FIELD OF THE INVENTION

The present invention relates generally to optical pumping of waveguides. More specifically, the invention relates to a pump combiner including a multi-clad waveguide.

BACKGROUND OF THE INVENTION

Amplifiers and lasers require high optical power to be injected within the region of the waveguide that includes the active medium that provides the optical gain, i.e. the gain medium. The output power of amplifiers and lasers is limited by the amount of optical power that can be injected, i.e. coupled, into the gain medium and by the size of the core of the waveguide.

In general, an optical fiber may be multi-mode or single-mode or few-moded like large mode area (LMA) fibers. A multi-mode or LMA fiber allows for more than one mode of the light wave, each mode travelling at a different phase velocity, to be confined to the core and guided along the fiber. A single-mode fiber supports only one transverse spatial mode at a frequency of interest. Given a sufficiently small core or a sufficiently small numerical aperture, it is possible to confine a single mode, the fundamental mode, to the core. Fundamental modes are preferred for many applications because higher beam quality and focusing properties are obtained, and the intensity distribution of the light wave emerging from the fiber is unchanged regardless of launch conditions and any disturbances of the fiber.

Multi-clad cladding-pumped waveguides enable higher pump power to be injected into the amplifiers and lasers. An increase in the cladding area provides a longer pump absorption length. However, a longer cavity length in turn gives rise to nonlinear effects such as stimulated Raman scattering and stimulated Brillouin scattering.

Another option is to increase the core diameter of the fiber along with a proportionally reduced numerical aperture of the fiber. However, the maximum core diameter is limited by any requirement of single-mode propagation in the core of the fiber and enhanced bending loss in low numerical aperture fibers.

Several different approaches have been disclosed in respect of efficiently inject pump light into the gain medium. US Patent No. 6,243,515 teaches an apparatus for optically pumping an optical fiber from the side that includes a grating formed in the flat surface of the optical fiber that diffracts a beam of pump light at an angle that is matched to the angle characterizing a propagating mode. US Patent No. 6,370,297 teaches physically coupling a multimode pump fiber to a double- clad optical fiber. US Patent No. 4,815,079 discloses an apparatus for coupling radiation into a single-mode core of an optical fiber laser. The single-mode core is disposed within a relatively large multimode cladding at a location that is displaced from the center of the cross-section of the cladding, and the cladding is surrounded by another layer to prevent radiation from propagating out of the cladding. US Patent No 5,864,644 discloses a tapered fiber bundle for coupling pump light to cladding-pumped fiber. US Patent No. 7,046,432 discloses an apparatus for optically pumping a clad amplifier fiber that includes transmission fibers arranged and configured to insert pump light from a pump light source such as a diode laser into the cladding of the amplifier fiber, wherein at least one of the transmission fibers is arranged to re-insert an unabsorbed portion of the pump light into the cladding for re-circulation into the amplifier core. Other known approaches include a method of side-coupling pump light by cutting V-grooves into the cladding of dual clad doped fiber. The cutting of the V-grooves however generally weakens the fiber. With the increased amount of optical power being injected, i.e. coupled, into the gain medium comes the increased risk of damage by residual or extraneous power making its way into the pump source.

With a typical pump combiner that uses a cladding-pumped waveguide 13 as shown in FIG. 3 [PRIOR ART], pump light 22 from a pump source is coupled into the cladding 19 of the waveguide 13 via one or more pump waveguides 26. Pump light 22 propagating in the cladding 19 is absorbed by the gain medium of the core thereby generating and/or amplifying a light signal 24 guided by the core. Any light signal that may leak from the core 15 into the cladding 19, for example occuring at a fiber splice junction or at the site of an imperfection in the fiber, can then travel within the cladding 19 back through the pump waveguide 26 and into the pump source, thus damaging the pump source. Replacement or repair of the pump source can be very costly. To prevent light damage to a pump source, one or more filters or optical isolators may be used. The filter(s) or isolators may be integrated into the pump source or placed at the entrance to the pump source. The use of filters or isolators can result in a more complex pump source setup and often adds to the cost of the pump source.

There is therefore a need for an apparatus for optically pumping a waveguide which minimizes the risk of damage by the residual or extraneous power to the pump source.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided a pump combiner that includes:

- a multi-clad waveguide comprising: o a core for guiding a light signal propagating therein; o a cladding surrounding the core; and o an intermediate region, disposed between the core and the cladding, for trapping light leaks escaping said core and preventing said light leaks from entering said cladding; and

- an optical arrangement configured to inject pump light from a pump source into the cladding of the multi-clad waveguide.

