Patent ID: 6839484

Claim:
Article comprising at least one optical fiber suitable for pulsed single-mode operation with electromagnetic energy of operating wavelength, “λ o ”, the major part of the fiber length being of guide strength, “Δ”, as measured at λ o and having a cross-sectional profile comprising a core and a cladding region, Characterized in That the said core region is of sufficient size for supporting at least two linearly polarized modes consisting of the fundamental mode together with from one to three higher-order modes of such energy, and in that the said fiber contains a continuous stream of longitudinally spaced perturbations over a substantial portion of its length, each perturbation constituting a local interruption in fiber guiding properties which induces mode coupling, perturbation spacing being such as to selectively couple said higher-order modes with a cladding mode, thereby resulting in selective extraction of said higher-order modes by radiation, and in which the cross-section profile of the said fiber additionally includes at least one radially positioned index-of-refraction feature for increasing the difference in effective index, Δn eff , between the said fundamental mode and a said higher-order mode, said feature introducing a radially-defined change in refractive index of peak value ≧25% Δ, relative to an unfeatured profile.