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Pulse Duration and Buildup Time

The pulse duration can be roughly estimated by considering the exponential rise of the power before gain saturation sets in. The power increases by a factor exp(1) within the time Trt / (g - l), and the FWHM pulse duration is several times this value, assuming that the decay after the pulse maximum is similarly fast, that is, l is roughly comparable with gi - l. For example, a numerical simulation for the case gi = 2l shows that the FWHM pulse duration is

It becomes apparent that particularly short pulses are obtained when the laser resonator is short and the initial gain is high. Pulses with durations far below 1 ns can be achieved with Q-switched Nd:YVO4 and Nd:GdVO4 microchip lasers, having a very small round-trip time and a high gain despite the small crystal thickness. For a given laser crystal and pump power, the gain can be increased, for example, by arranging for a smaller area of pump and laser beam in the crystal and, of course, by allowing for a long enough pumping time before triggering a pulse. Furthermore, the resonator losses should be roughly one half the initial gain.

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