Wavelength-agile diode-laser sensing strategies for monitoring gas properties in optically harsh flows: application in cesium-seeded pulse detonation engine

The rapid, broad wavelength scanning capabilities of advanced diode lasers allow extension of traditional diode-laser absorption techniques to high pressure, transient, and generally hostile environments. Here, we demonstrate this extension by applying a vertical cavity surface-emitting laser (VCSEL) to monitor gas temperature and pressure in a pulse detonation engine (PDE). Using aggressive injection current modulation, the VCSEL is scanned through a 10 cm(-1) spectral window at megahertz rates - roughly 10 times the scanning range and 1000 times the scanning rate of a conventional diode laser. The VCSEL probes absorption lineshapes of the similar to 852 nm D-2 transition of atomic Cs, seeded at similar to 5 ppm into the feedstock gases of a PDE. Using these lineshapes, detonated-gas temperature and pressure histories, spanning 2000 - 4000 K and 0.5 - 30 atm, respectively, are recorded with microsecond time response. The increasing availability of wavelength-agile diode lasers should support the development of similar sensors for other harsh flows, using other absorbers such as native H2O. (C) 2002 Optical Society of America.