Field of Invention

This invention relates to an amplifying optical device, and more especially, this invention relates to an amplifying optical device for providing optical amplification. The invention has relevance for lasers and optical amplifiers, and in particular for high power fibre lasers and fibre amplifiers.

Background to the Invention

High power fibre lasers have important industrial applications in laser marking, cutting, welding brazing and cladding, as well as in new applications such as additive manufacturing. High power fibre amplifiers may be deployed in high peak power pulsed systems, and in directed energy systems, in which the output beams from a plurality of fibre amplifiers are combined into a single beam. As the application areas of fibre lasers and fibre amplifiers expand, the average powers being emitted from each fibre (before beam combination) are increasing from around 100W to lkW, 2kW or higher. Unfortunately, as the powers increase, non-linear effects begin to have increasing importance, and ultimately limit the power that can be emitted from the fibre. This is particularly important for single mode optical fibres, or narrowband (<20GHz) amplifiers being developed for directed energy applications. These non-linear effects can be reduced by reducing the length of the amplifying optical fibres within the fibre laser or fibre amplifier, which length is primarily determined by the absorption length of the pump radiation that is coupled into the amplifying optical fibre. There is therefore a need to reduce the absorption length of amplifying optical fibres in high power fibre lasers and fibre amplifiers. An advantage of fibre lasers and fibre amplifiers over more traditional lasers and amplifiers such as ytterbium aluminium garnet (YAG) lasers and carbon dioxide (C0 ₂ ) lasers is wall plug efficiency. Wall plug efficiency is defined as the ratio of the output power emitted by the fibre laser to the electrical power required to drive the laser and its ancillary equipment such as water coolers or chillers. High wall plug efficiency is beneficial in industrial and directed energy applications. The wall plug efficiency can be increased by reducing the absorption length of the amplifying optical fibre. There is therefore a further need to reduce the absorption length of amplifying optical fibres in high power fibre lasers and fibre amplifiers.

Cladding pumping is described in "High Power Fiber Lasers: A Review", IEEE J. of Selected Topics in Quantum Electronics, September / October 2014, Zervas and Codemard, paper number 0904123. In conventional end-pumped optical fibre lasers, the absorption length of the amplifying optical fibre is reduced by introducing azimuthal asymmetries into the amplifying optical fibre, such as facets on its inner cladding. Such special geometries improve pump absorption, thereby reducing the absorption length of the amplifying optical fibre. Fibres with polygonal claddings are commonly used. However machining such facets introduces additional steps in the fibre manufacturing process which can increase pump light scattering (thus increasing losses and reducing wall plug efficiency) and make it more difficult for the fibre core to be maintained concentric with the surfaces of the outer cladding. There is therefore a need to reduce the absorption length of amplifying optical fibres without increasing scattering losses or manufacturing complexity.

PCT Patent Application no. WO00/67350 describes an optical fibre arrangement comprising at least two optical fibre sections, each having an outside longitudinally extending surface, and the outside longitudinally extending surfaces being in optical contact with each other. As described in the corresponding European Patent No. EP 1,873,874 Bl, the optical fibre arrangement can be used to make an amplifying optical device for providing optical

amplification. One of the optical fibre sections can be a pump optical fibre, and the other optical fibre section can be an amplifying optical fibre. Pump radiation input into the pump optical fibre is coupled across to the amplifying optical fibre along its length. Optical fibre arrangements having non-circular amplifying optical fibre or pump optical fibre are disclosed, but not for the purpose of improving pump absorption. Indeed, it is stated explicitly that the optical fibre arrangement eliminates the need for special geometries for improved pump absorption.

An aim of the present invention is to provide an amplifying optical device which reduces the above aforementioned problems.

The Invention:

Accordingly, in one non-limiting embodiment of the present invention, there is provided an amplifying optical device for providing optical amplification, the amplifying optical device comprising at least one pump for supplying pump radiation, at least one pump optical fibre and at least one amplifying optical fibre, and the amplifying optical device being such that:

• the pump optical fibre and the amplifying optical fibre are coated with a common coating and are in optical contact along a portion of their length,

• the pump optical fibre is separated from the amplifying optical fibre at two end

portions of the common coating,

• one of the separated end portions of the pump optical fibre is connected to the pump,

• the pump optical fibre and the amplifying optical fibre comprise glass,

• the amplifying optical fibre comprises a core and a cladding,

• the core is doped with at least one rare earth dopant, and the amplifying optical device being characterized in that:

• the pump optical fibre comprises a mode coupling means in the form of a variation of refractive index with azimuth along at least one circle of constant radius, which variation has at least one azimuthal harmonic having an amplitude greater than an amplitude of a first and a second azimuthal harmonic, and which thereby couples guided modes of the pump optical fibre to guided modes of the amplifying optical fibre that overlap and can be absorbed by the rare earth dopant in the core. The guided modes of the amplifying fibre referred to above are those guided by the cladding against the coating. These modes are for pump radiation for pumping the rare earth dopant in the core.

