JPH03232286A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To improve a semiconductor laser in safety by a method where an abnormality detection circuit and a check circuit which previously checks if the abnormality detection circuit works normally are provided. CONSTITUTION:A setting circuit 104 is connected to an abnormality detection circuit 105, and the abnormality detection circuit 105 detects the abnormality of an automatic light quantity control mechanism by comparing an upper limit signal with a monitored signal and outputs an abnormality signal LDOVER. The abnormality detection circuit 105 is connected to a check circuit 106, and when a laser diode 1a operates normally through the action of the automatic light quantity control mechanism, the upper limit signal is made to decrease to a certain value smaller than a reference signal responding to a check signal LDCHK to enable the detection circuit 105 to falsely output the abnormality signal LDOVER, whereby the function of the abnormality detection circuit 105 is periodically ascertained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser drive circuit, and particularly to an abnormality detection technique thereof.

[Conventional technology]

In recent years, high-performance semiconductor lasers have become available at low cost and are used as components for various information devices. For example, it can be used as a laser beam light source in the information reading head of a compact disc player, the information reading/writing head of an optical disc memory, the reading head of a barcode reader used in a PO8 terminal, a laser beam pointer, or a print head of a laser beam printer. Widely used. In these various information devices, the semiconductor laser drive circuit generally includes a laser beam light amount automatic control circuit to keep the laser beam light amount constant.

[Problem that the invention seeks to solve]

However, such an automatic light amount control circuit is composed of a plurality of circuit components, and cannot necessarily be said to be completely free of failures. In such a case, automatic light control will not work,
There is a risk that the semiconductor laser will emit excessive light. There is a problem in that if the excessive light emitted directly enters the human eye, it may cause blindness. Furthermore, in optical disk memories and laser beam printers, there is a problem in that there is a risk of information being destroyed if excessive light emission occurs.

[Means for solving problems]

In view of the above-mentioned problems of the conventional technology, an object of the present invention is to improve the safety of a semiconductor laser drive circuit. In order to achieve this objective, the present invention includes an abnormality detection circuit for detecting abnormalities in the automatic control circuit. A check circuit is also provided to check in advance whether this abnormality detection circuit can operate correctly when the automatic light amount control circuit is abnormal. By taking such double measures, the safety of the semiconductor laser drive circuit is significantly improved compared to the conventional one. Generally, the probability that an automatic light amount control circuit will fail is low, and an abnormality detection circuit will only operate infrequently. Therefore, it is extremely important to periodically check whether the abnormality detection circuit is operating correctly.

FIG. 1 shows the basic configuration of a semiconductor laser drive circuit according to the present invention. As shown in the figure, the semiconductor laser drive circuit has a laser package 1. This laser package 1 includes a semiconductor laser such as a laser diode 1a that emits a laser beam according to driving power, and a light receiving element such as a PI that photoelectrically converts the laser beam and outputs a corresponding electric signal.
It has a built-in N photodiode 1b. A monitor circuit 101 is connected to the photodiode 1b, which monitors the electrical signal and outputs a monitor signal according to fluctuations in the amount of laser beam light. The monitor circuit 101 has a control circuit 10
2 is connected, and compares the monitor signal with a predetermined reference signal and outputs a control signal according to the difference. Control circuit 1
A power circuit 103 is connected to 02, and supplies drive power to the laser diode 1a in accordance with a control signal so as to cancel out the difference between the monitor signal and the reference signal. These laser package 1, monitor circuit 101, control circuit 102, and power circuit 103 constitute an automatic light amount control mechanism consisting of a servo loop, and control the laser beam light amount of the laser diode 1a to be constant in a steady state.

The semiconductor laser drive circuit further includes a setting circuit 104. The setting circuit 104 sets an upper limit signal that is larger than the reference signal and has a magnitude that the monitor signal does not exceed under normal conditions. A reference signal is also set accordingly. An abnormality detection circuit 105 is connected to the setting circuit 104, and detects an abnormality in the automatic light amount control mechanism by comparing the upper limit signal and a monitor signal, and outputs an abnormality signal LDOVER. A check circuit 10B is connected to the abnormality detection circuit 105, and when the laser diode 1a is lit normally due to the action of the automatic light amount control mechanism, a check signal LDCHK is output.
In response to this, the operation of the abnormality detection circuit 105 is periodically checked by lowering the magnitude of the upper limit signal to below the reference signal and outputting a pseudo abnormality signal LDOVER.

