JPH08242031A - Semiconductor laser drive circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] To enable control of the emission intensity of a laser diode by an external input in a semiconductor laser drive circuit used for a light emitting section of a displacement sensor. [Structure] A power source is pulsed by a switching circuit 1, and the power source is supplied to a light emitting section 6 through a laser driving section 5.
The optical output of the laser diode LD is the photodiode PD.
Is monitored by, and the monitor voltage is monitored by the error amplification circuit 3
Be fed back to. The operational amplifier OP compares the reference signal of the reference voltage generating circuit 2 with the monitor voltage and gives an error signal to the transistor Tr1. At this time, when the control signal is input to the source of the FET, the value is added at the output end of the photodiode PD. Since the feedback circuit operates so that the added value here becomes equal to the reference signal, the intensity of the laser beam is decreased or increased by increasing or decreasing the control signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive circuit for controlling the intensity of a laser beam by an external input.

[0002]

2. Description of the Related Art Various displacement sensors are used to detect the position, movement, or type of a sensing object. The displacement sensor generally includes a light projecting unit that emits a laser beam and a light receiving unit that receives reflected light from a detection object.
A semiconductor laser is used in the light projecting section, and a semiconductor laser drive circuit is provided to drive the semiconductor laser.

The semiconductor laser drive circuit is also continuous light (CW light)
There are those that output pulse light and those that output pulsed light. In either case, in order to prevent a change in light output due to a rise in temperature of the laser diode LD, a photodiode PD is attached to the stem holding the laser diode LD, and the output current of the photodiode PD is monitored, so that the light output of the laser diode LD is monitored. A compensation circuit is provided to stabilize the.

FIG. 6 shows an example of a conventional semiconductor laser drive circuit having such a function. This semiconductor laser drive circuit is roughly divided into a switching circuit 1, a reference voltage generation circuit 2, an error amplification circuit 3, a charge / discharge circuit 4, a laser drive unit 5, and a light emitting unit 6.

The structure of the light emitting portion 6 to the switching circuit 1 will be described. The light emitting section 6 has a laser diode LD and a photodiode PD (light receiving element) attached to the same stem, a protection resistor R1 is connected to the cathode side of the laser diode LD, and a monitoring resistor R2 is connected to the anode side of the photodiode PD. Are connected. Both ends of these resistors R1 and R2 are connected to the GND line,
The anode side of the laser diode LD and the cathode side of the photodiode PD are commonly connected.

The laser drive section 5 is a circuit for supplying a drive current to the light emitting section 6, and is composed of an NPN type transistor Tr1. The emitter of the transistor Tr1 is the light emitting section 6
The collector is connected to the output end of the switching circuit 1 and the base is connected to the output end of the error amplification circuit 3. The charging / discharging circuit 4 is composed of a resistor R3 and a capacitor C1 connected in parallel, and is a circuit that regulates the rise and fall time constants of the pulse current of the laser diode LD by charging and discharging the capacitor C1.

The error amplification circuit 3 receives the monitor signal Vm obtained from both ends of the resistor R2 and the reference signal Vr generated by the reference voltage generation circuit 2, and amplifies the difference value between them to generate a correction signal. Circuit. The operational amplifier OP in the error amplifier circuit 3 has a monitor signal Vm at its non-inverting input terminal.
Is input and the reference signal Vr is input to the inverting input terminal, the difference value between them is amplified and the error signal Ve is supplied to the base of the transistor Tr2. NPN type transistor T
A resistor R4 and a diode D2 are connected in series between the collector of r2 and the output terminal of the switching circuit 1. The emitter of the transistor Tr2 is connected to the GND line via the resistor R5. The charge / discharge circuit 4 is connected between the cathode side of the diode D2 and the GND line. A noise preventing capacitor C2 is connected to the output terminal and the inverting input terminal of the operational amplifier OP.

The reference voltage generation circuit 2 is a circuit for generating a reference signal (pulse) Vr to the error amplification circuit 3. A resistor R8 is provided between the output end of the switching circuit 1 and the GND line.
And the constant voltage diode D3 are connected in series, and a voltage pulse having a peak value equal to the reference voltage of the constant voltage diode D3 is generated at both ends of the constant voltage diode D3. The voltage of this pulse is divided by the resistors R6 and R7, and is applied to the inverting input terminal of the operational amplifier OP as the reference signal Vr.

