Fuel vapors pose a fire hazard to crews aboard military vehicles, especially because they use a middle-distillate fuel that is similar to commercial kerosene. Now researchers at the US Army Research Laboratory have developed a sensing method that uses a laser-mixing technique to perform fuel vapor measurements quickly and accurately. The vapors from the middle-distillate fuel used in military vehicles are a significant fire hazard. Researchers at the US Army Research Laboratory have developed a laser-mixing technique that can detect fuel vapor quickly and cheaply. The Army has investigated several methods to rapidly detect fuel vapor, including Fourier transform infrared (FTIR) spectroscopy. The drawbacks to FTIR are the size of the instrumentation and its inability to sample in remote or confined locations. Another approach is to use electrothermal or electrochemical sensors, which are inexpensive but which can yield inaccurate results for some fuel products. "The sensor system we use is conceptually very similar to optical phone systems," explained Kevin L. McNesby, a researcher at the lab and co-author of a report that appeared in the Feb. 20 issue of Applied Optics. "We take several lasers, each emitting at a discrete wavelength corresponding to a known absorption in the target gas, and we turn each laser on and off at slightly different frequencies." The technique mixes the output of the lasers in a single optical fiber, which is terminated with a gradient index lens that collimates the light. The light then travels through free space without excessive divergence. After the light has traveled 10 to 100 cm, a mirror reflects it to a room-temperature photodetector, typically InGaAs for the spectral range of 1.5 to 1.8 µm. The electrical signal from the photodetector is demodulated, revealing the contribution to the signal from each wavelength of the light that was in the fiber. If the researchers know how much light was initially emitted, they can measure the wavelength-dependent absorption of the gas in its path and so determine the identity and quantity of any vapor that is present. Telecom by-products A technique based on infrared laser diodes has many benefits, McNesby explained. The sensors have high temporal resolution, and the lasers' microwatt output enables safe sampling. Moreover, because they are based on inexpensive fiber optics, lasers and detectors, a damaged module can be replaced cheaply. Oddly, the technology bucks the historical trend in photonics, where military advances trickle down to industry. "All these advantages we employ are by-products of the [telecommunications] industry," McNesby said, "which has driven the tech base for diode laser spectroscopic techniques."