Background Art It is known to make an optical fibre by forming a glass pre-form, which, on a large scale, has the structure of the required optical fibre, and then heating and drawing an optical fibre from the pre-form in a fibre drawing tower. Such a pre-form may comprise an elongate glass rod, which is several millimetres, or even several centimetres, in diameter and a metre or more in length. An optical fibre having a diameter in the region of a hundred microns and a length of many kilometres may be formed from the pre-form.

A drawing tower for drawing a pre-form into an optical fibre is standard apparatus. A simplified cross-section of a fibre drawing tower arrangement 100 is illustrated schematically, and not to scale, in Figure 1. The arrangement 100 includes: a chuck 105 at the top for holding a pre-form 110 ; a furnace 115 below the chuck 105 for heating the pre-form 110 ; and two abutting, co-planar rollers 120 below the furnace 115 for pulling, or drawing, fibre 110' from the pre-form 110. The components of the arrangement 100 are mounted on a frame, which is not shown. Other components, for example fibre cooling and fibre coating arrangements, are also not illustrated in Figure 1 for the sake of simplicity only.

The chuck 105 is mounted on a carriage (not shown) and moves (by motor drive) down the frame to feed the pre-form 110 into the furnace 115 at a relatively slow speed during a fibre drawing operation.

The furnace 115, which is stationary relative to the frame during fibre drawing, has upper and lower orifices, 125 & 130, to receive the pre-form and deliver the fibre, respectively. The furnace 115 also has three chambers: an upper chamber 135, a middle chamber 140 and a lower chamber 145, through which a pre-form 110 or drawn fibre 110' pass in use. Furnace elements 150, for heating the pre-form 110 during fibre drawing, are mounted within the middle chamber 140. The furnace elements 150, when heated, create a so-called hot zone in the middle chamber 140, which is where the pre-form 110 softens and deforms, tapering along a so-called neck-down region 155 from its initial diameter to the required optical fibre diameter.

The furnace elements 150 may be graphite elements heated by electric current or RF induction.

The steel rollers 120 are mounted below the furnace 15 and are motor-driven to pull, or draw, the fibre 110'from the lower orifice 130 of the furnace 115, at a relatively high speed, after initial drop-off of molten silica from the pre-form. The drawn fibre is wound on a spool (not shown) downstream of the rollers 120.

In use, the fibre drawing tower 100 is controlled by an electronic controller (not shown), which has two basic inputs: a desired fibre diameter, which is input by a user, and a pre-calibrated feedback signal from a transducer, which is arranged to monitor the actual diameter of the drawn optical fibre 110'as it emerges from the lower orifice 130 of the furnace 115. The control system is arranged to vary the draw speed of the fibre 110', by varying the speed of rotation of the rollers 120, in order to cause the actual fibre diameter to match the desired fibre diameter. For example, a slight increase in draw speed is applied by the controller to decrease the diameter of the drawn fibre 110'and a slight decrease in draw speed is applied to increase the diameter of the fibre.

It is typical for a fibre drawing tower 100 to be housed in a clean-room environment, which has a room pressure slightly above the atmospheric pressure outside the clean room.

As used herein, the term"atmospheric pressure"means the local atmospheric pressure, in the open air and in the immediate vicinity of the clean room.

As used herein, the term"over-pressure"will be used hereafter to describe an increased pressure relative to the surroundings and the term"under-pressure"will be used hereafter to describe a decreased pressure relative to the surroundings.

Hence, the room pressure in the clean room is at an over-pressure relative to the atmospheric pressure. This over-pressure prevents particulate contaminants from entering the clean-room. The over-pressure is generated by pumping filtered air into the clean room. A typical clean room over-pressure is in the region of 40 Pascals (Pa).

During a fibre drawing operation, the atmosphere inside the furnace 115 needs to be inert, to protect the heating elements at typical fibre drawing temperatures. This is typically achieved by generating an over-pressure in the furnace, relative to room pressure, using an inert gas.

The middle chamber 140 of the furnace 115 is maintained at an over-pressure by flooding the chamber with an inert gas, such as Argon, which does not react with the heating elements or the pre-form 110 at a typical fibre drawing temperature, for example, which is in

excess of 1700°C for silica glass. The over-pressure is maintained in the middle chamber 140 by a pressure regulator 170, which controls a flow of Argon into the chamber 140 from a pressurised source 160.

A flow of Argon into the middle chamber 140 is required during fibre drawing, in order to maintain the over-pressure, since the chamber 140 is not sealed. A typical flow rate of Argon into the middle chamber 140, in order to maintain an appropriate over-pressure, is around 5-10 litres per minute.

There are a number of pressure regulating arrangements that would be suitable for maintaining an over-pressure in the middle chamber 140, and the person skilled in this field would be able to select an appropriate arrangement.

The pressure differential between the over-pressure in the middle chamber 140 and the room pressure around the furnace is distributed, or stepped-down, across upper and lower pressure seals, which are associated with the upper and lower chambers 135,145.

The upper and lower pressure seals are created in the chambers by generating pressures in the chambers, which are between the over-pressure in the middle chamber 140 and the room pressure. The pressures are controlled by further pressure regulators 171,172, which control respective flows of Argon into the chambers 135,145 from the pressurised source 160.

