JP2941353B2 - 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. Semiconductor lasers are widely used as heads of various information devices. For example, it is widely used as a laser light source for an information read head of a compact disk player, an information read / write head of an optical disk memory, a read head of a bar code reader of a POS terminal, or a print head of a laser beam printer. In general, when a semiconductor laser is used as a light source of a head of an information device, a semiconductor laser drive circuit has an automatic light amount control mechanism in order to keep a laser beam light amount constant in a steady state.

[Problems to be solved by the invention]

However, when a failure occurs in the automatic light amount control mechanism, the light amount control of the laser beam stops working, and there is a risk that the semiconductor laser emits abnormal light. For example, when abnormal excessive light emission occurs in the optical disk memory, information written on the optical disk may be destroyed.
Also, if excessive light emission occurs in a bar code reader used for a POS terminal or a so-called pointer using the excellent directivity of a laser beam, there is a possibility that the light may accidentally enter the eyes of surrounding humans and cause blindness. Conversely, when abnormal under-emission occurs, the normal operation of the main unit is impeded. In the conventional semiconductor laser drive circuit, no countermeasures are taken against the failure of the automatic light quantity control mechanism, and there is a problem in safety and stability.

[Means to solve the problem]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the related art, it is a general object of the present invention to improve the safety and stability of a semiconductor laser driving circuit and prevent abnormal light emission of a semiconductor laser.

In general, the automatic light amount control mechanism includes a light receiving element for receiving a laser beam and a detection resistor for detecting an electric signal output from the light receiving element and outputting a monitor signal for automatic light amount control. The resistance of the detection resistor may change with time due to deterioration or the like. Accordingly, it is a characteristic object of the present invention to effectively detect an abnormality of the automatic light amount control mechanism based on the deterioration of the detection resistor and prevent abnormal light emission.

In order to achieve the above object, a semiconductor laser drive circuit according to the present invention has a basic configuration shown in FIG. That is, as shown in the figure, the semiconductor laser drive circuit has a laser 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. The photodiode 1b receives a laser beam emitted from the laser diode 1a, performs photoelectric conversion, 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. The monitor circuit 101 has a first detection resistor for detecting an electric signal, and outputs a first monitor signal based on the detection result. Further, a second detection resistor connected to the first detection resistor is provided for separately detecting the electric signal, and a second monitor signal correlated with the first monitor signal is output based on the detection result. The monitor circuit 101 has a control circuit 1
02 is connected, compares the first 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, the laser diode 1a, the photodiode 1b, the monitor circuit 101, the control circuit 102, and the power circuit 10
A servo loop is formed by 3 to automatically control the amount of laser beam. An abnormality detection circuit 104 is connected to the monitor circuit 101, and detects an abnormality by comparing a reference signal different from the reference signal with the second monitor signal, and outputs an abnormality signal LDNG.

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 is configured such that when the second monitor signal falls below the lower limit reference signal set to be smaller than the magnitude of the reference signal or when the upper limit reference signal is set to be larger than the magnitude of the reference signal. When the second monitor signal exceeds the threshold value, an abnormal signal is output to warn of an abnormality of the semiconductor laser drive circuit.

Additionally, the monitor circuit has first and second detection resistors connected in series with each other to detect the electrical signal;
The circuit includes a circuit that forms a first monitor signal according to a potential difference generated between both ends of the first detection resistor, and a circuit that forms a second monitor signal according to a potential difference generated between both ends of the second detection resistor.

[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 second monitor signal fluctuates significantly 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 the first monitor signal fluctuates due to a photodiode failure and there is a risk that the laser diode will emit excessive light through the servo loop. In particular, in the present invention, the first and second monitor signals are individually formed using two detection resistors connected to each other. Therefore, when the relative resistance ratio of the two detection resistors fluctuates due to deterioration with time or the like, it correlates with the first monitor signal for automatic light amount control,
Since the second monitor signal for abnormality detection fluctuates, an abnormality caused by the detection resistor can be effectively recognized.

〔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 showing a first embodiment of the semiconductor laser drive circuit according to the present invention. As shown, the semiconductor laser driving circuit has a laser package 1. This package contains the laser diode 1a
And its anode terminal is connected to the power supply line _Vcc.
It is connected to the. Further, a photodiode 1b is housed, and 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 variable first detection resistor 2 connected 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 the impedance, and outputs the first monitor signal V
Output _det1 . The voltage level of the first monitor signal V _det1 is proportional to the amount of light received by the photodiode 1b.

