Light transmission device

Abstract: This coupling means comprises a sleeve (50) containing a bore (52) in which is located a bundle of optical fibers (16), at least some of which are of compression-deformable material. The input ends of the fibers, which receive light from a light source, are located in a predetermined plane (67). The bore tapers from a relatively large diameter at a location in a second plane spaced from said predetermined plane to a relatively small diameter at said predetermined plane. The tapered bore is of such a size that the optical fibers therein are laterally compressed and the compression-deformable fibers are so deformed in transverse cross-section relative to their normal cross-sections that the fibers more completely fill the bore in its relatively small diameter region than in said second plane.

Description: CROSS-REFERENCE TO RELATED PATENT This application is related to the subject matter disclosed and claimed in U.S. Patent No. 4,811,172-Davenport and Hansler, assigned to the assignee of the present invention, which patent is incorporated by reference in the present application. TECHNICAL FIELD This invention relates to a lighting subsystem that comprises a light source and a bundle of optical fibers for transmitting light from the source and, more particularly, relates to coupling means for coupling light from the source into the input ends of the fibers with reduced loss of light. BACKGROUND A known type of lighting system, which is exemplified by the system disclosed in the aforesaid U.S. Patent No. 4,811,722 - Davenport et al, comprises a bundle of optical fibers each having an input end, a light source, and a reflector for receiving light from the source and reflecting such light from the source into the input ends of the optical fibers for transmission by the fibers. It is desirable in such a system that light from the source be coupled into the optical fibers with the least possible loss of light. One factor that can result in undesired losses is the presence of voids between the optical fibers in the bundle. Any light from the source entering these voids represents lost light which is not transmitted by the optical fibers. In one prior proposal (DE-A-2 733 100) there is disclosed a lighting subsystem in which a bundle of deformable optical fibres is introduced into a coupling device which tapers from a relatively large cross-sectional area to a relatively small cross-sectional area at the coupling end. The fibres are deformed in this device so that in transverse section at the coupling end the optical fibres substantially fill the coupling device. A similar arrangement is also shown in US-A-4 116 655. OBJECTS Our invention seeks to provide, between the light source and the input end of a bundle of optical fibers, coupling means that is capable of reducing the losses resulting from light from the source entering the usual voids between the fibers at the input end of the bundle where both deformable and non-deformable fibers are used together. According to the invention, there is provided a lighting subsystem comprising: (a) a bundle of optical fibers each having an input end for receiving light from a light source, the input ends of said fibers being located substantially in a predetermined plane; and (b) a coupling device positioned about said bundle adjacent said predetermined plane and comprising a member containing a bore for receiving said bundle, said bore tapering from a relatively large diameter at a location spaced from said predetermined plane to a relatively small diameter at said predetermined plane, said bore in a second plane parallel to and spaced from said predetermined plane having a circumference just large enough to surround said bundle without laterally compressing the fibers in said second plane; characterised in that said bundle of fibers includes a combination of a first group of fibers of non-compression-deformable material and a second group of fibers of compression-deformable material, each of said second group of fibers having a predetermined transverse cross-section when in a uncompressed state and (c) said bore in said predetermined plane is of such a size that the second group of optical fibers therein are laterally compressed and so deformed in transverse cross-section relative to their predetermined transverse cross-sections that the optical fibers more completely fill said bore in the relatively small diameter region of the bore than in said second plane, the total cross-sectional area of said fibers being substantially the same in said predetermined and second planes. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in greater detail, by way of example, with reference to the drawings in which:- Fig. 1 is a sectional side-elevational view of a prior art lighting subsystem comprising a light source and a bundle of optical fibers for transmitting light from the light source. Fig. 2 is an enlarged sectional view along the line 2-2 of Fig. 1. Fig. 3 is an enlarged sectional view of coupling means for coupling light from a source into the input end of a bundle of optical fibers. Fig. 4 is a sectional view along the line 4-4 of Fig. 3. Fig. 5 is a sectional view along the line 5-5 of Fig. 3. Fig. 6 is a side elevational view showing a modified coupling. Fig. 7 is a sectional view along the line 7-7 of Fig. 6. Fig. 8 is a sectional view along the line 8-8 of Fig. 6. Fig. 9 is a sectional view showing an embodiment of the invention including a bundle of optical fibers that includes compression-deformable fibers and also fibers that are not deformed by the compression forces involved. Fig. 10 is a sectional view of the bundle depicted in Fig. 9 taken at the input end of