IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982 305

Special Issue Papers

An Overview of the Modified Chemical Vapor Deposition (MCVD) Process and Performance

SUZANNE R. NAGEL, J. B. MAcCHESNEY, AND KENNETH L. WALKER

(Invited Paper)

Abstract–This paper reviews the MCVD process, with special emphasis FLOW METERS. MASS-FLOW CONTROLLERS on fiber design and material choices, understanding of mechanisms in- 02 AND MANIFOLO — ._—, r FUSEDSILICATUBE volved in the process, process improvements, and performance. ——.—— — o~ 02, ,02, , h+ , ‘ce2 MULTIBURNER oEPosITEOLAYER -. . -$:- ~ % TORCH I. INTRODUCTION OF CORE‘GLASS Ji&!lP’~’sIF4, 02 POC13 I GeC14 L%, SF6 OR CF4 -Hz INCE the initial suggestion by Kao and Hockman [1] in $C14 .— TRANSLATION s1966 that low loss silica-based fibers could be used as a BUBBLERS transmission medium in the near infrared frequency region, Fig. 1. Schematic diagram of MCVD apparatus. there has been tremendous progress in the development of processing techniques to fabricate lightguides which exhibit MCVD FLOW DIAGRAM low attenuation, high bandwidth, good dimensional control, and excellent mechanical properties. Several processing tech- TUBE CHARACTERIZATION niques have demonstrated excellent performance as well as TUBE CtEANING manufacturability: OVPO [2], MCVD [3], VAD [4], the TUBESET UP double crucible technique [5], and PCVD [6]. This paper HIGHTEMPERATUREFIREPOLISH- 1600-20000C will review the MCVD technique, focusing on the process CHEMICALREACTION-> 1300”C and mechanisms involved, as well as the resultant fiber per- NUCLEATIONAND GROWTHOF PARTICLES> 1300”C formance and manufacturability y. PARTICLEDEPOSITIONTWALL. TGAS The MCVD process was first reported by MacChesney et al. CONSOLIDATION 1500-1800”C TOSECOLLAPSE19OO-21OO”C [7], [8], and has been widely used because it was relatively PREFORMCHARACTERIZATION easy to implement. Subsequent studies have focused on FIBERDRAWINGAND COATING understanding the fundamental physics and chemistry of the PROOFTEST process, and establishing process controls and limits in order FIBERCHARACTERIZATION to realize the best geometric, optical, and mechanical perfor- Fig. 2. Processstepsfor preparing lightguides by the MCVD technique. mance. Upon the basis of this understanding,-. modification and scale -up of the process have been accomplished to improve Chemical reagents are first entrained in a gas stream in con- both cost effectiveness and manufacturability. The results of trolled amounts by either passing carrier gases such as 02, Ar, this evolutionary process and its impact on fabrication will be or He through liquid dopant sources, or by using gaseous reviewed. dopants. Halides which have reasonably high vapor pressures at room temperature are typically used and include SiCIQ, II. DESCRIPTION OF THE PROCESS SiF4, GeC14, BC13, BBr3, PC13, POC13, SF6, CF4, and CC12F2. The MCVD process is based on the high temperature oxida- The process takes advantage of the fact that the vapor tion of reagents inside a rotating tube which is heated by an pressure of these dopants is many orders of magnitude higher external heat source. A schematic of the process, as currently than any transition metal impurities which might be present practiced, is depicted in Fig. 1, while Fig, 2 shows a flow dtid- in the starting materials and cause absorption losses [9] ; gram of the various steps in the process. thus, the entrained dopants are of very high purity relative to such contaminants. Hydrogenated species, however, t ypically Manuscript received September 25, 1981; revisedDecember 1, 1981. have very similar vapor pressures as the dopants and are there- The authors are with Bell Laboratories, Murray Hill, NJ 07974. fore commonly purified to remove such contaminants. The

0018-9480/82 /0400 -0305 $00.75 @ 1982 IEEE 306 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982 photochlorination technique, reported by Barns et al. [10], [7] , [8], [16]. B,03 is added to decrease the viscosity and converts chemically bound hydrogen to HC1 which can then thus improve process temperatures, and decreases the refractive be easily separated. index [7], [8], [17] . Another additive, Pj 05 ,increases the in- The high purity gas mixture is injected into a rotating tube dex, and has the advantage of greatly reducing the viscosity and which is mounted in a glass working lathe and heated by a scattering of the resultant glasses [13] , [18] , [19] . Fluorine traversing oxyhydrogen torch in the same direction as gas can also be added to Si02 to reduce the index [20], [21]. flow. The reagents react by a homogeneous gas phase reaction The various dopants can be added in controlled amounts to at high temperatures to form glassy particles which are sub- make a variety of glass compositions which have refractive sequently deposited downstream of the hot zone. The heat indexes which are less than, equal to, or higher than SiOQ, from the moving torch fuses the material to form a transparent yet process at lower temperatures. Recently, considerable glassy film. Typical deposition temperatures are sufficiently work has been reported on using fluorine in combination with high to sinter the deposited material, but not so high as to other dopants to make a wide variety of single mode fiber cause distortions of the substrate tube. The torch is traversed designs [22] -[25] . repeatedly in order to build up, layer by Payer, the desired Typical systems for the core and deposited cladding of amount of material. The composition of the layers can be multimode and single mode fibers made by the MCVD process varied during each traversal to build up a graded index film. are shown in Fig. 3. The choice of composition affects both Typically, 30-100 layers are deposited. Fiber design and com- the optical properties of the resultant lightguide as well as positional requirements determine the exact deposition pro- the processibility. Historically, there has been an evolution of gram. The technique can thus be used to make both step and compositions used in MCVD. Initial multimode fibers were graded single mode and multimode structures. based on Ge02 -Si02 cores, with a thin Bz 03 -SiOa barrier As MCVD is currently practiced, a thin cladding layer is layer. Bz 03 was added to the core to reduce the processing first deposited. This serves as a barrier to diffusion of any temperatures and thus avoid tube shrinkage [7] , [8], [16]. impurities, such as OH, which could degrade the optical Payne and Gambling first reported on doping SiOz with PJ OS attenuation [8], [11 ] -[ 14] . [n addition, the deposited clad- [18], and fibers based on the GeOj -PJ 05 -SiOz system ding is a low loss material compared to the substrate tube; in were developed by Osanai [13] and Sommer et al. [19]. The both single mode and multimode fibers, a finite amount of substitution of Pz05 for Bz 03 resulted in many beneficial power is propagated in the cladding, depending on the fiber effects: lower fusion temperatures, thus easier processing; design. Sufficient low loss cladding is necessary to assure low lower ~ayleigh scattering, thus lower loss; and yet better attenuation [15 ] . After the cladding is deposited, the core long wavelength loss, due to the elimination of the boron material is laid down according to a programmed chemical associated infrared absorption tail. Similarly, early single delivery mte. The overall efficiency of material incorporation mode fibers were based on glasses in the SiOg-B203 system is typically between 40-70 percent, but the incorporation of and were quite suitable for shorter wavelength applications. GeOz is quite low relative to S,iOj. The chemistry and deposi- With increased interest in optimizing single mode fibers for tion mechanisms involved will be discussed in subsequent long wavelength (1.3 -1.55 #m) applications, boron has been sections. eliminated as a dopant, and a variety of structures based on Following deposition, the tube and deposit are collapsed to core and deposited cladding glasses in the system Si02 -Ge02 - a solid rod called a preform by heating to temperatures sufficient F-Pz 05 are being explored by various investigators [22] - [25]. to soften the substrate tube. The preform is then transferred The basic design of a lightguide consists of a core glass to a fiber drawing apparatus where it is drawn and coated. The surrounded by a cladding glass of lower refractive index n. structure of the fiber exactly replicates the preform. The numerical aperture NA is a function of the refractive index difference An between the core n, and cladding n2 [26] : III. MATERIAL CHOICES AND FIBER DESIGN

Three basic fiber types of current importance are step index NA=(n~ - n2)1/2 =nl @ multimode, graded index multimode, and single mode, with the latter two of greatest interest. There has been a historical where A = An/nl. evolution of both multimode and single mode fiber design, The fiber NA and core diameter 2a determine the number of both in terms of composition and dimensions. Design has been optical modes which propagate. These are related to the dictated by performance in such areas as attenuation, band- normalized frequency V, given by width, numerical aperture, bend sensitivity, splice loss, and mechanical properties, as welll as the cost of the final product. (2) The MCVD process has demonstrated tremendous flexibility in the ability to fabricate fibers of a wide variety of designs, compositions, and dimensions, and many of the fundamental where X = wavelength of light. studies have utilized fibers made by this process. To achieve single mode transmission in an idealized step Silica is the primary glass former used in lightguide fabrica- index fiber, V must be <2.405 [26] . The wavelength at which tion. Additions are made in order to alter its refractive index the fiber becomes single mode is termed the cutoff wave- and build a waveguide structure. Ge02 is the predominant length Ac. dopant for increasing the refractive index of Si02 in MCVD Attenuation is an important consideration in fiber design. NAGEL et al.: MODIFIED CHEMICAL VAPOR DEPOSITION 307

CAE DEPOSITEDCLAD RAYLEIGH SCATTERING VS A 20 o YOSHIDA ET AL (1977) A GAMBLING ET AL (1976) ● SCHROEOERET AL (1973) n HORIGUCHIET AL (1976) ■ 6LANKENSHIP ET AL (1979)

‘E F•P205 0S102 d. - Ge02-B203-3i02 ~ 1.6 g Ge02 ..$02 Sio* . z { Pzo~.s40~ -p=s ‘3 s? ~ Ge02-P205-SIOz 2 .- SIO* F* P206*Si02 z ~ / Fig. 3. Typical MCVJ) lightguide compositions for single mode and a multimode fiber.

