Sensitivity Pattern of Femtosecond Laser Micromachined and Plasma-Processed In-Fiber Mach-Zehnder Interferometers, as Applied to Small-Scale Refractive Index Sensing

A highly sensitive refractive index (RI) sensor based on an in-fiber Mach-Zehnder interferometer is discussed. The sensor is fabricated by femtosecond laser in a single-mode fiber. A cylindrical cavity is micromachined in the fiber cladding and partly in the fiber core, forming an interferometric structure. We have found that when the spectral response of the structure to RI is considered, a periodic pattern is observed and the sensitivity highly increases with RI and reaches over 23 000 nm/RIU within the RI range of 1.4200-1.4400 RIU. Moreover, during the measurement of cavities with diameters in the order of tens of micrometers, we found the cavity-filling process to be both difficult and time-consuming, especially when high-RI liquids are being investigated. We therefore applied reactive ion etching in oxygen plasma, which significantly changed the wettability of the cavity's surface and enabled fast cavity filling with any investigated liquid. Due to its ultra-high sensitivity and capability for investigating sub-nanoliter volumes of liquids, the sensor could be well suited for chemical and bio-sensing applications.