Pulsed laser clamps voltage when pulsed too fast

I bought a 50mW laser for use in data communications. I did not plan to use a current driver, just a resistor and mosfet. It's pulsed on/off about 10 times a second at 30mS pulses at 10% duty cycle, so not really on long enough for thermal runaway to be an issue. On the oscilloscope it gets full voltage on the first pulse then drops to a clamped very low voltage thereafter... I do not get it. Is there a standard safety feature or something obvious I'm missing?

You're leaving out an awful lot of information here. What laser diode? What current limit resistor? What voltage? What FET? What drives the FET? What does the gate waveform look like? Where are you scoping the voltage?