Modelling of Rib channel waveguides based on silicon-on-sapphire at 4.67 μm wavelength for evanescent field gas absorption sensor

The mid-IR spectrum contains absorption peaks for a wide variety of trace gases, for instance, N2O, CO, CO2, H2O, NO2, NH3, NO and CH4 etc. These gases strongly absorb in the mid-IR >2.5 μm spectral region due to their fundamental rotational and vibrational transitions. In this work, we propose a silicon-on-sapphire (SOS) rib waveguide design at 4.67 μm for carbon monoxide (CO) gas sensing based on the evanescent field absorption. The exponentially decaying evanescent fields in the lower index region of a waveguide offer a high perspective in the realization of a variety of sensors. Numerous waveguide dimensions are modelled and studied in order to achieve an optimal evanescent field ratio (EFR) with lowest possible total losses of the waveguides. The total losses of these waveguides are in the range of 2.59-3.89 dB and 1.51-1.76 dB for TE and TM modes, respectively.