Patent Publication Number: US-2016231509-A1

Title: Optical fiber assembly and method for making same

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the United States Government. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to optical assemblies for delivery of laser radiation. 
     II. Description of Related Art 
     In military applications, the irradiance or power density of a laser beam established on the target is the most essential factor for destroying the target. Generally, the better the beam quality and power of the laser beam, the higher the irradiance. For example, a laser beam having a power of 10 kilowatts can create the destructive action on a target in seconds, if focused into spot with diameter of 2-3 cm. 
     In recent years, the performance of lasers in military applications has improved dramatically owing to success in fiber lasers. Single mode fiber lasers have almost ideal quality of radiated beam. The wall-plug efficiency has reached the unparalleled level 40%. However, the current limiting factor for military laser applications is the limitation of the maximum power for one fiber laser. Currently, fiber lasers with high quality of emitting beam are limited to about 1 kilowatt in power. This power limitation, furthermore, is related mostly to the nonlinear effects of the laser beam. 
     In order to focus the laser beam on the target, a complex and expensive beam forming apparatus, such as a large mirror of high quality, has been previously required. The mitigation of beam degradation induced with atmospheric turbulences also requires expensive adaptive optics in order to achieve the necessary concentration of radiation on the target. There are also limitations related to the optical strength of the fiber optic interface which actually emits the laser beam due to its small core diameter, i.e. typically about 20 microns. This, however, may be mitigated by pining a coreless endcap to the emitting end of the fiber tip. The endcap allows one to reduce the optical strength on emitting interface (endcap facet) hundreds and thousands times to the non-dangerous power density level on facet. However the conventional endcaps have large weight and size. See for instance the U.S. patents
     U.S. Pat. No. 8,213,753 from Jul. 3, 2012, by Li et al. “System for delivering the output from an optical fibre”;   U.S. Pat. No. 8,355,608 from Jan. 15, 2013, by Hu “Method and apparatus for in-line fiber-cladding-light dissipation”;   U.S. Pat. No. 8,419,293 from Apr. 16, 2013, by Zerfas et at “Methods and apparatus related to a launch connector portion of a ureteroscope laser-energy-delivery device”;   U.S. Pat. No. 8,419,296 from Apr. 16, 2013, by Murayama et al. “Optical fiber structure, system for fabricating the same, and block-like chip for use therein”;   U.S. Pat. No. 8,457,461 from Jun. 04, 2013 by Ott “Fiber optic cable assembly and method of making the same”;   U.S. Pat. No. 8,511,401 from Aug. 20, 2013, by Zediker et al. “Method and apparatus for delivering high power laser energy over long distances”;   U.S. Pat. No. 8,724,945 from May 13, 2014, by Gapontsev et al, “High power fiber laser system with integrated termination block”.
 
Moreover, in many applications the endcap requires the external cooling to dissipate the parasitic radiation which appears due to reflection and scattering in endcap and nearest environment, for instance in material processing or surgery.
   

     The previously known optical fibers typically include an optical wave-guiding core which conveys the radiation from the laser. This core is encased in a cladding having smaller refractive index RI(clad) than refractive index of core RI(core). Cladding, in turn, is encased within a polymer coating typically having the refractive index RI(coat) smaller than cladding, RI(coat)&lt;RI(clad). However, the distal or free end of the optical fiber is subjected to high heat during the process of attaching (splicing) the endcap. This high heat may melt a portion of the polymer coating at the distal end of the optical fiber assembly. Therefore, the portion of polymer coating is typically stripped from fiber cladding before splicing the endcap. 
     The free or distal end of the optical fiber assembly delivering the relatively small power, not exceeding 1 W, can be typically mounted within a metal tubing which is normally optically isolated from the laser radiation which may propagate in cladding by the polymer coating owing to total internal reflection of cladding optical modes from boundary between cladding and polymer. However, when the polymer coating is removed at the distal end of the optical fiber assembly, e.g. by stripping some portion when attaching the endcap by thermal fusion, contact between the stripped portion of optical fiber and the metal tubing can occur. When this happens, light leakage can occur which results in heating of the metal tubing. This, in turn, reduces the overall efficiency of the optical fiber assembly and limits the power of the delivered radiation. 
