WO2015026998A1

WO2015026998A1 - Light-coupling apparatus and methods for light-diffusing optical fibers

Info

Publication number: WO2015026998A1
Application number: PCT/US2014/051972
Authority: WO
Inventors: Stephan Lvovich Logunov; Nikolay Timofeyevich TIMOFEEV
Original assignee: Corning Incorporated
Priority date: 2013-08-23
Filing date: 2014-08-21
Publication date: 2015-02-26
Also published as: US9158080B2; CN105531534A; US20150055915A1; KR20160048853A; EP3036476A1

Abstract

Light-coupling apparatus and methods for light-diffusing optical fibers are disclosed. The light-coupling apparatus includes a light-diffusing fiber bundle having an end section made up of tightly packed cores by removing the claddings. The spaces between the cores are filled with a material having a refractive index equal to or less than that of the cores. A light-emitting diode light source can be butt-coupled to the bundled-core end of the light-diffusing fiber bundle or can be coupled thereto via a reflective concentrator. A method of forming a flat and smooth end on a cleaved fiber that has a rough end is also disclosed.

Description

LIGHT-COUPLING APPARATUS AND METHODS FOR

LIGHT-DIFFUSING OPTICAL FIBERS

This application claims the benefit of priority under 35 U.S.C. § 120 of U.S. Application Serial No. 13/974580 filed on August 23, 2013 the content of which is relied upon and incorporated herein by reference in its entirety.

FIELD

[0001] The present disclosure relates to optical fiber connectors and to light-diffusing fibers, and in particular relates to light-light-coupling apparatus and methods for light-diffusing optical fibers.

[0002] The entire disclosure of any publication or patent document mentioned herein is incorporated by reference.

BACKGROUND

[0003] Optical fibers are used for a variety of applications where light needs to be delivered from a light source to a remote location. Optical telecommunication systems, for example, rely on a network of optical fibers to transmit light from a sendee provider to system end- users.

[0004] Optical fibers are typically designed to efficiently deliver light from one end of the fiber to the other end of the fiber (usually over long distances), which requires that very little light escapes from the sides of the typical fiber. Yet, there are a number of applications such as special lighting, signage, or biological applications, including bacteria growth and the production of hoto-bioenergy and biomass fuels, where select amounts of light need to be provided in an efficient manner to the specified areas.

[0005] To this end, light-diffusing optical fibers have been developed, such as described in US. Patent No. 7,450,806, and U.S. Pre-Grant Publications US2012/0275180,

US201 1/0122646, and US2013/0107565. Light-diffusing optical fibers are configured to scatter light out of the fiber along the length of the fiber. Light diffusing fibers are used in a variety of applications, such as light sources (see, e.g., WO 2013/055842 Al).

[0006] Light diffusing optical fibers can be optically coupled to a. laser-diode light source with a coupling efficiency of greater than 90% due to the small and relative low numerical aperture (NA) of laser diodes. However, for certain applications, it is preferred to use light- emitting diodes (LEDs) as the light source. The coupling of an LED to a light-diffusing optical fiber is challenging because the core of a light-diffusing fiber can have a diameter that ranges from 125 μιη to 250 μηι and has a NA of about 0.35, while an LED has a size of about 1 mm or larger and a large NA, e.g., from about 0.86 and up to about 1.

[0007] In an optical system, the etendue is conserved. In geometric terms relating to a light source and an optical fiber, if Ai and K₂ are respectively the size of a light source emitting area and the fiber core, and NAj and A2 are respectively the source and fiber numerical apertures, then Αι ·(ΝΑι)² = A^^NA?)^7'. Consequently, the optical coupling between a light- diffusing optical fiber and an LED light source would be unacceptably low due to the mismatch between numerical apertures and emitting/receiving areas.

SUMMARY

[0008] An aspect of the disclosure is an optical fiber apparatus. The optical fiber apparatus has a plurality of light-diffusing optical fibers each having a cylindrical glass core with a refractive index n 1, and a surrounding cladding with a refractive index ri2 < ni, wherein an end section of each light-diffusing optical fiber has the cladding removed to expose the core. The glass cores are arranged in a tight bundle with spaces between the cores. A transparent material of refractive index ΠΜ resides in the spaces, wherein ΠΜ≤ nj.

