Patent ID: 7526165

Claim:
An optical fibre having a longitudinal, optical axis, and a cross section perpendicular to the longitudinal axis, the optical fibre being adapted to guide light at an operating wavelength λ, the optical fibre comprising: a. a first core region disposed around the longitudinal, optical axis, the first core region exhibiting a predetermined refractive index profile n core-1 ; b. a second core region surrounding the first core region, the second core region exhibiting a predetermined refractive index profile n core-2 ; c. a cladding region surrounding the second core region and comprising a multitude of longitudinally extending spaced apart micro-structural holes disposed in a cladding material, the cladding material having a refractive index n clad , the holes having cross sectional dimensions d i (z) and mutual centre to centre distances Λ ij (z), z being a coordinate along the longitudinal axis of the optical fibre; d. a first fibre cross section having a first cross sectional area; e. a second fibre cross section having a second cross sectional area which is smaller than the first cross sectional area; f. the first and second fibre cross sections being separated by a tapered length of the optical fibre over which the cross-sectional physical dimensions of the fibre, including the micro-structural holes, are tapered down from the first to the second cross section; and wherein in the first and second cross sectional areas, the refractive index profiles n core-1 , n core-2 of the first and second core regions, the refractive index n clad of the cladding region and the cross sectional dimensions d i and mutual centre to centre distances Λ ij of the micro-structural holes in the first and second cross sectional areas are adapted, at the operating wavelength, to provide a mode field of a guided mode of the optical fibre with a diameter MFD 1 in the first cross section, and a mode field with a diameter MFD 2 in the second cross section, and wherein MFD 2 is larger than or equal to MFD 1 .