WO2007034186A1

WO2007034186A1 - Microstructured optical fibres and methods relating thereto

Info

Publication number: WO2007034186A1
Application number: PCT/GB2006/003505
Authority: WO
Inventors: Timothy Adam Birks; Kevin Lai; Sergio Gregorio Leon-Saval; William John Wadsworth; Agata Anna Witkowska
Original assignee: The University Of Bath
Priority date: 2005-09-23
Filing date: 2006-09-21
Publication date: 2007-03-29
Also published as: GB0519503D0

Abstract

A method of processing a microstructure fibre comprises (10) providing a microstructured fibre (10) comprising a solid matrix material defining a plurality of elongate holes 30 that extend along a first length of the fibre (10). A uniform pressure is applied to a first end-face of the fibre (10); and a second length of the fibre (10) that at least partially overlaps with the first length is heated. The fibre (10) is arranged such that one or more of the holes (30) experience an effective pressure that is different from the effective pressure experienced by the other holes (30), so that in the second length the one or more holes expand or contract relative to the other holes, and at least one hole expands relative to the whole fibre.

Description

Microstructured optical fibres and methods relating thereto .

The present invention relates to the field of optical fibres, in particular microstructured optical fibres (also called photonic crystal fibres (PCFs)). Microstructured fibres typically comprise a solid matrix material defining a plurality of elongate holes that extend along the length of the fibre. The fibre usually has a core, to which guided light is, to a greater or lesser extent confined. Confinement may be by any of several mechanisms . The earliest PCFs guided light by total internal reflection at the interface between a cladding region of elongate holes (the hole and solid matrix material in the cladding region together being of lower effective refractive index) and a solid core (of higher refractive index) . Later, guidance by containment by a photonic bandgap was reported, which permitted propagation of light in a core of a very low refractive index, such as an air core. Other guidance mechanisms have also been reported, relying on anti- resonance effects in the matrix material immediately adjacent to a hollow core.

In a "standard" or "stock" PCF, the array of holes in the cladding region is substantially uniform along the length of the fibre. Various methods have been proposed for providing cladding structures that are not uniform, either along their length or transversely or both.

WO00/49435 teaches heat treatment of PCFs after their fabrication, to alter the size of holes, and suggests pressurisation of holes during the heat treatment. Birks et al, ^λΛPhotonic Crystal Fibre Devices", Proc SPIE, 4943, 142-151 (2002) teach pressurisation of holes whilst heating and tapering a fibre on a fibre tapering rig. EC Magi et al, Opt. Express 12 (2004) 776 teach sealing the ends of a PCF to delay hole collapse during tapering. Fibre tapering has been used to change locally the core size and air-filling fraction (see JK Chandalia et al, IEEE PTL 13 (2001) 52; G Kakarantzas et al, Opt. Lett. 27 (2002) 1013). EP 1340725 A2 and WO03/080524 teach selective pressurisation of holes during the drawing of a PCF from a preform.

However, prior-art methods have either only enabled alteration of the size of all holes in a microstruetured fibre or, where selective pressurisation of individual holes is contemplated, either no method of achieving pressurisation is suggested or the suggested method is complex (such as the pressurisation of holes during drawing) . In all of the prior art methods, the shape of the core is little-changed by the process, deviating at most only slightly from six-fold rotational symmetry and an essentially circular aspect ratio.

In another aspect of the prior art, some dispersion compensators and other fibre devices are based on high- order mode (typically LP₀₂) propagation along a fibre. Mode converters - usually long-period gratings with a period matching the beat length between the modes - are needed at the input and output to couple light from and to the fundamental mode. The strength of a grating must be closely controlled to give complete coupling between the modes: over- or under-coupling leaves some residual power in the input mode, reducing the extinction ratio. Such a resonant device only works over a band of wavelengths, so it must be carefully designed and constructed for the operating wavelength to fall within this band. This also makes the device sensitive to environmental perturbations .

The invention aims to provide an improved method of controlling hole patterns in a microstructured fibre and microstructured fibres made by that method. The invention also aims to provide a method of controlling the shape of the core of a microstructured fibre and microstructured fibres made by that method. The invention also aims to provide optical devices incorporating microstructured fibres made by those methods .

A first aspect of the invention provides a method of processing a microstructure fibre comprising: (a) providing a microstructured fibre comprising a solid matrix material defining a plurality of elongate holes that extend along a first length of the fibre; (b) applying a uniform pressure to a first end-face of the fibre; and (c) heating a second length of the fibre that at least partially overlaps with the first length; wherein, the fibre is arranged such that one or more of the holes experience an effective pressure that is different from the effective pressure experienced by the other holes, so that in the second length the one or more holes expand or contract relative to the other holes, and at least one hole expands relative to the whole fibre.

It will be understood that, in contrast to prior-art methods of manufacturing a microstructured fibre that involve pressurisation of holes in a preform from which the fibre is being drawn, in the present invention a microstructured fibre is itself processed by pressurisation and heating; thus, the method is carried out on a pre-existing microstructured fibre, which has been manufactured prior to the method being carried out. It will further be understood that the first and second lengths at least partially overlap in the sense that at least part of the fibre is comprised in both the first and the second lengths.

It will further be understood that expansion of a hole relative to the whole fibre means that the hole in a cross-section of the fibre has expanded relative to the size of the whole fibre in that cross-section, or relative to the distance that separates the centres of two representative holes in the cross-section, or to the size that the hole would have if the heating process had caused only a change in the scale of the entire fibre and the hole had only changed size in proportion. The size of the whole fibre may have changed, for example as a result of stretching or tapering, which may be included as part of the method. (Tapering is a process in which a short length of fibre is permanently stretched and narrowed whilst being heated.)