the bundle, where the compression-deformable fibers have been deformed. DETAILED DESCRIPTION OF EMBODIMENTS Referring first to Fig. 1, the known lighting subsystem depicted therein comprises a light source 12, an ellipsoidal reflector 14, and a bundle 16 of optical fibers 18, each having an input end 19 facing the reflector. This subsystem corresponds to that disclosed in the aforesaid U.S. Patent No. 4,811,172; and, accordingly, the central portion of the light source 12 is located at one focal point 20 of the reflector, and the input end of the bundle 16 is located adjacent the other focal point of the ellipsoidal reflector 14. Most of the light rays emitted by the source 12, such as rays 22 and 24, are intercepted and reflected by the reflector 14 and enter the exposed faces, or input ends, 19 of the optical fibers 18 within the confines of an imaginary truncated cone having its axis colinear with the axis of bundle and having a vertex angle of about 76°. For coupling the bundle 16 of optical fibers to the light source 12, the subsystem of Fig. 1 utilizes a metal sleeve 30 that surrounds the bundle 16 and is suitably fixed to a housing 32 that surrounds the source 12. In a typical prior art subsystem, such as depicted in Fig. 1, the sleeve 30 has a bore uniform diameter for receiving the bundle 16 without deforming the fibers. Fig. 2 is an enlarged cross-sectional view along the line 2-2 of Fig. 1, showing the input end of the optical fiber bundle 16. Fig. 2 also illustrates a typical distribution at the input end of the optical fibers 18 within the bore 34 of sleeve 30. As shown in Fig. 2, each of the optical fibers comprises a core 36 of a light-transmissive material having a relatively high index of refraction and a surrounding coating, or cladding, 38 of a material having a relatively low index of refraction. It will be apparent that in Fig. 2 there are relatively large voids 40 between the optical fibers 18 and also between the optical fibers and bore 34 of the surrounding sleeve 30. The active light-transmitting area within the bore 34 is the total cross-sectional area of the cores 36 or, stated in another way, the cross-sectional area of the bore minus the cross-sectional area of the voids 40, minus the cross-sectional areas of the cladding 38. The light-gathering efficiency of this interface is directly Proportional to the ratio of the active light-transmissive area to the total cross-sectional area of the bore. Or stated in another way, the light losses at this interface are inversely proportional to this ratio. One known proposal for increasing this light-gathering efficiency and reducing the losses at this interface is shown in Figures 3 to 5. Here, such an increase is effected by using for the surrounding coupling a sleeve 50 that has a tapered bore 52 of such a size that the sleeve, adjacent its small-diameter or inner end, laterally compresses the optical fibers within the bore and causes the fibers at the exposed end of the bundle (Fig. 4) to deform to such an extent that at this exposed end the cross-sectional area of the voids (40) as a percentage of the total cross-sectional area is greatly reduced as compared to this percentage at the opposite end of the sleeve. More specifically, the optical fibers 18, both core and cladding, are of compression-deformable material, and the above-described compression produced by the surrounding tapered bore 52 causes the fibers to deform in such a way that they more completely fill the bore at the exposed ends of the fibers (i.e., in the plane 4-4 depicted in Fig. 4) than in a transverse plane (such as depicted in Fig. 5) spaced axially outward from plane 4-4, where the fibers are substantially uncompressed and still have their normal uncompressed cross-sectional configuration. Fig. 5 is taken in a plane where the bore 52 has tapered to the smallest diameter that it can be reduced to without compressing the bundle therein. It will be observed that the transverse cross-section of each fiber in the embodiment of Figs. 3-5 is changed from its substantially circular form in the plane of Fig.5 to a distinctly non-circular form in the plane of Fig. 4. It is noted that the material of the fibers is of such a character that the compressive forces developed in the bundle within the sleeve 50 do not substantially change the total cross-sectional area of the light-tranmissive core material of the bundle in any transverse plane. This total cross-sectional area remains substantially the same whether the bundle is uncompressed, as in Fig. 5, or compressed, as in Fig. 4. Accordingly, the compression does not substantially reduce the cross-sectional area of the core available for light transmission. In the embodiment of Figs. 3-5, and in all the other systems to be described, the optical fibers and their components are designated with the same reference numerals as used for corresponding elements in Figs. 1 and 2. Depending upon the number of fibers and their cross-sectional size, many different packing configurations are possible. As another example, reference may be had to Figures 6-8. In this case a sleeve 50 is included with a tapered bore 52 of circular transverse cross-sections. Fig. 7 is a sectional view taken along the line 7-7 where the bore 52 has tapered to the smallest diameter possible without compressing the fiber bundle therein. As shown in Fig.7, the bundle comprises a closely-packed hexagonal array of seven fibers 18 arranged in top and bottom horizontal rows, each of two fibers, and a third horizontal row of three fibers between the top and bottom rows. Voids 40 are present between the fibers and the fibers and the bore 52. By compressing this bundle of fibers radially inward, each