JPH01115181A - Drive circuit for semiconductor laser - Google Patents

Abstract

PURPOSE:To improve rising and falling characteristics of an optical output by providing a current waveform adjusting terminal in a buffer stage and adjusting the rising and falling waveforms of a driving signal according to the level of signal sent from the terminal. CONSTITUTION:The gates of FETs 22, 32 used as loads for the source follower FETs 21, 31 forming buffer stages 20, 30 are connected with a current waveform adjusting terminal. A current of FETs 22, 32 as the loads of follower output can be set to an adequate value by setting a voltage level of signal applied to the current waveform adjusting terminal to an adequate value. Therefore, the mutual conductance of FETs 21, 31 making the follower operation can be adjusted. The rising and falling waveforms of driving signal can be adjusted with the signal sent from the current waveform adjusting terminal. Thereby, the rising and falling characteristic of optical output can be improved while influence of alleviated vibration of a semiconductor laser 18 suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser drive circuit that drives a semiconductor laser to emit light.

[Description of prior art]

Semiconductor laser drive circuits are used in transmitters of optical communication systems. FIG. 4 is a diagram showing the configuration of a conventional example. In the figure, a field effect transistor (FET) 11 serving as a current source provides a bias current to the semiconductor laser LD, and an FET 12 serving as a current source applies a bias current to the semiconductor laser LD.
Give a modulating current to. Switching FET1. ．． 2
is for driving the semiconductor laser LD to emit light, and the input signal S
It is turned on and off complementarily by in'Sln.

These input signals S, , S are input signals SIN, SIN]
It is generated by a buffer stage 20.30 which inputs n In . Here, the buffer stages 20 and 30 each include FETs 21 and 31 that perform follower operations, and an FET that is used as a load.
It has ET22 and ET32.

In such a circuit, when the optical output from the semiconductor laser LD is coupled to an optical fiber and applied to an oscilloscope via an O/E (optical/electrical) converter, the optical output waveform can be observed.

FIG. 5 shows signal waveforms at each part of FIG. 4.

When there is a rectangular input signal as shown in FIG. 5(a), the drive current of the semiconductor laser LD also becomes rectangular as shown in FIG. 5(b), and a light output is obtained accordingly. However, semiconductor laser LDs generally have a phenomenon called relaxation oscillation, and even when driven by a rectangular signal, the optical output varies as shown in Figure 5 (c).
) vibrations as shown appear. This causes communication errors in optical communications.

Therefore, conventionally, as shown in FIG.
D is provided with a high frequency bypass circuit consisting of, for example, a resistor R and a capacitor C or CC.

In this way, the high frequency component of the input signal is bypassed, and therefore the driving current of the semiconductor laser LD also changes clearly according to the time constant determined by the values of R and C. For this reason, the relaxation oscillation that appears in the optical output of the semiconductor laser LD is reduced, for example, the optical output is reduced as shown in Fig. 5 (d
) can be a substantially rectangular waveform.

[Problem that the invention is trying to solve] However,
According to the prior art shown in FIG. 4, although the influence of relaxation oscillation can be suppressed to a small level, there is a problem in that it is not easy to determine the specific characteristics of the bypass circuit. This is because the characteristics of the semiconductor laser LD are not uniquely determined, and depending on these characteristics and compatibility with the drive circuit, the optimal resistance and
The capacitor must be identified.

Then, in the process of trial and error, the characteristics of the semiconductor laser deteriorate or are destroyed due to heat from the soldering iron or shock.

On the other hand, depending on the characteristics of the semiconductor laser, the optical output waveform may be as shown in FIG. 5(e), leading to communication errors in optical communications. However, there has been no particularly effective solution to this problem.

SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser drive circuit that improves the rise and fall characteristics of optical output while suppressing the effects of relaxation oscillation of the semiconductor laser.

[Means to solve the problem]

A semiconductor laser drive circuit according to the present invention drives a semiconductor laser supplied with a predetermined bias current to emit light by applying a drive signal from a buffer stage to a switching transistor connected in series to the semiconductor laser. The laser drive circuit is characterized in that the buffer stage has a current waveform adjustment terminal, and the rising and falling waveforms of the drive signal are adjusted by signals from this terminal.

