Abstract:
An apparatus for conducting a fusion process on a fiber includes a fiber chamber having coupled thereto a first fiber holder having a groove through which a fiber can be inserted into the fiber chamber and suspended therein, a filament chamber which maintains an inert and/or reducing atmosphere, a resistive filament movably supported in the filament chamber, a partition adjoining the fiber chamber and the filament chamber, the partition being provided with an orifice, a valve which selectively opens and closes the orifice, and a positioning device which moves the resistive filament between the filament chamber and the fiber chamber when the valve opens the orifice.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 09/832,668, filed Apr. 11, 2001, the disclosure of which is herein incorporated by reference. 
     
    
     BACKGROUND OF INVENTION  
       [0002]      FIG. 1  shows a micro-optic device  1  including micro-optic elements  2 , such as filters, polarizers, etc., aligned with an input fiber  3  and an output fiber  4 . A collimating lens  5 , such as a ball lens, graded-index (GRIN) lens, asphere lens, etc., is inserted between the input fiber  3  and the micro-optic elements  2 . A second collimating lens  6  is inserted between the micro-optic elements  2  and the output fiber  4 . Because optical fibers are divergent in nature, light  7  transmitted through the input fiber  3  diverges rapidly upon exiting the input fiber  3 . One function of the collimating lens  5  is to collimate the light  7  exiting the input fiber  3 . The collimated light passes through the micro-optic elements  2  and is gathered by the collimating lens  6 . The collimating lens  6  converges the light into the output fiber  4 . To ensure efficient optical coupling in the system, the collimating lenses  5 ,  6  must be properly aligned with the input and output fibers  3 ,  4  in three dimensions.  
         [0003]     Various mechanical methods for coupling lenses to optical fibers are known in the art.  FIG. 2  shows an example where an optical fiber  10  is mechanically coupled to a lens  12  by an alignment device  14 . A refractive-index matching agent  16  is disposed between the optical fiber  10  and lens  12  to minimize reflection of the light signal. Coupling the lens  12  to the optical fiber  10  in the manner shown in  FIG. 2  requires aligning the optical axis of the optical fiber  10  and the optical axis of the lens  12  at submicron level. This process can be very time consuming. Because the lens  12  is independent of the optical fiber  10  and must be precisely aligned with the optical fiber  10 , fabricating this type of fiber-optic system is expensive and may result in decreased efficiency in optical coupling. This is also true for independent lens systems that are attached to optical fibers by other means such as gluing. In the case of gluing, the materials used to bond the lens to the fiber can present reliability problems in terms of micro-movement of the lens and fiber in hostile operating conditions.  
         [0004]     U.S. Pat. No. 5,293,438 issued to Konno et al. proposes a solution that includes integrally forming a lens with an optical fiber using a fusion process. An optical fiber with an integrally formed lens is referred to as a lensed fiber.  FIG. 3  shows a lensed fiber  20  having an optical fiber  22  integrally formed with a spherical lens  26 . The lensed fiber  20  is made by fusion-splicing the optical fiber  22  to a rod  24  made of a lens material, such as silica or borosilicate. The fiber-to-rod splice is shown at  25 . To form the lens  26 , a fusion process is used to form a radius of curvature at the end of the rod  24  not spliced to the optical fiber  22 . One of the primary advantages of lensed fibers is simplified packaging because the lens is already aligned with and integrally formed with the fiber. Thus, there is no need for mechanically attaching or gluing the lens to the fiber. Also, a lensed fiber can be made in a wide range of sizes so that its spot size and working range can be tailored for a particular application. Lensed fiber consisting of planoconvex lens fusion-spliced to optical fiber has been proposed as a replacement for GRIN lens in micro-optic packages.  
