JPS59117336A - Driving circuit of laser diode - Google Patents

Abstract

PURPOSE:To obtain a constant optical output independently of temperature in driving a VSD LD (V-grooved substrate varied heterostructure laser diode) or the like by controlling a bias current and an input pulse amplitude inputted to a laser diode both in response to temperature. CONSTITUTION:A temperature is converted into a current value by a thermistor 16, a bias circuit 18 gives a proper bias current Ib is given to a laser diode 14 of the VSB LD or the like in response to this current value, a photodiode 20 receives a back light of the diode 14 and converts the light into a current and the result is given to a peak detecting circuit 22. The detecting circuit 22 compares a current value from the diode 20 with a reference value REF, and the compared value is inputted to a current variable amplifier 12 via a pulse current control circuit 24. Then, since the amplifier 12 gives a pulse input current of a proper amplitude, a constant optical output is obtained independently of temperature fluctuation .

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to a laser diode drive circuit for driving a laser diode whose threshold current and differential quantum efficiency both change with temperature.

(2) Background of the Invention Although SAS LD (self-aligned structure laser diode) has traditionally been known as a laser diode, VSB LD (V-groove substrate) has recently been attracting attention as an alternative to this. It is a peridot heterostructured laser diode).

(Refer to Electronics Letters, June 25, 1981, Vol. 17Al 3 PP465-467) Compared to conventional radar diodes such as SAS LDs, this VSB LD has a low value conversion current, so
It is suitable for reducing the power consumption of passive circuits, has a good hourglass-shaped response, has good high-speed modulation characteristics, and is suitable as a light source for optical transmission in the 1.3 ttnr band. However, in this VSB LD, not only the threshold flow but also the micromolecular 6-molecule efficiency changes depending on the temperature as described below. Thus, there is a need for a laser diode drive circuit in which both the threshold current and the differential quantum efficiency change with temperature.

(3) Prior Art and Problems Conventional laser diode drive circuits drive laser diodes such as SAS LDs in which only the threshold current changes with temperature.

FIG. 1 shows the current (r)-light output (L) characteristics of a conventional SAS LD. In the same figure, the curve (-+(Tt)
indicates the IL characteristic at temperature Tl, and C1(I
2) shows the IL characteristic at a temperature T2 higher than TI. The threshold current for obtaining a constant optical output LO is 1 at temperature T1, and 1 at temperature T2.
I2 is greater than 1. In this way, S.A.S.L.
In the case of D, the current at the threshold ' changes with the temperature change, but the ratio of the light output change to the current change, i.e.
Differential quantum efficiency remains almost constant with respect to temperature changes. Therefore, in order to obtain the optical output LO, when the temperature is T, the bias current is set to Ib1 and the input data I) is input to the input of the drive circuit.
If the temperature is T2, a bias current Ib2 larger than Ib1 is applied to input data DI with the same amplitude.
Just enter N. Note that the bias current of the 5AsLD is approximately 100 mA at 25°C.

FIG. 2 is a block diagram showing an example of a conventional laser diode drive circuit. In the same figure, flip-flop 1
Input data DIN and clock signal CL are input to 1,
Is the output °IA' or °'0" signal 1"? The current is applied to the current amplification circuit 2 and the first power of the comparison circuit 3. The output of the J-flow amplifier circuit 2 is input to the SAS LD 4. S
The backlight of the AS LD 4 is input to the photodiode 5, and its output is input to the second input of the comparator 3. Comparator 3 receives 7.0 from flip-flop 1. J, <current 11+ and current from photodiode 5 ■, are compared, and the bias circuit 6 is 'dill til L' according to the difference, thereby increasing bias current Ib when I, >T. , T, < I, the bias current b is reduced [7] in order to obtain a constant optical output LO that is not affected by temperature changes.

However, with the conventional circuit shown in FIG. 2, it is not possible to obtain a constant optical output for the laser diode having the more IL characteristics shown in FIG.

FIG. 3 is a graph showing the IL characteristics of the VSB LD. As shown in the figure, the differential quantum efficiency of the VSB LD, that is, the rate of change of optical output with respect to current, decreases as the temperature increases. Therefore, the slope of curve C2 (I2) is smaller than the slope of curve C2 (Tt).

Therefore, if only the bias current is changed using the conventional circuit shown in FIG. 2, a light output with a constant amplitude cannot be obtained. In the figure, the temperature T
The amplitude of the optical output Lot at , is the bias current Ib2
is larger than the amplitude of the optical output LO at temperature T2.

