Characterization of a waveguide Mach-Zehnder interferometer using PDMS as a cover layer

Abstract

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 1.1x10^-7π 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 1.3x10^-7π 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.