     Attempts have been made to recoat the portion of the coating removed during attachment of the endcap. However, such recoating of the distal end of the optical fiber assembly is a delicate process which can damage or contaminate the facet for the endcap. Any damage to the facet for the endcap necessarily reduces the overall efficiency of the light transmission by the optical fiber assembly sometimes causing the activation of burning the fiber core up to catastrophic damage of fiber laser or fiber amplifier. 
     In most military applications, a plurality of optical fiber assemblies are arranged together into arrays in order to achieve the necessary total power to destroy the target. Consequently, a mount is typically provided adjacent the distal end of each of the optical fibers. Each optical fiber with its surrounding metal tube extends through a passageway in the mount and the proximal end of tube is secured to the mount in an conventional fashion. Typically the laser beams emitted by facets of distal ends of fibers may independently deviate (wander) due to vibrations and atmospheric turbulence before these beams will reach the target. This wandering may misalign the overlapping of laser beams on the same spot of target, drastically reducing the radiance on target. To provide the permanent overlapping of laser beams on the target the distal end of tube can be moved in focal planes of collimating (focusing) lenses. Typical frequencies of movements of distal end should be of the order of thousand Hertz or higher to mitigate typically very fast beam wandering. Therefore, the movable part of tube together with distal end of fiber with endcap should have very small inertia, with weight typically in range of tens of milligrams. Such strict requirement to weight and size of movable parts of fiber assembly is in very strong contradiction to heavy and bulky endcaps commonly used for radiation with power of kW level. 
     The portion of the optical fiber extending from the mount and to the source of radiation, however, should be also very light. Additionally, this portion is very fragile and vulnerable to abrasion and mechanical stresses. Consequently, this portion of the optical fiber extending between the source of radiation or laser and the mount is encased in one or more protective sheaths. Typically the protective sheaths like metal semi-flexible conduits are thick and heavy. In case of plurality of fiber laser array the forces of gravity and stiffness of such sheaths may misalign the position of emitting fiber tips with additional misalignment of beams on a target. Some portion of optical fiber between laser source and mount may contain much lighter and flexible tubes/pipes/sheaths made from plastic. Previously, the distal ends of such plastic sheaths have been attached to the mounts by epoxy adhesive. One disadvantage, however, of using epoxy adhesive to secure the protective sheaths to the mounts is that, in the event that the optical fiber assembly must be replaced for maintenance or other reasons, it is difficult and time consuming to detach the adhesively secured protective sheaths from the mounts. 
     Since the optical fibers are so thin, the optical fibers can be easily stressed and deflected for many reasons, including gravity. Such stress on the fiber can adversely affect the overall efficiency and performance of the optical fiber assembly, for instance due to appearance of high-order modes in a single-mode fibers. 
     Consequently, it would be advantageous to protect the optical fiber at spaced distances between its mount and the laser radiation source by such protecting means as tubes, sheaths and conduits having very small weight to avoid the stresses induced by gravity and stiffness. However, there are no previously known ways to support the weight of the optical fibers in the previously known optical fiber assemblies. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides a method for making and optical fiber constructions which overcome the above-mentioned disadvantages of the previously known optical fiber assemblies. 
     In order to prevent contact between stripped portion of the optical fiber and its encasing metal tube adjacent the distal end of the fiber and to block the radiation leakage from cladding to inner space of said tube, the optical fiber is first extended out through the distal end of its supporting tube until all, or almost all, of the portion of the optical fiber stripped of the coating protrudes forwardly of its support tube. One or more drops of a curable material, such as an adhesive, is placed on the stripped portion of the optical fiber at longitudinally spaced positions. The curable material is then allowed to cure and form a solid but resilient and compressible material. The refractive index (RI) of said curable material (RI glue ) is substantially less than RI of cladding (RI clad ). Owing to the total internal reflection (TIR) the occasional radiation which may propagate in cladding (cladding optical modes) will be blocked from leaving the cladding. 
     After the adhesive has cured, the optical fiber is then retracted within its supporting tube. In doing so, the cured drops of the adhesive or other curable material are sandwiched in between the optical fiber and its support tube thus isolating the optical fiber from its support tube in the desired fashion. After the optical fiber has been retracted to its operational position, the adhesive or other sealant with refractive index RI glue  smaller than RI clad  is then applied to the distal end of the optical fiber support tube thus sealing the distal end of the support tube and the optical fiber together and preventing contact between the support tube and the optical fiber including the prevention of parasitic radiation leakage from fiber cladding into inner space of support tube. 