         [0005]     Fabrication of a lensed fiber generally involves four steps: (1) pre-positioning, (2) splicing, (3) taper-cutting, and (4) melting-back. Using the lensed fiber  20  in  FIG. 3  as an example, the pre-positioning step involves aligning the optical fiber  22  with the rod  24  such that one end of the optical fiber  22  is in opposing relation with one of the rod  24 . The splicing step involves pushing the opposing ends of the optical fiber  22  and the rod  24  together while heating them to fuse or melt the ends together. The taper-cutting step involves moving a heat source to a desired location along the rod  24  to taper the rod  24  to a desired length. The melting-back step involves moving the heat source back towards the splice  25 , i.e., the joint between the optical fiber  22  and the rod  24 , by a selected distance to form the lens  26 . The distance the heat source is moved back towards the splice  25  depends on the desired radius of curvature for the lens  26 . The closer the heat source is to the splice  25 , the larger the radius of curvature of the lens  26 .  
         [0006]     Fabrication of a lensed fiber, such as the lensed fiber  20  of  FIG. 3 , requires a uniform heat source to allow for a formation of a substantially perfectly spherical lens  26  at the end of the fiber  22 . One possible heat source is a standard fusion splicer with a tungsten filament.  FIG. 4  shows a cassette  30  used in a standard fusion splicer, such as one sold under the trade name FFS-2000 by Vytran Corporation of Morganville, N.J. The cassette  30  includes a tungsten filament loop  32 , which has been shown to provide exceptionally uniform heat that allows for the formation of a spherical lens with a symmetrical circular mode field. However, manufacturing lensed fibers using a standard fusion splicer, such as sold under the trade name FFS-2000 by Vytran Corporation, has not been practical because the lifetime of the filament of the fusion splicer is very short, at least in comparison to when the fusion splicer is used for fusion-splicing of fibers. The reasons for this short filament lifetime are discussed below.  
         [0007]     Filament powers required during fabrication of a lensed fiber are generally higher than the filament power required for standard fusion-splicing of fibers. For example, using a standard filament loop on a Vytran FFS-2000 splicer with a 15 Amp DC power supply, the filament powers required to fabricate a lensed fiber from an optical fiber, such as a Corning® SMF-28™ optical fiber, and a 200 micron diameter silica rod are 21 W for splicing, 26 W for taper cutting, and 31 W for melting back. On the other hand, the filament power required for standard fusion-splicing of optical fibers, such as a Coring® SMF-28™ optical fiber to another Corning® SMF-28™ optical fiber, is 21 W. Table 1 below shows typical filament powers required for fabrication of lensed fiber depending on rod material.  
                                               TABLE 1                           Filament powers required for fabrication of lensed fiber                Filament Power (Watts, W)                Process   Silica   B 2 O 3 —SiO 2     GeO 2 —SiO 2                         Splicing   21   18   19           Taper-cut   26   21   24           Melting-back   31   24   26                      
 
         [0008]     In addition, during fabrication of a lensed fiber, the filament is on much longer than when used to make a standard fiber-to-fiber splice. For example, the filament of the fusion splicer sold under the trade name FFS-2000 by Vytran Corporation is on an average of about 25 seconds when forming a lens using the method described above and only an average of 5 seconds when forming a standard fiber-to-fiber splice. Because the filament powers for lens formation are much higher and the filament stays on much longer, the lifetime of the filament is greatly reduced when used for lens fabrication. For example, while a filament, such as the filament of the fusion splicer sold under the trade name FFS-2000 by Vytran Corporation, can typically make around 500 fiber-to-fiber splices, it is typically only capable of making a maximum of about 80 lenses when silica is used as the lens material and about 150 lenses when borosilicate glass is used as the lens material.  
         [0009]     Another reason for a short filament lifetime using existing technology, such as the FFS-2000 fusion splicer sold by Vytran Corporation, is that the tungsten filament of the fusion splicer is exposed to air. In the current fusion processes, the filament loop, which is run with a DC current, sits inside a splice head that is completely open to air. Exposure of the tungsten filament to air results in tungsten oxidation. When the filament is used for splicing or making a lens, the filament is purged with argon at about 0.5 to 1 L/min. However, when the filament is not in use, it is exposed to air. Tungsten oxide has a much lower melting point than tungsten metal, which leads to constant evaporation of oxidized tungsten from the surface of the filament until the filament is so thin that it breaks.  
