JPH09283831A - Laser diode driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser diode driving circuit capable of detecting the bias current flowing in the laser diode, without performing a calculation work taking account of other components in the circuit. SOLUTION: A diode RC is inserted between a resistor R2 and collector terminal of a transistor TR3 so that the entire collector-emitter current flows in a laser diode LD. Hence, the bias current flowing in the laser diode LD is detected by measuring only the emitter voltage of the transistor TR3, without taking account other components in the circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter circuit in optical transmission, and more particularly to a laser diode drive circuit.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram showing a configuration example of a conventional laser diode drive circuit.

In this conventional example, as shown in FIG. 2, a constant current source 1 is used.
Of transistors TR1 and TR2 whose emitter terminals are connected to the constant current source 1, resistors R1 connected to the collector terminals of the transistors TR1, resistors R2 and R4 connected to the collector terminals of the transistors TR2, and resistors R2. The anode of the resistor R2 is connected between the side where the collector terminal of the transistor TR2 is not connected and the side where the collector terminal of the transistor TR1 of the resistor R1 is not connected and the side where the collector terminal of the transistor TR2 of the resistor R4 is not connected. Laser diode LD connected as a terminal
A transistor TR3 whose collector terminal is connected to the cathode terminal of the laser diode LD;
It is composed of a resistor R3 connected between the emitter terminal of R3 and ground.

A pulse signal for causing the laser diode LD to emit light is input from the anode terminal of the laser diode LD, and a base terminal of the transistor TR2 receives logical data for switching the supply of the pulse signal to the laser diode LD. The signal is also the transistor T
The inverted data signal for switching the supply of the pulse signal to the laser diode LD is input to the base terminal of R1 similarly to the logical data signal.

The operation of the laser diode drive circuit configured as described above will be described below.

First, when the data signal is at the H level, the transistor TR2 is turned on and the transistor TR1 is turned off.
The F state is entered, and a pulse current is supplied to the laser diode LD by a pulse signal input from the outside, and the laser diode LD emits light. Next, when the data signal is at the L level, the transistor TR1 is turned on and the transistor TR2 is turned off. Even if a pulse signal is input from the outside, the pulse current is not supplied to the laser diode LD and the LD is turned off.

Here, the resistor R2 is provided to adjust the ringing of the waveform generated when the frequency of the input pulse signal is high.

FIG. 3 shows the laser diode L shown in FIG.
It is a figure which shows the characteristic of D.

As shown in FIG. 3, a laser diode LD
Is controlled by the bias current I _B flowing through the laser diode LD, and the threshold I _th of the bias current I _B necessary for the laser diode LD to emit light is input from the base terminal of the transistor TR3. It is controlled by voltage. The bias current I _B is detected by measuring the emitter voltage at the emitter terminal of the transistor TR3.

The bias current I _B flowing through the laser diode LD is detected by the method described below.

The bias current flowing between the collector and the emitter of the transistor TR3 flows in the laser diode LD and also in the resistors R2 and R4.
If the current flowing through 2, R4 is I ₂ , then I _B + I ₂ , and the measured emitter voltage of the transistor TR3 is
R3 becomes _{× (I B + I 2)} .

Therefore, the voltage applied to the laser diode LD is the voltage measured at the emitter terminal of the transistor TR3 minus R3 × I ₂ , and then the bias current I _B flowing through the laser diode LD is detected.

FIG. 4 is a circuit diagram showing another configuration example of a conventional laser diode drive circuit. In this conventional example, as shown in FIG. 4, a capacitor C1 is connected in series to the side of the resistor R2 of the laser diode drive circuit shown in FIG. 2 to which the collector terminal of the transistor TR2 is not connected. , The bias current no longer flows through the resistor R2.

Therefore, only the voltage applied to the laser diode LD can be detected at the emitter terminal of the transistor TR3.

As another conventional technique, Japanese Patent Laid-Open No. 2-11
There is one described in Japanese Patent No. 2331, but in this conventional example, since an additional circuit for waveform adjustment is required, the above-described calculation is necessary when detecting the voltage applied to the laser diode. .

[0016]

In the laser diode drive circuit configured as described above, when the bias current flowing in the laser diode is detected, the calculation work that takes into consideration other components in the circuit is required. Because it is necessary
There is a problem that it takes time and effort.

When a capacitor is used, the current flowing through the resistor connected in parallel with the laser diode changes according to the mark ratio of the input data signal, so the voltage measured by the mark ratio changes. There is a problem that it does.

The present invention has been made in view of the problems of the above-described conventional technique, and the current flows to the laser diode without performing the calculation work in consideration of other components in the circuit. An object of the present invention is to provide a laser diode drive circuit that can detect a bias current.

[0019]

In order to achieve the above object, the present invention has a constant current source and a laser diode which emits light according to a pulse signal input from the outside, and the laser diode of the pulse signal is applied to the laser diode. Is a laser diode drive circuit for controlling a bias current flowing in the laser diode according to a control voltage input from the outside, the first and second transistors having emitter terminals connected to the constant current source. And a first resistor connected to the collector terminal of the first transistor,
A second resistor connected to the collector terminal of the second transistor, a diode whose cathode terminal is connected to the collector terminal of the second transistor, and a third resistor whose collector terminal is connected to the anode terminal of the diode. And a third resistor connected between the emitter terminal of the third transistor and ground, the laser diode is connected to the collector terminal of the second transistor of the second resistor. The diode side is connected as a cathode terminal between the side where the collector terminal of the first transistor of the first resistor is not connected and the anode terminal of the diode. A pulse signal for causing the laser diode to emit light is input from the anode terminal, and the pulse signal for the first and second transistors is input. The base terminal of the Njisuta first and second switching signal for switching the supply to the laser diode of the pulse signal is input respectively,
The bias current is detected by measuring an emitter voltage of the third transistor.