     The support tube for the optical fiber is secured to a mount associated with the optical fiber in any conventional fashion, such as by an adhesive. However, in order to protect the portion of the optical fiber extending from the mount and to the radiation source, at least one, and preferably two or more sheaths are provided around the optical fiber. 
     Unlike the previously known sheaths, however, in the present invention the distal end of the innermost sheath is positioned around the support tube for the optic fiber and is positioned within an outwardly flared cavity formed on the optical fiber mount. A radially inwardly compressible shimming lock is inserted into the mold cavity. Upon doing so, the coaction between the mold and the shimming lock causes the shimming lock to compress radially inwardly thus sandwiching the distal end of the inner protective sheath in between the shimming lock and the support tube for the optical fiber. However, when desired, the shimming lock may be easily and rapidly removed. 
     In order to attach an outer sheathing around the optic fiber between the mount and the laser radiation source, a distal end of the outer sheath is pressed over an outwardly flared portion of the mount which causes the distal end of the outer sheath to flare outwardly. A compression ring is then pressed over the outwardly flared portion of the outer sheath thus sandwiching the distal end of the outer sheath in between the compression ring and the mount thus securing them together. The appropriate cone-like shape of inner cavity, of outwardly flared portion of mount and inner surface of compression ring are promoting the easy and reliable attachment of inner and other light plastic sheaths to the mount. 
     An adapter for supporting the optical cable assembly at spaced locations is also provided. The adapter is generally cylindrical in shape having one end adapted for attachment to a structure, such as the mount for the optical fiber assembly. An outer protecting plastic sheath/tube is positioned within the other end of the adapter and surrounded by a pair of conical inserts which are radially movable relative to each other. Each insert includes a conical surface. 
     A nut is threadably mounted to the end of the adapter. This nut includes an inner conical surface which co-acts with the conical surface of the insert as the nut is tightened. Consequently, as the nut is tightened, the nut forces the inserts radially inwardly and towards the support tube. A resilient gasket is preferably provided between the inserts and the outer protecting sheath so that, upon tightening of the nut, the said outer sheath is firmly attached to the adapter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       A better understanding of the present invention will be had upon reference to the following detailed description when read in conjunction with the accompanying drawing, wherein like reference characters refer to like parts throughout the several views, and in which: 
         FIG. 1  is a fragmentary side longitudinal sectional view illustrating a preferred embodiment of the invention; 
         FIG. 2  is a view similar to  FIG. 1 , but illustrating the fiber optic retracted into its support tube; 
         FIG. 3  is a longitudinal sectional view illustrating the attachment system for the protective sheaths for the fiber optic assembly; 
         FIG. 4  is a view illustrating the shimming lock used to attach the protective sheaths; 
         FIG. 5  is a longitudinal sectional view of an adapter and associated fiber optic assembly; 
         FIG. 6  is a view similar to  FIG. 5 , but illustrating the adapter in its locked position; and 
         FIG. 7  is an elevational view of the locking insert used in connection with the adapter. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
     With reference first to  FIG. 1 , a portion of an optical fiber assembly  20  in accordance with the present invention is shown. The assembly  20  includes an optic fiber core  22  with typical diameter 10-30 μm which conveys the radiation from a laser  24  to a distal end  26  of the optic fiber core  22 . 
     A cladding  28  with typical diameter 100-500 μm and refractive index RI clad  smaller than refractive index of core RI core  is provided around the fiber core  22 . A polymer coating  30 , typically made from material having smaller refractive index RI coat  than refractive index of cladding RI clad , is then provided around the cladding  28 . 
     An endcap  32  is attached to the distal end  26  of the optic fiber  22  with cladding  28 . Since high heat is used to attach the endcap  32 , a portion  34  of the outer coating  30  is typically stripped away thus leaving a stripped portion  34  of the cladding  28 . 
