system is increased. In order to reduce this work man-hour, the same lot or the same model number in the manufacturing process of the laser diode is regarded as the same characteristic, and a means for measuring the characteristic of the representative laser diode and storing it in the memory can be applied. Although it is conceivable, the temperature characteristics of the laser diode are actually varied, and there is a problem that it cannot be controlled to maintain a desired extinction ratio. There is also a problem that it is not possible to cope with aging of the laser diode.

  In addition, the modulation signal is periodically changed by a minute value, the slope of the characteristic curve is obtained based on the average light output, and the bias current is controlled so that the desired slope characteristic is obtained, and the pilot tone signal is modulated with the modulation current. The conventional example of driving the laser diode in superimposition to extract the pilot tone signal component in the optical output signal and performing feedback control does not measure the individual characteristics of the laser diode, and changes in temperature and aging Can be controlled to maintain the desired extinction ratio, but the minute change component or pilot tone signal component of the modulation signal becomes an optical signal continuously superimposed on the main signal to be transmitted, and these Since this component corresponds to a noise component, there is a problem of reducing the margin for the pulse mask definition in the optical signal transmission system.

  The present invention solves the problems of the prior art, reduces the influence of noise components due to pilot tone signals, compensates for laser diode characteristic fluctuations due to temperature fluctuations, etc., and maintains a desired extinction ratio. For the purpose.

  A laser diode driving circuit according to the present invention includes a bias current constant current source for supplying a bias current to a laser diode, and a modulation current first constant current source for supplying a modulation current according to transmission data superimposed on the bias current. A laser diode driving circuit comprising: a second constant current source for modulating current that is supplied by adding a minute current to the modulating current; and a monitoring photodiode that monitors the optical output of the laser diode. A bias current control loop for controlling the constant current source for bias current so as to suppress fluctuations in the average value of the optical output power of the laser diode detected by the photo diode, and the bias current in the bias current control loop On the basis of a control state in which the value is changed by a predetermined value or more, the first constant current source for the modulation current The second constant current source for modulation current is connected to the laser diode, a minute current is superimposed on the modulation current and supplied to the laser diode, and the fluctuation of the average value of the optical output power of the laser diode is detected to detect the modulation A modulation current control loop that controls the first constant current source for current and the second constant current source for modulation current; and the connection of the second constant current source for modulation current to the first constant current source for modulation current. And a control circuit for controlling the separation.

  The control circuit detects a control state in which the bias current in the bias current control loop is changed by a predetermined value or more, and detects the modulation current second constant current source with respect to the modulation current first constant current source. The contact / separation is repeatedly controlled at a period slower than the transmission speed of the transmission data, and the contact / separation is continued from the first constant current source for modulation current after the repetition of the contact / separation for a predetermined period. 2 It has the structure which performs control which isolate | separates a constant current source.

  In addition, a control circuit is provided that can set the predetermined value for detecting a change in the bias current that is greater than or equal to a predetermined value or a time until the bias current is greater than or equal to the predetermined value.

  The minute current from the second constant current source for the modulation current is superimposed on the modulation current when the bias current by the bias current control loop is changed more than the change value, and the light output average is thereby increased. The modulation current control loop performs control based on the power fluctuation, and the noise component for the main optical signal due to the superposition of the minute current is connected to or separated from the modulation current second constant current source. This is only the control period to be performed, and the others are not superimposed, so that the pulse mask margin characteristic on the optical signal receiving side can be improved.

  Referring to FIG. 1, the laser diode driving circuit of the present invention is a bias current constant current source 11 for supplying a bias current to the laser diode 1, and a modulation for supplying a modulation current according to transmission data in a superimposed manner on the bias current. A laser including a first constant current source 12 for current, a second constant current source 13 for supplying a modulation current by adding a minute current to the modulation current, and a monitoring photodiode 2 for monitoring the optical output of the laser diode 1 A bias current control loop for controlling the constant current source for bias current 11 so as to suppress fluctuations in the average value of the optical output power of the laser diode 1 detected by the monitoring photodiode 2; Based on a control state in which the bias current in the bias current control loop is changed by a predetermined value or more, the first constant current source 12 for modulation current is Thus, the modulation current second constant current source 13 is connected and separated in parallel for a predetermined period, a minute current is superimposed on the modulation current and supplied to the laser diode 1, and the average value of the optical output power of the laser diode 1 is calculated. A modulation current control loop that detects the fluctuation and controls the first constant current source 12 for modulation current and the second constant current source 13 for modulation current, and a modulation current for the first constant current source 12 for modulation current And a control circuit 10 for controlling contact / separation of the second constant current source 13 with a switch 14.

