WO2004019092A1 - Photonic crystal fibre - Google Patents

Abstract

The present invention provides an improved large mode area, single mode or few mode optical fibre that is robust towards loss mechanisms such as bending losses and coupling losses. The optical fiber is aimed for use in transmission of light at at least one optical wavelength, λ. The optical fibre (10) has a longitudinal direction and a cross-section perpendicular thereto. The optical fiber comprises a core region (11) and a micro-structured cladding region. The cladding region surrounds the core region and it comprises micro-structured cladding features (12) that are arranged in a background cladding material (13). A plurality of the micro-structured cladding features has substantially similar size d - typical- ly the cladding features are circular in cross-section and d is equal to the diameter of the cladding features. At least a number of the cladding features are arranged proximal to said core region at a center-to-center cladding feature spacing, h, larger than 3 times λ. The core region is surrounded by more than six innermost cladding elements having a ratio d/h less than 0.45.

Description

PHOTONIC CRYSTAL FIBRE

DESCRIPTION

1. BACKGROUND OF THE INVENTION

The present invention relates to optical fibre waveguides, and use thereof in various applications, such as transmission optical fibres or speciality optical fibres in optical communication systems, and high power fibre lasers or amplifiers; in particular the invention relates to new designs of improved large mode area photonic crystal fibres that support a single mode or a few number of modes.

The Technical Field

Optical fibres with large mode areas and single mode or few mode operations are desired within a vast number of technical areas, ranging from optical communications, sensor technology, spectroscopy, imaging, lithography, medicine, material processing, micro-machining, and many others. Often it is desired to guide light through the optical fibre in a well-controlled manner, such as for example in a single mode or a few modes and at the same time with a high intensity. This may either be for continuous wave or pulsed operation.

For optical telecommunications, for example, it is desired to transmit light signals at high bit rates over long distances. This requires short light pulses of high intensity and optical transmission fibres wherein little or no pulse distortion occurs. The most common causes of pulse distortion are dispersion and/or non-linear effects. Therefore, as optical transmission systems are being pushed towards higher bit rates, there is a need for developing optical fibres with improved properties with respect to dispersion and non-linearities.

Today, dispersion effects in optical transmission links may be handled using various types of speciality compo- nents at discrete or extended lengths of a link - for example using dispersion and dispersion-slope compensating optical fibres.

Although non-linearities in optical fibres may be utilized for certain applications, such as for Raman amplification, there is a general interest in minimizing non-linear effects in optical fibres. A key to minimize non- linear effects is to increase the effective mode area of the optical fibre so that the power-density is lowered. There are, however, a number of problems that must be solved when trying to expand the mode field diameter of an optical fibre by increasing its core size. These problems relate mainly to multi-mode operation and practical robustness in terms of low susceptibility to longitudinal non-uniformities. (these being, mainly, bending and structure variations due to imperfections during manufacturing and/or cabling) .

Presently employed optical fibres in optical communica- tion systems operating at near-infrared wavelengths have, typically, a mode field diameter of up to around 10 μm - corresponding to an effective area of around 80 μm². Larger effective areas may, in theory, be realized, but generally effective areas of above 220 μm² are not pos- sible for practical fibres operating at near-infrared wavelengths due to the afore-mentioned issues and because such a fibre requires an extreme control of doping levels .

Within recent years a new type of optical fibre has been demonstrated that is capable of providing large effective areas for single or few mode operations. Fibres of this type comprise microstructured features that are elongated in the longitudinal direction of the fibres. These new fibres are referred to by a number of different names including photonic crystal fibres, microstructured fibres, holey fibres, photonic band gap fibres, and hole-assisted fibres. We shall here refer to a fibre of this type as photonic crystal fibre (PCF). Optical fibres without microstructured features shall be referred to as standard optical fibres (such as traditional optical fibres that have been used and developed over several decades) .

Although PCFs, in theory, may result in extremely large effective areas for single mode operation, e.g. areas of above 500 μm², also for PCFs it turns out that problems such as bending losses and coupling losses, in practice, put limits to the achievable effective areas (see for example Monro et al . , Optics Letters, Vol. 26, No. 14, 2001 for an analysis of effective area with respect to macro-bending losses) . However, due to the significantly different composition of PCFs, these limits are not the same as known from standard optical fibres. Generally, it turns out that PCFs provide improvements compared to standard optical fibres in terms of larger effective areas as well as in terms of more broadband single mode or few mode operation. Therefore, a large research interest has been paid to PCFs within the last few years from a number of very different technical areas (see afore-mentioned areas) .

Although existing PCFs and devices using such optical fibres have advantageous properties, it would generally be desirable to increase the effective areas (also known as mode area or effective mode area) that may be realized in practice. This application discloses new PCFs having designs that facilitate improved robustness for large effective areas. The designs disclosed in this application may be used generally for a broad range of large mode area PCFs and applications, including optical transmission fibres, high-power fibre lasers and amplifiers, fibres for transmission of visible to near-infrared wave- lengths in single mode or few mode operation, infrared optical fibres, etc.

Prior Art Disclosures

WO 99/00685 discloses a single mode PCF with a large core size and single mode operation. WO 99/00685 discloses how the core size of the PCF may be scaled to large dimensions while maintaining single mode operation.

T.A. Birks et al . , Electronics Letters, Vol. 34, No. 13, 1998 disclose a PCF comprising a core of uniform material, e.g. a solid silica core, and a cladding surrounding the core, said cladding comprising substantially perio- dically arranged air holes in a silica background material. The optical fibre is allegedly suitable for high power transmission uses such as e.g. laser transmission for surgery, gas sensors, and communication network. WO 02/39159 discloses PCFs with increased robustness to bending losses through the use of a microstructured cladding region with features of increasing size in directions away from the fibre centre, as well as PCFs utiliz- ing non-circular and non-equilateral outer shape of an outer cladding.

WO 02/14946 discloses a microstructured optical fibre comprising a solid core surrounded by a cladding of holes, said optical fibre being prepared by drawing preforms comprising capillaries of compound glasses including sulphides, halides, and heavy metal oxides.

WO 01/42831 discloses PCFs comprising a core region with a refractive index being between the refractive index of a cladding background material and cladding feature material (typically air holes) . The PCFs have a short-wavelength cut-off for the fundamental mode.

Wadsworth et al . , Conference on Lasers and Electro Optics, CLEO, 2001, paper CWC1, pp. 319 disclose a large mode area PCF laser. The PCF comprises a microstructured core region with 425 Yb doped elements placed in an irregular ensemble. A high number of small, doped ele- ments are used in order to avoid any doped region regions themselves to form waveguides.

2. DISCLOSURE OF THE INVENTION

Object of the Invention

It is an object of the present invention to seek to provide an improved large mode area, single mode or few mode optical fibres which is robust towards loss mechanisms such as bending losses and coupling losses.

It is a further object of the present invention to seek to provide use of such optical fibres in high power transmission of light, in particular use in optical communication systems, lasers and amplifiers, imaging, lithography, medical applications such as laser surgery, and in material processing applications such as laser cutting and micro-machining.

Further objects appear from the description elsewhere.

Solution According to the Invention

"Robust, large mode area, single mode or few mode optical fibre"

In an aspect, according to the present invention, these objects are fulfilled by an optical fibre for transmitting light of at least one predetermined free-space optical wavelength λ, which optical fibre has a longitudinal direction and a cross-section perpendicular thereto, and which optical fibre comprises:

(a) a core region for propagating the light to be transmitted in the longitudinal direction of the optical fibre; and

(b) a microstructured cladding region, said cladding region surrounding said core region and comprising microstructure cladding features arranged in a background cladding material, said microstructure cladding features having sizes d which are equal or different,

at least a number of said cladding features are being arranged proximal to said core region at a center-to-center cladding feature spacing Λ larger than 3 times λ, and

said core region being surrounded by more than six innermost cladding features having a ratio d/Λ less than 0.45,

whereby it is obtained that large mode area, single mode or few mode optical fibre having robust operation with respect to bending and coupling losses can be provided compared to prior art PCF.

