Mode-splitting cloning in birefringent fiber Bragg grating ring resonators.

In this Letter, we report the theoretical model and the experimental evidence of a mode-splitting cloning effect due to the resonant coupling between modes having different polarizations in weakly birefringent fiber Bragg grating (FBG) ring resonators. This modal coupling depends on the fiber birefringence and the FBG reflectivity. In the ideal case of the absence of birefringence, a single split-mode resonant structure can be observed in the resonator transmission spectrum due to the degeneracy removal of the two counter-propagating modes. In the presence of FBG birefringence, a secondary split doublet resulting in a clone of the initial one is generated. The described effect can be exploited for spectroscopic-sensing applications based on more complex split-mode dynamics.

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

The transmission spectrum of a ring resonator enclosing a π-phase shifted fiber Bragg grating (π-FBG) shows a spectral feature at the Bragg wavelength that is much sharper than resonance of the π-FBG alone, and that can be detected with a simple integrated cavity output technique. Hence, the resolution of any sensor based on the fitting of the π-FBG spectral profile can be largely improved by the proposed configuration at no additional fabrication costs and without altering the sensor robustness. A theoretical model shows that the resolution enhancement attainable in the proposed closed-loop geometry depends on the quality factor of the ring resonator. With a commercial grating in a medium-finesse ring, a spectral feature 12 times sharper than the π-FBG resonance is experimentally demonstrated. A larger enhancement is expected in a low-loss, polarization maintaining setup.

Fiber Bragg grating ring resonators under rotation for angular velocity sensing.

In this paper we investigate the possibility of using hybrid resonators based on fiber Bragg grating ring resonators (FBGRRs) and π-shifted FBGRRs (i.e., defective FBGRRs) as rotation sensitive elements for gyroscope applications. In particular, we model the conventional fiber Bragg grating (FBG) with the coupled mode theory by taking into account how the Sagnac effect, induced by the rotation, modifies the eigenvalues, the photonic band gap, and the spectral response of the FBG. Then, on the basis of the FBG model under rotation conditions, the spectral responses of the FBGRR and π-FBGRR have been evaluated, confirming that the Sagnac effect manifests itself with a spectral shift of the eigensolutions. This physical investigation can be exploited for opening new ways in the optical gyroscope platforms.

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

We demonstrate a strain sensor with very high sensitivity in the static and low frequency regime based on a fiber ring cavity that includes a π phase-shifted fiber Bragg grating. The grating acts as a partial reflector that couples the two counter-propagating cavity modes, generating a splitting of the resonant frequencies. The presence of a sharp transition within the π phase-shifted fiber Bragg grating's spectral transmittance makes this frequency splitting extremely sensitive to length, temperature, and the refractive index of the fiber in the region where the grating is written. The splitting variations caused by small mechanical deformations of the grating are tracked in real time by interrogating a cavity resonance with a locked-carrier scanning-sideband technique. The measurable strain range and bandwidth are characterized, and a resolution of 320 pϵ/Hz(1/2) at 0 Hz is experimentally demonstrated, the highest achieved to date with a fiber Bragg grating sensor.