GB2387666A

GB2387666A - Optic fibre core with interconnected elongate elements

Info

Publication number: GB2387666A
Application number: GB0209000A
Authority: GB
Inventors: Brian Joseph Mangan; Jonathan Cave Knight; Timothy Adam Birks; Philip St John Russell
Original assignee: Blazephotonics Ltd
Current assignee: Blazephotonics Ltd
Priority date: 2002-04-19
Filing date: 2002-04-19
Publication date: 2003-10-22
Also published as: GB0209000D0

Abstract

A system for propagating pulsed optical signals comprises one or more chromatically dispersive optical component(s) and a dispersion-compensating optical fibre 110. The dispersion-compensating optical fibre 110 has a cladding region 130 and a core region 120. The core region 120 has a plurality of interconnected, elongate elements 150, each having a shortest transverse dimension of less than one micron, and a plurality of elongate holes 170 that are arranged between the elongate elements 150, and which take up 50% or more of the cross-sectional area of the core region. The dispersion-compensating optical fibre has a length selected to compensate chromatic dispersion in the optical component(s). The system may include optic fibres and elongate air holes which surround the core region 120. The cladding region may be photonic so that light is confined to the core region 120 by photonic band-gap guidance.

Description

1 2387666

Improvements in and relating to optical fibres This invention relates to the field of optical fibres.

Optical fibres are important components of several 5 technologies, in particular telecommunications technology.

Optical fibres are usually made entirely from solid materials such as glass, and each fibre usually has the same cross-

sectional structure along its length. Transparent material in one part (usually the middle) of the cross-section has a 10 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, lS incorporating a controlled concentration of dopant, and have a circular outer boundary that is typically of diameter 125 microns. A problem with many systems for propagating pulsed optical signals is that different wavelength components of a 20 single pulse typically travel at different speeds in the optical system. That phenomenon, known as "chromatic dispersion" or just "dispersion", can cause pulse distortion and even break-up. In standard telecomms fibre, dispersion per km is low but, because very long lengths of fibre are 25 used, dispersion effects can accumulate and become problematic over those distances. In other systems, dispersion effects may be significant even over short propagation distances.

The effects of dispersion can be countered in a number of ways. A particularly important approach is to arrange the 30 system so that the propagating pulse propagates through a medium that exhibits dispersion of the opposite sign to that of the principal source of dispersion in the system. For example, light at 1.55 microns propagating through standard telecomms fibre experiences a dispersion of about + 20 ps nm 3S kml. (Dispersion having a positive sign is called positive dispersion or anomalous dispersion; dispersion having a

negative sign is called negative or normal dispersion.) Commercially available dispersion-compensating fibre has a dispersion of about -100 ps nm1 kml. Thus propagating a pulse through one unit length of dispersioncompensating fibre for 5 every five unit lengths of standard fibre it has passed through will provide a net dispersion of approximately zero.

Recently a new type of optical fibre has been developed known as a photonic crystal fibre (PCF), also known as a microstructured fibre or a holey fibre.

10 PCFs are fibres having a cladding region that comprises a plurality of elongate regions, running parallel to the longitudinal axis of the fibre, that are of a different refractive index from a matrix region in which they are embedded. The elongate regions are, in many cases, air-filled 15 holes, although they are in some cases solid regions or regions filled with a liquid or another gas.

The core of a PCF is a region having a different structure from the cladding region; it is often a region having no holes or a region having one or more extra holes.

20 Light is confined to the core of a PCF by the cladding through the action of one of two mechanisms. The first is closely related to the guidance mechanism of a standard fibre.

In this mechanism, the matrix regions and the elongate regions of the cladding have an 'effective' refractive index that is 25 less than the refractive index of the core region, so that total internal reflection occurs and traps light in the core.

