JPS63110685A - Drive circuit of light emitting element - Google Patents

Abstract

PURPOSE:To deep an optical output constant, even if its uniform deterioration develops by preparing a low pass filter, a switching element for driving a light emitting element as well as a switching element for compensating the optical output where ON-OFF controls are simultaneously carried out and an output terminal is connected to the other end of the low pass filter. CONSTITUTION:The and of a CR low pass filter is connected to a series circuit that is composed of a semiconductor laser LD, a transistor Q1 as well as a resistance R1 and a transistor Q3 for compensating an optical output is connected to the other end of the low pass filter. When an H-signal from a non- inversion output terminal of a comparator IC continues for a long time, an electrical potential VE obtained through the CR low pass filter becomes lower adequately. Then, a signal amplitude current supplied to the semiconductor laser LD increases as the time proceeds. Subsequently, when time passes long enough to show the uniform deterioration of the optical output of the semiconductor laser LD, as a necessary consequence considerable amounts of electric currents are supplied to the semiconductor laser LD and its laser LD keeps the optical outputs constant, regardless of its uniform deterioration.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a drive circuit for a light emitting element such as a semiconductor radar used as a light source for optical communication, optical information processing equipment, etc.

(Prior Art) Semiconductor lasers have established an extremely important position as key devices in optical information processing systems such as optical communications, pickups for optical disk devices, and light sources for laser printers. When used as a light source, it is necessary to keep the laser light output constant and modulate it at high speed.With semiconductor lasers, the light output can be easily modulated by modulating the drive current, which is a major feature that other lasers do not have. .

When directly pulse-driving a semiconductor laser, a method is generally adopted in which the DC bias current is set near the trigger threshold current and a signal pulse current is superimposed on this in order to suppress pattern effects caused by light emission delays. has been done. FIG. 2 is a circuit diagram showing an example of the configuration of such a drive circuit.

The configuration of the conventional circuit will be explained with reference to FIG. 2. Two transistors Q1. Q2 constitutes a differential circuit for switching a semiconductor laser (laser diode) LD, and a pulse current with an inverted phase is applied to each base from a comparator IC. The comparator IC is for shaping the waveform of the differential input and outputting it, and its non-inverting output terminal is connected to the automatic output adjustment circuit (APC>10.APClo adjusts the optical output fluctuation of the semiconductor laser LD) This adjustment is for transistor Q1
This is done by controlling the bias voltage between the base and collector of. Also, the transistor Q1. A resistor R1 commonly connected to the emitters of Q2 is connected to the negative power supply terminal.

Next, the operation of the conventional circuit shown in FIG. 2 will be explained.

When the non-inverted output of the comparator IC becomes H (high level) and is applied to the base of the transistor Q1,
Transistor Q1 turns on and the semiconductor laser LD
The current flowing through the light increases and the light becomes emissive. At this time, the comparator IC applied to the base of transistor Q2
Since the inverted output of Q2 is at L (low level), the transistor Q2 is kept at a high resistance and no current flows.

On the other hand, when the non-inverted output of the comparator IC goes from H to L and the inverted output goes from L to H, the transistor Q
1 changes from the on state to the off state, and the transistor Q2 changes from the off state to the on state.

Therefore, the current flows through the transistor Q2, so that the semiconductor laser LD is extinguished.

In this way, the output (control signal) from the comparator IC
Since the semiconductor laser LD can be made to emit and extinguish light depending on the situation, it can be used in optical communication systems and the like. However, the optical output of a semiconductor laser LD is not always constant with respect to temperature; for example, even if an optical output of 1 mW is obtained under conditions of 25°C, it will be much lower than this under conditions of 0°C. Only optical output can be obtained, and on the contrary, 50
Under conditions of .degree. C., a light output of significantly higher than 1 mW can be obtained. As described above, if the optical output of the semiconductor laser fluctuates due to changes in the ambient temperature, this will cause problems in the configuration of optical communication systems and the like.

Therefore, an APC circuit 10 as shown in FIG. 2 has been conventionally provided. This APC circuit 10 includes a semiconductor laser L
By detecting the optical output of D with a sensor (not shown) and changing the bias voltage between the base and collector of the transistor Q1 accordingly, the optical output is kept constant even if the ambient temperature changes. That is, by feeding back the optical output of the semiconductor laser LD to the bias of the transistor Q1, a constant optical output is realized under different temperature conditions.

