JP2000208866A - Laser drive current control circuit and ld protective circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To use commonly an LD protective circuit which prevents an LD(laser diode) from emitting excessively laser rays to each type of LD of anode common and cathode common. SOLUTION: An LD 11 of anode-common type or cathode-common type, a semiconductor switch, and a resistor are connected in series to form an LD current path, an automatic current control circuit 12 sets an LD current at a prescribed value turning the semiconductor switch ON or OFF so as to set the terminal voltage of the resistor at a prescribed value. When the emission power of the LD becomes larger than a prescribed value, the LD current-limiting circuit 16 makes a limiting current flow through the above resistor when the LD 11 is of anode-common type or makes a current equivalent to a limiting current flow out from the resistor when the LD 11 is of cathode-common type, by which a laser current is limited so as to lessen the LD 11 in emission power.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic current control circuit (ACC circuit) for controlling a laser current to a set value.
Protection circuit in LD driving section having ACC and ACC
Related to the laser drive current control circuit in the circuit,
The present invention relates to an LD protection circuit that protects an LD (laser diode) by preventing excessive emission of the LD (laser diode) at the time of low-temperature startup, and a laser drive current control circuit that can be applied to both anode common and cathode common LDs.

[0002]

2. Description of the Related Art In high-speed optical transmission, deterioration of transmission characteristics due to wavelength fluctuation (chirping) cannot be ignored. In addition, wavelength division multiplexing (Wavelength Division Multip
In the lexing method), wavelength stability becomes very important. For this reason, the LD driver is connected to the ACC circuit (Automati
c Current Control circuit) and ATC circuit (Automatic Tem
perature control circuit) to control the current of the LD (laser diode) to be a constant current value and the LD chip temperature (LD temperature) to be a constant temperature.

FIGS. 24A and 24B are configuration diagrams of an optical transmitter for digital optical communication. FIG. 24A shows an optical transmitter using an anode common type LD, and FIG. 24B shows an optical transmitter using a cathode common type LD. It is. In the figure, 1 is an LD driving unit, 1a is an anode common LD, 1b is a cathode common LD, 2 is an LD
An ACC circuit having an operation amplifier (OPamp) configuration for controlling a current to a set current value, and an ATC circuit 3 for controlling an LD temperature to a set value. Also, 4,
5 is an optical fiber, 6 is a clock C for the data signal DATA.
D-type flip-flop (D-FF) punched by LK, 7
Is a light intensity modulator drive circuit (DRV), and 8 is data "
A light intensity modulator (I) that modulates light intensity by 1 "," 0 "
M). LDs include an anode common type and a cathode common type, and the driving currents of the LDs have different directions. In the anode common type (FIG. 1A), the anode is connected to the ground, and it is necessary to discharge the drive current id from the LD 1a.
In (b)), the cathode is grounded, and the drive current id needs to be sucked into the LD 1b.

FIG. 25 shows an example of an ACC circuit.
Node common type ACC circuit, (b) Cathode common
ACC circuit. In FIG. 25 (a), 1
a is an anode common LD, R1 to R3 are resistors (resistance value is
r₁~ R_Three), TR1 is a transistor, IC1 is an operation
Comparator (current control circuit) with
You. LD 1a, transistor TR1, and resistor R1 are in series
Connected between the ground and the negative power supply-Vee
You. Id · r where id is the current flowing through LD 1a₁Is compa
It is input to the inverting terminal of the lator IC1. On the other hand,
The reference voltage divided by resistors R2 and R3 for the non-inverting terminal of the parator
Voltage V _REFIs entered. This ACC circuit has a resistor
R1 terminal voltage id · r₁Is the reference voltage V_REFTo be
On / off control of the transistor TR1 to reduce the LD current id
Set current value. That is, the partial pressure of the resistors R2 and R3
Voltage V obtained from_REFIs directly on both ends of the resistor R1
And the voltage is changed to a resistance value r.₁LD divided by
The current id flows through 1a. In other words, the resistance R1
Current source, and its current value (= V_REF/ R₁)become
Of the transistor TR1 with the comparator IC
By controlling with 1, it is possible to reduce temperature change.
A constant current value can be obtained.

[0005] In FIG. 25 (b), 1b cathode common LD, R4-R6 is the resistance (resistance r ₄ ~r _6), TR
Reference numeral 2 denotes a transistor, and IC2 denotes a comparator (current control circuit) composed of an operation amplifier. LD 1
b, the transistor TR2 and the resistor R4 are connected in series,
It is provided between the ground and the positive power supply + Vcc. LD1b
Id · r ₄ where id is the current flowing through the comparator IC2
Is input to the inverting terminal. On the other hand, the reference voltage V _REF divided by the resistors R5 and R6 is input to the non-inverting terminal of the comparator. The ACC circuit, the voltage between the terminals id · r ₄ of the resistor R4 is the set current value LD current id transistor TR2 on / off control to so that the reference voltage V _REF. That is, the value of the voltage V _{RE F} obtained by the partial pressure of the resistors R5, R6 is divided by the voltage becomes as voltage across the resistor R4 in resistance value r ₄ is the current id flowing through the LD 1b. In other words, the base of the transistor TR2 is controlled by the comparator IC2 such that the resistor R4 serves as a constant current source and always keeps the current value (= V _REF / r ₄ ), thereby obtaining a constant current value even with a temperature change. Can do things.

FIG. 26 shows an example of an ATC circuit.
D chip, 3a heats LD chip according to current direction,
A Peltier element for cooling, 3b is a thermistor having a negative resistance characteristic for detecting the temperature of the LD chip, and 3c is
Package containing D, Peltier element, and thermistor. 3d and 3e are resistors, 3f and 3g are PNP and NPN
Transistor 3h is a comparator. A voltage (voltage corresponding to the LD temperature) Vt obtained by voltage division by a thermistor and a resistor is input to an inverting terminal of the comparator, a reference voltage Vref is input to a non-inverting terminal, and an output terminal is connected to a base of each transistor. I have. The emitter of the PNP transistor 3f is connected to V +, the emitter of the NPN transistor 3g is connected to V-, and the collector of each transistor is connected to the Peltier element 3a.

When the temperature of the LD chip is low, the thermistor 3
b, the voltage Vt decreases, and Vt <Vref
And the output of the comparator 3h becomes positive. As a result, a current flows in a direction in which the transistor 3f is turned off and the transistor 3g is turned on to generate heat in the Peltier element, thereby heating the inside of the package and increasing the LD temperature. When the temperature of the LD chip rises, the resistance of the thermistor decreases and the voltage Vt
Increases, Vt> Vref, and the output of the comparator 3g becomes negative. As a result, the transistor 3f is turned on,
A current flows in a direction in which the transistor 3g turns off to cool the Peltier element, and the temperature of the LD decreases. As a result, LD
The temperature is controlled to reach the set temperature.

[0008]

FIG. 24 (a),
Power is supplied to the optical transmitter of (b) at a low temperature, and the LD 1a, 1
When b is driven, the LD may emit excessive light, and the LD itself may be damaged. This excessive light emission is due to the following reason. The LD has a temperature characteristic as shown in FIG. 27, and the power P required for flowing a constant LD current increases as the temperature decreases. In addition, the LD current becomes the set value by the ACC control.
Comparing the CC stabilization time with the stabilization time at which the LD temperature reaches the set temperature by ATC control, the ATC
The stabilization time is longer. For this reason, when the LD is driven by turning on the power at a low temperature, the LD current becomes the set value by the ACC control before the LD chip reaches the set temperature due to the delay of the ATC control as shown in FIG. As a result, the light emission power of the LD is increased, excessive light is emitted, and the LD
It causes its own characteristic deterioration and breakage. That is, although the LD current has reached the target value by the ACC circuit, the LD temperature has not reached the target value, so that the light emission exceeds the target value.
As described above, it is necessary to prevent excessive light emission of the LD at the time of low-temperature driving and to prevent the destruction and the characteristic deterioration of the LD.

As described above, LDs are classified into an anode common type and a cathode common type.
The comparators (current control circuits) IC1 and IC2 used in the C circuit (see FIGS. 25A and 25B) are different and must be separately designed and prepared. For this reason, it is convenient if the current control circuit used for each type of ACC circuit can be shared, and a cost reduction effect can be expected by sharing (LSI).

Incidentally, the minimum value of the laser current id controlled by the ACC circuit is 0 mA. The specification of the minimum value = 0 mA is a specification necessary for realizing a function (shutdown function) of completely stopping light emission of a laser required as an optical transmitter. For this reason, the ACC circuit for each type has a resistor R
1, it is necessary to control the voltage generated at both ends of R4 to 0 V, and the input voltage range of the operation amplifier of the shared comparator (current control circuit) is a positive / negative power supply voltage value (+ Vc
c, -Vee). That is, if the terminal voltage of the resistors R1 and R4 generated by the laser current id during normal light emission is represented by v, a comparator (current control circuit) to be shared
It is necessary to operate at least the input voltage in the voltage range of + Vcc to + Vcc-v and -Vee to -Vee + v shown in FIG. This input voltage range is
Resistor R1, the resistance value of R4 r _1, r is ₄ and can be alleviated to some extent by the increase, but lower voltage circuit power supply, control up and considering the values like resistance r ₁ of the current that can, r ₄ is desirably about several Ω to several tens of Ω, and there is a limit.

