Optical material and a method for its production

Abstract

A composite material for supporting propagation of light of a wavelength λ, comprising a plurality of cylinders each having a longitudinal axis, the cylinders being separated from each other by regions of a matrix material and the cylinders having their longitudinal axes substantially parallel to each other, and each cylinder having a diameter, in the plane perpendicular to the longitudinal axis, that is small enough for the composite material to be substantially optically homogeneous in respect of light of wavelength λ.

1 . A composite material for supporting propagation of light of a wavelength λ, comprising a plurality of cylinders each having a longitudinal axis, the cylinders being separated from each other by regions of a matrix material and the cylinders having their longitudinal axes substantially parallel to each other, and each cylinder having a diameter, in the plane perpendicular to the longitudinal axis, that is small enough for the composite material to be substantially optically homogeneous in respect of light of wavelength λ.  
     
     
         2 . A composite material as claimed in  claim 1 , in which the cylinders are solid.  
     
     
         3 . A composite material as claimed in  claim 1 , in which the cylinders comprise a dielectric material.  
     
     
         4 . A composite material as claimed in  claim 1 , in which the cylinders comprise a metal.  
     
     
         5 . A composite material as claimed in  claim 1 , in which at least some of the cylinders are holes defined by the matrix regions.  
     
     
         6 . A composite material as claimed in  claim 5 , in which at least some of the holes are filled with a fluid.  
     
     
         7 . A composite material as claimed in any preceding claim, in which the diameter of the cylinders is less than 1 micron.  
     
     
         8 . A composite material as claimed in any preceding claim, in which the refractive index of the cylinders is less than that of the regions of a matrix material.  
     
     
         9 . A composite material as claimed in any of  claims 1  to  7 , in which the refractive index of the cylinders is greater than that of the regions of a matrix material.  
     
     
         10 . A composite material as claimed in any preceding claim, in which the cylinders comprise an inner region having a first refractive index and an outer region having a second refractive index and surrounding the inner region so that the refractive index of the cylinder is an effective refractive index that is between the first and the second refractive indices.  
     
     
         11 . A composite material as claimed in  claim 10 , in which the effective refractive index is equal to the refractive index of the matrix regions.  
     
     
         12 . A composite material as claimed in any preceding claim, in which the cylinders are distributed in the matrix material at different densities in different parts of the material.  
     
     
         13 . A composite material as claimed in any preceding claim, in which the regions of a matrix material are not all of the same material.  
     
     
         14 . A composite material as claimed in any preceding claim, in which the cylinders are not all identical to each other.  
     
     
         15 . A composite material as claimed in  claim 14 , in which the cylinders are not all of the same refractive index.  
     
     
         16 . A composite material as claimed in  claim 14  or  claim 15 , in which the cylinders are not all of the same shape.  
     
     
         17 . A composite material as claimed in any of  claims 14  to  16 , in which the cylinders are not all of the same size.  
     
     
         18 . A composite material as claimed in any of  claims 14  to  17 , in which the cylinders are not all of the same material.  
     
     
         19 . A composite material as claimed in any preceding claim, in which at least some of the cylinders have been doped with a dopant.  
     
     
         20 . A composite material as claimed in any preceding claim, in which at least some of the regions of a matrix material have been doped with a dopant.  
     
     
         21 . A composite material as claimed in  claim 19  or  claim 20 , in which the doping results in a change in refractive index.  
     
     
         22 . A composite material as claimed in any of  claims 19  to  21 , in which at least some of the cylinders are doped with an optically active dopant.  
     
     
         23 . A composite material as claimed in  claim 22 , in which the optically active dopant renders the material photosensitive.  
     
     
         24 . A composite material as claimed in  claim 23 , in which the optically active dopant is germanium, boron, or tin.  
     
     
         25 . A composite material as claimed in  claim 23  or  claim 24 , in which not all of the cylinders are doped in the same manner, so that not all regions of the material are equally photosensitive.  
     
     
         26 . A composite material as claimed in  claim 22 , in which the optically active dopant provides optical gain.  
     
     
         27 . A composite material as claimed in  claim 26 , in which not all of the cylinders are doped with the same optically active dopant, so that gain is provided at two or more different wavelengths.  
     
     
         28 . A composite material as claimed in  claim 26  or  claim 27  in which the optically active dopant comprises ytterbium or erbium.  
     
     
         29 . A composite material as claimed in any preceding claim, having a selected effective-refractive-index profile in the plane perpendicular to the longitudinal axes.  
     
     
         30 . A composite material as claimed in  claim 29 , in which the selected effective-refractive-index profile is a gradual decrease in refractive index radially from the centre of the material.  
     
