WO2001023926A1 - Single mode optical waveguide fibre - Google Patents

Single mode optical waveguide fibre Download PDF

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Publication number
WO2001023926A1
WO2001023926A1 PCT/AU2000/000919 AU0000919W WO0123926A1 WO 2001023926 A1 WO2001023926 A1 WO 2001023926A1 AU 0000919 W AU0000919 W AU 0000919W WO 0123926 A1 WO0123926 A1 WO 0123926A1
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WO
WIPO (PCT)
Prior art keywords
fibre
optical waveguide
region
side core
waveguide fibre
Prior art date
Application number
PCT/AU2000/000919
Other languages
French (fr)
Inventor
John Love
Ronald Bailey
Simon Fleming
Mark Sceats
David Thorncraft
Peter Krug
Leon Poladian
Michael Large
Thomas Ryan
Original Assignee
The University Of Sydney
The Australian National University
Australian Photonics Pty Ltd
Korea Telecom Research And Development Group
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by The University Of Sydney, The Australian National University, Australian Photonics Pty Ltd, Korea Telecom Research And Development Group filed Critical The University Of Sydney
Priority to AU61408/00A priority Critical patent/AU767621B2/en
Publication of WO2001023926A1 publication Critical patent/WO2001023926A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03661—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 4 layers only
    • G02B6/03666—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 4 layers only arranged - + - +
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/018—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD] by glass deposition on a glass substrate, e.g. by inside-, modified-, plasma-, or plasma modified- chemical vapour deposition [ICVD, MCVD, PCVD, PMCVD], i.e. by thin layer coating on the inside or outside of a glass tube or on a glass rod
    • C03B37/01861—Means for changing or stabilising the diameter or form of tubes or rods
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03605—Highest refractive index not on central axis
    • G02B6/03611—Highest index adjacent to central axis region, e.g. annular core, coaxial ring, centreline depression affecting waveguiding
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03622—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only
    • G02B6/03627—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 2 layers only arranged - +
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
    • G02B6/03644—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only arranged - + -
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/10—Internal structure or shape details
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/10—Internal structure or shape details
    • C03B2203/22—Radial profile of refractive index, composition or softening point
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/10—Internal structure or shape details
    • C03B2203/22—Radial profile of refractive index, composition or softening point
    • C03B2203/24—Single mode [SM or monomode]
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02004—Optical fibres with cladding with or without a coating characterised by the core effective area or mode field radius
    • G02B6/02009—Large effective area or mode field radius, e.g. to reduce nonlinear effects in single mode fibres
    • G02B6/02014—Effective area greater than 60 square microns in the C band, i.e. 1530-1565 nm
    • G02B6/02019—Effective area greater than 90 square microns in the C band, i.e. 1530-1565 nm
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02214—Optical fibres with cladding with or without a coating tailored to obtain the desired dispersion, e.g. dispersion shifted, dispersion flattened
    • G02B6/02219—Characterised by the wavelength dispersion properties in the silica low loss window around 1550 nm, i.e. S, C, L and U bands from 1460-1675 nm
    • G02B6/02228—Dispersion flattened fibres, i.e. having a low dispersion variation over an extended wavelength range
    • G02B6/02238—Low dispersion slope fibres
    • G02B6/02242—Low dispersion slope fibres having a dispersion slope <0.06 ps/km/nm2

