Trace chemical vapor detection by photothermal interferometry

摘要

Photothermal interferometry has been demosntrated as a technique that can detect vapors with extremely high sensitivity (parts-per-trillion levels). Our present research uses a photothermal detection scheme that incorporates tunable sources and a modified Jamin interferometric design to provide high selectivity and sensitivity for orgnao-phosphate vapor detection. Two possible tunable excitation sources are being studied for this sensor technology, a tunable CO_2 laser and difference frequency mixing of a tunable NIR laser with a fixed wavelength NIR laser in a nonlinear crystal. The modified Jamin design imparts superior vibrational immunity by ensuring both interferometer eams encounter common optical elements. Examining the two complementary optical outputs of the interferometer, phase shifts on microradian levels have been detected. Trace chemical vapor detection is accomplished by introduicng the tunable excitation laser source across the path of one interferometer beam providing a phase shift due to absorptive heating. Prelimianry results indicated parts-per-billion level detection ofboth DMMP and DIMP using approx 400 mW of CO_2 laser power at appropriate wavelengths.