Design of an optical gas sensor based on chalcogenide (ChG) glass platform in the mid-infrared for detection of CO₂ and CO

In this work, a gas sensing system based on chalcogenide (ChG) glass platform in the mid-infrared (Mid-IR) region is modeled. The proposed gas sensing system composed of a linear tapers waveguide, ridge waveguide, a multimode interferometer (MMI) coupler and transducer arms. The components of the sensing system were simulated using FimmWave from Photon design. First, we determined the structural key parameters of the ridge waveguide that allow for single-mode propagation while maximizing the evanescent confinement field factor. The obtained results show that at the gas absorption wavelengths, \({\uplambda }_{{\text{CO}}_{2}}\) ${\mathrm{\lambda }}_{{\text{CO}}_2}$ = 4.26 µm and \({\uplambda }_{\text{CO}}\) ${\mathrm{\lambda }}_{\text{CO}}$ = 4.6 µm, the evanescent confinement field factor, reached 3.12% and 3.24%, respectively. For these operating wavelengths, a maximum transmission of 99.8% was achieved with a taper length of 450 µm. The footprint of the MMI coupler is 32 \(\times\) $\times$ 9975 µm². A Contrast of 16.6 dB and insertion losses of 2 dB and 2.87 dB were obtained at \({\uplambda }_{{\text{CO}}_{2}}\) ${\mathrm{\lambda }}_{{\text{CO}}_2}$ = 4.26 µm and \({\uplambda }_{\text{CO}}\) ${\mathrm{\lambda }}_{\text{CO}}$ = 4.6 µm respectively. The sensor performance was validated at 4.26 µm and 4.6 µm, respectively, giving a detection limit of 10.73 ppm for carbon dioxide (CO₂) at 4.26 µm and 138 ppm for carbon monoxide (CO) at 4.6 µm. A sensitivity of 3.02 mW.L/mol and 0.12 mW. L/mol, was achieved at the wavelenghts of interset. The obtained results of the sensor by the optimizations of its components serve to enhance a gas sensing system based on chalcogenide (ChG) glass platform.