Comparison of signals under top-hat and Gaussian beam excitations in surface thermal lens technique

A theoretical model for the surface thermal lens (STL) signal with modulated top-hat and Gaussian beam excitations is developed. For an optical coating sample, distributions of the temperature and surface deformation in both transient and quasi-steady states are deduced, and the STL amplitudes, corresponding to the alternating current (AC) deformation and direct current (DC) deformation, respectively, are defined. Numerical simulations and comparison results for the temperature and deformation demonstrate that there exist large differences in radial distributions between the AC and DC cases especially in the high modulation frequency range. The pulsed or AC STL amplitude under the top-hat beam excitation is approximately two times of that under the Gaussian beam excitation at the optimum detection distance in the high frequency, and correspondingly, the DC STL amplitude in quasi-steady state with top-hat beam excitation is only ~1.1 times of that with Gaussian beam excitation at the optimum detection distance. Influences of the heating-beam radius and modulation frequency on the STL amplitudes are also presented and compared. The application of the STL technique to the deformation measurement of an optical component is discussed.