The input voltage range of a general operation amplifier is from -Vee to + Vee, as shown in FIG.
cc-1.5 or -Vee + 1.5 to + Vcc, power supply level +
Even if a signal in the range of about 1.5 V from Vcc or -Vee is input, it cannot operate as an operation amplifier. this is,
This is a characteristic that is inevitably attached to an operation amplifier composed of a differential pair. That is, with a general operation amplifier, it is difficult to obtain the input voltage range (the voltage range shown in FIG. 29A) near the dual power supply voltage required for the operation amplifier of the shared comparator (current control circuit). is there.

In view of the above, it is an object of the present invention to prevent excessive light emission of an LD at the time of low-temperature driving and to prevent the LD from being destroyed and its characteristics from deteriorating. The object of the present invention is
The purpose is to monitor the light emission power (rear power) of D to prevent excessive light emission of the LD at the time of low-temperature driving, and to prevent destruction of the LD and deterioration of characteristics. SUMMARY OF THE INVENTION An object of the present invention is to monitor the temperature of an LD chip to prevent excessive light emission of the LD at the time of low-temperature driving, and prevent the LD from being broken and its characteristics from deteriorating. An object of the present invention is to monitor the elapsed time after turning on the power and to set the LD at the time of low-temperature driving.
To prevent excessive emission of light and to prevent LD breakdown and LD characteristic deterioration. An object of the present invention is to make it possible to share a comparator (current control circuit) used for each type of ACC circuit of anode common and cathode common. An object of the present invention is to make it possible to share an LD protection circuit for preventing excessive light emission of an LD for each of an anode common type and a cathode common type.

[0013]

(A) First principle of the present invention FIG. 1 is an explanatory diagram of the first principle of the present invention. 11 is LD
(Laser diode), 12 is an ACC circuit for controlling the LD current to be a set value, 13 is an ATC circuit for controlling the LD temperature to be constant, 14 is an optical fiber, 15
Is a power monitor circuit for monitoring the light emission power of the LD, which detects the power behind the LD (rear power BP) as the light emission power. The current limiting circuit 16a is a reference voltage generator which outputs a reference voltage Vr according to the set power, and 16b is a comparator which compares the voltage Vp according to the detected light emission power with the reference voltage Vr. Although not shown in the ACC circuit 12, a comparator (current control circuit) designed to be commonly used for each of the LD type of the anode common and the cathode common is used.

At the time of low-temperature driving, the LD current increases by the ACC control before reaching the set temperature due to the delay of the ATC control. When the light emission power increases and exceeds the set power, the LD current limiting circuit 16 causes the ACC circuit 12 to stop the ACC control to limit the LD current, thereby preventing the LD from emitting excessive light. In this state, if the temperature rises due to the ATC control and the light emission power of the LD falls below the set power, L
The D current limiting circuit 16 restores the ACC control function of the ACC circuit and controls the LD current so that the LD current becomes the set current value.
With the above, the light emission power (rear power) of the LD
Can be monitored to prevent excessive light emission of the LD at the time of low-temperature driving, thereby preventing the destruction of the LD and deterioration of characteristics.
Further, the LD current limiting circuit 16 for preventing excessive light emission of the LD is configured so as to be commonly used for each of the anode common type and the cathode common LD type. For example, when the light emission power is equal to or higher than the set value, (1) if the anode is the common LD, the current is supplied from the LD current limiting circuit 16 to the ACC circuit 12 to limit the LD current; The current to be supplied to the LD
Limit the current.

(B) Second principle of the present invention FIG. 2 is an explanatory diagram of the second principle of the present invention. 11 is LD
(Laser diode), 12 is an ACC circuit for controlling the LD current to be a set value, 13 is an ATC circuit for controlling the LD temperature to be constant, 14 is an optical fiber, 16
Is an LD current limiting circuit for limiting the LD current when the LD temperature is equal to or lower than the set temperature, and 21 is a temperature monitor circuit for monitoring the temperature of the LD chip. In LD current limiting circuit 16, 16a is a reference voltage generator, which outputs a reference voltage Vr corresponding to the set temperature, 16b is a comparator for comparing the voltage V _T with the reference voltage Vr corresponding to the detected temperature. Although not shown in the ACC circuit 12, a comparator (current control circuit) designed to be commonly used for each of the LD type of the anode common and the cathode common is used.

When the LD is driven at a low temperature, the LD temperature is initially lower than the set temperature. Therefore, the LD current limiting circuit 16 causes the ACC circuit 12 to stop the ACC control and limits the LD current. This can prevent excessive light emission of the LD at a low temperature. In this state, if the LD temperature rises by the ATC control and becomes equal to or higher than the set temperature, the LD does not emit excessive light even if the LD current increases. Therefore, the LD current limiting circuit 16 restores the ACC control function of the ACC circuit, and Becomes the set current value. By doing so, the LD temperature is monitored to prevent the LD from emitting excessive light at the time of low-temperature driving, and to destroy the LD,
It is possible to prevent deterioration of characteristics. Further, the LD current limiting circuit 16 for preventing excessive light emission of the LD is configured so as to be commonly used for each of the anode common type and the cathode common LD type. For example, when the LD temperature is equal to or lower than the set value, (1) if the anode common LD, the current is supplied from the LD current limiting circuit 16 to A
(2) In the case of a cathode common LD, a current to be supplied to the LD is supplied to the outside from the ACC circuit 12 to restrict the LD current.

(C) Third Principle of the Present Invention FIG. 3 is an explanatory diagram of the third principle of the present invention. 11 is LD
(Laser diode), 12 is an ACC circuit for controlling the LD current to be a set value, 13 is an ATC circuit for controlling the LD temperature to be constant, 14 is an optical fiber, 16
Indicates that the elapsed time since power-on has not reached the set time,
An LD current limiting circuit for stopping the ACC control to limit the LD current and recovering the ACC control and setting the LD current to a set value when the elapsed time after power-on reaches a set time. This is an elapsed time monitoring circuit that monitors the elapsed time, and is composed of, for example, a delay circuit (integrating circuit) in which the output voltage _VL increases according to the elapsed time after the power is turned on. In the LD current limiting circuit 16, reference numeral 16a denotes a reference voltage generator which outputs a reference voltage Vr according to a set time, and 16b denotes a comparator which compares a voltage _VL according to an elapsed time with the reference voltage Vr. Although not shown in the ACC circuit 12, a comparator (current control circuit) designed to be commonly used for each of the LD type of the anode common and the cathode common is used.

The LD current limiting circuit 16 stops the ACC control until the elapsed time after turning on the power reaches the set time, and
The current is limited, thereby preventing the LD from emitting excessive light at low temperatures. If the elapsed time exceeds the set time, ATC
Since the temperature is raised by the control and the LD does not emit excessive light even if the LD current is increased, the LD current limiting circuit 16
The ACC control function of the CC circuit 12 is restored to control the LD current to the set value. By doing so, it is possible to monitor the elapsed time after turning on the power and prevent excessive light emission of the LD at the time of low-temperature driving, thereby preventing destruction of the LD and deterioration of characteristics. Further, the LD current limiting circuit 16 for preventing excessive light emission of the LD is configured so as to be commonly used for each of the anode common type and the cathode common LD type. For example, until the elapsed time after power-on reaches the set time, (1) anode common L
In the case of D, the current is supplied from the LD current limiting circuit 16 to the ACC circuit 12 to limit the LD current. (2) In the case of the cathode common LD, the current to be supplied to the LD is supplied to the outside from the ACC circuit 12 to supply the LD current. Restrict.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) Embodiment in which light emission power is detected to prevent excessive light emission (a) First embodiment FIG. 4 detects light emission power (back power of LD) to prevent excessive light emission. FIG. 2 is a configuration diagram of a first embodiment, and the same parts as those in FIG. 1 are denoted by the same reference numerals. Reference numeral 11a denotes an anode common type LD, which is a Peltier element 17
And a thermistor 18 for detecting the temperature of the LD chip and a pin photodiode 19 for detecting the rear power (light intensity) of the LD, in a package PKG.

Reference numeral 12 denotes an ACC circuit which controls the LD current so that the LD current becomes a set current value. The ACC circuit 12 shown in FIG.
It has the same configuration as the circuit, and is connected to a current source (to be described later) 16d that appropriately supplies current to the resistor R1. When the backward power is equal to or less than the set power, no current flows from the current source 16d.
ACC control is performed so as to become the set current value. But,
If the rear power is equal to or higher than the set power, the current source 16d
More current flows into the resistor R1. For this reason, the ACC circuit 12 stops the ACC function, reduces the LD current id by the amount of the flow, and limits the LD current value. An ATC circuit 13 controls the LD temperature to be constant, and has the same configuration as the ATC circuit shown in FIG. Reference numeral 15 denotes a power monitor circuit that outputs a voltage signal (rear power monitor signal BP) according to the rear power using the pin photodiode 19, and 16 denotes an LD current that restricts the LD current when the rear power exceeds a set power. It is a limiting circuit.

In the LD current limiting circuit 16, reference numeral 16a denotes a reference voltage generating unit which divides the voltage by resistors R7 and R8 to output a reference voltage Vr, and 16b comprises an operation amplifier, and corresponds to the detected light emission power. A comparator having an operation amplifier configuration for comparing the voltage Vp with the reference voltage Vr, 16c is a voltage follower circuit, 16d is a current source, and is turned on when the rear power is larger than the set power (Vp <Vr), and the rear power is set. A transistor TR2 that is turned off when the power is smaller than the power (Vp ≧ Vr), and a resistor R connected between the emitter and the ground.
4 and the collector of the transistor TR2 is connected to the resistor R1 of the ACC circuit 12. The reference voltage Vr is set by using the fact that the forward power of the LD and the rear power BP of the LD are in a proportional relationship, and becomes the voltage of the rear power monitor signal BP corresponding to when the forward power is several times the normal power. Set as follows.