[0009] Another aspect of the disclosure is a light-coupling apparatus. The light-coupling apparatus includes the above-described optical fiber apparatus, which has an endface of a first size and shape. The light-coupling apparatus also includes an LED light source having an output end from which light is emitted and that has a second size and shape that is substantially the same as the first size and shape. The output end of the LED light source is interfaced with the endface of the optical fiber apparatus to establish a butt-coupling of the light from the LED into the optical fiber apparatus.

[0010] Another aspect of the disclosure is a light-coupling apparatus that includes the optical fiber apparatus as described above and having a numerical aperture NAB and an endface of a first size and a first shape. The light-coupling apparatus also includes a LED light source having a numerical aperture NA_S > NA_B, an output end of a second size smaller than the first size and having the substantially the first shape, wherein the output end emits light. The light-coupling apparatus also has a reflective concentrator operably disposed between the LED light source and the optical fiber apparatus so that at least a portion of the light from the LED light source reflects from the reflective concentrator and enters the endface of the optical fiber apparatus in a manner that substantially matches the numerical aperture NA_B of the optical fiber apparatus. The light-coupling apparatus operates, for example, at a wavelength(s) situated in from 300 nm to 2000 nm wavelength range (e.g., 350 nm-2000 nm). According to some embodiments, the optical fiber core is transparent at at least one wavelength λ situated in these wavelength range, and the coatings can also be transparent at at least one wavelength λ situated in that wavelength range.

[0011] Another aspect of the disclosure is a method of processing an end of a light- diffusing optical fiber having a core and a cladding surrounding the core. The method includes: cleaving the light-diffusing optical fiber to form a cleaved end wherein the core has a rough end; depositing an ultraviolet (UV)- curable material on the rough end; placing a non-stick UV-transparent plate against the UV-curable material and the cleaved end to form a flat surface of the UV-curable material; and illuminating the UV-curable material with UV light through the non-stick UV-transparent plate (transparent for UV wavelengths (e.g., 300nm to 450 nm, for example 3 10, 350 nm, 380 nm, or 400 nm) to cure the UV-curable material to form smooth and flat hardened surface at the cleaved end.

[0012] Another aspect of the disclosure is a light-coupling apparatus. The light coupling apparatus has a light-diffusing-fiber (LDF) bundle having a plurality of light-diffusing optical fibers each having a glass cylindrical core with a refractive index n i, and a surrounding cladding with a refractive index ti2 < ni, wherein an end section of each light-diffusing optical fiber has the cladding removed to expose the glass core. The glass cores are arranged in a tight bundle with spaces between the cores. A first transparent material of refractive index riMi resides in the spaces, wherein nui < nj. The LDF bundle has an endface of a first size and first shape. The apparatus also has a LED light source having an output end having a second size and second shape that substantially match the first size and shape and that emits light. The apparatus further includes a transparent rod having first and second ends and disposed between and in contact with the endface of the LEF bundle and the output end of the LED light source. The apparatus also includes a housing having an interior that contains the LED light source, the transparent rod and the endface of the LDF bundle. The transparent housing has a sidewall that is generally transparent to the light from the LED light source and that scatters a portion of the light that travels within the sidewall.

[0013] Another aspect of the disclosure is a method of coupling light from a LED into a plurality of light-diffusing optical fibers each having a glass core and a cladding with respective refractive indices ni and ti2. The method includes: removing a portion of the cladding from an end section of each of the plurality of optical fibers to form exposed cores having respective core ends; arranging the exposed glass cores in a tight bundle that has spaces between the tightly bundled cores; filling the spaces with a transparent material (i.e., a material that is transparent for a wavelength or wavelengths situated in 300nm -2000 nm wavelength range (e.g., 350 nm - 2000 nm range)) having a refractive index ΠΜΙ, wherein ΠΜΙ < m to define a light-diffusing core bundle having an endface that includes the core ends and a portion of the material; and directing light from the LED into the endface to travel within the cores and the material.

