Laser diode protecting circuit and laser driving current control circuit

Disclosed is a laser diode protecting circuit adapted to prevent a laser diode from producing an excessive emission when the laser diode is driven at low temperature, thereby assuring that the laser diode will not be damaged or degraded in terms of its characteristic. When the laser diode is started at low temperature, a laser diode protecting circuit has a power monitor circuit for monitoring backward power of the laser diode and a laser diode current limiting circuit for limiting the laser diode current when the backward power becomes equal to the set power. When the laser diode temperature subsequently rises and the backward power falls below the set power, an automatic current control circuit performs automatic current control in such a manner that the laser diode current attains a set current value. Alternatively, a temperature monitor circuit monitors the temperature of the laser diode and the laser diode current limiting circuit limits the laser diode current when the monitored temperature of the laser diode is less than the set temperature. When the laser diode temperature exceeds the set temperature, the automatic current control circuit performs automatic current control in such a manner that the laser diode attains the set current value.

BACKGROUND OF THE INVENTION
 This invention relates a laser diode protecting circuit in a laser diode
 drive having an automatic current control circuit (ACC circuit) for
 performing control in such a manner that laser diode current attains a set
 value, as well as to a laser driving current control circuit in the
 above-mentioned ACC circuit. More particularly, the invention relates to a
 laser diode protecting circuit for protecting a laser diode by preventing
 an excessive emission from the laser diode when the laser diode is started
 up at low temperatures, as well as to a laser driving current control
 circuit applicable also to laser diodes of both the common-anode and
 common-cathode types.
 A deterioration in transmission characteristics due to wavelength
 fluctuation (chirping) cannot be ignored in high-speed optical
 communications. In addition, wavelength stability is extremely important
 in wavelength division multiplexing. For these reasons the laser diode
 drive is constructed by combining an ACC circuit and an ATC (Automatic
 Temperature Control) circuit and control is performed in such a manner
 that the laser diode current will attain a constant current value and the
 laser diode chip temperature (laser diode temperature) a constant
 temperature.
 FIGS. 24A, 24B are block diagrams illustrating optical transmitters used in
 digital optical communication, in which FIG. 24A shows an optical
 transmitter using a laser diode of the common-anode type, and FIG. 24B
 shows an optical transmitter using a laser diode of the common-cathode
 type. Numeral 1 in these Figures denotes a laser diode drive, 1a a
 common-anode laser diode and 1b a common-cathode laser diode. Also shown
 are an ACC circuit 2, which is constituted by an operational amplifier (OP
 amp) for performing control in such a manner that the laser diode current
 attains a set current value, an ATC circuit 3 for performing control in
 such a manner that the laser diode temperature attains a set value,
 optical fibers 4, 5, a D-type flip-flop (D-FF) 6 for storing a data signal
 DATA in response to a clock CLK, and a drive circuit (DRV) 7 for a light
 intensity modulator (IM) 8, which modulates light intensity in accordance
 with the "1", "0" logic of the data. The laser diodes are of common-anode
 type and common-cathode type, the driving currents of which have different
 directions. The laser diode 1a of common-anode type (FIG. 24A) has its
 anode connected to ground, and it is required that a driving current id be
 expelled from the laser diode 1a. The laser diode 1b of common-cathode
 type (FIG. 24B) has its cathode connected to ground, and it is required
 that a driving current id be drawn in by the laser diode 1b.
 FIGS. 25A, 25B show examples of the ACC circuit 2, in which FIG. 25A shows
 an ACC circuit of common-anode type, and FIG. 25B shows an ACC circuit of
 common-cathode type.
 In FIG. 25A, the laser diode (LD) of common-anode type is indicated at 1a.
 The ACC circuit includes resistors R1-R3 having resistance values r.sub.1
 -r.sub.3, respectively, a transistor TR1 and a comparator (current control
 circuit) IC1 constituted by an operational amplifier. The laser diode 1a,
 transistor TR1 and resistor R1 are serially connected and provided between
 ground and a negative power source -Vee. If id represents a current that
 flows through the laser diode 1a, then id.cndot.r.sub.1 will enter the
 inverting input terminal of the comparator IC1. On the other hand, a
 reference voltage V.sub.REF, obtained by voltage division by the resistors
 R2, R3, enters the non-inverting input terminal of the comparator IC1. The
 ACC circuit 2 brings the laser diode current id into line with the set
 current value by controlling the on/off operation of the transistor TR1 in
 such a manner that the terminal voltage id.cndot.r.sub.1 across the
 resistor R1 becomes equal to the reference voltage V.sub.REF. More
 specifically, the voltage V.sub.REF obtained by voltage division by the
 resistors R2, R3 becomes the voltage across the resistor R1 and a value
 obtained by dividing this voltage by the resistance value r.sub.1 becomes
 the current id that flows through the laser diode 1a. In other words, the
 base of the transistor TR1 is controlled by the comparator IC1 in such a
 manner that the resistor R1 will serve as a constant-current source the
 current value of which will be V.sub.REF /r.sub.1 at all times, thereby
 making it possible to obtain a constant current value even when the
 temperature varies.
 In FIG. 25B, the laser diode (LD) of common-cathode type is indicated at
 1b. The ACC circuit includes resistors R4-R6 having resistance values
 r.sub.4 -r.sub.6, respectively, a transistor TR2 and a comparator (current
 control circuit) IC2 constituted by an operational amplifier. The laser
 diode 1b, transistor TR2 and resistor R4 are serially connected and
 provided between ground and a positive power source +Vcc. If id represents
 a current that flows through the laser diode 1b, then id.cndot.r.sub.4
 will enter the inverting input terminal of the comparator IC2. On the
 other hand, a reference voltage V.sub.REF, obtained by voltage division by
 the resistors R5, R6, enters the non-inverting input terminal of the
 comparator IC2. This ACC circuit brings the laser diode current id into
 line with the set current value by controlling the on/off operation of the
 transistor TR2 in such a manner that the terminal voltage id.cndot.r.sub.4
 across the resistor R4 becomes equal to the reference voltage V.sub.REF.
 More specifically, the voltage V.sub.REF obtained by voltage division by
 the resistors R5, R6 becomes the voltage across the resistor R4 and a
 value obtained by dividing this voltage by the resistance value r4 becomes
 the current id that flows through the laser diode 1b. In other words, the
 base of the transistor TR2 is controlled by the comparator IC2 in such a
 manner that the resistor R4 will serve as a constant-current source the
 current value of which will be V.sub.REF /r.sub.4 at all times, thereby
 making it possible to obtain a constant current value even when the
 temperature varies.
 FIG. 26 illustrates an example of the ATC circuit. The laser diode chip is
 shown at 1a. The ATC circuit includes a Peltier device 3a for heating or
 cooling the laser diode chip 1a depending upon the direction of the
 current, and a thermister 3b having a negative resistance characteristic
 for detecting the temperature of the laser diode chip 1a. The laser diode
 1a, Peltier device 3a and thermister 3b are accommodated in a package 3c.
 The ATC circuit further includes resistors 3d, 3e, PNP, NPN transistors
 3f, 3g and a comparator 3h. A voltage Vt (which conforms to the laser
 diode temperature) resulting from voltage division by the thermister 3b
 and resistor 3d is applied to the inverting input terminal of a comparator
 3h, and a reference voltage V.sub.REF is applied to the non-inverting
 input terminal of the comparator 3h. The output terminal of the comparator
 is connected to the bases of transistors 3f, 3g. The emitter of the PNP
 transistor 3f is connected to V+, the emitter of the NPN transistor 3g is
 connected to V-, and the collectors of these transistors are connected to
 the Peltier device 3a.
 When the laser diode chip is at a low temperature, the resistance of the
 thermister 3b increases, the voltage Vt decreases to establish the
 inequality Vt&lt;Vref and the output of the comparator 3h becomes positive.
 As a result, the transistor 3f is turned off and the transistor 3g is
 turned on so that a current flows in a direction that causes the heating
 of the Peltier device 3a, thereby heating the interior of the package 3c
 and raising the temperature of the laser diode. When the temperature of
 the laser diode chip rises, the resistance of the thermister 3b decreases
 and the voltage Vt increases to establish the inequality Vt&gt;Vref so that
 the output of the comparator 3g becomes negative. As a result, the
 transistor 3f is turned on and the transistor 3g is turned off so that a
 current flows in a direction that cools the Peltier device 3a, thereby
 lowering the temperature of the laser diode. The temperature of the laser
 diode is thus controlled so as to attain the set temperature.
 When power is introduced to the optical transmitters of FIGS. 24A and 24B
 at low temperatures to drive the laser diodes 1a, 1b, the laser diode
 emits radiation excessively and the laser diode itself may be damaged. The
 reason for the excessive emission is as follows: The laser diode has a
 temperature characteristic of the kind shown in FIG. 27. It will be
 understood that the lower the temperature, the greater the power P needed
 to pass a constant laser diode current. If ACC stabilization time at which
 the laser diode current attains the set value by ACC is compared with
 stabilization at which the laser diode temperature attains the set value
 by ATC, it will be seen that ATC stabilization time is longer than ACC
 stabilization time. Consequently, when the laser diode is driven by
 introducing power at low temperature, as shown in FIG. 28 the laser diode
 current attains the set value by ACC before the laser diode chip attains
 the fixed temperature owing to the delay involved in ATC, as a result of
 which the power of the emission from the laser diode increases and becomes
 so excessive as to degrade the characteristic of the laser diode and
 eventually destroy the same. In other words, though the laser diode
 current attains the target value owing to the ACC circuit, the laser diode
 temperature does not attain its target value. Accordingly, the laser diode
 produces an emission in excess of the target value. It is necessary to
 prevent the excessive emission from the laser diode at low driving
 temperatures so that the laser diode will not be destroyed or suffer
 degradation of its characteristics.
