JPH06260703A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To provide a semiconductor laser drive circuit with which the temperature characteristics of an output current can be selected. CONSTITUTION:The complementary electric signal applied to input terminals 2a and 2b is amplified by a differential amplifier 1 and inputted to the gate of FETs 4a and 4b. The light emission and the extinction of a semiconductor laser 5 is controlled by turning ON/OFF the current which is determined by the constant current circuit commonly connected to the source of the FETs 4a and 4b, on the left and right FET 4a and 4b. FET 8a, 8b and 8c are used for the constant current circuit. The FET 8a, 8b and 8c have different threshold voltages respectively, and the source terminal of one of them is connected to a power source 7. The temperature characteristics of output current of a driving circuit can be selected by the selection of the source terminal to be connected to the power source 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for a semiconductor laser used for optical communication or the like.

[0002]

2. Description of the Related Art In a signal format using optical pulses such as optical communication, a semiconductor laser is driven by a pulse current to generate an optical signal.

FIG. 5 is an equivalent circuit diagram of a conventional example of a semiconductor laser drive circuit. In the figure, 1 is a differential amplifier circuit, 2a, 2
b is an input terminal, 3a and 3b are output terminals, and 4a and 4b are F
ET, 5 are semiconductor lasers, 6 is a constant current circuit, and 7 is a power source. Complementary input pulse signals are applied to the input terminals 2a and 2b of the differential amplifier circuit 1. The output terminals 3a and 3b of the differential amplifier circuit 1 are connected to the gates of the FET pair 4a and 4b. The drain of the FET 4a is the semiconductor laser 5
Is connected to the reference potential (ground) via, and the drain of the FET 4b is directly connected to the reference potential. FET
The sources of the pairs 4a and 4b are the power supply 7 via the constant current circuit 6.
It is connected to the.

The complementary electric signals applied to the input terminals 2a and 2b are amplified by the differential amplifier 1 and the FET pair 4
It is input to the gates of a and 4b. This FET pair 4a, 4
The current I determined by the constant current circuit 6 commonly connected to the sources of b is distributed as the drain currents of the left and right FET pairs 4a and 4b according to the input signals of the FET pairs 4a and 4b,
When the FET 4a is on, a constant current flows through the semiconductor laser 5 to cause the semiconductor laser 5 to emit light. When the FET 4a is off, no current flows through the semiconductor laser 5 and the semiconductor laser 5 is extinguished. . In this way, the emission and extinction of the semiconductor laser 5 are controlled.

FIG. 6 is an explanatory diagram of a configuration example of the constant current circuit in FIG. FIG. 6A is a symbol representing the constant current circuit 6, and as shown in FIG. 6B, it can be realized by one FET 8, and the constant current circuit 6 is constant according to the gate-source voltage V _gs. The current value I _d is set. Since the drain current I _d of the FET 8 becomes the output current of the FET 8 of FIG. 6 as a constant current, the magnitude of the output current is adjusted by adjusting the gate-source voltage V _gs of the FET 8 of the constant current circuit. be able to.

As for the current-light output characteristics (IL characteristics) of the semiconductor laser, as shown in FIG. 7, the differential quantum efficiency (gradient of IL characteristics) decreases as the temperature rises, and the light output becomes Decrease. That is, it can be said that the semiconductor laser has a negative temperature characteristic. Therefore, the conventional semiconductor laser has a problem that the optical output is significantly reduced due to the temperature rise at the time of a large current.

Further, the temperature characteristics of the semiconductor laser, that is, the rate of fluctuation of the optical output due to temperature fluctuation greatly differs depending on the individual semiconductor laser. Therefore, the temperature characteristic required for the output current of the semiconductor laser drive circuit in order to keep the light output constant in a wide temperature range varies depending on the semiconductor laser used.

However, the conventional semiconductor laser drive circuit can obtain only a single temperature characteristic as shown in FIG.
Therefore, there is a problem in that the temperature characteristics of the semiconductor laser drive circuit are optimum for a certain semiconductor laser but cannot be suitable for other semiconductor lasers having different characteristics.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and provides a semiconductor laser drive circuit in which the temperature characteristic of the output current can be selected by selecting the wiring. That is the purpose.

