WO2003058310A2

WO2003058310A2 - Improvements in and relating to optical fibres

Info

Publication number: WO2003058310A2
Application number: PCT/GB2003/000022
Authority: WO
Inventors: William John Wadsworth; Jonathan Cave Knight; Timothy Adam Birks; Philip St.John Russell; George Kakarantzas
Original assignee: Blazephotonics Limited
Priority date: 2002-01-11
Filing date: 2003-01-07
Publication date: 2003-07-17
Also published as: AU2003201638A8; AU2003201638A1; WO2003058310A3; GB0200615D0

Abstract

An optical fibre and a method of processing the optical fibre are disclosed. The fibre comprises an inner rod (100) and an outer tube (140). The inner rod (100) is supported in the outer tube (140) by a plurality of webs (160). The webs (160) are sufficiently thin for the outer tube (140) to be cleavable without cleaving of the inner rod (100). The method comprises cleaving the outer tube (140) in a cleaving action that does not cleave the inner rod (100).

Description

Improvements in and relating to 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 .

A particular problem with standard fibres is that their optical properties can be sensitive to changes in the environment of the fibre. For example, long period fibre gratings are particularly sensitive to changes in the environment surrounding their cladding region. European Patent Application No. 0905834 (Lucent Technologies Inc.) and R.P. Espindola et al . , ^x External refractive index insensitive air-clad long period fibre grating' , Electron. Lett. 3_5, 327 (1999) each describe a fibre that is effectively substantially isolated from external perturbations. The fibre comprises a central rod supported by a plurality of webs within an outer cylinder. The central rod is made of silica and comprises a core region formed by germanium doping and an inner cladding region that surrounds the core region. The central rod is thus in essence a standard fibre. There are elongate air-filled holes between the webs around the central rod and those holes form an outer cladding region having a refractive index close to 1. The outer cladding region substantially isolates the central rod from changes in the environment outside of, or impinging on, the outer cylinder. As stated above, the Espindola paper is particularly concerned with long-period gratings. A long period grating (LPG) couples modes from the core of a fibre to the cladding at certain wavelengths; however the coupling is very sensitive to the modes of the cladding and hence to the environment surrounding the cladding. The air gap and outer support tube of the ^λ air-clad' fibre serve to remove the influence of the environment and so stabilize the LPG transmission/rejection characteristics .

Another form of prior art fibre having a double cladding region is a form of fibre laser (see, for example, J.K. Knight et al . , Electron. Lett. Vol. 37, No. 18 pplll6- 1117 (2001) ) . In such a device there are three different index regions: a central, high- index core, usually single mode and doped with absorbing and lasing material; surrounding that is an inner cladding of lower index which itself is surrounded by an outer cladding made, for example, of PTFE or the like. A (typically) highly multimode pump laser diode is coupled into the (typically) highly multimode inner cladding and the pump light is then absorbed in the central doped region so that the laser lases in a single mode in the central core. The outer cladding in such a fibre laser may also be made by suspending the inner cladding rod in air inside a tube by thin webs, instead of a solid cladding. A problem with existing double-clad and 'air-clad' fibres is that, although the provision of an air gap and an outer cylinder is advantageous for fibre lasers and for protecting the fibre from its external environment, the outer cylinder restricts access to the standard fibre forming the inner rod. The conventional method of gaining access to the inner rod is to cleave the fibre to provide an end facet that can be accessed. The fibre is scored with any suitable sharp instrument and then struck. The cleave propagates readily from the scribe mark on the outside of the support tube across the support struts and across the central rod.

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 they 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. 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. (The 'effective' refractive index of the cladding region can readily be calculate by a person skilled in the art; in general, its exact value depends upon the shape of the mode guided in the core but its value will be between that of the elongate regions and that of the matrix regions.)

