JP2965597B2 - Semiconductor laser drive circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive circuit, and more particularly to a technique for detecting an abnormality caused by a failure or deterioration of a semiconductor laser or a light receiving element used for laser output control.

[Conventional technology]

Conventionally, semiconductor lasers have been widely used as head components of various information devices. For example, it is used for an information read head of a compact disk player, an information read / write head of an optical disk memory, a bar code reader head built in a POS terminal device, or a print head of a laser beam printer.

[Problems to be solved by the invention]

In the various information devices described above, the semiconductor laser is the most important component, and performs correct information processing when the information device is operated despite the fact that the semiconductor laser does not function properly due to a failure or the like. There is a problem that things cannot be done. In particular, in the case of a semiconductor laser that emits infrared light, since it is not visible light, even if the semiconductor laser has failed or deteriorated, it cannot be easily recognized. When the optical disk memory or the laser beam printer is operated in a failure or deterioration state, input information may be lost.

Further, these information devices generally incorporate an automatic light amount control circuit including a light receiving element for detecting the light amount of the laser beam in order to keep the light amount of the laser beam constant in a steady state. If the semiconductor laser continues to be driven despite the failure or deterioration of the light receiving element, there is a risk that an excessively large drive current flows through the semiconductor laser and damages human eyes due to an abnormally high output laser beam. There is a problem.

[Means to solve the problem]

In view of the above-described problems of the conventional technology, the present invention can immediately detect an abnormal situation caused by a failure and deterioration of a semiconductor laser or a light receiving element, and can prevent an accident by warning a CPU, a peripheral device or an operator. It is an object to provide a semiconductor laser drive circuit. To achieve the above objectives,
The semiconductor laser drive circuit according to the present invention has the basic configuration shown in FIG. That is, as shown in the figure, the semiconductor laser drive circuit has a laser diode package 1, in which a semiconductor laser such as a laser diode 1a and a light receiving element such as a photodiode 1b are housed. Photodiode 1b is laser diode 1a
Receives a laser beam radiated from the device, photoelectrically converts the laser beam, and outputs a corresponding electric signal. A monitor circuit 101 is connected to the photodiode 1b, monitors an 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. A power circuit 103 is connected to the control circuit 102, and supplies drive power to the laser diode 1a so as to cancel the difference according to the control signal. Thus, a servo loop is formed by the laser diode 1a, the photodiode 1b, the monitor circuit 101, the control circuit 102, and the power circuit 103,
Automatic control of the amount of laser beam is performed. An abnormality detection circuit 104 is connected to the monitor circuit 101, detects an abnormality by comparing the monitor signal with a reference signal different from the reference signal, and outputs an abnormality signal LDNG. As shown in FIG. 2, the abnormality detection circuit includes a comparator 25, which is a comparison circuit that compares a reference signal and a monitor signal and outputs a state signal that is inverted when an abnormality is detected, and that responds to the inversion of the state signal. Is a circuit that is set to output abnormal signals continuously
An RS flip-flop 31 is provided.

Preferably, control circuit 102 includes a forced light-off circuit for forcibly turning off laser diode 1a in response to abnormal signal LDNG.

More preferably, the abnormality detection circuit 104 outputs an abnormality signal when the monitor signal falls below the reference signal set smaller than the magnitude of the reference signal to warn of an output decrease abnormality of the laser diode 1a or the photodiode 1b. It is like. In addition, the abnormality detection circuit 104 stops the abnormality detection operation during the starting period of the laser diode 1a in response to the lighting signal, thereby preventing erroneous detection.

[Action]

According to the present invention, an abnormality occurs in a laser diode, a photodiode or an automatic light amount control servo loop,
If the monitor signal fluctuates greatly compared to the reference signal, an abnormal signal is immediately output. This abnormal signal is output when the laser beam quantity decreases or disappears due to failure or deterioration of the laser diode. Alternatively, this signal is output when there is a risk that the laser diode will emit excessive light through the servo loop due to a decrease in the monitor signal due to the disconnection of the photodiode.

〔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 diode package 1. A laser diode 1a is housed in this package, and its anode terminal is connected to a power supply line _Vcc . In addition, the photodiode 1b is housed,
Its cathode terminal is connected to the power supply line _Vcc .

