Beam intensity profile tailoring with a composite, tapered optical fibre
A composite all optical-fibre based tapered photonic waveguide ( 110 ) including a single or multiple secondary waveguides ( 120 ) within or around a primary waveguide ( 118 ) is described. The composite optical fibre may also be termed abeam tailoring optical fibre (BT Fibre) ( 110 ). In use, at the larger secondary end ( 114 ) both the primary waveguide ( 118 ) and the secondary waveguide/s ( 120 ) may guide modes at a particular wavelength. However, at the same wavelength, adiabatically tapering down the waveguides ( 118, 120 ) reduces the dimensions of the secondary waveguide/s ( 120 ) such that all the secondary waveguide/s ( 120 ) become effectively non-guiding at the smaller primary end ( 112 ), whilst the primary waveguide ( 118 ) still guides. In other words, the composite optical fibre is a spatially modulating optical fibre ( 110 ).
1 . A composite optical waveguide comprising:
at least one primary waveguide; and
a secondary waveguide surrounding or within the at least one primary waveguide,
wherein the composite optical waveguide is tapered such that the at least one primary waveguide and the secondary waveguide taper together towards an end of the composite optical waveguide,
the taper of the at least one primary waveguide and the taper of the secondary waveguide cause a substantial transfer of an optical fundamental mode from one of the at least one primary waveguide and the secondary waveguide to the other of the at least one primary waveguide and the secondary waveguide, and
a beam intensity profile of the output optical mode is modified.
2 . A composite optical waveguide according to claim 1 , wherein the taper of the optical waveguide is slow and steady, so that the composite waveguide fulfils a length-scale and a weak-power criterion to avoid a coupling between the fundamental mode and higher-order modes.
3 . A composite optical waveguide according to claim 1 , wherein the taper of the composite optical waveguide includes at least one of a reduction in: a diameter of the composite waveguide, a diameter or a thickness of the at least one primary waveguide and a diameter or a thickness of the secondary waveguide.
4 . A composite optical waveguide according to claim 1 , wherein the taper of the composite optical waveguide is substantially adiabatic with respect to the propagation of the optical mode.
5 . A composite optical waveguide according to claim 1 , further comprising at least one further secondary waveguide surrounded by or within the at least one primary waveguide.
6 . A composite optical waveguide according to claim 1 , wherein a refractive index profile between the at least one primary waveguide and the secondary waveguide is at least one of a step-index and a graded-index.
7 . A composite optical waveguide according to claim 1 , wherein a refractive index of the at least one primary waveguide is less than a refractive index of the secondary waveguide.
8 . A composite optical waveguide according to claim 1 , wherein the composite optical waveguide is reciprocal with respect to an input end and an output end.
9 . A composite optical waveguide according to claim 1 , wherein a minimum refractive index difference (DN−Dn) is maintained between the at least one primary waveguide and the secondary waveguide, so that the secondary waveguide is an effective waveguide for at least one of a fixed secondary end or a taper length.
10 . A composite optical waveguide according to claim 1 , wherein at least two secondary waveguides are within the at least one primary waveguide,
whereby a single beam at a primary end of the composite optical waveguide is split into at least two corresponding beams outputted from a secondary end of the composite optical waveguide.
11 . A composite, optical fibre comprising of:
at least one primary waveguide,
at least one secondary waveguide, and
the at least one secondary waveguide surrounds or is within the primary waveguide,
wherein the composite optical fibre has a uniformly increasing diameter from one end to another such that the at least one primary waveguide and the at least one secondary waveguide each have a uniformly increasing diameter from one end of the composite optical fibre to another end of the composite optical fibre.
12 . A composite optical fibre according to claim 11 , wherein a longitudinal axis of the at least one primary waveguide and a longitudinal axis of the secondary waveguide are parallel.
13 . A composite optical fibre according to claim 11 , wherein a longitudinal axis of the at least one primary waveguide and a longitudinal axis of the secondary waveguide are coaxial.
14 . A composite optical fibre according to claim 11 , wherein the at least one primary waveguide has a transverse cross-sectional shape that is at least one of circular, elliptical and annular.
15 . A composite optical fibre according to claim 11 , wherein the at least one secondary waveguide has a transverse cross-sectional shape that is at least one of elliptical, circular and annular.
16 . A method of modifying a beam intensity profile of a laser beam by the steps of:
providing at least one secondary waveguide within or surrounded by at least one primary waveguide; and
tapering both the primary waveguide and the secondary waveguide together such that a laser beam mode is caused to be transferred from one waveguide to the other along the tapered section;
whereby a beam intensity profile of an input laser beam is modified.