JP2895143B2 - Semiconductor laser drive circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for a semiconductor laser, and more particularly to a technique for detecting an abnormality of the driving circuit.

[Conventional technology]

In recent years, high-performance semiconductor lasers can be supplied at low cost, and are used as components of various information devices. For example, it is widely used as a laser beam light source in information reading heads of compact disc players, information reading and writing heads of optical disc memories, reading heads of bar code readers used in POS terminals, laser beam pointers, and printing heads of laser beam printers. Used. In these various information devices, the drive circuit of the semiconductor laser generally has a laser beam light amount automatic control circuit to keep the laser beam light amount constant.

[Problems to be solved by the invention]

However, such an automatic light amount control circuit is composed of a plurality of circuit components, and it cannot be said that there is no failure. In such a case, the automatic light quantity control does not work, and the semiconductor laser may emit excessive light. There is a problem that if excessive light emission is directly incident on human eyes, blindness may occur. In addition, in an optical disk memory or a laser beam printer, there is a problem that when excessive light emission occurs, there is a risk that information is destroyed.

[Means to solve the problem]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the related art, an object of the present invention is to improve the safety of a semiconductor laser drive circuit. In order to achieve this object, the present invention includes an abnormality detection circuit for detecting an abnormality in the automatic control circuit. A check circuit is also provided for confirming in advance whether the abnormality detection circuit can operate properly when the automatic light amount control circuit is abnormal. By taking such a double measure, the safety of the semiconductor laser drive circuit is significantly improved as compared with the related art. Generally, the probability of the automatic light quantity control circuit causing a failure is low, and the abnormality detection circuit rarely operates. Therefore, it is extremely important to periodically check whether the abnormality detection circuit operates properly.

FIG. 1 shows a basic configuration of a semiconductor laser drive circuit according to the present invention. As shown, the semiconductor laser drive circuit has a laser package 1. The laser package 1 includes a semiconductor laser such as a laser diode 1a that emits a laser beam according to the driving power, and a light receiving element such as a PIN that photoelectrically converts the laser beam and outputs a corresponding electric signal.
Built-in photodiode 1b. Photodiode
A monitor circuit 101 is connected to 1b, monitors the electric signal, and outputs a monitor signal according to a change in the amount of laser beam. A control circuit 102 is connected to the monitor circuit 101, compares the monitor signal with a predetermined reference signal, and outputs a control signal according to the difference. The control circuit 102 has a power circuit 1
03 is connected, and the drive power is changed to a laser diode so as to cancel the difference between the monitor signal and the reference signal according to the control signal.
Supply 1a. These laser package 1, monitor circuit
101, the control circuit 102, and the power circuit 103 constitute an automatic light amount control mechanism including a servo loop, and perform control so that the laser beam amount of the laser diode 1a is 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 during normal operation. The reference signal is also set. Setting circuit
An abnormality detection circuit 105 is connected to 104, and detects an abnormality of the automatic light amount control mechanism by comparing the upper limit signal and the monitor signal, and outputs an abnormality signal LDOVER. Error detection circuit 10
A check circuit 106 is connected to 5, and when the laser diode 1a is normally lit by the action of the automatic light amount control mechanism, the magnitude of the upper limit signal is smaller than the magnitude of the reference signal in response to the check signal LDCHK. To pseudo, abnormal signal LD
The operation of the abnormality detection circuit 105 is periodically checked by outputting OVER.

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 includes a check signal
A switching circuit that switches the size of the upper limit signal to the size of the reference signal or less according to LDCHK, and a reset that resets the latch circuit in response to the release of the check signal LDOCHK and releases the abnormally output abnormal signal LDOVER Circuit. Finally, the semiconductor laser drive circuit preferably has a forced turn-off circuit for forcibly turning off the laser diode 1a in response to the abnormal signal LDOVER.

[Action]

According to the present invention, the abnormality detection circuit constantly monitors the monitor signal for the upper limit signal, and outputs an abnormal signal when the monitor signal exceeds the upper limit signal to prevent excessive emission of the laser diode 1a. I have. Further, the check circuit periodically checks the abnormality detection circuit 105 so that when an abnormality occurs in the automatic light amount control mechanism, the abnormality detection circuit can correctly detect the abnormality.

〔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 the semiconductor laser drive circuit according to the present invention. As shown, the semiconductor laser drive circuit has a laser package 1. In the laser package 1, a laser diode 1a and a photodiode 1b are built in close proximity to each other. The anode terminal of the laser diode 1a and the cathode terminal of the photodiode 1b are both connected to a power supply line _Vcc . Laser diode 1
a emits a laser beam in accordance with the drive current, and the photodiode 1b receives the emitted laser beam, performs photoelectric conversion, and outputs a corresponding current signal.

