Fiber devices and sensors based on multimode fiber Bragg gratings

摘要

New sensing and diagnostic methods are implemented with multimode fiber Bragg gratings (MMFBG) and methods for sensor addressing and multiplexing are described. Real time structural strain and vibration monitoring using optical time domain reflectometry (OTDR) is discussed. The first all-mode, wave optics calculations of the reflectivity of Bragg gratings in highly multimode optical fibers are presented, including the effects of individual mode power changes due to microbending transducer loss upstream from the grating. Two categories of multimode fiber gratings are possible, independent mode and mode coupling, and the fabrication differences between these types of gratings are described. The reflectivity properties of independent mode multimode fiber gratings are examined and compared to single mode fiber gratings. In particular, multimode fiber gratings offer added flexibility in grating design and performance characteristics compared to single mode fiber gratings, because the reflectivity response may be tuned by the spatial periodicity, length, core size, numerical aperture, and mode coupling characteristics of the grating.