Numerical aperture expansion in fiber optic devices
United States Patent 1191 Cole [ Apr. 1, 1975 NUMERICAL APERTURE EXPANSION IN FIBER OPTIC DEVICES [75] Inventor: Henry B. Cole, Woodstock, Conn.  
 [73] Assignee: American Optical Corporation.  
 Southbridge. Mass.  
 [22] Filed: Mar. 8. 1974 [2i] Appl. No.: 449,320  
 Related U.S. Application Data [63] Continuation of Scr. No. 277.702. Aug. 2. I972.  
 abandoned.  
 [52] 0.8. CI 350/96 B 5 l] Int. Cl. G02b 5/16 Field of Search 350/96 B. 96 C [56] References Cited UNITED STATES PATENTS 3.043.9l0 7/l962 Hicks 350/96 B X 3.142.235 7/l964 Sicgmund 350/96 B X 3.389.950 6/l968 Harper 350/96 B X Crawford 350/96 B Yamazaki et al. 350/96 B X Primary Examiner-David H. Rubin Attorney. Agent. or Firm--H. R. Berkenstock. .lr.; William C. Nealon [57] ABSTRACT A fiber optic device comprised of a pair of bundles of light-conducting optical fibers each having lightreceiving and light-emitting opposite end faces. A light-emitting face of one bundle and a light-receiving face of the other bundle are coupled together unsymmetrically with respect to axes of correspondingly interfacially related fibers of the two bundles wherewith light entering the uncoupled light-receiving face of the one bundle at a given numerical aperture will be emitted from the uncoupled light-emitting face of the other bundle at an angular spread of larger numerical aperture than that of the entering lightv 5 Claims, 9 Drawing Figures EBAPR 1 ms w UL:  
 Will NW NUMERICAL APERTURF. EXPANSION IN FIBER OPTIC DEVICES This is a continuation of application Ser. No. 277,702 filed Aug. 2. I972. now abandoned.  
 BACKGROUND OF THE INVENTION l. Field ofthe Invention Fiber optics with particular reference to the expan sion of numerical aperture in fiber optical devices.  
 2. Description of the Prior Art Fiber optic light and image-conducting devices commonly have relatively small output numerical apertures particularly when used as magnifiers and even more so when magnification is large. i.e.. when the extent of taper of the magnifying device is large.  
  Light traversing tapered optical fibers inherently continually reduces in numerical aperture as it traverses the fibers from their small ends toward their larger ends. Thus. in fiber optic magnifiers. the viewing angle of output becomes progressively restricted in size as the degree of magnification is increased. Furthermore. with the viewing angle centered on the axis of a fiber optic device as in the usual case. output of the device is awkwardly directed for some applications ofuse. Viewing from above or directly in front of an imageemitting device is often considerably less convenient than off-axis viewing.  
  Numerical aperture expansion in other than tapered fiber optic light and image-conducting devices. making wide angle and/or off-axis viewing possible, is highly advantageous. In fiberscopcs or in the control of beam spreading with fiber optics in laser or conventional liglit&#39;projecting lens systems for example. the capability of controlling light spreading and viewing angle direction has considerable utility. The present invention makes this possible.  
 SUMMARY OF THE INVENTION Controlled numerical aperture expansion and orientation of viewing angle in fiber optical light and image transmitting devices in accomplished according to this invention by off-axis coupling of fiber optic bundles wherewith the introduction of light from one bundle into another unsymmetrieally with respect to the axis of the other bundle results in a distribution or spreading of light through controlled, greater than the usual emitting aperture. Unsymmetrical off-axis coupling is accomplished by forming one or more of the coupled faces of the fiber optic devices oblique to the axes of fibers of at least one of the fiber bundles.  
  Details ofthe present invention will become apparent in the following description when taken in conjunction with the accompanying drawing;.  
 IN THE DRAWINGS FIG. I is a diagrammatic illustration of off-axis coupling of fiber optical bundles according to one aspect of the present invention;  
  FIG. 2 is a longitudinal cross-sectional view of an optical fiber wherein the behavior of light conducted therethrough is diagrammatically illustrated;  
  FIG. 3 is a greatly enlarged diagrammatic crosssectional view of a pair of interfacially coupled optical fibers which schematically illustrate principles of operation of the FIG. I embodiment of the invention. all cross-sectioning has been omitted for ease and clarity of illustration;  
 (ill  
  FIGS. 4 and S are diagrammatic cross-sectional views of interfacially coupled optical fibers illustrating modi fications of the FIG. I embodiment of the invention also with all cross-section lines omitted.  