(The 'effective' refractive index of the cladding region can readily be calculated by a person skilled in the art; its value will be between that of the elongate regions and that of 30 the matrix regions.) In the second mechanism, the arrangement of elongate regions in the cladding is periodic such that they exhibit a photonic band gap. (This phenomenon is analogous to the formation of electronic band gaps in semiconductors.) 35 Interference between light reflected from the elongate regions is such that there are certain bands of frequencies and

propagation constants that cannot propagate in the cladding.

The core of a PCF that guides by this mechanism forms a defect' in the periodic structure of the cladding; light can propagate in this defect region. Light is thus confined to s and propagates in the core of the PCF.

A number of other fibre structures have been proposed that include cladding regions that are comprised at least in part of air. For example, P. Kaiser and H.W. Astle, "Low-loss single-material fibers made from pure fused silica", Bell 10 System Tech J., vol. 53, pp 1021-1039 (1974) describes a silica fibre having a solid silica core, of diameter 25 microns, suspended in an air-filled region from a silica tube by a pair of thin webs. European Patent Application No. 0905834 (Lucent technologies, Inc.) describes a fibre having 15 multiple cladding regions, including a first outer cladding region, that comprises a plurality of large, elongate air holes, and that surrounds and shields an inner fibre, comprising an inner cladding region and a core region, from variations in a second outer cladding region.

20 United States Patent No. 5,155,792 (Hughes Aircraft Company) describes a fibre that guides light by total-internal reflection and which comprises a cladding region comprising a plurality of elongate holes and a core region that may also comprise a plurality of elongate holes and has a higher 25 effective refractive index than the cladding region. The Patent states at lines 40 to 45 of column 4, "if the dimensions of the passageways P' are chosen to be less than the shortest light wavelength of interest then the effective index of refraction of the cladding 130 will correspond to a 30 weighted average of the indices of refraction of air and of the medium of the rod structures 140"; thus the Patent teaches that, if the scale of the microstructure is smaller than the wavelength of light, an average effect of the microstructure is seen and the material can be treated as being homogeneous.

35 A similar fibre is described in International Application No. PCT/USO1/18444 (Gazillion Bits, Inc.).

An object of the invention is to provide improved ways of dispersing light and improved dispersion-compensation devices.

According to the invention there is provided a system for propagating pulsed optical signals, the system comprising one 5 or more chromatically dispersive optical component(s) and a dispersion-compensating optical fibre, the dispersion-

compensating optical fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected, elongate elements, each having a shortest 10 transverse dimension of less than one micron, and a plurality of elongate holes that are arranged between the elongate elements and which take up 50% or more of the cross-sectional area of the core region, the dispersion-compensating optical fibre having a length selected to compensate chromatic 15 dispersion in the optical component(s).

We have discovered that the approach taken in United States Patent No. 5, 155,792 (Hughes Aircraft Company) is over-

simplified. Although a microstructured material may be treated as being homogenous, as is done in that patent, and 20 only average properties considered, a fibre in which the core region comprises a plurality of elements each having a shortest transverse dimension of less than one micron and a plurality of elongate holes that are arranged between the elongate elements has interesting and useful dispersion 25 properties that result from the inhomogeneity of the structure. Specifically, light confined to the core of such a structure tends to concentrate in the elongate elements and avoid the holes. However, the degree to which the light is confined to the elongate elements varies with wavelength.

30 Longer wavelengths are less well confined to the elements and spread further into the holes than shorter wavelengths. The local effective refractive index experienced by the light thus varies; it is lower for longer wavelengths than for shorter.

This variation in effective refractive index can cause the 35 fibre to exhibit negative (normal) dispersion. We have found that this dispersion is very strong.

The invention may be better understood from the theoretical discussion of T.A. Birks, D. Mogilevtsev, J.C.

Knight and P. St.J. Russell, 'Dispersion Compensation Using SingleMaterial Fibres' IEEE Photonics Technology Letters, s Vol. 11, No. 6, pp 674- 676 (1999); that paper is hereby incorporated by reference into this specification. In the

paper, Birks et al. describe the dispersion properties of a single rod, surrounded entirely by air, of circular cross section and of diameter 0. 98 microns.