In the above configuration, when the power of the LD driving circuit is turned on at a low temperature, the voltage Vp of the rear power monitor signal BP becomes smaller than the reference voltage Vr until the temperature of the LD is stabilized (Vp < Vr), the output of the comparator 16b drops to a point where the transistor TR2 can be turned on. As a result, the voltage across the resistor R1 of the ACC circuit 12 is determined by the discharge current (limit current) i _L of the transistor TR2, and the LD current id decreases. That is, when the rear power (light emission power) increases and the voltage value Vp of the rear power monitor signal BP becomes lower than the reference voltage Vr (when the LD 11a tries to emit excessive light), the transistor TR2 turns on, and Vp = Vr
The LD current id is limited so as to satisfy the following condition, thereby preventing excessive light emission.

As described above, as shown in FIG. 5, in the low-temperature driving of the LD, the LD current becomes large by the ACC control before the LD temperature rises to the target temperature by the ATC control, so that the LD 11a tries to emit excessive light. Then (time t ₁ ), the LD drive circuit operates so that the light emission power becomes constant (APC mode: Automatic Power Control mode). Thereafter, when the temperature of the LD 11a is stabilized to the set temperature by the ATC control (time t ₂ ), the voltage Vp of the rear power monitor signal BP becomes higher than the reference voltage Vr, so that the output of the comparator 16b rises and the current source 16d
Transistor TR2 is turned off. As a result, the ACC circuit 12 restores the ACC function, and changes the LD current id to the resistance R2.
Control is performed so that the current value is set by the resistance ratio of R3 (ACC mode). This circuit configuration has an advantage that when the power is turned on at a low temperature, an operation of preventing excessive emission of the LD is performed, and at the same time, when the transistor TR2 is always turned on, it can be used as an APC circuit.

(B) Modified Example In the first embodiment shown in FIG. 4, an anode common LD is used, but it is also possible to use a cathode common LD. FIGS. 6A and 6B are configuration diagrams of an anode common type and a cathode common type for detecting excessive light emission by detecting light emission power. FIG. 6A is a configuration diagram of an anode common type, and FIG. 6B is a configuration diagram of a cathode common type. In the figure, 12 is an anode common type ACC circuit, and 12 'is a cathode common type ACC circuit.
The configuration shown in FIG. Reference numeral 15 denotes a power monitor, and 16 denotes an LD current limiting circuit. In the anode common type ACC circuit 12, the limiting current i _L is supplied to the resistor R1 to increase the terminal voltage of the resistor R1, thereby increasing the LD voltage.
Limit the current id. On the other hand, a cathode common type AC
In the C circuit 12 ', a part of the current flowing through the resistor R4 (the limiting current i _L ) is allowed to flow to the outside without flowing to the LD.
Limit the current id.

(C) Second Embodiment FIG. 7 is a block diagram of a second embodiment for detecting the light emission power (the rear power of the LD) to prevent excessive light emission. Have the same reference numerals. Reference numeral 11a denotes an anode common type LD, which is a Peltier element 17
And a thermistor 18 for detecting the temperature of the LD chip and a pin photodiode 19 for detecting the rear power (light intensity) of the LD, in a package PKG.

Reference numeral 12 denotes an ACC circuit which controls the LD current so that the LD current becomes a set current value. The ACC circuit 12 shown in FIG.
The circuit has the same configuration as the circuit, and the comparator IC1
Is connected to a reference voltage reduction circuit (described later) 16e. When the rear power is equal to or less than the set power, the reference voltage reducing circuit 16e outputs the reference voltage V of the comparator IC1.
_Does not reduce _REF . Therefore, the ACC circuit 12 sets the ACC so that the LD current id becomes the set current value (= V _REF / r ₁ ).
Perform control. However, when the rear power becomes equal to or higher than the set power, the reference voltage reducing circuit 16e reduces the reference voltage V _REF input to the non-inverting terminal of the comparator IC1. AC
Since the C circuit 12 controls the voltage between the terminals of the resistor R1 (= id · r ₁ ) when the LD current id flows to be equal to the reference voltage V _REF , the LD current id decreases as the reference voltage decreases. Limited. An ATC circuit 13 controls the LD temperature to be constant, and has the same configuration as the ATC circuit shown in FIG. Reference numeral 15 denotes a power monitor circuit that outputs a voltage signal (rear power monitor signal) corresponding to the rear power using the pin photodiode 19;
Reference numeral 6 denotes an LD current limiting circuit that limits the LD current when the rear power exceeds the set power.

In the LD current limiting circuit 16, reference numeral 16a denotes a reference voltage generating unit which divides the voltage by resistors R7 and R8 and outputs a reference voltage Vr, and 16b comprises an operation amplifier, and corresponds to the detected light emission power. A comparator 16e for comparing the voltage Vp with the reference voltage Vr, a reference voltage reducing circuit 16e having a diode configuration, and a comparator 16b
Is connected to the non-inverting input terminal of the comparator IC1 of the ACC circuit 12 via the diode DD with the polarity shown. When the backward power is larger than the set power (Vp <Vr), the diode DD is forward-biased.
For this reason, the reference voltage V _REF of the comparator IC1 of the ACC circuit 12 becomes a low potential which is higher by one diode than the output of the comparator 16b, and limits the LD current id. On the other hand, when the rear power is smaller than the set power
(Vp ≧ Vr), the diode DD is reverse biased.
For this reason, the reference voltage V _REF of the comparator IC1 of the ACC circuit 12 becomes a large value obtained by dividing the voltage by the resistors R2 and R3, and the ACC circuit 12 makes the id · r ₁ equal to the reference voltage V _REF , that is, the LD current id is controlled to be equal to V _REF / r _1.

The reference voltage Vr is set by utilizing the fact that the forward power of the LD and the rear power BP of the LD are in a proportional relationship, and the rear power monitor signal BP corresponding to when the forward power is several times the normal power is used. Set to voltage. In the above configuration, when the power of the LD drive circuit is turned on at a low temperature,
Until the temperature of the LD stabilizes, the voltage Vp of the rear power monitor signal BP becomes lower than the reference voltage Vr (Vp <Vr), the output of the comparator 16b drops to a point where the diode DD can be turned on, and the LD current id becomes Is determined not by the resistance division ratio but by the potential raised by one diode from the output of the comparator 16b.

That is, when the rear power (light emission power) increases and the voltage value Vp of the rear power monitor signal BP becomes smaller than the reference voltage Vr (when the LD 11a tries to emit excessive light), the diode DD is turned on (forward bias). ), Limit the LD current id (APC mode). Thereafter, when the temperature of the LD is stabilized at the set temperature by the ATC control, the voltage Vp of the rear power monitor signal BP becomes higher than the reference voltage Vr. Therefore, the comparator 16b
Output rises, the diode DD is turned off (reverse bias), and the LD current id has a current value determined by the normal resistance ratio of the resistors R2 and R3 (ACC mode). In the circuit configuration of the second embodiment, when the power is turned on at a low temperature, an operation of preventing excessive light emission of the LD is performed, and at the same time, the diode D
There is a merit that it can be used as an APC circuit if it is used with D always on. Although the anode common LD is used in the second embodiment of FIG. 7, the cathode common LD is used.
Can also be used.

(B) Embodiment for Detecting LD Temperature to Prevent Excessive Light Emission (a) First Embodiment FIG. 8 is a block diagram of a first embodiment for detecting the temperature of an LD chip to prevent excessive light emission. is there. The first embodiment has a configuration similar to that of the embodiment shown in FIG. 4 in which the light emission power is detected to prevent excessive light emission, and the same portions are denoted by the same reference numerals. The difference is that (1) the power monitor circuit 15 in FIG. 4 is deleted, and (2) a signal provided in the ATC circuit 13 and having a voltage corresponding to the LD temperature output from the temperature monitor circuit 21 (temperature monitor signal (TP) is input to the LD current limiting circuit 16, and (3) the reference voltage Vr is set to a voltage corresponding to a temperature several degrees lower than the target temperature of the ATC control.

The temperature monitor circuit 21 of the ATC circuit 13 monitors the chip temperature of the LD 11a, inputs a temperature monitor signal TP corresponding to the temperature to the voltage follower 16c, and outputs a voltage V corresponding to the LD temperature from the voltage follower 16c. _T is input to the comparator 16b. Comparator 16b compares the voltage V _T corresponding to the reference voltage Vr and LD temperature set by resistors R7, R8. The reference voltage Vr is set to a voltage value output from the temperature monitor circuit 21 at a temperature lower than a few degrees Celsius from the target temperature of the ATC control. Therefore, when the power supply of the LD driving unit at a low temperature, (between until the set temperature) the temperature of the LD 11a is until stabilized, the voltage V _T output from the temperature monitor circuit 21 is higher than the reference voltage Vr a negative direction of the potential (V _T <Vr). V _T <period Vr, the output of the comparator 16b, the transistor T of the current source 16d
R2 lowered until turned on, the current source 16d is flowing the current i _L in the resistance R1 of the ACC circuit 12. For this reason, the inverting input of the comparator IC1 becomes larger than _VREF which is the non-inverting input, the transistor TR1 turns off, and the LD current id
Becomes zero. As described above, while the temperature of the LD 11a is excessively high, the transistor TR2 is turned on and the LD
The current id is reduced to prevent the LD 11a from emitting excessive light, thereby preventing damage and characteristic deterioration (ACC stop mode).

