JPS6018982A - Driving system for semiconductor laser - Google Patents

Abstract

PURPOSE:To control bias current to the optimum value at each temperature even when the temperature is changed by a method wherein a means, by which pulse current becomes a resultant value consisting of a component, wherein the pulse current is proportioned to the bias current, and a component, wherein the pulse current is proportioned inversely to the bias current, is provided. CONSTITUTION:Bias current IB, which flows on a transistor TR Tr4, and current IP1, which flows on a transistor TR Tr3, have a proportional relation to each other, while current IP2, which flows on a transistor TR Tr5, and the bias current IB have an inversely proportional relation to each other. In this case, as pulse current IP is the sum of the current IP1 and the current IP2, the current IP is proportioned to the current IB during a time when the current IB is larger in case the characteristics between the bias current and the pulse current of a semiconductor laser LD are as illustrated in the diagram, but when the current IP2 is zeroed, the inclination between the current IB and the current IP changes, the current IP is indicated by the sum of the current IP1 and the current IP2, and the optimum relation between the bias current and the pulse current is indicated. Accordingly, when resistors R3', -R5' and R7', resistors R8-R10 and the reference voltage Vref have been set so as to match with the characteristics of the LD, the bias value at each temperature can be controlled to the optimum value.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention monitors a part of a single output, and controls the pulse current and bias current of a semiconductor laser (hereinafter referred to as LD) based on this monitor output level to control the temperature. This LD drive method is related to the LD intensity adjustment circuit that automatically controls the peak value of the light output to be constant even when the temperature changes, and the circuit size is small and the LD drive method can control the asymmetry flow to the optimum value at each temperature. 0(b) Technical Background Figure 1 shows the current flowing through the LD (I
) and the optical output (P), and FIG. 2 is a characteristic diagram showing the relationship between the optimum bias current (I n , ) and the pulse current (Ip) divided into each temperature.

When the temperature changes, the threshold current of the LD changes as shown in Figure 1, and the differential quantization efficiency of optical output (P) (ratio of change in optical output to change in LD applied current) also changes as shown in Figure 1Q. Change. For this reason, in the LD intensity modulation circuit, APC
(Automatic Power Control)
A circuit is used to control the peak value of the BU0 output to be constant as shown in A of FIG. 1 even if the temperature changes. As shown in the example in Figure 1, the current value of the LD when this constant value is set is 26 mA at 0°C, 30 mA at 25°C, and 5 mA at 25°C.
At 0°C, it is 48mA. Normally, the bias current is set as an optimum value to be a certain value lower than the threshold current or a certain ratio to the threshold current. In Figure 1, the dark value current is 11mA at 0℃, 16mA at 25℃, and 29mA at 50℃.
It is A. Therefore, the optimum bias current is 1 m for example.
If the value of A is low, the bias current is 10 mA at 0°C, 25
It becomes 15mA at ℃ and 28mA at 50℃. Therefore, the LD current (I) and the bias current (IB)' (r minus the value 0
The pulse current (Ip) is 16 mA at °C, 15 mA at 25 °C, and 20 mA at 50 °C. This bias current (I n
) and pulse current (II)) is shown in the second i.
LfA).

As can be seen from the second m(A), this relationship is not a straight line, but the pulse current (Ip) curves in the -F direction at lower temperatures. Furthermore, depending on the characteristics of the LD, the relationship between the bias current 1 (In) and the pulse current (Ip) may increase only a small amount at high temperatures, as shown in FIG. 2(B). Therefore, when controlling the peak value of the optical output to be constant even when the temperature changes, the bias current (In) should be set to the optimum value [This is the pulse current shown in Figure 2 (A).
) The change in temperature must be changed accordingly to match the characteristics of (B). This is because the bias current is different from the dark value current c) Conventional technology and problems Conventionally, in order to perform the above control, a temperature sensor was used to control the pulse current and bias current independently to the optimum value. However, this method requires a separate temperature IW sensor and circuits for controlling the pulse current and bias current, which has the disadvantage of increasing the circuit scale. There is also a method which will be explained below, but this also has the drawbacks which will be explained below.

Figure 3 is a circuit diagram of a conventional example of LD intensity variation [Route 91, No. 4]
The figure is a characteristic diagram showing the relationship between the bias force, the current (In), and the pulse current (Ip) of the circuit of FIG. 3.

In FIG. 3, l is an LD, R, .about.7 are resistors, L is a choke, Tr, .about.Trs are transistors, Vc 11 is a constant voltage, and -■ is a negative DC voltage.

The voltage Va in Figure 3 is the output voltage obtained by receiving the LDI output V:-t' with an element and comparing it with a reference voltage using a comparator in order to keep the optical output constant in the APC circuit. As the output decreases, the voltage Va increases.

In proportion to this voltage Vat, the LDI bias current Ie flowing through the transistor 1r4 and the transistor Trs
The pulse current Ip flowing through changes.