10 Preferably, the interconnected elongate elements have a shortest transverse dimension of less than 0.8 microns, more preferably less than 0.6 microns. Preferably, the holes take up more than 60% of the crosssection of the core region, more preferably more than 80, still more preferably more than 90.

15 Preferably, one of the chromatically dispersive optical component(s) is an optical fibre. More preferably, that optical fibre is a long-haul telecomms fibre.

The interconnected elongate elements in the core region may be interconnected elongate rods. Preferably, the elongate 20 rods are interconnected by thin webs. Preferably, the thin webs are sufficiently thin that they do not have any substantial effect on the dispersion properties of the fibre.

We have considered the dispersion properties of other sub-micron structures, including a slab (that is a rod of 25 rectangular crosssection) and have discovered that such structures also exhibit strong anomalous dispersion. For example, in the case light with a wavelength of 1550 nm in a single slab of silica surrounded by air, one finds that a 0. 3 micron thick slab provides a dispersion of -650 ps nm1 km1 for 30 both polarizations (TE and TM) and a 0.5 micron thick slab provides a dispersion of -1050 ps nm1 km1 for one polarization (TM). The interconnected elongate elements in the core region may be interconnected elongate slabs. Preferably, the as elongate slabs are directly interconnected.

A source of strong dispersion has many potential applications in systems for propagating pulsed optical signals. The system for propagating pulsed optical signals may be any system in which chromatic dispersion can be a s problem for propagating pulses. For example, the system may be a longhaul telecommunications system. Alternatively, the system may be an optical device, such as a laser or an amplifier. The chromatically dispersive optical component(s) may comprise at least one optical fibre. More preferably, the 10 optical fibre exhibits anomalous dispersion at a signal wavelength (for example, 1550 nm) and the compensating optical fibre exhibits normal dispersion at that wavelength. Still more preferably, the optical fibre is standard telecomms fibre. Preferably, the compensating optical fibre exhibits a 15 normal dispersion more negative than -100 ps nml km1, more preferably more negative than -200 ps nm1 kml, still more preferably more negative than-500 ps nm1 kml.

Any suitable cladding region may be provided. The fibre may guide by total internal reflection between the cladding 20 region and the core region (i.e. the cladding region may have a lower effective refractive index than the core region). For example, the cladding region may comprise an elongate air hole which substantially surrounds the core region. Thus, the core region may be suspended in the cladding region by a small 25 number of webs (e.g. 2 to 12 webs). Alternatively, the cladding region may comprise a plurality of elongate holes such that the cladding region has a lower effective refractive index than the core region.

Alternatively, the cladding region may be a solid region 30 of uniform refractive index that is lower than the effective refractive index of the core region.

Alternatively, the cladding region may be a photonic crystal such that light is confined to the core region by photonic band-gap guidance.

35 Also according to the invention there is provided a method of compensating chromatic dispersion in an optical

system, comprising propagating light in an optical fibre, the optical fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected, elongate elements, each having a shortest transverse dimension s of less than one micron, and a plurality of elongate holes that are arranged between the elongate elements and which take up 50% or more of the cross-sectional area of the core region.

Also according to the invention there is provided use of a fibre to disperse light chromatically, the fibre comprising 10 a cladding region and a core region, the core region comprising a plurality of interconnected, elongate elements, each having a shortest transverse dimension of less than one micron, and a plurality of elongate holes that are arranged between the elongate elements and which take up 50% or more of 15 the cross-sectional area of the core region.

Also according to the invention there is provided a dispersion compensator comprising an optical fibre, the optical fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected, 20 elongate elements, each having a shortest transverse dimension of less than one micron, and a plurality of elongate holes that are arranged between the elongate elements and which take up 50% or more of the cross-sectional area of the core region.