fiber is deformed into a more or less hexagonal cross-sectional shape with almost no voids between the fibers, as is depicted in Fig. 8. For other numbers and sizes of fibers, the symmetry of the compressed bundle may not be as great as shown in Fig. 8 and some voids will remain, but a substantial improvement in the percentage of the total area filled by active core area will still be obtained by compressing and deforming the fibers essentially as disclosed. The present invention deals with the problem of increasing the light-gathering efficiency and reducing losses at the interface where some of the fibers are of material that is substantially undeformable by the compressive forces involved. One embodiment of the invention is shown in Figures 9 and 10. In these Figures there is shown an array of four optical fibers (18) of a material that is compression-deformable and five optical fibers (60) of a core material, such as acrylic resin, that is substantially undeformed by the compressive stresses applied by the tapered sleeve 50, which corresponds to the tapered sleeve 50 of the first two embodiments. Fig. 9 shows the bundle in a region of the surrounding sleeve 50 where the bundle is in an uncompressed condition. Fig. 10 on the other hand, is a cross-sectional view taken at the small end of the tapered bore 52 of the sleeve, where the bundle is radially compressed and the deformable fibers are severely deformed by such compression. In the region depicted in Fig.10 the deformed fibers 18 substantially fill the portions of bore that are not occupied by the undeformed fibers 60. Examples of compression-deformable optical fibers are those sold by Lumenyte International Corp. of Cosa Mesa,CA as its "end light" optical fibers and those sold by Fibre Lite Corp., Dallas, TX as its END GLOW optical fibers. Examples of fibers that are not compression-deformable under these conditions are those sold by Edmund Scientific Co., Barrington, NJ., as ESKA acrylic optical fibers, manufactured by Mitsubishi Rayon Co.,Ltd. Another feature of our coupling device is its inclusion of a thin glass or plastic cover 65 (shown in Fig. 3) over the exposed ends of the optical fibers 18. This cover is attached to the exposed ends of the fibers by an optically clear cement 67 that fills any voids between the ends of the fibers and the glass cover. The glass cover 65 is also sealed by this cement to the exposed inner face of the metal sleeve 50 to form a hermetic seal between the glass cover and the sleeve 50. The cement selected for this application has an index of refraction that matches that of the optical fiber core material and also that of cover 65. To provide a hermetic seal about the entire coupling, an O-ring 70 is provided. When the sleeve 50 is threaded into place in its position of Fig. 3, the O-ring provides a high quality seal about the outer circular periphery of the sleeve. The O-ring is seated in an annular groove 73 in the internal surface of an outwardly-projecting circular ring 74 integral with the wall 32. It is to be understood that the illustrated seal (70-74) is exemplary of many different types of seals which are suitable for use in this location. An important function of the taper in the bore 52 is to enable the bundle of fibers (when it is inserted into the large end of the sleeve 50 during assembly of the coupling) to be fed naturally and gradually into its compressed state at the small end of the bore. It is not necessary that the taper extend all the way to the end of the bore, and as a matter of fact, our bore 52 includes a short length near its small end that is of uniform diameter. It is noted that each fiber includes a protective cover 76 surrounding its core and cladding. This cover, which is of a conventional design, is removed from the end portion of each fiber beginning just beyond the point where the fiber enters the sleeve 50. This cover-removal is carried out in such a way that the cladding 38 remains intact. At the outer end of the sleeve 50 a suitable cement, preferably a silicone cement shown at 78 in Fig. 3, is applied between the sleeve 50 and the fiber bundle. This cement flows into the voids in this region between the fibers and also into the voids between the fibers and the bore of the sleeve. The cement also flows into the space between the sleeve 50 and the protective covers 76. When the cement cures, it provides a seal at this outer end of the coupling which acts to prevent foreign substances from entering the coupling. The seal also provides a good mechanical bond between the fibers and the sleeve 50 that precludes the fibers from being easily pulled from the coupling. The above-described coupling is manufactured by first removing from the inner end of each fiber a section of the surrounding cover 76 while leaving the cladding 38 on the undividual fibers undisturbed, as already noted above. The bundle is then forced axially into the outer end of the sleeve 50, effecting the above-described compression and deformation of the optical fibers at the small end of the bore 52. The bundle is forced in this manner into the bore 52 until a short length of the bundle extends past the inner end of the sleeve 50. This protruding end is then trimmed off smoothly, flush with the inner end of the sleeve 50. Then the transparent window 65 is cemented over the end of the bundle and the inner end of the sleeve. After the above steps have been completed, the silicone cement 78 is applied to the outer end of the sleeve 50, following which it cures and forms the above described mechanically-strong seal for the coupling. Certain optical fiber materials that we may wish to utilize are not deformable in the manner described above unless they