"Robust operation"

It surprisingly turns out that by selecting:

at least a number of said cladding features to be arranged proximal to said core region at a center-to-center cladding feature spacing Λ larger than 3 times λ, and

surrounding said core region by more than six innermost cladding features having a ratio d/Λ less than 0.45,

it is obtained that the effective area and the MFD at λ is significantly enhanced for a triangular core optical fibre compared to prior art hexagonal core optical fibres, and that the triangular core design is less susceptible to longitudinal non-uniformity, i.e. it is more robust, than the hexagonal core design.

"Core region"

The core region is designed for propagating light to be transmitted in the longitudinal direction of the optical fibre, said propagation being effected by providing a refractive index difference between the core region and said a microstructured cladding region surrounding it. This refractive index difference may result by an effective refractive index difference between the core region and the microstructured cladding region.

The core region may take any shape suitable for the field distribution of the fundamental mode of the PCF and mode confinement by the inner most cladding features for providing a sufficiently low leakage of losses.

It has surprisingly turned out that a core region having a substantially triangular shape is particularly preferred whereby increased robustness of a single mode optical fibre is obtained.

The core region can be seen as being positively defined by the presence of innermost cladding features, i.e. more than six e.g. nine or twelve innermost cladding features, or as being negatively defined by the absence of a number of cladding features, e.g. three cladding features.

Accordingly, in a preferred embodiment said core region is formed by omission of three cladding features.

The area of the core region is selected according to the number of modes and level of intensity of the light to be transmitted. The cladding features determine it and their arrangement as further disclosed below.

In a preferred embodiment, said core region has a geo- metric core area of more than 120 μm², preferably more than 190 μm², most preferred more than 220 μm².

The core region may comprise any suitable material including a single material, a single material doped with dopants, a mixture of materials, a mixture of materials doped with dopants, and microstructured materials comprising core features.

In a preferred embodiment, said core region comprises at least one core feature, preferably three or more core features, most preferred more than four core features.

In further preferred embodiment, at least part of said core features is arranged to provide a substantially one- dimensional periodicity in a cross-section of said core region.

"Microstructured cladding region"

Generally, the number of innermost cladding features is more than 6, however, depending on the desired area of the core region, a larger number can be applied.

In a preferred embodiment, said number of innermost clad- ding features is equal to nine or twelve whereby optical fibres having particularly large mode field areas of more than 6 times λ can be provided while maintaining robust transmission of light. The size and arrangement of cladding features can vary within broad limits. In particular for confining propagation of light in the core region, size and arrangement of the innermost cladding features is selected.

It surprisingly turns out that for said innermost cladding features having a ratio d/Λ smaller than or about 0.30 it is ensured that only a limited number of modes are supported by the robust optical fibre.

In a preferred embodiment, the ratio d/Λ is smaller than or about 0.25 whereby a condition of single mode is ensured.

Generally, not all cladding features need be of the same dimensions. Typically the innermost cladding features are selected and have one size whereas the size of outer cladding features and there between may have different sizes .

In a preferred embodiment, a majority of said micro- structure cladding features have a ratio d/Λ of less than or about 0.30, preferably less than or about 0.25, most preferred in the range from 0.22 to 0.28 whereby it is obtained that cladding features arranged in the micro- structure may be present for different purposes, e.g. large area voids containing air or vacuum and smaller area solid features comprising doped materials.

In a particular preferred embodiment, substantially all microstructure cladding features have ratio d/Λ of less than or about 0.30, preferably less than or about 0.25, most preferred in the range from 0.22 to 0.28 whereby it is obtained that an optical fibre of substantially uni- form microstructured cladding can be obtained. Such a uniformity may, for example, be preferred for facilitate production of the optical fibre using only one type of capillary tubes for forming the microstructured cladding.

'Mode field diameter"

The mode field of the optical fibre is adapted to the application, e.g. to the wavelength to be transmitted by the optical fibre.

In a preferred embodiment, the optical fibre has a mode field diameter equal to or larger than 6λ whereby increased robustness compared to prior art microstructured fibres is obtained.

In another preferred embodiment, the optical fibre exhibits a parameter 0.7(3Λ-d) which is equal to or larger than 6λ whereby a simplified expression for the mode field diameter relationship and wavelength is obtained.

Generally, the cladding features can be arranged in any suitable structure that provides index guiding of the light in the core region.

In a preferred embodiment, said microstructure cladding features are arranged in a periodic structure.

In another preferred embodiment, said microstructure cladding features are arranged in concentric rings around said core region.

"Materials"

The optical fibre comprises material known in the art In a preferred embodiment, said core region, said micro- structured cladding region, or both, comprises silica whereby a well-known optical fibre material for which production techniques exists can be applied.

The optical fibre may, however, comprise different materials for its components.

Accordingly, in a preferred embodiment, said core region, said microstructured cladding region, or both, comprises silica and/or silica including one or more co-dopant materials, preferably a material selected from the group consisting of Ge, Al, B, or F (or other materials) , or a combination thereof.

Generally, the cladding region may comprise any suitable optical fibre material, however, specific properties may be obtained by selecting specific cladding features.

Accordingly, in a preferred embodiment, said micro- structure cladding features are selected from the group consisting of solids, liquids, voids, and combinations thereof.

In particular it is preferred that said voids comprises air, vacuum, liquids, solids, or a combination thereof.

In another preferred embodiment said voids are filled with liquid, or a polymer.

For certain application e.g. for lasers and amplifiers, the optical fibre comprises an active material for providing lasing action. Accordingly, in a preferred embodiment, said core region and/or at least a part of said cladding background material comprises an active material, preferably silica doped with a rare earth element, most preferred silica doped with Er, Yb, or Nd, or a combination thereof.

In another embodiment, said core region and/or at least a part of said cladding background material comprises co- dopant materials, preferably a material selected from the group consisting Ge, Al, B, or F, (or other materials) or a combination thereof.

"Predetermined wavelength"

In a particular application, the optical fibre used depends on the required wavelength or range of wavelength to be transmitted.

In a preferred embodiment, the predetermined wavelength λ is in the range from λi to λ₂, where λi is less than or equal to λ₂, and λi and λ₂ are in the range 0.1 μm to 2.0 μm.

Specifically, said λi and λ₂ are in the range from 0.4 μm to 1.7 μm, preferably in the range from 1.3 μm to 1.7 μm, most preferred in the range from 1.5 μm to 1.6 μm whereby the optical fibre may be used for applications using visible to near-infrared light.

In another preferred embodiment, Λ is larger than 3 times λ₂ whereby it is obtained that the optical fibre is more robust than prior art microstructure fibres in the whole wavelength range from λi to λ₂ (since λi is smaller than λ₂ and therefore it is also valid that Λ is larger than 3 times any wavelength down to λi) .

"Method of producing robust, large mode area PCF"

In another aspect, according to the present invention, there is provided a method of preparing an optical fibre the method comprising:

(a) providing three, preferably six or seven, rods to form a core region of a preform and surrounding said three, six or seven rods by elements of capillary and/or rods to form a cladding region;

(b) providing a preform of said preform elements by arranging said preform elements in a predetermined periodic structure, and optionally con- solidating said structure, preferably by a heat treatment; and

(c) drawing said preform to an optical fibre with predetermined dimension under a controlled heat treatment.

"Use of the optical fibre accroding to the invention"

In another aspect, according to the present invention, there is provided use of an optical fibre according to the present invention or an optical fibre produced in a method according to the invention in an optical communi- cation system, in an optical fibre laser, or in an optical fibre amplifier, or in one or more parts thereof.

