Abstract:
An apparatus and method for manufacturing an optical fiber preform from a tube of vitreous material. A holding device holds the tube during manufacture of the preform and a heater supplies the necessary heat energy for preform manufacturing. A diffuser is disposed adjacently to an end of the tube of vitreous material to trap and diffuse light radiation generated in the tube by the heater. The invention can be used in an apparatus for drawing an optical fiber from a preform. The diffuser traps and diffuses light radiation generated in the optical fiber preform by a fiber drawing oven.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to apparatus for manufacturing an optical fiber preform, the apparatus comprising a glassmakers&#39; lathe for holding a glass tube by means of chucks clamped to the two ends of the tube, gas feed means for injecting gaseous compounds into the inside of the tube via a leakproof connection at one end of the tube, and a source of the energy required for causing the gaseous compounds to be deposited inside the tube or for collapsing the tube after deposition has been completed. 
     BACKGROUND OF THE INVENTION 
     In such a device, heat energy from the source is transmitted to the leakproof connection via the corresponding end of the glass tube. This connection has a sealing gasket which is thus raised to a high temperature while the gaseous compounds are being deposited or while the tube is being collapsed. Unfortunately, it is very important for the gasket to maintain a good seal so as to prevent any risk of the gaseous compounds being exhausted into the medium surrounding the apparatus, and to avoid any contamination of the deposit by the surrounding medium. 
     A known solution to the problem of the leakproof connection between the gas feed means and the end of the tube being overheated consists in designing a gasket of a complex shape and in making it out of special materials so as to guarantee that the connection remains sealed at the temperatures generated by a blowtorch. Unfortunately, that solution increases the cost of manufacturing a preform. 
     Another known solution to the above problem consists in holding the glass tube on the lathe by means of a tubular endpiece. In this way, the end of the tube is located at a certain distance from the leakproof connection and the corresponding chuck which come into contact only with the tubular endpiece. The inside diameter of the endpiece is slightly greater than the outside diameter of the glass tube and it is assembled thereto by adhesive acting on an overlap zone where the glass tube is received in the tubular endpiece. This establishes a discontinuity which enables a large part of the heat energy to be dissipated by means of the end section of the glass tube. Nevertheless, that solution also increases cost due to making and assembling the endpiece. 
     After a preform has been subjected to an internal deposition method, similar problems arise when the preform is collapsed. At that time, the preform is held at its ends by the chucks. While the preform is being heated by the heater means for the purpose of collapsing it, the thermal radiation generated by the heater means heats the chucks that are holding the preform, thereby running a risk of the chucks becoming jammed. 
     Finally, similar problems arise when drawing an optical fiber from a preform. At that time, the preform is held vertically at one of its ends by means of a chuck. While the preform is being heated by the fiber-drawing oven, the thermal radiation it generates heats the chuck that is holding the preform and that also can run the risk of the chuck becoming jammed. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the invention is to solve the problem of the leakproof connection between the gas feed unit and the end of the glass tube overheating in apparatus for manufacturing a preform, and to do in a manner that combines effectiveness and low cost. 
     Another object of the invention is to solve the problem of overheating the means for holding a preform while it is being collapsed or while a fiber is being drawn therefrom. 
     To this end, the invention provides apparatus for manufacturing an optical fiber preform from a tube of vitreous material, the apparatus comprising 
     means for holding said tube during manufacture of said preform; and 
     heater means supplying said tube with the heat energy required for manufacturing said preform; and 
     diffuser means placed around at least one of the ends of said tube to trap and diffuse light radiation generated in said tube by said heater means. 
     The invention also provides an apparatus for drawing an optical fiber from a preform, the apparatus comprising 
     means for holding one end of said preform; and 
     means for lowering said preform as held by said end through a fiber drawing oven; and 
     diffuser means disposed around said end of said preform to trap and diffuse light radiation generated in said end by said fiber drawing oven. 