The mode coupling means may be one that preferentially increases the coupling of the guided modes of the pump optical fibre to the guided modes of the amplifying optical fibre that overlap the core compared to a pump fibre that does not include the mode coupling means. The mode coupling means may be such that at least 75% of the guided modes of the pump optical fibre couple to the guided modes of the amplifying optical fibre that overlap the core. The mode coupling means may be such that at least 90% of the guided modes of the pump optical fibre couple to the guided modes of the amplifying optical fibre that overlap the core.

An important advantage of the invention is that by increasing the coupling between the guided modes of the pump optical fibre and the guided modes of the amplifying optical fibre that overlap the core, the absorption length of the amplifying optical fibre is reduced. Shorter amplifying optical fibres, with corresponding lower losses, can be used to provide the same optical output power, but at greater wall plug efficiencies. Higher levels of pump absorption can also be achieved, thus enabling higher output powers to be obtainable before reaching power limitations caused by non-linear effects such as stimulated Brillouin scattering and stimulated Raman scattering. In addition, by introducing the mode coupling means into the pump optical fibre as opposed to the amplifying optical fibre, there is less mechanical processing needed in the manufacture of the amplifying optical fibre, and thus core concentricity is easier to ensure. Core concentricity is needed in order to splice optical fibres together with low optical loss. Low loss splices are important in order not to degrade wall plug efficiency, and to avoid thermal runaway in the splices which can lead to catastrophic damage.

The mode coupling means may comprise a non-circular cross section of the pump optical fibre. The non-circularity may comprise at least one facet on an outside surface of the pump optical fibre. The facet may be flat. The facet may be concave. Facets of other shapes may be employed.

The mode coupling means may comprise at least one doped-glass region in the pump optical fibre. The mode coupling means may comprise at least three doped-glass regions.

The mode coupling means may comprise at least one longitudinally extending hole in the pump optical fibre. The mode coupling means may comprise at least three longitudinally extending holes.

The variation of refractive index with azimuth may also be achieved via the photoelastic effect from thermal stress or from the application of mechanical stresses. The variation of refractive index may also be achieved by applying ultraviolet light, or optical radiation from ultrafast femtosecond lasers to the pump optical fibre. The variation can be uniform along the pump optical fibre, or be varied along the pump optical fibre. The pump optical fibre may have an m-fold azimuthal symmetry and wherein m is at least 3. The factor m may be a multiple of 4. The factor m may be equal to 8. The factor m may be a prime number.

The pump optical fibre may be such that it is non-circular and has no azimuthal symmetry.

The amplifying optical device can comprise a plurality of the pump coupling means at different circles of constant radius, and wherein each circle of constant radius has an azimuthal harmonic having an amplitude larger than the other azimuthal harmonics, and wherein the disposition of the mode coupling means is such that the azimuthal harmonic number having the largest amplitude increases with radius. The azimuthal harmonic having the largest amplitude corresponding to at least one of the circles of constant radius may be the second harmonic.

The cladding of the amplifying optical fibre may be a non-circular cladding.

The coating may be a polymer with a refractive index less than the refractive index of the cladding of the amplifying optical fibre, and less than the refractive index of the glass of the pump optical fibre.

The rare earth dopant may be selected from the group comprising Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium and Dysprosium, or may be doped with a transition metal or a semiconductor.

The pump optical fibre may have a substantially uniform refractive index across its glass cross-section.

The pump optical fibre may have a first diameter, and the amplifying optical fibre may have a second diameter which is less than the first diameter. The pump optical fibre and the amplifying optical fibre may comprise a length of composite optical fibre that is configured with a bend radius. The bend radius may vary along the length. Variations in bend radius along the length can improve the coupling between the guided modes of the pump optical fibre and the guided modes of the amplifying optical fibre that overlap the core, thus reducing the absorption length of the amplifying optical fibre.

The amplifying optical device may further comprise at least one optical element coupled along the length of the amplifying optical fibre, the optical element being selected from the group comprising a polariser, an isolator, a circulator, a grating, an optical fibre Bragg grating, a long-period grating, an acousto-optic modulator, an acousto-optic tuneable filter, an optical filter, a Kerr cell, a Pockels cell, a dispersive element, a non-linear dispersive element, an optical switch, a phase modulator, a Lithium Niobate modulator, and an optical crystal.

The grating may be selected from the group comprising a gain-flattened grating, a dispersion compensating grating, a gain-flattened, dispersion compensating grating, and a blazed grating.

The amplifying optical fibre may be circular with circular symmetry. At least one of the core and the cladding of the amplifying optical fibre may be tapered and spliced to a second optical fibre. Such an arrangement is advantageous over prior art cladding pumped fibres that have non-circular amplifying fibres, because tapering non-circular amplifying fibres typically increases optical attenuation of at least one of the pump radiation and the signal radiation. The circular symmetry of the amplifying optical fibre of the present invention avoids such losses.

