JPS6317581A - Driving circuit for light-emitting element - Google Patents

Abstract

PURPOSE:To inhibit the ringing due to the relaxation oscillation at the time a pulse modulation is applied to such a light-emitting element as a semiconductor laser and to remove the distortion of output pulse of the semiconductor laser by connecting a capacitor in parallel with a constant-current source. CONSTITUTION:When a high-frequency pulse is applied to the bases of transistors Q1 and Q2 through a comparator IC and a high-speed pulse modulation is applied to a semiconductor laser, high-frequency components appear by the relaxation oscillation of the semiconductor laser LD and could flow in a constant-current source transistor Q3 through the transistor Q1. But, as a capacitor C is connected between the common emitter connection of the transistors Q1 and Q2 and a negative power source, a large portion of the high-frequency components flows through this capacitor to the positive power source passing. Accordingly, it is eliminated that a signal amplitude current flowing through the constant-current source transistor Q3 becomes unstable by the high-frequency component. This prevent distortion of the photo output waveform of the semiconductor laser LD, which could be caused by ringing.

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 laser used as a light source for optical communication, optical information processing equipment, etc.

[Conventional technology]

Semiconductor lasers have established a very important position as key devices for optical information processing systems such as optical communications, pickups for optical disk devices, and light sources for laser printers.

When using semiconductor lasers as the light source for these systems,
It is necessary to keep the laser light output constant and modulate it at high speed.

Semiconductor lasers can easily modulate their optical output by modulating their 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 firing 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 differential input into a waveform and outputting it, and its output terminals are connected to a negative power supply via pull-down resistors R1 and R2, respectively. A transistor Q3 commonly connected to the emitters of drive transistors Q1 and Q2 constitutes a constant current source, and is connected to a negative power supply via a resistor R3. Further, a bias current supply circuit (not shown) (consisting of a transistor, a coil for blocking the flow of high-frequency current, etc.) is connected to the cathode of the semiconductor laser LD.

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 becomes H and the inverted output changes from L to 1-1, the transistor Q1 changes from the on state to the off state and the transistor Q2
changes from off state to on state.

Therefore, the current flows to the constant current transistor Q3 via the transistor Q2, so that the semiconductor laser LD is extinguished.

[Problem that the invention seeks to solve]

However, the above conventional circuit has the following problems. In other words, when high-speed modulation is applied using a high-frequency pulse from the comparator IC, a high-frequency component generated by relaxation oscillation (pulsation) of the semiconductor laser LD flows into the constant current transistor Q3, and therefore the current flowing through the constant current source transistor Q3 becomes unstable. It became. As a result, transient vibrations, ie, ringing, occur at the rising edge of the optical output waveform of the semiconductor laser LD, and distortion appears in the waveform.

FIG. 3 is a waveform diagram for explaining this.

When a driving pulse as shown in FIG. 3(a) is applied to the base of the transistor Q1 via the comparator IC, 1
~Langister Q1. In the common emitter of Q2, the third
As shown in Figure (b), a waveform in which high-frequency imaging is superimposed on a substantially constant current appears. Therefore, in the rising portion of the output waveform of the semiconductor laser LD, ringing causes large vibrations as shown in FIG. 3(C).

Such problems occur not only when driving semiconductor lasers but also when driving other light emitting elements such as light emitting diodes. Also, two transistors Ql.

This problem occurs not only in drive circuits in which a differential circuit is configured by Q2, but also in drive circuits in which only one transistor is configured.

Therefore, the present invention suppresses ringing due to relaxation oscillation when pulse modulation is applied to a light emitting element such as a semiconductor laser, and
It is an object of the present invention to provide a driving circuit for a light emitting element that can eliminate distortion of output pulses of a semiconductor laser.

[Means for solving problems]

In order to achieve the above object, a driving circuit for a light emitting element according to the present invention includes a semiconductor switching element, a light emitting element connected in series to the semiconductor switching element, and a constant current source connected in series to the semiconductor switching element and the light emitting element. A driving circuit for a light emitting element comprising: a constant current source and a capacitor connected in parallel with the constant current source.

[Effect]

According to the present invention, since the light emitting element drive circuit is configured as described above, the capacitor allows high frequency components to pass through (bypassing the constant current source), thus suppressing high frequency imaging from being applied to the constant current source. It works to stabilize the signal amplitude current.

〔Example〕

The present invention will now be described in detail with reference to FIGS. 1, 4 and 5 of the accompanying drawings.

