Pulsed mode thermal lens effect detection in the near field via thermally induced probe beam spatial phase modulation: a theory.

A Fresnel diffraction model for dual beam pulsed mode laser thermal lens effect detection is described. The model accommodates the effects of aberrations in the lens element introduced by departures in the sample's thermally induced refractive index profile from the ideal parabolic approximation. The model also accommodates probe and irradiation beams of arbitrary complex radius at the sample, and permits the computation of probe beam intensity profiles observed at arbitrary cell-detector distances. The theoretical basis for a new method of thermal lens effect detection is demonstrated in which the radial dimensions of the lens element are much smaller than the probe beam. Detection of the thermal blooming effect is achieved by a Fourier transform method which uses spatial frequency domain detection to measure thermally induced departures in the probe beam's intensity profile from the TEM(0,0) Gaussian mode structure, as the lens element forms. This strategy, combined with near field detection predicts a sensitivity enhancement by a factor of 60 relative to the conventional far field beam center measurement.