[fest]

I'm more interested in things like: if the diode is on for half the time (50% duty cycle), can I use 10mW? if I transmit a high frequency chirp with even less total duty cycle, can I use even more power?

Not that it really matters in my case: I will probably scrap the project when/if I make a good enough amplifier stage and phase detector for short range where 5mW should be enough.

[analog31]

Well, Hobbs is also the place to go for how to make that amplifier stage. He has some articles online for photodiode front ends, that are quite informative.

Admittedly I've never looked into those issues because the gadgets that I've helped design are all Class 1 -- the system is fully enclosed and interlocked.