The switching circuit 1 is a power source (voltage + Vc)
Is a circuit that opens and closes based on the input pulse SP, and is configured by a switching transistor Tr3 and a resistor R9. Here, for example, when the transistor Tr3 is a PNP type, when the input pulse SP is at L level, it is turned on, and the pulse voltage is supplied to the collector side via the resistor R9 connected to the emitter.

The operation of the conventional semiconductor laser drive circuit thus configured will be described. First, by switching the power supply voltage Vc by the transistor Tr3, a pulse voltage having the voltage Vc is supplied to the reference voltage generation circuit 2, the error amplification circuit 3, and the laser drive unit 5. Input pulse S
When P is at L level, the transistor Tr3 becomes conductive and the capacitor C1 of the charge / discharge circuit 4 is charged via the resistor R4 and the diode D2. The charging current at this time changes depending on the conduction state of the transistor Tr2. Now, assuming that the error signal Ve output from the operational amplifier OP is 0V,
The transistor Tr2 is turned off or operates with a bias current. In this case, the base voltage Vb1 of the transistor Tr1 after the completion of charging becomes the value of the following expression (1). Vb1 = (R3 + r2) / (R3 + R4 + r2) .Vc (1) where r2 is the forward resistance when the diode D2 is on.

The value of the voltage Vb1 in this case is a value for making the transistor Tr1 having an emitter load conductive, and a drive current is supplied to the laser diode LD. When the laser diode LD emits light, a monitor current Im proportional to the laser emission intensity flows through the photodiode PD. This current Im is converted into a voltage by the resistor R2, and the monitor signal Vm of the following equation (2) is input to the non-inverting input terminal of the operational amplifier OP. Vm = Im · R2 (2)

On the other hand, when the input pulse SP is at the L level, the reference signal Vr as shown in the following equation (3) is applied to the inverting input terminal of the operational amplifier OP. Vr = R6 / (R6 + R7) .Vz (3) where Vz is the reference voltage of the diode D3.

Generally, when the operational amplifier OP operates in a linear region with negative feedback applied through an external circuit, its output value is controlled so that the voltages at the non-inverting input terminal and the inverting input terminal become equal to each other. It On the other hand, the transistor Tr
Since 2 operates with the error signal Ve output from the operational amplifier OP, if the value of the error signal Ve is positive, the transistor Tr
The emitter current of 2 increases. If the value of the error signal Ve is negative, the emitter current of the transistor Tr2 decreases.
Therefore, the base voltage Vb1 of the transistor Tr1 drops if the value of the error signal Ve is positive, and rises if the value of the error signal Ve is negative. The positive value of the error signal Ve means that the radar diode LD
Of the error signal V is higher than the reference value.
A negative e value means that the emission intensity is lower than the reference value. Therefore, when the values of the reference signal Vr and the monitor signal Vm are balanced, the emission intensity of the laser diode LD is the reference signal Vr generated by the reference signal generation circuit 2.
Will be controlled to a value proportional to.

[0014]

As described above, the emission intensity of the laser diode LD is controlled to a constant value by keeping the value of the monitor current Im output from the photodiode PD constant. Laser diode LD here
Consider the case where the emission intensity of is changed. The surface state of the detection object at the displacement center changes variously depending on its type.
In particular, when the type of the detected object changes and the reflectance changes significantly, the emission intensity of the light emitting unit 6 must be adjusted each time. However, with the configuration shown in FIG. 6, in order to change the emission intensity of the laser diode LD, the value of the reference signal Vr input to the error amplification circuit 3 is changed or the monitor signal Vm of the photodiode PD is changed. You have to change the value.

Here, a method of changing the value of the reference signal Vr will be briefly described. The value of the reference signal Vr may be increased to increase the light emission intensity of the light emitting unit 6, and the value of the reference signal Vr may be decreased to reduce the light emission intensity. However, the voltage of the reference signal Vr is usually around 1V, and if it exceeds 1V, control becomes difficult. Further, if the voltage of the reference signal Vr is reduced, it becomes easy to be affected by the offset voltage of the operational amplifier OP. Therefore, when the emission intensity is controlled by changing the value of the reference signal Vr, the control range is limited.