This abnormality detection circuit includes a comparison circuit that outputs an inverted signal when the monitor signal exceeds the upper limit signal, and a latch circuit that stores the inverted signal and continuously outputs the abnormal signal. In addition, the check circuit outputs a check signal L
A switching circuit that switches the magnitude of the upper limit signal to the magnitude of the reference signal or less according to DCHK, and a check signal LDCHK.
and a reset circuit for resetting the latch circuit in response to the release of the error signal LDOVER and canceling the pseudo-output abnormality signal LDOVER. Finally, preferably, the semiconductor laser drive circuit includes a forced extinguishing circuit for forcibly extinguishing the laser diode 1a in response to the abnormal signal LDOVER.

[For production]

According to the present invention, the abnormality detection circuit always monitors the monitor signal with respect to the upper limit signal, and when the monitor signal exceeds the upper limit signal, outputs an abnormal signal to prevent the laser diode 1a from emitting excessive light. There is. Further, the check circuit periodically checks the abnormality detection circuit 105 so that the abnormality detection circuit can correctly detect an abnormality when an abnormality occurs in the automatic light amount control mechanism.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. FIG. 2 is a detailed circuit diagram of a semiconductor laser drive circuit according to the present invention. As shown in the figure, the semiconductor laser drive circuit has a laser package 1. A laser diode 1a and a photodiode 1b are built into the laser package 1 in close proximity to each other. Laser diode 1a
The anode terminal of the photodiode 1b and the cathode terminal of the photodiode 1b are both connected to the power supply line V.

C. The laser diode 1a emits a laser beam according to the drive current, and the photodiode 1b receives the emitted laser beam, photoelectrically converts it, and outputs a corresponding current signal.

In addition to the laser package 1, the semiconductor laser drive circuit includes:
It has a monitor circuit, a control circuit, and a power circuit, and together constitutes an automatic light amount control mechanism. The monitor circuit consists of a current-voltage conversion resistor 2 and a differential amplifier 3. A resistor 2 is connected between the anode terminal of the photodiode 1b and the ground line. Further, one end of a current-voltage conversion resistor 2 is connected to the positive input terminal of the differential amplifier 3, and the negative input terminal and output terminal are directly connected. This monitor circuit converts the current signal output from the photodiode 1b into a corresponding monitor signal, and this monitor signal changes in accordance with fluctuations in the amount of light output from the laser diode 1a.

The control circuit includes an integrating resistor 4, a differential amplifier 5, an integrating capacitor 6, and an analog switch 7.

Integrating resistor 4 is connected between the output terminal of differential amplifier 3 and the negative input terminal of differential amplifier 5. A reference signal having a predetermined reference voltage Vrer is applied to the positive input terminal of the differential amplifier 5.

Integrating capacitor 6 is connected between the negative input terminal and output terminal of differential amplifier 5. Moreover, the analog switch 7 is connected in parallel with the integrating capacitor 6, and the lighting signal LDON is connected in parallel with the integrating capacitor 6.
Its opening and closing is controlled by. A control circuit having such a configuration outputs a control signal having an output voltage according to the difference between the monitor signal and the reference signal.

The power circuit includes a pair of voltage dividing resistors 8 and 9 connected in series.
, a drive transistor 10, and a voltage-current conversion resistor 11. One voltage dividing resistor 8 is connected between the output terminal of the differential amplifier 5 and the base terminal of the drive transistor 10, and the other voltage dividing resistor 9 is connected between the base terminal of the driving transistor IO and the ground line. has been done.

The collector terminal of the drive transistor 10 is connected to the cathode terminal of the laser diode 1a, and the emitter terminal is grounded via the voltage-current conversion resistor 11. The power circuit having such a configuration supplies a driving current to the laser diode 1a in accordance with the control signal so as to cancel out the difference between the monitor signal and the reference signal.

The semiconductor laser drive circuit further includes a setting circuit, an abnormality detection circuit, and a check circuit. The setting circuit consists of three voltage dividing resistors 26, 27 and 28 connected in series. These voltage dividing resistors are connected in series between the power supply line and the ground line, and divide the power supply voltage vo0 according to their respective resistance ratios. A reference voltage Vre'r is set at the lower end of the central voltage dividing resistor 27, and is sent to the positive input terminal of the differential amplifier 5 as a reference signal. The upper end of the central resistor 27 sets an upper limit voltage and outputs an upper limit signal. This upper limit voltage is larger than the reference voltage and has a magnitude that the voltage of the monitor signal does not exceed under normal conditions, and is set, for example, 5 to 10% higher than the reference voltage.

The abnormality detection circuit consists of a comparator 33 forming a comparison circuit and an RS flip-flop 34 forming a latch circuit. A monitor signal is input to the positive input terminal of the comparator 33, and an upper limit signal is input to the negative input terminal via the resistor 35. The comparator 33 outputs an inverted signal LDOVERI when the monitor signal exceeds the upper limit signal.
Output.

