JPH0368184A - Semiconductor laser drive system - Google Patents

Abstract

PURPOSE:To modulate an optical output at a high speed by a method wherein the current source of a semiconductor laser is controlled by a negative feedback loop composed of an up/down counter and a multiplication type D/A converter, where the modulating signal is inputted into the D/A converter concerned as one of inputs. CONSTITUTION:A reference value Vref is set corresponding to a required optical output, which is inputted into the (+) input of a comparator 3. On the other hand, the output of a photodiode 2 which monitors the output of a laser diode 1 is inputted into the (-) input of the comparator 3. An up/down counter 11 counts up or down the output of an oscillator 12 corresponding to the input from the comparator 3. The output of the counter 11 is inputted into one input terminal of a multiplication type D/A converter 100. On the other hand, the modulated signal is inputted into the other input terminal of the D/A converter 100. By this setup, the output of the counter 100 is converted based on the modulating signal and a control signal used for a current source is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a semiconductor laser driving method that can modulate optical output at high speed.

<Prior art> A semiconductor laser obtains laser light by passing a forward current IF through a type of PN junction, and the forward current IP
The relationship between and the optical output Po is not linear, but the forward current IF
As the forward current Ith increases, laser oscillation starts at the threshold current 1th, and thereafter the optical output P increases as the forward current Ith increases.
O also increases. This characteristic is shown in FIG.

By the way, the threshold current Ith and the differential efficiency η (ratio of change in optical output Po to change in forward current IF) are not constant, but change depending on the ambient temperature and the individual.

For this reason, a device shown in FIG. 3 is generally used to drive such a semiconductor laser with a constant output. 1 is a semiconductor laser, which is driven by a current source 4 controlled by the output of an amplifier 3.

Further, the optical output of the semiconductor laser 1 is monitored by a photodiode 2, and the current from the photodiode 2 is connected to a resistor 5.
is converted to voltage by This voltage is input to one input terminal of an operational amplifier (hereinafter referred to as an operational amplifier) 3, a reference voltage V ref is input to an input terminal of the operational amplifier 3, and the output of the operational amplifier 3 causes an output current I of the current source 4 to be input.
P is controlled.

In this way, a negative feedback loop is constructed as a whole, and the optical output from the semiconductor laser I becomes a constant value corresponding to the reference voltage V ref.

FIG. 3 shows a configuration for constantly obtaining a constant optical output from the semiconductor laser 1, but in certain applications, it becomes necessary to turn on and off the constant optical output at high speed. In that case, for example, a drive device 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 capacitor 8, a high-speed current switch 6, and a buffer 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 output defined by the reference voltage V rer.

Next, turn switch 7 to oven. At this time, since the voltage is held by the capacitor 8, the current that drives the semiconductor laser 1 basically does not change.

Then, the current switch 6 is turned ON at high speed, and the current is set to 0FFt.

Then, the optical output from the semiconductor laser 1 is turned on and off.

However, in the driving device shown in FIG. 4, the voltage is held by the capacitor 8 to obtain a constant optical output.
Since the capacitor 8 inevitably discharges, the capacitor 8 cannot maintain a constant voltage, which is an analog quantity, for a long period of time, and therefore, there is a problem that a constant optical output cannot be obtained for a long period of time.

Therefore, the following driving device shown in FIG. 5 was devised to be able to maintain a constant optical output over a long period of time. FIG. 5 shows the up/down counter 11 except for the sample hold section 8.9 and buffer amplifier 9 in FIG.
, a D/A converter 10, and an oscillator 12 are introduced, and the operational amplifier 3 works as a comparator. The up/down counter 11, whose operation is controlled by the control signal sc, counts up the output pulses of the oscillator 12 when the output of the comparator 3 is ")(IGH", and vice versa.
LOW”, the count result is converted to a control signal for the current source 4 by the D/A converter IO, and the forward current IF is controlled by this control signal.
The laser diode l is driven by. As shown 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 is changed to the reference voltage V.
It becomes the value specified by ref. In the case of Figure 5, an error equal to one power count occurs, but since there is a part that becomes a digital code in the negative feedback loop, there is no problem even with long-term hold, and a constant optical output can be obtained for a long time. be able to.

<Problem to be solved by the invention> By the way, when changing the set light output frequently, for example,
When the drive device shown in Fig. 5 is used to drive a semiconductor laser used in a magneto-optical disk, the reference voltage V rer is changed each time it becomes necessary to change the optical output, and a negative feedback loop is established. This operation requires waiting until the optical output reaches a desired value, which is undesirable and time consuming.

In view of the above points, the present invention provides a semiconductor laser driving device that can turn on and off while keeping the optical output constant for a long time, and can quickly modulate the optical output. The purpose is to

<Means for Solving the Problems> In order to achieve the above object, the present invention includes a photoelectric conversion element that monitors the optical output of a semiconductor laser driven by a current source, and a reference value between the output of the photoelectric conversion element and a reference value. Along with a comparator to compare and count the output of the oscillator,
an up/down counter whose up/down is controlled by the output of the comparator; one input terminal receives a reference signal; the other input terminal receives the output of the up/down counter; When modulating the optical output using a multiplication type D/A converter that converts the output of the up/down counter into a control signal for the current source of the semiconductor laser, one of the multiplication type D/A converters is used. A modulation signal is input to the input terminal instead of the reference signal, and the D/A converter performs D/A conversion on the output of the up/down counter based on the modulation signal.

