Microstructured optical fibres

An optical fibre with a waveguide structure having a longitudinal direction, said optical fibre comprising:

• a core region extending along the longitudinal direction,
• a cladding region extending along the longitudinal direction, said cladding region comprising an at least substantially two-dimensionally periodic structure comprising:
• primary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the primary elements having a refractive index being lower than a refractive index of any material adjacent to the primary elements,
• secondary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the secondary elements having a refractive index being lower than a refractive index of any material adjacent to the secondary elements,
• wherein any area of any primary element is larger than any area of any secondary element, and wherein
• the periodic structure being, in a cross-section perpendicular to the longitudinal direction, defined by at least one unit cell, wherein, for each unit cell:
• the sum of the areas of secondary elements, which areas are comprised within a given unit cell, is larger than 0.09 times the area of that unit cell.

An optical fibre with a waveguide structure having a longitudinal direction, said optical fibre comprising:

• a core region extending along the longitudinal direction,
• a cladding region extending along the longitudinal direction, said cladding region comprising an at least substantially two-dimensionally periodic structure comprising:
• primary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the primary elements having a refractive index being lower than a refractive index of any material adjacent to the primary elements,
• secondary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the secondary elements having a refractive index being lower than a refractive index of any material adjacent to the primary elements,
• wherein any area of any primary element is larger than any area of any secondary element, and wherein, in a cross section perpendicular to the longitudinal direction
• the primary, elongated elements define a triangular structure,
• wherein the periodic structure being, in the cross-section, defined by at least one unit cell, wherein, for each unit cell:
• a first circle is defined as the largest circular area possible having a centre not positioned outside the unit cell and not enclosing any part of any primary, elongated elements, and wherein
• the centres of any of the secondary, elongated elements, in the cross section, do not coincide with the centre of the first circle.

A preform for manufacturing an optical fibre, the preform having a length in a longitudinal direction and a cross section perpendicular thereto, the preform comprising:

• primary, elongated elements each having a centre axis extending in the longitudinal direction of the preform, the primary elements having a length in the longitudinal direction being essentially the same as the length of the preform,
• inserted elements each extending in the longitudinal direction of the preform over a length being smaller than the length of the preform,
• the primary, elongated elements and the inserted elements form both a non-periodic structure and an at least substantially two-dimensionally periodic structure, the non-periodic structure being surrounded by the substantially two-dimensionally periodic structure,
• the periodic structure being, in the cross-section perpendicular to the longitudinal direction, defined by at least one unit cell, wherein, for each unit cell:
• a first circle is defined as the largest circular area possible having a centre not positioned outside the unit cell and not enclosing any part of any primary elements, the periphery of said first circle defining an inserted element.

A method for fabricating a preform, the preform having a length in a longitudinal direction and a cross section perpendicular thereto, the method comprising the steps of:

• providing a holder for the preform, the holder having a predetermined shape and elongated grooves at its inner surface, the grooves having a length in the longitudinal direction being essentially the same as the length of the preform.
• providing primary, elongated elements each having a centre axis extending in the longitudinal direction of the preform, the primary elements having a length in the longitudinal direction being essentially the same as the length of the preform,
• providing secondary elements each extending in the longitudinal direction over a length being smaller than the length of the preform, and
• positioning a plurality of secondary elements at essentially the same position along the longitudinal direction of the preform.