The monitor circuit further has a variable second detection resistor 63 connected in series between the photodiode 1b and the first detection resistor 2. One end of the resistor 63 is connected to the positive input terminal of the differential amplifier 64, and its negative input terminal and output terminal are connected to each other. Therefore, the amplifier 64 acts as a buffer, and its output terminal is connected to the first and second series detection resistors 2, 63.
Is output as a voltage equal to the potential difference appearing at both ends. The output terminal of the amplifier 64 is connected to a differential amplifier 65 via voltage dividing resistors 68 and 69.
And the output terminal of the amplifier 3 is connected to the negative input terminal of the differential amplifier 65 via a voltage dividing resistor 66. The negative input terminal and output terminal of this amplifier 65 are divided resistors.
They are connected to each other via 67. Therefore, the differential amplifier 65
The second monitor signal V _det2 having a voltage proportional to the potential difference generated between both ends of the second detection resistor 63 is output to the output terminal of the second detection resistor 63. If the resistance values of the voltage dividing resistors 66, 67, 68 and 69 are set to be equal and the resistance values of the two variable detection resistors 2 and 63 are adjusted to be equal, the first monitor signal V _det1 and the second monitor signal The magnitudes of V _det2 are equal.

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 _Vref 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 according to the difference between the first monitor signal V _det1 and the reference signal _Vref . 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 includes a forced light-off circuit comprising an AND gate 24 having two input terminals. A lighting signal ▲ ▼ for instructing lighting of the laser diode 1a is input to one inverting input terminal of the AND gate 24, and an abnormal signal LDNG is input to the other inverting input terminal. ing. The inverted output terminal is connected to the analog switch 7, and controls this conductive 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 grounded via the resistor 11.

The abnormality detection circuit includes a pair of window comparators 25 and
With 33. The second monitor signal V _det2 is input to the negative input terminal of one comparator 25, and the lower limit reference signal V _llt is input to the positive input terminal. By the voltage _dividing resistors 70, 71, 72 and 73 connected in series, the voltage level of the lower-limit reference signal _Vllt is set to a predetermined value lower than the voltage level of the reference signal _Vref . In the present embodiment, the magnitude of the lower-limit reference signal _Vllt is set to be 5% lower than the reference signal _Vref . The output terminal of the comparator 25 outputs a state signal ▼ whose voltage level is inverted according to the magnitude of the second monitor signal V _det2 and the lower limit reference signal V _llt . Anomaly detection circuit has three inputs and gate
Has 30. The output terminal of the comparator 25 is connected to the first input terminal of the AND gate 30, and the lighting signal LDON is input to the second input terminal via the inverter 58. The input terminal receives a lighting signal ▼ via an inverter 58 and a delay circuit 29. The output terminal of the AND gate 30 is connected to the set terminal S of the RS flip-flop 31 via the OR gate 74. A clear circuit 32 is connected to a reset terminal R of the RS flip-flop 31. When the power is turned on, the clear circuit 32
Is for resetting. The output terminal Q of the flip-flop 31 outputs an abnormal signal LDNG. In the embodiment described above, AND gates 24 and 30, delay circuit 29,
Although the inverter 58, the OR gate 74, and the flip-flop 31 are composed of individual circuit elements, they can also be composed of software by a microcomputer built in the semiconductor laser control mechanism.

The second monitor signal is input to the positive input terminal of the other comparator 33, and the upper limit reference signal V _ult is input to the negative input terminal.
Is entered. The upper limit reference signal V _ult is obtained from the upper end of the second resistor 71 among the series voltage _dividing resistors 70 to 73, and is set to be 5% to 10% higher than the reference signal V _ref obtained from the lower end. In a normal state, the value does not exceed the second monitor signal V _det2 . An inverted signal LDOVER is output to the output terminal of the comparator 33 when the second monitor signal V _det2 exceeds the upper limit reference signal V _ult . The set terminal S of the RS flip-flop 31 is connected to the output terminal of the comparator 33 via the OR gate 74. flip flop
31 stores the inverted signal LDOVER and outputs the abnormal signal LDON to the output terminal Q.
Output to