  FIG. 6 is a side elevational view of another modified form of a coupled fiber optical element:  
  FIG. 7 is an enlarged diagrammatic cross-sectional view (without cross-section lines) of a pair of interfacially coupled optical fibers schematically illustrating the function and operation of the FIG. 6 embodiment of the invention.  
  FIG. 8 is a side elevational view of still another modification of optical fiber components which are coupled together according to the present invention. and  
  FIG. 9 is an enlarged diagrammatic cross-sectional view of a pair of interfacially coupled optical fibers schematically illustrating principles of operation of the FIG. 8 embodiment of the invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I illustrates an interfacially coupled pair of fiber optic light-conducting components I0 and I2 wherewith numerical aperture expansion is accomplished according to one aspect of the invention. Components 10 and I2 each comprise a large number of lightconducting fibers all bundled tightly together with their corresponding opposite ends forming light-receiving and light-emitting opposite faces of each component. Fiber 14 of FIG. 2 illustrates. in greatly enlarged scale. the type of construction making up each of fibers 14a and I4! of components Ill and 12. In this connection. fiber I4 has a core I6 of a relatively high refractive index glass. plastic. quartz or other light-conducting core material. The core 16 is surrounded by a relatively thin cladding I8 of lower refractive index material which is intimately interfacially bonded to core 16. Light entering one end of fiber I4 within the critical angle of reflection of interface 20 between core I6 and cladding I8 will transfer through fiber 14 according to the well known principles of total internal reflection. Those interested in greater details of construction and basic principles of operation of clad optical fibers per se may refer to any one of U.S. Pat. Nos. 2.825.260; 2.992.5l6: or 3.037.24l.  
  With particular regard to matters of angular aperture or numerical aperture in optical fibers. it should be understood that the extreme meridianal ray which will be trapped in a fiber such as fiber 14 is found from:  
 n,, sin B n,. sin n,.  
 where n is the refractive index of the core of the fiber and 11.. is the refractive index of the fiber cladding.  
  To find the angle i (FIG. 2) of this extreme meridinal ray outside the fiber sin i= n,, sin r n sin (90-3) This is conventionally defined as the numerical aperture&#34; or NA of the fiber or any bundle of such fibers. e.g.. component Ill or l2. For different applications. fibers ha\ ing different combinations of refractive index may be used. For example. a relatively large NA can be accomplished with fiber I4 having an index of refraction for core [6 of 1.75 and an index of refraction n,. of 1.52 for cladding I8. This would give an NA of 0.86 wherein the steepest acceptance angle i would be approximately 59.4&#34;.  
  It is further pointed out relative to exemplary fiber I4 (FIG. 2) that the half angle 1&#39; of a cone of light entering one end of the fiber will be substantially duplicated at the exit end ofthe fiber (cg. as angle i,) when fiber [4 is of uniform cross-sectional siye throughout its length. The sines of angles i and 1&#39;, being the entrance and exit numerical apertures respectively will be referred to hereinafter as the NA in all remaining figures of the drawings. NA,- will indicate the acceptance numerical aperture for light entering a particular fiber or bundle of fibers (light in) and NA,. will indicate the emitting aperture or half spread of light emitted from a particular fiber or bundle thereof (light out).  
  Referring more particularly to the embodiment of the inventioin illustrated in FIGS. I and 3. it can be seen that the entrance numerical aperture NA,- of the unit 20 is expanded in size to NA,, at the uppermost exit end of the unit. This is accomplished by off-axis or unsymmetrical coupling of fibers I4u and M11 of components I and I2. In this particular case. component It) having light-receiving and light-emitting faces 22 and 24 which are parallel to each other and perpendicular to the axes of fibers I4!) is placed with face 22 coupled to light emitting face 26 of component Ill. Light-emitting face 26 of component It] is biased relative to the axes of fiber Mu at a precontrollcd angle leg. ll)&#34; to 15) as shown more clearly in FIG. 3. While FIG. 3 illustrates the function of only one pair of coupled single fibers 14a and I4h of unit 20. it should be understood that all correspondingly bundled together and interfacially coupled fibers of unit function in the same manner. Accordingly. like reference numerals in FIGS. I and 3 designate like parts.  
  It should be further understood that the change in NA at the coupling interface is dependent only on the angle between the two fiber axes. not on their posi&#39; tional alignment. Thus. although FIGS. 3, 4, S and 9 show perfect alignment between input and output fibers for ease of illustration. this positional alignment is not necessary. In most bundles such as I0 and I2. 32 and 34. 52 and 60, the alignment will be random with each fiber emitting light into or receiving light from more than one fiber of the coupled bundle.  