Definitions

  • This invention relates to a single mode optical waveguide fibre and preferably to an optical fibre of a type that exhibits low but non-zero dispersion at a wavelength ⁇ typically in the order of 1550 nm.
  • the optical fibre is, for convenience, referred to in this specification and more generally as a non-zero dispersion shifted fibre.
  • a conventional single mode fibre typically exhibits zero dispersion in the 1310 nm wavelength region, but high dispersion (in the order of -17 ps n ⁇ f " " " km -1 ) in the 1550 nm region.
  • SMF single mode fibre
  • Dispersion shifted fibre has been developed to take advantage of the inherently low attenuating properties of optical fibre at 1550 nm and the availability of fibre amplifiers, but dispersion shifted fibre exhibits enhanced non-linear effects such as four-wave mixing (FWM) and self- phase modulation (SPM) .
  • Non-zero dispersion shifted fibre has been developed to avoid the non-linear effects of the DSF fibre and for use in telecommunication systems that employ high power lasers, high bit rate transmissions and wavelength division multiplexing (WDM) .
  • Non-zero dispersion shifted fibre typically has a zero dispersion wavelength positioned slightly outside of the range 1530 nm to 1570 nm.
  • Prior art non-zero dispersion shifted fibres that have been sold commercially and described in the relevant literature have a central core region and at least one circularly symmetrical annular region positioned within the light guiding region of the fibres .
  • the central core region has an average refractive index which is different from that of the surrounding annular region and, in the case of a fibre having two annular regions, the outer annular region has an average refractive index that is higher than that of the inner annular region.
  • the average refractive index of the core region normally is greater than that of both of the annular regions .
  • the present invention has evolved from the development of a fibre geometry that permits a greater number of degrees of freedom to be exploited in the design of nonzero dispersion shifted optical waveguide fibre for use in various applications.
  • the present invention provides a single mode optical waveguide fibre having a light guiding region that includes a central core region, a surrounding region that surrounds the central core region, and at least three angularly separated regions disposed radially outwardly from the central core region.
  • the central core region has an average refractive index n 0
  • the surrounding region has an average refractive index n ⁇ no
  • each of the angularly separated regions has an average refractive index n 2 ⁇ n ⁇ .
  • the outwardly disposed, angularly separated regions may be considered as "side core regions" and are hereinafter referred to as such.
  • the side core regions may be composed of any transparent medium, such as silica or doped silica or may be formed as channel-like voids that extend parallel to the central core. In the latter case, the voids may be occupied by a vacuum or a gas or be filled with other transparent material .
  • the invention as above defined differs from known non- zero dispersion shifted fibres, in that the side core regions are provided in lieu of the annular regions that surround the central core in the known fibres.
  • the fibre in accordance with the present invention does not have circular symmetry in cross-section, although two or more of the side core regions may be positioned on a common notional circle.
  • the characteristics of the fibre in accordance with the present invention may be varied from one fibre to another by varying any one or more of the following elements in the fibre: (a) The average refractive index n 0 and the radial profile of the refractive index of the central core region of the fibre .
  • the side core regions preferably are positioned equi- angularly around the central core region and preferably have a common cross-sectional configuration.
  • the side core regions may be positioned and configured in an irregular manner, provided that the overall geometry does not give rise to unwanted artefacts, for example unwanted birefringence .