Definition of expressions

In the present context it is intended that the term "light" designates electromagnetic radiation, in particular light having a wave length in the range from 0.1 μm to 30 μm.

The term "size" is intended to designate a size parameter of a structural element of the optical fibre, e.g. the diameter of a cladding feature such as a hole.

The term "from a to b" is intended to mean the range from a- to b including a and b.

The term "substantially" is intended to mean being largely but not necessary wholly that which is specified.

In the present context we distinguish between the terms "refractive index" and "effective refractive index" , respectively. The term "refractive index" is intended to mean the conventional refractive index of a homogeneous material. Most relevant materials for optical fibre production (e.g. silica) may be considered substantially wavelength independent, or at least not strongly wave- length dependent. However, for non-homogeneous materials, such as microstructures, the effective refractive index is very dependent on the morphology of the material. Furthermore, the effective refractive index of a micro- structure is strongly wavelength dependent - much stronger than the individual refractive index of any of the materials constituting the components of the microstructure. Determination of the effective refractive index of a given microstructure at a given wavelength is known to those skilled in the art (see e.g. the method disclosed by Jouannopoulos et al, "Photonic Crystals", Princeton University Press, 1995, or the method disclosed by Broeng et al, Optical Fiber Technology, Vol. 5, pp. 305-330, 1999) .

It should be noted that for accurate determination of the effective refractive indices of microstructures usually a numerical method of solving Maxwell's equation on full vectorial form is required. A documented method is disclosed by Johnson et al . , Optics Express Vol. 8, no. 3, 173-190, 2001. In the long-wavelength regime, the effective refractive index is roughly identical to the weighted average of the refractive indices of the constituents of the material.

For microstructures, a directly measurable quantity is the so-called "filling fraction" which is defined as the ratio of the volume of disposed features, or micro- structure elements, in a microstructure relative to the total volume of a microstructure. For optical fibres that are invariant in the axial direction, the filling fraction may be determined from direct inspection of the cross-section of the optical fibre. An effective refractive index should generally be determined at a given operational wavelength or at a predetermined wavelength.

The term "effective area" is intended to designate the effective area of a fundamental mode at a given wavelength that is guided in the core of a fibre. The effec- tive area is defined as [see e.g. N.A. Mortensen, Optics Express, Vol. 10, pp. 341-348 (2002)]:

where I is the intensity distribution in the PCF cross section. The corresponding mode-field diameter (MFD) is defined from the relation:

A_ejr ≡ π(μVOI2)² .

The term "robust" is intended to mean low susceptibility to longitudinal non-uniformity, e.g. resulting in bending losses and coupling losses. Susceptibility to longitudi- nal perturbations and the consequences for scattering loss caused by e.g. by said bending losses and coupling losses can be expressed by the "coupling length" scale ^'

ξ ≡ 2π /{β - β_cl)

which is the beat length at a given wavelength between a fundamental mode and a lowest order cladding mode with propagation constants β and β_cl, respectively. The lowest order cladding mode is also known as the fundamental space filling mode of the cladding (see e.g. T.A. Birks et al . , Optics Letters, Vol. 22, pp. 961-963, 1997). If L_n is the length scale for longitudinal non-uniformities and λ the wavelength then a critical- loss criterion can be formulated as a competition of length scales [J.D. Love, IEE Proceedings - J. Optoelectronics, Vol. 136, pp. 225-228 (1989) ] ;

λ < L_n < ξ ("high" scattering loss), λ,ξ < L_n or L_n < λ,ξ ("low" scattering loss).

For PCFs the wavelength dependence of ξ generally gives rise to both a short-wavelength and long-wavelength loss edge. As an example, macro-bending induces longitudinal non-uniformities with L_n ∞ R^1/2 where R is the bend radius; for a given radius both a short-wavelength and long-wavelength loss-edge can be observed [see e.g. T.A. Birks et al . , Optics Letters, Vol. 22, pp. 961-963 (1997); and e.g. T. Sørensen et al . , Electronics Letters, Vol. 37, pp. 287-289 (2001)].

Thus, the susceptibility of a PCF to longitudinal non- uniformities can be expressed by the coupling length ξ; the lower a value of the coupling length the lower a loss can in general be expected.

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(a) shows schematically an example of a PCF with a design according to the present invention. Fig. 1(b) shows a microscope photograph of a produced fibre with such a design.

Fig. 2(a) shows experimentally observed near-field distribution of the fundamental mode of the fibre in Fig. 1(b) at a wavelength of 980 nm. Figs. 2(b) and 2(c) show simulated results of the same near-field for a linear and logarithmic intensity scale, respectively. Fig. 3(a) shows schematically a prior art PCF with a core that is formed by omitting one air hole. The core has a substantially hexagonal shape. In Fig. 3(b) is shown schematically a cross sectional design of an optical fibre according to the present invention, this PCF has a substantially triangular shaped core that may be formed by omitting three air holes. For illustrative purposes, Fig. 3(c) shows schematically three omitted cladding fea- tures. The three omitted features are placed substantially in a triangle.

Figs. 4(a) and 4 (b)_ show coupling length versus wavelength for PCFs with hexagonal (dashed lines) and trian- gular (solid lines) cores. In Fig. 4(a) the wavelength is normalized by the pitch and in Fig. 4(b) the wavelength is normalized by the edge-to-edge separation of the air holes .

Fig. 5(a) shows the effective area and Fig. 5(b) shows the mode-field diameter versus wavelength for PCFs with hexagonal (dashed lines) and triangular (solid lines) cores .

Fig. 6(a) shows the effective area versus coupling length and Fig. 6(b) shows the mode-field diameter versus coupling length for PCFs with hexagonal (dashed lines) and triangular (solid lines) cores.

Fig. 7 shows simulated MFD as function of pitch for triangular core design - as well as a simple rule-of- thumb calculation of MFD as function of pitch and hole size . Fig. 8(a) shows a definition of geometric core area of a hexagonal core PCF according to prior art and Fig. 8(b) shows a definition of geometric core area of a triangular core of a preferred embodiment of a PCF according to the present invention.

Fig. 9(a) shows further examples of preferred embodiments of PCFs according to the present invention having cladding features placed substantially periodically. Fig. 9(b) shows an example of a preferred optical fibre according to the present invention having cladding features placed in substantially concentric circles.

Fig. 10 shows cut-off properties of preferred embodiments of produced PCFs according to the present invention for different cladding feature sizes.

Fig. 11(a) shows the waveguide group-velocity dispersion versus wavelength, Fig. 11(b) shows the waveguide group- velocity dispersion versus effective area, Fig. 11(c) shows the waveguide group-velocity dispersion versus mode-field diameter, and Fig. 11(d) shows the waveguide group-velocity dispersion versus coupling length for preferred embodiments of PCFs with hexagonal (dashed lines) and triangular (solid lines) cores.

Fig. 12 shows an example of a preferred embodiment of a PCF according to the present invention comprising low- index type core features.

Fig. 13 shows mode index as function of normalized wavelength of the fundamental mode, the 2. order mode and the lowest order cladding mode of the PCF shown in Fig. 12. Fig. 14 shows a simulation of MFD as function of pitch for the PCF shown in Fig. 1(a), the PCF in Fig. 12 as well as for a prior art PCF with an un-doped hexagonal core .

Fig. 15(a) shows schematically a part of the inner cross- section of a preferred embodiment of a PCF according to the present invention, where the core region comprises three core elements each having microstructured features. Fig. 15(b) shows a close-up view on one of the core elements .

Fig. 16 shows a schematic example of a preferred embodiment of a PCF according to the present invention for use as cladding pumped fibre for laser or amplifier applications. The PCF comprises an air-clad region in an outer cladding.

