Title of Invention	A METHOD FOR FORMING OPTICAL FIBER PREFORMS
Abstract	A single plasma burner (4a) deposits soot material on multiple rotating parallel targets (2a, sb, sc). All of the targets are grown simultaneously. The result is multiple performs for optical fibers.

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MULTIPLE TORCH - MULTIPLE TARGET METHOD AND APPARATUS FOR PLASMA OUTSIDE CHEMICAL VAPOR DEPOSITION DESCRIPTION Field of the Invention [D001] This invention relates genet-ally to the field of optical fiber manufacture and, more particularly, to a method and apparatus for high rate processing of pure or doped silica tubes or fiber preforms for further processing into optical fibers Ovscfiptioa of the Related Art [HC02] Th^re are a variety of known apparatus and methods Jc;- forming synthetic silica "uhss or preforms with ,;* given cro^o sectional pronit; of tneu u apparatus of the "312 application. More particularly, the present inventors have identified a shortcoming with the existing art, which is that a low deposition rats is obtained when the target is at its smaller diameter, j,e.T during the early stages of deposition. Further, the present inventors have identified a need for improving the utilization of material over the utilization .attained by the existing deposition processes, SUMMARY OF THE INVENTION {0004} Aft object of the present invention is to solve the above-identified shortcoming of the prior ait's deposition rate. Another object is to improve the utilization of deposition material in the manufacturing of preforms. [0005] A general method according to the present invention for achieving these rnd otfier objectives utilizes one or more first diamenet piar.ma torches for depositing siiica onto one or- mo.'e targets to form a first inteirneaiate preform having.;* first diameter, followed by an arratfgQmerU cf or-e or mote \ ' second diameter plasma torches for depositing silica onto the tic&l intermediate preforms to form a succeeding intermediate preform or a final preform [0006] A first example method according to the invention comprises a mst step of simultaneously depositing glass, on a plurality of cylindrical targets to fornt a plurality of first intermediate preforms having a'first diameter, using a •'inst piasna torch, and a second step of simultaneously depositing glass on vhif? plurality of ftst intermediate preforms to fonrn £ plurality of preforms, using a ^econc plasma torch, wherein the first plasma torch has 3 first cotl diameter an;l the second plasma torch has a second coil diameter, the first coil ciki meter being greater than the second ceil diameter. [0C'07] A further embodiment of this invention is a method comprising a first step -of simultaneously depositing g!ass on a plurality of cylindrical targets to form a plurality of first intermediate preforms, usirk) a first plasma torch, a ■f second step of simultaneously depositing glass on the plurality cf first intermediate preforms to form a plurality of second intermediate preforms, using si second plasma torch, and a third step of simultaneously depositing glass on the plurality of second intermediate preforms to fonn a plurality of preforms, wherein the first plasma torch has a first coil diameter, the second plasma torch has a second coil diameter, and the third plasma torch has a third coil diameter, the third coil diameter being larger than the second coil d ameter and the second coil diameter being larger than the frrst coil dameter. BRIEF DESCRIPTION OF THE DRAWINGS [COOS] The foregoing and other objects, aspects, and advantages ws!! be better understood from the following description of preferred embodiments of the invention with reference to the drawings, in which: Fig 1 depicts an airangernent for carrying out a first step in a first example. mar.hod of the invention, using a single plasma torch for depositino silica onto three targets; Fiy. 2 depicts an arrangement for carrying out a second step »r, b.c first example rnethr>a of the invention, employing three plasma torches for depositing silica onto three targets; Ficj. 3 depicts fctn acrznaemvftt of torches for carrying out a second example method according to the invention, viewed from an X-axis direction; Fig. 4 shows the arrangement of torches according to Fig. 3 viewed fron- a Y- ayis direction; Fi;;u 5 is a method flow diagram for a;i example operation carded out using thir apparatus of Figs. 3 and 4, rkjs. 6A and 6B show a frst and second position aac arrangement of plasma ■{ojches c nd targets for a thiid example embodiment, having apparatus for controlled and movable spacing between adjacent targets; Figs, 7A and 7B show a first and second position of an arrangement of a first movable target spacing apparatus for the embodiment of Figs. 6A and 6B; Figs. 3A and SB show a first and second position of an arrangement of a second movable target spacing apparatus for the embodiment of Figs. 6A and 68; Fig, 9 is a method flow diagram for an example operation of the Figs. 6A and SB embodiment; fig. 10 is a top view of a variable diameter plasma torch configuration for carrying ou! one or more steps of methods of previously described urnbociiments; * Rg, 11 is a sectional view through the BB projection fines of Fig. 10; and i::ig. 12 is a method flow diagram for an example operation of the variable diameter plasma torch of Figs. 10 and 11. DETAILED DESCRIPTION OF THE INVENTION fOOOd] Table 1 below defines an optimal relationship between the target sizes and the diameters of the torch that has been identified by thetiv* .... ...