The semiconductor laser drive circuit further includes a monitor circuit, a control circuit, a power circuit, and an abnormality detection circuit.
The monitor circuit has a current-voltage conversion resistor 2 inserted between the anode terminal of the photodiode 1b and the ground line. This resistor 2 converts the photocurrent output from the photodiode 1b into a corresponding voltage. One end of the resistor 2 is connected to the positive input terminal of the differential amplifier 3. The negative input terminal and the output terminal of the differential amplifier 3 are connected. Therefore, the differential amplifier 3 acts as a buffer, converts impedance, and outputs a monitor signal to its output terminal. The voltage level of this monitor signal is photodiode 1b
Is proportional to the amount of received light.

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, and the integrating capacitor 6 is connected between the negative input terminal and the output terminal of the differential amplifier 5. . Further, a reference signal having a preset voltage is input to a positive input terminal of the differential amplifier 5. The resistor 4, the differential amplifier 5, and the integrating capacitor 6 constitute an integrating circuit and output a control signal corresponding to the difference between the monitor signal and the reference signal. Further, the analog switch 7 is inserted between the negative input terminal and the output terminal of the differential amplifier 5. in addition,
The control circuit comprises an AND gate 24 having two input terminals.
Forcibly turning off the light. A lighting signal ▲ ▼ for instructing lighting of the laser diode 1a is input to one input terminal of the AND gate 24,
An abnormal signal LDNG is input to the other input terminal. The output terminal is connected to the analog switch 7, and controls this conduction state.

The power circuit includes a pair of voltage dividing resistors 8 and 9, a driving transistor 10, and a voltage-current conversion resistor 11. The base terminal of the transistor 10 is connected to the output terminal of the differential amplifier 5 via one voltage dividing resistor 8, the collector terminal is connected to the cathode terminal of the laser diode 1a, and the emitter terminal is connected via the resistor 11. Grounded.

The abnormality detection circuit has a comparator 25. The monitor signal is input to the negative input terminal of the comparator 25, and the reference signal is input to the positive input terminal. The voltage level of the reference signal is set to a predetermined value lower than the voltage level of the reference signal by the voltage dividing resistors 26, 27, and 28 connected in series. In the present embodiment, the magnitude of the reference signal is set to be 5% lower than the reference signal. comparator
A state signal whose voltage level is inverted depending on the magnitude of the monitor signal and the reference signal is output to the output terminal 25.
▼ is output. The abnormality detection circuit further has a three-input AND gate 30. The output terminal of the comparator 25 is connected to the first input terminal of the AND gate 30, the lighting signal ▲ ▼ is input to the second inversion input terminal, and the third inversion input terminal Has a delay circuit 29
A lighting signal ▲ ▼ is input via the. The output terminal of the AND gate 30 is connected to the set terminal S of the RS flip-flop 31. A clear circuit 32 is connected to a reset terminal R of the RS flip-flop 31. The clear circuit 32 is for resetting the flip-flop 31 when the power is turned on.
The output terminal Q of the flip-flop 31 outputs an abnormal signal LDNG. In the embodiment described above, the AND gate 24
And 30, the delay circuit 29 and the flip-flop 31 are composed of individual circuit elements, but these can also be composed of software by a microcomputer built in the semiconductor laser control mechanism.

