Theoretical and Experimental Investigation of the Relationship between Aperture Dimension and Signal-to-Noise Optimization in Thermal Lens Spectroscopy

The effect of three types of experimental laser noise on conventional thermal lens spectroscopy are considered. Theoretical calculations of noise and signal-to-noise ratio as a function of aperture size are included to determine ideal aperture dimensions for systems limited by laser pointing instabilities, flicker noise, and/or shot noise. Experimentally determined noise curves are also presented and discussed in the context of the theoretical results. The results show that for most thermal lens configurations based on lock-in detection, optimum S/N is obtained at an aperture-to-beam-size ratio of 0.6.