CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims the benefit of priority to U.S. Provisional Patent Application serial no. 61/426,558, filed December 23, 2010, which is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] The present invention relates generally to optical waveguides for the transmission of electromagnetic energy. The present invention relates more particularly to optical fibers, optical fiber devices, and methods that can be used to remove cladding-propagating energy from a system.

2. Technical Background

[0003] Optical fiber lasers have many attractive properties that make them suitable for various industrial applications. Such properties can include one or more of good beam quality, easy thermal management, compact size, and good efficiency. Fiber lasers are therefore often preferred to conventional types of lasers, such as solid-state and gas lasers. Fiber lasers are able to produce optical output in the several kW range with excellent beam quality. Thus, these lasers can be used for macro-machining applications like welding and cutting of metal. Furthermore, fiber lasers lend themselves for operation with ultra-short pulses by a method of mode-locking, enabling them to be used in micro-machining applications as well.

[0004] Optical fiber amplifiers are useful in optical fiber telecommunications systems to allow for long-distance transmission of signals. While the optical fibers used to transmit signals have relatively low losses, over a long distance the signals can degrade; amplifiers are used periodically to increase the signal level.

[0005] In order to provide increased reliability of high power optical fiber lasers and amplifiers, it can be desirable to remove or substantially reduce the light propagating in the cladding of an optical fiber. Current state-of-the-art cladding light strippers rely on high refractive index polymeric materials to remove light from the cladding and dump it into a high thermal conductivity metal strip, where it is absorbed. As the amount of light increases from several Watts to tens and even hundreds of Watts, these polymer-based strippers can degrade quickly over time, acting as failure points in the system. This can give rise to fiber fuses that migrate back into the high-power pump and pre-amplification stages, causing catastrophic damage to the system. This problem is especially acute in 2 μιη optical systems, as 2 μιη light interacts strongly with water molecules present in the polymer. Even several Watts of 2 μιη light can be sufficient to completely degrade a polymer-based high-index epoxy material.

[0006] Accordingly, there remains a need for optical fibers, optical fiber devices, and methods that can be used to remove cladding-propagating energy from a system without suffering from the drawbacks or deficiencies of the prior art.

SUMMARY OF THE INVENTION

[0007] One aspect of the present invention is a rough-clad optical fiber section having a first end and a second end, the optical fiber section including

a core; and

a glass cladding disposed about the core, the cladding having an outer surface having an rms surface roughness of at least about 100 nm, the outer surface not being immediately surrounded by polymer layer.

[0008] Another aspect of the invention is a rough-clad optical fiber section having a first end and a second end, the optical fiber section for use with cladding-propagating light having a wavelength, the optical fiber section including

a core

a glass cladding disposed about the core, the cladding having an outer surface having an rms surface roughness of at least about 1/10 of the wavelength, the outer surface not being immediately surrounded by polymer layer.

[0009] Another aspect of the invention is an optical fiber including the rough-clad optical fiber section as described herein.

[0010] Another aspect of the invention is an optical fiber device including the optical fiber as described herein.

[001 1] Another aspect of the invention is a packaged optical fiber device comprising an optically absorptive package comprising a body, a first groove and a second groove in the body, and a cavity disposed along a path between the grooves; an optical fiber according to any of claims 8-14, comprising a first optical fiber section disposed in the first groove and a second optical fiber section disposed in the second groove, with the rough-clad optical fiber section disposed along the optical fiber between them and disposed in the cavity.

[0012] Another aspect of the invention is an optical fiber laser including an optical fiber or optical fiber device as described herein.

[0013] Another aspect of the invention is an optical fiber amplifier including an optical fiber or optical fiber device as described herein.

[0014] Another aspect of the invention is a method for stripping cladding-propagating light, the method including:

transmitting optical energy through a rough-clad optical fiber section or an optical fiber device as described herein, such that any cladding propagating light is substantially scattered out of the fiber and into the space surrounding the fiber.

[0015] In various aspects, the present invention can result in a number of benefits. For example, in certain embodiments, the optical fibers, devices and methods of the present invention provide a way for cladding-propagating light to be removed from a system without causing failure in the optical path.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGS. 1 and 2 are respectively a side schematic view and a schematic cross- sectional view of a rough-clad optical fiber section according to one embodiment of the invention;

[0017] FIG. 3 is a side schematic view of a rough-clad optical fiber section according to another embodiment of the invention;

[0018] FIG. 4 is a side schematic view of a rough-clad optical fiber section according to another embodiment of the invention;

[0019] FIG. 5 is a side schematic view of an optical fiber according to one embodiment of the invention; [0020] FIG. 6 is a side schematic view of an optical fiber including a splice according to one embodiment of the invention;

[0021] FIG. 7 is a cut-away side schematic view of an optical fiber device according to one embodiment of the invention;

[0022] FIGS. 8 and 9 are respectively partial top schematic and side cross sectional schematic views of a packaged optical fiber device according to one embodiment of the invention;

[0023] FIG. 10 is a schematic depiction of the experimental setup used in the

Example;

[0024] FIG. 1 1 is a schematic depiction of the packaged optical fiber device used in the Example;

[0025] FIG. 12 is a thermal image of the package of the packaged optical fiber device, at 15.5 A input current, as described in the Example; and

[0026] FIG. 13 is a set of pictures of the packaged optical fiber device after testing as described in the Example.