Next, there is a method of changing the value of the monitor signal Vm. Basically, the resistance R2 may be changed from a fixed resistance to a variable resistance. However, in this case, in order to change the resistance value of the resistor R2, the adjusting screw must be directly operated by hand, and there is a problem in the workability of adjustment in the semiconductor laser driving device attached to the small displacement sensor. It is also conceivable to control the voltage Vc of the power supply, but the power supply voltage used for the displacement sensor has a specified value, and adjustment in this part was practically impossible.

The present invention has been made in view of such conventional problems, and a compensation circuit for setting the emission intensity of the laser diode LD by an electric signal and for temperature compensation and stabilization of the emission intensity. It is an object of the present invention to realize a semiconductor laser driving circuit with a mark.

[0018]

According to the present invention, a laser light emitting element for outputting a laser beam, a light receiving element for monitoring the light emission intensity of the laser light emitting element, a laser driving section for driving the laser light emitting element, and a reference signal are generated. The reference voltage generator, the reference signal of the reference voltage generator, and the monitor signal of the light receiving element are input, the signals are differentially amplified, and the difference signal is output to the laser driver as a control signal. And a light emission intensity control signal for adjusting the light emission intensity of the laser light emitting element is input, and the light emission intensity control signal is superimposed on the monitor signal input to the error amplification unit. It is characterized in that a control unit is provided.

[0019]

According to the present invention having such a feature, when the laser light emitting element outputs the laser beam by the laser driving section, the light receiving element generates a monitor signal proportional to the light emission intensity of the laser light emitting element. When the reference signal of the reference voltage generator and the monitor signal of the light receiving element are input, the error amplifier differentially amplifies these signals and outputs the difference signal as a control signal to the laser driver. At this time, when the emission intensity control signal for adjusting the emission intensity of the laser light emitting element is input by the emission intensity control unit, the emission intensity control signal is superimposed on the monitor signal input to the error amplification unit. This makes it possible to control the light emission intensity of the laser light emitting element by an external input.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A semiconductor laser drive circuit according to a first embodiment of the present invention will be described with reference to FIGS. still,
The same blocks and the same signal names as those in the conventional example are designated by the same reference numerals, and detailed description thereof will be omitted. FIG. 1 is a block diagram showing the basic configuration of the semiconductor laser drive circuit of the first embodiment. As shown in this figure, the semiconductor laser drive circuit has a switching circuit 1, a reference voltage generation circuit 2, a laser drive unit 5, and a light emitting unit 6 as in the conventional example. The portion different from the conventional example is provided with a light emission intensity control unit 7A, and the output of the first control current Id is the photodiode P.
This is to be added to the monitor current Im of D, and the added current is given to the error amplification circuit 3 as the second control current Ip.

The emission intensity control section 7A is composed of a field effect transistor (hereinafter referred to as FET). The drain electrode of the FET is connected to the output terminal of the switching circuit 1,
The source electrode is connected to the adding circuit. Then, the emission intensity control signal Vg for adjusting the emission intensity to the gate electrode
Is entered.

FIG. 2 is a graph showing the input / output characteristics of the FET. As shown in the figure, the control current Id, which is the drain current, can be controlled by changing the control signal Vg, which is the gate-source voltage of the FET.

When the FET is off, the control current Id which is the drain current does not flow, and the following equation (4) is established. Ip = Im (4) When the FET is turned on and the control current Id flows and the monitor current changes to Im ′, the following equation (5) is established. Ip = Im '+ Id ... (5)

Output of reference voltage generating circuit 2 (reference signal V
As long as r) does not change, the value of the control current Ip does not change due to the operation principle of the error amplification circuit 3. Therefore, the monitor current Im 'is reduced by the control current Id as compared with Im as shown in the following equation (6). Im ′ = Ip−Id (6) Therefore, the emission intensity of the laser diode LD is controlled by the control current I.
It becomes smaller by the amount corresponding to d. Thus, the emission intensity of the laser diode LD can be controlled by changing the control signal Vg.

