WO2004001461A1

WO2004001461A1 - Improvements in and relating to microstructured optical fibres

Info

Publication number: WO2004001461A1
Application number: PCT/GB2003/002620
Authority: WO
Inventors: Abdel Fetah Benabid; Jonathan Cave Knight
Original assignee: Crystal Fibre A/S
Priority date: 2002-06-19
Filing date: 2003-06-18
Publication date: 2003-12-31
Also published as: AU2003236903A1; GB0214118D0

Abstract

An optical fibre comprises a core region and a cladding region (30), the core region (40) comprising an elongate hole (40), the cladding region (30) comprising a plurality of webs (50) that are elongate substantially flat walls that define a plurality of elongate holes (60,70). In a preferred embodiment, the cladding region (30) comprises a periodic structure characterised by a unit cell comprising webs defining an elongate hole (60) of hexagonal cross-section and two elongate holes (70) of triangular cross-section arranged adjacent to opposite sides of the hexagonal hole (60), such that each triangular hole (70) shares a web in common with the hexagonal hole (60).

Description

IMPROVEMENTS IN AND RELATING TO MICROSTRUCTURED OPTICAL FIBRES

This invention relates to the field of optical fibres. Optical fibres are important components of several technologies, in particular telecommunications technology.

Optical fibres are usually made entirely from solid materials such as glass, and each fibre usually has the same cross- sectional structure along its length. Transparent material in one part (usually the middle) of the cross-section has a higher refractive index than material in the rest of the cross-section and forms an optical core within which light is guided by total internal reflection. We refer to such a fibre as a conventional fibre or a standard fibre.

Most standard fibres are made from fused silica glass, incorporating a controlled concentration of dopant, and have a circular outer boundary that is typically of diameter 125 microns .

Recently a new type of optical fibre has been developed known as a photonic crystal fibre (PCF) , also known as a microstructured fibre or a holey fibre) .

PCFs are fibres having a cladding region that comprises a plurality of elongate regions, running parallel to the longitudinal axis of the fibre, that are of a different refractive index from a matrix region in which the elongate regions are embedded. The elongate regions are, in many cases, air-filled holes, although they are in some cases solid regions or regions filled with a liquid or another gas.

The core of a PCF is a region having a different structure from the cladding region; it is often a region having no holes or a region having one or more extra holes.

According to the prior art understanding of propagation in a PCF, light is confined to the core of a PCF by the cladding through the action of one of two mechanisms. The first is closely related to the guidance mechanism of a standard fibre. In this mechanism, the matrix regions and the elongate regions of the cladding have an ^effective' refractive index that is less than the refractive index of the core region, so that total internal reflection occurs and traps light in the core.

In the second mechanism, the arrangement of elongate regions in the cladding is periodic such that the cladding exhibits a photonic band gap. (This phenomenon is analogous to the formation of electronic band gaps in semiconductors.) Interference between light reflected from the elongate regions is such that there are certain bands of frequencies of light that cannot propagate in the cladding. In a PCF that guides by this mechanism, the core forms a efect' in the periodic structure of the cladding. Light can propagate in this defect region and light is thus confined to and propagates in the core of the PCF. The microstructure of the cladding determines the band-gaps of the fibre (i.e. its transmission bands) and their properties such as their frequency width, their locations (in a frequency-wavevector plot) and their polarisation properties.

A common PCF configuration comprises a cladding region comprising elongate holes of circular cross-section arranged on a triangular lattice. Such a configuration has been demonstrated to provide desirable optical properties, including enabling the guiding of light in a fibre core comprising an elongate hole (see R.F. Cregan et al . , Science Vol 285, No. 5433, ppl537 - 1539 (1999) ) . Photonic band-gap guidance in fibres having other cladding region configurations has also been demonstrated, for example a honeycomb (hexagonal) lattice of small, approximately triangular holes (see J.C. Knight et al., Science Vol. 282, No. 5393, pp 1476 - 1478 (1998)).