DETAILED DESCRIPTION OF THE INVENTION

[0027] In the following discussion it is assumed that the reader has the basic knowledge of the structure of optical fibers familiar to the person of skill in the art. Thus, the concepts of a fiber core, cladding, and coating are not discussed in detail. As is familiar to the person of skill in the art, the radiation generated in an active optical fiber device, such as a fiber laser or fiber amplifier, here referred to as the signal, propagates generally in the core of the fiber, the diameter of which is typically in the range of a few microns to a few tens of microns.

[0028] The terms "light" or "optical", as used herein, are used broadly as understood by one of ordinary skill in the art of optical waveguides, and are not to be limited as pertaining only to the visible range of wavelengths.

[0029] One embodiment of the invention is a rough-clad optical fiber section, for example, as shown in schematic side view in FIG. 1, and in schematic cross-sectional view in FIG. 2. Rough-clad optical fiber section 100 has a first end 102 and a second end 104. Rough-clad optical fiber section 100 includes a core 120, and a glass cladding 130 surrounding the core. The glass cladding can be single layered, as shown in FIG. 2, or can have multiple layers, as would be evident to the person of skill in the art. Notably, the glass cladding has an outer surface 132, which has a surface roughness that can operate to scatter the light propagating in the cladding of the fiber, as described in more detail below. The outer surface 132 is not immediately surrounded by a polymer layer.

[0030] In the embodiment of FIG. 1, the rough-clad optical fiber section 100 is part of a longer optical fiber, flanked on either end by optical fiber sections 112 and 114. In other embodiments, the rough-clad optical fiber section can be connected at one or both ends into some other optical component or device, or arranged in some other fashion.

[0031] The rough cladding can function to strip light from the cladding of the optical fiber section. Light propagating in the cladding can be scattered by the rough surface of the cladding, out of the fiber and into the space surrounding the fiber, where it can be, for example, absorbed by a material disposed near the rough-clad optical fiber section (e.g., an absorptive, thermally-conductive package). Notably, the absorption of the stripped energy can occur at a position isolated from the optical fiber section, and therefore any heat build-up will not cause fusion of the optical fiber itself.

Moreoever, the absorption of the stripped energy can occur over a larger surface area than the surface area of the optical fiber itself, resulting in relatively smaller heat build-up. Reduced heat build-up of the optical fiber can be especially desirable in polarization-maintaining fibers, as the polarization state of the fiber is generally rather temperature sensitive.

[0032] Accordingly, in one aspect of the invention, the rms (root mean square) surface roughness of the outer surface of the glass cladding is at least about 100 nm, at least about 200 nm, or even at least about 500 nm, at least about 1 μιη, at least about 2 μιη, or at least about 5 μιη. In another embodiment, the rms (root mean square) surface roughness of the outer surface of the glass cladding is at least about λ/10, at least about λ/5, or even at least about λ/2, in which λ is the wavelength of light to be propagating in the cladding. The surface roughness of the outer surface of the cladding can be selected to provide the desired level of scattering for the wavelength of light expected to be propagating in the cladding. For example, the light propagating in the cladding is often light used to pump the core of an adjoining active optical fiber; pump wavelengths often range from about 500 nm to about 4000 nm, depending on the composition of the active optical fiber and the particular absorption band desired to be pumped. In certain embodiments, the light to be propagated in the cladding has a wavelength in the range of 500 nm - 1500 nm. In other embodiments, the light to be propagated in the cladding has a wavelength in the range of 1500 nm - 3000 nm; use of a rough-clad optical fiber can be especially useful with such wavelengths. Generally larger surface roughnesses may be desirable for use with larger wavelengths. For example, in certain embodiments, the surface roughness of the outer surface of the cladding in the rough-clad optical fiber section is at least about 1000 nm, at least about 1500 nm, or even at least about 2000 nm, over the lengths described above. Desirably, the surface roughness is not substantially regular in dimension or pitch (e.g., as would be a surface grating).

[0033] Accordingly, another aspect of the invention is a method for stripping cladding-propagating light, including transmitting optical energy through a rough-clad optical fiber section as described herein, such that any cladding-propagating light is substantially scattered by the rough surface of the cladding, out of the fiber and into the space surrounding the fiber. The methods of this aspect of the invention can be performed using, for example, the rough-clad optical fiber sections, optical fiber devices described herein. Certain additional embodiments of methods are described further herein.