A concrete example of the semiconductor laser drive circuit having the above-described operation principle is shown in FIG. In this figure, the difference from FIG. 6 of the conventional example is that the above-mentioned FET is provided between the output end of the switching circuit 1 and the anode side of the photodiode PD. FE as in the case of FIG.
When the control signal Vg is applied to the gate electrode of T, the drain current flows when the switching circuit 1 is conducting. When this current is taken as the control current Id, the photodiode PD
Is added to the monitor current Im of, and is connected to GND via the resistor R2.
Flowing to the side. Therefore, the terminal voltage of the resistor R2 is set to the monitor voltage V
If m, this voltage is input to the non-inverting input terminal of the operational amplifier OP.

When the control signal Vg input from the outside is increased, the drain current of the FET is increased, so that the value of the monitor signal Vm input to the operational amplifier OP is expressed by the following equation (7). Vm = R2 · (Im + Id) (7) Since the feedback circuit operates so that the positive and negative inputs of the operational amplifier OP are the same as described above, the monitor signal Vm
The value of the error signal Ve output from the operational amplifier OP is increased as the value of is increased. Therefore, the base voltage of the transistor Tr1 drops, and the emission intensity of the laser diode LD drops. By changing the value of the control signal Vg in this manner, the light emission intensity of the light emitting unit 6 can be controlled. In the case of the present embodiment, the light emission intensity of the light emitting unit 6 decreases as the control signal Vg increases.

As described above, according to the first embodiment, the emission intensity can be controlled by operating the control signal Vg from a remote location, and it is not necessary to adjust the proximity sensor itself. Further, the circuit to be added is only the FET, and the conventional circuit having a high use record can be used as it is.

Next, a semiconductor laser drive circuit according to the second embodiment of the present invention will be described with reference to FIGS. The same blocks and the same signal names as those in the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted. FIG.
FIG. 6 is a block diagram showing a basic configuration of a semiconductor laser drive circuit of a second embodiment. As shown in this figure, the semiconductor laser drive circuit has a switching circuit 1, a reference voltage generation circuit 2, an error amplification circuit 3, a laser drive unit 5, a light emission unit 6, and a light emission intensity control unit 7B as in the first embodiment. It is the same.

The emission intensity control section 7B is composed of a FET,
The source electrode is connected to GND, and the drain electrode is connected to the non-inverting input terminal of the error amplification circuit 3. Further, a control signal Vh (emission intensity control signal) for controlling the emission intensity is input to the gate electrode. The first control current Id that is the drain current of the FET is subtracted from the monitor current Im obtained from the photodiode PD, and the second control current Ip that is the subtraction result is supplied to the error amplification circuit 3. This is a feature of this embodiment.

When the FET is off, the control current Id does not flow and the following equation (8) is established. Ip = Im (8) When the FET is turned on and the control current Id flows and the monitor current changes to Im ′, the following expression (9) is established. Ip = Im'-Id (9)

Output of the reference voltage generating circuit 2 (reference signal V
As long as r) does not change, the value of the control current Ip does not change due to the operation principle of the error amplification circuit 3. Therefore, the monitor current Im 'increases by the value of the control current Id as compared with Im, as shown in the following expression (10). Im ′ = Ip + Id (10) Therefore, the emission intensity of the laser diode LD is controlled by the control current I.
It is increased by a value corresponding to d.

A concrete example of the semiconductor laser drive circuit having the above-described operation principle is shown in FIG. In this figure, the parts different from FIG. 3 are the anode side of the photodiode PD and the G side.
That is, the FET and the resistor R10 are provided in series with the ND. Similar to FIG. 3, the control signal Vh is applied to the gate electrode of the FET.
Is given, as long as the switching circuit 1 is conducting,
The transistor Tr1 also becomes conductive. Photodiode PD
If the monitor current of Im is Im, a part of the current is FET
Flows toward. Therefore, the control current Ip flowing through the resistor R2
Decreases, and the terminal voltage (monitor voltage Vm) decreases.