         [0010]     Although other sources of heat, such as a CO 2  laser, may potentially be used for fabricating a lensed fiber, these sources have not been shown to provide heat that is sufficiently uniform and controlled to allow for the level of lens reproducibility necessary for production. On the other hand, filament loops, such as in fusion splicers, have been shown to achieve a select rate of 90% or better in the production of lensed fiber with a working distance of 4 mm when borosilicate glass is used as the lens material. The term “select rate” is the number of lenses that meets the specification. With a working distance of 4 mm, the size of lenses that can be made is limited. However, larger lenses can be made if the filament loop is made larger. Because filament lifetime is a major limitation on fabrication processes for lensed fiber, a new apparatus and method for increasing the lifetime of a filament is desired.  
       SUMMARY OF INVENTION  
       [0011]     In one aspect, the invention relates to an apparatus for conducting a fusion process on a fiber which includes a fiber chamber having coupled thereto a first fiber holder having a groove through which a fiber can be inserted into the fiber chamber and suspended therein, a filament chamber which maintains an inert and/or reducing atmosphere, a resistive filament movably supported in the filament chamber, a partition adjoining the fiber chamber and the filament chamber, the partition being provided with an orifice, a valve which selectively opens and closes the orifice, and a positioning device which moves the resistive filament between the filament chamber and the fiber chamber when the valve opens the orifice.  
         [0012]     In another aspect, the invention relates to a method of conducting a fusion process on a fiber which includes providing an inert and/or reducing atmosphere in a filament chamber in which a resistive filament is movably supported, inserting a fiber into a fiber chamber, opening an orifice provided in a partition adjoining the filament chamber and the fiber chamber, extending the filament through the orifice into the fiber chamber to conduct a fusion process on the fiber, retracting the filament through the orifice into the filament chamber, and closing the orifice.  
         [0013]     Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  is a schematic representation of a micro-optic device.  
         [0015]      FIG. 2  is a schematic of a prior art method for coupling a lens to an optical fiber.  
         [0016]      FIG. 3  is a schematic of a prior art lensed fiber.  
         [0017]      FIG. 4  shows a prior art fusion splicer with a filament loop.  
         [0018]      FIG. 5  is a front view of an apparatus for conducting a fusion process in accordance with one embodiment of the invention.  
         [0019]      FIGS. 6A and 6B  are cross-sections of the apparatus shown in  FIG. 5 .  
         [0020]      FIG. 7  is a front view of an apparatus for conducting a fusion process in accordance with another embodiment of the invention.  
         [0021]      FIG. 8  shows filament chambers mounted on a carousel. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]     The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail in order to not unnecessarily obscure the invention. The features and advantages of the invention may be better understood with reference to the drawings and discussions that follow.  
         [0023]      FIG. 5  shows an apparatus  100  for conducting a fusion process using a resistive filament as a heat source. The apparatus  100  typically provides advantages when the resistive filament is one made of a material that easily oxidizes, such as tungsten. The apparatus  100  may be used to fabricate a lensed fiber or splice fibers or taper a fiber. The apparatus  100  includes a fiber chamber  110  and a filament chamber  120 . The chambers  110 ,  120  are preferably fabricated from a corrosion-resistant material such as stainless steel. The fusion process takes place inside the fiber chamber  110 . In operation, fibers, such as fibers  114   a,    114   b,  are inserted in the fiber chamber  110  so that a fusion process can be performed on them. The fibers inserted in the fiber chamber  110  may be selected from optical fibers (with waveguide cores) and glass rods (without waveguide cores). A resistive filament ( 130  in  FIG. 6B ) is stored in an inert or reducing atmosphere inside the filament chamber  120  when it is not being used to perform a fusion process on fibers in the fiber chamber  110 . The resistive filament ( 130  in  FIG. 6B ) can be extended into the fiber chamber  110  as necessary to perform a fusion process on the fibers.  
