galvanometer beam-scanning system for laser fiber drawing

5
Galvanometer beam-scanning system for laser fiber drawing R. C. Oehrle A major difficulty in using a laser to draw optical fibers from a glass preform has been uniformally distrib- uting the laser's energy around the melt zone. Several systems have evolved in recent years, but to date the most successful technique has been the off-axis rotating lens system (RLS). The inability of this device to structure efficiently and dynamically the heat zone longitudinally along the preform has restricted its use to preform of less than 8-mm diameter. A new technique reported here employs two orthogonal mounted mirrors, driven by galvanometers to distribute the laser energy around the preform. This system can be retrofitted into the RLS to replace the rotating lens element. The new system, the galvanometer scanning system (GSS), operates at ten times the rotational speed of the RLS and can instantaneously modify the melt zone. The ability of the GSS to enlarge the melt zone reduces the vaporization rate at the surface of the preform permitting efficient use of higher laser power. Experiments indicate that fibers can be drawn from significantly larger preforms by using the expanded heat zone provided by the GSS. 1. Introduction Drawing silica fibers using a CO 2 laser offers several unique features and possibilities including (1) minimum contamination of the neckdown re- gion, (2) ease of start-up and shut-down, (3) controlled modulation of the neckdown region to provide diameter fluctuations suitable for mode conversion, (4) drawing fibers with elliptical cross sections. The basic components of a rotating lens laser fiber drawing system' are shown in Fig. 1. A CO 2 laser beam incident upon a rotating off-axis lens produces an an- nular pattern. This pattern is reflected off a mirror at 450 into a multifaceted conical reflector. The energy is distributed uniformally around a preform, which passes through the center of the conical reflector. Fi- bers, 100 ,m in diameter, with a standard deviation of 1% can be drawn at 1 m/sec from 7-mm fused silica preforms while maintaining drawing tensions below 25 Although capable of producing high strength fibers with good diameter uniformity, the rotating lens system (RLS) has one serious limitation that prevents the laser The author is with Western Electric Company, Inc., Engineering Research Center, Princeton, New Jersey 08540. Received 3 August 1978. 0003-6935/79/040496 -05$00.50/0. © 1979 Optical Society of America. from drawing fibers from large diameter preforms. The heat zone created by the rotating laser beam at any in- stant in time is circular. Therefore, the practical limit to the heat zone length is the diameter of the preform. Increasing the longitudinal length of the heat zone by enlarging the diameter of the laser beam is inefficient because of the followingtwo facts. Maximum absorp- tion of laser energy occurs when the beam is normal to the surface. Expanding the beam beyond the diameter of the preform obviously wastes energy. This report describes a galvanometer deflection system that ob- viates this problem and provides a unique capability to structure the heat zone at will. II. Experiences with the RLS When drawing 7-mm preforms at 1 m/sec, the maxi- mum power that can be used with the RLS is approxi- mately 270 W. Increasing the power above this level will result in excessive vaporization at the surface of the preform. 2 To draw larger than 7 mm and maintain a drawing tension below 25 g would require additional power, which would only result in an increase in the rate of vaporization. To break the 7-mm preform barrier would require increasing the heat zone along the lon- gitudinal axis of the preform, thus reducing the inten- sity below the vaporization level. The RLS has a heat zone ratio of one, with the heat zone ratio defined as the length of the heat zone divided by its width. A heat zone ratio of one or less produces a melt zone with a very sharply curved neckdown region, as can be seen in Fig. 2(A). A furnace drawn preform with a large heat zone ratio has a very gradual taper, which produces superior diameter stability while drawing fiber 3 [Fig. 2(D)]. 496 APPLIED OPTICS / Vol. 18, No. 4 / 15 February 1979

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Galvanometer beam-scanning system for laser fiber drawing

R. C. Oehrle

A major difficulty in using a laser to draw optical fibers from a glass preform has been uniformally distrib-uting the laser's energy around the melt zone. Several systems have evolved in recent years, but to date themost successful technique has been the off-axis rotating lens system (RLS). The inability of this device tostructure efficiently and dynamically the heat zone longitudinally along the preform has restricted its useto preform of less than 8-mm diameter. A new technique reported here employs two orthogonal mountedmirrors, driven by galvanometers to distribute the laser energy around the preform. This system can beretrofitted into the RLS to replace the rotating lens element. The new system, the galvanometer scanningsystem (GSS), operates at ten times the rotational speed of the RLS and can instantaneously modify themelt zone. The ability of the GSS to enlarge the melt zone reduces the vaporization rate at the surface ofthe preform permitting efficient use of higher laser power. Experiments indicate that fibers can be drawnfrom significantly larger preforms by using the expanded heat zone provided by the GSS.