The present invention also provides an optical fibre laser comprising the amplifying optical device of the invention and an optical feedback arrangement for promoting light generation within the optical fibre laser. Brief Description of the Drawings

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 shows an amplifying optical device according to the present invention;

Figure 2 shows a pump optical fibre having a longitudinally extending region of different refractive index such as a hole;

Figure 3 shows a pump optical fibre having an azimuthally asymmetric cladding;

Figure 4 shows an amplifying optical fibre having a polygonal cladding;

Figure 5 shows an amplifying optical device in the form of a laser;

Figure 6 shows an optical mode of an amplifying optical fibre;

Figure 7 shows another optical mode of an amplifying optical fibre;

Figure 8 shows the modal space of an amplifying optical fibre;

Figure 9 shows the modal space of a pump optical fibre and the modal space of an evanescently coupled amplifying optical fibre;

Figure 0 shows a pump optical fibre with four doped regions;

Figure 11 shows a four-sided pump optical fibre;

Figure 12 shows the variation of refractive index with azimuth in the fibres of Figures 10 and 11, and the strongest azimuthal harmonic;

Figure 13 shows an elliptical pump optical fibre not according to the present invention; Figure 14 shows a modal space mapping of an optical fibre;

Figure 15 shows coupling of groups of optical modes from a region IV to a region III; and

Figure 16 shows a non-circular pump optical fibre comprising a plurality of doped region. Preferred Embodiment

Referring to Figure 1, there is shown an amplifying optical device 10 for providing optical amplification. The amplifying optical device 10 comprising at least one pump 11 for supplying pump radiation, at least one pump optical fibre 1 and at least one amplifying optical fibre 2. The amplifying optical device 10 is such that the pump optical fibre 1 and the amplifying optical fibre 2 are coated with a common coating 3 and are in optical contact along a portion of their length 4. The pump optical fibre 1 is separated from the amplifying optical fibre 2 at two end portions 5, 6 of the common coating 3. One of the separated end portions 5 of the pump optical fibre 1 is connected to the pump 11. The pump optical fibre 1 and the amplifying optical fibre 2 comprise glass 7. The amplifying optical fibre 2 comprises a core 8 and a cladding 9. The core 8 is doped with at least one rare earth dopant 12. The amplifying optical device 10 is characterized in that: the pump optical fibre 1 comprises a mode coupling means 15 in the form of a variation of refractive index with azimuth along at least one circle 119 of constant radius, which variation has at least one azimuthal harmonic having an amplitude greater than an amplitude of a first and a second azimuthal harmonic, and which thereby couples guided modes of the pump optical fibre 1 to guided modes of the amplifying optical fibre 2 that overlap and can be absorbed by the rare earth dopant 12 in the core 8.

The guided modes of the amplifying fibre 2 referred to above are those guided by the cladding 9 against the coating 3. These modes are for pump radiation for pumping the rare earth dopant 12 in the core 8. The circle 119 is in the cross section of the pump optical fibre 1 , and is centered at the centre of mass of the pump optical fibre 1. Further details are described with reference to Figures 10 to 12.

The mode coupling means 15 may be one that preferentially increases the coupling of the guided modes of the pump optical fibre 1 to the guided modes of the amplifying optical fibre 2 that overlap the core 8 compared to a pump fibre that does not include the mode coupling means 15. The mode coupling means 15 may be such that at least 75% of the guided modes of the pump optical fibre 1 couple to the guided modes of the amplifying optical fibre 2 that overlap the core 8. The mode coupling means 15 may be such that at least 90% of the guided modes of the pump optical fibre 1 couple to the guided modes of the amplifying optical fibre 2 that overlap the core 8.

The pump optical fibre 1, the amplifying optical fibre 2, and the common coating 3 are preferably obtained from a composite optical fibre 17 by cutting the composite optical fibre 17 to length, and stripping off the common coating 3 from either end of the composite optical fibre 17. Techniques to manufacture such composite fibres that have round pump and amplifying optical fibres are described in United States Patent No. 6,826,335 which patent is hereby incorporated herein. The techniques include drawing a composite fibre using an optical fibre drawing tower, which tower has a graphite furnace, from a silica rod held adjacent to an optical fibre perform, and coating the composite fibre with a polymer during the drawing process. The optical fibre preform has the core 8 doped with rare earth dopant 12, and when drawn, forms the amplifying optical fibre 2. The silica rod, when drawn, forms the pump optical fibre 1.

The composite optical fibre 17 is characterized by a pump absorption length 16, which is the length required for pump radiation to be absorbed by the rare earth dopant 12. This is typically defined as the length required for 1/e (approximately 37%) of the pump radiation that is coupled into the composite fibre 17 to be absorbed, where "e" is Euler's number which is approximately equal to 2.71828.

