Hollow core optical fibre
A hollow core optical fibre comprises a tubular jacket; a cladding comprising a plurality of primary capillaries spaced apart from one another in a ring and each bonded to an inner surface of the jacket at a peripheral location around the circumference of the jacket; and a hollow core formed by a central void within the ring of primary capillaries; wherein the cladding further comprises, within each primary capillary, two secondary capillaries and no more, the two secondary capillaries spaced apart from one another and each bonded to an inner surface of the primary capillary at an azimuthal location around the circumference of the primary capillary which is displaced from the peripheral location of the primary capillary.
1. A hollow core optical fibre comprising:
a tubular jacket;
a cladding comprising a plurality of primary cells spaced apart from one another in a ring around an inner surface of the jacket with gaps between adjacent cells; and
a hollow core formed by a central void within the ring of primary cells;
wherein the cladding further comprises, inside each primary cell, two secondary cells and no more, the two secondary cells spaced apart from one another and a gap between the two secondary cells, neither secondary cell being radially aligned with the primary cell along a radius of the hollow core optical fibre.
2. A hollow core optical fibre according to claim 1 , in which, inside each primary cell, the secondary cells are spaced apart on either side of an azimuthal position of the primary cell inside the jacket.
3. A hollow core optical fibre according to claim 2 , in which each secondary cell has a contact point around the perimeter of the primary cell which has a displacement from the azimuthal position of the primary cell inside the jacket.
4. A hollow core optical fibre according to claim 2 , in which each secondary cell has a contact point around the perimeter of the primary cell which has a displacement from a contact point of the primary cell with the inner surface of the jacket.
5. A hollow core optical fibre according to claim 2 , in which each secondary cell has a contact point around a perimeter of the primary cell which is not coincident with a contact point of the primary cell with the inner surface of the jacket.
6. A hollow core optical fibre according to claim 2 , in which the secondary cells have equal and opposite displacements from the azimuthal position of the primary cell inside the jacket.
7. A hollow core optical fibre according to claim 2 , in which the secondary cells are misaligned from having equal and opposite displacements from the azimuthal position of the primary cell inside the jacket.
8. A hollow core optical fibre according to claim 1 , comprising four primary cells.
9. A hollow core optical fibre according to claim 1 , comprising five primary cells.
10. A hollow core optical fibre according to claim 1 , in which the primary cells are spaced apart by gaps d out in the range of 0<d out /λ≤4.5 and the secondary cells are spaced apart by gaps d in in the range of 0<d in /λ≤6.7, where λ is an optical wavelength of light which the hollow core optical fibre is configured to guide.
11. A hollow core optical fibre according to claim 1 , in which the primary cells are spaced apart by gaps d out in the range of 0<d out /λ≤3.1 and the secondary cells are spaced apart by gaps d in in the range of 0<d in /λ≤3.4, where λ is an optical wavelength of light which the hollow core optical fibre is configured to guide.
12. A hollow core optical fibre according to claim 1 , and having a level of optical propagation loss for guided light at a wavelength which the hollow core optical fibre is configured to guide which is 1 dB/km or less.
13. A hollow core optical fibre according to claim 1 , and having a level of optical propagation loss for guided light at a wavelength which the hollow core optical fibre is configured to guide which is 0.01 dB/km or less.
14. A preform or a cane for making a hollow core optical fibre which is configured to be drawn into a hollow core optical fibre according to claim 1 .