The core may include a gain medium and may or may not be uniform. The multi- clad waveguide may include more than one core. The intermediate region may include at least one layer disposed outward the core.

The multi-clad waveguide may be a photonic crystal fiber.

The multi-clad waveguide may be a photonic bandgap waveguide.

The optical arrangement may include at least one pump waveguide coupled to the cladding of the multi-clad waveguide, for example side-coupled and an angle longitudinally extending therealong.

In according with one embodiment, both the multi-clad waveguide and the pump waveguide may be an optical fiber.

The objects, advantages and other features of the present invention will become more apparent and be better understood upon reading of the following non- restrictive description of the preferred embodiments of the invention, given with reference to the accompanying drawings. The accompanying drawings are given purely for illustrative purposes and should not in any way be interpreted as limiting the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional representation of a pump combiner, according to an embodiment of the present invention.

FIG. 2A is a schematic cross-sectional representation of a multi-clad waveguide, according to an embodiment of the present invention.

FIG. 2B is a diagram of the refractive index profile along the diameter of a prior art waveguide and along the diameter of a multi-clad waveguide according to an embodiment of the present invention.

FIG. 2C is a diagram of the refractive index profile along the diameter of two multi- clad waveguides according to two embodiments of the present invention, showing the difference in the refractive index profile of the intermediate region.

FIG. 2D is a diagram of a refractive index profile of a multi-clad waveguide, according to an embodiment of the present invention.

FIG. 2E is a diagram of a refractive index profile of a multi-clad waveguide, according to an embodiment of the present invention.

FIG. 2F is a diagram of a refractive index profile of a multi-clad waveguide, according to another embodiment of the present invention.

FIG. 2G is a diagram of a refractive index profile of a multi-clad waveguide, according to another embodiment of the present invention, showing multiple cores and corresponding intermediate regions.

FIG. 2H is a diagram of a multi-clad waveguide according to an embodiment of the present invention, showing a microstructured intermediate region. FIG. 3 [PRIOR ART] is a schematic cross-sectional representation of a prior art pump combiner.

FIG. 4 is a schematic representation of a pump combiner, according to an embodiment of the present invention, showing a pump waveguide side-coupled to a pedestal waveguide.

FIG. 5 is a schematic representation of a pump combiner, according to an embodiment of the present invention, showing a pump waveguide side-tapered and fused to a multi-clad waveguide.

FIG. 6 is a schematic representation of a pump combiner, according to an embodiment of the present invention, showing a pump waveguide longitudinally fused to a multi-clad waveguide.

FIG. 7 is a schematic representation of a pump combiner, according to an embodiment of the present invention, showing a pump waveguide that includes a fiber bundle.

FIG. 8 is a schematic representation of a pump combiner, according to an embodiment of the present invention, showing a pump waveguide fused and tapered to the multi-clad waveguide.

FIG. 9 is a graph illustrating signal leakage back into a pump branch for a prior art standard cladding-pumped optical fiber, a pedestal multi-clad waveguide and a higher pedestal multi-clad waveguide.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be described more fully hereinafter with reference to the accompanying drawings, FIGs. 1 to 9, in which like numerals refer to like elements throughout. The present invention provides a pump combiner that injects optical pump power into a waveguide, and advantageously prevents or minimizes residual or extraneous power from entering into the pump source and causing damage thereto. Although the embodiments described herein below consider optical fiber arrangements, it will be understood that the waveguides used in the present invention are not limited to optical fibers and may be embodied by any appropriate light-guiding structures such as planar or channel waveguides.

Referring now to the drawings, FIG. 1 thereof illustrates a pump combiner 10 according to an embodiment of the invention. The pump combiner 10 includes a multi-clad waveguide 12 that has a core 14 for guiding a light signal 24 propagating therein, a cladding 18 surrounding the core 14, and an intermediate region 16 disposed between the core 14 and the cladding 18; and an optical arrangement 20 configured to inject pump light 22 from a pump source into the cladding of the multi-clad waveguide 12.