A Kagome structure is a well-defined structure that is known in solid-state physics; it is formed from overlapped ^λStars of David' . A theoretical study of Kagome photonic structures has been reported for slab waveguides (see J. B. Nielson, T. Sondergaard, S. E. Barkou et al . , "Two-dimensional Kagome structure, fundamental hexagonal photonic crystal configuration," Electron. Lett. 35 (20), 1736 (1999) and J. B. Nielson, T. Sondergaard, S. E. Barkou et al . , "Two- Dimensional Kagome Photonic Bandgap Waveguide," IEEE Photon. Technol. Lett. 12 (6), 630 (2000)). In that prior art, the rods/holes are located at the apexes of the Kagome lattice surrounded by a background material.

International Patent Application No. PCT/GB00/01934 (published as WO 00/72067) describes a solid-core PCF with a cladding structure having a distorted Kagome-like structure.

Douglas C. Allen et al . describe in ^λ Photonic crystal fibres: effective index and band-gap guidance' (CM Soukoulis (ed.), Photonic Crystals and Light Localisation in the 21^st Century, pp. 305-320, Kluwer Academic Publishers, Netherlands, 2001) describe a number of different band-gap guiding fibres, including, in Fig. 3 of that document, a hollow-core fibre having a cladding having a Kagome structure, with large substantially circular holes (some of which are distorted) surrounded by smaller interstitial holes.

R.F. Cregan et al . describe in ^λSingle Mode Photonic Band Gap Guidance of Light in Air', Science, Vol. 285 ppl537-1539 (1999) another hollow-core fibre that has a cladding having a Kagome structure resulting from holes surrounded by interstitial holes.

In International Patent Application No. PCT/DK99/00193, Broeng at al . describe a cladding structure based on an array of holes that are arranged at the vertices of a lattice defined by a ^regular hexagonal polygon and a regular triangle having a side length corresponding to that of the regular hexagonal polygon, and wherein hexagonal polygons exist, each side of which is shared with a triangle'. Broeng et al. identify that structure with the ^λso-called Kagome structure' .

In the Application, Broeng et al . identify nodes' and Veins' in PBG structures. The nodes and veins are higher- index areas in the fibre cross-section. Broeng et al . identify the separation and size of the nodes as being particularly important in determining the bandgap of a structure. They state that ^Λboth Kagome and Honeycomb structures have intrinsically larger nodes and relatively narrower veins than the triangular structures' . They go on to state that in the Kagome structure, 'the advantages of the hexagonal structure are maintained, and a further spacing of the high index areas defined in the hexagonals is provided via the triangles' . Thus the polygons identified in International Patent Application No. PCT/DK99/00193 define high-index areas, although the vertices of the polygons themselves are identified by low index areas.

In International Patent Application No. PCT/DK99/00279 Broeng et al . describe other structures that they identify as being Kagome structures. They state: ^λBoth Honeycomb and Kagome structures have intrinsically larger nodes and relatively narrower veins than the triangular structure. However, with respect to the second issue that must be addressed for the optimisation of the cladding structures for use in hollow core fibres, namely that the bandgaps must extend below the air line, neither realistic Honeycomb nor Kagome structures have been found to fulfil this necessary requirement. That no bandgaps have been found to extend below the air line for realistic Honeycomb and Kagome structures are attributed to the fact that these structures have intrinsically lower void filling fractions than triangular structures with voids of similar sizes.' Broeng et al . go on to reject their Kagome structure; they state that they ^xhave therefore realised that triangular-like arrangements of voids form the best basis for cladding structures, which are optimised for use in hollow- core PBG fibres.'

An object of the invention is to provide an optical fibre having an alternative structure having advantageous properties compared with prior art fibres and a method of making such a fibre.