         [0024]      FIG. 6A  shows a cross-section of the apparatus  100 . A partition  104  adjoins the fiber chamber  110  and the filament chamber  120 . The partition  104  includes an orifice  140  through which the resistive filament ( 130  in  FIG. 6B ) in the filament chamber  120  can be extended into the fiber chamber  110  and subsequently retracted into the filament chamber  120 . A valve  141  is provided at the orifice  140  to selectively open and close the orifice  140 . The valve  141  may be any mechanism capable of opening and closing the orifice  140 . Preferably, the valve  141  forms a substantially airtight seal between the fiber chamber  110  and the filament chamber  120  when in the closed position, thereby minimizing airflow from the fiber chamber  110  into the filament chamber  120  when the filament ( 130  in  FIG. 6B ) is stored in the filament chamber  120 . The valve  141  may be a sliding door or a hinged door, similar to those found in multiple-chamber glove boxes. Alternatively, the valve  141  may be a gate valve. Various types of gate valves suitable for use in the invention are available from, for example, MDC Vacuum Products Corporation, Hayward, Calif. Preferably, the valve  141  can be controlled to the open or closed position from the outside of the apparatus  100 , e.g., from a control system  150 .  
         [0025]     Fiber holders (or guides)  112   a,    112   b  are mounted in openings at the top and bottom of the fiber chamber  110 . (The fiber holders may also be an integral part of the fiber chamber.) The fiber holders  112   a,    112   b  have grooves  113   a,    113   b,  respectively, through which fibers, e.g., fibers  114   a,    114   b,  are inserted into the fiber chamber  110 . The grooves  113   a,    113   b  may be V-grooves. Alignment devices  115   a,    115   b  are coupled to the fiber chamber  110 . The alignment devices  115   a,    115   b  have bores which are aligned with the grooves  113   a,    113   b,  respectively, in the fiber holders  112   a,    112   b.  This allows fibers  114   a,    114   b  to be inserted through the alignment devices  115   a,    115   b  into the grooves  113   a,    113   b  and fiber chamber  110 . The alignment devices  115   a,    115   b  may include fiber grippers  117   a,    117   b,  e.g., gripping jaws, at the entrance of the bores to grip the fibers  114   a,    114   b  once a desired length of the fibers  114   a,    114   b  have been inserted into the fiber chamber  110 . Alternatively, the fiber grippers may be incorporated in the fiber holders  112   a,    112   b.  The alignment devices  115   a,    115   b  allow alignment of the fibers  114   a,    114   b  inside the fiber chamber  110 .  
         [0026]     In one embodiment, the alignment devices  115   a,    115   b  are xyz stages capable of translating the fibers  114   a,    114   b  in three dimensions. The xyz stages may be driven manually or automatically, e.g., using motors, such as DC or stepper motors or servomotors. The xyz stages may be compound stages or may be made of individual translation stages. A stage or actuator providing translation in fewer than three dimensions may also be used as the alignment devices  115   a,    115   b.  For example, adjusting the fibers  114   a,    114   b  along the y-axis only may suffice if the grooves  113   a,    113   b  are aligned with sufficient precision. The alignment devices  115   a,    115   b  may also incorporate tilt platforms to allow for angular adjustment of the fibers  114   a,    114   b.  The alignment devices  115   a,    115   b  may also incorporate a rotational stage or actuator which would allow the fibers  114   a,    114   b  to be rotated within the fiber chamber  110 . Alternatively, actuators may be provided separately from the alignment devices  115   a,    115   b  to selectively grip and rotate the fibers  114   a,    114   b.  The actuators may be mounted above and below the alignment devices  115   a,    115   b,  respectively. To give an idea of the working area in the fiber chamber  110 , the distance (d) between the fiber holders  112   a,    112   b,  would typically be on the order of 5 mm. Preferably, the alignment devices  115   a,    115   b  have positional accuracy and resolution in the micron or high sub-micron range, preferably 10-25 nm range.  
