Phase-shifted Bragg grating for high sensitivity measurements

In this contribution we report a complete characterization of a dynamic strain sensor with very high sensitivity. Such a sensor is based on a π-phase-shifted fiber Bragg grating (FBG) interrogated by a frequency modulated laser radiation emitted by an extended cavity diode laser (ECDL) . Pound-Drever-Hall technique was used to lock the laser emission frequency to the resonance peak of the FBG spectrum. Measuring the error signal, a maximum sensitivity of 5 pε/(Hz)1/2 is achieved for frequencies larger than 100 kHz. The 3-cm long homemade grating was fabricated by the continuous writing technique using an UV radiation from a frequency-doubled Ar-ion laser. The transmission spectrum of the π phase-shifted grating shows a sharp resonance peak (47 MHz FWHM) around the centre of the FBG reflection bandwidth. Used as frequency discriminator in a Pound-Drever-Hall system, the grating shows a frequency discriminator coefficient of ~32 nV/Hz. Assuming a white noise contribution this value corresponds to a frequency noise sensitivity of ~750 Hz/(Hz)1/2, corresponding to a strain sensitivity of 5 pε/(Hz)1/2 where the noise value is dominated by the laser frequency noise.