In the following description, the term "light" is used to refer to all electromagnetic radiation, including but not limited to visible light. Furthermore, the term "optical" is used to qualify all electromagnetic radiation, that is to say light in the visible spectrum and light in other wavelength (λ) ranges.

It should also be noted that although not shown in the drawings, additional outer cladding layers and/or a protective jacket may be provided around the waveguides without departing from the scope of the invention.

The multi-clad waveguide 12 may operate on a total internal reflection refractive- index guiding principle or a photonic-bandgap guiding principle or an antiresonant- reflection guiding principle, or any other appropriate guiding principle. In the case of light guidance by total internal reflection (TIR), when light crosses a boundary between materials with different refractive indices, the light beam is usually partially refracted and partially reflected at the boundary surface. However, when travels from a medium with a higher refractive index to one with a lower refractive index at an angle of incidence greater than a critical angle, the light will instead be totally reflected back internally. In this way, total internal reflection may be used to confine light in the multi-clad waveguide. In the case where the multi-clad waveguide is a photonic bandgap waveguide or fiber, light is confined within the waveguide (even in low-index or hollow regions) by at least one photonic bandgap; periodic dielectric material structures in the waveguide will block light of wavelength in the photonic band gap range while allowing other wavelengths to pass freely. In the case where the multi-clad waveguide uses antiresonant reflecting guidance, light confinement is realized by creating a Fabry-Perot reflector for the transverse component of the wavevector at the light signal wavelength. Of course, the multi-clad waveguide 12 may be embodied by any appropriate waveguide, including (but not limited to) a TIR-guidance waveguide, a photonic crystal fiber (a fiber microstructured from two or more materials most commonly arranged periodically, e.g. "holes" of one material embedded within a medium of another material or concentric rings of two or more materials), a photonic bandgap waveguide, an antiresonant reflection optical waveguide (ARROW), etc.

Referring to FIGs. 1 and 2A, the core 14 of the multi-clad waveguide 12 represents the waveguide section through which the light signal 24 is guidedly transmitted, attenuated or amplified. The core may be made of glass, for example silica glass or phosphosilicate glass, heavy metal soft glass, polymer, or any other appropriate material. The core may be doped. It may be doped with an optically active element. It is understood that the expression "optically active" in the sense of the invention refers to a material that interacts with light and which may be used to generate light, amplified or attenuated. In accordance with an embodiment of the invention, the core 14 may amplify the light signal 24. The core 14 of the multi- clad waveguide 12 may therefore include a gain medium doped with an optically active element that can be optically excited by the pump light 22 to provide optical gain for the light signal 24 confined to the core 14. In general, the optically active element may be a rare-earth element useful for the transfer of energy between the pump light and the light signal. The rare-earth dopant may include ytterbium, thulium, neodymium, bismuth, erbium or any combination thereof, including oxides thereof. Of course, other optically active elements may also be used, such as for example titanium or chromium. To reduce photodarkening effects that cause power degradation in amplifiers and lasers, the core may include phosphorus oxide. Adding phosphorous to the multi-clad waveguide advantageously increases the refractive index contrast, improves the resistance to bendings of the fiber, and allows for an increase in the saturation energy as well as the extraction energy of pulsed amplifiers and lasers. The core 14 is not restricted to being a solid core. It may be a hollow core, where the hollow core may be filled with gas (for example air) or a liquid.

Although the core 14 may preferably be single-mode, it is not restricted to being single-mode and may be few-moded or multi-mode. In fact, the multi-clad waveguide 12 is not limited to a single core waveguide, but may include multi-core waveguides as seen in FIG. 2G which shows a cross section of a multi-clad waveguide having multiple cores 14 and corresponding intermediate regions 16.

