WO2009080039A1

WO2009080039A1 - Optical combiner and method of producing the same

Info

Publication number: WO2009080039A1
Application number: PCT/DK2008/050304
Authority: WO
Inventors: Martin Dybendal Maack
Original assignee: Crystal Fibre A/S
Priority date: 2007-12-20
Filing date: 2008-12-12
Publication date: 2009-07-02

Abstract

The invention relates to a fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where said input section comprises a plurality of N individual optical fibers wherein the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section and Acore,n is reduced along at least a part of the intermediate section where said reduction is obtainable by removal of material from said core. Thereby improved properties of the combiner may be obtained.

Description

Optical combiner and method of producing the same

TECHNICAL FIELD

The invention relates to a fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where

said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area A_core,n,

said intermediate section being at least partly formed by said individual fibers merged to a fiber bundle, wherein

the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section.

BACKGROUND ART

Pump combiners with signal feed-through are becoming a key component in the development of high power fiber based lasers and amplifiers. For moderate signal mode-field diameters (MFDs) (such as MFD < 6 μm at a wavelength of about 1060nm) solutions with tapered (elongated) feed-through fibers are normally preferred due to their simplicity. These can be accomplished by either elongation of signal fibers with step-index cores or with more complex waveguide structures.

In the context of the present invention the term tapering may in principle comprise any method of altering the shape of the cross section of tapered objects, such as reduction of the cross-sectional area by elongation i.e. stretching or removal of material by e.g. etching.

In one example of a combiner with an elongated signal fiber feed-through, a number of pump fibers are bundled with a (central) signal fiber, fused together and elongated in order to reduce the cross-sectional area and provide a better fit to the pump guide area of a high NA (active) double-clad fiber . In another example, a number of pump fibers are bundled with a (central) signal fiber, fused together, and spliced to a double clad fiber (DCF) in such a way that the signal core in the fiber bundle is coupled to the signal core in the DCF. In a second step, the DCF is elongated to fit the pump guide area of a high NA (active) double-clad fiber.

With increasing signal power levels commonly follows the requirement for increasing the mode-field diameter. Signal fibers with large MFD (such as MFD > 6 μm at a wavelength of about 1060nm) may be realized by several different approaches. One approach is to make a low NA micro-structured core. Another approach is to use single-mode operation of multi-mode fibers through loss discrimination which is typically obtained through mode converters. In all these examples, mode spacing to higher-order modes, cladding modes, or radiation modes may inherently be small. Consequently, only small external perturbations may deteriorate the signal. Since elongation will inevitably disturb the waveguide, adiabatic tapering of the signal can be very difficult or even impossible to obtain.

DISCLOSURE OF INVENTION

Based on at least the above cited problems of the prior art an alternative method of multiplexing the light from several fibers is required. Accordingly, an object of the present invention is to provide an optical combiner, and a method of producing the same, with improved characteristics.

The objects of the invention are achieved by the invention described in the accompanying claims and as described in the following.

In one embodiment the present invention relates to a fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where

said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area A_COre,n, said intermediate section being at least partly formed by said individual fibers merged to a fiber bundle, wherein the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section and A_COr_e,n is reduced along at least a part of the intermediate section where said reduction is obtainable by removal of material from said core. By removing material from said core the area carrying the light originating from the pump fibers may be reduced leading to an increase intensity of the combined light originating from the pump fibers. Having the light originating from the pump fibers confined to a reduced area allows for coupling to a receiving fiber with a smaller core. In one example of an embodiment of the invention increase in intensity of the light from a set of pump fibers is obtained while one or more signal fibers extend through the intermediate section substantially unperturbed.

Combined light refers to light originating on the input side from two or more individual fibers coupled to the receiving fiber.

For simplicity, fibers having their cores extending through the intermediate section with the area, A_core,f, and/or shape of the core substantially constant are referred to a feed-through fiber and their core referred to as a feed-through core regardless of the intended application of the combiner. Similarly, light traveling in such feed-through fibers may be referred to as feed-through light. Other fibers are generally referred to as pump fibers. Prior to being merged all feed-through fibers and all pump fibers may have substantially identical or dissimilar optical properties, respectively. Furthermore, one or more pump fibers may have substantially identical optical properties with one or more feed-through fibers.

In one embodiment said set of individual fibers comprises one or more fibers which are multi-mode at the intended wavelength of operation. In one embodiment one or more pump fibers are multi-mode at the intended wavelength of operation.

In one embodiment said set of individual fibers comprises one or more fibers which are single-mode at the intended wavelength of operation. In one embodiment one or more feed-through fibers are multi-mode at the intended wavelength of operation. In one embodiment one or more feed-through fibers are single-mode at the intended wavelength of operation In the context of the present application merging of individual fibers refers to bringing the fiber in contact so that each of the individual fibers has a common interface with one or more of the other fibers over a section of length. The term comprises bringing the fiber in contact with each other as well as embodiments where the fibers are also adhesively joined. Adhesively joining comprises techniques such a fusing, cementing and gluing fibers together.

In one embodiment the invention relates to a combiner combining pump light where one or more the fibers carry a higher optical intensity so that it will be advantageous to preserve this light energy with greater care than for the remaining fibers. This could for example be for a fiber laser or amplifier system with separate pump sources. This could for example be a fiber laser or amplifier being pumped primarily by a high power remote pump and supplementary local pumps of lower power. Other examples included fiber lasers or amplifiers that are pumped at two or more wavelengths, where different power levels are available for the different wavelengths. In one embodiment, the invention relates to a combiner combining pump light where one or more the fibers have a lower numerical aperture than one or more of the remainder of fibers.

Examples of use of combiners includes a backward-pumped fiber amplifier with a combiner according to an embodiment of the present invention that is used to supply pump light to the amplifier and allow the (amplified) signal to be coupled out of the amplifier via a feed-through fiber of the combiner. As another example; a modular fiber laser or amplifier, where amplifier stages are cascaded and one or more combiners according to an embodiment of the present invention are used to provide signal light from one amplifier module to another.

