WO2004053550A1

WO2004053550A1 - Improvements relating to photonic crystal fibres

Info

Publication number: WO2004053550A1
Application number: PCT/DK2003/000843
Authority: WO
Inventors: Jes Broeng; Anders Petersson
Original assignee: Crystal Fibre A/S
Priority date: 2002-12-09
Filing date: 2003-12-09
Publication date: 2004-06-24
Also published as: AU2003302807A1

Abstract

A multi-mode optical fibre for guiding light, the optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising (a) a core region (10, 70, 74) for propagating the light to be guided in the longitudinal direction of the optical fibre; said core region having a minimum dimension (ρ) equal to or larger than 30 µm; (b) an inner cladding region, said inner cladding region surrounding said core region and comprising three or more voids or holes separated by a minimum distance (b) in the range from 2m to 10m, and being arranged in an inner cladding background material with refractive index n1; and (c) an outer cladding region with an outer cladding background material with refractive index n2, and a thickness (t), said thickness being equal to or larger than 10µm; said core region and said outer cladding region being connected by said inner cladding material; an optical fibre communication system or high power laser transmission system comprising such as fiber; a preform and method for its production. Further, an optical fiber for guiding light comprising a core region and a cladding region; said core region, and/or cladding region comprising ring-shaped regions or elements having refractive indices different from that of the background materials of said core region, and/or said cladding region.

Description

IMPROVEMENTS RELATING TO PHOTONIC CRYSTAL FIBRES

DESCRIPTION

1. BACKGROUND OF THE INVENTION

The present invention discloses photonic crystal fibres with large cores that may be used for delivery of high power light with good beam quality and/or for delivery of short pulses of light with little pulse spreading. The invention further relates to methods and preforms for making such fibres, and to systems or part thereof where such fibres are utilized. Further, the invention relates to photonic crystal fibres with effective index profiles that are accurately controlled using structuring of the core and/or cladding.

The Technical Field

Optical fibres are today used in numerous applications that span very diverse fields of optics. These fields include telecommunications, medicine, sensors, lasers, and many others .

Some of the first optical fibres were proposed in the 1960 's and 1970 's and were single material fibres (see for example US3434774; US3535017; US4046537; Kaiser et al., The Bell System Technical Journal, Vol. 52, No. 2, 1973; Marcatili, The Bell System Technical Journal, Vol. 53, No. 4, 1974; Kaiser et al . , The Bell System Technical Journal, Vol. 53, No. 6, 1974). Single material fibres were largely abandoned by the emergence of doped silica fibres, where a refractive index profile is defining a core and cladding in a solid fibre form. These solid fibres are typically made using silica-based glasses or polymers and are extensively used today. We shall refer to such solid fibres as ^λ standard optical fibres' .

The operation of the above-mentioned single material fibres is well described in the cited references - and the guiding mechanism is today known as a modified total internal reflection. Although largely abandoned during the 1980' s, a renaissance to single material fibres has recently appear by the emergence of so-called Photonic Crystal Fibres (PCF) (see e.g. Bjarklev, Broeng and Bjarklev, "Photonic Crystal Fibres", Kluwer Academic Press, 2003.). PCFs are also known as microstructured fibres, holey fibres, photonic bandgap fibres, hole- assisted optical fibres, single-material fibres - as well as other names may be used.

An important application area of optical fibres is high bandwidth optical fiber communication systems - typically for metro and/or local area networks (LAN) . For such applications, multi-mode fibres are typically used (see for example Agrawal, Fiber-Optic Communication systems, 2. edition, p. 29, Wiley-Interscience, 1997). Preferably, the multi-mode fibres have a graded index profile that is made in silica-based glasses or polymers. The graded index profile is commonly preferred in order to reduce inter-modal dispersion that is the main fibre parameter that limits the so-called Bandwidth-Length-product . The theory and design of graded-index profile for reducing inter-modal dispersion in standard optical fibres is well-described and understood in the art - see for example previous Agrawal-reference; Snyder and Love, Optical Waveguide Theory, Kluwer Academic Press, 1983 (for example pp. 55-61); or EP1199581. In EP1199581, White discloses Photonic Crystal Fibres (or microstructured fibres as White chooses to label the fibres) that has a graded effective refractive index profile in the core. White discloses a graded effective index profile that is obtained by arrangement of axially oriented elements (typically air holes/cylinders) in the core. The PCFs disclosed by White have a cladding region surrounding the core region, where the cladding region exhibits a refractive index less than the effective refractive index of the portion of the core immediately adjacent the cladding region, i.e., there exists an index step at the core/cladding interface. It is a disadvantage of the fibres disclosed by White that they have a cladding region that exhibits a refractive index less than the effective refractive index of the portion of the core immediately adjacent the cladding region. It is a further disadvantage of the fibres disclosed by White that the core and the cladding do not comprise solid material of similar refractive index. It is a further disadvantage of the fibres disclosed by White that they do not exhibit a low NA. It is a further disadvantage of the fibres disclosed by White that they do not disclose optical fibres with a reduced amount of solid material in the cladding region.

Other examples of optical fibres for reduced dispersion in multimode fibres include US3687514 and US3785718.

Apart from graded index profiles of the core region, it is also known from the prior art that scattering of modes

(such that coupling between bound/guided modes may take place) in a multi-mode optical fibre may be beneficial for increasing the bandwidth of an optical communication system - as for example Miller et al. US 3687514. The idea disclosed by Miller et al . is that the total energy of a pulse (such as a single bit in an optical signal) , which is distributed between several modes, may vary its distribution between the various modes of the fibre in the longitudinal direction of the fibre, such that the pulse travels at an average group velocity (the average being that of the group velocity of the individual modes) . This provides less pulse distortion than in the case of the energy traveling in non-coupled modes. Miller et al. discloses fibres or waveguides that comprises changes in the diameter of the fibres or waveguides to obtain such coupling/scattering. Miller et al. only discloses details about the optical fibres regarding the diameter of the fibres. Hence, it is a disadvantage that Miller et al . do not disclose other improvements with respect to bandwidth than varying the diameter of the optical fibre.

Further, it may be desired to provide multi-mode optical fibres with low NA. Lowering of the NA of an optical fibre, generally, reduces the number of modes that may be guided in the optical fibre. Hence, a reduced numerical aperture (NA) is advantageous in order to lower the number of modes - and thereby the inter-modal dispersion - of multi-mode optical fibres.

