Measurement of low-absorption coefficients using the beat frequency shift between transverse modes of a laser

A new method of absorption measurement of liquids is presented. This method takes advantage of the shift in beat frequency between a strong Gaussian mode, forming a thermal lens in the liquid, and a weak first transverse mode acting as a probe to detect the thermal lens. The light absorption coefficients of the liquids are directly related to the beat frequency changes that are measured on an RF spectrum analyzer. The approach is general in that it can be used to measure the focal length of any weak lens. It appears to be more sensitive by nearly an order of magnitude than the methods that measure change in spot size.