Q-switching is a method for generating high-intensity, short-pulse laser radiation; its fundamental operating principle is as follows: First, the gain medium is optically pumped, but the resonant cavity is maintained at a high-loss level (i.e., a low Q-factor); consequently, energy cannot be extracted in the form of laser light. The methods used to modulate this loss can be broadly categorized as either active or passive. Subsequently, the loss within the resonant cavity is abruptly reduced. At this juncture, the gain significantly exceeds the cavity loss, causing the intracavity power to grow exponentially-typically commencing from the faint fluorescence of the gain medium-until the gain saturates and the power begins to decay once again.
The generation of such a light pulse enables the extraction of the vast majority of the energy stored within the gain medium. To achieve high pulse energies, the gain medium must possess the capacity to store substantial energy; this necessitates a long upper-state lifetime, a high density of laser-active ions or atoms, and a gain efficiency that is not excessively high. This latter requirement is of critical importance; otherwise, amplified spontaneous emission (ASE) would limit energy storage, thereby necessitating an extremely high initial cavity loss to prevent premature laser oscillation. The most commonly employed gain media for Q-switched lasers are rare-earth-doped crystals and glasses; consequently, solid-state lasers represent the most prevalent type of Q-switched system. Nevertheless, fiber lasers can also be configured for Q-switched operation and, when coupled with fiber amplifiers, are capable of delivering exceptionally high average power.
Active vs. Passive Q-Switching: Active Q-switching typically incorporates an acousto-optic modulator within the resonant cavity to actively modulate cavity losses. Driven by an RF signal, the acousto-optic modulator causes the light beam to exit the resonant cavity via first-order diffraction, thereby introducing significant loss. A pulse is generated when the RF signal is momentarily switched off. To achieve a high repetition rate, the gain medium requires continuous pumping while the Q-switch is triggered repetitively. Conversely, to obtain maximum pulse energy, pulsed pumping (such as flashlamp pumping) combined with a low repetition rate is required.
Passive Q-switching employs a saturable absorber in place of an active modulator. For instance, an Nd:YAG laser can utilize a Cr⁴⁺:YAG crystal as a saturable absorber. While other saturable absorber crystals may be selected for different wavelengths, a Semiconductor Saturable Absorber Mirror (SESAM) is suitable for a wide range of operating wavelengths.