[Effect]

According to the configuration of the present invention, the rise of the drive signal and the waveform at the time of rise are adjusted by the signal from the current waveform adjustment terminal.

〔Example〕

Hereinafter, some embodiments of the present invention will be described with reference to FIGS. 1 to 3 of the accompanying drawings. In addition, in the description of the drawings, the same elements are given the same reference numerals, and redundant description will be omitted.

FIG. 1 is an explanatory diagram of the structure and operation of a first embodiment of the present invention. As shown in the figure, in this embodiment, the buffer stage 2
The gates of FETs 22 and 32 used as loads are connected to the current waveform adjustment terminal for the source follower FETs 21 and 31 constituting 0.30. By setting the voltage level of the signal applied to this current waveform adjustment terminal to an appropriate value, the current of the current source (FET 22, 32) that is the load of the follower output can be set to an appropriate value. Therefore, the mutual conductance of FET 21.31 that performs follower operation can be adjusted.

In this circuit, there is no need to provide a bypass circuit near the semiconductor laser LD, and the additional circuit is
Since it is provided in a part separate from D, it does not adversely affect the characteristics of the semiconductor laser LD, which is a weak element. In addition, since a current waveform adjustment terminal is provided and adjustment is made from the outside,
It can also be easily applied when the illustrated circuit is monolithically integrated.

Next, the operation of the circuit of the first embodiment will be explained with reference to FIGS. 2 and 5. Note that the function of the buffer stage 30 is similar to that of the buffer stage 20, so the buffer stage 20
will be explained only.

Now, when the level of the current waveform adjustment terminal is set to be the same as that of the power supply terminal (low) (as in the conventional example), the input pulse shown in Fig. 5(a) is applied to the non-inverting input terminal SIN. Assume that the drive pulse (2) shown in FIG. 5(b) (the waveform indicated by the dotted line in FIG. 2(a)) is obtained for the FETI. and,
Assume that the optical output A (the dotted line waveform in FIG. 2(b)) shown in FIG. 5(c) is obtained from the semiconductor laser LD.

At this time, the relaxation oscillation of the semiconductor laser LD is large,
In order to perform good optical communication, it is necessary to remove the vibration component appearing in this waveform. Therefore, when the level of the current waveform adjustment terminal is lowered, the current flowing through the FET 22 decreases, and the mutual conductance g of the FET 21 that operates as a follower decreases. As a result, the driving speed of the FETI that performs switching work is slowed down, and the driving pulse I
A changes from the dotted line waveform in FIG. 2(a) to the solid line waveform.

Then, the optical output AA of the semiconductor laser LD is also shown in Fig. 2(b).
The waveform of the dotted line changes to the waveform of the solid line, and relaxation oscillations are suppressed.

On the other hand, the driving pulse waveform shown in FIG. 5(b) (see FIG. 2(
When the optical output C shown in FIG. 5(e) (the dotted line waveform in FIG. 2(d)) is obtained by the dotted line waveform in FIG. That is, by setting the level of the current waveform adjustment terminal to be high, the current flowing through the FET 22 is increased. As a result, the mutual conductance g of the FET 21 that performs a follower operation increases, so that the driving speed of the FET 21 that performs a switching operation becomes faster. Then, the waveform of the driving pulse IC changes from the dotted line waveform in FIG. 2(c) to the solid line waveform, and the optical output CC of the semiconductor laser LD also changes to the second waveform.
The waveform shown by the dotted line in Figure (d) changes to the waveform shown by the solid line. Thereby, it is possible to improve the "abortion" of the rising and falling waveforms of the optical output.

Next, a second embodiment of the present invention will be described with reference to FIG.

In this example, FET 22 serves as a load for the follower output.
, 32 and the power supply terminal (low), a level shifting diode D is connected. With this circuit as well, the FETs 22 and 32 become variable current sources whose currents are controlled by the current waveform adjustment terminals. Therefore, by adjusting this current value -, it is possible to adjust the mutual conductance g of FETs 21 and 31 that perform follower operations, and adjust the drive speed of FET 1.2 that performs switching operations.

According to this second embodiment, the following special effects can be achieved.