Another important application of optical fibres is transmission of high power laser light. Also for such applications, inter-modal dispersion it is desired to be reduced such that short pulses of high power may be transmitted. Hence, graded index profiles in the core region are also desired for high power applications. The graded index profile, however, has the disadvantage that a high intensity in the fibre core may occur (typically in the centre where the refractive index profile has a maximum) . This high intensity may increase beyond a damage threshold of the fibre - and cause fatal break down of the fibre. In particular, graded- index profile may result in self- focusing effects that further increase the risk of fibre damage. Apart from reducing the inter-modal dispersion, it is, therefore, important to provide optical fibre with as high damage threshold as possible. This requires use of materials with as low level of impurities as possible (as damage tend to takes place as impurity sites) , and fibre designs with a low local intensity as possible. In order to limit the level of impurities, it is desired to provide optical fibre, where doping of for example silica is completely avoided. In order to provide as low local intensity as possible, it is desired to utilize optical fibres with large cores. For large core, however, a larger number of modes are typically confined, and the local intensity within the fibre core may at certain spatial region become significantly higher than the average intensity in the core. Therefore, it is desired to provide optical fibres with large core and as low number of modes as possible (since lower-order modes have 'larger' spatial distribution of their peak intensity). In order to achieve low number of modes, it is desirable to provide fibre designs with low NA and large core size.

In US 5418882, Ortiz discloses an optical fibre for high power laser transmission. The optical fibre disclosed by Ortiz has a combined step-index and graded index profile - labeled a partially graded optical fibre. It is a disadvantage of the optical fibre disclosed by Ortiz that the core region comprises a part with a graded index profile. It is a further disadvantage that partially graded optical fibre comprises region of different material, such as anhydrous, fused silica and anhydrous, fused silica doped with fluorine, boron or germanium. It is "a further disadvantage of the fibre disclosed by Ortiz, that the core - and in particular a centre of the core - comprises material with a different refractive index than a material of the cladding.

WO 00/49436 discloses a photonic crystal fibre comprising very regular thin-walled capillary structures and a very small guiding core. A fibre having an inner cladding of 10 m and less than 0.5 m thin walls comprising capillaries expanded holes surrounding a guiding core of 1 m, and an outer jacket is disclosed.

US 6,529,676 discloses a waveguide incorporating a tunable scattering material such as a liquid crystal, the waveguide comprising a core region and a solid or liquid material having tunable scattering cladding elements randomly dispersed therein. Specifically, a microstructured optical fibre having a germanium-doped core of diameter 10 m surrounded by six capillary air holes of diameter 40 m for receiving the tunable scattering material and forming a ring around a silica region of 32 m surrounding the core is disclosed; the silica region of the inner part of the cladding surrounding the core being connected to the outer part of the cladding by cladding material between the capillary air holes.

WO 02/39159 discloses single mode microstructured optical fibres having a specifically designed cladding providing single-mode guiding and low bending losses. A fibre with a filling fraction of 18% having a core of 4 m or larger and a bridging width of 1.1 m or larger is disclosed.

2. DISCLOSURE OF THE INVENTION

It is an object to provide a system or part thereof for transmission of optical signals at high bit-rates. It is further object to provide multi-mode optical fibres that may guide light at a wavelength in the range from 800 nm to 1300 nm with low inter-modal dispersions, such that a high bandwidth-length product is obtained.

It is a further object of the present invention to provide multi-mode optical fibres that have low NA and a large core sizes.

It is an object of the present invention to provide multi-mode optical fibres for high bandwidth optical fibre communication systems where there exist at least three interfaces between a core and a cladding with no index step. It turns out that such interfaces may provide increase mode scattering that provides increase bandwidth of the fibres, for example through scattering of skew and/or meridional rays .

It is a further object of the present invention to provide multi-mode optical fibres where the core and the cladding comprise similar material. In particular, it is an object of the present invention to provide single material, multi-mode optical fibres with high bandwidth.

It is a further object to provide such multi-mode fibres that have a low amount of solid material such that a reduced amount of raw material is required for fabrication of the multi-mode optical fibres.

It is a further object to provide fibre designs that are mechanically stable with respect to various types of handling. It is a further object of the present invention to provide optical fibres with core and/or cladding having accurately controlled effective index profiles. In particular, it is an object to provide single mode fibres with accurately controlled effective index profiles in order to realize large mode areas.

It is a further object to provide a laser system or part thereof for delivery of laser light through an optical fibre with a relatively large core and a high beam quality. The highest beam quality is naturally obtained for a single mode optical fibre. Hence, the various ideas and aspects of the present invention also apply to single mode optical fibres .

It is a further object of the present invention to provide use of optical fibres for delivery of light with a high beam quality in large cores .

It is a further object to provide preforms or parts thereof for making optical fibres according to the present invention.

It is a further object to provide methods for making optical fibres according to the present invention.

Further objects appear from the description elsewhere.

Solution According to the Invention

According to an aspect of the present invention, these objects are fulfilled by providing a multi-mode optical fibre for guiding light, the optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising (a) a core region for propagating the light to be guided in the longitudinal direction of the optical fibre; said core region having a minimum dimension equal to or larger than 30 μm;

(b) an inner cladding region, said inner cladding region surrounding said core region and comprising three or more voids or holes separated by a minimum distance in the range from 2 m to

10 m, and being arranged in an inner cladding background material with refractive index n₁₇ (and said inner cladding region having an effective refractive index, n_cladeff) ; and

(c) an outer cladding region with an outer cladding background material with refractive index n2, and a thickness (t) , said thickness being equal to or larger than 10 μm;

said core region and said outer cladding region being connected by said inner cladding material

whereby a multi-mode optical fibre is provided with a large core size, a low numerical aperture, a reduced amount of solid material in the inner cladding region, robustness for handling, and ability to scrambles modes guided in the fibre.

According to another aspect of the present invention, these objects are fulfilled by providing an optical fibre for guiding light, the optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising (a) a core region for propagating the light to be guided in the longitudinal direction of the optical fibre; said core region comprising a background material with refractive index n_core;

(b) a cladding region, said cladding region surrounding said core region and comprising an cladding background material with refractive index n_1#-

said core region, and/or cladding region comprising ring- shaped regions or elements having refractive indices different from that of the background materials of said core region, and/or said cladding region;

whereby a (single-mode or multi-mode) optical fibre is provided with an accurately controlled effective refractive index profile of the core and/or the cladding. The effective index profile can thus be controlled more accurately than the absolute control of the absolute re- fractive index values. Preferably, this control is used to realize single mode optical fibres with large core size.

Preferred embodiments are defined in the claims.

Other objects, features and advantages of the present invention will be more readily apparent from the detailed description of preferred embodiments set forth below, taken in conjunction with the accompanying drawings.

Definition of terms and expressions

In this application there is made a distinction between the term "refractive index" and the term "effective re- fractive index" . The refractive index is the conventional refractive index of a homogeneous material . For optical fibres of the present invention, the most important optical wavelengths are in the visible to near-infrared regime (wavelengths from approximately 400 nm to 2 μm) . In this wavelength range most relevant materials for fibre production (e.g. silica) may be considered mainly wavelength independent, or at least not strongly wavelength dependent. However, for non-homogeneous materials, such as fibres with voids or air holes, the effective refractive index may be very dependent on the morphology of the material. Furthermore, the effective refractive index of such a fibre may be strongly wavelength dependent. The procedure of determining the effective refrac- tive index at a given wavelength of a given fibre structure having voids or holes is well-known to those skilled in the art, as well as the art of computing advanced optical properties of such optical fibres (see e.g. Jouannopoulos et al, "Photonic Crystals", Princeton University Press, 1995; or afore-mentioned Bjarklev et al. reference). The simple term "index" may also be used in short without directly stating "refractive" - it should be understood that the term "index" in this context is meant to be equal to "refractive index" . The term "absolute" may also be used to underline the refractive index of the material itself. The absolute refractive index is therefore to be understood as equivalent to the refractive index. It should be clear that the absolute and effective refractive index may be similar in the case of uniform materials - or they may be different in the case of a micro- or nano-structured material.

