Thermal lens spectroscopy excited with two wavelengths

Abstract

There is a worldwide need for low cost devices with high sensitivity and selectivity for water monitoring and medical and biological analysis. Most commercial equipments suitable for this type of applications are of big dimensions and / or excessive costs. Other types of methods require sample pre-treatments or pre-concentrations making them time consuming. In this work we show the design of a compact low cost thermal lens system for water monitoring. The device is made up of two compact excitation lasers (pointers) of different wavelengths, 405 nm and 532 nm, modulated at low frequencies (4 and 7 Hz respectively), allowing differential measurements in real time. It also includes a red laser diode as probe which goes through the sample collinearly with the excitation beams. A small sample of the probe beam impinges on a 200μm core optic fiber and is guided to a photodiode. The detector signal is digitized by an Arduino microcontroller and sent to a PC to be analyzed. The signal processing is made in real time applying a Fast Fourier Transform to distinguish the signals that come from both excitation sources. A flow injection analysis system was implemented in order to speed up measurements. In this work we show how this system works using low toxicity dyes.