WO2003093884A2

WO2003093884A2 - A method and apparatus relating to optical fibres

Info

Publication number: WO2003093884A2
Application number: PCT/GB2003/001833
Authority: WO
Inventors: Jonathan Cave Knight; Philip St.John Russell
Original assignee: Crystal Fibre A/S
Priority date: 2002-04-30
Filing date: 2003-04-29
Publication date: 2003-11-13
Also published as: GB0209908D0; AU2003226571A8; WO2003093884A3; AU2003226571A1

Abstract

An optical fibre comprises a core region (20) and a cladding region (30), characterised in that the fibre includes a metallic or semiconducting region (40) in the core region (20) or the cladding region (30). The metallic or semiconducting regions may be embedded in an insulating matrix material (50). For example, the core region may comprise a semiconductor region that provides gain or a semiconductor region that is a saturable absorber. The fibre may be a photonic crystal fibre (PCF) (10). For example, inclusion of metallic or semiconducting regions 40 in a cladding region (30) of a PCF (10) may provide a more robust photonic band gap.

Description

A Method and Apparatus relating to optical fibres

This invention relates to the field of optical fibres. Single-mode and multimode optical fibres are widely used in applications such as telecommunications. Such fibres are usually made entirely from solid, insulating, dielectric 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 typically of diameter 125 microns. Standard fibres can be single-mode or multimode. They can have more than one core, and they can be polarisation- maintaining fibres . In the past few years a non-standard type of optical fibre has been demonstrated, called the photonic crystal fibre (PCF) [J. C. Knight et al . , Optics Letters v. 21 p. 203] ; such fibres have alternatively been called holey fibres or microstructured fibres. Typically, a PCF is made from a single, solid, insulating, dielectric material such as fused silica glass, within which is embedded an array of air holes. The holes run parallel to the fibre axis and extend the full length of the fibre. A region of solid material between holes, larger than neighbouring such regions, can act as a waveguiding fibre core. Light can be guided in this core in a manner analogous to total-internal-reflection guiding in standard fibres. One way to provide such an enlarged solid region in a fibre with an otherwise periodic array of holes is to omit one or more holes from the structure. However, the array of holes need not be periodic for total-internal- reflection guiding to take place; we nevertheless refer to such a fibre as a photonic-crystal fibre.

Another mechanism for guiding light in photonic-crystal fibres is based on photonic bandgap effects rather than total internal reflection. For example, light can be confined inside a hollow core (an enlarged air hole) by a suitably- designed array of smaller holes surrounding the core [R. F. Cregan et al . , Science v. 285 p. 1537] . True guidance in a hollow core is not possible at all in conventional fibres. PCFs can be fabricated by stacking glass elements (rods and tubes) on a macroscopic scale into the required pattern and shape, and holding them in place while fusing them together. This primary preform can then be drawn into a fibre, using the same type of fibre-drawing tower that is used to draw standard fibre from a standard-fibre preform. The primary preform can, for example, be formed from fused silica elements with a diameter of about 0.8 mm.

It is known that a large band-gap can be produced in a photonic crystal that comprises disconnected regions of higher refractive index embedded in a (interconnected) matrix material of a lower refractive index. However, the cladding regions of prior-art photonic crystal fibres typically form photonic crystals that comprise disconnected regions of lower refractive index (e.g. air) embedded in a matrix material of a higher refractive index (e.g. silica), which leads to narrower, and therefore less robust, bandga s .

An object of the invention is to provide an optical fibre having improved optical properties and a method of making the same.

According to the invention there is provided an optical fibre comprising a core region and a cladding region, characterised in that the fibre includes a metallic or semiconducting region in the core region or the cladding region.

As is well known (see, for example, Max Born and Emil Wolf, ^x Principles of Optics', 6th (Corrected) Edn. , Pergamon Press, Oxford, 1980, pp 611-615) , refractive index may be expressed as a complex value, n = n_reaι(l+ik) , where k is the attenuation index. The real part of the complex refractive index of metals and semiconductors is usually significantly higher than the corresponding value for insulators used in prior-art optical fibres. The imaginary part of the refractive index may also be large, leading to higher absorption. However, in many situations that increased absorption is not significant, for example if light in the fibre does not extend into the high-absorption regions, or if those regions are very small compared with the wavelength of light .