As appreciated within the field of microstructured fibres, the term "air holes" of the cladding and/or in the core may include holes or voids comprising a vacuum, gas or liquid, said holes or voids being fully or partly filled with a liquid or a gas after production of the microstructured optical fibre.

It is to be understood that the following detailed description is merely exemplary of the invention, and is intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying figures are included to pro- vide further understanding of the invention, and are incorporated in and constitute a part of the invention. The invention is not limited to the described examples. The figures illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.

3. BRIEF DESCRIPTION OF THE DRAWINGS

In the following, by way of examples only, the invention is further disclosed with detailed description of preferred embodiments . Reference is made to the drawings in which

Fig. 1 shows a schematic example of an optical fibre ac- cording to a preferred embodiment of the present invention.

Fig. 2 shows photo micrographs of three optical fibres according to preferred embodiments of the present inven- tion.

Fig. 3 shows schematically an effective refractive index profile of an optical fibre according to a preferred embodiment of the present invention. Fig. 4 shows measured NA of the optical fibres in Fig. 2.

Fig. 5a and 5b show schematically effective refractive index profiles of fibres according to preferred embodi- ments of the present invention, wherein the core has a graded absolute and/or effective refractive index profile.

Fig. 6a shows a schematic example of a core region of a fibre according to a preferred embodiment of the present invention. Fig. 6b shows schematically a (absolute) refractive index profile of through a section of a core region of a fibre according to a preferred embodiment of the present invention. The effective refractive index profile may be graded as shown schematically shown in Fig. 5a and 5b.

Fig. 7a shows another schematic example of a fibre according to a preferred embodiment of the present inven- tion. Fig. 7b shows yet another schematic example of a fibre according to a preferred embodiment of the present invention, wherein the core has a graded absolute or effective refractive index profile.

Fig. 8 shows a schematic example of a preform for producing a fibre according to a preferred embodiment of the present invention.

Fig. 9 shows a photo micrograph of a preform (or preform cane) according to a preferred embodiment of the present invention.

Fig. 10 shows a schematic example of another preform for producing a fibre according to a preferred embodiment of the present invention. Fig. 11 shows a schematic example of another preform for producing a fibre according to a preferred embodiment of the present invention.

Fig. 12 shows a schematic example of another preform for producing a fibre according to a preferred embodiment of the present invention, wherein the optical fibre comprises two different solid materials.

Fig. 13 shows schematically an optical communication system or high power laser transmission system that utilizes an optical fibre according to a preferred embodiment of the present invention.

4. DETAILED DESCRIPTION

Optical fibres according to the present invention have a longitudinal direction and a cross-section perpendicular thereto. The cross-section of a fibre may vary along its length, but is typically constant. Most references to physical fibre parameters - such as dimensions - and figures of fibre designs refer to a fibre cross-section.

A preferred embodiment an optical fibre according to the present invention is shown schematically in Fig. 1. At a cross-section, the fibre in Fig. 1 comprises a core region 10 and a cladding region comprising an inner cladding region and an outer cladding region 13. The inner cladding region comprises voids or holes 11 and a solid material 12. The solid material has a minimum dimension, b, and comprises material with a refractive index . The solid material 12 may be viewed as a bridge-type region that connects the outer cladding and the core. The bride- type region 12 may have a non-uniform width (typically, it may be broader near the core region) such that an appropriate length of the bridge region may be defined using the parameter L as shown in Fig. 1. The outer cladding region comprises a solid material of refractive in- dex n2. The optical fibre is characterized in that the core region has a minimum dimension, p, being equal to or larger than 30 μm, and b is in the range from 2.0 μm to 10.0 μm, and three or more holes or voids, such that there exists three or more bridge-type regions.

In order to lower losses of the optical fibre, it is generally desired to have the length, L, as long as possible. In practice, however, issues, such as cleaving, handling, cabling, amount of material used for fabrica- ting a fibre etc. puts limits on how long L should be.

In a preferred embodiment the bridge length L is in the range from 10 μm to 300 μm, preferably in the range from 40 μm to 100 μm thereby ensuring that an optical fibre with outer fibre diameter in the range from 125 μm to 250 μm can be obtained.

In preferred embodiments, a wall thickness, t, of the outer cladding region is around 10 μm or larger. Prefer- ably t is larger than 10 μm, such as larger than 20 μm or even larger in order to improve mechanical robustness of the fibre during various types of handling, such as cleaving, cabling, bending etc.

Fig. 2 shows photo micrographs of three different optical fibres according to preferred embodiments of the present invention. The fibres are produced using pure silica material, hence n_core = n_x = n₂. The fibre in top of the figure has the following physical design parameters: p is around 30 μm, b is around 3.0 μm, L is around 43 μm, and t is around 12 μm. The two other fibres have different dimension that may be deduced from the photos (it is mentioned that the b parameter is 2.0 μm and 4.8 μm for the fibre in the middle and bottom of the figure, respec- tively) .

Fibres according to the present invention may be made using a single solid material or using more materials. In the case of a single solid material, the fibre may guide light despite the fact that there is no index difference between a refractive index of the core region, n_core, and a refractive index, n_{l f} of the bridge-type region - as a result of the holes or voids .

"Model of NA calculation"

Key parameters of fibres according to the present invention, such as numerical aperture and effective index of the inner cladding region, may be determined from the refractive indices of the various materials of the fibre and the fibre morphology. Usually, numerical methods are required for accurate prediction of these properties (such as the methods referred to by Agrawal, Snyder and Love; or EP 1 119 581 mentioned previously) . A more simple description may, however, provide a first estimate of some important fibre properties. Using a simplifying assumption of infinitely long uniform bridge-type regions of width b, an effective refractive index of the inner cladding region, n_cladeff, at a given wavelength, λ, may be determined using theory of the fundamental mode in a planar waveguide. The theory of planar waveguides is for example described by Ramo, Whinnery, and Van Duzer, "Field and waves in communication electronics", Wiley, 3. edition., pp. 763-765, 1997. The following equation may be deduced for the fundamental mode: tan (hb/2 ) =q/h , ( 1 )

where h and q are given by :

q² = β² k. ( 2 ) h² = k, solid ( 3 )

where k_air is the wave number in the holes or voids (2π/λ) and k_solld is the wave number in the bridge-type material (2πn_x/λ)and β is the propagation constant of the fundamental mode. The effective refractive index of the inner cladding region may now be determined as :

n_clad;eff = β/k, where k = 2π/λ . (4)

If the PCF is assumed to act as a step-index fibre with core refractive index n_core, and the cladding is assumed uniform with refractive index n_cladeff, it is possible to approximate the NA of the PCF as:

NA² = n_core ²-n_{clad, eff} ² . ( 5 )

For a preferred embodiment shown in fig. 1, the above- described model is illustrated schematically in Fig. 3. The model has been found to provide a reasonable good qualitative description of the NA of various optical fibres according to preferred embodiments of the present invention. In general, however, the model is found to over-predict the NA - hence to under-predict n_cladeff.

