Physical sensing with optical-fiber ring, grating-based and coupled resonators
Fiber Bragg gratings (FBGs) possess an undisputed potential in the field of thermal and mechanical sensing owing to their intrinsic sensitivity, immunity to electro-magnetic interference and suitability of optical fibers to in-field monitoring of solid materials and large environments. In recent years, sensing schemes based on unconventional fiber structures have been developed, aiming at more sophisticated applications, such as FBG Fabry-Pérot resonators or fiber ring resonators that include a π phase-shifted Bragg grating (π-FBG). Some fo the developed sensors were exploited for real applications, such as acoustical instrument pick-ups, seismic accelerometers and, in a slightly modified version, fiber-based detectors of ionizing radiation for medical use.
On a parallel path, low-noise laser-based detection schemes, in most cases an extension of spectroscopic techniques, have been deployed to interrogate the above mentioned sensors, .
In a recent work, our group has demonstrated that deformations in the infrasonic range can be detected beyond the resolution of conventional techniques, using a semiconductor diode laser stabilized against an optical frequency comb for high-precision interrogation of a FBG Fabry-Pèrot cavity. A resolution level down to 10<sup>-13</sup> is achieved that is no longer hampered by laser noise or shot noise, intrinsic thermal phase fluctuations of the fibers being the only limit.
Despite the impressive strain resolution levels shown in previous works, both in the acoustic domain and the infrasonic frequency range, measurement of fully static deformations still remains extremely challenging for most cavity-based techniques. One major limitation comes from the ambient perturbations acting on the resonator.
In a recent experimetal activity, we analyzed in detail the performance of a fiber-optic strain sensor obtained by integrating a π-FBG in a closed fiber loop, which shows a much reduced sensitivity to external perturbations of the resonator. A full theoretical description of the spectral properties of a Fiber Bragg ring resonator (FBGRR) has been developed and recent experiments have shown the actual possibility to exploit FBG-based ring resonators for static strain measurements at unprecedented resolution levels, in the 10<sup>-12</sup> range.
Exploiting these resolution levels and the described sensor configurations, we have provided the evidence that fibers are suitable for detection of ionizing radiation, i.e. photons or electrons, in the Mev energy range. This paves the way to new generation of all-optical dosimeters for medical applications and radio-protection purposes. Work is still in progress towards new directions for the realization of portable devices, ultimately based on new materials combined to optical fibers.