GB2389915A

GB2389915A - Optic fibre with cladding region having rotational symmetry

Info

Publication number: GB2389915A
Application number: GB0214266A
Authority: GB
Inventors: Brian Joseph Mangan; Jonathan Cave Knight; Timothy Adam Birks; Philip St John Russell; Peter John Roberts
Original assignee: Blazephotonics Ltd
Current assignee: Blazephotonics Ltd
Priority date: 2002-06-20
Filing date: 2002-06-20
Publication date: 2003-12-24
Also published as: GB0214266D0

Abstract

An optical fibre (10), comprises a core region (40) and a first cladding region that, in a transverse cross-section of the fibre (10), is adjacent to and surrounds the core region (40). The first cladding region comprises a plurality of elongate elements (50,60) embedded in a matrix material (20). There is an azimuthal variation around the core region (40) of the elongate elements (50,60), and/or of the matrix material (20), in the first cladding region, such that the first cladding region has n-fold rotational symmetry, where n is at least 3. There may also be a second cladding layer (30), surrounding the first cladding region, but in which there is no azimuthal variation.

Description

1 2389915

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

Optical fibres are important components of several s 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, 15 incorporating a controlled concentration of dopant, and have a circular outer boundary that is typically of diameter 125 microns. Chromatic dispersion is the phenomenon of light signal components of different wavelengths travailing at different 20 group velocities. It is generally undesirable as it may cause pulse distortion and break up of signals. Although dispersion in standard telecomms fibre is relatively low, its effects can accumulate over the very long distances involved in many telecommunications applications and cause severe problems. In 25 order to avoid that, relatively short lengths of fibre exhibiting dispersion of the opposite sign to that of standard telecomms fibre are interspersed at intervals along the length of the standard fibre. The propagating light pulses thus pass along a system having a net dispersion close to zero.

30 Dispersion is said to be 'normal' when red wavelengths (i.e. longer wavelengths) travel faster than blue wavelengths (i.e. shorter wavelengths) and 'anomalous' when blue wavelengths travel faster than red wavelengths. Standard telecomms fibre exhibits anomalous dispersion of about + 20 ps 35 nml kml in the preferred telecomms window around 1550 nm.

The effects of dispersion can be countered in a number of ways. A particularly important approach is to arrange the system so that the propagating pulse propagates through a medium that exhibits dispersion of the opposite sign to that 5 of the principal source of dispersion in the system.

Commercially available dispersion compensating fibre has a dispersion of about -loo ps nml kml. Thus propagating pulses through one unit length of dispersion-compensating fibre for every five unit lengths of standard fibre it has passed 10 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.

PCFs are fibres having a cladding region that comprises a 15 plurality of elongate regions, running parallel to the longitudinal axis of the fibre, that are of a different refractive index from a matrix region in which they are embedded. The elongate regions are, in many cases, air-filled holes, although they are in some cases solid regions or 20 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 or different sized holes.

2s 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 30 less than the refractive index of the core region, so that total internal reflection occurs and traps light in the core.

"Effective refractive index" is a term well known in the field of photonic crystal fibres. The effective refractive

index of a cladding region consisting of elongate regions 35 embedded in a matrix material will be between the refractive index of the matrix material and the refractive index of the

material forming the elongate regions. Its actual value depends upon the mode distribution of the light guided in the fibre; its actual value may readily be calculated by persons skilled in the art. The calculation is based upon the method 5 of calculating effective refractive index that is described in T.A. Birks et al, Opt. Lett. Vol. 22, No. 13, pp 961-963 (1997). That paper describes the calculation for an infinite periodic structure; to calculate a local effective index in a real fibre, one merely takes the structure of the relevant 10 local region and extends it by tiling to fill an infinite plane. The method of Birks' paper may then be applied.

