Laser Diode driver circuit help
Laser diodes are notoriously unstable and easy to blow up. (It's not if, but when if you are designing drivers for them. I speak from bitter experience. It is best to develop and learn on cheap ones. Most suppliers of the high end ones will sell a driver with the diode, you buy a sealed unit.)
The photo diode and Q2 are negative feedback to reduce laser current if it gets too bright. Reducing Q2's current will reduce diode drive, so the photodiode has to decrease its voltage with more light.
Q4 is current limiting, it will start to shunt current away from the drive transistor at about 50mA.
One issue with laser diodes can be that absolutely any ringing in the leads going to them will create transients which will kill the diode after a few million pulses. It's often a good idea to limit switching slew rates to reduce this. That may well be what the CR network is about. Their time constant is about 150ns, so this circuit can probably pulse the diode down to a us or so.
EDIT - One comment about this circuit. Because of the horrible vulnerability of laser diodes to any transients (causing damage by overvoltage), it is not generally recommended to drive them with a transistor and a resistor as here. It might be OK if the diode is on the driver PCB (thus extremely short traces) but I ran into problems with leaded diodes where the leads were only about 4 or 5cm with such an approach, with lasers failing after many millions of pulses. We then went to another approach which was to drive the laser with a current source, always on, and then put a &quot;soft switch&quot; transistor in parallel with the laser, shunting current away from it to turn it off. By turning the shunt transistor on and off softly (say 200ns) we avoided diode failures. I can't say whether that is a commonly used approach, but it worked quite well for us.