In the second mechanism, the arrangement of elongate regions in the cladding is periodic such that they form 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 that cannot propagate in the cladding. The core of a PCF that guides by this mechanism forms a 'defect' in the periodic structure of the cladding; light can propagate in this defect region. Light is thus confined to and propagates in the core of the PCF. International Patent Application No. PCT/GB00/01249 (published as WO 00/60388, in the name of The Secretary of State for Defence et al) describes such a fibre in which the core is an empty region; that is, the PCF has a tubular structure and light is guided in the vacuum- or fluid-filled hollow centre of the tube. The term 'photonic crystal fibre' reflects the historical roots of the structure of the fibres; the fibres were developed with a view to demonstrating the band-gap guidance mechanism. However, we refer to all fibres having such elongate regions as photonic crystal fibres, even if they do not have band-gaps and guide by the first mechanism, index guiding. In particular, the term is not restricted to fibres having periodic arrangements of elongate regions in their claddings.

An object of the invention is to provide a method of processing an 'air-clad' optical fibre to provide improved access to the inner rod and an 'air-clad' optical fibre having an inner rod that is accessible by such a method. Such a fibre has many potential applications.

According to the invention there is provided a method of processing an optical fibre, the fibre comprising an inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs, the webs being sufficiently thin for the outer tube to be cleavable without cleaving of the inner rod, the method comprising cleaving the outer tube in a cleaving action that does not cleave the inner rod. The thinness of web required for the webs to be sufficiently thin will vary according to other parameters of particular embodiments of the invention, such as the material from which the fibre is made and the cross- sectional dimensions of the inner rod and the outer tube.

Preferably, the webs have a smallest transverse dimension of less than 1000 n . More preferably, the webs have a smallest cross-sectional dimension of between 100 nm and 1000 nm. Still more preferably, the webs have a smallest cross-sectional dimension of between 200 nm and 700 nm. Preferably, the method further comprises removing a portion of the outer tube to expose a portion of the inner rod.

A particular advantage of this method is that it is not necessary to touch the inner rod with anything. If a region of outer tube is removed from the end or middle of the fibre then, particularly if the removed length is short, the fibre may be held by the remaining outer tube during processing and may also be packaged in protective packaging to which it is fixed at the remaining outer tube.

Preferably, the method further comprises the step of polishing the exposed portion of the inner rod; for example, flame polishing may give a clean, smooth profile. Polishing the inner rod with, for example, a flame or arc or other heater may improve the properties of the fibre because, in practice, the inner rod will often not be smooth but will often have peaks where the webs were attached prior to cleaving of the outer tube. There may also be bits of broken webbing on the surface of the inner rod. Polishing will often help to remove such loose webbing and to round off the peaks and may also be used to smooth off other jagged edges and small cracks that may result from removal of the outer tube. Such polishing will often increase the strength of the rod, which is advantageous for further processing and use. For example, polishing will often improve the fibre for cleaving, end-pumping by laser light and processing for side pumping.

Preferably, the method further comprises the step of tapering the exposed portion of the inner rod. The portion of the outer tube may be removed from part way along the fibre. Such an arrangement would provide access to the inner rod at an intermediate point along the fibre. It may enable stripping of unwanted modes. The portion of outer tube may be removed in any appropriate way: for example, it may be demarcated by two cleaving actions and then: crushed; or rotated, to break the webs holding it in place, and then cracked; or pulled away from the inner rod by elements, which may be glued to its outer surface. Alternatively, the portion of the outer tube may be removed from an end of the fibre.

Preferably, the method further comprises cleaving the inner rod in another cleaving action.

Preferably, the inner rod is cleaved in a different position along the fibre from the position in which the outer tube is cleaved, such that a portion of the inner rod projects beyond a cleaved facet of the outer tube. Preferably, the method further comprises the step of splicing the projecting portion of the inner rod to another fibre; the projecting portion thus facilitates splicing of the fibre according to the invention. Preferably, the other fibre is an inner rod of a fibre comprising the inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs. Preferably, the splice is to a portion of the inner rod of the other fibre that projects beyond a cleaved facet of the outer tube of the other fibre. Thus two fibres each having projecting portions may be spliced and, preferably, packaged; such a structure is useful and would be difficult to fabricate without the projecting portions. Splicing the central rods of two similar or identical structures may thus give a structure with a region in the middle, instead of at the end, that is liberated. Preferably, the splice is further processed after it is formed, for example by flame- polishing, tapering or heat-treatment. Tapering and heat treatment may be used to provide mode control, for example as a mode filter in, for example, a laser or an amplifier. Tapering may be used to provide access to a mode in a core of the inner rod. Tapering may also be used to affect dispersion and non-linearity of a core of the inner rod. A liberated middle part may be processed to allow side pumping into the inner rod, which may form a laser or an amplifier.