Fig. 17 (a) shows a schematic example of a preferred em- bodiment of a preform for producing a fibre according to the present invention, where the core is placed substantially in the center of the PCF. Fig. 17 (b) shows a similar schematic example of a preferred embodiment of a preform, but with a core region placed in a non-central position in the PCF.

Fig. 18 shows experimental data of attenuation for a prior art PCF and a preferred embodiment of a PCF according to the present invention in the case where the two types of PCFs have substantially similar susceptibility to longitudinal non-uniformities. The PCF according to the present invention has a MFD of about 12.0 μm, whereas the prior art PCF has a MFD of around 10.5 μm. Fig. 19 shows a microscope photograph of a preferred embodiment of a produced PCF according to the present invention. The photograph shows that for real fibres some dimensions of geometric features in the cross-section may vary - such as the hole size that is seen to decrease in size away from the core.

4. DETAILED DESCRIPTION

Fig. 1 (a) schematically depicts the cross-section of an exemplary preferred embodiment of an optical fibre 10 according to the present invention. The optical fibre comprises a core region 11 for propagating the light to be transmitted in the longitudinal direction of the optical fibre, here a triangularly shaped core region, a microstructured cladding region surrounding said core region and comprising microstructured cladding elements 12, or so-called cladding features 12, here elongated cladding features of equal size the inner most of which shape the extent of the core region, said cladding features being placed in a background cladding material 13 in the cladding, and an over-cladding region 14.

In this embodiment, nine innermost cladding features surround the core region, here defining a substantially triangular shape that is formed by the position of the innermost cladding features.

Fig. 1(b) shows a microscope photograph of the cross-section of a preferred embodiment of a real PCF according to the present invention. The PCFs has a core region of similar material as the cladding background material - in this case pure silica. The optical fibre has a centre-to- centre cladding feature spacing, the so-called pitch, Λ, of about 6 μm and a cladding feature size, here hole diameter, d, of about 1.5 μm.

The optical fibre in Fig. 1(b) shows a cross-sectional view of a microscope photograph of a produced optical fibre that has been characterized experimentally to be single mode over a broad wavelength range of about 400 nm to 1700 nm.

Fig. 2 illustrates the field distribution of the fundamental mode of- the PCF shown in Fig. 1(b). At a wavelength of 980 nm, Fig. 2(a) shows the experimentally observed near-field.

Figs. 2(b) and 2(c) shows computer simulations of the same near-field on a linear and logarithmic intensity scale, respectively. The computer simulations were performed using a program described in previously mentioned Johnson et al . reference. The experimental and linear near-field intensities demonstrate a substantial triangular shape of the fundamental mode and a mode confinement defined substantially by nine innermost cladding features .

In Fig. 2 (c) , the logarithmic scaled graph facilitates observation of low-intensity parts of the mode field distribution. This graph confirms that the number of cladding feature layers surrounding the core contributes in providing low leakage losses. Consequently, in a prefer- red embodiment, the number of cladding feature layers is maximised for a given outer diameter of the PCF, typically 125 μm for optical fibres used in telecommunications.

Fig. 3(a) shows schematically an inner, cross-sectional part of a prior art PCF with a substantially hexagonal core 30. The core may be defined as a defect in the otherwise periodic cladding structure, where the defect is formed by omission of a single cladding feature. Alternatively, six innermost cladding features 31 can define the core. This type of PCF shall hereinafter be referred to as a hexagonal core PCF.

Fig. 3(b) shows similarly a schematic illustration of an inner, cross-sectional part of a preferred embodiment of a PCF according to the present invention - as shown in Fig. 1. The core region 35 may be seen as a defect being formed by omission of three cladding features 37 (see Fig. 3(c)) or as being defined by nine innermost cladding features 36. This type of PCF shall hereinafter be refer- red to as a triangular core PCF. Both optical fibres may have similar core materials 30,35 and similar cladding background materials 32,37. The two PCFs have cladding features that are positioned periodically (in this case in a close-packed arrangement, also known as a triangular structure or hexagonal structure) . The present invention is, however, not limited to periodic cladding structures - and examples of this are presented for the embodiment shown in Fig. 9(b) .

The PCF technology is known to have a high potential for the realization of highly -birefringent optical fibres

[see e.g. T.P. Hansen et al . , IEEE Photonics Technology

Letters, Vol. 13, pp. 588-590 (2001)]. However, for PCFs with an m-fold rotational symmetry it follows from sym- metry considerations and group theory that the wave guide modes occur in degenerate pairs if m > 2 [see M.J. Steel it al . , Optics Letters, Vol. 26, pp. 488-490 (2001) and references therein] . The ideal hexagonal core and triangular core optical fibre structures shown in Fig. 3(a) and 3(b), respectively, have m=6 and m=3. Consequently, these PCFs have no intrinsic birefringence. Birefringence may, however, result in realised optical fibres exhibiting structural symmetry breaking and/or stress- induced anisotropy.

In the prior art, it is known that the hole-diameter to pitch ratio d/Λ can be chosen such that a defect in the cladding structure supports only a single (doubly degenerate) mode for all wavelengths, i.e. endlessly single mode operation [T.A. Birks et al . , Optics Letters, Vol. 22, pp. 961-963 (1997)]. In the case of the defect being formed by omitting a single air hole, the hexagonal core PCF as described above, the PCF will be endlessly single- mode for d/Λ < 0.45 [see e . g. J. Broeng et al . , Optical Fiber Technology, Vol. 5, pp. 305-330 (1999)]. For larger air holes a second-order mode will be guided at short wavelengths and the corresponding cut-of wavelength increases with increasing air-hole size [N.A. Mortensen, Optics Express, Vol. 10, pp. 341-348 (2002)].

For a preferred embodiment of a triangular core PCF of pure silica PCF and air-filled cladding features, the optical fibre surprisingly turns out to be endlessly single-mode for a cladding feature size, here a hole size, d/Λ, of approximately 0.25. Many other materials than air may be used in the cladding features, such as gasses or vacuum that will have a refractive index close to that of air. For these materials, d/Λ = 0.25 will also be a representative value for endlessly single mode operation. For other materials of the cladding features, e.g. cladding features filled with a liquid or a polymer, the refractive index of the cladding feature material is larger than 1 (the refractive index of vacuum, air, or another gas), and a d/Λ value of larger than 0.25 may yield end- lessly single mode operation. In a preferred embodiment, the cladding features are made using glass of lower refractive index than the background material (see for example US 6 243 522 for a general description of all-glass microstructured fibres, including preferred materials and methods^' of their production/fabrication) , and d/Λ may be as large as 0.45 for endlessly single mode operation. Naturally, to allow a few number of modes, d/Λ may be even larger. Optical fibres according to the present invention may also be realized using polymer (s) as background material (see for example WO 02/16984 for general description of polymer microstructured fibres, including preferred materials and methods of their production/- fabrication) .

"Comparison between triangular core and hexagonal core PFCs"

In order to demonstrate the advantages for the herein- disclosed PCFs according to the present invention compared to prior art PCFs for large mode area applications, it is valuable first to analyse the MFD and effective area of the PCFs in Fig. 3.

The hexagonal core PCF has a core region with a physical dimension dc that is comparable to twice the pitch, i.e. dc = 2Λ. For this hexagonal core design, the MFD compares well to Λ [N.A. Mortensen, Optics Express, Vol. 10, pp. 341-348 (2002)] for large mode area applications, i.e. for optical fibres with an MFD of a magnitude of several times the free-space optical wavelength, λ, e.g. about 6λ or larger. In the case of the triangular core PCF, the physical dimension of the core region dc is of the order of three times the pitch, dc = 3Λ, and the MFD to pitch ratio, MFD/Λ is larger for this PCF than for the hexagonal core PCF.

