A Stable Evanescent Field-Based Microfiber Knot Resonator Refractive Index Sensor

An evanescent field-based microfiber knot resonator (MKR) refractive index sensor with long-term stability was proposed in this letter. To protect the microfiber from the mechanical and optical degradation and at the same time preserve the large evanescent waves, a dip-coating technique was presented to coat the MKR and a thin coating in the submicrometer thickness range was experimentally fabricated. The temporal stability of the coated MKR and its sensing properties in refractive index were, respectively, investigated. The results indicated that the coated MKR did not degrade over a long time, and the minimal detectable refractive index of \(2 \times 10 ^{-4}\) was successfully achieved.