Metals and semiconductors may each strongly exhibit a wide variety of interesting optical properties; the invention allows the incorporation of such properties, and of phenomenon based upon them, into an optical fibre. It will be understood that the metallic or semiconducting region is a region that consists only of metal or semiconductor or a region that includes sufficient metal or semiconductor (i.e. as a substantial, preferably major, fraction of the region's composition) such that the region behaves as if it is metallic or semiconducting. Thus, the region is not a region that contains relatively small quantities of metal or semiconductor atoms or ions, for example as dopants, but in which those quantities are not sufficient for the region itself to behave as if it is metallic or semiconducting. The region may be said to be a 'bulk' region (although it may have dimensions of the order of microns or smaller, in many cases much smaller than a wavelength of light that the fibre is to guide) . It may be that there are only one or two metallic or semiconducting regions. Alternatively, more such regions may be provided, preferably an array of such regions. Preferably, the metallic or semiconductor regions are embedded in an insulating matrix material . Preferably, the metallic or semiconductor regions have a largest transverse dimension that is smaller than a wavelength of light that the fibre guides; for example, it may be smaller than 2 microns, 1.5 microns, 1 micron, 0.5 microns or even 0.3 microns. The large absorption has a far less significant effect on propagating light when the absorbing regions are so small. Preferably, the cladding region comprises a plurality of the metallic or semiconducting regions. The effects of high absorption by the regions may be reduced in that arrangement because a light mode propagating in the core of the fibre will, in general, have only a small fraction of its energy in the cladding region.

Inclusion of a semiconducting region in the fibre may be particularly advantageous, because semiconductors are well known to exhibit many interesting properties resulting from their electronic band-gap structure. For example, a semiconductor may be used to provide gain by applying a voltage. Thus, the core region may comprise a semiconductor region that provides gain. Another interesting property of a semiconductor is that it may absorb light of a frequency corresponding to the electronic band-gap of the semiconductor, but that absorption may be significantly reduced at high light intensities, because the conducting band of the semiconductor is filled and further absorption is not possible. Thus, the fibre may comprise a semiconductor region that is a saturable absorber. Similarly, the fibre may comprise a semiconductor region that is switchable between a first state in which it is transparent to propagating light and a second state in which it is not transparent to the light.

Use of a semiconductor may be beneficial simply because it can behave as a simple dielectric material with a large refractive index under certain circumstances. For example, silicon is relatively non-absorbing and has a refractive index of 3.4 for light with a wavelength of 1.5μm.

The core region may have a lower effective refractive index than the cladding region. Preferably, the core region is an elongate hole in the fibre. The strong photonic band- gap provided by metals or semiconductors enables more robust guidance of light in a hollow-cored fibre than is possible using insulating materials such as are used in prior-art hollow-cored fibres. Alternatively, the core region may have a higher effective refractive index than the cladding region.

Preferably, the fibre is a photonic crystal fibre. In that case, the cladding region may comprise elongate regions of a first insulating material having a first refractive index embedded in a second insulating material having a second refractive index. Thus, for example, a photonic crystal may be formed by a plurality of elongate holes in an insulating, dielectric material. The metallic or semiconducting regions may form a photonic crystal structure in the cladding region. The use of metals or semiconductors to form a photonic-crystal region in a photonic crystal fibre is advantageous because the high real refractive index of those materials enables the creation of a robust photonic band-gap. The photonic band gap produced using metals or semiconductors can be particularly strong when the metallic or semiconducting regions are disconnected from each other. Preferably, the metallic or semiconducting regions are elongate along the direction of the longitudinal axis of the fibre. The core region may comprise a metallic and/or a semiconducting region. Preferably, the metallic and/or semiconducting region in the core region has a shortest transverse dimension that is similar to, or larger than, a wavelength of light that the fibre guides.

Alternatively, the metallic or semiconducting regions may be arranged in the cladding region in concentric rings about the core region. Preferably, there are concentric rings comprising alternating rings of metal or semiconductor and of insulating material. An arrangement of rings of alternating higher and lower refractive index is known to provide good confinement of guided light. Use of a metal or semiconductor produces significantly stronger confinement than using two insulators, such as alternate rings of doped and undoped silica.

