Detection of elemental mercury using a frequency-doubled diode laser with wavelength modulation spectroscopy

We demonstrate a new method for elemental mercury sensing by wavelength modulation spectroscopy (WMS) using a tunable ultraviolet laser generated through a process of second-harmonic generation (SHG). The WMS is implemented by fast modulating the injection current of the Fabry–Perot-type green diode laser equipped with a Littrow grating to increase the laser-mode power density. The technique of correlation spectroscopy is exploited to deal with the signal variations due to mode hops and guarantee the measurement accuracy. According to the performance evaluation, the SHG-WMS system exhibits a better sensitivity (0.15 µg/m 3 for 1-m pathlength with an integration time of 10 s) and a comparably high linearity ( R 2 = 0.9995 within the range of 60 µg/m 2 ) compared with the direct absorption scheme. The employment of WMS significantly simplifies the data processing for extraction of small mercury absorption signals from the large and complex SHG light background, and, thus, give robust measurement results. High-harmonic (4 f and 6 f ) detections are also carried out, showing a great potential for suppression of large residual amplitude modulation background. The proposed SHG-WMS system shows great promise for rapid and sensitive mercury sensing in industrial fields.