In the second mechanism, the arrangement of elongate regions in the cladding is periodic such that they form a photonic band gap. (This phenomenon is analogous to the 15 formation of electronic band gaps in semiconductors.) Interference between light reflected from the elongate regions is such that there are certain bands of frequencies that cannot propagate in the cladding. The core of a PCF that guides by this mechanism forms a 'defect' in the periodic 20 structure of the cladding; light can propagate in this defect region. Light is thus confined to and propagates in the core of the PCF.

The term 'photonic crystal fibre' reflects the historical roots of the structure of the fibres; the fibres were 25 developed with a view to demonstrating the band-gap guidance mechanism. However, we refer to all fibres having such elongate regions as photonic crystal fibres, even if they do not have band-gaps and guide by the first mechanism, index guiding. In particular, the term is not restricted to fibres 30 having periodic arrangements of elongate regions in their claddings. An object of the invention is to provide an improved method and apparatus for dispersing light and for dispersion compensation. 35 According to the invention there is provided an optical fibre, comprising a core region and a first cladding region

that, in a transverse cross-section of the fibre, is adjacent to and surrounds the core region, the first cladding region comprising a plurality of elongate elements embedded in a matrix material, characterized in that there is an azimuthal s variation around the core region of the elongate elements, and/or of the matrix material, in the first cladding region, such that the first cladding region has e-fold rotational symmetry, where n is at least 3.

We have discovered that provision of an azimuthal 10 variation in effective refractive index may provide more dispersion compared with a uniform azimuthal effective refractive index (of course, even for a uniform effective refractive index, the true refractive index will vary from point to point because of the presence of the elongate 15 elements in the matrix material). Because the fibre has at least a 3-fold rotational symmetry, it is not birefringent; that is advantageous in many applications.

Providing an azimuthal variation in the first cladding region provides an additional degree of freedom that can be 20 exploited by optimization of the fibre parameters to provide a fibre having a desired property, such as a particular dispersion or a particular dispersion slope. Compared with a fibre having a uniform arrangement of holes, a fibre having an azimuthal variation in a first cladding region (but the same 25 arrangement of holes elsewhere) may exhibit a larger dispersion and the exact azimuthal variation (for example, the exact sizes or shapes or material of the elongate elements) may be chosen in a particular embodiment to provide a particular dispersion slope. The parameters of the azimuthal 30 variation will usually be chosen by optimization of a computer model to provide a desired property in the final fibre r Thus, the fibre has additional flexibility of its design that may be exploited to compensate both dispersion and dispersion slope in optical systems. In contrast, prior art fibres do not

35 permit ready selection of both dispersion and dispersion slope.

( 5 The azimuthal variation in effective refractive index may be provided by any suitable means. For example, the azimuthal variation may result at least partly from a variation in the pitch of the arrangement of elongate elements. Preferably, 5 the azimuthal variation results at least partly from a variation in the cross-sectional area of the elongate elements. The azimuthal variation may result at least partly from a variation in the material of which the elongate elements or the matrix material is made.

10 Preferably, the elongate elements are holes.

Preferably, the first cladding region has three-fold rotational symmetry.

Preferably, the first cladding region is ring-shaped.

Preferably, the ring is a substantially circular ring.

15 Alternatively, the ring may be a substantially hexagonal ring.

The elongate elements may be arc-shaped. The arc-shaped elements may subtend an angle of 30 or more about the centre of the core region.

Preferably, the elongate elements have been selected to 20 provide dispersion compensation for an optical system.

Alternatively, the elongate elements have been selected to provide dispersion slope compensation for an optical system.

Selection of elongate elements that provide dispersion compensation and/or dispersion-slope compensation may readily 25 be achieved by optimizing parameters describing the elongate elements in a computer model of the dispersion properties of the fibre. Computer models of prior art photonic crystal

fibres are known in the art and the skilled person, with the benefit of knowledge of the invention, would readily adapt 30 such models to provide simulation of the effects of choosing different element parameters (e.g. diameter, pitch, refractive index). Whether or not a particular fibre has a particular dispersion or dispersion slope may readily be determined by direct measurement.

