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

Abstract

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 ⇠1900 nm/RIU via temperature measurements, which corroborates simulation results, thereby paving the way towards gas sensing applications.Index Terms-Integrated photonics, long period grating, refractometry, coupled-local-mode theory, silicon photonics.

Gas Sensor Based on Silicon Nitride Integrated Long Period Grating

“…The rate of energy exchange between the core and cladding modes upon resonance mainly depends on the modulation amplitude and optical field superposition at the modulated guide interfaces, as previously detailed in [9]. In refractometric sensing schemes employing LPGs, the presence of the analyte generally modifies the refractive index (RI) of the equivalent sensing layer which, in turn, modifies the propagation constants of the modes propagating in the waveguide.…”

“…where n g,x is the group index of the x th mode and n sens the RI of the sensing layer of the mode x. Based on (2) and as shown in [9], the LPG sensitivity is enhanced by improving the modal overlap of the modes to couple in the sensing region, which can substantially increase…”

“…In this work, we propose to use the planar multimode Long Period Grating (LPG) described in [9] as gas sensing refractometer. The integrated LPG operates on wavelengthselective coupling of two or more co-propagative modes between the core and cladding modes whose effective indices (EI) can be analyzed using the resonance spectral features.…”

High-sensitivity integrated SiN rib-waveguide long period grating refractometer

“…Table 1 summarizes the sensitivities and specific properties of our proposed structure in comparison with previously reported fabricated LPWGs. Other LPWG designs based on waveguide sensitivity enhancement [12,16] or the PMTP optimization technique [14] exhibit either limited sensitivity enhancement or measurement range, respectively. On the other hand, our proposed sensor demonstrates ultrahigh sensitivity that, more significantly, is found to remain constant over a broad measurement range.…”

“…Their simplicity of fabrication using fiber tapering, irradiation, corrugation, inscription, or arc discharge has also rendered them attractive for mass fabrication. Moreover, LPGs have recently been subject to increasing interest for implementation in integrated photonics [12][13][14][15][16] due to their very high sensitivity potential, small footprints, and specific features that can be exploited on a design-flexible platform. In fact, when compared to typical interferometers or resonators, which exhibit periodic spectral patterns [17], LPGs offer the particular ability to create a localized spectral response while having similar performance potential.…”