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 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.
2. A hollow core optical fibre according to claim 1 , comprising five primary capillaries.
3. A hollow core optical fibre according to claim 1 , comprising four, six, seven, eight, nine or ten primary capillaries.
4. A hollow core optical fibre according to claim 1 , in which the azimuthal locations of the two secondary capillaries have substantially equal and opposite angular displacements from the peripheral location.
5. A hollow core optical fibre according to claim 1 , in which the azimuthal locations of the two secondary capillaries have substantially equal and opposite angular displacements from a midpoint on the circumference of the primary capillary between the azimuthal locations, and the midpoint has an angular displacement from the peripheral location.
6. A hollow core optical fibre according to claim 5 , in which the midpoint has an angular displacement of 30 degrees or less.
7. A hollow core optical fibre according to claim 5 , in which the midpoint has an angular displacement of substantially 38 degrees or less.
8. A hollow core optical fibre according to claim 5 , in which the midpoint has an angular displacement of substantially 53 degrees or less.
9. A hollow core optical fibre according to claim 4 , in which the equal and opposite angular displacement of the azimuthal locations is in the range of 30 degrees to 142 degrees.
10. A hollow core optical fibre according to claim 4 , in which the equal and opposite angular displacement of the azimuthal locations is in the range of 75 degrees to 112 degrees.
11. A hollow core optical fibre according to claim 4 , in which the equal and opposite angular displacement of the azimuthal locations is in the range of 85 degree to 95 degrees.
12. A hollow core optical fibre according to claim 1 , in which the primary capillaries are spaced apart by gaps d out in the range of 0<d out /λ≤4.5 and the secondary capillaries 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.
13. A hollow core optical fibre according to claim 1 , in which the primary capillaries are spaced apart by gaps d out in the range of 0<d out /λ≤3.1 and the secondary capillaries 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.
14. A hollow core optical fibre according to claim 1 , in which the secondary capillaries have an inner radius r in which is at least 0.29 times an inner radius r out of the primary capillaries.
15. A hollow core optical fibre according to claim 1 , in which the secondary capillaries have an inner radius r in which is at least 0.38 times an inner radius r out of the primary capillaries.
16. 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.
17. 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.
18. 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 .
19. A preform or a cane for making a hollow core optical fibre, and comprising:
an outer tube for forming a jacket;
a plurality of primary tubes for forming primary capillaries to define a cladding of the fibre, the primary tubes spaced apart from one another in a ring around a central void to form a hollow core, each primary tube arranged against an inner surface of the outer tube at a peripheral location around the circumference of the outer tube; and
within each primary tube, two secondary tubes and no more, the secondary tubes spaced apart from one another and each arranged against an inner surface of the primary tube at an azimuthal location around the circumference of the primary tube which is displaced from the peripheral location of the primary tube.