[0034] The length of the rough-clad optical fiber section can be selected by the person of skill in the art to sufficiently couple light out of the cladding of the optical fiber. The efficiency of scattering will depend on a number of factors, including the numerical aperture of the cladding-propagating light, the diameter of the optical fiber, the surface roughness of the outer surface of the cladding, and the wavelength of the light. In certain embodiments, the surface roughness of the cladding is at least 500 nm over a length of at least about 5 mm of the rough-clad optical fiber section. For example, in certain embodiments, the surface roughness of the cladding can be at least about 100 nm, at least about 200 nm, or even at least about 500 nm, at least about 1 μηι, at least about 2 μηι, or at least about 5 μηι over a length of at least about 10 mm, at least about 15 mm, or even at least about 20 mm of the rough-clad optical fiber section. In other embodiments, the surface roughness of the cladding can be at least about λ/10, at least about λ/5, or even at least about λ/2 over a length of at least about 10 mm, at least about 15 mm, or even at least about 20 mm of the rough-clad optical fiber section. Longer lengths can also be desirably used. For example, for a rough- clad optical fiber section about 400 μιη in diameter, it can be desirable in certain embodiments to use a roughened length on the order of 50-60 mm.

[0035] The diameter of the optical fiber section can be selected to provide the desired balance between efficient scattering and adequate transmission of optical signals propagating in the core. Smaller diameters will provide increased scattering of the cladding-propagating light, and therefore can require a relatively shorter length of rough-clad fiber to achieve the desired stripping of cladding-propagating light.

Accordingly, in certain embodiments, the diameter of the rough-clad optical fiber section is no more than about 300 μιη, or even no more than about 200 μιη. For example, a rough-clad length on the order of 20-30 mm can in certain embodiments be sufficient to strip >95% of cladding-propagating light from an optical fiber section having a diameter of about 125 μιη. However, if the diameter of the rough-clad optical fiber section is too small, the light propagating in the core can also be scattered. Smaller diameter fibers can also be more difficult to roughen without optical loss or failure, especially by etching (as described below). Accordingly, in certain embodiments, the rough-clad optical fiber section has a diameter of at least about 50 μιη.

[0036] The rough-clad optical fiber section can have a different diameter than the remainder of the optical fiber of which is it a part. For example, as shown in side view in FIG. 3, rough-clad optical fiber section 300 has a diameter that is less than the diameter of the flanking optical fiber sections 310. The optical fiber section to be rough-clad can have its diameter reduced, for example, by etching down to the desired. As described in more detail below, etching can also provide the desired surface roughness. While the rough-clad optical fiber section of FIG. 3 is shown as having a sharp reduction at the ends of the rough-clad section, the person of skill in the art will recognize that the transition between larger and smaller diameters can be made gradually (e.g., adiabatically), and/or at a different position with respect to the rough-clad section (e.g., outside of the rough-clad section, or inside the rough-clad section).

[0037] The surface roughness of the outer surface of the rough-clad optical fiber section can be provided in a variety of ways. For example, in certain embodiments, the cladding has glass particles bound at its outer surface. For example, glass particles (e.g., fumed silica) can be bound to the outer surface of the cladding thermally or chemically. For example, calcium-assisted glass-to-glass bonding can be used to attach glass particles to the surface, for example, as described in P.B. Allen and D.T. Chiu, "Calcium-Assisted Glass-to-Glass Bonding for Fabrication of Glass Microfluidic Devices," Anal. Chem. 2008, 80, 7153-57, which is hereby incorporated herein by reference in its entirety. Sol-gel chemistry can also be used. In other embodiments, the outer surface of the cladding is formed from roughened cladding material. For example, etching with hydrofluoric acid (or a compound containing or generating fluoride) can be used to provide the desired surface roughness. Dilute hydrofluoric acid in a paste form can be used as the etching compound, for example, at concentrations in the range of 0.1-49% HF. The amount of HF present will determine the rate at which etching occurs as well as the roughness of the surface. HF solutions can also be used. Hydrogen fluoride vapor can also be used to etch the surface, for example by holding the fiber ~l-2 cm above a hydrofluoric acid container, preferably in a high flow exhaust hood in order to ensure a reasonable upward flow of vapor. Exposure to HF vapor for as little as five minutes can provide a uniformly frosted glass optical fiber surface with roughnesses in excess of about 1 μιη. Commercially available glass frosting etchants can also be used. Such substances typically include ammonium and/or sodium bifluorides. ARMOUR ETCH brand cream is an example of such a product, but it can take over ½ hour to provide a sufficiently rough surface. For example, the fiber can be etched with ARMOUR ETCH for two 45 minute periods, each followed by rinsing with deionized water and isopropanol. Alternatively, mechanical abrasion of the glass surface can be used to provide the desired surface roughness. For example, 0.5-10 μιη SiC, alumina, or diamond impregnated polishing paper or cloth can be wiped across the glass surface until it acquires a light frosty appearance as viewed under a low power microscope. The amplitude of the roughness will depend, inter alia, by the grit size of the abrasive used. Any combination of the above-described techniques can be used to provide rough-clad fibers. For example, in some embodiments, a chemical etch (e.g., use of vapor as described above) is used after a mechanical abrasion step.