In one embodiment all fibers of the input side of the intermediate section have their cores extending through the intermediate section with A_core substantially constant so that all fibers are feed-through fibers by the definition provided above. This may, for example, be preferred in order to combiner signal light from multiple signal sources.

In the present context A_core being substantially constant is taken to mean constant within less than 30%, such as within less then 20%, such as within less then 10%, such as within less then 5%, such as within less then 1 %, such as within less then 0.1 %, such as within less then 0.01 %.

In one embodiment the invention relates to a method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, the method comprises providing a set of N individual optical fibers, merging said set of N optical fibers on the input section of the combiner to obtain a bundle of fibers forming said intermediate section, said set of optical fibers comprising fibers having a core suitable for guiding light along a longitudinal direction of said fiber and said core having a cross sectional area A_COr_e,n , removing material from the core of at least one optical fiber, of said set of optical fibers, prior and/or post to said merging of the set of optical fibers so that the core of said fiber(s) extends through at least part of the intermediate section from input section to output section where A_COre,n is reduced along the direction from input section to output section.

In principle it may be possible to join the fiber sequentially along the longitudinal direction and it may be possible to remove material before all fibers are merged without parting from the scope of the invention.

In one embodiment the invention relates to a fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area Acore.n, said intermediate section is at least partly formed by said individual fibers adhesively joined in a fiber bundle, and the core of at least one of said individual fibers through at least part of the intermediate section from input section to output section wherein A_COr_e,n is reduced along at least a part of the intermediate section and said reduction is obtainable by removal of material from said adhesively joined intermediate section. In this way a reduction of the diameter of the fiber bundle is obtainable often substantially without perturbing fibers not located peripherally in the bundle. In one embodiment said removal of material from the fiber bundle comprises removal material from the core of at least one optical fiber.

Correspondingly, in one embodiment the invention relates to a method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, the method comprising providing a set N of individual optical fibers, merging said set of N individual optical fibers at the input section of the combiner to obtain a bundle of fibers forming the intermediate section between said input section and said output section, said set of fibers comprising fibers having a core suitable for guiding light along a longitudinal direction of said fiber, said core having a cross sectional area A_COr_e,n, adhesively joining the fibers of said intermediate section, removing material from the adhesively joined intermediate section.

In one embodiment the invention relates to a fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of said fiber, said core having cross sectional area A_core._n and a cross section with a cross sectional area A_cross,n said intermediate section is at least partly formed by said individual fibers merged to a fiber bundle, wherein the core of at least one first optical fiber of said individual optical fibers extends through at least part of the intermediate section from input section to output section where A_cross,n is reduced along at least a part of the intermediate section obtainable by removal of material, and at least one second optical fiber of said individual optical fibers extends through at least part of the intermediate section from input section to output section where A_core is reduced along at least a part of the intermediate section obtainable by elongation of said second optical fiber.

Correspondingly, in one embodiment the invention relates to a method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, the method comprising tapering a section of at least one first optical fiber by removal of material, tapering a section of at least one second optical fiber by elongation of said second optical fiber, forming a set of optical fibers comprising said first and second optical fibers, merging said set of optical fibers at the input section of the combiner to obtain a bundle of fibers forming an intermediate section between said input section and said output section said intermediate section comprising at least part of the tapered section of said first and second fibers.

In one embodiment the invention relates to a method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, the method comprising adhesively joining one or more optical fibers to a tube thereby forming a sleeve, fitting one or more inner fibers to the inside of said sleeve thereby forming the intermediate section of the combiner, optionally adhesively joining said inner fibers to said sleeve. In this manner a combiner may be produced where the shape and processing of one set of fibers, i.e. the one or more fibers adhesively joined with the tube, may be separated from another set of fiber, i.e. the one or more inner fibers. In one embodiment the method comprises tapering at least at part of said sleeve prior to said fitting to the sleeve. In one embodiment the method further comprises tapering one or more of said inner fibers prior to said fitting to the sleeve.

It should be noted that the phrase removing of material refers to the removal of material intended to guide light and/or to confine light to the core but not to removal of material solely intended to shield the fiber. Commonly glass fibers and polymer fibers are coated with a polymer coating shielding the fiber from mechanical stress. Stripping of at least some of this coating before splicing, or otherwise processing of optical fibers, is routinely performed in the art. However, for some fibers a glass core with a polymer cladding is applied. For such fibers, the part of the polymer arranged to actively confine light to the core is not considered to be part of the material solely intended to shield the fiber. In one embodiment, solely intended to shield means that in use the material comprises a field density originating from the light guided by the fiber of less than 5% of the maximum field density of the guided mode, such as less than 1 % of the maximum field density of the guided mode, such as less than 0.1 % of the maximum field density of the guided mode. Here the field density in material and the maximum field density of the guided mode are measured in the cross section perpendicular to the direction of propagation of said mode. In principle this applies as well for any types of coatings. In one embodiment the at least one of said individual fibers comprises a glass core surrounded by a polymer cladding. In one embodiment said fiber is a pump fiber.

In one embodiment the invention relates to a fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area Acore.n and a cross sectional shape Score said intermediate section is at least partly formed by said individual fibers merged to a fiber bundle, wherein the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section and A_core,n is reduced along at least a part of the intermediate section and S_COre is altered along at least a part of the intermediate section. Here such a change in shape may be caused by different materials or glass with varying concentration of dopants may respond differently to etching. In one embodiment Score alters substantially gradually along said part of the intermediate section which may be preferable to maintain substantially adiabatic intermediate section. In one embodiment S_COre is altered similar to a full moon approaching a new moon.