Preferably, the inner rod comprises a core region and a cladding region for confining light to the core region. The inner rod may thus form a standard fibre. Preferably, the cladding region comprises a plurality of elongate regions having a different refractive index from a matrix region in which they are embedded; thus the inner rod may be a photonic crystal fibre. The inner rod may be a tube. For example, the inner rod may form a photonic-crystal fibre that guides light in a hollow or fluid-filled core region. Alternatively, the inner rod may have no internal structure, such that it forms a multimode waveguide in conjunction with its surroundings (e.g. air); in that case the inner rod may be relatively small, for example with a cross-section of the order of 50 microns.

Preferably, the inner rod is heated to alter its optical properties and/or its physical properties. If the inner rod is a photonic crystal fibre in which the elongate regions are elongate holes then heat treatment may be used to alter the size of the holes, for example to change the mode properties of the fibre. If the inner rod is doped then heating may cause the dopants to diffuse into or out from the core and thus change the core mode properties. Preferably, the heating is used to increase the size of a core mode (for example, by decreasing the hole size of a PCF or by causing outward diffusion of dopants from the core of a standard fibre or PCF) . A large mode is advantageous in a fibre laser as end-face damage is often a limiting factor on power. Heating may be used to increase the strength of the fibre or as a polishing method.

Preferably, the method further comprises the step of passing a fluid between the inner rod and the outer tube. Preferably, the fluid is used to cool the inner rod or the outer tube. Alternatively, the fibre is used to sense a property of the fluid. Preferably, the fluid is passed in at a part of the fibre, away from the fibre end, from which the outer tube has been removed (such a fibre part may be formed, for example, by splicing exposed and cleaved inner rods of two fibres) . Such an arrangement may enable avoidance of contact between the fluid and the end face of the rod (for example, to avoid contamination or damage) . It may enable avoidance of introduction of fluid into holes in the inner rod, such as the holes that may be found if the inner rod is a photonic crystal fibre. Preferably, the fibre comprises a plurality of inner rods supported in the outer tube by a plurality of webs. Preferably, the inner rods are separated from each other by webs. Preferably, the method comprises cleaving the plurality of inner rods separately from the outer tube. Preferably, the plurality of inner rods are cleaved independently from each other. The inner rods may thus be cleaved at different lengths, independently heat-treated, spliced to fibres, spliced to each other, spliced to rods liberated from another similarly structured fibre or multiple similarly structured fibres each with one or more core rods. If spliced to rods from an identically structured fibre, rods from one fibre may be spliced to corresponding rods from the other fibre or to different rods from the other fibre. The different rods may have different properties from the properties of the corresponding rods. Preferably, prior to cleaving of the outer tube, the outer tube completely surrounds the inner rod. Alternatively, prior to cleaving of the outer tube, the outer tube may partially surround the inner rod; for example, the outer tube may be a C-shaped jacket.

Also according to the invention there is provided an optical fibre comprising an inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs, the webs being sufficiently thin for the outer tube to be cleavable without cleaving of the inner rod.

Provision of thin webs is a particular advantage compared with the structures described in European Patent Application No. 0905834 (Lucent Technologies Inc.) and R.P. Espindola et al . , 'External refractive index insensitive air-clad long period fibre grating', Electron. Lett. 3_5, 327 (1999) . In a structure, such as the prior-art structures, which have thick support webs, a cleave can propagate readily from a scribe mark on the outer tube, across the support webs and across the inner rod. That provides a conventional, reliable cleave of the whole fibre. However, when the struts are sufficiently thin, a cleave does not propagate well to the central rod. Rather, the outer tube cleaves and once it is loose it may then be pushed against the inner rod so that inner rod also breaks, if desired. Preferably the inner rod has a smallest cross-sectional dimension of more than five microns.

Preferably, the inner rod has an exposed portion not covered by the outer tube .

Preferably, the inner rod is tapered in the exposed portion.

The exposed portion may be part way along the fibre. Alternatively, the exposed portion may be at an end of the fibre such that the exposed portion forms a portion of the inner rod projecting from the outer tube. Preferably, the fibre is a solid of extrusion; that is, it has a cross-section that is substantially the same along an axis of symmetry (the fibre's longitudinal axis).