Thereafter, the LD 11a is controlled by ATC control.
Voltage V _T the temperature of the output from the temperature monitor circuit 21 increases and becomes the more the high potential reference voltage Vr, the output of the comparator 16b becomes positive, turns off the transistor TR2. As a result, thereafter, the ACC circuit 12 restores the ACC control function and sets the LD current id to a current value corresponding to the reference voltage V _REF set by the resistors R2 and R3 (ACC mode).

[0033] From the above, as shown in FIG. 9, the low-temperature driving of the LD, the number of the target temperature of the LD temperature ATC control ⁰
Until the temperature rises below C, the LD current id becomes zero,
The light emission power (light output) of the LD is also zero (ACC
Stop mode). That is, the LD current id becomes zero during the temperature at which the LD emits excessive light, thereby preventing the LD from emitting excessive light. Thereafter, when the LD temperature rises by the ATC control to reach a temperature equal to or lower than the target temperature of the ATC control, which is several ⁰ C (time t ₁ ), the ACC circuit recovers the ACC function and the LD current i.
d is controlled so as to be a current value set by the resistance ratio of the resistors R2 and R3 (ACC mode). As described above, since the ACC is performed after the temperature rises, the LD does not emit excessive light. In the above description, the LD current id is set to zero in the ACC stop mode, but may not be set to zero. That is, an LD current that does not cause excessive light emission may flow. As described above, the current of the LD is forcibly squeezed in the region where the excessive light emission is most likely to occur immediately after the power is turned on at a low temperature, thereby preventing damage to the LD and deterioration of the characteristics of the LD.

(B) Modified Example In the first embodiment shown in FIG. 8, an anode common LD is used, but it is also possible to use a cathode common LD. FIGS. 10A and 10B are configuration diagrams of an anode common type and a cathode common type for detecting an LD temperature to prevent excessive light emission. FIG. 10A is a configuration diagram of an anode common type, and FIG. 10B is a configuration diagram of a cathode common type. In the figure, 12 is an anode common type ACC circuit, and 12 'is a cathode common type ACC circuit.
The configuration shown in FIG. 16 is an LD current limiting circuit, and 21 is a temperature monitor. By pouring the anode common type of ACC circuit 12 to limit current i _L in the resistor R1 increases the terminal voltage of the resistor R1 limits the LD current. On the other hand, in the cathode common type ACC circuit 12 ', the LD current is limited by flowing a part of the current (limiting current i _L ) flowing through the resistor R4 to the outside without flowing to the LD.

(C) Second Embodiment FIG. 11 is a block diagram of a second embodiment in which the temperature of an LD chip is detected to prevent excessive light emission. The second embodiment has a configuration similar to that of the embodiment shown in FIG. 7 in which the light emission power is detected to prevent excessive light emission, and the same portions are denoted by the same reference numerals. The difference is that (1) the power monitor circuit 15 of FIG. 7 is deleted, and (2) the voltage V _T according to the LD temperature output from the temperature monitor circuit 21 provided in the ATC circuit 13 is changed to the LD current limiting circuit. 16 (the non-inverting terminal of the comparator 16c), and (3) the reference voltage Vr is set to a voltage corresponding to a temperature several degrees lower than the set temperature of the ATC control.

[0036] monitors the chip temperature of the LD 11a at a temperature monitor circuit 21 of the ATC circuit 13, and inputs a voltage signal V _T corresponding to the temperature to the comparator 16b. Comparator 16b compares the voltage V _T corresponding to the reference voltage Vr and LD temperature determined by the resistance R7, R8. The reference voltage Vr is set to a voltage value output from the temperature monitor circuit 21 at a temperature of several degrees C or less from the target temperature of the ATC control. Therefore, when the power supply of the LD driving unit at a low temperature, (between until the set temperature) the temperature of the LD 11a is until stabilized, the voltage V _T output from the temperature monitor circuit 21 is higher than the reference voltage Vr a negative direction of the potential (V _T <Vr). V _T <period Vr, LD current id decreases to place the output of the comparator 16b is capable of turning on the diode DD is not the resistance division ratio is determined by one diode up potential from the output of the comparator 16b, no excessive emission It is limited to a current value of the order.

As described above, the diode DD is turned on and the LD
The current id is reduced to prevent the LD 11a from emitting excessive light, thereby preventing damage and characteristic deterioration (ACC stop mode). Thereafter, the temperature of the LD 11a rises due to the ATC control, and the voltage V output from the temperature monitor circuit 21 is increased.
_{When T} becomes higher than the reference voltage Vr, the comparator 1
The output of 6b becomes positive, turning off the diode DD. As a result, the ACC circuit 12 thereafter recovers the ACC control function, and changes the LD current id to the reference voltage V set by the resistors R2 and R3.
A current value corresponding to _REF is set (ACC mode). FIG.
In the first embodiment, the anode common LD is used. However, it is also possible to use a cathode common LD.

(C) Example of Monitoring the Elapsed Time After Power-on to Prevent Excessive Light Emission (a) First Embodiment FIG. 12 shows the first example of monitoring the elapsed time after power-on to prevent excessive light emission. It is a lineblock diagram of an example. The first embodiment has a configuration similar to that of the embodiment shown in FIG. 4 in which the light emission power is detected to prevent excessive light emission, and the same portions are denoted by the same reference numerals. The difference is that (1) the power monitor circuit 15 and the voltage follower 16c in FIG. 4 are deleted, and (2) an elapsed time monitor circuit 31 for monitoring the elapsed time after the power is turned on is provided. That the voltage _VL corresponding to the elapsed time is input to the LD current limiting circuit 16 (the non-inverting terminal of the comparator 16b). (3) Until the LD temperature becomes substantially constant by the ATC control after the power is turned on. Is set as the reference voltage Vr.

The elapsed time monitor circuit 31 includes a resistor R10
And an integration circuit of a capacitor C1 and a power supply voltage is input. When the power of the LD driver is turned on at low temperature, the capacitor terminal voltage (output voltage V _L )
Rises exponentially with a time constant R10 · C1. The comparator 16b compares a reference voltage Vr determined by the resistors R7 and R8 with a voltage _VL according to an elapsed time after power-on. The reference voltage Vr is set to L by ATC control after the power is turned on.
This is a voltage corresponding to the time required for the temperature D to reach a substantially constant temperature. Therefore, until the temperature of the LD 11a is stabilized, the voltage V _L output from the time lapse monitor circuit 31 is output.
Becomes a potential in the negative direction from the reference voltage Vr (V _L <V
r).

During the period of V _L <Vr, the output of the comparator 16b falls until the transistor TR2 of the current source 16d can be turned on, and the current source 16d is connected to the resistor R1 of the ACC circuit 12.
A current i _L is supplied. Therefore, the comparator IC1
Becomes larger than _VREF which is a non-inverting input, the transistor TR1 is turned off, and the LD current id becomes zero. From the above, until the LD temperature becomes substantially constant, that is, L
The transistor TR2 is turned on to narrow the LD current id until a period during which D 11a is likely to emit excessive light elapses, so that the LD 11a does not emit excessive light, thereby preventing damage and characteristic deterioration (ACC stop mode). .

Thereafter, the LD 11a is controlled by ATC control.
Rises to a substantially constant temperature, the voltage _VL output from the time lapse monitor circuit 31 becomes higher in potential than the reference voltage Vr. When V _L > Vr, the comparator 16
The output of b becomes positive, turning off the transistor TR2. As a result, thereafter, the ACC circuit 12 restores the ACC control function and sets the LD current id to a current value corresponding to the reference voltage V _REF set by the resistors R2 and R3 (ACC mode).

As described above, as shown in FIG. 13, in the low-temperature driving of the LD, the LD current id becomes zero and the light emission power of the LD becomes zero until the LD temperature is stabilized (ACC).
Stop mode). That is, while the temperature of the LD is low enough to cause excessive light emission, the LD current id becomes zero,
Prevents excessive emission of LD. After a while, L is controlled by ATC control.
When the D temperature is stabilized (time t ₁ ), the ACC circuit 12
The CC function is restored, and the LD current id is controlled so as to have a current value set by the resistance ratio of the resistors R2 and R3 (ACC mode). As described above, since the ACC is performed after the temperature rises, the LD does not emit excessive light. In the above, AC
Although the LD current id is set to zero in the C stop mode, it need not be zero. That is, an LD current that does not cause excessive light emission may flow. As described above, the ACC circuit 12 does not operate for several seconds after the power is turned on. For this reason, at the time of low-temperature startup, first, the ATC circuit 13 operates to bring the temperature to a stable state. Then, the ACC circuit 12 starts operating after the temperature is stabilized. Therefore, excessive emission of the LD can be prevented.

(B) Modification of First Embodiment FIG. 14 shows a modification of the first embodiment, in which the elapsed time monitor circuit 31 after power-on is constituted by a counter and a DA converter. In the figure, 31a is an oscillator that oscillates at a constant frequency, 31b is a counter, and 31c is a DA converter that converts the count value of the counter into DA. After turning on the power, the counter 31b counts the pulses output from the oscillator 31a,
The DA converter 31c performs DA conversion of the count value of the counter, and outputs a voltage signal _VL that increases in proportion to the elapsed time.