1. Introduction

Drawing silica fibers using a CO2 laser offers severalunique features and possibilities including

(1) minimum contamination of the neckdown re-gion,

(2) ease of start-up and shut-down,(3) controlled modulation of the neckdown region

to provide diameter fluctuations suitable for modeconversion,

(4) drawing fibers with elliptical cross sections.The basic components of a rotating lens laser fiber

drawing system' are shown in Fig. 1. A CO2 laser beamincident upon a rotating off-axis lens produces an an-nular pattern. This pattern is reflected off a mirror at450 into a multifaceted conical reflector. The energyis distributed uniformally around a preform, whichpasses through the center of the conical reflector. Fi-bers, 100 ,m in diameter, with a standard deviation of1% can be drawn at 1 m/sec from 7-mm fused silicapreforms while maintaining drawing tensions below 25

Although capable of producing high strength fiberswith good diameter uniformity, the rotating lens system(RLS) has one serious limitation that prevents the laser

The author is with Western Electric Company, Inc., EngineeringResearch Center, Princeton, New Jersey 08540.

Received 3 August 1978.0003-6935/79/040496 -05$00.50/0.© 1979 Optical Society of America.

from drawing fibers from large diameter preforms. Theheat zone created by the rotating laser beam at any in-stant in time is circular. Therefore, the practical limitto the heat zone length is the diameter of the preform.Increasing the longitudinal length of the heat zone byenlarging the diameter of the laser beam is inefficientbecause of the following two facts. Maximum absorp-tion of laser energy occurs when the beam is normal tothe surface. Expanding the beam beyond the diameterof the preform obviously wastes energy. This reportdescribes a galvanometer deflection system that ob-viates this problem and provides a unique capability tostructure the heat zone at will.

II. Experiences with the RLS

When drawing 7-mm preforms at 1 m/sec, the maxi-mum power that can be used with the RLS is approxi-mately 270 W. Increasing the power above this levelwill result in excessive vaporization at the surface of thepreform.2 To draw larger than 7 mm and maintain adrawing tension below 25 g would require additionalpower, which would only result in an increase in the rateof vaporization. To break the 7-mm preform barrierwould require increasing the heat zone along the lon-gitudinal axis of the preform, thus reducing the inten-sity below the vaporization level.

The RLS has a heat zone ratio of one, with the heatzone ratio defined as the length of the heat zone dividedby its width. A heat zone ratio of one or less producesa melt zone with a very sharply curved neckdown region,as can be seen in Fig. 2(A). A furnace drawn preformwith a large heat zone ratio has a very gradual taper,which produces superior diameter stability whiledrawing fiber 3 [Fig. 2(D)].

496 APPLIED OPTICS / Vol. 18, No. 4 / 15 February 1979

PREFORM

ANNUAL

45° MIRROR

FIBER

I | ROTATING LENS

Fig. 1. Basic components of RLS.

A B C D

Fig. 2. Fiber drawn preforms.

GALVONOMETER IVERTICALGALVONOMETER

Fig. 3. Basic components of GSS.

NECK-DOWN REGION

MULT FACETREFLECTORF

||MINOR DIAMETERl|

MAJOR DIAMETER

Fig. 4. Cross section of conical reflector.

Another set of problems is caused by the lens rotation.The rotation speed of the lens in the RLS is typically50-60 Hz, and at this speed the laser beam circumro-tates the preform in 20 msec. Diameter fluctuationsare created in the fiber due to the cooling of the preformsurface after the beam has passed a given area. In-creasing the rotational speed of the lens will minimizethis effect but makes it difficult to keep mechanicalvibration from disturbing the melt zone, which resultsin fiber diameter fluctuations and also accelerates thewear on the bearings of the rotating lens mount. Fi-nally, cooling the rotating lens can be a major problemif high laser power is used. Thermal gradient due toabsorption in the lens can actually distort the surfacesand cause significant beam aberrations.