Further objects of the invention are achieved by the embodiments defined in the dependent claims and in the detailed description of the invention.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other stated features, integers, steps, components or groups thereof. DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, one advantageous application of the invention may be to have signal light guided in a fiber which extends through the intermediate section with A_core substantially constant, i.e. a feed-through fiber. In some applications the combiner may be applied to combine pump light and signal light into a double clad fiber intended to carry signal light in the inner core. In one embodiment the intermediate section comprises a centre fiber and said centre fiber is a feed-through fiber.

For some applications the inclusion of fibers with larger mode-field diameters (MFD) may be advantageous so that at least one of said set of individual fibers has a MFD of a guided mode with wavelength λ wherein (MFD/λ)>10, such as (MFD/λ)>12, such as (MFD/λ)>14, (MFD/λ)>16, such as (MFD/λ)>18, such as (MFD/λ)>20, such as (MFD/λ)>30, such as (MFD/λ)>40, such as (MFD/λ)>100, such as (MFD/λ)>200.

The inclusion of feed-through fibers in the fiber bundle may be particularly advantageous for fibers with larger MFD, so in one embodiment feed-through fiber has a MFD of a guided mode with wavelength λ wherein (MFD/λ)>10, such as (MFD/λ)>12, such as (MFD/λ)>14, (MFD/λ)>16, such as (MFD/λ)>18, such as (MFD/λ)>20, such as (MFD/λ)>30, such as (MFD/λ)>40, such as (MFD/λ)>100, such as (MFD/λ)>200.

In one embodiment one or more of the individual fibers are a pedestal fiber. A pedestal fiber comprises a pedestal region that surrounds the core, often the central core. For large mode area fibers, a pedestal region may be applied in order to reduce the central core NA and improve beam quality. Alternatively, a pedestal region may be applied in order to protect pump light sources from scattered signal light. In this case, the pedestal region may capture at least part of the scattered signal light such that less or no scattered signal light is coupled to the pump fibers. In one embodiment said pedestal fiber is a feed-through fiber.

In one embodiment the combiner will function to combine light into a receiving fiber suitable for either transporting the combined light and/or process said light such as amplifying all or parts of the combined light. Therefore, in one embodiment the output section comprises a cross section being suitable for optical connection to a receiving fiber with a receiving core having an area A_rθC In one embodiment the receiving fiber is a pedestal fiber which may be applied to improve the beam quality of the device in which the combiner is applied, such as a laser.

In one embodiment the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section and A_core,n is tapered along at least a part of the intermediate section wherein said tapering is more than or equal to 10%, such as more than or equal to 25%, such as more than or equal to 50%, such as more than or equal to 75%, such as equal to 100%. Such a reduction of the A_core,n may serve one or purposes, such a providing better adaptation to the receiving fiber by reducing the cross section of the intermediate section. Another purpose may be to improve intensity of the combined light as less material is available to guide. As mentioned above A_core,n may be tapered by 100%, so that material originating from the core of this fiber does not extend to the output section. In one embodiment light inserted into the input side of the combiner guided by such a fiber is found to be guided by material originating from other fibers of the fiber bundle, such as the cladding, and/or to be guided by one or more outer cladding regions surrounding at least part of the intermediate section. In one embodiment said tapering is at least partially performed by removal of material. This may have the advantage of allowing one or more of the remaining fibers to function as feed-through fibers which, depending on the embodiment, may be more difficult to obtain by e.g. elongation.

However, in principle the receiving fiber may have multiple receiving cores. In one embodiment the receiving core of the receiving fiber surrounds at least one inner core. In one embodiment the receiving fiber further comprises one or more outer cladding regions surrounding the receiving core. In the following the term double clad fiber refers to a fiber having at least one core surrounding at least one inner core so as to act at least partially as cladding for the said inner core. In one embodiment a feed-through fiber core substantially matches an inner core of the receiving fiber so that good coupling to this inner core of signal light may be provided. In one embodiment the term substantially matches refers to a spatial match in position and or dimensions to within less than 30%, such as less than 20%, such as less than 10%, such as less than 1 %. Often double clad fibers are designed to have a central inner core, so that in one embodiment the fiber bundle comprises a center fiber wherein said center fiber is a feed-through fiber in order to match spatially to said inner core. However, in principle the core of one or more feed-through fibers are off-set from the centre axis on the output section and/or the core may be off-set from the centre. Also, the combiner may comprise multiple feed-through fibers such as two or more feed-through fibers, such as three or more feed-through fibers, such as four or more feed-through fibers, such as 5 or more feed-through fibers, such as 6 or more feed-through fibers.

In one embodiment one or more of the fibers of the fiber bundle are adhesively joined, such as fused or cemented preferably with a resin having suitable optical properties. In one embodiment the resin is optically inactive, such as optically dark at one or more wavelengths for which the combiner is intended. One advantage of such a resin is that the intensity of the combined light may be improved as less material is available for guiding light in the combiner. In one embodiment less brightness is lost relative to not using said resin.

In one embodiment fusing is performed using standard fusing equipment, such as FFS-2000 or GPX-3000, produced by Vytran (NJ, USA). In one embodiment a tube is applied to hold the fiber bundle. In one embodiment said tube is applied prior to adhesively joining one or more fibers of the bundle. In one embodiment the tube is subsequently removed, such as by an etch. In one embodiment the tube is a glass tube, such as F300 or F500 fused silica tubes produced by Hereaus (Germany). The glass tubes may be drawn to reduced dimensions using fiber optical drawing tower. The process of drawing glass tubes to reduced dimensions is well known in the art of producing photonic crystal fibers and in the art of producing pure silica glass tubes for medical applications. In one embodiment the matrix is partly formed by a resin for adhesively joining the fiber bundle. In one embodiment the matrix comprises air holes, such as due to insufficient cladding material available during joining. In one embodiment the fiber bundle is supplemented by the addition of rods which carry material arranged to at least partly form a matrix when the bundle is fused or otherwise adhesively connected. In one embodiment the thickness of the cladding of the pump fibers is arranged to provide a desirable shape of the matrix, such as between pump fiber(s) and a feed through fiber. In one embodiment said desirable shape relates to the smallest thickness of the matrix between two fibers. In one embodiment said smallest thickness is less than 10 μm, such as less than 5 μm, such as less than 1 μm, such as less than 0.5 μm, such as less than 0.1 μm, such as less than 0.05 μm, such as less than 0.01 μm. In one embodiment said desirable shape relates to the length of matrix residing between two fibers. In one embodiment said length is less than 10 μm, such as less than 5 μm, such as less than 1 μm, such as less than 0.5 μm, such as less than 0.1 μm, such as less than 0.05 μm, such as less than 0.01 μm. In one embodiment a tube is applied to hold the individual fibers in the bundle. In one embodiment such a tube is removed after adhesively joining the fibers of the bundle. In one embodiment said tube provides material for forming of the matrix.

