JP2682419B2 - Semiconductor laser drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive circuit, and more particularly to a semiconductor laser drive circuit which simplifies a drive circuit for a bias current flowing through a semiconductor laser and can reduce the circuit scale and the circuit current.

[0002]

2. Description of the Related Art A conventional semiconductor laser drive circuit is composed of a circuit as shown in FIG. That is, the first transistor 8 having the semiconductor laser 14 as a collector load, the second transistor 9 forming a differential pair with the first transistor 8, the transistor 10 serving as a current source of the differential pair, and the current source. Drive current control circuit 13 for controlling the transistor 10 that becomes a bias current, a bias supply transistor 11 for supplying a bias current to the semiconductor laser 7, and a control circuit 12 for controlling the bias supply transistor 11.
It is composed of

In this conventional semiconductor laser drive circuit, the semiconductor laser 7 used has a constant optical output because the oscillation threshold current value Ith and the slope efficiency dQE change greatly as the temperature changes, as shown in FIG. Therefore, it is necessary to control the drive current Iop and the bias current Ib of the semiconductor laser 7. For this reason, conventionally, a drive current control circuit and a bias current control circuit that detect and operate a change in optical output are added.

On the other hand, in recent years, the characteristics of semiconductor lasers have been improved in order to enable application to optical subscriber systems which require operation in a bad temperature environment.
As for the umWell laser, as shown in FIG. 6, one having a very small change in the threshold current value Ith has been developed.

[0005]

However, when driving a semiconductor laser in which the change in the threshold current Ith with respect to the temperature is small, the conventional semiconductor laser driving circuit has a complicated control circuit, and the circuit scale is reduced and consumed. There is a problem that it is not suitable for application to optical subscriber systems that require power reduction.

An object of the present invention is to provide a semiconductor laser driving circuit which solves the above-mentioned problems.

[0007]

In order to achieve the above object, a semiconductor laser drive circuit of the present invention has a first transistor having a semiconductor laser as a collector load, and a differential pair configuration with the first transistor. A second transistor,
In a semiconductor laser drive circuit including a third transistor that serves as a current source of the differential pair and a drive current control circuit that controls the third transistor, between a collector of the first transistor and a power supply. The capacitance and resistance are loaded in series.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a semiconductor laser driving circuit of the present invention, FIG. 2 is a diagram showing a current flowing through a semiconductor laser in the semiconductor laser driving circuit of the present invention, and FIG. 3 is a semiconductor laser driving circuit of the present invention. FIG. 4 is a diagram showing the current flowing through the first transistor, and FIG. 4 is a diagram showing the current flowing through the capacitor and the resistor in the semiconductor laser drive circuit of the present invention.

The semiconductor laser 4 is connected as a collector load of the transistor 1. The transistor 2 forms a differential pair with the transistor 1, and the transistor 3 is connected as a current source of this differential pair. The transistor 3 controls the drive current Iop of the semiconductor laser 4, and a current control circuit is connected to the base of the transistor 3. The current control circuit 4 sets the optical output of the semiconductor laser 4.

The bias current Ib of the semiconductor laser 4 flows through the capacitor 5 and the resistor 6 connected to the collector of the transistor 1. The current value is determined by the potential difference ΔV of the collector of the transistor 1 when the semiconductor laser 4 emits light or not and the resistor 6. Assuming that the collector potential when the semiconductor laser 4 emits light is V (t0) and the collector potential when the semiconductor laser 4 does not emit light is V (t0), the current Ib flowing through the capacitor 5 and the resistor 6 is

[0012]

## EQU1 ## Bias current Ib flows for C
Determined by the R time constant.

Here, V (t0) is VF when the semiconductor laser 4 emits light, and V (t1) is VCC, so ΔV
Is equal to VF when the semiconductor laser 4 emits light. Therefore, if the drive current is increased to correct the optical output deterioration due to the temperature rise, VF increases by the current increase and ΔV increases. Therefore, the bias current Ib changes following the increase / decrease of the drive current.

[0015]

As described above, in the semiconductor laser drive circuit of the present invention, the bias current Ib of the semiconductor laser 4 is made to flow by the capacitor 5 and the resistor 6 added to the collector of the transistor 1, which is conventionally required. This eliminates the need for a bias current transistor and a bias current control circuit, and has the effect of simplifying the circuit and reducing power consumption. Further, since the bias current value can be controlled only by controlling the drive current, the bias current can be increased or decreased according to the drive current without individually controlling the bias current.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a semiconductor laser drive circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a current flowing through a semiconductor laser in a semiconductor laser drive circuit of the present invention.

FIG. 3 is a diagram showing a current flowing through a first transistor in the semiconductor laser drive circuit of the present invention.

FIG. 4 is a diagram showing a current flowing through a capacitor and a resistor in the semiconductor laser driving circuit of the present invention.

FIG. 5 is a photo-current characteristic of a conventional semiconductor laser.

FIG. 6 is a photo-current characteristic of a recent semiconductor laser.

FIG. 7 is a diagram showing a relationship between a drive current and a bias current according to the present invention.

FIG. 8 is a circuit diagram of a conventional semiconductor laser drive circuit.

[Explanation of symbols]

 1 First Transistor 2 Second Transistor 3 Third Transistor 4 Semiconductor Laser 5 Capacitance 6 Resistor 7 Drive Current Controller 8 First Transistor 9 Second Transistor 10 Third Transistor 11 Fourth Transistor 12 Bias Current Control unit 13 Drive current control unit 14 Semiconductor laser

Claims (1)

(57) [Claims] 1. A first transistor having a semiconductor laser as a collector load, a second transistor having a differential pair configuration with the first transistor, and a third transistor serving as a current source of the differential pair. And a drive current control circuit for controlling the third transistor, wherein a capacitor and a resistor are connected in series between the collector of the first transistor and a power supply. Semiconductor laser drive circuit.