  Opposite to the coupling of face 22 of component I2 and biased face 26 of component Ill. light-receiving face 28 is disposed perpendicularly to its axis. Lightentering face 28 of component I0. which face is a composite of all exposed ends of corresponding fibers 14a. is restricted in its numerical aperture to within an angular spread NA,-. This NA, may be established by the refractive index differential of cores I6a and claddings I80 of fibers 14a as was explained hercinabove with respect to fiber I4 of FIG. 2. or it may be limited to a lower value by other optical components. In traversing unit 20. however. the angular spread NA,- of this light is expanded to an increased output numerical aperture NA,, (FIG. 3). This numerical aperture expansion is di agrammatically illustrated in FIG. 3 by a tracing of ray 30 entering fiber I411 at NA,. Upon entering fiber I-Iu. ray 30 is refracted by the external air-to-core I6u rcfraetive index differential and then conducted by total internal reflection through fiber l4u as illustrated and explained hereinabove in connection \xith FIG. 2.  
  With faces 22 and 26 of fibers [40 and I4!) optically interfacially coupled. ray 30 emerges directly into ti ber 14b for continued conduction therethrough by total internal reflection. In fiber 14h. however. ray 30 becomes incident upon and reflected from the interface between the core and cladding of fiber I4h at a steeper angle than in fiber I40 and. consequently. with greater spread or NA,, upon emission from fiber 14b. The change from the less critical angle of incidence along the interface of fiber Mu to the steeper angle of incidence along the interface of fiber 14b is produced by the off-axis relationship of the two fibers [4a and I4!) resulting front coupling to the bias of face 26 of component 10.  
  It should be understood that by increasing or decreasing the angle of bias on face 26 of component I0, greater or lesser expansion of NA,, can also be accomplished. The extent of angle of bias of face 26 further determines the viewing angle orientation of light emitted upwardly from unit 20. This off-axis viewing is selectively angularly adjustable by control of the angle of bias of face 26.  
  It can be seen that factors controlling numerical aperture NA,, expansion and viewing angle orientation of fiber optic units such as unit 20 are relative values of refractive indices of cores and claddings of fibers I40 and I4!) and the degree of bias of face 26 relative to the axis of component 10.  
  While the unsymmetrical or off-axis coupling of faces 22 and 26 of unit 20 may be accomplished by biasing of face 26 only as just described. it should be understood that face 22 of component 13 may be biased instead. In FIG. 4. fiber 140, has opposite end faces 26 and 28 which are perpendicularly parallel to the fiber axis. Fiber 14h has face 22 disposed on a bias and opposite emitting face 24&#39; disposed perpendicularly to its axis.  
  A still further modification of unit 20 is illustrated in FIG. 5. Numerical aperture expansion in FIG. 5 is accomplished with fibers I4u, and I4h by substantially equally biasing the coupled faces 22&#34; and 26&#34;.  
  It is pointed out that. in all embodiments ofthe invention described hereinabove and those to be subse quently described. numerical aperture designations NA. and NA,, indicate half-angle spreads of cones of light which are emitted from the illustrated fibers and fiber units.  
  Refractive index differentials between core glasses of interfacially coupled optical fibers and bundles thereof can be utilized to effect numerical aperture expansion. FIGS. 6 and 7 are presented to illustrate this aspect of the invention.  
  In FIG. 6 fiber optical components 32 and 34 each made up of tightly bundled together optical fibers 36 and 38 respectively are interfacially coupled without angulation. Matching biases of end faces 40 and 42 are used to retain axial direction symmetry of adjoining fibers 36 and 38. With this arrangement. numerical aperture expansion NA. is accomplished as follows:  
  Ray of light 44 (FIG. 7) which represents an extreme ray of the entrance cone whose half angle is NA,. traverses fiber 360 by total internal reflection. Upon reaching face 40 of fiber 38a at interface 40-42. core material 46 of fiber 3811. being of lower refractive index than the core material 48 of fiber 36a. causes light ray 44 to refraet still further away from the common axes of fibers 36a and 38a. In so doing. ray 44 becomes incident upon the fiber 38a interface at a steeper angle for continued conduction through fiber 38a and emission therefrom at the expanded numerical aperture NA Thus. it can be seen that numerical expansion may be accomplished in straight coaxially aligned optical fibers and bundles thereof having biased coupling faces according to principles of the invention. While a flat interface 40-42 has been illustrated herein cylindrical. spherical or aspheric intcrfacial curvatures may be used to produce an effect upon the NA expansion which varies with the position of fibers 36, 38 in bundle 34.  