A set terminal of the RS flip-flop 34 is connected to an output terminal of the comparator 33. flip flop 3
4 stores the inverted signal and continuously outputs the abnormal signal LDOVER2 to the output terminal Q.

Finally, the check circuit consists of a switching circuit and a reset circuit. The switching circuit switches and maintains the magnitude of the upper limit signal below the magnitude of the reference signal in response to the check signal LDCHK, and confirms the operation of the abnormality detection circuit by outputting a pseudo abnormality signal LDOVER2. In addition, the reset circuit resets the flip-flop 34 in response to the release of the check signal LDCHK, and outputs a pseudo error signal LD.
Cancel OVER2. The switching circuit consists of a switching transistor 37 and resistors 38, 38 and 39.

A bias resistor 3 is connected to the base terminal of the switching transistor 37.
A check signal LDCHK is inputted through the comparator 8, the collector terminal is connected to the negative input terminal of the comparator 33 through the voltage dividing resistor 36, and the emitter terminal is grounded.

The reset circuit includes a differentiating circuit for forming a pulse in response to the fall of the check signal LDCHK, and an inverter for inverting the formed pulse and outputting a reset pulse signal. The differential circuit is composed of a capacitor 40 and resistors 41, 42 and 43 connected in series. The inverter is a transistor 44 and a resistor 45
It is composed of.

The base terminal of the transistor 44 is connected to the output terminal of the differentiating circuit, and the collector terminal is connected to the flip-flop 34.
It is connected to the reset terminal R of , and its emitter terminal is grounded.

Finally, the forced light-off circuit is composed of a two-man gate 24. One inverting input terminal has a lighting signal LDON.
is input, and the abnormal signal LDOVER2 is input to the other inverting input terminal.
The inverted output terminal is connected to an analog switch 7.

Next, the operation of the semiconductor laser drive circuit shown in FIG. 2 will be explained in detail. First, the operation of the automatic light amount control mechanism will be explained with reference to FIG. 3A. When the lighting signal LDON changes from a high level (hereinafter referred to as H level) to a low level (hereinafter referred to as L level), the output terminal of the AND gate 24 becomes L level and the analog switch 7 becomes non-conductive. As a result, the control circuit and the power circuit start operating and the laser diode 1a starts emitting light. A current signal proportional to the amount of laser beam is generated in the photodiode 1b. This current signal is converted into a voltage by a current-voltage conversion resistor 2, and further has its impedance converted by a differential amplifier 3, and then output as a monitor signal. Since the current signal starts to increase as the laser diode 1a turns on, the monitor signal also rises. Since an integrator is formed by the integrating resistor 4, the differential amplifier 5, and the integrating capacitor 6, the differential amplifier 5
The integration capacitor 6 is charged and discharged by the difference between the reference voltage vref applied to the positive input terminal of the monitor signal and the voltage of the monitor signal, and the output voltage of the differential amplifier 5 is determined according to this difference.

The output voltage of the differential amplifier 5 is divided by voltage dividing resistors 8 and 9,
A current is converted by a transistor IO and a resistor 11, and a drive current is supplied to the laser diode 1a to drive it. This drive current is supplied so as to cancel the difference between the reference voltage vr8r and the voltage of the monitor signal, so in a steady state, the voltage level of the monitor signal is equal to the reference voltage level as shown in the figure, and the voltage level of the monitor signal is equal to the reference voltage level as shown in the figure. Laser diode 1
A constant amount of laser beam light can be obtained regardless of some deterioration of a. Next, when the lighting signal LDON is switched from the L level to the H level to instruct the laser diode to turn off, the inverted output terminal of the AND gate 24 goes to the H level and the analog switch 7 becomes conductive. The charge accumulated in this integrated integration capacitor 6 is discharged and the differential amplifier 5
The output voltage of will be equal to the reference voltage.

Since the resistance values of the voltage dividing resistors 8 and 9 are set so that when this output voltage is divided by the voltage dividing resistors 8 and 9, the drive transistor 10 is not conductive, the laser diode 1
a is turned off.