The intrinsic loss ai of high silica glasses in the near infrared i “0 : ~~’o’ 2 region of the spectrum is composed of three components [26]: SIO~ (BULK) -S’” ~j = ~uv ~ ~IR + ffR (3) -“ ❑-P*05-3i02 = ultraviolet absorption, @ = infrared absorption, 0.5 I 1 , , where auv 0 0.5 1.0 1 and @R= Rayleigh scattering. A (%) RELATIVE TO 3i0 ~ Ultraviolet absorption is determined by the electronic band- Fig. 4. Rayleigh scattering coefficient as a function of normalized gap of a material and decays exponentially with increasing refractive index difference for various lightguide compositions. wavelength. The tail of the UV edge in glasses is almost negli- gibly small in the near infrared. In typical lightguide composi- lightguide glasses reported in the literature [33] - [37] as a tions, the UV absorption is due to Ge02, as reported by function of refractive index difference A relative to SiOz. Schultz [27]. The UV contribution to the loss ht any wave- These data clearly show the compositional sensitivity of length can be expressed as a function of the mole fraction x of scattering loss. Small additions of Pz 05 markedly reduce the GeO, [28]: Rayleigh scattering compared to Bz 03, as Sommer et al. [19] 154.2x 4.63 and Blankenship et al. [37] reported that higher levelsofP205 X 10-2 exp — (4) with decreased Ge02, to achieve the same A, even further re- auv = 46.6X +60 () A“ duces scattering. In general, the higher the A -of the fiber, The infrared absorption tail results from very strong cation- typically accomplished by increasing Ge02 levels, the higher oxygen vibrational modes of the glass lattice at long wave- the intrinsic loss levels. lengths. The fundamental vibrational bands of B-O, P-O, An intrinsic transmission window exists between the infrared Si-0, and Ge-O occur at 7.3, S.0, 9.2, and 11.0 Mm, respec- and ultraviolet losses, with the minimum loss occurring in the tively, in high silica glass and extend to shorter wavelengths 1.55 urn region for fibers lightly doped with Ge02 [28]. [29]. Izawa et al. [30] reported the absorption spectra for However, extrinsic loss mechanisms due to impurities, im- Ge02 -, P205 -, andB203 -doped fused silica over the 3-25 pm perfections, and fiber design in regard to bend sensitivity can range. The extension of the B203 tail is especially deleterious cause additional attenuation. to the loss, even at wavelengths as low as 1.2 #m, thereby Transition metal ions at the level of a few ppb’s can cause making it an unsuitable dopant for long wavelength applica- unacceptably high absorption losses in lightguides [I], [38]. tions. The IR edge for GeOz -doped silica was expressed by The optical absorption of 3d transition elements in fused Miya et al. [28] as silica was reported by Schultz [39]. In fibers prepared by MCVD, loss due to transition metal impurities is not a problem. -48.48 a~R =7.81 X 1011 X exp ~ . (5) Another source of absorption is due to hydroxyl ions in () the glass structure. In silica-based glasses,a strong fundamental This constituent makes a small contribution to the loss at Si-OH vibration occurs at ‘2.7 #m and results in a first and 1.6 pm. Irven [31] has recently reported on the effect of Pz 05 second overtone at 1.38 and 0.95 #m, respectively, and a in the 1.6 pm region and postulated that the added loss is combination overtone at 1.25 pm [40], [41]. For every ppm consistent with the P-O phonon absorption. The effect OH, losses of48 dB/km at 1.39 urn, 2.5 dB/km at 1.25 #m, and increases with increasing P2 05. 1.2 dB/km at 0.95pm are added; thus, very low levels of OH Rayleigh scattering results from compositional and density are necessary to achieve low loss at long wavelengths, especially fluctuations over distances much smaller than the wavelength because the peaks are relatively broad. The exact peak position of light, and decays with the fourth power of wavelength, as and width are a function of composition [32], with additions expressed by of GeOz moving the peak to longer wavelengths [42]. When Pz05 is added to Si02, a new and very broad band ~R =Ah-4 appears at 3.05 pm, attributed to the fundamental P-OH stretch where ~ is the Rayleigh scattering coefficient, expressed in [43]. The first overtone of this band is quite broad in the 1.5- units of dB/km . #m4. Fig. 4, after Olshansky [32], sum- 1.7 Mm region, and has been reported by Blankenship et al. marizes the Rayleigh scattering coefficients for a number of [37] and Edahiro et al. [44]. The higher either the PZOS or 308 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982

OH content, the greater the added loss. Thus, fibers for use The optimal a is thus a function of composition. In binary in this region of the spectrum must either have extremely low Pz OS-Si02 glass there is very little variation of aOPt with wave- OH levels or avoid Pz 05 as a dopant. length because of the similar material dispersion character- Other sources of loss in fibers are due to waveguide imper- istics [64]. Ge02 -Si02 compositions [32] , [57] have a very fections such as bubbles and defects, microbending or other strong dependence of aOPt on wavelength. Ternary GeOz - bend sensitivity due to fiber design or coating irregularities, Pz 05 -Si02 will fall somewhere between, depending on the and interracial scattering. Typically, such effects give rise to relative doping levels [19], [32] , [65] . Blankenshlp et al. loss which is independent of wavelength. [37] were able to use higher P, 0, levels in this system in Inada [45] proposed a graphical method to analyze fiber order to decrease the sharp dependence of bandwidth on wave- loss mechanisms by using the relationship that loss a is length. Marcatili [66] has studied the broader case where the profile dispersion is not constant, such as whenB203 is added ~ =A~-4 +B+ c(~) (7) to GeOz -SiOz, or for high NA fibers. Peterson et al. [67] have where A is the Rayleigh scattering coefficient, B is wavelength- used exact solutions to Maxwell’s equation to predict dis- independent loss, and C’(l) is any loss mechanism that has a persion effects. Marcuse and Presby analyzed the effect of per- wavelength-dependent component. By plotting a versus A-4, turbations in typical profiles of multimode MCVD fibers [68]. a straight line is obtained whose slope is A and intercept is 1?. The source width AA will also affect the bandwidth. Horiguchi Deviations from linearity are C(k), such as an OH peak. At et al. [69] reported the effect of AX on the maximum band- shorter wavelengths (< 0.7 pm), deviations have been observed width, and found that for a given profile a = 1.95, not only which have been attributed to a number of different effects: did the bandwidth decrease markedly with increased Al, but drawing induced coloration [46] , [47] , drawing induced loss the position of the maximum shifted t o longer wavelengths due [23], [48], and UV damage [49], [50]. Radiation can also to its effect on intermodal dispersion. In addition, for any A, affect the fiber loss, as reviewed by Friebele [51]. In single the further away from ho one is, the lower the bandwidth for mode fibers, loss beyond & can occur due to poor mode a given AA, i.e., the greater the effect of chromatic dispersion. confinement [24], [25 ] . All of the above factors affect the attainment of the theoret- Dispersion in fibers determines the bandwidth performance. ical bandwidth estimated for an optimal a profile. Fiber com- In multimode fibers there are two sources of dispersion: inter- position profile control and spectral width will determine the modal dispersion due to delay differences among the modes actual performance of MCVD lightguides. and chromatic dispersion resulting from the spectral width of In single mode lightguides the total dispersion is only due to the light source. The intermodal dispersion is controlled by chromatic dispersion, and is a function of the material dis- the shape of the refractive index profile, and can be minimized persion and waveguide dispersion [70], [71]. The waveguide by having a power-law shaped profile of the form [52] dispersion is due to the variation in the group velocity of the n(r) = n ~ [1 - A(r/a)*] (8) fundamental mode Vg with wavelength dug/dh. In turn, wave- where Q = profile parameter. guide dispersion is a function of the core radius, A and A. At The value of the optimal a for any wavelength of operation short wavelengths the material dispersion is dominant, but near is affected by the material dispersion of the lightguide [53]. 10 the contribution of waveguide dispersion approaches that Material dispersion J.f is due to the wavelength dependence of of material dispersion. At wavelengths greater than A., the the refractive index and is expressed by [54] two are opposite in sign, and thus can be counterbalanced to shift the zero total dispersion wavelength to longer wavelengths, (9) such as 1.55 Km where the loss is minimum [28] . Thus, by balancing these two effects, single mode fibers can be designed where c = speed of light. to have X. fall over a wide wavelength range. Intense investi- M is a function of glass composition, and for high silica com- gation of single mode fiber design to optimize the fiber per- positions, there is a zero dispersion wavelength ho which formance in regard to bandwidth, loss, mode confinement, bend corresponds to where d2 n/dA2 = O that is in the region of sensitivityy, and wavelength of operation have recently been re- 1.3 ~m [55] . Malitson [56] found X. = 1.274 ~m for fused ported [22] - [25] . Theoretical loss levels have been achieved in Si02. Additions of Ge02 increase A. [57] - [62]; Pz 05 is single mode fibers [28] , as well as very high bandwidths. not thought to shift A. [59] , [61] ; B203 [57] , [60] , [62] In addition to bandwidth and attenuation, fiber design must and F [63] doping slightly decrease A.. take into account other important parameters. For multimode Olshansky and Keck [53] , taking into account material dis- fibers, the dimensions and refractive index difference determine persion effects, predict an optimal a profile described by the sensitivityy to bends. Olshansky [72] has shown the micro- 12A bending sensitivity ~M to be ~.2-p. — (lo) 5“ (12) P is defined as the optimal profile dispersion parameter where a = core diameter and d = outer diameter. Splice loss (11) is predominantly affected by offset, and is proportional to (l/a)’”’ [73] . Fiber cost is related to the volume of material and NI is the group index = n ~ - l(dn/dA). They assumed which must be fabricated; thus, it is also a function of the constant profile dispersion across the’ core. fiber diameter. Based on such considerations, the standard NAGEL et al.: MODIFIED CHEMICAL VAPOR DEPOSITION 309