*«■" J as obtaining a high deposition rate, while preserving proper plasma gas fiow characteristics. This.relationship of target size to torch diameters provides a * * P d&posiibn rate of approximately 3 grams/minute or higher, white preserving adequate plasma gas flow characteristics, and is therefore used for the e; The present inventors also analyzed the deposition rate by using the following equation: Where: P is density of the glass which is a constant and is about 7,2 glee, D is the diameter of the target, At is the layer thickness for one deposition pass. [i}010] From the inventors' above equation (1), one of ordinary skiii can observe the problem of there being a low deposition rate when the target diameter is small. As aiso seen from equation (1 }r another controllable pesramerer is the deposition $poed.*»On its faee. the •mathematics pf^quatic»r ' . {) indicates that a faster deposition speed automatical?]/ means a higher deposition rate. However, in practice, deposition rate does not work according to t'nis equation alone. More particularly when the deposition speed alone is increased, without other adjustments being made, the thickness of the deposited layer ms> be reduced, and/or the deposited glass IT:ay be unconsolidated. [0011J the present invention so!ve.i these iir*ii ether problems by uiiLnng multiple torches and multiple targets, as described for tha example embodiments below, to maintain the teiationship between torch dtriirnfiter and target diameter within that represented by TabSe 1, [0812] in addition, particular example combinations and modification options relating to the plasma torch sire identified in the description betow. l,i the preferred embodiments of this invention the plasma torch 4a, and alt other plasma torches described herein include stafcilizer bars and injection ports (nozzles), labeled as item 58 and 60, respectively in the '970 application- The purpose and function of the stabilizer bars and injection ports 58 and 60 of the '970 application is described therein. [00183 Step or\e using the Fig, 1 apparatus will now be described. First, deposition begins with the single pilasma torch 4a4 having a diameter labeled DMTRA, traversing the length of the targets along the 2 axis and depositing class onto all three targets at once Before the deposition starts sach of the targets has an initial diameter 0 (start). For this example the plasma torch diameter DMTRA is 100 mm and D(start) is 25 mm- The center-io-center spacinc between the targets for step one is SP1. For this exampie SP1 it; 40 mm. The deposition of step one continues until the targets' diameter teaches a predetermined interim diameter, D(inter) An example D(inter) is 35 mm, wnich corresponds to the example O(start) and DMTRA values The total deposition-rate on the three tejgsfte for step one, using-ihetD(staft) ^nd • CMTRA and SPl values of this example, is more than 10 grams/mm. [0019j 'Referring to Fig. 2, a&&r the deposition orstep one achieve* Jha Oi'inter) target diameter, the plasms to»ch 4a of Pig, 1 is switched oft and tt ree new plasma torches, labeled as 4fa1, 4b2 and Ab3t are switched or,. Caeh of the plasma torches 4b1f 4b2 and 403 has a diameter DMTRB, which icr this ijxampie is 70 mm. The arrangement of the hree plasma torches with respect to the single plasma torch 4a is a matter of design choice An 6 ;£rnpte arrangement is to mount the three torches 4b1, 4b2 and 4b3 at positions displaced from the torch 4a in the Z-axis such that either group can bo turned on and operated without physical interference from the other fd('J20] Referring to Fig. 2, the arrangement of step two deposits g:ass onto the three targets 2 using the three plasma torches 4b 1. 4o2 and 4fc3 jr til the diameter D reaches a predetennined D(finai) which, for example, is 7f:mni. i .[0013] .Referring to Figs. 1 and 2 a first embodiment of this invention will bo described. The described example carries out a two step deposition process. Fig. 1 illustrates the apparatus for the first step, and Fig. 2 illustrates ■Ihe apparatus used for the second step. J0014J Referring to Fig 1, the apparatus and arrangement for the first r;tep comprises a single plasma torch 4A and three deposition targets 2at 2b and 2c, which are referenced collectively as tteni 2, each target held by a conventional chuck (not shown in Fig, 1) of a rotating glass working lathe, (not shown in Fig I), and rotated about the target's longitudinal axis, labeled AXa, AXb and AXc„ respectively, in the Fig. 1 arrangement, the spacing between ti e respective axes AXa - AXb is fixed. Therefore, the rotation may be carried out by a three spindle lathe, and suitable models of such are available from a number of commercial suppliers known io persons skill^cJJn the art ^ [Ml 5* Each of the longitudinal axes extends parade? to the reference axis keeled Z. The reference axis labeled Y is vertical to the iarget(s) 2, and V normal \o the 2 axis. From the perspective of an observer (net shown) facing tf e gia&s working tetfhejs;, the Y-axis is sn the up and down direction wiih re sped io the floor [not shown}- Tha X-axis is perpendicular to the Y2-plane and, from the perspective of the observer described for the Y-axis, the X-axis is in the in-and-out direction with respect to the target, normal to the Z axis. Tue described X, Y. 2 reference* axes apply to all other embodiments described