According to a first aspect of the invention there is provided an optical fibre comprising a core region and a cladding region, the core region comprising an elongate hole, the cladding region comprising a plurality of webs that are elongate substantially flat walls that define a plurality of elongate holes . A web may be substantially flat in the sense that two surfaces of the web form, in a transverse plane perpendicular to the elongate axis of the fibre, two lines that are parallel over at least 50% of the length of the web in that plane. The lines may be parallel over at least 75%, at least 85%, at least 90% or at least 95% of the length of the web in that plane. Thus, the web may be substantially rectangular in a transverse cross-section of the fibre.

The cladding region may comprise a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole. Alternatively, the periodic structure may be of some other form, for example based on a triangular lattice or a square lattice or a hexagonal lattice .

According to a second aspect of the invention there is provided an optical fibre comprising a core region and a cladding region, the cladding region comprising a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole.

The following discussion describes preferred features that may be found in fibres that embody the first aspect of the invention, the second aspect of the invention or both the first and second aspects of the invention (except where that is not consistent with the sense of the discussion) . Of course, the described features are optional rather than limiting. Nodes are formed where two or more of the webs meet in the cladding. The nodes may have a width that is less than 1.5 times the width of the webs . We have found that a fibre having a cladding structure with relatively small nodes may guide a significant amount of light even when the fibre is not refractive-index guiding or photonic band-gap guiding. Rather, the cladding region has a structure that has over a band of wavelengths a very low density of states for which propagation is supported. Guidance of light can occur in that band of wavelengths; although the guidance is lossy, the loss is far less than would be the case, say, for a waveguide made from a hole through a glass block, which has a relatively high density of states.

Thus, the cladding region may be arranged such that the fibre exhibits a low density of states allowed to propagate in the cladding region. That may be achieved as a result of the arrangement of the plurality of webs. Different examples of cladding regions may be modelled and their density of states along the light line determined by numerical simulation, in a manner known in the art. Thus, a cladding structure may be selected that has a sufficiently low (but not necessarily zero) density of states in the cladding region that a fibre made having that cladding structure guides light in its core region. The fibre may exhibit a bandgap, but it may be over a narrower band of wavelengths than the band of wavelengths over which the fibre guides light. Alternatively, the fibre may not exhibit a bandgap or may exhibit no significant bandgap. Any bandgap that does exist may, but need not, overlap with the region of low density of states that supports guidance.

Of course, the core-cladding boundary has different properties from the cladding in general (for example, webs in the core-cladding boundary usually connect to a different pattern of neighbouring webs from the pattern in which webs in the cladding connect). The core-cladding boundary may have a different structure from the cladding. The core boundary may be substantially smooth.

The particular periodic cladding described above may be identified as a form of Kagome lattice. However, whereas in prior art Kagome fibre cladding structures (such as those described in the International Patent Applications discussed above) , the triangles and hexagons of the Kagome pattern are abstract polygons defined by the positions of holes and high- index regions in the fibre cladding, in the fibre according to the invention the holes themselves actually have triangular and hexagonal cross-sections that create the Kagome pattern. Thus, in the present invention the structure may be made of fine silica membranes (forming the 'lines' of the Kagome lattice) surrounded by air.

Furthermore, we have found that fibres having a periodic cladding characterised by a unit cell of that form may provide a relatively low loss over a much wider bandwidth compared with prior art fibres characterised by other unit cells and having the same period. That discovery may be contrasted with the conclusion in International Patent Application No. PCT/DK99/00279, set out above, that Kagome structures are intrinsically inferior to triangular structures. The structure of the invention is, of course, physically different from the fibres described in that application.

The holes may fill the fibre cross-section to a filling fraction of 70% or more. According to prior-art understanding of light propagation in PCFs, larger filling fractions generally provide wider band-gaps and may also provide lower loss as the amount of solid material encountered by light during propagation through the fibre is reduced. The holes may fill the fibre cross-section to a filling fraction of 80% or more. Such high filling fractions may be achieved by the webs being thin. The webs may have a minimum transverse dimension of less than 10% of the pitch (that is, the centre-to-centre separation of two adjacent cells; i.e. the period of the periodic structure) . The webs may have a minimum transverse dimension of less than 4% of the pitch of the periodic structure.