 Further, the laser diodes are of the common-anode and common-cathode types,
 as mentioned above, the comparators (current control circuits) IC1, IC2
 used in the respective ACC circuits (see FIGS. 25A, 25B) are different and
 they must be designed and provided separately. It would be advantageous,
 therefore, if the ACC circuits of each type could make common use of a
 current control circuit, and a reduction in cost can be achieved by making
 common use of the current control circuits (i.e., by using LSI
 techniques).
 The minimum value of laser current id controlled by the ACC circuit is 0
 mA. This specification stipulating a minimum value of 0 mA is necessary
 for implementing a shut-down function, namely a function for halting
 completely the emission of laser light necessary for an optical
 transmitter. Consequently, it is required that the ACC circuits of both
 types perform control in such a manner that the voltage produced across
 the resistors R1, R4 is made 0 V, resulting in that it is required that
 the range of input voltages of the operational amplifier of the shared
 comparator (current control circuit) include the positive and negative
 power-source voltage values (+Vcc, -Vee). In other words, if v represents
 the terminal voltage of the resistors R1, R4 produced by the laser current
 id at the time of an ordinary emission, it is required that the
 operational amplifier of the shared comparator (current control circuit)
 operates at least at an input voltage within the voltage range of +Vcc to
 (+Vcc-v) or -Vee to (-Vee+v) shown at (a) of FIG. 29. This input voltage
 range can be relaxed to some extent by enlarging the resistance values
 r.sub.1, r.sub.4 of the resistors R1, R4, respectively. However, when such
 factors as a reduction in the voltage of the circuit power source and the
 maximum value of controllable current are taken into consideration, it is
 desired that the resistance values r.sub.1, r.sub.4 be several ohms to
 several tens of ohms. Hence, there is a limitation on how large r.sub.1,
 r.sub.4 can be made.
 The input voltage range of a typical operational amplifier is -Vee to
 (+Vcc-1.5) or (-Vee+1.5) to +Vcc, as shown at (b) or (c) of FIG. 29. The
 operational amplifier cannot operate when a signal within a range of about
 1.5 V from the +Vcc or -Vee of power supply level is input to the
 amplifier. This is a characteristic that inevitably accompanies an
 operational amplifier constituted by a differential pair. That is, with a
 typical operational amplifier, it is difficult to obtain an input voltage
 range [the voltage range shown at (a) of FIG. 29] in the vicinity of both
 power source voltages required for the operational amplifier of the shared
 comparator (current control circuit).
 SUMMARY OF THE INVENTION
 Accordingly, an object of the present invention is to prevent an excessive
 emission from a laser diode when the laser diode is driven at low
 temperature, thereby assuring that damage to the laser diode and
 deterioration of its characteristics will not occur.
 Another object of the present invention is to monitor emission power
 (backward power) of a laser diode to prevent an excessive emission from a
 laser diode when the laser diode is driven at low temperature, thereby
 assuring that damage to the laser diode and deterioration of its
 characteristics will not occur.
 Another object of the present invention is to monitor the temperature of a
 laser-diode chip to prevent an excessive emission from a laser diode when
 the laser diode is driven at low temperature, thereby assuring that damage
 to the laser diode and deterioration of its characteristics will not
 occur.
 A further object of the present invention is to monitor time that elapses
 from introduction of power to prevent an excessive emission from a laser
 diode when the laser diode is driven at low temperature, thereby assuring
 that damage to the laser diode and deterioration of its characteristics
 will not occur.
 Yet another object of the present invention is to make possible the common
 use of a comparator (current control circuit) employed in ACC circuits of
 both the common-anode and common-cathode types.
 Yet another object of the present invention is to arrange it so that a
 laser diode protecting circuit for preventing an excessive emission from a
 laser diode can be used commonly for laser diodes of both the common-anode
 and common-cathode types.
 In accordance with the present invention, the foregoing objects are
 attained by providing a laser diode protecting circuit of a laser diode
 drive having an automatic current control circuit for performing control
 in such a manner that laser diode current attains a set current value,
 comprising a power monitor circuit for monitoring emission power of a
 laser diode, and a laser diode current limiting circuit for halting
 automatic current control to limit laser diode current when the emission
 power exceeds a set value, and restoring automatic current control to make
 the laser diode current equal to the set current value when the emission
 power falls below the set value.
 Further, in accordance with the present invention, the following objects
 are attained by providing a laser diode protecting circuit of a laser
 diode drive having an automatic current control circuit for performing
 control in such a manner that laser diode current attains a set current
 value, comprising a temperature monitor circuit for monitoring temperature
 of a laser diode, and a laser diode current limiting circuit for halting
 automatic current control to limit laser diode current when the
 temperature of the laser diode is less than a set temperature, and
 restoring automatic current control to make the laser diode current equal
 to the set current value when the temperature of the laser diode is
 greater than the set temperature.
 Further, in accordance with the present invention, the following objects
 are attained by providing a laser diode protecting circuit of a laser
 diode drive having an automatic current control circuit for performing
 control in such a manner that laser diode current attains a set current
 value, comprising an elapsed-time monitor circuit for monitoring time that
 elapses from introduction of power, and a laser diode current limiting
 circuit for halting automatic current control to limit laser diode current
 when the time that elapses from introduction of power has not attained a
 set time, and restoring automatic current control to make the laser diode
 current equal to the set current value when the time that elapses from
 introduction of power has attained the set time.
 Further, in accordance with the present invention, the foregoing objects
 are attained by constructing an automatic current control circuit of a
 laser diode drive in such a manner that the circuit can be used commonly
 in laser diodes of both the common-anode and common-cathode types.
 Further, in accordance with the present invention, the foregoing objects
 are attained by constructing a laser diode current limiting circuit, which
 prevents an excessive emission from a laser diode, in such a manner that
 the circuit can be used commonly in laser diodes of both the common-anode
 and common-cathode types.
 Other features and advantages of the present invention will be apparent
 from the following description taken in conjunction with the accompanying
 drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 (A) Principles of the Present Invention
 (a) First principle of the present invention
 FIG. 1 is a block diagram useful in describing a first principle according
 to the present invention. Shown in FIG. 1 is a laser diode (LD) 11, an ACC
 circuit 12 for performing control in such a manner that the laser diode
 current attains a set current value, an ATC circuit 13 for performing
 control in such a manner that the laser diode temperature is rendered
 constant, an optical fiber 14, and a power monitor circuit 15 for
 monitoring emission power of the laser diode 11. The power monitor circuit
 15 detects the backward power (BP) of the laser diode as the emission
 power thereof. A laser diode current limiting circuit 16 limits the
 current of the laser diode when the emission power exceeds a set power and
 includes a reference voltage generator 16a for outputting a reference
 voltage Vr conforming to set power, and a comparator 16b for comparing a
 voltage Vp, which conforms to detected emission power, with the reference
 voltage Vr. Though not shown, a comparator (current control circuit)
 designed to be sharable by laser diodes of both common-anode and
 common-cathode types is used in the ACC circuit 12.
 When the laser diode is driven at low temperature and the laser diode
 current increases and approaches the set current value under ACC before
 the set temperature is attained owing to the lag associated with ATC, the
 emission power of the laser diode rises. When the emission power rises and
 surpasses the set power, the laser diode current limiting circuit 16
 causes the ACC circuit 12 to halt ACC and limit the laser diode current,
 thereby preventing an excessive emission from the laser diode. If the
 temperature rises and the emission power of the laser diode falls below
 the set power owing to ATC under these conditions, the laser diode current
 limiting circuit 16 restores the ACC function of the ACC circuit 12 so
 that the laser diode current attains the set current value. As a result of
 the foregoing operation, the emission power (backward power) of the laser
 diode is monitored and excessive emission from the laser diode when the
 diode is driven at low temperatures is prevented so that the laser diode
 will not be damaged or sustain a deterioration in characteristics.
 Further, the laser diode current limiting circuit 16 for preventing the
 excessive emission from the laser diode is constructed so as to be
 sharable by laser diodes of both common-anode and common-cathode types.
 For example, when emission power is greater than a set value, (1) laser
 diode current is limited by causing current to flow into the ACC circuit
 12 from the laser diode current limiting circuit 16 if the diode is of the
 common-anode type, and (2) laser diode current is limited by causing
 current that is to flow into the laser diode to flow to the outside from
 the ACC circuit 12.
 (b) Second principle of the present invention
 FIG. 2 is a block diagram useful in describing a second principle according
 to the present invention. Shown in FIG. 1 is the laser diode (LD) 11, the
 ACC circuit 12 for performing control in such a manner that the laser
 diode current attains a set current value, the ATC circuit 13 for
 performing control in such a manner that the laser diode temperature is
 rendered constant, the optical fiber 14, the laser diode current limiting
 circuit 16 for limiting the laser diode current when the laser diode
 temperature falls below the set temperature, and a temperature monitoring
 circuit 21 for monitoring the temperature of the laser diode chip. The
 laser diode current limiting circuit 16 includes the reference voltage
 generator 16a for outputting a reference voltage Vr conforming to set
 power, and the comparator 16b for comparing a voltage V.sub.T, which
 conforms to detected temperature, with the reference voltage Vr. Though
 not shown, a comparator (current control circuit) designed to be sharable
 by laser diodes of both common-anode and common-cathode types is used in
 the ACC circuit 12.
