Abstract:
An apparatus for transmitting laser light and redirecting the light laterally relative to an axis of the apparatus includes an optical fiber having a core and a cladding surrounding the core. The core terminates at a core end. The cladding terminates at a cladding end spaced from the core end to expose an exposed length of the core. A tubular member surrounds at least a distal portion of the fiber and has a closed distal end. The exposed length of the core is bent for the core end to oppose a side of said tubular member. The core end is bonded to the side of the tubular member. A seal creates a sealed volume of the tubular member surrounding said exposed length. The volume may contain a vacuum or a gas such as air.

Description:
I.  
     CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is a continuation-in-part of U.S. patent application Ser. No. 11/155,348 filed Jun. 17, 2005. 
     
    
     II.  
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention pertains to optical fibers for discharging laser energy laterally to an axis of the optical fiber. More particularly, this invention pertains to such an optical fiber probe and a method for making the same.  
         [0004]     2. Description of the Prior Art  
         [0005]     So called “side-firing” optical fiber probes discharge light laterally or transverse to a longitudinal axis of the optical fiber as opposed to discharging light from a laser tip in a direction substantially parallel or on axis of the optical fiber. An example of a side-firing optical fiber is shown in U.S. Pat. No. 4,785,815 to Cohen dated Nov. 22, 1988. Particularly, FIGS. 7 and 9 of the &#39;815 patent show optical fiber tips for discharging energy laterally relative to the axis of an optical fiber.  
         [0006]     Optical fibers are fragile when not protected by appropriate cladding, jacket and buffers. Currently, the construction of a side-firing optical fiber probe or device requires removal of these components and addition of other materials, a process which can be difficult or expensive to manufacture in a manner which preserves the desired optical qualities while avoiding damage to a fragile optical fiber during the assembly process. A more simple construction of a side-firing optical fiber is disclosed in U.S. Pat. No. 5,537,499 to Brekke, dated Jul. 16, 1996. As shown in FIGS. 7-11 of the &#39;499 patent, an optical fiber is placed within a tubular member formed of silica. The optical fiber has an inclined end surface within a gas filled chamber to cause reflection of light traveling along the axis of the optical fiber to exit the optical fiber tip transverse to the optical fiber axis. The optical fiber tip is fused to the silica of the tubular member to create a continuous material from the optical fiber tip through the silica tubular member to avoid alteration in an index of refraction throughout the light path.  
         [0007]     While the design of the &#39;499 patent is an efficient design for many applications, it has limitations. Specifically, the design of the &#39;499 patent is limited to a optical fiber having a cladding which can withstand the thermal energies required during the process of fusing the optical fiber tip to the silica tubular member. The fusion process results in a melting of the optical fiber in the silica tubular member to form a continuous material. This occurs at the melting point of fused silica, a temperature of about 1600° C. If the cladding of the optical fiber cannot withstand such temperatures, the cladding will melt resulting in at least a portion of the length of the optical fiber being unclad and thereby not reflective to incident internal energy. In the &#39;499 patent, such cladding is a so-called “doped fused silica cladding” which can withstand the temperatures of the welding process of the optical fiber tip to the silica tubular member.  
         [0008]     Optical fibers having doped fused silica cladding are acceptable for many applications. For most optical fibers, the doped fused silica layer is approximately 5% of core diameter or typically 20 microns in thickness. There is only a small index of refraction difference between the fused silica core of the optical fiber and the doped fused silica cladding. The critical angle of an optical fiber is determined by the index of refraction difference between its core and cladding. The numerical aperture is the square root of (n 1   2 −n 2   2 ) where n 1  is the index of refraction of the core and n 2  is the index of refraction for the cladding. The critical angle is defined as the maximum incidence angle from the centerline of an optical fiber for total internal reflection. The smaller the index of refraction difference between the core and cladding, the more collinear the laser light must be when entering the optical fiber. For most commercially available optical fibers using a fused silica core and a doped fused silica cladding, the critical angle of the optical fiber must be less than 13 degrees. A critical angle of less than 13 degrees corresponds to a numerical aperture of 0.22 (which is approximately the arcsine of the critical angle). Many commercially available optically pumped lamp lasers have very small divergence angles which are ideally suited for use with the design of the &#39;499 patent having doped silica cladding on a silica core optical fiber.  
