JPH0368183A - Semiconductor laser drive device - Google Patents

Abstract

PURPOSE:To enable an optical output to be modulated 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 D/A converter provided with an input latch. where the count value corresponding to two or more optical outputs of the counter concerned is stored in a memory device. 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 110 counts up or down the output of an oscillator 12 corresponding to the input from the comparator 3. The output concerned is stored in a memory element 13. The output of the counter 110 is inputted into a D/A converter 100 provided with a latch and converted into a control signal for a current source 4. In this constitution, when the optical output of the laser diode 1 is modulated, a prescribed count value stored in the element 13 is selected.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a semiconductor laser driving device 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.
The relationship between and the optical output Po is not linear, but the forward current IF
As the forward current 1th increases, laser oscillation starts at the threshold current 1th, and thereafter the optical output P increases as the forward current 1th increases.
o also increases. This characteristic is shown in FIG.

By the way, the threshold current 1th and the differential efficiency η (ratio of change in optical output PO with respect to change in @ direction current TF) 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;

Furthermore, the optical output of the semiconductor laser l is monitored by a photodiode 2, and the current from the photodiode 2 is passed through a resistor 5.
is converted to voltage by This voltage is input to the input terminal of an operational amplifier (hereinafter referred to as operational amplifier) 3, the reference voltage Vref is input to the input terminal of operational amplifier 3,
The output current IF of the current source 4 is controlled by the output of the operational amplifier 3.

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.

Although FIG. 3 shows a configuration for constantly obtaining a constant optical output from the semiconductor laser 1, 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 ref.

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 and off at high speed, and the pre-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 drive device shown in FIG. 5 was devised to be able to maintain a constant light 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 functions 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 "HIGH", and conversely counts up the output pulses of the oscillator 12 when the output of the comparator 3 is "HIGH". If so, count down. The count result is sent to the current source 4 by the D/A converter 10.
The forward current IP, which is converted into a control signal for and controlled by this control signal, causes the laser diode l
is driven. As shown in FIG. 5, when the current switches fall to the right, a negative feedback loop is formed as a whole, and the optical output becomes a value defined by the reference voltage V 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 method shown in Fig. 5 is used to drive a semiconductor laser used in a magneto-optical disk, the reference voltage Vref is changed each time it becomes necessary to change the optical output, and the negative feedback loop is activated. Therefore, it is necessary to wait until the optical output reaches a desired value, which is undesirable and takes time.

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> To achieve the above object, the present invention provides a photoelectric conversion element that monitors the optical output of a semiconductor laser driven by a current source, and a reference value for the output of the photoelectric conversion element. 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 and whose output is 3 states, and a D/A with an input latch that converts the output of the up/down counter into a control signal for the current source of the semiconductor laser.
A semiconductor laser driving device including a converter, further comprising a storage device that stores count values corresponding to a plurality of optical output values of the up/down counter and outputs them to the D/A converter. It is a feature.

<Operation> The 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, the output is "LO".
Suppose it becomes W”.

The up/down counter counts up or down the output of the oscillator depending on whether the output of the comparator is HIGH or LOW.
The signal is input to the converter and converted into a control signal for the current source, thereby controlling the forward current output from the current source. In this way, since the current source is controlled by a negative feedback loop that includes a D/A converter that latches the digital value, it is possible to turn the semiconductor laser on and off while keeping the optical output of the semiconductor laser constant for a long time. can.

Further, reference values corresponding to a plurality of optical outputs are set, and each count value of an up/down counter when the optical output is controlled to correspond to each of the reference values is stored in the storage device. When modulating the optical output of the semiconductor laser, the reference value of the negative feedback loop is set each time by selecting predetermined data stored in the storage device and inputting it to the latched D/A converter. Even if you don't fix it,
The optical output is modulated at high speed. Note that at this time, the up/down counter is in a high impedance state.

