JPH03232284A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To prevent abnormal operation from occurring by a method wherein an abnormality is detected when it occurs so as to output an emergency signal comparing the monitored signal with a reference signal when the monitored signal varies much as compared with a reference signal. CONSTITUTION:When an abnormality occurs in a laser diode 1a, a photodiode 1b, or an automatic light quantity control mechanism, the voltage of a monitored signal decreases sharply or gradually in level, and a state signal LDREADY is made to turn from a low level to a high level again due to the action of a comparator 25 when the monitored signal becomes lower than the reference voltage. The change of the state signal LDREADY in level is transmitted to the set terminal S of an RS flip-flop 31 through an opened AND gate 30, and an output signal which appears at an output terminal Q is outputted as an abnormality signal LDNG. This abnormality signal informs that an abnormality occurs in a host system or a peripheral equipment and is inputted into the inversion input terminal of an AND gate 24 which constitutes a forced stop circuit, an analog switch 7 becomes electrically conductive, a control circuit and a power circuit are made to stop operating, and the laser diode 1a is forcedly turned off.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a driving circuit for a semiconductor laser, and more particularly to a technique for detecting an abnormality caused by failure or deterioration of a semiconductor laser or a light receiving element used for laser output control.

[Conventional technology]

Semiconductor lasers have been widely used as head components of various information devices. For example, it is used in the information read head of a compact disc player, the information read/write head of an optical disc memory, the head of a barcode reader built into a PO8 terminal device, or the print head of a laser beam printer.

[Problem that the invention seeks to solve]

In the various information devices mentioned above, the semiconductor laser is the most important component, and if the information device is operated even though the semiconductor laser is not functioning properly due to failure or deterioration, it is necessary to perform correct information processing. The problem is that it cannot be done. In particular, in the case of a semiconductor laser that emits infrared rays, it is not visible light, so even if the semiconductor laser is broken or deteriorated, it cannot be easily recognized. If an optical disk memory or laser beam printer is operated in a malfunctioning or deteriorated state, input information may be lost.

Furthermore, in order to maintain the light intensity of the laser beam constant in a steady state, these information devices generally include an automatic light intensity control circuit that includes a light receiving element that detects the laser beam intensity.

If the semiconductor laser continues to be driven even though this photodetector has failed or deteriorated, an excessive drive current will flow through the semiconductor laser and there is a risk of damage to human eyes due to the abnormally high output laser beam. There is a problem.

[Means for solving problems]

In view of the above-mentioned problems of the conventional technology, the present invention is capable of immediately detecting an abnormal situation caused by failure or deterioration of a semiconductor laser or a light-receiving element, and warning the CPU, peripheral equipment, or operator to prevent accidents from occurring. The purpose is to provide a semiconductor laser drive circuit.

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 diode package 1, and the package 1 houses a semiconductor laser such as a laser diode 1a and a light receiving element such as a photodiode 1b. The photodiode 1b receives the laser beam emitted from the laser diode 1a, photoelectrically converts it, and outputs a corresponding electric signal. 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. Monitor circuit 101
A control circuit 102 is connected to the control circuit 102, which 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 driving power to the laser diode 1a so as to cancel out the difference according to the control signal. In this way, the laser diode la, photodiode lb, and monitor circuit 101
A servo loop is formed by a control circuit 102 and a power circuit 103 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 the monitor signal with a reference signal different from the reference signal No. 1, and outputs an abnormality signal LDNG.

Preferably, the control circuit 102 includes a forced extinguishing circuit for forcibly extinguishing the laser diode 1a in response to the abnormality signal LDNG.

More preferably, the abnormality detection circuit 104 outputs an abnormality signal when the monitor signal is lower than a reference signal set to be smaller than the reference signal, and outputs an abnormality signal to detect the abnormality in the laser diode 1a.
Or, it is designed to warn of an abnormality in the output drop of the photodiode 1b. In addition, the abnormality detection port 'jF1104 is adapted to stop the abnormality detection operation during the starting period of the laser diode 1a in response to the lighting signal LDON, thereby preventing erroneous detection.

[For production]

According to the present invention, if an abnormality occurs in the laser diode, photodiode, or automatic light amount control servo loop and the monitor signal fluctuates significantly compared to the reference signal, an abnormality signal is immediately output. This abnormality signal is output when the amount of laser beam light decreases or disappears due to failure or deterioration of the laser diode. Alternatively, it is output when there is a risk that the monitor signal will drop due to a photodiode failure and the laser diode will emit excessive light via the servo loop.

〔Example〕

Preferred embodiments of the present invention will be described in detail below 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 diode package 1. A laser diode 1a is housed in this package, and its anode terminal is connected to the power supply line VCC. Furthermore, 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 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. Further, the negative input terminal and output terminal of the differential amplifier 3 are connected.

Therefore, the differential amplifier 3 acts as a buffer, converts the impedance, and then outputs a monitor signal to its output terminal. The voltage level of this monitor signal is proportional to the amount of light received by the photodiode 1b.