         [0009]     In addition to so-called optically pumped lasers, direct diode lasers are becoming increasingly popular due to their lower cost, smaller physical size, higher efficiency and greater reliability. However, direct diode lasers suffer from poor beam quality. As a result, applications using direct diode lasers need optical fibers for delivering the laser energy which maintain high optical efficiency to provide adequate power to the optical fiber tip and accept a divergent beam significantly greater than commercially available side firing optical fibers which use optically efficient designs such as the &#39;499 patent.  
         [0010]     Commonly, the divergence angle of most direct diode lasers is approximately 22 degrees which requires an optical fiber with a numerical aperture of 0.37 to capture and transmit all incident energy. This is significantly greater than the maximum tolerable numerical aperture of commercially available fibers which use a design such as that of the &#39;499 patent containing a pure silica core optical fiber with a doped fused silica cladding. Accordingly, the use of such a direct diode laser with such a design results in a substantial loss of power during transmission of the laser energy along the optical fiber because the incidence angle of the laser is larger than the numerical aperture of the optical fiber.  
         [0011]     A higher numerical aperture would be possible with the design of the &#39;499 patent if the doped silica cladding were to be replaced with any one of a number of different commercially available plastic claddings having a higher index of refraction difference between the cladding and the pure silica core of the optical fiber. Unfortunately, such plastic claddings have melting temperatures significantly lower than that of the silica core. As a result, the fusion process described in the &#39;499 patent cannot be used with such optical fibers since, during the fusion process, a substantial length of the plastic cladding will melt leaving a substantial length of the optical fiber core unclad. This substantial length results in loss of laser energy. Since laser diodes already operate at relatively low power outputs, such a loss of energy is unacceptable for most applications.  
         [0012]     Commonly assigned and co-pending U.S. patent application Ser. No. 11/155,348 describes an improvement to the apparatus of the &#39;499 patent.  
       III.  
     SUMMARY OF THE INVENTION  
       [0013]     According to a preferred embodiment of the present invention, an apparatus is disclosed for transmitting laser light and redirecting the light laterally relative to an axis of the apparatus. The apparatus includes an optical fiber having a core and a cladding surrounding the core. The core terminates at a core end. The cladding terminates at a cladding end spaced from the core end to expose an exposed length of the core. A tubular member surrounds at least a distal portion of the fiber and has a closed distal end. The exposed length of the core is bent for the core end to oppose a side of said tubular member. The core end is bonded to the side of the tubular member. A seal creates a sealed volume (of vacuum or air or other gas) of the tubular member surrounding said exposed length. 
     
    
     IV.  
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is the view of FIG. 10 of U.S. Pat. No. 5,573,499 showing, in lateral cross section, an optical fiber fused to a surrounding tube, according to the teachings of the &#39;499 patent;  
         [0015]      FIG. 2  corresponds to FIG. 11 of the &#39;499 patent and is a view taken generally along lines  2 - 2  of  FIG. 1 ;  
         [0016]      FIG. 3  is the view of  FIG. 1  showing energy loss resulting from partial destruction of a cladding of a optical fiber of  FIG. 1 ;  
         [0017]      FIG. 4  is a view similar to that of  FIG. 1  and showing an improvement in a manufacturing process according to the present invention;  
         [0018]      FIG. 5  is a view taken along line  5 - 5  of  FIG. 4 ;  
         [0019]      FIG. 6  is a view similar to that of  FIG. 4  showing an alternative embodiment of the present invention which uses a plastic clad optical fiber and a thermal bond with a cap having substantially the same index of refraction as the cladding of the optical fiber;  
         [0020]      FIG. 7  is a view similar to that of  FIG. 4  showing a still further alternative embodiment of the present invention adapted to create a linear pattern of light energy from a distal end of a fiber;  
         [0021]      FIG. 8  is a side sectional view of an apparatus with a laser fiber bent within a tubular member;  
         [0022]      FIG. 9  is the view of  FIG. 8  showing an alternative embodiment;  
         [0023]      FIG. 10  is a bottom plan view of a side-firing fiber such as that of  FIG. 6  and illustrating an ellipsoid light discharge pattern; and  
         [0024]      FIG. 11  is a bottom plan view of a bent fiber such as that of  FIG. 8  and illustrating a circular light discharge pattern. 
     
    
     V.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     With reference now to the various drawing figures in which identical elements are numbered identically throughout, a description of a preferred embodiment of the present invention will now be provided. The complete disclosure including the specification and drawings of U.S. Pat. No. 5,537,499, to Brekke issued Jul. 16, 1996, is incorporated herein by reference as though set forth in full.  