The semiconductor laser drive circuit has a monitor circuit, a control circuit, and a power circuit in addition to the laser package 1, and together constitutes an automatic light amount control mechanism. The monitor circuit includes a current-voltage conversion resistor 2 and a differential amplifier 3. The resistor 2 is connected between the anode terminal of the photodiode 1b and the ground line. One end of the current-voltage conversion resistor 2 is connected to the positive input terminal of the differential amplifier 3, and the negative input terminal and the output terminal are directly connected. The monitor circuit converts a current signal output from the photodiode 1b into a corresponding monitor signal, and the monitor signal changes according to a change in the output light amount of 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. The integrating resistor 4 is connected between the output terminal of the differential amplifier 3 and the negative input terminal of the differential amplifier 5. A reference signal having a predetermined reference voltage _Vref is applied to a positive input terminal of the differential amplifier 5. The integrating capacitor 6 is connected between the negative input terminal and the output terminal of the differential amplifier 5. The analog switch 7 is connected in parallel with the integrating capacitor 6, and its opening / closing is controlled by a lighting signal ▼. The 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, a driving transistor 10, and a voltage-current conversion resistor 11 connected in series. One voltage dividing resistor 8 is connected between the output terminal of the differential amplifier 5 and the base terminal of the driving transistor 10, and the other voltage dividing resistor 9 is connected between the base terminal of the driving transistor 10 and the ground line. Have been. The collector terminal of the driving 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 drive current to the laser diode 1a so as to cancel the difference between the monitor signal and the reference signal according to the control signal.

The semiconductor laser drive circuit further includes a setting circuit, an abnormality detection circuit, and a check circuit. The setting circuit includes three series-connected voltage dividing resistors 26, 27 and 28. These voltage dividing resistors _divide the power supply voltage Vcc connected in series between the power supply line and the ground line according to the respective resistance ratios. The lower end of the center voltage _dividing resistor 27 is connected to the reference voltage V _ref.
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 does not exceed the voltage of the monitor signal in a normal state, and is set, for example, 5 to 10% higher than the reference voltage.

The abnormality detection circuit includes a comparator 33 forming a comparison circuit and an RS flip-flop 34 forming a latch circuit. The monitor signal is input to the positive input terminal of the comparator 33, and the upper limit signal is input to the negative input terminal via the resistor 35. The comparator 33 outputs the inverted signal LDOVER1 when the monitor signal exceeds the upper limit signal. The set terminal of the RS flip-flop 34 is connected to the output terminal of the comparator 33. The flip-flop 34 stores the inverted signal and continuously outputs the abnormal signal LDOVER2 to the output terminal Q.

Finally, the check circuit comprises a switching circuit and a reset circuit. The switching circuit switches the magnitude of the upper limit signal to the magnitude of the reference signal or less according to the check signal LDCHK, and confirms the operation of the abnormality detection circuit by pseudo-outputting the abnormality signal LDOVER2. Further, the reset circuit resets the flip-flop 34 in response to the release of the check signal LDCHK, and releases the abnormal signal LDOVER2 that has been pseudo-output. The switching circuit comprises a switching transistor 37 and resistors 36, 38 and 3
It consists of nine. The check signal LDCHK is input to the base terminal of the switching transistor 37 via the bias resistor 38, and the collector terminal is connected to the voltage dividing resistor 36.
Is connected to the negative input terminal of the comparator 33, and the emitter terminal is grounded. The reset circuit includes a differentiating circuit for forming a pulse in response to the falling of the check signal LDCHK, and an inverter for inverting the formed pulse and outputting a reset pulse signal. The differentiating circuit is a capacitor 40 and a resistor connected in series.
41, 42 and 43. The inverter includes a transistor 44 and a resistor 45. The base terminal of the transistor 44 is connected to the output terminal of the differentiating circuit, the collector terminal is connected to the reset terminal R of the flip-flop 34, and the emitter terminal is grounded.

Finally, the forcible light-off circuit comprises a two-input AND gate 24. Lighting signal ▲
▼ is input, and an abnormal signal LDOVER2 is input to the other inverted input terminal, and the inverted output terminal is connected to the analog switch 7.