  FIG. 1 is an explanatory diagram of Embodiment 1 of the present invention, where 1 is a laser diode, 2 is a monitoring photodiode, 3 is a transimpedance amplifier, 4 is a first comparator, 5 is a second comparator, and 6 is First counter, 7 second counter, 8 latch circuit, 9 adder, 10 control circuit, 11 constant current source for bias current, 12 first constant current source for modulation current, 13 for modulation current A second constant current source, 14 is a switch, C1 and C2 are capacitors, R1 to R6 are resistors, and Vcc is a power supply voltage. A transmission data application path for supplying a modulation current according to the transmission data to the laser diode 1 is not shown.

  The laser diode 1 is connected to a first constant current source 12 for modulation current and a second constant current source 13 for modulation current via a switch 14 and also to a constant current source 11 for bias current via a resistor R1. And the power supply voltage Vcc is applied. The monitor photodiode 2 detects an optical signal from the monitor side of the laser diode 1 or an optical signal branched from a part of the output light. The current flowing through the monitor photodiode 2 is converted by a transimpedance amplifier 3. The voltage is changed and input to the first comparator 4, and the power supply voltage Vcc is divided by the resistors R 2 and R 3 to be a reference voltage, and this reference voltage is input to the first comparator 4. The optical output monitor configuration including the transimpedance amplifier 3 outputs a detection value corresponding to an average value of optical output corresponding to high-speed transmission data.

  The comparison output signal of the first comparator 4 is input to the control circuit 10, input to the up / down terminal UP / DN of the first counter 6, the clock signal from the control circuit 10 is input to the clock terminal CLK, The counter counts up or down at the timing of the clock signal, and the count value is input to the bias current constant current source 11 from the output terminal OUT. In this case, the first counter 6 is configured to input a predetermined number of up-count or down-count output signals to the bias current constant current source 11 for current control, and the average value of the optical output power of the laser diode 1 is calculated. The bias current is controlled by detecting the fluctuation, and a bias current control loop is formed by a control loop including the comparator 4 and the first counter 6. When the clock signal input from the control circuit 10 to the clock terminal CLK of the first counter 6 is stopped, the count value at that time is maintained and output.

  The output signal of the transimpedance amplifier 3 is input to the latch circuit 8, and the control signal from the control circuit 10 is input to the latch terminal LATCH and latched. The input signal of the latch circuit 8 and the latch output signal are input to the adder 9 and added, and the added output signal is input to the second comparator 5. Further, the power supply voltage Vcc is divided by resistors R5 and R6 to obtain a reference voltage, which is input to the second comparator 5. The comparison output signal of the second comparator 5 is input to the up / down terminal UP / DN of the second counter 7, and the clock signal from the control circuit 10 is input to the clock terminal CLK. Then, the count value is input from the output terminal OUT to the control circuit 10, the modulation current first constant current source 12, and the modulation current second constant current source 13. When the second counter 7 also stops the clock signal input to the clock terminal CLK, it maintains the count value at that time and outputs it. The switch 14 is composed of a semiconductor element such as a transistor, and is controlled to be turned on and off by the control circuit 10. The current is superimposed and flows to the laser diode 1. The ON / OFF (contact / separation) cycle can be set to, for example, about several kHz, which is lower than the transmission data.

  A monitoring photodiode 1 that detects the optical output of the laser diode 1, a transimpedance amplifier 2, a latch circuit 8, an adder 9, a second comparator 5, a second counter 7, and a first modulation current. A control loop including the constant current source 12 and the second constant current source 13 for modulation current forms a modulation current control loop. The second constant current source 13 for modulation current has a configuration that supplies a smaller amount of current than the first constant current source 12 for modulation current, and the first constant for modulation current flowing in the laser diode 1 according to transmission data. This is for superimposing a minute variation on the modulation current from the current source 12. Only when the control in the bias current control loop has a fluctuation range of a predetermined value or more, the first constant current source 12 for modulation current is turned on by turning on the switch 14 for the second constant current source 13 for modulation current. Are connected in parallel to each other, and the on / off (contact / separation) of the switch 14 is controlled, and the period corresponds to a low frequency such as a pilot tone signal. It is also possible to adopt a configuration in which a minute modulation current is output by a low-frequency signal through a path that is not shown while the switch 14 is on.

