Abstract
Long-period fiber gratings (LPGs) are well known for their sensitivity to external influences, which make them interesting for a large number of sensing applications. For these applications, fibers with a high numerical aperture (i.e., fibers with highly germanium (Ge)-doped fused silica fiber cores) are more attractive since they are intrinsically photosensitive, as well as less sensitive to bend- and microbend-induced light attenuations. In this work, we introduce a novel method to inscribe LPGs into highly Ge-doped, single-mode fibers. By tapering the optical fiber, and thus, tailoring the effective indices of the core and cladding modes, for the first time, an LPG was inscribed into such fibers using the amplitude mask technique and a KrF excimer laser. Based on this novel method, sensitive LPG-based fiber optic sensors only a few millimeters in length can be incorporated in bend-insensitive fibers for use in various monitoring applications. Moreover, by applying the described inscription method, the LPG spectrum can be influenced and tailored according to the specific demands of a particular application.
Highlights
Long-period fiber gratings (LPGs) are applied in order to couple light between core and co-propagating cladding modes in single-mode optical fibers
We present for the first time a method to inscribe UV-induced LPGs in highly
SM1500 fibers from Fibercore were used for inscription of the LPGs, and a single single resonance loss dip with up to attenuation in the wavelength range between
Summary
Long-period fiber gratings (LPGs) are applied in order to couple light between core and co-propagating cladding modes in single-mode optical fibers. In terms of sensing applications, fibers with a high NA (i.e., fibers with highly Germanium (Ge)-doped fused silica fiber cores) are more attractive since they are less sensitive to bend/microbend-induced light attenuations, and are intrinsically photosensitive, and are suitable for the fabrication of fiber grating structures using UV excimer laser light. When applying single-mode optical fibers with high NAs for use in LPG applications, it follows from Equation (1), that due to the large difference in the effective refractive indices between the core and cladding modes, small grating periods are required for the co-propagating. Since metal masks with small periods are difficult to fabricate, and the durability of chrome masks to high-power UV light exposure is limited, the manufacturing of LPGs in highly Ge-doped, single-mode fibers has not been reported so far. This was followed by the measurement of the influence induced by the refractive index change in the surroundings
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