     Still referring to  FIG. 1 , the optic fiber with core  22  and with its cladding  28  and coating  30  is positioned within a support tube  36 . Typically the diameter of tube  36  is some hundreds of microns and weight of the tube in range of tens of milligrams. The support tube  36  extends through a mount  38  associated with the fiber optic core  22 , cladding  28 , and polymer coating  30 . 
     In order to prevent contact between the cladding  28  of the optic fiber assembly  20  and its support tube  36 , one or more drops  40  of a curable material, such as adhesive with refractive index RI glue  smaller than refractive index of cladding RI clad , are placed on the stripped portion  34  of the optic fiber assembly  20 . These drops  40  are allowed to cure thus forming a compressible, resilient, and solid material. Two or even more spaced drops of the curable material  40  can be placed or the stripped portion  34  of the fiber optic assembly  20 . 
     The optical fiber assembly  20  is first positioned relative to a support tube  36  so that the stripped portion  34  protrudes outwardly from the distal end of the support tube  36 . 
     With reference now to  FIG. 2 , after the curable material  40  has cured, most of the stripped portion  34  of the optical cable assembly  20  is retracted back into the support tube  36 . In doing so, the drops  40  of curable material compress in between the cladding  28  for the assembly  20  and the support tube  36  thus isolating the cladding  28  from the support tube  36  and effectively preventing contact of the cladding  28  with the support tube  36  and leakage of parasitic radiation from cladding owing to total internal reflection on boundary between adhesive and cladding. Typically the thickness of curable material layer  40  is in range of tens of microns, and the placing of drop of material  40  as well as retracting the assembly  20  back into supporting tube  36  is accomplished under microscope. 
     After retraction of the stripped portion  34  of the fiber optic assembly  20  into the support tube  36  to the position shown in  FIG. 2 , a further drop  42  of adhesive material is preferably applied to the distal end of the support tube  36  and the cladding  28 . The refractive index RI 42  of this drop adhesive  42  is less than RI clad  providing the optical insulation of cladding optical modes. This additional drop  42  not only ensures that the cable assembly  20  will not contact its support tube  36 , but also serves to support the cable assembly  20  against wobbling which might otherwise occur during high frequency movement in many kHz range of the endcap  32  of the type typically used to focus the plurality of laser beams on the target in military applications. A drop  43  of adhesive material is additionally applied to the proximal end  37  of support tube  36  to ensure the further stopping the fiber optic assembly  20  from wobbling inside of support tube  36 . The refractive index RI 43  of drop  43  is typically less than RI coat  of polymer coating  30 . 
     With reference now to  FIG. 3 , the mount  38  for the fiber optic assembly  20  includes a through passage  44  in which the support tube  36  extends and is secured in place by an adhesive  46  or screw. This passage  44 , furthermore, includes an outwardly flared cavity  51  at its proximal end. 
     In order to protect the fiber optic extending between the mount  38  and the laser radiation source  24 , a first protective sheath  50  has its distal end  52  positioned within the cavity  51  and thus around the support tube  36  for the cable assembly  20 . This protective sheath  50  has the refractive index RI 50  smaller than the refractive index of polymer coating  30  providing the additional protection from occasional optical leakages from polymer coating  30  owing to total internal reflection between polymer coating  30  and sheath  50 . Preferably the Teflon tube can be used due to that refractive index of Teflon ˜1.31 is among the smallest refractive indices for polymers. As best shown in  FIGS. 3 and 4 , a shimming lock  54  includes at least two, and preferably four or more, axially extending slots  56  which permit the shimming lock  54  to deflect radially inwardly. Consequently, upon insertion of the shimming lock  54  to the position shown in  FIG. 3 , the outwardly flared cavity  51  co-acts with the slotted end  56  of the shimming lock  54  thus causing fingers  58  on the shimming lock  54  caused by the slots  56  to deflect radially inwardly. In doing so, the shimming lock fingers  58  sandwich the free distal end  52  of the sheath  50  in between the shimming lock  54  and the support tube  36  for the optic fiber assembly  20 . Consequently, the sheath  50  is firmly, but removably, attached to the mount  38 . If required, the sheath  50  could be easily removed by simply removing the shimming lock  54 . 
     Typically the shimming lock has diameter around 1 mm and weight 30-50 milligram, providing non-significant weight load to fiber assembly. 