In one embodiment the refractive indices of two or more of the cladding of at least one feed through fiber, the core of at least one feed through fiber, the cladding of at least one pump fiber, and the core of at least one pump is arranged to light propagation from one to the other. In one embodiment the matrix is arranged to confine light to the core of at least one pump fiber or feed through fiber. In one embodiment two or more of the cladding of at least one feed through fiber, the core of at least one feed through fiber, the cladding of at least one pump fiber, and the core of at least one pump is separated by matrix sufficiently this to allow for a good coupling between the two. In one embodiment said good coupling is better than 10% per meter such as better than 20% per meter, such as better than 30% per meter, such as better than 40% per meter, such as better than 50% per meter, such as better than 60% per meter, such as better than 70% per meter, such as better than 80% per meter, such as better than 90% per meter, such as equal to a 100% per meter.

In one embodiment one or more outer cladding regions surround at least part of the intermediate section and or the output side. One function of such outer cladding regions may be to act as cladding for the light guided by cores of fibers arranged peripherally in the fiber bundle. In one embodiment said outer cladding region comprises an air clad. An air clad is a cladding comprising air holes, so that light experiences a low average refractive index. The air clad is typically surrounded by a solid material, such as fused silica. The overcladding is typically applied for mechanical protection. Often the term air clad is used, where it is to be understood that the air clad and a surrounding solid overcladding material is meant. More details describing the concept of an air clad may be found in US 5,907,652 and in international patent application WO03019257. In this way light may be relatively strongly confined due to a high difference in refractive index between the materials of the fiber, such as doped fused silica which may form the core, and air.

A cladding tube provides advantages in terms of physical protection of light guiding parts of the combiner (for example protection from contamination). This may provide an advantage in comparison other configurations of the combiner, where the light guiding parts have an interface to (open) air or are recoated with for example low-index polymer materials. Such (open) air or recoating are at risk of contamination and may cause reduced production yield, lifetime and power performance.

In one embodiment said cladding regions are provided at least partially by inserting the fiber bundle into a cladding tube comprising one or more cladding structures suitable for surrounding at least the output side of the combiner. This cladding tube may comprise one of the above mentioned cladding regions such as one or more air dads. In one embodiment one or more of the fibers of the bundle and/or the fiber bundle are adhesively connected to the cladding tube. In principle fibers may also be inserted into the cladding tube prior or simultaneously to merging into the fiber bundle so that said merging is performed in the cladding tube. In another embodiment outer cladding regions are formed by depositing suitable cladding material onto the fiber bundle, such as by dipping the fiber bundle into a liquid polymer with suitable optical properties. Similarly, in one embodiment at least part of the intermediate section comprises an outer polymer layer for mechanical shielding of the combiner.

In one embodiment the receiving fiber is a delivery fiber so the combiner is connected to a delivery fiber intended to guide the combined light. In one embodiment the receiving fiber is an active fiber. In one embodiment this active fiber may be arranged to be pumped by all or part of the combined light and/or amplify all or part of the combined light. Examples of such amplification include amplifying the overall level of the combined light, amplifying the overall level of the part(s) of the combined light e.g. by amplifying only part of the spectrum of the combined light, and amplifying a spatial part of the combined light e.g. in a designated core such as in a double clad fiber, and combinations thereof.

An active fiber may in principle may be any fiber arranged to amplify light. In one embodiment the active fiber comprises one or more cores capable of functioning as a gain medium. Such a gain medium is often, depending of the intended gain spectrum, doped with one or more active ions such as Yb, Er, Nd, Tm, Cr.

In one embodiment, the receiving fiber is a double clad fiber comprising one or more cores and the combiner is arranged to substantially match a feed-through fiber to one or more of said cores. In one embodiment the combiner is arranged to provide pump light into the one or more cores surrounding said cores arranged to receive light from the feed-through fibers. In one embodiment the pump light pumps active dopants of the said cores arranged to receive light from the feed-through fibers, such as in a double clad fiber where the inner core is doped with active ions. In one embodiment the receiving fiber is essentially free of active ions and may be implemented to transport the light from the feed- through fiber(s) as well as the light from the pump fibers. In one embodiment this transport is separated and in one embodiment this transport is overlapped, such as in a double clad fiber where light from the pump fiber(s) may be transported in a core surrounding that of the core(s) transporting the light from the feed- through fiber(s). It should be noted that sometimes in the art of double clad fiber the inner core is referred to as the core and the core surrounding this core (often suitable for guiding pump light) is referred to as the inner cladding.

In one embodiment the fibers of the fiber bundle are adhesively joined, such as through fusing or cementing with an optically suitable resin, i.e. a resin which is transparent at the wavelength(s) of the intended use of the combiner.

In one embodiment the output side of the combiner comprises a socket suitable for holding a receiving fiber. Such a socket may facilitate easy mating with a receiving fiber and it may allow the receiving fiber to be mated without splicing or other adhesive connection. In such embodiment it may be possible to exchange receiving fibers.