         [0027]     In one embodiment, the fiber chamber  110  includes one or more viewing ports  118 , such as fused silica windows. A viewing device  144 , such as camera, may be mounted at the viewing port  118  to capture images of the fibers  114   a,    114   b  in the fiber chamber  110 . For example, when the fibers  114   a,    114   b  are being aligned using the alignment devices  115   a,    115   b,  the image of the fibers  114   a,    114   b  inside the fiber chamber  110  may be captured through the viewing port  118  by the viewing device  144 . This image may then be supplied to the control system  150 , which will use the supplied data to control operation of the alignment devices  115   a,    115   b.  The viewing device  144  may also be used to capture the image of a lens while forming the lens from a fiber inserted in the fiber chamber  110 . The captured lens image can be sent to the control system  150 , which may include an algorithm for measuring the dimension of the lens from the lens image. As will be further discussed below, the measured dimensions of the lens can be used to control positioning of the filament ( 130  in  FIG. 6B ) with respect to the fibers  114   a,    114   b  during manufacture of the lensed fiber.  
         [0028]      FIGS. 6A and 6B  show a filament support structure  132  disposed in the filament chamber  120 . The filament support structure  132  comprises a head  133  which holds a filament cassette  135 . The filament cassette  135  comprises an insulating plate  138  and electrodes  137  that extend through the insulating plate  138 . One end of the electrodes  137  is coupled to a power supply (not shown) through, for example, leads ( 142  in  FIG. 6A ). The other end of the electrodes  137  is coupled to the filament ( 130  in  FIG. 6B ). The electrodes  137  support and provide power to the filament  130 . In one embodiment, the filament  130  is made of tungsten. In one embodiment, the filament  130  is a loop or is generally circular so that it uniformly distributes heat about the diameters of the fibers  114   a,    114   b  during a fusion process. The filament support structure  132  is movable between the filament chamber  120  and the fiber chamber  110  through the orifice  140 . In one embodiment, a positioning device  134 , such as an xyz stage or yz stage or a linear translation stage or actuator, is coupled to the filament support structure  132  to move the filament support structure  132  such that the filament  130  is positioned in the fiber chamber  110  and to provide controllable alignment of the filament  130  with the fibers in the fiber chamber  110 .  
         [0029]     The positioning device  134  may be operated manually or may be automated, e.g., driven by one or more motors. The positioning device  134  may receive control signals from the control system  150 , where the control system may generate control signals in response to images captured through the viewing ports  118  in the fiber chamber  110 . Preferably, the positioning device  134  has positional accuracy and resolution in the micron or high sub-micron range, preferably 10-25 nm range. In one embodiment, an optical sensor  136  is coupled to the filament support structure  132  to detect (or measure) a gap ( 139  in  FIG. 6A ) between the fibers  114   a,    114   b  to ensure, for example, that the filament  130  is centered at the gap ( 139  in  FIG. 6A ) prior to fusion-splicing of the fibers  114   a,    114   b.    
         [0030]     In operation, the filament chamber  120  maintains an inert or reducing atmosphere so that oxidation of the resistive filament  130  is reduced. Thus, storing the resistive filament  130  in the filament chamber  120  when not in use can prolong the lifetime of the resistive filament  130 . The inert or reducing atmosphere may be achieved as follows: once the filament support structure  132  (with the filament) is disposed in the filament chamber  120 , a vacuum pump (not shown) may be coupled to a port  122  in the filament chamber  120  to evacuate or pump down the filament chamber  120 . A gas source (not shown) may then be coupled to a port  124  in the filament chamber  120  to supply an inert gas, such as argon, or a mixture of inert gas and reducing agent, such as argon with several percent of hydrogen, into the filament chamber  120 . Baffles  126  may be provided at the ports  122 ,  124  to impede flow of gas into and out of the filament chamber  120 . Mass flow controls (not shown) may be provided as necessary to control flow of gas into and out of the filament chamber  120 . The orifice  140  is preferably closed while providing the inert or reducing atmosphere in the filament chamber  120 .  
