This invention relates to the field of optical materials.
U.S. Pat. No. 5,155,792 teaches an optical fibre having a low index of refraction. In one embodiment described in the Patent, the cladding of the fibre comprises a uniform array of tube structures each being of a diameter less than the shortest wavelength of light guided by the fibre. The core may be formed from a rod or from several rods of uniform refractive index. In another embodiment, the core of the fibre also comprises a uniform array of tube structures each being of a diameter less than the shortest wavelength of light guided by the fibre, the core having a higher refractive index than the cladding but nevertheless a uniform and low refractive index.
However, that U.S. Patent does not contemplate the possibility of engineering the optical properties of a material other than by providing a uniform and low refractive index.
An object of the invention is to provide an optical material having optical properties that can readily be engineered. An object of the invention is to provide a method of engineering such properties.
According to the invention there is provided 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.
Such an arrangement of cylinders is referred to below as “nanostructure”, although the cylinders may, in some circumstances, be of dimensions and/or separations of a larger scale than nanometers.
Substantially optically homogeneous material causes no significant perturbation of the transverse optical field profile of light propagating in the composite material. Of course, whether or not the cylinders cause significant perturbation of the transverse optical field profile of propagating light is a question that may be determined experimentally. However, one way to quantify the effect of the cylinders is to calculate a modified version of the well-known V-parameter (also known as the V-value or normalised frequency). In a simple case of cylinders of cross-sectional area A and of a material of refractive index ncylinder
embedded in a matrix material of refractive index nmatrix
, the modified V-value V is given by
The modified V-value gives a measure of how confined to the cylinders is the transverse optical field. If the V-value is small (less than 1, for example) then the optical field is not well confined and the cylinders will not cause a significant perturbation. Ever smaller values of V are in general preferable, for example V<0.6 or even V<0.4.
In more complicated embodiments of the invention, such as, for example, when each cylinder comprises an inner region of refractive index n1
and area A1
, surrounded by an annular outer region of refractive index n2
and area A2
, one may calculate a “collective V-value” Vc
. For p regions embedded, directly or indirectly, in a matrix material, each region being of refractive index np
and area Ap
, we define
Again, if the v-value is small, then the optical field is not well confined and the embedded regions will not cause a significant perturbation of the transverse profile of an optical field of wavelength λ.
In some circumstances, such as when a waveguide is defined in the material, another indication of the effect the cylinders would have on an optical field can be obtained by evaluating a transverse component k sin θ of the wavevector k for light propagating at an angle θ to the longitudinal axis of the cylinders. That component gives an effective transverse wavelength
which will in general be greater than λ. The material will be substantially optically homogeneous if the diameter of the cylinders is small compared with λeff.
As light propagating in the material will in general see only an average of the optical properties of the cylinders and the matrix regions, the optical properties of the material may be engineered by appropriate choice of cylinder and matrix properties and by the arrangement of the cylinders in the matrix. A wide range of properties may be introduced, for example by using cylinders comprising different materials in different parts of the material or by doping the cylinders.
Preferably, the composite material is arranged to support propagation of light in a direction substantially parallel to the longitudinal axes. Preferably, the cylinders are solid. The cylinders may comprise a dielectric material. The cylinders may comprise a metal. At least some of the cylinders may be holes defined by the matrix regions. The holes may be filled with a fluid; such an arrangement of holes filled with a gas, in particular a gas other than air, or a liquid may be used to provide a nonlinear material. Preferably, the material is arranged to guide light propagating in a direction not in the plane perpendicular to the longitudinal axis.
It will be understood that the word “cylinder” refers to any three-dimensional figure of uniform cross-section, which cross-section is not necessarily circular. Preferably, the cylinders are of circular cross section.
Preferably, the diameter of the cylinders is less than 1 micron.
The refractive index of the cylinders may be less than that of the regions of a matrix material. The refractive index of the cylinders may be greater than that of the regions of a matrix material. Alterations to the refractive indices may be achieved in different ways, some of which are discussed below.
Preferably, the cylinders comprise an inner region having a first refractive index ninner
and an outer region having a second refractive index nouter
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. If the diameter of the cylinders is small compared with the wavelength of propagating light, and if the absolute variation of refractive index is small, the effective refractive index neff cyl
will be given approximately by
where Ainner and Aouter are the cross-sectional areas respectively of the inner and outer regions of each cylinder. Equation (4) is an approximation given for the purpose of aiding understanding of the invention; in general, expressions for the effective refractive index are not so simple but may readily be calculated by a person skilled in the art.
The effective refractive index may be equal to the refractive index of the matrix regions. It is thus possible to use different materials in the inner region, the outer region and the matrix region, each of which materials may have different, desirable properties, but still present a uniform effective refractive index to propagating light.