(C) Another Modified Example of the First Embodiment In the first embodiment shown in FIG. 12, an anode common LD is used. However, it is also possible to use a cathode common LD. FIGS. 15A and 15B are configuration diagrams of an anode common type and a cathode common type for detecting an elapsed time to prevent excessive light emission. FIG. 15A is a configuration diagram of an anode common type, and FIG.
It is a block diagram of a cathode common type. 25, reference numeral 12 denotes an anode common type ACC circuit, and reference numeral 12 'denotes a cathode common type ACC circuit.
It has the configuration shown in FIGS. 16 is an LD current limiting circuit, and 31 is an elapsed time monitor. By pouring the anode common type of ACC circuit 12 to limit current i _L in the resistor R1 increases the terminal voltage of the resistor R1 limits the LD current. On the other hand, in the cathode common type ACC circuit 12 ', the LD current is limited by flowing a part of the current (limiting current i _L ) flowing through the resistor R4 to the outside without flowing to the LD.

(D) Second Embodiment FIG. 16 is a block diagram of a second embodiment in which an elapsed time after power-on is monitored to prevent excessive light emission. The second embodiment has a configuration similar to that of the embodiment shown in FIG. 7 in which the light emission power is detected to prevent excessive light emission, and the same portions are denoted by the same reference numerals. The difference is that (1) the power monitor circuit 15 in FIG. 7 is deleted, and (2) an elapsed time monitor circuit 31 for monitoring the elapsed time after power-on is provided. until LD current limiting circuit 16 a voltage V _L corresponding to the points that are input to the (forward rotation terminal of the comparator 16b), (3) LD temperature becomes substantially constant temperature (stabilized) at ATC control after power Is set as the reference voltage Vr.

The elapsed time monitor circuit 31 includes a resistor R10
And an integration circuit of a capacitor C1 and a power supply voltage is input. When the power of the LD driver is turned on at low temperature, the capacitor terminal voltage (output voltage V _L )
Rises exponentially with a time constant R10 · C1. The comparator 16b compares a reference voltage Vr determined by the resistors R7 and R8 with a voltage _VL according to an elapsed time after power-on. The reference voltage Vr is set to L by ATC control after the power is turned on.
This is a voltage corresponding to the time required for the temperature D to reach a substantially constant temperature. Therefore, until the temperature of the LD 11a is stabilized, the voltage V _L output from the time lapse monitor circuit 31 is output.
Becomes a potential in the negative direction from the reference voltage Vr (V _L <V
r). During the period of _VL <Vr, the output of the comparator 16b drops to the point where the diode DD can be turned on, and the LD current id is determined by the potential raised by one diode from the output of the comparator 16b, and is limited to a current value that does not cause excessive light emission. You. As described above, while the LD 11a is at a low temperature at which excessive light emission is possible, the diode DD
Is turned on to reduce the LD current id to prevent the LD 11a from emitting excessive light, thereby preventing its breakage and characteristic deterioration (AC
C stop mode).

Thereafter, the LD 11a is controlled by ATC control.
Rises to a substantially constant temperature, the voltage _VL output from the time lapse monitor circuit 31 becomes higher in potential than the reference voltage Vr. When V _L > Vr, the comparator 16
The output of b becomes positive, and the diode DD is turned off. As a result, the ACC circuit 12 thereafter recovers the ACC control function, and changes the LD current id to the reference voltage V set by the resistors R2 and R3.
A current value corresponding to _REF is set (ACC mode). As described above, since the ACC circuit 12 does not operate for a few seconds after the power is turned on, the ATC circuit 13 operates first and the temperature becomes stable at the time of low-temperature startup. Then, since the ACC circuit 12 starts operating after the temperature is stabilized, excessive light emission of the LD can be prevented.

(E) Modification of Second Embodiment FIG. 17 shows a modification of the second embodiment, in which the elapsed time monitor circuit 31 after power-on is composed of a counter and a DA converter. In the figure, 31a is an oscillator that oscillates at a constant frequency, 31b is a counter, and 31c is a DA converter that converts the count value of the counter into DA. Counter 31b
After the power is turned on, the pulse output from the oscillator 31a is counted, and the DA converter 31c converts the count value of the counter from DA to DA, and outputs a voltage signal _VL that increases in proportion to the elapsed time. Although the anode common LD is used in the embodiment shown in FIG. 16, it is also possible to use a cathode common LD.

(D) Shared type comparator (laser current control circuit) (a) Configuration FIG. 18 (a) shows a comparator (laser current control circuit) ICC that can be commonly used in both ACC circuits of the anode common LD and the cathode common LD. FIG.
This shared-type comparator ICC can be commonly used as the comparators IC1 and IC2 in FIGS. 25 (a) and 25 (b).
FIG. 18B is a series circuit of an anode common type ACC circuit (FIG. 25A) inserted between the ground and the negative power supply by connecting the anode common LD 11a, the transistor TR1, and the resistor R1 in series. c) is the cathode common L
This is a series circuit of a cathode common type ACC circuit (FIG. 25B) inserted between the ground and the positive power supply by connecting D11b, transistor TR2, and resistor R4 in series.

The first input terminal T _R of the shared comparator ICC
Is _supplied with a reference voltage V _REF , and the second input terminal _TF is supplied with L
The terminal voltage V _F of the resistor R1 or resistor R4 caused by D current id is input as a feedback signal. Also, the output terminal Tout
Is connected to the base terminal of the transistor TR1 of the anode common series circuit or the base terminal of the transistor TR2 of the cathode common series circuit. Shared comparator
In ICC, OPamp is an operation amplifier and a resistor R
a and Rb are attenuation resistors, and the other resistors and transistors constitute an emitter follower. That is, two transistors TR _iK and TR _iA having the same suffix number and two resistors R _iK and R _iA (i = 1, 2, 3)
Constitute a complementary emitter follower circuit. The first complementary emitter follower circuits of suffixes 1 and enter the reference voltage V _REF, the emitter follower circuit of the second complementary suffix 2 type feedback voltage V _F is,
The output signal of the operation amplifier OPamp is input to the third complementary emitter follower circuit of the suffix 3. Note that an element with a suffix A means an element that operates when driving the anode common LD, and an element with a suffix K means that it operates when driving the cathode common LD.

(B) Complementary emitter follower circuit FIG. 19A is a diagram for explaining the operation of a complementary emitter follower circuit, and two transistors TR _iK and T having different polarities.
The emitters and bases of R _iA are connected to each other, the collector of npn transistor TR _iK is connected to the positive power supply + Vcc, the collector of pnp transistor TR _iA is connected to the negative power supply -Vee, and the base has a positive or negative input. A signal is applied. When a positive polarity signal is input to the base, the npn transistor TR _iK turns on and the pnp transistor T
R _iA is turned off, is constituted emitter follower circuit of gain 1 comprising a resistor R _iK emitter resistor (b), the
A positive polarity output signal having the same amplitude as the input signal appears at the output terminal OUT. When a negative signal is input to the base, np
The n-transistor TR _iK is turned off, the pnp transistor TR _iA is turned on, and an emitter follower circuit having a gain of 1 is formed as shown in (c), with the resistor R _{iA serving} as an emitter resistance. The output terminal OUT has the same amplitude as the input signal. A negative output signal appears.

(C) Operation of Shared Type Comparator When controlling the drive current of the anode common LD, the input terminals T _R and T _{F of} the shared type comparator ICC (base terminals of the first and second complementary emitter follower circuits) the polarity of the reference voltage V _REF and the feedback voltage V _F to be input into the negative. Therefore, when controlling the drive current of the anode common LD, the first and second
Operation amplifier complementary emitter follower circuit operates as shown in FIG. 19 (c), via respective damping resistors Ra, and Rb negative polarity reference voltage V _REF and the feedback voltage V _F of the
Input to the non-inverting input terminal and inverting input terminal of OPamp.
When controlling the drive current of the cathode common LD, the input terminals T _R and T _F (first and second terminals) of the common-type comparator ICC are controlled.
The polarity of the reference voltage V _REF and the feedback voltage V _F to be input to the base terminal) complementary emitter follower circuit is positive. Therefore, at the time of driving current control of the cathode common LD,
The second complementary emitter follower circuit operates as shown in FIG. 19B, and has a positive reference voltage V _REF and a feedback voltage V _F.
Is input to the non-inverting input terminal and the inverting input terminal of the operation amplifier OPamp via the attenuation resistors Ra and Rb, respectively.

The attenuation resistors Ra and Rb convert the input signal level to a level within the operation range of the operation amplifier OPamp. That is, even if the amplitude of the input signal (the reference voltage V _REF and the feedback voltage V _F ) is + Vcc to -Vee, the operable input range of the operation amplifier OPamp (+ Vcc-1.5)
Convert to the level of ~ (-Vee + 1.5).

The operational amplifier OPamp outputs + Vcc if the non-inverted input signal is larger than the inverted input signal, and outputs -Vee if it is smaller. Therefore, the anode common LD
When controlling the drive current id of 11a, the drive current
If id is equal to or less than the set value, V _F <V _{REF and} the output signal of the operation amplifier OPamp becomes + Vcc. As a result, the transistor TR _{3a of} the third emitter follower circuit
Is turned on, a high-level signal is output from the output terminal Tout, the transistor TR1 of the anode common series circuit is turned on, and the drive current id of the anode common LD 11a increases. On the other hand, if the drive current id is equal to or more than the set value, V _F > V
_REF , and the output signal of the operation amplifier OPamp is-
Vee. As a result, the transistor TR _{3b of} the third emitter follower circuit is turned on, a low-level signal is output from the output terminal Tout, the transistor TR1 of the anode common series circuit is turned off, and the drive current id of the anode common LD 11a decreases. .

