Definition: the generation of light pulses (flashes)
Categories: light pulses, methods
Author: Dr. Rüdiger Paschotta
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Short or ultrashort light pulses can in principle be generated by starting with a continuous light source and using a fast modulator, which lets the light pass only for a short period of time. However, such a method is not efficient, since most of the light will be lost at the modulator, and also the pulse duration is limited by the speed (bandwidth) of the modulator.
Pulses with much higher energies and much shorter durations can be generated in pulsed lasers. The most frequently used methods are:
- Pulsed pumping: if a solid-state laser, for example, is pumped with a flash lamp, it will naturally generate light pulses. However, those will be relatively long, e.g. with durations in the microsecond or millisecond region.
- Q switching: this method allows the generation of energetic pulses with energies of millijoules or more, durations typically in the nanosecond range, and repetition rates between a few hertz and many kilohertz. Q-switched lasers find widespread industrial application.
- Mode locking in active or passive form is used for generating ultrashort pulses (with typical durations between 30 fs and 30 ps), having megahertz or gigahertz pulse repetition rates and moderate pulse energies (typically picojoules to nanojoules, sometimes microjoules). Higher pulse energies combined with lower repetition rates can be generated with cavity-dumped mode-locked lasers and with regenerative amplifiers, particularly with chirped-pulse amplification or divided-pulse amplification. It is also possible to reduce the repetition rate with a pulse picker. This is an optical switch which transmits only every <$N$>th pulse, before one amplifies these pulses, e.g. in a fiber amplifier.
- Cavity dumping can be used for nanosecond pulses, sometimes in combination with Q switching, but also for ultrashort pulses with mode-locked lasers (see above).
- Gain switching, most frequently applied to semiconductor lasers, where nanosecond or picosecond pulses are formed by quickly switching the optical gain via the pump power (→ picosecond diode lasers). The same principle is usually used in quasi-continuous-wave lasers, although with much longer pulse durations.
There are other methods, which are less frequently used. Some examples are:
- A kind of opto-electronic oscillator consists of a gain-switched laser diode, where the optical pulses are fed into a long optical fiber, then sent to a photodetector, and the photodetector signal is used to electrically control the diode. Although the basic principle of pulse generation is then still gain switching, the optical delay line determines the pulse repetition rate and serves to achieve a very low timing jitter.
- For very high pulse repetition rates, a beat signal can be generated by superimposing the outputs e.g. of two single-frequency laser diodes with significantly different optical frequencies, and subsequently “sharpening” the beat signal by four-wave mixing in an optical fiber. In that way, pulse trains with high contrast are obtained at extremely high pulse repetition rates.
See also: light pulses, double pulses, ultrashort pulses, pulsed lasers, nanosecond lasers, picosecond lasers, femtosecond lasers, modes of laser operation, Q switching, mode locking, cavity dumping, regenerative amplifiers, chirped-pulse amplification, gain switching, quasi-continuous-wave operation
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