Also according to the invention there is provided a method of manufacturing an optical fibre, comprising drawing an optical fibre from a preform, characterised in that the method also comprises the steps: (a) including a metal or semiconductor in the preform; and

(b) melting the metal or semiconductor after it is included in the preform.

The metal or semiconductor should be carefully chosen to melt, but not boil, at the temperature at which the fibre is drawn. It is also important that the metal does not expand or contract during heating or cooling so much as to disrupt the structural integrity of the drawn fibre (however, for small filaments of metal or semiconductor, thermal expansion/contraction is not generally a problem) . An example of a suitable choice of metal is copper used with borosilicate tubes. An example of a suitable choice of semiconductor is silicon used with silica tubes. The preform may comprise a plurality of tubes. The preform may comprise a plurality of elongate holes. The metal/semiconductor may be in a hole in the preform or in a tube. The metal/semiconductor may be in powder form. Alternatively, a wire of the metal or semiconductor may be introduced into the hole. Preferably, the wire is fed into the hole at a higher rate than the rate at which the preform or tube is fed. The wire may not perfectly fill the hole at first, in which case feeding the wire into the preform or tube at a faster rate than the preform or tube is fed ensures that there is sufficient metal to fully fill the holes. Alternatively, the metal or semiconductor is introduced into the hole by deposition from a gas. Preferably the gas is produced from a container. The container may be held inside or upstream of the preform or tube . The gas may be produced from a container suspended at the necking point of the preform or tube during drawing.

Preferably, the metal or semiconductor is melted whilst the fibre is being drawn. Preferably, the metal or semiconductor is introduced into a hole in the preform after the preform has been partially drawn. For example, a first preform may be drawn to form a second preform having a transverse dimension of 0.5 mm to 10 mm (preferably, 1 mm to 5 mm) . Alternatively, the metal or semiconductor may be introduced into the hole before the preform is drawn.

Preferably, the metal or semiconductor is melted using radio-frequency (RF) electromagnetic radiation. A coil may be used (in a manner known in the art) to produce RF radiation of suitable frequency and intensity to melt the metal or semiconductor. Heat generated in the melting may be sufficient to soften the insulator. However, for larger volumes of insulator, further heating may be necessary to soften the insulator sufficiently for it to be drawn. It is important not to boil the metal during the application of the further heat. Preferably, the insulator is softened prior to drawing using a laser, such as a carbon-dioxide laser.

The metal or semiconductor may be introduced into an interstitial hole between a plurality of the tubes.

The method may include the steps of providing a capillary tube, introducing the metal or semiconductor into a hole in the capillary tube (by any suitable method, such as those described above) and then introducing the capillary tube into the preform.

Preferably, the preform has been drawn from another preform; thus the earlier preform may comprise a bundle of rods and/or tubes and the later preform may be monolithic; the capillary tube may then be introduced into a hole in the preform.

Also according to the invention there is provided a method of manufacturing an optical fibre, comprising:

(a) forming a preform including a plurality of elongate holes; (b) drawing the preform down to form a (preferably monolithic) preform cane;

(c) drawing further (preferably fine) capillaries (preferably from the same glassy material as the preform) , each capillary including a semiconductor or metal core; (d) introducing the capillaries into the holes in the preform; and

(e) drawing the monolithic preform down to form the final fibre.

Also according to the invention there is provided a transmission line for carrying data between a transmitter and a receiver, the transmission line including along at least part of its length such a fibre.

Also according to the invention there is provided data conditioned by having been transmitted through such a fibre. As in any transmission system, data that is carried by the system acquires a characteristic ^λ signature' determined by a transfer function of the system. By characterising the system transfer function sufficiently accurately, using known techniques, it is possible to match a model of the input data, operated on by the transfer function, with real data that is output (or received) from the transmission system.