The drive current of the cathode common LD 11b
When controlling the id, if the drive current id is equal to or less than the set value, V _F > V _REF and the operation amplifier OPam
The output signal of p is -Vee. As a result, the transistor TR _{3b of} the third emitter follower circuit is turned on, a low-level signal is output from the output terminal Tout, and the transistor TR2 of the cathode common series circuit is turned on to increase the drive current id of the cathode common LD 11b. . On the other hand, drive current
If id is equal to or larger than the set value, V _F <V _{REF and} the output signal of the operation amplifier OPamp becomes + Vcc. As a result, the transistor TR _{3a of} the third emitter follower circuit
Is turned on, a high-level signal is output from the output terminal Tout, the transistor TR2 of the cathode common series circuit is turned off, and the drive current id of the cathode common LD 11b decreases.

As described above, the shared-type comparator ICC of the present invention adds an emitter follower circuit in which transistors having different polarities are complementarily connected to each of the input stage and the output stage of a general OPamp, so that the input voltage is positive. One transistor is turned on with a forward bias and the other transistor is turned off with a reverse bias between a case near a power supply and a case near a negative power supply. Further, the level of the emitter follower output is shifted using an attenuator so that the output falls within a general OPamp input voltage range. By using such a common comparator ICC for the ACC circuit,
The drive current of the anode common LD and the cathode common LD can be controlled only by changing the reference voltage generator and the series circuit. In the above description, the power supply voltages are + Vcc and -Vee, but one of them may be grounded. That is, the combination of parallel voltages is represented by + Vcc, 0 (= −
Vee), or 0 (= + Vcc) or -Vee, enables detection of the target operation (input closer to either + Vcc or -Vee).

(E) Shared LD Current Limiting Circuit An embodiment for detecting light emission power and preventing excessive light emission (FIG. 4)
7 to FIG. 7), an embodiment for detecting the LD temperature to prevent excessive light emission (FIGS. 4 to 11), and an embodiment for monitoring the power-on elapsed time to prevent excessive light emission (FIGS. 12 to 17). In this embodiment, the drive current of one of the anode common LD and the cathode common LD is limited so as not to emit light excessively. Therefore, the LD current limiting circuit is connected to the anode common LD.
It is convenient if it can be shared by both the cathode common LD and the cathode common LD.

(A) Shared LD current limiting circuit for detecting light emission power and preventing excessive light emission FIG. 20 shows an LD for detecting light emission power and preventing excessive light emission
The current limiting circuit consists of an anode common LD and a cathode common L
FIG. 11 is a configuration diagram of a shared LD current limiting circuit that can be shared by D, 11 is an anode common or cathode common type LD, 12 is an ACC circuit that controls the LD current to be a set value, and 13 is an LD temperature. ATC circuit that controls so that is constant, 14 is an optical fiber, 15 is an LD
A power monitor circuit that monitors the power behind the LD (rear power BP) as the light emission power. Reference numeral 16 denotes a shared LD that limits the LD current when the light emission power exceeds a set power. It is a current limiting circuit.

In the shared type LD current limiting circuit 16, 1
Reference numeral 6a denotes a reference voltage generator which outputs a reference voltage Vr according to the set power, 16b a comparator for comparing a voltage Vp corresponding to the detected light emission power with the reference voltage Vr, and 16c a light emission power equal to or higher than the set power. when it is, and outputs a constant limit current i _L, when emission power is below the set value, limits the current control unit that limits current to 0, 16d
Denotes an LD type detector for detecting whether the LD 11 is of the anode common type or the cathode common type, and 16e.
Is input a limiting current i _L , (1) if the LD 11 is an anode common type, the limiting current flows into the ACC circuit 12, and (2) if the LD 11 is a cathode common type,
This is a limiting current direction switching unit that performs control to cause a current corresponding to the limiting current to flow from the ACC circuit 12.

[0060] As described in FIG. 6, to increase the terminal voltage of the resistor R1 by pouring the ACC circuit 12, the limiting current i _L of the anode-common type resistor R1 LD
Limit the current. On the other hand, a cathode common type ACC
The circuit 12 'limits the LD current by flowing a part of the current (limiting current i _L ) flowing through the resistor R4 to the outside without flowing to the LD. Therefore, the shared LD current limiting circuit 16
Controls the ACC circuit so that the limiting current i _L flows into the ACC circuit if it is an anode common type, and the limiting current i. The details of the LD type detection 16d and the limited current direction switching unit 16e will be described later.

(B) Shared LD Current Limiting Circuit for Detecting LD Temperature and Preventing Excessive Light Emission FIG. 21 shows an LD current limiting circuit for detecting LD temperature and preventing excessive light emission, using an anode common LD and a cathode common LD.
FIG. 11 is a configuration diagram of a shared type LD current limiting circuit that can be shared by the LD, 11 is an LD of an anode common or cathode common type, 12 is an ACC circuit that controls the LD current to a set value, and 13 is an LD temperature. ATC circuit for controlling to be constant, 14 for optical fiber, 16 for LD
An LD current limiting circuit for limiting the LD current when the temperature is equal to or lower than the set temperature, and a temperature monitor circuit 21 for monitoring the temperature of the LD chip. In LD current limiting circuit 16, 16a is a reference voltage generator, which outputs a reference voltage Vr corresponding to the set temperature, 16b compares the voltage V _T with the reference voltage Vr corresponding to the detected temperature comparator 1
6c is a constant limiting current i _L when the LD temperature is below the set value.
And a limit current control unit that sets the limit current to 0 when the LD temperature is equal to or higher than a set value. 16d is an LD type detection unit that detects whether the LD 11 is an anode common type or a cathode common type. When the current i _L is input, (1) if the LD 11 is an anode common type, the limiting current flows into the ACC circuit 12, and (2) if the LD 11 is a cathode common type, the current corresponding to the limiting current is converted to AC.
It is a limiting current direction switching unit that performs control flowing out of the C circuit 12.

As described with reference to FIG. 10, in the ACC circuit 12 of the anode common type, the terminal voltage of the resistor R1 is increased by flowing the limiting current i _L to the resistor R1, thereby increasing the terminal voltage of the resistor R1.
Limit D current. On the other hand, a cathode common type AC
In the C circuit 12 ', a part of the current flowing through the resistor R4 (the limiting current i _L ) is allowed to flow to the outside without flowing to the LD.
Limit the current. Therefore, the shared LD current limiting circuit 16
Controls the ACC circuit so that the limiting current i _L flows into the ACC circuit if it is an anode common type, and the limiting current i.

(C) Shared LD Current Limiting Circuit for Monitoring the Elapsed Time of Power-on to Prevent Excessive Light Emission FIG. 22 shows an LD current limiting circuit for monitoring the elapsed time of power-on and preventing excessive light emission. FIG. 11 is a configuration diagram of a shared LD current limiting circuit that can be shared by a cathode common LD, 11 is an anode common or cathode common type LD, 12 is an ACC circuit for controlling the LD current to a set value, and 13 is ATC circuit for controlling the LD temperature to be constant, 14 for an optical fiber, 16 for stopping the ACC control when the elapsed time after power-on has not reached the set time to limit the LD current, and after power-on When the elapsed time reaches the set time, the ACC control is restored to set the LD current to the set value.
Reference numeral 1 denotes an elapsed time monitoring circuit for monitoring an elapsed time after the power is turned on, and includes, for example, a delay circuit (integrating circuit) in which the output voltage _VL increases according to the elapsed time after the power is turned on.

In the LD current limiting circuit 16, reference numeral 16a denotes a reference voltage generator, and the reference voltage Vr corresponds to a set time.
Outputs a, 16b compares the voltage V _L and the reference voltage Vr with the elapse time comparator, 16c when the elapsed time has not reached the set time, outputs a constant limit current i _L, the elapsed time When the set time is exceeded, the limit current is set to 0
A limiting current control unit, 16d is an LD type detecting unit for detecting whether the LD 11 is an anode common type or a cathode common type, and 16e is supplied with a limiting current i _L. (1) If the LD 11 is an anode common type If the LD 11 is of the cathode common type, the current corresponding to the limited current is supplied to the ACC circuit 12.
It is a limiting current direction switching unit that performs control flowing out of the C circuit 12.

As described with reference to FIG. 15, in the ACC circuit 12 of the anode common type, the terminal voltage of the resistor R1 is increased by flowing the limiting current i _L to the resistor R1.
Limit D current. On the other hand, a cathode common type AC
In the C circuit 12 ', a part of the current flowing through the resistor R4 (the limiting current i _L ) is allowed to flow to the outside without flowing to the LD.
Limit the current. Therefore, the shared LD current limiting circuit 16
Controls the ACC circuit so that the limiting current i _L flows into the ACC circuit if it is an anode common type, and the limiting current i.

(D) Partial Detailed Circuit of Shared LD Current Limiting Circuit FIG. 23 shows a shared LD current limiting circuit 16 (FIGS. 20 to 2).
FIG. 3 is a partially detailed circuit diagram of 2), showing an LD type detection unit 16d.
And the details of the limited current direction switching unit 16e. (d-1) LD Type Detector The LD type detector 16d is formed of a complementary emitter follower circuit. That is, the emitters and bases of _two transistors T ₁ and T _{2 having} different polarities are connected to each other, the collector of the npn transistor T ₁ is connected to the positive power supply + Vcc, and the collector of the pnp transistor T ₂ is connected to the negative power supply -Ve.
e, and a positive or negative input signal is applied to the base. When a positive polarity signal is input to the base, the npn transistor T ₁ is turned on, and the pnp transistor T ₂
There was off, the resistor R ₁ is configured the emitter follower circuit of gain 1 as the emitter resistance, a positive output signal of the input signal and the same amplitude are inputted to the next stage of the limiting current direction switching unit 16e. When a negative signal is input to the base, np
n transistor T ₁ is turned off, pnp transistor T ₂ is turned on, resistance emitter follower circuit of gain 1 R ₂ is an emitter resistor is configured, the input signal and the negative polarity of the output signal is next limit current of the same amplitude Input to the direction switching unit 16e.