[0014] Another aspect of the disclosure is an optical fiber apparatus that includes a plurality of light-diffusing optical fibers arranged in a bundle. Each optical fiber has a numerical aperture (NA), a cylindrical glass core with a refractive index n i, and a surrounding cladding with a refractive index ti₂ < ni. Each end section of each light-diffusing optical fiber has the cladding removed to expose the core. The exposed glass cores at the end sections of the optical fibers are fused together to form a solid-glass fused-core section at an end of the bundle. A lower- index clad material surrounds the fused-core section of the bundle to define a fused-core NA that substantially matches the single-fiber NA.

[0015] Additional features and advantages are be set forth in the Detailed Description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the Detailed Description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which:

[0017] FIG. 1 is a front elevated view of an example light-diffusing optical fiber (LDF);

[0018] FIG. 2 is a front-on view of an example LDF bundle made up of nine LDFs, where the bundle includes a transparent material within the spaces between the fibers;

[0019] FIGS. 3A and 3B are schematic side views of an example light-coupling apparatus that includes an LED light source and the LDF bundle of FIG. 2; [0020] FIG. 4A is a side view of an LDF wherein an end portion of the cladding has been removed to expose the core in the process of forming an LDF core bundle at the end of an LDF bundle;

[0021] FIG. 4B is an end view of an example LDF bundle that includes an LDF core bundle end section;

[0022] FIGS. 4C and 4D are cross-sectional views of example LDF bundles that include an LDF core bundle end section formed by fusing the exposed cores at the end of each LDF optical fiber;

[0023] FIGS. 5 A and FIG. 5B are similar to FIG. 3 A and 3B and illustrate an example light-coupling apparatus wherein the LDF bundle includes the LDF core bundle into which light from the LED source is coupled;

[0024] FIG. 5C is similar to FIGS. 5 A and 5B, and illustrates an example wherein the LDF bundle is a solid piece of glass formed from the fused cores as illustrated in FIGS. 4C and 4D.

[0025] FIG. 5D shows an example light-coupling apparatus similar to that of FIG. 5B, and shows the light portion in the form of light rays whose paths were calculated using numerical simulation;

[0026] FIG. 6 illustrates an example embodiment of a light-coupling apparatus that employs a reflective concentrator to match the numerical aperture of the LED source to the LDF bundle;

[0027] FIG. 7 is a plot of the coupling efficiency CE versus the refractive index n oi the material that resides in the spaces between the LDF fiber cores, for the light-coupling apparatus of FIG. 5D (circles) and the light-coupling apparatus of FIG. 6 (squares);

[0028] FIG. 8A is a close-up view of the end of an LDF fiber that has been cleaved, showing how the core is fractured, leaving a rough end that requires polishing;

[0029] FIG. 8B shows the LDF fiber of FIG. 8A, where a UV-curable material is provided on the rough end and a transparent plate is used to form a flat surface on the material as the material is cured with ultraviolet (UV) light directed through the transparent plate;

[0030] FIG. 8C shows the result of the process illustrated in FIG. 8B, wherein the material has a flat surface, thereby obviating the need to polish the LDF fiber;

[0031] FIG. 9A is an elevated side view of an example light-coupling apparatus that employs a transparent housing; [0032] FIG. 9B is a cross-sectional view of an example of the light-coupling apparatus of FIG. 9A, wherein the housing interior defines a gap that is filled with a gas such as air;

[0033] FIG. 9C is similar to FIG. 9A, but wherein transparent material fills the gap;

[0034] FIG. 9D is similar to FIG. 9B, but wherein the LDF bundle does not include the LDF core bundle end section; and

[0035] FIG. 9E is similar to FIG. 9D and illustrates an example wherein the LDF bundle of FIG. 4D that has a fused-core section is employed in the light-coupling apparatus.

DETAILED DESCRIPTION

[0036] Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure.

[0037] The claims as set forth below are incorporated into and constitute part of this Detailed Description.