The hexagonal cross-section may be a regular hexagon. The triangular cross-section may be an equilateral triangle.

In the second aspect of the invention (as well as in the first), the core region may comprise an elongate hole. A particular advantage of fibres guiding by mechanisms other than total internal reflection is that such fibres may be arranged to guide light in an air hole. Guiding in air is advantageous in many applications, because it significantly reduces losses and permits propagation of high intensity light without the nonlinear effects that may be experienced in propagation through solid material. The elongate hole of the core (or, indeed, holes of the cladding) may be filled with a gas with which propagating light interacts.

The hole may have a smallest transverse dimension that is larger than the period of the lattice. The hole may have a smallest transverse dimension that is larger than twice the period of the lattice.

The optical fibre may further comprise a jacket region that surrounds the cladding region. Provision of such a jacket region provides durability and ruggedness for the fibre, which otherwise may be rather fragile.

According to a third aspect of the invention there is provided a method of manufacturing an optical fibre, comprising providing a preform comprising dielectric material defining a plurality of holes and drawing the fibre from the preform in one or more draws whilst controlling pressure in the holes such that the dielectric material deforms to form a plurality of webs that are elongate substantially flat walls that define a plurality of elongate holes in the drawn fibre.

The dielectric material may comprise a bundle of tubes and the method may then comprise drawing the fibre from the preform in one or more draws whilst controlling pressure in and/or around the tubes such that the tubes deform to form the plurality of webs .

The dielectric material may deform to form a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section, arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole. According to a fourth aspect of the invention there is provided a method of manufacturing an optical fibre, comprising providing a preform comprising a plurality of tubes and drawing the fibre from the preform in one or more draws whilst controlling pressure in and/or around the tubes such that the tubes deform to form a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section, arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole.

The following discussion describes preferred features that may be found in fibres that embody the third aspect of the invention, the fourth aspect of the invention or both the third and fourth aspects of the invention (except where that is not consistent with the sense of the discussion) . Of course, the described features are optional rather than limiting.

Without pressure control, surface tension would tend to collapse the holes during the draw. If the pressure around a tube is lower than the pressure of the hole in the tube, then that hole will tend to expand. Controlling pressure in and/or around the holes or tubes thus enables the desired periodic structure to be produced.

We have found that fibre that has a relatively low loss over a much wider bandwidth compared with prior art fibres having other unit cells may be manufactured by the method. Similarly, it may be easier to fabricate a fibre having a selected bandgap or transmissivity or loss using the method. For example, a pitch Λ of 1-2 microns is required to provide a bandgap between 600 nm and 700 nm in a fibre having a cladding comprising an array of holes of circular cross-section arranged on a triangular lattice. In contrast, a pitch Λ of 4-5 microns is required to provide a similar transmission region in a fibre according to the invention. It is significantly easier to fabricate a fibre having a larger rather than a smaller pitch and thus the fibre according to the invention is more easily fabricated than a fibre having a similar transmission region in its spectrum but based on a triangular lattice. Of course, the need for pressure control adds some complication to the manufacturing procedure but the skilled person may readily provide a suitable pressure-control arrangement and, once that is provided, the fabrication tolerances required to fabricate the larger-pitch fibre are relaxed compared with the smaller- pitch fibre .

The method may also provide a very high air-filling factor with a small pitch and high degree of regularity.

The tubes may be thin-walled. The interplay between pressurisation and surface tension may be more readily optimised to give the desired unit-cell structure by using thin-walled tubes. The tubes may have an inner diameter : outer diameter ratio that is greater than 0.90; the diameter may be greater than 0.95.

