Gain-switching is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, of the order of picoseconds (10−12 s).
When the carrier density exceeds that value, the ensuing stimulated emission results in the generation of many photons.
So the carrier density eventually falls back to below lasing threshold which results in the termination of the optical output.
If the injection current is rapidly switched off at the proper time, for example using a step recovery diode circuit, a single 50 ps light pulse can be generated.
The term gain-switching derives from the fact that the optical gain is negative when carrier density or pump intensity in the active region of the device is below threshold, and switches to a positive value when carrier density or the pump intensity exceeds the lasing threshold.