In the APC circuit, if the optical output is set to a constant value as shown in FIG. 1(a), the current (I) flowing through the LDI increases as the temperature rises, as explained earlier. (50℃48mA, 25℃30
mA, 26 mA at 0°C) This current I, for example, at 50°C, the optimum bias current (In) of 28 mA and the pulse current (Ip)
+20/mA, and when the temperature changes [7], resistors R8 to Rg are adjusted to match the characteristic @i shown in Figure 2.
Assuming that R7 is adjusted, in the circuit shown in Figure 3, the bias current (In) and the pulse current Ip are proportional to the voltage Va, so the bias current (ITI) and the pulse current (I
The relationship p) is a proportional relationship as shown in Figure 4 t, and the second
Compared to Figure (A), the pulse current becomes smaller at low temperatures (e.g. 0°C), and the bias current does not reach its optimal value, and compared to Figure 2 (R), the pulse current increases at high temperatures. It is not the optimal value.

The circuit shown in FIG. 3 has such a drawback.

(dl) OBJECTS OF THE INVENTION In view of the above-mentioned drawbacks, it is an object of the present invention to provide an LD driving system which has a small circuit scale and is capable of controlling the bias current to an optimum value at each temperature.

(e) Disadvantages of the Invention In order to achieve the above object, the present invention, in an LD intensity modulation circuit that controls the peak value of optical output to be constant, divides the pulse current into a component proportional to the bias current and a component inversely proportional to the bias current. The present invention is characterized in that the bias current can be controlled to an optimum value in response to temperature changes.

(f) Embodiment of the Invention An embodiment of the invention will be described below with reference to the drawings. Fifth
Figure 6 is a circuit diagram of an LD intensity modulation circuit according to an embodiment of the present invention.
The figure is a characteristic diagram showing the relationship between the bias current (IB) and the pulse current (IB) of the circuit of FIG.

Items in FIG. 5 that have the same functions as those in FIG. 3 are indicated by the same symbol.

"rs + T r6 is a transistor, and Ra to R+o are resistors, which together constitute a differential amplifier. Nu R3'
-4,', R7' is a resistance, and Vref is a reference voltage.

The bias current (I B) flowing through the transistor Tr4 and the current Ip1 flowing through the transistor Tr s are in a proportional relationship with the voltage Va explained earlier, and the bias current (I
The relationship between n) and current IP+ is shown in Figure 6 (A) and (B).
pl) As shown, there is a proportional relationship. Also transistor'r
The relationship between the current Ip2 flowing through r5 and the voltages obtained by subtracting the reference voltage Vref from the voltage Va does not change while the subtracted voltage is small, but becomes an inversely proportional relationship when it exceeds that value. That is, the bias current -(In) has the same relationship as above. Figure 2 (A) (B) by changing the reference potential Vref
Characteristics [The relationship when they are matched is shown in Figure 6 (A)
B) Ip.

It is shown in Since the pulse current Ip is the sum of the current Ip+ and the current IPy, in the case of Fig. 6 (A), while the bias current is large (when the temperature is high), the pulse current ITI is proportional to the bias current. The slope of the pulse current ILI changes near where the current IT't becomes 0, and when the bias current becomes less than this, it becomes the sum of the pulse current Ip+ and Ipt, and the optimum bias current and pulse current shown in Fig. 2 (A) are obtained. The relationship t is approximated.

In the case of Fig. 6(B), the pulse current Ip is proportional to the bias current while the bias current is small, but as it becomes large, the amount of increase decreases, and the optimum bias current and pulse current shown in Fig. 2(B) are obtained. Approximate the relationship. Therefore, using this characteristic, resistors R3' to R6' are set.

If R7', resistors R8 to 11o, and reference voltage Vref are determined for each LD so as to match the characteristics shown in FIG. Even if the temperature changes, the bias current at each temperature can be controlled to the optimum value. Jin's circuit shown in FIG. 5 only adds a differential amplifier to the circuit G shown in FIG. 3, and its circuit scale is smaller than the conventional circuit that uses temperature control.

(g) Effects of the Invention As explained in more detail, according to the present invention, when controlling the peak value of the output at a constant value even when the temperature changes, the circuit scale is small and the bias current at each temperature is reduced. This has the effect of being able to control it to an optimal value.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between the current flowing through a semiconductor laser and the optical output when the temperature changes, Figure 2 is a characteristic diagram showing the relationship between the optimum bias current and pulse current at each temperature, and Figure 3 is a characteristic diagram showing the relationship between the optimum bias current and pulse current at each temperature. is a circuit diagram of a conventional semiconductor laser modulation circuit, FIG. 4 is a characteristic diagram showing the relationship between bias current and pulse current of the circuit in FIG. 3, and FIG. 5 is a diagram of a semiconductor laser according to an embodiment of the present invention. FIG. 6, a circuit diagram of the intensity modulation circuit, is a characteristic diagram showing the relationship between bias current and pulse current of the circuit of FIG. In the figure, 1 is a semiconductor laser, R+ -Rho + Rs'
~RB'+R7' is a resistor, Try-Tr6 is a transistor, and L is a choke. Akira 1121-1. . 7□2''4 Bay 7th'6L geL (xgrain-/v47λ' current = 18 3rd figure - 0 bias voltage jt (Ia) -

Claims (1)

[Claims] A part of the optical output is monitored, and this monitor output level controls the laser pulse current and current of the laser diode so that the peak value of the optical output remains constant even when the temperature changes. In the semiconductor laser intensity modulation circuit to be controlled, a means is provided to make the Luss current a composite current of a component proportional to the current and a component inversely proportional to the bias current at each temperature G. A semiconductor laser drive system characterized by the ability to control the current to an optimum value.