JPS5989482A - Bias circuit for semiconductor laser - Google Patents

Abstract

PURPOSE:To limit bias currents flowed through the semiconductor laser by using control voltage for controlling bias currents for generating discharge voltage. CONSTITUTION:When control voltage Vc is applied to amplifiers 3, 6, bias voltage Vb and discharge voltage VL are each generated. When the gains of the amplifiers 3 and 6 are equal, voltage Vb and VL are equal, a transistor 7 is turned OFF, and voltage Vb is not limited by voltage VL. When voltage Vb exceeds Vb+Vbe for some causes though voltage Vc is constant, the transistor 7 is turned ON, and voltage Vb does not rise further. Bias currents I'b flowing at that time reaches (VL+ v'be)/RL. When the gains of the amplifiers 3 and 6 are equal, bias currents Ib are limited when they increase by v'be/RL (RL is the emitter resistance of a transistor 2).

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to semiconductor lasers (hereinafter referred to as
This relates to a bias circuit for a LD (abbreviated as LD).

Conventional Structure and Problems When using an LD as a light source, the bias circuit of the LD requires measures to limit the bias current in order to protect the LD from excessive current due to surges and the like. The LD bias circuit as described above will be described below with reference to the drawings.

FIG. 1 shows an example of the configuration of a conventional LD bias circuit. In Figure 1, 1 is an LD, 2 is a transistor that flows a bias current Ib to LDl, 3 is an amplifier that generates a bias voltage b from a controlled voltage source, and 4 is a constant voltage L
The operation of the LD bias circuit configured as shown in FIG. 5 will be explained below.

Control voltage ■ from the control voltage source to amplifier 3. When Vb is applied, a bias voltage vb is generated and applied to the base electrode of the transistor 2. At this time, a bias current flows. If this bias voltage vb is lower than the constant voltage vL created by the Zener diode, the diode 5 is reverse biased, so it is in the off state and the bias voltage Vb'fl'1ffitJ is limited. Never. However, when vb becomes higher than the constant voltage vL due to some reason such as external noise or surge, diode 5 is forward biased and turns on, and bias voltage vb never exceeds the inherent potential difference of diode 5. Therefore, no excessive current flows through LDl.

The resistor R and the amplifier 3 are used to prevent the voltage from affecting the control voltage source when the bias voltage vb rises instantaneously. Note that the amplifier is used to arbitrarily change the voltage level and is not an essential requirement.

In the above configuration, when determining the amount of current to limit,
Variations in the threshold current of LDl pose a problem. The threshold current differs depending on the structure of LD1. Current commercially available devices have a variation of 20 to 100 mA. Further, for LDl having the same structure, a variation of 1.4 to 1.7 times is common. For this reason, the magnitude of the limiting current must be determined depending on each LDl used. Therefore, in the conventional case, it was necessary to change circuit constants such as replacing the Zener diode 4, and adjustment and maintenance were time-consuming.

OBJECTS OF THE INVENTION In view of the above drawbacks, the present invention provides an LD bias circuit that can limit the bias current Ib regardless of variations in the threshold current of the LD1 used.

using a common limited voltage source to control the LD, and interposing an amplifier between the power source and the base of the transistor connected in series with the LD;
A limiting voltage generating means having a diode functional element as a component is connected in parallel to the amplifier, and when the base potential of the transistor becomes higher than a predetermined value, the diode functional element is operated to raise the base potential. It is a restriction.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 shows the configuration of a circuit in one embodiment of the present invention. In FIG. 2, the same elements as in FIG. 1 are given the same numbers. 1 is an LD, 2 is a transistor, 3 is an amplifier circuit, 6 is a second amplifier, and 7 is a transistor.

The amplifier 3 generates a bias voltage vb from a control voltage vc applied from a control voltage source. This bias voltage vb is applied to the base electrode of transistor 2. A bias current flows through LDl in accordance with a bias voltage vb applied to the base electrode of transistor 2.

The amplifier 6 receives the control voltage V. Create the limit voltage vL from . The emitter electrode of the transistor is connected to the output terminal of the amplifier 3, and the base electrode is connected to the output terminal of the amplifier 6.

Regarding the LD bias circuit configured as above,
Let's explain its operation. When the control voltage vc is applied to the amplifier circuit 3 and the amplifiers, a bias voltage b and a limiting voltage vL are respectively created. Considering the case where the gains of amplifiers 3 and 6 are equal, the bias voltage vb and the limit voltage vL are equal. At this time, the transistor 7 is in an off state, and the bias voltage vb is not limited by the limit voltage vL.

Therefore, a bias voltage vb is applied to the base electrode of the transistor 2, and a bias voltage corresponding to that voltage flows to ff1I and LD1. If the potential difference between the base and emitter of the transistor 20 is Z'be, and the emitter resistance of the transistor 2 is RL, then the bias current flowing through the LDl is more than = (vb Z'be)/RL.

Here, the control voltage V. When bias voltage vb exceeds Cvb +vbe ) for some reason even though Cvb is constant, transistor 7 is turned on, so bias voltage vb does not increase any further. If the potential difference between the base and emitter of the transistor 70 is Z''be, the bias current 'b flowing at this time becomes 'b - (vL+Z/b8]/RL. When the gains of amplifiers 3 and 6 are equal, the bias current When "b e / u L increases, the limiting current value of the control bias current Ib is determined by the gains of amplifiers 3 and 6, the emitter resistance RL of transistor 2, the base-emitter potential difference vbe of transistor 2, and the base of transistor 7.・It goes without saying that it is determined by the potential difference between the emitters, v', 8, etc.