     The heat energy supplied by the heater means propagates along the tube or the preform towards its ends mainly in the form of light radiation. The heat energy which is thus trapped and diffused by the diffuser means is therefore not transmitted to the leakproof connection or to the supporting chuck, thereby contributing to reducing the heating thereof. 
     Advantageously, the diffuser means has a refractive index close to that of the tube. By way of example, it can likewise be made of a vitreous material. This makes it possible to obtain good transmission of the radiation between the tube and the diffuser means. 
     Also advantageously, a liquid film can be inserted between the diffuser means and the tube. The light radiation passes from the tube to the diffuser means via the liquid film, and the diffuser means diffuses it to the surrounding medium. This increases the amount of light radiation that is transmitted from the tube to the diffuser means. The liquid film can have a refractive index that is close to that of the tube and of the diffuser means. 
     The invention makes it quick and easy to place the diffuser means on the glass tube, thereby making savings in equipment costs and in machine time costs. In addition, the risk of breaking the glass tube while the diffuser means is being mounted is considerably reduced compared with sticking on a tubular endpiece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will appear on reading the following description of an embodiment of the invention as shown by the drawings. In the figures: 
     FIG. 1 is a diagram showing apparatus of the invention for manufacturing a preform; 
     FIG. 2 is an enlarged view of FIG. 1 showing the diffuser means of the invention; and 
     FIG. 3 is a diagram showing apparatus of the invention for drawing a fiber from a preform. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A method of manufacturing a preform, e.g. the method known under the initials MCVD (for modified chemical vapor deposition) is implemented by means of apparatus comprising, FIG. 1, a glassmakers&#39; lathe  1  which holds a silica-based tube  3  via two chucks  5 A and  5 B clamped onto the two ends  3 A and  3 B of the glass tube  3 , and which cause it to rotate, R, about its axis. A gas feeder  7  serves to inject oxygen O 2  charged with the vapors of compounds such as silicon tetrachloride SiCl 4  and germanium tetrachloride GeCl 4 , for example, to be injected into the inside of the silica tube  3 . A blowtorch  9  heats the silica tube  3  and moves in translation T therealong in the same direction as the direction in which the gaseous compounds flow. In FIG. 1, this direction is from the end  3 A clamped in the chuck  5 A towards the end  3 B clamped in the chuck  5 B. On coming close to the end  3 B, the blowtorch  9  returns quickly to its starting point to begin a new pass. 
     The blowtorch  9  supplies the heat energy required for depositing gaseous compounds inside the glass tube. In the heating zone closest to the blowtorch  9 , the gaseous compounds react, forming particles which then deposit on the inside wall of the silica tube  3  downstream from the blowtorch  9  in the gas flow direction. The deposited particles are then vitrified by the blowtorch  9  as it moves along the silica tube  3 . 
     At the end of deposition, the silica tube  3  has a cladding precursor formed by the first-deposited layers and a core precursor formed by the last-deposited layers. The gas feeder  7  is then turned off so as to stop injecting gas. The operation of collapsing the silica tube  3  then begins, during which the blowtorch  9  raises the temperature of the silica tube  3  so as to soften the silica and cause the tube to shrink. After a “starting” pass, the blowtorch  9  is moved very slowly so as to close up the silica tube in the form of a solid preform, which preform may optionally be recharged, i.e. additional silica-based layers may optionally be deposited thereon. 
     Communication between the gas feeder  7  and the silica tube  3  takes place at the end  3 A of the tube via a leakproof connection  13  which includes an  0 -ring gasket  13 A. It is very important for the gasket to be leakproof so as to prevent any risk of the gaseous compounds being rejected into the medium surrounding the apparatus and conversely any risk of the MCVD deposition being contaminated by the surrounding medium. 
     In the invention, a diffuser in the form of a circularly cylindrical ring  15  are placed around the silica tube  3  close to the leakproof connection  13 , downstream from the chuck  5 A clamped to the end  3 A of the silica tube  3 . As mentioned above, the ring  15  can itself be made of silica so as to have a refractive index that is substantially equal to that of the tube  3  so that the change of index at the interface is negligible. 