[0038] One application for a rough-clad optical fiber section is in the neighborhood of a splice. When optical fibers are spliced together to form a continuous optical fiber, some fraction of the light propagating in the core is lost to the cladding. This effect can be, for example, as high as 5-10%, and is higher when the spliced optical fibers are dissimilar (e.g., in core size, numerical aperture, and/or index profile). In some conventional systems, this light is stripped from the cladding using a high-index polymer coating over the splice, which is heat-sunk to a metal package. For a system operating in the kW range, the power dumped into the polymer coating can be on the order of several tens of Watts, even up to about 100 W. Polymeric materials are very susceptible to damage when subjected to such power levels over such a small area (i.e., the surface area around the spliced fiber). Use of a rough cladding surface in the neighborhood of the splice point can scatter light out of the cladding and away from the spliced fibers, as described above.

[0039] Accordingly, in certain embodiments, the rough-clad optical fiber section includes a fiber splice. The fiber splice can join two very similar fibers, or, in other embodiments, two substantially dissimilar fibers. Accordingly, in some

embodiments, the rough-clad optical fiber section includes a first rough-clad optical fiber subsection on one side of the splice, and a second rough-clad optical fiber subsection on the other side of the splice, the two rough-clad optical fiber subsections having substantially different lengths. The splice need not be placed symmetrically between the ends of the rough-clad optical fiber section. In certain embodiments, the splice is substantially nearer one end of the rough-clad optical fiber section than the other. Desirably, the distance in the direction of light propagation along the rough- clad optical fiber section between the splice and an end of the rough-clad optical fiber section over which the surface roughness of the cladding is at least 500 nm is at least about 5 mm. For example, the surface roughness of the cladding can be at least 500 nm over a length of at least about 10 mm, at least about 15 mm, or even at least about 20 mm along the rough-clad optical fiber section in the direction of light propagation between the splice and an end of the rough-clad optical fiber section. [0040] One embodiment is shown in side schematic view in FIG. 4. Rough-clad optical fiber section 400 includes a splice 470, which is disposed between rough-clad optical fiber subsections 407 and 409. In this embodiment, rough-clad optical fiber subsections 407 and 409 are similar in diameter, but they can be dissimilar in other properties (e.g., as described above). In this embodiment, the direction of propagation is from left to right; and the majority of the rough-clad section is to the right of the splice (i.e., after the splice as light propagates).

[0041] Of course, in certain situations, a splice can also deliver some light to the cladding such that it is propagating in a direction opposite the light in the core.

Accordingly, in some embodiments, it can be desirable for there to be some distance of rough-clad optical fiber section before the splice (i.e., in the direction of propagation, to the left of the splice in FIG. 4. The length of this distance can be as described above (e.g., surface roughness of at least 500 nm over a length of about 5 mm).

[0042] Splices can be formed, for example, in a conventional manner, e.g., by abutting cleaved or polished end-faces of the optical fibers to be spliced, and heating to fuse them to one another. In certain embodiments, the splicing can be performed before the forming of the roughened surface, so as not to degrade the roughness during the splicing operation.

[0043] Another aspect of the invention is an optical fiber including a rough-clad optical fiber section as described herein. For example, one embodiment of the invention is an optical fiber comprising a rough-clad optical fiber section as described herein, and an optical fiber section (e.g., not rough-clad) extending from the first end of the rough-clad section. The optical fiber can further comprise a second optical fiber section (e.g., not rough-clad) extending from the second end of the rough-clad section. Such an optical fiber is shown in FIG. 1; first optical fiber section 1 12 extends from the first end 102 of rough-clad optical fiber section 100, while second optical fiber section 1 14 extends from the second end 104 of rough-clad optical fiber section 100. In certain embodiments, at least one (in some embodiments, both) of the optical fiber sections that extend from the rough-clad optical fiber section is not coated with a polymer, at least for a distance of about 2 mm from the rough-clad optical fiber section. In certain embodiments, the uncoated distance is at least about 5 mm, or even at least about 10 mm. An uncoated section of optical fiber can isolate any optical fiber coatings from the scattered light, and thereby provide for acceptably minimal absorption by the coatings of the light scattered from the rough-clad optical fiber section. Accordingly, use of an uncoated section of optical fiber can allow for the use of optical fiber coatings in other parts of the optical fiber, without long-term degradation of the coatings.