[0010]

The present invention relates to a drive circuit for a semiconductor laser, which is driven by FETs which are connected to a constant current circuit and which are operated differentially. In the invention according to claim 1, the constant current is provided. The circuit includes a plurality of FETs provided so as to be selectively connected and having different threshold voltages. In the invention according to claim 2, the constant current circuit selectively operates. A plurality of FETs having different gate widths are provided so as to be connected to each other. In the invention according to claim 3, the constant current circuit is selectively connected to a source circuit of the FETs. In the invention according to claim 4, there is provided a diode provided as described above.
The constant current circuit has a resistance provided so as to be selectively connected to a source circuit of an FET, and in the invention according to claim 5, the constant current circuit is an FET. The temperature characteristic compensating circuit is provided so as to be selectively connected between the gate terminal and the constant voltage source.

[0011]

The temperature characteristics of the drain current of the FET 8 shown in FIG. 6 have the following characteristics. The smaller the gate-source voltage V _gs , the stronger the positive temperature characteristic. That is, the drain current increases as the temperature rises. If the gate-source voltage V _gs is constant, the FET
The larger the threshold voltage V _{th of} 8 is, the stronger the positive temperature characteristic is.

Further, as the means for adjusting the temperature characteristic of the drain current by connecting the other circuit element to the FET 8, the following may be mentioned. By inserting the diode 9 between the source terminal of the FET 8 and the power supply, a strong positive temperature characteristic can be obtained. The FET8 of FIG. 9 is the FET8 of FIG.
Equivalent to. The voltage V _d applied to the diode 9 decreases as the temperature rises when the drain current I _d is constant. Therefore, the voltage V between the gate terminal of the FET8 and the power supply is
_{If g} is kept constant, the gate-source voltage V of FET8
_gs increases with increasing temperature and increases the drain current I _d . By inserting the resistance element 10 between the source terminal of the FET 8 and the power supply, a strong negative temperature characteristic can be obtained. The resistance value r of the resistance element 10 generally increases as the temperature rises. For this reason, the voltage V _r applied to the resistor increases, and if the voltage V _g between the gate terminal of the FET 8 and the power supply is kept constant, the gate-source voltage V _gs of the FET 8 decreases as the temperature rises, Drain current I
decrease _d . By connecting the temperature compensation circuit 11 to the gate terminal of the FET 8 as shown in FIG. 11, stronger temperature characteristics can be obtained. The temperature compensation circuit 11 generates a voltage V _O according to the voltage V _s from the outside and applies it to the gate of the FET 8. By setting the temperature compensating circuit 11 so that this voltage V _g increases as the temperature rises when the voltage V _s is constant, the gate-source voltage V _{gs of the} FET 8 increases and the drain current increases. Increase.

According to the present invention, the temperature characteristics of the semiconductor laser can be compensated by providing a plurality of circuits capable of providing the above-mentioned temperature characteristics and selecting them.

[0014]

1 is an equivalent circuit diagram of a semiconductor laser drive circuit according to a first embodiment of the present invention. In the figure, the same parts as those in FIG. 8a, 8b,
8c is a FET. These FETs 8a, 8b, 8c are
The threshold voltages are different from each other. The drain terminal and the gate terminal of each FET are connected to each other, and the source terminal is separated. One of the source terminals is connected to the power supply 7, and the temperature characteristic of the output current of the drive circuit can be selected. The selection of the source terminal connected to the power supply 7 is determined by the temperature characteristics of the semiconductor laser 5. The selected source terminal is connected by an appropriate means such as soldering to the pad of the IC. You may make it selectable with a DIP switch or the like.

In the first embodiment, the FETs 8a, 8b, 8
For c, those having different threshold voltages were used, but the present invention is not limited to this. As a second embodiment, FETs having different gate widths may be used. The circuit configuration in this case may be similar to that of FIG. 1, and the temperature characteristic of the output current of the drive circuit can be selected by selecting the source terminal.

FIG. 2 is an equivalent circuit diagram of the third embodiment of the semiconductor laser drive circuit of the present invention. In the figure, the same parts as those in FIG. 8 is FE
T and 9 are diodes. The positive electrode of the diode 9 is F
It is connected to the source terminal of ET8. Therefore, F
The source of the ET 8 can be selected to be directly connected to the power supply 7 or connected via the diode 9, and the temperature characteristic of the drive circuit can be selected according to the temperature characteristic of the diode 9. Although one is shown as the diode 9,
Needless to say, a plurality of devices may be connected so that they can be selected.