 When the laser diode is driven at low temperatures, the temperature of the
 laser diode initially is less than the set temperature. Consequently, the
 laser diode current limiting circuit 16 causes the ACC circuit 12 to halt
 ACC and limit the laser diode current, thereby preventing an excessive
 emission from the laser diode at low temperatures. If the temperature of
 the laser diode rises and exceeds the set temperature owing to ATC under
 these conditions, the laser diode 11 will not emit light excessively even
 though the laser diode current increases. The limiting circuit 16,
 therefore, restores the ACC function of the ACC circuit 12 so that the
 laser diode current attains the set current value. As a result of the
 foregoing operation, the laser diode temperature is monitored and
 excessive emission from the laser diode when the diode is driven at low
 temperatures is prevented so that the laser diode will not be damaged or
 sustain a deterioration in characteristics.
 Further, the laser diode current limiting circuit 16 for preventing the
 excessive emission from the laser diode is constructed so as to be
 sharable by laser diodes of both common-anode and common-cathode types.
 For example, when laser diode temperature is less than a set value, (1)
 laser diode current is limited by causing current to flow into the ACC
 circuit 12 from the laser diode current limiting circuit 16 if the diode
 is of the common-anode type, and (2) laser diode current is limited by
 causing current that is to flow into the laser diode to flow to the
 outside from the ACC circuit 12.
 (c) Third principle of the present invention
 FIG. 3 is a block diagram useful in describing a third principle according
 to the present invention. Shown in FIG. 3 is the laser diode (LD) 11, the
 ACC circuit 12 for performing control in such a manner that the laser
 diode current attains a set current value, the ATC circuit 13 for
 performing control in such a manner that the laser diode temperature is
 rendered constant, the optical fiber 14, the laser diode current limiting
 circuit 16 for halting ACC to limit the laser diode current when time that
 elapses from introduction of power reaches a set time, and an elapsed-time
 monitor circuit 31 for monitoring time that elapses from introduction of
 power. For example, the elapsed-time monitor circuit 31 is constituted by
 a delay circuit (integrator circuit) in which output voltage V.sub.L
 increases as time elapses from the moment power is introduced. The laser
 diode current limiting circuit 16 includes the reference voltage generator
 16a for outputting a reference voltage Vr conforming to set time, and the
 comparator 16b for comparing the voltage V.sub.L, which conforms to the
 aforementioned elapsed time, with the reference voltage Vr. Though not
 shown, a comparator (current control circuit) designed to be sharable by
 laser diodes of both common-anode and common-cathode types is used in the
 ACC circuit 12.
 The laser diode current limiting circuit 16 halts ACC to limit the laser
 diode current until the elapsed time from introduction of power reaches
 the set value, thereby preventing an excessive emission from the laser
 diode. When the elapsed time surpasses the set time, the temperature rises
 owing to ATC and the laser diode 11 will not emit light excessively even
 though the laser diode current increases. The limiting circuit 16,
 therefore, restores the ACC function of the ACC circuit 12 so that the
 laser diode current is regulated to the set value. As a result of the
 foregoing operation, elapsed time from introduction of power is monitored
 and excessive emission from the laser diode when the diode is driven at
 low temperatures is prevented so that the laser diode will not be damaged
 or sustain a deterioration in characteristics.
 Further, the laser diode current limiting circuit 16 for preventing the
 excessive emission from the laser diode is constructed so as to be
 sharable by laser diodes of both common-anode and common-cathode types.
 For example, when elapsed time following introduction of power has not
 attained a set time, (1) laser diode current is limited by causing current
 to flow into the ACC circuit 12 from the laser diode current limiting
 circuit 16 if the diode is of the common-anode type, and (2) laser diode
 current is limited by causing current that is to flow into the laser diode
 to flow to the outside from the ACC circuit 12.
 (B) Embodiments for Preventing Excessive Emission by detecting emission
 power
 (a) First embodiment
 FIG. 4 is a diagram showing the construction of a first embodiment in which
 excessive emission is prevented by detecting emission power (backward
 power) of the laser diode. Components identical with those shown in FIG. 1
 are designated by identical reference characters. The laser diode (LD) 11a
 of common-anode type is housed in a package PKG along with a Peltier
 device 17, a thermister 18 for detecting the temperature of the laser
 diode chip and a PIN photodiode 19 for detecting the backward power
 (optical intensity) of the laser diode.
 The ACC circuit 12, which is for performing control in such a manner that
 the laser diode current will attain the set current value, has a
 construction identical with that of the ACC circuit shown in FIG. 25A and
 is connected to a current source 16d (described later) that passes a
 current into the resistor R1. If the backward power is less than the set
 power, there is no inflow of current from the current source 16d and
 therefore the ACC circuit 12 performs ACC in such a manner that the laser
 diode current id attains the set current value. If the backward power is
 greater than the set power, on the other hand, current flows into the
 resistor R1 from the current source 16d. Consequently, the ACC circuit 12
 halts the ACC function and reduces the laser diode current id by an amount
 commensurate with the amount of current inflow, thereby limiting the
 current value of the laser diode.
 The ATC circuit 13, which performs control in such a manner that the laser
 diode temperature is rendered constant, has a construction identical with
 that of the ATC circuit shown in FIG. 26. The power monitor circuit 15
 uses the PIN photodiode 19 to output a voltage signal (referred to as a
 "backward power monitor signal BP") conforming to backward power. The
 laser diode current limiting circuit 16 limits the laser diode current
 when backward power exceeds the set power.
 The laser diode current limiting circuit 16 includes the reference voltage
 generator 16a for outputting the reference voltage Vr through voltage
 division by resistors R7, R8, the comparator 16b constituted by an
 operational amplifier for comparing the voltage Vp dependent upon detected
 emission power (the larger the detected power, the smaller the voltage Vp)
 with the reference voltage Vr, a voltage follower circuit 16c and the
 current source 16d. The current source 16d has a transistor TR2 that turns
 on when the backward power is greater than the set power (i.e., when Vp&lt;Vr
 holds) and turns off when the backward power is less than the set power
 (i.e., when Vp.gtoreq.Vr holds), and a resistor R4 connected across the
 collector of transistor TR2 and ground. The emitter of the transistor TR2
 is connected to the resistor R1 of ACC circuit 12.
 By utilizing the fact that the forward power of the laser diode and the
 backward power BP of the laser diode are proportionally related, the
 reference voltage Vr is set in advance so as to attain a backward power
 monitor signal voltage corresponding to that which would prevail when the
 forward power is some multiple of the usual value.
 When power is introduced to the laser diode drive circuit at low
 temperatures in the arrangement described above, the voltage Vp of the
 backward power monitor signal BP is smaller than the reference voltage Vr
 (Vp&lt;Vr) during the time that temperature of the laser diode is
 stabilizing. The output of the comparator 16b declines until the
 transistor TR2 turns on. As a result, the voltage across the resistor R1
 of the ACC circuit 12 is decided by the collector current (the limit
 current) i.sub.L of transistor TR2 and the laser diode current id
 decreases. In other words, when the backward power (the emission power)
 increases and the voltage value Vp of the backward power monitor signal BP
 becomes smaller than the reference voltage Vr (i.e., when the laser diode
 11a is about to emit excessively), the transistor TR2 turns on and the
 laser diode current id is limited to establish the relation Vp=Vr, thereby
 preventing the excessive emission.
 The operation described above is such that when the laser diode is driven
 at low temperatures, the laser diode current takes on a large value under
 ACC before the laser diode temperature rises to the target temperature
 under ATC, as shown in FIG. 5. As a result, when the laser diode 11a is
 about to emit light excessively (time t.sub.1), the laser diode drive
 circuit functions in such a manner that the emission power is rendered
 constant. This is the APC (Automatic Power Control) mode. When the
 temperature of the laser diode 11a subsequently stabilizes at the set
 temperature owing to ATC (time t.sub.2), the voltage Vp of the backward
 power monitor signal BP becomes higher than the reference voltage Vr.
 Consequently, the output of the comparator 16b rises and the transistor
 TR2 of the current source 16d turns off. As a result, the ACC circuit 12
 restores the ACC function and performs control in such a manner that the
 laser diode current id takes on a current value determined by the
 resistance ratio of the resistors R2, R3. This is the ACC mode.
 This circuit arrangement is advantageous in that it functions to prevent an
 excessive emission from the laser diode in a case where power is
 introduced at low temperature and, at the same time, can be employed as an
 APC circuit as well when it is used with the transistor TR2 in the ON
 state at times.
 (b) Modification
 Though a common-anode laser diode is used in the first embodiment of FIG.
 4, it is also possible to adopt an arrangement using a common-cathode
 laser diode. FIGS. 6A, 6B are diagrams showing arrangements of
 common-anode type and common-cathode type for preventing excessive
 emission by detecting emission power. FIG. 6A is a diagram showing the
 arrangement of common-anode type, and FIG. 6B is a diagram showing the
 arrangement of common-cathode type. Shown in these Figures are an ACC
 circuit 12 of common-anode type and an ACC circuit 12' of common-cathode
 type, the constructions of which are illustrated in FIGS. 25A and 25B,
 respectively. Numeral 15 denotes the power monitor and 16 the laser diode
 current limiting circuit.
 With the ACC circuit 12 of common-anode type, the limit current i.sub.L is
 passed into the resistor R1, thereby enlarging the terminal voltage of the
 resistor R1 and limiting the laser diode current id. With the ACC circuit
 12' of common-cathode type, on the other hand, the laser current id is
 limited by causing some of the current (the limit current i.sub.L) that
 flows through the resistor R4 to flow to the outside without passing
 through the laser diode.