The one or more holes may expand or contract in area relative to the other holes by a factor of at least 1.5, 2, 4 or 10. The one or more holes may collapse completely.

All of the holes in the fibre (prior to the method being carried out) may be the same size. In that case, surface-tension effects for each hole will be the same. That is an example of a case in which the different effective pressures will be achieved by subjecting different holes to different hydrostatic pressures.

Alternatively, at least one hole may be a different size.

The term "effective pressure" is used herein to refer to the pressure in a hole including the effects of surface tension forces at the boundary of the hole as well as the hydrostatic pressure of the gas in the hole. For example, for a circular hole of diameter d the effective pressure P_st due to surface tension is

P_st=2γld

where γ is the surface tension of the matrix

material. If the matrix material is fused silica, then P_st in bar is approximately 6 divided by the diameter in microns. Thus holes of different sizes filled with gas to the same hydrostatic pressure will experience different effective pressures due to the different contributions from surface tension forces.

As indicated above, the terms "expand" and "contract" as used herein refer to relative changes in hole sizes, as well as absolute changes. Changes in hole size are expected to be relative when the fibre is tapered during heating, and may be absolute when it is not.

The one or more holes may be larger or smaller than the other holes, such that the effective pressure in the one or more holes including the effects of surface tension forces is, respectively, smaller or larger than the effective pressure in the other holes . A given hydrostatic pressure will produce a lower effective pressure in a larger hole than in a smaller hole because the contribution from surface tension in a larger hole will be smaller than the contribution from surface tension in a smaller hole. Hence, for the same hydrostatic pressure, a larger hole will tend to expand more than a smaller hole; indeed, a larger hole may expand while a smaller hole shrinks and a small hole may collapse completely as a result of the different effective pressures.

The one or more holes may be arranged to be at a different hydrostatic (or internal) pressure from the hydrostatic (or internal) pressure of the other holes.

The one or more holes may be sealed at at least a first end, such that the other holes are pressurised in step (b) to a different effective pressure from the pressure inside the sealed holes even though the uniform gas pressure is applied to the end-face of the fibre and such that in the heated length the pressurised holes expand (or at least partially collapse) and the sealed holes at least partially collapse (or expand) .

The sealed holes will tend to expand (relative to the other holes, as discussed above) if the applied uniform pressure is less than ambient pressure and contract if it is greater.

The fibre (prior to the method being carried out) may have one or more cores . The fibre may guide light in a solid core or in a hollow core. Alternatively, the fibre may have no cores .

The holes may form in the transverse cross-section of the fibre a lattice, which may for example be triangular or square. The holes may be arranged in concentric rings, which may be circular.

The holes may contribute to confinement of light to a core of the fibre. The holes may provide substantially all the confinement. The fibre may be a PCF: it may guide light by index-guiding, by photonic-bandgap confinement or by another mechanism.

The fibre may be endlessly single-mode.

The fibre may be made of a glass, for example silica, or of some other substance, for example a polymer. The holes may be arranged in a pattern having 6-fold rotation symmetry. Alternatively, the holes may be arranged in a pattern having no more than 2-fold rotation symmetry; the fibre may then be birefringent .

At least one of the one or more holes may be sealed, for example with a liquid insert, a solid insert or a gel insert. The insert may be fluid when applied to the hole and then solidify. The insert may be an adhesive, which may be cured by heat or ultraviolet light.

The holes to be sealed in the fibre may be smaller than the other holes .

The insert (which may be an insert material that becomes the insert, such as a curable adhesive, as discussed above, or a liquid) may be applied by attaching it to a tip and then positioning the tip adjacent to the hole so that the insert is brought into contact with the hole. The tip may be positioned adjacent to the hole using a micropositioning stage.

The hole may be inflated prior to insertion of the insert and then cleaved to provide an inflated end, to facilitate the insertion. The fibre end-face then has larger holes (as described by WJ Wadsworth et al, Opt. Express 13 (2005) 6541) to which it is easier to apply an insert. At least one of the one or more holes may be sealed by heating to collapse at least partially the end of the hole (of course, the heating may be carried out anywhere beyond the length of the fibre that is heated in step (c) of the method, so that the collapse occurs somewhere between that heat-treated region and the very end of the fibre; the term "end" should be understood in that way throughout this specification, except where context or sense dictates otherwise.). The collapse may be carried out whilst the hole is at ambient pressure (i.e. primarily under surface tension) ; alternatively, the collapse may be preceded by at least partial evacuation.

At least one hole may be left unsealed. At least one hole may be only partially sealed. At least 2 holes may be sealed.

At least some of the holes may be sealed at a second end of the fibre. All of the holes at one end of the fibre may be sealed. The holes may be sealed the same pattern at the first and the second end; i.e. all sealed holes may be sealed at both ends . Alternatively, some holes may be sealed at one end but not the other. The holes may be sealed at a first end in a pattern that is the complement of the pattern of holes sealed at the other end, so that every hole is sealed at only one end. A first pressure may be applied to a first end of the fibre and a second, different, pressure applied to the other end of the fibre.

At least one of the holes may be sufficiently close to the outer surface of the fibre that explosion of the at least one hole during the heating step opens a pathway to the space outside the fibre.

In the fibre manufactured by the method, at least one hole may be completely collapsed. At least one hole may be not collapsed. All sealed holes may have collapsed; all unsealed holes may have not collapsed.