Preferably, the outer tube has an outer diameter of between 50 microns and 1 mm, but may, of course, be larger. Fibre lasers often require relatively large diameters but the outer tube may have a relatively small outer diameter, for example of between 50 microns and 200 microns.

Preferably, the inner rod has a cleaved facet in the exposed portion such that the exposed portion forms a portion of the inner rod projecting from the outer tube.

Preferably the fibre further comprises another fibre to which the projecting portion of the inner rod is connected by a splice at the cleaved facet. Preferably, the other fibre is an inner rod of a fibre comprising the inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs. Preferably, the splice is to a projecting portion of the inner rod of the other fibre . Preferably, the inner rod comprises a core region and a cladding region for confining light to the core region. The inner rod may thus form a standard fibre. Preferably, the cladding region comprises a plurality of elongate regions having a different refractive index from a matrix region in which they are embedded. Thus, the inner rod may form a photonic crystal fibre. The inner rod may be a tube. For example, the inner rod may form a photonic-crystal fibre that guides light in a hollow or fluid-filled core region. Alternatively, the inner rod may have no internal structure, such that it forms a multimode waveguide in conjunction with its surroundings (e.g. air); in that case the inner rod may be relatively small, for example with a cross-section of the order of 50 microns. Alternatively, the inner rod may comprise a plurality of elongate regions that form a photonic crystal not having a core region. Preferably, a region of the inner rod has been doped with an active ion. Preferably, the fibre is, or is part of, a fibre laser or amplifier. The part of the fibre in which the inner rod is covered by an outer tube provides an advantageous structure for a fibre laser. The inner rod may have a doped core surrounded by a cladding region, which may or may not be microstructured. Pump light may be confined in the cladding by an air gap between the inner rod and the outer tube. Thus, a "double-clad" fibre laser or amplifier is provided, in which the cladding region forms an inner cladding region and the air gap provides an outer cladding region. Such a structure permits construction of "double- clad" fibre lasers and amplifiers having an inner cladding with a large numerical aperture, which may enhance efficiency. In a double-clad laser the absorption of light from the inner cladding is to a great extent controlled by the area overlap of the core and inner cladding, so the inner cladding should be small. However if the inner cladding is small it has few modes, unless it has a high numerical aperture. However, provision of an air region, which has very low index, provides an inner cladding region with a high NA, as required.

Provision of an exposed portion of the inner rod is also particularly advantageous in a fibre laser or amplifier. A good cleave of the inner rod permits end- pumping of the inner rod. Alternatively, the pump light is introduced into the inner rod by side pumping the exposed portion; in that case the end face would generally also be cleaved in order to enable signal light to exit cleanly from the inner rod.

Of course, the inner rod and/or the outer tube may be of non-circular cross-section. The inner rod may be off- centre relative to the tube. Preferably, the outer tube completely surrounds the inner rod at some point along the length of the fibre. However, the outer tube may only partially surround the inner rod; for example, the outer tube may be a C-shaped jacket in at least one transverse cross-section of the fibre.

Preferably, the fibre comprises a plurality of inner rods supported in the outer tube by a plurality of webs.

Preferably, the inner rods are separated from each other by webs .

Also according to the invention there is provided a fibre coupler comprising a first fibre and a second fibre, the first fibre being a fibre as claimed in claim 18 and being fused, at the exposed portion of the inner rod, to the second fibre.

Also according to the invention there is provided a fluid monitor comprising a fibre as described above as being according to 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 schematic perspective view of a preform for use in the manufacture of a fibre according to the invention;

Fig. 2 is a schematic perspective view of a second preform for use in the manufacture of a fibre according to the invention; Fig. 3 is a schematic cross-sectional view of a fibre according to the invention, drawn from the preform of Fig. 1, prior to cleaving;

Fig. 4 is a schematic perspective view of the fibre of Fig. 3 after cleaving according to the invention; Fig. 5 is an optical micrograph of a first example of a fibre such as that of Fig. 3 after cleaving;

Fig. 6 is an optical micrograph of a second example of a fibre such as that of Fig. 3 after cleaving;