Next, the operation of the semiconductor laser drive circuit shown in FIG. 2 will be described in detail. First, the operation of the automatic light amount control mechanism will be described with reference to FIG. 3A. When the lighting signal ▼ 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 is turned off. 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 the current-voltage conversion resistor 2, and the impedance is converted by the differential amplifier 3 before being output as a monitor signal. Since the current signal starts to increase with the turning on of the laser diode 1a, the monitor signal also rises. Since the integrating resistor 4, the differential amplifier 5 and the integrating capacitor 6 form an integrator, 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 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 the voltage dividing resistors 8 and 9, the current is converted by the transistor 10 and the resistor 11, and the driving current is supplied to the laser diode 1 a for driving. Since this drive current is supplied so as to cancel the difference between the reference voltage _Vref and the voltage of the monitor signal, in a steady state, the voltage level of the monitor signal becomes equal to the reference voltage level as shown in FIG. Alternatively, a constant amount of laser beam can be obtained irrespective of some deterioration of the laser diode 1a. Next, when the lighting signal 信号 is switched from the L level to the H level to instruct the laser diode to be turned off, the inverted output terminal of the AND gate 24 becomes the H level, and the analog switch 7 becomes conductive. As a result, the electric charge stored in the integrating capacitor 6 is discharged, and the output voltage of the differential amplifier 5 becomes equal to the reference voltage. This output voltage is divided by the voltage dividing resistors 8 and 9
Since the resistance values of the voltage-dividing resistors 8 and 9 are set so that the driving transistor 10 does not conduct when the voltage is divided by
The laser diode 1a is turned off.

Next, the operation of the abnormality detection circuit will be described with reference to FIG. 3A. As described above, the voltage level of the monitor signal is held at the reference voltage level when the automatic light amount control mechanism is operating normally in the steady state. comparator
The monitor signal held at the reference voltage level in the steady state is supplied to the positive input terminal of the
The upper limit signal is supplied to the negative input terminal of 33. As shown in the figure, the upper limit voltage level of the upper limit signal is set higher than the reference voltage level, and is set to a level that the monitor signal does not exceed in a normal state.
If an excessive light emission of the laser diode 1a occurs due to an abnormality of the automatic light amount control mechanism or a short circuit abnormality of the light amount monitoring photodiode 1b occurs, 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. Flip-flop when H level is reached even for a moment
The signal 34 is inverted, and the abnormal signal LDOVER2 appearing at the output terminal Q of the flip-flop 34 changes from the L level to the H level to warn the peripheral upper device, lower device or operator of the occurrence of the abnormal state. At the same time, when the abnormal signal LDOVER2 becomes H level, the AND gate 24 is shut off and the lighting signal ▲
Regardless of the state of ▼, the analog switch 7 is placed in the conductive state, the operation of the automatic light amount control mechanism is stopped, and the laser diode
1a is forcibly turned off.

Finally, the operation of the check circuit will be described with reference to FIG. 3B. The check signal LDCHK is L during the check period.
The level is switched from the level to the H level. The check operation of the abnormality detection circuit is performed in a state where the automatic light amount control mechanism is operating normally, that is, in a state where the monitor signal is held at the reference voltage level, and may be performed after the laser diode is turned on, or may be periodically performed by programming. It may be performed in a specific manner. When the check signal LDCHK rises, the switching transistor 37 conducts, and the current flowing through the series resistors 26, 27 and 28 in the setting circuit flows through the resistors 26, 35 and 36.
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 below 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 LDOVER1 becomes L
The level changes from the level to the H level. Since the flip-flop 34 is inverted according to this change, the abnormal signal LDOVER2 is also inverted from L level to H level. By confirming the inversion of the abnormality signal LDOVER2, it can be understood that the abnormality detection circuit operates normally. As described above, when the abnormal signal LDOVER2 becomes H level, the laser diode 1a is forcibly turned off. As a result, the voltage immediately drops to the voltage level of the monitor signal. As a result, the voltage level of the monitor signal falls below the temporarily switched upper limit voltage level, and the inverted signal LDOVER1 appearing at the output terminal of the comparator 33 returns to the L level. However, flip-flops
By the operation of 34, the abnormal signal LDOVER2 is continuously stored at the H level. Check period ends, check signal
When LDCHK returns from the H level to the L level, the reset circuit operates to reset the flip-flop. As a result, the abnormal signal LDOVER2 also returns to the L level and shifts to a normal operation state.

Finally, a failure analysis program for an abnormality detection circuit using a check circuit will be described with reference to FIG. When the check operation is started, first, the lighting signal ▲ ▼ becomes L
Level, and the laser diode (hereinafter referred to as LD) lights up. After the light emission of the LD is delayed until reaching the steady state, the check signal LDCHK is switched to the H level, and the flip-flop 34 is set. At this time, the voltage level of the abnormal signal LDOVER2 is checked. When the signal remains at the L level, the voltage level of the inverted signal LDOVER1 is further checked. As a result, when the inverted signal LDOVER1 is at the H level, it is determined that the flip-flop 34 is defective. Conversely, L
When the level is at the level, it is determined that the comparator 33 is defective or the LD does not emit light. On the other hand, abnormal signal
When LDOVER2 is at H level, LD is forcibly turned off. After a lapse of a predetermined delay time, the voltage level of the inverted signal LDOVER1 is checked. When the level is at the H level, it is determined that the forced turn-off is not performed and the AND gate 24 is defective. On the other hand, when the inverted signal LDOVER1 has returned to the L level, the check signal LDCHK is subsequently returned to the L level. As a result, the flip-flop 34 is reset.
Subsequently, the voltage level of the abnormal signal LDOVER2 is checked. When the level remains at the H level, it is determined that the flip-flop 34 is defective or the reset circuit is defective. On the other hand, when returning to the L level, the forcible light is released.
LD turns on again. Finally, change the lighting signal ▲ ▼ to H
The level is set to the level, the LD is turned off by normal operation, and the check operation is completed.