Next, the operation of the abnormality detection circuit will be explained with reference to FIG. 3A. As described above, when the automatic light amount control mechanism is operating normally in a steady state, the voltage level of the monitor signal is maintained at the reference voltage level. Comparator 3
A monitor signal maintained at a reference voltage level in a steady state is supplied to the positive input terminal of the comparator 33, and an upper limit signal is supplied to the negative input terminal of the comparator 33. As shown in the figure, the upper limit voltage level of this upper limit signal is set higher than the reference voltage level, and is set at a level that the monitor signal does not exceed under normal conditions. Now, if the laser diode 1a emits excessive light due to an abnormality in the automatic light amount control mechanism, or if a short-circuit abnormality occurs in the light amount monitoring photodiode 1b, the voltage level of the monitor signal increases. When the voltage level of the monitor signal exceeds the upper limit voltage level, the inverted signal LDOVER1 appearing at the output terminal of the comparator 33 is inverted from L level to H level. - When the level goes high even momentarily, the flip-flop 34 is inverted, and the abnormal signal LDOVER2 appearing at the output terminal Q of the flip-flop 34 changes from the low level to the high level, warning nearby higher-level equipment, lower-level equipment, or the operator that an abnormal state has occurred. do. At the same time, when the abnormality signal LDOVER2 becomes H level, the AND gate 24 is cut off, the analog switch 7 is placed in a conductive state regardless of the state of the lighting signal LDON, the operation of the automatic light amount control mechanism is stopped, and the laser diode 1a is forcibly turned off. be done.

Finally, the operation of the check circuit will be explained with reference to FIG. 3B. Check signal LDCHK is switched from L level to H level during the check period. The abnormality detection circuit check operation is performed when the automatic light intensity control mechanism is operating normally, that is, when the monitor signal is maintained at the reference voltage level, and may be performed after the laser diode is turned on, or periodically by programming. It may be done intentionally. When the check signal LDCHK rises, the switching transistor 37 becomes conductive, and the series resistor 26.
The current flowing through 27 and 28 is connected to resistors 26.35 and 3.
It becomes like flowing through 6. At this time, since the combined resistance value of the resistors 35 and 36 is set smaller than the combined resistance value of the resistors 27 and 28, the upper limit voltage level is switched to be lower than the reference voltage level. As a result, the voltage level of the monitor signal held at the reference voltage level exceeds the switched upper limit voltage level, and a pseudo abnormal state appears.

As a result, the output level of the comparator 33 is inverted, and the inverted signal LDOVERI changes from L level to H level. Since the flip-flop 34 is inverted in response to this change, the abnormal signal LDOVER2 is also inverted from L level to H level. By confirming the inversion of this abnormality signal LDOVER2, it can be seen that the abnormality detection circuit operates normally.

As described above, when the abnormality signal LDOVER2 becomes H level, the laser diode 1a is forcibly turned off.

As a result, the voltage level of the monitor signal immediately drops to 0 level. Therefore, the voltage level of the monitor signal falls below the temporarily switched upper limit voltage level, and the inverted signal LDOVER appearing at the output terminal of the comparator 33
I returns to L level.

However, due to the action of the flip-flop 34, the abnormal signal LDOVER2 is still stored at the H level. When the check period ends and the check signal LDCHK returns from the H level to the L level, the reset circuit operates and resets the flip-flop 34. As a result,
The abnormality signal LDOVER2 also returns to the L level and shifts to the normal operating state.

Finally, a failure analysis program for an abnormality detection circuit using a check circuit will be explained with reference to FIG.

When starting the check operation, first the lighting signal LDO
A relay laser diode (hereinafter referred to as LD) with N at L level lights up. After delaying the light emission of the LD until it reaches a steady state, the check signal LDCHK is switched to H level and the flip-flop 34 is set. At this time, the voltage level of the abnormal signal LDOVER2 is checked. When it remains at L level, the inverted signal LDOVER
Check the voltage level of I. As a result, the inverted signal L
When DOVERI is at H level, flip-flop 3
4 is determined to be defective. Conversely, when it is at L level, it is determined that the comparator 33 is defective or that the LD is not emitting light. On the other hand, the abnormal signal LDOVER
When 2 is at H level, the LD is forcibly turned off. After a predetermined delay time has elapsed, the voltage level of the inverted signal LDOVERI is checked. When it is at H level, forced light-off is not executed and it is determined that the AND gate 24 is defective. On the other hand, when the inverted signal LDOVERI has returned to the L level, the check signal LDCHK is subsequently returned to the L level. As a result, flip-flop 34 is reset. Subsequently, the voltage level of the abnormal signal LDOVER2 is checked. When it remains at H level, it is determined that either the flip-flop 34 is defective or the reset circuit is defective. On the other hand, when the level returns to L, the forced extinguishing is canceled and the LD is turned on again. lastly,
The lighting signal LDON is set to H level, and the LD is turned off by normal operation to complete the check operation.

