Abstract:
An optical fiber having a thin layer of gold positioned between the core and cladding. The gold layer is vacuum deposited on a rotating clean glass rod which will become the fiber core. The rod is inserted into a tube that will form the cladding of the fiber. The tube is sealed and placed in a hot tin bath inside a stainless steel pressure chamber that is pressurized and heated to collapse the cladding around the gold-coated core, thereby forming a fiber perform that may be pulled to form the gold metal cylinder fiber of the present invention. The fiber may be cleaved at one end and etched to expose a gold cylinder, thereby forming a highly responsive sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/715,537, filed Sep. 9, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to optical fibers and, more specifically, to an optical fiber having improved characteristics.  
         [0004]     2. Description of the Related Art  
         [0005]     An optical fiber is a cylindrical dielectric waveguide, usually made of glass, that transmits light along its axis by the process of total internal reflection. The fiber generally consists of a denser core surrounded by a cladding layer and is made by constructing a large-diameter preform that is pulled to form a long, thin optical fiber. Although optical fibers are used primarily for the transmission of communications, optical fibers have been used as sensors to measure strain, temperature, pressure and other parameters. The light absorption spectra and light intensity dependence of conventional optical fibers, however, limit their utility for such applications.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     It is therefore a principal object and advantage of the present invention to provide an optical fiber having an improved light absorption spectrum.  
         [0007]     It is an additional object and advantage of the present invention to provide an optical fiber having improved light intensity dependence.  
         [0008]     It is a further object and advantage of the present invention to provide an optical fiber that may be used as a sensor.  
         [0009]     It is another object and advantage of the present invention to provide an process for manufacturing improved optical fibers.  
         [0010]     In accordance with the foregoing objects and advantages, the present invention comprises an optical fiber having a thin layer of gold positioned between the core and cladding. The gold layer is vacuum deposited on a rotating clean glass rod which will become the fiber core. The rod is inserted into a tube that will form the cladding of the fiber. The tube is sealed and placed in a hot tin bath inside a stainless steel pressure chamber that is pressurized and heated to collapse the cladding around the gold-coated core, thereby forming a fiber perform that may be pulled to form the gold metal cylinder fiber of the present invention. The fiber may be cleaved at one end and etched to expose a gold cylinder, thereby forming a highly responsive sensor having various uses. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a perspective view of a gold metal fiber according to the present invention.  
         [0013]      FIG. 2  is schematic of a hot tin bath according to the present invention.  
         [0014]      FIG. 3  is a perspective view of a pressure vessel according to the present invention.  
         [0015]      FIG. 4  is a schematic of an ampoule according to the present invention.  
         [0016]      FIG. 5  is a schematic of a fiber drawing tower according to the present invention.  
         [0017]      FIG. 6  is a perspective view of a first embodiment of a gold metal fiber sensor according to the present invention.  
         [0018]      FIG. 7  is a perspective view of a second embodiment of a gold metal fiber sensor according to the present invention.  
         [0019]      FIG. 8  is a schematic of a sensing process according to the present invention.  
         [0020]      FIG. 9  is a schematic of a gold metal fiber sensor system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Referring now to the drawings, wherein like reference numerals refer to like parts throughout, there is seen in  FIG. 1  an optical fiber  10  according to the present invention. Fiber  10  comprises a glass core  12 , a glass cladding  14 , and a layer of gold  16  disposed between core  12  and cladding  14 .  
         [0022]     Fabrication of fiber  10  starts with the vacuum deposition of a gold film  18  on a rotating 0.5 mm diameter glass rod  20 . Before deposition, glass rod  20  is thoroughly cleaned. Glass rod  20  rotates during the deposition process so that a uniform gold metal film  18  is deposited thereon. Glass rod  20  eventually becomes core  12  of fiber  10 . The same gold metal film  18  may be deposited on flat glass pieces so that it may be measured. The thickness of the gold metal film  18  on rod  20  is equal to 1/π times the thickness of the deposited film on flat glass pieces. Thus, it is possible to determine the thickness of the deposited gold metal film  18  on core rod  20 .  
         [0023]     Coated glass rod  20  is removed from the vacuum system and inserted into a glass tube  22  having a 1.5 mm inside diameter and a 6.3 mm outside diameter that has been sealed at one end. Tube  22  will eventually form cladding  14  of fiber  10 . The gold will not be affected by transporting it through the air between the vacuum system and cladding tube  22 . Before core rod  20  is placed into cladding tube  22 , cladding tube  22  is thoroughly cleaned.  
         [0024]     A Corning type 7056 glass with a softening point of 702° C. and an index of refraction of 1.487 may be used for core rod  20  and a Corning type 7052 glass with a softening point of 712° C. and an index of refraction of 1.484 for the cladding tube  22 . Alternatively, Corning type 7440 (“Pyrex”) glass with a softening point of 821° C. and an index of refraction of 1.474 may be used for both the core rod  20  and cladding  22 . In the later case, gold film  18  would have to provide some guiding.  
         [0025]     After placing coated core rod  20  into cladding tube  22 , cladding tube  22  is evacuated and sealed to form an ampoule  24 . Ampoule  24  is placed into a boat  26  filled with tin (Sn) solder  28 , as seen in  FIG. 2 . Boat  26  is placed into a stainless steel pressure chamber  30  is then closed and pressurized to about 27 atmospheres (about 400 lbs per square inch) before preheating in a preheat furnace (not shown) to a temperature of 300 degrees Celsius to melt Sn solder  28 , as seen in  FIG. 3 . Ampoule  24  floats in the liquid Sn solder  28 , heating it uniformly along its length.  
