In-Fiber Capillary-Based Micro Fabry-Perot Interferometer Strain Sensor

Air-cavity fiber Fabry-Perot interferometers (FFPIs), composed by a micrometric section of capillary fiber (CF) spliced between two single-mode fibers (SMFs), were fabricated and tested to longitudinal strain using a high-precision, semi-automatic, and reconfigurable mechatronic platform. It was observed that strain sensitivity increased from 2.11 to 27.5 pm/με, to our knowledge one of the largest ever reported for a single FFPI, as the length of the air cavity decreased from 270.9 to 2.189 μm. The characteristics of the reflected spectra of the FFPIs fabricated, high contrast and large free spectral range (FSR), made feasible the intensity-monitoring of strain. A remarkable sensitivity of 3.51 nW/με was obtained with an air-cavity of 16.79 μm. The intensity interrogation of the FFPI allows the reduction of the sensor cost, making the proposed strain sensor very competitive in a wide range of practical applications.