WO2009029987A1

WO2009029987A1 - A method for making microstructured fibres sensitive to the external environment

Info

Publication number: WO2009029987A1
Application number: PCT/AU2008/001314
Authority: WO
Inventors: Maryanne Candida Jane Large; Cristiano Monteriro De Barros Cordeiro; Felicity Cox
Original assignee: University of Sydney
Current assignee: University of Sydney
Priority date: 2007-09-04
Filing date: 2008-09-04
Publication date: 2009-03-12
Anticipated expiration: 2010-03-04

Abstract

The present invention relates to the design and manufacture of microstructured optical fibres. One aspect of the invention provides a method of forming a microstructured optical fibre, the method comprising forming one or more apertures at predetermined locations in a preform, the one or more apertures formed at an angle to the axis of the preform, and subsequently drawing the preform to form a length of optical fibre.

Description

A METHOD FOR MAKING MICROSTRUCTURED FIBRES SENSITIVE TO THE

EXTERNAL ENVIRONMENT Field of the Invention

The present invention relates to the design and manufacture of microstructured optical fibres.

Background of the Invention

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. The core of microstructured optical fibre is generally a pure material whilst the cladding is composed of a two-dimensional microstructure consisting of two or more substances. For the most common case of a solid-core, the cladding consists of an array of air holes in a solid material that run longitudinally along the length of the fiber. The cladding is thus characterized by an average refractive index value less than that of the solid material, allowing the fibre to guide by total internal reflection. The solid material is usually silica or polymer although other materials may be used. Similarly, the holes of the cladding are not restricted to being air-filled, and can be filled with any material that preserves the lower index condition of the cladding. The optical properties of the microstructured cladding, and thus those of the entire waveguide, depend on the optical properties of these constituent materials as well as the relative amount and arrangement of each. The corollary of this is that the constituent materials have a direct effect on the guidance properties of the fibre and it is this intrinsic link between materials and optical properties, coupled with the ability to freely exchange at least one of these materials via the microstructure, which make MOFs such interesting vessels for chemical sensing.

In recent times there has been increasing interest in making fibres that allow the core or cladding (or both) to be easily accessed through the side rather than through the endface. Opening the side of a fibre in this way does not necessarily affect or destroy the ability of the fibre to guide light, although losses may be increased. To date this approach has been explored by making side holes in microstructured fibres after the fibre has been drawn.

Advantageously, making the interior of the fibre accessible allows several things: a large number of sensing experiments have been enabled in microstructured fibres, because of their ability to guide light in low refractive index materials, through the photonic bandgap effect, evanescent fields, or liquid core effective index guiding. Other experiments using these fluids would include, but not be limited to, non-linear optical effects, and electrooptic effects, processes in which tunability is introduced by for example heating or applying a voltage to a fluid and hence changing its optical properties. These experiments require use the fact that the holes in the microstructure can guide material as well as light. Critical to their operation is the ability to guide light and materials simultaneously. However in most cases loading the fluid into the fibre is both time consuming (usually requiring pressure as well as capillary action) and difficult. In addition, the loading of the material is normally done via the same fibre end as is used for coupling light in and out of the fibre. Most frequently, a specially designed loading system has to be designed, and a rapid response time to changes in the fluid is impossible for this reason, prohibiting for example the use of these fibres for effective online measurements. Similarly, there is increasing interest in using light to control the motion of particles or atoms. As for the fluid examples, there are practical difficulties associated with the simultaneous launching of light and particles into the fibre.

It is therefore an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. Summary of the Invention

To this end, a first aspect of the present invention provides a method of forming a microstructured optical fibre, said method comprising forming one or more apertures at predetermined locations in a preform, said one or more apertures formed at an angle to the axis of the preform, and subsequently drawing said preform to form a length of optical fibre.

Preferably, said apertures extend into the preform at an angle to the direction in which the preform is to be drawn.

Preferably, said preform is formed from an optically suitable polymer material. Preferably, said aperture is in the form of a hole or slot which is introduced into the preform of a microstructured fibre.

Preferably, the axis of said aperture is perpendicular to the direction of the fibre axis.

Preferably, said aperture is in the form of a hole or slot which extends longitudinally along the fibre axis.

Preferably, said aperture permits a portion of said microstructure to be open to the external environment when said preform is drawn to a fibre.

A second aspect of the present invention provides a microstructured optical fibre formed in accordance with the method of the first aspect of the invention. In one embodiment, said aperture is in the form of a hole or slot which is introduced into the preform of a microstructured fibre. The direction of the hole or slot may be perpendicular or at a more glacing angle to the direction of the fibre axis. The hole or slot allows some portion of the microstructure (core, cladding, or both) to be open to the external environment when the preform is drawn to fibre.