[0044] For example, in one embodiment, an optical fiber includes a rough-clad optical fiber section as described above, flanked on at least one end by an uncoated optical fiber section, which in turn is flanked by a coated optical fiber section. One embodiment is shown in schematic side view in FIG. 5. Optical fiber 550 includes a rough-clad optical fiber section 500 having a first end 502 and a second end 504; a first coated optical fiber section 566a and a first uncoated optical fiber section 562a disposed directly between the first end 502 of the rough-clad optical fiber section 500 and the first coated optical fiber section 566a. For example, the first coated optical fiber section 566a includes a core, a glass cladding surrounding the core, and a polymer coating surrounding the glass cladding, and has a first end 567a. The first uncoated optical fiber section 562a also includes a core and a glass cladding surrounding the core, but it substantially lacks a polymer coating disposed about the glass cladding. The first uncoated optical fiber section 562a has a first end 563a and a second end 564a. The first end 563a of the first uncoated optical fiber section 562a is coupled to the first end 502 of the rough-clad optical fiber section; and the second end 564a of the first uncoated optical fiber section 562a is coupled to the first end 567a of the first coated optical fiber section 566a. In certain embodiments, the distance along the optical fiber from the rough-clad optical fiber section to the polymer coating of the first coated optical fiber section (i.e., the length of the first uncoated optical fiber section) is at least about 2 mm. In some embodiments, the distance is at least about 5 mm, or even at least about 10 mm.

[0045] In certain embodiments, for example as shown in FIG. 5, the optical fiber further includes a second uncoated optical fiber section coupled between the second end of the rough clad optical fiber section and a coated optical fiber section. As shown in FIG. 5, optical fiber 550 also includes a second coated optical fiber section 566b and a second uncoated optical fiber section 562b disposed directly between the second end 504 of the rough-clad optical fiber section 500 and the second coated optical fiber section 566b. For example, the second coated optical fiber section 566b includes a core, a glass cladding surrounding the core, and a polymer coating surrounding the glass cladding, and has a first end 567b. The second uncoated optical fiber section 562b also includes a core and a glass cladding surrounding the core, but it substantially lacks a polymer coating disposed about the glass cladding. The second uncoated optical fiber section 562b has a first end 563b and a second end 564b. The first end 563b of the second uncoated optical fiber section 562b is coupled to the second end 504 of the rough-clad optical fiber section; and the second end 564b of the second uncoated optical fiber section 562b is coupled to the first end 567b of the second coated optical fiber section 566b. In certain embodiments, the distance along the optical fiber from the rough-clad optical fiber section to the polymer coating of the second coated optical fiber section (i.e., the length of the second uncoated optical fiber section) is at least about 2 mm. In some embodiments, the distance is at least about 5 mm, or even at least about 10 mm.

[0046] In certain embodiments, the optical fiber includes a splice. For example, the optical fiber can include a splice in the rough-clad optical fiber section, as described above. In other embodiments, the splice can be located near, but not in the rough-clad optical fiber section. For example, in certain embodiments, the splice can be within 50 mm, within 20 mm, or even within 10 mm of the rough-clad optical fiber section. Desirably, the optical fiber is not coated with a polymer coating between the splice and the rough-clad optical fiber section. Desirably, the splice is located before the rough-clad optical fiber section (i.e., in the direction of propagation of the core- propagating signal). Accordingly, light sent into the cladding by the splice and propagating in the direction of the signal will encounter the rough-clad optical fiber section and be scattered out of the fiber. As mentioned above, a splice can back- reflect some light into the cladding; accordingly, in certain situations, it can be desirable to additionally have a rough-clad optical fiber section disposed before the splice (i.e., in the direction of signal propagation). Use of a rough-clad optical fiber section slightly removed from the splice can be advantaged, in that the splicing operation will not degrade the roughening, so pre-roughened fibers can be used. In fact, either or both optical fibers to be spliced can be roughened all the way out to their ends; even if the splicing operation degrades the roughness, there will remain a rough-clad optical fiber section on one or both sides of the splice.

[0047] One embodiment is shown in FIG. 6. Optical fiber 650 includes a rough-clad optical fiber section 600, and an uncoated optical fiber section 662 coupled thereto. The uncoated optical fiber section 662 includes a splice 670, and the rough-clad optical fiber section 600 is disposed after the splice (i.e., in the direction of propagation). In the embodiment of FIG. 6, the optical fiber also includes a second rough-clad optical fiber section 601, disposed in the uncoated optical fiber section 662, before the splice (i.e., in the direction of propagation), to strip any back-scattered light. Notably, the second rough-clad optical fiber section 601 is shorter in length than the rough clad optical-fiber section 600, because relatively less light will generally be back-reflected in the cladding than will be propagating in the cladding the same direction as the core-propagating signal.