Light-diffusing fiber

[0038] FIG. 1 is a front elevated view of an example light-diffusing optical fiber ("LDF") 10. LDF 10 has a cylindrical core 12 of diameter dl surrounded by an annular cladding 14 that has an outer surface 16 and an inner diameter dl (i.e., the same as the core) and an outer diameter d2. LDF 10 has a central axis Ap. In an example, LDF core 12 has inner and outer regions (not shown), with the outer region being annular and having randomly arranged and randomly sized voids therein. In an example, cladding 14 can include inner and outer regions, not shown. In an example, the core diameter dl can range from 125 μηι to 250 μηι, and the cladding thickness d2 can range from 15 to 100 microns. LDF core 12 has a retractive index «_/ while LDF cladding 14 has a refractive index «₂₎ wherein « > «2- LDF 10 as an endface 18, which is preferably flat and in a plane perpendicular to central axis Ap. In the case where LDF core and/or cladding have different (i.e., multiple) sections, indices «_/ and ri2 can be representative of some select portion of the core and cladding, respectively. LDF bundle

[0039] FIG. 2 is a front-on view of an example LDF bundle apparatus ("LDF bundle") 20 made up of nine LDFs 10. In an example, LDF bundle 20 includes spaces 24 between the bundled LDFs 10. In an example, spaces 24 are filed with air, while in another example the spaces are filled with a transparent material 26 of refractive index ΠΜΙ at a wavelength λ, where the material is transparent for at least that wavelength and 300 nm < λ <2000nm. In an example, material 26 is a UV-curable material such as a UV-curable polymer, e.g. a fluoroacrylate polymer. In one example, ΠΜΙ≤ni, while in another example ti2≤ ΠΜΙ≤ni. LDF bundle 20 has an endface 28 defined at least in part by LDF endfaces 18. Endface 28 is also referred to as the input face 28 when light is inputted into LDF bundle 20 at the endface. Coupling Assemblies

[0040] FIGS. 3 A and 3B illustrate an example light-coupling apparatus 40 that includes an LED light source ("LED source") 50 and the LDF bundle 20 of FIG. 2. LED source 50 a light-source axis As and that emits light 52 at a source numerical aperture NAs. LDF bundle 20 has a numerical aperture NA_B, wherein NA_S > NA_B. In an example, NA_B is about 0.35 and As is in the range from about 0.86 to about 1. FIG. 3 A shows an angle 9s that light rays make with respect to a line parallel to light-source axis AS. The LED source numerical aperture NA_S = «o'sin(9s), where no is the refractive index of the surrounding medium, which is assumed to be air, in which case no = 1. For LDF 10, NA_F = {m² - n ) ^1/2 . LDF bundle 20 has a numerical aperture NA_B = NAF.

[0041] LED source 50 has a front end (i.e., an output end) 58 having a size that corresponds to the light-emitting portion of the output end, and which in an example has a dimension (e.g., a diameter Ds for a circular output end) of about 1 mm. In an example, endface 28 of LDF bundle 20 has a size that substantially matches the size and shape of output end 58 of LED light source 50. In an example, endface 28 is generally circular and has a diameter D_B that in an example (e.g., for butt-coupling) is configured so that D_s = D_B.

[0042] FIG. 3B shows the output end 58 light source 50 interfaced with the input end 28 of LDF bundle 20. In this configuration, a portion 52P of light 52 from LED light source 50 falls within the numerical aperture NA_B and travels down LDF fibers 10 as well as material 26. Light portion 52P travels as guided modes in LDF fibers 10, as well as in the cladding and through spaces 24, and through material 26 in the case where the spaces are filled with the material.

[0043] Simulations were carried out for the coupling configuration of FIG. 3B for an LDF bundle 20 made up of nine LDFs 10, and wherein cladding 14 included two layers, with the inner layer having a low index clad (n2i = 1.35) and the outer layer having an index «22 > «2; at a wavelength λ situated in a 300-2000 nm range. The diameter dl of core 12 was taken as dl = 140 microns, while the outside fiber diameter d2 was taken as &₂ = 250 microns. [0044] The simulations showed that the amount of optical power carried by LDF fibers 10 in cores 12 and claddings 14 of LDFs 10 by light portion 52P was about 36% of the optical power outputted by LED source 50, i.e., the coupling efficiency CE was 0.36. The actual optical power carried by cores 10 is about half of this amount, i.e., 18% of the total outputted optical power. In this embodiment, all simulations were performed at 500 nm wavelength, but similar results are obtained for wavelengths λ situated within the 350 nm to 2000 nm range.