An important advantage of the invention is that by increasing the coupling between the guided modes of the pump optical fibre 1 and the guided modes of the amplifying optical fibre 2 that overlap the core 8, the pump absorption length 16 of the composite fibre 17 is reduced. Consequently, it is possible to reduce the length 4 of the composite fibre 17 while maintaining the same or attaining even higher levels of pump absorption. Shorter amplifying optical fibres 2, with corresponding lower losses, can be used to provide the same optical output power, but at greater wall plug efficiencies. Higher levels of pump absorption can also be achieved, thus enabling higher output powers to be obtainable before reaching power limitations caused by nonlinear effects such as stimulated Brillouin scattering and stimulated Raman scattering.

Surprisingly, this technique enables a typical length 4 of the composite fibre 17 required to absorb 12dB to 15dB of pump radiation to be reduced from around 15 to 20m down to 10m or less. The invention enables higher optical powers and greater wall plug efficiencies without the onset of non-linearities such as stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS).

In addition, by introducing the mode coupling means 15 into the pump optical fibre 1 as opposed to the amplifying optical fibre 2, there is less mechanical processing needed in the manufacture of the amplifying optical fibre 2, and thus core concentricity is easier to ensure. Core concentricity is needed in order to splice optical fibres together with low optical loss, particularly in manufacturing environments where repeatability of reliable processes is vital. Low loss optical splices are important in order not to degrade wall plug efficiency, and to avoid thermal runaway in the splices which can lead to catastrophic damage.

The amplifying optical fibre 2 is preferably a large mode area fibre. Such a fibre has a multimode core 8 which can be operated as a single mode optical fibre, thus reducing non-linear effects and improving beam quality. Alternatively or additionally, the core 8 can be a segmented core, can comprise rings of different refractive indices, or can be a microstructured core. These designs are known to increase the mode area of the fundamental mode of the amplifying fibre 2.

The mode coupling means 15 shown in Figure 1 comprises azimuthal variations around the circumference of the pump optical fibre 1. The pump optical fibre 1 has eight facets 18 around its outer surface 19. The facets 18 are shown as flat, but they may be convex or concave. The pump optical fibre 1 can have a non-circular cross section and a diameter 1001 that is azimuthally independent; such a shape is known as a Reuleaux polygon, or curve of constant width, and may have certain advantages in minimizing splice losses when splicing to circular fibres using standard splicing equipment especially in fibre laser manufacturing environments. Facets can be machined onto silica glass rods using carbon dioxide lasers, or industrial grinding machines. Alternatively or additionally, facets can be incorporated directly into silica glass rods during their manufacture. Thus square, hexagonal, octagonal or other cross-sectional shaped rods can be formed directly.

Figure 2 shows a composite fibre 20 that has a mode coupling means 15 comprising a formation 21 which provides a variation in refractive index. The formation 21 extends along the length of the pump optical fibre 1. The formation 21 may comprise a longitudinally extending region, that can be a hole, a void, or a region of doped glass such as a borosilicate-doped stress rod. Preferably the refractive index of the formation 21 is less than the refractive index of the surrounding glass 7. The composite fibre 20 is shown as also having optional facets 18 on its outer surface 19.

The pump optical fibre 1 shown with reference to Figure 1 has 8-fold azimuthal symmetry. The pump optical fibre 12 shown with reference to Figure 2 has an outer surface 19 that also has 8-fold azimuthal symmetry. In general, the azimuthal symmetry is an m-fold azimuthal symmetry, wherein m is at least 3. The factor m can be a multiple of 4. The factor m can be equal to 8. Preferably the factor m is equal to 16. Alternatively, the factor m may be a prime number.

Figure 3 shows a composite optical fibre 30 that comprises a pump optical fibre 31 that is not circular and has no azimuthal symmetry.

Figure 4 shows a composite optical fibre 40 that comprises an amplifying optical fibre 41 that has a non-circular cladding 9. This can increase mode coupling further, but has the disadvantage that it is more difficult to ensure the core 8 is concentric with the cladding 9.

Referring to Figures 1 to 4, the coating 3 may be a polymer or a glass with a refractive index less than the refractive index of the cladding 9 of the amplifying optical fibre 2, and less than the refractive index of the glass 7 of the pump optical fibre 1. Preferably the polymer is a silicone. Silicones are known to have excellent thermal properties, an important property for use in high power lasers and amplifiers. Alternatively, the polymer can be an acrylate such as an ultra-violet or thermally cured acrylate.

The rare earth dopant 12 can be selected from the group comprising Ytterbium, Erbium, Neodymium, Praseodymium, Thulium, Samarium, Holmium, and Dysprosium, or may be doped with a transition metal or a semiconductor. The pump optical fibre 1 may have has a substantially uniform refractive index across its glass cross-section.