 (c) Second embodiment
 FIG. 7 is a diagram showing a second embodiment in which excessive emission
 is prevented by detecting emission power (backward power) of the laser
 diode. Components identical with those shown in FIG. 1 are designated by
 identical reference characters. Here the laser diode (LD) 11a of
 common-anode type is housed in the package PKG along with the Peltier
 device 17, the thermister 18 for detecting the temperature of the laser
 diode chip and the PIN photodiode 19 for detecting the backward power
 (optical intensity) of the laser diode.
 The ACC circuit 12, which is for performing control in such a manner that
 the laser diode current will attain the set current value, has a
 construction identical with that of the ACC circuit shown in FIG. 25A. In
 addition, the output terminal of a reference voltage reducing circuit
 (described later) 16e is connected to the non-inverting input terminal of
 the comparator IC1. If the backward power is less than the set power, the
 reference voltage reducing circuit 16e does not reduce the reference
 voltage V.sub.REF of the comparator IC1. Consequently, the ACC circuit 12
 performs ACC in such a manner that the laser diode current id attains the
 set current value (=V.sub.REF /r1). If the backward power is greater than
 the set power, on the other hand, the reference voltage reducing circuit
 16e reduces the reference voltage V.sub.REF input to the non-inverting
 input terminal of the comparator IC1. The ACC circuit 12 performs control
 in such a manner that the terminal voltage (=id.cndot.r1) of the resistor
 R1 that prevailed at flow of the laser diode current id becomes equal to
 the reference voltage V.sub.REF. When the reference voltage decreases,
 therefore, the laser diode current id is limited to a small value. The ATC
 circuit 13 performs control so as to render the laser diode temperature
 constant and has a construction identical with that of the ATC circuit
 shown in FIG. 26. The power monitor circuit 15 uses the PIN photodiode 19
 to output a voltage signal (the backward power monitor signal) conforming
 to backward power. The laser diode current limiting circuit 16 limits the
 laser diode current when backward power exceeds the set power.
 The laser diode current limiting circuit 16 includes the reference voltage
 generator 16a for outputting the reference voltage Vr through voltage
 division by resistors R7, R7, the comparator 16b constituted by an
 operational amplifier for comparing the voltage Vp dependent upon detected
 emission power with the reference voltage Vr, and the reference voltage
 reducing circuit 16e, which is constituted by a diode (DD). The reference
 voltage reducing circuit 16e is connected across the output terminal of
 the comparator 16b and the non-inverting input terminal of the comparator
 IC1 via the diode DD in the polarity shown.
 The diode DD is forward biased when the backward power is greater than the
 set power (Vp&lt;Vr). Consequently, the reference voltage VREF of the
 comparator IC1 in the ACC circuit 12 takes on a low potential higher than
 that of the output of comparator 16b by the voltage across the diode,
 whereby the laser diode current id is limited. If the backward power is
 less than the set power, on the other hand (Vp.gtoreq.Vr), the diode DD is
 reverse biased and, hence, the reference voltage VREF of the comparator
 IC1 of ACC circuit 12 takes on a large value obtained by voltage division
 by the resistors R2, R3. The ACC circuit 12 performs control in such a
 manner that id.cndot.r1 becomes equal to the reference voltage V.sub.REF,
 i.e., in such a manner that the laser diode current id becomes equal to
 V.sub.REF /r1.
 By utilizing the fact that the forward power of the laser diode and the
 backward power BP of the laser diode are proportionally related, the
 reference voltage Vr is set in advance so as to attain a backward power
 monitor signal voltage corresponding to that which would prevail when the
 forward power is some multiple of the usual value.
 When power is introduced to the laser diode drive circuit at low
 temperatures in the arrangement described above, the voltage Vp of the
 backward power monitor signal BP is smaller than the reference voltage Vr
 (Vp&lt;Vr) during the time that temperature of the laser diode is
 stabilizing. The output of the comparator 16b declines until the diode DD
 turns on, and the laser diode current id is decided not by the resistance
 dividing ratio but by a low potential higher than that of the output of
 comparator 16b by the voltage across the diode.
 More specifically, when the backward power (emission power) rises and the
 voltage value Vp of the backward power monitor signal BP falls below the
 reference voltage Vr (i.e., when the laser diode 11a is about to emit
 light excessively), the diode DD turns on (is forward biased) to limit the
 laser diode current id. This is the APC (Automatic Power Control) mode.
 When the temperature of the laser diode 11a subsequently stabilizes at the
 set temperature owing to ATC, the voltage Vp of the backward power monitor
 signal BP becomes higher than the reference voltage Vr. Consequently, the
 output of the comparator 16b rises, the diode DD turns off (is reversed
 biased) and the laser diode current id takes on a current value determined
 by the resistance ratio of the resistors R2, R3. This is the ACC mode.
 This circuit arrangement is advantageous in that it functions to prevent an
 excessive emission from the laser diode in a case where power is
 introduced at low temperature and, at the same time, can be employed as an
 APC circuit as well when it is used with the diode DD in the ON state at
 times.
 Though a common-anode laser diode is used in the second embodiment of FIG.
 7, it is also possible to adopt an arrangement using a common-cathode
 laser diode.
 (C) Embodiments for Preventing Excessive Emission by Detecting Laser Diode
 Temperature
 (a) First embodiment
 FIG. 8 is a diagram showing the construction of a first embodiment in which
 excessive emission is prevented by detecting the temperature of the laser
 diode chip. This embodiment has a construction similar to that of the
 embodiment of FIG. 4 in which an excessive emission is prevented by
 detecting emission power. Components identical with those shown in FIG. 4
 are designated by identical reference characters. This embodiment differs
 in the following respects:
 (1) The power monitor circuit 15 of FIG. 4 is deleted.
 (2) The input to the laser diode current limiting circuit 16 is a signal (a
 temperature monitor signal TP), which conforms to the laser diode
 temperature, output by the temperature monitor circuit 21 provided in the
 ATC circuit 13.
 (3) The reference voltage Vr is set to a voltage that corresponds to a
 temperature several degrees Centigrade lower than the target temperature
 of ATC.
 The chip temperature of the laser diode 11a is monitored by the temperature
 monitoring circuit 21, the temperature monitor signal TP conforming to
 this temperature is input to the voltage follower 16c, and a voltage
 V.sub.T conforming to the laser diode temperature enters the comparator
 16b from the voltage follower 16c. The comparator 16b compares the
 reference voltage Vr, which is decided by the resistors R7, R8, with the
 voltage V.sub.T conforming to the laser diode temperature. The reference
 voltage Vr is set to a voltage value output by the temperature monitoring
 circuit 21 at a temperature several degrees Centigrade lower than the
 target temperature of ATC. Accordingly, when power is introduced to the
 laser diode drive circuit at low temperatures, the voltage V.sub.T output
 by the temperature monitoring circuit 21 takes on a potential in the
 negative direction with respect to the reference voltage Vr (V.sub.T &lt;Vr)
 during the time that the temperature of the laser diode is stabilizing
 (i.e., until the set temperature is attained). During the time that
 V.sub.T &lt;Vr holds, the output of the comparator 16b declines until the
 transistor TR2 of the current source 16d is capable of turning on. The
 current source 16d passes a current i.sub.L into the resistor R1 of the
 ACC circuit 12. Consequently, the inverting input of the comparator IC1
 becomes greater than V.sub.REF, which is at the non-inverting input
 terminal of the comparator, the transistor TR1 turns off and the laser
 diode current id becomes zero. Thus, during the time that the laser diode
 11a is at a temperature that would result in an excessive emission, the
 transistor TR2 is rendered conductive to reduce the laser diode current
 id, as a result of which the laser diode 11a does not emit light
 excessively. Damage to the laser diode 11a is thus prevented and so is
 deterioration of the laser diode characteristics. This is the ACC
 termination mode.
 If the temperature of the laser diode 11a subsequently rises owing to ATC
 and the voltage VT output by the temperature monitoring circuit 21 attains
 a potential higher than that of the reference voltage Vr, the output of
 the comparator 16b becomes positive and the transistor TR2 turns off. As a
 result, the ACC circuit 12 subsequently restores the ACC function so that
 the laser diode current id will take on a current value conforming to the
 reference voltage V.sub.REF decided by the resistors R2, R3. This is the
 ACC mode.
 The operation described above is such that when the laser diode is driven
 at low temperatures, the laser diode current id is zero and so is the
 emission power (optical output) of the laser diode until the laser diode
 temperature rises to a temperature several degrees Centigrade lower than
 the target temperature of ATC, as shown in FIG. 9. This is the ACC
 termination mode. In other words, during the time that the laser diode is
 at a temperature that would result in an excessive emission, the laser
 diode current id is zero and an excessive emission from the laser diode is
 prevented. When the temperature of the laser diode 11a subsequently rises
 under ATC and attains a temperature several degrees Centigrade lower than
 the target temperature of ATC (time t.sub.1), the ACC circuit 12 restores
 the ACC function and performs control in such a manner that the laser
 diode current id takes on a current value determined by the resistance
 ratio of the resistors R2, R3. This is the ACC mode. Since ACC is thus
 performed after the temperature rises, an excessive laser diode emission
 does not occur. It should be noted that although the laser diode current
 id is made zero in the ACC termination mode in the foregoing description,
 this is not an essential requisite. In other words, it will suffice to
 pass such a laser diode current that does not cause the laser diode to
 emit light excessively.
 By virtue of the foregoing operation, the laser diode current is forcibly
 reduced in a region wherein an excessive emission is most likely to occur
 immediately after the introduction of power at low temperature.