multimode fiber design has evolved to have a 50 ~m core, of 02 flow. Oxidation of the individual reagents was examined, 125 Mm OD with a fiber iVxt = 0.20-0.23 [74], [75]. The and for both cases they calculated that the equilibrium was optimal single mode fiber design is still under evaluation. reached at high temperatures, according to the reactions .

IV. TUBING CONSIDERATIONS AND SETUP SiC14 + Oz =+Si02 + 2C12 (13) The MCVD process starts with a tube, typically fused silica, GeClq +02 * Ge02 + 2C12. (14) although Vycor can also be used. The tube ultimately becomes For SiC14, the oxidation was complete, whereas for GeC14, the outer cladding in the resultant Iightguide. Early MCVD the amount of GeOz formed, as described by equilibrium configurations used small thin-walled tubes, such as 13 mm ID X calculations, was a strong function of excess oxygen. When 15 mm OD. The current standard process uses a 19 mm ID X GeC14 is oxidized in the presence of SiC14, they calculated 25 mm OD tube. the conversion of GeC14 to Ge02 to be controlled by the de- The tube is first dimensionally characterized for overall size, pendence of equilibrium on temperature and the ratio of con- end-to -end variations, and dimensional uniformity in regard centrations in the gaseous phase. Experimentally, they de- to siding and ellipticity since all of these can affect the dimen- termined only 27 percent Ge02 conversion at 2000 K. sional quality of the final product. The quality of the starting Wood et al. [81] have studied the germanium chemistry tube is important since the existence of bubbles, inclusions, or experimentally using infrared spectroscopic analysis of the other defects can affect both the strength [76] and optical effluent gases, and concluded that low Ge02 incorporation is attenuation of the final product. The most commonly used due to unfavorable equilibria in the reaction hot zone. They substrate tube is Heralux waveguide grade TO-8. examined conditions of GeC14 = 0.05 g/rein, SiC14 = 0.5 g/rein, The tube is cleaned before use with typical cleaning pro- 02 = 1500 cm3 /rein (Fig. 5). Below 1300 K, no reaction cedures, employing both a decreasing step as well as an acid occurs. Between 1300 and 1600 K, reaction commences for etch, This procedure assures removal of any debris or con- SiCl~ conversion to silicon oxychlorides. Above 1800 K, all taminants that could lead to bubbles or impurity effects. SiC14 is oxidized to SiOj. For GeC14, it only reacts partially The setup of the tube is important for controlled MCVD to form Ge02 up to temperatures of’1 800 K as determined processing; it is mounted in a glass working lathe which has by the effluent GeC14. However, above 1800 K the GeC14 precisely aligned chucks, and is connected to the chemical in the effluent increases, indicating less Ge02 formation (Fig. delivery system by means of a swivel device. This must stay 5). They explained this by considering the equilibria of the leak-tight during all phases of the operation to avoid the in- reactions [13], ![14] which have equilibrium constants gress of wet room air. The tube is aligned so it runs true, a critical step in the subsequent high temperature processing. K1 = asi#2 C12 lpSiC14pOq (15) It is then flared and connected to a larger exit tube designed to K2 = a&02 P2clz /PGeC14pO~ (16) collect unincorporated particles; a stress-free joint is essential. The larger exit tube is coupled to’s scrubber designed to remove where asio2, a@02 = activities of species in solid reaction chlorine and particles. product (these are taken as equal to the mole fraction of the The tube is then fire polished at very high temperatures (up constituent in the solid) and P = partial pressure of component. to 2000”C) in order to remove surface irregularities and shrink The equilibrium for SiCIJ is far to the right at high tempera- any tubing bubbles. An oxyhydrogen torch is most commonly tures (Kl = 6 X 104 at 1800 K); thus, all SiC14 is converted used as the external heat source, and the tube temperature to Si02. However, for the GeC14 equilibrium, K2 = 0.36 at is monitored with an optical pyrometer. The signal from this 1600 K and 0.17 at 2000 K, and the equilibrium is shifted to can be used to feedback to a flame control module. Under the left by the large amounts of C12 produced in the oxidation optimal conditions, tube temperature is controlled to *2°C of SiC14. Thus, as temperature is raised, less Ge02 is formed during each traversal pass. and more GeC14 is detected in the effluent (Fig. 5). The minimum in the effluent partial pressure of GeC14 at’1 800 K V. CHEMISTRY marks the transition from the reaction being rate limited tQ Various aspects of the chemistry play a key role in determining being equilibrium limited. MCVD is typically operated at the characteristics of the final fiber product. Very little has temperatures >1800 K or in the equilibrium-limited regime. been reported on the chemistry involved in the MCVD process. Wood et al. [81] also showed that SiBr4 could be substi- Powers [77] and French et al. [78] studied the kinetics of tuted for SiC14, resulting in a decrease in C12 concentration in the oxidation of various reagents such as SiC14, GeC14, POC13, the MCVD atmosphere, thus increasing the extent of GeC14 SiBr4, and BC13 in the gaseous phase at high temperatures. reaction. They found that the fraction of GC02 incorporated They concluded that under normal MCVD operating conditions was increased by greater than 3X, thus providing further the reaction will go to completion. Wood et al. [79] reported experimental proof that the incorporation efficiency for Ge02 on a technique to study reactions by direct infrared absorption is largely controlled by equilibrium. spectrophotometry of the flowing gas system in MCVD. They Thus, equilibrium considerations are important in terms of found that the oxidation of SiC14 resulted in oxychloride relating the input chemical delivery program to the amount of formation at intermediate temperatures (900-1 10O°C), but dopant incorporated in the resultant preform. The addition at higher temperatures converted entirely to Si02. of POC13 and/or fluorine dopants to the MCVD gas stream Kleinert et al. [80] have examined the equilibria of the further complicates the equilibrium chemistry and is currently oxidation of SiClq and GeCIJ at high temperatures as a function being evaluated [82]. 310 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982

