JP2002353554A - Optical-output control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical-output control circuit which can respond at a high speed. SOLUTION: A constant-current power-supply circuit CC1, which is operated by an external control signal, is installed at the optical-output control circuit in a conventional laser diode, and a capacitor C1 in a filter circuit FC1 is charged only for a fixed time at the output operation of an operational amplifier AMP1. Consequently, a gradual increase in the optical output of an LD 1 will not be generated, the capacitor C1 in the filter circuit FC1 is charged quickly corresponding nearly to the transient period portion of a change in an output voltage, and the optical-output control circuit can respond at a high speed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a light output control circuit.

[0002]

2. Description of the Related Art FIG. 6 shows a conventional laser diode (LD).
It is a control circuit.

[0003] This LD control circuit is a control circuit for controlling the optical output power of the LD 1, and the monitor beam P of the LD 1 is controlled.
2, an inverting input terminal is connected to the anode of PD1, and a non-inverting input terminal is connected to the operational amplifier AMP1 as a comparator connected to the anode of the reference voltage source E2, and to the output terminal of the operational amplifier AMP1. It comprises a filter circuit FC1 and a transistor Q1 whose base is connected to the output terminal of the filter circuit FC, whose emitter is grounded via a resistor R0, and whose collector is connected to the cathode of LD1.

The filter circuit FC1 includes an operational amplifier AMP
1 and a resistor R2 connected between the output terminal of the transistor Q1 and the base of the transistor Q1, and a capacitor C1 having one end grounded and the other end connected between the base of the transistor Q1 and the resistor R2. This is for removing a damped oscillation component that occurs transiently at the time of output of the operational amplifier AMP1. E1 is a power source for the LD1 and PD1, and R1 is a bias resistor. Also, PD1 and LD
1 constitute an LD module M1.

When this control circuit operates, a part of the optical power P1 (monitor beam P2) output from the LD 1 becomes
The light / current is converted by PD1 in the LD module M1 (I
1) flows through the resistor R1. The voltage V1 (= I1 × R1) generated at both ends of the resistor R1 is compared with the reference voltage source E2 by the operational amplifier AMP1, and after the difference is amplified (V1
2) A transistor Q1 for controlling a current flowing through LD1
(V3). Since the current I2 is proportional to the voltage V2, if the light output of the LD1 is smaller than a predetermined value, the voltage V2
Becomes higher, the current I2 increases, and operates to increase the light output.

On the contrary, if the light output of the LD 1 is large, the voltage V2 becomes low, the current I2 decreases, and the operation is performed so as to reduce the light output.

By repeating the above operation, LD1
Has a constant light output. This operation is shown in the characteristics when there is no filter circuit in FIG.

FIG. 7 is a graph showing the LD control voltage / light output characteristics. The horizontal axis shows the time axis, and the vertical axis shows the light output axis or control voltage axis of the LD.

[0009]

In FIG. 7, the time T1 required for the optical output to settle to a constant value while oscillating becomes a factor in transmission of a signal (occurrence of transmission error). Therefore, a filter circuit FC1 is inserted between the operational amplifier AMP1 and the transistor Q1 so that the light output has a characteristic of a gradual simple increase tendency (FIG. 6).

However, the characteristic that the light output gradually increases has a problem that it cannot be applied to an application requiring a response in a time period of T1 or less.

It is an object of the present invention to solve the above-mentioned problems and to provide an optical output control circuit capable of high-speed response.

[0012]

In order to achieve the above object, an optical output control circuit according to the present invention comprises a photodiode for detecting a monitor beam of a laser diode and a photodiode for comparing an output voltage of the photodiode with a reference voltage. Output control circuit, comprising: a filter, a filter circuit for removing a damped oscillation component generated transiently at the time of output of the comparator, and a transistor for controlling the optical output power of the laser diode according to the output voltage from the filter circuit. Wherein a constant current power supply circuit is provided for supplying a current to the filter circuit for a certain period of time when the comparator outputs in response to an external control signal.

In addition to the above configuration, the filter circuit of the optical output control circuit of the present invention has a capacitor for absorbing a damped oscillation component that occurs transiently at the time of output from the comparator. It is preferable to supply a current that allows the capacitor to be charged without excess or deficiency for a time corresponding to the transition period of the output voltage.

In addition to the above configuration, it is preferable that the comparator of the light output control circuit of the present invention be operated when an external control signal is input, and be stopped when there is no external control signal.