 (b) Modification
 Though a common-anode laser diode is used in the first embodiment of FIG.
 8, it is also possible to adopt an arrangement using a common-cathode
 laser diode. FIGS. 10A, 10B are diagrams showing arrangements of
 common-anode type and common-cathode type for preventing excessive
 emission by detecting laser diode temperature. FIG. 10A is a diagram
 showing the arrangement of common-anode type, and FIG. 10B is a diagram
 showing the arrangement of common-cathode type. Shown in these Figures are
 the ACC circuit 12 of common-anode type and the ACC circuit 12' of
 common-cathode type, the constructions of which are illustrated in FIGS.
 25A and 25B, respectively. Numeral 16 denotes the laser diode current
 limiting circuit and 21 the temperature monitor.
 With the ACC circuit 12 of common-anode type, the limit current i.sub.L is
 passed into the resistor R1, thereby enlarging the terminal voltage of the
 resistor R1 and limiting the laser diode current. With the ACC circuit 12'
 of common-cathode type, on the other hand, the laser current is limited by
 causing some of the current (the limit current i.sub.L) that flows through
 the resistor R4 to flow to the outside without passing through the laser
 diode.
 (c) Second embodiment
 FIG. 11 is a diagram showing the construction of a second embodiment in
 which excessive emission is prevented by detecting the temperature of the
 laser diode chip. This embodiment has a construction similar to that of
 the embodiment of FIG. 7 in which an excessive emission is prevented by
 detecting emission power. Components identical with those shown in FIG. 7
 are designated by identical reference characters. This embodiment differs
 in the following respects:
 (1) The power monitor circuit 15 of FIG. 7 is deleted.
 (2) The input to the laser diode current limiting circuit 16 (the
 non-inverting input terminal of the comparator 16b) is the voltage
 V.sub.T, which conforms to the laser diode temperature, output by the
 temperature monitor circuit 21 provided in the ATC circuit 13.
 (3) The reference voltage Vr is set to a voltage that corresponds to a
 temperature several degrees Centigrade lower than the target temperature
 of ATC.
 The chip temperature of the laser diode 11a is monitored by the temperature
 monitoring circuit 21 and the voltage V.sub.T conforming to the laser
 diode temperature enters the comparator 16b. The comparator 16b compares
 the reference voltage Vr, which is decided by the resistors R7, R8, with
 the voltage V.sub.T conforming to the laser diode temperature. The
 reference voltage Vr is set to a voltage value output by the temperature
 monitoring circuit 21 at a temperature several degrees Centigrade lower
 than the target temperature of ATC. Accordingly, when power is introduced
 to the laser diode drive circuit at low temperatures, the voltage V.sub.T
 output by the temperature monitoring circuit 21 takes on a potential in
 the negative direction with respect to the reference voltage Vr (V.sub.T
 &lt;Vr) during the time that temperature of the laser diode 11a is
 stabilizing (i.e., until the set temperature is attained). During the time
 that V.sub.T &lt;Vr holds, the output of the comparator 16b declines until
 the diode DD of the current source 16d is capable of turning on. The laser
 diode current id is decided not by the resistance dividing ratio but by a
 low potential higher than that of the output of comparator 16b by the
 voltage across the diode. Thus the laser diode current is limited to such
 a current value that will not result in excessive emission.
 Thus, during the time that the laser diode 11a is at a low temperature that
 would result in an excessive emission, the diode DD is rendered conductive
 to reduce the laser diode current id, as a result of which the laser diode
 11a does not emit excessively. Damage to the laser diode 11a is thus
 prevented and so is deterioration of the laser diode characteristics. This
 is the ACC termination mode.
 If the temperature of the laser diode 11a subsequently rises owing to ATC
 and the voltage VT output by the temperature monitoring circuit 21 attains
 a potential higher than that of the reference voltage Vr, the output of
 the comparator 16b becomes positive and the diode DD turns off. As a
 result, the ACC circuit 12 subsequently restores the ACC function so that
 the laser diode current id will take on a current value conforming to the
 reference voltage VREF decided by the resistors R2, R3. This is the ACC
 mode.
 Though a common-anode laser diode is used in the second embodiment of FIG.
 11, it is also possible to adopt an arrangement using a common-cathode
 laser diode.
 (D) Embodiments for Preventing Excessive Emission by Monitoring Time that
 Elapses from Introduction of Power
 (a) First embodiment
 FIG. 12 is a diagram showing the construction of first embodiment in which
 excessive emission is prevented by monitoring time that elapses from
 introduction of power. This embodiment has a construction similar to that
 of the embodiment of FIG. 4 in which an excessive emission is prevented by
 detecting emission power. Components identical with those shown in FIG. 4
 are designated by identical reference characters. This embodiment differs
 in the following respects:
 (1) The power monitor circuit 15 and voltage follower 16c of FIG. 4 is
 deleted.
 (2) The elapsed-time monitor circuit 31 is provided for monitoring time
 that elapses from introduction of power and the input to the laser diode
 current limiting circuit 16 (the non-inverting terminal of the comparator
 16b) is a reference voltage V.sub.L, which conforms to elapsed time,
 output by the elapsed-time monitor circuit 31.
 (3) A voltage corresponding to the time needed for the laser diode
 temperature to attain a substantially constant temperature by ATC
 following the introduction of power is set as a reference voltage Vr.
 The elapsed-time monitor circuit 31 is constituted by an integrator circuit
 composed of a resistor R10 and a capacitor C1 and has a power supply
 voltage applied thereto. If power is introduced to the laser diode drive
 at low temperature, the capacitor terminal voltage (output voltage
 V.sub.L) rises exponentially at a time constant R10.cndot.C1 from the
 moment power introduction. The comparator 16b compares the reference
 voltage Vr, which is decided by the resistor R7, R8, with the reference
 voltage V.sub.L that conforms to elapsed time measured from the moment of
 power introduction. The reference voltage Vr is a voltage that corresponds
 to the time needed for the laser diode temperature to attain a
 substantially constant temperature by ATC following the introduction of
 power. Consequently, the voltage V.sub.L output by the elapsed-time
 monitor circuit 31 takes on a potential in the negative direction with
 respect to the reference voltage Vr (V.sub.L &lt;Vr) during the time that the
 temperature of the laser diode 11a is stabilizing.
 During the time that V.sub.L &lt;Vr holds, the output of the comparator 16b
 declines until the transistor TR2 of the current source 16d is capable of
 turning on. The current source 16d passes a current into the resistor R1
 of the ACC circuit 12. Consequently, the inverting input of the comparator
 IC1 becomes greater than V.sub.REF, which is at the non-inverting input
 terminal of the comparator, the transistor TR1 turns off and the laser
 diode current id becomes zero. Thus, the transistor TR2 is driven into
 conduction to reduce the laser diode current id until the laser diode
 temperature becomes substantially constant, i.e., until the elapse of a
 period of time in which an excessive emission from the laser diode 11a is
 possible. Damage to the laser diode 11a is thus prevented and so is
 deterioration of the laser diode characteristics. This is the ACC
 termination mode.
 If the temperature of the laser diode 11a subsequently rises and attains a
 substantially constant temperature owing to ATC, the voltage V.sub.L
 output by the elapsed-time monitor circuit 31 attains a potential higher
 than that of the reference voltage Vr. If the relation V.sub.L &gt;Vr is
 established, the output of the comparator 16b becomes positive and the
 transistor TR2 turns off. As a result, the ACC circuit 12 subsequently
 restores the ACC function so that the laser diode current id will take on
 a current value conforming to the reference voltage V.sub.REF decided by
 the resistors R2, R3. This is the ACC mode.
 The operation described above is such that when the laser diode is driven
 at low temperatures, the laser diode current id is zero and so is the
 emission power of the laser diode until the laser diode temperature
 stabilizes, as shown in FIG. 13. This is the ACC termination mode. In
 other words, during the time that the laser diode is at a low temperature
 that would result in an excessive emission, the laser diode current id is
 zero and an excessive emission from the laser diode is prevented. When the
 temperature of the laser diode 11a subsequently stabilizes under ATC (time
 t.sub.1), the ACC circuit 12 restores the ACC function and performs
 control in such a manner that the laser diode current id takes on a
 current value determined by the resistance ratio of the resistors R2, R3.
 This is the ACC mode. Since ACC is thus performed after the temperature
 rises, an excessive laser diode emission does not occur. It should be
 noted that although the laser diode current id is made zero in the ACC
 termination mode in the foregoing description, this is not an essential
 requisite. In other words, it will suffice to pass such a laser diode
 current that does not cause the laser diode to emit light excessively.
 By virtue of the foregoing operation, the ACC circuit 12 does not operate
 for several seconds following the introduction of power. At
 low-temperature start-up, therefore, first the ATC circuit 13 functions to
 stabilize temperature. Once temperature has stabilized, the ACC circuit 12
 begins operating. This makes it possible to prevent an excessive emission
 from the laser diode.
 (b) Modification of first embodiment
 FIG. 14, which illustrates a modification of the first embodiment, is an
 example in which the circuit 31 for monitoring elapse of time from
 introduction of power is constituted by a counter and a DA converter. More
 specifically, the elapsed-time monitor circuit 31 includes an oscillator
 31a which oscillates at a constant frequency, a counter 31b and a DA
 converter 31c for converting the digital count from the counter 31b to an
 analog signal. The counter 31b counts pulses output by the oscillator 31a
 after power is introduced, and the DA converter 31c subjects the count
 from the counter to digital-to-analog conversion to output a voltage
 signal V.sub.L that increases in proportion to elapsed time.
