What is Gain Switching in Lasers?

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- GoPhotonics

Feb 2, 2023

Gain switching is a method to generate short optical pulses in the order of picoseconds (10-12) in a laser by modulating the pump power or laser gain. An electrical or optical pumping can be provided for gain-switching. 

In this method, the optical cavity gain changes rapidly from a low value to a high value. When a high pump power is given, it takes time for the laser emission to happen since population inversion needs time and the laser always starts with a weak fluorescence light. At this instant, the gain in the active medium is low. This light is then amplified in a number of resonator round trips and therefore some amount of energy is stored in the gain medium and hence gain increases. The stored energy is extracted in the form of short optical pulses and this technique is called gain switching in lasers. These output pulses will be in the form of spikes. The pulse build-up time and the pump pulse duration are shorter when the pump pulse energy is higher. Therefore, the obtained pulses can be shorter than the pump pulse duration and the upper laser lifetime of the gain medium. By using a longer laser resonator, the pulse build-up time can be increased which also increases the duration of the output pulse.

Figure 1: Output pulse obtained when the pump is OFF and ON

The output pulse obtained when a pump is turned from an OFF state to an ON state is shown in figure 1. Initially, when the pump is OFF, there is no gain in the laser medium. That is, only loss exists there. When suddenly, the pump is turned ON, the oscillation and amplification of light pulses start building up which increases the gain and it will be more than the loss. If the pump is turned OFF, the gain will reduce automatically and energy goes to zero till the pump is switched ON again.

The semiconductor lasers directly modulate the laser output by modulating the injection current passing through it. In a semiconductor laser, the active region is a p-n junction. It is injected with electrons or carriers that will increase the carrier density from a low value to a high value above the lasing threshold. When carrier density rises above this threshold value, stimulated emission occurs and photons are generated. Carrier depletion occurs faster as a result of stimulated emission than the injected rate causing the carrier density to fall below the lasing threshold and stops the optical output. This process may repeat by increasing the carrier density again if the carrier injection has not stopped during this period.

Figure 2: The output spike obtained in gain-switching a solid-state laser when the fixed pump power is given

A single output pulse obtained by carefully adjusting the laser parameters and the pump pulse is shown in figure 2. And figure 3 shows the output of a gain-switched solid-state laser when the pump pulse energy is increased further. This increase in pump pulse energy leads to premature emission of the laser pulse which leads to the generation of a second pulse.

Figure 3: The output obtained when the pump power is increased further


Gain switching is used in different lasers such as:

  • Dye laser: They are pumped with a pulsed nitrogen UV laser or an excimer laser
  • Gas laser: Short current pulses are used to pump TEA carbon dioxide lasers and nanosecond pulses possibly shorter than the applied current pulses are emitted by them.
  • Solid-State laser: These lasers emit nanosecond pulses when pumped optically for a few microseconds. Since Q-switching is more powerful, gain switching is done less frequently in these lasers.
  • Laser diodes: When laser diodes are operated with short current pulses or continuously modulated signals, light pulses with few nanoseconds or even picoseconds are emitted with pulse repetition rates of up to several gigahertz. These laser diodes are used for telecom applications. Gain-switched diode lasers can also be used as seed lasers in optical parametric amplifiers.
  • Fiber lasers: Gain-switched fiber lasers have advantages such as high efficiency, single-mode operation, maintenance-free, etc. They are having wide spectral coverage, narrow bandwidth, simple geometry, and high pulse energy. Laser diodes are used to pump these fiber lasers.