According to the present invention, a conventional laser diode light output control circuit is provided with a constant current power supply circuit operated by an external control signal, and charges a capacitor of a filter circuit for a fixed time when a comparator outputs. Accordingly, the optical output of the laser diode does not increase gently, and the capacitor of the filter circuit is charged faster by the transition period of the output voltage change, thereby enabling a high-speed response.

[0016]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing one embodiment of the light output control circuit of the present invention. FIG. 2A is a diagram showing control signals of the light output control circuit shown in FIG.
2B is a diagram showing a charging current of the light output control circuit shown in FIG. 1, and FIG. 2C is a diagram showing a control voltage of the light output control circuit shown in FIG. The same members as those in the conventional example shown in FIG.

The LD control circuit shown in FIG. 1 is a control circuit for controlling the optical output power of the LD 1. The PD 1 receives the monitor beam P2 of the LD 1, the inverting input terminal is connected to the anode of the PD 1, and the non-inverting input terminal is connected. The terminal is the reference voltage source E2
The operational amplifier AMP1 connected to the anode of the first amplifier and the filter circuit FC1 connected to the output terminal of the operational amplifier AMP1
And an NPN transistor (a PNP transistor may be used) Q1 having a base connected to the output terminal of the filter circuit FC1, an emitter grounded via a resistor R0, and a collector connected to the cathode of the LD1. An output terminal is connected to an output terminal of the filter circuit FC1, and the charging circuit (constant current source circuit CC1) is operated by a control signal from the outside. Filter circuit FC1
Is the output terminal of the operational amplifier AMP1 and the transistor Q1.
And a capacitor C1 connected at one end to the ground and at the other end between the base of the transistor Q1 and the resistor R2.
The operational amplifier AMP1 is controlled to be ON / OFF by an external control signal.

When a control signal is externally input to the optical output control circuit shown in FIG. 1 (FIG. 2A), the operational amplifier AMP
1 and the charging circuit CC1 starts charging the capacitor C1 (FIG. 2 (b)), and the control voltage V3 is set to the transistor Q1.
1 (FIG. 2C). As a result, the time required for the control voltage to reach V3 from 0 is reduced from T1 to T
Reduced to 2.

The details will be described below.

Assuming that a current for charging the capacitor C1 via the resistor R2 is I3, a charging time T1 is expressed by the following equation (1).

[0022]

T1 = V3 × C1 / I3 (sec) The charging time T1 is determined by the amount of charging current to the capacitor C1.

Therefore, in the present invention, a charging circuit for charging the capacitor C1 for a certain period of time is added in accordance with the timing of starting the light emission of the LD1 with the charging current I4 from outside. Assuming that the time when the charging of the capacitor C1 is completed is T2, the charging completion time T2 is expressed by the following equation (2).

[0024]

## EQU2 ## T2 = V3.times.C1 / (I3 + I4) (sec) As shown in FIG. 3, the time can be reduced (the circuit operation can be speeded up).

FIG. 3 is a characteristic diagram of the control voltage and the LD light output of the light output control circuit shown in FIG. 1, and shows the I4 optimum value characteristic of the charging current. In FIG. 3, the horizontal axis indicates the time axis, and the vertical axis indicates the control voltage axis and the LD light output axis.

The optimum value of the charging current I4 from the outside can be obtained from Equation 3 once the saturation time T2 is determined.

[0027]

I4 = (V3.times.C1 / T2) -I3 (A) The characteristics at the time of overcharging and undercharging by the charging current I4 are also shown in FIG.

[0028]

FIG. 4 is a block diagram showing an embodiment of a light output control circuit according to the present invention.

This light output control circuit is an LD module 1
And a conventional LD control circuit including an operational amplifier AMP1, a transistor Q1 and the like, and a charging circuit CC1 which starts charging the capacitor C1 by an external control signal. That is, the present optical output control circuit comprises a PD1 for receiving the monitor beam P2 of the LD1, an operational amplifier AMP1 having an inverted input terminal connected to the anode of the PD1, and a non-inverted input terminal connected to the anode of the reference voltage source E2.
A filter circuit FC1 connected to the output terminal of the operational amplifier AMP1, an NPN type having a base connected to the output terminal of the filter circuit FC1, an emitter grounded via a resistor R0, and a collector connected to the cathode of the LD1. A transistor (a PNP transistor may be used) Q1 and a charging circuit (constant current source circuit) CC1 having an output terminal connected to an output terminal of the filter circuit FC1 and operated by an external control signal. I have.

