Fiber laser sensor using the intermodal beating firequency to interrogate fiber Bragg gratings

We discuss a fiber Bragg grating laser sensor in which the measurement of an intermodal beating frequency is used for the Bragg gratings (BG) interrogation. The linear cavity of the laser is formed by one reference BG at one side and one or several sensing BGs at the opposite side of the cavity. For this report we used two sensing BGs. In the laser with one BG at each cavity side the beating frequency is defined by the distance between BGs. If two sensing BGs are used, the beating frequency is defined by both the distances between the reference BG and each of the sensing BGs and by a ratio between sensing BGs reflections at the laser wavelength. So, when the reflection of sensing BGs is shifted by temperature or strain, the beating frequency is changed. We discuss the experimental results obtained for the laser sensor with the total cavity length equal to 4277 m; the distance between the sensing BGs was 47 m. A RF spectrum analyzer with 100-kHz bandwidth was used that allows the measurement of three first harmonics. We found that the beating frequency can be moved from the value corresponding to the 4277-m cavity to the value corresponding to the 4230-m cavity by shifting the maximum reflection of the sensing BGs. The narrowest spectrum equal to 52 Hz was obtained when the tuned delay was incorporated into the cavity. A shift of the maximum BG reflection less then 0.05 nm was detected.