 (c) Other modification of first embodiment
 Though a common-anode laser diode is used in the first embodiment of FIG.
 12, it is also possible to adopt an arrangement using a common-cathode
 laser diode. FIGS. 15A, 15B are diagrams showing arrangements of
 common-anode type and common-cathode type for preventing excessive
 emission by detecting elapsed time. FIG. 15A is a diagram showing the
 arrangement of common-anode type, and FIG. 15B is a diagram showing the
 arrangement of common-cathode type. Shown in these Figures are the ACC
 circuit 12 of common-anode type and the ACC circuit 12' of common-cathode
 type, the constructions of which are illustrated in FIGS. 25A and 25B,
 respectively. Numeral 16 denotes the laser diode current limiting circuit
 and 31 the temperature monitor.
 With the ACC circuit 12 of common-anode type, the limit current i.sub.L is
 passed into the resistor R1, thereby enlarging the terminal voltage of the
 resistor R1 and limiting the laser diode current. With the ACC circuit 12'
 of common-cathode type, on the other hand, the laser current is limited by
 causing some of the current (the limit current i.sub.L) that flows through
 the resistor R4 to flow to the outside without passing through the laser
 diode.
 (d) Second embodiment
 FIG. 16 is a diagram showing the construction of a second embodiment in
 which excessive emission is prevented by monitoring elapse of time from
 introduction of power. This embodiment has a construction similar to that
 of the embodiment of FIG. 7 in which an excessive emission is prevented by
 detecting emission power. Components identical with those shown in FIG. 7
 are designated by identical reference characters. This embodiment differs
 in the following respects:
 (1) The power monitor circuit 15 of FIG. 7 is deleted.
 (2) The elapsed-time monitor circuit 31 is provided for monitoring time
 that elapses from introduction of power and the input to the laser diode
 current limiting circuit 16 (the non-inverting terminal of the comparator
 16b) is a reference voltage V.sub.L, which conforms to elapsed time,
 output by the elapsed-time monitor circuit 31.
 (3) A voltage corresponding to the time needed for the laser diode
 temperature to attain a substantially constant temperature by ATC
 following the introduction of power is set as a reference voltage Vr.
 The elapsed-time monitor circuit 31 is constituted by an integrator circuit
 composed of a resistor R10 and a capacitor C1 and has a power supply
 voltage applied thereto. If power is introduced to the laser diode drive
 at low temperature, the capacitor terminal voltage (output voltage
 V.sub.L) rises exponentially at a time constant R10.cndot.C1 from the
 moment of power introduction. The comparator 16b compares the reference
 voltage Vr, which is decided by the resistor R7, R8, with the reference
 voltage V.sub.L that conforms to elapsed time measured from the moment of
 power introduction. The reference voltage Vr is a voltage that corresponds
 to the time needed for the laser diode temperature to attain a
 substantially constant temperature by ATC following the introduction of
 power. Consequently, the voltage V.sub.L output by the elapsed-time
 monitor circuit 31 takes on a potential in the negative direction with
 respect to the reference voltage Vr (V.sub.L &lt;Vr) during the time that the
 temperature of the laser diode 11a is stabilizing.
 During the time that V.sub.L &lt;Vr holds, the output of the comparator 16b
 declines until the diode DD is capable of turning on. The laser diode
 current id is decided by a low potential higher than that of the output of
 comparator 16b by the voltage across the diode, whereby the laser diode
 current id is limited. Thus, the diode DD is driven into conduction to
 reduce the laser diode current id during the period of low temperature
 over which it is likely that the laser diode 11a will emit light
 excessively. This is the ACC termination mode.
 If the temperature of the laser diode 11a subsequently rises and attains a
 substantially constant temperature owing to ATC, the voltage V.sub.L
 output by the elapsed-time monitor circuit 31 attains a potential higher
 than that of the reference voltage Vr. If the relation V.sub.L &gt;Vr is
 established, the output of the comparator 16b becomes positive and the
 diode DD turns off. As a result, the ACC circuit 12 subsequently restores
 the ACC function so that the laser diode current id will take on a current
 value conforming to the reference voltage V.sub.REF decided by the
 resistors R2, R3. This is the ACC mode.
 By virtue of the foregoing operation, the ACC circuit 12 does not operate
 for several seconds following the introduction of power. At
 low-temperature start-up, therefore, first the ATC circuit 13 functions to
 stabilize temperature. Once temperature has stabilized, the ACC circuit 12
 begins operating. This makes it possible to prevent an excessive emission
 from the laser diode.
 (e) Modification of second embodiment
 FIG. 17, which illustrates a modification of the second embodiment, is an
 example in which the circuit 31 for monitoring elapse of time from
 introduction of power is constituted by a counter and a DA converter. More
 specifically, the elapsed-time monitor circuit 31 includes the oscillator
 31a which oscillates at a constant frequency, the counter 31b and the DA
 converter 31c for converting the digital count from the counter 31b to an
 analog signal. The counter 31b counts pulses output by the oscillator 31a
 after power is introduced, and the DA converter 31c subjects the count
 from the counter to digital-to-analog conversion to output a voltage
 signal V.sub.L that increases in proportion to elapsed time.
 Though a common-anode laser diode is used in the second embodiment of FIG.
 16, it is also possible to adopt an arrangement using a common-cathode
 laser diode.
 (E) Shared Comparator (Laser Current Control Circuit)
 (a) Construction
 FIG. 18A is a diagram showing the construction of a comparator (laser
 current control circuit) ICC sharable in ACC circuits of laser diodes of
 both the common-anode and common-cathode types. The shared comparator ICC
 can be made common use of as the comparators IC1, IC2 of FIGS. 25A and
 25B. FIG. 18B shows a series circuit of the ACC circuit (FIG. 25A) of
 common-anode type in which the common-anode laser diode 11a, transistor
 TR1 and resistor R1 are serially connected and inserted between ground and
 the negative power source. FIG. 18C shows a series circuit of the ACC
 circuit (FIG. 25B) of common-anode type in which the common-cathode laser
 diode 11b, transistor TR2 and resistor R4 are serially connected and
 inserted between ground and the positive power source.
 The shared comparator ICC has a first input terminal T.sub.R to which the
 reference V.sub.REF is input, and a second input terminal T.sub.F to which
 the terminal voltage V.sub.F of resistor R1 or R4, produced by the laser
 diode current id, is input as a feedback signal. The shared comparator ICC
 has an output terminal Tout connected to the base terminal of the
 transistor TR1 of the common-anode series circuit or to the base terminal
 of the transistor TR2 of the common-cathode series circuit.
 The shared comparator ICC has an operational amplifier Opamp and resistors
 Ra, Rb, which are for attenuation. The other resistors and transistors
 construct emitter followers. More specifically, complementary emitter
 follower circuits are constructed by two transistors TR.sub.iK, TR.sub.iA
 and two resistors R.sub.iK, R.sub.iA (i=1, 2, 3) having identical suffix
 numbers. The reference voltage V.sub.REF is input to a first complementary
 emitter follower circuit of suffix 1, the feedback voltage V.sub.F is
 input to a second complementary emitter follower circuit of suffix 2, and
 the output signal of the operational amplifier OPamp is input to a third
 complementary emitter follower circuit of suffix 3. Elements having the
 suffix A operate when the common-anode laser diode is driven, and elements
 having the suffix K operate when the common-cathode laser diode is driven.
 (b) Complementary emitter follower circuit
 FIG. 19A is a diagram useful in describing the operation of the
 complementary emitter follower circuit. The two transistors TR.sub.iK,
 TR.sub.iA of different polarities have their emitters connected together
 as well as their bases, the collector of the npn transistor TR.sub.iK is
 connected to the positive power source +Vcc, the collector of the pnp
 transistor TR.sub.iA is connected to the negative power source -Vee, and a
 positive or negative input signal is applied to the bases of these
 transistors. When a signal having positive polarity is input to the bases,
 the npn transistor TR.sub.iK turns on and the pnp transistor TR.sub.iA
 turns off, thereby constructing an emitter follower circuit of gain 1, in
 which the resistor R.sub.iK serves as the emitter resistor, as shown in
 FIG. 19B. An output signal of positive polarity having the same amplitude
 as that of the input signal appears at the output terminal OUT. When a
 signal having negative polarity is input to the bases, the npn transistor
 TR.sub.iK turns off and the pnp transistor TR.sub.iA turns on, thereby
 constructing an emitter follower circuit of gain 1, in which the resistor
 R.sub.iA serves as the emitter resistor, as shown in FIG. 19C. An output
 signal of negative polarity having the same amplitude as that of the input
 signal appears at the output terminal OUT.
 (c) Operation of shared comparator
 In a case where the driving current of the common-anode laser diode is
 controlled, the polarities of the reference voltage V.sub.REF and feedback
 voltage V.sub.F input to the input terminals T.sub.R, T.sub.F (the base
 terminals of the first and second complementary emitter follower circuits)
 of the shared comparator ICC are positive. When the driving current of the
 common-anode laser diode is controlled, therefore, the first and second
 complementary emitter follower circuits operate as shown in FIG. 19C, and
 the reference voltage V.sub.REF and feedback voltage V.sub.F of negative
 polarity are input to the non-inverting input terminal and inverting input
 terminal of the operational amplifier OPamp via the attenuation resistors
 Ra, Rb, respectively.