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

Fig. 1 is a transverse cross-sectional view of a fibre according to the invention;

Fig. 2 is a transverse cross-sectional view of a second fibre according to the invention; Fig. 3 is a transverse cross- sectional view of a third fibre according to the invention;

Fig. 4 is a transverse cross-sectional view of a fourth fibre according to the invention;

Fig. 5 shows steps in a method according to the invention;

Fig. 6 shows a step in another method according to the invention;

Fig. 7 shows a step in another method according to the invention; Fig. 8 shows a device according to the invention, incorporating a saturable absorber.

An optical fibre 10 (Fig. 1) , comprises a core region 20 and a cladding region 30. The core region 20 is an elongate hole. The cladding region 30 is formed of elongate holes 60 embedded in a matrix of silica 50. The elongate holes 60 and silica regions 50 were formed from a bundle of tubes that are fused together and drawn into the fibre 10, in the usual way. The tubes were of circular cross-section and therefore tile imperfectly; the imperfect tiling results in interstitial regions 40 in the drawn fibre 10. Those regions 40 are filled with silicon.

The holes 60 are of circular cross-section and are arranged, in the fibre's transverse cross-section, in a triangular lattice pattern having a pitch (i.e. nearest neighbour separation distance) of 1.7 microns. The holes fill the cladding region to a filling fraction of 0.735. The core region 20 is of substantially circular cross-section and has a radius of 2.4 microns. The silicon interstitial regions 40 fill the cladding region to a filling fraction of 0.0726 microns. Thus, a graphite matrix of silicon is overlaid on the triangular air-hole array.

This particular arrangement of silicon in silica provides a band-gap between 1.4 microns and 1.9 microns. Light having a wavelength between those wavelengths is guided in core region 20 by photonic band-gap guidance.

A second optical fibre 110 (Fig. 2) , comprises a core region 120 and a cladding region 130. The core region 120 is an elongate hole. The cladding region 130 is formed of elongate regions of copper 140 embedded in a matrix of borosilicate 150.

The copper regions 140 are of circular cross-section of radius 0.2 microns and are arranged, in the fibre's transverse cross-section, in a triangular lattice pattern having a pitch (i.e. nearest neighbour separation distance) of 1.7 microns. The core region 120 is of substantially circular cross-section and has a radius of 2.4 microns. Use of copper in borosilicate has the potential to provide a photonic crystal fibre that guides light over a significantly larger range of wavelengths than prior-art air-in-silica photonic crystal fibres.

A third optical fibre 200 (Fig. 3) , comprises a core region 220 and a cladding region 230. The cladding region 230 is formed of a triangular-lattice arrangement of elongate holes 260 embedded in a matrix of silica 250. In this case, there are no interstitial holes between holes 260, as the gaps between tubes forming holes 260 and silica matrix 250 have collapsed during drawing. In this embodiment, core 220 comprises seven elongate silicon regions 240.

The presence of the silicon regions 240 in the core region 220 and holes 260 in cladding region 230 causes the core region 220 to have an effective refractive index that is significantly higher than the effective refractive index of the cladding region 230. Light is therefore guided in the core region 220 by total internal reflection.

The holes 260 are of circular cross-section of radius 0.8 microns and are arranged, in the fibre's transverse cross-section, in a triangular lattice pattern having a pitch (i.e. nearest neighbour separation distance) of 1.7 microns. The silicon rods in core region 220 have a transverse dimension of 0.2 microns; such a small size reduces absorption losses in the fibre.

A fourth fibre 300 (Fig. 4) , comprises a core region 320 and a cladding region 330 comprising concentric rings 340, 350. Rings 340 are of silicon and have a thickness of 100 nm. Rings 350, which separate rings 340, are of silica and have a thickness of 1 micron.

The concentric-ring structure of fibre 300 benefits particularly from the use of silicon, which, being a semiconductor, has a high refractive index. The structure provides strong confinement of light to core region 320 over a wide range of wavelengths .

A method of making a fibre such as fibre 110 is shown in Fig. 5. In the first illustrated step (Fig. 5(a)), borosilicate tubes 400 are bundled together to form a triangular lattice pattern. The tubes are placed inside a large jacketing tube 410. The tubes 400 and jacket 410 are fused and drawn (on a standard fibre drawing tower) into a preform 420 (Fig. 5(b)) . The preform 420 has a diameter of about 5 mm and the holes of tubes 400 form elongate holes 430 embedded in a borosilicate matrix 440 in the preform 420.