The method further comprises placing the preform or bundle in a jacketing tube, which may be a tube of silica. Drawing the fibre without a jacket may be difficult because at a slightly elevated temperature the holes collapse completely and the fibre becomes very fine. Even if the draw is successful, the resultant fibre is likely to be very fragile, making its use a tedious task. As indicated above, use of a jacketing tube provides mechanical protection for the fibre, making the fibre more robust and flexible. The jacket may have a thickness of between 30 mm and 50 mm.

Even with a jacketing tube, drawing a fibre having the desired structure may be difficult because the structure may be deformed by the presence of the jacket. Use of a jacketing tube however provides the further advantage that it defines a volume, around the preform or bundle, which may be connected to a pressure controller. Thus, the method may further comprise reducing pressure in the space between the bundle and the preform or tube during the draw. Reduction of the pressure of the volume will tend to encourage expansion of the holes. The above problems with the draw are thus solved by a simple technique, which is nonetheless very efficient in producing well preserved structural regularity and very high air-filling factor, as well as being robust and easy to use. The jacketed fibre may be coated.

The preform comprising the plurality of tubes may be fused and may be drawn into an intermediate preform prior to the drawing of the fibre. Thus, for example a bundle of rods having a bundle diameter about 4 mm may be drawn into an intermediate preform of 1 mm diameter, which may then be drawn into a fibre of conventional size (175 μm to 125 μm) .

The fibre may be drawn at a temperature of between 1880 k and 1940 k. The pressure around the preform or bundle may be between -0.40 bar and -0.60 bar during the draw. The preform may be fed at a speed between 35 mm/min and 50 mm/min during drawing of the fibre. The fibre may be pulled from the preform at a speed of between 20 mm/min and 30 mm/min.

The fibre manufactured by the method of the third or fourth aspect of the invention may be a fibre according to the first or second aspect of the invention.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

Fig. 1 is a transverse cross-section through a fibre according to the invention;

Fig. 2 is the primitive unit cell of the cladding of Fig. 1;

Fig. 3 is a perspective view of a first preform for use in an example of a method according to the invention; Fig. 4 is a perspective view of a second preform drawn from the preform of Fig. 3 and inserted into a jacketing tube;

Fig. 5 is a perspective view of a preform assembly, including the preform and jacketing tube of Fig. 4, for use in a method according to the invention; Fig. 6 is a schematic view of the preform assembly of Fig. 5 being drawn into a fibre according to the invention; Fig. 7 is a set of three scanning electron microscope photographs, at different magnifications, of an example of a fibre according to the invention;

Fig. 8 is a set of two optical microscope photographs showing light propagating in the fibre of Fig. 7;

Fig. 9 is plot of the variation of power attenuation with wavelength for light propagating through the fibre shown in Figs. 7 and 8;

Figs. 10 to 14 are photographs of further fibres according to the invention;

Fig. 15 is plot of the variation of (a) transmission and (b) loss with wavelength for light propagating through an example of a fibre according to the invention;

Fig. 16 is a plot of the variation with (β-k)Λ and kΛ of the density of states for which propagation is allowed in a fibre according to the invention having a Kagome cladding based on a hexagonal hole with its parallel sides spaced by 0.05 microns (the density of states is normalised over the density of states in a vacuum) ; Fig. 17 is a plot of the variation of density of states with kΛ along the light line of Fig. 16 (i.e. along the line β=k) .

Fibre 10 (Fig. 1) is an example of a fibre according to the invention. Fibre 10 comprises jacket region 20 (which provides the fibre with mechanical strength and durability) cladding region 30 and core 40.

