In-Fiber Transmission Filters with Broad Stopbands using Chirped Bragg Gratings

Abstract

Many optical fiber applications, such as wavelength division multiplexing/demultiplexing, frequency-guiding in soliton systems, and the filtering out of spontaneous emission noise from fiber amplifiers require transmission band-pass filters with a wide range of finesses and free spectral ranges. Bragg gratings written directly into the optical fiber by direct UV exposure [1] show particular promise in this area and there have been many reported techniques to date proposed for fabricating fiber grating transmission filters (e.g. [2–7]) Most of these techniques however do not allow independent arbitrary selection of the pass and stop band characteristics thus limiting their usefulness for many applications. While the recently reported wide stop band transmission filters based on resonant chirped grating Moiré and Fabry-Perot designs [2,3] do allow a large degree of control of the pass and stop band characteristics, in practice the parameters of the filter including transmissivity, transmission wavelength and linewidth are difficult to tailor during fabrication. Where multiple pass bands are required the number of passbands and the free spectral range are inherently fixed by the phase shift between the two interacting gratings.