I surprisingly turns out that broadening of the width of the bridge-region near the core region acts to lower the

NA of the fibre - by an increase in effective index area near the core region (area is here to be understood when considering a cross-section of a fibre) ; it is therefore advantageous to provide means for broadening the effective index area of this region. This may for example be done using a broadened geometric shape of the bridge- region near the core region (as apparent from Fig. 2, bottom figure) and/or this may be done using more than a single material for the bridge-type region. An example of the latter is discussed in connection with Fig. 10.

Considering the three fibres in Fig. 2, Fig. 4 shows their measured NA as a function of wavelength for different lengths of fibre. A slight decrease in NA over longer lengths are observed - and is attributed to higher order modes being stripped off as they travel along the fibre length. In Fig. 4, solid (two series of data placed in the upper part of the figure) , dashed (two series of data placed in the medium part of the figure) and dashed- dotted lines (two series of data placed in the lower part of the figure) represent b-values of 2.0 μm, 3.0 μm, and 4.8 μm, respectively; and diamond-points represent fibre lengths of 2 metres, whereas circular-points represent fibre lengths of more than hundred metres. As seen the bridge thickness significantly affects the NA values and consequently the performance of the fibre for various applications (including the bandwidth-length product for multi-mode optical fibre communication, as shall be demonstrated) .

As a first approximation, the fibre in Fig. 1 may be viewed as an equivalent step-index standard fibre having an effective refractive index profile as schematically shown in Fig. 3. An advantage of fibres according to various preferred embodiments of the present invention is that the effective index step between n_core and n_clad>eff may, in principle, be made infinitely small and thereby provide lower NA, than what can typically be realized using standard fibre manufacturing techniques (such as cylindrical vapour deposition techniques) . For various applications at wavelengths from visible to near-infrared (about 400 nm to 2.0 μm) , it turns out that large core multi-mode fibres (p equal to or larger than 30 μm, or even equal to or larger than 50 μm, or even equal to or larger than 100 μm) may be realized using low NA values (lower than 0.10, such as lower than 0.08, or even lower than 0.07, or lower than 0.06, or lower than 0.05) may be realized using b larger than 2.0 μm, such as b larger than 5.0 μm, such as b larger than 6.0 μm, or even larger.

An optical fibre with such large core and low NA may be utilized in numerous applications. In particular, such an optical fibre may be utilized in delivery of optical power with a good beam quality. The beam quality is often expressed using a parameter M² that is desired to be as low as possible (ideally M² = 1 as for a Gaussian beam profile) . M² may be approximated as:

M² = x(pπNA)/2λ, (6)

where x is a parameter that approximately describes the relative difference between the core diameter and the spot-size of the multi-mode fibre (x = spot-size/p) . For the fibres according to the present invention, we shall assume x equal to one (a lower value may be more appropriate for few or single mode PCFs, but assuming α=l should provide a worst-case estimation for the here- disclosed fibres and applications, where low M² values are desired) . As an example, consider a pure silica optical fibre according to a preferred embodiment of the present invention as shown schematically in Fig. 1. The fibre has p equal to 100 μm and is used for delivery of light with a wavelength around 1.0 μm. For b of around 4.8 μm, the fibre has an NA of around 0.08 (extrapolation from Fig. 3) , and an M² value of around 6. It may be argued that an NA value of around 0.08 may be obtained using for example a pure silica core fibre and a solid F-doped silica clad- ding having an index step of around 2.0*10^"3. An important advantage of the here-disclosed optical fibres is, however, that a substantial part of the optical fibre is hollow (the inner cladding comprises voids or holes) thereby reducing the amount of raw material required for producing the fibre. The amount of reduced raw material for fibres according to preferred embodiments of the present invention is equal to the void fraction of the fibre. From geometrical considerations, the air or void filling fraction, f, (the amount of void in a fibre) may be found to be approximately equal to:

f = (π[(L+p/2) -(p/2)²] -nbL)/(π(t+L+p/2)²) . (7)

As an example, a fibre with L = 40 μm, r = 30 μm, b = 5.0 μm, t = 10 μm, and n = 7 gives f equal 8796-1400/13273 to 0.55. Therefore, about 55% of raw material is saved as compared to fabrication of solid cladding multi-mode optical fibres. Naturally, the various combinations of parameters in Eq. (7) for fibres according to various preferred embodiments may provide various values of f . In preferred embodiments, f is larger than 20%, such as larger than 50%, such as larger than 70%. Hence, fibres according to the present invention may, therefore, become cheaper to fabricate than standard optical fibres through the use of less raw material. Furthermore, since a number of preferred embodiments of the present invention are fibre realized using a single material, costly processes related to production of doped glasses or polymers with different refractive indices are eliminated. This further underlines the potential cost-reducing aspects of the present invention for multi-mode optical fibres. For b larger than 4.8 μm, such as for example b of around 6.0 μm or larger, even lower NAs may be achieved. While opti- cal fibres, generally, becomes more susceptible to external issues (such as micro-bending losses) for lower NAs, the present invention provides means for reducing the degrading influence of such issues through adjusting (increasing) the parameter L. As L is increased, the fibres may, however, become more fragile mechanically and more difficult to cleave - in particular in the case of a low number of bridge-type region, such as three to six regions. Therefore, in a further preferred embodiment, the number of bridge-type regions is increased to provide more mechanical stability, such as more than six, preferably more than ten bridging regions exists .

Having addressed one important application of the here- disclosed optical fibres, a second important application shall be disclosed. Multi-mode optical fibres with large cores are also of significant interest in communication systems, where optical information should be transmitted from one point to another. In particular, multi-mode optical fibres (both glass-based and polymer-based) are finding extensive use in metro and local area networks where typical signal wavelengths is around 850 nm and around 1300 nm. There are a number of key issues for fibres in such applications; one of them being a large core size, such that relatively simple coupling to the fibres is possible; another issue being low inter-modal dispersion, such that high bit rates may be transmitted over relatively long distances (typically a few hundred meters to a few kilometers) . As shall be demonstrated, the present invention provides optical fibres that are well suited for metro and LAN applications as well as a number of important improvements that provides optical fibres that have transmission properties that may resemble those of single mode optical fibres in terms of low dispersion.

