... would like to see rise-decay times of less than half a millisecond.
... using 5 ms pulses at 10-20 Hz ...
An unmodulated laser is most stable, adding a chopper-wheel might offer a solution here.
100mm diam.
900 rpm = 15Hz
open for 27 of 360 degrees = 5mSec
1mm beam diameter will get you 0.2mSec rise/fall times