Furthermore, the base electrode of transistor 2 and the transistor 7
It is also possible to set a limiting current value that is higher than the bias current by inserting one or more elements 5 that maintain a specific potential, such as a diode, Zener diode, or transistor, between the emitter electrodes of the bias current.

FIG. 3 shows a second embodiment of the invention,
Elements common to those in FIG. 2 are given the same numbers. 1 is LD,
2 is a transistor, 3 is an amplifier circuit, and 5 is a diode.

Resistors R1 and R2 divide the control voltage v0 and connect the diode 5.
Form a voltage divider circuit to apply to the voltage. bias voltage v
Even if b becomes high for some reason, the control voltage V. Since the voltage divided by resistors R1 and R2 remains constant, the diode 6 becomes forward biased, and current flows to the ground via the diode 5 and resistor R2, increasing the bias voltage ■ The increase in b is limited. At this time, the current flowing through the resistor R2 increases and the divided voltage increases, but this does not pose any practical problem if the resistance value is appropriately selected. Therefore, even with a simple structure as shown in this embodiment, effects close to those of the first embodiment can be obtained.

In addition, 1. In both embodiments, an amplifier 3 is interposed between the current control transistor 2 of the LD1 and the control voltage source, but it is basically unnecessary if a resistor is provided. However, if an amplifier is used, the control voltage can be set more widely.

FIG. 4 shows a third embodiment of the present invention.
Elements common to those in FIG. 2 are given the same numbers. 1 is LD,
2 and 7 are transistors, 3 is an amplifier circuit, 6 is an amplifier circuit,
8 is a photoelectric conversion circuit.

The photoelectric conversion circuit 8 detects the optical output of the LDl and generates an electrical output voltage. 8 is a control voltage generation source, which supplies the control voltage V to the amplifier circuits 3 and 6. is added. Amplifier 3 and amplifier 6 have equal gains.

A reference voltage vr is applied to both. The first amplifier 3 compares and amplifies the reference voltage vr and the output voltage V' of the photoelectric conversion circuit 8, creates a bias voltage vb, and applies it to the base electrode of the transistor 2, thereby controlling the optical output of L'D1 at a constant level. ing. At this time, the Wako output with the high gain of the amplifier 3 becomes stable. When the optical output of LDl is constant, V' is also approximately - constant. If a voltage vf equal to this v′o is applied in advance to the second amplifier, the bias voltage vb and the second
The limiting voltage vL, which is the output of the amplifier 6, becomes approximately equal, and the bias bias voltage vb is limited by the effect described in the embodiment of FIG. In this embodiment, the limiting current value can be changed by changing the constant voltage Vf applied to the amplifier 6, and when replacing the LDl with a different threshold current, the reference voltage vr can be adjusted to obtain the required optical output. All you have to do is set it, and at this time you can set the limit current value at the same time.

The differential quantum efficiency of LIlz also varies like the threshold current, but in LDl of the same structure, there is no variation as much as the threshold current. A function that controls the output light of the LDl so that it is always constant during normal operation, and an abnormality in this control function system suppresses the increase in the bias current of the LDl.
It also has a control function and produces stable LD operation effects.

Effects of the Invention As described above, by using the control voltage for controlling the bias current flowing through the LD to generate the limiting voltage, the required bias current can be obtained regardless of the variation in the threshold current of the LD. It is possible to set the control voltage as shown in FIG.
This can prevent excessive current from flowing through D.

[Brief explanation of the drawing]

FIG. 4 is a configuration diagram of a different embodiment; 1...LD, 2.7...Transistor, 3,6 Amplifier circuit, 4...Zener diode, 5...Diode, 8... ...Photoelectric conversion circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure V Figure 3 Figure 4

Claims (1)

[Claims] (1) A first transistor for causing a bias current to flow through the semiconductor laser by applying a bias voltage to a base electrode, a control voltage source, and a device for supplying a bias voltage to the semiconductor laser. A resistor connected between the transistor and a control voltage source, and a limit voltage generating means connected from the control voltage source to the base electrode via a diode functional element are provided, and the bias voltage exceeds the limit voltage. When a semiconductor laser bias circuit limits an increase in the bias voltage. 3. The semiconductor laser bias circuit according to claim 1, further comprising: a control voltage level adjusting amplifier connected in series with the resistor between the resistor and the control voltage source. (3) The semiconductor laser according to claim 1 or 2, wherein the limited voltage generating means includes a second transistor and a second amplifier, and the base voltage of the first transistor is controlled by the output of the amplifier. bias circuit. (79) The bias circuit for a semiconductor laser according to claim 1, wherein the limit voltage generating means comprises a voltage dividing circuit and a diode, and the diode is connected between the voltage dividing circuit and the base of the first transistor. (5) The control voltage source includes a reference voltage source, a photoelectric conversion output of light emission from the semiconductor laser, and a constant voltage source equal to a control voltage when the semiconductor laser operates with a predetermined optical output;
2. The semiconductor laser bias circuit according to claim 1, wherein the first amplifier receives the reference voltage and the photoelectric conversion output, and the second amplifier circuit receives the reference voltage and the constant voltage.