     With reference now particularly to  FIG. 3 , an outer sheath  60  is also provided around the inner sheath  50  for additional protection of the optic fiber assembly  20 . This outer sheath  60  is pressed over an outwardly flared portion  62  of the mount  38 . In doing so, a distal end  64  of the outer sheath  60  is flared outwardly. In order to lock the distal end  64  of the outer sheath  60  to the mount  38 , a compression ring  66  having an inner surface, which matches the outer surface  62  of the mount, is compressed against the mount  38  thus sandwiching the distal end  64  of the outer sheath  60  in between the compression ring  66  and the mount  38 . In doing so, the outer sheath  60  is firmly, but removably, secured to the mount  38 . Typically the compression ring  66  has diameter 3-5 mm and weight 50-100 milligrams, providing non-significant gravity load on fiber assembly. 
     With reference now to  FIGS. 5 and 6 , the outer plastic sheath  80  is added above of plastic sheath  60  for increase of overall robustness and further protection of fiber optic assembly. Optionally, an aramid (Kevlar) layer  68  is contained in between the sheath  60  and outer sheath  80  as it made for instance in commonly used furcation tubing in fiber optic industry. The aramid (Kevlar) layer  68  provides a further protection of fiber assembly  20 . 
     Still with reference to  FIGS. 5 and 6 , an adapter  70  which may be used to provide structural support of the optical fiber assembly  20  at any position along its length is shown. This adapter  70  firmly connects the outer plastic sheath  80  with mount  38  by gentle clamping of outer sheath  80  without stresses to inner sheaths  60  and  50 , hence without stress to fiber  20 . The adapter  70  includes a generally tubular and cylindrical body  71  thus forming a through passageway  75  through which the fiber assembly  73  extends (including fiber core  20 , cladding  28 , polymer coating  30 , Teflon tube  50 , furcation tubing  60 - 68 - 80 ). One end  72  of the adapter body  71  is attached to a structure  74 , such as the mount  38 , in any conventional fashion, such as by a threaded connection as shown in  FIGS. 5 and 6 . Other mechanisms for securing the adapter  70  to the structure  74  may alternatively be used. 
     Still referring to  FIGS. 5-7 , a pair of inserts  76  and  78  is disposed around an outer sheath  80  for the fiber assembly  73 . These inserts  76  and  78 , furthermore, are movable radially relative to each other. 
     Optionally, a compressible gasket  82  is positioned around the outer sheath  80  in alignment with the inserts  76  and  78 . The inserts  76  and  78  are, furthermore, at least partially contained within an end  84  of the adapter body  71 . In addition, the inserts  76  and  78  each includes a longitudinally extending slot(s)  86  which cooperate(s) with tabs  88  on the adapter body  71  to prevent rotation of the inserts  76  and  78  relative to the adapter  70 . 
     As best shown in  FIGS. 5 and 6 , a nut  90  threadably engages the adapter body  71 . This nut  90  includes an annular conical surface  92  which cooperates with a conical surface  94  on the inserts  76  and  78 . Consequently, upon tightening of the nut  90 , the nut  90  moves the inserts  76  and  78  radially inwardly from the position shown in  FIG. 5  to the position as shown in  FIG. 6 . This in turn compresses the gasket  82  against the outer sheath  80  and firmly secures the outer sheath  80  to the adapter  70 . Typically the weight of adapter is of the order of 2-3 grams, which is small fraction ˜10% of weight of mount  38 . 
     Similar adapter  70 -R in reverse orientation (“second adapter”) can be used for protection of outer sheath  80  on a remaining distance from adapter  70  to radiation source  24 . The end  72 -R of second adapter is connected to distal end of metal conduit above (outwardly) the outer plastic sheath  80 . When the nut  90 -R of second adapter  70 -R is not tightened the proximal end of metal conduit can be easily glided and attached to radiation source  24 . After tightening the nut  90 -R in second adapter  70 -R the distal end of metal conduit is firmly attached to plastic sheath  80  in position, where fiber assembly  20 , inner Teflon tube  50  and inner sheath  60  are in stress-free condition from endcap  32  to laser  24 . 
     From the foregoing, it can be seen that the present invention provides several improvements over the previously known cable assemblies of the type used in military applications. Having described my invention, however, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.