Now, actually, the output signal of the first or second emitter follower circuit of the shared comparator ICC is input to the base of the LD type detection section 16d. When the common comparator ICC is used in the ACC circuit of the anode common LD, the first or second emitter follower circuit inputs a negative signal to the base of the LD type detection unit 16d. If the LD is an anode common type, the signal of the negative polarity is switched to the limited current direction switching unit 16e.
To enter. When the common comparator ICC is used in the ACC circuit of the cathode common LD, the first or second emitter follower circuit inputs a positive signal to the base of the LD type detection unit 16d. The section inputs a signal of positive polarity to the limited current direction switching section 16e if the LD is a cathode common type.

(D-2) Limited Current Direction Switching Unit In the limited current direction switching unit 16e, a transistor
T_Three, T_FourForm a differential pair, and one transistor T_ThreeTo
Is the input of the output signal of the LD type detector 16d,
Transistor T_FourThe base of the resistor R_Three, R_FourPartial pressure
A constant voltage is input. This differential pair has a small base voltage
Output from the limiting current control unit 16c to the other transistor
Limiting current i_LFlow. Transistor T_Five, T₆Is the first mosquito
The rent mirror circuit is connected to the transistor T₇, T₈Is the second mosquito
The rent mirror circuit is connected to the transistor T ₉, T_TenIs the third
Construct a current mirror circuit. Output terminal To is ACC
Feedback voltage input of the shared comparator ICC that constitutes the circuit
Terminal T_FAnd resistor R1 (anode common type
) Or resistor R4 (for cathode common type)
Connected.

[0069] (d-3) Operation LD type detection unit 16d, since LD outputs a negative signal if the common-anode type, the transistor T ₃ is turned on, the limit current i _L flows through the transistor T ₅ . The first current mirror causes the limiting current i _L to flow through the transistors T ₆ and T ₇ , and the second current mirror causes the limiting current to flow through the transistor T ₈ . That is, if the LD is of the anode common type, the limiting current i _L flows into the resistor R ₁ of the ACC circuit. On the other hand, the LD type detector 16
d, since LD outputs a positive signal if the cathode common type, the transistor T ₄ is turned on, the limit current i _L flows through the transistor T _9, the limit current i _L is the transistor by a third current mirror flowing through the T _10. That is, if the LD is a cathode common type, the limiting current i
_L flows out of the ACC circuit. As described above, the present invention has been described with reference to the embodiments. However, the present invention can be variously modified in accordance with the gist of the present invention described in the claims, and the present invention does not exclude these.

[0070]

As described above, according to the present invention, the light emission power (rear power) of the LD is monitored, and when the rear power exceeds the set power, the LD current is limited thereafter.
When the D temperature rises and the rear power falls below the set power, the LD current is set to the set current value by ACC control, so that the LD current can be limited when the LD temperature is low,
Thus, excessive emission of the LD can be prevented, and destruction of the LD and deterioration of characteristics can be prevented.

According to the present invention, the LD temperature is monitored, and when the LD temperature is lower than the set temperature, the LD current is limited. When the LD temperature becomes equal to or higher than the set temperature, the LD current is reduced by the ACC control. , Excessive emission of the LD during low-temperature driving can be prevented, and destruction of the LD and deterioration of characteristics can be prevented. According to the present invention, the elapsed time after the power is turned on is monitored, and when the elapsed time does not exceed the set time, the LD current is limited to zero or a low current value. Since the ACC control was performed so as to be equal to the set current value,
By setting the set time to the time from when the power is turned on until the LD temperature becomes substantially constant (the time until the LD temperature becomes stable), the LD current can be limited when the LD temperature is low. Can prevent excessive light emission of LD
Can be prevented from destruction and characteristic deterioration.

According to the present invention, since the comparator (current control circuit) used for each type of ACC circuit of the anode common and the cathode common is shared, a cost reduction effect can be expected. According to the present invention, an LD protection circuit for preventing excessive light emission of an LD can be shared by each type of an anode common and a cathode common, so that a cost reduction effect can be expected.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a first principle of the present invention.

FIG. 2 is a diagram illustrating a second principle of the present invention.

FIG. 3 is a diagram illustrating a third principle of the present invention.

FIG. 4 is a diagram illustrating a first method of detecting light emission power to prevent excessive light emission.
This is an example.

FIG. 5 is an operation explanatory diagram at the time of low-temperature startup of the first embodiment.

FIG. 6 shows a first method of detecting light emission power to prevent excessive light emission.
It is a block diagram of the anode common type and cathode common type of an Example.

FIG. 7 shows a second method for detecting light emission power to prevent excessive light emission.
This is an example.

FIG. 8 is a first embodiment for detecting an LD temperature to prevent excessive light emission.

9 is an operation explanatory diagram of the first embodiment of FIG. 7 at the time of low-temperature startup.

FIG. 10 shows a first method of detecting an LD temperature to prevent excessive light emission.
It is a block diagram of the anode common type and cathode common type of an Example.

FIG. 11 shows a second method of detecting an LD temperature to prevent excessive light emission.
This is an example.

FIG. 12 is a first embodiment in which the power-on elapsed time is monitored to prevent excessive light emission.

FIG. 13 is an explanatory diagram of the operation at the time of low-temperature startup of the power-on elapsed time monitor type.

FIG. 14 is a modification of the first embodiment.

FIG. 15 is a configuration diagram of an anode common type and a cathode common type according to the first embodiment for monitoring the elapsed time of power-on to prevent excessive light emission.

FIG. 16 shows a second embodiment in which the power-on elapsed time is monitored to prevent excessive light emission.

FIG. 17 is a modification of the second embodiment.

FIG. 18 is a circuit diagram of a laser drive current control circuit (shared type comparator).

FIG. 19 is a diagram illustrating the operation of a complementary emitter follower.

FIG. 20 is a configuration diagram of a shared LD current limiting circuit that detects light emission power and prevents excessive light emission.

FIG. 21 is a configuration diagram of a shared LD current limiting circuit that detects LD temperature and prevents excessive light emission.

FIG. 22 is a configuration diagram of a shared-type LD current limiting circuit that monitors the power-on elapsed time to prevent excessive light emission.

FIG. 23 is a partial detailed circuit diagram of a shared LD current limiting circuit.

FIG. 24 is a block diagram of an optical transmitter.

FIG. 25 is an example of an ACC circuit.

FIG. 26 is an example of an ATC circuit.

FIG. 27 is an explanatory diagram of a temperature characteristic of an LD.

FIG. 28 is a diagram illustrating the operation of the LD at the time of low-temperature startup.

FIG. 29 is an explanatory diagram of an input voltage range of an operation amplifier that can be used for current control of the anode common LD and the cathode common LD.

[Description of Signs] 11. LD (laser diode) 12. ACC circuit 13. ATC circuit 14. Optical fiber 15. Power monitor circuit 16. LD current limiting circuit 16a Reference voltage generator 16b・ Comparator 21 ・ ・ Temperature monitor circuit 31 ・ ・ Elapsed time monitor circuit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takahisa Nagakubo 2-3-9 Shin-Yokohama, Kohoku-ku, Yokohama, Kanagawa Prefecture In-house Fujitsu Digital Technology Co., Ltd. 4-1-1 1-1 Fujitsu Limited (72) Inventor Motoyoshi Sekiya 4-1-1 1-1 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa F-term within Fujitsu Limited 5F073 EA03 GA21 GA38

Claims (10)

