L3-A

Basic Principles of Q-Switching
Q-Switching is a mode of operating a laser in which energy is stored in the laser material during pumping in the form of atoms in the upper laser level and suddenly released in a single, short burst. This is accomplished by changing the feedback of the laser cavity. During pumping the HR mirror is effectively removed from the system, preventing lasing. After a large amount of energy has been stored in the active medium, the HR mirror is returned to proper alignment and operation, and most of the stored energy emerges in a single, short pulse. The extremely short, high-energy output pulse will occur at a predictable time. This makes the Q-switched laser an ideal transmitter source for rangefinders and surveillance radar. Q-switched lasers are also used to produce rapid, localized heating in materials. If the enormously high peak power in the output pulse is concentrated or focused, the intensity of the focused beam represents sufficiently high electromagnetic field strengths to ionize the air and cause "breakdown"—just like a high dc voltage between a spark gap.
Q-switched lasers can be classified for study according to two criteria:
1. Whether they have a continuous or pulsed pumping source.
2. The methods and equipment used to "Q-switch" them.
The first portion of this discussion will describe the theory of Q-switching using a pulsed pumping source for the laser amplifier. The discussion will then apply this technique to continuously pumped laser systems. The latter portion of the discussion briefly describes several Q-switching techniques.
Figure 1 shows a typical time sequence of the generation of a Q-switched pulse. Lasing action is disabled in the cavity by a low Q of the cavity. Toward the end of the flashlamp pulse, when the inversion has reached its peak value, the Q of the resonator is switched to some high value. At this point a photon flux starts to build up in the cavity and a Q switch pulse is emitted. As illustrated in Fig. 1, the emission of the Q-switched laser pulse does not occur until after an appreciable delay, during which time the radiation density builds up exponentially from noise