Abstract:We report a methane sensor based on an integrated MachZehnder interferometer, which is cladded by a styrene-acrylonitrile film incorporating cryptophane-A. Cryptophane-A is a supramolecular compound able to selectively trap methane, and its presence in the cladding leads to a 17-fold sensitivity enhancement. Our approach, based on 3 cmlong low-loss Si 3 N 4 rib waveguides, results in a detection limit as low as 17 ppm. This is 1-2 orders of magnitude lower than typically achieved with chip-scale low-cost sensors.

The dependence of propagation losses on waveguide width and polarization is measured for strip and rib waveguides made of tantalum pentoxide. For strip waveguides, the propagation losses are found to increase rapidly for widths <3 µm. The losses were significantly smaller for rib than for strip waveguides, as expected. A method is developed for estimating the dependence of propagation losses on waveguide width. The method is based on approximating sidewall imperfections with an area with complex refractive index in a 2D model and showed a good agreement with the measured dependence on waveguide width. The method is also used to predict that propagation losses will decrease rapidly for rib heights less than 20 nm for TM-polarization.

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A Mach-Zehnder interferometer made with shallow rib waveguides is studied experimentally and using simulations. The rib-height giving single-mode guidance is found as function of core thickness and polarization. Devices have been made using shallow rib waveguides (5 nm rib height) in silicon nitride. The sensitivity and the limit of detection (LOD) is studied experimentally regarding the length of the sensing window and for two cover media: water with hydrochloric acid (HCl) and polydimethylsiloxane (PDMS). For HCl-solution, which is the standard method for testing Mach-Zehnder interferometers, the measured sensitivity and LOD was 13000π rad/RIU and 1.1 × 10-7 π RIU, respectively, for a 2 cm long sensing window. This is comparable to the best results reported previously. With PDMS as the cover medium, the temperature of the device was increased in order to measure the sensitivity. This is a new approach that makes it possible to measure the sensitivity with a solid cover medium which has a relatively high temperature coefficient for the refractive index. Measured sensitivity and LOD was 15200π rad/RIU and 1.3 × 10-7 π RIU, respectively, again for a 2 cm long sensing window. Measured sensitivities agreed with simulations and increased linearly with sensing length as expected. However, the LOD showed a minimum for 2 cm sensing length. This was mainly due to increased noise for 3 cm sensing length, both for HCl-solution and PDMS. With higher sensitivity and similar LOD for PDMS compared to HCl-solution, it is concluded that using the temperature dependence of PDMS is a good alternative for testing Mach-Zehnder interferometers.

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