[0048] Coated optical fiber sections can be disposed after a rough-clad optical fiber section disposed after the splice, and/or before a rough-clad optical fiber section disposed before a splice, separated from the rough-clad optical fiber section by uncoated optical fiber sections as described above, for example, with reference to FIG. 5.

[0049] In certain embodiments of the optical fibers described herein, it can be desirable to provide a substantially opaque (i.e., to the radiation to be used in the optical fiber) coating or covering over at least part of any polymeric coatings on the fiber. The opaque coating or covering can protect the underlying polymer coating from light- induced heat increase and/or damage. While the opaque coating or covering may itself experience an increase in temperature, the heat will be confined to the outer part of the optical fiber, and can be effectively removed to the environment.

[0050] Another aspect of the invention is an optical fiber device comprising an optical fiber as described herein.

[0051] In one embodiment, the optical fiber device includes an optical fiber as described above, with the rough-clad optical fiber section disposed within a glass capillary. In certain desirable embodiments, the rough-clad optical fiber section is not in substantial contact with the inner surface of the capillary. Use of a capillary can provide an additional degree of mechanical robustness to the optical fiber. Moreover, the capillary can also protect the surface of the rough-clad optical fiber section from absorbing debris and/or particles. Absorbance of scattered light by materials on the surface of rough-clad optical fiber section would cause an undesirable heat increase on the fiber itself, which, as discussed above, can lead to fiber fusion and degradation of optical properties. If absorbing debris is on the capillary, however, any heat increase due to absorbance of scattered light will be substantially isolated from the optical fiber itself, and much less likely to cause any damage thereto.

[0052] One embodiment is shown in cut-away side schematic view in FIG. 7. Optical fiber device 780 includes an optical fiber 750, which includes a rough-clad optical fiber section 700 as described herein. The rough-clad optical fiber section (along with some of the optical fiber on either side of it) is disposed within a glass capillary 782. An adhesive 783 is used at either end of the capillary to hold the fiber in place; as the person of skill would appreciate, glass frit could alternatively be used. A little tension can be placed on the optical fiber in order to keep the rough-clad optical fiber section from contacting the inner capillary walls. The adhesive or glass frit is desirably at least about 2 mm from the rough-clad optical fiber section. In some embodiments, the adhesive is at least about 5 mm, or even at least about 10 mm from the rough-clad optical fiber section. The adhesive or glass frit desirably has a refractive index substantially less than that of the cladding (e.g., at least about 0.01 less, at least about 0.02 less, or even at least about 0.04 less than that of the cladding, as measured at the wavelength to be scattered). For example, a fluoroacrylate adhesive can be used. The adhesive or glass frit can adhere a polymer-coated section of the optical fiber to the capillary; in such cases, the polymer coating desirably extends into the capillary past the adhesive by no more than about 5 mm.

[0053] In another embodiment, the optical fiber device includes optically absorptive material not in substantial contact with the rough-clad optical fiber section, and disposed so as to absorb at least 70% of the light scattered from the rough-clad optical fiber section when in use. For example, the optically absorptive material can substantially surround the rough-clad optical fiber section. In certain embodiments, the optically absorptive material is disposed so as to absorb at least 80%, at least 90%, or even at least 95% of the light scattered from the rough-clad optical fiber section. The optically absorptive material can be, for example, metal (e.g., the inner surface of a metal package in which the optical fiber is packaged). The metal is desirably of relatively high thermal conductivity. Particular materials include copper or aluminum, for example with a blackened nickel or blackened chrome surface finish. The optically absorptive material is desirably not polymeric. The optically absorptive material can in some embodiments be externally cooled, either passively (e.g., by a heat sink or radiating fins), or actively (e.g., with a fluid, a fan, or thermoelectrically).

[0054] Another aspect of the invention is a packaged optical fiber device. The packaged device includes an optically absorptive package including a body, a first groove and a second groove in the body, and a cavity disposed along a path between the grooves; and an optical fiber as described herein, including a first optical fiber section disposed in the first groove and a second optical fiber section disposed in the second groove, with the rough-clad optical fiber section disposed along the optical fiber between them and disposed in the cavity. The optically absorptive package can be formed from an optically absorptive material, as described above, and can be configured to absorb at least 70%, at least 80%, at least 90%, or even at least 95% of the light scattered from the rough-clad optical fiber section, as described above. The entire package need not be optically absorptive, of course, as long as the regions adjacent the rough-clad optical fiber are sufficiently absorptive to absorb scattered light as described herein. The rough-clad optical fiber section can be disposed within a glass capillary, as described herein. The inventors estimate that a water-cooled copper package with dimensions 13 cm long by 1.5 cm x 1.5 cm can dissipate over 100 W of scattered light power without significant temperature increases at the thermally weakest points in the device— the ends of the package, where polymeric adhesives are used to adhere the optical fiber to the package.