Ill. Galvanometer Scanning System

A. Basic Operation

An alternative to the RLS has been developed thateliminates many of the limitations mentioned above.Figure 3 is a diagram of the galvanometer scanningsystem (GSS). This technique employs two orthogo-

MULTIFACED nally mounted galvanometers that deflect low-massberyllium mirrors. A laser beam reflecting from the twomirrors, controlled by sinusodial voltages separated bya 90-degree phase shift, will produce an annular pattern.The low mass of the beryllium mirrors permits scanning

45° REFLECTOR speeds greater than 500 Hz or ten times the rotationalspeed of the RLS. Changing either the amplitude orfrequency of the sinusodial carrier voltage to the gal-vanometers will vary the diameter of the annular ring.As can be seen in Fig. 4, decreasing the diameter of theannular pattern results in the beam reflecting higher inthe conical reflector, moving the heat zone along thepreform perpendicular to the rotational direction. Thescanning of the rotating beam along the length of thepreform increases the area which reduces the powerdensity for a given power level.

15 February 1979 / Vol. 18, No. 4 / APPLIED OPTICS 497

DRIVEAMP GALVANOMETER

CARRIEROSCILLATOR A.700 0.30; VERT. MIRROR

A.70 X° HORZ. MIRROR

MODULATIONOSCILLATOR

Fig. 5. Electronic block diagram.

The galvanometers used were model G-302 purchasedfrom General Scanning. Manufacturers specificationsindicate that the G-302 is capable of a 20 peak-to-peakscanning range. The galvanometers evaluated couldscan 6 peak-to-peak without any noticeable visualdistortion in the annular ring. Originally berylliumcopper mirrors with a total mass of 18 g were installedon the galvanometers. The heavy copper mirrors limitthe carrier frequency to 320 Hz with a modulating fre-quency of 18 Hz. The beryllium copper mirrors werereplaced with beryllium mirrors with a reduction inmass of the mirror and mount to 2.8 g. The resonantfrequency was increased to 500 Hz, which permitted acarrier frequency of 550 Hz and a modulation frequencyof 100 Hz. Higher carrier frequencies can be obtainedat the expense of the modulating frequency since thebandwidth is equal to the sum of the carrier frequencyplus or minus the modulating frequency.

The diameter of the annular ring can be varied bymodulating either the frequency or the amplitude of thecarrier. However, modulating the frequency of thecarrier provides a larger range of diameter variationswhile maintaining orbicularity of the annular ring.Frequency modulation of the carrier can be obtainedconveniently by using a Wavetex model 132 generator,which has a voltage control oscillator input. The elec-tronic components needed to drive the galvanometerare shown in Fig. 5.

The data presented in this report were taken usinga 550-Hz carrier and a 100-Hz modulation frequency.These values were selected arbitrarily and do not nec-essarily represent the optimum operating parametersfor the galvanometers. Galvanometers that operate athigher resonant frequencies are available which allowhigher modulation frequencies. This will permit moreexotic modulating waveforms if desired. Feedbackgalvanometers, which provide a signal proportional tothe mirror position, can maintain a constant mirrorscanning amplitude while changing the carrier fre-quency. This would allow the carrier frequency andmodulating frequencies to be changed with a minimumof disturbance in the heat zone position. This featurewould be useful in determining an optimum heat ratiofor various preform sizes.

The original beryllium copper mirrors used in theGSS were glued to the mirror mounts with an epoxycement. Measurements indicated that the temperatureof the mirror mounts was reaching 1000C, which is nearthe softening point of epoxy. A jet of cool air was di-

rected to the back of the mirror mount during laserscanning. This cooling technique permits the use ofhigher laser power and minimizes the deterioration ofthe scanning mirror caused by excessive heat. Devicessuch as the vortex tube supplied with ordinary com-pressed air can provide super cool air to the berylliummirrors. The good thermal conductivity of the beryl-lium would help maintain mirror surface temperaturesat tolerable levels. However, unless the mirrors arecooled, the heat absorbed by them is conducted to thegalvanometer through the shaft and mount resulting inscanning instabilities.

