Laser Drawing of Optical Fibers

U. C. Paek

A system consisting of a cw CO2 laser and an ellipsoidal reflector was developed in order to draw fused

silica fibers from bulk material. The system was used to obtain unclad fused silica fibers (Suprasil 2)

having a total transmission loss of less than 100 dB/km at 6328 A. Preforms (rod in tube) have also been

drawn to fibers of long lengths with diameters as small as 10 gm and variations within 5%. Analysis of

the fiber drawing process established relations that allow determination of the design parameters for fiber

drawing systems. Experimental results are given that support these relationships.

IntroductionLow loss glass fibers with less than 10 dB/km loss

have been considered attractive for use in a varietyof optical transmission systems. Fibers with loss aslow as 4 dB/km at 1.06-gm wavelength have beenreported.1 There appears to be an intense effort toreduce fiber loss further and develope processes forthe economical and reliable manufacture of long fi-bers. Such efforts include the development of lowloss materials and preform structures and the tech-niques for drawing the preforms into long fiberswithout contaminating the fiber or otherwise in-creasing its loss.

We describe in this report the initial work on thedevelopment of a versatile fiber drawing process witha CO2 laser. It is expected that the process will besuitable for a variety of preform configurations andfor fused silica as well as lower softening point glass-es. The drawing process has been used to draw un-clad fused silica fiber (Suprasil 2) with a loss of lessthan 100 dB/km at 6328 A. This loss value was thelimit of our detection for the length of fiber mea-sured and lower loss is probable. For these mea-surements a mode-stripping technique was used.2' 3

The fiber diameter was 100 Aim.Many heating methods have been used for fiber

drawing, including the oxyhydrogen torch, inductionheating, resistance heating, and plasma heating. ACO2 laser was chosen as the heat source to producefiber,4 since it has unique capabilities. The mainadvantage in using the laser is that its energy can beeasily controlled to quickly raise the material's tem-

The author is with Western Company, Inc., Engineering Re-

search Center, P.O. Box 900, Princeton, New Jersey 08540.Received 7 December 1973.

perature to the required point while not introducingimpurities to the workpiece or fiber during process-ing. This may be of major importance, since suit-able fiber core materials5' 6 such as Suprasil W-1 (aproduct of Amersil, Inc.) must have metallic impuri-ties less than 1 ppm and an OH content of approxi-mately 5 ppm. The rapid energy control availablewith CO2 lasers allows for fiber diameter feedbackcontrol systems.

A laser system capable of drawing fibers from nu-merous low loss materials with softening points rang-ing to above 1580'C was designed and built. It con-sists of a CO2 laser (50 W), an ellipsoidal reflector,and a mechanism capable of feeding material andtaking up a fiber.

The ratio of these two parameters (feeding andtakeup speed) determines the size of a fiber in draw-ing operation. As yet there is no agreement on thefiber dimension to be used in communication sys-tems.7'8

ApparatusAs shown in Fig. 1, a beam from the 50-W CO2

laser (Coherent Radiation Model 42) is collimated bya beam expander and focused with a germaniumlens. Between the lens and reflector, a jet nozzleblows air into the cavity of the reflector along thebeam axis. This stream not only prevents any silicavapor generated at the focused area from depositingon the reflecting surface but also provides a con-trolled atmosphere.

An ellipsoidal reflector, typically having a majoraxis of 13.6 cm and a minor axis of 5 cm, is cut per-pendicular to the major axis at one focal point F1 .Two 0.65-cm-diameter holes at the top and bottomare positioned so that a line drawn through theircenters is perpendicular to the major axis and passesthrough focal point F2. The reflector is mountedand adjusted so that its major axis is aligned in the

June 1974 / Vol. 13, No. 6 / APPLIED OPTICS 1 383

RACK AND PINION

PREFORM ROD

(~~~~~~~~~ FNFIBER

TAKE-UP REEL

Fig. 1. Schematic fiber drawing system.

beam axis. Focal point F1 is easily adjusted to coin-cide with the focused spot of the lens. Accordingly,the laser energy is recollected at focal point F2,where the workpiece is introduced. The reflectingsurface is made of electroplated and vacuum-depos-ited gold.9

The fiber drawing operation can be briefly de-scribed by the following procedures (Fig. 1). A fusedsilica or glass rod (or rod-and-tube combination) isfed into the top port of the reflector (with rack andpinion) and through the bottom port until it extendsto about 2.5 cm below the port. The laser is thenturned on and a portion of the rod at point F2 isheated to the softening point. The end of the rodthat extends out of the reflector is pulled down andsecured on the takeup reel. The fiber is then contin-uously drawn by rotation of the takeup reel. A pho-tograph of the setup is shown in Fig. 2, and includesthe CO2 laser, reflector, drawing machine, and dis-play system.