  • the optical fibre in accordance with the present invention most preferably has at least four equi-angularly positioned side core regions, and all of the side core regions preferably have a common cross-sectional size and configuration. Furthermore, each of the side core regions, when composed of silica or doped silica preferably has a generally arcuate configuration.
  • the voids preferably are formed as holes that surround and extend parallel to the core region. The holes preferably have approximately circular cross-section.
  • Figure 1 shows a diagrammatic (idealised) representation of the cross-section of an optical fibre that incorporates side core regions.
  • Figures 2A and 2B show refractive index profiles that are applicable to the optical fibre shown in Figure 1 and as seen in the directions of section planes A-A and B-B in Figure 1.
  • Figure 3 shows a cross-sectional representation of an optical fibre that has been designed with side core regions to exhibit a very small dispersion slope over the wavelength region 1530 to 1570 nm.
  • Figures 4A and 4B show refractive index profiles that are applicable to the optical fibre shown in Figure 3 and as seen in the directions of section planes A-A and B-B in Figure 3.
  • Figure 5 shows a cross-sectional representation of an optical ' fibre that has been designed with side core regions to exhibit a non-linear effective area approaching 100 ⁇ m 2 .
  • Figures 6A to 6B show refractive index profiles that are applicable to the optical fibre shown in Figure 5 and as seen in the directions of section planes A-A and B-B in Figure 5.
  • Figures 7 and 8 show cross-sectional representations of optical fibres that incorporate side core regions in the form of channel-like voids.
  • Figure 9 shows graphs of group velocity dispersion (GVD) against wavelength for the optical fibres of Figures 7 and 8 as compared with the GVD/wavelength graph of a "standard" optical fibre.
  • GVD group velocity dispersion
  • Figure 1 shows a diagrammatic representation of the cross-section of one form of an optical fibre that embodies the present invention.
  • the diameter of a cladding portion 10 of the fibre will typically have a diameter in the order of 3 Ox that of a central core region 11 of the fibre.
  • the region of the fibre through which a major portion of transmitted light is guided may be considered for convenience as being bounded by the inner margin 12 of the cladding 10 in the case of the fibre as illustrated in Figure 1. More specifically, the light guiding region includes the central core region 11 and four angularly spaced side core regions 13, each of which is disposed radially outwardly from the central core 11.
  • the central core region 11 is located within a core- surrounding region 14 which extends outwardly to the inner margin 12 of the cladding and, as illustrated, the side core regions 13 are disposed within the core-surrounding region 14.
  • the boundary 12 between the core surrounding region 14 and the cladding 10 may not be delineated clearly and that the side cores 13 may be disposed at least partially within the cladding 10 of the fibre, as in the fibre that is illustrated in Figure 5.
  • the light guiding region may extend into the cladding 10 and need not be bounded by the inner margin 12 of the cladding.
  • the central core region 11 and the side core regions 13 may have average refractive indexes n Q and n 2 that are enhanced relative to that of undoped silica, and the core surrounding region 14 may have an average refractive index ni that is depressed relative to that of undoped silica.
  • index relationships are indicated in Figures 2A and 2B in respect of the fibre cross-section that is illustrated in Figure 1.
  • the fibre has four equi-angularly spaced side core regions 13 , although it will be understood that the fibre may be fabricated with three or more side core regions.
  • each of the side core regions 13 has a generally arcuate cross-sectional configuration.
  • the refractive index profiles of the above described fibre, as seen in the directions of section planes A-A and B-B, are shown in Figures 2A and 2B.
  • the fibre as illustrated in Figure 1 may be manufactured in various ways, one of which is described briefly as follows by way of example.