Referring to Figure 1, the pump optical fibre 1 is shown as having a first diameter 1001, and the amplifying optical fibre 2 is shown as having a second diameter 1002. Preferably the second diameter 1002 is less than the first diameter 1001. This is because it is desirable to use as little glass cross-sectional area as possible in the composite fibre 17 in order to maintain the brightness of the pump radiation emitted from the pump 11. If the brightness were to reduce as a result of using a larger second diameter 1002, the pump absorption length 16 would increase. It should be noted that the first diameter 1001 of the pump optical fibre 1 is preferably equal to, or slightly larger than, the diameter of a fibre pigtail (not shown) from the pump 1 1 or a pump combiner (not shown) that combines pump radiation from a plurality of pumps.

With reference to Figure 1, the amplifying optical device 10 can comprise a seed laser 1010 for providing a signal that is amplified by the composite fibre 17. The amplifying optical device 10 is then in the form of a master oscillator power amplifier.

Figure 5 shows an amplifying optical device 50 that comprises the amplifying optical device 10 and an optical element 51 coupled along the length of the amplifying optical fibre 2. The optical element 51 can be selected from the group comprising a polariser, an isolator, a circulator, a grating, an optical fibre Bragg grating, a long-period grating, an acousto-optic modulator, an acousto-optic tuneable filter, an optical filter, a Kerr cell, a Pockels cell, a dispersive element, a non-linear dispersive element, an optical switch, a phase modulator, a Lithium Niobate modulator, and an optical crystal. Preferably, the optical element 51 is a fibre Bragg grating, which may be selected from the group comprising a gain-flattened grating, a dispersion compensating grating, a gain- flattened, dispersion compensating grating, and a blazed grating.

The amplifying optical device 50 shown with reference to Figure 5 is shown in the form of an optical fibre laser comprising the amplifying optical device 10 as described with reference to Figures 1 to 4, and an optical feedback arrangement 52 and 53 for promoting light generation within the optical fibre laser. The optical feedback arrangement 52 and 53 preferably comprises fibre Bragg gratings.

One of the pumps 11 is shown as comprising a plurality of pumps 54, each having a fibre pig tail 55, and whose outputs are combined in a fibre combiner 56 that is coupled to the pump optical fibre 1, typically with a fusion splice (not shown). The pumps 54, 11 shown with reference to Figures 1 and 5 can be semiconductor laser diodes in the form of single emitters, or modules comprising many single emitters, diode bars, or diode stacks. The pumps 54, 11 can also be fibre lasers.

The amplifying optical device 50 is shown having a splice 59 between the amplifying fibre 2 and a second fibre 58. The splice 59 is preferably a fusion splice. The amplifying fibre 2 may be tapered in the splice 59. Having the mode coupling means 15 in the pump fibre 1 and not the amplifying fibre 2 enables splices with low optical loss to be made.

The amplifying optical device 50 that is in the form of a laser can be used as the seed laser 1010 in Figure 1. The apparatus of Figure 1 may also include the splice 59.

Without wishing to limit the scope of the invention, it is believed that the enhanced mode coupling achieved with the invention arises because of increased scattering or coupling between the modes of the pump optical fibre 1 and the amplifying optical fibre 2. The following discussion, which is given by way of example only, is based on typical single-mode optical amplifier fibres having a core radius of approximately 5 micron and cladding diameter of approximately 125 micron surrounded by a coating (not shown) having a lower refractive index than the refractive index of the glass of the amplifying fibre's cladding 9. The optical modes that will be described with reference to Figures 6 and 7 are the guided modes of the cladding 9 which guides against the coating 3. The numerical aperture of the cladding in typical amplifying fibres is typically greater than 0.2, and can be greater than 0.4.

Figure 6 shows the amplifying optical fibre 2, together with an optical mode 60 that is guided by the cladding 9. The optical mode 60 comprises a number of lobes 61 illustrating where the intensity of the optical mode 60 has a local maximum. There are sixteen lobes 61 around the azimuth 65 of the fibre 2, and four lobes 61 along its radius 66. Following normal convention, the optical mode 60 is an LP _p>q mode, where 2p is the number of lobes 61 around the azimuth 65, and q is the number of lobes 61 along the radius 66. The mode 60 shown is the LP ₈ ,4 mode, as there are 16 lobes 61 around the azimuth 65, and four lobes 61 along the radius 66. There are no lobes 61 that have a local maximum inside the core 8, and consequently, the optical mode 60 does not overlap the core 8.

Figure 7 shows an optical mode 70 that is the LP _4;6 mode. The mode 70 has eight lobes 61 around its azimuth, and six lobes 61 along its radius 66. There are lobes 61 that have a local maximum inside the core 8, and consequently, the optical mode 70 is one that does overlap the core 8.

