1.8 μm vertical-cavity surface-emitting laser absorption measurements of HCl, H₂O and CH₄

A 1.8 μm wavelength single frequency InGaAIAs-InP vertical-cavity surface-emitting laser (VCSEL) was used successfully for the first time for high resolution absorption spectroscopic measurements. As a demonstration of the multi-species measurement capability, spectra of HCl, CH₄ and H₂O gases were measured at pressures ranging from 0.07 to 1.5 bar and compared with calculations based on the HITRAN2000 database. In general, good agreement was observed. The laser threshold was 0.9 mA, the temperature tuning rate was 0.125 nm K^-1 (0.38 cm^-1 K^-1) and the current tuning rate was 0.9 nm mA^-1 (2.75 cm^-1 mA^-1). A continuous mode-hop-free single frequency current tuning range of 8.4 cm^-1 (2.8 nm) was achieved using a 0-4.1 mA driving current. Single frequency VCSELs show a variety of advantages compared to edge emitting semiconductor lasers, such as wide current tuning range, very high tuning speeds (MHz) and therefore very good time resolution, little susceptibility to optical feedback and low manufacturing costs. The availability of long wavelength single frequency VCSELs (1.3-2μm) should make VCSEL the preferred choice for diode-laser gas sensing and process control applications and help to significantly expand the application fields for infrared diode-laser gas sensors.