For more direct comparison of MFD and coupling length properties of the two above-described PCFs, it is valuable to utilize an independent length scale (for normali- zation) that the two structures have in common; namely, the free-space wavelength, λ. The following analysis shall be performed for both the hexagonal and triangular core PCF in the case of maximum hole size, d/Λ, for endlessly single mode operation, since largest possible hole sizes are generally preferred in order to reduce leakage losses. Hence, d/Λ = 0.45 will be used for the hexagonal PCF and d/Λ = 0.25 will be used for the triangular PCF. Both optical fibres will be assumed to be single material optical fibres that are realized in pure silica, and a refractive index of 1.444 will be used as a representative value. The cladding features are assumed to be air (or vacuum) with a refractive index of 1.

Fig. (a) shows the normalized coupling length, ξ/λ, as a function of normalized wavelength, λ/Λ, for a prior art hexagonal core PCF (dashed line) and for a triangular core preferred PCF (solid line) according to the present invention. Here it is observed that there is an increased coupling length for a given wavelength for a triangular core PCF corresponding to an increased susceptibility to longitudinal non-uniformities for the triangular defect (optical fibre according to the present invention) compared to the hexagonal defect (prior art fibre) of similar pitch. To a person skilled in the art, this increased susceptibility to longitudinal non-uniformities is - apparently a less attractive characteristic of the triangular core PCF which might be expected due to the lower air filling fraction of the triangular core PCF. However, since the two optical fibres have significantly different MFDs for similar pitch, such a conclusion is, in fact, generally not valid. For large mode area fibre optical applications with similar MFDs this shall be demonstrated later (for the discussion of Fig. 6).

In Fig. 4(b) is shown the same data of Fig. 4(a), but with the wavelength normalized by the edge-to-edge separation Λ-d of the cladding features, here air holes; re- scaling makes the two curves coincide in the short wavelength limit. Fig. 4(b) shows that Λ-d, rather than the air-filling fraction, is a parameter to minimize when designing low-loss large-mode area PCFs.

In Fig. 5 is shown the effective area and the MFD as a function of normalized wavelength. As seen, the effective area is significantly enhanced for the optical fibre according the present invention (triangular core) as compared to the prior art fibre (hexagonal core) .

"Robustness of large mode area triangular core design'

Comparing Figs. 4 and 5, it is found for both the hexagonal core PCF and the triangular core PCF that their ef- fective areas are increased at the prize of an increased susceptibility to longitudinal non-uniformities. However, for large-mode area applications the triangular defect design is superior to that with a hexagonal defect. This is illustrated in Fig. 6 where the data of Figs. 4 and 5 are combined. For MFD/λ > 6 (relevant for large-mode area applications) the triangular core PCF design is, surprisingly, found in general to be less susceptible to longitudinal non-uniformities compared to the hexagonal core PCF design. Hence, for operation, e.g., at a wavelength of 1.55 μm, an optical fibre according to the present invention is less susceptible to longitudinal non-uniformities for MFD larger than approximately 9 μm.

From Fig. 6 it is also directly observed that for even larger MFD, such as MFD equal to or larger than lOλ, 15λ, 20λ, 25λ, and 30λ, the fibre according to this preferred embodiment is further advantageous compared to prior art fibres. Hence for e.g. λ equal to about 1 μm, it is preferred that MFD is equal to or larger than 10 μm, 15 μm, 20 μm, 25 μm, and 30 μm. Equivalently, it is further advantageous to use fibre according to this preferred embodiment compared to prior art fibres for very large MFD- values, such as MFD larger than 40 μm, 50 μm, and 60 μm, hence for e.g. λ equal to about 1 μm and MFD larger than 40 μm, 50 μm, and 60 μm.

"Comparison examples - increased effective areas for same robustness"

As a first example we consider a wavelength of 630 nm and a prior art PCF with a hexagonal core with a MFD of 10.0 μm corresponding to an effective area of 78.5 μm². According to Fig. 6, the here-disclosed PCF with a triangular core can provide an effective area of 90.8 μm² (and a MFD of 10.8 μm) , but still with the same susceptibility to longitudinal non-uniformities as for the PCF with a hexagonal core. This corresponds to an increase in effective area by close to 16%. As a second example we consider a wavelength of 1550 nm and a PCF with a hexagonal core and a MFD of 15 μm corresponding to an effective area of 176.7 μm2, the corresponding effective area of the PCF with a triangular core becomes 193.7 μm², i.e. an increase by close to 10%.

As a third example we consider a wavelength of 1550 nm and a PCF with a hexagonal core and a MFD of 20 μm corresponding to an effective area of 314.2 μm², the corre- sponding effective area of the PCF with a triangular core becomes 360.6 μm², i.e. an increase by close to 15%.

"Quantification of geometrical features"

In order to quantify the geometrical features of the triangular core optical fibre in the range where this optical fibre design turns out to be superior to the prior art, hexagonal core PCF, Fig. 7 shows a computer simulation of MFD as a function of pitch, Λ, for the triangu- lar PCF of Fig. 4-6 at a wavelength of 1.1 μm. For Λ larger than λ (Λ larger than 1.1 μm) , the fibre is found to have MFD that is close to 2Λ. Hence, as a first rule-of- thumb, the triangular core PCF may be found to be advantageous in the case of Λ larger than about 3λ. As another simple rule-of-thumb, the MFD is found to be well described by the expression MFD = 0.7(3Λ-d), where d should be taken for the innermost cladding features. This expression takes into account the size of the innermost cladding. Both simple rule-of-thumbs have been found suitable in the case of single to few mode PCFs according to the present invention. In the case of a few modes PCF, the MFD is defined using the fundamental mode.

Alternatively, to quantify the geometric features of the present invention, it is valuable to consider a geometri- cally defined core area. From Fig. 1, it is seen that a fibre according to the present invention has core with larger area relative to the width Λ-d compared to a hexagonal core, prior art PCF. In order to quantify this property for the new designs of the present invention, it is useful to define a geometric core area A_geo of PCFs as illustrated in Figs. 8(a) and 8(b) (this area is not identical to the effective mode area A_eff that is used elsewhere in the application) . As shown in Fig. 8 (a) , using a polygon 80 expanded by centres of innermost cladding features 81 that surround the core region and subtracting the area of the part of the innermost cladding feature within this polygon, a core area A_geo,_hex can be defined for the hexagonal core PCF equal to

Similary, as shown in Fig. 8 (b) , a polygon 82 expanded by centres of innermost cladding features 83 for the triangular core PCF has six sides of non-equal lengths, but the core region may still be considered as substantially triangular. The core area A_geo,_trι of the triangular core PCF may, thus, be found from geometrical considerations to be equal to:

A gσeeoo,. trni =

For comparison, for two fibres with Λ = 6.0 μm and d = 1.5 μm (d/Λ = 0.25 and Λ-d = 4.5 μm) , the triangular core design provides an area of about 195 μm², whereas the hexagonal core design provides a core area of about 90 μm². Hence, these expressions quantify the signifi- cantly larger core area of the triangular core design compared to the prior art, hexagonal core for similar Λ- d.

As the hexagonal core design, however, may tolerate a larger d/Λ for similar number of supported modes, it may be more relevant to compare the two designs, in the case where the hexagonal core design has d /Λ = 0.45, but maintains Λ-d of 4.5 μm (hence, having Λ = 8.2 μm and d = 3.7 μm) . In this case, the hexagonal core design has a core area of about 143 μm² and, therefore, still less than for the triangular core design.

The reason for the larger core area relative to Λ-d for the triangular core PCF as compared to the hexagonal core PCF is the number of innermost cladding features surrounding the core region. Hence, according to a preferred embodiment of the present invention, PCF core designs providing more than 6, such as 9 or 12 innermost cladding features may be advantageous for large mode area PCF applications .