Typical flow rates of Argon into the upper and lower chambers are in the region of 2-5 litres per minute.

For convenience of understanding herein, in the diagram in Figure 1 and in similar diagrams described hereafter, a pressure level generated by a pressure regulator (see for example pressure regulator 171) is indicated by an arrow within a circle. Increased clockwise rotation of the arrow within the circle, away from the top centre position in the circle, indicates a pressure above room pressure. Likewise, increased counter-clockwise rotation of the arrow, away from the top centre position in the circle, indicates a pressure below room pressure. The degree of rotation of any particular arrow is intended to provide a relative measure only, compared with other regulators, and is not intended to indicate pressure level as such.

The over-pressures created in the different chambers of the furnace 115 are relatively low, for example, in the region of a few Pa.

Considering (1) a typical over-pressure in the middle chamber 140 of the furnace relative to the clean room and (2) a typical over-pressure in the clean room relative to the

atmospheric pressure, a typical over-pressure in the middle chamber relative to atmospheric pressure is likely to be in the region of 40-100 Pa.

Of course, it is not essential to draw fibres in an over-pressurised clean room, in which case an over-pressure in the middle chamber of a furnace, relative to atmospheric pressure, may only be a few Pa.

Accordingly, fibre drawing conditions in which the ambient pressure in the hot zone of a furnace is, relative to atmospheric pressure, at an over-pressure in the region of 0-100 Pa will hereafter be referred to as"standard fibre drawing conditions".

Over recent years, a new class of optical fibres, known as a photonic crystal fibres, or holey fibres, has been developed.

Photonic crystal fibres typically have a core region surrounded by a micro-structured cladding region. The cladding region is micro-structured in that it has regions of low refractive index interspersed with regions of high refractive index.

There are two main kinds of photonic crystal fibre; those with a core region which has a high refractive index compared with the cladding and those with a core region which has a low refractive index compared with the cladding. The low refractive index regions, in the core region or the cladding, may be air holes, holes under vacuum or filled with gas other than air, a liquid or another low refractive index material (or combinations of any of the foregoing).

The core region of a photonic crystal fibre typically forms a so-called defect in the cladding structure, which may have, but need not have, a periodic structure.

Indeed, in practice, it is highly unlikely that a photonic crystal fibre comprises a 'perfect'structure, due to accidental or even deliberate imperfections being introduced during its manufacture and/or perturbations being introduced by virtue of the presence of the core defect. As used herein, therefore, any reference to"array", "periodic","lattice", or the like, imports the likelihood of imperfection.

Light guidance in known photonic crystal fibres is typically by one of two guidance mechanisms, although there may be other mechanisms which act instead of or as well as these two mechanisms.

The first light guidance mechanism, which typically applies to photonic crystal fibres having a relatively high refractive index core region, is total internal reflection. Such fibres are analogous to standard optical fibres. However, photonic crystal fibres offer far greater control over their transmission characteristics and find application as single mode fibres over

a wide range of wavelengths and core diameters as demonstrated in W099/00685 (Birks et al.) and birefringent fibres as demonstrated in WO00/49436 (Russell et a/.). The particular arrangement, and refractive indices, of the low and high refractive index regions in the cladding provide the cladding with a so-called effective refractive index, which also depends on the wavelength of propagating light.

The second light guidance mechanism, which typically applies to photonic crystal fibres having a relatively low refractive index core region, is a so-called photonic bandgap (PBG). Such fibres, incorporating an air core surrounded by a lattice of air holes, were first proposed in 1995.

In, for example, "Full 2-D photonic bandgaps in silica/air structures", Birks et al., Electronics Letters, 26 October 1995, Vol. 31, No. 22, pp. 1941-1942, it was proposed that a PBG may be created in an optical fibre by providing a dielectric cladding structure, which has a refractive index that varies periodically between high and low index regions, and a core defect in the cladding structure in the form of a hollow core. In the proposed cladding structure, periodicity was provided by an array of air holes that extended through a silica glass matrix material to provide a PBG structure through which certain wavelengths of light could not pass. It was proposed that light coupled into the hollow core defect would be unable to escape into the cladding due to the PBG and, thus, the light would remain localised in the core defect.

An exemplary hollow core photonic crystal fibre 200 is shown in transverse cross- section in Figure 2. The darker regions are fused silica glass and the lighter regions are air holes. It can be seen that the fibre 200 has a relatively large core region 210 and a microstructured cladding region 220, comprising a triangular array of generally hexagonal air holes in a silica matrix. The core region 210 has an area that is approximately equal to seven air holes-an inner hole and the six surrounding air holes. The microstructured cladding region 220 is surrounded by a relatively thick region 230 of fused silica, which is truncated only by the edges of the photograph.

A photonic crystal fibre as shown in Figure 2 is typically drawn, using standard fibre drawing conditions in a standard fibre drawing tower, from a pre-form having a micro- structure that is the same, on a macro scale, as in the desired photonic crystal fibre. The pre- form may be made by known stack-and-draw or extrusion methods.