[0055] One embodiment is shown in partial top schematic view in FIG. 8, and in side cross-sectional schematic view in FIG. 9. Packaged optical fiber device 890 includes an optically absorptive package 891. The package includes a first groove 892, a second groove 893, and a cavity 894 disposed on a path between the grooves. In the example of FIGS. 8 and 9, the grooves are disposed along a common line with the cavity between them. Of course, in other embodiments, the grooves and the cavity can be differently disposed. For example, the grooves can be disposed on the same side of the package, with the cavity extending out from the grooves; an optical fiber can curve to turn back on itself in the cavity. In this case, the path between the grooves is U-shaped; the cavity still lies on the path between the grooves. Returning to FIGS. 8 and 9, the optical fiber 850 includes a rough-clad optical fiber section 800 disposed in the cavity; desirably, it does not contact the package. Adhesive 883 affixes the optical fiber in the grooves. In the embodiment of FIGS. 8 and 9, the optical fiber includes a first coated optical fiber section and a first uncoated optical fiber section, as well as a second coated optical fiber section and a second uncoated optical fiber section, substantially as described above with respect to FIG. 5. In certain embodiments, the first and second coated optical fiber sections extend into the cavity of the package no more than about 5 mm (i.e., distances 897 in FIG. 8). The side cross-sectional schematic view also depicts a lid 898 that closes the grooves and the cavity.

[0056] In certain embodiments, the optical fiber device or packaged optical fiber device described above further comprises an active optical fiber, configured to be cladding-pumped, optically coupled to the optical fiber comprising the rough-clad optical fiber section (desirably without any non-fiber optical components

therebetween). For example, the rough-clad optical fiber section can be spliced to an end of the active optical fiber. The wavelength of the light used in the cladding pumping can be used to determine the surface roughness of the rough-clad optical fiber section. For example, in certain embodiments, the surface roughness of the rough-clad optical fiber section can be 1/10, 1/5, or even 1/2 of the wavelength of the light used in the cladding pumping.

[0057] Another aspect of the invention is an optical fiber laser or optical fiber amplifier comprising a rough-clad optical fiber section, optical fiber, optical fiber device or packaged optical fiber device as described herein. The rough-clad optical fiber section, optical fiber, optical fiber device and packaged optical fiber devices can be used at points in the system where it is desired to strip light out of a fiber cladding. The rough-clad optical fiber section can be used, for example, to strip cladding- propagating pump radiation from an optical fiber (e.g., at a position remote from an active fiber). [0058] Other aspects of the invention provide methods for making the rough-clad optical fiber sections and devices of the present invention. For example, in one embodiment, a method for making a rough-clad optical fiber section as described herein comprises providing an optical fiber having a core and glass cladding disposed about the core, the cladding having an outer surface not immediately surrounded by a polymer layer. A polymer coating can, for example, be stripped from the cladding in a section of optical fiber to be rough-clad. The outer surface of the cladding can be roughened to an rms surface roughness of at least about 100 nm, at least about 200 nm, or even at least about 500 nm, at least about 1 μιη, at least about 2 μιη, or at least about 5 μιη, as described above. For example, the outer surface of the cladding can be roughened by binding glass particles at its outer surface, as described above. In other embodiments, the cladding material itself can be roughened. For example, the roughening can be performed by contacting the outer surface of the cladding with HF or a compound containing or generating fluoride, as described above. The roughening can also be performed by mechanical abrasion, as described above. Moreover, any combination of the above-described techniques can be used to roughen the fiber. For example, in some embodiments, a chemical etch (e.g., use of vapor as described above) is used after a mechanical abrasion step, as described above. The method can be performed to provide the rough-clad optical fiber sections having the lengths and roughnesses as described herein.

[0059] In certain embodiments, the method includes reducing the diameter of the optical fiber section to be rough-clad before roughening the surface of its outer cladding. Such embodiments can be used to make the rough-clad optical fiber sections described above with reference to FIG. 3.

[0060] In certain embodiments, the section that is rough-clad is formed by splicing two optical fibers together, to form devices such as those described with reference to FIG. 4.

[0061] In certain embodiments, the method involves splicing an optical fiber including a rough-clad optical fiber section to another optical fiber. Such methods can be used to make optical fibers as described above with reference to FIG. 6. For example, in certain embodiments, the splice is within 50 mm, within 20 mm, or even within 10 mm of the rough-clad optical fiber section. [0062] In one embodiment, the method includes disposing the rough-clad optical fiber section within a glass capillary to make an optical fiber device. Such a method can be used to make devices such as those described above with reference to FIG. 7.