     A liquid is inserted between the glass ring and the tube so as to form a film  17  (see FIG.  2 ). The heat energy supplied by the blowtorch  9  during deposition or during collapsing propagates along the silica tube  3  towards its ends  3 A and  3 B mainly in the form of light radiation. By means of the liquid film  17 , this light radiation passes from the tube  3  to the ring  15  which diffuses it into the surrounding medium. The heat energy that is trapped and diffused in this way by the silica ring  15  is therefore not transmitted to the leakproof connection  13 , thereby contributing to reducing heating thereof. The presence of the liquid film  17  increases the transmission of the light radiation between the two parts. 
     The liquid film  17  can likewise have a refractive index that is substantially equal to that of the glass ring  15  and of the glass tube  3 . As can be seen in FIG. 2, the ring  15  is held in place relative to the tube  3  by means of two O-rings  19  which also hold the liquid film  17  between the ring  15  and the tube  3 . By using lip gaskets, mounted in appropriate housings formed in the silica ring  15 , it is possible to guarantee constant clearance relative to the silica tube  3 . The liquid film  17  is distributed uniformly at the interface between the two pieces, thereby further contributing to good transmission of light radiation. 
     In a variant embodiment of the invention, the liquid film  17  is inserted in contact with a surface  15 I of the ring  15  which is carefully polished so as to further increase the transmission of light radiation through the interface between the ring and the tube. Provision is also made to frost the side faces  15 A and  15 B and the outside face  15 E of the silica ring  15  so as to increase the diffusion of light radiation in the form of heat into the medium surrounding the apparatus for manufacturing the preform. 
     Provision can also be made to cool the silica ring  15  by means of a compressed air blower  21  for maintaining the liquid film  17  at a temperature such that its refractive index remains substantially constant and equal to that of the ring  15  and of the tube  3 . A deflector  23  prevents the cold air delivered by the blower  21  from disturbing heating of the silica tube  3  by the blowtorch  9 . 
     The invention is also applicable to apparatus for drawing an optical fiber from a preform. Apparatus of this type comprises (see FIG. 3) lowering means  25  for lowering a preform  27  through a fiber drawing oven  9 ′. The lowering means  25  comprise a blind chuck  5 ′ which is mounted to move in translation parallel to a fiber drawing axis L so as to displace the preform  27  by means of a glass bar  3 ′ that is clamped at one end in the chuck  5 ′ and welded at its other end to the preform  27 . 
     According to the invention, a glass ring  15 ′ is placed around the glass bar  3 ′ and located close to the chuck  5 ′ to trap and diffuse light radiation generated in the glass bar  3 ′ by the fiber drawing oven  9 ′. 
     As mentioned above, the ring  15 ′ placed around the bar  3 ′ has a refractive index that is substantially equal to that of the bar  3 ′ so that the change of index at the interface is negligible. A liquid is then inserted between the glass ring and the bar  3 ′ so as to form a film  17 . The heat energy supplied by the oven  9 ′ propagates along the bar  3 ′ mainly in the form of light radiation. By means of a liquid film  17 , this light radiation is transmitted from the bar  3 ′ to the ring  15 ′ which diffuses it into the surrounding medium. The heat energy which is thus trapped and diffused by the silica ring  15 ′ is therefore not transmitted to the blind chuck  5 ′, thereby contributing to reducing its heating and to preventing any risk of it jamming. It is therefore no longer necessary to use a blind chuck that is made of refractory steel. 
     The above-described variants apply in like manner to apparatus of the invention for drawing a glass fiber from a preform. 
     The invention is not limited to the embodiments described above. 
     In particular, the space between the diffuser means and the preform need not be occupied by a liquid film but can be occupied by the ambient medium. Under such circumstances, it is preferable to ensure that the diffuser means fits on the preform with small tolerance. 
     In another alternative, the liquid film could be replaced by an index-matching gel.