Validation of wavelength modulation spectroscopy techniques for oxygen concentration measurement

Tunable Diode Laser Absorption Spectroscopy (TDLAS) based gas sensing technology, has widespread applications, ranging from use in biomedical industry to aerospace applications. An experimental set-up was established to measure concentration of oxygen gas in the range 0-100% using TDLAS technique. The oxygen absorption at 760.241 nm was scanned with a tunable DFB laser, and wavelength modulation spectroscopy was used to obtain the harmonics (1f, 2f, 3f & 4f) of the oxygen absorption signal. The modulation parameters such as the modulation voltage, modulation frequency, reference phase, time constant of lock in amplifier, the tuning voltage, and the tuning frequency were optimized to obtain the harmonics of high amplitude and narrow half width. Keeping the experimental parameters constant, the oxygen concentration measurements were obtained by the following three methodologies, viz, (i) using only the 2nd harmonic, (ii) using the 2nd and 4th harmonics and, (iii) using the 1st and 2nd harmonics. The results of these measurements were compared and the merits and drawbacks of these methods are discussed.