Abstract:
An apparatus and process for fabricating tubing used in the making of chalcogenide fibers. The apparatus features a three-sectioned melt/spin ampoule that allows for fabricating the chalcogenide tubing without introducing impurities and contaminants, in a self-contained apparatus.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fabricating waveguide materials for the fiber-based materials industry and, more particularly, to a novel ampoule that provides for a contaminant-free, chalcogenide glass tubing fabrication process. 
     BACKGROUND OF THE INVENTION 
     In the fabrication of waveguiding fibers for tele-communications, chalcogenide glasses (non-oxide glasses, based on sulfur, selenium and tellurium) are candidate materials for providing low loss transmission in the near to mid infrared. Moreover, rare-earth doped chalcogenide glasses are useful materials for the fabrication of efficient optical amplifiers and/or lasers. In particular, Pr-doped, (GeAs) sulfide glasses are considered promising materials for providing gain in the range of 1,300 nm, as taught by B. G. Aitken et al. in U.S. Pat. No. 5,389,584 for Ga- AND/OR In- CONTAINING AsGe SULFIDE GLASSES. At present, the best fiber design consists of a Pr-doped, Ga-codoped, GeAs sulfide core that is surrounded by a lower index GePAs or GeAs sulfide cladding, as taught by B. G. Aitken et al. in copending patent application, Serial No. 09/530,831, filed on May 3, 2000, and hereby incorporated by reference. 
     In order to fabricate rod-in-tube preforms of this material or other chalcogenide glasses that can be redrawn into fibers, a necessary first step requires making tubing from the selected cladding glass. The current method used to produce such tubing consists of a two step process. First, the batch material is loaded into a melting ampoule, and then melted and quenched to form a solid cylindrical glass rod. Then, the glass rod is loaded into a forming ampoule, melted, spun cast upon a mechanical lathe and quenched into glass tubing. 
     The solid glass rod can become contaminated during its removal from the “melting” ampoule, and also while transferring and loading it into the “forming” ampoule. For example, the surface of the pristine, solid glass rod can be contaminated with air-borne particles, silica pieces from the fractured “melting” ampoule, and other impurities. These adulterants can cause partial devitrification of the cladding glass during preform redraw, which results in fiber attenuation losses that are much greater than the theoretical minimum. 
     As a practical matter, a 1,300 nm amplifier fiber must have an attenuation loss no greater than 1 dB/m. Other applications, such as sensing or laser-power delivery, require even lower loss. Therefore, it would be immensely beneficial to eliminate any chance of introducing potential impurities during the chalcogenide fiber fabrication process. 
     The present invention reflects the discovery that spun-cast tubing of chalcogenide glass to be used as cladding or fiber can be fabricated directly from batch materials in a single apparatus process. The new, single apparatus process eliminates the transferring step. 
     The current invention provides a novel ampoule design that combines the melting and spinning steps within the ampoule, and hence prevents introduction of impurities. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an apparatus and method for fabricating chalcogenide glass tubing to be used in making fibers. The apparatus comprises a new melt/spin ampoule for melting and forming chalcogenide glass materials. The ampoule comprises three sections. The first section comprises a loading chamber that is open at one end and attached to a second chamber at its other end. The loading chamber is provided with a small external flange, whose orientation is coplanar with a bent drainage tube that is disposed in the second melting This second section is attached to, but separated from the third section (a forming or spin casting channel) by a septum that is fitted with the drainage tube. The drainage tube is centered in the septum and extends into the melting chamber. The tube is bent in an arc that terminates close to the side wall of the melting chamber. 
     The requisite raw materials for making the chalcogenide tubing, including but not limited to elemental Ge, As, P and S, are introduced into the open end of the first ampoule section. The loaded ampoule is evacuated and then sealed by heating the wall of the first section to collapse the tubing. The sealed ampoule is then placed in a rocking furnace in such a way that the external flange on the loading chamber and, hence, the open end of the drainage tube is located in an upright position. This procedure ensures that, at high temperature, and with the rocking motion (approximately ±30° from horizontal) of the furnace, molten glass is prevented from flowing into the forming chamber. 
     The ampoule is then heated to produce a homogeneous chalcogenide melt in the melting chamber. The heated ampoule containing the chalcogenide melt is then transferred to a vertical furnace operating at approximately the final melting temperature. The ampoule is suspended with an orientation that allows the chalcogenide melt to drain into the forming chamber. 