Some parts of the material may have different properties from other parts of the material. The cylinders may be distributed in the matrix material at different densities in different parts of the material. It may be that the regions of a matrix material are not all of the same material and/or that the cylinders are not all identical to each other; for example, it may be that the cylinders are not all of the same refractive index, shape, size and/or material.
Preferably, at least some of the cylinders have been doped with a dopant. At least some of the regions of a matrix material may have been doped with a dopant. Doping is a convenient way to change the optical properties of a material; stresses in the material may be much less than if two different materials are used and fused together.
Preferably, the doping results in a change in refractive index. Doping can then be used to provide a selected effective refractive index of the material and, for example, to introduce refractive-index structures into the material.
Preferably, at least some of the cylinders are doped with an optically active dopant.
Preferably, the optically active dopant renders the material photosensitive; examples of suitable dopants would be germanium, boron, or tin. It may be that not all of the cylinders are doped in the same manner, so that not all regions of the material are equally photosensitive. Such a material could be incorporated into, for example, multicore fibre.
The optically active dopant may provide optical gain; for example, the optically active dopant may comprise ytterbium or erbium. It may be that not all of the cylinders are doped with the same optically active dopant, so that gain is provided at two or more different wavelengths; thus “dual wavelength” operation may be provided.
Preferably, the composite material has a selected effective-refractive-index profile in the plane perpendicular to the longitudinal axes. Such a selected effective-refractive-index profile may be provided, for example, by altering the refractive index of a bulk material by any of the means described above.
Preferably, the selected effective-refractive-index profile is a gradual decrease in refractive index radially from the centre of the material. Such a graded-index structure may be a useful waveguiding structure.
Nanostructuring according to the invention is a convenient means of introducing other effective-refractive-index profiles into the material. For example, the selected effective-refractive-index profile may define a regular polygon. Alternatively, the selected effective-refractive-index profile may define a non-regular figure.
Preferably, 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. Preferably, 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 (for example, a 25 micron diameter core with a refractive index of 1.5 would support propagation of light of wavelength 1.5 microns in a single mode if the refractive index difference is less than 0.0007). Ever larger cores may be preferable in many applications; preferably, the core is larger than 15 microns, more preferably larger than 20 microns and still more preferably larger than 25 microns in its smallest transverse dimension.
The refractive-index profile may result in additional changes in the optical properties of the material; for example, it may define an optically dispersive material.
The composite material may exhibit birefringence due to mechanical stresses between the cylinders and the regions of a matrix material. Birefringence will generally result if the material has no rotational symmetry higher than two-fold rotational symmetry, about any longitudinal axis Thus, the material may have two-fold rotational symmetry or no rotational symmetry about such an axis. The rotational symmetry of the material may result at least partly from the shape of the cylinders. The rotational symmetry of the material may result at least partly from the distribution of the cylinders.
Also according to the invention there is provided an optical amplifier comprising a composite material as described above as being according to the invention. Preferably, the optical amplifier is a large-mode-area amplifier. Also according to the invention there is provided a laser including such an optical amplifier.
Also according to the invention there is provided a method of providing a selected optical property in a composite material for supporting propagation of light of a wavelength λ comprising: (i) 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; (ii) forming a bundle of the rods, in which the rods have their longitudinal axes substantially parallel to each other, and (iii) drawing an elongate, fused rod from the bundle; cutting the fused rod into a plurality of lengths; and (iv) 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.
Thus, the optical properties of the composite material are fixed by arranging macroscopic objects and then drawing reduces the dimensions of the structure until the material becomes substantially optically homogeneous.
Preferably, the bundle of rods is enclosed in a jacket in step (ii). An advantage of using a jacket is that the spaces between the canes may be evacuated to eliminate unwanted air holes that would otherwise be formed in the drawing process.
Also according to the invention there is provided 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 such a method. Preferably, the method includes the step of enclosing the preform in a jacket prior to drawing the fibre.
Preferably, 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.
Also according to the invention there is provided an optical fibre comprising a composite material as described above as being according to the invention. Preferably, the fibre is a photonic crystal fibre. Photonic crystal fibres are also known as microstructured fibres. Such fibres typically have a cladding comprising an array of elongate holes. If the effective refractive index of the cladding is lower than that of the core then a photonic-crystal fibre generally guides light by total internal reflection. Otherwise, a photonic-crystal fibre has its elongate holes arranged in a periodic lattice that excludes light at certain frequencies, forming a photonic band-gap. The core in such a fibre acts as a defect that breaks the periodicity of the lattice and allows propagation of light at frequencies in the photonic band-gap; the light is thereby confined to the core.
Also according to the invention, there is provided a fibre amplifier comprising such a fibre. Also according to the invention, there is provided a fibre laser comprising such an amplifier.
Also according to the invention, there is provided a method of transmitting light in a composite material described above as being according to the invention.