Typically, standard optical fibres for metro and LAN applications have core sizes of around 50 μm to 65 μm. From the previous discussion, it should be clear that the here-disclosed optical fibres may provide large core sizes. With respect to high bit rate transmission over given distances, multi-mode fibres are often characterized by a so-called bandwidth-length product, BL, that describes over how long a distance a signal may be transmitted at a given bit rate (or vice versa) for the speci- fie fibre. For a step-index fibre, BL may be expressed as (see afore-mentioned Agrawal-reference, Eq. (2.1.6)):

BL < (n_clad/n_core ²) ( c/Δ) , ( 8 )

where Δ is equal to n_core/ (n_core-n_clad) . For a step-index fibre with a pure silica core (n_core of around 1.450) and a down-doped homogeneous cladding (n_clad around 1.447), an NA of around 0.09 and Δ of around 2.0*10^"3 may be achieved. Such a fibre will have BL limited at around 100 (Mb/s)-km.

For optical fibres having an inner cladding comprising low-index elements, as for the here-disclosed optical fibres, Eq. (8) may be approximate by: BL < (n_{clad, eff}/n_core ²) (c/Δ_eff ) , ( 9)

where Δ_eff is equal n_core/ (n_core-n_clad<eff) .

From the simplified step-index fibre analogy model (see previous description) , a similar limit of BL from Eq. (9) of around 100 (Mb/s) -km should be expected for a PCF according to the present invention having a NA of around 0.09. It turns out that fibres according to the present invention provide a higher BL product through their ability to scramble modes guided through the fibre. There exists several means for obtaining this scrambling, as reflected by the various preferred embodiments of the present invention. In the previous examples of fibres according to the preferred embodiments of the present invention, the number of holes or voids surrounding the core is six, but other numbers of holes or voids are also covered by the present invention. In particular, it is preferred that the number of holes or voids is equal to three, five, or seven, or another prime number. As mentioned, it is generally desired to scramble the different modes of multi-mode fibred in order to lower the effects of inter-modal dispersion, such that a higher bandwidth- length product may be achieved. It turns out that the voids or holes play an important role for scrambling guided modes of the fibre - in particular so-called skew rays - and that a prime number is particularly advantageous for scrambling the modes or rays. In particular, it is an advantageous that there exist a number, n, of interfaces between the core region and the cladding region where no index difference occurs - or rather that there are a number, n, of connection regions where the core and cladding region connects to each other without any index step (such as the regions where the bridge-type regions is in contact with the core region and the bridge-type region similar material as comprised in the core region) . Also for scrambling of skew-rays, the core region may comprise a single or a number of elements - such as for example a single hole, void or solid low- index element - or the core region may comprise a few holes, voids, and/or low-index elements that are placed substantially a long a line or another arrangement. Also the shape of the core region plays an important role for scrambling the modes or rays. It has previously be de- scribed that it is an advantage for lowering the NA that the bridge-type regions have a broaden width near the core region. It further turns out that a broadened width of the bride-type region near the core region provides increased mode scrambling and, therefore, is further advantageous for increasing the BL product . Preferably the bridge-type regions have a width of more than 1.2b near the core region, such as more than 2b. The bridge width may further be broadened through the use of a non- uniform refractive index profile of the bridge-type re- gion. For example, an inner part of the bridge-type region (the part not being adjacent to the holes or voids) may have a refractive index equal to n_core, whereas the parts of the bridge-type material adjacent to the holes or voids may have a lower refractive index. An example of this shall be discussed in more detail in connection with preforms for fabricating fibres according to preferred embodiments of the present invention.

It is well-known in the field of multi-mode optical fibres that graded-index profiles provides a higher bandwidth-length product than step-index fibres. As a first approximation assuming a ray-tracing model as fully and accurately describing the propagation of modes in the fibres, an optimum refractive index profile having a substantially parabolic shape is found (expressed as the profile parameter equal to 2(1-Δ) as described by Agrawal) . The substantially parabolic profile ensures that all modes in the fibre travel with similar group velocity through the fibre. In this ideal case, BL may be expressed as :

BL < 8c/ (n_coreΔ²) . ( 10 )

State-of-the-art multi-mode fibres for high bit rate applications include Infinicor 600 from Corning and Max- Cap 300 from Draka. These fibres have core diameters of 50 μm and outer fibre diameters of 125 μm, and they address the same applications as the here-disclosed fibres. Both fibres have a numerical aperture of around 0.20 and a graded index profile to provide low inter-modal dispersion. The specified BL values of the two fibres are around 600 (Mb/s) -km and 1500 (Mb/s) -km, for the Corning and the Draka fibre, respectively (the BL product may also be stated in units MHz*km, that is similar to the here used units) . The refractive index of profile of the two fibres are not disclosed, but assuming n_core of around 1.480, Eq. (10) predicts a BL product of around 20 (Gb/s) -km (Δ = 9.1*10^~3). The discrepancy between the specified BL values of the Corning and the Draka fibres are mainly related to inaccuracies during the manufacturing process (the Draka fibre thus providing the better refractive index profile control) .

For optical fibres having an inner cladding comprising low-index elements, as for the here-disclosed optical fibres, Eq. (10) may be approximate by:

BL < 8c/ (n_coreΔ_eff ²) • ( H) The present invention covers preferred embodiments, where the core region has a graded refractive index profile, as schematically shown in Fig. 5a and 5b. The graded refractive index profile may be obtained using a doped core profile having an absolute refractive index profile with a substantially parabolic profile. The graded profile may also be obtained using an effective refractive index variation, as for example obtained using low- and/or high-index cylinders in the core region. It turns out that a desired effective refractive index profile may advantageously be realised using ring-shaped regions 61, annular (preferably coaxial) regions, within the core region 50 - as illustrated schematically in Fig. 6a. Preferably, these ring-shaped regions have a lower refrac- tive index than a background material of the core region 62 - as schematically illustrated in Fig. 6b. It is an advantage that the core region comprising rings-shaped elements preferably can be made using MCVD or other method of realizing doped fibres that is suitable for circular symmetric geometry. Alternatively to using ring- shaped regions, elements such as isolated cylinders may be used. The ring shaped regions may be substantially increasing in density in radial distance from the core centre. Alternatively, the refractive index of the ring- shaped regions or elements may decrease in radial direction from the core centre. It turns out that the use of ring-shaped regions or elements allows to realized graded effective refractive index profiles that have a lower effective index difference between the high and low values of the profile. This allows realization of multi- mode fibres with low Δeff that again is advantageous in order to increase BL.

Furthermore, it turns out that the use of ring-shaped regions or elements within the core region provides an increased control of the refractive index profile - as compared to absolute refractive index profiles .

Alternatively, the use of ring-shaped regions or elements may be applied in a cladding region, or the use of ring- shaped regions or elements may be used in both a core and a cladding.