Other examples of optical fibres according to the present invention are shown in Fig. 9, where Fig. 9(a) shows ex- amples of PCFs according to the present invention having substantially periodic cladding features, and Fig. 9(b) shows an example of a PCF 90 having non-periodic cladding features 92 - in this example the cladding features are placed on substantially concentric circles 93 surrounding the core region 91.

For other embodiments of the present invention, such as the designs schematically shown in Fig. 9, a different maximum hole size for endlessly single mode operation may be found. The maximum hole size for the designs shown in Fig. 9(a) are indicated in the figure. These hole sizes, d/Λ, are -0.18, -0.15, and -0.15 for the PCFs with 4, 6, and 7 missing cladding features forming the core region, respectively.

Generally, in the case of other fibre materials or material compositions, for example polymer, non-silica glasses or crystalline materials with a different refractive index than pure silica, the endlessly single mode crite- rion in terms of hole size may be different.

Although, the above description has been based on single- mode PCF, also PCFs that guide a few number of modes are within the scope of the present invention.

For hole sizes larger than d/Λ = 0.25 the triangular core PCF may support one or more higher order modes, but it may also be operated in single mode regime (such as known from standard fibres). Fig. 10 shows cut-off pro- perties of produced PCFs with a triangular design for different cladding feature sizes. Two PCFs are characterized experimental, where both PCFs have Λ = 3.53 mm, but their hole size, d/Λ, is varying from 0.268 to 0.273 and 0.253 to 0.263, respectively. The variation in hole size is mainly a result of structure variations during production. As seen from the figure, the PCF with larger hole sizes may be operated in a single mode for wavelengths larger than approximately 990 nm, and the PCF with smaller holes may be operated in a single mode for wavelengths longer than approximately 830 nm. For a PCF with d/Λ larger than 0.25, the cut-off wavelength may be scaled by scaling the pitch of the structure, such that for larger pitch, a cut-off at longer wavelengths may be realized. Hence, for single mode operation at a given wavelength, a larger hole size than d/Λ = 0.25 may be tolerated provided the pitch is kept below a given value. For certain applications, however, it may be tolerated to operate the optical fibre in a multi-mode regime, similar to the method for example described in US 6 400 866, where two or more optical signals are transmitted at the same optical wavelength, but in two or more different modes of the optical fibre.

For other applications, such as for example optical fibre lasers, mode selection may allow the laser to operate in a fundamental mode, although the optical fibre, in theory, supports several modes. For example, different loss levels of the modes may perform selection. Hence, the present invention is not restricted to fibres with d/Λ of 0.25 or less.

"Use in communication systems"

PCFs according to the present invention may be used in optical communication systems. For such applications, dispersion is an important optical fibre parameter and Fig. 11 facilitates to design a PCF according to the present invention with specific dispersion properties. The dispersion results are obtained using computer simulations for a triangular PCF of pure silica material and with d/Λ = 0.25. In Fig. 11(a) shows the waveguide group-velocity dispersion versus wavelength, Fig. 11(b) shows the waveguide group-velocity dispersion versus effective area, Fig. 11(c) shows the waveguide group- velocity dispersion versus mode-field diameter, and Fig. 11(d) shows the waveguide group-velocity dispersion versus coupling length for PCFs with hexagonal (dashed lines) and triangular (solid lines) cores. The waveguide dispersion has been calculated in the case of no material dispersion. For a real optical fibre of a given material, the total fibre dispersion may as a good approximation be determined as the sum of material dispersion and waveguide dispersion. Hence, for a single material, e.g. pure silica PCF, as an example of a preferred embodiment ac- cording to the present invention that is designed to operate at λ = 1.55 μm with a MFD of about 25 μm, the optical fibre will have a waveguide dispersion of about 3 ps/nm/km and a total dispersion of about 20 ps/nm/km.

"Non-single material optical fibres"

The present invention also covers optical fibres of non- single materials, such as fibres comprising various types of dopants in the cladding region or parts thereof, and/- or in the core region or parts thereof. Also the present invention covers optical fibres where the cladding features comprise various types of materials, such as polymers, semiconductors, crystalline materials, etc.

For example, various doping profiles may be used in the fibre core to provide further flexibility in tailoring dispersion and/or cut-off properties of the fibres. Dopants may also be used to provide active regions in the PCFs, for example for making optical fibres for lasers and amplifiers applications.

Fig. 12 shows an example of a preferred embodiment of a PCF according to the present invention comprising low- index type core features. The low-index features may, for example, be introduced to control the cut-off properties of the optical fibre. The cut-off properties of a specific PCF with a design as in Fig. 12 are illustrated in Fig. 13. Fig. 13 shows mode index as function of normalized wavelength of fundamental, 2. order and lowest order cladding mode. As seen from the figure, the fibre has a cut-off of the first higher order mode (2. order mode) at a normalized wavelength, λ/Λ, of about 0.085. The optical fibre has furthermore a cut-off of the fundamental mode at a normalized wavelength of about 0.050. As an example, a PCF with Λ of 20.0 μm, will, therefore, be single mode at a wavelength of 1.0 μm to 1.7 μm. The PCF simulated in Fig. 13 has d/Λ of 0.30. Although, this hole size is larger than the endlessly single mode limit for a single material PCF, the fibre is observed to cut-off even the fundamental due to the low-index features in the core.

Fig. 14 shows a simulation of MFD as function of pitch for the PCF in Fig. 13, for the PCF in Fig. 1, panel (a), and for a prior art PCF with an undoped hexagonal core. The hole size d/Λ of the three fibres is 0.30, 0.22 and 0.45, respectively. As seen from the figure, MFD is significantly larger for a given pitch for the two triangular core PCF compared to the prior art PCF. Apart from the previously mentioned advantages of the here-disclosed PCFs, a further advantage may be realized from Fig. 2 panel (c) and Fig. 14. For real PCFs there is a finite outer diameter, hence there will in practice be a limited number of cladding feature layers that may be utilized. It is generally desired to realize a large number of layers, as the more layers or rings of air holes in the cladding region lowers the leakage loss [T.P. White et al., Optics Letters, Vol. 26, pp. 1660-1662 (2001)]. Typically for realizing low-loss large mode area PCFs 6- 10 rings or more are usually needed. However, due to the bounds on the outer diameter (usually 125 microns) the maximal number of rings and the MFD are linked. It is thus an advantage to be able to realize a given MFD with as low as possible a pitch. Hence, from Fig. 14 it is found that the smaller pitch required for the fibre ac- cording to the present invention compared to the prior art fibre, that more rings or layers of microstructured features may be placed about the core for a given outer fibre diameter - thereby providing improved means for reducing leakage losses. As an example, a PCF with MFD of 20 mm requires a L of about 9.5 μm and 15.0 μm for the triangular and hexagonal core PCF, respectively. Hence, the triangular core PCF requires an outer diameter of about 185 μm for about 6 layers of cladding features about the core, whereas the hexagonal core PCF requires an outer diameter of about 250 μm for 6 layers. The core region is approximated to a diameter of about 30 μm and the overcladding region to a radial width of about 20 μm (in total contributing to 40 μm of the outer diameter) . From a fibre cost point of view, the (smaller) triangular core PCF, therefore, requires less glass for production. Hence, the triangular PCF may prove a cheaper optical fibre to produce in terms of raw material. The present invention, therefore, also provides new, large mode area PCFs that may be cheaper to produce that prior art PCFs.

"High-power optical fibre laser and amplifiers'

For other applications, such as high-power fibre laser and amplifiers, it may be advantageous to tailor the ef- fective refractive index of the core region as a function of wavelength. This may, for example, be done using a number of microstructured features in the core region as sown schematically in Fig. 15. In fig. 15(a), the figure shows schematically a part 150 of the inner cross-section of a PCF according to the present invention, where the core region is surrounded by nine innermost cladding features 151 and it comprises three core elements 152 each having microstructured features 152. Fig 15(b) shows a close-up view on one of the core elements 155 to il- lustrated that a core element may comprise a large number of high-index elements 156 and low-index elements 157. The low-index and/or the high-index elements may optionally be of similar material as the core and/or cladding background material, or they may be of a different mate- rial with similar refractive index as one or both background materials. In the example in Fig. 15, the high- index features are positioned in substantially hexagonal concentric layers of high- and low-index type of silica rods .