35 Preferably, the fibre further comprises a second cladding region that is adjacent to and substantially surrounds the

first cladding region. The strongest effect on the dispersion properties of the fibre will be had by the first cladding region; that is by the region closest to the core region.

However, the arrangement of parts of the cladding further from S the core region may also affect the dispersion properties of the fibre and properties of those regions are therefore further parameters that may be optimized to provide a particular property Preferably, the second cladding region comprises elongate elements embedded in a matrix material.

10 Preferably, there is azimuthal variation around the first cladding region of the elongate elements, and/or of the matrix material, in the second cladding region, such that the second cladding region has m-fold rotational symmetry, where m is at least 3.

15 The variation may be different from the variation in the first region.

It may be that m equals n. At least some of the elongate elements of the second cladding region may be of a different cross-sectional area from any of the elongate elements in the 20 first cladding region.

Preferably, the elongate elements of the second region are in the same arrangement as the elongate elements in the first region except that at least some elongate elements in corresponding positions in each region are of a different 25 cross-sectional area.

There may be regions of the cladding still further out from the core that affect the properties of the fibre.

Preferably, the fibre further comprises one or more further cladding regions that each substantially surround an adjacent 30 cladding region. Preferably, the further cladding region(s) comprise elongate elements embedded in a matrix material.

Preferably, there is/are azimuthal variation(s) of the elongate elements, and/or of the matrix material, in the further cladding region(s) such that the effective refractive 35 index of each of the further cladding regions varies azimuthally around its adjacent region and such that the

( further cladding regions each have a rotational symmetry that is at least three-fold.

Provision of second or further cladding regions provides still further degrees of freedom in fibre design, facilitating s optimization for a particular fibre property or set of properties. Also according to the invention there is provided method of chromatically dispersing light, comprising propagating the light along a fibre as described above as being according to 10 the invention.

Also according to the invention there is provided a chromatic-dispersion compensation module, comprising a fibre as described above as being according to the invention.

Also according to the invention there is provided use of 15 a fibre, as described above as being according to the invention, for dispersing light.

Also according to the invention, there is provided a method of manufacturing an optical fibre comprising:(i) providing a computer simulation of dispersion in an optical 20 fibre; (ii) varying one or more parameters of the simulation until the simulation indicates that the fibre will exhibit a desired amount of dispersion and/or dispersion slope; and (iii) manufacturing the fibre having the parameters that the simulation indicates will provide the desired amount of 25 dispersion and/or dispersion slope.

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 transverse cross-section of a fibre according 30 to the invention; Fig. 2 is a contour plot of the mode distribution at 1.55 microns in the fibre of Fig. 1; Fig. 3 is a plot of dispersion D (in ps nm1 km1) in the fibre of Fig. 1 for wavelengths between 1.5 microns and 1.6 35 microns;

Fig. 4 is a transverse cross-section of a second fibre according to the invention (the scale is in microns).

Fibre 10 (Fig. 1) is a photonic crystal fibre comprising a cladding region formed from solid silica 20 and elongate 5 holes 30, which run parallel to the longitudinal axis of the fibre. Holes 30 in the cladding region are arranged on a triangular lattice having a pitch (nearest neighbour separation) of 1 micron Holes 30 have a diameter:pitch ratio of 0.75:1. A core region 40 is defined by the absence of a 10 hole from a lattice site near the centre of the fibre 10. A first cladding region is formed by six holes 50, 60, immediately adjacent to the core region 40, which are on sites of the triangular cladding lattice but are of a different diameter than the other holes 30 in the cladding. Holes 50 15 are of a larger diameter than holes 30 (diameter:pitch = 0.95:1) and holes 60 are of a smaller diameter than holes 30 (diameter:pitch = 0.65:1) . Holes 50 and 60 are arranged alternately around the core region 40, so that each hole 50 has two neighbours that are holes 60 and vice versa (each of 20 holes 50, 60 also has three neighbours that are holes 30; the sixth neighbouring site on the triangular lattice is the site of core 40, where there is no hole). Holes 30 form a second cladding region, surrounding the first cladding region, but in which there is no azimuthal variation.