Laser Intensity Distribution Around The Workpiece

As shown in Fig. 3, one end of the ellipsoidal re-flector is cut at the focal plane located at x = -cperpendicular to the major axis to introduce thelaser beam into the reflector cavity with a lens. Thebeam that is refocused through reflection will con-tribute the most heat to the workpiece. On theother hand, the beam that strikes the rod directly issubstantially defocused and will not appreciably af-fect the material, and it can therefore be neglectedwhen calculating the intensity distribution aroundthe rod.

In order to derive the intensity distribution, onecan assume that a Gaussian beam Io exp (-w 2 /h2 ) isfocused at the focal point F by a lens (with focallength f) where h is a beam radius and w is the radi-al variable that denotes distance from the propagat-ing axis. The beam arriving at the differential arcds from the point F will be reflected with anamount of R (reflectivity in percent) and reach thepoint F2. It can be seen in Fig. 3 that the arc dsforms a viewing angle d with an origin of F2 . FromFig. 3 we can relate I to the beam reflected from ds

towards F2, where I is defined as intensity distribu-tion over (d1/2) -do, the differential arc of the rod.

RfIo(r2/r)d0Sec2Oexp$(f /h)[y/(x + )]21 = Il(d,/2)do,where r = [(c + x)2 + y2 ]1/2 , r2 = [(c - x) 2 + y2 ]1/2

and w/h = (f/h)[y/(x + c)]. The ellipse is describedby (x2 /a2 ) + (y2 /b 2 ) = 1, where a and b representsemimajor and minor axes. Therefore,

(I1/I0) = (2Rf/d1A[(1 - X) 2 + y, 2 ]1 /2[(1 + X1 )2 + y, 2]1"2 /

(1 + x)1exp[-(f/h)2[y2/(1 + X)2], (1)where x1 = x/c and y = y/c. Equation (1) indi-cates that a Gaussian beam with its amplitude o isrefocused on the surface of the rod through reflec-tion.

Figure 4(a) shows the upper half portion of the in-tensity distribution over the rod, indicated by theshaded area in the center of the polar coordinate,since Eq. (1) is symmetric about the x axis. In otherwords, when the rod is placed right at focal point F2 ,it will be heated with the intensity distribution asindicated in Fig. 4(a). Thus, the portion between350 and 165° is expected to be much more softened

Fig. 2. Fiber drawing setup.

LIoexp(- W

2/h2)

a

Fig. 3. Description of a beam from a lens, refocused by a reflec-tor to a rod.

1384 APPLIED OPTICS / Vol. 13, No. 6 / June 1974

400 1400

200 1600

00 0 1800

(a) R~IROD

(b)

Fig. 4. (a) Angular distribution of laser intensity around thesurface of a rod by a reflector (all data used for calculations are h= 0.5 cm, f = 2.5 cm, a = 6.8 cm, b = 2.5 cm, and R = 0.95).

(b) Effect on the rod due to angular distribution (a).

takeup speeds. By applying the mass conservationlaw (Fig. 6) while assuming identical densities forpreform and fiber, we can establish the followingrelation:

Vf d,2 = Vtd 2 or V, = Vf(d / d2), (2)

where d, is the diameter of the rod, d2 is the desiredfiber diameter, V is the feeding speed, and Vt thetakeup speed. From Eq. (2), one can easily computethe pulling rate corresponding to a feeding speed fora given ratio of fiber to preform.

From 0.1-cm or 0.2-cm rods, fibers were drawndown to less than 10 gm in diameter. It was foundthat the diameter variation was within 5%. Largerdiameter fibers have also been drawn, with diame-ters exceeding 250 ,gm.

Power Requirements

When the rod at the focused area F2 reaches thesoftening point, the fiber is drawn with an appropri-ate feeding speed Vf for a given laser power P. Tofind correlation between Vr and P, it is necessary towrite a governing equation for the model shown inFig. 7. In this case, the heat leaves the shaded areadue to convection and thermal radiation while thelaser beam continuously irradiates the area to main-tain the softening temperature T5. Then the heatconduction equation'0 for steady state can be writ-ten as

-Vf(TM/S) = K(82T/aS 2) - (4HT/pCpd1), (3)

where K = k/pCp, k is thermal conductivity, p isdensity, Cp is specific heat, di is diameter of a rod,and S the distance defined in Fig. 7. The total heattransfer coefficient is defined as

H = h + hr.

(a) 23X

(4)

(b) 23X

Fig. 5. (a) Rod is heated around its periphery.during drawing processes.