As discussed above, the elements may be slabs.

25 Preferably, the slabs are directly interconnected.

Alternatively, the elements may be rods. Preferably, the rods are interconnected by webs.

Also according to the invention there is provided a method of manufacturing a dispersive optical fibre, the method 30 comprising providing a bundle comprising a plurality of tubes and drawing the bundle into a fibre, the method including the step of changing the pressure in at least some of the tubes to produce a drawn fibre comprising a cladding region and a core region, the core region comprising a plurality of 3s interconnected, elongate elements, each having a shortest transverse dimension of less than one micron, and a plurality

of elongate holes that are arranged between the elongate elements and which take up 50% or more of the cross-sectional area of the core region and result from the tubes.

The holes are defined by a plurality of tubes in the 5 preform and the interconnected elements may be a plurality of elongate slabs that result from material from the tubes.

Alternatively, the holes are defined by a plurality of tubes in the preform and the preform further comprises a plurality of canes and the interconnected elements are a 10 plurality of rods resulting from the plurality of canes.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 is an example of a telecommunications system 15 according to the invention; Fig. 2 is a cross-sectional view of (a) a first optical fibre for use in the telecommunications system of Fig. 1 and (b) a preform for drawing the fibre of (a); Fig. 3 is a cross-sectional view of (a) a second optical 20 fibre for use in the telecommunications system of Fig. 1 and (b) a preform for drawing the fibre of (a).

A system for propagating pulsed optical signals is provided in the form of a long-haul telecommunications system (Fig. 1). The general form of such systems is widely known 25 and typically comprises a transmitter 10 for generating pulsed optical signals, a receiver 50, positioned several tens or even hundreds of kilometres from the transmitter 10, for receiving the signals and lengths of standard telecomms optical fibre 20, 30, 40 which connects the transmitter 10 30 from the receiver 50. Standard telecomms fibre is of high optical quality but, when signals are transmitted over many kilometres, loss and dispersion accumulates. (The loss of typical telecomms fibre is about 0.2 dB km1 and the dispersion is about 20 ps nm1 kml.). Losses and dispersion would cause 35 unacceptable deterioration of the transmitted signal for

transmission over long distances and so repeaters 60, 70 are provided at intervals between fibres 20, 30, 40.

Each repeater comprises a preamplifier 80, a dispersion compensation device 90, and an amplifier 100. A signal 5 transmitted along fibre 20 from transmitter 10 is amplified by the preamplifier 80. Dispersion compensation device 90 disperses the signal by an amount equal in magnitude but opposite in sign to the dispersion of fibre 20.

Dispersion compensation device 90 comprises a relatively 10 short length (compared with the length of fibres 20, 30, 40) of dispersioncompensating fibre 110 (Fig. 2(a)). Fibre 110 comprises a core region 120, a cladding region 130 and a jacket region 140.

Jacket region 140, which surrounds the cladding region 15 130, is made of solid silica.

Core region 120 comprises nineteen elongate silica rods 150, of circular cross-section, which are arranged in a triangular lattice pattern. The rods each have a diameter of less than one micron. Each rod 150 is connected to its 20 neighbours by thin silica webs 160. Rods 150 and webs 160 define elongate holes 170.

Light propagating in the fibre 110 is confined to the core region 120 by cladding region 130. Cladding region 130 is a hexagonal lattice formed of silica webs 180, which define 25 holes 190. The presence of solid rods 150 in the core region results in the effective refractive index of the core region 120 being higher than that of cladding region 130 and so light is generally confined to the core region 120 by total internal reflection. 30 Fibre 110 is highly dispersive because of the sub-micron scale internal structure of its core region 120. Light propagating in the fibre is confined to the vicinity of silica rods 150 but, because the rods are so small, the light spreads out into the air holes 170 surrounding each rod 150. However, 35 the light is not distributed sufficiently evenly across the whole core region 120 for core region 120 to be treated as a

homogeneous region. Rather, the partial localization of light to rods 150 results in micron scale variations in the light mode in the fibre core 120. Webs 160 are sufficiently narrow relative to rods 150 to be ignored to a first approximation.