Whenever other devices are interfaced at an end of a fibre, loss is introduced. A significant advantage of the present invention is that it enables improved coupling with devices such as, for example, laser diodes. An advantageous application of the method is in the production of fibres which can be coupled with devices such as laser devices without the need for additional micro-optical elements which are more expensive and prone to greater loss . Using a complicated anamorphic optical system to transform a diode laser's highly elliptical laser output mode into a circular mode suitable for low- loss coupling into a standard fibre is expensive, fragile and causes loss. Using the invention, the fibre itself can incorporate the mode-shape transformer seamlessly. Thus PCFs made by the invention may be used (with effectively seamless integration) as low loss devices such as couplers, spectral filters and non-linear optical elements .

Furthermore the fibre (before it is processed according to the method) may be stock microstructured fibre and the cost of processing such fibre will generally be significantly cheaper than purchasing and inserting separate devices .

The size and/or shape of a core of the fibre may be modified by the difference in effective pressure in the holes. That may be done, for example, by blocking (say) 2 holes in a line either side of the core which might create for example a 5:1 aspect ratio for the core.

Thus at least one of the sealed holes may be adjacent to a core of the fibre. One or more of the sealed holes adjacent to the core may collapse, in a pattern that forms a length of enlarged core.

The sealed holes may be arranged in a straight line through a core of the fibre. The line may be one row wide.

There may be at least one blocked hole that has more blocked holes adjacent to it than has a core of the fibre. The enlarged core may be six-fold symmetric in transverse cross-section. Alternatively, the enlarged core may be elliptical in transverse cross-section. It is understood that the shape of the core is unlikely to be a true ellipse, and that "elliptical" here means without more than two-fold rotation symmetry. The ellipticity of the core, for example its aspect ratio, may be represented by that of the ellipse fitted entirely within the core such that the harmonic mean of the semi-major and semi- minor axes is a maximum. The aspect ratio of the elliptical core may be at least 1.5, 2, 3, 5, 8 or 15. The core may be birefringent .

The sealed holes may form a pattern, in the first end of the fibre, that has 6-fold rotation symmetry about a core. Alternatively, the pattern may have less than 6- fold rotation symmetry. It may be that the pattern has no more than 2-fold rotational symmetry about a core of the fibre. It may be that the pattern has no rotation symmetry. The pattern may have reflection symmetry.

Alternatively, a transformation to a ring-shaped core may be provided.

The technique also enables additional cores to be created at defined locations along a fibre. The collapse of a sealed hole may form a new core in the fibre. As indicated above, it may be that the fibre, prior to processing according to the method, has no core. The new core may be optically isolated from an original core or another new core; alternatively, the new core may optically interact with an original core or another new core.

At least one hole may have exploded, such that the matrix wall separating it from an adjacent hole or the space outside the fibre is broken, providing an open path between the hole and the adjacent hole or the space outside the fibre. A connected chain of holes may have exploded. The explosion of the holes may form a continuous path for gases to pass between the space outside the fibre to the neighbourhood of the core of the fibre.

The uniform pressure may be applied for example by compressed air, nitrogen, argon or helium. The pressure may be increased above ambient, for example to at least 1, 2, 5 or 10 bar above. The pressure may be increased above the pressure needed to balance surface tension.

Alternatively, the pressure may be reduced below ambient.

The pressurisation may be achieved by attaching a first end of the fibre to a sealed chamber to which pressurised gas is applied. The second (other) end of the fibre may be left open to the atmosphere. The length of fibre between the heated region and the second end may be longer than length of fibre between the heated region and the first end; for example, the ratio of lengths may be at least 2, 5, 10, 20, 50 or 100. This allows the hydrostatic pressure in an unsealed hole to be proportionately closer to the pressure applied to the first end than to the pressure at the second end.

Alternatively, holes at the other end of the fibre may be completely sealed, or both ends may be pressurised. If both ends are pressurised, it may be to same pressure or a different pressure. The pressure may be a constant pressure; alternatively, the pressure may be varied during the process.

The length of fibre may be heated using any suitable means, for example a flame, an electric resistance heater, an electric arc, a laser beam. It may be heated on for example a fusion splicer or a taper rig. It may be heated using a static heat source or a heat source that moves along the fibre. For example, the heat source may sweep back and forth along the fibre. The sweep distance may be constant or change from one sweep to another. The final sweep distance may be less than the initial sweep W

16

distance, which may form a gradual transition in the degree of hole inflation along the fibre. The sweep distance may be less than: 10, 5, 2, 1 or 0.5 cm. The sweep distance may be more than: 0.5, 1, 2, 5 or 10 cm.

The fibre may be stretched while it is being heated. It may be slightly stretched just to keep it under tension while being heated. The portion of the fibre being heated may for example be extended by less than: 10%, 5%, 2% or 1% of the original length of the portion being heated. Alternatively the fibre may be stretched to significantly reduce the cross-sectional area of matrix material, for example to less than: 60%, 30%, 10%, 3%, or 1% of its initial value. The fibre may be stretched after an initial inflation process without stretching. The inflating pressure may be maintained during stretching; alternatively, the pressure may be released before stretching.

The heating process may be carried out at a plurality of separated points along the length of the fibre, such that the one or more holes change size relative to the other holes to form the same transverse pattern at different points along the length of the fibre. This allows several sections of the fibre to be identically modified even when the process of sealing the holes is undertaken only once. Alternatively the sealed holes may collapse to form different transverse patterns at different points along the length of the fibre. There may be overlap regions in which different transverse patterns overlap; for example, a first length may have a core in a first place in the fibre cross-section, a second core in a second place, and an overlap in which the core is in both places. The fibre may be heated and stretched at least 2, 5, 10, or even 30 times at different points along the fibre with or without a pressurisation step and with or without the sealing of holes .

The fibre manufactured by the method may afterwards be cleaved in the heated length and may be spliced to a conventional fibre. The conventional fibre may be spliced at the untreated end or the treated end. The mode mismatch loss to the conventional fibre is preferably less than 6, 3, 1, 0.5, or 0.2 dB. An unheated length of fibre may be cleaved and spliced to a conventional fibre.