(b) Heating zone

than the front and back portion (between 0°-35° and165°-180°). This nonuniform melting has been ob-served, as shown in Fig. 4(b). However, the heatdistribution can be made essentially uniform aroundthe periphery by a slight defocusing adjustment ofthe position of focal point F2 relative to the rod whilethe heated portion of the rod is observed through the0.32-cm inspection hole in the reflector surface or aTV monitor display. One finally obtains the resultshown in Fig. 5.

When uniform heating is achieved, as in Fig. 5, afiber is continuously drawn from a preform with itssize dependent on the ratio between the feeding and

LASER- HEAREA FIBER

Vt

Fig. 6. Necking-down region of a rod.

June 1974 / Vol. 13, No. 6 / APPLIED OPTICS 1385

LASER BEAM

Ih V S

hC

-vta S

to unity, and is the Stefan-Boltzmann constant,5.6696 x 10- 5 erg/sec cm2 K4 .

The solution to Eq. (3) with boundary condition(5) is

/ROD

A d

z

Fig. 7. A model of fiber drawing.

FEEDING SPEED, V(cm/sec)

Fig. 8. Analytical and experimental results for relation betweenlaser intensity vs feeding speed Vt (di = 0.2 cm, H = 10-2 cal/

cm2 sec 'C, q = 0.5, and fused quartz used).

where he and h, represent convective and radiativeheat transfer coefficients. The boundary conditionsare

T = T S = 0, T = 0, S . (5)In addition, an equation is needed to describe the

variable Vf in Eq (3) that can be obtained by takingthe energy balance at S = 0, neglecting conductionloss through the fiber:

-k(aT/OS) = qI - HT - VpCpTs, (6)

where -q is the loss in the optical system (in percent-age) and I represents the laser intensity (W/cm2).

In the case of no air jet, an average value of freeconvection is calculated to be 0.35 x 10-3 cal/seccm2 K and will be higher when the air jet is activat-ed. The coefficient h is defined as EaT8

3 where E

is the emissivity of the rod, which is normally equal

T= Ts exp(- + [( ) 16H 1/2

Finally, we established the relation of Vt and I bysubstituting Eq. (7) into (6). Then it becomes

q = 3pCpVfT, + HT, + (T,/2)[(pCpVf)2

+ (16Hk/d, )]/2. (8)

Equation (8) indicates the relationship among therequired power, feeding speed, and diameter of thepreform. The power P = IA, where A is the cross-sectional area of the rod. In addition, it explainsphysically that sensible heat corresponding to thefeeding speed of the rod should be added to the heatloss due to conduction, convection, and radiation tomaintain drawing action. Otherwise, the fiber willbreak. Theoretical [Eq. (8)] and experimental re-sults are given in Fig. 8, with good agreement be-tween the two.

Conclusions

It has been demonstrated that a CO2 laser (50 W)with an ellipsoidal reflector system is capable ofdrawing clad and unclad fibers of long lengths, withdiameters down to 10 um, and diameter variationswithin 5%. For unclad fiber, losses below 100dB/km have been measured at the wavelength of6328 A. We have observed that laser power from 20to 35 W was required to draw fiber from a 2-mm-di-ameter rod. The power levels were determined bythe feeding speeds, which were in the range of0.002-0.02 cm/sec. The correlation between the re-quired laser power and feeding speed has been estab-lished, and good agreement between experimentalresults and theoretical calculations was obtained.

The author would like to thank A. L. Weaver forhis assistance in the experiments. He would alsolike to acknowledge many helpful discussions with P.Kaiser.References1. D. B. Keck, R. D. Maurer, and P. C. Schultz, Appl. Phys.

Lett., 14, 307 (1973).2. P. Kaiser, Appl. Phys. Lett. 23, 45 (1973).3. P. Kaiser, BTL, Crawford Hill; private communication.4. R. E. Jaeger, Metallurgical Society of AIME meeting on Prep-

aration and Properties of Electronic Materials, Las Vegas,August, 1973.

5. Optical Fused Quartz and Fused Silica, a catalog publishedby Amersil, Inc.

6. D. A. Pinnow and T. C. Rich, OSA Topical Meeting Digest onIntegrated Optics, TUA-4, Las Vegas, February, 1972.

7. C. A. Burrus, OSA Topical Meeting Digest on Integrated Op-tics, WB-3, Las Vegas, February, 1972.

8. S. Saito, IEEE Trans. COM-20, 725 (1972).9. W. M. Toscano, ERC-Western Electric Co., Princeton, N. J.;

private communications.10. H. S. Carslow and J. C. Jaeger, Conduction of Heat in Solids

(Oxford University Press, New York, 1971), p. 387.

1386 APPLIED OPTICS / Vol. 13, No. 6 / June 1974

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