3-10 percent C12 present due to the oxidation of the chloride —— reactants. Wood et al. [84], [85] and LeSergent et al. [86] have shown that the chemical equilibrium results in almost all the hydrogen present being converted to HC1 which is not in- corporated into the glass network. The level of hydrogen contamination is reduced by a factor of -4000. However, during collapse, chlorine is typically not present, and signifi- cant amounts of OH can be incorporated, as reported by Pearson [87]. Three approaches can therefore be taken to reduce OH contamination in MCVD: to reduce the H20 concentration by careful processing and purification of chemicals; increasing the partial pressure of C12 such as by adding it during deposition L?L’l’I or collapse; or lowering the 02 partial pressure such as by RT 11001200 1400 1600 1800 2000 2200 T(KI using an inert gas or lower excess Oz flows. Fig. 5. MCVD effluent compositions for reactions at various tempera- Walker et al. [83] have reported that the use of a 20 percent tures, starting composition: GeC14 = 0.05 g/rein; SiC14 = 0.5 g/rein; chlorine atmosphere during collapse reduces the OH peak by 02 = 1500 cm3/min. Dotted line indicates initial partial pressures. >twofold for multimode fiber fabrication and >tenfold during single mode fabrication, which agrees well with the equilibrium Another important aspect of MCVD chemistry is the incor- predictions. Ainslie et al. [88] have also used this approach. poration of the impurity OH. The reduction of OH in optical Fig. 6 shows the dependence of the SiOH concentration in fibers to very low levels has been a key to the realization of the resultant glass as a function of the typical PO, and PC,, close to theoretical attenuation levels in the 1.3- 1.7 pm conditions which occur during deposition, soot consolidation, region of the spectrum. The sources of OH contamination and and collapse with and without chlorine when 10 ppm H20 is the chemistry of OH incorporation must be understood and in the starting gas. The beneficial effect of chlorine during controlled in MCVD in order to achieve these very low levels, collapse is clearly demonstrated. In MCVD, practiced with and considerable advances have been reported by many CIZ collapse techniques, the residual water is believed to be workers. incorporated during deposition, resulting in ‘0.04 dB/km/ In MCVD, OH contamination has been identified as coming ppm HZO in the gas stream at 1.39 ym. from three sources [13]: thermal diffusion of OH from the VI. THERMOPHORESM starting substrate tube into the core during processing; in- corporation of OH from impurities in the starting reagents and Thermophoresis has been conclusively established as the carrier oxygen gas; and contamination which results from particulate deposition mechanism in MCVD [89]- [92]. The leaks in the chemical delivery system or swivel during process- basic mechanism of thermophoresis is that a suspended particle ing. This last source of contamination can be eliminated by in a temperature gradient experiences a net force in the direc- very careful construction and frequent leak testing of the tion of decreasing temperature, resulting from the fact that equipment, as well as through careful processing. molecules impacting the particle on opposite sides have differ- Diffusion of OH into the active optical region of the fiber ent average velocities due to the temperature gradient. has been studied by a number of investigators [11] - [14] and Walker et al. [90] have reported a mathematical model for can be eliminated by deposition of sufficiently thick low OH thermophoretic deposition in which they assessedthe quantita- cladding. The remaining source of OH is that which comes in tive importance of thermophoresis in MCVD by considering a during processing either through the gas supply, reactants, or simple idealization of the process, and found that it predicted leaks in the system, and is determined by the chemistry in- deposition efficiencies of the same magnitude as had been volved in the MCVD reaction [83]. The OH level in the fiber experimentally observed. A basic feature of the mathematical is controlled by the reaction model was that the deposition efficiency under normal MCVD operating conditions was a function of the equilibrium tem- H20+C1Z *2HC1+ *02 (17) perature Te at which the gas and tube wall equilibrate down- which has an equilibrium constant K: stream of the hot zone and the temperature at which the chemical reaction to form particles TFx~ occurs. The deposi- ~ = (PHC,)’ (Po2)’/2 tion efficiency e was given by e x 0.8 [1 - (Te/Twn)] where (18) (PH20)(PC,2) the temperature is in degrees Kelvin. Subsequent modification of the mathematical model was where P = partial pressure of the component. made in order to compare experimentally measured efficien- The amount of OH incorporated into the glass COH is cies under a variety of different experimental conditions to described by theoretical predictions [91 ], [92]. This involved using measured outside wall temperature profiles, realistic effective (19) reaction temperatures, and other relevant operating parameters to compute the temperature field within the tube and the sub- During the deposition phase of MCVD, there is typically sequent particle trajectories which resulted from this tempera- NAGEL et al.: MODIFIED CHEMICAL VAPOR DEPOSITION 311

10ppm H20 IN THE (cm) ~ MCVO COLLAPSE 10.5 STARTINGGAS o 10 20 30 40 50 60 67 } CONDITIONS

10-6

10.7-

} gy:N::~j:q:[

-ih

:2 ~~ 7 ,08 ,Oe ,04 ,02 ,00 10-2 ,0-4 Plo2WP’cf2 ISOTHERMS ToRcH Fig. 7. Example of the temperature field inside MCVD substrate tube Fig. 6. Equilibrium concentration of SiOH as a function of typical when heated by an external torch. partial pressures of 02 and C12 during MCVD processing for a given initial H2 O concentration (10 ppm). (cm) 10 20 30 40 50 60 67 1,00 ture field. They then could predict efficiencies. Fig, 7 shows 0.8 one case of the temperature field within the tube, while Fig. 8 0.6 .= shows the resultant trajectories. As cool gas enters the hot 0.4 zone it begins to heat, and at any point where the temperature 0.2 o T> Tn~, a particle is formed at a particular radial position. As the gas and particle continue to flow through the hot zone, the gas heats further because the wall temperatures in the hot zone are greater than Tnn, resulting in the particle moving toward the center of the tube. Further downstream the wall

temperature is less than the gas temperature, resulting in the zONE particle reversing direction and moving toward the tube wall. Fig. 8. Particle trajectories resulting from temperature field of Fig. 7. Certain trajectories near the wall result in particle deposition, while other particles nearer the center are swept out of the \ tube. Excellent agreement between the predicted and experi- mentally observed efficiencies for deposition of pure SiOz established thermophoresis as the predominant mechanism for deposition. ‘R= The model was also used to establish two distinct operating BASE LINE CASE; COMPLETEREACTION; regimes in MCVD related to flow rate and “thermal configura- E IS DEPOSITIONLIMITED; REACTANTSWITHIN tion [92]. Under normal operating conditions, the chemical Rd PROOUCEPARTICULATEEXHAUST ——. reaction goes to completion within the entire tube because the ,~ o ------(b) ““7 temperature of the gas stream exceeds the temperature neces- R,- sary for reaction [Fig. 9(a)]. Iir this case the process is limited R~-T ---- \ by the transport of particles to the tube wall, with particles at /f 1 positions r> Rd depositing. Efficiency is described as e ~ I I INCOMPLETEREACTION 0.8 (1 - Te/Tnn) and is most sensitive to T.. The value of T, E IS DEPOSITIONLIMITEO;REACTANTSBETWEEN will depend strongly on the torch traverse length, the torch Rr AND Rd PROOUCEPARTICULATEEXHAUST traverse velocity, the ambient temperature, and the tube wall (c) thickness, but is only weakly dependent on the flow rate and tube radius. ,> , At higher flow rates, there is a region in the center of the !’ ‘Rg__...:1 J tube where the temperature does not reach Tn~, and a critical INCOMP.REACT.;E IS REACTIONLIMITEO; radius RF exists such that for radial positions r > RY, reaction NO PARTICULATEEXHAUST and particle formation occur, and for positions r < RV, there is EVOLUTIONOF TRAJECTORYPATTERNS WITH INCREASINGFLOW RATE no particle formation. If R, < Rd, efficiency is limited by Fig. 9. Trajectory patterns asa function of increasedflow. particle transport, as shown in Fig. 9(b). Fig. 9(c) illustrates the case of reaction-limited deposition where all particles formed are deposited, but particle formation is limited by in- Operation in the deposition-limited regime is preferred for complete reaction. Deposition is then extremely sensitive to maximum process control, and can be assured by using lower the hot zone length, gas flow rate, gas thermal diffusivity, and total flow rate Q; adding helium to the gas stream to increase especially the internal wall temperature, making controlled the thermal diffusivity a, and using a broader hot zone, deposition very difficult. thereby more effectively heating the gases; or by operating 312 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982

