Theory of thermal lens signal on optical film surface excited by CW modulated hat beam

Theoretical model of surface thermal lens excited by flat-topped beam in optical thin film measurement

However, the output of high-power laser is usually the superposition of multiple modes, not the Gaussian distribution (TEM00). So it is very convenient to add an aperture to get a hat beam.

In previous surface thermal lens devices, Gaussian beams are mostly used as excitation light. However, due to the multimode output of high power laser, it is difficult to obtain strict fundamental mode Gaussian beam. Adding a diaphragm with a certain size in front of the laser can realize a flat-topped beam, which reduces the requirements for laser beam quality and is more conducive to actual measurement.