Next, the operation of the semiconductor laser drive circuit shown in FIG. 2 will be described. First, the automatic light amount control operation will be described. When the lighting signal ▲ ▼ becomes low level, the analog switch 7 becomes non-conductive via the AND gate 24, the control circuit and the power circuit start operating, the laser diode 1a emits light, and a photocurrent proportional to the amount of the laser beam is generated. It occurs in the photodiode 1b. This photocurrent is converted into a corresponding voltage by the current-voltage conversion resistor 2 and subjected to impedance conversion by the differential amplifier 3, and then appears as a monitor signal at the output terminal of the differential amplifier 3. Therefore, this monitor signal is a signal having a voltage proportional to the amount of light received by the photodiode 1b. An integrator is formed by the integrating resistor 4, the differential amplifier 5, and the integrating capacitor 6, and the reference signal _Vref applied to the positive input terminal of the differential amplifier 5 and the monitor signal applied to the negative input terminal of the differential amplifier 5. The integration capacitor 6 is charged and discharged according to the voltage difference. As a result, a control signal having an output voltage corresponding to the difference is output to the output terminal of the differential amplifier 5. The output voltage is divided by the voltage dividing resistors 8 and 9 and the current is converted by the driving transistor 10 to drive the laser diode 1a. Since this drive current is supplied to the laser diode 1a so as to cancel the voltage difference between the monitor signal and the reference signal, in a steady state, the voltage of the monitor signal and the voltage of the reference signal become equal, and when the ambient temperature changes or Irrespective of the slight deterioration of the laser beam, the laser beam light amount is controlled to be constant. Also, lighting signal ▲
When ▼ is inverted from the low level to the high level, the analog switch 7 becomes conductive through the AND gate 24,
The charge stored in the integrating capacitor 6 is discharged.
As a result, the output voltage of the control signal becomes equal to the reference voltage _Vref . The laser diode 1a is turned off because the resistance ratio of the resistors 8 and 9 is set so that when the output voltage equal to this reference voltage is divided by the voltage dividing resistors 8 and 9, the driving transistor 10 is not conductive. Is done.

Next, the operation of the abnormality detection circuit which is a characteristic component of the present invention will be described in detail with reference to the timing chart shown in FIG. First, the lighting signal 信号 changes from the high level to the low level, and instructs lighting of the laser diode 1a. The lighting signal ▼ is directly applied to one inverting input terminal of the AND gate 30. Further, the lighting signal ▼ is delayed by a predetermined time, for example, 1 ms, via a delay circuit 29, and is applied as a delay signal to the other inverting input terminal of the AND gate 30. This delay time is longer than the time required for the automatic light quantity control to function normally and for the laser beam quantity to become steady. For example, the delay time is set to 1 ms because the time constant of the integrating circuit is about 500 μs. As a result, the AND gate 30 is opened 1 ms after instructing to turn on the laser diode. When the drive current is supplied and the laser diode starts to emit light in response to the inversion of the lighting signal ▼, the voltage level of the monitor signal starts to increase. When the voltage level of the monitor signal exceeds the reference voltage level, the state signal ▲ which is the output signal of the comparator 25
▼ changes from high level to low level. This change indicates that the light emission of the laser diode 1a is shifting from a transient state to a steady state. Therefore, when the status signal goes low, the central or peripheral device will enter a normal operating state. As described above, since the inversion of the state signal ▲ ▼ normally occurs within a preset delay time, this inversion change does not pass through the AND gate 30 and changes to the output level of the SR flip-flop 31. There is no. After the voltage of the monitor signal exceeds the reference voltage level, it continues to rise by the automatic light quantity control and reaches the reference voltage level _Vref which is set about 5% higher than the reference voltage level, thereby achieving a steady state. At this point, since the delay time has elapsed, the AND gate 30 is opened, and the abnormality detection circuit also shifts to the differential state.

With the passage of time, an abnormality may occur in the laser diode 1a, the photodiode 1b, or the automatic light amount control mechanism. When this abnormality is a failure or a remarkable deterioration of the laser diode 1a, a failure of the automatic light amount control mechanism, or a failure or disconnection of the photodiode 1b, the voltage level of the monitor signal is sharply or gradually. To decline. If this decrease exceeds the controllable range of the automatic light quantity control mechanism, the automatic light quantity control mechanism no longer functions, and the voltage level of the monitor signal leaves the reference voltage level and falls further below the reference voltage level. . When the voltage level of the monitor signal falls below the reference voltage level, the status signal
By the action of 25, the level is inverted again from the low level to the high level.
As a result, the state signal ▼ indicates that the semiconductor laser drive circuit is no longer in a normal state.
At the same time, the inversion of the status signal is open AND gate
The signal is transmitted to the set terminal S of the RS flip-flop 31 via 30. As a result, the output terminal Q of the RS flip-flop 31
Is changed from a low level to a high level, and is output as an abnormal signal LDNG. The abnormality signal LDNG is a signal that indicates that an abnormality has occurred in an upper system such as an operator, a CPU, or a peripheral device. Furthermore, the abnormal signal LD
NG is applied to the inverting input terminal of the AND gate 24 constituting the forced light-off circuit, the analog switch 7 becomes conductive, the operation of the control circuit and the power circuit is stopped, and the laser diode 1a is forcibly turned off. As a result, when the abnormality is caused not by the laser diode but by the photodiode, the risk of the laser diode emitting excessive light due to the inability to control the automatic light amount can be prevented beforehand.

