Tunable diode laser spectroscopy of ethylene oxide near 1693 nm

We describe the application of the vertical-cavity surface-emitting laser (VCSEL) to absorption spectroscopy of the ethylene oxide (EO) Q-branch near 1693 nm. The VCSEL was electrically scanned over spectral intervals of up to ∼13 cm⁻¹ at a 1 kHz repetition rate. A methane absorption line at 5903.3 cm⁻¹ and Fabry–Pérot etalon with a free spectral range of 0.4 cm⁻¹ were used to calibrate frequency scale. The EO was mixed with ambient air, and total gas pressure and mixing ratios were varied from a few mbar to 1 bar and from ∼10² to 10⁵ parts per million, respectively. A rapid roll off of EO absorbance at 5903.7 cm⁻¹ was observed at gas pressures below ∼0.5 bar. A linear dependence of EO peak absorbance on mixing ratio was found at a total gas pressure of 1 bar. We conclude that monolithic VCSELs operating near 1693 nm could be used in EO sensors with a detection limit in the ppb range.