The introduction of the hole or slot at the preform stage has a number of advantages over attempting to do so at the fibre stage, including: a) It is easier to drill a hole in the preform, the hole can be much bigger than it would be in the fibre. In particular, it is possible to obtain an extended slot rather than an isolated hole. It is possible to do this in glass mid polymer. This greatly enhances the ability of the fibre to be filled quickly. If more than one section is required to be open (for example, 2 points to provide a flow circuit), the use of the long slot rather than the hole is clear- rather than making separate holes, a slot can be made, and some region of the slot sealed to produce separate holes. b) Potentially it is easier to remove dust created by the drilling with pressurized air and with the drawing itself. It is also possible to use a laser drilling and hot wire. Further, making the hole prior to drawing makes it possible to use the draw process to improve the smoothness of the hole through heat polishing and the draw process. This is significant, as it will affect both the optical properties and the flow properties. c) The hole or slot does not have to pass directly and/or perpendicularly into the fibre- it may take a less direct path, and in particular the hole or slot may enter the preform (and hence later the fibre) at a more glancing angle (i.e. the hole is not perpendicular to the fibre axis, but enters the fibre at a glancing angle. This has many advantages- allowing for example light (rather than a material) to be launched into a hollow core fibre, or material to be launched with almost the correct direction.

Note that in the context of the present invention the term "preform" is to be taken to mean any structure that is later drawn to smaller dimensions- and could also apply to structure that is later tapered (rather than drawn which implies a steady state processes). Brief Description of the Drawings

The invention will now be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates slotted microstructured polymer optical fibres (mPOF) with

(a) a 3-hole design and (b) a 5-hole design. In each case the fibre diameter is approximately 140μm.

Figure 2 illustrates a slotted mPOF with varying slot sizes. The slots were formed by drilling 1.0 mm, 1.6 mm and 2.5 mm holes into the intermediate preform. Detailed Description of the Invention

The present invention is directed to a way of making microstructured optical fibres sensitive to the external environment by opening a hole or slit in the microstructure so that external material such as fluids (both liquids or gases) or particles (including atoms) may be introduced into the fibre, either entering the core, or cladding, or both. The side hole or slot in a hollow core fibre may also allow beams, such as highly directed lasers, to freely impinge on the interior of the fibre without being required to pass through the microstructured cladding. Advantageously, this greatly improves the writing of gratings in the fibre. Grating writing is usually done using a focused laser beam to modify the refractive index of the fibre core through photochemical, photophysical or other physical changes in material. However the presence of the microstructured cladding is known to interfere with this process by scattering the incident light. An extended side hole or slot would thus allow a more direct access to the core. Similarly, a close access to core would allow materials to be deposited (such as metals, or nanoparticles) in a way that would affect the transmission in the core, or allow enable other processes to be applied to the core. These include: applying an electric field, enhanced surface effects such as surface plasmon resonances or the surface enhanced Raman effect, magneto-optic effects, or a localised fluorescence exitation. The present invention makes it possible to make a fibre that constrains light, but not fluids or other material. This allows the fibre to be sensitive to the ambient environment (eg: gases or liquids), while still acting as a waveguide. Thus, the light used to probe the liquid will be guided, allowing the traditional advantages of fibre sensors- and in particular the long interaction lengths needed for sensitive detection through for example spectroscopic means. More generally, it would allow the separate launching of material (such as particles or fluids) and radiation (such as light).

In this case the side hole was introduced before the preform was drawn to fibre. A hole was drilled in the preform, and this lengthened to a slot during the draw process. This means that the holes used can be relatively large (a simpler process to make) and long lengths can be made without affecting the structural integrity of the preform. This also means that the draw process will result in relatively smooth walled holes. Alternatively, a slot may be formed in the preform. Small holes can be made by mechanical drilling, laser drilling, or in, in the case of polymers, by a hot wire technique. In such fibres, the light is confined by total internal reflection and so the presence of the hole will not dramatically affect the guidance (though the field will be localised more in the liquid filled higher index region). This is less obvious with bandgap fibres where although the bandgap will in principle still guide the fibres, the presence of the side hole will still the loss in that physical region.

This effect can be reduced by using an angled side hole so that all regions of the fibre retain some reflective layers. Importantly this also improves the mechanical stability of the structure. Alternatively, a hole may be introduced at the capillary stacking stage (holes being introduced into individual capillaries being slacked), so that the hole inwards through the structure to the core progressed layer by layer through the stack.

The present invention enables the fabrication of a laterally accessible mPOF device whereby fluid from the bulk can access the holes of the microstructured cladding along the length of the fibre. This is facilitated by a slot that is oriented laterally with respect to the fibre axis. Entry and exit of fluid through the endfaces of the fibre is avoided. Two examples of mPOFs with lateral slots are shown in Figure 1. The diameter of the fibres in Figure 1. is approximately 140 μm.

