Experimental evidence of temporal and spatial incoherencies of Q-switched Nd:YAG nanosecond laser pulses

Seeded nanosecond Q-switched Nd:YAG lasers working with an unstable resonator and a variable reflectivity mirror are widely used, for they represent useful sources for stable and repeatable light-matter interaction experiments. Moreover, in most setups, the fundamental wavelength is converted to higher harmonics. When the injection seeder is turned off, random longitudinal mode beating occurs in the cavity, resulting in strong variations of the temporal profile of the pulses. The generated spikes can then be ten times higher than the maximum of equivalent seeded pulses. This strong temporal incoherence is shown to engender spatial incoherence in the focal plane of such unseeded pulses leading to an instantaneous angular displacement of tens of A mu rad. This effect is even more pronounced after frequency conversion.