The use of ring-shaped regions or elements provides an increased control of the refractive index profile for any refractive index profile of an optical fibre. Hence, the present invention also relates to single mode optical fibres. In particular, the control of effective index profile may be employed for single mode fibres in the same manner as discussed for multi-mode optical fibres. In a preferred embodiment, the present invention relates to a single mode optical fibre with a large core size. Preferably, a core size of 15 μm or more, more preferably around 20 μm, around 30 μm, or even larger is achieved. Various types of claddings may be used for the single mode fibre; including solid claddings, claddings comprising holes or voids, double cladding, etc.

Intuitively, the control is increase proportionally to A_core/A_coreelement, where A_core is the cross-sectional area of the core region and A_coreelement is the cross-sectional area of the ring-shaped elements and/or cylinders. Hence, the present invention provides multi-mode fibres with highly controllable effective refractive index profiles. Espe- cially, highly controllable effective refractive index profiles that may provide low Δ_eff values. In particular, for applications of multi-mode fibres in metro and LAN networks, such properties are highly desirable in order to increase BL. The present inventions, therefore, in- eludes various preferred embodiments wherein the core region has a graded effective index profile. Especially, the present invention discloses preferred embodiments of multi-mode fibres having substantially parabolic graded effective index profiles. Further preferred embodiments provide fibres with graded effective index profiles and low Δ_eff.

Preferably, the graded effective index profile has a value that matches the effective refractive index value of the inner cladding at the outer part of the core region - as indicated schematically in Fig. 5a. However, it is also within the present invention that the effective refractive index profile of the core region does not match the effective refractive index of the inner clad- ding region - as schematically illustrated in Fig. 5b.

As an example of a PCF according to a preferred embodiment having b adapted to provide NA of 0.08 at a given wavelength (adjustable through the b-parameter as pre- viously discussed) , and a substantially parabolic absolute and/or effective refractive index profile in the core region, Eq. (11) predicts that the fibre may have BL limited at around 700 (Gb/s) -km (the following parameter were used for the calculation of Eq. (11) : n_core = 1.450 and Δ_eff = 1.5*10 (determined from the expression Δ O₍ff

^(NA/n_core)²) ) . This limit is seen to be around 35 times higher than the theoretical limit found for the two afore-mentioned state-of-the-art multi-mode fibres - and around 450 times higher than for the prior art fibres. Due to the previously described advantageous of the here- disclosed PCFs with respect to mode scrambling, and the increased control of the graded effective refractive index profile, the optical fibre disclosed by the present invention provides a BL product that is significantly closer to the theoretical limit of the BL product. Hence, the present invention provides optical fibres that have significantly higher BL product than prior art multi-mode fibres for applications in metro and LAN networks.

The ideas of the present invention may also be utilized for PCFs having several layers of holes or voids surrounding the core region. Examples of other preferred embodiments of the present invention are shown schematically in Fig. 7a and 7b. The fibres are characterized by a core region 70 having a smallest dimension, p, equal to or larger than 30 μm, and a minimum distance between nearest neighboring holes or voids 71 that is in the range from 2.0 μm to 10.0 μm. The fibre may be a single material fibre such that a material of the core region 70 and a background material 72 of the inner cladding region and a material of the outer cladding region 73. Alternatively, the fibre may, for example, comprise a graded absolute or effective refractive index profile in the core region 74 - as shown schematically in Fig. 7b. Preferably, the core is surrounded by a large number of holes or voids such as at least 18 holes or voids in order to realize a relatively large core diameter (such as more than 30 μm, such as more than 50 μm, such as more than 100 μm) for hole or voids separation in the range from 2.0 μm to 10.0 μm.

Optical fibres according to the present invention may be fabricated using a preform as shown schematically in Fig. 8. The preform comprises a central rod 80 to substan- tially form the core region in the final fibre. The core rod may be single material or may a refractive index profile. For example, the central rod may have an absolute refractive index profile - such as a substantially parabolic refractive index profile realized using for example an MCVD process. Alternatively, the central rod may com- prises a refractive index profile that provides a graded effective refractive index profile - as for example an effective index profile obtained using ring-shaped regions (see Fig. 6a for an example of a refractive index profile that may provide a graded effective index profile) . Surrounding the central rod is a number, n, of tubes 81. The tubes 81 and the central rod 80 are placed in an overcladding tube 82. The material of the preform elements may be silica-based glasses or polymer (s) .

As an example, the preform elements may have dimensions 4.9 mm of the outer diameter of the central rod 80, outer diameter 4.9 mm and inner diameter 3.9 mm of the tubes 81, and outer diameter of 20.0 mm and inner diameter 14.9 mm of the overcladding tube 82. The length of the preform elements is 1.0 meter. The preform may be drawn or stretched to an intermediate step using a furnace in a preform lathe or on a drawing tower, where the preform is heated to a temperature of around 1950 degrees Celsius. The tubes may be pressurized during stretching or drawing and the voids between the tubes may be collapsed be applying a vacuum inside the overcladding tube. In this manner one or more new preforms, also referred to as a preform canes, may be realized. Fig. 9 shows a photo micrograph of a preform cane that has an outer dimension of around 5.0 mm. This holes or voids originating from the holes or voids in the preform tubes 81 are seen to have obtained a non-circular form and bridge-type regions have formed in the preform cane. These bridge-type regions connect the outer cladding to the core region. The preform cane may be drawn into fibre using a conventional fibre drawing tower - and be used to fabricate fibres as for example shown in Fig. 2. The parameter b may be controlled during fibre drawing by applying a pressure to the holes or voids of the preform cane. The specific physical design parameters of the final PCF may, thus, be obtained by careful design of the preform (including selection of preform elements of specific material (s) and refractive index profiles) . The mean of pres- surizing the preform during one or more step of the fibre production provides further means for tailoring the various fibre parameters to specific desires dimensions. In particular, a person skilled in the art of making PCFs would know how to adjust the various design and drawing parameters to obtain the here-disclosed fibre parameters using the above description.

It should be emphasised that in order to optimise various properties of a PCF, it is preferred that there is flexi- bility in tuning the core refractive index profile. Since the central core rod is chosen or selected separately from the cladding tubes 81 and the overcladding tube 82, any given rod with a desired profile may be inserted in the preform.

In preferred embodiments, the optical fibre is made using silica based glasses, such that certain parts of the preform elements are realized using pure silica and other parts are realized using doped silica. Various dopants may be used to provide a given refractive index level or profile as well as active dopants may be used to provide fibre for e.g. amplifying or lasing applications.

In order to stabilize the drawing of the preform, it may be preferred to use additional stuffing or buffering elements to further fill the overcladding tube (not shown in Fig. 8) .