In other preferred embodiments, the core comprises micro- structured features that are placed in a substantially one dimensionally periodic structure. This arrangement may, for example, be accomplished by having low-index and high-index core features placed in a layered manner as described in Danish Patent Application PA 2002 00787 (the priority of which is being claimed in PCT/DK03/00345) that is incorporated herein by reference.

"Cladding pumped lasers and amplifiers"

Fig. 16 shows a schematic example of a preferred embodiment of a PCF according to the present invention that may be used as a cladding pumped fibre for laser or amplifier applications. The PCF comprises an air-clad region in an outer cladding. The PCF 160 comprises a triangular core design with three microstructured core elements 161 and an inner microstructured cladding region 162 and an outer cladding region comprising an air-clad region 163.

"Method of production"

In order to produce optical fibres according to the present invention, methods well known for production/fabri- cation of PCFs or optical fibres with multiple cores may be employed with modification further discussed herein below - see for example Patent Abstracts of Japan, appl. no. 06 250 491; Patent Abstracts of Japan, appl. no.

58 090 313 Patent Abstracts of Japan, appl. no. 55 117 209 Patent Abstracts of Japan, appl. no. 54 081 518 US 5 155 792; WO 02/26648, US 5 471 553,

US 4 551 162, WO 02/14946, EP 0 905 834, US 5 802 236.

A number of these methods are based on stacking of rods or canes to form a preform and drawing this into optical fibre using a conventional drawing tower. Examples of preforms for optical fibres according to the present invention are illustrated in Fig. 17. The preform 170 in Fig. 17(a) comprises three pure silica rods 171 that will form the core. These rods are surrounded by a number of silica tubes 172 that will form a periodic structure of the inner cladding region. The stack of rods and tubes is placed in an overcladding tube 173. The preform may further comprise various types of filling/stuffing/buffer elements, such as for example rods to fill out the gaps 174.

The overcladding tube usually acts as an outer cladding region providing a desired outer diameter of the final fibre as well as mechanical robustness of the fibre. Other types of outer claddings are also covered by the present invention, such as outer claddings comprising an air-clad layer and a solid part - as for example described in US 5 907 652 the content of which regarding further information on drawing optical fibres with micro- structured features is incorporated be reference. During fibre drawing, a lower than atmospheric pressure may be apply inside the overcladding tube in order for the small interstitial voids between the rods and tubes to col- lapse. In this manner, the rods and tubes may form a close packed, periodic arrangement. The preform in Fig. 17 comprises a relatively low number of periods surrounding the core. Preferably, a larger number of layers is employed in order to reduce various loss mechanisms. The preform may preferably be processed prior to fibre drawing where the preform is for example placed in a lathe and heated to a temperature such that the overcladding tube collapses and fixates the rods and tubes. The method of stacking rods may also be applied, using appropriate modifications, to other material systems, such as for example polymers or non-silica glasses.

Fig. 17(b) shows a similar schematic example as in Fig. 17(a), but for a preform where the core region 175 is placed in a non-central position in the PCF. Such a position may be preferred for improved pump absorption in cladding pumped fibre applications.

In order to realize microstructured core regions, as shown in, for example, Fig. 15 and 16, it may be advantageous for to pre-produce the element (s) of the core region by stacking high- and low-index features in a regular manner inside an overcladding tube. This overcladding tube may then be drawn to a (solid) core rod with the inner structure being regular in at least a part of the cross-section. The resulting core rod may hereafter be used in a preform for a microstructured fibre in a manner as described above.

The preform may be prepared by controlled heat treatment, optionally under pressure and/or vacuum of the capillary tubes and the interstitial voids between the tubes. A skilled person would know how to calibrate the parameters of the preparation, e.g. the temperature, pressure, vac- uum, with respect to the glass of the capillaries ap- plied, e.g. its thickness, viscosity, softness, etc., see e.g. WO 02/14946.

For non-silica based PCFs, other principles than the ones described above may be used for production. In the case of crystalline or poly-crystalline materials, such as for example a silver halide, e.g. a solution of AgClo.₅Bro.5A a combination of methods known in the art may be used. Preform elements of silver halide materials can be extruded by a modified extrusion method disclosed in US 5 342 022

Example 1 "VIS to NIR transmission optical fibre"

In the following, a preferred embodiment of a robust large mode area optical fibre for visible to near-infrared light transmission according to the present invention and its production is described.

An optical microscope photograph of the exemplary PCF is shown in Fig. 1(b) . The PCF is made from pure silica and the cladding features are air-filled voids. The core region has a substantially triangular shape. The core region may be viewed as formed by omission of three cladding features. Surrounding the core region - or a centre of the core region - are nine innermost cladding features. The PCF comprises cladding features with a pitch, Λ, of about 6 μm and with a diameter, d, of about 1.5 μm. Hence, the PCF comprises cladding features with a relative diameter, d/Λ, of about 0.25. Specifically, the PCF has nine innermost cladding features with d/Λ of about 0.25. At a free-space wavelength, λ, of 1.55 μm, the PCF has MFD of about 12 μm. MFD is approximately equal to 2Λ or approximately equal to 0.7(3Λ-d) . Hence, at λ of 1.55 μm, the PCF has MFD of more than 6λ. Alternatively, at λ of 1.55 μm, the PCF has Λ of more than 3λ.. Alter- natively, at λ of 1.55 μm, the PCF has 0.7(3Λ-d) of more than 6λ. In fact, for a free-space wavelength, λ, shorter than about 2.0 μm, such as λ from 0.4 μm to 1.7 μm, the

PCF has MFD of more than 6λ, Λ of more than 3λ, and 0.7(3Λ-d) of more than 6λ.

The produced optical fibre transmits light robustly from visible wavelengths of at least 600 nm to at least 1.6 μm.

The optical fibre was prepared by a method as described above, wherein packaging of preform elements was adapted to provide the preferred design of core region and micro- structure cladding elements in a background cladding material, here a triangular core of silica surrounded by a silica background material with a closed packed periodic structure of microstructure cladding elements in form of holes, said triangular core being surrounded by nine innermost cladding element, and about 8 layers of cladding elements.

A preform was prepared by stacking 249 capillary tubes of silica (glass tubes of silica material with trademark F300 from Hereaus) ) to form the cladding region and three rods of silica (glass rods of silica material with trademark F300 from Hereaus) to form the core region as shown in Fig. 17 (a) . The capillary tubes have an outer diameter of 1.7 mm and an inner diameter of 0.6 mm. The rods have similar outer diameter as the capillary tubes. The capil- lary tubes and rods where stacked in a close packed manner and placed in an overcladding tube of silica (trademark F300 from Hereaus) with inner dimensions of 30 mm and outer diameter of 35 mm. A number of smaller rods of silica where used to partially fill gaps between the stack of capillary tubes/rods and the overcladding tube. Dimensions of the capillary tubes, the rods, and the overcladding tube varied within normal production limits known to a skilled person, e.g. generally up to a few percent. Less variability can be obtained by individual selection of the preform elements/components.

The preform was placed in a conventional drawing tower and vacuum was applied to the "interstitial region" between the capillary tubes. The preform was drawn to a "preform cane" with an outer diameter of 5.7 mm. An atmospheric pressure in the capillary tubes was used to prevent collapse and keep them open during drawing. The preform cane was afterwards overcladded with a tube of quartz glass having an outer diameter of 10.0 mm and an inner diameter of 6.0 mm.