[0045] FIG. 4A is a side view of LDF 10 wherein an end portion of the cladding 14 has been removed to expose the core 12 in the process of forming an LDF core bundle at the end of an LDF bundle. FIG. 4B is an end-on view similar to FIG. 2 of LDF bundle 20 that has an LDF core bundle end section ("LDF core bundle") 30. LDF spaces 24 are filled with material 26, wherein m < nu < ri₂ In this case, nineteen cores 12 can be packed into same 1 mm diameter DB that included only nine cores when entire LDF fibers 10 were used at the end of LDF bundle 20. FIG. 4B shows endface 28 as having a generally circular shape as indicated by the dashed-line circle 29, thought the actual shape is more hexagonal.

[0046] FIG. 4C is a cut-away view of an example embodiment illustrates an example embodiment wherein the LDF core bundle 30 of LDF bundle 20 is formed by fusing cores 12 to form a solid-glass fused-core section 32 at the end portion of the LDF bundle. This fused- core embodiment obviates the need for material 26 residing between cores 12, as shown in FIG. 4B. FIG. 4D is similar to FIG. 4C and shows lower index clad material 26 surrounding the fused-core section 32 of fused-core LDF core bundle 30 so that the numerical aperture AB of LDF bundle 20 substantially matches the single-fiber numerical aperture NAp.

[0047] FIGS. 5A and 5B are similar to FIG. 3A and 3B but wherein LDF bundle includes LDF core bundle 30. FIG. 5C is similar to FIGS. 5A and 5B, and illustrates an example wherein the LDF bundle is a solid piece of glass defined by fused-core section 32, as illustrated in FIGS. 4C and 4D.

[0048] FIG. 5D shows light-coupling apparatus 40 similar to that of FIG. 5B, but that shows the LDF core bundle 30 at the end of LDF bundle 20, and also shows light 52 and light portion 52P in the form of light rays with paths calculated using numerical simulation.

[0049] The numerical aperture AB of LDF core bundle 30 is well-approximated by the relationship AB = (tii² - M )¹'², since material 26 can be thought of as taking the place of the individual cladding layers 14 and replacing them with a single, thin cladding layer for all of exposed cores 12. This calculation of NA_B also holds for the fused-core embodiment of LDF bundle 20 of FIG. 4D.

[0050] For the condition NAs = NAB, a coupling efficiency CE = 92% can be obtained. However, NAs > NAB, and in an example NAs ~ 0.86 and NAB ~ 0.48, and the losses due to the difference in the numerical apertures is approximated by [NA_B/NA_S]², which is ~ 0.31, so that the actual maximum coupling efficiency CE that can be obtained is estimated by:

CE = (0.92 (0.31) ~ 0.29.

[0051] In an example, it is thus desirable to try to match the numerical apertures NAs and NAB of the LED light source 50 and the LDF bundle 20. FIG. 6 illustrates an example embodiment of a light-coupling apparatus 40 that includes LED light source 50, a reflective concentrator 1 10, and either LDF bundle 20 or LDF core bundle 30. In an example, reflective concentrator 1 10 comprises a compact reflector having, for example, a parabolic or a conic shape.

[0052] Reflective concentrator 110 is operably arranged and configured to receive light 52 from LED source 60 over the range of NAs and direct the receive light to LDF core bundle 30 to substantially within the numerical aperture NAB. The light-coupling apparatus 40 enables LED source 50 to be relatively small (i.e., having a diameter Ds < DB) because reflective concentrator 110 serves to match NAs to NAB at the expense of the beam diameter increase, or a reduction of the LED source size (i.e., diameter D_s), depending on the size of D_B.

[0053] For light-coupling apparatus 40 according to FIG. 6 that utilizes LDF bundle 20 without LDF core bundle 30, the coupling efficiency improves from 0.35 to 0.46, an increase of about 25%. For a light-coupling apparatus 40 according to FIG. 6 that utilizes LDF core bundle 30 with material 26, the coupling efficiency CE was measured at 0.57.