(Problem to be Solved by the Invention) However, in such a conventional circuit, since the signal swing current I is constant,
If the signal to is kept in the T1 state for a long period of time, so-called uniform deterioration may occur in the semiconductor laser LD. This occurs regardless of the above-mentioned changes in the ambient temperature, and regardless of the type of DLD (dark line) or end face deterioration, and is caused by an increase in the temperature of the common laser due to long-term light emission from the semiconductor laser LD. This results in a -like reduction in light output.

The state of reduction in optical output shown in FIG. 4 will be explained.

Such a reduction in optical output causes various problems in the configuration of optical communication systems and the like.

Therefore, the present invention provides a light emitting element that can maintain a constant optical output even when an H signal is continuously input to a semiconductor switching element such as a transistor, thereby causing the light emitting element such as a semiconductor laser to continuously emit light. The purpose of this invention is to provide a drive circuit for the following.

[Means for solving problems]

A driving circuit for a light emitting element according to the present invention includes a series circuit of a light emitting element and a switching element for driving the light emitting element, a control means for controlling on/off of the switching element for driving the light emitting element, and a circuit for supplying current to the series circuit. A driving circuit for a light emitting element comprising a current source, a low pass filter having one end connected between the DC circuit and one terminal of the current source, and a control means that simultaneously turns on a switching element for driving the light emitting element. It is characterized by comprising an optical output compensation switching element which is turned off and whose output terminal is connected to the other end of the low-pass filter.

[Effect]

Since the light emitting element drive circuit according to the present invention is configured as described above, the optical output compensation switching element works to turn on at the same time when the light emitting element driving switching element turns on, and the low-pass filter operates to control the optical output. It works to remove the high frequency component of the output terminal voltage of the compensation switching element and apply it to the connection point between the series circuit and the current source. Therefore, when the DC circuit remains energized for a long time, the connection between the DC circuit and the current source It acts to lower the potential at a point.

(Example〕 Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a circuit diagram of a drive circuit according to one embodiment. The difference between this and the conventional example shown in Fig. 2 is that the semiconductor laser L
One end of a CR low-pass filter is connected to a series circuit consisting of D, transistor Q1, and resistor R1, and a transistor Q3 for optical output compensation is connected to the other end of this low-pass filter. The CR low-pass filter is composed of a resistor R2 connected between the collector of the transistor Q3 and the resistor R1, and a capacitor C connected between the resistor R2 and the negative power terminal. The collector of the transistor Q3 is connected to the positive power supply terminal via the resistor R3, the emitter is connected to the negative power supply terminal, and the base is connected to the non-inverting output terminal of the comparator IC.

Next, the operation of the circuit of the above embodiment will be explained. When the non-inverted output of the comparator IC changes from L to H (,:) as shown in FIG. 3(a), the transistor Q1 for driving the light emitting element (semiconductor laser LD) is turned on, and the semiconductor laser LD starts emitting light. At this time, the transistor Q3 for optical output compensation is also turned on, so its collector potential VC decreases as shown in FIG. ), thereby changing the potential VE on the drive circuit side.

By the way, the H from the non-inverting output terminal of comparator ■C
While the period of the signal is short, the potential V obtained through the CR low-pass filter decreases only slightly, as is clear from the waveform in FIG. 3(C). Therefore, the signal amplitude current supplied to the semiconductor laser LD hardly changes. As a result, for a short period in which the optical output of the semiconductor laser LD does not show uniform deterioration, a constant current is supplied to the semiconductor laser LD, and a constant optical output is maintained.

On the other hand, when the signal No. 11 from the non-inverting output terminal of the comparator IC continues for a long time, the potential ■E that is 1q through the CR low-pass filter is moderately low, as is clear from the waveform in Figure 3 (C). Become. Therefore, the semiconductor laser LD
The signal amplitude current supplied to the signal increases over time. As a result, after a long enough period of time that the optical output of the semiconductor laser LD exhibits uniform deterioration, the semiconductor laser LD
Since more current will be supplied to the semiconductor laser LD, the semiconductor laser LD will maintain a constant optical output regardless of uniform deterioration.

