Photonic bandgap optical waveguide with anti-resonant core boundary
Improved photonic band-gap optical fibre The present invention relates in particular to improved photonic band-gap optical fibres that can confine light to a core region of the fibre by the action of both a photonic band-gap cladding and an antiresonant core boundary, at the interface between the core and cladding. According to embodiments of the present invention, a fibre has a core, comprising an elongate region of relatively low refractive index, a photonic bandgap structure arranged to provide a photonic bandgap over a range of wavelengths of light including an operating wavelength of light, the structure, in a transverse cross section of the waveguide, surrounding the core and comprising elongate relatively low refractive index regions interspersed with elongate relatively high refractive index regions and a relatively high refractive index boundary at the interface between the core defect and the photonic bandgap structure, the boundary having a thickness around the core such that the boundary is substantially anti-resonant at the operating wavelength of the fibre. In preferred embodiments, the core boundary is a relatively constant thickness region of glass around a hollow core.
1. An optical waveguide, comprising:
a core, comprising an elongate region of relatively low refractive index;
a photonic bandgap structure arranged to provide a photonic bandgap over a range of wavelengths of light, the structure, in a transverse cross section of the waveguide, surrounding the core and comprising elongate relatively low refractive index regions interspersed with elongate relatively high refractive index regions; and
a relatively high refractive index boundary at the interface between the core and the photonic bandgap structure, the boundary having a thickness around the core such that, in use, light guided by the waveguide is guided in a transverse mode providing an F-factor of less than 0.23 μm −1 for an operating wavelength of 1.55 μm, less than an equivalent F-factor value scaled for a different operating wavelength or less than 0.7Λ −1 for structures having a periodic cladding and a pitch Λ.
2. A waveguide according to claim 1 , in which the boundary is anti-resonant at an operating wavelength of light.
3. A waveguide as claimed in claim 1 , in which the boundary has a substantially constant thickness around the core.
4. A waveguide as claimed in claim 1 , in which the boundary has a thickness that varies around the core, wherein the core boundary has a thickness t around at least a fraction y of the boundary, where y>0.5.
5. A waveguide as claimed in claim 1 , in which the boundary comprises, in the transverse cross-section, a plurality of relatively high refractive index boundary veins connected end-to-end around the boundary between neighbouring boundary nodes, each boundary vein being connected between a leading boundary node and a following boundary node, with no nodes in between, and each boundary node being connected between two boundary veins and to a relatively high refractive index region of the photonic bandgap structure.
6. A waveguide according to claim 5 , wherein each boundary vein has a characteristic thickness substantially at the mid-point between the two boundary nodes to which it is connected.
7. A waveguide according to claim 5 , wherein the characteristic thickness of at least one boundary vein is at least 110% of the characteristic thickness of a plurality of the veins in the array of veins in the photonic band-gap structure.
8. A waveguide as claimed in claim 1 , in which the array has a characteristic primitive unit cell and a pitch Λ.
9. A waveguide as claimed in claim 8 , in which the boundary has a thickness t, wherein, t=uΛ for a fraction y of the boundary, where u>0.06 and y>0.5.
10. A waveguide as claimed in claims 1 , in which the core boundary has a thickness t defined by
a
λ
4
n
HI
2
-
n
LO
2
≤
t
≤
b
λ
4
n
HI
2
-
n
LO
2
,
where a=0.5 and b=1.75 and n HI and n LOW are the refractive indices of the boundary and of the relatively low refractive index region of the core, respectively.
11. A waveguide according to claim 1 , in which the boundary is anti-resonant at an operating wavelength of light.
12. A waveguide as claimed in claim 1 , in which the boundary has a substantially constant thickness around the core.
13. A waveguide as claimed in claim 1 , in which the boundary has a thickness that varies around the core, wherein the core boundary has a thickness t around at least a fraction y of the boundary, where y>0.5.
14. A waveguide as claimed in claim 1 , in which the array has a characteristic primitive unit cell and a pitch Λ.
15. A waveguide as claimed in claim 1 , in which the core boundary has a thickness t defined by
a
λ
4
n
HI
2
-
n
LO
2
≤
t
≤
b
λ
4
n
HI
2
-
n
LO
2
,
where a=0.5 and b=1.75 and n HI and n LOW are the refractive indices of the boundary and of the relatively low refractive index region of the core, respectively.
16. A waveguide as claimed in claim 1 , in which the boundary has a thickness such that, in use, light guided by the waveguide is guided in a transverse mode in which, in the transverse cross-section, more than 95% of the guided light is in the regions of relatively low refractive index in the waveguide.
17. A waveguide as claimed in claim 1 , in which the boundary has a thickness such that, in use, light guided by the waveguide is guided in a transverse mode in which, in the transverse cross-section, more than 1% of the guided light is in the regions of relatively low refractive index in the photonic bandgap structure.
18. A waveguide as claimed in claim 1 , in which the boundary has a thickness such that, in use, light guided by the waveguide is guided in a transverse mode in which, in the transverse cross-section, more than 50% of the guided light is in the region of relatively low refractive index in the core.