Gas temperature measurements using widely tunable long-wavelength VCSEL

Abstract

A method for gas temperature measurements with a widely tunable laser diode is presented. The method involves rapidly switching the laser frequency between two distantly spaced absorption lines chosen for optical thermometry. Direct absorption spectroscopy using a single-mode VCSEL was employed to probe the R10 and R22 lines of the 2ν₁+2ν₂ ⁰+ν₃ combination band of CO₂ near 6355.9 and 6363.7 cm^-1 sequentially. A specially designed 0.5-m cryogenic gas cell was filled with 10 mbar CO₂ at room temperature and cooled to 150 K with liquid N₂. The VCSEL was modulated with a 10-kHz ramp superimposed on a 1-kHz square waveform to scan two 0.04 cm^-1 intervals sequentially. The gas temperatures obtained with the VCSEL in the 150–300 K range are in a good agreement with those derived from gas pressure ratios. The maximum relative error of temperature measurements using the VCSEL was ± 3%. A compact VCSEL-based sensor can be developed for gas temperature and concentration measurements in the Martian atmosphere. The method proposed can be used for many applications including in situ monitoring of combustion processes.