FIG. 5 is a schematic sectional view of a bar code reader using a laser light source incorporating the semiconductor laser drive circuit shown in FIG. The bar code reader has a casing 51.
A laser light source 52 is built in the casing 51. When the automatic light amount control mechanism operates normally by inputting a lighting signal, a laser beam with a constant light amount is emitted. The laser beam is reflected by the polygon mirror 54 rotated by the scan motor 53 in a scanning manner, and is irradiated via the reflection mirror 55 so as to cross a bar code 56 attached to the surface of the article. The light reversed from the barcode 56 is condensed by a condenser lens 57 via a reflection mirror 55 and a polygon mirror 54, and received by a light receiving sensor 59 via another reflection mirror 58. The bar code 56 can be read by analyzing the intensity distribution of the received light amount. 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 monitoring photodiode, the abnormality detection circuit immediately operates to forcibly activate the laser diode. The lights are turned off to prevent accidents caused by excessive light emission. In addition, since the operation of this abnormality detection circuit is periodically checked by the check circuit, when an abnormality occurs in the semiconductor laser drive circuit, the abnormality detection operation can always be performed without fail.

〔The invention's effect〕

As described 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 immediately performed, and excessive emission of the semiconductor laser can be prevented beforehand. This has the effect. Furthermore, a pseudo abnormal state can be safely created without excessive emission of the semiconductor laser, and the operation of the abnormality detection circuit can be periodically checked, so that the abnormal state of the semiconductor laser drive circuit can be reliably detected. This has the effect of significantly improving safety.

[Brief description of the drawings]

1 is a configuration diagram of a semiconductor laser drive circuit, FIG. 2 is a detailed circuit diagram of the semiconductor laser drive circuit, FIG. 3A is an operation timing chart of the semiconductor laser drive circuit, and FIG. 3B is a check of the semiconductor laser drive circuit. FIG. 4 is a check flowchart of the semiconductor laser drive circuit, and FIG. 5 is a schematic sectional view of a bar code reader using the semiconductor laser drive circuit. DESCRIPTION OF SYMBOLS 1 ... Laser package 1a ... Laser diode 1b ...... Photodiode 2 ...... Current-voltage conversion resistance 3 ... Differential amplifier 4 ... Integration resistance 5 ... Differential amplifier 6 ... Integration capacitor 7 ... Analog Switch 10: Driving transistor, 24: AND gate 26, 27 and 28: Dividing resistor, 33: Comparator 34: RS flip-flop 35, 36 ... Dividing resistor 37: Switching transistor, 101 ... Monitor circuit 102: Control circuit 103: Power circuit 104: Setting circuit 105: Abnormality detection circuit 106: Check circuit

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shinichi Sato 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Tomoyuki Kashiwazaki 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited ( 56) References JP-A-58-140652 (JP, A) JP-A-57-177738 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/096 H01S 3/133

Claims (4)

(57) [Claims] 1. A semiconductor laser that emits a laser beam in accordance with drive power, a photodetector that photoelectrically converts the laser beam and outputs a corresponding electric signal, and a monitor that monitors the electric signal and responds to a change in the amount of laser beam. A monitor circuit for outputting a signal, a control circuit for comparing the monitor signal with a predetermined reference signal and outputting a control signal corresponding to the difference, and a power for supplying driving power to the semiconductor laser so as to cancel the difference according to the control signal A circuit, a setting circuit for setting an upper limit signal having a magnitude larger than the reference signal and not exceeding the monitor signal in a normal state, and detecting an abnormality by comparing the upper limit signal with the monitor signal to output an abnormal signal. An abnormality detection circuit and by lowering the magnitude of the upper limit signal to less than the magnitude of the reference signal when the semiconductor laser is operating normally, and outputting a pseudo abnormal signal Semiconductor laser drive circuit consisting of a check circuit for checking the operation of the ordinary detection circuit. 2. The semiconductor laser drive circuit according to claim 1, further comprising a forced turn-off circuit for forcibly turning off the semiconductor laser in response to the abnormal signal. 3. The 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. Item 2. A semiconductor laser drive circuit according to item 1. 4. A check circuit for switching the magnitude of an upper limit signal to a magnitude equal to or less than the magnitude of a reference signal in response to a check signal, and resetting a latch circuit in response to release of the check signal to output a pseudo signal. 4. The semiconductor laser drive circuit according to claim 3, further comprising a reset circuit for canceling the abnormal signal.