Fig. 7 is an optical micrograph showing enlarged a portion of the cross-section of Fig. 6; Fig. 8 is an optical micrograph showing a side view of an example of a fibre after cleaving according to the invention;

Fig. 9 consists of optical micrographs showing side views of the central rod of the fibre of Fig. 8, (a) prior to heat treatment and (b) after heat treatment; the left- hand images in both (a) and (b) are focused on the centre of the top of the rod, whereas the right-hand images are focused on the edge of the rod; Fig. 10 is a schematic cross-sectional view of a coupler incorporating a fibre according to the invention;

Fig. 11 is a schematic cross-sectional view of a monitoring device incorporating a fibre according to the invention. An example of a fibre according to the invention is drawn from a preform 10 (Fig. 1) . The preform comprises a bundle of silica tubes 20, 25 of external diameter 1.5mm. The bundle of silica tubes comprises two types of tubes: an outer ring of tubes 25 with thin walls and an inner region of tubes 20 with relatively thicker walls. At the centre of the bundle is a solid silica rod 30. The bundle is held together in a large-diameter silica tube 40.

Fibre 110 (Fig. 3) is drawn from preform 10 on a drawing rig of the standard type used to draw optical fibres. The drawing process causes the silica of cane 30, tubes 20, 25 and tube 40 to fuse. Tubes 25 form a ring of large holes 150 separated by silica webs 160. The webs 160 link an inner rod 100, which results from the rods 20 and the cane 30, to an outer tube 140, resulting from tube 40. Inner rod 100 is a photonic crystal fibre and comprises a cladding region, comprising a plurality of elongate holes 120 resulting from the internal cavities of tubes 20, surrounding a solid silica core region 130, resulting from cane 30. The thickness of the webs 160 is determined by the thickness of the walls of the tubes 25 in preform 10 and the distance between holes 120 of the fibre. The thickness of the webs 160 may be reduced and their length increased by stretching. This is achieved by drawing preform 10 in a first draw to a cane of a few millimetres diameter, and then drawing this cane in a second draw inside a loose-fitting thick-walled tube and evacuating the gap between the cane and the tube. This stretches the cane to fill the tube, the stretching being taken up by the thinnest parts of the structure, which in this case are the webs 160 which become longer and hence thinner.

In this example of a method according to the invention, the outer tube 140 is cleaved as follows. A scratch is scored on outer tube 140 with a ceramic tile. Fibre 110 is gently bent by hand to propagate a crack across and around the outer tube. Because of the thinness of the internal silica webs 160, the cleave does not propagate to the inner rod 100. Bending of the fibre in different directions is continued until a portion of outer tube 140 is detached.

Care is taken not to bend the fibre far enough to damage the inner rod 100. The outer tube 140 is then pulled off. A slight twisting motion often helps to free tube 140 from rod 110 by breaking the webs 160 inside the cleaved-off length of tube 140. After removal of the portion of outer tube

140, central rod 100 protrudes from remaining tube 140 (Fig. 4) . Webs 160 often remain at least partially attached to inner rod 100 even after the outer tube 140 is removed. Webs 160 can be removed from inner rod 100 by flame treatment, as described below.

In an alternative embodiment of the invention (not illustrated) , tubes 20 are replaced with solid rods and central rod 30 is doped to have a raised refractive index relative to the other solid rods. The preform is then drawn into a standard fibre. A further alternative preform (Fig. 2) has a large solid central rod 70, which in this embodiment has a raised index core 80 (as in a conventional preform) (although in an alternative embodiment it has no internal structure) . Rod 70 is surrounded by small silica tubes 75 and the whole is enclosed in a large diameter silica tube 90.

Fibre 210 (Fig. 5) is an example of a fibre such as that in Fig. 1, cleaved in order to provide an image of the fibre cross section. The cleave is almost flat across the whole structure. Inner rod 200 has a diameter of 100 microns and is supported in outer tube 240 by twelve webs 260, arranged in pairs at six sites approximately evenly distributed around the inner rod 200, each of approximately 700 microns in width. Webs 260 define (between pairs) large elongate holes 250 and (within pairs) smaller elongate holes 255. The inner rod 200 contains an array of small (compared with holes 250, 255) elongate holes 220 forming a photonic crystal region. The region's diameter of 100 microns spans nine periods of an hexagonal array of holes with a pitch (hole to nearest hole separation) of 12 microns.

