JPH03232285A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To improve a semiconductor laser in safety and to prevent it from excessively emitting light by a method wherein an abnormality signal is outputted and sent to a host system or a subordinate system when troubles occur in an automatic light quantity control mechanism to induce an over light emission. CONSTITUTION:When a laser diode excessively emits laser light due to the malfunction of an automatic light quantity control mechanism, the voltage of the monitored signal is made to increase in level exceeding an upper voltage limit, an inverted signal appearing at the output terminal of a comparator 33 is made to turn high in level, a flip-flop is inverted, and an abnormality signal LDOVER is made to turn high in level. In result, the change of the abnormality signal in level is transmitted to the outside, and when the abnormality signal LDOVER is made to change from a low level to a high level, an AND gate 24 is forcedly closed and an analog switch is put in an ON-state independent of a lighting signal LDON, the automatic light quantity mechanism stops operating, and a laser diode 1a is forcedly turned off.

Description

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

[Conventional technology]

In recent years, high-performance semiconductor lasers have become available at low cost. Semiconductor lasers are widely used as heads of various information devices.

For example, information read head of compact disc player, information read/write head of optical disc memory, PO5
It is widely used as a laser light source for read heads of terminal barcode readers or print heads of laser beam printers.

Generally, when a semiconductor laser is used as a light source for a head of an information device, the semiconductor laser drive circuit has an automatic light amount control mechanism built-in in order to keep the amount of laser beam constant in a steady state.

[Problem that the invention seeks to solve]

However, if a failure occurs in the automatic light amount control mechanism, there is a risk that the light amount control of the laser beam will not work and the semiconductor laser will emit excessive light.

For example, in an optical disk memory, if such excessive light emission occurs, there is a risk that information written on the previous disk may be destroyed. Furthermore, if such excessive light emission occurs in a barcode league used in a PO8 terminal or a so-called pointer that utilizes the excellent directivity of a laser beam, there is a risk that the light may accidentally enter the eyes of people nearby and cause blindness. In conventional semiconductor laser drive circuits, no measures have been taken against failures of the automatic light amount control mechanism, resulting in safety problems.

Means for Solving Problem C] In view of the problems of the prior art described above, the present invention aims to improve the safety of a semiconductor laser drive circuit and prevent excessive light emission of the semiconductor laser.

In order to achieve the above object, a semiconductor laser drive circuit according to the present invention has a basic configuration as shown in FIG. As shown in the figure, the semiconductor laser drive circuit has a laser diode package 1.

The laser diode package 1 includes a semiconductor laser such as a laser diode 1a that emits a laser beam according to driving power, and a light receiving element such as a PIN photodiode 1 that photoelectrically converts the laser beam and outputs a corresponding electric signal.
It has a built-in b.

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. A control circuit 102 is connected to the monitor circuit 101, 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 semiconductor laser so as to cancel out the difference according to the control signal. The laser diode package 1, monitor circuit 101, control circuit 102, and power circuit 103 described above form a so-called servo loop and constitute an automatic light amount control mechanism that keeps the laser beam light amount constant in a steady state. Further, the semiconductor laser drive circuit is set to supply an upper limit signal that is larger than the reference signal and has a magnitude that the monitor signal will not exceed under normal conditions, that is, when the automatic light amount control mechanism has no failure and is operating stably. It has a circuit 104. Setting circuit 10
4 is connected to an abnormality detection circuit 105, which detects an abnormality by comparing the upper limit signal and a monitor signal and outputs an abnormality signal LDOVER. This abnormality detection circuit includes a comparison circuit that outputs an inverted signal when the monitor signal exceeds the upper limit signal, and a latch circuit that stores the inverted signal and continuously outputs the abnormal signal. The semiconductor laser drive circuit includes a forced extinguishing circuit for forcibly extinguishing the semiconductor laser in response to an abnormality signal.

[For production]

According to the present invention, the abnormality detection circuit operates so that the monitor signal outputs a special abnormality signal in which the monitor signal exceeds the upper limit signal set to be larger than the reference signal. Therefore, when some kind of abnormality occurs in the automatic light amount control mechanism, and the abnormality causes the laser diode to emit too much light instead of under-emitting light or extinguishing it, an abnormality signal is output. Therefore, by sending this abnormal signal to the surrounding higher-level equipment or lower-level equipment,
Accidents caused by excessive light emission can be prevented.

〔Example〕

Hereinafter, preferred embodiments of a semiconductor laser drive circuit according to the present invention will be described in detail with reference to the drawings.

FIG. 2 is a detailed circuit diagram of one embodiment. As shown,
The semiconductor laser drive circuit is a laser diode package,
It consists of a monitor circuit, a control circuit, a power circuit, a setting circuit, an abnormality detection circuit, and a forced light-off circuit.

The laser diode package 1 includes a laser diode 1a that emits a laser beam according to a drive current, and a photodiode 1b that photoelectrically converts the laser beam and outputs a corresponding current signal. Laser diode 1a
The anode terminal of is connected to the power supply line vcc,
The cathode terminal of the photodiode 1b is also connected to the power supply line Vc.
connected to c.

