An experiment concept to measure stratospheric trace constituents by laser heterodyne spectroscopy

Laser heterodyne spectroscopy (LHS) techniques with semiconductor laser local oscillators (LO) in the 3–30 μm range have the potential to measure radical gas species in the stratosphere. The goal of this experiment is to measure radical gases from Spacelab, including ClO, ClONO₂, HO₂, H₂O₂, N₂O₅, and HOCl in solar occulation with vertical resolution ≦2km and vertical range from 10 to 70 km. Sensitivity analyses have been performed on ClO and O₃ to determine design criteria for the LHS instrument. Results show that O₃ and ClO vertical profiles can be measured with an accuracy ≧95% and ≧80%, respectively, over the total profile. These accuracies require the LO to maintain the following characteristics: frequency stability (Δf _w≦20 MHz), single-mode power (P _LO≧500 μW), and minimum frequency drift (≦5 MHz). Laboratory heterodyne measurements performed with semiconductor lasers generated the same shot-noise photocurrent as CO₂ lasers, for comparable single-mode power. “Excess-noise” regions were identified, but could be wavelength controlled by fine control of operating temperature and injection current. Doppler-shift effects and limited solar occultation measurement times due to Spacelab orbits should pose minimum mission constraints on the experiment.