         [0000]     A. Teachings of the Prior Art  
         [0026]     In order to facilitate an understanding of the present invention, an initial description will be presented of a prior art optical fiber combination as taught in U.S. Pat. No. 5,537,499. The text of this section is taken substantially from the &#39;499 patent.  FIGS. 1 and 2  are reproductions of Figures of 10 and 11 of the &#39;499 patent. The figures show a side firing laser optical fiber apparatus  113 .  
         [0027]     The apparatus  113  has an elongated flexible optical fiber  117  terminating at an inclined end surface  118 . The optical fiber  117  has a pure silica optical fiber core surrounded with a fluorine doped fused silica cladding  119 . A sleeve  120  of plastic material covers the cladding  119 . It will be noted that the sleeve  120  is spaced from the end surface  118 .  
         [0028]     The cladding  119  is enclosed within a jacket (not shown) of plastic material, such as Teflon. The surface  118  has a generally oval polished shape. According to the &#39;499 patent, a diamond-tipped abrasive tool, a carbon dioxide laser tightly focused or excimer laser can be used to polish the surface  118 .  
         [0029]     The surface  118  is inclined forwardly at an angle 37° relative to the longitudinal axis of the optical fiber  117 . Such angle can be between 37 to 45° relative to the longitudinal axis of the optical fiber  117 , or such other angles as may be suitable for a particular application. When the angle of the surface  118  is 37°, reflected light will emerge at approximately 70° in air with an associated divergence.  
         [0030]     A tubular layer of silica cladding  119  surrounds the core of the optical fiber  117  to protect the core and maintain the laser light within the optical fiber  117 . A transparent capsule of tubular member  122  of silica having a closed convex curved end  123  is located about the distal end of the optical fiber  117  to enclose the distal end of the optical fiber within an air chamber  124 . The distal end of the optical fiber  117  is surrounded by air chamber  124 . Member  122  is a silica cylindrical tubular member made of silica material the same as or similar to the silica material of optical fiber  117 .  
         [0031]     The distal end of optical fiber  117  is united at  125  to the adjacent inside wall of silica tubular member  122 . The silica materials of optical fiber  117  and tubular member  122  are fused with localized heat. As shown in  FIG. 7  of the &#39;499 patent, the heat required to cause the fusion of the silica materials of optical fiber  117  and tubular member  122  is in the range of 1400° C. to 1700° C.  
         [0032]     As described in the &#39;499 patent, an infrared laser beam is directed through an optical lens which concentrates the laser beam on the surface of silica tubular member  122 . The heat from the laser beam is conducted through the silica of tubular member  122  toward the distal end of optical fiber  117 . The high temperature heat radiates across the air gap and melts the silica of the optical fiber core as well as the silica of tubular member  122 . The opposing silica materials of optical fiber  117  and tubular member  122  are melted and fused together as shown in FIGS. 8-11 of the &#39;499 patent.  
         [0033]     Referring to  FIGS. 1 and 2  (which correspond to FIGS. 10 and 11 of the &#39;499 patent), light or laser beam  130  generated by a laser axially propagates down optical fiber  117 . When light  130  encounters a change in refractive index, it undergoes total internal reflection (TIR). The index of refraction change redirects the light energy laterally as indicated by arrows  131 . The angle of polished surface  118  being 37 degrees relative to the longitudinal axis of optical fiber  117  results in almost total internal reflection of light  130  as redirected light  131  at an angle of approximately 70 degrees relative to the longitudinal axis of optical fiber  117 .  
         [0034]     Light  131  is efficiently redirected laterally through the distal end of optical fiber  117 , the fused area  125  and silica tubular member  122 . Optical fiber  117 , fused area  125  and silica tubular member  122 , being of substantially the same silica materials, do not produce changes in the refractive indices and thereby do not produce reflected light nor secondary light.  
         [0000]     B. Limitations of the Prior Art Design  
         [0035]     As previously described, the construction of  FIGS. 1 and 2  are necessarily limited to use with lasers having good beam quality capable of launching a numerical aperture of 0.22 or less. For use with diode lasers (having a numerical aperture of 0.37 or greater), the doped silica cladding  119  can not be used since too great of a power loss occurs as a result of transmission loss of the energy along the optical fiber escaping through to the cladding  119 .  
         [0036]     Plastic claddings provide the necessary cladding for such an energy source. Examples of such plastic claddings are Ceramoptec Optran HUV/ of CeramOptec Industries, Inc., 515A Shaker Road, East Longmeadow, Mass., USA 01028 (www.ceramoptec.com) and FiberTech VIS/IR of Fibertech USA, Inc., 4111 East Valley Auto Drive, Suite 104, Mesa, Ariz., USA 85206 (www.us-fibertech.com). However, plastic claddings have a substantially lower melting temperature (about 85° C.) than silica. This precludes their efficient use in the manufacturing process described with reference to  FIGS. 1 and 2 .  