     The hot ampoule is then removed from the furnace, attached to a lathe, and spun in order to form the chalcogenide glass tubing. The ampoule is then removed from the lathe and quenched. This causes the chalcogenide glass tubing to delaminate from the ampoule wall, and prevents cracking as the ampoule cools to ambient temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawing, when considered in conjunction with the subsequent detailed description, in which: 
     THE FIGURE illustrates a schematic, front view of the ampoule used for fabricating chalcogenide glass, in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally speaking, the invention features a novel apparatus for fabricating chalcogenide tubing (e.g., sulfide tubing). The apparatus features a fused quartz, three-sectioned, melt/spin ampoule for fabricating the sulfide tubing without contamination, in a self-contained apparatus. It should be understood, however, that other materials can be used to form the ampoule  10 , such as Pyrex, Vycor, fused silica, or the like. 
     Now referring to THE FIGURE, the melt/spin ampoule  10  of this invention is shown. As aforementioned, the new melt/spin ampoule  10  is preferentially made from fused quartz or fused silica tubing. The ampoule  10  has three chambers  12 ,  14 , and  16 , respectively, and is 40 cm in overall length. The first section, or loading and sealing chamber  12 , is 15 cm long, made from 10 mm ID×16 mm OD tubing. This section  12  is open at its end  18 , and attached at its other end  20  to the second chamber  14 . The loading chamber  12  is provided with a small external flange  15 , that is oriented to be coplanar with a bent drainage tube  17  so that the apparatus  10  is oriented in the furnace in such a way as to prevent liquid from entering the tube  17  during the melting step. The drainage tube  17  is 4 mm ID×6 mm OD. 
     The second section  14  (melting chamber) is 10 cm long and fabricated from 26 mm ID×30 mm OD tubing. It is designed to accommodate a 35 to 45 gm batch of materials. Section  14  is attached to, but separated from the third section  16  (forming or spin casting chamber), by a septum  19 , that is fitted with the drainage tube  17 . The drainage tube  17  is centered in the septum  19 , and extends into the melting chamber  14 . The drainage tube  17  is bent in a 90° arc that terminates about 3 mm from the side wall  21  of the melting chamber  14 . The forming chamber  16  is 15 cm long and is made from 10 mm ID×16 mm OD tubing. The end  23  of this tube  16  is closed. 
     The requisite raw materials for making the sulfide tubing (e.g., elemental Ge, As, P and S) are introduced into the open end  18  of the first ampoule section  12  using a funnel (not shown). A plug (not shown) can be inserted into the proximal end of ampoule  10  to prevent backstreaming of batch material during evacuation and sealing. The materials-loaded ampoule  10  is evacuated to ≦(less than or equal to) 10 −3  Torr, and then sealed by heating the wall  22  of the first section  12  to collapse the tubing. The sealed ampoule  10  is then placed in a rocking furnace (not shown) in such a way that the external flange  15  on the loading chamber  12  and, hence, the open end  24  of the drainage tube  17  is located in an upright position. This procedure ensures that, at high temperature, and with the rocking motion (±30° from horizontal) of the furnace, molten glass is prevented from flowing into the melting chamber  14 . 
     In the preferred embodiment, the ampoule  10  is then heated to produce a homogeneous molten chalcogenide glass in the melting chamber  14 . The ampoule  10  is initially heated to approximately 200° C., at a rate of approximately 6.7° C./min. Following a thirty-minute hold at approximately 200° C., the ampoule is heated to approximately 600° C., at a rate of about 0.67° C./min. After a ten-hour hold at 600° C., the ampoule  10  is heated to approximately 800° C., at a rate of about 0.33° C./min. Following a ten-hour hold at 800° C., the ampoule  10  is brought to the final melting temperature of approximately 875° C. at an approximate rate of 0.13° C./min. The melting schedule is completed after about thirty hours at the final melting temperature. 
     The heated ampoule  10  is then transferred to a vertical furnace (not shown) operating at approximately the final melting temperature. The vertical furnace temperature should be sufficient to avoid crystallization and to provide fluidity to the melt for drainage. The ampoule  10  is suspended with an orientation that allows the molten glass to drain into the melting chamber  14 . After approximately twenty hours, the viscous, molten glass drains from the melting chamber  14 . The hot ampoule  10  is then attached to a lathe (not shown), and spun at about 2,800 rpm for about four to five minutes, in order to form the chalcogenide glass tubing. An insulating holder or fixture (not shown) can be attached to the distal end  23  of the hot ampoule  10  in order to retain the ampoule  10  without causing slippage or stress while spinning. 
     The ampoule  10  is then removed from the lathe, and quenched by gradual immersion into a water bath. It should be understood that other quenching methods, well known in the art, can be used. This causes the chalcogenide glass tubing to delaminate from the ampoule wall  25  in the spin section  16 , and prevents cracking as the ampoule cools to ambient temperature. The inventive technique has successfully fabricated 15 cm lengths of 10 mm OD GeAs and GeAsP sulfide glass tubing with a wall thickness between approximately 1.5 and 2.7 mm. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims