Elemental mercury sensing by synchronously sweeping two multimode diode lasers

We propose a sum-frequency-generation (SFG) laser-based elemental mercury sensing method by mixing two low-cost multimode diode lasers (MDLs). The wavelengths of the two MDLs are synchronously scanned, which enlarges the whole coverage range of wavelength and improves the measurement stability. Correlation spectroscopy was used to eliminate the impact of environmental change and enhance and trace the absorption signal of the sample accurately. A novel data processing method was employed to extract the weak absorption signals from the background efficiently. A sensitivity of ${0.1}\;\unicode{x00B5} {{\rm g/m}^3}$0.1µg/m (11 ppt) was achieved for 1-m path length and 10-s integration time. The sensing range was efficiently increased up to ${200}\;\unicode{x00B5} {{\rm g/m}^3}$200µg/m using a calibration curve based on a new mathematical analytical formula. Real-time monitoring of the mercury volatilization and diffusion process was experimentally demonstrated with a time resolution of 10 s. The performance of the system shows great practical value for the detection of elemental mercury in industrial applications.