JP2661373B2 - Laser diode 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 laser diode driving circuit, and more particularly to a laser diode driving 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 driving 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
The magnitude of the pulse current output from the laser drive circuit 2 is controlled by a signal input from the pulse current adjustment terminal 5.

FIG. 4 (FIGS. 4a to 4d) shows a conventional resistor shown in FIG. 3, in which a resistor for simulation is connected in place of the laser diode 1, and its terminal voltage, that is, a pulse current flowing through the resistor (FIG. 2 is an example of an electrical output waveform obtained by observing a logical signal. At both the high level and the low level of the logic signal represented by the pulse waveform, the terminal voltage of the resistor (proportional to the pulse current) is increased from FIG. 4A to FIG. 4D (vertical axis, horizontal axis in each figure). The axis is the same scale). As apparent from FIG. 4, the ringing is different depending on the magnitude of the pulse current (terminal voltage of the resistor). That is, in FIG. 4d, the pulse transfer to the inside of the low level is about 10% of the logic amplitude (point a), whereas in FIG. 4a, it reaches 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 driving circuit which outputs a pulse current having a small ringing and a good output waveform.

[0006]

SUMMARY OF THE INVENTION A laser diode driving circuit according to the present invention is provided with a diode driving circuit for outputting a pulse current signal to a laser diode for outputting pulsed optical power in response to a pulse current input. A pulse current control circuit that controls a current value of the pulse current by 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 by the pulse current adjustment signal. .

[0007]

Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing 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 for outputting a control signal for changing the band of an output signal of the diode drive circuit 2 in accordance with an input signal to a pulse current adjusting terminal 5.
Is provided.

Here, when the bandwidth of the output signal of the diode drive circuit 2 decreases, the rise time tr and the fall time tf of the output pulse current waveform increase,
Accordingly, ringing can 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 an input signal to the pulse current adjustment terminal 5 for adjusting the pulse current.

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

This circuit comprises an NPN transistor Q1 having a collector connected to the cathode of the laser diode 1 and a base connected to the pulse signal input terminal 6a, and a collector connected to a resistor RE.
An NPN transistor Q2 whose base is connected to the pulse signal input terminal 6b, whose emitter is connected to the emitter of the NPN transistor Q1, and whose collector is connected to the emitter of the NPN transistor Q1 and whose emitter is connected to the negative power supply 7 via the resistor RE4. NPN transistor Q3, and an NP whose collector and base are connected to the base of NPN transistor Q3 and whose emitter is connected to negative power supply 7 via resistor RE5.
An N transistor Q4, an NPN transistor Q5 having a collector grounded via a resistor RE1 and a base connected to the collector of the NPN transistor Q1, and a collector grounded via a resistor 2 and having a base connected to the collector of the NPN transistor Q2 and having an emitter NPN An NPN transistor Q6 connected to the emitter of the transistor Q5, an NPN transistor Q7 having a base grounded via the resistor RE9, a collector connected to the emitter of the NPN transistor Q5 and an emitter connected to the negative power supply 7 via the resistor RE6, and a base; An NPN transistor Q8 having a collector connected to the ground via a resistor RE7, a collector connected to the base of the NPN transistor Q7, and an emitter connected to the negative power supply 7 via 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.

The base of the NPN transistor Q8 is a pulse current adjusting terminal 5. Then, 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.
The anode of the laser diode 1 is grounded.

Here, the differential circuit constituted by the NPN transistors Q1, Q2 and Q3 constitutes the 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 a current flowing through the laser diode 1 by changing the resistance R7 of the resistor RE7, that is, N
The magnitude of the current I1 flowing through the emitter of the PN transistor Q3 is adjusted.

Also, NPN transistors Q5, Q6 and Q
7 constitute the band control circuit 4. That is, the signal band of the pulse current output to the laser diode 1 is governed by the capacitance value connected to the collectors of the NPN transistors Q1 and Q2. This capacitance value is determined by the current I3 flowing through the emitter of the NPN transistor Q7. Change. That is, the NPN transistor Q
1 is connected to the capacitor C
This is obtained by multiplying the capacitance value C1 of A1 by the gain of the NPN transistor Q5. NPN transistor Q8, resistor RE9
And a circuit configured by RE10, the voltage of the pulse voltage adjustment terminal 4 is used as an input signal, and the voltage Vc of the resistor RE9 generated thereby causes the NPN transistor Q7
Is controlled. That is, the band control circuit 4
And a circuit that takes 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 obtained. Since the capacitance value CT is obtained by multiplying the capacitance C1 of the capacitor CA1 by the gain of the NPN transistor Q5, the expression (1) is established.

