Abstract

In real-world experiments with chirp-compensated ultrashort-pulse lasers in the sub-20 fs range, the beam parameters strongly depend on the adjustment state of the laser system. To characterize the radial homogeneity of the temporal coherence of femtosecond laser beams, spatially resolving measuring technique is necessary. Matrix-shaped thin-film microoptical devices deliver a spatial resolution given by their pitch. Three types of multichannel interference experiments with pulses of 12.5…30 fs emitted from a chirp-compensated Ti:sapphire laser system have been performed with cellular arrangements of uniform as well as non-uniform thin-film microlenses. By the help of a BBO-crystal (100 μm thickness), nonlinear conversion of the interference patterns was realized by SHG. With orthogonal arrays of photolithographically produced resist microlenses on a thin quartz substrate, characteristic Newton s fringes were generated as shown in Fig. la. Because of the sensitivity of a Fabry-Perot etalon against the coherence time [2], the interferometric single-shot mapping of the beam contains information about the spatial homogeneity of the temporal coherence. Contrast and spatial frequency were analysed with image processing tools. To visualize the spatially resolved contrast, a cooccurrence matrix was used (Fig.1b). The standard deviation of the average contrast of 135 elements was found to be about 26%. The central dip should be caused by maximum spectral broadening corresponding to the shortest coherence time. With arrays of thin-film micro-axicons of Gaussian phase profile, free-space Bessel beam intensity fringes can be detected [3] which also carry spatially resolved coherence information. In the case of femtosecond self-imaging of an array-shaped phase-object, the coherence information is dissipated in lateral direction (by diffraction and refraction) so that the spatial resolution decreases with the order of the Talbot image. The bandpass properties of microoptical components given by the small apertures have to be taken into account as a limiting factor for multichannel interferometry of ultrashort pulse lasers. For all investigated types of interferometers, the intensity contrast depends not only on the spectral bandwidth but also on travel time effects. The separation of spectral interference, travel time effects and pulse deformations by filter functions of the device is all but trivial. First numerical simulations have been started.

© 2001 EPS