B. Galvanometer Stability

The following procedure was used to determine thestability of the galvanometer scanning deflection sys-tem. A He-Ne laser aligned coaxial with the CO2 laserwas added to the system as an alignment aid. An an-nular ring formed by only the He-Ne was projected ona frosted screen 85 cm from the galvanometer mirrors.A Reticon LC600 line scanner was mounted to measurethe displacement of the image on the opposite side ofthe screen. Random readings taken during a 5-h periodindicated a movement of only 0.8 mrad in the projectedimage. The frequency stability of the Wavetex model132 generator was measured at the same time and foundto be within 0.2%. The same signal after amplificationby the A-700 General Scanning amplifier varied 0.3%.

C. Additional Modifications

Several other design changes are possible. using theGSS that can modify the heat zone permitting higherlaser power. The present multifaceted reflectors weredesigned with an 80° cone angle. A 1-cm change in theannular ring diameter produces approximately a 1-cmexcursion along the vertical direction of the preform.Changing the angle of the multifaceted reflector to 1270would double the vertical scanning distance the heatzone would traverse along the preform. The sides of thefaceted reflector could also be curved to produce anonlinear scanning rate in the vertical direction. Aproperly designed curvature would create an optimizedheat distribution to conform with the geometry of themelt zone.

The heat zone ratio can also be enhanced by changingthe vertical galvanometer flat mirror to a concave orconvex mirror. Selecting the proper focal length willproduce an elliptical beam with the major axis of theellipse parallel to the axis of the preform. These me-chanical modifications minimize the constraints asso-ciated with the finite bandwidth of the galvanome-ters.

IV. Results

The GSS was evaluated by recording on-line diametermeasurements using a forward laser scattering tech-nique.4 The 512 diode array detector in the system iscapable of 0.5 /,m resolution. A commercial real timeanalyzer (RTA) processed the diameter data. The RTAanalyzes the diameter fluctuations into its spectralcomponents, and a spectrum is computed using diam-eter measurements taken every 500 sec over ap-

498 APPLIED OPTICS / Vol. 18, No. 4 / 15 February 1979

Io -10,

cm,

10 -r

0 - t

10-"

0.0001 0.001 0.01 0.1 1.0 10.0 100.0

cm-

Fig. 6. Averaged power spectral density for RLS.

10-10-5

10-6I

cm, 10- '

1 0-1

l o-, F

10-5

cm, 1l0° ,

10-'cm0 10-110-

0.0001 0.001 0.01 0.1 1.0 10.0 100.0

cm-

Fig. 7. Averaged power spectral density for GSS (withoutmodulation).

cm-

Fig. 8. Averaged powerlspectral density for GSS (withmodulation).

105.8

104.9

104.0 Am

103.2

0 5562 11129 16692 22254

cm

Fig. 9. 3 a Diameter variations for 222.4M of fiber.

Fig. 11. GSS expanded heat zone.

116.3

113.9

111.4 i

109.0

106.6

fi"'1 "'1'

0 5640 11284 16924

cm

Fig. 10. 3 a Diameter variations for 225.6M.

22564

Fig. 12. RLS heat zone.

15 February 1979 / Vol. 18, No. 4 / APPLIED OPTICS 499

8 -s

-^ ^ .. | . . v . s . . . s . . .. | . . .. i . . ^

- - - - - -

proximately 200 m of fiber.5 Figure 6 is a typical powerdensity plot for data gathered using a rotating-lenssystem. The peaks that appear at 0.57 cm-' and higherare the result of the rotational speed of the lens. Figure7 is a power density plot for a galvanometer scanningsystem, which shows the absence of the rotating peakscreated by a RLS. The GSS operating at rotationalspeeds of 500 Hz cannot cause peaks in the spectrumbecause the drawing dynamics will not respond tovariations much above 100 Hz. Figure 8 is a spectralplot for the GSS operating at a carrier frequency of 550Hz and a modulation of 100 Hz. There is very littledifference between the spectrum with and withoutmodulation.

Fiber diameter variations average 0.8% during eightsample runs, typically 200 m long, and drawn from TO-8preforms. Figure 9 is the 3 sigma diameter variationplot for the best run of the eight sample groups. Thestandard deviation for this 222-m sample was 0.56%,which is near the resolution limits of the measurementsystem as mentioned above.