herein. [0CH6J The plasma torch 4a is mounted Ijeiow the targets 2 on a laihe carriage attachment (not shown), such thai the plasma torch as rncvaole toward arid away from the targets 2 in a direction parallel to the reverence axis aisled Y The lathe carriage (not shown) is movable in the Z-diret:tion along !i:ho (engti of the targets 2 by a lead screw (not shown) of the lathe (not :;howny. |'0017J An exampie plasma torch 4a is in accordance with that described by the copending '970 application, which is hereby Incorporated by reference. [0021J The average deposition rate during step two of this example, for each target, is about 10 gram/min- This second step therefore achieves a total deposition rate for the three targets of about 30 g/min. J0022] The two-step process of Figs, f and 2 is described only as an example of the present invention, and Is not a limit on the number of steps or on the lumber of different diameters of torches. For example, if a final target diameter D(finaf) of 100 mm instead of 75 mm were desired, then a third step (not shown) would be earned out using three more plasma torches (not shown), each having a diameter of 100 mm- Further. Fig, 2 shows step two being carried out with three torches, 4b1tf 4b2 and 4b3, mounted on a single target-rotating equipment (not shown). Alternatively, the second step can be carried out by mounting the three individual targets 2 formed by tne first step onto three separate lathes (not shown). Each of the three lathes (not shown) would have a single torch similar to any of 4b 1 „ 4b2 and 4b3. This alternative means lor carrying out the second step would achieve, at each piece of e tjufpment^an average deposition -rate of>10 grafris/fnin'for tftfe fi^al taVg&T" d amcter of 75 mm. Second .Exalte Embodiment £00233 Referring to Figs. 3, 4 anc' 5, a second example apparatus and m&hod :>f this invention will be described. Rgiernng first: to Figs. 3 and 4, thin, example arranges four plasma torches, labeled as4Ci, 4C2, 403 and 4C4, displace j from one another aiong the"Z-axis, each pointed toward two targets 2 n the Y-axts direction. The leftmost plasma torch 4C1 has a diameter Df4Cl) which is the smallest of the four, followed in iiioraadng order by xh& diameters D(4C2), p(4C3) and D{4C4}of the torches 4C2, 4C3 and 4C4, respectively. For this example, 0(401)* 60 mm, D(4C2) ••- 80 mm. D(4C3) * J00 mm, and D(4C4) -. 120 mm. £04124] Each of the two targets 2m and 2n has an initial diameiej (not labeled) which, for this example, is 25 mm, and each is supported within a lathe chuck {not jahown) or equivalent target rotation apparatus. As depicted In Fig. 4, the two targets 2 extend parallel to one another in the X-Z plane, substantially symmetric about the center axis C of the torches. (0025] Referring to Fig. 5, an example method carried out with an apparatus according to Figs. 3 and 4 will be described. {0020] The first step of this example, labeled as 100 in the flow chart of Fig. 5, oegins by depositing glass on the two targets 2n and 2m using the smallest diameter torch 4C1. The deposition continues until step 102 detects the tub 2 diameter D having reached a predetermined first intermediate value D(inter'l) which, for this example, is 35 mm. The total deposition rate on the two targets 2 during step one of this example is more than 10 grams/min, [0027] Next, at step 104, the smallest plasma torch 4C1 is switched off and the next larger, adjacent plasma torch 4C2 is switched on, and deposition commences with the torch 4C2. As stated above, for this example the c lameter D(4C2) of the 4C2 plasma torch is 80 mm. as compared to the 60 rim diameter D(4C1) of the piasma torch used for the first deposition step. The step 104 deposition continues until step 106 detects the tube diameter D teaching a predetermined .s.eqec«Jj!7terme- which, in,view, -. c4 the torch diameters .of this example, is 45 mm. The iotai deposition rate on the two targets 2 during step 104 is more than 16 g/rnir,. The increased rate results, in significant part, from the larger diameter of the plasma torch 4C2, vhk:h better matches the starting diameter, i.e., 35 mm, of the tubes 2, than any of the other three plasma torches. [IT028] Ne*t at step 106 of Fig. 5, "he plasma torch 4C2 Is switched oft and th next larger diameter plasma to*ch 4C3 with 3 diameter D(4C3) of, for tnis example 100 mm, is switched an snd lis&d to deposit glass on the targets 2. The step 103 deposition continues until step 110 delects the tube diameter D leaching a predetermines third intermediate vaiue D(inter3) which, for this examplo, is 60 mm The process ttien ends at stop 110. Tha total deposition rate on the two targets 2 during step 10Sf using the torch and target parameters identified for is more than 20 grarns/min. [0029J Next, at step 112, the piasma torch 4C3 is switched off and the next k'.ryer diameter piasma torch 4C4 with a diameter 0(4C4) of. for this'example 120 mm, is switched on and used to deposit glass on the targets 2. The step 112 deposition continues until step 114 detects the,tube diameter D reaching a final value D{fxna!) which, for this example, is 70 mm. The tola! deposition rate on the two targets 2 during step 112 is more than 26 g/min. |0030] The described apparatus of Figs. 3 and 4 and the described method of Fig. 5 are for purposes of example only. The multiple totch process can be continued, with additional torches (not shown), or with another target-rotation and torch apparatus (not shown) until any desired target diameter n> reached 10031 j The described apparatus of Figs. 3 and 4 uses a single high frequency generator (not shown) to supply the required power to ali of the described pfasma torches. An example generator is a variable power Model No. IG 120/5(100 from Fritz Huttinger Electronic GmbH ot Germany, outputting tip to 120 kW at a frequency of 5.00 MHz. {+/■- 0.13 Mi-ti) to energize the plasma torches. The example generator is driven with a commercially available 50 H2t 3-phase 380 V power supply Xbtrd-E^nj3Je.EmbodifItt3jit [0032] Refemng to Figs. 6A through 9, another embodiment of the ir'vsntim will be described. Referring firsi to Fig. 6A the general feature of this emfctodiment is ihat multiple targets, in this example the number being three, labeled as 2xr 2y and 2zt respectively, ate each mounted within a respective rotating drive means, such as a lathe (not shown) The lathes are controllably moved apart with respect to one another in the X direction, by .. apparatus described.in reference to Figs. 7A through 8B- The movement of the lathes progressively separates centeMo-center spacing SP2 of the targets 2x, 2y and 2z as their diameters increase from the deposited glass, ■thereby maintaining the spacing SP2 between the adjacent targets- as the I;JUSS is deposed. [M33] Fig. SA shows a starting position of the targets 2. and Fig, 68 shows a second and wider centeMo-center spacing SP2. Referring to Fig, :>A, in :he starling position of the targets a single torch 4D1 is employed. An example torch 4D1 has a diameter D(4R)of 100 mm, corresponding to an example starting diameter of 25 mm for the three targets 2. Referring to Fig i3B¥ a second center-to-center spacing SP2 position corresponds to a target diameter of 35 mm. For the Fig. 6B position two torches 4D1 and 4D2 are employed, each having, for the 35 mm target diameter, a diameter D(4R) of 100 mm. 10034] Two example mechanisms and apparatus for the feature generally described above by Figs. 6A and 6B are shown, the first by Figs, 7A and *7B» ;ind tht: second by Figs, BA and 8B. (0035] Ficjs, 7A and 78 depict a first mechanism for translations! movement of three lathes, 6A - 6C, relative to one another in the X direction, with th« two foures; showing the same mechanism In a first and a second I iosition, respectively. Each of the three lathes 6A - SC is * commercially available glass-working lathe such as, for example, the units sold by Arnold™, Heathway™ or Litton™, with a support platform modified as described arvd depicted. The l&h&s $A-6C'each cdVipriso a pair dr rotating chucks, which a/e housed at the headstock labeled 5Af 58v and 5C. respectively, and also tht? taiistock iabeted 7A. 78 a.id 7C, respectively, for supporting the targets. The spindle driveis are tor rotating tne chucks, labeled 1GA, 1GB, and 10C. (30363 As shown in Figs. 7A and 78, the center laths 6A is mounted on ptatfomi 12, lathe 68 on plalrorrn 14, and lathe 6C on platform 16. Patforrr; 12 is shown as the larger structure because, in addition to being the mounting support for lathe? 6A, it te the major support'for the ether two platforms 14 and 16. TN? platform 14 and p'atfcrm 1S each have cooperating way guides (not shown) which engage with and slide along the ways 18 m the X direction, ] he machine ways 16 may be V-shaped, inverted V-shaped, round, or of any other of the plurality of well-known way configurations extant in the art of machine tools, in the example shown in Fig. 7A, two machine ways t&are mounted to a top Surface (not labeled) of the platform 12. The above described structure- and arrangement of the platforms 12, 14, ana XQ \$ readily implemented by one of ordinary skill in the art using commercially available machine tool mounting hardware. * |; [0(139] Referring to Fig. 9 an example operation ol the system of Figs. 6A an:! 6B, using the lathe arrangement of Figs, 7A and 78, will be described. [0( jncreas'ng diameter of the targets 2 and outputs a corresponding value of the signal S>. Step 204compares the signal S to a value PV1 representing the spacing position of the lathes 6A, 63 and 6C which, in turn, indicates the center !o-centei spacing between the targets. The comparison identifies if the targets av% spaced adequately from one another in consideration of their increased diameter from the deposition- tf the answer at step 204 is "yes", the process goes to step 206 compares the S value, representing the target diameter, to determine of the targets have reached their final processing diameter. If the answer is "no" (which is expected at the beginning of the processing), the process lops back to step 202 and continues deposition. [0041] If the answer at step 204 is "no", meaning that the targets are no longer adequately spaced, the process goes to step 208 where the first and second lathe position drives 22 and 26 rotate the first and second lead screws 24 and 23, respectively, which moves the platforms 14 and 16 away from the center platform 12. This moves lathes 68 and 6C apart from (tie center lathe 6A The value of PV1 is updated accordingly to se^efct ttoe now spacing position of the lathes. [4 042] The first and second lathe position drives are preferably programmed to effect a stepped motion of the lathes, wherein the lathes GB arid 6C are moved to an incremental next position each time the target diameter senaor 20 detects the target 2 diameters reaching a next predetermined value. In addition, the first