Unlike a usual pre-form for a standard optical fibre, a photonic crystal fibre pre-form typically has holes running along its length. These holes shrink down with the rest of the pre-

form to form the holes that run through a respective photonic crystal fibre. The holes typically form the cladding microstructure and may also form a hollow core.

During the drawing of a photonic crystal fibre, it is typically necessary to maintain the pressure in the holes at an over-pressure relative to the ambient pressure around the pre-form in the hot zone, in order that the holes do not collapse due to the surface tension of molten glass. Depending on hole radius, typical hole over-pressures are in the region of 5-20 kPa, bearing in mind that larger holes need a lower pressure than smaller holes to keep them open during drawing. Indeed, hole size may be varied by varying the pressure applied to the holes, for example as described in WO00/49436 (Russell et al.).

Disclosure of the Invention In arriving at the invention, the present inventors have identified that it is advantageous, in certain situations at least, to use fibre drawing conditions that are not, so- called, standard fibre drawing conditions.

According to a first aspect, the present invention provides a method of drawing an optical fibre or the like from an elongate pre-form, the fibre and the pre-form having at least one hole running along their lengths, the method including the steps of heating the pre-form and drawing the fibre from the pre-form while controlling the ambient pressure around the pre-form to be significantly higher or significantly lower than it would be under standard fibre drawing conditions.

For example, the ambient pressure around the pre-form may be at an over-pressure, relative to atmospheric pressure, of at least 150Pa, 200Pa, 500Pa, lkPa, 2kPa, 5kPa, lOkPa, 20kPa, 50kPa, 100kPa, 110kPa, 120kPa, 150kPa, 200kPa, 400kPa, 600kPa, IMPa or even higher, for example above IOMPa.

The method may also include the step of controlling the hole pressure in the pre-form to be higher than the ambient pressure around the pre-form. However, this is not essential.

For example, controlling the pressure in the pre-form hole may not be necessary if the ambient pressure around the pre-form is below room pressure. Typically, whether or not the hole pressure is controlled, the hole pressure is at least sufficient to prevent the hole from collapsing during drawing so that the fibre includes a respective hole along at least a part of its length.

As used herein, the phrase"optical fibre or the like"is intended to encompass a photonic crystal fibre. More broadly, however, the term is intended to include any filament that is drawn from a pre-form in a similar manner to an optical fibre.

The hole may run along the entire length of the fibre or only a part of the length thereof.

The ambient pressure may be that pressure which surrounds the entire pre-form during a fibre drawing operation. Alternatively, the ambient pressure may be that which surrounds only a part of the pre-form which is in the fibre drawing furnace and which deforms due to being heated and drawn.

The hole pressure in the drawn fibre may be controlled to be significantly different from the value that would result from being drawn under standard fibre drawing conditions.

The ambient pressure around the pre-form may be controlled to be significantly higher than it would be under standard fibre drawing conditions.

The hole pressure in the drawn fibre may be controlled to be significantly higher than the value that would result from being drawn under standard fibre drawing conditions. For example, hole pressure in the drawn fibre may be controlled to be equal to or above atmospheric pressure. In order to achieve this, the ambient pressure around the pre-form may be controlled to be significantly higher than it would be under standard fibre drawing conditions. For example, the hole pressure may exceed atmospheric pressure by at least 50kPa, 100kPa, 110kPa, 120kPa, 150kPa, 200kPa, 400kPa, 600kPa, lMPa or even higher.

At extremely high ambient pressures, for example above lMPa, the surface chemistry of the pre-form material may be significantly affected, such that the surface properties of the hole (or holes) in the fibre may be modified. For example, we believe that at such high ambient pressures, surface roughness of the hole (s) in the fibre may be reduced compared with similar fibres that are drawn under standard fibre drawing conditions.

Advantageously, for a fibre that is drawn with a hole having an over-pressure relative to the atmospheric pressure, any net flow of gas will be out of the fibre, at least until the hole pressure equalises with its surroundings, which may take in the order of hours, days, months or even years, as will be described below.

In contrast, under standard fibre drawing conditions, the inventors have found that photonic crystal fibres are likely to be produced with under-pressures, relative to atmospheric pressure, in their holes. The effect of this can be disadvantageous, since net flow of air, containing humidity and contaminants, is from the atmosphere into any open holes.

Patent applications W003/032039 (Skovgaard et al.) and W003/032018 (Patlakh et al.) describe that ingress into the ends of photonic crystal fibres of humidity, and air carrying other contaminants, is undesirable. Both patent applications attribute this ingress to capillary action and suggest plugging or hermetically sealing the ends of the fibres to minimise such ingress. The present inventors believe that the observed ingress of humidity and air-born contaminants is at least partly due to a negative pressure differential between the holes and the outside atmosphere.

Clearly, embodiments of the present invention may find application in addition to, or instead of, the proposals that are in the foregoing two patent applications.

In alternative embodiments of the present invention, the ambient pressure around the pre-form is controlled to be significantly lower than it would be under standard fibre drawing conditions. For example, the ambient pressure around the pre-form may be at an under- pressure, relative to atmospheric pressure, of 50Pa, 100Pa, 200Pa, 500Pa, lkPa, 2kPa, 5kPa, 10kPa, 20kPa, 50kPa, 90kPa, or at an even higher under-pressure. In some embodiments, the absolute ambient pressure around the pre-form may approach zero.