[0054] FIG. 7 is a plot of the material refractive index ΠΜΙ VS. the coupling efficiency CE for the example coupling assemblies 40 shown in FIG. 5D (circles) and in FIG. 6 (squares). The plot shows an optimal coupling efficiency CE =0.35 for the light-coupling apparatus 40 of FIG. 5D (butt coupling) when the material index ΠΜΙ < for M; = 1.45. For ΠΜΙ > ni, then the coupling efficiency CE efficiency drops.

[0055] For the light-coupling apparatus of FIG. 6 that employs a smaller LED source 60 in combination with reflective concentrator 1 10, the coupling efficiency CE = 0.57 when nui≤ n l but drops steeply for ΠΜΙ > ni. [0056] Because the amount of optical power emitted by an LED is proportional to its size D_s, the butt-coupling configuration for light-coupling apparatus 40 can be used in cases where the most optical power needs to be coupled into LDF bundle 20 with less regard to coupling efficiency. On the other hand, the reflective concentrator configuration of light- coupling apparatus 40 can be used when high coupling efficiency is called for.

[0057] For example, if LED source 50 has a diameter D_s = 300 μιη and emits 1 mW power, then 0.57 mW can be coupled into LDF core bundle 30 having a diameter DB = 900 μιη and material 26 with < nj. When LED source 50 has a size Ds = 900 μιη and emits 9 mW, and the butt-coupling configuration of light-coupling apparatus 40 is employed, then

(9 mW)-(0.35) = 3.2 mW can be coupled to bundle, which is about six times more than for the reflective concentrator configuration using the smaller LED source.

Preparing the endface of the LDF bundle

[0058] FIG. 8A is a close-up end view of an example LDF 10 that has been cleaved. The LDF fiber 10 shows core 12 having a rough end 13. Usually, cleaved LDFs 10 have to be polished so that rough end 13 becomes a smooth end. The polishing process is usually slow and adds expense to the overall process of forming LDF bundle 20.

[0059] FIG. 8B is similar to FIG. 8 A and shows a material 15 added to rough end 13, wherein material 15 has a refractive index «;j that is relatively low, e.g., about the same as the core index n i. An example material 15 is a UV-curable polymer or glue, e.g., a fluoroacrylate polymer. In an example illustrated in FIG. 8B, a flat fiber endface 18 can be accomplished by depositing material 15 onto rough end 13 , and then pressing the endface and the material against a flat, non-stick transparent plate 150. The material 15 is then irradiated with UV light 160 to cure the material and form a flat, smooth hardened surface 17. The result of the process is a smooth endface 18 at core 12, is shown in FIG. 8C.

Light-coupling apparatus with transparent housing

[0060] FIG. 9 A is an elevated view of an example light-coupling apparatus 200 that includes a transparent housing 200. Also shown in FIG. 9A is a power supply 202 for LED source 50, which is contained within housing 200, as discussed below. In another example, power supply (e.g., a battery) resides within housing 200.

[0061] FIG. 9B is a cross-sectional view of an example light-coupling apparatus 200 wherein LED source 50 is coupled to LDF bundle 20 via a transparent rod 210, such as a glass rod. In an example, transparent rod 210 is substantially non-scattering. Light-coupling apparatus 200 includes a transparent housing 220 that has an endwall 224 and a sidewall 226 with an outer surface 228. In an example, sidewall 228 has the shape of a cylindrical tube. Endwall 226 and sidewall 228 define a housing interior 232 that contains LED source 50, transparent rod 210 and an end portion of LED bundle 20, which in an example include LDF core bundle 30. Transparent rod has ends 212 and 214, and in an example embodiment output end 58 of LED source 50 is butt-coupled to end 212 of transparent rod 210 while endface 28 of LDF core bundle 30 is butt-coupled to end 214, as shown. The unoccupied portion of interior 232 defines a gap 234. In an example, gap 234 is filled with a gas (e.g., air), as shown in FIG. 9A.

[0062] In the operation of light-coupling apparatus 200, light portion 52P from LED source 50 enters end 212 of transparent rod 220 and travels therethrough to end 214 without substantial scattering. Some of light portion 52P is then coupled into LDF core bundle 30, while some of light portion 52 traveling at higher angles is trapped in sidewall 226 and travels therein by total internal reflection, while some of light portion 52 travels straight through the sidewall. In an example, housing 220 is made of glass, polymer, plastic or thermoplastic (e.g., PMMA) material, and is configured to be light diffusing or light scattering, e.g., to diffuse or scatter light 52. In an example, the light-scattering properties of housing 220 are due to scattering centers (e.g., Ti02) within the material that makes up sidewall 24.