Fig. 10 shows a schematic example of another preform for producing a fibre according to a preferred embodiment of the present invention, wherein the optical fibre comprises two different solid materials. The preform comprises a central rod 100, cladding tubes comprising an outer part 101 of material with refractive index n_x and an inner part 102 of refractive index n_11# and an overcladding tube 103. In preferred embodiments, n_lx is lower than n₁₇ such that a PCF may be drawn from the preform, wherein the bridge-type region 12 has a refractive index profile with a maximum in the centre of the bridge-type region - as previously mentioned in relation to broadening the bridge-type regions. The broadening of the bridge-type region is the case of non-uniform refractive index of the bridge-type material should be seen in relation to the material with refractive index n_x being adjacent to holes or voids. The higher refractive index n_lx compared to 1.0 (that of the holes or voids) allows increasing the b-parameter while maintaining a given NA - as compared to uniform bridge-type regions of refractive index n_x .

Fig. 11 shows a schematic example of another preform for producing a fibre according to a preferred embodiment of the present invention. The preform comprises a central rod 110 wherein longitudinal grooves 111 are formed. The preform further comprises an overcladding tube 112. The longitudinal grooves may be formed using a laser etching process - as described in details in WO02072489. During stretching or drawing of the preform in Fig. 11 to a preform cane, a controlled vacuum may be applied inside the overcladding tube to seal the central rod to the overcladding tube without collapsing the grooves completely. The preform cane may thereafter be drawn into fibres as described for the preform in Fig. 8. The central rod may be single material or may comprise a desired absolute and/or effective refractive index profile. Alternatively, a preform as shown schematically in Fig. 12 may be used for fabricating optical fibres according to the various preferred embodiments of the present invention. The preform in Fig. 12 comprises a central rod 120, ^"and inner cladding tube 121 having elongated grooves 122, and an overcladding tube 123. As previously described the central rod 120 may have a desired refractive index profile.

Fig. 13 shows a schematic example of a fibre optical communication or a high power laser transmission system utilizing an optical fibre according to a preferred embodiment of the present invention. In the case of a fibre optical communication system, the system includes a sender 130 of an optical signal (typically a laser source) , an optical fibre 131 according to a preferred embodiment of the present invention, and a receiver 132. In the case of a high power laser transmission system, the laser 130 emits light that is coupled into the opti- cal fibre 131. The light transmitted through the fibre may for example be used to laser process an article 132, such as an article that is being etched or machined by the laser light. The article may for example be a metallic object or a polymeric object. The communication or laser system would typically include a large number of sub-components, such as collimators, lenses, fibre holders, and various control equipment, such as power meters etc .

The control of effective refractive index profile - by use of ring-shaped elements and/or cylinders - may alternatively be used to realize other effective index profiles than graded index profiles, such as step-index, asymmetric, depressed-cladding, W-profiles or any other profile known from optical fibre, as well as novel profiles may be imagined.

The control of effective refractive index profile - by use of ring-shaped elements and/or cylinders - may also be used for single mode fibres. Hence, the advantages of controlling accurately effective index profiles may be used for single mode, few mode, and highly multi-mode fibres .

In a preferred embodiment, ring-shaped elements and/or cylinders are used to form the core of a single or fewmode optical fibre with a core size of around 10 μm or more, such as around 15 μm, around 20 μm, or around 30 μm. In further preferred embodiment, the effective index profile is controlled using ring-shaped elements and/or cylinders to provide a single mode optical fibre. Preferably, a large mode area, single mode optical fibre with a core size of around 10 times or more an optical wave- length of light guided through the fibre. For example, asingle mode fibre guiding light at around 1.5 μm with a core size of around 15 μm or more can be achieved.

While the invention has been particularly shown and de- scribed with reference to particular embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, and it is intended that such changes come within the scope of the following claims.

Claims

IMPROVEMENTS RELATING TO PHOTONIC CRYSTAL FIBRES CLAIMS

1. A multi-mode optical fibre for guiding light, the optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising

(a) a core region (10,70,74) for propagating the light to be guided in the longitudinal direction of the optical fibre; said core region having a minimum dimension (p) equal to or larger than 30 μm;

(b) an inner cladding region, said inner cladding region surrounding said core region and comprising three or more voids or holes separated by a minimum distance (b) in the range from 2 m to 10 m, and being arranged in an inner cladding background material with refractive index n₁- and

said core region and said outer cladding region being connected by said inner cladding material.

2. The optical fibre according to claim 1 wherein said number of voids or holes in said inner cladding material are forming elongated bridge-type regions separating said voids or holes, and said core region and said outer cladding region being connected by said bridge-type regions.

3. The optical fibre according to claim 1 or 2 wherein said bridge-type region has a length (L) in the range from 10 μm to 300 μm, preferably in the range from 40 μm to 100 μm.

4. The optical fibre according to any one of claim 1-3 wherein said minimum dimension p is in the range from 50 μm to 65 μm, and b in the range from 4 μm to 10 μm, preferably in the range from 6 μm to 7 μm.

5. The optical fibre according to any one of claims 1-4 wherein said fibre is a single material fibre exhibiting a bit-rate bandwidth-width-length product, BL, of more than 100 Mbit/s*km.

6. The optical fibre according to any one of claims 1-5 wherein said fibre is a single material fibre, preferably a pure silica optical fibre or a polymer optical fibre.

7. The optical fibre according to any one of claims 1-6 wherein said core region comprises material with a refractive index n_core, and n_core is equal to n_x .

8. The optical fibre according to any one of claims 1-4 wherein said core region has a graded absolute refractive index profile, so that the optical fibre exhibits a bit- rate bandwidth-width-length product, BL, of more than 1000 Mbit/s*km.

9. The optical fibre according to any one of claims 1-4 wherein said core region comprises a number of core element providing a graded effective refractive index profile, so that the optical fibre exhibits a bit-rate bandwidth-width-length product, BL, of more than 1000 Mbit/s*km.

10. The optical fibre according to claim 9 wherein said inner cladding region has an effective refractive index, n_cladeff; and said graded effective refractive index profile of the core has an effective refractive index value being equal to n_cladeff adjacent to said inner cladding holes or voids.

11. The optical fibre according to claim 9 wherein said inner cladding region has an effective refractive index, ⁿ _ci_d,_eff/' ^anc3- said graded effective refractive index profile of the core has an effective refractive index value being different from n_cladeff adjacent to said inner cladding holes or voids .

12. The optical fibre according to any one of claims 9-11 wherein said core elements are ring-shaped elements (61) having a common center axis .

13. The optical fibre according to claim 12 wherein a majority of said ring-shaped elements have refractive index, n_ring, being less than n_core.

1 . The optical fibre according to claim 12 or 13 wherein all ring-shaped elements have a similar refractive index, n_rlng, being less than n_core.

15. The optical fibre according to claim 12, wherein a majority of said ring-shaped elements has refractive index, n_ring, being larger than n_core.

16. The optical fibre according to claim 12 or 13 wherein all ring-shaped elements have a similar refractive index, n_ring, being larger than n_core.

17. The optical fibre according to any one of claims 12-

16 wherein said ring-shaped elements have decreasing refractive index as a function of radial distance from a center of the optical fibre.

18. The optical fibre according to any one of claims 12-

17 wherein said ring-shaped elements increasing size or width as a function of radial distance from a center of the optical fibre.

