JPH04239188A - Laser diode driving circuit - Google Patents

Abstract

PURPOSE:To provide a laser diode driving circuit which outputs a pulse current of good output waveform and of little ringing. CONSTITUTION:A diode driving circuit which outputs a signal of a pulse current to a laser diode which outputs pulse-like photoelectric power by input of pulse current, a pulse current control circuit which controls a current value of a driving pulse current by input pulse current adjusting signal, and a band control circuit which controls an output signal band of a diode driving circuit in accordance with a driving pulse current value through pulse current adjusting signal are provided.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode drive circuit, and more particularly to a laser diode drive circuit that requires high-speed operation.

0002

2. Description of the Related Art A conventional technique will be described with reference to a block diagram of a conventional laser diode drive circuit shown in FIG.

The laser diode 1 has an anode grounded and a cathode connected to a signal output terminal of a diode drive circuit 2. Then, it is driven by the pulse current output by the diode drive circuit 2, and outputs pulsed optical power according to the logic signal input to the pulse signal input terminal 6 of the diode drive circuit 2. The pulse current control circuit 3 is
The magnitude of the pulse current output by the laser drive circuit 2 is controlled by a signal input from the pulse current adjustment terminal 5.

FIG. 4 (FIG. 4a to FIG. 4d) shows that in the conventional example shown in FIG. 3, a resistor for simulation is connected in place of the laser diode 1, and its terminal voltage, that is, the pulse current flowing through the resistor ( This is an example of an electrical output waveform observed using a logic signal. The terminal voltage of the resistor (proportional to the pulse current) for both the high level and low level of the logic signal shown by the pulse waveform increases as we move from Figure 4a to Figure 4d (in each figure, the vertical axis and horizontal axis (same scale for both axes). As is clear from FIG. 4, the appearance of ringing differs depending on the magnitude of the pulse current (terminal voltage of the resistor). That is, in FIG. 4d, the inward pulse transfer of the low level is about 10% of the logic amplitude (point a), whereas in FIG. 4a, it is nearly 50% (point b). Therefore, when the pulse current is small, it is necessary to reduce the ringing.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a laser diode drive circuit that outputs a pulse current with less ringing and a good output waveform.

[0006]

[Means for Solving the Problems] A laser diode drive circuit of the present invention includes a diode drive circuit that outputs a pulsed current signal to a laser diode that outputs pulsed optical power in response to an input pulsed current; A pulse current control circuit that controls the current value of the pulse current using a pulse current adjustment signal, and a band control circuit that controls an output signal band of the diode drive circuit according to the pulse current value using the pulse current adjustment signal. .

[0007]

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

FIG. 1 is a block diagram of one embodiment of a laser diode drive circuit according to the present invention. This example is
In addition to the conventional circuit shown in FIG. 1, a band control circuit 4 outputs a control signal that changes the band of the output signal of the diode drive circuit 2 based on an input signal to the pulse current adjustment terminal 5.
is provided.

Here, when the bandwidth of the output signal of the diode drive circuit 2 decreases, the rise time tr and fall time tf of the output pulse current waveform increase,
Accordingly, ringing can also be reduced. That is, when the pulse current is small, the band of the output signal of the laser diode drive circuit 2 is reduced to reduce pulse ringing. This band control operation is performed by the band control circuit 4 in synchronization with the input signal to the pulse current adjustment terminal 5 that adjusts the pulse current.

FIG. 2 is a circuit diagram showing details of the laser diode drive circuit shown in FIG. 1.

This circuit consists of an NPN transistor Q1 whose collector is connected to the cathode of the laser diode 1 and whose base is connected to the pulse signal input terminal 6a, and whose collector is connected to the resistor RE.
3, the base is connected to the pulse signal input terminal 6b, the emitter is connected to the emitter of the NPN transistor Q1, and the collector is connected to the emitter of the NPN transistor Q1, and the emitter is connected to the negative power supply 7 through the resistor RE4. an NP transistor Q3 whose collector and base are connected to the base of the NPN transistor Q3 and whose emitter is connected to the negative power supply 7 via a resistor RE5.
an NPN transistor Q4, an NPN transistor Q5 whose collector is grounded via a resistor RE1 and whose base is connected to the collector of an NPN transistor Q1, and an NPN transistor Q5 whose collector is grounded via a resistor 2 and whose base is connected to the collector of an NPN transistor Q2. an NPN transistor Q6 connected to the emitter of the transistor Q5, an NPN transistor Q7 whose base is grounded via a resistor RE9, whose collector is connected to the emitter of the NPN transistor Q5, and whose emitter is connected to the negative power supply 7 via a resistor RE6; is grounded through a resistor RE7, and whose collector is connected to the base of the NPN transistor Q7 and whose emitter is connected to the negative power supply 7 through a resistor RE10,
Collector of transistor Q4 and NPN transistor Q8
, a capacitor CA1 connected between the collector and base of the NPN transistor Q5, and a capacitor CA2 connected between the collector and base of the NPN transistor Q6.

Note that the base of the NPN transistor Q8 serves as the pulse current adjustment terminal 5. The terminal voltage Vc of the resistor RE7 obtained by changing the resistance value R7 of the resistor RE7, which is a variable resistor, is used as a control input signal for controlling the pulse current value and the band of the output signal. Further, the anode of the laser diode 1 is grounded.

Here, a differential circuit constituted by NPN transistors Q1, Q2, and Q3 constitutes a diode drive circuit 2. A current mirror circuit composed of an NPN transistor Q4 and an NPN transistor Q3 constitutes a pulse current control circuit 3, and by changing the resistance value R7 of the resistor RE7, the current flowing through the laser diode 1, that is, N
The magnitude of the current I1 flowing through the emitter of the PN transistor Q3 is adjusted.