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, and integrating capacitor 6 is connected between the negative input terminal and output terminal of differential amplifier 5. . Furthermore, a reference signal having a preset voltage is input to the positive input terminal of the differential amplifier 5. These resistor 4, differential amplifier 5, and integrating capacitor 6 constitute an integrating circuit and output a control signal according to the difference between the monitor signal and the reference signal. Furthermore, an analog sincheon is inserted between the negative input terminal and the output terminal of the differential amplifier 5. In addition, this control circuit includes an AND gate 24 having two input terminals.
It includes a forced lights-off circuit consisting of: and gate 24
A lighting signal LDON instructing the laser diode 1a to turn on is manually input to one input terminal of the
An abnormality signal LDNG is input to the other input terminal. The output terminal is connected to an analog switch 7 to control this conduction state.

The power circuit includes a pair of voltage dividing resistors 8 and 9, a driving transistor 10, and a voltage-current converting 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 to the resistor Il. is grounded through.

The abnormality detection circuit has a comparator 25. A monitor signal is input to the negative input terminal of the comparator 25,
A 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 28, 27 and 28 connected in series. In this embodiment, the magnitude of the reference signal is set to be 5% lower than the reference signal. A state signal LDREADY is output to the output terminal of the comparator 25, the voltage level of which is inverted depending on the magnitude relationship between the monitor signal and the reference signal. The abnormality detection circuit further includes a three-man gate 30. The output terminal of the comparator 25 is connected to the first input terminal of the AND gate 30, the lighting signal LDON is input to the second inverting input terminal, and the lighting signal LDON is input to the third inverting input terminal. is such that the lighting signal LDON is inputted via the delay circuit 29, and the output terminal of the AND gate 30 is connected to R.
A set terminal S of the S flip-flop 31 is connected. Also, the reset terminal -r- of the RS flip-flop 31
A clear circuit 32 is connected to R. This clear circuit 32 is for resetting the flip-flop 31 when the power is turned on. Furi TubuFuro Tubu 31
An abnormality signal LDNG is output to the output terminal Q of.

In the embodiment described above, AND gates 24 and 30
, the delay circuit 29, and the flip-flop 31 are constructed from individual circuit elements, but these can also be constructed using software using a microcomputer built into the semiconductor laser control mechanism.

Next, the operation of the semiconductor laser drive circuit shown in FIG. 2 will be explained. First, automatic light amount control operation will be explained. When the lighting signal LDON becomes a 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 laser beam is generated. This occurs in diode 1b. This photocurrent is converted into a corresponding voltage by the current-voltage conversion resistor 2, and after impedance conversion is performed by the differential amplifier 3, it becomes a monitor signal and appears 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 an integrating resistor 4, a differential amplifier 5, and an integrating capacitor 6, and the voltage of 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. The integrating capacitor 6 is charged and discharged according to the 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. This output voltage is divided by voltage dividing resistors 8 and 9, and current is converted by a drive transistor 10 to drive the laser diode 1a. This drive current is supplied to the laser diode 1a so as to cancel out the voltage difference between the monitor signal and the reference signal, so in a steady state, the voltage of the monitor signal and the voltage of the reference signal are equal.
Regardless of changes in ambient temperature or slight deterioration of the laser diode, the amount of laser beam light is controlled to be constant. Furthermore, when the lighting signal LDON is inverted from a low level to a high level, the analog switch 7 becomes conductive via the AND gate 24, and the charge accumulated in the integrating capacitor 6 is discharged. As a result, the output voltage of the control signal is the reference voltage V
It becomes equal to ref'. 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 drive transistor 10 becomes non-conductive, so the laser diode 1a turns off. be done.

Next, the operation of the abnormality detection circuit, which is a characteristic component of the present invention, will be explained in detail with reference to the timing chart shown in FIG. First, the lighting signal LDON changes from high level to low level to instruct the laser diode 1a to light up. This lighting signal LDON is directly applied to one inverting input terminal of the AND gate 30. Furthermore, this lighting signal LDON is passed through a delay circuit 29 for a predetermined period of time, for example, 1 seconds.
The delayed signal is delayed by the amount of time and is applied to the other inverting input terminal of the AND gate 30. This delay time is set to be longer than the time required for the automatic light amount control to function normally and for the laser beam light amount to become steady. For example, since the time constant of the integrating circuit is about 500 μs, the delay time is set to 1 μS. As a result, the AND gate 30 is opened 1ws after the instruction to turn on the laser diode is given. Now, in response to the inversion of the lighting signal LDON, when a drive current is supplied and the laser diode begins to emit light, the voltage level of the monitor signal begins to rise. Then, when the voltage level of the monitor signal exceeds the reference voltage level, the state i LDRE, which is the output signal of the comparator 25,
ADY changes from high level to low level. This change indicates that the light emission of the laser diode 1a is transitioning from a transient state to a steady state. Therefore, when the status signal goes low, the solid device or peripheral device enters a normal operating state. As mentioned above, since the inversion of the state signal LDREADY normally occurs within a preset delay time, this inversion change is caused by the AND gate 3.
It never passes through 0, and the output level of the SR flip-flop 31 does not change. After the voltage of the monitor signal exceeds the reference voltage level, it continues to rise by automatic light amount control and reaches the reference voltage level V ref, which is set approximately 5% higher than the reference voltage level, to achieve a steady state. at this point
Since the delay time has also elapsed, the AND gate 30 is opened and the abnormality detection circuit also shifts to the differential state.