  • the fibre will be drawn from a preform that is fabricated using modified chemical vapour deposition of required material within an undoped silica tube. Portions of the preform corresponding to the side core regions 13 will be formed by depositing doped silica to a required thickness within the silica tube and by etching away portions of the deposited material to leave four equi- spaced longitudinally extending lands of the doped silica. Thereafter, further layers of differently doped silica will be deposited within the tube, including over the lands, to form the core-surrounding region 14 and the central core region 11 of the fibre to be drawn from the preform. Finally, the entire structure, including the deposited material, will be collapsed in the usual manner to form a solid preform from which the fibre may be drawn.
  • Figure 3 shows a diagrammatic representation of the cross-section of a second form of optical fibre that embodies the features of the present invention. This is similar to that shown in Figure 1 and like reference numerals are employed to indicate like elements .
  • the fibre as represented in Figures 3 and 4 exhibits a substantially constant dispersion across the EDFA band, and properties of the fibre at a wavelength of 1550 nm are summarised as follows : Dispersion +3.41 ps n ⁇ rf 1 k ⁇ rf 1
  • Figure 5 shows a diagrammatic representation of the cross-section of a third form of optical fibre that embodies the features of the invention. Here again, this is somewhat similar to that shown in Figure 1 and. like reference numerals are employed to identify like elements .
  • Diameter (12) of core-surrounding region 14 10.6 ⁇ m
  • Dimension of each side core 13 3.39 x 3.84 ⁇ Radial displacement of each side core 14.0 ⁇ m Refractive index of cladding 10 1.444
  • Refractive index peak of core region 11 1.455 Refractive index peak of side cores 13 1.459 (graded) Refractive index peak of core-surrounding region 14 1.441
  • the refractive index profiles of the fibre of Figure 5, as seen in the directions of section planes A-A and B-B, are shown in Figures 6A and 6B respectively.
  • the fibre as represented in Figures 5 and 6 has a nonlinear mode area of 85 ⁇ m 2 , and the properties of the fibre at a wavelength of 1550 nm are summarised as follows:
  • the fibre as represented in Figures 5 and 6 has a Petermann II area much smaller than the non-linear area. This facilitates low bend losses and permits the splicing of the fibre to a standard single mode fibre with low loss, typically less than 0.5dB.
  • Figure 7 shows a diagrammatic representation of the cross-section of an alternative form of optical fibre that embodies the features of the invention. This shares some of the features of the fibre that is illustrated in Figure 1 and, to a certain extent, like reference numerals are employed to identify like elements.
  • the side core regions 13 in the previously described embodiments are composed of silica which is doped to establish refractive index peaks in the order of 1.454 to 1.459
  • the side core regions 13 in the Figure 7 embodiment comprise channel-like voids or, expressed in an alternative way, longitudinally extending holes 13.
  • Six holes 13 are positioned geometrically on the apexes of a notional hexagon at a radial distance in the order of 1.7 ⁇ m from the axial centre of the fibre. Each hole has a diameter in the order of 1.5 ⁇ m.
  • FIG 8 shows a variation of the fibre which is illustrated in Figure 7 and in which two concentrically disposed hexagonal arrays of holes 13a and 13b are provided in the light guiding region of the fibre.
  • each of the holes 13 has a diameter in the order of 1.5 ⁇ m.
  • the inner array 13a of holes is radially disposed 1.6 ⁇ m from the centre of the fibre and the outer array 13b of holes is radially disposed 3.2 ⁇ m from the centre of the fibre.
  • FIG. 9 shows graphs of group velocity dispersion (GVD) against wavelength for the fibres of Figures 7 and 8 and, purely for comparison, for a standard single mode fibre. It is apparent from the graphs of Figure 9 that little benefit is derived from the provision of concentric arrays of holes as compared with the single array in the fibre as shown in Figure 7.
  • VTD group velocity dispersion
  • optical fibres as previously described in the specification and illustrated in the drawings are but a few of a vast number of fibres that may be produced, to meet various requirements, by varying one or more of the characteristic features of the invention as defined in the following claims

