Photothermal Interferometry Gas Sensor Based on the First Harmonic Signal

Photothermal interferometry (PTI) incorporated with wavelength modulation spectroscopy is a typical ultrasensitive spectroscopic technique for trace gas detection. In this paper, to our best knowledge, we demonstrate that the first harmonic signal is the best choice for a PTI spectroscopy gas sensing system for the first time. Both the theoretical and experimental results confirm that the first harmonics signal has the largest peak-to-peak amplitude in all harmonic signals. The noise measurement indicates that the all harmonic signals nearly have the same noise level. The detection limit in terms of noise equivalent concentration is improved by 30%, from 114 to 78 ppb ( ${\text{9}}\,\times \,{\text{10}}^{-8}\,{\text{cm}}^{-1}$ in absorption coefficient) by use of the first harmonic signal instead of the second harmonic signal.