5 The microstructure of core 120 results in fibre 110 being highly dispersive. Light of longer wavelengths is less well confined to the silica rods 150 than light of shorter wavelengths. Thus, broadly speaking, the refractive index seen' by longer wavelengths is closer to that of air (i.e. 10 unity) than the refractive index seen by shorter wavelengths.

This variation is such that light signals propagating in fibre 110 thus experience normal dispersion. The length of fibre 110 is chosen to provide sufficient normal dispersion to compensate for the anomalous dispersion of fibre 20; fibre 110 15 is tens of times shorter than fibre 110 because it is tens of times as dispersive.

Fibre 110 is readily manufactured from the preform 210 of Fig. 2(b). A bundle of canes 250 and capillary tubes 260, 265 is provided within a large diameter tube 240. Canes 250 and 20 tubes 260 are arranged to form the core of the final fibre 110. Tubes 265 are arranged around canes 250 and tubes 260 to form the cladding region of the final fibre 110. Because the tubes 260, 265 and the cane 250 are of circular cross-section, interstitial holes 220 exist between them where they do not 25 tile perfectly. The canes 250, tubes 260, 265 and tube 250 are heated and fused together to form the preform 210. Tubes 260, 265 are pressurized. Interstitial holes 220 are left open to the atmosphere. Fibre 110 is then drawn from preform 210 on a drawing rig, in the usual way that a standard fibre 30 is drawn from a preform. During the drawing process, interstitial holes 220 collapse and the holes at the centre of tubes 260, 265 expand so that webs 160, 180 are formed from the silica of adjacent tubes.

In an alternative arrangement of the telecommunications 3s system, dispersion compensation device 90 in repeater 60 compensates for the dispersion of half of fibre 30 (pre

propagation compensation) as well as for that of fibre 20 (postpropagation compensation); similarly, dispersion compensation device 90 in repeater 70 compensates for half of fibre 30 and all of fibre 40.

5 In an alternative embodiment of the invention, dispersion compensation device 90 has the structure shown in Fig. 3 (a).

Fibre 310 has core region 320, cladding region 330 and jacket region 340. Core region 320 has no rods 150 but instead has interconnected slabs 360, which are arranged on a hexagonal 10 lattice and define elongate holes 370. Slabs 360 are larger than webs 380 but are still smaller than one micron.

Cladding region 330 comprises longer and thinner slabs 380, which define larger air holes 390. Again, the refractive index of the cladding region 330 is lower than that of the 15 core region 320 and so light is confined to the core region 320 by total internal reflection.

Fibre 310 provides normal dispersion by the same mechanism as fibre 110, but in this case light is guided in the vicinity of slabs 360 (in contrast, webs 160 were too 20 narrow to guide significant amounts of light in fibre 110). A variation with wavelength in the overlap of the guided mode with air regions 370 provides normal dispersion in a similar manner to the behaviour of light guided in the rods 150 of fibre 110.

25 Fibre 310 is drawn from a preform 410 (Fig. 3 (b)) in a similar manner to the drawing of fibre 110 from preform 210, described above. Preform 410 comprises a bundle of tubes 460, 465, separated by interstitial holes 420. In preform 210, tubes 460 are thicker than tubes 465.

Claims

Claims

1. A system for propagating pulsed optical signals, the system comprising one or more chromatically dispersive optical 5 component(s) and a dispersion-compensating optical fibre, the dispersion-compensating optical fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected, elongate elements, each having a shortest transverse dimension of less than one micron, and a 10 plurality of elongate holes that are arranged between the elongate elements, and which take up 50% or more of the cross-

sectional area of the core region, the dispersion-compensating optical fibre having a length selected to compensate chromatic dispersion in the optical component(s).

