Light-induced thermoelastic spectroscopy by employing the first harmonic phase angle method

A highly sensitive gas sensor based on light-induced thermoelastic spectroscopy (LITES) with the first harmonic phase angle(1f-PA) wavelength modulation technique is reported. In this research, methane (CH4) is selected as the target gas, and the detection target wavelength is close to 1653.72 nm. Modulated laser beam passes through a 35 cm absorption cell and is focused on the root of the quartz tuning crystal fork (QCTF). The retrieved in-phase and out-of-phase first harmonic (1f ) components are processed with 1f-PA method. With this detection method, the amplitudes of the 1f-PA have demonstrated immunity to light intensity dithering as well as demodulation phase shift. The fluctuation of 1f-PA amplitudes is less than 0.5% while the light intensity varies between 2.11 mW and 10.20 mW. Also, the 1f-PA amplitudes have ranged within 0.4% while the demodulated phase shifted from 0 degrees to 360 degrees. In addition, the system also demonstrates an excellent linear response. While the concentration varies from 200 ppm to 10,000 ppm, the linearity coefficient (R2) is 0.999. According to Allan deviation analysis, the lowest detection limit is 2.85 ppm, and the corresponding root normalized noise equivalent absorption coefficient (NNEA) is 3.24 x 10-8 cm-1W Hz. With the 1f-PA method, the precise gas concentration which is immune to both light intensity dithering and phase shift can be obtained conveniently for LITES. It has shown great advantages and application prospects in long-term field applications.