Antiresonant hollow core fibre, preform therefor and method of fabrication

1. A shaped tube for use as a component in the fabrication of an antiresonant hollow core optical fibre, the shaped tube having a side wall with a transverse cross-sectional shape comprising a number of major curved portions alternating with the same number of minor substantially straight portions, each curved portion having an inwardly curving shape, and each straight portion being equidistant from a central longitudinal axis of the shaped tube.

2. The shaped tube according to claim 1 , in which the inwardly curving shape is symmetrical about a radius from the central longitudinal axis to a midpoint between two straight portions one either side of the curved portion.

3. The shaped tube according to claim 2 , in which the inwardly curving shape is a semi-circle, a semi-ellipse, an arc of a circle or an arc of an ellipse.

4. The shaped tube according to claim 1 , in which each curved portion has the same inwardly curving shape.

5. The shaped tube according to claim 1 , in which each curved portion has the same separation from the central longitudinal axis.

6. The shaped tube according to claim 1 , in which the number of curved portions is in the range of from two curved portions to twelve curved portions.

7. The shaped tube according to claim 1 , in which the shaped tube is formed from a glass material or a polymer material.

8. The preform for an antiresonant hollow core optical fibre, comprising a shaped tube according to claim 1 secured inside an outer jacket tube of glass material or polymer material such that the straight portions of the shaped tube are in contact or near-contact with an inner surface of the outer jacket tube.

9. The preform according to claim 8 , further comprising at least one additional shaped tube in a nested arrangement with the said shaped tube, each of the shaped tubes having curved portions with a different inwardly curving shape and/or a different length from the curved portions of the other shaped tube or tubes, and each of the shaped tubes having the same number of curved portions.

10. The cane for an antiresonant hollow core optical fibre drawn from a preform according to claim 8 .

11. The antiresonant hollow core optical fibre drawn from a preform according to claim 8 .

12. The method of fabricating a preform for an antiresonant hollow core optical fibre comprising:

providing a shaped tube according to claim 1 ;

inserting the shaped tube into an outer jacket tube of glass material or polymer material such that there is contact or near-contact between the straight portions of the shaped tube and an inner surface of the outer jacket tube; and

securing the shaped tube inside the outer jacket tube.

13. The method according to claim 12 , further comprising:

providing at least one additional shaped tube where each of the shaped tubes has curved portions with a different inwardly curving shape from the curved portions of the other shaped tubes or tubes, and the same number of curved portions; and

inserting the shaped tubes into one another to provide a nested group of shaped tubes inside the outer jacket tube.

14. The method of fabricating a cane for an antiresonant hollow core optical fibre, comprising fabricating a preform according to the method of claim 12 , and drawing the preform into a cane.

15. The method of fabricating an antiresonant hollow core optical fibre, comprising fabricating a preform according to the method of claim 12 , and drawing the preform into an optical fibre.

16. The method according to claim 12 , in which providing the shaped tube or tubes comprises fabricating the or each shaped tube by heating and reforming a cylindrical glass tube over a former with an outer shape corresponding to the desired cross-sectional shape of the shaped tube.

17. The method according to claim 12 , in which providing the shaped tube or tubes comprises fabricating the or each shaped tube by depositing glass soot onto a rotating mandrel with an outer shape corresponding to the desired cross-sectional shape of the tube to form a shaped tube of glass soot, and sintering the shaped tube of glass soot to form a shaped tube of consolidated glass material.

18. The method according to claim 12 , in which providing the shaped tube or tubes comprises fabricating the or each shaped tube by extruding molten glass through a die shaped to correspond to the desired cross-sectional shape of the shaped tube.

19. The method according to claim 12 , in which providing the shaped tube or tubes comprises fabricating the or each shaped tube using three-dimensional printing.

20. A nested antiresonant nodeless hollow core optical fibre, or a preform or a cane for drawing into a nested antiresonant nodeless hollow core optical fibre, comprising:

a cylindrical elongate outer jacket of glass material or polymer material;

a cladding structure comprising a plurality of cladding elements of glass material or polymer material spaced apart around an inner surface of the outer jacket and defining elongate cladding voids, each cladding element comprising a first inwardly curving wall extending from the inner surface towards a central longitudinal axis of the outer jacket and back to the inner surface along a first curved shape, and a second inwardly curving wall extending from the inner surface towards the central longitudinal axis and back to the inner surface along a second curved shape which has a different shape and/or a different length from the first curved shape; and

an elongate hollow core region along the central longitudinal axis and bounded by negative curvatures of the first inwardly curving walls of the cladding elements.

21. The antiresonant hollow core optical fibre, a preform or a cane according to claim 20 , in which each cladding element further comprises one or more additional inwardly curving walls extending from the inner surface towards the central longitudinal axis and back to the inner surface along a curved shape different in shape and/or length from the first curved shape and the second curved shape.