In the third illustrated step of the method (Fig. 5(c)), copper wires 450 of radius 100 microns are inserted into each of the holes 430 except the central hole 460 in the preform 420. (Of course, wires of any suitable metal or semiconductor can be used. )

In the final step of the method illustrated in Fig. 5 (Fig. 5(d)), preform 420 is secured in the fibre drawing tower. A radio-frequency electromagnetic wave generator 500 is used to melt the copper wires 450 by irradiation through the preform 420. The hot molten copper formed from wires 450 softens the borosilicate matrix 440 of preform 420; however, the total volume of susceptible metal is too low too sufficiently soften all of the glass volume and carbon- dioxide laser light 510 is used to further soften the borosilicate 440 until it is sufficiently soft to be drawn into fibre 10, in the usual way onto drum 520.

The method shown in Fig. 5 makes a fibre such as fibre 110, but the method may readily be adapted to make other fibres according to the invention. For example, fibre 10 may be made by inserting wires of silicon between silica tubes at step (a) of the method of Fig. 5. Fibre 200 may be made by inserting silicon wires into silica tubes and then drawing the filled tubes down to form canes having one-seventh the cross-sectional area of the tubes forming the cladding region 230. Seven such canes then take the place of one tube in a preform, similar to preform 420, used to form fibre 200.

Fibre 300 may be made using a Modified Chemical Vapour Deposition (MCVD) lathe, by alternately depositing, on the inside of a tube, layers of silica soot (deposited by known methods) and layers of silicon. The silicon layers are formed directly on the inside of the tube using vapour deposition from an electrically-heated crucible (as in Fig. 7) or by carrying silicon vapour, using nitrogen as a carrier gas, from a flame-heated bulb (which contains silicon) at an end of the tube .

In other examples of methods according to the invention (Figs. 6 and 7), a metal or semiconductor is introduced into a silica tube, which is then included in a bundle of tubes and/or rods that is drawn into a fibre.

In one example (Fig. 6) , a crucible 640 containing solid silicon is suspended in a relatively large diameter silica tube 630 (sufficiently large to receive the crucible) . The tube 630 is placed on a standard fibre drawing tower and heated, by furnace elements 610, and drawn, by drawing mechanism 620.

Heating by furnace 610 produces silicon vapour 650 from the silicon in furnace 640. Part of vapour 650 is deposited on tube 630 in a deposition zone 660 at the point at which tube 630 necks during drawing. Element 600, drawn from tube 630, therefore comprises a silica outer region and a silicon inner region (depending on the rate of vaporisation of silicon and the rate of drawing of element 600, element 600 may be a solid rod or a tube still containing some air at its centre) .

Drawn element 600 is then cut into a number of shorter elements, which are bundled, together with capillary tubes, to form a preform which is then drawn into an optical fibre. The deposited silicon melts during that second draw and flows to form elongate, solid silicon regions in the drawn fibre. In an alternative deposition method (Fig. 7) , a silica tube 700 (of a length and diameter suitable for direct inclusion in a preform bundle) is held in a glass lathe 710. A crucible 720 containing silicon is inserted into tube 700 and heated electrically. Meanwhile, gas 730 is passed along tube 700 and it draws silicon vapour off crucible 720. The gas and silica vapour then exits tube 700 as exhaust gases 740. However, some of the silicon is deposited on the walls of tube 700 between crucible 720 and the end of tube 700 from which gases 740 exhaust. When a desired amount of silicon has built up in the tube 700, tube 700 is bundled into a preform and a fibre including the silicon is drawn as described above . An example of an optical device including a fibre according to the invention is the device of Fig. 8. A standard Erbium-doped fibre laser 810 is spliced at splice 820 to a fibre 800 that includes InP in its core. Light generated in laser 810 passes through fibre 800, is reflected at mirror 830, and passes back into laser 810. Fibre 800 acts as a saturable absorber, due to the presence of InP in the core. Thus, fibre 800 exhibits an absorption that flattens off when the intensity of propagating light reaches a certain level. As is well known, such a device can be used to modelock a laser to produce ultrashort pulses.

