CONCEPT   OF  Q-SWITCHING

         The most simplistic method that can be envisioned to produce a pulsed laser is to switch the gain of the medium on and off. This can be accomplished easily by switching the pump energy to the laser medium. In the case of a gas laser, the current through the discharge may be controlled easily, or in the case of an optically pumped laser such as a YAG, the lamp may be turned on and off as required. By switching the pump energy to the medium on and off, gain inside the laser is also switched on and off. When pump energy is sufficient to allow laser gain to exceed the threshold, an output beam appears. The problem with this scheme is that the output pulses will be quite rounded, since there is a delay as population inversion and hence gain builds in the laser; this also sets limits on the pulse length and repetition rate for the laser.

Figure 1.1. Precision thick-film resistor trimming by a YAG laser.

This technique is commonly used with semiconductor lasers, in which the laser diode current is modulated to control the laser output.

      In a Q-switching technique, the laser output is switched by controlling loss within the laser cavity as outlined in Figure 1.2. More correctly, Q-switching is loss switching in which a loss is inserted into the cavity, thus spoiling it for laser action. In the simplest manner, a Q-switch can be though of as an optical gate blocking the optical path to one cavity mirror and hence causing laser action to cease. In reality, it is not necessary to block the optical path completely. Simply inserting a loss high enough to raise the lasing threshold beyond the maximum gain of the laser is sufficient. When the Q-switch is on, it blocks the intracavity beam. This state is called a low-Q state, meaning that the quality factor, or Q, of the cavity (which measures the ability of a laser cavity to act as a resonator) is ruined. With the switch off, losses in the cavity are reduced, and the cavity is resonant a high-Q state. During the time when the Q factor of the cavity is low, the laser is not oscillating and hence has no usable output beam, but the pump energy continues to drive the laser medium. In this state population inversion continues to build and an inversion much larger than normally possible builds in the laser gain medium. When the Q of the cavity is restored in such a condition, the laser begins to oscillate immediately and the large population inversion releases energy in the laser medium in one enormous pulse. This technique allows energy to be stored in the laser medium during the low-Q state and released in a single massive pulse. The peak power of the pulse is much larger than is possible with gain switching, which for a CW laser, yields a peak power equal to the CW output power of the laser.

Figure 1.2. Q-switching a laser.