JPS63254778A - Semiconductor-laser driving system - Google Patents

Abstract

PURPOSE:To improve the control accuracy of an optical output, by providing a nonlinear D/A converter in a feedback loop for obtaining the constant optical output. CONSTITUTION:A nonlinear D/A converter in a feedback loop for obtaining a constant optical output is provided. Namely, when it is considered that a digital code is an input into the D/A converter and a forward current IF is an output, the D/A converter 10, in which the quantizing width of a part where the current IP does not reach a laser oscillating current Ith is large and the quantizing width is small when the current IP is higher than the current Ith, is used. Thus the quantizing width of an optical output P0 can be made small. Even if the D/A converter having the same number of bits is used, highly accurate control can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for driving a semiconductor laser.

<Prior art> A semiconductor laser obtains laser light by passing a forward current IF through a type of PN junction, and the IF and optical output P are
The relationship with o is not linear; as IF is increased, laser oscillation begins at a certain current 1th, and thereafter, as IF increases, laser light output P increases.

will also increase. This characteristic is shown in FIG.

However, the ratio of change in Po to change in Ip, called 1th or differential efficiency η, is not constant and changes depending on the ambient temperature and the individual.

In order to drive such a semiconductor laser with a constant optical output, a method shown in FIG. 3 is generally used. 1 is a semiconductor laser, which is driven by a current source 4 controlled by the output of an amplifier 3; The light output is also monitored by a photodiode 2 and converted into a voltage by a resistor 5 in FIG. This voltage is input to the amplifier 3, and the reference voltage Vref is applied to one input of the amplifier 3, which constitutes a negative feedback loop as a whole, and the way the light is output is a constant determined by the reference voltage. Becomes a value.

Although FIG. 3 shows a configuration for constantly obtaining a constant light output, in certain applications, it becomes necessary to turn on and off the constant light output at high speed. In that case, for example, the configuration shown in FIG. 4 is used. FIG. 4 shows the configuration of FIG. 3 with the addition of a sample and hold circuit composed of an analog switch 7 and a capacitance 8, a high-speed current switch 6, and a backup amplifier 9. The operation of FIG. 4 is as follows. First, the current switch 6 is turned to the right side and the switch 7 is turned on to obtain a constant optical output defined by the reference voltage Vref. Next, turn 7 into the oven.

At this time, since the voltage is held by the hold capacitor 8, the current that drives the semiconductor laser does not change.

Then, the current switch 6 is turned on and off at high speed to turn on and off the optical output. In the case of the configuration shown in FIG. 4, since the voltage held by the capacitor 8 is an analog quantity, it is difficult to maintain a constant value for a long time. The following driving method shown in FIG. 5 was designed to keep the optical output constant over a long period of time.

In FIG. 5, an up/down counter II, a D/A converter 10, and an oscillator 12 are introduced, except for the sample-and-hold section and buffer amplifier of FIG. 4, and the amplifier 3 functions as a comparator. Up/Down-Counter 11
When the output of comparator 3 is 'HIGH', the output pulse of oscillator I2 is counted up and conversely 'to'
In the case of w', the Q count result that counts down is D/A
It is converted into an IF by the converter 0 and drives the laser diode 1. In FIG. 5, when the current switch 6 falls to the right, a negative feedback loop is formed as a whole, and the optical output becomes a value defined by the reference voltage Vref. 'In the case of FIG. 5, an error equivalent to one force count occurs, but since there is a part that becomes a digital code in the feedback loop, there is no problem even when holding for a long time. However, as mentioned above, since the IF-Po characteristics of the semiconductor laser are not 17 degrees near, a high-resolution D/A converter is required to obtain a constant optical output with high precision.

<Problems to be Solved by the Invention> In the system shown in FIG. 5, laser oscillation does not occur if the IP is less than 1th due to the ll--Po characteristics of the semiconductor laser. Therefore, a region where the IF is 1th or less cannot be used for control. Therefore, the quantization error of PO becomes larger than the quantization error of IF. The present invention aims to solve the above-mentioned problems and improve the control accuracy of Po.

Means to solve problems〉 The D/A converter shown in FIG. 5 is made nonlinear.

<Example> To explain in more detail, when considering the digital code as the input of a D/A converter and the IF as the output, the quantization width of the part where IF is less than Ith is large, and IF is 1th.
In the above, a D/A converter having a small quantization width is used. For example, with the same number of bits as the D/A converter with the characteristics shown in FIG. The relationship between F is shown in Figure 1 (,)
When using a D/A converter like this, the quantization width of Po can be made smaller than when using a D/A converter having the characteristics shown in FIG. Even if an A converter is used, more accurate control is possible. Figure 6 (
b) is the relationship between the digital code and the optical output Po when a conventional linear/A converter is used;
Since the digital code portion corresponding to h or less does not contribute much to the optical output, the actually effective codes are limited and the effective resolution is lowered. On the other hand, in the case of FIG. 1(b), the number of digital codes corresponding to 1th or less is small due to its nonlinearity, and the number of codes that can be effectively used for controlling the optical output PO is large.

<Effects of the Invention> Since a system with the same performance can be constructed using a simpler and easier to implement low-resolution D/A converter, the cost of the product can be reduced. Furthermore, it becomes easier to integrate the system into an integrated circuit.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of the relationship between the digital code, forward current IF, and optical output Pa when the nonlinear A/A converter is used in FIG. 5, and FIG. A diagram showing the relationship between forward current IF and optical output Po, FIG. 3 is a diagram showing an example of a semiconductor laser drive circuit block for obtaining constant optical output, and FIG. 4 is a diagram showing the relationship between forward current IF and optical output Po. Figure 5 shows an example of a semiconductor laser drive circuit block for high-speed output switching.
The figure shows an example of a semiconductor laser drive circuit block for performing high-speed switching with a constant optical output, in which a D/A converter is inserted with loose optical output setting feedback. When using A converter,
Digital code, forward current IF and optical output Pa
FIG. Explanation of symbols 1... Semiconductor laser (laser diode), 2...
Photodiode for optical output monitor, 3...Amplifier, 4...
...Current source, 5...Resistor, 6...High speed current switch, 7...Analog switch, 8...Capacitance for voltage hold, 9...Buffer amplifier, 10...D/ A
Converter, 11...Up/down counter, 12.
...Oscillator. Agent Patent Attorney Takeshi Sugiyama (and 1 other person) Figure 2 Figure 3! $4 Figure z Figure 5 Child°゛Igital Food Q) Army (b) 6 Figure

Claims (1)

[Claims] 1. A semiconductor laser driving system characterized by having a nonlinear D/A converter in the feedback loop for obtaining a constant optical output.