[0063] FIG. 9C is similar to FIG. 9B and includes a material 266 of refractive index nu₂ within gap 234. In an example, housing 220 has a refractive index ΠΗ, wherein ΠΗ > n 2 and fiM2 < ni, where m is the (base) refractive index of core 12. In an example, material 266 serves as an adhesive that maintains LED source 50, transparent rod 210 and LDF bundle 20 in relative alignment and fixed within housing 200. In an example, material 266 is the same as material 26 so that ΠΜΙ = «Μ2·

[0064] FIG. 9D is similar to FIG. 9C, and illustrates an example embodiment of light- coupling apparatus 200 wherein LDF bundle 20 does not include LDF core bundle 30 so that light portion is coupled directly into core 12 and cladding 14 of LDFs 10, as well as into material 26 that resides in spaces 24 between the LDFs.

[0065] FIG. 9E is similar to FIG. 9D and illustrates an example wherein the LDF bundle 20 of FIG. 4D that included fused-core 32 is employed in light-coupling apparatus 200. Fused- core 32 is similar to transparent rod 210. In FIG. 9E, material 266 replaces the material 26 shown in the embodiment shown in FIG. 4D. [0066] According to some embodiments a method of processing an end of a light-diffusing optical fiber having a core and a cladding surrounding the core, comprises the following steps: (a) cleaving the light-diffusing optical fiber to form a cleaved end, wherein the core has a rough end; (b) depositing an ultraviolet (UV)- curable material on the rough end; (c) placing a non-stick UV-transparent plate (i.e., a plate transparent at a wavelength situated in 300-450nm range) against the UV-curable material and the cleaved end to form a flat surface of the UV-curable material; and (d) illuminating the UV-curable material with UV light through the non-stick UV-transparent plate to cure the UV-curable material to form smooth and flat hardened surface at the cleaved end. In some embodiments the UV-curable material comprises fluoracrylate. In some embodiments the fiber core has a first refractive index and the UV-cured material has a second refractive index substantially the same as the first refractive index.

[0067] it will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.

Claims

What is claimed is:

1. An optical fiber apparatus, comprising:

a plurality of light-diffusing optical fibers each having a cylindrical glass core with a refractive index n i, and a surrounding cladding with a refractive index ti2 < ni, wherein an end section of each light-diffusing optical fiber has the cladding removed to expose the core; the glass cores being arranged in a tight bundle with spaces between the cores; and a transparent materialof refractive index ΠΜ residing in the spaces, wherein « ≤ said material being transparent for at least one wavelength situated in a 350 nm to 2000 nm wavelength range.

2. The optical fiber apparatus according to claim 1, wherein the material comprises an ultraviolet- (UV)-curable adhesive.

3. The optical fiber apparatus according to claim 1, wherein the material comprises fluoroacrylate.

4. A light-coupling apparatus, comprising:

the optical fiber apparatus according to claim 1, 2, or 3, having an endface of a first size and shape;

a light-emitting diode (LED) light source having an output end from which light is emitted and that has a second size and shape that is substantially the same as the first size and shape; and

wherein the output end of the LED light source is interfaced with the endface of the optical fiber apparatus to establish a butt-coupling of the light from the LED into the optical fiber apparatus.

5. A light-coupling apparatus, comprising:

the optical fiber apparatus according to claim 1, 2, or 3, having a numerical aperture AB and an endface of a first size and a first shape;

a light-emitting diode (LED) light source having a numerical aperture As > NAB, an output end of a second size smaller than the first size and having the substantially the first shape, wherein the output end emits light; and a reflective concentrator operably disposed between the LED light source and the optical fiber apparatus so that at least a portion of the light from the LED light source reflects from the reflective concentrator and enters the endface of the optical fiber apparatus in a manner that substantially matches the numerical aperture AB of the optical fiber apparatus.

6. The light coupling apparatus according to claim 5, wherein the reflective concentrator has either a parabolic shape or a conical shape.

