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This PhD thesis addresses several aspects of macrobending loss in single-mode optical fibers. The context for the work is the development of an edge filter for a rapid wavelength measurement system. Based on scalar approximation theory and the beam propagation method, a simulation platform is developed and used to predict the macrobending loss of singlemode fiber with single or multiple coating layers. A single-mode step index fiber macrobending loss edge filter is investigated theoretically in terms of baseline loss, discrimination range and polarization dependent loss (PDL). The theoretical analysis is supported by a range of experimental investigations and results. The bend loss characteristics of a conventional SMF28 based edge filter are investigated. The bend loss characteristics of a high bend loss fiber based edge filter are also investigated for comparison. It is shown that while the high-bend loss fiber based filter requires significantly fewer bend turns and exhibits a lower PDL, temperature dependence can be a major issue. A fiber selection method, specifically for an edge filter application, is reported and using this method a fiber is selected for a fiber bend loss edge filter that shows significantly improved spectral response (measured baseline loss is 5.09 dB and discrimination range is 17.42 dB) coupled to a lower average PDL and simpler fabrication technique. A theoretical model for macrobending-induced temperature dependent loss (TDL) for a fiber with dual coating layers is presented, with good agreement demonstrated between theoretical calculations and experimental results. The impact of temperature on two examples of an all-fiber based edge filter is also investigated theoretically and experimentally and using the developed model, it is shown that it is possible to predict the impact of temperature variations on an all-fiber based edge filter. The effect of temperature on polarization dependent loss is presented theoretically and experimentally as well. The impact of fiber manufacturing tolerances is also investigated and it is shown that variations in the numerical aperture and fiber core radius can have significant impact on the spectral response of an edge filter. A simple bend radius tuning technique is proposed to mitigate the effect of fiber manufacturing tolerances. A complete methodology for the design of a fiber based edge filter is presented, starting with the task of selecting suitable fibers and then considering the design of a fiber bending loss edge filter. Through the comparison of the performances of two previously developed types of fiber edge filters, as an example, a Corning SMF28e fiber with 900 μm jacket is selected as a tradeoff between SMF28 and 1060XP fiber and an edge filter design is undertaken. Finally, a subsidiary objective of this thesis is to explore novel applications for macrobending fiber based optical sensing devices. In this context, a macrobending fiber based refractometer sensor is studied theoretically, and a new theoretical model based on a 3-dimensional full-vectorial finite difference beam propagation method for accurately predicting the fiber bending loss is also presented.
Wang, P. (2008) Development of fiber bend loss edge filter. Doctoral Thesis, Technological University Dublin. doi:10.21427/D7502H