In the context of the present application removal of material could in principle be obtained by any process capable of removing glass material. In one embodiment this process is an etch process. In one embodiment this etch process is a wet etch process. In one embodiment this wet etch process is a wet etch process comprising etching by hydrofluoric acid (HF) acid. Other process suitable for removing class comprises etching using HF-like acids, buffered HF, laser sputtering or laser ablation

BRIEF DESCRIPTION OF DRAWINGS The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

Fig. 1a shows a number of pump fibers bundled with a central feed-through fiber.

Fig.1b shows a fiber bundle after being fused into a connected element

Fig. 1c shows a fused fiber bundle being etched in an acid solution to provide a tapered shape. Tapering is in this embodiment achieved by lifting the sample out of the acid during this etch process.

Fig. 1d shows a fused combiner with cross-sections of fibers at non-fused and fused positions. Fig. 1e shows a the etched fiber bundle inserted into a tapered cladding tube.

Fig. 1f shows a tapered etched fiber bundle inserted into a tapered air-clad tube.

Fig. 1g shows a fiber bundle fused to a cladding tube after being spliced to a suitable double clad receiving fiber.

Fig. 1h shows an alternative embodiment to that of Fig. 1g without a cladding tube.

Fig. H shows a fiber bundle where a feed-through fiber has been etched prior to bundling, such as to have substantially the same diameter as the individual pump fibers.

Fig. 2a shows individual pump fibers pre-tapered (or etched) and a feed-through fiber is etched to have the same outer diameter profile as the pump fibers, such as etched as in Fig. 1c.

Fig. 2b shows tapered fibers bundled with a feed-through fiber.

Fig. 2c shows the bundle of Fig. 2c after being fused together.

Fig. 2d shows the bundle of Fig. 2c inserted into a tapered cladding tube.

Fig. 2e shows the bundle and cladding tube of Fig. 2d fused together and cleaved through the fused cross section.

Fig. 2f: shows a double-clad receiving fiber spliced onto the combiner of Fig. 2e.

Fig. 3a shows bundling of a tube and pump fibers.

Fig. 3b shows the pump fibers and tube of Fig. 3a fused together.

Fig. 3c shows a feed-through fiber wet etched to have the same profile as the inner channel in the hollow element of after it has been tapered by elongation as shown in Fig. 3d

Fig. 3d shows the etched feed-through fiber of Fig. 3d inserted into the tapered hollow element of Fig. 3c. Fig. 3e shows combiner derived from the insertion according to Fig. 3d

Fig. 3f shows the fused element of Fig. 3e inserted into a tapered cladding tube comprising a cladding structure.

Fig. 3h shows the elements of Fig. 3f fused and cleaved to obtain a facet suitable for splicing to a receiving fiber.

Fig. 3i shows the fused combiner of Fig. 3h spliced to a double clad fiber.

The figures are schematic and simplified for clarity and just show details which are essential to the understanding of the invention, while other details are left out. The figures represent schematic and simplified examples and are not meant to limit the scope of the invention. Throughout, the same reference numerals are used for identical or corresponding parts. Identical or corresponding parts may not be numbered in all figures.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description.

MODE(S) FOR CARRYING OUT THE INVENTION In the following the invention will be discussed by way of examples. While these examples relate to a combiner with a single central feed-through fiber, other configurations are possible without parting from the scope of the invention as discussed above. Unless otherwise clear, discussion regarding individual features will relate to that feature and is not limited to the specific example.

Example 1

This example illustrates one embodiment of the invention and an embodiment of its method of production. Fig. 1 a shows a fiber bundle 11 with a central feed- through fiber 2 having a core 3 and a cladding 103 bundled with a number of pump fibers 1 having cores 101 and cladding 102. In Fig. 1 b the same fiber bundle 11 has been adhesively joined by fusing along the section 4. In this embodiment the section 4 has been adhesively joined by fusing which may be preferable as transitions between fibers may be reduced or eliminated. Fig. 1 c shows an exemplary removal of material from the fiber bundle via a wet etch 6. Here removal of material is performed by pulling the fiber bundle from a wet etch 6, such as a solution comprising HF acid. In the shown embodiment the fiber bundle 11 has been tapered by reduction of the cross section by removal of material. In one embodiment the taper has been sufficient to remove part of the core of one or more of the outer fibers so that A_COr_e,n of one or more fibers is reduced in the direction from input section to output section and said reduction is obtainable by removal of material from said adhesively joined intermediate section. The fused section 4 now forms an intermediate section 5 of the combiner 12. In Fig. 1 e the input side is set to be between the planes marked by 21 and 22, respectively. Correspondingly the output side is set to be between the planes 24 and 25. Cross sections 26, 27 and 28 are shown along different parts of the combiner. The cross section 26 shows a cross section of an un- fused part of the fiber bundle wherein cross sections of the pump fibers 1 are shown surrounding the feed through fiber 2. The cross section 27 shows a cross section of a fused part of the fiber bundle showing the cores of the pump fiber 101 and the core of the feed-through fiber 3. The cladding of the pump fibers has been redistributed to form at part of a matrix 29 around the cores of the pump fibers as well as around the cladding 103 of the feed-through fiber. In the shown embodiment the matrix forms a round cross section however it may form other shapes such a as a flower shape e.g. following the outline of the cores of the pump fibers 101. In this example the cladding 103 of the feed through fiber has assumed a hexagonal like shape due to surface tensions in the fiber bundle. In one embodiment the cladding 103 of the feed-through fiber substantially matches the refractive index of the core 101 of the pump fiber which may enable better coupling of light from the said cores 101 to said cladding 103. In one embodiment having coupling between the cladding 103 and the cores 101 allows for a reduction of the cores 101 as shown in the cross section 29. Fig. 1 e illustrates the insertion of the combiner into an optional cladding tube 7 comprising a cladding region 15 and Fig. 1f shows the combiner 12 optionally adhesively connected, such as fused, to the cladding tube 7 allowing the cladding tube to form part of the combiner. In Fig. 1 g the combiner has been cleaved along the line 8 and subsequently spliced to a double clad fiber 9, such as a delivery or active fiber. The cross section 30 shows a cross section through a cross section of the combiner comprising part originating from the cladding tube. Here the cross section 29 has been surrounded by an air clad 15 as well as an outer cladding 107. The cross section 31 shows a cross section of the delivery fiber being a double clad fiber comprising a core 32 surrounded by an inner cladding 33 which again is surrounded by an air clad 34 and an outer cladding. In this embodiment the inner cladding 33 is arranged to guide pump light whereas the core 32 is arranged to guide light from the feed through fiber. In one embodiment the core 32 is an active core which may receive energy pump light guided by the inner core 33.

