L3-C

Q-Switching CW Lasers
CW solid-state and molecular gas lasers may be pumped continuously and repetitively Q-switched to produce a regular train of output pulses. The most common examples of this are CW Nd:YAG lasers and Class I CW CO2 lasers. Since the pumping rate is much lower for CW systems, the maximum stored energy is also lower, and the resulting peak power is lower (typically 103 to 104 watts).
Nd:YAG is by far the most common Q-switched, CW pumped laser system. Such systems can typically produce several thousand pulses per second without degradation of the pulse energy. Typical pulse durations for such a system range from several hundred nanoseconds to a few microseconds.

Methods of Q-Switching
Several techniques have been used for Q-switching lasers. Each has its advantages, disadvantages, and specific applications. The four most important Q-switch types are discussed in this section. While many Q-switch characteristics are important to specific applications, the following four characteristics are generally most important:
1. Dynamic loss is the maximum loss introduced in the optical cavity when the Q-switch shutter is "closed." Ideally the dynamic loss should be 100% to ensure that lasing cannot occur until the Q-switch is opened.
2. Insertion loss is the minimum loss introduced by the presence of the Q-switch in the "open" condition. Ideally this is zero, but most Q-switches include optical surfaces that introduce reflection and scattering losses.
3. Switching time is the time necessary for the Q-switch to open. Faster switching times result in shorter, higher-peak-power pulses because the switch can become fully open before lasing has a significant effect on the population inversion. Slower switches allow significant amounts of the stored energy to be depleted before the Q-switch is fully opened. This lowers the maximum loop gain of the system and "stretches out" the output pulse.
4. Synchronization is an indication of how well the laser output can be timed with external events. Some Q-switches allow precise control of when the output pulse occurs. Others offer virtually no control at all.