The term "mode that overlaps the core" means that the optical mode has at least one lobe 61 having a local maximum that is inside the core 8 such as the mode 70 shown in Figure 7. Figure 8 shows a depiction of the LP _{p q} optical modes plotted as the radial mode number q against the azimuthal mode number p. Typical pump optical fibres 1 have many more optical modes than shown. Only modes having lower-order mode numbers have been shown for clarity. The LP ₈₄ mode 60 and the LP _4>6 modes 70 are highlighted. Also shown is a line 83 that separates modes that are in Region 1 81 and which overlap the core 8, from modes that are in Region II 82 and which do not overlap the core 8. The exact position of the line 83 will depend upon the relative sizes of the core 8 and the cladding 9, as well as refractive indices of the core 8, the cladding 9, and the coating 3.

Figure 9 depicts the modes of the pump optical fibre 1 and the modes of the amplifying optical fibre 2 alongside each other. The pump optical fibre 1 does not have a separate glass core. When the pump optical fibre 1 and the amplifying optical fibre 2 are placed alongside each other, optical modes having similar azimuthal symmetries will couple together evanescently, and the strength of such coupling increases and can be become resonant when the propagation constants β (and hence effective refractive indices) become equal to each other. The consequence of this is that there will exist a region III 91 in the mode space of the pump optical fibre 1 that couples to Region 1 81 of the mode space of the amplifying optical fibre 2, and thus the coupled power will overlap the core 8 and be absorbed. However, there will also exist a region IV 92 in the mode space of the pump optical fibre 1 that couples to Region II 82 of the mode space of the amplifying optical fibre 2, and thus the coupled power will not overlap the core 8 and will not be absorbed. Region III 91 is shown separated from Region IV 92 by the line 93. In practice, the lines 83 and 93 may not be linear as shown, but may be curved. The purpose of the pump coupling means 15 is to couple optical modes of the pump optical fibre 1 in Region IV 92 to optical modes in Region III 91 that evanescently couple to optical modes of the amplifying fibre 2 in Region 1 81 that overlap the core 8, and which will be absorbed by the rare earth dopant 12. The mode coupling means 15 thus increases the overall mode coupling between the modes of the pump optical fibre 1 and the modes of the amplifying optical fibre 2 that are absorbed by the rare earth dopant 12, and thus reduces the absorption length 16 of the composite fibre 17 shown in Figure 1.

Figure 10 shows a pump optical fibre 101 that has a normalized outer radius r = 1, and regions 102 that create variations in refractive index between radii r < b and r > a. The regions 102 can be longitudinally extending holes. The regions 102 can be doped regions comprising glass such as silica glass doped with dopants such as oxides of boron, germania, phosphorus and aluminium. Such doped regions can be fabricated by inserting doped glass rods into

longitudinally extending holes in a silica glass rod, which holes can either be formed during manufacture of the glass rods, or drilled subsequently, for example by an ultrasonic drill. The pump optical fibre 101 and the amplifying optical fibre 2 are surrounded by the coating 3 of Figure 1, which is not shown for reasons of clarity.

Figure 11 shows a pump optical fibre 111 that has a square cross section. The pump optical fibre 111 and the amplifying optical fibre 2 are surrounded by the coating 3, which may be glass, or may be a polymer.

In each of Figures 10 and 11, the refractive index 100 of the pump optical fibre 101 and the refractive index 100 of the pump optical fibre 111 (including its coating 3), has a variation 121 in azimuth 0 that varies in circles 119 of constant radius r between radii a and b. The refractive index variation 121 with azimuth Θ along circles of constant radius r 119 measured from the centre of mass 105 of the pump optical fibres 101, 111 is shown in Figure 12 by the solid line. The refractive index variation 121 resembles a square wave with four cycles per revolution. The refractive index variation 121 can be described by the Fourier Series:

where am(r) and b _m (r) are the amplitudes of the m azimuthal harmonics of as a function of

radius. These are given by:

By symmetry, the b _m are all zero, as are the a _m coefficients where m is an odd number, and where m = 2, 6, 10, 14 etc. The only non-zero coefficients are m = 4, 8, 12, 16 etc. Figure 12 shows the largest harmonic 120, and the second largest harmonic 122, which in the examples shown in Figures 10 to 12 are the fourth harmonic a ₄ and the eight harmonic a ₃ respectively.

The modes will only couple if certain symmetry conditions are met. Approximating the pump optical fibre 1 as a perturbation of a step index circular fibre having a circular glass core of radius r=l surrounded by the polymer cladding, the coupling strength C between the

mode and the LP _p2 , _q 2 mode caused by each of the harmonic terms in turn is proportional to the following integral (where the normalization has been omitted for simplicity):

where J _p i is the Bessel function of order is the Bessel function of order

and U ₂ are the eigenvalues of the wave equation corresponding to these guided modes. By symmetry, the coupling strength is only non-zero . So for example, the can only couple to the ₂ modes, where are integers. Similar equations can

be derived for coupling of guided modes caused by the term of the above Fourier Series.