FIG. 4 is a schematic sectional view showing a bar code reader using a laser light source having a built-in semiconductor laser drive circuit according to the present invention, and shows one application example of the present invention.
As shown, the bar code reader has a laser light source 42 housed in a casing 41. This laser light source 42 is the first
It has a circuit configuration as shown in the figure, and includes a laser diode, an automatic light amount control mechanism, and an abnormality detection circuit.
The laser beam emitted from the laser light source 42 is scanned and reflected by a polygon mirror 44 rotated by a scan motor 43, and then applied to a bar code 46 attached to the surface of the article via a reflection mirror 45. . Barcode 46
After scanning, the reflected light travels backward, is condensed by the condenser lens 47, and is received by the light receiving sensor 49 via the reflection mirror 48. The bar code 46 is read by analyzing a fluctuation component included in the received light. In such a barcode reader, when a laser diode or a photodiode incorporated in the laser light source 42 fails or deteriorates and an abnormal state occurs, an abnormal signal is output. In response to this abnormal signal, the bar code reader stops reading operation. For this reason, it is possible to prevent a possibility that the barcode reader continues reading and erroneous detection occurs even though the laser diode is significantly deteriorated and normal reading becomes impossible. Or,
When a photodiode has a derailment failure, it is possible to prevent an accident such that an excessive laser beam radiated from the laser diode erroneously irradiates an operator or a customer's eyes.

〔The invention's effect〕

As described above, according to the present invention, in a semiconductor laser driving circuit having an automatic light amount control mechanism, an abnormal signal is output immediately when a laser diode or a laser beam light amount monitoring photodiode has deteriorated or failed. As a result, there is an effect that it is possible to prevent a malfunction of a device using the laser diode, and to prevent an accident caused by excessive emission of the laser diode.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of a semiconductor laser driving circuit, and FIG.
FIG. 3 is a detailed circuit diagram of the semiconductor laser drive circuit, FIG. 3 is a timing chart of the semiconductor laser drive circuit, and FIG. 4 is a schematic cross-sectional view of a bar code reader incorporating the semiconductor laser drive circuit. DESCRIPTION OF SYMBOLS 1 ... Laser diode 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 ... Drive transistor, 24 ... And gate 25 ... Comparator, 29 ... Delay circuit 30 ... And gate 31 ... RS flip-flop 101 ... Monitor circuit, 102 ... Control circuit 103 ... Power circuit , 104 …… Abnormality detection circuit

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shinichi Sato 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Ichiro Shinoda 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Fujitsu Limited ( 56) references Patent Sho 64-76546 (JP, A) JP Akira 64-14982 (JP, A) JitsuHiraku Akira 58-140652 (JP, U) (58 ) investigated the field (Int.Cl. ^6, (DB name) H01S 3/096 H01S 3/18

Claims (4)

(57) [Claims] A semiconductor laser that emits a laser beam;
A photodetector that photoelectrically converts a laser beam and outputs a corresponding electric signal, a monitor circuit that monitors the electric signal and outputs a monitor signal according to a change in the amount of laser beam light, compares the monitor signal with a reference signal, and compares the difference with the reference signal. A control circuit that outputs a corresponding control signal, a power circuit that supplies driving power to the semiconductor laser so as to cancel the difference according to the control signal, and an abnormality by comparing a monitor signal with a reference signal different from the reference signal. A semiconductor laser drive circuit comprising: an abnormality detection circuit that detects and outputs an abnormality signal; wherein the abnormality detection circuit compares a reference signal and a monitor signal and outputs a state signal that is inverted when an abnormality is detected; and A circuit which is set in response to the inversion of the state signal and continuously outputs an abnormal signal. 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 outputs an abnormality signal when a monitor signal falls below a reference signal set to be smaller than a reference signal to indicate an abnormality in output reduction of a semiconductor laser or a light receiving element. 2. The semiconductor laser drive circuit according to claim 1, comprising: 4. The semiconductor laser drive circuit according to claim 1, wherein said abnormality detection circuit includes a circuit for stopping an abnormality detection operation during a startup period of the semiconductor laser.