25 Fibre 10 is manufactured in the way that photonic crystal fibres are usually manufactured. A preform is made by bundling a plurality of silica tubes around a solid silica cane and fusing the tubes and the cane together. The canes and the tubes are arranged on a triangular lattice that is a 30 large-scale version of fibre 10. The central holes of the tubes form holes 30 in the fibre 10 and the cane forms core 40. At the sites in the bundle corresponding to the sites in the fibre 10 of holes 50 and 60, tubes having the same external diameter as the other tubes and the cane, but having 35 larger and smaller bore diameters respectively, are provided.

Fibre 10 is drawn from the preform on a standard fibre-drawing

rig in the same way as a standard fibre is drawn from a preform. The dispersion properties of fibre 10 can be understood to a first approximation from the changes in the light 5 distribution 80 in a mode 70 guided in the fibre core 40 at shorter and longer wavelengths.

At shorter wavelengths, the light mode 70 is more tightly confined near the centre of core 40. The mode 70 has a rounded triangular shape, with light less well confined in the 10 directions of each of the three smaller holes 60 than in the directions of larger holes 50 (such that smaller holes 60 are in the direction of the vertices of the triangle).

At longer wavelengths, the mode 70 is generally more spread out and less well confined to fibre core 40 and there 15 is a significant amount of light spreading into holes 60; thus holes 60 confine light to the core region 40 less well.

At shorter wavelengths, the refractive index seen by mode 80 is close to that of silica, as the light is largely confined to silica regions. As the light spreads 20 significantly into holes 60 at longer wavelengths, the effective refractive index seen by mode 40 is reduced by the effect of the air-filed holes 60. Thus, longer wavelengths travel faster than shorter wavelengths because longer wavelengths see a lower refractive index. Moreover, fibre 10 25 exhibits a variation in group velocity with wavelength that results in normal dispersion, as is shown in Fig. 3, which shows dispersion D as a function of wavelength A. The dispersion D is normal (negative in sign) for all wavelengths between 1.5 microns and 1.6 microns. The 30 dispersion increases in magnitude from about -380 ps nml km at 1.5 microns to about -520 ps nml km1 at 1.6 microns. (In comparison, commercially available dispersion-compensating fibre exhibits a dispersion of about -100 ps nm1 kml.) The fibre thus provides a useful source of dispersion for 35 use, for example, in compensating the dispersion of standard telecomms fibre. The relative dispersion slope at 1.55

microns is 0.00335, which is appropriate for compensating SMF 28 (a commonly used telecomms fibre).

In other examples of embodiments of the invention, azimuthal variations are provided in regions further out from 5 core 40 than the inner cladding region of holes 50, 60.

In the fibre of Fig. 4, for example, core 140 is again surrounded by three largest holes 150, embedded in matrix material 120, which alternate azimuthally around core 140 with three smaller holes 160. However, outside the ring of holes 10 150, 160 there is a second ring, comprising six smallest holes 110 and six holes 130, which are of a similar size to, but slightly smaller than, holes 160 and which are larger than holes 110. Holes 110 and holes 130 alternate azimuthally around the core 140, forming a pattern having six-fold 15 symmetry. Outside the second cladding region comprising holes 110, 130, a third cladding region is provided, comprising holes 170 and holes 180. Holes 170 are larger than holes 160 but smaller than holes 150. Holes 180 are smaller then holes 130 but larger than holes 110. Three rings of holes 110, 130 20 are provided, with holes 110 and holes 130 alternating azimuthally within each ring. In this embodiment, all of the holes 130-180 are embedded in a uniform silica matrix 120.

However, in some other embodiments (not illustrated), the azimuthal variation is in the matrix material.

