Investigation On The Gaseous Mercury Monitoring Technology Based On Multimode Diode Laser

Mercury is a poisonous heavy metal element.If it is over discharged,it will not only exert a great influence on industry and agriculture,but also threaten people's health.Mercury pollution is rather serious in China,so a few years ago,provisions have been issued to control mercury emissions.Therefore,effective monitoring of mercury,especially gaseous mercury,is becoming more and more important.In recent years,with the development of photoelectric technology,many chemical monitoring methods are replaced by simple,efficient,low cost optical monitoring methods.And in optical monitoring methods,the optical spectroscopy monitoring has become the mainstream direction.This subject is mainly aimed at elemental mercury,which includes gaseous mercury in the atmosphere and factory emissions.The monitoring of gaseous mercury not only helps to improve mercury removal efficiency,but also has important significance for environmental pollution monitoring and improvement.This paper demonstrates second harmonic generation(SHG)and sum-frequency generation(SFG)based methods to produce multimode ultraviolet radiation at 253.7nm by using tunable multimode semiconductor diode laser combined with correlation spectroscopy technique.Through automatic peak detection and generalized linear fitting algorithm of the sample in the gas chamber to get the concentration of mercury,and realize the continuous monitoring of mercury.The absorption spectrum of mercury is narrow and it has strong absorption of 253.7 nm light.In frequency doubling experiment,the center wavelength of tunable multimode green semiconductor diode laser is 505 nm.By controlling the current and temperature of the laser and adding an external cavity,we finally got 507.3 nm green multimode light,and thus got 253.7 nm multimode UV light through frequency doubling effect by using BBO crystal.The experimental results show that the intensity of the UV light is16.6 nW and in the condition of 1 m path,4 s integration time,the mercury concentration measurement sensitivity reaches 0.07 ?g/m~3.The linear dynamic range of mercury concentration is about 0~45 ?g/m~3.At the same time,gas selectivity and practicability of the system were tested.Finally,the continuous monitoring of volatilization and diffusion of mercury have been finished.In the sum-frequency generation experiment,we used tunable multi-mode red semiconductor diode laser with center wavelength of 687 nm and tunable multimode blue semiconductor diode laser with center wavelength of 401 nm,through SFG produced by BBO crystal,got 253.7 nm UV light.Scan signals were applied both on red and blue laser,which is called dual wavelength scanning.The experimental results show that the intensity of the UV light is 1.0 nW and at 1 m path,10 s integration time,the mercury concentration measurement sensitivity reaches 0.13 ?g/m~3.The linear dynamic range of mercury concentration is about 0~36 ?g/m~3.The corresponding analytical formulas of both linear range and nonlinear range weregiven.By changing the temperature of two lasers,we evaluated system's stability and finished continuous monitoring of volatilization and diffusion of mercury.