Integrated Width-Modulated SiN Long Period Grating Designed for Refractometric Applications

A novel integrated photonic structure based on Long Period Waveguide Gratings (LPWGs) relying on channel-width modulation is proposed and tested for refractometric applications. These LPWGs have been fabricated through a Silicon Nitride process and experimentally characterized in terms of both surface and bulk sensitivities. Surface sensing configuration is first achieved by propagating one of the coupling optical modes into an epoxy-based negative photoresist SU8 cladding region that is in contact with the analyte via its outer surface. We subsequently show that the proposed LPWGs cladding layer can be advantageously replaced by a gas-porous polymeric bulk layer such as Styrene-co-AcryloNitrile (SAN) as the cladding region to be directly sensed to anticipate future gas sensing applications. Here, bulk sensing is optimized by increasing the analyte's influence on the modal propagation constants as it is demonstrated to be currently the most promising solution to effectively enhance the figure of merit of long period gratings of given lengths. Using varying water-glycerol mixtures, the surface sensitivity of these LPWGs has been measured at up to 240 nm per RI unit (RIU) that is in agreement with simulation. In addition, the bulk sensitivity has been indirectly estimated to be similar to 1900 nm/RIU via temperature measurements, which corroborates simulation results, thereby paving the way towards gas sensing applications.