[0014] Also, NPN transistors Q5, Q6 and Q
7 constitutes the band control circuit 4. That is, the signal band of the pulse current output to the laser diode 1 is controlled by the capacitance value connected to the collectors of the NPN transistors Q1 and Q2, but this capacitance value is controlled by the current I3 flowing through the emitter of the NPN transistor Q7. Change. That is, NPN transistor Q
The capacitance value CT connected to the collector of 1 is the capacitor C
It is obtained by multiplying the capacitance value C1 of A1 by the gain of the NPN transistor Q5. NPN transistor Q8, resistor RE9
The circuit constituted by RE10 and RE10 uses the voltage at the pulse voltage adjustment terminal 4 as an input signal, and the resulting voltage Vc at the resistor RE9 causes the NPN transistor Q7 to
The emitter current I3 is controlled. That is, the band control circuit 4
This is a circuit that serves as an interface between the band control circuit 4 and the pulse voltage adjustment terminal 5.

In FIG. 2, the capacitance value CT connected to the collector of the NPN transistor Q1 will be determined. Since the capacitance value CT is obtained by multiplying the capacitance C1 of the capacitor CA1 by the gain of the NPN transistor Q5, equation (1) holds true.

CT≒C1×R1/(kT/qI3)...(1) Here,
R1: Resistance value of resistor RE1 (hereinafter, Rx is the resistance value of resistor REx) k: Boltzmann constant q: unit charge of electrons If the potential of the base of NPN transistor Q8 is Vc, then formulas (2) and (3) hold. do.

Vc=R7×[(VEE+VBE4)/(R5+R
7+R8)]...(2) Here, VBE4: Base-emitter potential of NPN transistor Q4 (emitter side is referenced) VEE: Power supply voltage I3=[R9×(-Vc+VBE8+VEE)/R
10-VEE]/R6...(3) From the above formulas (1) to (3), the capacitance value CT
It is possible to calculate

Further, when calculating the pulse current Ip flowing through the laser diode 1, it becomes as shown in equation (4).

Ip=R5×[(VEE+VBE4)/(R5+R
7+R8)]/R6...(4) (1) ~ (4) As is clear from the formula, the resistance R
When the resistance value R7 of E7 is increased, the laser diode 1
The current Ip flowing in decreases, and the output optical power decreases. On the other hand, since the current I3 increases, the capacitance value CT increases, and since this is applied in parallel to the laser diode 1, the NP
The signal band of current Ip, which is the output signal of N transistor Q1, decreases. That is, when the current Ip flowing through the laser diode 1 is reduced by adjusting the resistor RE7, the output signal band is simultaneously reduced. Conversely, when the current Ip increases, the output signal band increases. In this way, the output signal band can be automatically optimized according to the output pulse current value.

[0020]

As explained above, according to the present invention, at the same time as adjusting the magnitude of the laser diode drive pulse current, the band of the output signal of the laser diode drive circuit is automatically optimized according to the pulse current value. Because it is possible to
This has the advantage of being able to provide a laser diode drive circuit that outputs a current with a good output waveform and minimizes the influence of ringing.

[Brief explanation of the drawing]

FIG. 1: A first laser diode drive circuit according to the present invention.
FIG. 2 is a block diagram of an embodiment of the invention.

FIG. 2 is a detailed circuit diagram of the first embodiment.

FIG. 3 is a block diagram of a conventional example.

FIG. 4 is a waveform diagram of a pulse output signal in a conventional example.

[Explanation of symbols]

1 Laser diode 2 Diode drive circuit 3 Pulse current control circuit 4 Bandwidth control circuit 5 Pulse current adjustment terminals 6, 6a, 6b Pulse signal input terminal 7
Negative power supply CA1, CA2 Capacitor Q1~Q8 NPN transistor RE1~RE1
0 resistance

Claims (2)

[Claims] 1. A diode drive circuit that outputs a pulsed current signal to a laser diode that outputs pulsed optical power by inputting a pulsed current, and controlling the current value of the pulsed current by an inputted pulsed current adjustment signal. and a band control circuit that controls an output signal band of the diode drive circuit according to the pulse current value using the pulse current adjustment signal. 2. The diode drive circuit includes first and second NPN transistors inputting logic signal pulses to their respective bases, and the collector of the first NPN transistor is connected to the cathode of the laser diode. a differential circuit in which the collector of the NPN transistor is grounded via a first resistor, and a third NPN transistor in which the collector is connected to the emitters of the two NPN transistors and the emitter is connected to a negative power supply; The current control circuit inputs a control signal generated by inputting the pulse current adjustment signal to the base of the third NPN transistor to control its emitter current, and the band control circuit has a collector connected to a second resistor. a fourth NPN transistor, the base of which is grounded through the transistor, the base of which is connected to the collector of the first NPN transistor, and the first capacitor is connected between the collector and the base; and the collector is grounded through a fourth resistor. The base of the second NPN transistor is connected to the collector of the second NPN transistor, and the second transistor is connected between the collector and the base.
a fifth NPN transistor, the collector of which is connected to the emitters of the fourth and fifth NPN transistors, and the emitter of which is connected to the negative power supply via a second resistor; a transistor, and outputs a signal to the base of the sixth NPN transistor by inputting the pulse current adjustment signal and controlling the emitter current of the sixth NPN transistor in a direction opposite to the increase/decrease in the emitter current of the third NPN transistor. 2. The laser diode drive circuit according to claim 1, further comprising an interface circuit that provides a laser diode drive circuit.