This new production preform was then drawn into a PCF. A vacuum was applied to the gap between the overcladding tube and the preform cane in order to seal the overclad- ding tube and the preform rod together during drawing. The pressure was applied to the inside of the capillary tubes of the preform cane by mounting a pressure chamber on the top end of the preform cane and the pressure was controlled to yield a final size to pitch ratio of the cladding elements, here holes, of d/Λ of about 0.25. The drawing temperature was about 1900 degrees Celcius and the drawing speed was around 30 m/min. The PCF was coated with an acrylate material using a standard UV-curing coating technique. The hole size of the PCF was inspected visually during drawing of the fibre using an optical microscope. At the desired dimensions of the drawn optical fibre, the optical fibre was wound up using conventional optical fibre spools. Generally, the specifically desired dimensions of the core region, the micro structure features, and their center-to-center distance are achieved by calibration of production/manufacturing parameters such as temperatures, fibre pulling speed and preform feeding rates, etc. for producing PCFs and preforms comprising capillary tubes, rods and cladding tubes of given dimensions all of which production/manufacturing parameters and preform element dimensions are known to and/or can be assessed e.g. ex- perimentally by a person skilled in the art.

Fig. 18 shows experimental data of attenuation for the above disclosed and produced triangular core PCF with d/Λ = 0.25 and a prior art hexagonal core PCF with d/Λ = 0.45. The two fibres have substantially similar attenuation curves for the investigated wavelengths. This imply that they have similar sensitivity to longitudinal changes, i.e. that they have similar susceptibilities to longitudinal non-uniformities. However, the triangular core PCF according to the present invention has a larger mode area MFD of about 12.0 μm, whereas the prior art PCF has a smaller MFD of around 10.5 μm. During the attenuation measurements both PCFs were spooled on optical fibre drums with a radius of 32 cm.

Fig. 19 shows another microscope photograph of a preferred embodiment of a produced PCF according to the present invention. The photograph shows that for real optical fibres some variation of dimensions of geometric features in the cross-section can occur. For example the hole size can decrease in size away from the core. Generally, the further away from the core, a feature in a PCF is placed, the smaller is the influence of that feature on the performance of the PCF (in terms of e.g. leakage losses, dispersion, etc.). For the PCF in Fig. 19, it is observed that the largest size variation is for the outermost cladding features, whereas the nine innermost cladding features have a smaller variation. Generally, it is desired to most accurately control the innermost cladding features and a larger variation may be tolerated for the outer cladding features.

It will be apparent to those skilled in the art that various modifications and variations of the present invention can be made without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

PHOTONIC CRYSTAL FIBRECLAIMS

1. An optical fibre for transmitting light of at least one predetermined wavelength λ, the optical fibre (10) having a longitudinal direction and a cross-section per- ^■ pendicular thereto, the optical fibre comprising:

(a) a core region (11) for propagating the light to be transmitted in the longitudinal direction of the optical fibre; and

(b) a microstructured cladding region, said cladding region surrounding said core region and comprising microstructure cladding features (12) arranged in a background cladding material (13),

said microstructure cladding features having sizes d which are equal or different,

at least a number of said cladding features are being arranged proximal to said core region at a center-to-center cladding feature spacing Λ larger than 3 times λ, and

said core region being surrounded by more than six innermost cladding features having a ratio d/Λ less than 0.45.

2. The optical^' fibre according to claim 1 wherein said number of innermost cladding features is equal to nine or twelve .

3. The optical fibre according to claim 1 or 2 wherein said innermost cladding features have a ratio d/Λ smaller than or about 0.30, preferably smaller than or about 0.25.

4. An optical fibre according to any of the claims 1 or 3 wherein said core region has a substantially triangular shape .

5. An optical fibre according to any of the claims 1 or 4 wherein said core region is formed by omission of three cladding features.

6. An optical fibre according to any of the claims 1 to 5 wherein a majority of said microstructure cladding fea- tures have a ratio d/Λ of less than or about 0.30, preferably less than or about 0.25, most preferred in the range from 0.22 to 0.28.

7. An optical fibre according to any of the claims 1 to 6 wherein substantially all microstructure cladding features have ratio d/Λ of less than or about 0.30, preferably less than or about 0.25, most preferred in the range from 0.22 to 0.28.

8. An optical fibre according to any of the claims 1 to 7 the optical fibre having a mode field diameter equal to or larger than 6λ, preferably equal to or larger than 9 μm.

9. The optical fibre according to any of the claims 1 to

8 wherein 0.7(3Λ-d) is equal to or larger than 6λ, preferably equal to or larger than 9 μm.

10. An optical fibre according to any of the claims 1 to

9 wherein said microstructure cladding features are arranged in a periodic structure.

11. An optical fibre according to any of the claims 1 to 9 wherein said microstructure cladding features are arranged in concentric rings around said core region.

12. An optical fibre according to any of the claims 1 to 11 wherein said core region, said microstructured clad- ding region, or both, comprises silica.

13. An optical fibre according to claims 12 wherein said core region, said microstructured cladding region, or both, comprises silica and/or silica including one or more co-dopant materials, preferably a material selected from the group consisting of Ge, Al, B, or F (or other materials), or a combination thereof.

14. An optical fibre according to any of the claims 1 to 13 wherein said microstructure cladding features are selected from the group consisting of solids, liquids, voids, and combinations thereof.

15. The optical fibre according to claim 14 wherein said voids comprises air, vacuum, liquids, solids, or a combination thereof.

16. An optical fibre according to any of the claims 1 to 15 wherein said voids are filled with liquid, or a polymer.

17. An optical fibre according to any of the claims 1 to 16 wherein the predetermined wavelength λ is in the range from λi to λ₂, where λi is less than or equal to λ₂, and λi and λ₂ are in the range 0.1 μm to 2.0 μm.

18. The optical fibre according to claims 17 wherein said λi and λ₂ are in the range from 0.4 μm to 1.7 μm, preferably in the range from 1.3 μm to 1.7 μm, most prefer- red in the range from 1.5 μm to 1.6 μm.

19. An optical fibre according to claim 17 or 18 wherein Λ is larger than 3 times λ₂.

20. An optical fibre according to any of the claims 1 to

19 wherein said core region has a geometric core area of more than 120 μm², preferably more than 190 μm², most preferred more than 220 μm².

21. An optical fibre according to any of the claims 1 to

20 wherein said core region comprises at least one core feature, preferably three or more core features, most preferred more than four core features.

22. An optical fibre according to any of the claims 1 to

21 wherein said core region and/or at least a part of said cladding background material comprises an active material, preferably silica doped with a rare earth element, most preferred silica doped with Er, Yb, or Nd, or a combination thereof.

23. The optical fibre according to claims 28 wherein said core region and/or at least a part of said cladding background material comprises co-dopant materials, preferably a material selected from the group consisting Ge, Al, B, or F, (or other materials) or a combination thereof.

24. The optical fibre according to claim 22 or 23 wherein at least part of said core features are arranged to provide a substantially one-dimensional periodicity in a cross-section of said core region.

25. A method of producing a microstructured optical fibre, the method comprising:

(a) providing three, preferably six or seven, rods

(171) to form a core region of a preform (170) and surrounding said three, six or seven rods by elements of capillary (172) and/or rods to form a cladding region;

(b) providing a preform (170) of said preform elements by arranging said preform elements in a pre- determined periodic structure, and optionally consolidating said structure, preferably by a heat treatment; and

(c) drawing said preform to an optical fibre with predetermined dimension under a controlled heat treatment .

26. Use of an optical fibre as defined to any of the claims 1-24, or an optical fibre produced in a method as defined in claim 25, in an optical communication system, in an optical fibre laser, in an optical fibre amplifier, or in one or more parts thereof.