The monitor circuit includes a current-voltage conversion resistor 2 and a differential amplifier 3. One end of resistor 2 is photodiode 1
b, and the other end is grounded. Further, one end of the resistor 2 is connected to a positive input terminal of a differential amplifier 3, and its negative input terminal and output terminal are connected to each other. The current signal flowing through the current-voltage conversion resistor 2 is converted into a current-voltage signal and becomes a monitor signal, which is sent to the differential amplifier 3.
output to the output terminal. Therefore, this monitor signal is a signal that corresponds to fluctuations in the amount of laser beam light.

The control circuit is composed of 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 at the front stage and the negative input terminal of differential amplifier 5 at the rear stage. A reference signal having a predetermined reference voltage Vrer is input to the positive input terminal of the differential amplifier 5. Integrating capacitor 6 is connected between the negative input terminal and output terminal of differential amplifier 5. Further, the analog switch 7 is connected in parallel to the integrating capacitor 6, and its opening/closing is controlled by the lighting signal LDON. With this configuration, a control signal having an output voltage level corresponding to the difference between the reference signal and the monitor signal is output to the output terminal of the differential amplifier 5.

The power circuit includes a pair of series-connected voltage dividing resistors 8 and 9,
It is composed of a drive transistor IO and a voltage-current conversion resistor 11. One voltage dividing resistor 8 is connected between the output terminal of the differential amplifier 5 and the base terminal of the transistor 1o, and the other voltage dividing resistor 9 is connected between the base terminal and the ground line.

The collector terminal of the drive transistor 1O is connected to the cathode terminal of the laser diode 1a, and the emitter terminal of the transistor 10 is grounded via a resistor 11. With this configuration, the drive transistor 10 supplies a drive current to the laser diode 1a in accordance with the control signal so as to cancel out the difference between the monitor signal and the reference signal. As a result, automatic light amount control is performed using a servo loop.

The setting circuit is composed of three voltage dividing resistors 26, 27 and 28 connected in series between the power supply line Vcc and the ground line. The voltage Vref' appearing at the lower end of the voltage dividing resistor 27 in the middle is applied to the positive input terminal of the differential amplifier 5 as a reference signal. Further, the voltage Vult appearing at the upper end of this voltage dividing resistor 27 is output as an upper limit signal. As is clear from the figure, according to the potential difference across the voltage dividing resistor 27, 1,
The upper limit voltage Vult is set higher than the reference voltage V rer. This difference is determined in advance by appropriately setting the resistance value of the voltage dividing resistor 27. That is, the upper limit voltage Vult is set so that the monitor signal does not exceed it when the automatic light amount control mechanism is operating under normal conditions. It is usually preferable to set the difference between the upper limit voltage Vult and the reference voltage V ref' to 5 to 10%.

The abnormality detection circuit includes a comparator such as a comparator 33, a latch circuit such as an RS flip-flop 34, and a clear circuit 32. A monitor signal is input to the positive input terminal of the comparator 33, and an upper limit signal is input to the negative input terminal.

Therefore, an inverted signal is output to the output terminal of the comparator 33 when the monitor signal exceeds the upper limit signal. R
The set terminal S of the S flip-flop 34 is connected to the output terminal of the comparator 33, and the reset terminal R is connected to the clear circuit 32.

The flip-flop 34 stores the inverted signal and outputs the abnormal signal LD.
Output OVER to output terminal Q. The clear circuit 32 has a function of resetting the flip-flop 34 and returning the abnormality detection circuit to its initial state when the power is turned on.

Finally, the forced light-off circuit is composed of a two-man gate 24. The lighting signal LDON is input to one inverting input terminal of the AND gate 24, and the abnormal signal LDOVER is input to the other inverting input terminal. Therefore, when the abnormal signal LDOVER is input, the AND gate 24 is closed and the analog switch 7 becomes conductive regardless of the lighting signal LDON, and the control circuit and power circuit stop operating, so the laser diode 1a is forced to close. The lights are turned off. In this example,
The forced light-off circuit and the abnormality detection circuit are constructed of circuit elements such as the AND gate 24 and the flip-flop 34, but it goes without saying that similar functions can be constructed using software using a microcomputer.

Next, the operation of the embodiment shown in FIG. 2 will be described in detail with reference to the timing chart of the semiconductor laser drive circuit shown in FIG. First, the operation of the automatic light amount control mechanism will be explained. When lighting the laser diode 1a, the lighting signal LDON changes from high level to low level. As a result, the inverted output terminal of the AND gate 24 switches from high level to low level, and the analog switch 7 becomes non-conductive. As a result, the control circuit and the power circuit start operating and supply drive current to the laser diode 1a to start emitting light. When the laser diode 1a starts emitting light, a current signal proportional to the amount of laser beam is sent to the photodiode 1b.
occurs in This current signal is converted into a voltage signal by a current-voltage converting resistor 2, and further has its impedance converted by a differential amplifier 3, and then output as a monitor signal to its output terminal. Differential amplifier 3 acts as a buffer. This monitor signal is inputted via the integrating resistor 4 to the negative input terminal of the differential amplifier 5 at the subsequent stage.

