Protecting LEDs driven by pulse generator via coax cable
Assuming you're pulsing it fairly hard, if you're only ever going to use a source with a \$50\Omega\$ source impedance and a \$50\Omega\$ cable, then the LED is as protected as it's going to get unless you do something fancy with an RC network at the LED -- and if you do that, you'll just be protecting it against over-long pulses, and you'll be really complicating your life as far as acceptable duty cycles and the interaction between duty cycle, frequency, and light output.
If you're pulsing it hard (i.e., if you're operating it at above it's designed maximum current rating) then anything that you do to &quot;protect&quot; the LED will fail to protect it if you accidentally give it too high of a duty cycle.
C'mon -- it's a physics lab.  You're expected to blow up electronics components.
OTOH, if you're not pushing much current through it, then some series resistance will help stabilize the LED current, because diode voltages depend heavily on everything (temperature, part to part variation, possibly the phase of the moon, aging, etc.).  Having a series resistor puts more drop across the resistor, which has the effect of moderating the influence of he diode voltage on the current level.
Reflections shouldn't be a problem.  The voltage wave will travel down the cable, hit the LED and be reflected, travel back up to the source and -- because the source is matched to the cable -- be absorbed.  Your LED will just see a constant voltage going through a constant \$50\Omega\$ resistance.