and second lathe position drives an:- preferably progfarnmod to diancj? the position ci lathes 6B and 6C at the end of, rather than during a deposition pass. [0043] After step 208 moves the aihes 68 and 6C, step 210 compares the $ signal to a first torch control parameter T to determine if the diameter D(4R) of the single torch 4D1 iz sufficient to cover all three targets 2. If the answer is "Yes*", the process loops back to step 202 and deposition proceeds with tc^ch 401. If the answer at step 210 is "No**, meaning that one torch 4D1 alone cannot efficiently deposit glass on all three targets, the process goes to step 212 and tin? next torch, in this case another torch 4D2 arrantted next to th ■> first larch 4D1 as In Fig, 66, is switched on- Associated with step 212, the center-to -center spacing of the two torches are automatically adjusted to ensure proper coverage of all the targets 2. The process then Ux>ps back to step 202 and continues the deposition with the two activated torches 4D1 and 4D2, The process continues, with step 208 increasing the center-to-center spacing between the targets to compensate for their increasing diameter, and with steps 210 and 212 activating and positioning additional torches as needed, until, step 206 detects that the targets have reached their desired final diameter The process then goes to step 214 ami ends. ;[0O44] The overall deposition rate using the apparatus of Figs. 6A and 6B> •and the particular example lathe arrangement of Figs. 7 A and 7Q is very iiirnilair to that of Example 1. The process at step 212 continues until the targets reach the predetermined final target diameter D. |O045]| The lathes 6A-6C of Figs. 7A and 7b are conventional giass working lathes having modified supports as described above. Accordingly, weft of the lathes 6A-6C has an individual spindle drive, with the three drives . beingJabeled IDA 'through 10C. Fiys.. SA and B^show two positions of-tiie same apparatus which, as wiil be described below, removes the redundant c rives *0A through 10G and, still further, removes the lead screws 24 and 2i for ease of description the apparatus of Figs. 8A and 3E are projections in* the AA direction of Figs. 7A and 7B, with iikfc structure labeled by identical numbers. (0046) Referring to Fig. SA, the platforms 14 *and 16 are supported on the ways 18 of Figs.. 7A and 7B (not shown in Fig. 8A) on the top of platform 12, a- described for .the Third Example Embodiment A lathe motor 30 having a drive sprocket 32 is mounted in a support 34 in a cooperative and movable arrangement with respect to vertical guide slot 36. each of the lathe chucks 5A through 5C of Figs. 7A and 7B, which are not shown in Figs, 6A and 8B, h;is a sprocket, labeled as 9A through 9C, respectively. A drive chain 34 extends around the driving sprocket 32 and the three lathe sprockets 9A through 9C. Accordingly, the single lathe motor 30 provides the rotational drive for all of the lathe chucks 5A through 5C. A conventional servo drive trv:rf. Shown), which is readily seiecteote? from commercially available units, by a person of ordinary skill in the art, controliably positions, in a vertical direction, the lathe motor 30 in response to the above-described control signal 8. A bias spring 40 is arranged between a center structure 12a of the platfoim 12 and the platform 14, and a bias spring 42 is arranged between the center structure 12a and the platform 16. Bias springs 40 and 42 urge platforms. 14. and 16 in the X direction away from platform 12. Platform 12 is fixed, as described in reference to Figs. 7A and 7B. 100471 Fig. BA shows the rotating drive motor at its lowest vertical position. At this lowest posftion the tension of the drive chain 34 pulls the outer sprockets 9B and 9C toward the inner sprocket 9A, against the force of bias springs 40 and 42. Accordingly, the lathe chucks 5B and 5C are at their closest position relative to the center lathe chuck 5A. [0048] The deposition process begins with the apparatus in the position shown by Fig. 8A, using the same plasma torch apparatus as described in reference to fig, 7A of the Third Example Embodiment. As the deposition continues the diameter of ttie.t$rgets 2 increases. Targe*diameter sensor 20 ^Mtpubs a control signal 8 indicative of the target 2 diameter, as described above, which is received by the servo drive 33. The above-described servo ■drive, in response, moves the lathe- motor 30 to a higher position. As the lathe motor 30 moves upward there is resulting alack in the drive chain 34. The bias springs 40 and 42 take up the slack by urging the platforms 14 and 16 away from lo take up the slack, whereupon the platforms 14 and 16 assume? a position spaced further from the center platform 12 in the X direction. The deposition continues until the targets reach another designed diameter, whereupon the servo drive movgs the lathe motor 30 to a next upper position. As described above, the bias springs 40 and 42 correspondingly urg& the platforms 14 and 16 to a next outward position with respect to the center platform 12. The process of detecting the target diameter and moving the lathe motor 30 upward in response continues until tiie final designed diameter is reached. Referring to Fig. SB, an example position of the lathe motor 30 and the platforms 14 and 16, When deposition has obtained the final designed diameter is shown. Ths total moving distance of the rotating drive motor 30 controls the overall traversal of the outskje