         [0031]     Preferably, the fiber chamber  110  also maintains an inert atmosphere, at least around the filament  130 , when the filament  130  is being used for a fusion process in the fiber chamber  110 . To achieve this, the fiber chamber  110  may be filled with an inert gas, such as argon, or an inert gas with a reducing agent, such as argon with several percent hydrogen. The fiber chamber  110  may include a port  116  that may be coupled to an inert gas source. The fiber chamber  110  may also include a separate port (not shown) that may be coupled to a vacuum pump (not shown). The vacuum pump may be used to evacuate or pump down the fiber chamber  110  prior to pumping the inert gas into the fiber chamber  110 . The fiber chamber  110  is always leaky because of the need to continually load fibers into the fiber chamber  110  and remove fiber and lenses from the fiber chamber  110 . To minimize air flow into the fiber chamber  110 , the fiber chamber  110  is preferably maintained at a positive pressure by supplying the inert gas to the fiber chamber  110  at a higher pressure than ambient pressure.  
         [0032]     When the apparatus  100  is used for fabricating a lensed fiber, one of the fibers  114   a,    114   b  is an optical fiber and the other of the fibers is a glass rod made of a lens material such as silica or borosilicate. For example, the fiber  114   a  could be the optical fiber and the fiber  114   b  could be the glass rod made of a lens material. As can be seen in the drawing, the fiber  114   b  has a larger diameter than the fiber  114   a;  however, this is not a requirement. The fiber  114   b  may have the same diameter as the fiber  114   a  or a smaller diameter than the fiber  114   a.  Referring to  FIG. 6A , to begin fabrication of the lensed fiber, the fibers  114   a,    114   b  are inserted into the fiber chamber  110  and aligned using the alignment devices  115   a,    115   b.  At this time, the orifice  140  in the partition  104  between the filament chamber  120  and the fiber chamber  110  is closed so that the fibers  114   a,    114   b  can be inserted into the fiber chamber  110  without exposing the filament ( 130  in  FIG. 6B ) to air. As the fibers  114   a,    114   b  are inserted into and aligned within the fiber chamber  110  (or after aligning the fibers inside the fiber chamber), the fiber chamber  110  is purged with an inert gas (or an inert gas mixed with a reducing gas), which may be supplied through the port  116 . The valve  141  is then operated to open the orifice  140 , permitting the filament support structure  132  to move into the fiber chamber  110 . To prolong the lifetime of the filament, the fiber chamber  110  is preferably always purged with an inert gas prior to opening the orifice  140  and extending the filament support structure  132  (with the filament) into the fiber chamber  110 .  
         [0033]     When the filament ( 130  in  FIG. 6B ) is in the fiber chamber  110 , power is supplied to the filament ( 130  in  FIG. 6B ) to form the lensed fiber. To form the lensed fiber, the fiber  114   a  and lens material rod  114   b  are spliced by pushing their opposing ends together while being heated by the filament ( 130  in  FIG. 6B ). After splicing, the filament ( 130  in  FIG. 6B ) is moved by a desired distance along the lens material rod  114   b  to taper (or cut) the lens material rod  114   b  to a desired length. After tapering the lens material rod  114   b,  the filament ( 130  in  FIG. 6B ) is moved towards the splice, i.e., the joint formed between the fiber  114   a  and the lens material rod  114   b,  by a distance that depends on the desired radius of curvature of the lens to be formed on the lens material rod  114   b.  In general, the closer the filament ( 130  in  FIG. 6B ) is to the splice, the smaller the radius of curvature of the lens formed. After the lensed fiber is formed, the filament support structure  132  is retracted back into the filament chamber  120 , and the orifice  140  is closed to preserve the inert atmosphere in the filament chamber  120 . The lensed fiber is then removed from the fiber chamber  110 , and the process is repeated again for fabrication of other lensed fibers.  
