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Thermally tapered optical fiber components such as couplers, wavelength division de/multiplexers require a precise analysis of the relationship between technological and device parameters. Over the past 15 years there have been a lot of studies devoted to the familiar "fused biconical taper coupler" and similar fused bitapered optical components usually fabricated with microbumers. The main drawback with this fabrication method is that the temperature cannot be monitored during the fiber- coupler fabrication and hence the fabrication conditions are not controlled by physical parameters such as the fusion temperature and the fusion time of the fibers. Recently, to solve this problem an interesting method using a microheater has been proposed which allows wide and stable heat regions in order to obtain couplers with smooth and gradual taper shapes for low excess losses. Using this fabrication method, we propose in this paper a precise model for predicting the shape of thermally tapered fibers and for analyzing the influence of tapering on the sintering of fibers during the fabrication of fused bitapered optical components. The model has been developed and verified by preliminary experimental measurements in order to relate technological parameters to the characteristics of fused optical fiber components. Much of the literature published previously about this problem still lacks of generality and often approximates the taper geometry with simple analytic function and considers that tapering and sintering (fusion) phenomena are uncoupled. The basic approach is first to fuse the fibers and then elongate the fused fibers while considering that the degree of fusion is conserved during the tapering step. This hypothesis can be justified if the sintering is executed at high temperature ( above 1500°C) and elongation at a lower temperature (about 1300°C). But if sintering and tapering are executed at a similar temperature, or more generally if fibers are elongated at high temperature (typically 1500°C) it appears necessary to consider both sintering and tapering as coupled phenomena. Given that most glasses can be modeled as Newtonian liquids, we have simplified the basic model of nearly unidirectional viscous dominated extensional flow by using a simple rheological model to describe the process of pulling fibers in the range of their softening temperature. But the salient feature of our model is that it allows also a description of the degree of fusion by taking into account of surface tension forces and clearly demonstrates a coupling between the tapering and sintering phenomena.
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