Abstract:
A method of incorporating within a glass optical waveguide a material of interest having a property of interest that would be neutralized by exposure to molten glass includes combining pieces of a light-transmissive first glass with the material of interest. The combined first glass and material of interest are shaped within a container and heated to a temperature sufficiently high to cause the glass pieces and material of interest to mutually coalesce and form a light-transmissive core rod, but not high enough that the first glass melts and neutralizes the property of interest. A cladding tube is heated and fused about the core rod to define a mono rod. An optical waveguide through which light propagates by internal reflection, and which incorporates the material of interest, is defined when the cladding tube comprises a glass that renders the cladding of lower refractive index than the core rod.

Description:
PROVISIONAL PRIORITY CLAIM 
       [0001]    Priority based on Provisional Application Ser. No. 61/459,324 filed Dec. 10, 2010, and entitled “GLASS OPTICAL WAVEGUIDES INCORPORATING UNDISSOLVED MATERIALS OF INTEREST AND METHODS OF FABRICATING THE SAME” is claimed. The entirety of the disclosure of the previous provisional application, including the drawings, is incorporated herein by reference as if set forth fully in the present application. 
     
    
     BACKGROUND 
       [0002]    Embodiments and implementations of the present invention relate generally to internally-reflecting waveguides, such as optical fibers, and to waveguide arrays fabricated by adjacently fusing a plurality of such waveguides. 
         [0003]    Waveguide arrays such as optical fiber faceplates are widely implemented in imaging applications such as, by way of non-limiting example, x-ray imagers. In various such devices, one face of the faceplate is coated with a scintillating material that, when impinged upon by x-rays, emits radiation in another region of the electromagnetic spectrum (e.g., the visible region). In some cases, a scintillating material is incorporated within the constituent optical fibers of a waveguide array by coating at least one of the core bars (or rods) and the interior of the cladding tubes with the scintillating material before heating and drawing these components to form the “mono rods” that are eventually bundled, heated and drawn to form fused waveguide arrays. 
         [0004]    While methods such as those described above have proven useful in incorporating a scintillating material, or other material of interest, into the constituent waveguide elements of waveguide arrays, these methods result in the limited inclusion of the material of interest at the end faces of the constituent waveguide elements (e.g., optical fibers) and/or the interfaces between the mutually fused cores and claddings. It has been observed that the inclusion of a more even cross-sectional distribution of a selected material of interest within the cores of individual waveguide elements (e.g., optical fibers) would substantially enhance the performance of the specialized waveguide arrays in which these constituent elements are incorporated. However, the inclusion of any one of numerous selected materials of interest into the molten-glass batches used to form the core bars attendant to optical fiber fabrication is rendered impracticable by the fact that these materials, which are often of crystalline structure, readily dissolve in molten glass. With the dissolution of a material of interest comes the loss of the special property for which that material is being incorporated. 
         [0005]    Accordingly, there exists a need for a method of fabricating a glass-based, internally-reflecting optical waveguide having a light-transmissive core with a cross-sectional distribution of a material of interest characterized by a property of interest that would be lost in molten glass. 
       SUMMARY 
       [0006]    Implementations of the present invention are applicable to the fabrication of glass optical waveguides incorporating, in an undissolved state, a material of interest that is soluble in molten glass. Various aspects of the inventive process address the formation of an optical waveguide including a core with a relatively uniform cross-sectional distribution of the undissolved material of interest. Examples of optical waveguides that can be fabricated through alternative implementations of the inventive process are flexible and fused optical fibers and various light-transmitting and image-transmitting matrices (or arrays) made from a plurality of individual optical fibers (“monofibers”). More specifically, various implementations are applicable to the fabrication of specialized optical fiber image-conducting devices including, by way of non-limiting example, (i) elongated fused image bundles, including “straight-throughs,” image-rotating bundles (e.g., inverters), and fused tapers such as magnifiers/reducers and (ii) non-elongated image conductors such as optical fiber faceplates. Common to the fabrication of the aforementioned devices is the formation of monofibers, each of which monofibers is created by heating and collapsing a cladding tube around a core rod in accordance with the well-known rod-in-tube method. Plural monofibers are subsequently bundled, heated and drawn to form what is commonly referred to as a “multi.” Plural “multies” can then be subsequently bundled, heated and drawn to form a “multi-multi.” 