The hole pressure in the drawn fibre may be controlled to be below atmospheric pressure.

While a fibre drawn under standard fibre drawing conditions is likely to have under- pressurised holes, the present inventors suggest that fibres having even greater under- pressures in their holes may find useful application. For example, a reduced pressure is associated with reduced loss, for example due to Rayleigh scattering and absorption of light by gas molecules. The latter advantage might, for example, be significant when considering transmission of light in the ultra-violet region of the electromagnetic spectrum. In addition, it may be advantageous to reduce interactions between light and gas molecules for short pulse delivery applications, where reducing non-linear effects may be highly desirable.

Some embodiments may include the step of pressurising the hole in the pre-form with a gas that is highly diffusive through the fibre material. While fused silica glass is substantially impermeable to many gasses, for example Nitrogen, which is commonly used to pressurise pre-form holes, and Oxygen, it is permeable to other gases, for example Hydrogen, Helium and Neon. Specific consideration of the permeability of silica to various gases is beyond the scope of the present invention and the reader is referred for further information to alternative literature, for example"Properties and Structure of Vitreous Silica II", R Bruckner, Journal of Non-Crystaline Solids 5 (1971) ppl77-216.

After a photonic crystal fibre has been formed, a highly diffusive gas such as Helium will diffuse out of the holes. However, as air (apart from the relatively tiny components of highly diffusive gas, such as Helium) cannot readily diffuse back through the silica and into the holes, the holes, if sealed at their ends from the outside atmosphere, will eventually reduce in pressure and approach vacuum. The speed of evacuation by diffusion may be increased by heating the fibre.

More broadly, it is anticipated that drawing a photonic crystal fibre, while pressurising the holes with a highly diffusive gas, may prove beneficial as a way of reducing the pressure, and even evacuating the holes, even in the absence of varying the ambient pressure.

The pre-form and respective fibre may contain a plurality of holes along their respective lengths. Then, the pressures in a plurality of the pre-form holes may be controlled to be higher than the ambient pressure around the pre-form.

In some embodiments, the pressure in at least one pre-form hole may be controlled to be different from the pressure in at least one other pre-form hole. For example, the pressure in a pre-form hole having a relatively large diameter may be controlled to be lower than the pressure in a pre-form hole having a relatively small diameter.

A hole having a relatively large diameter may form a core region of the fibre and a plurality of holes having relatively small diameters may form a microstructured cladding region of the fibre.

According to a second aspect, the present invention provides a method, generally as described above, for forming a photonic crystal fibre.

According to a third aspect, the present invention provides a photonic crystal fibre made by the preceding method.

According to a fourth aspect, the present invention provides apparatus for heating an elongate pre-form, the pre-form having a hole along its length, from which an optical fibre or the like, also having a hole along its length, is drawn, the apparatus having a hot zone for heating the pre-form and a chamber, around the hot zone, within which the ambient pressure is controllable to be significantly higher or significantly lower than it would be under standard fibre drawing conditions.

The apparatus may comprise a fibre drawing furnace. The chamber may be in the fibre drawing furnace or enclosing the fibre drawing furnace.

According to a fifth embodiment, the present invention provides a method of forming an optical fibre, or the like, having a hole along its length, the method including the steps of

providing an elongate pre-form, having a respective hole along its length, and heating and drawing a fibre from the pre-form while pressurising the hole in the pre-form with a gas, which is highly diffusive in the pre-form material. Such a method may be used on its own, or in combination with varying the ambient pressure around the pre-form during the fibre drawing operation, to make a fibre in which the hole de-pressurises over a period of time due to diffusion of the gas out of the hole. Eventually, the hole may de-pressurise to be significantly below atmospheric pressure, possibly approaching vacuum.

According to a sixth embodiment, the present invention provides a photonic crystal fibre, including at least one hole along its length, which is drawn with an over-pressure in the hole relative to atmospheric pressure.

Other aspects and embodiments of the present invention will become apparent from the following description and drawings.

Brief Description of the Drawings The present invention will now be described by way of example only with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram, which illustrates a fibre drawing tower arrangement, as known in the prior art; Figure 2 is a scanning electron micrograph of a transverse cross section of a hollow core photonic crystal fibre, which has a microstructured array of air holes in a cladding region around the core; Figure 3 is a schematic diagram, which illustrates a fibre drawing tower arrangement suitable for use in drawing a hollow core photonic crystal fibre, which has an over-pressure in its core and cladding holes relative to atmospheric pressure; Figure 4 is a schematic diagram, which illustrates an alternative fibre drawing tower arrangement suitable for use in drawing a hollow core photonic crystal fibre, which has an over-pressure in its core and cladding holes relative to atmospheric pressure; Figure 5 is a schematic diagram, which illustrates a further alternative fibre drawing tower arrangement suitable for use in drawing a hollow core photonic crystal fibre, which has an over-pressure in its core and cladding holes relative to atmospheric pressure; and Figure 6 is a schematic diagram, which illustrates a fibre drawing tower arrangement suitable for use in drawing a hollow core photonic crystal fibre, which has an under-pressure in its core and cladding holes relative to atmospheric pressure.

