A novel wavelength modulation spectroscopy gas sensing technique with an ultra-compressed wavelength scanning bandwidth.

In a wavelength modulation spectroscopy (WMS) gas sensing system, a scanning ramp combined with a high frequency sinusoidal signal is applied to drive the laser source. Generally, a wide wavelength scanning bandwidth realized by voltage scanning ramp is required to fully cover the target gas absorption profile. In this paper, a novel WMS-based strategy is proposed and verified in a CH detection system. The wavelength scanning bandwidth is compressed from ∼659 pm to ∼166 pm, even narrower than the half width at full height (HWFM) of the CH absorption profile. In addition, the second harmonic signal that induced by the absorption is increased threefold by virtue of making full use of the dynamic range of the preamplifier circuit, and the waveform distortion that comes from the residual amplitude modulation (RAM) effect is eliminated as well. Benefiting from the compressed driving current range, the thermal stability of the laser diode is improved from the original level of 0.5 °C to 0.1 °C. As a result, a linear sensitivity of 75.2 ppb is achieved within 0-3000 ppm CH concentration range at 12.7 s time constant.