Field distributions and far-field patterns (FFP's) of the eigenmodes of thermally diffused expanded core (TEC) fibers are clarified by numerical analysis. The spot size of the TEC fiber is estimated by using the FFP that can be measured more easily than the near-field pattern (NFP). Splice losses of two TEC fibers are analyzed numerically considering a single or composite effect of gap, offset, and tilt. The results are useful for estimating isolations of fiber-integrated optical isolators and insertion losses of other fiber-integrated type optical devices. Splice losses considering multiple reflection between two endfaces of the TEC fibers with gaps are also analyzed numerically. This is useful for the design of fiber-integrated tunable wavelength filters.
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