Best Mode For Carrying Out the Invention. & Industrial Applicability Before considering embodiments of the present invention in detail, it is worth considering why a typical hollow core photonic crystal fibre, which is produced under standard fibre drawing conditions, is likely to exhibit an under-pressure in its core and cladding holes. The reason can be explained qualitatively as follows.

In the hot zone of a fibre drawing furnace, at fibre drawing temperatures, a silica photonic crystal fibre pre-form, and the gas in the pre-form holes, can reach temperatures around 2100°C, which is above the softening point of silica. The pressures in holes in the pre-form are maintained above the ambient pressure of the pre-form by a pressure controlling arrangement, so that the holes do not close during the drawing process.

As the pre-form melts in the hot zone and is drawn into a fibre, it and its holes reduce in diameter along a neck-down region until they reach the required diameter. In the hot zone, the gas pressure in the holes remains generally at the over-pressure provided by the pressure controller. However, the temperatures of the fibre and the gas in the holes drop rapidly as they move away from the hot zone. Therefore, according to the well-known Ideal Gas Law, PV = nRT Equation 1 where P is gas pressure, V is gas volume, n is the amount of gas, R is the gas constant and T is gas temperature, the pressure of the gas in the holes tends to drop proportionally with the drop in absolute temperature.

As the pressure in the holes drops, at least some gas is drawn into the holes from the hot zone, with the pressure controller compensating for any associated drop in pressure in the hot zone. However, the flow rate of the gas into the holes is not sufficient to compensate for the significant drop in pressure experienced by the gas as it cools. In effect, the gas cannot be sucked from the hot zone and into the holes fast enough to compensate for the drop in pressure, due to a combination of the small bore size of the holes, the drawing speed of the fibre and the viscosity of the gas; bearing in mind also that the gas in the hot zone is upwards of three times as viscous as it is at room temperature. The result is that the holes in the fibre, when cooled to room temperature, are at an under-pressure relative to atmospheric pressure.

The present inventors have found that, due to this under-pressure, humidity is drawn into the ends of open holes in the fibres more rapidly than would be expected due to capillary

action or diffusion alone. This has been seen to cause significant degradation in transmission performance of the fibres. Cutting back the fibres to remove the ends containing humidity improves transmission performance for a short time, but only until more humidity is drawn into the holes.

The benefit of producing a fibre with an over-pressure in the holes, and the resistance experienced by gas being drawn from a hot zone into the holes of a cooling fibre, can be appreciated qualitatively by considering Poiseuille flow Q of a viscous fluid in a circular cross-section channel. For example, the rate of flow Q (volume/time) of a gas in a round capillary due to a pressure p applied to one end of the capillary is defined by: Q=Òr4/8# dp/dx Equation 2 where r (which equals d/2) is the radius of the capillary, dp/dx is the pressure gradient along the capillary and # is the viscosity of the gas.

Assuming a uniform pressure gradient and a pressure difference AP between the ends of a capillary of length 1, where zIP is small compared to the ambient pressure Po (so that variations in density of the gas due to the pressure differential may be ignored), the equation for Q can be written in the form: Q=Òr4/8# ~P/l Equation 3 From this equation, the amount of time # it takes to purge the capillary along distance l may be defined as: <BR> <BR> <BR> <BR> <BR> <BR> 2<BR> ~=32#l2/~Pd2 Equation 4 Although ~P is assumed small, this equation nevertheless provides an order of magnitude for time T even if AP is not small. For example, using Equation 4, it is found that it

takes in the order of three hours to purge Nitrogen through a one metre long capillary, with a bore diameter of one micron, given a pressure differential AP of 50kPa.

Clearly, Equation 4 scales readily for the case where the capillaries are holes running along the length of a photonic crystal fibre.

For example, a PBG fibre with a hollow core region may have a core diameter of around 101lu. It would take on the order of four years to purge Nitrogen through a lkm fibre of this kind with a pressure differential AP of 50kPa.

Of course, creating a gas over-pressure in a capillary and allowing the over-pressure to equalise with the atmospheric pressure is not the same as purging gas through that capillary.

However, it is suggested that the times associated with the two systems are of the same order of magnitude.

Modified fibre drawing apparatus, suitable for producing a hollow core photonic crystal fibre with a microstructured air hole cladding, in which there is an over-pressure in the holes, is shown in Figure 3.

The apparatus comprises a fibre drawing tower arrangement 300 substantially as shown in Figure 1. A chuck 305, holding a photonic crystal fibre pre-form 310, a furnace 315 and rollers 320 are shown.

The pre-form 310 has a hollow, relatively large diameter inner region 311 and an array of relatively small diameter holes 312 in an outer region. Only two holes in the outer region are shown for reasons of clarity. The hollow inner region 311 becomes the core of a resulting photonic crystal fibre 310'and the array of holes 312 in the outer region becomes the cladding.

A brass cylinder 313 is sealed, for example with epoxy resin, to the top of the pre- form 310.

The inner hole 311 of the pre-form 310 is connected, through the chamber 313, via a gas line 380 and pressure regulator 381 to a pressurised cylinder 390 of Nitrogen. The array of holes 312 in the outer region of the pre-form 310 open into the brass cylinder, which is connected via a second gas line 382 and pressure regulator 383 to the pressurised cylinder 390 of Nitrogen. The gas lines pass through respective orifices in the chuck 305.

In this way, the holes in the pre-form 310 can be pressurised to appropriate levels during a fibre drawing operation.

The furnace 300 in Figure 3 has an upper orifice 325 into which the pre-form 310 is received and a lower orifice 330 from which the fibre 310'emerges.

The upper orifice 325 is reduced in size so that it is close to a sliding fit with the pre- form 310. The lower orifice, which is associated with a lower chamber 347, comprises an iris 348a, which may be opened and closed, manually or, optionally, by motor drive. In open state (as shown), the iris 348a has a diameter similar to the diameter of the lower orifice 130 in Figure 1. The iris 348a may be closed so that its diameter is close to the diameter of the optical fibre 310'to be drawn.

The furnace 315 has a middle chamber 340 and three chambers 333-335 above and three chambers 345-347 below the middle chamber 340. The middle chamber 340 and the other two lower chambers 345,346 also each comprise an iris 348b-348d, so that their respective lower orifices can be varied in diameter. As shown, irises 348b-348d are closed.

The upper chambers 333-335 have orifices which are also a near sliding fit with the pre-form 310.

As in Figure 1, the middle chamber houses heating elements 350. The upper, lower and middle chambers are each independently coupled to a pressurised Argon gas supply 360 via respective gas lines and pressure regulators 370-376.

In use, the furnace 315 is heated to operating temperature and the pre-form 310 is fed into the hot zone of the middle chamber 340 in the normal way. The Nitrogen feeds into the pre-form 310 are set to keep the pre-form holes open during the fibre drawing operation. In addition, the Argon feeds are set to create an initial over-pressure in the furnace 315.

The fibre drawing operation begins with all irises 348a-348d fully open to accommodate an initial pre-form drop-off, which passes through the lower orifices. After drop off, when the fibre 310'is being drawn by the rollers 320, the irises 348a-348d are closed so that their diameters are close to that of the fibre 310'being drawn.

Since the pre-form 310 is a near sliding fit with the orifices of the upper chambers 333-335, only a small volume of Argon gas escapes upwards and out of the furnace 310.

Escape of some Argon out of the gaps between the closed irises 348a-348d and the fibre 310' occurs and is beneficial because gas cushions are generated, which prevent the fibre from coming into contact with the inner edges of the irises.

The pressure regulators 370-376 act to reduce flow of Argon into the chambers when the irises 348a-348d are closed, since gas flow out of the furnace is reduced.

Next, the pressure regulators 370-376 are re-set to pre-calibrated levels to generate the required ambient pressure in the middle chamber 340, which is typically significantly above the ambient pressure around the furnace 315. The pressures in the upper and lower chambers

are also regulated to create gas seals, which distribute across the chambers the pressure differential between the middle chamber 340 and the ambient pressure outside the furnace 315.

In addition, the Nitrogen pressures applied to the pre-form holes are increased to maintain an over-pressure with respect to the ambient pressure around the pre-form 310 in the hot zone of the middle chamber 340, in order to keep the pre-form holes open during the fibre drawing operation. For example, if the ambient pressure in the middle chamber 340 is increased to 600kPa, then the pressures applied to the pre-form holes are increased by 600kPa above their initial levels before the irises were closed.

The degree by which the ambient pressure in the middle chamber 340 needs to be increased in order to ensure a resulting fibre 310'is made with an over-pressure in the holes may be quantified either theoretically or experimentally.

In general, the pressure drop in the pre-form holes as the gas therein cools may be calculated theoretically, by taking into account ambient pressure, hole size, draw speed, temperature drop and variation in gas viscosity as the gas cools.

As a simple approximation, it may be assumed that the gas pressure in the photonic crystal fibre holes, at room temperature, is six times lower than at atmospheric pressure. This approximation is made on the basis that gas in the holes cools from 1800°C to 20°C, which is approximately equivalent to a six times absolute temperature drop and a corresponding six times drop in pressure.

Therefore, assuming the simple approximation is correct, the ambient pressure in the middle chamber needs to be increased by more than 600kPa to generate a photonic crystal fibre having over-pressures in the holes.

However, the approximation assumes that no gas from the hot zone is drawn into the holes as the fibre is drawn and the gas in the holes cools. Of course, in practice, at least some gas is drawn into the holes, as mentioned above, which means the under-pressure in the holes would be less than six times lower than atmospheric pressure.

Accordingly, in practice, although the ambient pressure in the middle chamber would not necessarily need to be as high as 600kPa to generate an over-pressure in the photonic crystal fibre holes, such a high pressure could be used to guarantee there is an over-pressure.

An under-pressure in a photonic crystal fibre, which is drawn under standard fibre drawing conditions, can be determined experimentally in a number of ways.

