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

We propose a second harmonic phase angle method (WMS-
$\theta _{2f}$
) based on wavelength modulation spectroscopy for trace gas detection. WMS-
$\theta _{2f}$
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-
$\theta _{2f}$
method experimentally and theoretically, respectively.