Cladding region 30 comprises a plurality of elongate (along the axis of the fibre) and substantially flat webs 50 (in the cross-section shown in Fig. 1, webs 50 are thin lines) . Webs 50 form a Kagome lattice in the transverse cross-section of the fibre. The webs 50 define hexagonal holes 60 and triangular holes 70; the pattern can be viewed as a number of 'Stars of David' translated relative to each other so that the triangles of adjacent stars fully overlap. More formally, the pattern can be described by a primitive unit cell such as that of Fig. 2, which comprises an elongate hole 60' of hexagonal cross-section and two elongate holes 70', 70'' of triangular cross-section arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web 50', 50'' in common with the hexagonal hole. The configuration of cladding region 30 can be recovered by tiling the unit cell of Fig. 2, so that adjacent unit cells are perfectly abutted to each other without overlap. The pitch or period Λ of the cladding is the distance between corresponding points in adjacent unit cells. Core 40 is an elongate hole of hexagonal cross-section. It is formed by the omission of such webs from the centre of fibre 10 as would define seven further hexagonal holes and intervening triangular holes in the cladding pattern.

Fibre 10 may be fabricated from preforms 200, 300 (Figs. 3 and 4) similar to the preforms used to fabricate prior art microstructured fibres. The preform 200 comprises a bundle of capillaries 230 each of outer diameter 1 mm and inner diameter 0.93 mm. (Capillaries 230 are themselves drawn down from tubes of outer diameter 18 cm.) At the centre of the bundle of capillaries 230, seven capillaries are missing, such that surrounding capillaries 230 define a hole 240 that forms core 40 in fibre 10. (Of course, in alternative embodiments larger or smaller cores may be provided by the omission of more or fewer capillaries.) (In order to maintain the structural integrity of the bundle during the draw, short canes (not shown) are positioned at each end of the bundle in hole 240. Those canes act as spacers that keep open hole 240 during the draw.)

Each capillary 230 has a hole 260 at its centre. Away from the edges of the bundle, adjacent capillaries 230 define interstitial holes 270. Thus holes 260 form hexagonal holes 60 and holes 270 form triangular holes 70 in fibre 10. The solid parts 250 of tubes 230 form webs 50 and tube 220 forms jacket region 20. Preform 200 is partially drawn into intermediate preform in the form of cane 300. The drawing is much less than that used to draw a fibre (the diameter of the cane is about 1 mm) and is used primarily to form capillaries 230 into a monolithic structure (Fig. 4) . Holes 260 form holes 360 in preform 300. Interstitial holes 270 form holes 370. Solid silica regions 250 form matrix regions 350. Central hole 240 forms hole 340. Intermediate preform 300 is inserted into a silica jacketing tube 320 prior to drawing into the fibre itself.

The drawing technique employed to draw the fibre involves regulating a negative pressure in the space between the jacket 320 and the intermediate preform 300 (Fig. 5) . A metallic cap 410 with an outlet 420 for connection to a vacuum pump (not shown) is glued onto one end of jacket 320. The intermediate preform 300 passes through the cap 410.

Assembly 400, comprising cane 300, jacket 320 and cap 410 is mounted in the usual way in a drawing tower of the type used to draw standard fibres. Outlet 420 is then connected to a vacuum pump and the space between preform cane 300 and jacket 320 is evacuated. As in the usual manner of drawing a fibre from a preform, drawing tower furnace 510 softens the silica of the assembly 400 and fibre 500 is drawn from the lower end of assembly 400 (Fig. 6). Pressure in the space between cane 300 and jacket 320 is regulated throughout the draw. Assembly 400 is fed into furnace 510 at speed v_f . Fibre 500 is drawn at speed v_p. The choice of the right value of both the pressure and the temperature is very important to the success of the drawing and they both depend on the thickness of the jacket 320. We have fabricated a Kagome hollow-core photonic crystal fibre; Fig. 7 is a set of micrographs of the fibre. The principal elements of the fibre (the jacket, the Kagome lattice structure and the large air core) are visible in Fig. 7 (a) . The large air core is shown in more detail in Fig. 7(b), in which distortions of the cladding lattice near the air core are visible. A portion of the lattice structure having only insignificant distortions is shown in still more detail in Fig. 7 (c) . The fibre of Fig. 7 was fabricated according to the following fabrication parameters:

