JP2003101127A - Semiconductor laser driver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser driver which is capable of high-speed switching, by improving the rise and fall of optical output, even if the internal impedance of a semiconductor laser is large. SOLUTION: In the laser driver which lets a current flow or applies voltage from the anode side of the semiconductor laser, a resistor is provided in parallel with the semiconductor laser. Moreover, in this laser driver, a constant current circuit is provided in parallel with the semiconductor laser.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to driving a semiconductor laser.

[0002]

2. Description of the Related Art As shown in FIG. 5, a semiconductor laser generally has a pin photodiode PD for an optical monitor provided with a cathode of the semiconductor laser as a common. Some have not been done.
The output of the PD is connected to the light quantity monitor circuit 10 and is used for controlling the light quantity of the LD 1. As shown in FIG. 6, in order to drive the semiconductor laser LD1, generally, the base current Ib of the transistor TR1 is controlled,
A current (Id) greater than or equal to the threshold current (Ith) at which the semiconductor laser LD1 emits laser light is supplied.

Further, since the semiconductor laser has an abrupt increase in internal resistance in a region lower than the threshold current Ith, the collector-emitter impedance of TR1 is high in order to flow the current from 0 mA to Id to the semiconductor laser LD1. Therefore, in order to pass the drive current, it is necessary to have a considerably low impedance at the beginning, which makes high-speed switching difficult. Therefore, in order to turn on / off the laser emission at high speed, the current I at which the semiconductor laser LD1 starts emitting light
A current (bias current Ibias) slightly smaller than th is supplied in advance. By doing so, the internal resistance of the semiconductor laser is lowered, and high-speed switching of laser emission becomes possible.

[0004]

Normally, in a light emitting region, a semiconductor laser has an internal resistance of about 10Ω and a stray capacitance of about several tens of pF. Therefore, in the circuit shown in FIG. 5, the bias current Ibias described above is applied. By flowing, it was possible to drive for several ns of rising and falling. However, in recent years, the speed of communication using semiconductor lasers, laser beam printers, etc. has increased, and the number of semiconductor laser beams has been increased. For example, the light sources are arranged in a two-dimensional array of 3 × 3 (9). Surface emitting lasers have begun to be used. In the case of this surface emitting laser, the internal resistance is 100Ω to 1 even in the light emitting region.
KΩ and stray capacitance are several pF to several tens of pF. Therefore, when the drive as shown in FIG. 5 is performed, the impedance of the semiconductor laser LD1 is high. Therefore, it becomes difficult to drive the semiconductor laser LD1 with a rise and fall of several nanoseconds, which causes a problem that high-speed switching cannot be performed.

[0005]

To solve the above problems, a resistor or a constant current circuit is provided in parallel with the semiconductor laser so as to reduce the apparent impedance of the semiconductor laser when viewed from the driving side.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a driving circuit for a semiconductor laser according to the present invention. LD1 represents one semiconductor laser or one semiconductor laser in a semiconductor laser array composed of a plurality of semiconductor lasers. A PNP transistor TR1 for driving the semiconductor laser LD1
Is provided on the anode side of the semiconductor laser LD1. The transistor TR1 performs a constant current switching operation for controlling the collector current Ic with the base current Ib.

The cathode of the semiconductor laser LD1 is grounded to GND. A resistor R1 is connected in parallel between the anode and cathode terminals of the semiconductor laser LD1. Here, the collector current Ic supplies the current Ild for driving the semiconductor laser LD1 and the current Ir flowing through the resistor R1 connected in parallel with the semiconductor laser LD1. As for the Ild flowing through the semiconductor laser LD1, the bias current Ibias flows as shown in FIG. 6 when not emitting light, and the drive current Id flows when emitting light. That is, the collector current Ic flowing through the transistor TR1 is Ir + Ibias when light is not emitted.
Flows, and a current of Ir + Id flows when light is emitted. A current determined by the resistor R1 constantly flows through Ir.

When looking at the output from the transistor TR1, even if the internal resistance of the semiconductor laser LD1 is several hundred Ω, if the resistance R1 connected in parallel is set to the internal resistance of the semiconductor laser LD1 or less, at least the output impedance is The resistance value is 1/2 or less for only the semiconductor laser LD1. Therefore, the change in impedance for controlling the current between the collector and the emitter of the transistor TR1 can be suppressed small, and high-speed switching is possible.

FIG. 2 shows another embodiment according to the present invention. A current control circuit 1 is provided instead of the resistor R1 in FIG. Although the basic operation is the same as described above, a description thereof will be omitted. However, as compared with the case where the resistor R1 is provided in parallel with the semiconductor laser LD1 in FIG. 1, when the voltage between the semiconductor lasers LD1 changes, that is, the collector current When Ic changes,
Although the current Ir flowing through the resistor R1 changes, the current control circuit 1 makes it possible to control the current flowing through I1 so that it is always constant.

Therefore, the current Ild of the semiconductor laser LD1
However, since it directly depends on the change in the collector current Ic, the control becomes easy.

FIGS. 3 and 4 show an embodiment in which the current control by the transistor TR1 of FIGS. 1 and 2 is the voltage control by the buffer amplifier 2. The voltage applied to the semiconductor laser LD1 is determined by the voltage Vin input to the buffer amplifier 2. Semiconductor laser L
The light output of D1 is determined by the voltage-light quantity relationship of the semiconductor laser. That is, the current values of Ild, Ir, or I1 are, of course, similar to those in FIGS. 1 and 2 if the light amount is the same.

[0012]

According to the present invention, even if the internal impedance of the semiconductor laser is large, the rise and fall of the optical output is good, and high-speed switching laser driving is possible. As a result, for example, in a laser printer, higher speed printing and higher density are possible.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a semiconductor laser driving device according to the present invention.

FIG. 2 is a circuit diagram of a semiconductor laser driving device according to the present invention.

FIG. 3 is a circuit diagram of a semiconductor laser driving device according to the present invention.

FIG. 4 is a circuit diagram of a semiconductor laser driving device according to the present invention.

FIG. 5 is a circuit diagram showing a conventional example.

FIG. 6 is a characteristic diagram of a semiconductor laser.

[Explanation of symbols]

1 Current control circuit 2 buffer amplifier 10 Light intensity monitor circuit R1 resistance TR1 transistor LD1 semiconductor laser

Claims (2)

[Claims] 1. A semiconductor laser driving device in which a resistor is provided in parallel with a semiconductor laser in a laser driving device for supplying a current or applying a voltage from the anode side of the semiconductor laser. 2. A semiconductor laser driving device in which a constant current circuit is provided in parallel with the semiconductor laser in a laser driving device for supplying a current or applying a voltage from the anode side of the semiconductor laser.