         [0007]    It is important to establish that, throughout the present specification, optical elements employed in the fabrication of individual waveguides and arrays are variously referred to as “rods,” “filaments” or “fibers,” for example. Although the terms “filament,” “fiber,” “rod,” “cane,” “bar,” and the like, may be regarded by a reader as having somewhat different meanings, these terms are used interchangeably for purposes of the specification and claims. Accordingly, for example, a description of the fabrication of a “rod” is considered a disclosing description for the fabrication of a “filament” and a “fiber,” by way of non-limiting example. Additionally, no particular cross-sectional geometry is implied in describing, for example, rods and tubes. Accordingly, it is to be understood that tubes and rods used to form monofiber rods in accordance with implementations and embodiments described herein can be of any cross-sectional geometry including, for example, round, square, regular-polygonal (e.g., pentagonal, hexagonal, octagonal, etc.) and irregular polygonal. The broad interpretative scope attributed to each of these terms, and similar terms, applies equally within the specification and claims. 
         [0008]    In accordance with a first illustrative implementation, a plurality of elongated glass filaments is provided. Each filament has opposed first and second ends and a side surface extending between the first and second ends. A material of interest is bound to the side surface of each filament. In one illustrative version, the binding of the material of interest to the filaments is achieved by coating the filaments with an adhesive binder (i.e., a bonding agent) and adhering the material of interest thereto. At least two filaments to which the material of interest has been bound are then bundled in side-by-side relationship in order to form a filament bundle. The filament bundle is heated and drawn through, for example, an optical fiber drawing tower in order to form a rod incorporating a cross-sectional distribution of the material of interest. 
         [0009]    In light of aforementioned limitations associated with previous methods relative to the incorporation of certain materials of interest in glass-based optical waveguides, and glass matrices or arrays fabricated from the same, various implementations of the present invention are well-suited for the inclusion, within glass optical waveguides, of crystalline materials that are susceptible to dissolution in molten glass, by way of non-limiting example. More generally, irrespective of whether a material of interest is itself dissolved, a desired property of interest might be lost from that material of interest if it were introduced into molten glass during the formation of “core bars” or “core rods” configured for use in the early stages of a standard “rod-in-tube” optical fiber fabrication process. Versions of the current process obviate the exposure of the material of interest to molten glass by heating bundled glass filaments coated with the material of interest to a temperature sufficiently high to cause the filaments to coalesce (e.g., mutually fuse), but not so high that the glass melts and the material of interest dissolves therein or the property of interest thereof is otherwise neutralized or lost. 
         [0010]    In an alternative implementation, filaments coated with the selected material of interest are bundled inside a containment tube defined by a wall having inner and outer surfaces. The filament bundle and the containment tube are then heated and drawn. As the containment tube and filament bundle are heated and drawn, the coated filaments mutually fuse to form a core rod around which the containment tube collapses and fuses. In at least one version, the material of interest is also bound to the inside surface of the containment tube prior to heating and drawing. 
         [0011]    In some versions of a method including the use of a containment tube, the filaments are fabricated from a first glass having a first refractive index and the containment tube is fabricated from a second glass having a second refractive index lower in magnitude than the first refractive index such that the waveguide resulting from heating, drawing and fusing the containment tube and filaments transmits electromagnetic energy by internal reflection. In such a case, the containment tube serves as both (i) a container or sleeve in which the filaments are bundled prior to heating and drawing and (ii) an optical cladding material that facilitates internal reflection of electromagnetic energy after heating and drawing. Accordingly, where applicable and appropriate as indicated by context, the terms “containment tube” and “cladding tube” may be used interchangeably in any of the summary, detailed description, and claims. As an alternative to the bundling of coated filaments in a containment tube, it will be readily appreciated that a set of bundled filaments can be heated and drawn to form a fused core rod and then, subsequently, one or more such fused core rods can be introduced into a cladding tube for heating and drawing to form an internally-reflecting optical waveguide. 
