Results of experimental studies of multimode fiber Bragg gratings on multimode fibers

Nowadays fiber Bragg gratings (FBGs) are widely used in various devices and applications of fiber optics. They become a basic component for fiber optic sensor networks. The most of those both commercial and prototype fiber optic sensor systems are based on well-researched either singlemode or multimode regime of signal transmission over optical fibers. However some works are concerned with researches of FBGs written on silica multimode optical fibers (MMF) for various photonics and fiber optics applications. Thus, FBGs written on graded-index MMFs are used in optical filters of fiber optic data transmission systems 10G designed for local area networks and radio-over-fiber systems covering micro- and pico-cells. Some works are focused on FBGs written on step-index MMFs. Slanted or tilted multimode FBGs are also widely used in vibration, temperature, deformation etc. fiber optic sensors. Another group of publications is devoted to conventional FBGs on silica telecommunication graded-index MMFs 50/125 as well as 62.5/125 and experimental approbation of various fiber optic sensors based on them. However, the most part of mentioned papers are focused on just analysis of FBG spectral response during laser-excited optical signal propagation that corresponds to a few-mode regime. Few-mode regime adds a new another one dimension to parameter space that is associated with guided modes of particular order which limited number (from two to a few dozens) transfer the most part of optical signal power over tested optical fiber. Optical fiber operating in a few-mode regime of data transmission is interferometer that in combination with FBG can be considered as an alternative basis for design of elements with new unique properties. This work is concerned with experimental researches of few-mode regimes of FBG operating during transferring of laser based optical signals. We tested FBGs written on silica graded-index multimode fibers 50/125 Cat. OM2+/OM3 with Bragg wavelength 1550 nm by using them in a set of developed experimental schemes based on excitation of multimode fibers by corresponding laser sources. The researches were also focused on analysis of both spectral and pulse responses under changing of selected mode mixing and power diffusion processes due to tension and/or stress local and distributed action to FBG or sensor fiber. Results of spectral and pulse response measurements at the output of schemes with installed described FBGs are represented.