For example, a test length of 100m of fibre is drawn and both ends are sealed, for example as described in either patent application W003/032039 (Skovgaard et al.) and W003/032018 (Patlakh et al.), referenced above. The fibre is then placed in a container of gas, for example Neon, which has a well-documented absorption spectrum, and one end of the fibre is cleaved, to remove the seal. The fibre is left in the container of gas for a fixed period of time, for example one hour or one day. Then, the fibre is removed from the container, the cleaved end is quickly re-sealed, and the distance the Neon has travelled up the holes of the fibre is directly measured while illuminating the fibre with a Neon electric discharge lamp.

Although Neon is colourless under normal lighting conditions, when illuminated with light from a Neon lamp, the Neon gas absorbs some of the light and appears visibly dark. The approximate under-pressure, Ap, may then be calculated by substituting appropriate values into Equation 4 above.

The distance the Neon travels along the fibre, and hence the under-pressure, will typically be different for the core and cladding holes due to their diameter difference. The holes in which the Neon has travelled the greatest distance, indicating the more significant under-pressure, should be used for Equation 4.

The value of Ap indicates by how much the ambient pressure in the middle chamber needs to be increased to achieve a pressure in the holes, which is equal to atmospheric pressure. The ambient pressure in the middle chamber should be increased by an additional amount if photonic crystal fibre holes with over-pressures relative to atmospheric pressure are required.

Another way of measuring the gas pressure in a fibre is by drawing the fibre while pressurising the pre-form holes with a gas that has a well-defined absorption spectrum at a telecoms wavelength. Suitable gases are Acetylene (C2H2) or Hydrogen Cyanide (HCN), which may be used as wavelength references for calibrating telecommunications components; see, for example, "Wavelength Accuracy in WDM: Techniques and standards for component characterisation", OFC 2002, invited paper ThCl, pp. 391-393. These gases have numerous molecular absorption transitions that can be observed directly over a range of 1510-1565nm.

These transitions vary in position and intensity sensitively with gas pressure.

As such, light from, for example, a Helium-Neon laser operating at 1523nm is coupled into one end of the fibre and coupled back out from the other end into a suitable detector. The characteristics of the absorption spectrum of the detected light can then be used to determine the gas pressure in the holes.

The furnace arrangement illustrated in Figure 3 can be adapted for generating various different over-pressures in the holes in a photonic crystal fibre. For example, it may be found that as many as three gas seal chambers above and below the middle chamber are not necessary, when only relatively low ambient pressures are required in the middle chamber. In contrast, the number of upper and/or lower chambers may need to be increased for relatively high ambient pressures. Of course, the exact configuration of the furnace can be determined by experiment.

An alternative arrangement for generating a high ambient pressure around a photonic crystal fibre pre-form is illustrated in Figure 4. The apparatus 400 is similar to the apparatus illustrated in Figure 1. However, in Figure 4, the pre-form 410 has holes, which are pressurised in a similar manner to the arrangement in Figure 3. A significant difference is that a chuck 405 and furnace 415 are enclosed in a pressure controlled vessel 498. In use, the pressure controlled vessel 498 is sealed apart from a small orifice at its base, through which rollers 420 draw a resulting fibre 410'. The orifice is a small source of pressure loss from the vessel and the loss is compensated for by a pressurised Argon source 460 and respective pressure regulator 470. The Argon source 460 may be a simple pressurised cylinder or, for ambient pressures that exceed typical pressurised cylinder pressures, may include a gas pump.

The pressure loss caused by the orifice through which the fibre emerges at the base of the foregoing pressure controlled vessel 498 can be decreased significantly by increasing the effective length of the orifice, for example by using a capillary tube. With reference again to Equation 4, the time it takes for gas to escape through an orifice of effective length 1 increases in proportion with 12. In other words, the effect of doubling the length of the orifice reduces the speed the gas travels through the orifice by a factor of four.

The pressure controlled vessel 498 has an access door (not shown) to permit a pre- form 410 to be fixed into the chuck 405 and to accommodate pre-form drop-off, after which the door is closed, thereby forming the required pressure seal.

Alternatively, as shown in Figure 5, the entire fibre drawing tower arrangement is sealed in a pressure controlled vessel 598. In all other respects, the arrangement is the same as that illustrated in Figure 4.

It will be appreciated that the arrangements illustrated in Figures 4 and 5 do not require independent pressure control of upper and lower chambers of a furnace. This is

because the required ambient pressure around the pre-form is generated within the pressure controlled vessels.

In some embodiments, a clean room containing the fibre drawing tower is configured to accommodate increased room over-pressures. This obviates embodiments where chambers or vessels around chambers need to be pressurised.

The present inventors suggest that drawing fibre under extremely high ambient pressures, for example in excess of lOMPa, might have an effect on the surface chemistry of glass as it is drawn into a photonic crystal fibre. This may be advantageous for controlling surface roughness in the holes, for example due to surface waves, which are generated during fibre drawing and freeze into the surface of the glass when it cools. Such surface roughness may be found to contribute to unwanted surface scattering and mode coupling.

The fibre drawing arrangement 600 illustrated in Figure 6 produces photonic crystal fibres with significant under-pressures in their holes. The arrangement is similar to the arrangement illustrated in Figure 3, and only the differences between the two arrangements will be described hereafter, for the sake of brevity of description herein.