The radial mode numbers for each of these modes will correspond to the mode

that has a similar propagation constant. As discussed previously, if the propagation constants of the unperturbed mode and perturbed modes are affected equally, then the mode will couple

to the mode and the mode. However, the perturbation may affect modes with

different radial mode numbers differently. The change in propagation constant can be estimated using perturbation theory.

Referring again to Figures 6 and 7, the fourth harmonic in the refractive index

variation 121 will couple the ₄ mode 60 shown in Figure 6 to the mode 70 shown in

Figure 7 because the difference in the azimuthal mode numbers is four. This is shown in Figure 9 where the mode 60 is shown being coupled to the . Significantly, the mode 70 lies in Region III 91, which corresponds to an optical mode of the pump optical fibre 1 that will couple evanescently to an optical mode in the amplifying fibre 2 in Region 1 81. Modes in Region I 81 of Figure 9 overlay the core 8 of the amplifying fibre 1 and are absorbed by the rare earth dopant 12. Similarly, the next strongest azimuthal harmonic 122 shown in Figure 12 (Fourier component m=8) of the pump optical fibres 101 and 111 of Figures 10 and 11 will couple the mode 85 which also lies in Region III 91 of Figure 9. This mode 85 when coupled evanescently across to the amplifying optical fibre 2 of Figure 1 will overlap the core 8.

The LP ₈ ,4 mode 60 will also couple to the mode 95 shown in Figure 9 via the

strongest m=4 harmonic 120. As can be seen by Figure 9, this mode coupling is undesirable as it moves the mode further away from Region III 91. Comparing the mode coupling means 15 in Figures 10 and 1 1, the m=4 harmonic 120 will be stronger at higher radii in Figure 1 1 than in Figure 10 because the mode coupling means 15 is at the outside surface of the pump optical fibre 1 11. resides closer to the outside surface of the pump optical fibre 111. Thus the coupling between the mode 95 and the mode 60 will be stronger in Figure

I I than in Figure 10. By choosing where to place the mode coupling means 15 within the pump fibre 1, it is therefore possible to preferentially couple optical modes towards the desired Region

III 91 and thereby preferentially couple pump radiation in the form of optical modes guided by the pump optical fibre 1 to optical modes of the amplifying optical fibre 2 that overlap its core 8.

Figure 13 shows an elliptical pump optical fibre 131 surrounded by the coating 3 having a lower refractive index than the refractive index of the pump optical fibre 131 and the amplifying optical fibre 2. Analyzing the refractive index variation 121 reveals that the largest harmonic is the second harmonic, corresponding to the Fourier term As shown in

Figure 14, the mode 60 couples to the mode 141 which will not overlap the core 8 of

the amplifying optical fibre 2 when coupled across evanescently. It therefore takes a correspondingly greater number of mode couplings to shift modes that do not overlap the core 8 to modes that overlap the core 8 in the example shown in Figure 13 than the examples shown in Figures 10 and 11. Moreover, coupling can also occur in the reverse direction, from the

mode 60 into the LPio,3 mode 142. Both these effects explain the experimentally-observed low coupling efficiency and longer absorption lengths 16 when using elliptically-shaped pump optical fibres. There is poor coupling of the modes of the elliptical pump optical fibre 131 and modes of the amplifying optical fibre 2 that overlap the core 8.

Mode coupling will be strongest between optical modes with similar propagation constants β. In the above discussion, it was assumed that the modes of the pump optical fibre 1 were similar to the modes in a circular step index fibre. Thus mode couples to the mode because these modes have similar propagation constants to each other in circular step index fibres. This assumption can be removed by treating the refractive index variation 121 as being a perturbation of the equivalent step index fibre. Using the weakly guided

approximation, the change in propagation constant β caused by the perturbation δη(Γ,θ) is given by:

where βο and ψο are the propagation constant and field of the optical mode of the unperturbed (circular step index) fibre, is the propagation constant of the optical mode of the perturbed

fibre, and the integration is performed over the cross-sectional area A of the unperturbed optical fibre. It is assumed that the fields of the optical modes of the perturbed and unperturbed fibres are the same. Thus the stronger the overlap of the field ψ ₀ with the perturbation 8n(r,0), the greater the reduction in propagation constant β caused by the perturbation. If the perturbation is mainly negative, the perturbation will cause a reduction in the propagation constant.

The mode will overlap more strongly with the coating 3 surrounding the pump optical

fibre 1 than the mode because it has more lobes near the outside of the fibre. Therefore the reduction in propagation constant for the mode will be greater than the reduction in

propagation constant for the L mode. The implication is that the optical modes will couple to

optical modes having higher order radial mode numbers because optical modes having larger radial mode numbers have smaller effective refractive indices. For example, the mode

discussed previously will couple more strongly into the mode, or optical

modes with even higher order radial mode numbers, than into the mode described with

reference to Figures 6 to 8.

