Abstract:
An optical fiber comprising non-silica, specialty glass that has multiple fiber cores arranged in a square registered array. The fiber cores are “registered” meaning that the array location of any fiber core is constant throughout the entire length of the fiber, including both ends. Optical fiber bundles are fabricated by combining multiple multi-core IR fibers with square-registration. Also disclosed is the related method for making the optical fiber.

Description:
PRIORITY CLAIM 
       [0001]    This Application claims priority from U.S. Provisional Application No. 61/665,446 filed on Jun. 28, 2012 by Dan Gibson et al., entitled “Multi-Core Optical Fibers for IR Image Transmission,” the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to infrared fiber optics and, more specifically, to infrared sensing and imaging in the 2-12 μm region. 
         [0004]    2. Description of the Prior Art 
         [0005]    Imaging fibers and fiber bundles have been demonstrated for visible light using silica glass, silicate glasses and polymers and for infrared light using chalcogenide, fluoride and germanate glasses as well as metal-coated hollow waveguides. See, e.g., Gibson et al., “Transmission properties of hollow glass waveguides,” Proc. SPIE, vol. 3849, pp. 143 (1999); Wang et al., “Fabrication and experimental observation of monolithic multi-air-core fiber array for image transmission,” Opt. Exp., vol. 16, pp. 7703-7708 (2008); Sanghera et al., “Infrared fiber imager,” U.S. Pat. No. 6,175,678 B1 (2001); and Saito, “Method of producing infrared image guide,” U.S. Pat. No. 4,652,288 (1987), the entire contents of each are incorporated herein by reference. 
         [0006]    Fiber bundles are an assembly of individual fibers, where the fibers have a core and at least one cladding, and usually a protective coating. The individual fibers comprising a bundle are fused to each other, through adhesives, thermal bonding, mechanical or other means, at the bundle ends, and non-fused, or separate from each other, in between. Typically the fused length of the bundle is short (&lt;5 cm) relative to the bundle length (1-10 m or more) and the entire bundle is encased in a protective sheath. Oftentimes, the fibers, where they are not fused together, are separated by either air or a lubricating gel for mechanical protection. Fiber bundles can be characterized as coherent, where the individual fibers are spatially registered at both ends for image transfer applications, or incoherent, where the fibers are not intentionally registered for applications where spatial resolution is not important like illumination or power transmission. The most common arrangement of assembly in a fiber bundle is hexagonal packing to maximize the active area of the bundle. Imaging fibers serve a purpose similar to coherent fiber bundles, but are characterized as a single fiber having multiple cores. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an optical fiber comprising non-silica, specialty glass that has multiple fiber cores arranged in a square registered array. The fiber cores are “registered” meaning that the array location of any fiber core is constant throughout the entire length of the fiber, including both ends. Infrared imaging fiber bundles have been fabricated from single-core IR fibers, but this is the first coherent multi-core IR fiber with square-registration for IR imaging. Also disclosed is the related method for making the optical fiber. 
         [0008]    The IR imaging fibers described in this disclosure are unique with several novel features. The fibers are coherent and square-registered with consistent inter-core spacing, even across fiber joints when used in fiber bundles. This allows for direct coupling of the imaging fiber to emitter arrays or detector arrays, which also have consistent array pitch. Systems under test using these fibers may couple 1 emitter directly to 1 fiber core, or 1 emitter to 4 cores, etc. 
         [0009]    Additionally, the fibers employ a cross-talk reducing inter-core barrier to eliminate cross-talk between cores. Multi-core imaging fibers have an inherent trade-off between throughput and blur (crosstalk). Throughput is maximized when the core is large, but large cores necessitate small inter-core spacing which induces crosstalk. By introducing a very thin barrier in the cladding, between adjacent cores, crosstalk is practically zero in the fibers of the present invention, despite the rather large cores. The thin polymer barrier also serves to protect the outer surface of the fibers, enhancing durability and flexibility for fiber bundles. 