 In a case where the driving current of the common-cathode laser diode is
 controlled, the polarities of the reference voltage V.sub.REF and feedback
 voltage V.sub.F input to the input terminals T.sub.R, T.sub.F (the base
 terminals of the first and second complementary emitter follower circuits)
 of the shared comparator ICC are positive. When the driving current of the
 common-cathode laser diode is controlled, therefore, the first and second
 complementary emitter follower circuits operate as shown in FIG. 19B, and
 the reference voltage V.sub.REF and feedback voltage V.sub.F of positive
 polarity are input to the non-inverting input terminal and inverting input
 terminal of the operational amplifier OPamp via the attenuation resistors
 Ra, Rb, respectively.
 The attenuation resistors Ra, Rb convert the input signal level to a level
 that falls within the operating range of the operational amplifier OPamp.
 That is, even if the amplitude of the input signal (the reference voltage
 V.sub.REF and feedback voltage V.sub.F) is +Vcc to -Vee, the level is
 converted to a level within the input range (+Vcc-1.5) to (-Vee+1.5) at
 which the operational amplifier OPamp can operate.
 The operational amplifier OPamp outputs +Vcc if the non-inverting input
 signal is greater than the inverting input signal and -Vee if the converse
 is true. Accordingly, in the case where the driving current id of the
 common-anode laser diode 11a is controlled, V.sub.F &lt;V.sub.REF holds and
 the output signal of the operational amplifier OPamp becomes +Vcc if the
 driving current id is less than the set value. As a result, the transistor
 TR.sub.3a of the third emitter follower circuit turns on, a high-level
 signal is output by the output terminal Tout, the transistor TR1 of the
 common-anode series circuit turns on and the driving current id of the
 common-anode laser diode 11a increases. If the driving current id is
 greater than the set value, on the other hand, V.sub.F &gt;V.sub.REF holds
 and the output signal of the operational amplifier OPamp becomes -Vee. As
 a result, the transistor TR.sub.3b of the third emitter follower circuit
 turns on, a low-level signal is output by the output terminal Tout, the
 transistor TR1 of the common-anode series circuit turns off and the
 driving current id of the common-anode laser diode 11a decreases.
 In the case where the driving current id of the common-anode laser diode 1b
 is controlled, V.sub.F &gt;V.sub.REF holds and the output signal of the
 operational amplifier OPamp becomes -Vee if the driving current id is less
 than the set value. As a result, the transistor TR.sub.3b of the third
 emitter follower circuit turns on, a low-level signal is output by the
 output terminal Tout, the transistor TR2 of the common-cathode series
 circuit turns on and the driving current id of the common-cathode laser
 diode 11b increases. If the driving current id is greater than the set
 value, on the other hand, V.sub.F &lt;V.sub.REF holds and the output signal
 of the operational amplifier OPamp becomes +Vcc. As a result, the
 transistor TR.sub.3a of the third emitter follower circuit turns on, a
 high-level signal is output by the output terminal Tout, the transistor
 TR2 of the common-cathode series circuit turns off and the driving current
 id of the common-cathode laser diode 11b decreases.
 The shared comparator ICC of the present invention is constructed in such a
 manner that emitter follower circuits obtained by connecting transistors
 of different polarities in complementary fashion are added on to the input
 and output stages of a typical operational amplifier OPamp, with one
 transistor being forward biased and turning on and the other transistor
 being reverse biased and turning off depending upon whether the input
 voltage is in the vicinity of the positive power source voltage or
 negative power source voltage. Furthermore, a level shift is performed in
 such a manner that the emitter follower output will fall within the input
 voltage range of the typical operational amplifier OPamp. By using such a
 shared comparator ICC in an ACC circuit, the driving currents of a
 common-anode laser diode and common-cathode laser diode can be controlled
 merely by changing the reference voltage generating portion and series
 circuit.
 Though the foregoing description relates to a case where the power source
 voltages are +Vcc and -Vee, one of these can be made ground. That is, by
 making the combination of parallel voltages +Vcc, 0 (=-Vee) or 0 (=+Vcc),
 -Vee, it is possible to detect on which side (+Vcc, -Vee) an input is
 nearest.
 (F) Shared Laser Diode Current Limiting Circuit
 The embodiment (FIGS. 4 to 7) for preventing excessive emission by
 detecting emission power, the embodiment (FIGS. 4 to 11) for preventing
 excessive emission by detecting laser diode temperature, and the
 embodiment (FIGS. 12 to 17) for preventing excessive emission by
 monitoring elapsed time following introduction of power are embodiments in
 which the laser diode driving current of the common-anode laser diode or
 common-cathode laser diode is limited in such a manner that an excessive
 emission will not occur. Accordingly, it would be convenient if the laser
 diode current limiting circuit could be made sharable by both the
 common-anode laser diode and common-cathode laser diode, and such sharing
 would make it possible to lower cost.
 (a) Shared laser diode current limiting circuit for preventing excessive
 emission by detecting emission power
 FIG. 20 is a diagram showing the construction of a shared laser diode
 current limiting circuit so adapted that a laser diode current limiting
 circuit for preventing excessive emission by detecting emission power is
 made sharable by both a common-anode laser diode and common-cathode laser
 diode. Shown in FIG. 20 are the laser diode 11, which is of common-anode
 or common-cathode type, the ACC circuit 12 for performing control in such
 a manner that the laser diode current attains a set value, the ATC circuit
 13 for performing control in such a manner that the laser diode
 temperature is rendered constant, the optical fiber 14, and the power
 monitor circuit 15 for monitoring emission power of the laser diode 11.
 The power monitor circuit 15 detects the backward power (BP) of the laser
 diode as the emission power thereof. Numeral 16 denotes the shared laser
 diode current limiting circuit for controlling laser diode current when
 the emission power exceeds a set power.
 The shared laser diode current limiting circuit 16 includes the reference
 voltage generator 16a for outputting the reference voltage Vr conforming
 to set power, the comparator 16b for comparing the voltage Vp, which
 conforms to detected emission power, with the reference voltage Vr, the
 limit-current controller 16c for outputting a constant limit current
 i.sub.L when emission power surpasses the set value and for making the
 limit current zero when the emission power falls below the set value, a
 laser-diode type detector 16d for detecting whether the laser diode 11 is
 of the common-anode type or common-cathode type, and a limit-current
 direction changeover unit 16e, to which the limit current i.sub.L is
 applied, for performing control in such a manner that (1) when the laser
 diode 11 is of the common-anode type, the limit current flows into the ACC
 circuit 12, and (2) when the laser diode 11 is of the common-cathode type,
 a current equivalent to the limit current flows out of the ACC circuit 12.
 As described above in connection with FIG. 6, the common-anode ACC circuit
 12 is such that the limit current i.sub.L is caused to flow into the
 resistor R1, thereby enlarging the terminal voltage of the resistor R1 and
 limiting the laser diode current. On the other hand, the common-cathode
 ACC circuit 12' is such that some (the limit current i.sub.L) of the
 current that flows through the resistor R4 is caused to flow to the
 outside without passing through the laser diode, thereby limiting the
 laser diode current.
 Accordingly, the shared laser diode current limiting circuit 16 performs
 control in such a manner that the limit current i.sub.L is caused to flow
 into the ACC circuit if the ACC circuit is of the common-anode type, and
 in such a manner that the ACC circuit splits the limit current if the ACC
 circuit is of the common-cathode type.
 The details of the laser diode current limiting circuit 16 and
 limit-current direction changeover unit 16e.
 (b) Shared laser diode current limiting circuit for preventing excessive
 emission by detecting laser diode temperature
 FIG. 21 is a diagram showing the construction of a shared laser diode
 current limiting circuit so adapted that a laser diode current limiting
 circuit for preventing excessive emission by detecting laser diode
 temperature is made sharable by both a common-anode laser diode and
 common-cathode laser diode. Shown in FIG. 21 are the laser diode 11, which
 is of common-anode or common-cathode type, the ACC circuit 12 for
 performing control in such a manner that the laser diode current attains a
 set value, the ATC circuit 13 for performing control in such a manner that
 the laser diode temperature is rendered constant, and the optical fiber
 14. Numeral 16 denotes the laser diode current limiting circuit, which is
 for limiting laser diode current when the laser diode temperature is less
 than a set temperature. Numeral 21 denotes the temperature monitoring
 circuit, which is for monitoring the temperature of the laser diode chip.
 The laser diode current limiting circuit 16 includes the reference voltage
 generator 16a for outputting the reference voltage Vr conforming to a set
 temperature, the comparator 16b for comparing the voltage V.sub.T, which
 conforms to detected temperature, with the reference voltage Vr, the
 limit-current controller 16c for outputting a constant limit current
 i.sub.L when laser diode temperature is less than the set value and for
 making the limit current zero when the laser diode temperature is greater
 than the set value, the laser-diode type detector 16d for detecting
 whether the laser diode 11 is of the common-anode type or common-cathode
 type, and the limit-current direction changeover unit 16e, to which the
 limit current i.sub.L is applied, for performing control in such a manner
 that (1) when the laser diode 11 is of the common-anode type, the limit
 current flows into the ACC circuit 12, and (2) when the laser diode 11 is
 of the common-cathode type, a current equivalent to the limit current
 flows out of the ACC circuit 12.
 As described above in connection with FIG. 10, the common-anode ACC circuit
 12 is such that the limit current i.sub.L is caused to flow into the
 resistor R1, thereby enlarging the terminal voltage of the resistor R1 and
 limiting the laser diode current. On the other hand, the common-cathode
 ACC circuit 12' is such that some (the limit current i.sub.L) of the
 current that flows through the resistor R4 is caused to flow to the
 outside without passing through the laser diode, thereby limiting the
 laser diode current.
 Accordingly, the shared laser diode current limiting circuit 16 performs
 control in such a manner that the limit current i.sub.L is caused to flow
 into the ACC circuit if the ACC circuit is of the common-anode type, and
 in such a manner that the ACC circuit splits the limit current if the ACC
 circuit is of the common-cathode type.