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Abstract

A single mode optical waveguide fibre is disclosed which preferably is formed to exhibit low but non-zero dispersion at a wavelength μ in the order of 1550 nm. The fibre has a light guiding region that includes a central core region (11), a core-surrounding region (14) and at least three angularly separated side core regions (13) that are disposed radially outwardly from the central core region. The central core region (11) has an average refractive region n0, the surrounding region (14) has an average refractive index n1<n0, and each of the angularly separated regions (13) have an average refractive index n2≠n1.

Description

SINGLE MODE OPTICAL WAVEGUIDE FIBRE
FIELD OF THE INVENTION
This invention relates to a single mode optical waveguide fibre and preferably to an optical fibre of a type that exhibits low but non-zero dispersion at a wavelength λ typically in the order of 1550 nm. The optical fibre is, for convenience, referred to in this specification and more generally as a non-zero dispersion shifted fibre.
BACKGROUND OF THE INVENTION
A conventional single mode fibre (SMF) typically exhibits zero dispersion in the 1310 nm wavelength region, but high dispersion (in the order of -17 ps nπf"""km-1) in the 1550 nm region. In this specification the convention that assumes SMF has negative dispersion at λ = 1550 nm is adopted.
Dispersion shifted fibre (DSF) has been developed to take advantage of the inherently low attenuating properties of optical fibre at 1550 nm and the availability of fibre amplifiers, but dispersion shifted fibre exhibits enhanced non-linear effects such as four-wave mixing (FWM) and self- phase modulation (SPM) . Non-zero dispersion shifted fibre (NZDSF) has been developed to avoid the non-linear effects of the DSF fibre and for use in telecommunication systems that employ high power lasers, high bit rate transmissions and wavelength division multiplexing (WDM) . Non-zero dispersion shifted fibre typically has a zero dispersion wavelength positioned slightly outside of the range 1530 nm to 1570 nm.
Prior art non-zero dispersion shifted fibres that have been sold commercially and described in the relevant literature have a central core region and at least one circularly symmetrical annular region positioned within the light guiding region of the fibres . The central core region has an average refractive index which is different from that of the surrounding annular region and, in the case of a fibre having two annular regions, the outer annular region has an average refractive index that is higher than that of the inner annular region. The average refractive index of the core region normally is greater than that of both of the annular regions .
SUMMARY OF THE INVENTION The present invention has evolved from the development of a fibre geometry that permits a greater number of degrees of freedom to be exploited in the design of nonzero dispersion shifted optical waveguide fibre for use in various applications. Broadly defined, the present invention provides a single mode optical waveguide fibre having a light guiding region that includes a central core region, a surrounding region that surrounds the central core region, and at least three angularly separated regions disposed radially outwardly from the central core region. The central core region has an average refractive index n0, the surrounding region has an average refractive index nι<no, and each of the angularly separated regions has an average refractive index n2≠nχ . The outwardly disposed, angularly separated regions may be considered as "side core regions" and are hereinafter referred to as such.
The side core regions may be composed of any transparent medium, such as silica or doped silica or may be formed as channel-like voids that extend parallel to the central core. In the latter case, the voids may be occupied by a vacuum or a gas or be filled with other transparent material .
The invention as above defined differs from known non- zero dispersion shifted fibres, in that the side core regions are provided in lieu of the annular regions that surround the central core in the known fibres. Thus, the fibre in accordance with the present invention does not have circular symmetry in cross-section, although two or more of the side core regions may be positioned on a common notional circle.
The characteristics of the fibre in accordance with the present invention may be varied from one fibre to another by varying any one or more of the following elements in the fibre: (a) The average refractive index n0 and the radial profile of the refractive index of the central core region of the fibre .
(b) The cross-sectional area of the central core region of the fibre. (c) The average refractive index ni and the radial profile of the refractive index of the region surrounding the central core region of the fibre.
(d) The cross-sectional area of the region surrounding the central core region of the fibre. (e) The average refractive index n2 and the radial and circumferential profiles of the refractive index of the side core regions of the fibre.
(f) The cross-sectional area of each of the side core regions of the fibre. (g) The configuration of each of the side core regions of the fibre.
(h) The number of the side core regions in the fibre.
(i) The spatial relationship of the side core regions in the fibre.
PREFERRED FEATURES OF THE INVENTION
The side core regions preferably are positioned equi- angularly around the central core region and preferably have a common cross-sectional configuration. However, the side core regions may be positioned and configured in an irregular manner, provided that the overall geometry does not give rise to unwanted artefacts, for example unwanted birefringence .
