This mode is achieved by placing an optical accessory inside the resonator, at the side of the laser crystal, whose goal is to pulse optically the light (Siegman 1986; Goldman 1967; Raulin and Karsai 2011). It is generally used in crystal lasers such as ruby, alexandrite, and Nd:YAG, described below. The goal is to accumulate the laser energy at very high levels and release it at extremely rapid pulses. The result is a very high-peak-power laser pulse (often higher than the common pulse), which can penetrate deep into the tissue, with minimal side effects. Then a shockwave-induced mechanical action caused by the impact of the laser pulse onto the target tissue causes its fragmentation. In the long and Quasi-CW pulsed modes, the effect is purely thermal.

The Q-Switch can be passive , when using a crystal called “saturable absorber” that produces rapid pulses, or active , when using an electronic modulator crystal called “Pockels cell.”

Passive systems using the saturable absorber are generally simpler and more compact resulting in smaller portable devices or systems installed into handpieces incorporated to a platform. They are more limited as it is not possible to control ef iciently the stability of the fast pulse; the crystal is sensitive to higher energies, which limits the maximum working energy; and the application spot size is limited to a few millimeters (1–3 mm). They also fail to achieve high repetition rates of pulses (high frequencies), working in a maximum of 2–3 Hz. The active Q-Switch uses a Pockels cell which is a crystal subjected to a high electric frequency and is electronically controlled to produce a very fast and stable light switching effect. The result is faster pulses with very high peak powers that are not possible with passive systems. Thus, they can handle high energy, larger spot sizes (10 mm), and faster repetition frequencies of 2–20 Hz. Equipment with active Q-Switch allow the device to be turned off, and thus the laser can also work in the Quasi-CW mode, with micropulse, giving greater  lexibility to the system (Fig. 9).

Fig. 9 Diagram of a Nd:YAG laser with Q-Switch (QS). M1 is the 100 % mirror; M2 is the output coupler

The classic application is in tattoo removal and the treatment of pigmented skin lesions such as dark circles, postin lammatory hyperpigmentation, and melasma (Goldman 1967; Reid and Muller 1978; Raulin et al. 1998; Chang et al. 1996; Shimbashi et al. 1997; Reid et al. 1983, 1990; Stafford et al. 1995; Ogata 1997; Chan et al. 1999; Jeong et al. 2008; Mun et al. 2010) (Fig. 10).

Fig. 10 Laser tattoo removal