         [0037]     This disadvantage is shown with reference to  FIG. 3 . In  FIG. 3 , all elements in common with those of  FIGS. 1 and 2  are numbered identically with the addition of an apostrophe to distinguish the embodiments. Accordingly, not all elements will be separately described except to the extent they different from those in  FIGS. 1 and 2 .  
         [0038]      FIG. 3  illustrates the optical fiber  117 ′ identical to the optical fiber  117 , except that the cladding  119 ′ is a plastic cladding. A representative example of such a cladding is the afore-mentioned FiberTech VIS/IR with a hard polymer cladding with a melting point of 85° C.  
         [0039]     With a plastic cladding, the optical fiber  117 ′ may efficiently transport laser energy from a diode laser and having a numerical aperture of 0.37. However, during the fusion process described with reference to  FIGS. 1 and 2 , the cladding  119 ′ in close proximity to the fused area  125 ′ will melt exposing a length L of the cylindrical wall of the optical fiber core  117 ′. Due to such exposure, light  131   a ′ exits the core prematurely, resulting is a substantial power loss. With lower power direct diode lasers, such a power loss is unacceptable for most commercial applications.  
         [0000]     C. Teachings of the Parent Application  
         [0040]     The text of this section is taken substantially from parent application U.S. Ser. No. 11/155,348 filed Jun. 17, 2005.  
         [0041]     The design limitations of  FIGS. 1-3  are overcome with the present invention, as will now be described with reference to  FIGS. 4 and 5 . In  FIGS. 4 and 5 , all elements in common with those of the previously described embodiments are numbered identically within the addition of a double apostrophe to distinguish the embodiments and are not separately described except as necessary to distinguish the embodiments.  
         [0042]     An optical fiber  117 ″ of pure silica core is provided with a plastic cladding  119 ″ such as FiberTech VIS/IR. The plastic cladding  119 ″ on the silica core  117 ″ provides efficient transport of laser energy with a numerical aperture of 0.37 or greater. This permits efficient use of the apparatus  113 ″ with a direct diode laser energy source.  
         [0043]     The optical fiber  117 ″ is surrounded by a silica tubular member  122 ″ with a silica cap  123 ″ to surround the inclined surface  118 ″ of the optical fiber distal end with an air chamber  124 ″. At the end portion of the wall of the optical fiber  117 ″ (i.e., at the intersection of the optical fiber wall and inclined surface  118 ″ near the acute angled point of the inclined surface  118 ″), a portion of the cladding  119 ″ is removed along a length L 2 . The portion of the optical fiber wall along the length L 2  faces an opposing surface of the silica tubular member  122 ″.  
         [0044]     An adhesive layer  126 ″ is positioned between the wall of the optical fiber  117 ″ and the silica tubular member  122 ″ along length L 2 . The reminder of the cladding  119 ″ extends up to the adhesive layer  126 ″.  
         [0045]     The adhesive layer  126 ″ is selected to have an index of refraction which substantially matches the index of refraction of the optical fiber core  117 ″ and the silica tubular member  122 ″. As a result, there is little or no power loss for light passing through between the core  117 ″ and the adhesive  126 ″ or between the adhesive  126 ″ and the tubular member  122 ″. Adhesives  126 ″ having an index of refraction to match the silica of the core  117 ″ and the silica tubular member  122 ″ are commercially available. An example of such is Optocast™ 3580 adhesive by Electronic Materials Inc., 1814 Airport Road, Breckenridge, Colo., USA, 80424.  
         [0046]     It will be noted that by using an index-matching adhesive  126 ″, index matching in made between the optical fiber  117 ″ and the tubing  122 ″ in a manner to obtain the benefits of the fusion of the prior art, but avoiding a process requiring application of heat. By avoiding application of heat, the cladding  119 ″ is not destroyed by thermal energy, and remains intact throughout the length of the optical fiber  117 ″ and up to and abutting the adhesive layer  126 ″. As a result, there is little or no loss of scattered light through the wall of the optical fiber  117 ″ as described with reference to  FIG. 3 . Manufacturing efficiencies associated with the prior art of  FIGS. 1 and 2  can be achieved as well as providing for an optical fiber of plastic cladding  119 ″, which can accommodate a much greater numerical aperture than that limited by the doped silica cladding of the prior art.  