         [0010]    The fibers and bundles of the present invention have distinct advantages over the infrared fiber imaging bundle comprised of many individual fibers each having a single core as described in U.S. Pat. No. 6,175,678 to Sanghera et al. The multi-core fibers have a high core packing density, many cores per unit area, which enables high optical throughput and direct 1-to-1 coupling to sensor and emitter arrays. The fibers of the present invention have a polymer film woven into the cladding matrix that permits larger active area (large core diameter to core pitch ratio) by reducing cross-talk. Additionally, the fibers of the present invention can function as modules in larger fiber bundles. Moreover, the method for making the fibers of the present invention is more amenable to square registered arrays than the round outer-shaped fibers described by Sanghera, et al. due to the square outer-shaped preforms and fibers. 
         [0011]    These and other features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description, appended claims, and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram of the cross section of a fiber.  FIG. 1(   a ) shows a general view, and  FIG. 1(   b ) shows a detailed view of the portion of  FIG. 1(   a ) within the dashed lines. 
           [0013]      FIG. 2  shows a fiber having a proximal end and a distal end and includes a cross sectional diagram of each end. 
           [0014]      FIG. 3  is a diagram of the cross section of a fiber showing a polymer webbing between each adjacent core and around the outer surface of the fiber. 
           [0015]      FIG. 4  shows a fiber bundle consisting of nine multi-core square-registered coherent imaging fibers, each comprising 25 cores and includes a cross sectional diagram of each end of the fiber bundle. 
           [0016]      FIG. 5(   a ) shows an optical micrograph of an illuminated end face of a 64-core, square registered coherent IR imaging fiber.  FIG. 5(   b ) shows the cross-talk for this fiber. 
           [0017]      FIG. 6(   a ) shows an optical micrograph of an illuminated end face of a 64-core, square registered coherent IR imaging fiber with crosstalk reducing barrier.  FIG. 6(   b ) shows the cross-talk for this fiber. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The IR imaging fiber of the present invention and the method to make it are novel and have unique features. The fiber is comprised of a non-silica glass, specifically a chalcogenide glass, and more specifically an arsenic sulfide-based glass. 
         [0019]    As shown in  FIG. 1 , the fiber has a square cross sectional shape  101  invariable in shape and dimension along the fiber length. The fiber has multiple fiber cores  102  arranged in a regular rectangular lattice, running the entire length of the fiber. The spacing between any two adjacent cores  103  is constant and double the distance  104  between any core along the fiber perimeter  105  and the outer surface of the fiber  106 . The cores  102  may be round, approximately round, square, or some other shape. 
         [0020]    As shown in  FIG. 2 , the fiber has a proximal end  201  and a distal end  202 . The cores of the fiber are coherently registered such that each core (e.g.  203 ,  204 ,  205 ) is in the same relative position at the proximal end  201 , the distal end  202 , and everywhere along the length of the fiber between the ends. 
         [0021]    In some embodiments, the fiber has a polymer webbing  301  between each adjacent core and around the outer surface of the fiber as shown in  FIG. 3 . This polymer serves to mechanically protect the outer surface and strengthen the fiber. It also reduces cross-talk by absorbing any light leaking from one core thus preventing it from entering another core. In a preferred embodiment, the polymer is polyethersulfone. 
         [0022]    As shown in  FIG. 4 , the imaging fibers of the present invention can function as building blocks for a larger IR imaging fiber bundle  401 . The fiber bundle in  FIG. 4  consists of nine multi-core square-registered coherent imaging fibers, each comprising 25 cores. The fiber bundle is also square registered and coherent, meaning that the individual fibers are in the same relative spatial position and rotation at the proximal  402  and distal  403  ends. By fusing the imaging fibers over a short length  404 ,  405  at the ends only, large bundles are possible while maintaining flexibility. Since the inter-core spacing within the fiber  103  is exactly double the core to fiber perimeter distance  104 , bundles assembled from this fiber have a consistent inter-core spacing across the entire array, including near the fiber joints  407 . 
