Detection and measurement of middle-distillate fuel vapors by use of tunable diode lasers

A sensor for the rapid (10-ms response time) measurement of vapors from the hydrocarbon-based fuels JP-8, DF-2, and gasoline is described. The sensor is based on a previously reported laser-mixing technique that uses two tuna ble diode lasers emitting in the near-infrared spectral region [Appl. Opt. 39, 5006 (2000)] to measure concentrations of gases that have unstructured absorption spectra. The fiber-mixed laser beam consists of two wavelengths: one that is absorbed by the fuel vapor and one that is not absorbed. Sinus oidally modulating the power of the two lasers at the same frequency but 18 0 degrees out of phase allows a sinusoidal signal to be generated at the de tector (when the target gas is present in the line of sight). The signal am plitude, measured by use of standard phase-sensitive detection techniques, is proportional to the fuel-vapor concentration. Limits of detection at roo m temperature are reported for the vapors of the three fuels studied. Impro vements to he incorporated into the next generation of the sensor are discu ssed. OCIS codes: 120.0120, 120.1880, 140.2020, 280.0280.