A second aspect of the invention provides a method of manufacturing a microstructured fibre comprising: (a) providing a microstructured fibre comprising a solid matrix material defining a plurality of elongate holes that extend along a length of the fibre; (b) sealing one or more of the holes at at least a first end; (c) uniformly pressurising one end of the fibre; and (d) heating a length of the fibre, so that in the heated length the pressurised holes expand and the sealed holes at least partially collapse.

A third aspect of the invention provides a microstruetured optical fibre comprising a solid matrix material defining a plurality of elongate holes that extend along a length of the fibre and a core that extends along a length of the fibre and is arranged to guide light, the core having, in cross-section, a first shape in a first length of the fibre and a second shape in a second length of the fibre, the first and second shapes being ellipses of different eccentricity and the first and second lengths being separated by a transition length.

Although elliptical cores of changing eccentricity are known in prior-art fused couplers (in which two microstructured fibres are joined at a common waist, to give a coupler region with four fibre "ports") , we are not aware of any previously known microstructured fibre having an elliptical core with an eccentricity that changes along its own length.

The first and second shapes may be ellipses with major and/or minor axes differing (between the first and second shapes) by more than: 5%, 10%, 20%, or even 50% The transition length may be less than: 1 m, 50 cm or 10 cm.

The method and/or fibre according to the invention may be used for example to provide coupling to the planar core of a laser diode, in the manufacture of integrated optical components, or to form an optical device such as a spectral filter or sensor.

A fourth aspect of the invention provides an optical device including a fibre according to claim 20 or a fibre that has been processed by a method according to any of claims 1 to 20.

It will be appreciated that features of the present invention described in relation to any of the above- described aspects of the present invention are equally applicable to any of the other aspect of the present 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 pair of SEM images (to the same scale) : (a) being an endlessly single-mode microstruetured fibre (fibre A) and (b) being a section of cobweb PCF formed by inflating the fibre shown in (a) . Fig. 2 shows the fibre of Fig. 1 (a) before and (b) after inflation and tapering, according to a example embodiment of the method of the invention, with one hole blocked next to the core.

Fig. 3 is a pair of near-field images at the output of the fibre of Fig. 2, (a) with the input fibre present, and (b) with the input fibre removed and a defocused launch so that cladding features are illuminated.

Fig. 4 shows polarisation beating measured in light transmitted through the fibre of Fig. 2 (b) .

Fig. 5 is three optical micrographs (to the same scale) of another fibre that is an example embodiment of the invention, showing the endface of the fibre (a) before and (b) & (c) after inflation.

Fig. 6 is a schematic longitudinal cross-section of a PCF according to another example embodiment of the invention.

Fig. 7 is four images relating to the fibre of Fig. l(a) : (a) an SEM image of another portion of the fibre, with 2 holes plugged each side of the core along the line indicated, according to an example embodiment of the invention (b) an SEM image of that portion after inflation and (c) an SEM image after tapering, (d) output far- and (inset) near-field images at 1000 run, not to scale but oriented the same way.

Fig. 8 is another portion of the fibre of Fig. l(a) after inflation according to another example embodiment of the invention to give an annular core, (b) its output near-field image at 1000 nm, not to scale, (c) after inflation to give an expanded core, with (inset) the near field image for visible light, not to scale, and (d) after inflation to give a separate second core.

Fig 9 is the fibre of Fig. 1 (a) before and (b) after inflation according to another example embodiment of the invention and (c) output images of far-field for visible light and (inset) near-field at 1000 nm, not to scale.

Fig. 10 is output far-field images at 658 nm from (a) a laser and (b) the rectangular core of a fibre according to an example embodiment of the invention.

Fig. 11 is a (top) schematic longitudinal cross- section of an inflation-based mode converter, according to an example embodiment of the invention, showing core areas in dark grey and (bottom) optical micrographs of the holey region in the cleaved fibre in each of sections A-D, to the same scale.

Fig. 12 is (top) measured output near- (left) and far- (right) field intensity patterns at the wavelength of 1000 run, not to scale and (bottom) corresponding calculated LP02 mode patterns of a weakly-guiding circular core in a fibre according to another example embodiment of the invention.

Fig. 13 is measured output near-field images from that fibre at wavelengths of (a) 633 nm, (b) 1000 nm and (c) 1200 nm.

In an example embodiment of a method according to the invention, differential hole pressurisation and heat treatment were used to change radically the core shape along a piece of non-birefringent photonic crystal fibre, forming a low-loss transition to a highly-birefringent core with a sub-millimetre beatlength. Selected holes at the end of the fibre were blocked before pressure was applied. Part of the fibre was heated and tapered while the air holes were pressurised. Where the fibre was heated, unpressurised holes collapsed as their neighbours expanded, leaving a region of solid silica in the fibre's cross-section. In particular, by blocking one of the holes next to the core of an endlessly single-mode PCF, collapse of that hole added solid material to the side of the fibre's core to give it a pronounced elliptical aspect ratio and hence form-birefringence (which could be enhanced by tapering the core down in size) . That was then tapered in the usual way while retaining the cobweb structure, to give a low-loss transition to a small highly-nonlinear core.

Fig. 1 shows the effects of heating a PCF with hole pressurisation when none of the holes are blocked. An endlessly single-mode PCF 10 is shown in Fig. l(a), with a non-birefringent -12 μm diameter core 20. The PCF 10 is commercially available (ESM12-01, made by BlazePhotonics Ltd, now sold by Crystal Fibre A/S and Thorlabs Inc.) and is splice-compatible with SMF-28 telecoms fibre. Though hexagonal in shape, the core 20 is "circular" in the sense that it has a 1:1 aspect ratio, high rotational symmetry and negligible birefringence. As can be seen in Fig. l(b), after treatment holes 30 have expanded to form a cobweb-like structure 40 with a much smaller core 50.