The fibre had an air-filling factor of approximately 85% . The Kagome fibre exhibited different dispersion properties compared with a PCF having a conventional triangular structure of the same pitch Λ. In particular, the Kagome fibre had bandwidth covering over 1 unit of normalised frequency (kΛ), while the maximum bandwidth in the triangular structure is about 0.1-0.2. Furthermore, the bandwidth was localised at high-normalised frequency (kΛ being about 32-35) . Those features provide a very- broad transmission spectrum for a relatively large pitch (4-5 μm) , which is significantly easier to fabricate than the much smaller pitch that would be required to provide similar properties in a conventional triangular PCF. Light was transmitted down the fibre of Fig. 7. Fig. 8 is a pair of optical micrographs showing the light trapped in the air core (as observed under an optical microscope) . The light transmitted in the core was white in colour, showing that the transmitted spectrum was broad.

The measured loss of the fibre of Fig. 7 is shown in Fig. 9. The large peak 600 at around 1400 nm is attributed to OH absorption from water at the surface of the air holes 60,70. Excluding the water peak, the loss is under 4 dB/m over the whole spectrum shown (wavelengths of 400 nm to 1600 nm) . (The measured transmission spectrum was limited to between 400 nm and 600 nm by the detection system used; the fibre's actual transmission spectrum extends beyond those wavelengths.) The loss is less than 2 dB/m over a wide range of wavelengths and less than 1.5 dB/m at 1300nm, which is a wavelength of interest . The parameters chosen for the draw were optimised for the particular capillaries 230 and jacket 320 used. Changing the wall thickness of jacket 320, for example, would require a corresponding change in the negative pressure applied to the space between cane 300 and jacket 320, as the inertia and the surface tension would then be different. Of course, the skilled person would be able to determine suitable parameters without due burden by trial and error for a particular combination of capillaries and jacket.

In the heat zone in furnace 510, different effects are in action, in particular mechanical effects such as surface tension and the feeding-pulling forces and thermodynamical effects such as phase transitions. There is interplay between those effects, such that a variation in one affects one or more of the others. In order to preserve a high air-filling-factor structure when using a different jacket wall thickness, one has to keep surface tension low in order to prevent hole collapse. Surface tension of the jacket is proportional to the amount of work W (here heat) that the cane has undergone and is inversely proportional to the surface area A of the cane 300; that relationship can be written as:

W σ =

Equation 1

If we consider the silica as a fluid, the surface tension can also be written as:

^{σ ∞}

Pvapor f ^{x τ}" ∞ {Pi d j ^{x τ}" E quati on 2 which implies that the surface tension varies as the density of the glass to the power of 4. Hence, it is desirable to reduce that density in the preliminary drawing of stack 200 into cane 300 and to use thin-walled capillaries 230 for the stack.

The diameter of the fibre 500 depends on the feeding v_f _ ^VJ and pulling v_p speeds; the ratio , between the fibre-pulling speed and the cane-feeding speed determines the outer diameter

of the fibre 500. The difference ^VP ^{~~ v}f ~ ^vf determines the length of time that the assembly 400 spends in the heat zone.

Thus v_f determines the amount of energy received by the part of the assembly 400 in the heat zone (i.e. W) . With that information, one can adjust the feeding speed, the temperature and the negative pressure between the jacket 320 and the stack 300 in order to provide a desired surface tension for given preform parameters. The pulling speed is then adjusted to give the desired outer diameter for the fibre 500.

Figs. 10 to 14 are photographs of further examples of fibres having Kagome cladding structures. Fig. 10(b) shows in more detail the structure of the cladding of the fibre shown in Fig. 10 (a) .

Fig. 15 is plot of the variation of (a) transmission and (b) loss with wavelength for light propagating through the fibre that is shown in Fig. 12. The loss spectrum shows an improvement over the spectrum of Fig. 9, with attenuation of less than 0.5 dB/m over a broad band of wavelengths (between 800 nm and 1350 nm) and less than 2 dB/m over substantially the whole wavelength range shown (400 nm to 1700 nm) , except for at the water absorption peak around 1400 nm. This is a remarkably wide band of wavelengths for such a low loss.