Function> The optical output of the semiconductor laser is monitored by a photoelectric conversion element, and the output of the photoelectric conversion element is compared with a reference value representing a desired optical output by a comparator. Now, for example, when the optical output of the semiconductor laser is smaller than the desired optical output, the output of the comparator is "HIGH", and when it is larger, it is "LOW".
”.

The up/down counter is HI of the output of the comparator.
The output of the oscillator is counted up or down depending on GH'' or LOW''.

The count value of this up/down counter is a multiplication type D
The reference signal input to the other input terminal of the /A converter is converted into a control signal for the current source based on the reference signal input to the one input terminal, and the forward current output from the current source is controlled. In this way, the current source is controlled by a negative feedback loop that includes an up/down counter and a multiplication type D/A converter, so by having the up/down counter hold its output digital value, the light of the semiconductor laser can be controlled. The semiconductor laser can be turned on and off while keeping the output constant for a long time.

Next, when modulating the optical output of the semiconductor laser, by inputting the modulation signal instead of the reference signal to the multiplier type D/A converter, the D/A converter will up/down based on this modulation signal. The output of the counter is D/A converted and input to the current source. Therefore, the optical output can be modulated at high speed without having to reset the reference value input to the comparator of the negative feedback loop and operate the negative feedback loop.

<Example> Hereinafter, the present invention will be explained in detail with reference to illustrated embodiments.

Although FIG. 1 shows the configuration of this embodiment, the D/A converter IO in FIG. 5 is a multiplication type D/A converter 100,
A reference signal or a modulation signal is input to one input terminal of the up/down counter 11, and the output of the up/down counter 11 is input to the other input terminal. The rest of the configuration of this embodiment is the same as the configuration shown in FIG.
Identical constituent parts are given the same reference numbers and explanations will be omitted.

First, a reference value Vref is set according to the desired optical output and inputted to the ten inputs of the comparator 3. - On the other hand, the output of the laser diode 1 is monitored by a photodiode 2, and the output of the photodiode 2 is converted into a voltage by a resistor 5.
It is input to one input of the comparator 3. Comparator 3
outputs “LOW” when the optical output of laser diode 1 is larger than the desired optical output, and outputs “HI” when it is smaller.
The up/down counter 11 counts up or down the output of the oscillator 12 according to the input of "HIGH" or "LOW" from the comparator 3. The count output from the up/down counter 11 The value is input to one input terminal of the multiplication type D/A converter 100, and this count value is converted into a control signal for the current source 4 based on the reference signal Ref input to the other input terminal, The forward current IF output from the current source 4 is controlled. By holding the count value that can be output from the up/down counter 11, the laser diode 1 can maintain its optical output constant for a long time. , the high-speed current switch 6 can be switched on and off.

Next, when modulating the optical output of the laser diode 1,
A modulation signal is input to one input terminal of the multiplication type D/A converter 100 instead of the reference signal Ref.

Then, the count value output from the up/down counter 11 is D/A converted in the multiplication type D/A converter 100 based on the modulation signal, and from another perspective, the count value is modulated with the modulation signal and The signal is D/A converted, a control signal is created, and the signal is input to the current source 4. In this way, without having to reset the reference value V ref to be input to the comparator 3 of the negative feedback loop, you can simply input the modulation signal to the input terminal that was inputting the reference signal of the multiplication type D/A converter +00. By doing so, the optical output is modulated at high speed.

<Effects of the Invention> As is clear from the above, the semiconductor laser driving device of the present invention uses a negative feedback loop including an up/down counter and a multiplication type D/A converter to control the counting of the up/down counter. Since the value is D/A converted by the multiplication type D/A converter based on the reference signal or modulation signal input to one of its input terminals, the count value is held and the optical output is kept constant for a long time. The semiconductor laser can be turned on and off while being held, and the optical output can be quickly modulated by simply inputting the modulation signal to the multiplication type D/A converter.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the configuration of the semiconductor laser driving device of the present invention, Fig. 2 is a diagram showing the relationship between the forward current IP of the laser diode and the optical output PO, and Fig. 3 is a diagram showing the relationship between the forward current IP of the laser diode and the optical output PO. Figure 4 is a block diagram of a semiconductor laser driver for high-speed switching of constant output using a sample-and-hold circuit, and Figure 5 is a feedback loop for setting the optical output. niD
1 is a block diagram of a semiconductor laser driving device for performing high-speed switching with a constant optical output by inserting a /A converter; FIG. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 2... Photodiode for optical output monitor, 3... Amplifier, 4... Current source, 5... Resistor, 6... High speed current switch, 7... Analog switch , 8... Voltage holding capacitor, 9... Buffer amplifier, lO... D/A converter, 1
1... Up/down counter, 12... Oscillator,
ioo...multiplying type D/A converter. Patent applicant: Sharp Corporation Agent
Patent attorney: Suedou Bo and one other figure 3 2 ref 4

Claims (1)

[Claims] (1) A photoelectric conversion element that monitors the optical output of a semiconductor laser driven by a current source, a comparator that compares the output of the photoelectric conversion element with a reference value, and an output of the comparator that counts the output of the oscillator. an up/down counter whose up/down is controlled by;
One input terminal receives a reference signal, the other input terminal receives the output of the up/down counter, and based on the reference signal, the output of the up/down counter is used as a control signal for the current source of the semiconductor laser. When modulating the optical output using a multiplication type D/A converter, inputting a modulation signal instead of the reference signal to one input terminal of the multiplication type D/A converter,
A driving method for a semiconductor laser, characterized in that the D/A converter performs D/A conversion on the output of the up/down counter based on the modulation signal.