The first significant difference is that a middle chamber 640 of the furnace 615 is connected, via a pressure regulator 670, to a vacuum pump arrangement 662, instead of being connected to a gas supply.

In addition, of three upper chambers 633-635, the lower two 634,635 are connected to the vacuum pump arrangement 662 via respective pressure regulators 672,673. Likewise, of three lower chambers 645-647, the upper two 645,646 are connected to the vacuum pump arrangement 662, via respective pressure regulators 674,675.

The uppermost chamber 671 and the lowest chamber 647 are connected via respective pressure regulators 671,676, to a pressurised Argon gas supply 660.

In use, the gas pressure in the middle chamber 640 is reduced to the required level by the vacuum pump 662. The pressure differential between the middle chamber and the outside ambient pressure is distributed over the two upper chambers 634,635 and the two lower chambers 645,646, which act as gas seals, by reducing their respective pressures by lesser amounts, but still to levels below the ambient pressure outside the furnace 615.

The uppermost chamber 633 and the lowest chamber 647 are flooded with Argon to generate an over-pressure relative to both the surrounding ambient atmosphere and the other chambers. The over-pressures are sufficient to satisfy any gas demand generated by leakage

of gas, through the other chambers, into the vacuum arrangement 662. An inert atmosphere is thereby maintained in the middle chamber 640.

A pressurised Nitrogen supply 690 is adjusted so that pre-form holes remain at an over-pressure relative to the ambient pressure in the middle chamber 640. If the ambient pressure in the middle chamber 640 is significantly below the ambient pressure outside the furnace 615, then the over-pressure in the pre-form holes is also likely to be below the ambient pressure around the furnace.

The arrangement illustrated in Figure 6 may be adapted for working under increasingly low pressures, for example, by adding additional upper and lower gas seal chambers.

Alternative apparatus for generating a low ambient pressure around a photonic crystal fibre pre-form comprises a standard furnace and chuck arrangement enclosed in a pressure controlled vessel, similar to the arrangement illustrated in Figure 4, with a vacuum arrangement in place of the Argon source. Alternatively, the entire tower may be enclosed in a pressure controlled vessel, similar to the arrangement illustrated in Figure 5, or a clean room.

In the embodiments described so far, the pre-form holes have been pressurised with Nitrogen. As has been described above, the holes may instead be pressurised with a relatively highly diffusive gas such as Hydrogen, Helium or Neon. After the fibre has been drawn, the Helium in the holes diffuses out of the holes through the silica walls, thereby generating increased under-pressures in the holes. Given sufficient time, which may be decreased by heating a fibre, the holes will approach vacuum, assuming the ends of the holes are sealed.

Using a highly diffusive gas to pressurise the holes may be a step that is applied instead of, or in addition to, the step of varying the ambient pressure around the pre-form during a fibre drawing operation to make a fibre with initial under-pressures in the holes.

Indeed, the present inventors believe that use of a highly dispersive gas to pressurise the holes of a photonic crystal fibre, even using otherwise standard fibre drawing conditions, would generate a photonic crystal fibre with holes under a near vacuum.

As has been indicated, the under or over-pressures in the holes of a photonic crystal fibre tend to vary inversely with hole diameter. This is in part due to different sizes of hole being pressurised to different levels during drawing of the fibre and also due to more gas from the hot zone region being drawn into larger bore holes as the gas already in the holes cools.

For example, for a photonic crystal fibre that is made with an over-pressure in the holes, a large core will tend to have a lower over-pressure than small cladding holes. This is due to it being easier to draw gas from the hot zone into the relatively large diameter core hole as the gas cools away from the hot zone. In this case, it may be advantageous to pressurise the holes during the draw with a gas that is diffusive, but not highly diffusive, for example Argon. As such, the gas would tend to diffuse from the cladding holes into the core, through the relatively very fine glass walls, far faster than it would be able to diffuse out of the fibre through relatively thick over-cladding layers. In this way, any pressure differential between the core and cladding holes decreases relatively quickly compared with the time it takes for the net over-pressure to reduce due to diffusion of gas out of the fibre. It may also be beneficial to store fibres in an atmosphere containing the same diffusive gas that is used to pressurise the holes. In this way, the over-pressures would not decrease as rapidly, since net diffusion of gas out of the fibre would be reduced by the effect of the gas diffusing in.

The skilled person will appreciate that there may be many other arrangements of apparatus suitable for drawing photonic crystal fibres, or any other kind of optical or non- optical fibre, under significantly higher or lower ambient pressures. The embodiments described herein are merely exemplary and are in no way intended to limit the scope of the present invention. The skilled person would be able to design and build apparatus appropriate for any particular pressure requirements.

The exemplary embodiments illustrate ways in which holes in a fibre may be made under or over-pressurised. It will be appreciated that the teachings herein relate more broadly to drawing fibres under any fibre drawing conditions, in which ambient pressure around the pre-form at least in a hot zone is non-standard, irrespective of the resulting pressure in any holes in a fibre. For example, the resulting pressure may be higher than, lower than or equal to atmospheric pressure.