JPH0513849A - Ld driving circuit - Google Patents

Abstract

PURPOSE:To optimize the value of a bias current by controlling a pulse current and the bias current. CONSTITUTION:Feedback of a bias current is constituted to keep the optical power of a laser diode 6 constant. The output current of a pulse current generating circuit 3 is changed by controlling an integrator 12. The peak value of a current wherein a pulse current is added to a bias current does not changed, when the bias current is smaller than the threshold current by which light is generated from a laser diode 6. When the bias current is larger than the threshold current, one-half value of the change of the pulse current changes. A comparator 11 detects the above difference and controls the integrator 12. When the bias current is smaller than the threshold current, the pulse current is decreased, and the bias current is increased. When the bias current is larger than the threshold current, the pulse current is increased, and the bias current is decreased. Thereby the value of the bias current of the laser diode can be set to be the optimum value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission section in optical communication, and more particularly to an LD for driving a laser diode (LD).
Regarding driver circuit.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a conventional LD driver circuit. In the figure, 1 is a terminal for inputting data, 2 is a terminal for inputting a pulse current control signal for driving an LD, 3 is no current when the input data is at a low level, and 3 is a input when the input data is at a high level. A pulse current generation circuit for driving an LD, which flows a current value corresponding to a pulse current control voltage input, 4 is a terminal to which a bias current control signal for driving the LD is input, 5 is a bias current generation circuit for driving the LD, 6
Is a laser diode (LD) whose cathode is connected to the output terminals of the pulse current generating circuit 3 and bias current generating circuit 5 and whose anode is connected to the power supply terminal 7.

Next, the operation of the LD driver circuit having the above configuration will be described. First, when the data input to the terminal 1 is at a low level, the bias current of the LD 6 increases the electric / optical conversion speed of the LD 6, and _thus is set to a threshold current I _{Bth at} which the LD 6 starts to output laser light. Next, terminal 1
With the data being input to the terminal 6, the voltage of the pulse current control signal input to the terminal 2 is set so that the optical output of the LD 6 becomes a specified value. The LD 6 can convert the level / low level to a high level / low level of light at a fast response speed, and the light output can be set to a specified value.

[0004]

Since the conventional LD driver circuit described above does not have a circuit for detecting and correcting the variation of the light emission power of the LD, the characteristics of the drive current and the light emission power of the LD are temperature. When it changes due to fluctuation, the value of the threshold current _IBth of the LD also changes, so the bias current of the LD deviates from the optimum value, the electrical / optical conversion speed of the LD slows down, and the high / low level of light is increased. However, there is a problem that the extinction ratio, which is the power ratio of, deteriorates.

[0005]

An LD driver circuit according to the present invention includes a pulse current generation circuit for outputting a pulse current for LD driving corresponding to a pulse current control voltage for LD driving according to a data input, and an LD driving circuit. Bias current generating circuit that generates a bias current corresponding to the pulse current control voltage of, and the current output obtained by adding the output of the pulse current generating circuit and the output of the bias current generating circuit, and the peak value of the added current output as the voltage value. A peak current detection circuit having a converted voltage output, a laser diode whose cathode terminal is connected to the current output of the peak current detection circuit and whose anode terminal is connected to the power supply terminal, and whose cathode terminal is connected to the power supply terminal A photodiode that monitors the optical output, and a current source that receives the anode terminal output of this photodiode. A conversion circuit, a low-pass filter to which the output of the current-voltage conversion circuit is input, and a bias current control circuit which outputs the bias current control voltage with the output of the low-pass filter as a first input and the bias current control voltage as a second input. And a voltage output of the peak current detection circuit as an input, and a hold circuit that holds the voltage output of the peak current detection circuit at the change point of the clock signal, and a voltage output of the peak current detection circuit as the first input and hold The output of the circuit is used as the second input, the subtraction circuit for subtracting the two outputs the absolute value, and the output of this subtraction circuit as the input,
A comparator circuit that compares the voltage of the threshold voltage input terminal with the output of this comparator circuit as an input, and at the transition point of the input of the clock signal, a constant voltage is integrated depending on whether the output of the comparator circuit is high level or low level, and the pulse current And an integrator that outputs a control voltage.

[0006]

According to the present invention, the bias current of the laser diode can always be maintained at the optimum value even when the characteristics of the drive current and the light emission power of the laser diode change due to temperature fluctuations.

