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Diffractive Elements Inscribed at End-Fiber Surface by Femtosecond Laser

In this paper, diffractive elements in one and two dimensions have been inscribed at the end-face of single and multicore optical fibers using femtosecond lasers. Different sensing applications have been also presented on the basis of their projected far-field patterns. The manufacturing of 2D structures was achieved using two approaches: line writing and its equivalent point-by-point technique in order to speed up the process. Specific properties of both the techniques have been exploited by inscribing 2-direction and circular gratings. When diffraction grating is inscribed at the end-face of a four core fiber, the 2D interference pattern is transferred into diffraction orders with the possibility of decoupling the interferometric contribution of selected core. Particularly, with a circular diffraction grating, the multicore far-field pattern exhibits two perpendicular and independent interference patterns, as only opposite cores are interfering with each other. These patterns exhibit period and, hence, sensitivity with phase perturbation, proportional to diffraction order angle. The 2D diffraction grating inscribed in the single mode fiber has been employed for bidimensional displacement insensitive sensing, exhibiting linear response, while aforementioned structures written in multicore fiber have been proposed for bending vector sensing, thereby showing higher sensitivity than that of its pristine counterpart.