15 2. A system as claimed in claim 1, in which the chromatically dispersive optical component(s) comprises at least one optical fibre.

3. A system as claimed in claim 2, in which at least one optical fibre exhibits anomalous dispersion at a signal 20 wavelength and the compensating optical fibre exhibits normal dispersion at that wavelength.

4. A system as claimed in claim 3, in which at least one optical fibre is standard telecomms fibre.

5. A system as claimed in any of claims 1 to 4, in which the 25 cladding region comprises an elongate air hole which substantially surrounds the core region.

6. A system as claimed in any of claims 1 to 4, in which the cladding region comprises a plurality of elongate holes such that the cladding region has a lower effective refractive 30 index than the core region.

7. A system as claimed in any of claims 1 to 6, in which the cladding region is a solid region of uniform refractive index that is lower than the effective refractive index of the core region.

8. A system as claimed in any of claims 1 to 4, in which the cladding region is a photonic crystal such that light is confined to the core region by photonic band-gap guidance.

9. A system as claimed in any of claims 1 to 8, in which the s interconnected elongate elements in the core region are interconnected elongate rods.

10. A system as claimed in claim 9 in which the elongate rods are interconnected by thin webs.

11. A system as claimed in any of claims 1 to 8, in which the 10 interconnected elongate elements in the core region are interconnected elongate slabs.

12. A system as claimed in claim 11, in which the elongate slabs are directly interconnected.

13. A system as claimed in any of claims 1 to 12, which is a 15 long-haul telecommunications system.

14. A system as claimed in any of claims 1 to 12, which is an optical device.

15. A method of compensating chromatic dispersion in an optical system, comprising propagating light in an optical 20 fibre, the optical fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected, elongate elements, each having a shortest transverse dimension of less than one micron, and a plurality of elongate holes that are arranged between the elongate 25 elements and which take up 50 or more of the cross-sectional area of the core region.

16. Use of a fibre to disperse light chromatically, the fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected, elongate 30 elements, each having a shortest transverse dimension of less than one micron, and a plurality of elongate holes that are arranged between the elongate elements and which take up 50% or more of the cross-sectional area of the core region.

17. A dispersion compensator comprising an optical fibre, the 3s optical fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected,

elongate elements, each having a shortest transverse dimension of less than one micron, and a plurality of elongate holes that are arranged between the elongate elements and which take up 50% or more of the crosssectional area of the core region.

5 18. An optical fibre as claimed in claim 17, in which the elements are slabs.

19. An optical fibre as claimed in claim 18, in which the slabs are directly interconnected.

20. An optical fibre as claimed in claim 19, in which the 10 elements are rods.

21. An optical fibre as claimed in claim 20, in which the rods are interconnected by webs.

22. A method of manufacturing a dispersive optical fibre, the method comprising providing a preform comprising a plurality 15 of holes and drawing the preform into a fibre, the method including the step of changing gas pressure in at least some of the holes to produce a drawn fibre comprising a cladding region and a core region, the core region comprising a plurality of interconnected, elongate elements, each having a 20 shortest transverse dimension of less than one micron, and a plurality of elongate holes that are arranged between the elongate elements and which take up 50% or more of the cross-

sectional area of the core region and result from the pressurized tubes.

25 23. A method as claimed in claim 22, in which the holes are defined by a plurality of tubes in the preform and the interconnected elements are a plurality of elongate slabs that result from material from the tubes.

24. A method as claimed in claim 22, in which the holes are 30 defined by a plurality of tubes in the preform and the preform further comprises a plurality of canes and the interconnected elements are a plurality of rods resulting from the plurality of canes.

25. A method substantially as herein described with reference 35 to the accompanying drawings.

ED 26. A fibre substantially as herein described, with reference to the accompanying drawings.