Optical Pulses with durations of picoseconds or less.
Optical pulses as generated in mode-locked lasers can be extremely short, particularly for passive Mode locking. There is no commonly accepted definition of “ultrashort”, but usually this label applies to pulses IF their pulse duration is at most a few tens of picoseconds, and often in the Range of femtoseconds.
Note that ultrashort pulses should not be called “ultrafast” â€“ they are not faster (do not have a higher velocity) than longer pulses. They do, however, allow to investigate ultrafast processes, and can be used for FAST optical Data transmission. In the latter case, “fast” means a high data rate, not actually a high velocity.
Ultrashort pulses are usually generated with passively mode-locked lasers, but sometimes also with Optical Parametric Amplifiers (possibly using a supercontinuum as input) or with free Electron lasers. It is also possible to start with somewhat longer pulses and apply some method of pulse compression. The article on Ultrafast Lasers lists some important areas of ultrashort pulse generation, including the generation of few-cycle pulses, where the pulse duration is only a small multiple of an optical cycle