Localized strain sensing with fiber Bragg-grating ring cavities

Abstract

We report the theoretical description and the experimental demonstration of an optical resonator formed by inserting a Fiber Bragg Grating (FBG) in a closed fiber loop. The spectral characteristics of such a resonator strongly depend on the reflectivity of the FBG. In the wavelength region where the FBG reflectivity R is negligible, the system behaves like a conventional ring resonator. On the other hand, when R is not vanishing, a split-mode structure can be observed, associated to the degeneracy removal of two counterpropagating resonant modes. The magnitude of the mode splitting can be used to sense small variations of the FBG physical parameters, such as length, temperature or group index. An example of strain sensing with this setup is reported, showing that the mode splitting is sensitive to a mechanical strain applied to the FBG, while it is almost insensitive to a strain applied to any other point of the resonator. This peculiar feature allows to perform cavity-enhanced, local strain measurements with a reduced sensitivity to environmental perturbations, which represents an important improvement in the framework of the fiber-optic sensors.

Split-mode fiber Bragg grating sensor for high-resolution static strain measurements

“…As already shown in [12], the split resonances exhibited by this resonator make it a good candidate as a strain sensor. In fact, because of the long fiber loop, these resonances can be extremely narrow.…”

“…While a full theoretical description of the spectral properties of FBGRRs can be found in [12,13], the general behavior of these devices is recalled in the following. At wavelengths where the FBG reflectivity R ∼ 0, the loop behaves like a conventional ring resonator, with the optical power circulating only in the direction of excitation and an output spectrum formed by equally-spaced resonances.…”

“…The two counterpropagating modes are coupled by their continuous exchange of energy via the FBG element, resulting in a frequency splitting, S, of the cavity resonances. For a resonance at wavelength λ, in the small coupling approximation, the splitting is given by [12,13]:…”

Impedance self-matching ultra-narrow linewidth fiber resonator by use of a tunable π-phase-shifted FBG

“…have attained ~0.5 pm (60 MHz) wavelength resolution 13 . Recently, cavity enhanced methodology has been realized by fabricating a FBG based ring resonator 14, 15 , or more often, by fabricating a fiber based Fabry-Perot interferometer 16–19 , where it is easy to obtain a high wavelength resolution cavity (~1 pm, 125 MHz for a 12 mm long cavity typically). In cavity enhanced methodology, an improvement of wavelength resolution can be achieved by increasing the cavity finesse or the cavity length.…”

Enhanced spectral response of π-phase shifted fiber Bragg gratings in closed-loop configuration

“…As already described in [9], in a ring resonator the grating acts as a coupling element for the two counterpropagating modes, giving rise to a transmission spectrum formed by equally spaced split resonances. In correspondence of the π-FBG reflectivity drop, the two wings of these split-resonances are partially overlapped, and their interference leads to an extremely narrow notch figure in the transmission spectrum.…”