         [0034]     The viewing device  144  may also be used to capture the image of the lens while forming the lens to measure the dimensions of the lens after forming the lens. In general, it has been determined that the filament ( 130  in  FIG. 6B ) makes lenses with very reproducible radius of curvature when borosilicate glass is used. However, the length of the lens may need to be adjusted periodically using an algorithm that calculates the desired length and determines the position the filament ( 130  in  FIG. 6B ) should move to during the taper cut to make the correct length. The algorithm may be used to control the positioning device  134  coupled to the filament support structure  132 .  
         [0035]     In one embodiment, the position the filament ( 130  in  FIG. 6B ) should move to during a taper-cut step is adjusted based on measurement of thickness of the previous lens. In this embodiment, the adjustment is done so that the ratio of the thickness of the lens to the radius of curvature of the lens is substantially constant, as shown by the following equation:  
               T   new     =       T   old     +         (         T   measured       R   measured       -       T   target       R   target         )     ·     R   measured       F               (   1   )             
 
 where T new  is the adjusted number of taper cut steps for the next lens to be made, T old  is the number of taper cut steps used in making the previous lens, T measured  is the measured thickness of the lens, R measured  is the measured radius of curvature of the lens, T target  is the target thickness of the lens, R target  is the target radius of curvature of the lens, and F is the dampened step size of the splice head  133  moving along the fiber-optic axis. Dampening is determined experimentally to achieve a stable process. Typically, the ratio T target /R target  is about 3.5. Equation (1) above may be used to control the positioning device  134  coupled to the filament support structure  132 . 
 
         [0036]     Those skilled in the art will appreciate that various modifications can be made to the apparatus  100  shown in  FIGS. 5, 6A , and  6 B that are within the scope of the invention. For example, as shown in  FIG. 7 , the fiber chamber  110  and filament chamber  120  may be structurally independent chambers, i.e., not placed immediately adjacent to each other. The filament chamber  120  and fiber chamber  110  may be connected to a passage  146 . One end of the passage  146  would communicate with the fiber chamber  110  through an aperture (not shown) in the fiber chamber  110 , and the other end of the passage  146  would communicate with the filament chamber  112  through an aperture (not shown) in the filament chamber  112 . The filament support structure ( 132  in  FIG. 6B ) could then pass through the passage  146  into the fiber chamber  110 . One or both of the chambers  110 ,  120  may include a door (not shown) or gate valve adapted to selectively block the corresponding aperture (not shown) so that the filament chamber  120  can be selectively isolated from the fiber chamber  110 , such as during loading and unloading of fibers  114   a,    114   b  in the fiber chamber  110 . Alternatively, a door, valve, or other closable device may be disposed in the passage  146 .  
         [0037]     In another embodiment, to facilitate removal of the filament ( 130  in  FIG. 6B ) when burnt out, the filament support structure ( 132  in  FIG. 6B ) and positioning device ( 134  in  FIG. 6B ) can be attached to a flange (not shown). The flange (not shown) may then be mounted on the filament chamber ( 120  in  FIG. 6B ). When it is desired to change the filament, the flange can be quickly removed from the filament chamber and replaced with another flange that already has a filament support structure and a new filament and a positioning device attached to it. Alternatively, as shown in  FIG. 8 , multiple filament chambers  120  may be loaded on a turntable or carousel  148 , or the like. Any one of the filament chambers  120  may be connected to the fiber chamber  110  at any given time while any burnt-out filaments are replaced in the other filament chambers  120 .  
         [0038]     The invention typically provides the following advantages. Storing the filament in an inert and/or reducing atmosphere when not in use prolongs the lifetime of the filament. The lifetime of the filament is further prolonged by purging the fiber chamber with an inert and/or reducing gas prior to extending the filament into the fiber chamber for a fusion process. The apparatus allows automation of the lens fabrication process. The configuration of the apparatus can be adjusted as necessary to allow for fabrication of larger lenses.  
         [0039]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.