Theoretical study of an evanescent optical integrated sensor for multipurpose detection of gases and liquids in the Mid-Infrared

Abstract

International audienceA theoretical study of evanescent optical sensor for multipurpose detection in the Mid-Infrared of gases and pollutants in water is presented in this paper. The opto-geometrical parameters of the transducers - ridge waveguides - have been optimized in order to obtain the highest evanescent power factor for monomodal propagation in the Mid-Infrared. The highest sensitivity has been obtained for a configuration with an optimal length of waveguide L-ops = 4.3 cm for intrinsic propagation loss equal to 1 dB/cm. Then a spiral waveguide configuration is suggested to obtain this optical length path in a monolithic structure. A numerical example is also included using a ridge waveguide based on chalcogenide glasses (GeSbSe). In case of gas detection, a generic calculation of the minima concentrations to be detected as a function of the molar absorption for any working wavelength is presented. Extremely low limits of detection can be achieved due to the strong absorption coefficients of gases and chemical species in the Mid-Infrared spectral range, 268 ppb in case of carbon dioxide at lambda =4.3 p.m, 1.848 ppm and 781 ppb for methane at lambda=3.31 pm and at lambda=7.66 pm respectively. For the pollutants detection in water, an improvement of the integrated structure has been proposed to avoid water absorption in this spectral region by deposing a polymer (PIB) as waveguide superstrate, thus the limit of detection for toluene is 26 ppb at lambda=6.68 pm. These concentration minima that could be detected by the Mid-IR sensor are lower than the threshold limit values determined in the international environmental and health standards. Hence this integrated optical sensor may be considered as an attractive support tool in monitoring environmental and health fields. (C) 2016 Elsevier B.V. All rights reserved

Mid-infrared integrated photonics on silicon: a perspective

“…Given an NEP of 0.69 pW/Hz 1/2 and a factor of 166 / 2 6.4 = SNR enhancement from the Fellgett's advantage [213], the detector-limited noise-equivalent absorbance (NEA, given as the minimum detectable absorbance Γα′L) is NEP/ (0.18 μW)/6.4 = 6.0 × 10 −7 Hz −1/2 . Such an impressive NEA, while quoted in previous on-chip sensor performance projections [214], is challenging to attain in practice [215]. Empirically, NEA down to 8.5 × 10 −4 Hz −1/2 has been experimentally measured in on-chip waveguide sensors [216].…”

Experimental design approach for deposition optimization of RF sputtered chalcogenide thin films devoted to environmental optical sensors

“…The optical waveguiding requires selenide layers with few µm thicknesses 10, 11 , consequently a long deposition time and/or a high deposition rate are required. Thus, it is quite important to control that any compositional drift during the deposition will be observed.…”

“…Indeed, to develop an optical integrated system for mid-IR sensors which requires precise opto-geometric characteristics of the chalcogenide layers 10 , it is essential to adequately determine the influence of deposition parameters. By the role discrimination of each factor and their possible correlation, it will be possible to control thin film characteristics such as thickness, refractive index, roughness, or chemical composition.…”

“…In the ternary Ge-Sb-Se system, pseudo-binary (GeSe 2 ) 100−x (Sb 2 Se 3 ) x (x = 5, 10, 20, 30, 40, 50 and 60) glasses were already studied for photonics applications concerning nonlinear optics or sensors 10, 11, 35–40 . For this study, two nominal compositions, Ge 28.1 Sb 6.3 Se 65.6 (x = 10) and Ge 12.5 Sb 25 Se 62.5 (x = 50), were selected for their excellent mid-IR transparency, high stability against crystallization and refractive index contrast allowing optical waveguiding in mid-IR 10, 11, 39 .…”

“…For this study, two nominal compositions, Ge 28.1 Sb 6.3 Se 65.6 (x = 10) and Ge 12.5 Sb 25 Se 62.5 (x = 50), were selected for their excellent mid-IR transparency, high stability against crystallization and refractive index contrast allowing optical waveguiding in mid-IR 10, 11, 39 . The fabrication of these two different chalcogenide films has been optimized by the experimental design approach in order to fully meet the criteria for producing optical components devoted to optical bio-chemical sensor applications, for which controlling the layer homogeneity is fundamental for thicknesses of few µm (from 1 to 6 µm).…”