[0007]

1 is a block diagram showing an embodiment of an LD driver circuit according to the present invention. In the figure, 8 is a terminal for inputting a threshold voltage, 9 is a terminal for inputting a clock signal, 10 is an input of the following hold circuit to the first input terminal, and the following is a peak current to the second input terminal. A subtractor circuit to which the output of the detection circuit is input, 11 is a comparator to which the threshold voltage is input to the first input terminal via the terminal 8 and is an input to which the output of the subtraction circuit 10 is input to the second input terminal, and 12 is the first A clock signal is input to the input terminal and an output of the comparator 11 is input to the second input terminal. A clock input to the first input terminal of 13 and a peak current detection circuit described below to the second input terminal. Hold circuit to which the output of 1 is input, 1
4, the bias current control signal is input to the first input terminal,
It is a bias current control circuit in which the output of the following low-pass filter is input to the second input terminal.

The output of the pulse current generating circuit 3 is input to the first input terminal 15, the output of the bias current generating circuit 5 is input to the second input terminal, and the first output terminal is connected to the cathode of the LD 6. The second output terminal is the second output of the subtraction circuit 10.
Of the peak current detection circuit connected to the second input terminal of the hold circuit 13 and the input terminal of the photodiode, 16 is a photodiode whose cathode is connected to the power supply terminal 7 and monitors the light emission power of the LD 6, and 17 is the output of the photodiode 16. A current-voltage conversion circuit for converting a current into a voltage and outputting the voltage, and a low-pass filter 18 for receiving an output of the current-voltage conversion circuit 17 and removing an AC component.

Next, the operation of the LD driver circuit having the above configuration will be described. First, when the data input to the terminal 1 is at a low level, the pulse current generating circuit 3 receives the data at the low level, so that no output current flows. Therefore, since the peak current detection circuit 15 inputs only the output of the bias current generation circuit 5, only the output current of the bias current generation circuit 5 flows to the LD 6 and laser light is output. Therefore, the photodiode 16 monitors the laser light output from the LD 6 and outputs a current value proportional to the output of the laser light. Then, the current-voltage conversion circuit 17
Converts the output current of the photodiode 16 into a voltage and outputs the voltage. Then, the low-pass filter 18 removes the AC component of this voltage and outputs it to the bias current control circuit 14.

Then, the bias current control circuit 14 controls the bias current of the bias current generating circuit 5 by the difference voltage between the output voltage of the low pass filter 18 and the bias current control input voltage input to the terminal 4. Feedback is applied so that the output voltage of the low pass filter 18 becomes equal to the bias current control input voltage input to the terminal 4. In this state, the bias current control input voltage input to the terminal 4 is adjusted so that the light emission power of the LD 6 becomes a specified value.

Next, when high-level and low-level data pulses are input to the terminal 1, the pulse current output from the pulse current generation circuit 3 is input to the peak current detection circuit 15 together with the output current of the bias current generation circuit 5, LD is added
6 and drives this LD6. Therefore, the photodiode 16 monitors the LD light emission power of the LD 6, and the monitor current includes a current corresponding to the pulse current of the pulse current generating circuit 5 as a ripple component. Since this ripple component also includes DC component, the input voltage of the bias current control circuit 14 becomes higher than the input voltage of the bias current control input to the terminal 4. Therefore, the output of the bias current control circuit 14 controls the bias current generating circuit 5 to reduce the LD drive bias current, and feedback is performed so that the light emission power of the LD 6 becomes a specified value.

On the other hand, the hold circuit 13 holds the output voltage of the peak current detection circuit 15 at the change point of the clock signal input to the terminal 9. Then, the subtraction circuit 10 subtracts the output voltage of the peak current detection circuit 15 from the output voltage of the hold circuit 13, and calculates the absolute value of the difference voltage by the comparator 11.
Output to. This comparator 11 is the subtraction circuit 10
The absolute value V _D of the output voltage difference between the output voltage and the threshold voltage V _S input to the terminal 8 is compared, and the absolute value V _{D of the} voltage difference is high (V _D > V _S
), A low level voltage is output and the absolute value of the difference voltage V _D
Is low (V _D <V _S ), a high level voltage is output.

At the changing point of the clock signal input to the terminal 9, if the output of the comparator 11 is high level, the integrator 12 lowers the output voltage by a certain level, and if the output of the comparator 11 is low level, the integration is performed. The device 12 raises the output voltage by a certain level and controls the pulse current control voltage input of the pulse current generation circuit 3.

Next, the output pulse current value of the pulse current generating circuit 3 is selected by selecting the frequency at which the AC component of the data pulse input to the terminal 1 is sufficiently attenuated by the low-pass filter 18 as the frequency of the clock signal input to the terminal 9. I _P is the peak current detection circuit 15 for converting the peak value obtained by adding the output current of the output current and the bias current generating circuit 5 of the pulse current generating circuit 3 in the case of ± [Delta] I _P varies by the control of the integrator 12 to the voltage value The absolute value of the variation amount of the voltage output can be compared with the threshold voltage input to the terminal 8 for each cycle of the clock frequency.