An integrator is formed by an integrating resistor 4, a differential amplifier 5, and an integrating capacitor 6, and depending on the difference between the reference voltage V ref' applied to the positive input terminal of the differential amplifier 5 and the voltage of the monitor signal, , the integrating capacitor 6 is charged and discharged. As a result, the output voltage of the control signal appearing at the output terminal of the differential amplifier 5 is determined according to the difference. This output voltage is divided by voltage dividing resistors 8 and 9, and further converted into a current by a driving transistor 1O and a resistor 11 to drive a laser diode 1a. Since the power circuit supplies a drive current to the laser diode 1a so as to cancel out the difference between the monitor signal and the reference signal, the monitor signal gradually increases due to the automatic light amount control action after the laser diode starts lighting. In a steady state, the voltage level of the monitor signal is the reference voltage V.
ref', and a constant amount of laser beam can be ensured regardless of changes in ambient temperature or slight deterioration of the laser diode. Next, when the laser diode is turned off, the lighting signal LDON changes from a low level to a high level. As a result, the inverted output terminal of the AND gate 24 becomes high level, making the analog switch 7 conductive and discharging the charge accumulated in the integrating capacitor 6. As a result, the output voltage of the differential amplifier 5 becomes equal to the reference voltage V rer. When this output voltage is divided by a pair of voltage dividing resistors 8 and 9, the resistance values of the voltage dividing resistors 8 and 9 are set so that the driving transistor 10 is not conductive, so the driving transistor 10 does not operate. Laser diode 1a is turned off.

Next, the operation of the abnormality detection circuit will be explained. As described above, when the automatic light amount control mechanism is operating normally, the voltage level of the monitor signal is equal to the reference voltage V ref. This monitor signal is input to the positive input terminal of the comparator 33, and the upper limit voltage Vult is applied to the negative input terminal. This upper limit voltage V ult is the reference voltage V ref
' is set so high that it does not exceed it under normal conditions.

Therefore, the inverted signal appearing at the output terminal of the comparator 33 is held at a low level under normal conditions. However, if the laser diode emits excessive light due to an abnormality in the automatic light intensity control mechanism, or if a short circuit occurs in the photodiode for monitoring the laser beam light intensity, the voltage level of the monitor signal may exceed the upper limit voltage level. The inverted signal that rises and appears at the output terminal of comparator 33 is inverted to a high level. - When the inversion signal becomes high level even momentarily, the flip-flop 34 is inverted and the abnormality signal LDOVER changes from low level to high level. As a result, the abnormal state that occurs in the automatic light amount control mechanism is transmitted to the outside as a change in the level of the abnormal signal. In addition, when the abnormal signal LDOVER changes from a low level to a high level, the AND gate 24 is forcibly closed and the analog switch 7 is placed in a conductive state regardless of the lighting signal LDON. 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, if an abnormal situation occurs even temporarily, the inverted signal changes, and by latching the change, the abnormal signal is continuously output. Therefore, abnormalities can be detected and effective countermeasures can be taken before a serious accident occurs.

Finally, FIG. 4 shows an application example of a barcode reader using a laser light source incorporating a semiconductor laser drive device according to the present invention. For purposes of illustration, the barcode reader has a casing 41.

The casing 41 has a built-in laser light source 42 operated by a semiconductor laser drive circuit including an abnormality detection circuit similar to that shown in FIG. The laser beam emitted in response to the lighting signal LDON is scanned and reflected by a polygon mirror 44 rotated by a scan motor 43, and then passes through a reflection mirror 45 to a bar code attached to the surface of the article. Irradiate across 46. The light that returns after irradiating the barcode 46 is reflected backward by the reflecting mirror 45 and the polygon mirror 44, is focused by the condensing lens 47, and is then received by the light receiving sensor 49 via the reflecting mirror 48. Ru. By analyzing the intensity distribution of the received light, the barcode 4B can be read. During such a reading operation, if the relay laser diode emits excessive light due to an abnormality in the automatic light amount control mechanism of the semiconductor laser drive circuit, or if a short-circuit failure occurs in the light amount monitoring photodiode, an abnormality signal is immediately generated. Once this is output, the operation of the barcode reader is stopped to prevent erroneous barcode detection. In addition, since the laser diode is forcibly turned off in response to an abnormal signal, accidents such as blindness can be prevented.

〔Effect of the invention〕

As explained above, according to the present invention, when the laser diode emits excessive light even temporarily due to a failure of the automatic light amount control mechanism, or when the photodiode for monitoring the laser beam light amount causes a short circuit abnormality, even momentarily, By configuring the structure to immediately output an abnormality signal in response to these abnormalities, there is an effect that the safety of the semiconductor laser can be improved.

[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 8.9... Voltage dividing resistor l
O... Drive transistor 24... AND gate 2B, 27 and 28... Voltage dividing resistor 33... Comparator 34... RS flip-flop 101... Monitor circuit 102... Control circuit 03... - Power circuit 104...Setting circuit 05...Abnormality detection circuit Applicant: Copal Corporation (one other person) Configuration block diagram of semiconductor laser drive circuit Figure 1 Timing chart of semiconductor laser drive circuit

Claims (1)

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