First, since the diode D provides a level shift of about 0.6 volts, there is no need to provide a separate power supply for setting the level of the current waveform adjustment terminal. In other words, the source level of FET22.32 is set to the power supply terminal (low)
The level of the current waveform adjustment terminal can be increased by 0.6 volts with respect to the level of the current waveform adjustment terminal (high) and the power terminal (high).
This can be obtained by dividing the resistance between

Second, the FETs that make up the circuit are normally-on type D
- It becomes easy to configure with FETs.

That is, in a D-FET, the source and drain are turned on even if they are at the same level, so if the diode D is not provided, the current value cannot be sufficiently reduced when the D-FET is used. Therefore, by level shifting with diode D, the current waveform adjustment terminal can be lowered by 0.6 volts with respect to the source of FET22゜32 even when a separate power supply is not provided, so follower operation can be performed by reducing the current value. D
- It is possible to reduce the mutual conductance g of the FET and suppress relaxation oscillations.

By doing so, when a D-FET is used, the element can be made smaller and the parasitic capacitance can be reduced, and therefore it is suitable for high-speed operation. In addition, the D-FET is a normally-off type E-FET.
Since the temperature dependence is smaller than that of FET, the circuit shown in FIG. 3 is also superior in this respect.

The present invention is not limited to the above embodiments, and various modifications are possible.

For example, since no high-frequency current flows in the circuit that sets the level of the current waveform adjustment terminal, it is possible to use an inexpensive element with poor high-frequency characteristics or a variable resistor. Also, a buffer stage 20.3 that performs a follower operation
A diode or the like may be connected in order to provide level shift function to 0. Furthermore, a waveform rectification circuit or an amplitude amplification circuit may be connected before the buffer stage.

Furthermore, the transistors constituting the circuit are not limited to FETs, but may be bipolar transistors or the like. In this case, by adjusting the base current of the transistor serving as the load and making the current value of the current source variable, the common emitter current amplification factor hr of the transistor operating as a follower can be increased.
8, and thus the driving speed of the transistor that performs the switching operation.

〔Effect of the invention〕

According to the present invention, the rise and rise waveform of the drive signal are adjusted by the signal from the voltage waveform adjustment terminal. Therefore, it is possible to improve the rise and fall characteristics of the optical output while suppressing the influence of relaxation oscillation of the semiconductor laser.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a semiconductor laser drive circuit according to a first embodiment of the present invention, FIG. 2 is a waveform diagram showing the operation of the circuit of FIG. 1, and FIG. 3 is a circuit diagram of a semiconductor laser drive circuit according to a second embodiment of the present invention. FIG. 4 is a circuit diagram of a semiconductor laser drive circuit, FIG. 4 is a circuit diagram of a conventional example, and FIG. 5 is a waveform diagram showing its operation. 20.30... Buffer stage, FETI, 2... Switching transistor, FET21, 31... Transistor that operates as a follower, FET22, 32... Transistor serving as load for follower output, LD... Semiconductor laser , D...Level shift diode. Patent Applicant Sumitomo Electric Industries Co., Ltd. Representative Patent Attorney Yoshiki Hase Waveform of conventional example Fig. 5 Optical output AA Waveform of embodiment Fig. 2 Procedure Shinichi Masaaki June 1986 -

Claims (1)

[Claims] 1. A semiconductor laser to which a predetermined bias current is applied,
In a drive circuit for a semiconductor laser that drives light emission by applying a drive signal from a buffer stage to a switching transistor connected in series with the semiconductor laser, the buffer stage has a current waveform adjustment terminal, and the buffer stage has a current waveform adjustment terminal, and a signal from this terminal is applied to a switching transistor connected in series to the semiconductor laser. A semiconductor laser drive circuit characterized in that rising and falling waveforms of the drive signal are adjusted depending on a level. 2. The buffer stage is an emitter follower or source follower circuit having a current source as a load, and the current value of the current source is adjusted by the signal level from the current waveform adjustment terminal. Range 1
A driving circuit for the semiconductor laser described in . 3. The semiconductor laser drive circuit according to claim 2, wherein the current source is connected to a power source via a level shifting diode. 4. The semiconductor laser drive circuit according to claim 1, wherein the switching transistor and the buffer stage are monolithically integrated.