Adhesive can be disposed at one or both ends of the capillary to hold the fiber in place, as described above.

[0063] In one embodiment, the method includes disposing an optically absorptive material not in substantial contact with the rough-clad optical fiber section, so as to absorb at least 70% of the light scattered from the rough-clad optical fiber section when in use. Particular materials and configurations can be as described above.

[0064] The invention is further described by the following non-limiting Example.

Example

[0065] A schematic depiction of the experimental setup is provided in FIGS. 10 and 11. As a control, pump radiation (936 nm) was launched into a pigtailed pump/signal combiner (i.e., before splicing the LMA fiber thereto) to determine the coupled power as a function of laser current ("Po" and "Current" in Table 1).

[0066] The experimental set-up of FIG. 10 was then constructed. A rough-clad optical fiber section was formed in a large mode area optical fiber (400 μιη diameter; 25 μιη core diameter, available from Nufern as LMA-YDF-25/400-VIII) by stripping about 65 mm of coating off of the LMA fiber. About 50 mm of the stripped length was roughened. The optical fiber was held in a standard package with epoxy (Epo- Tek), as shown in FIG. 11. The pump/signal combiner was spliced (with a polymeric recoat) to an end of the LMA fiber. Output pump power was measured at the other end of the LMA fiber.

[0067] Pump radiation was launched through the pump/signal combiner into the cladding of the LMA fiber. The output power ("Pi") and the temperature of the package body ("Tbody") was measured, as well as the temperature of the recoated splice ("TspHce"). Notably, as the current increased, so did the power launched through the pump/signal combiner and into the LMA fiber. The temperature of the recoat splice rose only minimally as the power increased, suggesting that only very little pump energy is lost at the splice. In contrast, the temperature of the package body increased very significantly with launched power, demonstrating that pump was scattered from the rough-clad fiber section, and absorbed by the package body. FIG. 12 is a thermal image of the package at 15.5 A input current, showing a controlled release of energy from the LMA fiber and into the package. Pictures of the packaged section of the fiber are provided in FIG. 13; notably, there is no damage evident at the polymer coatings or at the epoxy used to hold the LMA fiber in place.

Table 1

Current (A) Po (W) Tsplice ( C) Tbody (°C) Pi (W)

23.5 216.9 22.0 163.3 1.1

24 223.2 22.0 166.1 0.7

24.5 227.5 22.2 169.7 1.3

25 232.1 22.3 172.9 1.1

[0068] In the claims as well as in the specification above all transitional phrases such as "comprising", "including", "carrying", "having", "containing", "involving" and the like are understood to be open-ended. Only the transitional phrases "consisting of and "consisting essentially of shall be closed or semi-closed transitional phrases, respectively, as set forth in the U.S. Patent Office Manual of Patent Examining Procedure § 21 11.03, 8th Edition, 8th Revision.

[0069] It is understood that the use of the term "a", "an" or "one" herein, including in the appended claims, is open ended and means "at least one" or "one or more", unless expressly defined otherwise. The occasional use of the terms herein "at least one" or "one or more" to improve clarity and to remind of the open nature of "one" or similar terms shall not be taken to imply that the use of the terms "a", "an" or "one" alone in other instances herein is closed and hence limited to the singular. Similarly, the use of "a part of, "at least a part of or similar phrases (e.g., "at least a portion of) shall not be taken to mean that the absence of such a phrase elsewhere is somehow limiting.

[0070] Subsequent reference to the phrase "at least one", such as in the phrase "said at least one", to specify, for example, an attribute of the limitation to which "at least one" initially referred is not to be interpreted as requiring that the specification must apply to each and every instance of the limitation, should more than one be under consideration in determining whether the claim reads on an article, composition, machine or process, unless it is specifically recited in the claim that the further specification so applies.

[0071] The use of "or", as in "A or B", shall not be read as an "exclusive or" logic relationship that excludes from its purview the combination of A and B. Rather, "or" is intended to be open, and include all permutations, including, for example A without B; B without A; and A and B together, and as any other open recitation, does not exclude other features in addition to A and B. [0072] Any of the features described above in conjunction with any one aspect described above can be combined with a practice of the invention according to any other of the aspects described above, as is evident to one of ordinary skill who studies the disclosure herein.

[0073] Those of ordinary skill in the art will recognize or be able to ascertain using no more than routine experimentation many equivalents to the specific embodiments of the invention described herein. It is therefore to be understood that the foregoing embodiments are presented by way of example only and that within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. The present invention is directed to each individual feature, system, material and/or method described herein. In addition, any combination of two or more such features, systems, materials and/or methods, if such features, systems, materials and/or methods are not expressly taught as mutually inconsistent, is included within the scope of the present invention.