         [0012]    Optical waveguides incorporating alternative materials exhibiting disparate properties are of interest to persons involved in various fields of research, science and medicine. Accordingly, alternative versions of the inventive method can be used to incorporate into a glass optical waveguide materials of interest including, but not limited to, at least one of (i) a scintillator material, (ii) a metal, (iii) a refractory material, and (iv) an absorber material configured to absorb electromagnetic energy within a predetermined wavelength range. The use of scintillator materials is of particular interest in the fabrication of detectors used to indirectly detect the presence of energy within a first or primary wavelength range of interest by detecting, instead, energy within a secondary wavelength range of interest that is emitted from a scintillator material when energy within the primary wavelength range impinges upon the scintillator material. Specific scintillator materials emit photons visible to a human eye when impinged upon by x-ray photons and, therefore, such a material is useful in the fabrication of an x-ray detector for use in the medical industry, by way of non-limiting example. Examples of materials that scintillate when impinged upon by x-rays include compounds or mixtures containing at least one of (i) erbium (Er), (ii) cerium (Ce), (iii) lutetium (Lu) and (iv) gadolinium (Gd). Illustrative lutetium-containing materials include compounds of the form Lu x Si y O z  and Lu x O y , wherein “x” and “y” are variables. An example of a scintillating material containing gadolinium is Gd 2 O 2 S. BGO (bismuth germinate Bi 4 Ge 3 O 12 ) and LYSO (cerium-doped lutetium yttrium orthosilicate) can also be used as the scintillating material of interest. Although a limited set of non-limiting illustrative materials of interest has been provided, it is to be expressly understood that implementations of the invention are more generally concerned with methodologies of including a material of interest within an optical waveguide, and not with the specific material of interest. 
         [0013]    As is generally known, the relative refractive indices of core rods and claddings are important factors in forming internally-reflecting waveguides. Accordingly, in various versions, the material of interest used, for example, in coating filaments or shards, or mixing with powderized glass, is index-matched to the core glass. That is, care is taken to match as closely as practicable the refractive index of the material of interest with the refractive index of the core glass. Similarly, in some situations, such as when the inside surface of a cladding tube is coated with a material or interest, the refractive index of the material of interest might be matched with the refractive index of the glass from which the cladding tube is formed. 
         [0014]    Pursuant to the fabrication of various image-conducting, waveguide array products, plural mono-rod waveguides are bundled for heating and drawing in a “multi-draw” step. Once heated and drawn, the plural mono-rod waveguides are adjacently fused in a product that is, in the optical fiber industry, commonly referred to as a “multi”. In accordance with some alternative versions, plural “multies” are bundled for heating and drawing to form a “multi-multi” array. Multi arrays and multi-multi arrays formed in accordance with various implementations of the present invention can be treated, machined, polished and handled to form fused array products already known in the optical fiber industry. 
         [0015]    Although the formation of optical core rods from filaments coated with a material of interest is conducive to a particularly effective implementation of the present invention, alternative versions form core rods by combining a material of interest with alternatively shaped glass pieces. In one version, glass pulverized into particles such as coarse fragments, shards or fine powder is coated with the material of interest and then the coated glass particles are heated and drawn to form an optical core rod. Such glass particles are introduced into a glass cladding tube for heating and drawing into a fused mono rod in which the particles are mutually fused within and to the cladding tube. 
         [0016]    In a second version alternative to the use of coated glass filaments, a containment tube is at least partially filled with a plurality of plate-like glass pieces. Each glass plate has opposed first and second faces, at least one of which is coated with the material of interest. The glass plates are stacked within the cladding tube such that their faces extend along planes orthogonal to the longitudinal axis of the cladding tube. By way of illustrative example, in a case in which the cladding tube is cylindrical, the glass plates are disc-shaped and stacked within the tube in a manner analogous to which rolled coins are contained within a paper or shrink-wrap plastic sheath. With the glass plates stacked within the tube, the plates and cladding tube are heated and drawn such that the plates mutually fuse to form an optical core rod and the cladding tube collapses and fuses around the core rod. 
         [0017]    In a third version alternative to the use of coated glass filaments, an optical core rod, or constituent filaments used in the subsequent formation of an optical core rod, are fabricated by combining pulverized (e.g., powderized) glass with pulverized material of interest and then sintering the particulated mixture. As previously suggested relative to one illustrative version, the sintering can occur as the mixed particles are drawn in a containment tube. In an alternative version, mixed particles of glass and material of interest are combined in a mold to form a rod or filament. In one specific example of the latter version, the rod or filament is formed using hot isostatic pressing. Regardless, at least one of such rods or filaments is then situated in a containment tube for heating and drawing in the general manner hereinbefore described. 
