W. Schade, U. Willer, I. Kostjucenko, C. Bohling, T. Zentgraf, and D. Scheel, "Evanescent-field laser sensor for in-situ monitoring of volcano gas emissions," in Laser Applications to Chemical and Environmental Analysis, A. Sawchuk, ed., Vol. 69 of OSA Trends in Optics and Photonics (Optical Society of America, 2002), paper SaB4.