FIG. 5 is a schematic cross-sectional view of a barcode reader using a laser light source incorporating the semiconductor laser drive circuit shown in FIG. 2. The barcode reader has a casing 51.

A laser light source 52 is built into the casing 51.

When the automatic light amount control mechanism operates normally by inputting the lighting signal, a laser beam of a constant amount of light is emitted. This laser beam is scanned and reflected by a polygon mirror 54 rotated by a scan motor 53, and is reflected by a reflection mirror 54.
5, the barcode 56 attached to the surface of the article is irradiated across the barcode 56. The light traveling backward from the barcode 56 is condensed by a condensing lens 57 via a reflecting mirror 55 and a polygon mirror 54, and then received by a light receiving sensor 59 via another reflecting mirror 58.

By analyzing the intensity distribution of the amount of light received, bar food 5B
can be read. In such a barcode reader, if an abnormality occurs in the automatic light amount control mechanism built in the laser light source 52 or a short-circuit abnormality occurs in the monitor photodiode, the abnormality detection circuit immediately operates and forces the laser diode. This prevents accidents caused by excessive light emission. Further, since the operation of this abnormality detection circuit is periodically checked by a check circuit, if an abnormality occurs in the semiconductor laser drive circuit, the abnormality detection operation can always be carried out without fail.

〔Effect of the invention〕

As explained above, according to the present invention, when an abnormality occurs in the automatic light amount control mechanism or the monitoring photodiode in the semiconductor laser drive circuit, the abnormality detection operation is performed immediately, and excessive light emission of the semiconductor laser can be prevented. There is an effect.

Furthermore, it is possible to safely create a pseudo abnormal state without causing the semiconductor laser to emit excessive light, and to periodically check the operation of the abnormality detection circuit, making it possible to reliably detect abnormal states in the semiconductor laser drive circuit. This has the effect of significantly improving safety.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of the semiconductor laser drive circuit, Figure 2 is a detailed circuit diagram of the semiconductor laser drive circuit, Figure 3A is an operation timing chart of the semiconductor laser drive circuit, and Figure 3B is a check of the semiconductor laser drive circuit. Timing chart, 4th
The figure is a check flowchart of the semiconductor laser drive circuit.
and FIG. 5 is a schematic cross-sectional view of a barcode reader using a semiconductor laser drive circuit. 1...Laser package 1a...Laser diode lb...Photodiode 2...Current-voltage conversion resistor 3...Differential amplifier 4...
... Integrating resistor 5... Differential amplifier 6...
Integrating capacitor 7... Analog switch 10... Drive transistor 24... AND gate 2B, 27 and 28... Voltage dividing resistor 33... Comparator 34... RS flip-flop 35, 3Ei... Min. Piezoresistor 37...Switching transistor 101...Monitor circuit 102...Control circuit 103...Power circuit 104...Setting circuit 105...Abnormality detection circuit 10B...Check circuit

Claims (1)

[Claims] 1. A semiconductor laser that emits a laser beam in accordance with driving power, a light receiving element that photoelectrically converts the laser beam and outputs a corresponding electric signal, and a light receiving element that monitors the electric signal and responds to fluctuations in the amount of laser beam light. a monitor circuit that outputs a corresponding monitor signal; a control circuit that compares the monitor signal with a predetermined reference signal and outputs a control signal according to the difference; and a control circuit that outputs a control signal according to the difference between the monitor signal and a predetermined reference signal; A power circuit that supplies power, a setting circuit that sets an upper limit signal that is larger than a reference signal and has a magnitude that the monitor signal does not exceed during normal operation, and detects an abnormality by comparing the upper limit signal and the monitor signal and generates an abnormal signal. The operation of the abnormality detection circuit is confirmed by lowering the magnitude of the upper limit signal below the reference signal magnitude and outputting a pseudo abnormality signal when the semiconductor laser is normally lit. A semiconductor laser drive circuit consisting of a check circuit for 2. The semiconductor laser drive circuit according to claim 1, further comprising a forced extinguishing circuit for forcibly extinguishing the semiconductor laser in response to an abnormal signal. 3. The abnormality detection circuit according to claim 1, comprising a comparison circuit that outputs an inverted signal when the monitor signal exceeds the upper limit signal, and a latch circuit that stores the inverted signal and continuously outputs the abnormal signal. The semiconductor laser drive circuit described. 4. The check circuit includes a switching circuit that switches the magnitude of the upper limit signal to be equal to or less than the magnitude of the reference signal in response to the check signal, and a pseudo abnormal signal that resets the latch circuit in response to cancellation of the check signal. 4. The semiconductor laser drive circuit according to claim 3, further comprising a reset circuit for canceling.