Abstract

The Balloon-borne Laser In-Situ Sensor (BLISS) instrument is a microprocessor controlled high-resolution absorption spectrometer operating in the mid-infrared wavelength region.¹ Tunable lead-salt diode lasers provide infrared radiation in selected wavelength regions for sensitive derivative absorption spectroscopy over a 1-km round-trip path. The TDL radiation is directed to a retroreflector which is lowered up to 500 m below the instrument gondola. A HeNe laser and co-aligned TV camera with CID imaging are used for retroreflector tracking. The instrument operates with three TDLs, and the capability exits to measure five stratospheric species simultaneously: NO, NO₂, HNO₃, O₃, and H₂O. A selected TDL is frequency-modulated at 2 kHz and scanned in wavelength over a spectral feature of the species of interest. An onboard cell filled with a fraction of a torr of the gas to be monitored is used to obtain a reference spectrum, for line identification and wavelength calibration. The returned signal from the lowered retroreflector is demodulated so that the 4-kHz second-derivative signal may be compared with the total returned power which is simultaneously measured by a low-frequency chop/demodulation. From a knowledge of the linestrength, lineshape, stratospheric temperature and pressure, the modulation/ demodulation parameters, and the pathlength used, the mixing ratio of the species may be extracted from the measured absorptance.

© 1985 Optical Society of America