In this work, we present a concept and method to fabricate miniature, high-quality optical fiber interferometers for sensing applications. The sensitive part of our devices consists of an off-center polymer micro-cap bonded on the end of a single mode fiber (SMF). The SMF end can be cleaved with a small angle or polished flat. In our devices, the reflection from the interface between the polymer and the external medium can be adjusted with an axial misalignment between the core of the SMF and the polymer micro-cap. This allows us to control the fringe contrast of the interference pattern. We fabricated several samples and they were tested as temperature, refractive index, and humidity sensors. In each case, our sensors were compared with commercial ones. Our results suggest that our devices are as accurate and sensitive as wellcalibrated electronic sensors. The sensitivities were found to be 270 pm/°C for temperature sensing and 0.04 RH% for humidity sensing. As refractometers, our interferometers have a resolution of 10<sup>-4</sup> over a broad measuring range that goes from 1 to 1.54. Some applications of the devices here proposed can be monitoring of temperature or humidity in small spaces and refractive index of liquids inside of micro-fluidic channels. An advantage of our sensors is their broad operating wavelength range (from 800 to 1600 nm, approximately). In addition, the thermo-optic or thermal-expansion coefficients of polymers can be tailored. This may allow, for example, to tailor the performance of our sensor depending on the parameter to be sensed.
Based on the attractive elasto-optic properties of single-mode microstructured polymer optical fibres (SM mPOFs) reported elsewhere,10 mode polarisation may be used as sensing probe for several parameters of interest like mechanical pressure. We report on a simplified detection scheme that does not require measuring the actual polarisation state of the light emerging from the mPOF. A polariser and a photodetector in a proper configuration are only required. The detected light intensity shows a high linearity with applied force to the mPOF.
The polarisation state of light may be exploited in single-mode polymer mPOFs for sensing purposes.
The bend-induced birefringence varies linearly with the inverse square of the bend radius, whereas the
twist-induced polarisation rotation varies linearly with the bre twist angle. Both e ects are highly reproducible and show higher sensitivity than their glass counterparts.