  The latch circuit 8 of the modulation current control loop latches the output signal of the transimpedance amplifier 3 at the timing of the control signal from the control circuit 10, and the latch output and the signal input to the latch circuit 8 at the next timing The adder 9 obtains a difference value and determines whether the difference value is larger than the reference voltage, that is, whether the optical output power of the laser diode 1 is increasing, decreasing, or unchanged. In the case of an increase tendency, the second counter 7 counts up, in the case of a decrease tendency, a down count, and in the case of no change, the control content is not counted. The count value of the second counter 7 is input to the control circuit 10 and is input to the modulation current first constant current source 12 and the modulation current second constant current source 13 as a control signal for increasing or decreasing the modulation current. .

  When there is no temperature fluctuation of the laser diode 1 and the optical output power does not change, the control circuit 10 continues to turn off the switch 14. Also, a predetermined bias current is supplied from the bias current constant current source 11, and the transmission data is input to the modulation current first constant current source 12 in order to control the modulation current through a path not shown in the figure. The modulation current according to “1” and “0” of the current flows through the laser diode 1, and the corresponding optical output is transmitted from the laser diode 1. In this case, since no control signal is input from the output terminal OUT of the first counter 6 to the constant current source 11 for bias current, the bias current at the time of initial setting is supplied to the laser diode 1. Further, since the control circuit 10 controls the switch 14 to be in the OFF state, the modulation current corresponding to the transmission data is supplied to the laser diode 1 from the first constant current source 12 for modulation current.

  When the threshold value of the laser diode 1 increases due to the temperature rise, the average value of the light output of the laser diode 1 decreases. As a result, the comparison output signal of the first comparator 4 has a negative polarity, the first counter 6 counts down, and the bias current is increased by controlling the constant current source 11 for bias current by down-counting a predetermined value. Let On the contrary, when the temperature decreases, the threshold value of the laser diode 1 decreases, so the average value of the optical output of the laser diode 1 increases, the comparison output signal of the first comparator 4 becomes positive, and the first counter 6 The bias current from the bias current constant current source 11 is reduced by counting up. The bias current is controlled so as to suppress a change in the average optical output value due to a temperature change or a characteristic change.

  When such control of the bias current control loop is performed, whether or not the change of the average value of the optical output in the predetermined time interval has changed beyond the reference value by the latch circuit 8 and the adder 9. Is detected by the second comparator 5, and it is determined by counting by the second counter 7 whether or not it is continuously increasing or decreasing over a predetermined period. If there is a change, the control circuit 10 is notified. At the same time, a modulation current change control signal is applied to the first constant current source 12 for modulation current and the second constant current source 13 for modulation current, and the control circuit 10 controls the switch 14 to be turned on. As a result, a small current according to a low-frequency signal such as a pilot tone signal from the modulation current second constant current source 13 is superimposed on the modulation current supplied from the modulation current first constant current source 12 to the laser diode 1. Is done. Accordingly, the ratio of the change ΔPav in the average optical output and the change ΔIm in the modulation current is controlled to be a reference value, and the extinction ratio can be controlled to be maintained at a predetermined value.

  FIG. 2 is a main part flowchart of the first embodiment of the present invention, showing the processing of the bias current control loop in FIG. 1 by a subroutine (S15) and the processing of the modulation current control loop by a subroutine (S21). Then, the subroutine (S15) obtains the average value Pav of the optical output power of the laser diode 1 (S16), and compares this average value Pav with the reference value Pavref (S17). That is, the first comparator 4 compares the average value Pav obtained by the transimpedance amplifier 3 with the reference value Pavref obtained by dividing the resistances R2 and R3. If the comparison result shows a decrease, the threshold value of the laser diode 1 has increased due to a temperature rise or the like, and the bias current constant current source 11 is controlled to increase the bias current (S18). If the average value Pav decreases compared to the reference value Pavref, the bias current is reduced (S19). And the process which transfers to a subroutine (S21) is performed (S20). When the average value Pav is not changed compared to the reference value Pavref, the increase / decrease control of the bias current is not performed. In step (S17), if there is no change, the process returns to the first step (S15).

  The subroutine (S21) is executed with the control of the increase / decrease of the bias current as a trigger, and the increase ΔIm of the modulation current of the laser diode 1 from the second constant current source 13 for the modulation current is changed to the first for the modulation current. The superimposed current is supplied to the modulation current from the constant current source 12 (S22). Then, the sum (Pav + ΔPav) of the average value Pav of the optical output power of the laser diode 1 and the increase ΔPav corresponding to the increase ΔIm of the modulation current is obtained as the output of the transimpedance amplifier 3 (S23). An increase ΔPav is calculated by calculating (Pav + ΔPav) − (Pav) (S24), the increase ΔPav is compared with the reference value ΔPavref by the second comparator 5 (S25), and the increase ΔPav is the reference value ΔPavref. In the following case, the increment ΔIm from the second constant current source 13 for modulation current and the modulation current from the first constant current source 12 for modulation current are increased (S26). On the contrary, when the increase ΔPav is equal to or larger than the reference value ΔPavref, the increase ΔIm from the second constant current source 13 for modulation current and the modulation current from the first constant current source 12 for modulation current are decreased (S27). ). And the process which returns to a subroutine (S15) is performed (S28). In step (S25), if there is no change, the process proceeds to step (S28).