The optical fibre in accordance with the present invention most preferably has at least four equi-angularly positioned side core regions, and all of the side core regions preferably have a common cross-sectional size and configuration. Furthermore, each of the side core regions, when composed of silica or doped silica preferably has a generally arcuate configuration. In the case of an optical fibre having side core regions formed as channel-like voids, the voids preferably are formed as holes that surround and extend parallel to the core region. The holes preferably have approximately circular cross-section. The invention will be more fully understood from the following description of preferred embodiments of single mode non-zero dispersion shifted optical fibres and a preferred method of forming a preform from which optical fibre may be drawn. The description is provided with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings -
Figure 1 shows a diagrammatic (idealised) representation of the cross-section of an optical fibre that incorporates side core regions.
Figures 2A and 2B show refractive index profiles that are applicable to the optical fibre shown in Figure 1 and as seen in the directions of section planes A-A and B-B in Figure 1.
Figure 3 shows a cross-sectional representation of an optical fibre that has been designed with side core regions to exhibit a very small dispersion slope over the wavelength region 1530 to 1570 nm. Figures 4A and 4B show refractive index profiles that are applicable to the optical fibre shown in Figure 3 and as seen in the directions of section planes A-A and B-B in Figure 3.
Figure 5 shows a cross-sectional representation of an optical' fibre that has been designed with side core regions to exhibit a non-linear effective area approaching 100 μm2. Figures 6A to 6B show refractive index profiles that are applicable to the optical fibre shown in Figure 5 and as seen in the directions of section planes A-A and B-B in Figure 5. Figures 7 and 8 show cross-sectional representations of optical fibres that incorporate side core regions in the form of channel-like voids.
Figure 9 shows graphs of group velocity dispersion (GVD) against wavelength for the optical fibres of Figures 7 and 8 as compared with the GVD/wavelength graph of a "standard" optical fibre.
DETAILED DESCRIPTION OF THE INVENTION
In making reference to the drawings, Figure 1 shows a diagrammatic representation of the cross-section of one form of an optical fibre that embodies the present invention. However, it will be understood that the various concentric regions that are shown in Figure 1 are not drawn to scale. The diameter of a cladding portion 10 of the fibre will typically have a diameter in the order of 3 Ox that of a central core region 11 of the fibre.
The region of the fibre through which a major portion of transmitted light is guided (herein referred to as "the light guiding region" ) may be considered for convenience as being bounded by the inner margin 12 of the cladding 10 in the case of the fibre as illustrated in Figure 1. More specifically, the light guiding region includes the central core region 11 and four angularly spaced side core regions 13, each of which is disposed radially outwardly from the central core 11. The central core region 11 is located within a core- surrounding region 14 which extends outwardly to the inner margin 12 of the cladding and, as illustrated, the side core regions 13 are disposed within the core-surrounding region 14. However, it should be understood that the boundary 12 between the core surrounding region 14 and the cladding 10 may not be delineated clearly and that the side cores 13 may be disposed at least partially within the cladding 10 of the fibre, as in the fibre that is illustrated in Figure 5. With this in mind it will be understood that the light guiding region may extend into the cladding 10 and need not be bounded by the inner margin 12 of the cladding.
The relationship of the refractive indexes of the various regions of the optical fibre will be dependent upon the characteristics required of the fibre for any given application. However, as an example, the central core region 11 and the side core regions 13 may have average refractive indexes nQ and n2 that are enhanced relative to that of undoped silica, and the core surrounding region 14 may have an average refractive index ni that is depressed relative to that of undoped silica. These index relationships are indicated in Figures 2A and 2B in respect of the fibre cross-section that is illustrated in Figure 1. The fibre has four equi-angularly spaced side core regions 13 , although it will be understood that the fibre may be fabricated with three or more side core regions. Again depending on the characteristics required of the fibre, the side core regions 13 will normally be disposed on a common circle, that is at equal radial distances from the axis of the fibre, and the side core regions 13 will normally have substantially the same cross-sectional configurations. As illustrated, each of the side core regions 13 has a generally arcuate cross-sectional configuration. The refractive index profiles of the above described fibre, as seen in the directions of section planes A-A and B-B, are shown in Figures 2A and 2B.
The fibre as illustrated in Figure 1 may be manufactured in various ways, one of which is described briefly as follows by way of example.
