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

The transmission spectrum of a ring resonator enclosing a ?-phase shifted fiber Bragg grating (?-FBG) shows a spectral feature at the Bragg wavelength that is much sharper than resonance of the ?-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 ?-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 ?-FBG resonance is experimentally demonstrated. A larger enhancement is expected in a low-loss, polarization maintaining setup.