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In this paper coupling of linearly polarized (LP) modes (forming an orthogonal basis of mode division multiplexing systems, MDM) caused by fiber cracks and cleavages is considered. Fiber cracks as well as bends and curves are unavoidable deformations and defects appearing during fiber exploitation. The physical origin of these cracks may vary: from technological flaws to exploitation factors including different stresses, temperature deviations, curves, longitudinal tension, pressure etc. The most common effect that cracks provide to classical multiplexed signals (WDM, FDM) is additional losses and reflection which can be measured by conventional reflectometer but it can be shown that any crack is itself also an additional transversal refraction index perturbation leading to the mode coupling and therefore to excitation of spurious modes. Consequently, in terms of MDM systems mode coupling becomes one of the important factors limiting MDM systems performance, so investigation of non-avoidable factors influence is a significant task: this aspect is a key factor of MDM systems implementation. Coupling of LP modes due propagation through fiber Bragg gratings (FBG) is also considered. FBG are widely used in a lot of telecom applications including PONs, AONs, DWDM systems, etc., so investigation of FBG influence on mode coupling is significant due to commercialization and establishment of MDM-systems.
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