The density of states in an example Kagome cladding structure is shown in Fig. 16. In the plot, darker regions correspond to a lower density of states for the given values of modified propagation constant (β-k)A (where β is the propagation constant) and normalised frequency kΛ. (β-k)Λ is used rather than βΛ as an axis in Fig. 16 as the light line then corresponds to the kΛ axis (i.e. (β-k)Λ=0 or β=k) . On the image, the black region to the top left is the cladding cut- off; there are no states in this region. The black line that starts at the top at kΛ=20 and follows a diagonal path down and to the left is the fundamental bandgap. It is very narrow. However there is a wide region along the light line in the region 26≤kΛ≤40 that has a very low density of states. Fig. 17 shows the variation in density of states along the light line. As can be seen, the density of states appears not to drop to zero at any point along the light line (or if it does, it does over only a very narrow region too narrow to be resolved in the plot) . It is therefore very unlikely that the broad band of wavelengths guided in the fibre (the broad low- loss region in Fig. 15) results from band-gap guidance. As the core of the fibre has a lower refractive index than the cladding, light is not guided by refractive-index guidance.

We believe that guidance occurs at normalised frequencies for which the density of states is low but not necessarily zero. We believe that the guidance in those circumstances is lossy but the loss is extremely gradual at least in part due to the low density of states. The fibre thus effectively behaves as if it has a bandgap but in reality guides in a region in which there are supported states but very few of them.

Claims

1. An optical fibre comprising a core region and a cladding region, the core region comprising an elongate hole, the cladding region comprising a plurality of webs that are elongate substantially flat walls that define a plurality of elongate holes.

2. An optical fibre as claimed in claim 1, in which the holes fill the fibre cross-section with a filling fraction of 75% or more .

3. An optical fibre as claimed in claim 1 or claim 2, in which the cladding region comprises a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole.

4. An optical fibre as claimed in claim 3, in which the hexagonal cross-section is a regular hexagon.

5. An optical fibre as claimed in claim 3 or claim 4, in which, the triangular cross-section is an equilateral triangle.

6. An optical fibre as claimed in any preceding claim, in which the core region is an elongate hole.

7. An optical fibre as claimed in any of claims 3 to 6, in which the hole comprised in the core region has a smallest transverse dimension that is larger than the period of the lattice .

8. An optical fibre as claimed in any preceding claim, further comprising a jacket region that surrounds the cladding region.

9. A method of manufacturing an optical fibre, comprising providing a preform comprising dielectric material defining a plurality of holes and drawing the fibre from the preform in one or more draws whilst controlling pressure in the holes such that the dielectric material deforms to form a plurality of webs that are elongate substantially flat walls that define a plurality of elongate holes in the drawn fibre.

10. A method as claimed in claim 9, in which the dielectric material comprises a bundle of tubes and the method comprises drawing the fibre from the preform in one or more draws whilst controlling pressure in and/or around the tubes such that the tubes deform to form the plurality of webs.

11. A method as claimed in claim 9 or claim 10, in which the tubes are thin-walled.

12. A method as claimed in any of claims 9 to claim 11, in which the dielectric material deforms to form a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section, arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole.

13. A method as claimed in any of claims 9 to claim 12, further comprising placing the preform in a jacketing tube.

14. A method as claimed in claim 13, further comprising evacuating the space between the preform and the tube.

15. An optical fibre comprising a core region and a cladding region, the cladding region comprising a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole .

16. A method of manufacturing an optical fibre, comprising providing a preform comprising a bundle of tubes and drawing the fibre from the preform in one or more draws whilst controlling pressure in and/or around the tubes such that the tubes deform to form a periodic structure characterised by a unit cell comprising webs defining an elongate hole of hexagonal cross-section and two elongate holes of triangular cross-section, arranged adjacent to opposite sides of the hexagonal hole, such that each triangular hole shares a web in common with the hexagonal hole .