The charging circuit CC1 includes a one-shot IC1 composed of a one-shot multivibrator that outputs one pulse having a fixed time width when a control signal is input, and a resistor R3 connected to the output terminal of the one-shot IC1. PNP connected to the power supply E1 via the pull-up resistor R4 to the emitter, the collector is grounded via the resistor R6, and connected to the output terminal (base of the transistor Q1) of the filter circuit FC1. (A NPN transistor may be used instead of a PNP transistor). Note that the resistor R5 and the capacitor C2 determine the time constant (pulse width) of the one-shot IC1.

When a control signal is input to the optical output control circuit, the operational amplifier AMP1 is activated, and the LD1, LD1,
A closed circuit of the PD1, the operational amplifier AMP1, the resistor R2, and the transistor Q1 is formed, and the charging current I3 is immediately supplied from the operational amplifier AMP1 to the capacitor C1 so that the optical output of the LD1 becomes a predetermined optical output, and the charging is started. You. At the same time, in the charging circuit CC1, a time (T) determined by the capacitor C2 and the resistor R5 is triggered by the control signal.
2) A pulse signal is output for minutes and input to the transistor Q2. The voltage / current conversion is performed by the transistor Q2, and the current I
4 is supplied to the capacitor C1 to start charging.

As described above, by charging the capacitor C1 with the two circuits, the LD light output power is controlled to have a constant value in a shorter time than in the conventional case.

In this embodiment, the charging current (I
4) was a constant continuous (FIG. 5 (a)).
As a modified example, time control of the current value as shown in (b) and (c) can be considered. That is, even if the charging current is pulsed (FIG. 5B), the charging current is triangular (FIG. 5B).
(C)).

FIG. 5A is a diagram showing a waveform of a charging current from the charging circuit shown in FIG. 1, and FIG.
FIG. 5C is a diagram showing a modification of the charging current from the charging circuit shown in FIG. 5, and FIG. 5C is a diagram showing another modification of the charging current from the charging circuit shown in FIG. 5A to 5C, the horizontal axis represents the time axis, and the vertical axis represents the charging current axis.

[0035]

In summary, according to the present invention, the following excellent effects are exhibited.

It is possible to provide a light output control circuit capable of high-speed response.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a light output control circuit of the present invention.

2A is a diagram showing a control signal of the light output control circuit shown in FIG. 1; FIG. 2B is a diagram showing a charging current of the light output control circuit shown in FIG. 1; 2) is a diagram showing a control voltage of the light output control circuit shown in FIG.

FIG. 3 shows a control voltage and L of the light output control circuit shown in FIG.
It is a characteristic view of D light output.

FIG. 4 is a block diagram showing one embodiment of a light output control circuit of the present invention.

5A is a diagram showing a waveform of a charging current from the charging circuit shown in FIG. 1, and FIG. 5B is a diagram showing a modification of a charging current from the charging circuit shown in FIG. , (C) shows FIG.
FIG. 10 is a diagram showing another modification of the charging current from the charging circuit shown in FIG.

FIG. 6 shows a conventional laser diode (LD) control circuit.

FIG. 7 is a diagram showing LD control voltage / light output characteristics.

[Explanation of symbols]

 AMP1 Operational amplifier (comparator) C1, C2 Capacitor CC1 Charging circuit (constant current source circuit) E1 Power supply E2 Reference voltage source LD1 Laser diode PD1 Photodiode Q1, Q2 Transistors R0 to R6 Resistors

Claims (3)

[Claims] 1. A photodiode for detecting a monitor beam of a laser diode, a comparator for comparing an output voltage of the photodiode with a reference voltage, and a damped oscillation component transiently generated at the time of output of the comparator is removed. And a transistor for controlling the optical output power of the laser diode in accordance with the output voltage from the filter circuit. An optical output control circuit comprising a constant current power supply circuit for supplying current to the circuit. 2. The method according to claim 1, wherein the filter circuit includes a capacitor for absorbing a damped oscillation component generated transiently at the time of output of the comparator, and the constant current power supply circuit operates during a transition period of the output voltage of the comparator. 2. The light output control circuit according to claim 1, wherein the capacitor supplies a current capable of charging the capacitor for an appropriate amount of time. 3. The light output control circuit according to claim 1, wherein the comparator is activated when the external control signal is input, and is stopped when there is no external control signal.