19. The optical fibre according to any one of claims 1- 4, and 7-10 wherein said core region comprises material with a refractive index n_core, and n_core is smaller, equal to, or larger than n2.

20. The optical fibre according to any one of claims 1-4, and 7-19 wherein the bridge-type region between two adjacent holes or voids in a plane (A-A Fig. 6a) has a non-uniform refractive index profile.

21. The optical fibre according to claim 20 wherein the refractive index profile has a maximum value being equal to n_core.

22. The optical fibre according to any one of claims 1-21 wherein said fibre comprises a number, n, of bridge-type regions that have an increased width near or at a connecting region between said bridge-type regions and the core region.

23. The optical fibre according to any one of claims 1-22 wherein said optical fibre comprises a number, n, of interfaces between holes or voids in the inner cladding and the core region, said interfaces having a curvature that is directed away from the core center so that scrambling of skew rays is enhanced.

24. The optical fibre according to any one of claims 1-21 wherein n is in the range from 4 to 10, such as equal to 4, 5, 6, 7, 8, 9, or 10.

25. The optical fibre according to any one of claims 1-24 wherein upon launching multiple modes of light of a wavelength in the range from 400 nm to 2000 nm, such as from 800 nm to 1600 nm, or from 800 nm to 1300 nm, a non- circular shape of at least three interfaces between holes or voids of the inner cladding and the core region provide mode scrambling to reduce or eliminate skew rays, such that BL is increased.

26. The optical fibre according to any one of claims 1-22 wherein upon launching multiple modes of light of a wavelength in the range from 400 nm to 2000 nm, such as from 800 nm to 1600 nm, or from 800 nm to 1300 nm, an increased width of at least three bridge-type regions provide mode scrambling to reduce or eliminate skew rays .

27. The optical fibre according to any one of claims 1- 4,6,7,22-26 wherein said optical fibre is a single mate- rial optical fibre such that the optical fibre is capable of transmitting high power laser light with good beam quality, such as M² of less than 15, such as M² less than 6.

28. The optical fibre according to any one of claims 1- 4,6,7,22-26 wherein said optical fibre is a single material optical fibre such that the optical fibre is capable of transmitting high power laser light, and p is larger than 400 μm, preferably larger than 600 μm, and t equal to or larger than 20 μm.

29. The optical fibre according to any one of claims 1-28 wherein said optical fibre comprises a fraction, f, of holes or voids that is equal to or larger than 20%, preferably f is equal to or larger than 50 %, more preferably f is equal to or larger than 70 %, so that the optical fibre can be produced with a reduced amount of raw material .

30. The optical fibre according to any one of claims 1-29 wherein t is larger than 25 μm, preferably larger than 50 μm, so that the optical fibre has an increased robustness for handling.

31. An optical fibre communication system or a high power laser transmission system comprising an optical fibre according to any one of claims 1-30.

32. An optical fibre communication system comprising: a source for generating at least one wavelength of light in the range from 400 nm to 2000 nm, preferably from 800 nm to 1600 nm, more preferably from 800 nm to 1300 nm; and

an optical fibre according to any one of claims 1-27 wherein multiple modes of light are launched into the optical fibre, wherein rays of guided light are being scrambled during propagating through the optical fibre by inner interfaces between inner cladding holes or voids and the core region to increase BL of the system.

33. A preform or parts thereof for producing an optical fibre according to any one of claims 1-30, the preform comprising:

(a) a center rod; and

(b) a number, n, of cladding tubes surrounding said center rod, and an overcladding tube surrounding said tubes and center rod;

wherein n is equal to or larger than three and the center rod has a uniform, graded or effectively graded refractive index profile.

34. A preform or parts thereof according to claim 33 wherein said center rod is a single material rod and said cladding tubes and overcladding tube are made from a similar material as the center rod.

35. A method of producing an optical fibre according to any one of claims 1-30, said method comprising the steps:

(a) providing a preform according to claims 33 or 34, having a center rod, a number, n, of clad- ding tubes, and an overcladding tube;

(b) stretching said preform to during a heat treatment at around 1900 degrees Celsius, such as to provide at least one preform cane; (c) during stretching to provide a vacuum inside said overcladding tube, and a pressure to said cladding tubes;

(d) drawing said preform cane to optical fibre using a conventional optical fibre drawing tower;

(e) optionally, controlling a pressure in holes or voids of said preform cane to obtain desired physical dimensions of the optical fibre.

36. An optical fibre for guiding light, the optical fibre having an axial direction and a cross section perpendicular to said axial direction, the optical fibre comprising

(a) a core region for propagating the light to be guided in the longitudinal direction of the optical fibre; said core region comprising a background material with refractive index n_core;

(b) a cladding region, said cladding region surrounding said core region and comprising an cladding background material with refractive index n_ιr-

said core region, and/or cladding region comprising ring- shaped regions or elements having refractive indices different from that of the background materials of said core region, and/or said cladding region.

37. The optical fibre according to claim 36 wherein said ring-shaped regions or elements (61) of said core region and/or said cladding region have a common center axis.

38. The optical fibre according to claims 36 or 37 wherein a majority of said ring-shaped elements have refrac- tive index, n_ring, being less than n_core.

39. The optical fibre according to any one of claims 36- 38 wherein all ring-shaped elements have a similar refractive index, n_ring, being less than n_core.

40. The optical fibre according to claims 36 or 37 wherein a majority of said ring-shaped elements has refractive index, n_ring, being larger than n_core.

41. The optical fibre according to claims 36 or 37 wherein all ring-shaped elements have a similar refractive index, n_ring, being larger than n_core.

42. The optical fibre according to any one of claims 36- 41 wherein said ring-shaped elements have decreasing refractive index as a function of radial distance from a center of the optical fibre.

43. The optical fibre according to any one of claims 36- 42 wherein said ring-shaped elements increasing size or width as a function of radial distance from a center of the optical fibre.

44. The optical fibre according to any one of claims 36- 43 wherein said core region has a minimum dimension (p) equal to or larger than 10 μm.

45. The optical fibre according to any one of claims 36-

43 wherein said core region has a minimum dimension (p) equal to or larger than 15 μm, preferably larger than 20 μm, or larger than 30 μm.

46. The optical fibre according to any one of claims 36-

44 wherein said core region and said cladding region are adapted to allow multi-mode propagation of the light to be guided .

47. The optical fibre according to any one of claims 36- 43 and 45 wherein said core region and said cladding region are adapted to allow single-mode propagation of the light to be guided.

48. The optical fibre according to any one of claims 36- 47 wherein said cladding region comprises

(a) an inner cladding region, said inner cladding region surrounding said core region and comprising three or more voids or holes separated by a minimum distance (b) in the range from 2 m to 10 m, and being arranged in an cladding background material with refractive index n_x ; and

(b) an outer cladding region with an outer cladding background material with refractive index n₂, and a thickness (t) , said thickness being equal to or larger than 10 μm.