Q-Switch

Q switching is a technique that adjusts the Q factor of the laser resonator by adjusting the intracavity losses to obtain high-energy, short optical pulses (rather than ultra-short) optical pulses from lasers. This technique is primarily used for solid-state body lasers to generate nanosecond pulses of high energy and peak power.

Description of the generation process of Q-switch pulse:

• Initially, the resonator losses were kept at a high level (there are both active and passive Q-switched techniques), making it impossible to generate a laser. The energy of the gain medium is rapidly increased and accumulated by means of a pumping mechanism. If the pumping energy is sufficient, the stored energy is usually limited only by spontaneous radiation (especially continuous pumping) and in other cases (with sufficiently large gain) by parasitic lasers.
• Then, the loss of the resonator is suddenly reduced to a very small value, so the power of the laser radiation increases rapidly in the laser resonator. This process usually starts with spontaneously emitted noise, which is amplified to macroscopic power levels over hundreds or thousands of resonator round trips.
• Once the power in the cavity reaches saturation of the gain medium, the gain of the order of magnitude of energy begins to saturate. The pulse peak is reached when the gain is equal to the loss of the resonator itself.

The huge amount of power in the chamber that exists at this point leads to further depletion of the energy stored during the power decay. In general, the energy extracted after the pulse maximum is similar to the energy extracted before the pulse maximum, and the resulting pulse shape is approximately symmetrical.

The pulse duration achieved with a Q switch is typically in the nanosecond range (equivalent to a round-trip of several resonators) and is typically much higher than the round-trip time of a resonator. The resulting pulse energy is typically higher than the saturation energy of the gain medium, and even small lasers can reach the millijoule range. Peak power can be orders of magnitude higher than power during CW operation. Even for lasers of medium size and moderately focused beams, the peak intensity is sufficient for photobreakdown in air.

In most cases, Q-switched lasers produce regular pulse trains with repeated Q-switching. The pulse repetition rate is usually in the range of 1-100kHz, sometimes higher. Passive Q-switched microchip lasers have pulse durations well below 1 nanosecond and repetition rates of up to several megahertz, while large (often amplified) laser systems can deliver pulses with thousands of joule energy and durations in the nanosecond range.

Lasers that apply Q-switched technology are known as Q-switched lasers.