with higher wall temperatures. In this regime, efficiency case, bubbles degrade optical performance. This sensitivity to changes only slightly with flow rate due to changes in Te, and bubble growth is a function of the chemical composition of deposition takes place over a length L. proportional to Q/a. the material, the size of the closed pore (which is determined Variations in T, from one end of the tube to the other can by the details of the deposition conditions), and the tempera- result in variations in deposit thickness and thus Te must be ture. Therefore, proper consolidation of MCVD films requires controlled. In addition, at the starting point of deposition optimization of operating conditions to avoid regimes where there is inherent entry taper which corresponds to the value of bubble growth can occur. one deposition length L. Entry taper can be minimized by low The details of the consolidation have some practical implica- Q and/or high a. tions for proper MCVD processing as well as on optimizing and The understanding of the thermophoretic mechanism has scaling up the process. Conditions which minimize the size of been a key to subsequent scale-up, high rate deposition, and initial void regions as well as the void fraction will enhance the optimization of the MCVD process. consolidation rate. It has been shown that under certain operating conditions [93] the addition of helium promotes VII. CONSOLIDATION proper consolidation. The formation of bubbles at high tem- In MCVD, the consolidation mechanism has been experi- peratures limits the maximum operating temperature; there- mentally identified as viscous sintering [93]. The rate of con- fore, in order to assure complete consolidation, particularly at solidation was shown to be inversely proportional to the higher deposition rates, the sintering time must be increased modified capillary number C where such as by using a broader hot zone. Lastly, there is no funda- &(l - CO)113 mental limit on the thickness of a layer which can be success- c. fully consolidated as long as the proper time-temperature at~ relationship is realized. v = glass viscosity VIII. COLLAPSE 10 = size of initial void regions The collapse phase of MCVD has a number of practical co = initial void fraction implications, and is therefore important to understand. The u = surface tension collapse rate is an important component in the total preform t~ = characteristic sintering time (= LH/ W where LH = hot processing time, and thus affects process economics. The zone length, W = torch traverse velocity). collapse stability affects the dimensional performance of the The sintering rate is therefore very sensitive to temperature, final product. Finally, this high temperature phase of the the chemical composition of the particles, primarily due to the process affects the resultant fiber refractive index profile by strong dependence of glass viscosity on these variables, and on enhancement of a refractive index dip in the center of the the gas thermal properties. fiber due to vaporization of volatile components such as Ge02 Walker et al. [93] showed that the consolidation of ger- and PZ05, and therefore can result in degradation of fiber manium borosilicate films proceeded both axially due to bandwidth [95]. progressive heat treatment, and radially due to a composition Both Lewis [96] and Kirchoff [97] have reported on the gradient across the layer, as shown in Fig. 10. Densification collapse of viscous tubes. The collapse rate is related to the is most rapid at the position closest to the tube wall where the viscosity of both the core and cladding material, the composite GeOz content is highest and thus viscosity lowest. Kosinski tube dimensions, the surface tension of the materials, and the et al. [94] have shown that consolidation of Ge02-P205-Si02 pressure difference between the inside and outside of the col- films proceed by the same mechanism, but that the deposition lapsing tube, The collapse rate R is roughly proportional to conditions affect the compositional variation in each layer. ~QPo -Pi+ ((J/RID)+ (IJ/Ro~) Consolidation of pure SiOz films was examined, and was q(T, C, t) shown to consolidate uniformly due to the lack of composi- tion gradient. where Pi = internal pressure in tube, P. = outside pressure on In all cases, consolidation results from a sintering process in tube due to torch, a = surface tension of glass, RID,OD = inside which the energy required for mass transport is provided by a and outside radius of tube, respectively, and q = viscosity as a decrease in surface energy due to a decrease in surface area. function of temperature T, composition C, and time t. The initial stages of consolidation proceed via the formation Extensive modeling of the details of collapse of MCVD and growth of necks between adjoining particles where the composite tubes has recently been accomplished by Geyling radius of curvature is smallest, and further densification results and Walker [98]. Some basic conclusions on collapse rate can in smooth networks of bridges which grow together slowly to be drawn. The surface tension of glass is only a weak function form a pore-free glass layer. of temperature, and therefore cannot be varied in order to en- The irregularly shaped closed pores present during consolida- hance the collapse rate. As the tube dimensions become tion demonstrate that diffusion of gas out of the pores is smaller, the surface tension term will become a larger compo- faster than the transport of glass as the pores shrink. Walker nent in determining the collapse rate at a given AP. The et al. [93] identified conditions which can lead to formation viscosity of the tube and deposit is determined not only by the of bubbles in MCVD fabrication, either through incomplete composition, but also by the details of the hot zone geometry consolidation or excessive deposition temperatures; in either during collapse. Because viscosity decreases with increased NAGEL et al.: MODIFIED CHEMICAL VAPOR DEPOSITION 313

Fig. 10. SEM micrographs of the consolidation of a particulate layer in the MCVD process- showing both surface (top) and cross-sectional views (bottom).

temperature, very high collapse temperatures favor fast col- STABILITYW PRESSUREDIFFERENCE A P (DYNESlcm7) lapse rates, but practical limitations are imposed due to fluid -1oo 0 100 200 1 I flow of the heated rotating tube, as well as chemical composi- I I tion considerations such as those imposed by GeO volatization 20 t d at the internal surface of the collapsing preform. Broader hot zones can be employed to increase collapse rate by virtue of the time-temperature effect of viscosity on the collapse rate, “‘but again, practical limitations are imposed by heating an ex- tended region of the tube into a regime where fluid flow can cause distortions in the collapse geometry, Nagel et al. [99] have reported on the optimization of collapse conditions to I / I considerably reduce the collapse time in standard MCVD multimode preform fabrication. Decreases in the collapse rate are only significant if coupled with the fabrication of preforms of good dimensional circu- larity. Geyling and Walker [98] have also modeled collapse -0.04 0 stability. A key result in their study was the dependence of 0.04 0.08 :Ua P (INCHESH * O) collapse stability on the’ initial tube geometry (wall thickness Fig. 11. Preform core and cladding ellipticity as a function of pressure variations and ellipticity) and on the pressure difference be- difference during collapse. tween the inside and outside of the tube. Fig. 11 shows the dependence of the final core and cladding ellipticity on AP during collapse. The outside pressure is a torch stagnation The work of Geyling and Walker [98] demonstrates that in pressure which results from the flow of gases as high velocities order to realize a stable geometric collapse, tube quality and during collapse and is a function of torch design and flow pressure difference must be considered. rate. The inside pressure is determined by the details of the During the collapse phase of” MCVD, very high temperatures collapse procedure used. If collapse is performed with the enhance the reaction tube open to atmosphere, tube geometry and flow rate will Ge02 +GeO(T) + + 02. (22) determine the pressure. In the case of pressurized collapse, the tube is sealed off at one end (corresponding to the right- Similarly, both P205 and SiOz can undergo volatization pro- hand erid in Fig. 1) and is accomplished by heating while cesses. The net effect is to cause a change in the composition traversing in the opposite direction of deposition. The inside of the deposited materials. Two schemes have been proposed pressure can be regulated by means of a pressure-metering to compensate for these effects. The first, proposed by device. Pressure control was first effectively used by French Akamatsu et al. [101], uses a flowing controlled amount of and Tasker [100] to fabricate fibers of excellent circularity. GeC14 during collapse to compensate for the burnoff of Ge02. 314 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982

This technique can be used quite successfully; however, its Em. ~ PREFORMFEED MECHANISM control during collapse is difficult and is markedly affected by the specific details of any given collapse procedure, particu- ZIRCONIAINDUCTION larly when Clz or SOCIZ are employed as drying agents. As a result, the practical implementation of this technique is deter- mined by empirical conditions defined by the particular pre- form fabricator. Another technique involves etching, which has been described by Hopland [102], and offers another COATING _ COATING alternative in processing. It also depends on an experimental APPLICATOR definition of specific conditions in order to minimize the I effect of the refractive index dip. CURING FURNACE Optimization of the collapse phase in MCVD thus involves OR LAMPS use of good geometric quality tubes, proper control of the 1 CAPSTAN length and temperature of the hot zone for a given tube geometry and relative clad/core composition and geometry, .@ control of the pressure difference, and control of the chemistry I 11 I I at the inner surface of the collapsing tube. !&-L Fig. 12. Schematic of state-of-the-art fiber drawing appmatus.

1X. FIBER DRAWING

X. PROCESSING CONTROL Significant advances have been made in fiber drawing tech- nology over the past few years, independent of the processing The implementation of MCVD into manufacture requires technique used to prepare the fiber precursor. The fiber draw- material and process controls in both the preform fabrication ing step determines the overall diameter, its variation, and to and fiber draw stages of the process. an extent, the fiber strength; it can also affect transmission For maximum control of preform processing, Runk [107] properties. reported that the following parameters should be monitored Fig. 12 shows a typical state-of-the-art fiber drawing ap- and controlled. paratus, capable of producing strong low-loss fibers with Geometric: protective coatings and excellent diameter control. Blyler and 1) the geometry of the support tube, including its diameters, DiMarcello have recently reviewed the important factors in- ovality, siding, and bow, volved in the fiber draw process [103]. 2) the mechanics of the setup procedures, DiMarcello et al. [104] reported on the drawing of long- 3) the mechanics for eliminating the initial stress and/or length high-strength fiber. His results show that as the fiber bow in the support tube, proof test level increases, the longest continuous length which 4) longitudinal and radial differential thermal expansion can be drawn decreases. Important furnace operating parame- effects, ters relating to convective flow through the furnace were de- 5) process temperature variations, and fined, thus indicating the potential for achieving high yields 6) heat source alignment. for long-length high-strength prooftesting. Optical: An important advance reported recently by Smithgall and 1) precursor purity, Frazee [105] involved use of an automatic detection and 2) dopant (type and quantity), centering technique for fibers within the applied protective 3) dopant profile, coating. This technique allows fiber coating concentricity to 4) deposition rate, be tightly controlled without disturbing the coating process, 5) support tube/deposition interface, and thus assuring no deleterious effects on the strength of the fiber 6) preform station hardware. or on coating-induced effects on the loss or bandwidth. Mechanical: A key factor to the economic drawing and coating is fiber 1) support tube quality, draw speed. Pack and Schroeder [106] have reported the 2) support tube and preform handling, and capability of drawing fibers at speeds >5 m/s compared to the 3) preform station environment. current standard 0.5- 1.0 m/s. This required a tall draw tower In order to control tube geometry during deposition of to control the fiber temperature entering the coating cup and highly viscous compositions which require high processing avoid menicus collapse, a special applicator to minimize shear temperatures and lead to tube distortions, several workers as well as draw tension, and a high-power UV-curing system to [108], [109] have reported the use of pressurizing devices to assure proper photopolymerization. prevent premature collapse of the tube. Most of the Iightguide Further work continues in the areas of long-length high- performance parameters (such as attenuation, numerical aper- strength fiber, fiber draw speed increases, and development of ture, bandwidth, relative core/clad dimensions, and fiber and coatings which result in good bend sensitivity protection and core ellipticity) will be determined during this phase of the to protect the fiber from strength degradation. operation. NAGEL et al.: MODIFIED CHEMICAL VAPOR DEPOSITION 315