As explained above, when the period of the H level of the non-inverted output of the comparator IC is short, the semiconductor laser LD emits light for only a short period of time, so uniform deterioration does not occur. In this state, the potential VE hardly changes, so a constant current is supplied to the semiconductor laser LD and a constant optical output is maintained.

On the other hand, when the H period of the non-inverted output of the comparator IC becomes long, the semiconductor laser LD emits light for a long time, causing -like deterioration.

However, in this state, the potential VE is lowered by the action of the transistor Q3 for light emission output compensation and the CR low-pass filter, so more current is supplied to the semiconductor laser LD, and therefore the uniformity due to the temperature rise of the co-photographer is reduced. A constant light output can be obtained regardless of deterioration.

FIG. 4(b) is an explanatory diagram of the optical output according to this embodiment.

According to the conventional circuit, the optical output decreases with the passage of time as shown in FIG. 4(a), but according to the circuit of this embodiment,
As shown in FIG. 4(b), the decrease in optical output is compensated for. Such a change in optical output is caused by, for example, a semiconductor laser L.
This can be easily confirmed by detecting the optical output of D with an O/E device and observing it with an oscilloscope or the like.

The present invention is not limited to the above embodiments, and various modifications are possible. For example, the light emitting element is not limited to a semiconductor laser, but may be a light emitting diode or the like. In addition, APC to remove the influence of ambient temperature
It is not essential to connect the circuit, and the low-pass filter is not limited to the CR type, but any filter that removes high frequency components may be used. In general, the switching element is not limited to a bipolar transistor, but may also be a field effect transistor (FET) or the like. Furthermore, in the embodiment, the semiconductor laser is driven by a differential circuit of two transistors, but the light emitting element may be driven by on/off control of a single transistor or the like.

〔Effect of the invention〕

As described above in detail, according to the present invention, the driving circuit for the light emitting element is configured as described above, so that the switching element for optical output compensation is turned on at the same time when the switching element for driving the light emitting element is turned on, The low-pass filter works to remove the high frequency component of the output terminal voltage of the switching element for optical output compensation and apply it to the connection point between the series circuit and the current source. This acts to lower the potential at the connection point, so the signal of 1 is continuously input to a semiconductor switching element such as a transistor, which causes a light emitting element such as a semiconductor laser to emit light continuously, resulting in so-called --like deterioration. Even when this occurs, the optical output can be kept constant.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram of an example of a conventional circuit, FIG. 3 is an explanatory diagram of voltage waveforms at each part of the circuit shown in FIG. FIG. 2 is an explanatory diagram of the optical output of the circuit shown in FIG. 1 and FIG. 2; LP...Semiconductor laser, Ql...Transistor for driving light emitting element, Q3...Transistor for optical output compensation, R
2...Resistor for low-pass filter, C...Capacitor for low-pass filter. Fig. 1 Conventional drive circuit diagram Fig. 2 An explanatory diagram of potential changes in each part Fig. 3 An explanatory diagram of changes in optical power Fig. 4

Claims (1)

[Scope of Claims] 1. A series circuit of a light emitting element and a switching element for driving the light emitting element, a control means for controlling on/off of the switching element for driving the light emitting element, and a current source supplying current to the series circuit. A light-emitting element drive circuit comprising: a low-pass filter having one end connected between the DC circuit and one terminal of the current source; and a low-pass filter that is turned on and off at the same time as the light-emitting element driving switching element by the control means. A driving circuit for a light emitting element, comprising: a switching element for optical output compensation which is controlled and whose output terminal is connected to the other end of the low-pass filter. 2. The DC circuit has a resistance means connected between one terminal of the current source, and one end of the low-pass filter is connected between one terminal of the current source and the resistance means. A driving circuit for a light emitting element according to claim 1. 3. The light emitting element driving circuit according to claim 1, wherein the light emitting element driving switching element and the optical output compensation switching element are transistors each inputting an output signal of the control means as a base. 4. The light emitting element drive circuit according to claim 1, wherein the low pass filter is a CR low pass filter.