lathe chucks 5B and 5C, (0040) The arrangement of the lathe motor 30, support 34t guide slot 36 and servo drive 38 is for purposes of example only. Many alternative arrangements can be seen by one of ordinary skill upon reading this description* For example, the lathe motor may be mounted on a pivoting swing arm (not shown), which is moved about a pivot point (not shown) in an arc fashion. [0050] Still further, if greater precision of positioning the platforms 14 and 16 is desired, the first and second lead screws 24 and 28, and drives 22 and 26 described in reference to Figs. 7A and 7B may be retained. In this case, the vertical movement of the lathe motor 30 by the servo drive 38 must be synchronized with the rotation of the lead screws 24 and 28 to maintain proper tension of the drive chain 34 Fourth .Example Embodiment [0051] The above-described embodiments are shown, for purposes of description, as employing various arrangements of and sequences of ceposit ng grass; using fixed diameter plasma torches. Referring to Ficjs. iG and 11 a variable diameter plasma torch 80, which significantly reduces the hardware over the fixed diameter piasma torch xrrmigements described for tne previous embodiments, will be described.. Fig. 10 is a top view of the variable diameter plasma torch 60, and Fig. 11 is a cross-sectional view through the projection line BB of Fig. 10. The variable diameter plasma torch . 60 sho*Arn at Figs. 10 and 11 can be directly substituted for the four torches 4Ct, 4C2t 4C3, and 4C4 employed in the example Second Embodiment. {0052] Referring to Fig. 10, the variable diameter torch 60 comprises an inner tube 62, and four concentric quartz glass tubes labeled as £4, 66. 68, and 70. A copper conductor coil 72 surrounds the outer quartz tube 70. The diameter of ring 64 is labeled D(64), and the diameters of rings 66,68 and 70 ar» labe&d 0(66), D(68) and D(70), respectively. [0053] Two example structures for the variable torch 60 will be described, tsach example being a substitute for all four torches 4C1,4C2,4C3, and 4C4 of Embodiment 2. [0(354] In a first example structure, which provides a precision control of the tubas heights, each of the tubes 64,66 and 68 fs independently movable in the s;xial or height direction AX. The torch rings 64, 66 and 68 are selectively positioned in the height direction AX, by ring steppers (not shown) to chainge the diameter of the torch. The present inventors have identified a preferred precision of the torch ring positioning and, hence, for the ring steppers, for the example ring dimensions 0(64), D(66). D(68) and D(70) described above, to be approximately 0,1 mm. The ring steppers (not shown) are conventional, commercially available precision stepper motors and associated precision drive mechanisms, using a conventional commercially available microprocessor-based controller unit (not shown), all of these being readily selected and configured in accordance with standard criteria and methods well known to persons of skill in the art to **>**'"*** *K;^ ;«i,«.«*:^ relates. [I3055] The second example structure for the variable torch 60 features fitted heights for the ring tube, which simpler than tho first and, tot some applications, may be preferred. However, the height of the inside ring 64 will be fow£f thar: the outer ones. It wi!i atow the proper mixture of the pSasnia gas aiv' achieve the desired the flow condition. Table 2 provides an example s difference. More particularly, a key^operatlon parameter is a constant Surface Velocity over the torch, where Surface Velocity is defined as: [0057] Cross Sectional Area is the cross sectional area of the torch, where the plasma gasses are actually flowing. [0058] By controlling the Surface Velocity a smooth operation, and more importantly, the quality of the deposited glass can be maintained. The present inventors have found that for purposes of this inversion an optimum Surface Velocity is approximately 35 meters/minute (m/m). Since the Surface Velocity must be maintained constant at or near the predetennined optimal value, the ratio of the total flow rate F to the cross sectional area A must be kept constant. However, the cross section area of the torch cannot remain constant if fuft coverage of the iargr>t(s) is to be maintained and, as seen from Hgs. 10 and 11, the cross section area is made larger by sequentially enabling the torch rings 64, 66, 68 and 70, as described below'. Therefore, *' the only available variable in Equation (2) is the total flow rate- The apparatus of this embodiment, as described in further detail below, uses a mass flow controller (MFC>tc vary the tolai flow rate, thereby maintaining the SSurtaoe Velocity at a predetermined constant value. The MFC fc; this example embodiment is a commercially available ur it, from suppliers including, but not limited to, Tylan General™, Unit Instalments™, MKSTft\ and Pwa™. As shown in Fig.11, the MFCs 80,82,84,86 are used to control the p:iasma gas foaorch rings 64,66,68, and 70. respectively. [0059] !n continuing the above example, to maintain the constant surface velocity of 35 m/m MFC 80 is initially set, for torch ring 64, at 100 aters/minute (l/m). When torch ring 66 is activated, MFC 80 wil! maintain the same 100 l/m flow rate and MFC 82 will have a flow of 76 l/m. When torch ring 68 is activated, MFCs 80 and 82 still has the same 100 and 75 l/m flow rate respectively but MFC 84 will haye a flow rate of 100 l/m. When the last torch ring, 