         [0018]    As those of ordinary skill in the relevant art(s) will appreciate, care must be taken to prevent trapping gas (e.g., air) within a mono rod as it is heated and drawn. In implementations in which filaments are used to form core rods, gas is forced from between filaments, and out an end of the containment tube, as the filament-in-tube assembly is drawn. The forcing out of gas from between filaments is aided by the fact that the filaments are mutually aligned along the direction in which the filaments and tube are heated and drawn (i.e., along the “draw axis”). However, where glass particles or stacked glass plates are used, trapped gas may have to be forced out laterally, and then longitudinally, relative to the draw axis. Accordingly, a higher draw temperature and higher vacuum might be necessary in implementations in which the core rods are not formed from filaments. 
         [0019]    Representative implementations and embodiments are more completely described and depicted in the following detailed description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows a plurality of elongated glass filaments; 
           [0021]      FIG. 2  depicts the filaments of  FIG. 1  coated with a material of interest; 
           [0022]      FIG. 3A  shows a plurality of coated filaments, such as the filaments of  FIG. 2 , situated within a containment tube; 
           [0023]      FIG. 3B  depicts a fused mono rod formed by heating and drawing the bundled filaments and containment tube of  FIG. 3A ; 
           [0024]      FIG. 4A  depicts a plurality of adjacently bundled filaments that have been coated with a material of interest; 
           [0025]      FIG. 4B  depicts a core rod fabricated by heating the bundled filaments; 
           [0026]      FIG. 4C  illustrates the introduction into a cladding tube of the core rod of  FIG. 4B ; 
           [0027]      FIG. 4D  shows a fused mono rod formed by heating and drawing the core rod and cladding tube of  FIG. 4C ; 
           [0028]      FIG. 5  shows a plurality of rod-like optical waveguides bundled for heating and drawing in a fiber drawing tower; 
           [0029]      FIG. 5A  is a representative section of an illustrative fused multi-array fabricated by heating and drawing bundled the bundled optical waveguides of  FIG. 5 ; 
           [0030]      FIG. 5B  shows plural multi-arrays made bundled for heating and drawing; 
           [0031]      FIG. 5C  is a representative section of an illustrative fused multi-multi array fabricated by heating and drawing plural multi-arrays such as the multi-arrays of  FIGS. 5A and 5B ; 
           [0032]      FIG. 6A  depicts a plurality of plate-like glass pieces each of which is at least partially coated with a material of interest; 
           [0033]      FIG. 6B  depicts a plurality of plate-like glass pieces, such as those shown on  FIG. 6A , stacked within a glass containment tube; 
           [0034]      FIG. 6C  shows a fused mono rod formed by heating and drawing the stacked glass pieces and containment tube of  FIG. 6B ; 
           [0035]      FIG. 7A  schematically depicts the introduction of a particle mixture containing particles of glass and particles of a material of interest into a mold; 
           [0036]      FIG. 7B  show the mold of  FIG. 7A  in an open state; 
           [0037]      FIG. 7C  depicts a core rod formed by heating the particle mixture of  FIG. 7A  within the mold of  FIGS. 7A and 7B  such that the particles of glass and material of interest mutually coalesce; 
           [0038]      FIG. 8A  schematically depicts the introduction of a particle mixture containing particles of glass and particles of a material of interest into a containment tube; and 
           [0039]      FIG. 8B  shows a fused mono rod formed by heating and drawing the particle mixture and containment tube of  FIG. 8A . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    The following description of illustrative embodiments of glass-based optical waveguides, waveguide arrays and methods of fabricating the same is demonstrative in nature and is not intended to limit the invention or its application of uses. The various implementations, aspects, versions and embodiments described in the summary and detailed description are in the nature of non-limiting examples falling within the scope of the appended claims and do not serve to define the maximum scope of the claims. 