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

FIG. 1 shows the configuration of this embodiment, in which the up/down counter 11 in FIG. The output of the up/down counter 110 and the input of the D/A converter 100 are connected to the storage element 13, and the output of the up/down counter 110 corresponding to a plurality of optical output values, that is, the D/A converter I00, is connected to the output of the up/down counter 110 and the input of the D/A converter 100.
The input of the A converter 100 can be stored in the memory 13 as a digital code. The rest of the configuration of this embodiment is similar to the configuration shown in FIG. 5, and the same components are given the same reference numerals and the explanation will be omitted.

In the above configuration, first, a reference value V ref is set according to the desired optical output and input to 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, which is then manually applied to a comparator 3. The comparator 3 outputs "LOW" when the optical output of the laser diode 1 is larger than the desired optical output, and outputs "HIGH" when it is smaller. The up/down counter 11O receives “i” from the comparator 3.
(The oscillator 12
Count up or down the output of The count value output from the up/down counter 110 is stored in the storage element 13. Further, the output of the up/down counter 110 is input to the latch-equipped D/A converter 100, where it is converted into a control signal for the current source 4, and the forward current IP output from the current source 4 is controlled. In this way, since the current source 4 is controlled by the negative feedback loop including the D/A converter lOO that latches the count value from the up/down counter 11.0, the laser diode l
After that, the high-speed current switch 6 can be turned on and off while keeping the output constant for a long time.

Further, a reference value V ref corresponding to a plurality of optical outputs is set respectively, and each count value of the up/down counter 110 when the optical output is controlled to correspond to each of the reference values is stored in the memory element 13. Store. When modulating the optical output of the laser diode 1, a predetermined count value stored in the memory element 13 is selected and the latched D/D/
Input to A converter 100. Then, the count value is latched by the D/A converter 100, and a control signal is input to the current source 4 according to the count value.

In this way, the optical output can be modulated at high speed by simply selecting data stored in the storage element 13 without having to reset the reference value Vref of the negative feedback loop each time. Note that at this time, the up/down counter 110
is in a high impedance state.

Note that such control over the current source 4 is as follows:
Since the optical output is basically controlled by a negative feedback loop, the forward current IF of the laser diode l - the optical output P
o The characteristic need not be linear; it only needs to be monotonic.

<Effects of the Invention> As is clear from the above, the semiconductor laser driving device of the present invention includes a negative feedback loop including a 3-state up/down counter and a D/A converter with a latch, and the up/down Since it is equipped with a storage device that stores the output of the down counter and outputs it to the D/A converter with a latch, the semiconductor laser can be turned on and off while keeping the optical output constant for a long time, and it is possible to modulate the optical output. can be done promptly. In addition, since the digital data that performs negative feedback control is stored in the storage device, even if the forward current IF - optical output Po characteristic of the oscillation region of the semiconductor laser is not linear, if it is monotonic, the specified light can be quickly adjusted. You can get the output.

[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 IF of the laser diode and the optical output Po, and Fig. 3 is a diagram showing the relationship between the forward current IF of the laser diode and the optical output Po. Figure 4 is a block diagram of a semiconductor laser driver for high-speed switching with a constant optical 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 in which a /A converter is inserted; FIG. ■... 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, IO... D/A converter, 11
・・Up/down counter, 12・・Oscillator, 13・
...Storage element, 100...D/A converter with input latch, +10...
3-state output up/down counter. Patent applicant: Sharp Corporation Agent
Patent attorney Previously mentioned Ao and 1 other person 2 Figure 2] +10 force (B) Nitto-Kun IF Figure 3 Vref Figure 4 Figure 5 3-1-Kun 0 rater 4-11 Den:, Kudo 5 --One-year spoon anti-61-4 Kokonoe Δ overturn r, fb sui soke

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. up/down is controlled by , and the output is 3-state up/down.
A semiconductor laser driving device comprising a down counter and a D/A converter with an input latch that converts an output of the up/down counter into a control signal for a current source of the semiconductor laser, comprising: a plurality of up/down counters; 1. A driving device for a semiconductor laser, comprising a storage device that stores a count value corresponding to an optical output value and outputs it to the D/A converter.