FIG. 1 is a circuit diagram showing the circuit configuration of one embodiment. The difference from the conventional circuit shown in FIG. 2 is that the transistor Q1. A capacitor C for cutting high frequency components is connected between the common emitter of Q2 and the negative power supply.

The capacitance of the capacitor C should be an appropriate size to pass the high frequency component that appears due to the relaxation vibration of the semiconductor laser LD, and specifically, it is determined according to the characteristic values of the transistors forming the circuit, the value of the resistance, etc. It will be done.

Next, the action will be explained. In Figure 1, comparator I
A high-frequency pulse is applied from C to the bases of transistors Q1 and Q2, and high-speed pulse modulation is applied to the semiconductor laser. In this way, a high frequency component appears due to the relaxation oscillation of the semiconductor laser LD, and this tends to flow into the constant current source transistor Q3 via the transistor Q1. However, transistor Q1. Since a capacitor C is connected between the common emitter of Q2 and the negative power supply, most of the high frequency components flow through this capacitor C to the negative power supply. Therefore, the signal (width current) flowing through the constant current source transistor Q3 does not become unstable due to high frequency components, and as a result, distortion of the optical output waveform of the semiconductor laser LD due to ringing can be prevented.

Briefly, the inventor conducted the following experiment in order to confirm the effectiveness of the above example. That is, as shown in FIG. 4, the emitted light from the semiconductor laser LD was coupled to the optical fiber 1, and the transmitted laser light was converted into an electrical signal by the O/E converter 2 and observed with the oscilloscope 3. Note that the semiconductor laser LD is made of 1.2 μm InGaAs.
A P laser was used, and a 220F capacitor C was used.

FIG. 5 is a waveform diagram of each part of the circuit shown in FIG. 4. That is, a driving pulse as shown in FIG. 5(a) is applied to the transistor Q1. When applied to the base of Q2, transistor Q
1. The current waveform appearing at the common emitter of Q2 is shown in Figure 5 (b
). As a result, it can be seen that the high frequency components are considerably suppressed compared to the waveform of the conventional example shown in FIG. 3, which was tested under the same conditions except for capacitor C. FIG. 5(C) shows the optical output waveform of the semiconductor laser LD observed with the oscilloscope 3. As is clear from a comparison with the conventional example shown in FIG. 3(C), vibrations at the rising edge of the pulse are sufficiently suppressed.

The present invention is not limited to the above embodiments, and various modifications are possible. For example, the invention can be applied not only to circuits that drive semiconductor lasers, but also to circuits that drive other light emitting elements such as light emitting diodes. Further, the present invention can be widely applied not only to a current changeover switch type drive circuit using a differential circuit as in the embodiment, but also to a drive circuit not using a differential circuit, an emitter follower type, and the like. However, since the effects of the present invention are particularly noticeable in high-speed pulse modulation, it goes without saying that it is particularly suitable for current changeover switch type devices such as the embodiment.

In general, the transistors Q1, 02, etc. are not limited to bipolar transistors, but may be MOS transistors, etc., and the capacitors may be configured with MOS capacitors.

〔Effect of the invention〕

As described above, according to the present invention, since a capacitor is connected in parallel to a constant current source that supplies a driving current to a light emitting element such as a semiconductor laser, high frequency components flow into the transistors etc. that constitute the constant current source. This can prevent the signal amplitude current from becoming unstable, and therefore has the effect of suppressing vibrations due to ringing appearing in the optical output waveform of the light emitting element.

The invention is particularly advantageous when performing high speed pulse modulation.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a semiconductor laser drive circuit according to an embodiment of the present invention, Fig. 2 is a circuit diagram of a conventional semiconductor laser drive circuit, and Fig. 3 shows the operation of the conventional example shown in Fig. 2. Figure 4 is a signal waveform diagram to explain, and Figure 4 is a circuit diagram when an optical fiber etc. is connected to evaluate the characteristics of the drive circuit in Figure 1. Figure 5 is a signal waveform diagram to explain the operation of Figure 4. be. 1...Optical fiber, LD...semiconductor laser, IC/
... Comparator, C... Capacitor. Patent applicant Sumitomo Electric Industries, Ltd. Applicant's agent Yoshiki Hase Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Scope of Claims] 1. In a driving circuit for a light emitting element, comprising a semiconductor switching element, a light emitting element connected in series to the semiconductor switching element, and a constant current source connected in series to the semiconductor switching element and the light emitting element. , A driving circuit for a light emitting element, characterized in that a capacitor is connected in parallel with the constant current source. 2. The light emitting device driving circuit according to claim 1, wherein the semiconductor switching element is a transistor, and the capacitor is connected between the emitter of the transistor and a negative power source.