The fiber drawing phase of the operation plays a major role They also implemented a faster collapse procedure by careful in determining the fiber diameter and its control as well as tube alignment, adjustment of the hot zone length and uni- strength. Runk [107] reported that the following components formity, proper balance of the internal pressure relative to the are critical in the fiber drawing manufacturing environment: torch pressure, and control of the tube viscosity. Preforms 1) the basic structure and precision drawing mechanics, capable of yielding 15 km were made in less than 7 h, and 2) a high temperature, clean heat source with appropriate exhibited very low loss and OH (0.5 dB/km at 1.3 ~m, 0.5 temperature control, dB/km OH peak height at 1.39 flm), bandwidth as high as 1.8 3) diagnostic devices including precise, high rate diameter GHz “ km, and good dimensional control. measurement, Simpson et al. [120], [121] and Tsukamoto et al. [122] 4) a diameter feedback control system, and have fabricated fibers at very high deposition rates (up to 1.3 5) an appropriate coating apparatus. g/rein) by modification of the standard process. Simpson et al. Very high strength fiber has been drawn using the zirconia used very high flow rates, a thinner walled silica support tube, induction furnace [76], [1 10], a controlled atmosphere [76], and a very broad hot zone (accomplished by use of two [103], careful preform handling [76], particulate free coating torches). This was coupled with the use of water cooling of material [76], and a noncontacting coating applicator [1 11]. the support tube downstream of the hot zone to increase the Excellent diameter control is achieved by using a diameter deposition efficiency, thereby minimizing T. and shortening feedback control system [11 2], and isolating the draw process the deposition length over which particle deposited. They from disturbances [1 13]. found that the length of the water-cooled region effectively determined the deposition length L, and hence the entry XI. MCVD SCALE-UP taper as well. They were able to further reduce entry taper by The fabrication rate of preforms is an important factor af- use of helium to increase the thermal diffusivity of the gas fecting fiber cost. Early MCVD processes operated at deposi- stream. Fig. 13 shows the preform taper as a function of the tion rates of 0.05-0.1 g/rein which was much higher than various experimental conditions. Water cooling and helium conventional’ CVD schemes for making fiber [114], [115]. resulted in small entry taper, highest rates, and uniform end-to- Since that time, attention has been focused on understanding end dimensions. Using this technique, they have achieved the fundamental mechanisms involved in MCVD in order to losses as low as 2.8 dB/km at 0.82 and 0.7 dB/km at 1.3 pm scale up the process without a degradation in the optical and and deposition rates on the order of 1 g/rein. Tsukamoto et al. dimensional quality of the resultant fiber. In addition to in- [122] have reported use of similar modifications to achieve creasing the deposition rate, accompanying decreases in the deposition rates up to 1.3 g/rein. They used an overcladding collapse time are necessary to fully optimize the process rate. technique to obtain the proper clad/core diameter and were Initially the emphasis was to use MCVD to make low-loss able to draw 40 km of 50pm core 125 pm OD fiber. high-bandwidth fibers, and typically low deposition rates Pearson [123] reported fabrication of single mode fiber at (<0.25 g/rein) were used. In fact, it was believed that higher deposition rates of 0.5 g/rein in a 19 mm ID X 25 mm OD rates degraded these properties; Osanai [13] reported that tube. Losses of 0.52 dB/km at 1.3pm and 0.28 dB/km at 1.55 fiber loss increased with deposited layer thickness greater than pm were achieved. By using an overcladding technique, he also -10 Mm. Akamatsu et al. [1 16] and O’Connor et al. [1 17] was able to produce a preform capable of yielding 40 km of reported improved deposition rates by adding helium to the fiber. gas stream which enabled them to deposit and fuse thicker The evolution of increased deposition rates is shown by layers. Yoshida et al. [1 18] reported higher deposition rates Fig. 14, after Cohen [124]. These have resulted in substan- (0.37 g/rein) without the use of helium, accomplished by tial increases in fiber fabrication rate, which is defined as the higher traverse speeds in order to minimize the layer thickness. length of fiber produced for each hour of preform processing Nagel et al. [91] reported high deposition rates without a time, including setup, deposition, collapse, and removal. Esti- loss penalty by use of a broad hot zone which allowed layers mates for the increase in fabrication rate if overcladding tech- to fuse properly. Subsequent work by Nagel and Saifi [1 19] niques [11 6] were used are also shown. The plasma-enhanced reported very low losses (0.5 dB/km at 1.3 pm) over the MCVD technique [125], to be discussed in the next section, 0.7- 1.7 ~m wavelength region at deposition rates up to 0.34 has resulted in considerable increases in fabrication rate. g/rein and layer thickness of 16 pm/pass. Further work is being directed at optimizing high deposition The understanding of the thermophoretic deposition mecha- rate fabrication as well as optimizing and reducing the time for nism and viscous sintering fusion mechanism has allowed the collapse of scaled-up preforms, consistent with the attainment process to be further scaled up without a loss in reaction in- of loss, bandwidth, and dimensional performance. corporation efficiency or improper fusion. . Nagel et al. [99] recently reported on a reduced cycle MCVD XII. PLASMA-ENHANCED MCVD process, optimized for fabrication of preforms from a 19 mm X 25 mm tube. By adjusting the flow rate, hot zone A number of investigators have used plasma-activated pro- length, and temperature to provide complete reaction, proper cesses to initiate chemical reactions within a tube in order to sintering, minimum entry taper, and avoidance of particle prepare lightguides. Both microwave plasmas” (PCVD) [6], agglomeration, deposition rates of 0.5 g/rein were achieved. [126], [127] and RF plasmas have been studied. In PCVD, it 316 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982

ROTATING PREFORMTAPER ~ 1.2 SILICA TUSE WATER COOLING r E

~ 0.6 m,WATER ANo HELluM 5 g 0.4 . ,HELluM, No WATER G . =WATER,NO HELIUM 2 0.2 ● . No WATER,No HELIUM I I\/ 021HZ - 0+ INOEPENOENTLYTRAVERSING 051015202530354045 505560 FLAME COIL ANO FLAME AXIAL POSITION(CM) Fig. 15. Schematic of the plasma-enhanced MCVD process. Fig. 13. Comparison of taper and core uniformity in high rate MCVD preforms prepared under different operating Conditions. have initial densities -65 percent theoretical. All conventional

RATEINCREASESIN MCVO MCVD dopants can be used. I I Fleming and O’Connor [13 1] have discussed the importance of centering the fireball, as well as the experimental conditions which affect its stability. The RF power is supplied to the coil at 3-5 MHz, resulting in typical power into the plasma of FABRICATIONRATE -12 kW. The plasma fireball operates at -104 “C; the high ~-W/OVER CLAODING lSSS@FAGRlcATlON RATE temperature of the plasma results in a large temperature ~- OPPOSITIONRATE , gradient, thus providing a large thermophoretic driving force 4 – 7’ Ii?l>/ for efficient deposition. Flowing water continuously cools the g surface of the substrate tube in the region of the plasma, thus 35 % preventing hot spots which can lead to tube distortion. This s ?. cooling also enhances plasma centering, and it minimizes Te to further enhance efficiency. A secondary heat source is used to

1 consolidate the deposited layers. A key feature of the technique is the separation of the de-