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 to a corresponding voltage by the first detection resistor 2 and the differential amplifier 3
After the impedance conversion, the first monitor signal V _det1 appears at the output terminal of the differential amplifier 3. Therefore, the first monitor signal is a signal having a voltage proportional to the amount of light received by the photodiode 1b. The level of the steady drive power obtained by the automatic light amount control operation can be adjusted by manually adjusting the resistance value of the variable detection resistor 2. That is, the power of the laser diode 1a can be adjusted. An integrator is formed by the integrating resistor 4, the differential amplifier 5, and the integrating capacitor 6, and a reference signal _Vref applied to a positive input terminal of the differential amplifier 5 and a first monitor applied to a negative input terminal. The integration capacitor 6 is charged and discharged according to the voltage difference of the signal V _det1 . 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 the steady state, the first monitor signal V _det1
Is equal to the voltage of the reference signal _{Vref, so that} the amount of laser beam light is controlled to be constant irrespective of changes in the ambient temperature and slight deterioration of the laser diode. 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 signal _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 abnormality detection operation will be described in detail with reference to the timing charts shown in FIGS. As shown in FIG. 3, first, the lighting signal ▼ changes from the high level to the low level to instruct the lighting of the laser diode 1a. The lighting signal ▲ ▼ is applied to one input terminal of the AND gate 30. Furthermore, this lighting signal ▲
▼ is delayed by a predetermined time, for example, 1 ms, via the delay circuit 29 and becomes a delay signal which is applied to the other 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 time constant of the integration circuit is 500 μs
Therefore, the delay time is set to 1 ms. As a result, the AND gate 30 is opened 1 ms after instructing to turn on the laser diode. Now, lighting signal ▲
When the driving current is supplied and the laser diode starts to emit light in response to the inversion of ▼, the voltage level of the second monitor signal V _det2 starts to increase. Note that the second
_{Is adjusted so that} the magnitude of the second monitor signal V _det2 becomes equal to the reference signal _Vref in the steady state of the automatic light quantity control. That is, the abnormality detection level adjustment. When the voltage level of the second monitor signal V _det2 exceeds the lower-limit reference voltage level V _llt , the comparator
The state signal ▲ ▼ which is the output signal of 25 changes from the high level to the 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, this status signal ▲
Since the inversion of ▼ normally occurs within a preset delay time, this inversion does not pass through the AND gate 30, and the output level of the SR flip-flop 31 does not change. After the voltage of the second monitor signal V _det2 exceeds the lower-limit reference voltage level V _llt , it continues to rise by the automatic light amount control and reaches the reference voltage level V _ref set about 5% higher _{than the} lower-limit reference voltage level V _llt . Achieve a steady state. At this point, since the delay time has elapsed, the AND gate 30 is opened, and the abnormality detection circuit shifts to the operating 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 the 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 second monitor signal V _det2 becomes Decreases sharply or to removal. If this decrease exceeds the controllable range of the automatic light quantity control mechanism, the automatic light quantity control mechanism no longer functions, the voltage level of the second monitor signal V _{det2 departs from} the reference voltage level _Vref , and the lower limit reference voltage It falls further below the level _Vllt . When the voltage level of the second monitor signal falls below the lower-limit reference voltage level, the state signal ▼ is again inverted from the low level to the high level by the operation of the comparator 25. 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 change of the status signal is RS via the open AND gate 30
The signal is transmitted to the set terminal S of the flip-flop 31. As a result, the output signal appearing at the output terminal Q of the RS flip-flop 31 changes 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. Further, the abnormal signal LDNG is applied to the inverting input terminal of the AND gate 24 which constitutes 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. You. As a result, it is possible to prevent the risk of excessive emission of light from the laser diode due to the inability to automatically control the amount of light.