  As described above, in the steady state, the modulation current according to the transmission data is supplied to the laser diode 1, and the second constant current source 13 for modulation current controls the switch 14 to the OFF state, and the laser The state is separated from the diode 1. In this state, the noise component due to the increase ΔIm that increases or decreases according to the low frequency is not included in the optical output signal, and does not adversely affect the optical signal receiving side. Further, the change in the average value of the optical output power due to the characteristic variation of the laser diode 1 is detected, the bias current constant current source 11 is controlled, the bias current is increased or decreased, and the control of the bias current is used as a trigger. The second constant current source 13 for modulation current is controlled, and the increase ΔIm of the modulation current is superimposed on the modulation current from the first constant current source 12 for modulation current and supplied to the laser diode 1, and the light output characteristic curve Thus, the modulation current is controlled so that the slope of becomes the reference value.

  FIG. 3 is an explanatory diagram of Embodiment 2 of the present invention. The same reference numerals as those in FIG. 1 denote the same names, 29 denotes an integrator, and external settings 1 and 2 are input to the control circuit 10. In addition, the output signal of the first counter 6 is input to the integrator 29 of the control circuit 10. This integrator 29 integrates the increase and decrease of the bias current by the first counter 6, compares the integration result with the set value by the external setting 1, and when the integration result exceeds the set value, the modulation current by the modulation current control loop Transition to control. The external setting 2 is for setting a time for comparison with the accumulated time of the operation of the modulation current control loop.

  FIG. 4 is a main part flowchart of the second embodiment of the present invention, including subroutines (S30) and (S38). In the subroutine (S30) for the bias current control loop, the light output average of the laser diode 1 is shown. The value Pav is obtained (S31), and the average value Pav is compared with the reference value Pavref (S32). That is, the first comparator 4 compares the average value Pav obtained by the transimpedance amplifier 3 with the reference value Pavref obtained by dividing the resistances R2 and R3. If the comparison result shows a decrease, this is a case where the threshold of the laser diode 1 has increased due to a temperature rise or the like, and the bias current constant current source 11 is controlled to increase the bias current (S33). When the average value Pav decreases compared to the reference value Pavref, the bias current is reduced (S34).

  Next, the accumulated increase / decrease value of the output of the first counter 6 by the integrator 29 of the control circuit 10 is compared with the set value by the external setting 1 (S35). If the cumulative increase / decrease value is smaller than the set value, that is, even if the bias current is controlled, the process returns to the first step (S30) (S36). If the cumulative increase / decrease value is larger than the set value, that is, if the bias current is continuously controlled, the process proceeds to a subroutine (S38) (S37).

  In the subroutine (S38) for the modulation current control loop, an increase ΔIm of the modulation current of the laser diode 1 from the second constant current source 13 for modulation current according to a low frequency signal such as a pilot tone signal is calculated as the modulation current. The superposed current is supplied to the modulation current from the first constant current source 12 (S39). Then, the sum (Pav + ΔPav) of the average value Pav of the optical output power of the laser diode 1 and the increase ΔPav corresponding to the increase ΔIm of the modulation current is obtained as the output of the transimpedance amplifier 3 (S40). An increase ΔPav is calculated by calculating (Pav + ΔPav) − (Pav) (S41), the increase ΔPav is compared with the reference value ΔPavref by the second comparator 5 (S42), and the optical output power of the laser diode 1 is calculated. When the increase ΔPav of the average value Pav is smaller than the reference average value increase ΔPavre, the increase ΔIm of the modulation current from the second constant current source 13 for modulation current and the first constant current source 12 for modulation current If the modulation current Im is increased (S43) and the increase ΔPav is larger than the reference value ΔPavref, the modulation current Im Reducing the pressure component ΔIm modulation current Im (S44).

  The modulation current control accumulated time by the integrator 29 of the control circuit 10 is compared with the set time by the external setting 2 (S45). If the accumulated modulation current control time is shorter than the set time, the process returns to the subroutine (S38), and the control of the modulation current control loop is continued. The process proceeds to a subroutine (S30) of control processing of the current control loop. If the comparison result is the same in step (S42), the process can proceed to step (S45) or the first step (S38).