FIG. 4 is an equivalent circuit diagram of a fourth embodiment of the semiconductor laser drive circuit of the present invention. In the figure, the same parts as those in FIG. 10 is a resistance. The resistor 10 is connected to the source terminal of the FET 8 and tapped from the intermediate point. Therefore,
The source of the FET 8 can be directly connected to the power supply 7 or to a selected tap of the resistor 10,
The temperature characteristic of the resistor 10 allows the temperature characteristic of the drive circuit to be selected. The resistor 10 is not limited to having a tap,
A single one may be used. Further, it may be possible to select by using those having different temperature coefficients.

FIG. 4 is an equivalent circuit diagram of a semiconductor laser drive circuit according to a fifth embodiment of the present invention. In the figure, the same parts as those in FIG. Reference numeral 11 is a temperature compensation circuit. A constant voltage V _{s is} applied to the gate terminal of the FET8.
Can be applied directly or via the temperature compensating circuit 11 to select the temperature characteristic of the output current. Although the temperature compensation circuit 11 uses a diode and a resistor, the temperature compensation circuit 11 is not limited to this. Alternatively, a plurality of temperature compensation circuits may be used for selection.

In the above-described embodiment, the characteristics and number of selectable circuits are designed in consideration of the range of temperature characteristics of the semiconductor lasers to be connected. Also, the selection does not have to be alternative and multiple terminals may be connected.

[0020]

As is apparent from the above description, according to the present invention, the temperature characteristic of the output current of the semiconductor laser drive circuit can be adjusted according to its application. As a result, it is possible to obtain the optimum temperature characteristic of the output current of the semiconductor laser drive circuit for keeping the optical output of the various semiconductor lasers constant over a wide temperature range.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of a first embodiment of a semiconductor laser drive circuit of the present invention.

FIG. 2 is an equivalent circuit diagram of a third embodiment of a semiconductor laser drive circuit of the present invention.

FIG. 3 is an equivalent circuit diagram of a fourth embodiment of the semiconductor laser drive circuit of the present invention.

FIG. 4 is an equivalent circuit diagram of a semiconductor laser drive circuit according to a fifth embodiment of the present invention.

FIG. 5 is an equivalent circuit diagram of a conventional example of a semiconductor laser drive circuit.

6 is an explanatory diagram of a configuration example of a constant current circuit in FIG.

FIG. 7 is a diagram showing characteristics of a semiconductor laser.

FIG. 8 is a diagram showing characteristics of a semiconductor laser drive circuit.

FIG. 9

FIG. 11 is an explanatory view of the operation of the present invention.

[Explanation of symbols]

 1 differential amplifier circuit 4a, 4b FET 5 semiconductor laser 6 constant current circuit 7 power supply 8 FET 9 diode 10 resistor 11 temperature compensation circuit

Claims (5)

[Claims] 1. In a drive circuit of a semiconductor laser connected to a constant current circuit and driven by an FET that operates differentially, the constant current circuit is provided with a plurality of threshold voltages which are provided so as to be selectively connected. 2. A semiconductor laser drive circuit having the FET of FIG. 2. In a drive circuit of a semiconductor laser connected to a constant current circuit and driven by FETs that operate differentially, the constant current circuit is provided with a plurality of gate widths which are provided so as to be selectively connected. 2. A semiconductor laser drive circuit having the FET of FIG. 3. A semiconductor laser drive circuit connected to a constant current circuit and driven by a FET that operates differentially, wherein the constant current circuit is a diode provided so as to be selectively connected to a source circuit of the FET. A semiconductor laser drive circuit comprising: 4. A semiconductor laser drive circuit connected to a constant current circuit and driven by an FET that operates differentially, wherein the constant current circuit is provided so as to be selectively connected to a source circuit of the FET. A semiconductor laser drive circuit having a resistance. 5. A semiconductor laser drive circuit driven by an FET that is connected to a constant current circuit and operates differentially, wherein the constant current circuit selectively connects between a gate terminal of the FET and a constant voltage source. A semiconductor laser drive circuit having a temperature characteristic compensation circuit provided so as to be connected.