Microstructures in microstructured optical fibers become radically more complicated. These microstructures offer the possibilities of better detection and control of linear and nonlinear properties of the guiding medium, which were not possible to achieve before. Mechanical cleavage is the dominant method used to cut fibers in the field and in the laboratory. The major shortcomming of such microstructured fibers is that they can not be easily cleaved using the mechanical method; therefore their extensive application faces a great challenge. In the present study, six different double-clad microstructured optical fibers are studied. Their cleaved surfaces have been precisely examined and the zones containing useful information are identified, localized and classified. In order to get a better understanding of the mechanical cleavage method, the theories of fracture propagation in fragile materials, Griffith's and Inglis' theories, are studied. The first one is based on energetic considerations and the second one is based on material resistance. To verify these theories, the suggested methods to measure the parameters related to material failure are applied to the available optical fibers. Those parameters are fracture toughness, KIC, and material resistance, sigmamax. In order to get meaningful results these two theories should be merged. The effects of the crack origins, the applied stress and the body shape are studied. By merging two theories the imperfections on the cleaved surfaces are explained. Each imperfection zone is discussed individually and qualitative explanations are provided. The three major contributions of the present study in the field of microstructured optical fibers are: (1) to observe and analyze the cleaved surface imperfections of microstructured optical fibers, (2) to define a criterion for crack propagation in a microstructured medium, and (3) to propose a method to design a microstructured optical fiber robust to the mechanical cleavage. The latest can be used in order to inspect the robustness of the before-made microstructured optical fibers. It imposes the distance and the form of air capillaries.
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