In a first example method according to the invention, a fibre 10 of the type shown in Fig. l(a) was pressurised and heated as described above, save that all the holes 30 at one end were collapsed and sealed using a fusion splicer and, at the other end, one hole 35 of the holes 30 next to the core 20 was sealed with glue. The glue was dispensed with a microscopically-positioned tip, and cured with UV light. Dry nitrogen at a pressure of 5.3 bar was supplied to this end of the fibre to pressurise all the holes except the hole 35 blocked with glue, which remained at atmospheric pressure.

A section of the pressurised fibre 10 was stripped of its coating and tapered down using an oxy-butane flame on a fibre tapering rig. The taper ratio was 4:1, meaning that the fibre ' s diameter would have reduced by this ratio if there had been no changes in relative hole sizes; i.e. the area of glass in the fibre's cross-section was reduced by 16:1. The tapered waist was then cleaved, leaving a 3 cm transition between ~1 m of the original (input) fibre 10 at one end and 9 cm of inflated-and-tapered (output) fibre 60 at the other end (fibres 10 and 60 formed a continuous fibre; it will be understood that the reference numeral 10 is henceforth used to refer to an unprocessed portion of the continuous fibre and the reference numeral 60 is used to refer to a processed portion) .

Fig. 2 (b) is an optical micrograph of the cleaved waist, and Fig. 2 (a) is the original fibre to the same scale. The pressurised holes 30 have inflated in a similar way to the Fig. 1 example, but the unpressurised hole 35 collapsed completely. The "circular" core 20 at the centre of the original fibre 10 reduced in size by the tapering and became "elliptical" by the addition of solid material from the collapse of the adjacent hole 35. The dimensions of the core 70 in the post-processing fibre 60 are now roughly 4.6 x 2.1 urn, an aspect ratio exceeding 2:1. (It can be seen that 3 others of holes 30 also collapsed, unintentionally, forming new cores 72, 74, 76 in the fibre 60. However, they were well-enough isolated to have no observable effect on propagation in the central core 70. We believe that the unwanted cores 72, 74, 76 formed as a result of water condensing on the fibre end- face prior to processing) .

Fig. 3 (a) is a near-field image of the output for 1430 nm light at the input. Only the central core was lit up, no light being evident in parasitic cores. The near- field pattern, and the simple symmetrical centrally-peaked far-field pattern we observed for red light (not shown) , indicate that the light was in the fundamental mode even though the core was almost certainly multimode.

Fig. 3 (b) was obtained, for the purpose of comparison, with Fg. 3 (a) . Input fibre 10 was removed and defocused light was launched into fibre 60, so that cladding features 80 are illuminated.

The transition loss of the structure was measured by the cut-back technique to be <0.4 dB at 1550 nm wavelength. To characterise the birefringence of the output fibre 60, polarisation beating with wavelength was recorded by- launching polarised light from a broadband source into the input fibre 10, and passing the output light to an optical spectrum analyser via a polariser at 45² to the optical axes of the fibre 60. Fig. 4 is the output spectrum.

The spectral beat period Δλ is related to the beatlength L_B and the fibre length L by

λ dL

L.-±L 'B,

^UB λ _LL_B dλ

The factor in brackets it is of order unity, so since the output fibre is 9 cm long and Δλ = 5.95 nm from Fig. 4, the equation gives an estimate for the beatlength of -390 μm. This compares well with the value of 280 μm calculated for a 4.6 x 2.1 μm rectangular silica core in air (AW Snyder et al, J. Opt. Soc. Am. A 3 (1986) 6008). (This value is highly sensitive to the dimensions, which can be only imperfectly known from our optical micrograph.) Repeating the measurement with the input fibre 10 alone confirmed that it did not contribute significantly to the birefringence.

The simple periodic form of Fig. 4 also indicates that the output is in a single mode. If more than one mode had been excited to a significant extent, its different birefringence would result in a more complicated beating spectrum.

Fibre tapers can help to couple light between optical components with different mode sizes. A gradual transition in core size along the fibre allows a low-loss connection to be made between the two dissimilar modes. However, few fibre transitions in mode shape instead of (or as well as) mode size have been reported. As described above, we have made a 3-cm low-loss taper transition in a PCF, where the core shape changes from "circular" (6-fold rotational symmetry) without birefringence to "elliptical" (2-fold rotation symmetry or less) with an aspect ratio exceeding 2:1 and a very short birefringence beatlength of about half a millimetre. The insertion loss of <0.4 dB is low, and the transition was made from widely-available stock PCF 10 by a simple tapering technique. This is the first time that such a radical transition in core shape has been reported in PCFs. As well as showing how strong variations in birefringence can be achieved, our result is also a demonstration of a general new technique with a much wider applicability. For example, additional cores can be created at defined locations along the fibre: monolithic versions of concatenated narrowband filters are possible where directional coupling occurs to an intermittent second core created periodically along the fibre (applying K Okamoto et al, Electron. Lett. 22 (1986) 211) . Formation of an elongated slab-like core by blocking a straight row of holes centred on the core can provide mode-matching to diode lasers and integrated-optic devices, as discussed further below. It is even possible to transform to a ring- shaped core by blocking a ring of holes, as also discussed further below.

Following our demonstration of the transition from non-birefringent to birefringent fibre, we used the above- described post-processing techniques to change the shape of the core of a PCF along its length in other ways. We formed a variety of such transitions, including one to a rectangular core of 5:1 aspect ratio, and demonstrated how they can act as the all-fibre equivalent of anamorphic optics for the low-loss coupling of diode lasers.