VHR RFP position and sintering steps. It has allowed stable operation of MCVO MCVO 1980 1980 the plasma throughout deposition, and realizationof very high Fig. 14. Comparison of deposition rates and preform fabrication rate deposition rates and efficiencies without substrate tube for laboratory MCVD processes: Early [7] , standard [ 119] , high rate distortion. [99], very high rate [122], and RF plasma MCVD [125]. Tremendous progress has been made in the preparation of is difficult to achieve deposition rates over 1.0 g/rein because high-quality high-rate preforms. Recent work has incorporated of the low pressures and nature of the process. RF plasma OH removal techniques to realize excellent long wavelength processes which operate at or near atmospheric pressure performance. Fig. 16 shows the best reported loss spectra for achieve high reaction rates and efficiencies. However, when multimode [133] and single mode [134] lightguides prepared the plasma is used to both initiate the chemical reaction and at deposition rates of 3.5 g/rein. These losses are comparable to sinter the deposited material, plasma instabilities are to the best obtained by standard MCVD, thus providing thought to have limited process performance [128]- [1 30]. further documentation that there is no attenuation penalty The current plasma-enhanced MCVD technique [125], when high rate processes are optimized. [131]- [134], a variation on conventional MCVD, embodies Fleming and Raju [133] also reported the effect of tube many improvements which have led to the fabrication of high- quality on optical attenuation results. Availability of tubes performance fibers at deposition rates> 3.5 g/rein. of proper dimensions which have the equivalent performance Fig. 15 shows a scherrtatic of plasma-enhanced MCVD. The of tubes used in standard MCVD is the key to implementation technique uses an oxygen RF plasma which is operated at of this technique. atmospheric pressure, and is centered in the substrate tube Future studies are aimed at further optimization of the with an annulus of> 1 cm between the visible edge of the fire- process. The very high deposition rates make the technique ball and the tube. In the initial plasma-enhanced MCVD work, particularly suitable for single mode lightguide preparation the tube was traversed relative to a stationary RF coil and where large amounts of deposited cladding are necessary in sintering torch, which limited the attainable traverse rate order to realize optimal properties. [13 1]. Subsequent modification by Fleming [132] used a stationary tube and simultaneously traversed the coil and torch XIII. MCVD PERFORMANCE at identical rates. This increased the attainable traverse rate The MCVD process has been used since its first announce- as well as the length of usable preform. Further modification ment [7] to make very high performance fiber. Steady im- was recently reported by Fleming and Raju [133] in which provement has been made in the important areas of optical they iridependently traversed the plasma and heat source attenuation, fiber bandwidth, fiber strength, control of fiber which allowed independent optimization of the deposition and bandwidth, fiber strength, control of fiber drawing and coat- consolidation step. ing, and in the increasingly important area of single mode Typical plasma operation involves use of a tube with an fiber preparation. Fabrication of the latter for long distance, ID> 40 mm, such as 40 mm ID X 44 or 46 mm OD tubes. high bit rate systems where loss, bandwidth, and bend sensi- Deposition rates up to 5 g/rein have been reported [133]. The tivity are critical, requires careful optimization and control fused layers deposited are typically 45 to 79 pm thick and of the process. NAGEL et al.: MODIFIED CHEMICAL VAPOR DEPOSITION 317

100 I I I 1 I I I 1 1 100 80 : $0 >. ‘ ‘-TT— 7 60 — MULTIMOOE --- SINGLEMOOE 40 ~. x,, 2.0 ‘, $ Z,o _ z ‘\ ~ < -,~., o g 1.0 -., - 0.8 ‘., /“l g 06 4 ~,_) i, 0.4 ‘c t ‘\\,<,/’ 0.4 – — 0.75 -.-040 dSlkm ‘\._+./’ 02 --0.55 —4060 0.2 – –- 030 — 046 dS/km o,lo~ --070 dOlkm 022 dS/km 16 17 01 I I 1- 0.7 80 09 10 1.1 12 1.3 14 15 16 17 WAVELENGTH(pm) Fig. 16. Loss spectra for single mode and multimode fiber prepared WAVELENGTH(pm) by the plasma-enhanced MCVD process at 3.5 g/rein. Fig. 17. Loss spectra for laboratory multimode and single mode fiber. Fig. 17 shows state-of-the-art loss curves for MCVD as prac- ticed at deposition rates of 0.5 g/rein. Multimode 0.23 NA 200 Ge02-P205-S102125Pm/50~ m graded index germanium phosphosilicate fibers [135] show 10.0- BANOWIOTHAT 0.S2 ~ m =2211 MHz km 8.0 - fiber losses near the intrinsic level for this composition: 2.8 120 ~ m =977 MHz km 6.0 a =1.88 dB/km at 0.82 #m, 0.45 dB/km at 1.3 urn, 0.35 dB/l& at 40 - F A=1,27 < 1.5 pm, and OH peak height of 0.5 dB/km (~10 ppb OH). . Fibers of this type exhibit bandwidths> 1 GHz “ ,jun and have ~’ been reported to have bandwidth as high as 1.8 (3Hz “ km [99] . a 1.0 - 0.8 - MCVD can also be used to make high-rate high-NA fiber with 08 - A losses as low as 0.7 dB/km at 1.3 Mm [135]. Single mode fibers 0.4 0.81d81km prepared at high a deposition rate also show excellent loss per- 0,2 formance, reported by Pearson [123] and Lazay et al. [24], 0.6 0.9 1.0 11 12 1.3 1.4 15 as well as very low dispersion. WAVELENGTH(~ m) Fig. 18. Loss and bandwidth data for multimode fiber optimized for MCVD has been demonstrated to be capable of producing operation at two wavelengths. very high bandwidth. Lin et al. [136] reported measuring bandwidths for two different multimode 0.23 NA MCVD SPECTRALPLOTS OF WECOTYPICAL ANO fibers which had maximum measured values of 4.3 GHz “ km HIGH 13UALITYFIBER 5.0 at 1.25 ~m and 4.7 GHz . km at 1.29 ~m, thus demonstrating WECOHIGH QUALITY that very high bandwidths, although still well below the theo- 4,0 h \ retical vahre, can be achieved. Fig. 18 shows fiber prepared by Partus [137], optimized for two wavelengths, which exhibits excellent loss at both 0.82 and 1.30 Mm, as well as high bandwidth at both wave- lengths. It was not necessary to use the approach of Blanken- ship et al. [32], [37] in which a very high phosphorous level !k was used to achieve this result; thus, very low loss levels in the o 0 0.8 0.9 1.0 1.1 12 1.3 1.4 1.5 long wavelength region were maintained. WAVELENGTli (~ h) Due to the excellent performance of MCVD fiber, a commit- Fig. 19. Loss spectra for a Western Electric typical fiber and a high- ment to large+cale manufacture using MCVD has been made quality fiber. by Western Electric. Pilot plant production at Atlanta’s Prod- uct Engineering Control Center (PECC) has culminated in the propagation phenomena are being actively pursued by a large design tid implementation of MCVD into production. Fibers number of investigators. To date, MCVD has been successfully produced on a 5-day, 3-shift, 24-h operation by hourly employ- used to fabricate very high quality single mode fibers in the ees on multimachine assignment i.how excellent performance. laboratory, and it is anticipated that further improvements Large-scale production of 0.23 NA, 125 urn OD, 50 Km core will occur. graded index fiber for use at two wavelengths, representing 30000 km of fiber show median values of loss at 0.825 pm= XIV. PROCESS COMPAEUSONS 3.41 dB/km and at 1.3 = 1.20 dB/km; bandwidth at 0.825 pm = Three major fiber fabrication techniques, outside vapor phase 825 MHz “ km and at 1.3 urn = 735 MHz “ km; and median oxidation (OVPO) [2], [139], [140], modified chemical OH peak height= 2.86 dB/km [138] . vapor deposition (MCVD), and vapor-phase axial deposition Fig. 19 shows the loss spectra for the typical and high-quality (VAD) [4], [141] - [144], have emerged as viable processes Western Electric product. This type of performance is com- for economic production of a high-quality multimode and parable to the best laboratory results. Additional manufac- single mode fiber suitable for long transmission distances at turing experience expected to further improve loss and band- both short (0.8-0.9 Mm) and long (1.3- 1.6 pm) wavelengths. width of production fiber. Direct melt fibers [5] have also demonstrated excellent short Manufacturing performance of single mode fiber cannot be wavelength performance, but are not suitable for long wave- assessedat this time. Studies on fiber design and fundamental length operation due to water and boron absorption, and 318 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982

TABLE I PROCESSFEATURES

(2,139,140) (4,141-144) OVPO MCVD VAD

COMMON s@2’G’42 Si02, Ge02 Si_02, Ge02 DOPANTS P205, B203 ‘2 °5’B203’F P21J5,B203

REACTION Flame hydrolysis Oxidation of Flame hydrolysis MECSANISM of halides halides of halides

DEPOSITION External lateral Internal depos~- End on deposition PRINCIPLE deposition of tion of oxide of oxide particles oxide particles particles on on rotating bait on rotating inside tube wall rod mandrel

DEPOSITION First core, then Cladding, then core Core and cladding FEATURES cladding depos - deposited; tube can be deposited i ted; Tube over- inherent to simultaneously; cladding can be process & provides tube can be used used for addi- cladding mat’ 1; can to overclad tional cladding overclad with another tube

PROCESS Multiskep process Multlstep process Multistep process STEPS involving depos i- involving simul - involving deposition tion of soot, taneous deposition consolidation & removal o f mandre 1, & consolidation, dehydration to form consolidation & collapse to form a blank, elongation, dehydration to preform, fiber overcladding, and form blank with drawing fiber drawing. Can or without hole, potentially do all fiber drawing steps simultaneously & continuously.

