Backward Wave Couplers And Reflectors

Abstract

The analysis and experimental results for weakly perturbed, narrow band, backward wave couplers and fibre reflectors, will be presented.Conventional, co-directional fibre couplers -polished, fused or coaxial -can be designed to demonstrate a sufficient wavelength dependence for some filtering and multiplexing applications.While coupled bandwidths can be sufficiently reduced by weakening the coupling between the guides and hence increasing the interaction (beat) length, useful limits of a few nms are reached for interaction lengths of a few cms.These limitations in coupled bandwidth are considerably transformed for contra -directional coupling provided by a periodic perturbation of suitable pitch (half the guide wavelength) imposed on the coupler.The inverse relationship between bandwidth and interaction length still holds, but the magnitudes are substantially improved.Calculations show that we may expect sub -Angstrom bandwidths for interaction lengths in the order of 1cm.These extremely narrow band effects depend on the successful fabrication of regular, weak, periodic perturbations over relatively long fibre lengths.We have fabricated long gratings (-1cm) of periodicity -0.27pm on D-shaped elliptical cored fibres as a first step towards narrow band couplers. These structures can be investigated by observing reflection characteristics at -800nm and the latest results will be described.

Improved coupled-mode theory for codirectionally and contradirectionally coupled waveguide arrays

“…A solution to Eq. (4) is now attempted in the form of a sum of forward-and backward-traveling local normal modes, namely, as (6) Here AFm and Agi are the amplitudes of the forward-and backward-traveling modes, respectively, in the ith guide.…”

“…The most common configuration uses a grating overlay on half of a polished fiber coupler, 3 " but graitings have also been made directly on a D fiber. 6 The use of a polished coupler has also permitted the fabrication of a track-changing reflector 7 ' 8 suggested in Refs. 9 and 10.…”

“…Most recently, a threeguide device has been shown to provide useful powerdivision functions, 5 but arrays of arbitrary size have also been studied in distributed-feedback laser geometries.' 6 Almost all analyses have been performed with the use of the coupled-mode theory, and representative treatments of single-guide structures can be found in Refs. [17][18][19][20].…”