JPS63254779A - Semiconductor-laser driving system - Google Patents

Abstract

PURPOSE:To improve the control accuracy of an optical output, by providing a D/A converter having an offset function in a feedback loop for obtaining the constant optical output. CONSTITUTION:A D/A converter 10 having an offset function is provided in a feedback loop for obtaining a constant optical output loop. Namely, when it is considered that a digital code is an input into the D/A converter 10 and a forward current IF is an output, the D/A converter system, in which an offset current IOS flows, is used for the current IF. Thus the quantizing width of an optical output PO 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 Ith, and thereafter, as IF increases, laser output PO also increases. This characteristic is shown in FIG.

However, the ratio of change in PO to change in IF, called xth and 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 whole constitutes a negative feedback loop. ◇A reference voltage V ref is applied to one input of the amplifier 3, and the optical output is eventually kept constant as defined 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 buffer ring 9. The operation of FIG. 4 is as follows. First, the current switch 6 is turned on the right side and the switch 7 is turned on to obtain a constant optical output defined by the reference voltage Vref. Next, open 7.

At this time, the voltage is held by the hold capacitor 8, so 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.
Turn off. 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 11, a D/A converter 10, and an oscillator 12 are introduced, except for the buffer amplifier in the sample and hold section 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 it is 'LO'.
If W', count down. The count result is D/A
It is converted into IP by a converter 10 and drives a laser diode l. 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 corresponding to one force count occurs, but since there is a portion 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 near IJ, a high-resolution D/A converter is required to obtain a constant optical output with high precision.

<Problem that the invention seeks to solve> In the system of FIG. 5, fF-P of the semiconductor laser.

According to the characteristics, if IF is less than 1th, laser emission loss will not occur. Therefore, an area with an IP of 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 Pa.

Means for Solving the Problems> The D/A converter shown in FIG. 5 is made to have an offset.

<Example> To explain in more detail, if we consider the digital code as the input of a D/A converter and the IF as the output, even if the digital code becomes zero, the IF is a D△ conversion in which a certain offset current IQs flows. Use the system. For example, when using a D/A converter with the same number of bits as the D/A converter with the characteristics shown in Fig. 6, and the relationship between the digital code and IF as shown in Fig. 1 (,), the quantization width of Po can be made smaller than when a D/A converter having the characteristics shown in FIG. 6 is used, and even if a D/A converter with the same number of bits is used, more accurate control is possible. Figure 6(b) shows the normal D/
Digital code and optical output P when using A converter
Regarding the relationship with O, since the digital code portion corresponding to 1th or less does not contribute to the optical output, the actually effective codes are limited and the effective resolution is lowered. On the other hand, in the case of Figure 1, there is no digital code corresponding to ith or lower (Ios > 1th + Figure 1 (b))
or less (tth > Ios + FIG. 1(C)), so the number of codes that can be effectively used to control the optical output PO increases.

<Effects of the Invention> Since a system with the same performance can be configured 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, the head current IF, and the optical output PO when a D/A converter with an offset is used in FIG. 5, and FIG. A diagram showing the relationship between the head direction current IP of the diode and the optical output Po, FIG. 3 is a diagram showing an example of a semiconductor laser drive circuit block for obtaining a constant optical output, and FIG. Figure 5 is a diagram showing an example of a semiconductor laser drive circuit block for high-speed switching with constant optical output.
FIG. 6 is a diagram showing an example of a semiconductor laser drive circuit block for performing high-speed switching with a constant optical output in which a /A converter is inserted. FIG. 3 is a diagram showing the relationship between a digital code, forward current IF, and optical output Po. Explanation of symbols l... 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, ■... Up/down counter, 12...
・Oscillator. Agent Patent Attorney Tsuyoshi Sugiyama (and others!) ) 6! ! l

Claims (1)

[Claims] 1. A semiconductor laser driving system characterized in that a D/A converter with an offset is provided in a feedback loop for obtaining a constant optical output.