Figure 15 shows a group of optical modes 151 which have similar propagation constants to the group of group of optical modes 152 in a circular step index fibre. If the outside glass surface of the pump optical fibre 1 is octagonal, then the optical modes 151 will overlap more strongly with the lower refractive index coating 3 than the optical modes 154. The optical modes 151 will thus couple to the group of optical modes 153, which in the example shown, are in Region III 91, and will not couple as strongly to the group of optical modes 152 that are in Region IV 92. Thus when coupled across to the amplifying optical fibre 2, the optical modes 153 will overlap its core 8. This should be compared to the group of optical modes 154 with higher radial mode numbers than the group of optical modes 151. These modes 154 will not overlap as much with the lower refractive index coating 3. Thus the coupling will be to the group of optical modes 155 in region III 91, which will have substantially the same propagation constants as in the unperturbed, circular step-index fibre.

Results for various amplifying optical devices 10 shown in Figure 1 have been obtained, in which the pump optical fibre 1 was a polygon with eight, nine, twelve or and sixteen sides. The largest azimuthal harmonic 120 in the refractive index variation 121 was therefore eight, nine, twelve and sixteen respectively. Each experiment revealed higher pump efficiency than obtained in prior art fibres comprising pump optical fibres with circular or elliptical cross- sections. The typical increase in efficiency was 20%. The pump radiation was absorbed by the core 8 of the amplifying optical fibres 2 more quickly, thus enabling the amplifying optical device 10 to be shorter. In addition, shorter devices enable higher peak powers to be obtained before the onset of non-linear optical effects.

Comparing the mode coupling that can be achieved with outside shape variations with variations in internal refractive index, it is clear that higher mode overlaps with a greater number of guided modes can be achieved with internal variations, particularly as the harmonic content of the circles increases.

By way of example, Figure 16 shows an optical fibre 160 comprising regions 161 and 162 having different refractive indices, and an outside surface 163 that is non-circular. The regions 161, 162 can be doped regions, stress rods, or microstructured regions, and each may contain at least one longitudinally extending hole. The regions 161, 162 can be circular or non circular. The optical fibre 160 is shown as being twelve sided with twelve facets 18, but other cross-sectional shapes are possible. The regions 161 are located closer to the centre of the optical fibre 160 than the regions 162. The second azimuthal harmonic (Fourier coefficient equal to two) is the largest harmonic of the azimuthal variation in refractive index 100 around the circle 164 that intercept the regions 161. The fourth azimuthal harmonic (Fourier coefficient equal to four) is the largest harmonic of the azimuthal variation in refractive index 100 around the circle 165 that intercept the regions 162. The twelfth azimuthal harmonic (Fourier coefficient equal to twelve) is the largest harmonic of the azimuthal variation in refractive index 100 around the circle 166 that intercept both the coating 3 and the facets 18. Optical modes with low azimuthal mode number tend to have most of their optical power nearer the centre of the pump optical fibre 1. Conversely optical modes with higher azimuthal mode number tend to have most of their optical power nearer the outside of the pump optical fibre 1. Thus it is advantageous for the strongest harmonic content of the mode coupling means 15 to have a higher order harmonic near the outside of the pump optical fibre 1 than nearer the centre of the pump optical fibre 1. The strongest harmonic content in the pump optical fibre 1 is order two near the centre, increasing to order four, and then increasing to order twelve with increasing radius. Such an increase is consistent with the relative disposition of optical power in the guided modes of the pump optical fibre 1 and is preferred in order to couple a larger proportion of the optical modes in Region IV 92 to optical modes in Region III 91. The optical fibre 160 can be used as the pump optical fibre 1 in Figure 1. Alternatively or additionally, it can be used as the amplifying optical fibre 2 in Figure 1 if the optical fibre 160 also includes a core 8. Other dispositions and combinations of the facets 18 and doped regions 161 and 162 are also possible, and the number, placements, and orientations of the facets 18 and the regions 161, 162 can be adjusted to optimize different designs.

Various composite fibres 17, 20, 30 and 40 have been described with reference to Figures 1, 2, 3 and 4 respectively. These composite fibres 17, 20, 30, 40 can each be used in the embodiments shown with reference to Figures 1 and 5 and in all embodiments of the invention. It is preferred that the composite fibres 17, 20, 30 and 40 are coiled in the amplifying optical devices 10, 50, preferably as a planar coil that can be coiled in a circle, but is more preferably coiled in a race track or dumbbell fashion. Such arrangements assist in coupling of the pump radiation from the pump optical fibre 1 to the amplifying optical fibre 2.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications and additional components may be provided to enhance performance. Individual components shown in the drawings are not limited to use in their drawings and they may be used in other drawings and in all aspects of the invention.

The present invention extends to the above-mentioned features taken in isolation or in any combination.