The Fresnel diffraction integral is used directly to describe the thermal lens (TL) effect with a mode-mismatched collinear configuration. The TL amplitudes obtained with Gaussian, Airy, and top-hat beam excitations are computed and compared. Numerical results for beam geometries optimized for both near- and far-field detection schemes are presented, and the analytical results developed by Bialkowski and Chartier [Appl. Opt. <b>36,</b> 6711 (1997)] for a Gaussian beam TL effect are summarized in simplified form. Both the numerical and the analytical results demonstrate that, under a beam geometry optimized for either near- or far-field detection, the Gaussian beam TL experiment has approximately the same maximum signal amplitude as does the photothermal-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 computation results further demonstrate that the optimum beam geometry and the TL amplitude depend largely on the excitation-beam profile. The top-hat beam TL experiment is approximately twice as sensitive as the Gaussian beam TL scheme.
© 1999 Optical Society of America
(050.1940) Diffraction and gratings : Diffraction
(120.6810) Instrumentation, measurement, and metrology : Thermal effects
(300.1030) Spectroscopy : Absorption
(300.6430) Spectroscopy : Spectroscopy, photothermal
Bincheng Li and Eberhard Welsch, "Probe-Beam Diffraction in a Pulsed Top-Hat Beam Thermal Lens with a Mode-Mismatched Configuration," Appl. Opt. 38, 5241-5249 (1999)