I am using a GaAs photodiode to pick off a small amount of the light of a high-powered infrared laser, supply this to a PID loop and thus have a way to stabilize the intensity of the laser. The thing is now that the optical power has to be changed continuously over up to three orders of magnitude (40W to ~10mW) by applying a voltage ramp to the setpoint input of the PID controller. Unfortunately, I could not manage to find a set of parameters for the PID loop for which it shows a reasonable response over the whole operating range.
While reading about the issue I came across what are called Programmable/Variable Gain (Transimpedance) Amplifiers. What do you guys think about supplying the voltage ramp to the gain input of the programmable TIA instead and keeping the setpoint at the PID regulator constant? This way, the PID regulator has a signal to work with that has a constant magnitude and - beware my limited EE knowledge - the system should be able to regulate the power better over this broad range? Or is there anyone that has ever used one of these before, perhaps for a similar application?
Of course, I welcome all other suggestions about ways to get a reasonably high signal out of a photodiode over such a broad range of optical intensities.