OH CONTROL Achieved by dehydra- Achieved through Achieved by dehy - tion in controlled leak tight system, dration in con- C12 atmosphere controlled cherni - trolled C!2 atmos - cal purity, C9.+ uhere coll~pse ~echnfque - PROFILE & Achieved by control- Achieved bv Achieved in radial REFRACTIVE ling and- varying controllin~ and plane by control of INDEX chemical composi - varying chemical many process varia - CONTROL tion of each composition of bles such as torch individual each individual deslgq and posi- deposited layer; deposited layer; tioning: Ha/O, ratio . . ty~ically 206 core ty~ically 50-cOre substrate tempera- layers layers ture, etc.

therefore will not be considered here. The three techniques TABLE II PERFORMANCEPARAMETERS are all based on vapor phase reactions to form high silica glass structures. This section will compare some key features of (2,139,140) (4,141-144) OVPO MCVD JmlJ these processes and review their relative performance. Table I itemizes aspects of the processes which distinguish STD PLASMA NA— the three techniques, while Table II summarizes performance ‘lyplcal 0.2 0.23 0.23 0.2 Max. Reported 0.3 0.38 NA 0.32 parameters which have been reported. DEP. RATE (g/rein. ) All three techniques use GeOz and P20~ dopants to increase Typical 1. 5-2 0. 35-0.5 3.5 0. 3-0.5 the refractive index relative to Si02 and B203 to decrease the Max . Reported 4 1.3 5.0 2 index. However, fluorine doping, which also decreases the index without deleterious absorption at long wavelengths EFFICIENCY (%) 50 50 80 60 associated with B203, has become increasingly important, MAX . BLANK 40 40 >20 100 particularly in single mode structures where it permits greater Length Reported fiber design flexibility. The flame hydrolysis reaction in OVPO (km) and VAD, favoring formation of HF, is thought to prevent BEST MULT lMODE fluorine incorporation. RESULTS FOR FIBER NA 0.21 0.23 0.2 0.21 Flame hydrolysis also results in high OH levels in the depos- Loss (dB/km) 0.82um 2.5 2.6 2.8 2.5 ited material in OVPO and VAD. However, because the depos- 1.3 0.7 0.45 0.7 0.42 1.5-l .6um 0.8 0.35 0.5 0.31 ited material is consolidated either as a second step or in a Bandwidth (GHz. Km) >3 5 NA 6.5 M~n. OH(ppb) 3 1 separate zone from deposition, the sintering conditions can be 10 10 optimized for dehydration by control of the sintering atmo- sphere. Low partial pressures of 02, and a Clz atmosphere dur- tion and consolidation occur simultaneously and in the same ing consolidation minimize the OH in the resultant fiber. Thus, atmosphere; therefore, the amount of OH incorporated is the hydrogen contamination levels in the starting materials is related to the hydrogen impurity levels present during deposi- not important in OVPO and VAD. In contrast, MCVD deposi- tion and collaspe, as well as the chlorine-oxygen ratio during NAGEL et al.: MODIFIED CHEMICAL VAPOR DEPOSITION 31[9 these phases of processing. Low hydrogen impurity levels in are typical, and the collapse phase of the process represents a the starting materials are required for low OH levels. In OVPO, larger portion of the preform preparation time. Overcladdin,g if a central hole is present during fiber drawing, control of the can be used to provide additional cladding at these high deposit- atmosphere to prevent OH contamination is also important. ion rates. AIl the techniques have achieved low OH, the lowest reported In contrast, in OVPO as it is currently practiced for multi- in VAD (Table II). mode fiber preparation, both core and cladding materials are Profile control is the key for good bandwidth performance deposited, typically at 2.0 g/rein. After deposition, the man- in the resultant fiber. In both OVPO and MCVD, the resultant drel must be removed from the blank which is then consoli- fiber NA and refractive index profile shape is achieved by vary- dated and dehydrated in a controlled atmosphere at an opti- ing the chemical delivery dopant ratios on each successive mum rate for obtaining a bubble-free, low OH glass with or, deposition pass. Relative deposition efficiency of the dopants more recently, without a hole, depending on the process details. must be taken into account to achieve the desired refractive Improved process rates are possible by use of higher deposition index and profile. Profile changes can be easily made by rates, overcladding, or by simultaneously sintering and drawing changing the chemical delivery program. A central refractive fiber in the fiber drawing furnace. Information on single mode index dip can be present in each process, as well as a ring struc- fabrication by this technique is not available. ture corresponding to individual deposited layers. In VAD, In VAD, the core and cladding material can, in principle, be the refractive index profile is achieved in the radial deposition deposited simultaneously by use of multiple torches. For plane primarily due to the spatial distribution of dopants in multimode fiber preparation, deposition of the core material the flame. The torch design and position, oxyhydrogen ratio as well as some cladding is typically done at 0.3-0.5 g/rein/ of the flame, flame temperature, dopant ratio injected through torch. The resultant soot blank can be simultaneously consoli- each torch nozzle, temperature distribution on the deposition dated and dehydrated in another zone as the growing blank is surface, pressure in the deposition vessel, rotation speed and undrawn through a sintering heat source, allowing for continu- position of the porous preform end, and other process variables ous blank making. Alternatively it may be consolidated off determine the resultant composition profile in the deposited line. Typically, the sintered blank is then elongated, overclad material. A refractive index dip in the center is not present. with a silica tube, then drawn into fiber. Preparation of single Profile changes are achieved by a complex interaction of all mode fiber preforms, where a large amount of deposited clad- the process variables; furthermore, in both OVPO and VAD ding is necessary, is accomplished by use of multiple torches. the resultant concentration profile can also be altered due to In order to compare process economics, simple comparison dopant sublimation during the consolidation phase of the of deposition rates is not sufficient. For instance, if a prefabri- process, and thus must be accounted for in achieving the desired cated tube provides additional cladding, the total volume of profile. deposited material required to produce a given volume of fibe r Each technique has produced high bandwidth fiber (Table is reduced. If a number of process steps are performed simul- II); however, sufficient data to make a relative bandwidth taneously then the process rate will be determined by the rate reproducibility y comparison are not available. limiting step. The number of process steps and their complex- Attenuation characteristics achieved by all three techniques it y affect equipment requirements, labor cost, and product are excellent (Table II). As currently practiced, processing yield. conditions which prevent transition metal impurity contamina- All three processes are potentially capable of producing high tion as well as scattering losses due to imperfections are not quality fiber at reasonable yield. The process rates are being limiting. Further improvements in process rate, however, must constantly improved, although these improvements and the be made without attenuation penalties, as well as with good process yield are, in general, proprietary. Further improvements bandwidth and dimensional control. in these processes are anticipated. The predominance of one or Assuming similar process yields, the relative process econom- more of these fabrication techniques will therefore depend onl ics will be greatly influenced by the process rate and its com- the ability to implement process rate improvements while main- plexity. All three processes, as currently practiced, consist of taining a high yield in production. two major steps: preform or blank preparation and fiber draw- ing from the bkink. The rate of fiber drawing for all processes XV. CONCLUSION can be the same, although in OVPO when the blank contains a Since the announcement of MCVD in 1974, steady progress central hole, the draw process must be optimized for its has been made in fiber performance and process improvements elimination. that have resulted in the large-scale manufacture of fiber by In standard MCVD, preform preparation time is determined this technique. Many systems have been installed by Western by both the deposition and collapse rates. Core deposition Electric and other companies throughout the world using fibers rates of 0.35-0.5 g/rein are typical. The cladding, which in made by this process, and system performance has been excel- multimode fiber comprises 84 percent of the fiber volume, is lent. Further improvements in the practice of this technique predominantly provided by the substrate tube. For single mode are expected to result ~neven better performance and economy. fiber preparation, although a large amount of low-loss deposited cladding is required, the bulk of the cladding is also provided ACKNOWLEDGE ENT by the original support tube. Higher deposition rates, fast The authors would like to thank their many colleagues at collapse schemes, and overcladding with a second substrate Bell Laboratories, Western Electric Engineering Research Cen- tube can further improve the process rate. In the RF-plasma- ter, and WE-PECC for their input and contributions to this enhanced MCVD configuration, deposition rates of 3.5 g/rein work. They are especially indebted to Western Electric per- 320 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-30, NO. 4, APRIL 1982 sonnel for their willingness to quote their data, as well as for pean Conf. Opt. Fiber Commun., Copenhagen, Denmark, 1981, their ongoing cooperation. paper 4.3-1. [26] D. Marcuse, D. Gloge, and E.A.J. Marcatili, “Guiding properties of fibers,” in Optical Fiber Telecommunications, S. E. Miller REFERENCES and A. G. Chynoweth, Eds., 1979, pp. 37-100. [27] P. C. Schultz, “Ultraviolet absorption of titanium and germanium [1] K. C. Kao and G. A. Hockman, “Dielectric-fiber surface wave- in fused silica,” in Proc. 11th Int. Congr. Glass, Prague, Czecho- guides for opticrd frequencies,” Proc. IEE, vol. 113, no. 7, p. slovakia, vol. 3, pp. 155-163, 1977. 1151, 1966. [28] V. Miya, Y. Terunuma, T. Hosaka, and T. 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