         [0047]      FIG. 6  illustrates an embodiment to permit use of the manufacturing process of U.S. Pat. No. 5,537,499, to Brekke but avoiding the premature loss of energy due to melting of a plastic cladding. In  FIG. 6 , elements in common with previously described embodiments are numbered identically with the addition of three apostrophes to distinguish the embodiments. To the extent those elements materially differ from previous embodiments in structure, materials or method of manufacture, they are separately described in the following description of  FIG. 6 . Otherwise, no additional description is necessary.  
         [0048]     In  FIG. 6 , an optical fiber  117 ′″ of silica core is provided with a plastic cladding  119 ′″ such as FiberTech VIS/IR as previously described. Instead of surrounding the fiber  117 ′″ with a silica tubular member and a silica cap as previously described, the fiber  117 ′″ is surrounded by a silica tubular member  122 ′″ and a silica cap  123 ′″. The silica tubular member  122 ′″ and cap  123 ′″ are formed from a doped fused silica having an index of refraction substantially identical to the index of refraction of the cladding  119 ′″.  
         [0049]     In the embodiment of  FIG. 6 , the fiber end is not adhered to the silica tubular member using an adhesive as described with reference to the embodiments shown in  FIGS. 4 and 5 . Instead, the doped fused silica tubular member  122 ″ is fused and bonded to the fiber  117 ′″ at  125 ′″ as described in U.S. Pat. No. 5,537,499 to Brekke. The reference numeral  125 ′″ illustrates area of welding the material of the fiber core  117 ′″ and the silica tubular member  122 ″. During this fusion, any cladding material in the area melts and evaporates and does not materially comprise part of the material of area  125 ′″. This fusion process partially melts the plastic cladding  119 ′″ (as described with reference to  FIG. 3 ) leaving an unclad length L′″. Light  131   a ′″ which escapes the fiber  117 ′″ along length L′″ is reflected back into the fiber  117 ′″.  
         [0050]     In  FIG. 6 , where the cladding  119 ′″ along length L′″ has been destroyed by the heat but proximal to the beginning of the angled surface  118 ′″ of the optical fiber, the light  131   a ′″ will be reflected back toward the center of the fiber  117 ′″ because the incidence angle of the light  131   a ′″ at the silica tubular member  122 ′″ is less than the critical angle. Once the light hits the angled surface  118 ′″ and is reflected toward the side of the optical fiber, the incidence angle is greater than the critical angle and the light  131 ′″ passes out the fiber.  
         [0051]      FIG. 7  illustrates a still further alternative embodiment of the present adapted to create a linear pattern of light energy from a distal end of a fiber. In  FIG. 7 , elements in common with  FIGS. 1 and 2  are numbered identically with the addition of “a” to distinguish the embodiments. To the extent those elements materially differ from previous embodiments in structure, materials or method of manufacture, they are separately described in the following description of  FIG. 7 . Otherwise, no additional description is necessary.  
         [0052]     The embodiment of  FIG. 7  illustrates a fiber manufactured with the thermal fusion process of U.S. Pat. No. 5,537,499 to Brekke. It will be appreciated the novel structure of  FIG. 7  could be incorporated into a fiber manufactured according to the embodiment described with reference to  FIGS. 4 and 5 .  
         [0053]     In  FIG. 7 , multiple sloped surfaces  118   a   1  are formed in the core  117   a  proximal to the sloped surface  118   a.  The sloped surfaces  118   a   1  are formed by creating notches in the fiber. The sloped surfaces  118   a   1  are bounded by air layers  124   a   1 . The sloped surfaces  118   a   1  have the same angle to the fiber axis as the distal inclined surface  118   a.  Therefore light  131   a   1  exits the fiber  117   a  from the sloped surfaces  118   a   1  at the same angle as light  131   a  from inclined surface  118   a.  Since this light  131   a  passes the cladding at an angle greater than a critical angle of the cladding  119   a,  the light  131   a   1  is not reflected back into the fiber  117   a.    
         [0000]     D. Improvements of the Present Application  
         [0054]     A further improvement over the prior art is shown in  FIG. 8 . An optical fiber  200  has a fused silica core  202  surrounded by a cladding  204 . The core  202  terminates at a distal end  206 . The cladding  204  terminates at and end  208  spaced from core end  206  to reveal an exposed length L of core  202  without cladding. As shown, the exposed length of core  202  is formed into a 90 degree bend of radius R′. The radius R′ is greater than a minimum radius that will provide for total internal reflection (i.e., must be greater than a minimum which will cause light to exceed the critical angle).  
