How to use a photodiode over a broad range of optical powers

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.

• How is the PID regulator implemented? Analog only or is the control digital? What is the response from your photodiode sensor that goes to your PID? Could you split the beam? That way you could use more photodiodes each with a good gain for a specific subset of the output power. Although using a single photodiode should be possible. How much light actually reaches the sensor when the laser is at a constant 40W or 10mW? Nov 20, 2017 at 23:23
• What is the dynamic range of the photo-generated currents? You've mentioned the optical power range (almost four orders of dynamic range), but I'd like to know what this means for your "pick off a small amount of the light" process in terms of current. Also, can you consider the idea of simply putting the photogenerated light through a logarithmic amplifier (which, if you aren't terribly picky, could just be a BJT collector and you examine the base voltage that results?)
– jonk
Nov 21, 2017 at 0:08
• The PID is completely analog. While I don't have the current values at hand, it is currently set up such that the Photodiode output is given to an I-to-V current preamp, and the gain is set such that 40W of laser power equals the maximum voltage of 5V. I could surely split the beams and use multiple photodiodes, but how would I switch between the PDs when applying a linear ramp between the full range of optical power, and the PID lock is still maintained? Nov 22, 2017 at 2:46
• Use as high of a reverse bias across the PD as you can. This will improve your S/N ratio so that at the lowest laser intensity, you'll be considerably above opamp bias and offset voltages. 4 orders of magnitude is a reasonable dynamic range to achieve with a PD transimpedance amplifier. Jan 29 at 19:55