Fibre 310 (Figs. 6 and 7) is a fibre similar to fibre 210. The diameter of inner rod 300 is 140 microns and it is suspended from outer tube 340 by twenty-three webs 360, which are approximately evenly distributed around rod 300 and define large elongate air holes 350. Webs 360 are of widths ranging from 0.35 microns to 0.6 microns. Inner rod 300 is, in this example, a pure silica rod. The measured numerical aperture of this fibre is 0.38 for light of 633 nm and is thus predicted to be nearly 0.5 for light of 980 nm. It is expected that still better numerical apertures could be achieved in a symmetrical structure with webs all of 0.35 microns or less thickness.

Inner rod 300 has been broken at a similar length to the outer tube 340 but, whilst the outer tube is relatively cleanly cleaved (it is smooth in the 2 o'clock to 7 o'clock direction) , the central rod 300 is roughly broken, where it has been cleaved in a relatively uncontrolled second cleaving action using the outer tube cleaved in a first cleaving action. Fig. 8 is an optical micrograph of an example of a fibre according to the invention, in which approximately 40 mm inner rod 400 is exposed where a portion of outer tube 440 has been removed in a first cleaving action, as described above . The surface of rod 400 is uneven after cleaving and shards of webbing remain attached to it (Fig. 9 (a) ) . Heat treatment with an oxygen/butane microflame removes many of the webbing remnants and greatly improves the smoothness of the surface (Fig. 9 (b) ) . The invention makes possible a wide range of devices.

An example is an optical coupler (Fig. 10) . The outer tubes 540, 640 of two fibres 510, 610 (each fibre comprising two fibres spliced to form a single fibre) , isolate two optical fibres in the form of inner rods 500, 600 from changes to their external environment.

Fibres 510, 610 are made as follows. A portion of outer tube is removed from about two centimetres of the end of four fibres. The portions are removed by scribing each fibre and working open cracks by the gentle bending, as described above. Once the portion of outer tube to be removed has been separated from the rest of the outer tube, that portion may be broken off the fibre. The end of the inner rods is left exposed where the outer tube has been removed. The exposed ends of pairs of the four rods are then spliced to form the two fibres 540, 640 having two continuous inner rods 510, 610. Rods 510, 610 are surrounded by outer tubes 540, 640 except for in the region near the splices, where the inner rod is exposed.

A fused coupler 590 is formed between the exposed lengths of inner rods 500, 600 by heating, tapering and fusing, in the usual way. The coupler 590 and the exposed portions of inner rods 500, 600 are encased in sealed packaging 695 that ensures that isolation from the external environment, provided elsewhere by outer tubes 540, 640, is maintained around coupler 695.

The invention also enables the manufacture of fluid monitoring units such as that of Fig. 11. Optical fibres in the form of inner rods 700, 800 of fibres 710, 810 are exposed by scribing and removing a length of outer cladding 740, 840, as described above. Inner rods 700, 800 are themselves then cleaved to provide clean end-facets. The rods 700, 800 are then heated and fused at their end-facets to form splice 790.

Splice 790 and the exposed portions of inner rods 700 and 800 are encased in airtight sleeve 770. Sleeve 770 includes inlet 780. A fluid to be monitored is pumped into inlet 780. The fluid propagates into the sleeve 770 and thence along the large holes 750, 850 formed in fibres 710, 810 between inner rods 700, 800 and outer tubes 740, 840. Light guided in inner rods 700, 800 is used to monitor properties of the fluid.

Claims

1. A method of processing an optical fibre, the fibre comprising an inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs, the webs being sufficiently thin for the outer tube to be cleavable without cleaving of the inner rod, the method comprising cleaving the outer tube in a cleaving action that does not cleave the inner rod.

2. A method as claimed in claim 1, further comprising removing a portion of the outer tube to expose a portion of the inner rod.

3. A method as claimed in claim 2, further comprising the step of polishing the exposed portion of the inner rod.

4. A method as claimed in claim 2 or claim 3, further comprising the step of tapering the exposed portion of the inner rod.

