Probe-beam diffraction in a pulsed top-hat beam thermal lens with a mode-mismatched configuration

The Fresnel diffraction integral is used directly to describe the thermal l ens (TL) effect with a mode-mismatched collinear configuration. The TL ampl itudes obtained with Gaussian, Airy, and top-hat beam excitations are compu ted and compared. Numerical results for beam geometries optimized for both near- and far-field detection schemes are presented, and the analytical res ults developed by Bi-alkowski and Chartier [Appl. Opt. 36, 6711 (1997)] for a Gaussian beam TL effect are summarized in simplified form. Both the nume rical and the analytical results demonstrate that, under a beam geometry op timized for either near- or far-field detection, the Gaussian beam TL exper iment has approximately the same maximum signal amplitude as does the photo thermal-interference scheme. A comparison between the optimum near- and far -field detection beam geometries indicates that a practical mode-mismatched TL instrument should be based on the far-field detection geometry. The com putation results further demonstrate that the optimum beam geometry and the TL amplitude depend largely on the excitation-beam profile. The top-hat be am TL experiment is approximately twice as sensitive as the Gaussian beam T L scheme. (C) 1999 Optical Society of America. OCIS codes: 050.1940, 120.68 10, 300.1030, 300.6430.