Moreover, the core of the fiber need not be radially uniform. The refractive index of the core may change in a gradual or stepped fashion. When increasing the concentration of rare earth dopants, the refractive index of the core increases and the numerical aperture of the inner core increases, and thus a smaller inner core must be used in order to have a singlemode inner core. The outer region of the core may have a lower refractive index than an inner region. To decrease or increase the refractive index of the core, the core may be doped with suitable dopants, for example, Al, La, Lu, P, Ge, Ti, F, B, or oxides thereof, or combinations thereof. Of course, one or more sub-regions of the core may have no dopant at all and consist of pure silica. The intermediate region 16 of the multi-clad waveguide 12 is the region, disposed between the core 14 and the cladding 18, used for trapping light leaks escaping the core 14 and preventing the light leaks from entering the cladding 18 and from there entering the optical arrangement 20 and damaging the pump source. In a pump combiner using a prior art standard cladding-pumped waveguide 13 as shown in FIG. 3, the light 24 which may leak from the core 15, enters the pump cladding 19 (for example at the boundary 21 ) where it can travel through the pump waveguide 26 back to the pump source and damage the pump source. With a multi-clad waveguide 12 as shown in FIG. 1 , the intermediate region 16 acts as a buffer zone between the core 12 and the cladding 18, preventing the light signal 24, which may have escaped from the core 14 or been introduced from an input splice 40 to the multi-clad waveguide 12 or any other mechanism in the combiner 10, from traveling into the cladding 18 and thus entering the pump source where it may cause damage to the pump source. The errant light signal 24 may even be made to re-enter the core 14 thus also aiding to maintain the power transmission efficiency of the pump combiner 10.

Referring to FIGs. 2A to 2G, the multi-clad waveguide 12 - unlike the typical prior art cladding-pumped waveguide - has the intermediate region 16 disposed between the core 14 and the cladding 18. The intermediate region 16 may include one or more layers, as depicted by the phantom lines in FIG. 2A. The intermediate region 16 may have a single layer 16A disposed about the core 14 or a first layer 16A disposed outward the core 14 and one or more additional layers 16B disposed outward over the first layer 16A, as best seen in FIGs. 2A and 2B. Furthermore, the multi-clad waveguide 12 may include more than one separate intermediate region 16. For example, in the case of more than one core, there may be an intermediate region 16 associated with each core 14, as seen in FIG. 2G. Of course, the intermediate region 16 is not limited to a particular geometry, but may cover any geometry that provides favourable guiding properties in terms of efficient coupling of pump power to the intermediate waveguide and pump power mode mixing. For example, the intermediate region 16 may have a pentagonal cross-section and the core 14 may have a circular cross-section, in accordance with an embodiment, the core 14 may be centered in order to facilitate splicing to another waveguide and the intermediate region 16 may be asymmetrically located with respect to the center of the multi-clad waveguide 12 to provide pump power mode mixing.

One or more of the layer(s) of the intermediate region 16 may be made of a material having a refractive index or an effective refractive index lower than the refractive index or effective refractive index of the core 14 (as seen in FIGs. 2B, 2C, 2D and 2E) or higher than the refractive index or effective refractive index of the core 14 (as seen in FIG. 2F). The intermediate region may also have a refractive index that is higher than the refractive index of the cladding 18, as seen for example in the embodiments of FIGs. 2B, 2C, 2E and 2F, or lower than the refractive index of the cladding 18, as illustrated for example in the embodiment of FIG. 2D which shows a depressed intermediate region 16.

Of course, as one skilled in the art would appreciate, the material used for the intermediate region 16 may be tailored in view of the light confinement and guidance principle in operation in the multi-clad waveguide 12. For example, the intermediate region 16 may include at least one layer having a refractive index or an effective refractive index higher than a refractive index of the cladding 18 so that light escaping from the core 14 may be totally internally reflected back into the intermediate region 16 and thus prevented from entering the cladding 18. In accordance with another embodiment, the intermediate region 16 may have a refractive index or an effective refractive index lower than a refractive index of the cladding 18. In such a case, the intermediate region 16 may be made of a photonic bandgap material that has at least one bandgap for stopping the light signal 24 escaping from the core 14 of the multi-clad waveguide 12 from entering the cladding 18, and thus making its way to the pump source. The intermediate region may be microstructured, and may for example have a refractive index profile as seen in FIG. 2H. In another alternative, the intermediate region may be hollow or filled with air and the cladding may be made of a photonic bandgap material for stopping the light signal 24 escaping from the core 14 from entering the cladding 18. In such an alternative, the cladding may be appropriately microstructured. In yet another alternative, light confinement is realized by including an antiresonant reflecting dielectric layer between the pumped layer of cladding 18 and the core to create a Fabry-Perot reflector for the transverse component of the wavevector at the light signal wavelength. This layer may be disposed in the intermediate region 16. Generally, the layer(s) of the intermediate region 16 may be made of any appropriate material, including (but not limited to) silica glass doped with suitable dopants, for example, Al, La, Lu, P, Ge, Ti, F, B, or oxides thereof, or combinations thereof. Of course, one or more layers or microstructures of the intermediate region 16 may have no dopant at all and consist of pure silica.

