Unbalanced Mach–Zehnder interferometer fabricated by femtosecond laser direct writing for sensing of urea concentrations via evanescent field interaction

Mach–Zehnder interferometers utilizing evanescent field interactions, fabricated via femtosecond laser direct writing, have demonstrated high efficacy in refractive index sensing. In this work, we introduce the first unbalanced Mach–Zehnder interferometer with evanescent field interaction for sensing urea concentrations, crafted using this technique with unique 3D writing capabilities. The interferometer was precisely inscribed in a fused silica substrate, rotated by 5°, positioning the sensing arm close to the surface in a region approximately 6 mm long. Calibration was conducted using Cargille refractive index liquids, revealing a maximum sensitivity of 249.27 nm/RIU at an index of 1.429. Urea solutions were prepared in ultrapure water for a concentration range from 1×10−15 M to 1×10−1 M. The sensor demonstrated a sensitivity of 0.43 nm/M, a limit of detection of approximately 0.42 pM, and a limit of quantification of about 21.73 pM. This detection limit is noteworthy as it rivals the capabilities of fiber optic sensors, yet our device offers the added benefits of easier handling, label-free detection, and seamless integration with fiber optic systems. Overall, this advancement holds significant potential for studying urea at low concentrations in aqueous solutions, with possible applications in laboratory analysis, environmental monitoring, and the field of health.