Laser driver diode design for fast switching times which can be controlled via computer

Question

I am trying to create a laser driver circuit that can allow me to drive a laser with relatively low operating current and voltage with fast switching times and can be controlled via a pulse blaster (Pulse Blaster PB12-100-4K).

laser to be driven is a Pigtailed single fiber laser Thorlabs LP520-SF15.

Operating current is 88.5 mA Operating Voltage is 6.7 V

Absolute Maximum Reverse voltage is 2V

The threshold current is 32 mA

I understand that Laser drivers should provide constant current, but I wanted to ask that in providing constant current does the voltage not matter, meaning any voltage applied is fine as long as the current stays in the limit?

How does one create such a circuit so that it can be connected with the pulse blaster and have sufficiently fast rise times?

Rise times should be in the range of 1∼10 μs

current accuracy as close as possible and noise also as low.

Looking for good schematic suggestions.

Answer 1

I understand that Laser drivers should provide constant current, but I wanted to ask that in providing constant current does the voltage not matter, meaning any voltage applied is fine as long as the current stays in the limit?

You also need to make sure the voltage does not exceed the rated voltage of the part, typically a high current laser diode will have a diode curve that accepts large amounts of current so it would be difficult to exceed the voltage at the rated current and the datasheet should reflect this. Keep in mind that its usually heat that kills parts so by current limiting the power is also limited. If the power stays within the recommended limits the temperature will also.

How does one create such a circuit so that it can be connected with the pulse blaster and have sufficiently fast rise times?

Use a mosfet with a really fast switching time, make sure the gate is being driven very fast (under 1us, shoot for 10's of ns)

Answer 2

in providing constant current does the voltage not matter, meaning any voltage applied is fine as long as the current stays in the limit?

The laser itself has a characteristic I-V curve. You can't provide a given current without also providing the corresponding voltage that depends on the laser.

However if you are thinking of just using a constant-voltage supply circuit there are a couple of problems:

1. The I-V curve is very steep. A small error in the applied voltage will cause a large error in the current, and this can easily lead to damaging the laser.

2. The I-V curve will shift as the laser heats and cools so the correct voltage applied when the laser first turns on will be the wrong voltage a few milliseconds later when the laser has warmed up due to self-heating.

If you use a good constant-current supply circuit it will provide the current you design it for even if the load changes to demand a different voltage to achieve that current.

Answer 3

To provide fast current rise time (1-50 us), pulsed laser drivers usually use circuits on a linear active element with current feedback. Usually the active element is a MOSFET in linear operation mode. The buffer voltage, in the linear mode of operation of the transistor, is limited only by heating the transistor. Because in the linear mode of operation the transistor will get very hot. And the greater the difference between the voltage drop on the laser diode and the buffer voltage, the stronger the active element will warm up. You can read more about driver topologies in the article "Passive and Active Topologies Investigation for LED Driver Circuits" by Salvatore Musumeci

reference design: https://www.ti.com/lit/ml/ssqw095/ssqw095.pdf