Evanescent wave long-period fiber grating within D-shaped optical fibers for high sensitivity refractive index detection

By now long-period fiber gratings (LPFGs) represent a well-assessed in-fiber technology. Their peculiar spectral properties and the intrinsic surrounding refractive index (SRI) sensitivity, together with the well known advantages of optical fibers, make these structures one of the most powerful tools, especially in the sensing field. Unfortunately, their main limitation relies on the need of opportunely coating the glass substrate when significant sensitivity enhancement and/or specific functionalization are required. In this work, the possibility to realize a self-functionalized and high sensitivity LPFG is demonstrated. The principle of operation is based on the exploitation of the evanescent wave interaction of the propagating light with a periodically patterned overlay. A D-shaped optical fiber is used as fiber substrate, that is made SRI sensitive by means of wet chemical etching based on hydrofluoric acid. Due to its peculiar geometry the process results easier in comparison with that to perform with standard single-mode fibers. In addition the chemical step can be stopped once the desired sensitivity is achieved. Successively a high refractive index (HRI) overlay is deposited on the flat surface of the fiber. Finally the HRI layer is periodically patterned by means of laser micromachining technique. The spectral characteristics of the fabricated device, together with the flexibility of the adopted fabrication method could allow the realization of innovative LPFGs to be adopted as technological platform for a multitude of chemical sensing applications, depending on the nature of the material deposited onto the flat surface of the etched D-fiber.