86, is used, the MFC 80a82,ancl 84 will hava ihe same flow rate of 100, 75 and 100 l/m. respectively, but the MFC 86 will need a flow rate of 120 t/rn. 3y using these flow rate**, the surface velocity will be maintained constant about 35 m/m. r0O6O) Referring to Fig. 10, the depicted example variable torch 60 further comprises two or more nozzles 74, located in the same plane, wfoh two being used lor the particular example depicted. The nozzles 74 are positioned opposite to each other, as shown, or at other predetermined angles (not ■shown). The vertical position of the nozzles 74 is in accordance with the description of the same as recited by the '970 application. The nozzles 74 are mounted to a precision micromovement device (not shown) and driven m the radial direction R labeled in the figure by a conventional commercially available stepping motor (not shown). An example stepping motor is a type PD 42-18,35 made by the RK Rose+Kriegel GmbH & Co.KG™. together with ; in associated precision micromovement device available from the same supplier. Equivalent models of stepping motors and precision inict'omovement devices are available .from various commercial suppliers known to persons of skill in the art. [0061] The nozzles 74 are connected Dy flexible tubings (not shown) to t^e rigad main gas deliver lines (not shown), the flexible tubings having sufficient slack to accommodate the full range of nozzle 74 motion. {^062} The abo»-e-identified control unit controls the nozzle stepper tc selectively position the opening- (not labeled) of each of the nozzles 72 to d lo-catior around the circumference of a selected tube from among tubes 64, €8,68 and 70. The selected tube corresponds to configures the desired diameter of the torch 60. [i)063] The described structure lor controlling th& position of the nozzles 74 in th-s radial direction ft permits tie same nozzles 74 to be used for ai! of the concentric quartz glass tubes 64, 66, 68 and 70, i.e., for ail of the different torch diameters. [•13064] Referring to Fig. 12, an example process using the variable diameter torch 60 is described. Unless otherwise: described, the procoss is that described for Example Embodiments Two and Three, with ihe single variable torcn fcu simstituted for the multiple torches or the previously described embodiments. More specifically, at step 300, one target 2 is installed in each of lathes 6A and 613 of the apparatus Figs. 7A and 7B or of Figs. 3A and S8. Each target 2 has a starting diameter of 25 mm. Next, at step 302, the nozzles 74 of Fig. 10 are moved to enable the inner most quart? tube, or torch ring. £4- For this example the diameter D(64) of the torch ring 64 is f>0 mm. Next, at step 304 deposition begins onto the two targets 2. Step 306 compares the diameter of the targets to the their center-to-center spacing to determine if the spacing is adequate, as described for step 204 of Fig. 9* based on the output S of the target sensor 22 and the lathe position value PVt, if the answer is "no*, the process goes to step 308 and repositions th** lathes 6A and 6B, as described for step 208 of Pig. 9. After step 338, the process goes to step 312 and determines if the to;di 6ft is configured tc> adequately cover the targets, Th£ determination ss made ba:-&d on the target diameter signal S and the csruer-tc-center spacing indicated by PV1, end a torch threshold T whteh.is set according tc which of the nngs64, 66, 68 and 70 is activated. For tU& first loop throuyh the process of Fig. 12 the threshold T set to correspond t-j the innermost torch ring 64. For this example the fust threshold,, is being 35 mm. If step 312 identifies that the torch 60 doe;; rot have adequate diameter, the process goes to step to step .314 to switch over to the larger torch ring 66. [0065j To caay out step 314 the microprocessor-based controller issues a sequence of changing commands (not shown) in response to signals from the 'target siensoir 22. One command slops the source chemical flow lo the nozzles by closing the chemical flow valves (not shown) and, at the same time, opens the valve for a purge gas, such as air or nitrogen. Another command to the designated MFC sets the chemical flow rate to a des.red value, which increases the plasma gas flow but maintain the constant surface-velocity. . Another command causes the nozzle stepper to retract the ro?zies 74 backwards in the R direction to match the 80 mm torch ring 63. The plasma torch 60 is then re-started with the 80 mm torch ring 66. the torch threshold T is updated to reflect the 80 mm diameter, the process loops back to step and deposition is resumed. |[006fi| When step 312 detects that the 80 mm diameter of the torch ring 66 is inadequate, the process gees to step 314 and switches over to the next larger ring 63 (100 mm). Similarly, when step 312 detects that the 100 mm diameter of the torch ring 68 is inadequate, the process goes to step 314 and switches over to the largest (for this example) ring 68 (120 mm) 10067] Step 310 in a manner similar to step 206 of Fig. 9 detects when the target has reached its final process diameter, shown as S = PFinal, where Jpon the process goes to step 316 and ends. [0068] The variable torch 60 shown at Figs 10 ami 11 is for purposes of example only. For example, more than four concentric tubes or rings, i.e., 64, •56, 68 and 70, could be used, [C0S9] To have a smooth opeiation, the retractable nozzles G2 should :"jave precision motion control, typicatiy in the range of 0.1 mm. Further, a .