  In tapered fiber optical bundles there is the peculiar problem of numerical aperture continually decreasing as light is transmitted from a small end to a larger end of the bundle. This, however. may be compensated for according to the present invention by expanding the emitting numerical aperture of such a unit as depicted in FIGS. 8 and 9.  
  Fiber optic unit 50 comprises a major element 52 of tightly bundled tapered optical fibers 54 having their smaller ends grouped together as a light-receiving face 56 and their opposite larger ends grouped together as a light-emitting face 58. Face 58 is biased at angle a relative to the central axis of element 52. Light-receiving element 60 of unit 50 comprises a tight bundle of parallcl cylindrical light-conducting fibers 62. Element 60 has oppositely disposed light-receiving and lightemitting faces 64 and 68 respectively which are each perpendicular to the axes of fibers 62. in unit 50. the  
 numerical aperture NA, of light entering face 56 which inherently diminishes with the conduction of light through tapered element 52 is expanded to an extent partially. fully or overcompensating for the reduction in the output numerical aperture NA of unit 50 with fiber optical element 60 as follows:  
  Referring more particularly to HO. 9, the diminishing numerical aperture NA,- of an extreme ray of light 70 entering tapered fiber 54 is illustrated. Angle NA,, illustrates the diminished numerical aperture output of fiber 54 which is expanded by component 60 having fibers 62. one of which is illustrated in FIG. 9.  
  With ray 70 entering fiber 62 at angle NA,, relative to the axis of fiber 54. the off-axis relationship of fiber 62 causes this ray to become incident upon its interface at a steeper angle of reflection thereby increasing its numerical aperture in fiber 62. This results in expansion of the output numerical aperture NA It should be understood that with variation in the selection of core and cladding glasses for fibers 54 and 62 and/or diffcrences in the angle 01 of interface 58-64, NA,, may be expanded still further or reduced in degree. as desired.  
 I claim;  
  I. A fiber optic device having light-receiving and lightcmitting oppositely disposed faces. one face having a different numerical aperture than the other. said device comprising:  
 a coupled together pair of bundles of tightly juxtapotil sitioned light-conducting fibers. the fibers of each of said bundles individually comprising a core of high refractive index material and a cladding of material having a lower index of refraction than said core \vherevvith opposite end faces of said fibers are individually provided with a numerical aperture determined by the difference in refractive index between said core and cladding and said end faces in each of said bundles collectively providing the respective bundle faces with corresponding numerical apertures.  
 the fibers of one of said bundles having an appreciably larger numerical aperture than the fibers of the remaining bundle; and  
 the plane of a light&#39;receiving face of one bundle and that of a light emitting face of the other bundle cutting corrcsponding fibers of said pair on a bias and said biasing faces being optically joined together to further expand said differential of numerical aperture of said oppositely disposed light recciving and light-emitting faces of said device.  
  2. A fiber optic device having light-conducting and light-emitting oppositely disposed faces. one face having a different numerical aperture than the other. said device comprising:  
 a coupled together pair of bundles of tightly juxtapositioned lighbconducting fibers, the fibers of said bundles individually comprising a core of high refractive index material and a cladding of material having a lower index of refraction than said core whcrewith opposite end faces of said fibers are individually provided with a numerical aperture dc termined by the difference in refractive index between said core and cladding and said end faces in each of said bundles collectively providing the respective bundle faces with corresponding numerical apertures:  
 each of the fibers of one of said bundles being longitudinally tapered thereby providing the bundle with opposite end faces of different sizes.  
 the other bundle having opposite end faces of substantially equal size; and  
 the larger of said faces of said bundle of tapered fibers and one face of said other bundle of fibers being optically interfacially coupled together. at least one of said coupled faces being cut on a bias across its respective fibers to render the uncoupled oppositely disposed faces of said bundles of different numerical aperture.  
  3. A fiber optic device according to claim 2 wherein the larger of said faces of said bundle of tapered fibers is cut on a bias across its respective fibers at said interfacial coupling of said pair of bundles.  
  4. A fiber optic device according to claim 2 wherein one face of said bundle having opposite end faces of substantially equal sizes is cut on a bias across its respective fibers.  
  5. A fiber optic device according to claim 2 wherein both the large face of said bundle of tapered fibers and said one face of said other bundle are cut on a bias across their respective fibers.  
  k l l