5. A method as claimed in any of claims 2 to 4, in which the portion of the outer tube is removed from part way along the fibre.

6. A method as claimed in any of claims 2 to 4, in which the portion of the outer tube is removed from an end of the fibre

7. A method as claimed in any preceding claim, further comprising cleaving the inner rod in another cleaving action.

8. A method as claimed in claim 7, in which the inner rod is cleaved in a different position along the fibre from the position in which the outer tube is cleaved, such that a portion of the inner rod projects beyond a cleaved facet of the outer tube.

9. A method as claimed claim 8, further comprising the step of splicing the projecting portion of the inner rod to another fibre.

10. A method as claimed claim 9, in which the other fibre is an inner rod of a fibre comprising the inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs.

11. A method as claimed in claim 10, in which the splice is to a portion of the inner rod of the other fibre that projects beyond a cleaved facet of the outer tube of the other fibre.

12. A method as claimed in any of claims 9 to 11, in which the splice is further processed after it is formed.

13. A method as claimed in any preceding claim, in which the inner rod is heated to alter its optical properties and/or its physical properties.

14. A method as claimed in any preceding claim, further comprising the step of passing a fluid between the inner rod and the outer tube.

15. A method as claimed in claim 14, in which the fluid is used to cool the inner rod or the outer tube.

16. A method as claimed in claim 14, in which the fibre is used to sense a property of the fluid.

17. A method as claimed in any preceding claim, in which the fibre comprises a plurality of inner rods supported in the outer tube by a plurality of webs.

18. A method as claimed in claim 17, comprising cleaving the plurality of inner rods separately from the outer tube.

19. A method as claimed in claim 18, in which the plurality of inner rods are cleaved independently from each other.

20. An optical fibre comprising an inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs, the webs being sufficiently thin for the outer tube to be cleavable without cleaving of the inner rod.

21. An optical fibre as claimed in claim 20, in which the inner rod has an exposed portion not covered by the outer tube.

22. A fibre as claimed in claim 21, in which the inner rod is tapered in the exposed portion.

23. A fibre as claimed in claim 21 or claim 22, in which the exposed portion is part way along the fibre.

24. A fibre as claimed in claim 21 or claim 22, in which the exposed portion is at an end of the fibre, such that the exposed portion forms a portion of the inner rod projecting from the outer tube .

25. A fibre as claimed in claim 24, in which the inner rod has a cleaved facet in the exposed portion such that the exposed portion forms a portion of the inner rod projecting from the outer tube .

26. A fibre as claimed in claim 25, further comprising another fibre to which the projecting portion of the inner rod is connected by a splice at the cleaved facet.

27. A fibre as claimed in claim 26, in which the other fibre is an inner rod of a fibre comprising the inner rod and an outer tube, the inner rod being supported in the outer tube by a plurality of webs.

28. A fibre as claimed in claim 27, in which the splice is to a projecting portion of the inner rod of the other fibre.

29. A fibre as claimed in any of claims 20 to 28, in which the inner rod comprises a core region and a cladding region for confining light to the core region.

30. A fibre as claimed in claim 29, in which the cladding region comprises a plurality of elongate regions having a different refractive index from a matrix region in which they are embedded.

31. A fibre as claimed in any of claims 20 to 30, in which the inner rod is a tube.

32. A fibre as claimed in any of claims 20 to 28, in which the inner rod has no internal structure .

33. A fibre as claimed in any of claims 20 to 28, in which the inner rod comprises a plurality of elongate regions which form a photonic crystal not having a core region.

34. A fibre as claimed in any of claims 20 to 33, in which a region of the inner rod has been doped with an active ion.

35. A fibre as claimed in claim 34, which is, or is part of, a fibre laser.

36. A fibre as claimed in claim 34 or 35 when dependent on claim 21, in which pump light is introduced into the inner rod by side pumping the exposed portion.

37. A fibre as claimed in any preceding claim, in which the outer tube partially surrounds the inner rod in a transverse cross-section of the fibre.

38. A fibre coupler comprising a first fibre and a second fibre, the first fibre being a fibre as claimed in claim 21 and being fused, at the exposed portion of the inner rod, to the second fibre.

39. A fluid monitor comprising a fibre as claimed in any of claims 20 to 37.