In a second experiment, a fibre 100 was provided in which there were two holes 110, 120, on opposite sides of a central core 130, which were larger than the other holes 140 in the fibre 100 (Fig. 5 (a) ) . The fibre 100 was then inflated by uniform application of a pressure of 10 bar to one end of the fibre 100 after the other end had been sealed by collapsing the holes 140 in a fusion splicer. A section of the fibre 100 was then heated on a tapering rig. The fibre 100 was however not stretched. In Fig. 5, all 3 photos (a, b, and c) are taken to the same scale. The outer diameter is about 125 microns. In Fig. 5 (a) and (b) , the fibre 100 is illuminated by reflected light; Fig. 5(c) is the same fibre as shown in Fig. 5(b) but illuminated by transmitted light. The effect of inflation is to enlarge all the holes 140, but the two bigger holes 110, 120 next to the core 130 are enlarged more than the others (Fig. 5 (b) ) . The result is that the roughly- circular core 130 in the initial fibre is squashed and made into a much more "rectangular" core 150. Fig. 5(c) shows where transmitted light concentrates in use. Note in particular the high concentration of light in the fibre core 150.

In each of the above examples, the shape of the core 50, 130 of the fibre 10, 100 is changed only in the region in and near where heat is applied. Consequently, the shape of the core 50, 130 is changed on a centimetre length scale. Of course, changes on longer length scales may be achieved using the invention but a particular advantage of the invention is that it enables changes over short length scales . In another set of experiments (Fig. 6) , we inflated a PCF 200 heated on a tapering rig, by pressurising the holes from one end 210 while all holes 215 at the other end 220 were sealed. Shortening the hot zone 225 as inflation proceeded yielded a section 230 of fibre with enlarged holes, connected to the original uninflated fibre 200 at both ends by gradual transitions 240, 250. Conventional tapering was used to reduce the scale of the structure. We used two different methods of differential hole pressurisation to change the core shape: (1) applying different gas pressures to different holes, and (2) exploiting surface tension to achieve different effective pressures in holes of different sizes.

In the first method, selected holes 260 were plugged with glue 270 at the end 220 where pressure was to be applied (all the holes 215 at the other end 210 being sealed as before) . The plug of glue 270 kept the selected holes 260 at atmospheric pressure while their (open) neighbours 280 become pressurised (Fig. 6) . When the fibre 200 was heated, each plugged hole 260 collapsed as its neighbours 280 expanded, leaving a new solid unit cell 290 in the fibre's cross-section. When increased pressure P was then applied to that end 220, the plugged holes 260 remained at atmospheric pressure P_atm- The pressurised open holes 280 inflated on heating, while plugged holes 260 collapsed to add solid material to the core 205 of fibre 200. Two gradual transitions 240, 250 along the fibre 200 were formed by varying the hot-zone length.

In another experiment, 4 holes 310 were plugged (as described above) along a line 320 through the core 330 at the end of another length of fibre A 300 (Fig. 7 (b) ) . On heating 17 cm of the fibre 300 at 3 bar pressure, the plugged holes 310 collapsed to add 4 solid unit-cells to the core 330 and form an enlarged core 340 having a "rectangular" 5:1 aspect ratio (Fg. 7 (b) ) . That section was then tapered normally to reduce its diameter and cleaved to access the rectangular core 340 (Fig. 7(c)). Near- and far-field images of the light from this core show the high aspect ratio and modal purity of the light (Fig. 7(d)). The insertion loss of the entire structure before cleaving was 0.2 dB at 1550 nm, a loss of 0.1 dB for each circular-to-rectangular transition after cleaving and confirming adiabatic single-mode propagation throughout .

Three further experiments demonstrated the versatility of this technique. We plugged 5 holes so that they and the core formed a ring with an open hole 410 in the middle. Inflation gave low-loss transitions (0.25 dB each at 1550 run) to an annular core 400 (Pig. 8 (a)) and hence a ring-shaped mode, Fig. 8 (b) .

We formed a simple beam expander (0.2 dB loss at 1550 nm) with an enlarged core 500 by inflation after the 6 holes around the core had been plugged, Fig. 8(c) .

Inflation with an isolated plugged hole formed a localised section of fibre with two cores 600, Fig. 8(d).

In the second method, we exploited the pressure-like effect of surface tension. The effective inward pressure

due to surface tension γ in a hole of diameter d is P_st = 2

γ / d, so different-sized holes at the same hydrostatic (gas) pressure have different effective pressures, bigger holes expanding relative to smaller ones . Dramatic changes in core shape are therefore possible without plugging any holes if the initial fibre has a suitable distribution of hole sizes . The hydrostatic pressure common to all holes changes the "bias point" (eg whether two holes shrink at different rates or one expands while the other shrinks) , giving an independent way to control the size of any one hole.

To test this without making a fibre specially, an existing polarisation-maintaining PCF (Fig. 9 (b) ) with two holes 700 bigger than the others (fibre B) was pressurised to 7 bar and inflated with no holes plugged. The bigger holes 700 enlarged more and squashed the core 710, giving it and its mode patterns an elongated rectangular shape (Fig. 9 (b) & (c)). The transition loss was 0.03 dB at 1550 nm.

Such transitions to rectangular cores (by either method) can provide low-loss coupling of light between fibres and diode lasers or integrated optic waveguides with non-circular modes . PCFs with rectangular cores have also been proposed for sensing applications, and annular cores for dispersion compensation and large-mode-area polarisation-maintaining fibres. Producing parasitic cores in well-defined sections could lead to simple concatenated directional-coupler filters . In all cases the plugging of selected holes in an endface need only be done once for a whole length of fibre to give thousands of transitions in 1 km of PCF. Alternatively, effective pressure control via hole size eliminates the need to plug holes at all .