■rireferred.embcdirnent includes a feedback loop (#ot shcyvn) for monitoring ihe coupJing oi plasma j-ower and oomp^nsoting th& changes in the induction ! L) during the -switch-over steps and during the entire deposition operation. The change of the inductance (L) will change the frequency (f) of the plasma •generator, and they are related by this relationship: where C is a capacitance inherent in the power supply tc the ioich. During the operation, the capacitance C is constant. When the inductance L changes; H wall change the frequency, A feedback loop will detect and automatically adjust the capacitance and maintain the constant frequency. J0070J The advantages of the variable torch 60 are that it requires one ci-enerator and one torch only. In addition, during the operation, there is no r\iaed or" switchover of the torches and of the generator. The variable torch 60 clso requires substantially less space than the multiple torch apparatus of the previously described embodiment. The space saving clso rn$are> there is no need to increase the length of the lathe to accommodate multiple torches. {007-IJ it is to be understood that the present invention is described above in reference to specific embodiments, which are for purposes of example only, and that the invention is not limited to the specific arrangement, or configuration described hereinabove or shown in the drawings, but also comprises the various modifications readily apparent to one skilled in the art upon reading this specification, as defined by the broadest scope of the appended claims 1, « method for forming optical liber preforms, mpri$ing steps of: providing a first plasma torch having a coi! for coupling pfasina energy. :*>aid coil having a first diameter; providing a plurality of targets, each having a longitudinal axis, arranged such that said longitudinal axes are separated from one another hy a spacing distance normal to said longitudinal axes, rotating said plurality of targets simultaneously about mutually parallel Mxes of rotation; depositing s glass simultaneously on said plurality of targets, using 5: aid fimt plasma torch, to form a plurality of intermediate preforms each having a first pteform diameter; providing a plurality of second plasma torches, each having a coil for coupling plasma energy, each of said coite having a second diameter, said second diameter being smaller than said first diameter; and depositing a glass on said plurality of intermediate preforms using said plurality of second plasma torches to form a corresponding plurality of final 1 reforms. 2 A method according lo claim 1, wherein a sum of said spacing distances of-£ilf adjacent pairs of said targets is less than said first coil diameter 3 A method for forming optical fit»&r preforms, comprising steps oJ. ptoviding a plurality of targets; rotating said plurality of targets simultaneous 'loout mutually parallel axes of rotation, providing a first plasma torch having a coii for coupling plasma energy, said coil having a first diameter, depositing gtess on said target* by moving said Jir^t plasma-torch along said targets parallel lo said axes of rotation; providing a second plasma torch having a coil for coupling piasma energy over a portion of each of said plurality of targets, said coil having a second diameter, said second diameter being greater than said first diameter, and depositing glass on said targets by moving said second plasma torch along said targets parallel to said axes of rotation. 4. A method for forming optical fiber preforms, comprising steps of: providing a first plasma torch having a coil for coupling plasma energy. said coil having a first diameter, providing a plurality of targets, each having a longitudinal axis, .riirangsd such that said longitudinal axes are separated from one another by a spacing distance normal to said longitudinal axes; rotating said plurality of targets simultaneo* •*u' ->^™»* ****»■ mc^riiwa longitudinal axes; depositing a glass simultaneously oa said plurality of targets, using a plasma torch; detecting a diameter of one or more of said targets: increasing said spacing in response to said detected diameter, ana depositing a glass simultaneously on said plurality cf targets with their longitudinal axes separated from one another by said increased spacing. 6. A method for fowling optical fiber preforms, comprising steps of; providing one plasma torch having a coil for coupling plasma energy . s-iid col having a fixed diameter; providing a plurality of targets, each having a longitudinal axis. a ranged such that said longitudinal axes are separated front one another by a spacir g distance normal to said longitudinal axes; rotating said plurality of targets simultaneously about their respective longitudinal axes; depositing a glass simultaneously on.said plurality of targets, using & phf;ma torch; detecting a diameter of one or more of said targets; increasing said spacing in response to said detected diameter, modifyfng the plasma torch by increasing the cross section area of the plasma torch and keeping the surface velocity constant; and depositing a glass simultaneously on said plurality of targets with their longitudinal axes separated from one another by said increased spacing. 6. A method for forming optical fiber preforms, substantially as hereinabove described and illustrated with reference to the accompanying drawings.

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