Theoretical and experimental investigation of on-chip mid-infrared chalcogenide waveguide CH4 sensor based on wavelength modulation spectroscopy

Compared with direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS) with good ability of suppressing noise is rarely used in waveguide sensor. The influence of waveguide parameters on WMS sensing performance is required to clarify in both theory and experiment. Three mid-infrared waveguide methane (CH4) sensors (trapezoid, rectangular, suspended) were proposed and optimized for WMS simulation. Theoretical formulation and the experimentally derived sensing noise were used to numerically evaluate the key figure of merit more accurately. Simulation results were presented to show the influence of waveguide parameters on sensing performance. Two chalcogenide on magnesium fluoride (ChG-on-MgF2) trapezoid waveguide CH4 sensors were fabricated using lift-off method, and CH4 measurement was carried out using WMS. The waveguide loss was measured to be 1.52 dB/cm, leading to an optimal sensor length of 2.7 cm. The limit of detection (LoD) of CH4 for the 1cm- and 2cm-long waveguide sensors are 0.68% and 0.17% with an averaging time of 0.2 s. The experimentally achieved second harmonic (2f) signal amplitude and LoD show good agreement with the theoretical results, verifying the feasibility of the design and analysis model of the WMS-based waveguide sensor. Due to the use of WMS and a reduction of waveguide loss from 3.6 to 1.52 dB/cm, the LoD of the 2cm-long ChG-on-MgF2 sensor is 24 times lower than our previously reported 2cm-long ChG-on-SiO2 sensor based on DAS. This work provides a systematic guidance for the design of waveguide gas sensor based on WMS and contributes to improving the sensitivity of on-chip gas sensing.