As seen in FIG. 2B, the refractive index profile of a pedestal waveguide (so-called owing to the step-like pedestal profile of its refractive index), an embodiment of the multi-clad waveguide 12 - in comparison to that of a typical cladding-pumped optical fiber - allows for high pump power to be injected into a larger numerical aperture while also allowing for a low numerical aperture of the core 14. In addition, the lower numerical aperture of the core 14 implies increased mode area of the modes propagating in the waveguide 12 and therefore decreased non-linear effects, reduced number of modes and therefore improved beam quality, and reduced amplified spontaneous emission (ASE) and therefore enhanced efficiency. An improved beam quality is well known to have a lower M ² , lower BPP parameter, more Gaussian-like pulse shape, and better focusing properties. Advantageously, the intermediate region 16 is useful in trapping some of the pump light 22 injected into the cladding 18 of the multi-clad waveguide 12 into a smaller section for accelerating the absorption of pump light 22. Furthermore, by using an appropriate diameter of the intermediate region 16, higher resistance to power losses owing to bending of the waveguide may be obtained. Referring to FIGs. 1 , and 2A to 2G, the multi-clad waveguide 12 may generally include a single-mode (or a few mode or even multimode) core 14, an intermediate region 16 disposed around the core 14, and a cladding 18 surrounding the intermediate region 16, wherein the cladding 18 generally supports multi-mode pumping light. The cladding represents the waveguide section into which the pump light 22 is injected. The cladding 18 may include one or more layers. It may have an index of refraction that is lower than that of the intermediate region 16 so as to prevent light leaks from entering the pumped layer of the cladding 18 by using total internal reflection, for example. Alternatively, it may be made of a photonic bandgap material, which has at least one photonic bandgap for blocking an errant light signal 24. The cladding 18 may be microstructured to obtain the appropriate effective refractive index or bandgap. The cladding 18 may also have an antiresonant reflecting dielectric layer between the pumped layer of cladding 18 and the intermediate region 16 to create a Fabry-Perot reflector for the transverse component of the wavevector at the wavelength of light signal 24. It may include an outer cladding 18B, as seen in FIGs. 2A, 2C and 2D, which may be a silica cladding, a fluorinated silica cladding, an air-silica microstructure or a low-index polymer cladding, with an effective refractive index less than that of the cladding region 18 and providing a large numerical aperture and guiding properties for the cladding 18. The cladding 18 may include more than one layer for limiting the pump power density that interacts with the intermediate region 16. These layers may also aid in the proper guidance of the pump light.

Pump light 22 may be coupled into the intermediate region 16 due to its large cross sectional area and high numerical aperture compared to that of the core 14.

Propagating in the intermediate region 16, pump light may be coupled into the core 14, for example by being absorbed by the rare-earth ions in the core 14, generating or amplifying light signals 24 in the core 14. Coupling of the pump light

22 into the core 14 can depend on the geometry of the multi-clad waveguide 12 and may be roughly proportional to the core area-to-intermediate region area ratio, the larger the area of the intermediate region the smaller absorption coefficient at a fixed core area.

Any part of the multi-clad waveguide 12 may be polarizing maintaining by using an appropriate configuration: elliptical core, elliptical cladding, D-shaped, a panda, a bow-tie, etc. The multi-clad waveguide may preferably consist of an optical fiber, but may be any appropriate waveguide and it need not be limited to a fiber form. Moreover, one or more regions of the multi-clad waveguide may be made of a material other than glass such as a metal, a liquid, a gas or even vacuum.

The optical arrangement 20 of the pump combiner 10 of the present invention, for example as seen in FIG. 1 , is configured to inject pump light 22 from a pump source into the cladding 18 of the multi-clad waveguide 12. It may have at least one pump waveguide 26 coupled to the cladding 18 of the multi-clad waveguide 12. The pump waveguide 26 may be made of any appropriate material for injecting the pump light 22 to the multi-clad waveguide 12, and is preferably made of material that does not absorb the pump light. It may be single-mode or multi- mode. It may be single-clad or multi-clad, and may be embodied by an optical fiber. The optical arrangement may include several pump waveguides associated with a single pump source or several different pump sources, for coupling in greater amounts of pump power. Any appropriate pump source may be used, for example diode lasers, flash lamps, etc. Advantageously, several lower-cost lower- power pump sources may be used to inject pump light into the multi-clad waveguide of the pump combiner.

