JPH0321089A - Laser diode drive circuit - Google Patents

Abstract

PURPOSE:To prevent the generation of excessive current, deterioration and breakdown of a diode by installing a protective circuit to the output of an error amplifier. CONSTITUTION:A laser diode 1, a monitor PD, an error amplifier 2, a protective circuit 3, and a bias current generation circuit 4 constitute a negative feedback loop and operate so that the light output of the laser diode may be constant. As negative feedback control is not exercised while the power is supplied, the output voltage 20 of the error amplifier 2 may exceed an optimum value. The protective circuit 30 protects bias current control voltage 30 so that it may not exceed the optimum value even when output voltage 20 rises rapidly, thereby preventing excessive bias current from flowing to the laser diode 1. It is, therefore, possible to prevent excessive electric current from being supplied to the laser diode 1 and to protects the laser diode 1 against its deterioration and breakdown as well.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application in Industry] The present invention relates to a driving circuit for a semiconductor laser diode, and more particularly, to a circuit that generates stable optical output as a light source for an optical communication system.

[Conventional technology]

Conventionally, a method shown in FIG. 2 has been used as a drive circuit for a semiconductor laser diode. That is, the optical output of the laser diode 1 is detected by a monitor photodiode (hereinafter referred to as monitor PD), compared with the reference voltage and amplified by the error amplifier 3, negative feedback is applied so that the optical output is constant, and the bias current is A generating circuit 4 supplies a bias current. On the other hand, a modulation current generation circuit 5 supplies a modulation current according to the signal input. With this method, an optical output of constant power can be obtained depending on the signal input. Regarding this type of method, Japanese Patent Application Laid-Open No. 51-1-14. 7 9 8 There is No. 4.

[Problem to be solved by the invention]

In the above conventional technology, when there is a sudden disturbance such as turning on or turning off the power, an excessive bias current may flow until the negative feedback loop that controls the bias current is activated and stabilized. There is a risk of deterioration or destruction. The purpose of this invention is to provide a protection circuit in the negative feedback loop to prevent the generation of excessive current and protect the laser diode.

[Means to solve the problem]

In order to achieve the above purpose, the time constant of the negative feedback loop is made larger than the time constant of the rise of the power supply voltage so that the bias current does not flow excessively, and the bias current control voltage is also clamped to prevent it from exceeding the appropriate level. A protective circuit with functions has been installed.

[For production]

Among the circuits that generate bias current and modulation current to drive the laser diode, the bias current generation circuit that forms a negative feedback loop that keeps the optical output constant generates current according to a control voltage that increases and decreases depending on the optical output. do. In such a laser diode drive circuit, a protection circuit is provided at the control voltage output section, the time constant is set large to prevent the control voltage from rising suddenly when the power is turned on, and a clamp is installed to prevent the control voltage from becoming higher than necessary. do. This prevents excessive bias current from flowing and can prevent damage to the laser diode.

[Example] Hereinafter, a practical example of the present invention will be explained with reference to FIG. This embodiment includes a semiconductor laser diode 1, an error amplifier 2 that compares and amplifies the output of a monitor PD that detects the optical output of the laser diode with a reference voltage, and a protection circuit 3 that stabilizes the output of the amplifier. , a bias current generation circuit 4 that supplies a DC bias current to the laser diode 1 according to the protection circuit output 30, and a modulation current generation circuit 5 that supplies a modulation current according to the signal input. Among these, the laser diode, monitor PD, error amplifier 2,
The protection circuit 3 and the bias current generation circuit 4 form a negative feedback loop and operate so that the optical output of the laser diode is constant. In the circuit shown in this figure, since negative feedback loop control is not engaged when the power is turned on, the output voltage 20 of the error amplifier 2 may exceed an appropriate value. The protection circuit 3 protects the bias current control voltage 30 from exceeding an appropriate value even if the output voltage 20 rises rapidly, and prevents an excessive bias current from flowing through the laser diode.

Next, the protection circuit 3 in the first embodiment will be explained in more detail with reference to FIG. The output voltage 20 of the error amplifier 2, which is composed of a differential amplifier and an emitter follower, may rise rapidly due to initial conditions at power-on or transient phenomena at power-up.

In order to prevent this sudden rise, the time constant is increased using the resistor 31 and the capacitor 32, and the output voltage 3 is increased using the Zener diode 33.
Clamp 0. In this manner, the bias current control voltage 30 gradually converges and does not exceed an appropriate value even if there is a disturbance such as power-on due to the protection circuit 3 made up of a resistor, a capacitor, and a Zener diode.

Further, in a laser diode drive circuit that uses a negative feedback loop to control the modulation current amplitude to keep the optical output constant, the same effect can be obtained by providing a protection circuit in the modulation current generation section.

〔Effect of the invention〕

According to the present invention, it is possible to prevent excessive current from being supplied to the laser diode, which has the effect of preventing deterioration, destruction, etc. of the laser diode.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram showing an example of a conventional system, and FIG. 3 is a detailed circuit diagram for explaining FIG. 1. 1... Laser diode, 2... Error amplifier, 3...
- Protection circuit, 4... Bias current generation circuit, 5... Modulation current generation circuit.

Claims (1)

[Claims] 1. A detector that detects the optical output of the laser diode, an error amplification section that compares and amplifies the output of the detector with a reference voltage, and supplies a driving current to the laser diode according to the output of the error amplification section. 1. A laser diode drive circuit comprising a current supply section, wherein a protection circuit is provided at the output of the error amplification section.