Gain switching is a method for pulse generation by quickly modulating the laser gain via the pump power. If a high pump power is suddenly applied to a laser, laser emission sets in only with a certain delay, as it starts with weak fluorescence light, which first needs to be amplified in a number of resonator round trips. Therefore, some amount of energy can be stored in the gain medium, which is subsequently extracted in the form of a short pulse. The pulse obtained can be shorter than the pump pulse and also shorter than the upper-state lifetime; the dynamics are essentially as in the phenomenon of spiking, where the pump power is applied for a short enough time to generate only a first spike.
The higher the pump pulse energy, the shorter is the pulse build-up time, and consequently the required pump pulse duration. The pulse build-up time of a gain-switched laser can be increased by using a longer laser resonator, but this also increases the output pulse duration.
The pump power may be completely switched off between the pulses, or may be kept at a level just below the laser threshold.
Gain switching can be applied to various types of lasers:
- Some gas lasers can be operated with a pulsed excitation current. This is often done with TEA carbon dioxide lasers (CO2 lasers), which are suitable only for pulsed operation.
- It is also possible to operate a laser diode with short current pulses, or with a continuously modulated signal. This leads to pulses with durations of a few nanosecond or even down to a few tens of picoseconds, and with pulse repetition rates up to several gigahertz, as used in telecom applications. With additional pulse compression, one can reduce the pulse durations further to below 4 ps . In contrast to mode-locked lasers, gain-switched lasers easily allow one to adjust the pulse repetition rate in a wide range, since it can be controlled with an electronic driver without changing the laser resonator setup. Also, they are simpler and more compact. However, the timing jitter and also fluctuations of other pulse parameters are larger than for a mode-locked laser.
- In other cases, a pulsed laser is used for optically pumping a gain-switched laser. For example, nitrogen ultraviolet lasers or excimer lasers can be used for pulsed pumping of dye lasers. Also, a solid-state laser can emit a single spike if it is pumped only for up to a few microseconds. The pulse duration may be of the order of tens to hundreds of nanoseconds.
An interesting approach to the generation of energetic nanosecond pulses is to combine a gain-switched laser diode with a fiber amplifier or an optical parametric amplifier. In this way, pulses with microjoule or millijoule energies can be generated. This method provides a high flexibility in terms of pulse duration, shape and repetition rate.
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