Q-switch
A Q-switch is used to increase the pulse power of a laser by shortening the output pulse duration. The function of a Q-switch is to prevent lasing in a laser oscillator while the gain medium is being pumped above the threshold value for lasing. Q-switching is a common and effective technique to achieve optical pulses with short duration, high repetition rate and high peak power. This process allows energy storage within the lasing medium prior to activation of the switch. Q-switching may be accomplished "actively" or "passively." Active Q-switches require electrical stimulation in order to function properly while passive Q-switches do not require electrical stimulation for proper operation. A passive Q-switch uses some form of light absorbing material that saturates when the gain inside the cavity exceeds a certain level. The dye then quickly drops below its saturation level and oscillation stops. The process automatically repeats to produce successive laser output pulses without the need for any external energy or control. Typical passive Q-switches that are saturable absorbers include dyes, dye films and crystals that contain saturable color center defects. Acousto-optic Q-switching uses an acousto-optic modulator to produce an rf acoustic wave in order to Bragg-diffract the light out of the cavity. This technique is simple and operates well at a kilohertz repetition rate, but it has a very slow opening time and does not work well with high-gain lasers. An electro-optic Q-switching uses an electro-optic crystal and is fast and precise, thus providing control over the peak output pulse width independent of output pulse frequency, it has some disadvantages. However, the electro-optic crystal is subject to optical damage at high intensities.
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