         [0041]    In conjunction with  FIGS. 1 through 8B , there are described alternative illustrative methods of fabricating a glass-based optical waveguide internally incorporating, in an undissolved state, a material of interest that is soluble in molten glass or otherwise possesses a property of interest that would be lost were the material of interest exposed to molten glass, irrespective of whether the material of interest dissolves therein. Shown in  FIG. 1  is a plurality of glass filaments  110 , each of which filaments  110  has longitudinally opposed first and second ends  114  and  116  and a side surface  118  extending between the first and second ends  114  and  116 . As depicted in  FIG. 2 , the side surface  118  of each filament  110  is treated (e.g., coated) with a material of interest M OI  having at least one property of interest P OI . In alternative implementations, at least two filaments  110  to which the material of interest M OI  has been applied are adjacently bundled in side-by-side relationship in order to form a filament bundle  140 . In one version, the coated filaments  110  are bundled within a containment tube  160 , as shown, for example, in  FIG. 3A . The containment tube  160  has opposed first and second ends  164  and  166  and is defined by a side wall  168  having inner and outer side surfaces  169 ; and  169   o . Optionally, the material of interest M OI  is applied to the inner side surface  169   i  of the containment tube  160 , in addition to the filaments  110 . As depicted schematically between  FIGS. 3A and 3B , the filament bundle  140  and containment tube  160  are heated and drawn through, for example, an optical fiber drawing tower which, owing to the ubiquitous use of such towers in the relevant art(s), is not shown. As the containment tube  160  and filament bundle  140  are heated and drawn, the coated filaments  110  mutually fuse to form a core rod  150  around which the containment tube  160  collapses and fuses to form a fused mono rod  180 , as shown in  FIG. 3B . 
         [0042]    In another, similar version, discussed with illustrative reference to  FIGS. 4A through 4D , glass filaments  110  are coated with a material of interest M OI  and bundled to form a filament bundle  140  in the general manner shown in  FIG. 4A . The bundled filaments  110  are then heated and drawn to form a fused core rod  150  such as that shown in  FIG. 4B . Referring to  FIG. 4C , in a procedure analogous to the well-known rod-in-tube method, the core rod  150  is axially introduced into a cladding tube  160  and the cladding tube  160  and core rod  150  are heated and drawn such that the cladding tube  160  collapses and fuses around the core rod  150  to form a fused mono rod  180 , as shown in  FIG. 4D . As with the version of  FIGS. 3A and 3B , the inner surface  169   i  of the containment tube can be coated with the material of interest M OI  prior to heating and drawing the core rod  150  and containment tube  160 . 
         [0043]    In alternative implementations, including those depicted in  FIGS. 3A through 3B  and  4 A through  4 D, the bundled filaments  110 , and resulting core rod  150 , are fabricated from a light-transmissive first glass G 1  having a first refractive index n 1  and the containment tube  160  is fabricated from a light-transmissive second glass G 2  having a second refractive index n 2  lower in magnitude than the first refractive index n 1  such that the fused mono rod  180  formed by heating, drawing and mutually fusing the containment tube  160  and filaments  110  is an optical waveguide  190  that transmits electromagnetic energy by internal reflection. 
         [0044]    In order to form an image-conducting waveguide array (e.g., optical fiber array), plural mono rods  180  in the form of waveguides  190  incorporating the material of interest M OI  are bundled, as shown in  FIG. 5 , for heating and drawing in a “multi-draw” step. After heating and drawing, the plural waveguides  190  are adjacently fused in a product referred to as a “multi” or multi array  200 , an illustrative section of which is shown in  FIG. 5A . Consistent with multi-draw steps associated with traditional fiber drawing methods, the cladding tubes  160  in the bundle in  FIG. 5  are, in the product of  FIG. 5A , fused into a contiguous glass matrix within which the optical core rods  150  are retained in mutually fixed positions. In accordance with some alternative versions, represented by  FIGS. 5B and 5C , plural multi arrays  200  are bundled for heating and drawing to form a “multi-multi” array  300 . 