[0015] Then, immediately after the bias current is the output current of the generator 5 bias current I _B in the case larger than the threshold current I _Bth is the output pulse current is changed from I _P to I _P + [Delta] I _P, LD6 is I _P + I _B + ΔI _P, driven by a current of I _B (if it is driven by the I _B, emits light), the light power corresponding to [Delta] I _P / 2 is increased,
Acts feedback to the bias current generating circuit 5, bias current is stabilized to the value of I _B -ΔI _P / 2, the peak value of the current for driving the LD6 is _{I B + I P + ΔI P} / 2 becomes stable, LD6 The peak value of the current for driving is I _B + I _P + ΔI
_P / 2 and the voltage output of the peak current detection circuit 15 is Δ
Only the current value corresponding to _IP / 2 fluctuates. Then, the bias current I _B which is the output current of the bias current generating circuit 5
But immediately when the threshold current I _Bth smaller than the output pulse current is changed from I _P to I _P + [Delta] I _P, LD6 is I _P
+ I _B + ΔI _P, the current is driven in a value of I _B (if it is driven by the I _B, does not emit light), the light power corresponding to [Delta] I _P / 2 is increased, return to the bias current generating circuit , The bias current becomes stable at the value of I _B −ΔI _P , the peak value of the current driving the LD 6 becomes I _B + I _P , and the voltage output of the peak current detection circuit 15 does not change.

Therefore, if the output voltage fluctuation amount of the voltage output when the current output of the peak current detection circuit 15 fluctuates by ΔI _P / 2 is ΔV _BP / 2, the input voltage V _th of the threshold voltage input terminal 8 is 0. By setting <V _th <ΔV _BP / 2, when the bias current I _B is larger than the threshold current I _Bth , the voltage fluctuation rate of the voltage output of the peak current detection circuit 15 becomes ΔV _BP / 2, so I _B Decreases and the threshold current I
_As it approaches _Bth , I _P increases so that the optical power does not fluctuate. When the bias current I _B is smaller than the threshold current I _{B th} , the voltage fluctuation amount of the voltage output of the peak current detection circuit 15 becomes almost 0 or 0, so that I _B increases,
When approaching the threshold current I _Bth , I _P decreases so that the optical power does not fluctuate. The bias current I _B is always the threshold current I
L so that the value is close to _Bth and the optical power is the specified value.
D6 can be driven.

[0017]

As described above in detail, according to the LD driver circuit of the present invention, the photodiode for monitoring the optical output of the laser diode and the output of the photodiode keeps the optical power of the laser diode constant. A feedback circuit including a bias current control circuit and a bias current generation circuit, and a peak current detection circuit that outputs a peak value by adding the output of the bias current generation circuit and the output of the pulse current generation circuit. The drive current is controlled by a comparator that compares the peak value output from this peak current detection circuit with the threshold current emitted by the laser diode, and a control circuit that controls the increase and decrease of the bias current and the pulse current by the output of this comparator. And the emission power characteristics change due to temperature fluctuations, and the threshold current of the laser diode If the value has changed, even if the characteristics of the LD drive current and LD emission power in the region driving current is greater than the threshold current is changed, the emission power is constant, the light emitting diode of the bias current value I _B and the threshold current I _Bth Since the laser diode can be made to follow a value very close to, the laser diode can always be driven at high speed, the extinction ratio hardly deteriorates, and the number of adjustment points is small.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an LD driver circuit according to the present invention.

FIG. 2 is a block diagram showing a conventional LD driver circuit.

[Explanation of symbols]

 3 pulse current generation circuit 5 bias current generation circuit 6 laser diode 10 subtraction circuit 11 comparator 12 integrator 13 hold circuit 14 bias current control circuit 15 peak current detection circuit 16 photodiode 17 current voltage conversion circuit 18 low-pass filter

Claims (1)

Claim: What is claimed is: 1. A photodiode for monitoring the optical output of a laser diode, and a bias current control means for converting the output current of the photodiode into a voltage and then converting the voltage with the bias current control voltage. The bias current of the laser diode is controlled so that the laser power becomes equal, and the feedback current is fed back by the bias current generating means so that the optical power of the laser diode becomes constant, and the pulse current and bias current generated from the pulse current generating circuit are generated. The bias value output from the circuit is added to output a peak value, and the peak value output from this adding means and the threshold current emitted by the laser diode are compared by a comparator. When the bias current is smaller than the threshold current, the addition is performed. Since the peak value output from the means does not change, The pulse current generating circuit is controlled so as to increase the current and decrease the pulse current, and when the bias current is larger than the threshold current, the peak value output from the adding means changes by half the change in the pulse current. And a control circuit for controlling the pulse current generating circuit so as to increase the pulse current and increase the pulse current.