are at an elevated temperature. Our invention in its broader aspects comprehends heating the end of the bundle to develop the desired elevated temperatures before the bundle is forced into the sleeve 50. Then the bundle is forced into the sleeve 50 as above described to produce the desired deformation of the optical fibers. Another way of terminating the bundle 16 after it has been forced into sleeve 50, as above described, is to freeze the bundle end with liquid nitrogen or the like and then score-snap it off. Thereafter, the end of the bundle can be polished with abrasives in order to obtain the desired smoothness of the bundle and fiber end faces. Such polishing can also be used after the above-described trimming step. Still another way of terminating the bundle is to pre-cut each fiber to an appropriate length, then bundle together the ends of the pre-cut fibers, then insert the bundled-together ends into the bore 52, and then adjust the ends for alignment, as by slightly withdrawing any excessively protruding fiber. This technique is especially suitable for the form of Figs. 9 and 10, which comprises both deformable and non-deformable fibers. While we have illustrated our invention in connection with a coupling sleeve (50 or 80) that has a bore of substantially circular transverse cross-section, it is to be understood that our invention in its broader aspects comprehends the use of a coupling sleeve in which the passage through the sleeve is non-circular in transverse cross-section, e.g., oval or rectangular. While as above-described, the protective covers (76) on the ends of the optical fibers are removed, leaving the cladding (38) in place on these ends, in certain applications the cladding is also removed from the end portions of the fibers located within the tapered bore (52). This removal of the cladding enables the filling of the small diameter end of the sleeve even more completely with active core area, thus further increasing the packing fraction and the resulting light-gathering efficiency at the input end of the coupling.

Claims: A lighting subsystem comprising: (a) a bundle (16) of optical fibers (18, 60) each having an input end (19) for receiving light from a light source (20), the input ends of said fibers being located substantially in a predetermined plane, and (b) a coupling device positioned about said bundle (16) adjacent said predetermined plane and comprising a member (50) containing a bore (52) for receiving said bundle (16), said bore (52) tapering from a relatively large diameter at a location spaced from said predetermined plane to a relatively small diameter at said predetermined plane, said bore (52) in a second plane parallel to and spaced from said predetermined plane having a circumference just large enough to surround said bundle (16) without laterally compressing the fibers (18, 60) in said second plane; characterised in that (c) said bundle of fibers includes a combination of a first group of fibers (60) of non-compression-deformable material and a second group of fibers (18) of compression-deformable material, each of said second group of fibers (18) having a predetermined transverse cross-section when in an uncompressed state and (d) said bore (52) in said predetermined plane is of such a size that the second group of optical fibers (18) therein are laterally compressed and so deformed in transverse cross-section relative to their predetermined transverse cross-sections that the optical fibers (18, 60) more completely fill said bore (52) in the relatively small diameter region of the bore than in said second plane; the total cross-sectional area of said fibers (18, 60) being substantially the same in said predetermined and second planes. The lighting subsystem of claim 1, in which each of said optical fibers (18,60) comprises a core and cladding surrounding the core, at least some of said cores being free of said cladding in the relatively small-diameter region of said bore. The lighting subsystem of claim 1 or 2, in which the compression deformable fibers (18) in said predetermined plane partly envelope non-deformable fibers (60), leaving a much smaller percentage of the bore (52) cross-section void in said predetermined plane than is the case in said second plane. A lighting subsystem as defined in any one of claims 1 to 3, and further comprising a transparent cover (65) for the input end of said bundle (18,60) and cement (67) attaching said cover to said bundle input end, said cover (65) and cement (67) each having an index of refraction substantially matching that of the light transmissive core material of said optical fibers (18,60). A lighting subsystem as defined in any one of claims 1 to 4, in which said member (50) has an end opposite to said predetermined plane through which said optical fibers (18, 60) enter said bore (52); and adhesive within said bore (52) at said opposite end for bonding said fibers (18, 60) to said bore (52) and to each other and for providing a seal at said opposite end for blocking the entry of foreign material into said bore. The lighting subsystem of any one of claims 1 to 5, in which each of said compression-deformable fibers (18) has a substantially circular transverse cross-section in its uncompressed state and a distinctly non-circular transverse cross-section in the relatively small diameter region of the bore (52).

Inventors: DAVENPORT JOHN MARTIN, FINCH WILLIAM WALTER, HANSLER RICHARD LOWELL, HENKES JOHN LAWRENCE, JOHNSON JEFFREY DONALD, DAVENPORT, JOHN MARTIN, FINCH, WILLIAM WALTER, HANSLER, RICHARD LOWELL, HENKES, JOHN LAWRENCE, JOHNSON, JEFFREY DONALD
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1991
Main classification: G02B6
IPC codes: G02B6