Application of a widely electrically tunable diode laser to chemical gas sensing with quartz-enhanced photoacoustic spectroscopy

A near-infrared diode laser with sample-grating distributed Bragg reflectors was used as a widely tunable spectroscopic source for multispecies chemical sensing. Quartz-enhanced photoacoustic spectroscopy was utilized to obtain high absorption sensitivity in a compact gas cell. ${\mathrm{CO}}_2$, ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{C}}_2{\mathrm{H}}_2$, and ${\mathrm{NH}}_3$ were monitored. A noise equivalent sensitivity of $8\times {10}^{-9} {\mathrm{cm}}^{-1} {\mathrm{W}}^{-1} {\mathrm{Hz}}^{-1/2}$ for ${\mathrm{NH}}_3$ detection was achieved, which corresponds to a ${\mathrm{NH}}_3$ mixing ratio of 4.4 parts in ${10}^6$ by volume (ppmv) with a 1-s time constant and available 5.2-mW optical power in the gas cell.