Next, an operation related to the other comparator 33 of the abnormality detection circuit will be described with reference to FIG. As mentioned above,
When the automatic light quantity control mechanism is operating normally, the voltage level of the second monitor signal is equal to the reference voltage level _Vref . The second monitor signal is input to the positive input terminal of the comparator 33.
V _det2 is input and the upper limit reference signal V
_ult is applied. The upper limit reference signal V _ult is set so high that it does not exceed the reference voltage V _{ref in} a normal state. Therefore, the inverted signal LDOVER appearing at the output terminal of the comparator 33 is normally kept at a low level. However, if the laser diode excessively emits light due to an abnormality in the automatic light amount control mechanism, or if a short circuit abnormality occurs in the laser beam light amount monitoring photodiode, the second monitor signal V
_{The det2} voltage level rises _{above the} upper-limit reference voltage level V _ult and the inverted signal LD appearing at the output terminal of the comparator 33
OVOR is inverted to a high level. If the inverted signal goes high even for a moment, the flip-flop 31 is output via the OR gate 74.
Is inverted, and the abnormal signal LDNG changes from a low level to a high level. As a result, an abnormal state occurring in the automatic light amount control mechanism is transmitted to the outside as a level change of the abnormal signal. In addition, when the abnormal signal LDON changes from the low level to the high level, the AND gate 24 is forcibly closed and the lighting signal ▲
Regardless of ▼, the analog switch 7 is placed in a conductive state. As a result, the automatic light amount control mechanism stops operating, and the laser diode 1a is forcibly turned off. That is, in the above-described embodiment, when an abnormal situation occurs even temporarily, the inverted signal changes and the change is latched, whereby the abnormal signal is continuously output. Therefore, before a serious accident occurs, an abnormality can be detected and effective measures can be taken.

By the way, in the present invention, the first monitor signal and the second monitor signal are generated using the first and second detection resistors, respectively. The first monitor signal is used for automatic light quantity control, and the second monitor signal is used for abnormality detection. The first and second detection resistors are connected to each other. If one of the first and second detection resistors causes a disconnection, a short circuit, or a resistance value variation failure due to aging or the like, the relative resistance ratio of the pair of detection resistors varies and the first and second detection resistors change. In addition to the monitor signal, the second monitor signal changes up and down. When this change exceeds the lower or upper limit reference voltage, an abnormal signal is output. Therefore, a failure of the detection resistor can be effectively detected. The probability of both sensing resistors failing at the same time is extremely low,
The safety of the semiconductor laser drive circuit can be further improved by judging the failure by observing the relative resistance change between the two.
In the steady state, the first monitor signal V _det1 is adjusted to be equal to the reference signal V _ref , and the second monitor signal V _det2 is also adjusted to be equal to the reference voltage V _ref in advance. Therefore, it is conceivable that the automatic light amount control and the abnormality detection are performed using a common monitor signal without using separate monitor signals. However, in this case, even if the value of the detection resistor fluctuates, the level of the monitor signal is kept constant by the automatic light amount control action, so that there is a disadvantage that the abnormality cannot be detected.

FIG. 5 shows a second example of the semiconductor laser drive circuit according to the present invention.
FIG. 3 is a circuit diagram showing an example of the embodiment. Hereinafter, the first type shown in FIG.
The same parts as those of the embodiment are denoted by the same reference numerals. Therefore, description of common points is omitted, and only different points will be described. In this example, the second detection resistor 63 is a fixed resistor, and a variable resistor 75 is connected to the output terminal of the differential amplifier 65. The level of the second monitor signal V _det2 is adjusted to the reference voltage _Vref by adjusting the variable resistor 75. In this example, the fluctuation of the resistance value of the first main variable detection resistor 2 is monitored based on the more stable fixed resistance value of the second sub detection resistor 63.

FIG. 6 shows a third embodiment of the semiconductor laser drive circuit according to the present invention.
FIG. 3 is a circuit diagram showing an example of the embodiment. In this embodiment, a differential amplifier is constituted by a pair of non-inverting amplifiers 3 and 64, and outputs a first monitor signal V _det1 and a second monitor signal V _det2 , respectively. Therefore, as compared with the first and second embodiments, the differential amplifier
65 can be omitted. In this case, it is desirable that the voltage dividing resistors 66, 67, 68 and 69 have the same resistance value. However, if you want to have a relative gain between the two monitor signals,
And 68 are set to have the same resistance, and resistors 66 and 69 are set to have the same resistance. Further, in this embodiment, in order to further stabilize the monitor circuit, the pair of detection resistors 2 and 63 are both fixed resistors, and a variable resistor 76 is connected to the output terminal of one of the amplifiers 3 instead. The monitor signal V _det1 is adjusted, the power of the laser diode is adjusted, and the other amplifier is adjusted.
Connect a variable resistor 75 to the output terminal of the second monitor signal V
_The adjustment of _det2 is adjusted.