7. A method of processing an end of a light-diffusing optical fiber having a core and a cladding surrounding the core, comprising:

cleaving the light-diffusing optical fiber to form a cleaved end, wherein the core has a rough end;

depositing an ultraviolet (UV)- curable material on the rough end;

placing a non-stick UV-transparent plate against the UV-curable material and the cleaved end to form a flat surface of the UV-curable material; and

illuminating the UV-curable material with UV light through the non-stick UV- transparent plate to cure the UV-curable material to form smooth and flat hardened surface at the cleaved end.

8. The method according to claim 7, wherein the UV-curable material comprises fluoracrylate.

9. The method according to claim 7, wherein the core has a first refractive index and the UV-cured material has a second refractive index substantially the same as the first refractive index.

10. A light-coupling apparatus comprising:

a light-diffusing- fiber (LDF) bundle having a plurality of light-diffusing optical fibers of claim 1, 2, or 3, with the LDF bundle having an endface of a first size and first shape; a light-emitting-diode (LED) light source providing light in at least one wavelength situated in a 300 nm to 2000 nm wavelength range, said light-emitting-diode (LED) light source having an output end having a second size and second shape that substantially match the first size and shape and that emits light;

a transparent rod having first and second ends and disposed between and in contact with the endface of the LEF bundle and the output end of the LED light source, wherein said rod is transparent for said at least one wavelength situated in a 300 nm to 2000 nm wavelength range; and

a housing having an interior that contains the LED light source, the transparent rod and the endface of the LDF bundle, the housing having a sidewall that is generally transparent to the light from the LED light source and that scatters a portion of the light that travels within the sidewall.

1 1. The light-coupling apparatus according to claim 10, wherein the housing interior includes a gap defined by the sidewall, the transparent rod and the bundled cores.

12. The light-coupling apparatus according to claim 10 or 1 1, wherein the housing sidewall has a refractive index ΠΗ, and the gap is filled with a second material having a refractive index ΠΜ2, wherein ΠΗ > u2-

13. The light-coupling apparatus according to claim 12, wherein the first material and the second material are the same material.

14. The light-coupling apparatus according to claim 13, wherein said same material comprises fluoroacrylate.

15. The light-coupling apparatus according to claim 10, wherein the transparent rod comprises glass.

16. A method of coupling light from a light-emitting diode (LED) providing light in in a wavelength λ situated in a 300 nm -2000 nm range into a plurality of light-diffusing optical fibers each having a glass core and a cladding with respective refractive indices ni and w_¾ comprising: removing a portion of the cladding from an end section of each of the plurality of optical fibers to form exposed cores having respective core ends;

arranging the exposed glass cores in a tight bundle that has spaces between the tightly bundled cores;

filling the spaces with a transparent material having a refractive index ΠΜΙ, wherein nui≤ ni to define a light-diffusing core bundle having an endface that includes the core ends and a portion of the material; and

directing light from the LED into the endface to travel within the cores and the material.

17. A method according to claim 16, wherein the endface has a first size and a first shape, wherein the LED has an output end with a second size and a second shape, and wherein the first size and first shape is substantially the same as the second size and the second shape, and including butt-coupling the LED output end to the endface of the light-diffusing core bundle.

18. A method according to claim 16, wherein the endface has a first size and a first shape, wherein the LED has an output end with a second size and a second shape, with the second size being smaller than the first size and the second shape being substantially the same as the first shape, and further including directing the light from the LED into the endface via a reflective concentrator and within a numerical aperture of the light-diffusing core bundle.

19. An optical fiber apparatus, comprising:

a plurality of light-diffusing optical fibers arranged in a bundle, with each optical fiber having a numerical aperture (NA), a cylindrical glass core with a refractive index nj, and a surrounding cladding with a refractive index ri2 < ni, wherein an end section of each light- diffusing optical fiber has the cladding removed to expose the core;

the exposed glass cores at the end sections of the optical fibers being fused together to form a solid-glass, fused-core section at an end of the bundle; and

a lower-index clad material surrounding the fused-core section of the bundle to define a fused-core NA that substantially matches the single-fiber NA.

20. The optical fiber apparatus of claim 19, further comprising a light source optically coupled to an input end of the fused-core section.