Figure 10 shows results of a sample run taken withoutfeedback control. The long term variations shown inthis plot are caused by laser power instabilities due totemperature changes in the laser cooling water. Withfeedback control it is possible to minimize these varia-tions to tolerable limits as can be seen in Figure 9.

Figures 11 and 12 show two photographs taken withthe same time exposure and at the same distance. Thepicture (Fig. 11) was taken while drawing fiber from a7-mm preform using an expanded heat zone ratio of 2.0.The preform can be clearly seen in the center of theconical reflector. The photograph at the bottom wastaken without the expanded heat zone using the samepower. The intensity is too high to observe the shapeor position of the preform. Rapid vaporization is alsooccurring but cannot be seen. These two pictures il-lustrate the ability of the GSS to lower the power den-sity at the melt zone by expanding the heat zone. Theneckdown region or melt zone of four preforms areshown in Fig. 2. The large preform [Fig. 2(D)] wasdrawn in an induction furnace. The gradual taper ofthe melt zone is typical of a furnace drawn preform witha large heat zone ratio. The preform marked C is a5-mm preform drawn with a GSS and a heat zone ratioof 2.4. The taper is very similar to that of the furnacedrawn preform. Preform B is a 7-mm GSS preformwith a slightly smaller heat zone ratio. The taper is notas smooth as the 5-mm preform because a larger heatzone ratio requires additional power, which could notbe provided by the CRL Q-41 laser. Preform A on theleft was drawn with a RLS. The sharply curved neck-down region is typical of low ratio drawn preforms. Thewhite ring at the top of the melt zone was caused byvaporization. A ring can be seen on the two GSS pre-forms in the center of the photograph, but the amountof vaporization is greatly reduced. This probably can

be eliminated by applying a more optimized scanningwaveform to the galvanometer rather than the trian-gular waveform used to sweep the heat zone in the ver-tical direction. The galvanometers having a finite re-sponse will not respond exactly to a triangular waveformwhich results in a longer dwell time at the top of the heatzone. The redesign of the angle of the conical reflectorand galvanometers with a higher natural resonant fre-quency could minimize this problem.

V. Summary

Power limitations of the CRL Q-41 (300-W) laserprevented drawing fibers from 7-mm preforms using anexpanded heat zone. With a heat zone ratio of 1, ap-proximately 270 W are required to draw fiber at 1 m/secwhile maintaining a tension of 25 g. To double the heatzone would require twice the power, which is beyond thecapacity of the Q-41 laser. To demonstrate the abilityof the GSS to minimize vaporization, experiments wereperformed using a 5-mm preform so that sufficientpower was available to expand the heat zone. With theRLS vaporization could be observed when the powerreached 100 W. Expanding the heat zone ratio to 2.4with the GSS permitted raising the power to the fullcapacity of the Q-41 without observing vaporization. Inboth experiments the drawing parameters were iden-tical. Although higher powers are required whiledrawing with an expanded heat zone, the GSS allowsincreased time for the energy to permeate the interiorvolume of the preform. This results in operating atlower surface temperatures and a more uniform tem-perature gradient across the preform. It was not pos-sible to determine the improvement that could be ob-tained in faster drawing speed since the laser was op-erating at maximum power. Increasing the drawingspeed without an increase in power would result in in-creasing tension, which would eventually break thefiber. Had sufficient power been available it seemsprobable that a significant increase in drawing speedscould have been obtained.

Thus it can be concluded that the GSS has eliminatedmany of the problems associated with the RLS. Itprovides greater flexibility over the heat zone geometryallowing the use of increased laser power.

References1. R. E. Jaeger, Am. Ceram. Bull. 52, 704 (1976).2. M: A. Saifi, in Digest of Topical Meeting on Optical Fiber

Transmission II (Optical Society of America, Washington, D.C.,1977), paper TUC2-1.

3. U. C. Paek and R. B. Runk, in Digest of Topical Meeting on Op-tical Fiber Transmission II (Optical Society of America, Wash-ington, D.C., 1977), paper TUC-1.

4. D. H. Smithgall, L. S. Watkins, and R. E. Frazee, Jr., Appl. Opt.16, 2395 (1977).

5. P. H. Krawarik, in Topical Meeting on Optical Fiber TransmissionI (Optical Society of America, Washington, D.C., 1975), paperPD1.

500 APPLIED OPTICS / Vol. 18, No. 4 / 15 February 1979