CT ≒ C1 × R1 / (kT / qI3) (1) where, R1: resistance value of resistance RE1 (hereinafter, Rx is resistance value of resistance REx) k: Boltzmann constant q: unit charge of electrons NPN Assuming that the potential at the base of the transistor Q8 is Vc, the equations (2) and (3) hold.

Vc = R7 × [(VEE + VBE4) / (R5 + R7 + R8)] (2) where, VBE4: base-emitter potential of NPN transistor Q4 (based on the emitter side) VEE: power supply voltage I3 = [R9 × (−Vc + VBE8 + VEE) / R10−VEE] / R6 (3) The capacitance value CT can be calculated by the above equations (1) to (3).

Further, when the pulse current Ip flowing through the laser diode 1 is calculated, the following equation (4) is obtained.

Ip = R5 × [(VEE + VBE4) / (R5 + R7 + R8)] / R6 (4) (1) to (4) As is clear from the equations (1) to (4), when the resistance R7 of the resistor RE7 is increased, the laser diode 1 The flowing current Ip decreases, and the output optical power decreases. on the other hand,
Since the current I3 increases, the capacitance value CT increases, which is added in parallel to the laser diode 1, so that the signal band of the current Ip, which is the output signal of the NPN transistor Q1, decreases. That is, when the current Ip flowing through the laser diode 1 decreases due to the adjustment of the resistor RE7, the output signal band decreases at the same time. Conversely, when the current Ip increases, the output signal band increases. Thus, the output signal band can be automatically optimized according to the output pulse current value.

[0020]

As described above, according to the present invention, the bandwidth of the output signal of the laser diode drive circuit is automatically and optimally adjusted according to the pulse current value while adjusting the magnitude of the drive pulse current of the laser diode. Therefore, there is an effect that it is possible to provide a laser diode driving circuit that outputs a current having a good output waveform with the influence of ringing minimized.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of a laser diode driving circuit according to the present invention.
FIG. 4 is a block diagram of an embodiment of FIG.

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 conventional pulse output signal.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 laser diode 2 diode drive circuit 3 pulse current control circuit 4 band control circuit 5 pulse current adjustment terminal 6, 6a, 6b pulse signal input terminal 7 negative power supply CA1, CA2 capacitor Q1 to Q8 NPN transistor RE1 to RE10 resistor

Claims (2)

(57) [Claims] 1. A diode drive circuit that outputs a pulse current signal to a laser diode that outputs pulsed optical power by inputting a pulse current, and a current value of the pulse current is controlled by an input pulse current adjustment signal. A laser diode driving circuit, comprising: a pulse current control circuit for controlling the pulse current adjustment signal; and a band control circuit for controlling an output signal band of the diode drive circuit according to the pulse current value according to the pulse current adjustment signal. 2. The diode driving circuit according to claim 1, wherein said diode driving circuit comprises a first and a second NPN transistor for inputting a logic signal pulse to each base, and a collector of said first NPN transistor is connected to a cathode of said laser diode by said second. A differential circuit in which a collector of the NPN transistor is grounded via a first resistor, and a third NPN transistor whose collector is connected to the emitters of the two NPN transistors and the emitters are connected to a negative power supply; The current control circuit inputs a control signal generated by the input of the pulse current adjustment signal to a base of the third NPN transistor to control an emitter current of the third NPN transistor. And a base connected to the collector of the first NPN transistor, and a collector and a base. The first capacitor is connected to the fourth NPN transistor and a collector is between the fourth base is grounded through a resistor connected to the collector of the second NPN transistor and the collector and the base second
And a sixth NPN transistor having a collector connected to the emitters of the fourth and fifth NPN transistors and having an emitter connected to the negative power supply via a second resistor. A transistor and the pulse current adjustment signal, and the sixth NPN transistor is connected to the sixth NPN transistor in a direction opposite to the increase or decrease of the emitter current.
2. The laser diode driving circuit according to claim 1, further comprising: an interface circuit for outputting a signal for controlling an emitter current of said NPN transistor to a base of said sixth NPN transistor.