Laser heat treatments driven by integrated beams: role of irradiation nonuniformities.

An analysis is given of how nonuniformities in the laser beam intensity translate into variations on the induced temperature distribution on an irradiated sample. The study involves materials with different thermal conductivities. By use of a reshaped irradiating beam obtained with a multifaceted integrating mirror, a three-dimensional numerical calculation allows us to establish both surface and in-depth temperature distributions. The results show that in the case of materials such as glass (i.e., with low thermal conductivity) large thermal gradients occur both on the surface and in depth during irradiation. However, the lateral heat flow is high enough to strongly reduce the surface gradients as soon as the laser irradiation ends. Conversely, in good thermal conductors such as nickel, the laser intensity nonuniformities induce a thermal peaking of the surface with lateral thermal gradients that are by no means negligible. Experimental evidence during laser glass polishing that confirms the numerical assessments are also provided.

Two-faceted mirror for active integration of coherent high-power laser beams.

A new integration method suited for spatially coherent high-power laser beams is demonstrated. The integrator system is based on a mirror with two facets, one of which can vibrate under the action of a piezoelectric translator. After reflection in the faceted mirror, the beam intensity distribution is modified to obtain greater uniformity. However, because of the coherence of the reflected beamlets, this distribution is affected by an interference pattern. The active integration consists of a periodic displacement of the moving facet that causes the interference pattern to vibrate, and its contribution to the intensity profile therefore averages out (fringe visibility within a 5% range). The combination of a faceted mirror and a simple imaging system results in an intensity profile with good uniformity over large spot sizes. Both simulated and experimental results are presented, the latter showing that a final uniformity within a 10% range can be achieved and it is limited mainly by diffraction at the edges of the facets.

Optical glass polishing by controlled laser surface-heat treatment.

It is shown that optical surfaces traditionally ground in conventional glasses with high coefficients of thermal expansion may be polished by irradiation with a space- and time-controlled uniform CO(2) laser beam. Comparisons of a theoretical simulation model of the laser-driven heating process with the experimental results allow us to determine the conditions for successful and reliable use of this technique. The technique can be applied indiscriminately to preheated samples made of different glasses, with any topography, and, of any size in a limited range that depends only on the available laser power.