High-Resolution and Low-Loss All-Fiber Spectral Filters Based on Biconic Tapers

A family of narrowband spectral filters based on biconic fiber tapers is investigated. These filters were made of highly-depressed-cladding single-mode fibers by a heat-and-pulling process, and using a cylindrical-tube graphite heater. The evolution of the taper profiles during the pulling of different elongations was modeled by a coupled system of partial differential equations governing the mass and axial momentum conservation. The optical responses were modeled by using the mode coupling theory. Theoretical results, ranging from prediction of the taper profiles to optical responses of these filters-transmission losses, free spectral range, and isolation-show strong accordance with experimental ones.