Optical EngineeringGas phase chemical analysis using long-wavelength vertical-cavity surface-emitting lasers
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A method of gas phase chemical analysis using direct absorption spectroscopy is implemented with long-wavelength vertical-cavity surface-emitting lasers (VCSELs) operating near 1577 nm. The method is based on a linear dependence of widths of collisionally broadened absorption lines on gas pressure. It is shown that the absolute gas concentrations in multicomponent gas mixtures can be extracted from the line widths of all compounds measured simultaneously. The concentrations of both absorbing and nonabsorbing compounds are extracted from the results of simultaneous measurements of peak absorption and line width of the absorbing compound. Long-wavelength VCSELs with a buried tunnel junction (Vertilas, Germany) are used, for the first time, for multispecies and trace gas detection. Continuous single-mode tuning of the VCSELs up to 30 cm–1 is achieved with temperature and injection current varied in the range 0 to 50 °C and 1.3 to 6.5 mA, respectively. A fractional absorption of ~10–4 (600 ppm of CO2 in air) is measured with a single-beam spectroscope. The method described can be used for laser chemical analysis of gas mixtures with relatively high concentrations of target compounds and for open-path trace gas sensing. Compact sensors based on long-wavelength VCSELs can be developed for environmental and industrial gas monitoring.