  As described above, the optical output average power control by the bias current control and the extinction ratio control by the modulation current control are switched using the increase / decrease control of the bias current as a trigger, thereby stabilizing both control characteristics and the main light signal. It is possible to reduce the influence of the superimposition of the noise component due to the minute change of the modulation current on the as much as possible, and to enable stable optical signal transmission.

  FIG. 5 is an explanatory diagram of an example of the control operation. The minimum control step of the bias current Ib and the modulation current Im is 0.1 mA, the set value of the increase in the bias current by the external setting 1 is ± 5 steps, and the external setting. When the bias current fluctuation is 0.5 mA due to the temperature rise with the set time by 2 set to 1 second, the bias current control loop operates in the direction in which the bias current Ib increases in the period Ta, and time t1, t2 , T3, t4, t5, the bias current constant current source 11 is controlled so that the bias current Ib increases. In this case, since the set value for the increase in the bias current is set to ± 5 steps, the control is performed so as to maintain the bias current after the 5-step increase control, and the modulation current control loop is controlled. Transition.

By shifting to the modulation current control, the switch 14 is turned on at time t6, and the current ΔIm from the modulation current second constant current source 14 is changed to the modulation current Im from the modulation current first constant current source 12. Is supplied to the laser diode 1 in a superimposed manner. As a result, the comparison between the change ΔPav of the average value Pab of the optical output power P of the laser diode 1 and the reference change ΔPavref is finished at time t7, and the switch 4 is turned off. This comparison result is reflected at time t8 and the modulation current increases. At the next time t9, the switch 14 is turned on again, the modulation current Im and the increase current ΔIm are controlled corresponding to the change, and the change ΔPav of the average value Pab of the optical output power P by the time t10, The comparison with the reference change ΔPavref is finished, and the switch 14 is turned off. This comparison result is a modulation current reflected between times t10 and t11. When such a control has passed the period Tb, that is, the set time of the external setting 2, the period Tc is shifted to the control of the next bias current control loop, and the same control as the period Ta is performed.

It is explanatory drawing of Example 1 of this invention. It is a principal part flowchart of Example 1 of this invention. It is explanatory drawing of Example 2 of this invention. It is a principal part flowchart of Example 2 of this invention. It is operation | movement explanatory drawing of Example 2 of this invention. It is characteristic explanatory drawing of a laser diode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser diode 2 Photodiode for monitoring 3 Transimpedance amplifier 4 1st comparator 5 2nd comparator 6 1st counter 7 2nd counter 8 Latch circuit 9 Adder 10 Control circuit 11 Constant current source for bias current 12 For modulation current First constant current source 13 Second constant current source for modulation current 14 Switch 29 Accumulator C1, C2 Capacitor R1-R6 Resistance Vcc Power supply voltage

Claims (3)

A constant current source for bias current that supplies a bias current to the laser diode, a first constant current source for modulation current that supplies a modulation current according to transmission data superimposed on the bias current, and a small current for the modulation current In a laser diode driving circuit including a second constant current source for modulation current to be additionally supplied and a monitoring photodiode for monitoring the optical output of the laser diode,
A bias current control loop for controlling the constant current source for bias current so as to suppress fluctuations in the average value of the optical output power of the laser diode detected by the monitoring photodiode;
Based on a control state in which the bias current in the bias current control loop is changed by a predetermined value or more, the second constant current for modulation current is parallel to the first constant current source for modulation current in parallel for a predetermined period. A power source is connected, a small current is superimposed on a modulation current and supplied to the laser diode, a variation in the average value of the optical output power of the laser diode is detected, and the first constant current source for the modulation current and the A modulation current control loop for controlling the second constant current source for modulation current;
And a control circuit for controlling contact and separation of the second constant current source for modulation current with respect to the first constant current source for modulation current.   The control circuit detects a control state in which the bias current in the bias current control loop is changed by a predetermined value or more, and connects the second constant current source for modulation current to the first constant current source for modulation current. The separation is repeatedly controlled at a cycle that is lower than the transmission speed of the transmission data, and the repetition of the contact / separation is continued for a predetermined period, and then the modulation current second constant is supplied from the modulation current first constant current source. 2. The laser diode driving circuit according to claim 1, wherein the laser diode driving circuit has a configuration for performing control for separating the constant current source.   2. The control circuit according to claim 1, further comprising a control circuit configured to allow an external setting of the predetermined value for detecting a change in the bias current that is equal to or greater than a predetermined value or a time until the bias current is equal to or greater than the predetermined value. Laser diode drive circuit.

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