         [0055]     A preferred method for forming the bent, exposed length of core, the fiber  200  is heated approximately to the softening point of the fused silica. Heating can be by using a CO 2  laser or other suitable thermal method.  
         [0056]     By thermally shaping the optical fiber  200 , the glass core  202  has little or no residual stress in the bent state allowing much tighter bends to be achieved in comparison to a mechanically bent fiber. By reducing the radius of curvature R with such method, the distance D from the axis A of the straight portion of the fiber  200  to the core tip  206  can be minimized. As will be apparent, this reduces the overall thickness of a containment tube  210   
         [0057]     The heating process destroys the lower temperature cladding  204 . This creates the exposed length L of core  202 . An exposed length L of core  202  is a zone having a potential for transmitted laser energy to escape through the side of the optical fiber core  202 . However, as discussed previously, air is a suitable cladding material having an index of refraction of about 1.0.  
         [0058]     To maintain an encapsulating air layer around the optical fiber core  202  in the presence of other medium, the optical fiber is contained within a tubular element  210  made from a fused silica glass capsule. A distal end  212  of tube  210  is closed. The fiber  200  is axially aligned in tube  210  with core end  206  abutting an interior surface of a side wall of the tube  210 .  
         [0059]     The tubular member  210  is thermally fused and bonded to the optical fiber core  202  at end  206  as described in U.S. Pat. No. 5,537,499 to Brekke. A fused region is illustrated at  220 . The fused region  220  maintains a scatter free interface because it eliminates any change in index of refraction along the path of laser energy.  
         [0060]     An adhesive seal  230  surrounds the clad portion of the fiber  200  proximally spaced from the cladding end  208 . The seal  230  seals between opposing surfaces of the cladding  204  and tube  210  to define a sealed volume  240  containing air or other medium having a low index of refraction. Air has an index of 1.00. The cladding has an index of 1.42 and the core has an index of 1.44.  
         [0061]     An alternative embodiment is shown in  FIG. 9 . Elements in  FIG. 9  in common with  FIG. 8  are numbered identically with the addition of an apostrophe to distinguish embodiments. Unless otherwise described, such elements function the same as in  FIG. 8  and are not separately described.  
         [0062]     In  FIG. 8 , the longitudinal axis A of the fiber  200  is collinear with a longitudinal axis of the tubular member  210 . In  FIG. 9 , the fiber axis A′ is offset from the axis of the tubular member  212 ′. Unlike the embodiment of  FIG. 8 , the fiber  200 ′ does not pass centrally through the seal  230 ′. Instead a portion  230   a ′ of the seal  230 ′ is larger on a same side of the fiber  200 ′ as the fiber end  206 ′. A diametrically opposite side of the fiber  200 ′ is spaced from the tubular member  210 ′ by a narrower seal portion  230   b ′ resulting in a narrow gap G′ between the fiber  200 ′ and tubular member  212 ′ on this side.  
         [0063]     By minimizing the gap G′ by offsetting the axis A′ of the optical fiber  200 ′, the thickness T′ of the tubular member  210 ′ can be minimized. For any given radius R′, a minimum distance D for a fiber with discharge axis A 1 ′ perpendicular to axis A′ is fixed.  
         [0064]     While the preferred embodiment shows a 90 degree bend with an axis A 1 , A 1 ′ of the bent core perpendicular to axis A, A′, a less degree of bending could suffice. The main advantage of the thermally bent fiber is this angle is independent of any other optical considerations other than those associated with the internal reflections at the core/cladding interface. For the side firing design (e.g.,  FIG. 6 ), this angle is limited by the index of refraction difference of the materials and other optical considerations at the angled surface which generally limit the maximum angle to something less than 90 degrees. For mechanically bent fibers, this angle is determined by maximum mechanical stress that the fiber can endure which is a function of the degree of bending of the fiber. In the design of the present invention, the fiber is not mechanically bent.  
         [0065]     Another advantage of the thermally bent fibers is the energy profile leaving the glass capsule  210 ,  210 ′.  FIG. 10  shows the geometric shape of the fiber/capsule interface  125 ′″ which is an ellipsoid. For a thermally bent fiber with a 90 degree angle the geometric shape of the interface  206 ,  206 ′ is a circle. The profile of light exiting the capsule will have a similar geometric pattern as that of the interface. Profile distortions can reduce energy density and coherency of the light energy.  
         [0066]     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.