Characterisation, wavefront reconstruction, and propagation of ultra-broadband laser pulses from Hartmann-Shack measurements

Abstract

Experiments in laser physics often require more comprehensive information about a beam than can be extracted from temporal and spatial profile measurement alone. In particular, the wavefront has considerable effect on both irradiance and phase distribution near focus, and thus large impact on the efficiency of non-linear coherent processes such as generation of higher harmonics from femtosecond ultra-short laser pulses. Here we present Hartmann-Shack wavefront measurements of ultra-broadband laser pulses with a spectral bandwidth of >190 THz, which are produced by focusing amplified pulses from a 20 fs Ti:Sapphire oscillator-amplifier system into an Argon filled hollow fibre of 400 μm diameter. After re-compression the pulses were analyzed with the Hartmann-Shack sensor, both at a distance of 140 cm behind the fibre exit and after reflection from a concave mirror (f = 100 mm). Measurements of the overall polychromatic wavefront are faced to a couple of quasi-monochromatic ones covering the whole spectrum. Incoherent superposition of the spectral components yields excellent agreement to the measured overall wavefront, showing that the total wavefront can be sensed reliably by a single measurement. Furthermore, comparison of numerically propagated and measured wavefronts shows good agreement for different spectral components: the measured overall wavefront fits, within sensor accuracy, the numerically propagated one obtained by incoherent superposition of its quasi-monochromatic parts. Drawbacks and opportunities of the Hartmann-Shack technique in ultra-short pulse sensing are briefly discussed.