Fabrication of intrinsic Fabry–Pérot sensors through proton irradiation

We report on a new technique to fabricate intrinsic fiber Fabry–Pérot interferometric sensors by exposing partially shielded end-faces of cleaved optical fibers to 27 MeV proton irradiation. The dimension of the shield that was placed in front of the cleaved optical fiber array was determined using a simulation package (Stopping Range of Ions in Matter). The simulation package was used to predict the stopping range of the protons in SMF-28 silica optical fibers. The shield consisted of an aluminum wedge that was placed in intimate contact with an array of coplanar cleaved optical fiber end-faces. The wedge profile was designed to influence the stopping distance and the longitudinal straggling range of the ions. The effect of the irradiation on the optical fibers was studied in situ, and the formation of interference fringes was observed when the fibers were interrogated using a MicronOptics SM130 interrogator. The interference fringes were seen to still be present several days after irradiation, and so, it was concluded that an intrinsic fiber Fabry–Pérot interferometer had been formed. The temperature sensitivity of the newly fabricated sensor was determined by immersing it in liquid nitrogen and separately by heating it in an air-circulating oven. The temperature sensitivity was calculated to be approximately 7.91 pm/°C. Optical fiber Fabry–Pérot sensors fabricated through intrinsic refractive index modulation, where the sensing element is the same thickness as the fiber and no external cavity exists, are inherently less affected by environmental refractive index fluctuations, offering an advantage over traditional cavity-based Fabry–Pérot designs. This study was undertaken to demonstrate that intrinsic fiber Fabry–Pérot etalons can be created using proton irradiation to modulate the refractive index.