SPIE’s International Symposium on Optical Science and Technology, 7—11 July 2002, Seattle, WA, Diode Lasers and Applications, Paper Number 4817-11. 1 Ultra-Sensitive Gas Detection Using Diode Lasers And Resonant Photoacoustic Spectroscopy Michael E. Webber, Michael B. Pushkarsky and C. Kumar N. Patel Pranalytica, Inc., 1101 Colorado Ave., Santa Monica, CA 90401 T: (310) 458-2624, F: (310) 458-0171, E: webber@pranalytica.com ABSTRACT A novel trace-gas sensor system has been developed based on resonant photoacoustics, wavelength modulation spectroscopy, near-infrared diode lasers and optical fiber amplifiers that can achieve parts-per-billion sensitivity with a ten centimeter long sample cell and standard commercially-available optical components. An optical fiber amplifier with 500 mW output power is used to increase the photoacoustic signal by a factor of 25, and wavelength modulation spectroscopy is used to minimize the interfering background signal from window absorption in the sample cell, thereby improving the overall detection limit. This sensor is demonstrated with a diode laser operating near 1532 nm for detection of ammonia that achieves an ultimate sensitivity of less than 6 parts-per-billion. The minimum detectable fractional optical density, α min l, is 1.8×10 -8 , the minimum detectable absorption coefficient, α min , is 9.5×10 -10 cm -1 , and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5×10 -9 Wcm -1 /√Hz. These measurements represent the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers. Keywords: Diode laser, ammonia, photoacoustic spectroscopy, resonance, fiber amplifier, gas detection 1. INTRODUCTION Significant demand exists for gas-sensing techniques that are fast, sensitive, selective and compact, and laser-based approaches are attractive because they offer the promise of meeting these criteria. For these systems, diode lasers are popular as sources because they have the beneficial features of narrow linewidth, continuous tunability, compact size, cost-effectiveness, and compatibility with optical fibers, which enables convenient alignment and multiplexing. However, the most popular telecom lasers operate at near infrared wavelengths (generally 1.5-1.65 µm), and therefore overlap with the spectra of vibrational overtones, which are in general much weaker than the spectra resulting from fundamental vibrations. To compensate for these small absorption cross-sections, many researchers have turned to more sensitive techniques that work well in the near-infrared such as cavity ring-down and its various off-shoots, auto- balancing, and frequency modulation (FM) spectroscopy. Photoacoustic spectroscopy, which is also known to be very sensitive for detecting fractional absorbance, has many other attractive features, including compact cell size, ruggedness, simple optical alignment, and inexpensive hearing-aid microphones as the transducers. However, this technique is power-dependent, and thus has been mostly deployed using CO and CO 2 lasers. Despite the many attributes afforded by diode lasers, their low output powers (typically less than 25 mW) have prevented their widespread use for photoacoustic spectrometers, and in fact, the authors are aware of less than a dozen published articles to date on this topic, starting with the first introduction by Feher, et al. in 1994. 1—7