Over time, the laser diode la.

An abnormality may occur in the photodiode 1b or the automatic light amount control mechanism. If this abnormality is due to a failure or significant deterioration of the laser diode 1a, a failure of the automatic light amount control mechanism, or a failure of the photodiode 1a,
In the case of a failure or disconnection of circuit b, the voltage level of the monitor signal drops suddenly or gradually. If this drop exceeds the controllable range of the automatic light amount control mechanism, the automatic light amount control mechanism will no longer function, and the voltage level of the monitor signal will leave the reference voltage level and fall further below the reference voltage level. . When the voltage level of the monitor signal falls below the reference voltage level, the state signal LDREADY is again inverted from a low level to a high level by the action of the comparator 25. As a result, the state signal LDREADY indicates that the semiconductor laser drive circuit is no longer in a normal state.

At the same time, the inverse change of the state signal is passed through the open AND gate 30 to the set terminal S of the RS flip-flop 31.
can be conveyed to. 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. This abnormality signal LDNG is a signal that informs the operator, a host system such as a CPU, or a peripheral device that an abnormality has occurred. Furthermore, the abnormality signal LDNG is applied to the inverting input terminal of the AND gate 24 constituting the forced extinguishing 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 extinguished. Ru. As a result, even if the cause of the abnormality is not the laser diode but the photodiode, it is possible to prevent the laser diode from emitting excessive light due to inability to automatically control the amount of light.

FIG. 4 is a schematic cross-sectional view showing a barcode reader using a laser light source incorporating a semiconductor laser drive circuit according to the present invention, and represents one application example of the present invention. As shown in the figure, the barcode reader has a laser light source 42 housed in a casing 41. This laser light source 42 has a circuit configuration as shown in FIG. 1, and includes a laser diode, an automatic light amount control mechanism, and an abnormality detection circuit.

A laser beam emitted from a laser light source 42 is transmitted to a polygon mirror 44 which is rotated by a scan motor 43.
After being reflected in a scanning manner by a mirror 45, the light is irradiated onto a barcode 46 attached to the surface of the article via a reflecting mirror 45. After scanning the barcode 46, the reflected light travels backward, is focused by a condenser lens 47, and is then received by a light receiving sensor 49 via a reflecting mirror 48. The barcode 46 is read by analyzing the fluctuation components contained in the received light. In such a barcode reader, a laser light source 4
If the laser diode or photodiode built in 2 breaks down or deteriorates and an abnormal state occurs, an abnormal signal is output. In response to this abnormal signal, the barcode reader stops reading operations. This prevents the risk of erroneous detection if the barcode reader continues reading even though the laser diode has deteriorated significantly and normal reading is no longer possible. Alternatively, in the event of a disconnection failure in the photodiode, it is possible to prevent an accident in which an excessive laser beam emitted from the laser diode accidentally irradiates the operator's or customer's eyes.

〔Effect of the invention〕

As explained above, according to the present invention, a semiconductor laser drive circuit having an automatic light amount control mechanism is configured to immediately output an abnormality signal when a laser diode or a photodiode for monitoring laser beam light amount deteriorates or breaks down. As a result, malfunctions of devices using laser diodes can be prevented, and accidents caused by excessive light emission of laser diodes can be prevented.

[Brief explanation of drawings]

Figure 1 is a block diagram of the semiconductor laser drive circuit;
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 barcode reader incorporating the semiconductor laser drive circuit. 1...Laser diode package 1a...Laser diode 1b...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 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 Applicant: Konokuru Co., Ltd. (1 other person) Configuration prop diagram of semiconductor laser drive circuit Figure 1

Claims (1)

[Claims] 1. A semiconductor laser that emits a laser beam, a light receiving element that photoelectrically converts the laser beam and outputs a corresponding electric signal, and a monitor signal that monitors the electric signal and outputs a monitor signal according to fluctuations in the amount of laser beam light. a control circuit that compares the monitor signal and a reference signal and outputs a control signal according to the difference; a power circuit that supplies driving power to the semiconductor laser to cancel the difference according to the control signal; A semiconductor laser drive circuit consisting of an abnormality detection circuit that detects an abnormality and outputs an abnormal signal by comparing a reference signal different from the signal and a monitor signal. 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 includes a circuit that outputs an abnormal signal when the monitor signal is lower than the reference signal, which is set to be smaller than the reference signal, to indicate an abnormality in the output of the semiconductor laser or the light receiving element. 2. The semiconductor laser drive circuit according to claim 1. 4. The semiconductor laser drive circuit according to claim 1, wherein the abnormality detection circuit includes a circuit that stops the abnormality detection operation during a starting period of the semiconductor laser.