The fibre will be drawn from a preform that is fabricated using modified chemical vapour deposition of required material within an undoped silica tube. Portions of the preform corresponding to the side core regions 13 will be formed by depositing doped silica to a required thickness within the silica tube and by etching away portions of the deposited material to leave four equi- spaced longitudinally extending lands of the doped silica. Thereafter, further layers of differently doped silica will be deposited within the tube, including over the lands, to form the core-surrounding region 14 and the central core region 11 of the fibre to be drawn from the preform. Finally, the entire structure, including the deposited material, will be collapsed in the usual manner to form a solid preform from which the fibre may be drawn.
Figure 3 shows a diagrammatic representation of the cross-section of a second form of optical fibre that embodies the features of the present invention. This is similar to that shown in Figure 1 and like reference numerals are employed to indicate like elements .
Characteristic features of the fibre as illustrated in Figure 3 are summarised as follows: Diameter of cladding 10 125 μm Diameter of central core region 11 8.4 μ
Diameter (12) of core-surrounding region 14 20 μ
Dimension of each side region core 13 1.72 x 6.36 μ
Radial displacement of each side core region 8.0 μ Refractive index peak of cladding 10 1.444 Refractive index peak of central core region 11 1.454 Refractive index peak of side core regions 13 1.454 (uniform) Refractive index peak of core-surrounding region 14 1.441
The refractive index profiles of the fibre of Figure 3 as seen in the directions of section planes A-A and B-B are shown in Figures 4A and 4B respectively.
The fibre as represented in Figures 3 and 4 exhibits a substantially constant dispersion across the EDFA band, and properties of the fibre at a wavelength of 1550 nm are summarised as follows : Dispersion +3.41 ps nιrf1kιrf1
Dispersion slope -0.004 ps nm~2km_1
Cutoff wavelength 1420 nm
Peter ann II area 36.4 μm2
Non-linear area 35.2 μm2 The fibre as represented in Figures 3 and 4 exhibits a dispersion of +3.57 ps rrrf1km"1 at λ = 1530 and +3.35 ps nm"1]™"1 at λ = 1570.
Figure 5 shows a diagrammatic representation of the cross-section of a third form of optical fibre that embodies the features of the invention. Here again, this is somewhat similar to that shown in Figure 1 and. like reference numerals are employed to identify like elements .
Characteristic features of the fibre as illustrated in Figure 5 are summarised as : Diameter of cladding 10 125 μm
Diameter of core region 11 6.3 μm
Diameter (12) of core-surrounding region 14 10.6 μm Dimension of each side core 13 3.39 x 3.84 μ Radial displacement of each side core 14.0 μm Refractive index of cladding 10 1.444
Refractive index peak of core region 11 1.455 Refractive index peak of side cores 13 1.459 (graded) Refractive index peak of core-surrounding region 14 1.441 The refractive index profiles of the fibre of Figure 5, as seen in the directions of section planes A-A and B-B, are shown in Figures 6A and 6B respectively.
The fibre as represented in Figures 5 and 6 has a nonlinear mode area of 85 μm2 , and the properties of the fibre at a wavelength of 1550 nm are summarised as follows:
Dispersion -2.56 ps nm"1km~1
Dispersion slope +0.083 ps nm"2km"1
Cutoff wavelength 1271 nm Petermann II area 51.4 μm2
Non-linear area 85.4 μm2
It is to be observed that the fibre as represented in Figures 5 and 6 has a Petermann II area much smaller than the non-linear area. This facilitates low bend losses and permits the splicing of the fibre to a standard single mode fibre with low loss, typically less than 0.5dB.
Figure 7 shows a diagrammatic representation of the cross-section of an alternative form of optical fibre that embodies the features of the invention. This shares some of the features of the fibre that is illustrated in Figure 1 and, to a certain extent, like reference numerals are employed to identify like elements. However, whereas the side core regions 13 in the previously described embodiments are composed of silica which is doped to establish refractive index peaks in the order of 1.454 to 1.459, the side core regions 13 in the Figure 7 embodiment comprise channel-like voids or, expressed in an alternative way, longitudinally extending holes 13.
Six holes 13 are positioned geometrically on the apexes of a notional hexagon at a radial distance in the order of 1.7 μm from the axial centre of the fibre. Each hole has a diameter in the order of 1.5 μm.
Figure 8 shows a variation of the fibre which is illustrated in Figure 7 and in which two concentrically disposed hexagonal arrays of holes 13a and 13b are provided in the light guiding region of the fibre. In this case each of the holes 13 has a diameter in the order of 1.5 μm. The inner array 13a of holes is radially disposed 1.6 μm from the centre of the fibre and the outer array 13b of holes is radially disposed 3.2 μm from the centre of the fibre.
Figure 9 shows graphs of group velocity dispersion (GVD) against wavelength for the fibres of Figures 7 and 8 and, purely for comparison, for a standard single mode fibre. It is apparent from the graphs of Figure 9 that little benefit is derived from the provision of concentric arrays of holes as compared with the single array in the fibre as shown in Figure 7.
The optical fibres as previously described in the specification and illustrated in the drawings are but a few of a vast number of fibres that may be produced, to meet various requirements, by varying one or more of the characteristic features of the invention as defined in the following claims

