Second Harmonic Phase Angle Method Based on WMS for Background-Free Gas Detection

Abstract

We propose a second harmonic phase angle method (WMS- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta _{2f}$</tex-math></inline-formula> ) based on wavelength modulation spectroscopy for trace gas detection. WMS- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta _{2f}$</tex-math></inline-formula> exhibits the advantages of background-free and immunity to light intensity fluctuations. The detection sensitivity is 1-2 orders of magnitude higher than that of the first harmonic phase angle detection method. Moreover, there is room for further improvement in sensitivity by optimizing the linearity of the light intensity modulation of the laser. We verified the feasibility of the WMS- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\theta _{2f}$</tex-math></inline-formula> method experimentally and theoretically, respectively.