         [0045]    As indicated in the summary, included within the scope and contemplation of the invention are implementations in which core rods and mono rods are formed by a method other than bundling, heating and drawing constituent filaments pre-treated with a material of interest M OI . According to one alternative implementation, described with initial reference to  FIG. 6A , a plurality of plate-like glass pieces  410  (alternatively, “glass plates  410 ”) is provided. Each glass plate  410  has opposed first and second faces  412   a  and  412   b , at least one of which is coated with the material of interest M OI . As illustratively depicted in  FIG. 6B , the coated glass plates  410  are adjacently stacked, in face-to-face relationship, within a containment tube  460  such that their faces  412   a  and  412   b  extend along planes (implied, but not shown) orthogonal to a containment-tube axis A CT  along which the length of the containment tube  460  extends. In the example of  FIG. 6B , the containment tube  460  is cylindrical. Accordingly, the glass plates  410  are disc-shaped and stacked within the tube  460  in a manner analogous to which rolled coins are contained within a paper or shrink-wrap-plastic sheath. 
         [0046]    As depicted schematically between  FIGS. 6B and 6C , the stacked glass plates  410  and containment tube  460  are heated and drawn through, for example, an optical fiber drawing tower, which is not shown for the reason previously stated. As the containment tube  460  and glass plates  410  are heated and drawn, the glass plates  410  mutually fuse to form a core rod  450  around which the containment tube  460  collapses and fuses to form a fused mono rod  480 , as shown in  FIG. 6C . In one illustrative implementation, the stacked glass plates  410 , and resulting core rod  450 , are fabricated from a light-transmissive first glass G 1  having a first refractive index n 1  and the containment tube  460  is fabricated from a light-transmissive second glass G 2  having a second refractive index n 2  lower in magnitude than the first refractive index n 1  such that the resultant fused mono rod  480  is an optical waveguide  490  that transmits electromagnetic energy by internal reflection. 
         [0047]    An alternative implementation involves the formation of an optical core rod, or the formation of constituent filaments used in the subsequent formation of an optical core rod, in a container such as a mold. For purposes of explanation, the illustrative fabrication of a core rod  550  is described with reference to  FIGS. 7A through 7C . As shown in  FIG. 7A , glass particles  510  and particles of a material of interest M OI  are combined to form a particle mixture  540 . The particle mixture  540  is introduced into a mold  570 . In one version, the particle mixture  540  is sintered within the mold  570  at a temperature sufficiently high that the glass particles  510  and the particles of the material of interest M OI  mutually coalesce, but not so high that the glass particles  510  melt and neutralize the property of interest P OI  for which the material of interest M OI  is being incorporated. In an alternative implementation, the particle mixture  540  is subjected to hot isostatic pressing. While isostatic pressing is frequently implemented within a chamber in which the object being formed is surrounded by a fluid, for illustrative purposes, this can still be regarded as a type of “molding” process and, therefore, reference to the mold  570  generically illustrated in  FIG. 7A  is sufficient to support a description of hot isostatic pressing as well as sintering. 
         [0048]    With reference to  FIGS. 7B and 7C , once the particle mixture  540  has sufficiently coalesced to define a self-supporting core rod  550 , the mold  570  is opened and the core rod  550  is removed. The core rod  550  can then be subsequently processed in, for example, a manner analogous to the manner in which the core rod  150  of  FIGS. 4B through 4D  is processed in order to form a fused mono rod  180 . 
         [0049]    In still an additional version described with initial reference to  FIG. 8A , glass particles  610  are combined with particles of a material of interest M OI  having a property of interest P OI  in order to form a particle mixture  640 . The particle mixture  640  is introduced into a containment tube  660 . As depicted schematically between  FIGS. 8A and 8B , the particle mixture  640  and containment tube  660  are heated and drawn. As the containment tube  660  and particle mixture  640  are heated and drawn, the particle mixture  640  coalesces to form a core rod  650  around which the containment tube  660  collapses and fuses to form a fused mono rod  680 , as shown in  FIG. 8B . In one illustrative implementation, the resulting core rod  650  is comprises a light-transmissive first glass G 1  having a first refractive index n 1  and the containment tube  660  is fabricated from a light-transmissive second glass G 2  having a second refractive index n 2  lower in magnitude than the first refractive index n 1  such that the resultant fused mono rod  680  is an optical waveguide  690  that transmits electromagnetic energy by internal reflection. 
         [0050]    The foregoing is considered to be illustrative of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired that the foregoing limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be resorted to that appropriately fall within the scope of the invention as expressed in the appended claims.