Fig. 1 h shows an alternative embodiment to that of Fig. 1 g; however, here without the cladding tube. In this embodiment the pump light is at least partly guided by the index difference between the cores 101 and the ambient air.

Fig. 1 i shows the fiber bundle 1 1 similar to that of Fig. 1 a of one embodiment of the invention, where the diameter of the central fiber 2 has been pre-tapered to ensure comparable diameters for all fibers in the bundle which may facilitate a more compact packing of the bundle. For many applications the feed-through fiber may have been produced with a relative thick cladding region, to provide mechanical screening from bending as bend loss may be critical. Fibers with a relatively low NA, often used to carry light with a relatively large MFD, are often sensitive to bend loss.

In many embodiments all pump fibers will be of identical type as these often carry light having similar properties. Accordingly, only the feed-through fiber(s) may require pre-tapering to provide similar diameter between the fibers in the fiber bundle. However, in principle one or more of the pump fibers may have been pre-tapered prior to merging with the remaining individual fiber with or without a pre-tapertion of the central fiber. In one embodiment all pump fibers have been pre-tapered. In one embodiment all fibers have comparable diameters to facilitate easy packing. Pre-tapehng of one or more fibers may have the advantage that the area of the core of the cross section of the output side not originating from a fiber core is reduced. In use, this may provide the advantage that the brightness of the combined light is increased relative to a combiner where the core is not reduced. Providing that the fiber bundle has comparable diameters may provide closer packing and therefore less space between the fibers. Such space is often preferably eliminated in an adhesive joining of the fibers by the addition of a resin and/or deformation of the fibers which may have detrimental effect on the performance of the final combiner.

In general the term taper, such as in pre-tapering, refers to a reduction in cross- sectional area. A reduction may e.g. be a scaling of the cross section, such a seen in a taper formed by stretching a heated section of a fiber or a removal of material along the perimeter of the fiber.

Example 2

Fig. 2a shows examples of a pump fiber 1 which has been pre-tapered by removal of material or by scaling (such as by elongation) of the cross section along the longitudinal direction in the section 14. An example of a feed-through fiber 2 is also shown. Similar to the previous figures, the feed-through fiber 2 is shown with a core 3, whereas the fiber core of the pump fiber 1 has been omitted from the figure for clarity. The feed-through fiber 2 is shown with an optional taper along the section 13. A signal core must extend through the intermediate section of the final combiner with A_core substantially constant. Therefore, in one embodiment, the taper in section 13 is performed by removal of material. This taper may be advantageous to pack with pre-tapered pump fibers and/or to improve intensity for the combined light when the combiner is in use.

Fig. 2b shows a fiber bundle 1 1 comprising pump and feed-through fibers according to Fig. 2a. The bundle 11 now forms a combiner 12 with an intermediate section 5 where two or more fibers are pre-tapered prior to joining. In one embodiment the intermediate section is provided by a flattened end (e.g. by cleaved) to provide easier coupling to a receiving fiber. Optionally the fiber bundle 11 may be adhesively joined along the section 4 as shown in Fig. 2c. The fiber bundle 1 1 may also be inserted into an optional cladding tube 7 comprising a cladding region 15 as shown in Fig. 2d. In Fig. 2e the combiner 12 is shown in an embodiment where the fiber bundle 1 1 is optionally adhesively connected to the cladding tube 7 and where the output side of the intermediate section 5 has been cleaved along the line 8 to provide a flat output side. In Fig. 2f the combiner of Fig. 2e is shown spliced to a receiving fiber 9.

Example 3 Fig. 3a shows a fiber bundle 1 1 of fibers 1 joined to a tube 16 forming a sleeve 17. In one embodiment said fibers comprise one or more pump fibers. In one embodiment said fibers comprise one or more feed-through fibers. In one embodiment one or more of said fibers are tapered prior to joining said tube. Similarly, in one embodiment said tube may be tapered by removal of material and/or elongation prior to being joined with said fibers. In the embodiment shown fibers are joined to the outside of the tube; however, pump fibers may also or solely be joined to the inside of the tube. Fig. 3b shows the optional adhesive joining of the tube 16 and the feed-through fibers 1 here shown as a fuse along the section 4. Fig. 3c shows a feed-through fiber 2 being tapered via a wet etch 6 so as to fit into the hollow sleeve 17 (cf.Fig. 3d). In one embodiment the taper is performed by removal of material in order to preserve the shape of the core 3 of the feed through fiber 2. Fig. 3d shows the feed-through fiber 2 being mounted inside the sleeve which has been tapered via elongation or stretching,; however, in one embodiment two or more feed-through fibers and/or pump fibers may be mounted inside the sleeve. Fig. 3e shows the tube-fiber 17 assembly of Fig. 3c adhesively joined with the feed-through fiber 2 of Fig. 3d now forming a combiner 12. The cross sections 37 to 39 show cross section of different parts of the combiner. In this embodiment the cores 101 of the pump fibers are oval due to the fusing to the tube and the matrix 29 is formed at least part of material from the tube and cladding 102 of the pump fibers. Similarly to the previous examples figures 3f to 3g show the optional inclusion of a cladding tube 7, a cleave 8 suitable for connection to a receiving fiber. The invention is defined by the features of the independent claim(s). Preferred embodiments are defined in the dependent claims. Any reference numerals in the claims are intended to be non-limiting for their scope.

Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.

Claims

1. A fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where

said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area A_COr_e,n,

the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section and Acore.n is reduced along at least a part of the intermediate section where said reduction is obtainable by removal of material from said core.

2 The combiner of claim 1 wherein said output section comprises a cross section being suitable for optical connection to a receiving fiber with a receiving core having an area A_rθC

3. The combiner of any of the claims 2 to 4 wherein the receiving core of the receiving fiber surrounds at least one inner core.

4. The combiner of claim 5 wherein the receiving fiber is a double clad fiber.

5. The combiner of any of the claims 2 to 6 wherein the receiving fiber is a delivery fiber.

6. The combiner of any of the claims 2 to 7 wherein the receiving fiber is an active fiber.

7. The combiner of claims 1 or 2 where the input section of the intermediate section comprises a feed-through fiber wherein the core of said feed- through fiber extends through the intermediate section with A_core,f substantially constant.

8. The combiner of claim 3 wherein the intermediate section comprises a centre fiber and said centre fiber is a feed-through fiber.

9. The combiner of any of the preceding claims where at least one of said set of individual fibers has a mode-field-diameter (MFD) of a guided mode with wavelength λ wherein (MFD/λ)>10, such as (MFD/λ)>12, such as (MFD/λ)>14, (MFD/λ)>16, such as (MFD/λ)>18, such as (MFD/λ)>20, such as (MFD/λ)>30, such as (MFD/λ)>40, such as (MFD/λ)>100, such as (MFD/λ)>200.

10. The combiner of claims 8 to 9 where said feed-through fiber has a mode- field-diameter (MFD) of a guided mode with wavelength λ wherein (MFD/λ)>10, such as (MFD/λ)>12, such as (MFD/λ)>14, (MFD/λ)>16, such as (MFD/λ)>18, such as (MFD/λ)>20, such as (MFD/λ)>30, such as (MFD/λ)>40, such as (MFD/λ)>100.

11. The combiner of claim any of the claims 7 to 10 wherein a core of a feed- through fiber substantially matches an inner core of the receiving fiber.

12. The combiner of claim any of the preceding claims wherein one or more outer cladding regions surrounds at least part of the intermediate section.

13. The combiner of claim 12 wherein said outer cladding region comprises an air clad.

14. The combiner of any of the preceding claims where the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section and A_core,n is tapered along at least a part of the intermediate section wherein said tapering is more than or equal to 10%, such as more than or equal to 25%, such as more than or equal to 50%, such as more than or equal to 75%, such as equal to 100%.

15. The combiner of any of the preceding claims wherein said tapering is at least partially obtainable by removal of material.

16. The combiner of any of the preceding claims comprising two or more feed-through fibers, such as three or more feed-through fibers, such as four or more feed-through fibers, such as 5 or more feed-through fibers, such as 6 or more feed-through fibers.

17. The combiner of any of the claims 7 to 16 the core of one or more feed- through fibers are off-set from the centre axis on the output section.

18. The combiner of any of the preceding claims wherein at least one of said individual fibers comprises a glass core surrounded by a polymer cladding.

19. The combiner of claim 18 wherein said fiber is a pump fiber.

20. The combiner of any of the preceding claims wherein one or more of the fibers of the fiber bundle are adhesively joined

21. The combiner of claim 20 wherein adhesively joined comprises one or more of fused together, cemented together via an optically dark resin and cemented together via an optically transparent resin.

22. The combiner of any of the preceding claims wherein one or more of said individual fibers are a pedestal fiber.

23. The combiner of claim 22 wherein said pedestal fiber is a feed-through fiber.

24. The combiner of any of the claims 2 to 23 wherein the receiving fiber is a pedestal fiber.

25. The combiner of any of the preceding claims wherein at least part of the intermediate region comprises an outer polymer layer.

26. A method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, the method comprising

. providing a set of N individual optical fibers,

. merging said set of N optical fibers on the input section of the combiner to obtain a bundle of fibers forming said intermediate section, said set of optical fibers comprising fibers having a core suitable for guiding light along a longitudinal direction of said fiber and said core having a cross sectional area A_core,n

• removing material from the core of at least one optical fiber, of said set of optical fibers, prior and/or post to said merging of the set of optical fibers so that the core of said fiber(s) extends through at least part of the intermediate section from input section to output section where A_COre,n is reduced along the direction from input section to output section.

27. The method of claim 26 comprises the adhesively joining one or more of the fibers of the fiber bundle.

28. The method of claim 27 wherein adhesively joining comprises one or more of fusing the fibers together, cementing the fibers together using an optically dark resin and cementing the fibers together using an optically transparent resin.

29. The method according to any of the preceding claims wherein said fiber bundle is inserted into a cladding tube.

30. The method of claim 29 wherein said cladding tube comprises one or more cladding structures.

31. The method according to claim 30 wherein said cladding tube comprises an air clad.

32. The method according to claim 30 or 31 where one or more of the fibers of the bundle and/or the fiber bundle are adhesively connected to the cladding tube.

33. The method according to any of the claims 26 to 32 where the set of individual optical fibers comprises one or more feed-through fibers wherein the core of said feed-through fiber extends through the intermediate section with A_core,f substantially constant.

34. The method according to any of the claims 27 to 34 comprising removal of material from said adhesively joined intermediate section.

35. The method according to any of the claims 27 to 34 wherein said A_COr_e,n is reduced in the direction from input section to output section and said reduction is obtainable by removal of material from said adhesively joined intermediate section.

36. The method according to claim 35 comprising pre-tapering one or more feed-through fibers of said individual fibers by removal of material prior to merging said feed-through fibers with any of the remaining individual fibers.

37. The method according to any of the claims 27 to claim 36 comprising pre- tapering two or more of said individual fibers prior to forming said intermediate section.