         [0023]    These fibers are fabricated using a multi-step extrusion and preform-draw process. First, cladding tubes with a square outer shape and a single round hole are extruded from an IR transparent glass. The tube width is approximately 10-20 mm and the hole is approximately 8-18 mm in diameter. 
         [0024]    Second, a solid, round IR glass core rod is cast, for example in a silica ampoule. The diameter of the core rod (approximately 7.9-17.9 mm) is slightly smaller than the hole of the cladding tube. The glass comprising the core rod has a slightly different composition than the glass comprising the cladding tube, such that it has a larger refractive index. This index contrast determines the numerical aperture of the imaging fiber. 
         [0025]    Third, the core rod is inserted into the cladding tube, forming a core-clad preform assembly. At this time, a thin (about 10-100 μm thick) layer of polymer film may be applied to the outer surface of the cladding tube and become a part of the core-clad preform assembly, if it is desired to have a cross-talk reducer in the final fiber. The core-clad preform assembly is now consolidated by fusing the components at an elevated temperature. A self-squaring press may be used during this step to ensure the outer shape of the core-clad preform does not deform, or for correcting the outer shape of an imperfect preform. A vacuum may optionally be used during this step to ensure no gaps at the core-clad interface or the clad-polymer interface. 
         [0026]    Fourth, the consolidated core-clad preform is stretched into cane, for example on a fiber optic draw tower, to widths smaller than the preform (around 0.5-2 mm). 
         [0027]    Fifth, short lengths (about 4-40 cm) of cane are assembled into a registered preform by stacking them in a squaring press. Care is taken to not impart any twist or crossing among the canes. 
         [0028]    Sixth, the registered preform is consolidated by simultaneously heating and pressing the preform. The pressing is best done using a self-squaring press and applying equal force from all 4 sides of the square registered preform. The ends of the preform may be constrained, but pressing on the ends is not required. 
         [0029]    Seventh, the consolidated registered preform is drawn on a fiber optic draw tower into a coherent imaging fiber using standard fiber drawing practices. The fiber typically has a width of about 100-1000 μm. 
       Example 1 
       [0030]    Example 1 is a 25-core, square-registered coherent IR imaging fiber and is shown schematically in  FIGS. 1 and 2 . The individual cores  102  are comprised of As-39%-S-61% glass and are surrounded by a continuous glass cladding matrix comprised of As-38%-S-62% glass. The core diameter is approximately 40 μm. The core pitch, the center-to-center spacing between cores is approximately 50 μm. The fiber width is approximately 250 μm. 
       Example 2 
       [0031]    Example 2 is a 25-core, square-registered coherent IR imaging fiber with cross-talk reducing barrier  301 , the cross-section of which is shown schematically in  FIG. 3 . This barrier is comprised of a polymer film, specifically polyethersulfone (PES) and is approximately 0.5 μm thick. The individual cores are comprised of As-39%-S-61% glass and are surrounded by a cladding comprised of As-38%-S-62% glass. The core diameter is approximately 30 μm. The core pitch is approximately 42 μm. The fiber width is approximately 210 μm. 
       Example 3 
       [0032]    Example 3 is a 64-core, square registered coherent IR imaging fiber, an optical micrograph of an illuminated end face is shown in  FIG. 5(   a ). The cores are comprised of As-39%-S-61% glass and have diameters between 18 μm and 20 μm. The core pitch ranges from 38 μm-40 μm. The cladding is a continuous matrix comprised of As-38%-S-62% glass. The end face of this fiber measures 316 μm×325 μm. The cross-talk for this fiber is shown in  FIG. 5(   b ). 
       Example 4 
       [0033]    Example 4 is a 64-core, square registered coherent IR imaging fiber with crosstalk reducing barrier, an optical micrograph of an illuminated end face is shown in  FIG. 6(   a ). The diameters of the individual cores measure 18 μm-20 μm, and the core pitch ranges from 38 μm-40 μm. The end face of this fiber measures 316 μm×325 μm. The cross-talk for this fiber is &lt;1% and is shown in  FIG. 6(   b ). 
         [0034]    The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular.