JPS62122189A - Driving circuit for semiconductor laser - Google Patents

Abstract

PURPOSE:To form the titled driving circuit, which is superior in responsibility and also, is simpler in circuit configuration and at a lower cost by a method wherein, when the peak value of the photo output of the semiconductor laser fluctuates, the level of the DC bias current is made to change in such a way that the peak value becomes constant. CONSTITUTION:The luminous output of a semiconductor laser LD is received by a pin photo diode PD and is converted into a voltage by a variable resistor VR. The voltage between both ends of this variable resistor VR becomes a comparison voltage at a connecting part B. When this comparison voltage becomes larger compared to the reference voltage at a connecting part A, the output of an operational amplifier OPI is increased. Hereby, the base voltage of a transistor Q1 is boosted and a DC bias current Idc is decreased. With this, the driving current Id of the semiconductor laser LD including the DC bias current Idc is also decreased. When the driving current Id is decreased, the photo output Po of the semiconductor laser LD is reduced. Thus, the photo output Po of the laser LD is controlled to be constant.

Description

[Detailed description of the invention] (Technical field) The present invention relates to a semiconductor laser drive circuit.

(Prior Art) FIG. 3 is an optical output-forward current characteristic diagram of a semiconductor laser. In FIG. 3, the horizontal axis represents the forward current If of the semiconductor laser.
, and the vertical axis indicates the optical output Po of the semiconductor laser. a is the temperature of the case housing the semiconductor laser (ambient temperature) T is 0°C, b is 25°C, C is 50°
This is the characteristic of the optical output Po of the semiconductor laser when C. r
tht indicates the threshold current rth of the semiconductor laser at 0° C., I tht at 25° C., and ■th3 at 50° C., respectively.

Incidentally, it is necessary to drive the semiconductor laser so that the peak value of its optical output PO remains constant. However, as is clear from FIG. 3, the optical output-forward current characteristic of the semiconductor laser is nonlinear. Further, in the semiconductor laser, the threshold current 1th and the differential efficiency (-ΔPo/ΔIf) change as the ambient temperature changes. Therefore, it is necessary to take such changes into account when driving a semiconductor laser.

FIG. 4 is a diagram for explaining a case where an optical output Po having a constant peak value is obtained by driving a semiconductor laser with a constant drive current Id in advance using a conventional semiconductor laser drive circuit. In FIG. 4, the semiconductor laser has a drive current of 1 d whose level is constant along the time axis.
is driven by.

This drive current 1d is in the form of a pulse current.

A semiconductor laser driven by a conventional drive circuit has a threshold current Ith and a characteristic of optical output Po as shown by a characteristic line d with respect to optical output Po - forward current If. At the driving current 1d, this semiconductor laser outputs an optical output Po whose peak value is constant along the time axis.

In this drive circuit, the drive current for the semiconductor laser is 1d.
, no DC bias current is superimposed on it. Therefore, the optical output PO of the semiconductor laser has poor rise and fall characteristics. Also, optical output P with respect to drive current Id
It is also inferior to the responsiveness of o. Furthermore, since it is necessary to apply a large drive current Id in the form of a pulse current, there is a problem that the cost of the drive circuit is high.

FIG. 5 shows a fourth example of a conventional semiconductor laser drive circuit.
FIG. In FIG. 5, parts corresponding to those in FIG. 4 are given the same reference numerals. In FIG. 5, the drive current Id for driving the semiconductor laser is obtained by superimposing a DC bias current Idc and a pulse current 1p. The maximum level of the DC bias current Idc corresponds to the threshold current 1th of the semiconductor laser. A drive circuit that provides such a drive current Id to the semiconductor laser has excellent responsiveness because the DC bias current 1dc corresponds to the threshold current 1th of the semiconductor laser.

Furthermore, since the pulse current rp may be made small, the cost of the drive circuit is reduced.

However, in order to keep the optical output PO constant, it is necessary to change not only the DC bias current IdC that constitutes the drive current 1d but also the pulse current II). Changing the pulse current ip in this way not only makes the drive circuit very complicated, but also has the problem that it is also very difficult to adjust each current level of the DC bias current 1 dc and the pulse current rp.

(Object of the Invention) It is an object of the present invention to solve these problems and to improve responsiveness, and to provide a simple circuit configuration that is inexpensive and consistent.

(Structure of the Invention) The semiconductor laser drive circuit of the present invention includes a first circuit means for generating a DC bias current that is at a maximum level or below a threshold current of the semiconductor laser, and a superimposed level of the DC bias current that is equal to or lower than the threshold current of the semiconductor laser. and a second circuit means for generating a pulse current having a constant level as well as a pulse current exceeding a threshold current. When the peak value of the laser light output fluctuates,
The level of the DC bias current is varied so that its peak value remains constant.

(Embodiment) FIG. 1 is a circuit diagram of a semiconductor laser drive circuit according to an embodiment of the present invention. In FIG. 1, LD is a semiconductor laser. The semiconductor laser LD is housed in a case C together with the bin photodiode PD, with its cathode connected to the cathode of the bin photodiode PD. The cathode of the semiconductor laser LD and the cathode of the bin photodiode PD are both grounded.

The transistor Q1 is configured to generate a collector current as a DC bias current rdc.

The transistor Q2 is configured to generate a collector current as a pulse current Ip. Both transistors Ql,
The collectors of Q2 are commonly connected. The anodes of the semiconductor laser LD are connected to the common connection point. The DC bias current WEIdc and the pulse current Ip are superimposed at their common connection point and are applied to the semiconductor ray 'J'LD as a drive current 1d (forward current ■r).

The semiconductor laser LD changes its light emission output in response to its drive current 1d. The light emission output from this semiconductor laser LD is received by a bin photodiode PD 4-L.

The cathode of the semiconductor laser LD is connected to the inverting input terminal (-) of the operational amplifier OP via a resistor R1. This connection point is called A. The anode of the bin photodiode PD is connected to the normal input terminal (+) of the operational amplifier OP. This connection point is called B.

A variable resistor V is connected to the anode of the bin photodiode PD.
R is connected.

The output terminal of operational amplifier OP is connected to the base of transistor Ql via resistor R2.

The voltage applied to the inverting input terminal (-) of the operational amplifier OP, that is, the voltage at the connection point A, becomes the reference voltage. The voltage applied to the non-inverting input terminal (+) of the operational amplifier OP, that is, the voltage at the connection point B, becomes the comparison voltage.

In such a configuration, the light emission output of the semiconductor laser LD is received by the bin photodiode PD, and the light emission output from the semiconductor laser LD is received by the variable resistor V.
It is converted into voltage at R. The voltage across the variable resistor VR becomes the comparison voltage at the connection point B. Therefore, for example, as the previous output Po of the semiconductor laser LD increases, this comparison voltage 4 increases. When this comparison voltage becomes larger than the basic voltage at the connection point A, the output of the operational amplifier OP becomes larger. This results in transistor Q
The base voltage of 1 increases, and the DC bias current Idc decreases. Then, the drive current Id of the semiconductor laser LD including the DC bias current Idc also decreases. Drive current I
When d decreases, the prior output Po of the semiconductor laser LD decreases. In this way, the optical output Po of the semiconductor laser LD is controlled to be constant.

The first circuit means described in the claims above includes a transistor Q1, a bin photodiode PD, a variable resistor VR, an operational amplifier OP, and the like. In this case, the first circuit means sets the DC bias current IdC so that the maximum level of the DC bias current Idc is equal to or lower than the threshold current rth of the semiconductor laser LD.

An emitter resistor R3 is connected between the emitter of the transistor Q2 and the DC power supply +Vcc.

A parallel circuit of a Zener diode ZD and a resistor R4 is connected between the base of the transistor Q2 and the DC power supply +Vcc. The base of the transistor Q2 is connected to the collector of the transistor Q3 via a resistor R5. A pulse voltage application terminal IN is connected to the base of the transistor Q3 via a resistor R6.

In such a configuration, a pulse voltage is applied to the base of the transistor Q3 via the pulse voltage application terminal IN. Transistor Q3 turns on and off in response to this pulse voltage. When transistor Q3 is turned on, Zener diode ZD is made conductive. In this case, since the voltage C across the conductive Zener diode ZD is constant, the level of the emitter current flowing through the emitter resistor R3 is constant. Furthermore, if the base current is ignored, the emitter current becomes equal to the pulse current Ip, which is the collector current. Therefore, the level of the pulse current ■p remains constant. Such a transistor Q2. Q3, Zener diode ZD, etc. constitute the second circuit means described in the above claims.

This second circuit means controls the pulse current Ip so that the superimposition level when the pulse current Ip is superimposed on the DC bias current taC exceeds the threshold current 1th of the semiconductor laser LD.
The level of p is set.

FIG. 2 is a diagram corresponding to FIG. 4 and FIG. 5 according to an embodiment of the present invention. As shown in FIG. 2, the DC bias current 1dc is less than the threshold current 1th of the semiconductor laser LD. In addition, the pulse current Ip is the DC bias current ■dC
When the current is superimposed on the threshold current t'th, the threshold current t'th is exceeded. Furthermore, even if the relationship between the optical output Po and forward current (f) of the semiconductor laser LD changes due to the influence of the ambient temperature as shown in Figure 3, the DC bias current Idc will not respond to the change. Therefore, the optical output Po from the semiconductor laser LD is always constant.

(Effects of the Invention) As described above, according to the present invention, since it has the above configuration,
As the ambient temperature of the semiconductor laser changes, the DC bias current changes accordingly, so that the semiconductor laser always outputs a constant optical output.

In addition, the DC bias current is set to be below the threshold current of the semiconductor laser, and when the pulse current is superimposed with the DC bias current, the superimposition level is set to be above the threshold current of the semiconductor laser, so the levels of the DC bias current and pulse current are adjusted. Not only is this simple, but the cost is also low because the circuit configuration is simple.

[Brief explanation of drawings]

1 is a circuit diagram of a semiconductor laser drive circuit according to an embodiment of the present invention, FIG. 2 is a diagram for explaining the optical output of a semiconductor laser by the drive circuit of FIG. 1, and FIG. 3 is a diagram of a semiconductor laser drive circuit according to an embodiment of the present invention. Characteristic diagrams of optical output versus forward current, FIG. 4 is a diagram corresponding to FIG. 2 for a conventional drive circuit, and FIG. 5 is a diagram corresponding to FIG. 2 for another conventional drive circuit. Ql, Q2 are transistors, LD is a semiconductor laser, OP
is an operational amplifier.

Claims (1)

[Claims] (1) A first circuit means for generating a DC bias current whose maximum level is below the threshold current of the semiconductor laser, and a pulse current whose superimposition level with the DC bias current exceeds the threshold current of the semiconductor laser and whose level is constant. and a second circuit means for generating a pulse current, a current obtained by superimposing the pulse current on the DC bias current is applied to the semiconductor laser as a drive current, and when the peak value of the optical output of the semiconductor laser fluctuates, the peak value A semiconductor laser drive circuit characterized in that the level of the DC bias current changes so that the value remains constant.