     
         31 . A composite material as clamed in  claim 29 , in which the selected effective-refractive-index profile defines a regular polygon.  
     
     
         32 . A composite material as claimed in  claim 29 , in which the selected effective-refractive-index profile defines a non-regular figure.  
     
     
         33 . A composite material as claimed in any of  claims 29  to  32 , in which the selected effective-refractive-index profile defines an optically dispersive material.  
     
     
         34 . A composite material as claimed in any of  claims 29  to  32 , in which the selected effective-refractive-index profile defines a waveguide including a core having an effective refractive index that is larger than the effective refractive index of material surrounding the core.  
     
     
         35 . A composite material as claimed in  claim 34 , in which the core is larger than 10 microns in its smallest transverse dimension and the difference between the effective refractive index of the core and the effective refractive index of the material surrounding the core is sufficiently small for the waveguide to support single-mode propagation of light.  
     
     
         36 . A composite material as claimed in any preceding claim, which exhibits birefringence due to mechanical stresses between the cylinders and regions of a matrix material.  
     
     
         37 . A composite material as claimed in any preceding claim, in which the material has no rotational symmetry higher than two-fold rotational symmetry about any longitudinal axis, so the composite material is birefringent.  
     
     
         38 . A composite material as claimed in  claim 37 , in which the material has no rotational symmetry about any longitudinal axis.  
     
     
         39 . A composite material as claimed in  claim 37 , in which the material has two-fold rotational symmetry about a longitudinal axis.  
     
     
         40 . A composite material as claimed in any of  claims 37  to  39 , in which the rotational symmetry of the material results at least partly from the distribution of the cylinders.  
     
     
         41 . A composite material as claimed in any of  claims 37  to  40 , in which the rotational symmetry of the material results at least partly from the shape of the cylinders.  
     
     
         42 . A composite material as claimed in any of  claims 1  to  40 , in which the cylinders are of circular cross-section.  
     
     
         43 . An optical amplifier comprising a composite material as claimed in any preceding claim.  
     
     
         44 . An optical amplifier as claimed in  claim 43 , which is a large-mode-area amplifier.  
     
     
         45 . A laser including an optical amplifier as claimed in  claim 43  or  claim 44 .  
     
     
         46 . A method of providing a selected optical property in a composite material for supporting propagation of light of a wavelength λ comprising: 
 providing a plurality of rods each having a longitudinal axis, a inner region of a first solid material and an outer region of a second solid material and surrounding the inner region;  
 forming a bundle of the rods, in which the rods have their longitudinal axes substantially parallel to each other, and drawing an elongate, fused rod from the bundle;  
 cutting the fused rod into a plurality of lengths; and  
 repeating steps (ii) and (iii) at least once by forming the bundle from the lengths of fused rod and drawing a further, elongate fused rod from the bundle; 
 wherein, the inner regions of the rods used in step (i) form, in the further, elongate fused rod produced at the end of step (iv), a plurality of solid cylinders each having a longitudinal axis, the cylinders being separated from each other by regions of a matrix material and the cylinders having their longitudinal axes substantially parallel to each other, and each cylinder having a diameter, in the plane perpendicular to the longitudinal axis, that is small enough for the composite material to be substantially optically homogeneous in respect of light of wavelength λ.  
 
 
     
     
         47 . A method as claimed in  claim 46 , in which the bundle of rods is enclosed in a jacket in step (ii).  
     
     
         48 . A method of manufacturing an optical fibre, including the step of drawing an optical fibre from a preform comprising at least one rod manufactured by the method according to  claim 46  or  claim 47 .  
     
     
         49 . A method as claimed in  claim 48 , including the step of enclosing the preform in a jacket prior to drawing the fibre.  
     
     
         50 . A method as claimed in any of  claims 46  to  49 , in which in at least some of the rods the inner region has a first refractive index and the outer region has a second, different refractive index.  
     
     
         51 . An optical fibre comprising a composite material according to any of  claims 1  to  42 .  
     
     
         52 . An optical fibre as claimed in  claim 51  and being a photonic crystal fibre.  
     
     
         53 . A fibre amplifier comprising a fibre according to  claim 51  or  claim 52 .  
     
     
         54 . A fibre laser comprising an amplifier according to  claim 53 .  
     
     
         55 . A method of transmitting light in a composite material according to any of  claims 1  to  42 .  
     
     
         56 . A method of manufacturing an optical material substantially as herein described with reference to the accompanying drawings.  
     
     
         57 . An optical material substantially as herein described with reference to the accompanying drawings.