Further, the first switching signal is a signal for not supplying the pulse signal to the laser diode, and the second switching signal is the supply of the pulse signal to the laser diode. Is a signal for performing.

(Operation) In the present invention configured as described above, when the laser diode emits light, all the current flowing between the collector and the emitter of the third transistor flows to the laser diode, so that the other current in the circuit is reduced. The bias current flowing in the laser diode is detected only by measuring the emitter voltage of the third transistor, without taking into account the components.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a laser diode drive circuit of the present invention.

In this embodiment, as shown in FIG. 1, a constant current source 1
A transistor TR1 and a second transistor TR2 which are first transistors whose emitter terminals are connected to the constant current source 1, and a resistor R1 which is a first resistor connected to the collector terminal of the transistor TR1. A resistor R2, which is a second resistor connected to the collector terminal of the transistor TR2, a diode RC whose cathode terminal is connected to the collector terminal of the transistor TR2, and a third resistor whose collector terminal is connected to the anode terminal of the diode RC. A transistor TR3 which is a transistor, a resistor R3 which is a third resistor connected between the emitter terminal of the transistor TR3 and the ground, a side of the resistor R2 where the collector terminal of the transistor TR2 is not connected and a transistor TR1 of the resistor R1. The side where the collector terminal is not connected and the diode RC Diode RC between the node terminal
Laser diode L connected with the side as the cathode terminal
D.

A pulse signal for causing the laser diode LD to emit light is input from the anode terminal of the laser diode LD, and a second terminal for switching the supply of the pulse signal to the laser diode LD is input to the base terminal of the transistor TR2. Of the logic data signal which is a switching signal of the laser diode L of the pulse signal to the base terminal of the transistor TR1 similarly to the logic data signal.
The inverted data signals, which are the first switching signals for switching the supply to D, are input.

The operation of the laser diode drive circuit configured as described above will be described below.

First, when the data signal is at the H level, the transistor TR2 is turned on and the transistor TR1 is turned off.
The F state is entered, and a pulse current is supplied to the laser diode LD by a pulse signal input from the outside, and the laser diode LD emits light.

Next, when the data signal is at L level, the transistor TR1 is in the ON state and the transistor TR2 is in the OF state.
In the F state, the pulse current is not supplied to the laser diode LD even if a pulse signal is input from the outside, and the LD is turned off.

The bias current I _B controlling the output of the laser diode LD is detected by measuring the emitter voltage at the emitter terminal of the transistor TR3. The bias current flowing between the collector and the emitter of the transistor TR3 is , The diode RC does not flow into the resistor R2 but only into the laser diode LD.

Therefore, the emitter voltage measured at the emitter terminal of the transistor TR3 becomes equal to the voltage of the bias current I _B flowing through the laser diode LD, and it is necessary to carry out the calculation work in consideration of other components in the circuit. Disappears.

[0031]

As described above, in the present invention,
When a diode is provided between the second resistor and the collector terminal of the third transistor and the laser diode emits light,
Since all the current flowing between the collector and the emitter of the third transistor is configured to flow in the laser diode, only by measuring the emitter voltage of the third transistor without considering other components in the circuit, Bias current flowing in the laser diode can be detected, and power consumption can be reduced.

Further, since it is not necessary to provide additional capacitance, there is no change in bias due to the mark ratio.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a laser diode drive circuit of the present invention.

FIG. 2 is a circuit diagram showing a configuration example of a conventional laser diode drive circuit.

FIG. 3 is a diagram showing characteristics of the laser diode LD shown in FIG.

FIG. 4 is a circuit diagram showing another configuration example of a conventional laser diode drive circuit.

[Explanation of symbols]

 1 constant current source LD laser diode R1 to R4 resistance RC diode TR1 to TR3 transistor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04B 10/14 10/04 10/06

Claims (2)

[Claims] 1. A constant current source and a laser diode that emits light in response to a pulse signal input from the outside. The supply of the pulse signal to the laser diode is controlled and the control voltage is input from the outside to control the supply of the pulse signal. A laser diode driving circuit for controlling a bias current flowing through a laser diode, wherein first and second emitter terminals are connected to the constant current source.
A first resistor connected to the collector terminal of the first transistor, a second resistor connected to the collector terminal of the second transistor, and a cathode connected to the collector terminal of the second transistor. A diode having a terminal connected thereto, a third transistor having a collector terminal connected to an anode terminal of the diode, and a third resistor connected between an emitter terminal of the third transistor and ground, In the laser diode, the side of the second resistor to which the collector terminal of the second transistor is not connected and the side of the first resistor to which the collector terminal of the first transistor is not connected are The diode side is connected as a cathode terminal between an anode terminal and the anode terminal of the laser diode. Pulse signal for causing the laser diode to emit light is input from the first and second switching signals for switching the supply of the pulse signal to the laser diode to the base terminals of the first and second transistors. Respectively, and the bias current is detected by measuring the emitter voltage of the third transistor. 2. The laser diode drive circuit according to claim 1, wherein the first switching signal is a signal for not supplying the pulse signal to the laser diode, and the second switching signal. Is a signal for supplying the pulse signal to the laser diode.