UV absorption spectroscopy in optically dense fireballs using achromatic frequency doubling of a broadband modeless dye laser

Optical diagnostics of fireballs, such as in high explosives and metal dust cloud combustion, are typically done in emission due to the intense luminosity, but because of the optical thickness of the fireball, the emission measurement is typically only representative of information near the surface of the fireball. Absorption techniques provide a path averaged measurement and give information about the inside of the fireball. Intense pulsed laser sources can penetrate an optically dense medium and still transmit enough light to disperse into a well-resolved spectrum, from which concentration and temperature information is extracted. In this study, the technique is extended to the UV spectral region by using a broadband frequency doubling technique. The output of a Nd:YAG(355 nm) pumped modeless dye laser can be spectrally dispersed using prisms and focused onto a type-I BBO crystal so that the angular dispersion of the light matches the external phase matching angle of the crystal for each wavelength in the laser. The generated dispersed UV light is recombined by another set of dispersive elements to generate a collimated beam of broadband UV light for the absorption measurement. The system is demonstrated using NO absorption in an electric discharge in the vicinity of 226 nm.