38. The method according to claim 37 comprising pre-tapering one or more feed-through fibers by removal of material.

39. The method according to claim 37 and/or 38 comprising pre-tapering one or more pump fibers by removal of material and/or elongation.

40. The method according to any of the claims 27 to 39 comprising joining one or more fibers to a tube, such as a glass tube, forming a sleeve and comprising fitting one or more inner fibers to the inside of said sleeve thereby forming said intermediate section.

41. The method according to claim 40 comprising pre-tapering one or more pump fibers by removal of material and/or elongation prior to being joined with said tube.

42. The method according to claim 40 and/or 41 comprising pre-tapering one or more feed-through fibers by removal of material prior to being joined with said tube.

43. The method according to any of the claims 40 to 42 where said tube is tapered by removal of material and/or elongation prior to being joined with said fibers.

44. The method according to claim 43 comprising mounting said inner fiber post a tapering of the sleeve.

45. The method according to claim 44 comprising reducing said inner fiber to substantially match the inner shape of the sleeve.

46. The method according to any of the claims 26 to 45 wherein removal of material is performed by one or more of the group of etching, etching by hydrofluoric acid, using HF-like acids, buffered HF, laser sputtering or laser ablation.

47. The method according to any of the claims 26 to 46 comprising mating the output section of the combiner to a receiving fiber.

48. The method according to any of the claims 26 to 47 comprising coating at least part of the intermediate section with a polymeric coating.

49. The method according to any of the claims 26 to 48 wherein the fiber optic combiner further comprises any of the features of claims 1 to 25.

50. A fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area A_COr_e,n, and a cross section with a cross sectional area A_cross,n

said intermediate section is at least partly formed by said individual fibers adhesively joined in a fiber bundle, and

the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section wherein A_COre,n is reduced along at least a part of the intermediate section and said reduction is obtainable by removal of material from said adhesively joined intermediate section.

51. The fiber optic combiner of claim 50 further comprises any of the features of claims 1 to 49.

52. A method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, the method comprising

. providing a set of N individual optical fibers,

• merging said set of N individual optical fibers at the input section of the combiner to obtain a bundle of fibers forming the intermediate section between said input section and said output section, said set of individual fiber comprising fibers having a core suitable for guiding light along a longitudinal direction of said fiber said core having a cross sectional area Acore.n, and a cross section with a cross sectional area A_cross,n

• adhesively joining the fibers of said intermediate section,

• removing material from the adhesively joined intermediate section.

53. The method of claim 52 further comprises any of the features of claims 1 to 49.

54. A fibre optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where

said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along to a longitudinal direction of said fiber said core having cross sectional area A_COre,n and a cross section with a cross sectional area A_cross,n

said intermediate section is at least partly formed by said individual fibers merged to a fiber bundle, wherein

the core of at least one first optical fiber of said individual optical fibers extends through at least part of the intermediate section from input section to output section where A_cross,n is reduced along at least a part of the intermediate section obtainable by removal of material, and

at least one second optical fiber of said individual optical fibers extends through at least part of the intermediate section from input section to output section where A_core is reduced along at least a part of the intermediate section obtainable by elongation of said second optical fiber.

55. The fiber optic combiner of claim 54 further comprises any of the features of claims 1 to 49.

56. A method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, the method comprising

• tapering a section of at least one first optical fiber by removal of material,

• tapering a section of at least one second optical fiber by elongation of said second optical fiber,

• forming a set of optical fibers comprising said first and second optical fibers, • merging said set of optical fibers at the input section of the combiner to obtain a bundle of fibers forming an intermediate section between said input section and said output section, said intermediate section comprising at least part of the tapered section of said first and second fibers.

57. The method of claim 56 further comprises any of the features of claims 1 to 49.

58. A method of producing a fiber optic combiner having an input section and an output section separated in a longitudinal direction of the combiner by an intermediate section, said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area A_COr_e,n, the method comprising

• adhesively joining one or more optical fibers to a tube thereby forming a sleeve,

• fitting one or more inner fibers to the inside of said sleeve thereby forming the intermediate section of the combiner,

• optionally adhesively joining said inner fibers to said sleeve.

59. The method of claim 58 further comprises tapering at least a part of said sleeve prior to said fitting to the sleeve.

60. The method of claim 59 further comprises tapering one or more of said inner fibers prior to said fitting to the sleeve.

61. A fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area A_COr_e,n,

said intermediate section is obtainable by a method according to any of claims 58 to 60.

62. A fiber optic combiner having an input section and an output section, separated in a longitudinal direction of the combiner by an intermediate section, where said input section comprises a plurality of N individual optical fibers each having a core suitable for guiding light along a longitudinal direction of the fiber, said core having a cross sectional area

Acore.n and a cross sectional shape S_core said intermediate section is at least partly formed by said individual fibers merged to a fiber bundle, wherein the core of at least one of said individual fibers extends through at least part of the intermediate section from input section to output section and A_COre,n is reduced along at least a part of the intermediate section and S_COre is altered along at least a part of the intermediate section.

63. The combiner of claim 62 wherein S_COre alters substantially gradually along said part of the intermediate section.

64. The combiner of claim 62 or 63 wherein S_∞rθ is altered similar to a full moon approaching a new moon.

65. The combiner of any of the claims 62 to 64 further comprises any of the features of claims 1 to 61.

66. An optical system comprising a fiber optic combiner according to any of the claims 1 to 61

67. An optical device comprising a fiber optic combiner according to any of the claims 1 to 61.

68. The optical device of claim 67 selected from the group of a fiber amplifier or a fiber laser.

69. Use of the optical system of claim 66 for laser materials processing or for medical applications, such as marking, trimming, drilling, cutting, ablating, dicing, melting, shaping, surgery, cosmetics.

71. Use of the optical device of claim 67 for light generation applications, such as frequency sum generation by nonlinear effects, including super continuum frequency doubling, frequency tripling, etc.