Claims

1. An optical fibre, comprising a core region and a first cladding region that, in a transverse cross-section of the 5 fibre, is adjacent to and surrounds the core region, the first cladding region comprising a plurality of elongate elements embedded in a matrix material, characterized in that there is an azimuthal variation around the core region of the elongate elements, and/or of the matrix material, in the first cladding 10 region, such that the first cladding region has e-fold rotational symmetry, where n is at least 3.

2. An optical fibre as claimed in claim 1, in which the azimuthal variation results at least partly from a variation in the cross-sectional area of the elongate elements.

15

3. An optical fibre as claimed in claim 1 or claim 2, in which the azimuthal variation results at least partly from a variation in the pitch of the arrangement of elongate elements.

4. An optical fibre as claimed in any preceding claim, in 20 which the azimuthal variation results at least partly from a variation in the material of which the elongate elements or the matrix material is made.

5. An optical fibre as claimed in any preceding claim, in which the first cladding region has three-fold rotational 2S symmetry.

6. An optical fibre as claimed in any preceding claim in which the first cladding region is ring-shaped.

7. An optical fibre as claimed in claim 6, in which the ring is a substantially circular ring.

30

8. An optical fibre as claimed in claim 6, in which the ring is a substantially hexagonal ring.

9. An optical fibre as claimed in any preceding claim, further comprising a second cladding region that is adjacent to and substantially surrounds the first cladding region.

10. An optical fibre as claimed in claim 9, in which the second cladding region comprises a plurality of elongate elements embedded in a matrix material.

11. An optical fibre as claimed in claim 10, in which there 5 is azimuthal variation around the first cladding region of the elongate elements, and/or of the matrix material, in the second cladding region, such that the second cladding region has m-fold rotational symmetry, where m is at least 3.

32. An optical fibre as claimed in claim 11, in which m 0 equals n.

13. An optical fibre as claimed in claim 11 or claim 12, in which at least some of the elongate elements of the second cladding region are of a different cross-sectional area from any of the elongate elements in the first region.

15

14. An optical fibre as claimed in any of claims 11 to 13, in which the elongate elements of the second cladding region are in the same arrangement as the elongate elements in the first cladding region except that at least some elongate elements in corresponding positions in each cladding region are of a 20 different cross-sectional area.

15. An optical fibre as claimed in any of claims 9 to 14, further comprising one or more further cladding region(s) that each substantially surround an adjacent cladding region.

16. An optical fibre as claimed in claim 15, in which the 25 further cladding region(s) comprise(s) a plurality of elongate elements embedded in a matrix material.

17. An optical fibre as claimed in claim 16, in which there is/are azimuthal variation(s) of the elongate elements, and/or of the matrix material, in the further cladding region(s) such 30 that the effective refractive index of each of the further cladding regions varies azimuthally around its adjacent region and such that the further cladding regions each have a rotational symmetry that is at least three-fold.

18. An optical fibre as claimed in any preceding claim, in 35 which the elongate elements are arc-shaped.

(

19. An optical fibre as claimed in claim 18, in which the arc-shaped elements subtend an angle of 30 or more about the centre of the core region.

20. An optical fibre as claimed in any preceding claim, in 5 which the elongate elements have been selected to provide dispersion compensation for an optical system.

21. An optical fibre as claimed in any preceding claim, in which the elongate elements have been selected to provide dispersion-slope compensation for an optical system.

lo

22. A method of chromatically dispersing light, comprising propagating the light along a fibre as claimed in any of claims 1 to 21.

23. A chromatic-dispersion compensation module, comprising a fibre as claimed in any of claims 1 to 21.

15

24. Use of a fibre according to any of claims 1 to 21 for dispersing light.

25. A method of manufacturing an optical fibre comprising: (i) providing a computer simulation of dispersion in an optical fibre according to any of claims 1 to 21; 20 (ii) varying one or more parameters of the simulation until the simulation indicates that the fibre will exhibit a desired amount of dispersion and/or dispersion slope; and (iii) manufacturing the fibre having the parameters that the simulation indicates will provide the desired amount of 25 dispersion and/or dispersion slope.

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

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