Xiutao Lou

Detection of elemental mercury by multimode diode laser correlation spectroscopy

We demonstrate a method for elemental mercury detection based on correlation spectroscopy employing UV laser radiation generated by sum-frequency mixing of two visible multimode diode lasers. Resonance matching of the multimode UV laser is achieved in a wide wavelength range and with good tolerance for various operating conditions. Large mode-hops provide an off-resonance baseline, eliminating interferences from other gas species with broadband absorption. A sensitivity of 1 μg/m3 is obtained for a 1-m path length and 30-s integration time. The performance of the system shows promise for mercury monitoring in industrial applications.

Gas detection by correlation spectroscopy employing a multimode diode laser

A gas sensor based on the gas-correlation technique has been developed using a multimode diode laser (MDL) in a dual-beam detection scheme. Measurement of CO(2) mixed with CO as an interfering gas is successfully demonstrated using a 1570 nm tunable MDL. Despite overlapping absorption spectra and occasional mode hops, the interfering signals can be effectively excluded by a statistical procedure including correlation analysis and outlier identification. The gas concentration is retrieved from several pair-correlated signals by a linear-regression scheme, yielding a reliable and accurate measurement. This demonstrates the utility of the unsophisticated MDLs as novel light sources for gas detection applications.

Oxygen measurement by multimode diode lasers employing gas correlation spectroscopy

Multimode diode laser (MDL)-based correlation spectroscopy (COSPEC) was used to measure oxygen in ambient air, thereby employing a diode laser (DL) having an emission spectrum that overlaps the oxygen absorption lines of the A band. A sensitivity of 700 ppm m was achieved with good accuracy (2%) and linearity (R(2)=0.999). For comparison, measurements of ambient oxygen were also performed by tunable DL absorption spectroscopy (TDLAS) technique employing a vertical cavity surface emitting laser. We demonstrate that, despite slightly degraded sensitivity, the MDL-based COSPEC-based oxygen sensor has the advantages of high stability, low cost, ease-of-use, and relaxed requirements in component selection and instrument buildup compared with the TDLAS-based instrument.

Continuous monitoring of elemental mercury employing low-cost multimode diode lasers

A continuous monitoring system for elemental mercury detection utilizing low-cost multi-mode diode lasers is demonstrated. Ultraviolet light with a central wavelength at 253.7 nm is generated through sum-frequency generation. Correlation spectroscopy is employed to trace the signals of the target gas by using a reference gas cell. Algorithms including automatic peak searching and multiple-linear-regression are employed to retrieve the mercury concentration, and long term measurements are performed. For a path length of 1 m, a sensitivity of 0.12 µg m−3 (13.4 ppt) is achieved for a 60 s integration time, and the linear dynamic range is about 0 ~ 60 µg m−3. Experimental results show that the present system has great potential for industrial applications.

Temperature-Corrected Oxygen Detection Based on Multi-Mode Diode Laser Correlation Spectroscopy

Temperature-corrected oxygen measurements were performed by using multi-mode diode laser correlation spectroscopy at temperatures ranging between 300 and 473 K. The experiments simulate in situ monitoring of oxygen in coal-combustion exhaust gases at the tail of the flue. A linear relationship with a correlation coefficient of -0.999 was found between the evaluated concentration and the gas temperature. Temperature effects were either auto-corrected by keeping the reference gas at the same conditions as the sample gas, or rectified by using a predetermined effective temperature-correction coefficient calibrated for a range of absorption wavelengths. Relative standard deviations of the temperature-correction coefficient calibrated for a range of absorption wavelengths. Relative standard deviations of the temperature-corrected oxygen concentrations obtained by different schemes and at various temperatures were estimated, yielding a measurement precision of 0.6%.

Detection of gaseous elemental mercury using a frequency-doubled green diode laser.

We demonstrate a second-harmonic-generation (SHG) based method for the detection of gaseous elemental mercury by using a newly available green diode laser. Multimode ultraviolet radiation at 253.7 nm is generated through a process of SHG. Correlation spectroscopy is introduced into the scheme to guarantee the measurement accuracy. The limit of detection achieved is 0.6 μg/m3 (0.07 ppb) for 1-m pathlength and 10-s integration time. The measurement accuracy is estimated to be 1.2%. The linear response range is estimated to be 0~60 μg/m2 (6.7 ppb·m), within which the linearity error is less than 1%. Real-time monitoring of mercury volatilization is demonstrated with a time resolution of 1 s. The results of performance characterization show that the proposed method has great potentials for mercury sensing in environmental and industrial fields.