[Claims] 1. An anode common type or cathode common type laser diode (LD), a semiconductor switch and a resistor are connected in series, and feedback control is performed so that a terminal voltage of the resistor generated by a laser drive current becomes equal to a set voltage. A laser drive current control circuit for setting the laser drive current to a set value, wherein two semiconductor elements having different polarities are complementarily connected to each other to form an emitter follower regardless of which semiconductor element is turned on, and A first emitter follower section to which the semiconductor element having two different polarities is complementarily connected, an emitter follower is formed even when any of the semiconductor elements is turned on, and a feedback voltage corresponding to the terminal voltage is provided. A second emitter follower that is input, an attenuator that attenuates an output voltage of the first and second emitter followers, An operation amplifier for inputting the set voltage and the feedback voltage attenuated by the attenuator to the inverting input terminal and the non-inverting input terminal, respectively, and outputting a signal for turning on / off the semiconductor switch based on the magnitude of the two voltage signals; A third emitter follower unit that complementarily connects two semiconductor elements having different polarities, forms an emitter follower when any of the semiconductor elements is turned on, and receives an output signal of an operation amplifier; A laser drive current control circuit, wherein the semiconductor switch is turned on / off by an output signal of an emitter follower unit. 2. An automatic current control circuit for controlling an LD current to a set value, a power monitor circuit for monitoring a light emission power of an LD (laser diode), and when the light emission power becomes equal to or more than a set value, An LD current limiting circuit that stops the automatic current control and limits the LD current, and recovers the automatic current control and sets the LD current to the set current value when the light emission power falls below the set value. Wherein the automatic current control circuit comprises a series circuit in which an anode common type or cathode common type LD, a semiconductor switch and a resistor are connected in series and inserted between a power supply and a ground, and two semiconductor elements having different polarities are complementary to each other. A first emitter follower section, which is connected to the other, and forms an emitter follower regardless of which semiconductor element is turned on, and receives a set voltage; A second emitter follower section in which semiconductor elements having different polarities are complementarily connected, an emitter follower is formed regardless of which semiconductor element is turned on, and a terminal voltage of the resistor generated by an LD current is input as a feedback voltage. An attenuator for attenuating the output voltages of the first and second emitter followers; a set voltage and a feedback voltage attenuated by the attenuator are input to an inverting input terminal and a non-inverting input terminal, respectively; An operation amplifier for outputting a signal for turning on / off the semiconductor switch based on the magnitude, two semiconductor elements having different polarities are complementarily connected to each other, and an emitter follower is formed regardless of which semiconductor element is turned on; A third emitter follower section to which an output signal of the amplifier is inputted, wherein LD protection circuit, characterized in that the set value LD current semiconductor switch is turned on / off. 3. An LD type detection unit for detecting whether an LD is an anode common type or a cathode common type based on an output voltage value of the first or second emitter follower unit. A comparison circuit for comparing the light emission power with a set value; a limit current control unit that outputs a constant current limit when the light emission power is equal to or more than the set value, and sets the current limit to 0 when the light emission power is equal to or less than the set value. The limiting current is input, and the limiting current is supplied to the resistor via a feedback voltage input terminal of the second emitter follower unit based on the LD type, or a current corresponding to the limiting current is fed back from the resistor. 3. The LD protection circuit according to claim 2, further comprising: a limiting current direction switching unit that performs control of flowing out through a voltage input terminal. 4. An automatic current control circuit for controlling an LD current to a set value, a power monitor circuit for monitoring a light emission power of an LD (laser diode), and when the light emission power becomes equal to or more than a set value, An LD current limiting circuit that stops the automatic current control and limits the LD current, and recovers the automatic current control and sets the LD current to the set current value when the light emission power falls below the set value. In the automatic current control circuit, a series circuit purchased between a power supply and a ground by connecting an anode common type or cathode common type LD, a semiconductor switch and a resistor in series, and the terminal voltage of the resistor becomes a set voltage And a control circuit for controlling the semiconductor switch to set the LD current to a set current value, wherein the LD current limiting circuit has an LD common anode type. LD type detector that detects whether the light source is of the cathode common type, a comparison circuit that compares the light emission power with the set value, and outputs a current limit according to the light power when the light emission power exceeds the set value. A limit current control unit that sets a limit current to 0 when the value is equal to or less than a set value; the limit current is input; if the LD is an anode common type, the limit current flows into the resistor of the automatic current control circuit; An LD protection circuit, comprising: a limited current direction switching unit that causes a current corresponding to the limited current to flow from the resistor in the case of a common type. 5. An automatic current control circuit for controlling an LD current to a set value, a temperature monitor circuit for monitoring a temperature of an LD (laser diode), and an automatic current control when the temperature of the LD is lower than a set temperature. Stopping the LD current, and when the temperature of the LD is equal to or higher than the set temperature, recovering the automatic current control and setting the LD current to the set current value. The current control circuit is a series circuit purchased between the power supply and ground by connecting an anode common type or cathode common type LD, a semiconductor switch and a resistor in series, and two semiconductor elements with different polarities are connected in a complementary manner. A first emitter follower section to which a set voltage is inputted regardless of which semiconductor element is turned on; Elements are connected in a complementary manner, an emitter follower is formed regardless of which semiconductor element is turned on, and a second emitter follower section to which a terminal voltage of the resistor is input as a feedback voltage; An attenuating unit for attenuating the output voltage of the emitter follower unit, a set voltage and a feedback voltage attenuated by the attenuating unit are input to an inverting input terminal and a non-inverting input terminal, respectively, and the semiconductor switch is controlled based on the magnitude of the two voltage signals. An operational amplifier that outputs a signal to turn on / off, two semiconductor elements having different polarities are complementarily connected, and an emitter follower is formed even when either semiconductor element is turned on, and an output signal of the operational amplifier is input. A third emitter follower section, and the semiconductor switch is turned on / off by an output signal of the third emitter follower section to control an LD power supply. LD protection circuit, characterized in that the set value. 6. An LD type detection unit for detecting whether an LD is an anode common type or a cathode common type based on an output voltage value of the first or second emitter follower unit. A comparison circuit that compares the LD temperature with a set value. When the LD temperature is equal to or lower than the set value, outputs a limit current according to the LD temperature, and when the LD temperature is equal to or higher than the set value, sets a limit current to 0. The limiting current is input, and based on the LD type, the limiting current flows into the resistor via a feedback voltage input terminal of the second emitter follower unit, or a current corresponding to the limiting current flows from the resistor. 6. The LD protection circuit according to claim 5, further comprising: a limiting current direction switching unit that controls to flow out through the feedback voltage input terminal. 7. An automatic current control circuit for controlling an LD current to a set value, a temperature monitor circuit for monitoring a temperature of an LD (laser diode), and an automatic current control when the temperature of the LD is lower than a set temperature. Stopping the LD current, and when the temperature of the LD is equal to or higher than the set temperature, recovering the automatic current control and setting the LD current to the set current value. The current control circuit is a series circuit purchased between the power supply and ground by connecting an anode common type or cathode common type LD, a semiconductor switch and a resistor in series, and a semiconductor switch so that the terminal voltage of the resistor becomes the set voltage. And a control circuit for controlling the LD current to set the LD current to a set current value, wherein the LD current limiting circuit comprises: an LD having an anode common type or a cathode common LD type detection unit for detecting whether a type, comparator circuit for comparing the LD temperature and the set value, when LD temperature is below the set value, and outputs the constant current limit,
When the LD temperature is equal to or higher than the set value, a limiting current control unit that sets the limiting current to 0. An LD protection circuit, comprising: a limiting current direction switching unit that causes a current corresponding to the limiting current to flow from the resistor if the LD is a cathode common type. 8. An automatic current control circuit for controlling an LD current to a set value, an elapsed time monitor circuit for monitoring an elapsed time after power-on, and an elapsed time after power-on does not reach the set time. When automatic current control is stopped, L
An LD current limiting circuit for limiting the D current and recovering the automatic current control when the elapsed time after turning on the power reaches the set time to set the LD current to the set current value. The current control circuit is a series circuit in which an anode common type or cathode common type LD, a semiconductor switch, and a resistor are connected in series and inserted between a power supply and a ground. Two semiconductor elements having different polarities are connected complementarily. A first emitter follower section to which a set voltage is input, regardless of which semiconductor element is turned on, and two semiconductor elements having different polarities are connected complementarily, and any semiconductor element is turned on. A second emitter follower unit that constitutes an emitter follower and receives the terminal voltage of the resistor as a feedback voltage; An attenuating unit for attenuating the output voltage of the tta follower unit, a set voltage and a feedback voltage attenuated by the attenuating unit are input to an inverting input terminal and a non-inverting input terminal, respectively, and the semiconductor switch is turned on based on the magnitude of both voltage signals. An operational amplifier for outputting a signal for turning on / off, a semiconductor element having two different polarities being connected in a complementary manner, forming an emitter follower regardless of which semiconductor element is turned on, and receiving an output signal of the operation amplifier. An LD protection circuit comprising: three emitter followers, wherein the semiconductor switch is turned on / off by an output signal of the third emitter follower to set an LD current to a set value. 9. An LD type detection unit for detecting whether an LD is an anode common type or a cathode common type based on an output voltage value of the first or second emitter follower unit. A comparison circuit that compares the elapsed time with the set time. Limit current control that outputs a constant current limit when the elapsed time has not reached the set time, and sets the current limit to 0 when the elapsed time exceeds the set time. The limiting current is input, and based on the LD type, the limiting current flows into the resistor via a feedback voltage input terminal of the second emitter follower unit, or a current corresponding to the limiting current flows from the resistor. 9. The LD protection circuit according to claim 8, further comprising: a limiting current direction switching unit that performs control for flowing out through the feedback voltage input terminal. 10. An automatic current control circuit for controlling an LD current to a set value, an elapsed time monitor circuit for monitoring an elapsed time after power-on, and an elapsed time after power-on does not reach the set time. The automatic current control is stopped to limit the LD current, and when the elapsed time after turning on the power reaches the set time, the automatic current control is restored to set the LD current to the set current value. In the LD protection circuit provided, the automatic current control circuit comprises a series circuit purchased between a power supply and a ground by connecting an anode common type or cathode common type LD, a semiconductor switch, and a resistor in series, and a terminal voltage of the resistor. And a control circuit that controls a semiconductor switch so that the LD becomes a set voltage so as to set the LD current to a set current value. LD type detection unit that detects whether the lamp is of the common or cathode common type, a comparison circuit that compares the elapsed time with the set time, and outputs a constant current limit when the elapsed time has not reached the set time. When a set time is exceeded, a limiting current control unit that sets a limiting current to 0. The limiting current is input, and if the LD is an anode common type, the limiting current flows into the resistor of the automatic current control circuit, and the LD An LD protection circuit, comprising: a limiting current direction switching unit that causes a current corresponding to the limiting current to flow from the resistor in the case of a cathode common type.