 (c) Shared laser diode current limiting circuit for preventing excessive
 emission by monitoring elapsed time following introduction of power
 FIG. 22 is a diagram showing the construction of a shared laser diode
 current limiting circuit so adapted that a laser diode current limiting
 circuit for preventing excessive emission by monitoring elapsed time
 following introduction of power is made sharable by both a common-anode
 laser diode and common-cathode laser diode. Shown in FIG. 22 are the laser
 diode 11, which is of common-anode or common-cathode type, the ACC circuit
 12 for performing control in such a manner that the laser diode current
 attains a set value, the ATC circuit 13 for performing control in such a
 manner that the laser diode temperature is rendered constant, and the
 optical fiber 14. Numeral 16 denotes the laser diode current limiting
 circuit which, when elapsed time from introduction of power has not
 attained a set time, halts ACC control and limits laser diode current,
 and, when elapsed time from introduction of power has attained the set
 time, restores ACC control and makes the laser diode current equal to the
 set value. Numeral 31 denotes the elapsed-time monitor circuit 31 for
 monitoring time that elapses from introduction of power. For example, the
 elapsed-time monitor circuit 31 is constituted by a delay circuit
 (integrator circuit) in which output voltage V.sub.L increases as time
 elapses from the moment power is introduced.
 The laser diode current limiting circuit 16 includes the reference voltage
 generator 16a for outputting the reference voltage Vr conforming to a set
 temperature, the comparator 16b for comparing the voltage V.sub.L, which
 conforms to elapsed time, with the reference voltage Vr, the limit-current
 controller 16c for outputting a constant limit current i.sub.L when
 elapsed time has not attained the set value and for making the limit
 current zero when the elapsed time has exceeded the set time, the
 laser-diode type detector 16d for detecting whether the laser diode 11 is
 of the common-anode type or common-cathode type, and the limit-current
 direction changeover unit 16e, to which the limit current i.sub.L is
 applied, for performing control in such a manner that (1) when the laser
 diode 11 is of the common-anode type, the limit current flows into the ACC
 circuit 12, and (2) when the laser diode 11 is of the common-cathode type,
 a current equivalent to the limit current flows out of the ACC circuit 12.
 As described above in connection with FIG. 15, the common-anode ACC circuit
 12 is such that the limit current i.sub.L is caused to flow into the
 resistor R1, thereby enlarging the terminal voltage of the resistor R1 and
 limiting the laser diode current. On the other hand, the common-cathode
 ACC circuit 12' is such that some (the limit current i.sub.L) of the
 current that flows through the resistor R4 is caused to flow to the
 outside without passing through the laser diode, thereby limiting the
 laser diode current.
 Accordingly, the shared laser diode current limiting circuit 16 performs
 control in such a manner that the limit current i.sub.L is caused to flow
 into the ACC circuit if the ACC circuit is of the common-anode type, and
 in such a manner that the ACC circuit splits the limit current if the ACC
 circuit is of the common-cathode type.
 (d) Circuit details of part of shared laser diode current limiting circuit
 FIG. 23 is a detailed circuit diagram of part of the shared laser diode
 current limiting circuit 16 (FIGS. 20 to 22) and illustrates the details
 of the laser-diode type detector 16d and limit-current direction
 changeover unit 16e.
 (d-1) Laser-diode type detector
 The laser-diode type detector 16d is constituted by a complementary emitter
 follower circuit. More specifically, two transistors T.sub.1, T.sub.2 of
 different polarities have their emitters connected together as well as
 their bases, the collector of the npn transistor T.sub.1 is connected to
 the positive power source +Vcc, the collector of the pnp transistor
 T.sub.2 is connected to the negative power source -Vee, and a positive or
 negative input signal is applied to the bases of these transistors. When a
 signal having positive polarity is input to the bases, the npn transistor
 T.sub.1 turns on and the pnp transistor T.sub.2 turns off, thereby
 constructing an emitter follower circuit of gain 1, in which the resistor
 R.sub.1 serves as the emitter resistor. An output signal of positive
 polarity having the same amplitude as that of the input signal enters the
 limit-current direction changeover unit 16e, which is the next stage. When
 a signal having negative polarity is input to the bases, the npn
 transistor T.sub.1 turns off and the pnp transistor T.sub.2 turns on,
 thereby constructing an emitter follower circuit of gain 1, in which the
 resistor R.sub.2 serves as the emitter resistor. An output signal of
 negative polarity having the same amplitude as that of the input signal
 enters the limit-current direction changeover unit 16e, which is the next
 stage.
 In actuality, the output signal of the first or second emitter follower
 circuit of the shared comparator ICC enters the bases of the laser-diode
 type detector 16d. When the shared comparator ICC is used in the ACC
 circuit of the common-anode laser diode, the first or second emitter
 follower circuit inputs a signal of negative polarity to the bases of the
 laser-diode type detector 16d. The laser-diode type detector, therefore,
 inputs a signal of negative polarity to the limit-current direction
 changeover unit 16e if the laser diode is of common-anode type. When the
 shared comparator ICC is used in the ACC circuit of the common-cathode
 laser diode, the first or second emitter follower circuit inputs a signal
 of positive polarity to the bases of the laser-diode type detector 16d.
 The laser-diode type detector, therefore, inputs a signal of positive
 polarity to the limit-current direction changeover unit 16e if the laser
 diode is of common-cathode type.
 (d-2) Limit-current direction changeover unit
 The limit-current direction changeover unit 16e has transistors T.sub.3,
 T.sub.4 constructing a differential pair. The output signal of the
 laser-diode type detector 16d enters one transistor T.sub.3 and a constant
 voltage obtained through voltage division by resistors R3, R4 enters the
 base of the other transistor T.sub.4. This differential pair passes the
 limit current i.sub.L, which is output by the limit-current controller
 16c, into the transistor having the smaller base voltage. Transistors
 T.sub.5, T.sub.6 construct a first current mirror circuit, transistors
 T.sub.7, T.sub.8 a second current mirror circuit and transistors T.sub.9,
 T.sub.10 a third current mirror circuit. An output terminal T.sub.0 is
 connected to a feedback-voltage input terminal T.sub.F of the shared
 comparator ICC constructing the ACC circuit and to the resistor R1 (in
 case of the common-anode type) or resistor R4 (in case of the
 common-cathode type).
 (d-3) Operation
 Since the laser-diode type detector 16d outputs a signal of negative
 polarity if the laser diode is of the common-anode type, the transistor
 T.sub.3 turns on and the limit current i.sub.L flows through the
 transistor T.sub.5. The limit current i.sub.L flows through the
 transistors T.sub.6, T.sub.7 owing to the first current mirror, and the
 limit current flows into the transistor T.sub.8 owing to the second
 current mirror. More specifically, if the laser diode is of the
 common-anode type, the limit current i.sub.L flows into the resistor R1 of
 the ACC circuit. On the other hand, since the laser-diode type detector
 16d outputs a signal of positive polarity if the laser diode is of the
 common-cathode type, the transistor T.sub.4 turns on, the limit current
 i.sub.L flows through the transistor T.sub.9 and the limit current i.sub.L
 flows through the transistor T.sub.10 owing to the third current mirror.
 More specifically, if the laser diode is of the common-cathode type, the
 limit current i.sub.L flows out of the ACC circuit.
 Thus, in accordance with the present invention, the power (backward power)
 of a laser diode is monitored and the laser diode current is limited when
 the backward power exceeds a set value. When the laser diode temperature
 subsequently rises and the backward power falls below the set power,
 automatic current control is applied so that the laser diode will take on
 a set current value. As a result, the laser diode current can be limited
 when the laser diode is at a low temperature. This makes it possible to
 prevent an excessive light emission from the laser diode so that the laser
 diode will not destroyed or suffer a deterioration in characteristics.
 In accordance with the present invention, laser diode temperature is
 monitored and the laser diode current is limited when the laser diode
 temperature is less than a set temperature. When the laser diode
 temperature rises and surpasses the set temperature, automatic current
 control is applied so that the laser diode will take on the set current
 value. As a result, an excessive emission from the laser diode at low
 temperature can be prevented. This assures that neither damage to the
 laser diode nor deterioration of the laser diode characteristics will
 occur.
 In accordance with the present invention, time that elapses from
 introduction of power is monitored. If the elapsed time has not exceeded a
 set time, the laser diode current is limited to zero or to a low current
 value. After the elapsed time exceeds the set time, automatic current
 control is applied so that the laser diode current will become equal to
 the set current value. As a result, the set time is made the time required
 for the laser diode temperature to attain a substantially constant
 temperature after power is introduced (i.e., the time needed for the laser
 diode temperature to stabilize). As a result, the laser diode current is
 limited when the laser diode is at a low temperature. This makes it
 possible to prevent an excessive emission of light from the laser diode
 and assure that neither damage to the laser diode nor deterioration of the
 laser diode characteristics will occur.
 In accordance with the present invention, common use is made of a
 comparator (current control circuit) employed in ACC circuits of both the
 common-anode and common-cathode types. This has the effect of lowering
 cost.
 In accordance with the present invention, a laser diode protecting circuit
 for preventing an excessive emission from a laser diode can be used
 commonly for laser diodes of both the common-anode and common-cathode
 types. This also has the effect of lowering cost.
 As many apparently widely different embodiments of the present invention
 can be made without departing from the spirit and scope thereof, it is to
 be understood that the invention is not limited to the specific
 embodiments thereof except as defined in the appended claims.