FIG. 7 shows a fourth embodiment of the semiconductor laser drive circuit according to the present invention.
FIG. 3 is a circuit diagram showing an example of the embodiment. Although the first to third embodiments are circuits operated by a single power supply, the present embodiment has a circuit configuration applicable when a positive and negative power supply is used. One amplifier 3 outputs a first monitor signal V _det1 having a voltage equal to a voltage drop generated when a photocurrent output from the photodiode 1b flows through the first detection resistor 2, and the other amplifier 64 outputs a first monitor signal V _det1. The second monitor signal V _det2 having a voltage equal to the voltage drop generated when the current flows through the second detection resistor 63 is output. Therefore, no additional differential amplifier 65 is required as in the third embodiment.

FIG. 8 is a schematic sectional view showing a bar code reader using a laser light source incorporating a 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, which is circuited 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 bar code reader, when an automatic light amount control mechanism including 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. Alternatively, it is possible to prevent an accident in which an excessive laser beam radiated from the laser diode erroneously irradiates the eyes of an operator or a customer when the photodiode breaks.

〔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 immediately output when an automatic light amount control mechanism including a laser diode or a laser beam light amount monitoring photodiode is deteriorated or malfunctions. Output to prevent malfunctions of devices that use laser diodes,
There is an effect that an accident caused by excessive light emission of the laser diode can be prevented beforehand. In particular, by using a plurality of detection resistors, there is an effect that an abnormality based on a failure of the detection resistor itself can be detected.

[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 showing a first embodiment of the semiconductor laser drive circuit, FIGS. 3 and 4 are timing charts of the semiconductor laser drive circuit, and FIGS. 5 to 7 are second to second embodiments of the semiconductor laser drive circuit. FIG. 8 is a detailed circuit diagram showing a fourth embodiment, and FIG. 8 is a schematic sectional view of a bar code reader having a built-in semiconductor laser drive circuit. DESCRIPTION OF SYMBOLS 1 ... Laser package 1a ... Laser diode 1b ... Photodiode 2 ... First detection resistance 3, ... Amplifier 4 ... Integration resistance 5, ... Amplifier 6 ... Integration capacitor 7 ... Analog switch 10 ... Driving transistor, 24 AND gate 25 Comparator 29 Delay circuit 30 AND gate 31 RS flip-flop 33 Comparator 63 Second detection resistor 64 Amplifier 65 Difference Dynamic amplifier 101 Monitor circuit 102 Control circuit 103 Power circuit 104 Abnormality detection circuit

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-64-57688 (JP, A) JP-A-64-59878 (JP, A) JP-A-2-196482 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) H01S 3/18

Claims (5)

(57) [Claims] A semiconductor laser that emits a laser beam;
A light-receiving element for photoelectrically converting the laser beam and outputting a corresponding electric signal; and a first monitor signal having a first detection resistor for detecting the electric signal, the first monitor signal corresponding to a change in the amount of the laser beam based on the detection result. And outputs a second monitor signal correlated with the first monitor signal based on a detection result of the second detection resistor connected to the first detection resistor to separately detect the electric signal. A monitor circuit that compares the first monitor signal with the reference signal and outputs a control signal according to the difference; a power circuit that supplies drive power to the semiconductor laser so as to cancel the difference according to the control signal; A semiconductor laser drive circuit comprising: an abnormality detection circuit that detects an abnormality by comparing a second monitor signal with a reference signal different from the reference signal and outputs an abnormality 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 according to claim 1, wherein the lower limit reference signal set to be smaller than the magnitude of the reference signal is lower than the second monitor signal or an upper limit set to be larger than the magnitude of the reference signal. 2. The semiconductor laser drive circuit according to claim 1, further comprising a circuit for outputting an abnormal signal when the second monitor signal exceeds the reference signal. 4. The monitor circuit has first and second detection resistors connected in series with each other to detect the electric signal, and the first and second detection resistors are connected in accordance with a potential difference generated between both ends of the first detection resistor. 1
2. The semiconductor laser drive circuit according to claim 1, further comprising: a circuit for forming a monitor signal; and a circuit for forming a second monitor signal according to a potential difference generated between both ends of the second detection resistor. 5. The circuit for forming the first monitor signal has first adjusting means for adjusting the magnitude of the first monitor signal to adjust the driving power, and the circuit for forming the second monitor signal includes: 5. The semiconductor laser drive circuit according to claim 4, further comprising a second adjusting unit that adjusts the magnitude of the second monitor signal according to the adjusted reference signal.