Claims

1. An optical fibre comprising a core region and a cladding region, characterised in that the fibre includes a metallic or semiconducting region in the core region or the cladding region.

2. A fibre as claimed in claim 1, in which the metallic or semiconducting regions are embedded in an insulating matrix material .

3. A fibre as claimed in claim 1 or claim 2, in which the cladding region comprises a plurality of the metallic or semiconducting regions.

4. A fibre as claimed in any preceding claim, in which the core region comprises a semiconductor region that provides gain.

5. A fibre as claimed in any preceding claim, comprising a semiconductor region that is a saturable absorber.

6. A fibre as claimed in any preceding claim, comprising a semiconductor region that is switchable between a first state in which it is transparent to propagating light and a second state in which it is not transparent to the light.

7. A fibre as claimed in any preceding claim, comprising a semiconductor region that acts as a dielectric with a large refractive index.

8. A fibre as claimed in any preceding claim, in which the core region has a higher effective refractive index than the cladding region.

9. A fibre as claimed in any of claims 1 to 6, in which the core region has a lower effective refractive index than the cladding region.

10. A fibre as claimed in claim 9 when dependent on any of claims 1 to 3, 5 or 6, in which the core region is an elongate hole in the fibre.

11. A fibre as claimed in any preceding claim, which is a photonic crystal fibre.

12. A fibre as claimed in claim 11, in which the cladding region comprises elongate regions of a first insulating material having a first refractive index embedded in a second insulating material having a second refractive index.

13. A fibre as claimed in claim 11 or claim 12, in which the metallic or semiconducting regions form a photonic crystal structure in the cladding region.

14. A fibre as claimed in claim 13, in which the metallic or semiconducting regions are elongate along the direction of the longitudinal axis of the fibre.

15. A fibre as claimed in any of claims 1 to 9, in which the metallic or semiconducting regions are arranged in the cladding region in concentric rings about the core region.

16. A fibre as claimed in claim 15, in which the concentric rings comprise alternating rings of metal or semiconductor and of insulating material.

17. A method of manufacturing an optical fibre, comprising drawing an optical fibre from a preform, characterised in that the method also comprises the steps: (a) including a metal or semiconductor in the preform; and

(b) melting the metal or semiconductor after it is included in the preform.

18. A method as claimed in claim 17, in which the preform comprises a plurality of tubes.

19. A method as claimed in claim 18, in which the metal or semiconductor is introduced into an interstitial hole between a plurality of the tubes.

20. A method as claimed in any of claims 17 to 19, in which the preform comprises a plurality of elongate holes.

21. A method as claimed in any of claims 17 to 20, in which the metal/semiconductor is in a hole in the preform or in a tube.

22. A method as claimed in claim 21, in which the metal/semiconductor is in powder form.

23. A method as claimed in claim 21, in which a wire of the metal or semiconductor is introduced into the hole.

24. A method as claimed in claim 23, in which the wire is fed into the hole at a higher rate than the rate at which the preform or tube is drawn.

25. A method as claimed in any of claims 21 to 24, in which the metal or semiconductor is introduced into the hole by deposition from a gas.

26. A method as claimed in claim 25, in which the gas is produced from a container held inside the preform or tube.

27. A method as claimed in claim 26, in which the gas is produced from a container suspended at the necking point of the preform or tube during drawing.

28. A method as claimed in any of claims 17 to 27, in which the metal or semiconductor is melted whilst the fibre is being drawn.

29. A method as claimed in any of claims 17 to 28, in which the metal or semiconductor is introduced into a hole in the preform after the preform has been partially drawn.

30. A method as claimed in any of claims 21 to 28, in which the metal or semiconductor is introduced into the hole before the preform is drawn.

31. A method as claimed in any of claims 21 to 30, in which the metal or semiconductor is melted using radio-frequency electromagnetic radiation.

32. A method as claimed in any of claims 21 to 31, in which the material is softened prior to drawing using a laser.

33. A transmission line for carrying data between a transmitter and a receiver, the transmission line including along at least part of its length a fibre as claimed in any of claims 1 to 16.

34. Data conditioned by having been transmitted through a fibre as claimed in any of claims 1 to 16.