Although it is possible to create lateral access to the microstructure of a fibre through laser drilling or by bursting through the fibre wall through the simultaneous application of heat and pressure, such techniques produce a hole rather than a continuous slot and so the long diffusion times associated with analytes moving through the cladding capillaries remains. Furthermore, the incorporation of lateral holes at the fibre rather than an earlier stage can lead to significant losses, e.g. by disturbance of the core in the drilling process or deformation of structure due to the application of heat and pressure. An advantageous feature of both slotted fibres and those with holes into the microstructure is that since the analyte accesses the fibre laterally, such devices can be potentially spliced to conventional fibres without loss of access to the microstructure. Fabrication of the slotted devices requires only minimal deviation from standard drawing procedures as the only additional step was drilling lateral holes into the preform before being drawn to fibre. For the fabrication of mPOF, the process begins with an 8 cm preform that is drawn down to an intermediate cane of ~0.6 cm diameter and then sleeved. Holes are then drilled into the sleeved cane, perpendicular to the cane axis. These holes intersect with a single air hole and do not impact the core. Cooling fluid is used when drilling the lateral holes and this is cleaned by rinsing both the lateral and microstructured holes with water and then blasting with compressed air. This cane is then drawn to fibre, with diameter -150 μm, and each drilled lateral hole of the cane is stretched to form a slot in the fibre. Shown in Figure 1 are two examples of the exposed core mPOF. In Figure Ia, the lateral hole has been drilled into one of the holes of a 3 -hole suspended core design, and in Figure Ib, it has been drilled into one of the holes of a 5-hole design. It is also possible to drill through more than one ring of holes to reach the innermost ring, although disruption of the cladding in such a way may change the guidance properties of the fibre.

In the case of a lmm diameter hole drilled into the 0.8 cm cane, 5-6 m of 150 μm slotted mPOF with an approximately uniform diameter was produced. This length is increased by increasing the diameter of the drilled hole in the cane. Some fluctuation in fibre diameter was observed during the draw process, related to heat transport with and without an open slot. The result was that the slotted regions of fibre have a diameter about 70 μm smaller than the regions without a slot. However, the diameter within a single region i.e. with or without a slot, remains constant.

Significantly longer lengths of the slotted fibre were also produced by drilling ~15 overlapping holes in the cane. This formed a slot in the cane itself, and hence an extended slot in the fibre. Minimal fluctuations in fibre diameter were observed when drawing the cane with a slot rather than holes, and steady-state draw conditions were achieved. Other possible methods of forming a slot in the cane are through laser or hot-wire cutting. The size of the hole drilled in the cane corresponds well to the slot width in the fibre. Shown below, in Figure 2, are fibres in which holes of 1.0 mm, 1.6 mm, and 2.5 mm diameter were drilled. The diameter of the three fibres shown is approximately 150 μm, and all fibres were drawn under the same conditions. Ranging from a narrow channel to an open bay, varying the width of the slot can change the rate that new chemical species diffuse into the sensing region.

It can be seen from Figures 1 and 2 that the microstructure experienced minimal, if any, distortion due to the presence of the lateral slot. The microstructure hole that connects to the slot is approximately the same size and shape as the other microstructure holes despite being open on one side. The slot remains open along its length, and the sides of the slot sustain an approximately parallel orientation. It appears that the existence of the slot induces no additional losses to the fibre.

A MOF with a laterally exposed core along the length of the fibre has been fabricated for the first time without compromising the strength of the waveguide. The slot was simple to fabricate and the slotted mPOF was drawn under relatively standard draw conditions. Both 3- and 5-hole versions of the slotted mPOF were fabricated. Control over the width of the access slot was demonstrated through the choice of hole diameter drilled in the mPOF cane. Control of the length of the slot was also achieved by the choice of hole size in the cane and more significantly by fabricating an extended slot in the cane, which was drawn to long lengths of the laterally accessible fibre. It is possible to achieve steady state drawing if the holes in the cane are replaced by a slot that runs along the entire length of the cane, whilst maintaining the integrity of the microstructure.

Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. A method of forming a microstructured optical fibre, said method comprising forming one or more apertures at predetermined locations in a preform, said one or more apertures formed at an angle to the axis of the preform, and subsequently drawing said preform to form a length of optical fibre.

2. A method of forming a microstructured optical fibre as claimed in claim 1, wherein said apertures extend into the preform at an angle to the direction in which the preform is to be drawn.

3. A method of forming a microstructured optical fibre as claimed in claim 1 or 2, wherein said preform is formed from an optically suitable polymer material.

4. A method of forming a microstructured optical fibre as claimed in any one of claims 1 to 3, wherein said aperture is in the form of a hole or slot which is introduced into the preform of a microstructured fibre.

5. A method of forming a microstructured optical fibre as claimed in any one of claims 1 to 4, wherein the axis of said aperture is perpendicular to the direction of the fibre axis.

6. A method of forming a microstructured optical fibre as claimed in any one of claims 1 to 5, wherein said aperture is in the form of a hole or slot which extends longitudinally along the fibre axis.

7. A method of forming a microstructured optical fibre as claimed in any one of claims 1 to 6, wherein said aperture permits a portion of said microstructure to be open to the external environment when said preform is drawn to a fibre.

8. A microstructured optical fibre formed in accordance with the method as defined in any one of claims 1 to 8.

9. A method of forming a microstructured optical fibre substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

10. A microstructured optical fibre formed in accordance with the method substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.