(4) Purpose of the Invention In view of the problems in the prior art described above, the purpose of the present invention is to drive a laser diode, such as a VSB LD, in which not only the threshold current but also the differential quantum efficiency changes depending on the temperature, so as to be independent of the temperature. An object of the present invention is to provide a laser diode drive circuit that can obtain a constant amplitude optical output.

(5) Structure of the Invention The gist of the present invention to achieve the above-mentioned object is to provide a laser diode drive circuit that drives a laser diode so that the optical output amplitude is always constant. An amplifier, a photodetection element that converts the optical output of the laser diode into an electrical signal, a current discount means for controlling the amplification factor of the variable amplifier according to the difference between the output current of the photodetection element and a reference current, and temperature detection. and bias current control means for controlling the bias current of the laser diode according to the output of the temperature detection means.

(6) Embodiments of the Invention Hereinafter, embodiments of the present invention will be explained with reference to FIGS. 4 and 5.

FIG. 4 is a block diagram showing a laser diode drive circuit according to an embodiment of the present invention, and FIG. 5 is a waveform diagram for explaining the operation of the circuit of FIG. 4. In FIG. 4, input data D4 and a clock signal CL are input to the flip-flop 10,
Its output is given to a variable current amplifier 12. Control of the bias current (1b) with respect to temperature changes is performed by a temperature detection means such as a thermistor 16 and a bias circuit 18. The amplitude of the optical output with respect to temperature changes is controlled by a photodiode 20, a peak detector 22, and a current control circuit 24. That is, it is converted into a current value by the thermistor 16, and according to this current value, the bias circuit 18 gives an appropriate bias current (i) to the laser diode 14 such as a VSB LD, and also applies an appropriate bias current (i) to the photodiode 20. receives the backlight from the laser diode 14, converts it into a current, and supplies it to the peak detection circuit 22. The peak detection circuit 22 compares the current value from the photodiode 20 with the reference value REF" and provides the comparison value to the pulse current control circuit 24. The pulse current control circuit 24 controls the current value according to the comparison value. It is input to the control terminal of the variable amplifier 12.The variable current amplifier 12 applies a pulse input current of an appropriate amplitude to the laser diode 1 according to this control current.
Give to 4. In this way, as shown in FIG. 5, when the temperature is T1, the bias current, current b, output from the bias circuit 18 is Ibl, and the input current output from the variable amplifier 12 is Ibl. The amplitude is AM, but as the temperature increases to T2, the bias current becomes T52, and the amplitude of the input current increases with AM2. Bias circuit 18, pulse current limit 1 circuit 24
, and the variable current amplifier 12 includes a laser diode 14
Light output from A? It is designed so that the amplitude of the pulse is always constant.

The bias current of VSBLD is approximately 20 m at 25°C.
A is smaller than that of the SAS LD, and therefore, as described above, the power consumption of one driving circuit can be reduced.

(7) As described in detail, by controlling both the bias current and the input pulse amplitude input to the laser diode according to the temperature according to the present invention, the threshold current can be adjusted according to the temperature as in 9. VSBLD. In addition, a laser diode drive circuit is obtained which can drive a laser diode 1' whose differential efficiency changes and obtain a constant optical output regardless of temperature.

[Brief explanation of the drawing]

Figure 1 shows the current-light output characteristics of a conventional SAS LD box; Figure 2 is a block diagram showing an example of a conventional laser diode drive circuit; Figure 3 shows the current-light output characteristics of a VSBLD. A graph that explains the problems of conventional circuits,
FIG. 4 is a block diagram showing a laser diode drive circuit according to an embodiment of the present invention, and FIG. 5 is a waveform diagram for explaining the operation of the circuit of FIG. 4. DESCRIPTION OF SYMBOLS 10... Flip 70 tripod, 12... Variable current amplifier, 14... Laser diode, 16... Thermistor, 18... Bias circuit, 20... Photodiode, 22... Peak detection Circuit, 24... Pulse current control circuit. Patent applicant Fujitsu Limited Patent agent Akira Aoki Patent attorney Kazuyuki Nishidate 1) Yukio Patent attorney Akira Yamaguchi Figure 1 Figure 2 b J b Figure 3 Figure 4 H Baboon Figure 5 192-

Claims (1)

[Claims] 1. The laser diode is controlled so that the optical output voltage is always constant. In the laser diode 9 drive circuit, a variable amplifier provides a shaft current to the laser diode, and a variable amplifier that controls the optical output of the laser diode. a photodetection element for converting into an optical signal, a current control means for controlling the amplification factor of the variable amplifier according to the difference between the output current of the photodetection element and a reference current, a temperature detection means, and an output of the temperature detection means. A radar diode drive circuit comprising bias current control means for controlling the bias current of the radar diode accordingly.