To demonstrate improved coupling from a diode laser, we inflated and tapered fibre A to give a 3 : 1 rectangular core (by plugging two holes) and an outer diameter of 60 μm. The aspect ratio of the core and its output light (Fig. 10 (b) ) roughly matched the ratio of divergence angles specified for the laser (Fig. 10 (a)). The laser's visible output (λ = 658 run) simplified alignment but raised the transition loss to 0.9 dB; the light must spread further in the transition relative to its wavelength, making adiabaticity harder to achieve.

Light was coupled from the laser to the rectangular core using a spherical coupling lens, then to the circular core via the transition. We coupled 12.5 mW of the 22.4 mW laser output (measured before the lens) through to the circular core, a coupling efficiency of 56%. That compared to 42% to a conventional fibre (single-mode at 658 run) after similarly optimising the alignment. Our transition improved coupling despite the relatively high loss of the transition; without that loss the efficiency would have been -70%, which we would expect to approach for longer wavelengths. By further tapering (and inflating enough to suppress leakage loss) , butting the fibre end against the diode laser emitter should be sufficient without the need for a lens at all.

Thus, low-loss anamorphic core shape transitions along photonic crystal fibres were made by two alternative forms of differential pressurisation. Large changes of aspect ratio of up to 5:1 were possible with losses as low as 0.03 dB, assisting the interfacing of diode lasers, integrated optics and other components with non-circular modes. Significant improvement in coupling efficiency from a diode laser was demonstrated.

We have also made an all-fibre mode converter based on PCF transitions . The converter acts like a null fused coupler (that is, a coupler in which two dissimilar fibre cores are fused into a common waveguide (the coupler waist) along a gradual adiabatic transition) . Light in the smaller core, with the smaller propagation constant, evolves into the second-order mode of the common waveguide. Likewise, in our mode converter a new and larger core is introduced beside an original single-mode core. If the cores are well-separated the presence of the new core does not affect light in the original core at all, but does "re-label" its mode to become the second mode of the composite structure comprising both cores. That change of identity means the place where the second core appears is, strictly speaking, a 100% mode converter; that is, maximally non-adiabatic without being lossy. Though merely a pedantic point at first glance, that has practical consequences if the structure is then adiabatically tapered to combine the cores, because the light must stay in the second mode through to the output waveguide. Because our mode converter relies on such adiabatic propagation rather than resonant coupling, the extinction ratio is determined only by the adiabaticity of the transition. It should therefore be effective at all wavelengths, and environmentally stable.

The mode converter was made by differential hole inflation of an existing off-the-shelf PCF 800 with a central single-mode core 810, Fig. HA. To make the device, we first plugged the second ring of 12 holes 820 around the core 810 with glue at one end, pressurised the fibre end to raise the pressure in the other holes 830 (so that only the 12 plugged holes 820 remained at atmospheric pressure) , and heated 10 cm of the fibre 800 on a tapering rig. The pressurised holes 830 inflated but the plugged holes 820 collapsed to form a new annular core 840 around the original core 810, Fig. HB. The gap 850 between the cores (the first ring of 6 holes) was big enough for coupling between the cores to be minimal. The transition between the sections with and without the annular core 840 was abrupt, i.e. "splice-like" not "taper-like".

The 6 holes in gap 850 between the cores were then plugged with glue at the end, and the inflation technique applied again to collapse them along 5 cm of the already- processed fibre 860. The result was a twice-processed section of fibre 870 with a single large core 880, Fig. HD. This time the transition between the once- and twice-processed sections (860 and 870) was gradual, i.e. "taper-like" not "splice-like", Fig. HC. The fibre was cleaved in the middle of the twice-processed section 870, giving two mode converters. Each mode converter was continuous with no splices.

Thus, in use, input light travels along (A) original input fibre with a central core, (B) fibre with an annular core around the central core, (C) a gradual transition where the holes between the cores diminish and vanish, and (D) output fibre with a single enlarged core.

Coupling from LΕ_lm modes with 1 = 0 to modes with 1 ≠ 0 is forbidden by symmetry, so we need consider only adiabaticity among the 2 = 0 modes: the "second mode" is the LPo₂ mode. Light in the LP₀I mode in the central core 810 of section A does not notice the sudden appearance of the optically-isolated annular core 840. However, the annular core 840 is much bigger, making its guided mode the fundamental mode of section B. The light, being in the central core 810, is therefore in the second mode of section B. If the transition C is gradual enough to be adiabatic, the light stays in the second mode through to the single enlarged core 880 in section D, where it becomes a conventional LP₀₂ mode. Thus the whole device is an LPoi to LP₀2 mode converter. The lengths of sections A, B, C and D in the final device were roughly 1 m, 1 cm, 3 cm and 4 cm respectively.

A very low loss of just 0.1 dB at 1000 nm wavelength was measured from the input in section A to the output in section D by the cut-back technique, with index-matching gel applied to strip away any light in cladding modes. Measured output near- and far-field intensity patterns are shown in Fig. 12, along with calculated LP₀₂ patterns for a circular step-index weakly-guiding core. The measured patterns have similar "sombrero hat" distributions to the calculated patterns, albeit with distortions due to the hexagonal rather than circular core shape. In both cases the central peak in the near-field patterns is much more intense than the outer ring, whereas in the far-field patterns they have similar intensities. As expected if the output light is in one mode of the fibre, the measured patterns did not noticeably change when section D was mechanically disturbed.

Fig. 13 shows output near-field images at three widely-spaced wavelengths . The mode converter behaved in roughly the same way across the whole wavelength range. Measurements of the minimum intensity in the dark ring between the lobes in the far-field pattern put a lower bound of -15 dB on the extinction ratio of the mode converter. However, the minimum intensity in the dark ring of an LP₀₂ mode is only zero in the scalar (weakly- guiding) limit, in contrast to that of the LPn mode which is zero by symmetry. With the index step of silica to air, the minimum intensity of even a pure LP₀₂ mode can be of similar magnitude to that we measured. The extinction ratio of our device was therefore probably better than 15 dB, but we cannot quantify it further at this stage.

Thus, differential hole inflation was used to make an LPoi to LP₀₂ converter from stock PCF, also with a broad range of operating wavelengths and a loss of just 0.1 dB. By optical reciprocity, the devices should also work in reverse. Our choice of modes was to some extent arbitrary, and the method could be used to make converters for other modes : in the first case by introducing more than two carefully-chosen and carefully-positioned SMFs; in the second case by collapsing different patterns of holes with different symmetries. Such mode converters may be used for higher-order mode dispersion compensators.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers of features of the invention that are described as preferable advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. For that reason, reference should be made to the claims for determining the true scope of the present invention.

Claims

1. A method of processing a microstructure fibre comprising:

(a) providing a microstructured fibre comprising a solid matrix material defining a plurality of elongate holes that extend along a first length of the fibre;

(b) applying a uniform pressure to a first end-face of the fibre; and

(c) heating a second length of the fibre that at least partially overlaps with the first length; wherein, the fibre is arranged such that one or more of the holes experience an effective pressure that is different from the effective pressure experienced by the other holes, so that in the second length the one or more holes expand or contract relative to the other holes, and at least one hole expands relative to the whole fibre.

2. A method as claimed in claim 1, in which the one or more holes are larger or smaller than the other holes, such that the effective pressure due to surface tension in the one or more holes is, respectively, smaller or larger than the effective pressure due to surface tension in the other holes.

3. A method as claimed in claim 1 or claim 2, in which the one or more holes are arranged to be at a different hydrostatic pressure from the hydrostatic pressure of the other holes .

4. A method as claimed in claim 3, in which the one or more holes are sealed at at least a first end, such the other holes are pressurised in step (b) to a different effective pressure from the pressure inside the sealed holes, so that in the heated length the pressurised holes expand (or at least partially collapse) and the sealed holes at least partially collapse (or expand) .

5. A method as claimed in claim 4, in which at least one of the one or more holes is sealed with a liquid insert.

6. A method as claimed in claim 4 or claim 5, in which at least one of the one or more holes is sealed with a solid insert.

7. A method as claimed in claim 4 or 5, in which at least one of the one or more holes is sealed with a gel insert .

8 A method as claimed in claim 4 or 5, in which at least one of the one or more holes is sealed with a liquid that is then solidified.

9. A method as claimed in any of claims 5 to 8, in which the insert is applied by attaching it to a tip and then positioning the tip adjacent to the hole.

10. A method as claimed in any of claims 5 to 9, in which the hole is inflated at its end prior to insertion of the insert .

11. A method as claimed in any of claims 4 to 10, in which at least one of the one or more holes is sealed by heating to collapse the end of the hole.

12. A method as claimed in any of claims 4 to 11, in which at least some of the sealed holes are also blocked at a second end.

13. A method as claimed in any of claims 4 to 12, in which at least one of the sealed holes is sufficiently close to the outer surface of the fibre that the at least one hole explodes during the heating step.

14. A method as claimed in any preceding claim, in which at least one of the one or more holes is adjacent to a core of the fibre.

15. A method as claimed in claim 13, in which the at least one sealed hole adjacent to the core collapses under the pressurisation and heating, in a pattern that forms an enlarged core.

16. A method as claimed in claim 14, in which the enlarged core is elliptical in transverse cross-section.

17. A method as claimed in claim 16, in which the aspect ratio of the elliptical core is greater than 1.5.

18. A method as claimed in any preceding claim, in which the one or more holes form a pattern in the transverse cross-section of the fibre that has no more than 2-fold rotational symmetry about a core of the fibre.

19. A method as claimed in any preceding claim, in which the collapse of at least one of the one or more holes forms a new core in the fibre.

20. A method as claimed in claim 19, in which the fibre comprises a pre-existing core, before it is processed according to the method, and the new core has a greater cross-sectional area than the pre-existing core.

21. A method as claimed in claim 20, in which the new core surrounds the old core.

22. A method as claimed in any of claims 19 to 21, in which the fibre comprises a pre-existing core, before it is processed according to the method, and in which a length of the fibre containing the new core is further processed to collapse at least one hole between the new core and the pre-existing core, such that a combined core is formed.

23. A method as claimed in claim 22, in which the collapse of the at least one hole is effected gradually as a transition over a length of the fibre.

24. A method as claimed in claim 19, in which the fibre has no core before it is processed according to the method.

25. A method as claimed in any preceding claim, in which the heating process is carried out at a plurality of separated points along the length of the fibre.

26. A mode converter comprising a fibre processed according to the method of claim 22 or 23.

27. A microstructured optical fibre comprising a solid matrix material defining a plurality of elongate holes that extend along a length of the fibre and a core that extends along a length of the fibre and is arranged to guide light, the core having, in cross-section, a first shape in a first length of the fibre and a second shape in a second length of the fibre, the first and second shapes being ellipses of different eccentricity and the first and second lengths being separated by a transition length.

28. A fibre as claimed in claim 1, in which the transition length is less than 1 m.

29. An optical device including a fibre according to claim 27 or a fibre that has been processed by a method according to any of claims 1 to 25.