         [0026]     Stainless steel pressure chamber  30  is then moved into a second furnace (not shown) set to a temperature of 630 degrees Celsius (for the lower temperature type 7052 or 7056 glass). When ampoule  24  reaches a temperature of 620 degrees Celsius, cladding tube  22  collapses onto the core rod  20 , trapping the thin gold film  18  between them, as seen in  FIG. 4 . Collapsed ampoule  24  should have a diameter of 6.139 mm. Note that the collapsing occurs at a temperature well below the softening temperature of the glass. While the collapsing process is not visible while ampoule  24  is in pressure chamber  30 , theory suggests that the collapsing process occurs very rapidly once the glass has reached the proper temperature because, at that temperature, the glass has the proper viscosity for collapse.  
         [0027]     The collapsing of ampoule  24  forms a fiber perform  36 . Pressure chamber  30  containing perform  36  is slowly returned to preheat furnace  32 . This movement should take 4 to 6 hours, thereby annealing perform  36  in the process. The glass cools while floating in the liquid Sn. This assures that perform  36  remains straight while the glass hardens.  
         [0028]     Preform  36  is preferably about 20 cm long at this point. Next, glass handles (not shown) that are each about 30 cm long are attached to each end of perform  36 . Preform  36  is mounted in a fiber drawing tower  40 , as seen in  FIG. 9 . Fiber  10  is then drawn from perform  36  in a fiber pulling tower, as seen in  FIG. 5 . A force of 200 grams is applied to fiber  10  during the pulling process. Fiber  10  is pulled at a temperature of 630 degrees Celsius for the type 7052 or 7056 glass with the lower softening point. At this temperature, the glass has a higher viscosity than the gold. Thus, the gold is extruded from a thickness of 0.1 μm in preform  36  to a thickness of 2.06 nm in fiber  10  by the surrounding glass. If the viscosity of the glass is less than the viscosity of the gold, the gold will tear during the fiber pulling process. The 0.5 mm diameter core of the preform is drawn to a 10.03 μm core in fiber  10  and the 6.139 mm outside diameter of preform  36  becomes the 126.4 μm outside diameter of fiber  10 .  
         [0029]     Fiber  10  according to the present invention may be used as a sensor. Since one can etch glass without etching the gold metal layer positioned between core  12  and cladding  14 , it is possible to construct a fiber  10  with a protruding very thin hollow gold cylinder  42 , as seen in  FIG. 6 . These devices are made by first fabricating fiber  10  with a gold layer between core  12  and cladding  14 , as explained above. An end of fiber  10  is then cleaved to obtain a flat uniform surface. Next, the cleaved end of fiber  10  is submerged in hydrofluoric acid. The acid will etch away some of the glass, leaving a protruding hollow gold cylinder  42 .  
         [0030]     An alternate arrangement is to fabricate a fiber  10  with two or more thin gold cylindrical arc sections  44  at the core cladding boundary. One end of fiber  10  is then cleaved to obtain a flat uniform surface. Next, the cleaved end of fiber  10  is submerged in hydrofluoric acid. The acid will etch away some of the glass leaving protruding gold cylindrical arc sections  44 , as seen in  FIG. 7 .  
         [0031]     The protruding gold cylinder  42  or cylindrical arc sections  44  of fiber  10  may be used as strain or fluid flow sensors. Since the very thin sections are easily deflected these can be very sensitive detectors. These devices can be also used as pressure sensors. They will sense any pressure induced strain in a material in which they are imbedded.  
         [0032]     The lowest order modes propagate through gold layer  16  and the glass immediately next to gold layer  16 . Thus, any light reflected from the end with the protruding gold structures  42  or  44  will be very sensitive. Nanometer scale structures, such as molecules, can be loaded onto the protruding gold sections  42  or  44  of the fiber. A light beam can be sent through fiber  10 . Some of the light will be reflected from the fiber end containing the gold structures  42  or  44 . Since light propagates through gold film  16  it will carry back information about the material placed on gold structures  42  or  44 .  
         [0033]     The molecules and the protruding gold structures  42  and  44  can also be subjected to electrical, magnetic or stress fields and the change do to these effects can be analyzed by the reflected light. The sample molecules can be loaded onto fiber  10  by coating a glass slide  48  with a thin film of suspension containing the molecules to be tested. The protruding gold structures  42  or  44  of fiber  10  are dipped carefully into the suspension film on glass slide  48 , as seen in  FIG. 8 . Before measuring, fiber  10  is pulled away from the glass slide, thereby depositing a small amount of the suspension with the molecules on the gold structures  42  or  44  protruding from the fiber.  
         [0034]     A sensor system  50  using a gold metal fiber  10  according to the present invention is seen in  FIG. 9 . Light from a laser  52  propagates along fiber  54  to a sensor  56  (i.e., fiber  10  having extending cylindrical gold cylinder or cylindrical sections) through a directional coupler  58 . The reflected light from sensor  56  is directed by directional coupler  58  to a detector  60 . If the distance between directional coupler  58  and sensor  56  is long, a standard single mode fiber may be used between a short piece of fiber  10  and directional coupler  58 .