Claims

THE CLAIMS
1. A single mode optical waveguide fibre having a light guiding region that includes a central core region, a surrounding region that surrounds the central core region and at least three angularly separated side core regions that are disposed radially outwardly from the central core region; the central core region having an average refractive index no, the surrounding region having a refractive index ni < n0, and each of the side core regions having an average refractive index n2 ≠ ni .
2. The optical waveguide fibre as claimed in claim 1 wherein each of the side core regions comprises a transparent optical medium.
3. The optical waveguide fibre as claimed in claim 2 wherein each of the side core regions is composed of doped silica.
4. The optical waveguide fibre as claimed in claim 1 wherein each of the side core regions is formed as a longitudinally extending channel-like void.
5. The optical waveguide fibre as claimed in claim 4 wherein each channel-like void is filled with a t . o .m.
6. The optical waveguide fibre as claimed in any one of claims 1 to 5 wherein at least four of the side core regions are disposed radially about the central core region.
7. The optical waveguide fibre as claimed in any one of claims 1 to 6 wherein the side core regions are positioned equi-angularly about the central core region.
8. The optical waveguide fibre as claimed in any one of claims 1 to 7 wherein the side core regions have a common cross-sectional size and configuration.
9. The optical waveguide fibre as claimed in claim 3 wherein each of the side core regions has a generally arcuate cross-sectional configuration.
10. The optical waveguide fibre as claimed in claim 3 wherein each of the side core regions has a generally rectangular cross-sectional configuration.
11. The optical waveguide fibre as claimed in claim 4 wherein each of the side core regions is in the form of a hole having a cross-section that is approximately circular.
12. The optical waveguide fibre as claimed in claim 11 wherein the side core regions are disposed in a ring that surrounds the central core region.
13. The optical waveguide fibre as claimed in claim 11 wherein the side core regions are disposed in two concentric rings that surround the central core region.
14. The optical waveguide fibre as claimed in any one of the preceding claims when in the form of a fibre having a doped silica core, that incorporates the central core region and the surrounding region, and a silica cladding.
15. The optical waveguide fibre as claimed in claim 14 wherein the side core regions are located within the surrounding region.
16. The optical waveguide fibre as claimed in claim 14 wherein the side core regions are located at least in part within the silica cladding.
17. The optical waveguide fibre as claimed in any one of the preceding claims wherein the central core region and the side core regions have average refractive indexes in n2 that are enhanced relative to that of undoped silica and wherein the surrounding region has an average refractive index ni that is depressed relative to that of undoped silica.
18. The optical waveguide fibre substantially as hereinbefore described with reference to Figures 1 and 2, Figures 3 and 4, Figures 5 and 6, Figures 7 and 9 or Figures 8 and 9 of the accompanying drawings .
PCT/AU2000/000919 1999-09-30 2000-08-02 Single mode optical waveguide fibre WO2001023926A1 (en)

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AUPQ3207A AUPQ320799A0 (en) 1999-09-30 1999-09-30 Non-zero dispersion shifted optical fibre

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JPH05341147A (en) * 1992-06-12 1993-12-24 Asahi Chem Ind Co Ltd Multi-core type single mode optical fiber and transmission using it
US5734773A (en) * 1994-05-24 1998-03-31 Asahi Kasei Kogyo Kabushiki Kaisha Multicore plastic optical fiber for light signal transmission
US5802236A (en) * 1997-02-14 1998-09-01 Lucent Technologies Inc. Article comprising a micro-structured optical fiber, and method of making such fiber
WO1999000685A1 (en) * 1997-06-26 1999-01-07 The Secretary Of State For Defence Single mode optical fibre

Patent Citations (5)

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Publication number Priority date Publication date Assignee Title
US4913521A (en) * 1987-12-04 1990-04-03 Nippon Telegraph And Telephone Corporation Single-polarization optical fiber
JPH05341147A (en) * 1992-06-12 1993-12-24 Asahi Chem Ind Co Ltd Multi-core type single mode optical fiber and transmission using it
US5734773A (en) * 1994-05-24 1998-03-31 Asahi Kasei Kogyo Kabushiki Kaisha Multicore plastic optical fiber for light signal transmission
US5802236A (en) * 1997-02-14 1998-09-01 Lucent Technologies Inc. Article comprising a micro-structured optical fiber, and method of making such fiber
WO1999000685A1 (en) * 1997-06-26 1999-01-07 The Secretary Of State For Defence Single mode optical fibre

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