When the control signal Vh input from the outside is increased, the source current of the FET is increased. Therefore, the value of the monitor signal Vm input to the operational amplifier OP is as follows (1)
It becomes like the formula 1). Vm = R2 · (Im−Id) (11) As described above, since the feedback circuit operates so that the positive and negative inputs of the operational amplifier OP are the same, the monitor signal V
When the value of m decreases, the value of the error signal Ve output from the operational amplifier OP decreases. Therefore, the base voltage of the transistor Tr1 rises and the light emission intensity of the laser diode LD increases. Therefore, the emission intensity of the light emitting unit 6 can be controlled by changing the value of the control signal Vh.
In the case of the present embodiment, the relationship between the value of the control signal Vh and the light emission intensity of the light emitting section 6 is almost directly proportional.

In the above-mentioned first and second embodiments, the case where the laser diode LD is pulse-driven by the operation of the switching circuit 1 has been described, but the same effect can be obtained also in the semiconductor laser drive circuit of the DC modulation system. . in this case,
It can be considered to be equivalent to the operation when the input pulse SP of the switching circuit 1 becomes L level, and the DC emission intensity of the laser diode LD can be similarly controlled by the control signals Vg and Vh. Therefore, the switching circuit 1 is not an essential component of the present invention.

[0035]

As described above, according to the present invention, the light emission intensity of the laser light emitting element can be controlled by an external input signal only by adding a light emission intensity control unit to a part of the conventional semiconductor laser drive circuit. You can In this case, the emission intensity of the laser light emitting element can be adjusted in the increasing direction and the decreasing direction without being affected by the offset voltage of the error amplification section. Therefore, when this semiconductor laser drive circuit is used for a displacement sensor,
The intensity of the laser emitting element can be adjusted remotely in accordance with the property of the reflecting surface of the sensing object.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of a semiconductor laser drive circuit according to a first embodiment of the present invention.

FIG. 2 is an input / output characteristic diagram of an FET used in each example of the present invention.

FIG. 3 is a circuit diagram showing a specific example of the semiconductor laser drive circuit of the first embodiment.

FIG. 4 is a basic configuration diagram of a semiconductor laser drive circuit according to a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing a specific example of a semiconductor laser drive circuit according to a second embodiment.

FIG. 6 is a circuit diagram showing a configuration of a conventional semiconductor laser drive circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 switching circuit 2 reference voltage generation circuit 3 error amplification circuit 4 charge / discharge circuit 5 laser drive section 6 light emitting section 7A, 7B emission intensity control section R1 to R10 resistors C1 and C2 capacitors Tr1 to Tr3 transistor LD laser diode PD photodiode D3 constant Voltage diode Vm monitor signal Vr reference signal Ve error signal Vg, Vh control signal Id first control current Ip second control current Im monitor current

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Zhang Tateiwa, Omron Co., Ltd. 10 Hanazono Dodo-cho, Ukyo-ku, Kyoto City, Kyoto Prefecture

Claims (3)

[Claims] 1. A laser light emitting element that emits laser light, a light receiving element that monitors the light emission intensity of the laser light emitting element, a laser drive unit that drives the laser light emitting element, and a reference voltage generation unit that generates a reference signal. A reference signal of the reference voltage generator and a monitor signal of the light receiving element are input, the signals are differentially amplified, and an error amplifier that outputs the difference signal to the laser driver as a control signal, In a semiconductor laser drive circuit comprising: a light emission intensity control signal for adjusting a light emission intensity of the laser light emitting element is input, and a light emission intensity control for superimposing the light emission intensity control signal on a monitor signal input to the error amplification unit. A semiconductor laser drive circuit having a portion. 2. The light emission intensity control unit includes a field effect transistor having a drain electrode connected to a power supply line of the laser drive unit and a source electrode connected to a monitor terminal of the light receiving element. A light emission intensity control signal for adjusting the light emission intensity of the laser light emitting element is input to a gate electrode of a transistor, and the light emission intensity control signal is added to a monitor signal input to the error amplification unit. Item 2. The semiconductor laser drive circuit according to item 1. 3. The light emission intensity control unit includes a field effect transistor having a drain electrode connected to a monitor output end of the light receiving element and a source electrode connected to a ground line of the laser driving unit. 2. The semiconductor device according to claim 1, wherein a light emission intensity control signal for adjusting the light emission intensity of the laser light emitting element is input to the gate electrode of the type transistor, and the light emission intensity control signal is subtracted from the monitor signal. Laser drive circuit.