The pump waveguide may be single-mode, supporting one propagation mode, or multi-mode, supporting many propagation modes. It may be side-coupled to the cladding of the multi-clad waveguide along a length of the multi-clad waveguide for cladding pumping the multi-clad waveguide. In accordance with an embodiment of the invention shown in FIG. 4, there are two pump waveguides 26 angle coupled to the cladding 18 of the multi-clad waveguide 12 to form branches thereof. Each pump waveguide 26 may be fused to the cladding 18 using any appropriate technique. In accordance with another embodiment shown in FIG. 5, the pump waveguide 26 may be fused to the cladding 18 of the multi-clad waveguide 12 forming a fused region 28 that tapers to the diameter of the multi-clad waveguide 12, thus providing a more efficient injection of the pump light 22 into the cladding 18 of the multi-clad waveguide 12. In yet another embodiment shown in FIG. 6, the pump waveguide 26 may be longitudinally fused to the cladding 18 along a length of the multi-clad waveguide 12.

In accordance with another embodiment shown in FIG. 7, the optical arrangement 20 includes a fiber bundle 30 spliced to the multi-clad waveguide 12 for coupling pump light 22 into the cladding 18 of the multi-clad waveguide 12. The fiber bundle 30 is made up of a number of pump waveguides 26, which may be single mode fibers but are preferably multi-mode fibers, that preferably converge to a tapered end 32 which is then fused, or coupled by any other appropriate means, to the cladding 18 of the multi-clad waveguide 12 of the pump combiner 10. In accordance with another embodiment shown in FIG. 8, the optical arrangement 20 may include at least one pump waveguide 26 fused to the the multi-clad waveguide 12 to form a bundle 3OA. Preferably, the multi-clad waveguide is centred within the bundle. The bundle 3OA may then be tapered to a reduced cross-section 32A by melting and drawing the bundle 3OA into a single fiber 50.

The pump combiner 10 may be used to generate or amplify a light signal. In accordance with one embodiment, a light signal 24 (for example, a light signal from a standard prior art optical fiber or a degraded light signal from an external optical device connected to the multi-clad waveguide 12 of the pump combiner 10) is introduced into the core 14 of the multi-clad waveguide 12. Pump light 22 may then be injected into the cladding 18 of the multi-clad waveguide 12, guided to the intermediate region 16 adjacent to the core where it is used to create a population inversion inside the gain medium of the core 14, thereby amplifying the light signal 24. In accordance with another embodiment, pump light 22 may be injected into the cladding 18 of the multi-clad waveguide 12, guided to the intermediate region 16 adjacent to the core where it is used to create a population inversion inside the core 14, thereby simply generating the light signal 24. Advantageously, the pump combiner 10 of the present invention provides increased isolation from light damage by preventing any light leaks from the core 14 from reaching the cladding 18 and hence damaging the pump source. Of course, the output of the pump combiner, whether made with a doped or undoped fiber, may be connected to some other optical device, for example a fiber amplifier for further amplifying the light signal 24.

FIG. 9 is a graph of signal power leakage into the pump waveguide 26 in relation to the output signal power of an amplifier for a pump combiner using a standard cladding-pumped fiber according to prior art and shown in FIG. 3 [PRIOR ART], a multi-clad waveguide fiber with a pedestal refractive index profile according to an embodiment of the invention (for example, as shown in FIG. 2C), and a multi-clad waveguide fiber with a higher pedestal refractive index profile according to another embodiment of the invention (for example, also as shown in FIG. 2C) . As can be seen, for the prior art arrangement which uses a typical cladding-pumped fiber, the amount of light signal leaking, entering, into the pump waveguide increases significantly with increasing output power of the amplifier, whereas for the present multi-clad arrangement the light signal leakage for a given output power is much lower and the rate of increase with output power is much slower. Therefore, the multi-clad arrangement of the pump combiner of the present invention presents an obvious advantage over a pump combiner which does not use such a multi-clad arrangement.

Of course, numerous modifications could be made to any of the embodiments described above without departing from the scope of the present invention as defined in the appended claims.