Temperature dependence of the extinction coefficientof fused silica for CO2 laser wavelengths

Anthony D. McLachlan and Fred P. Meyer

The mechanism of the interaction of CO2 laser radiation with fused silica is determined by the absorptiondepth of the radiation in the material. The extinction coefficient of a number of high-purity fused silicasamples has been measured by the transmission method by fabricating samples 30 ,um in thickness. Resultsobtained agree with the latest reported values. In addition, samples were heated by an auxiliary CO2 laserand the extinction coefficient determined as a function of temperature for six CO2 laser lines. No significantdifference in the extinction coefficient was observed for samples from different makers of the high-puritysilica. Measurements were also conducted on a silica-rich glass Vycor and a significant difference wasobserved.

1. Introduction

In studying the interaction of CO2 laser radiationwith pure fused silica or silica-rich glasses it is impor-tant to know the absorption coefficient a of the radia-tion in the material. The value of a, which is related tothe extinction coefficient k by a = 47rk/X determinesthe depth to which the radiation is absorbed in thesilica. If the absorption occurs essentially at the sur-face, the heat flow and thermally induced stress analy-sis may be treated in a relatively simple way by theassumption of surface heating. However, if absorp-tion occurs over a significant depth, the heat flow andresultant stress analysis require considerable modifi-cation to cope with the absorption in the bulk of thematerial.

Literature sources'" report values for the extinctioncoefficient k which are generally derived from reflec-tion measurements on fused silica. These values showmarked discrepancies and it was the aim of the report-ed work in this paper to measure the extinction coeffi-cient accurately and, in addition, to measure its tem-perature dependence for temperatures up to 18000C.No adequate information on the temperature depen-dence of the extinction coefficient was available.

A. D. McLachlan is with Materials Research Laboratories, De-fence Science & Technology Organisation, P.O. Box 50, Ascot Vale,Victoria 3032, Australia. Fred P. Meyer is with U.S. Army Materi-als Technology Laboratory, Watertown, Massachusetts 02172-0001.

Received 24 December 1986.

II. Experimental

Contrary to most methods used previously,1 4 theexperimental procedure described in this paper con-sisted of fabricating samples of pure fused silica andsilica-rich glass which were thin enough to allow signif-icant transmission of radiation in the 10-12-,gm wave-length band. From a determination of the transmit-tance of the sample it is possible to calculate theextinction coefficient. Samples were required to be25-mm diameter and 25-50 m thick with surfaceswhich were parallel to within a fringe of visible light.A range of samples was prepared from high-purityfused silica and silica-rich glasses obtained from anumber of well-known suppliers as detailed in Table I.The techniques used in the preparation of the samplesconsisted of careful application of standard opticalpolishing procedures as fully reported elsewhere.5The spectral transmittance at 250 C of a typical sample(26.1-gim Suprasil), measured on a Perkin-Elmer mod-el 580B spectrophotometer, is shown in Fig. 1. Sincethe surfaces were formed parallel to within one fringeof visible light, interference fringes occur between thetwo surfaces of the sample in the wavelength bandswhere there is little absorption (Fig. 1). The thicknessof the sample was measured to an accuracy > ,4m by areflecting autocollimation technique on an optical flat.It was also determined by measuring the interferencefringe spacing (Fig. 1) and application of the formula

t= 1 ,2ndlv

where t is the thickness of the sample, v is the fringespacing in wavenumbers, and n is the refractive indexcorresponding to the wavelength at which the fringe

1728 APPLIED OPTICS / Vol. 26, No. 9 / 1 May 1987

Table 1. Details of Makers and Thickness of Samples Prepared In theInvestigation

ThicknessMaker Grade (pm)

Heraeus-Amersil, Inc. T20 Suprasil 26.1Sayreville, NJ 08872USA

Dynasil Corp. of America Dynasil 6000 64.4Berlin, NJ 08009USA

Corning Glass Works Fused silica 7940 38.7Corning, NY 14830USA

Corning Glass Works Vycor 7913 46.1Corning, NY 14830USA

Nippon Silica Glass Co., Ltd. NSG-OY 26.3Somerville, NJ 08876USA

uJ0zI-

F 0.5

z0)M

0 L2.5 4 6 8 10 12

WAVELENGTH (pm)

Fig. 1. Spectrophotometer transmission spectra of a sample of purefused silica (Suprasil), 26 am thick.

spacing was determined. Since the refractive index isknown to a good accuracy up to 4 gum,6 the thicknesscan be readily determined. The values so obtained bythe two methods differed by <1gm.

Samples were carefully located in a strain-freemount which permitted convenient routine handlingof the samples. The spectrophotometer was fittedwith a heated stage so that measurements could beconducted for sample temperatures from 25 to 1750C.To facilitate measurements at higher temperatures,and as an independent measurement of the transmit-tance of the samples, an experimental facility was alsoset up (Fig. 2). In this arrangement, the choppedoutput of a line-tunable CO2 probe laser (Spectra-Physics model 941) was directed through the sampleand onto a pyroelectric detector (with tuned amplifier)mounted in an integrating sphere. The CO2 laser wasoperated in the TEMoo mode and produced a Gaussianbeam whose lie radius at the sample was measured tobe 4.5 mm with a power output of -1-3 W. The poweroutput stability of the laser was monitored by a sepa-rate power meter via a ZnSe beam splitter and its

Fig. 2. Schematic diagram of the experimental arrangement fortransmission measurement of fused silica at elevated temperatures.

wavelength, which was line selectable from 9.6 to 10.6gim, was measured by a CO2 spectrum analyzer (Opti-cal Engineering, Inc. model 16-A).

The sample mount was devised so that the samplecould be quickly translated into the beam. Monitor-ing of the power and wavelength output of the laserensured that stable experimental conditions appliedduring the measurement. The transmittance of thefused silica sample was measured by determining theratio of the detector signal with and without the pres-ence of the sample in the probe laser beam.

The CO2 laser radiation (1-3-W power) on the thinsilica samples caused a temperature rise of typically1500C and was observed with a pyrometer (Ircon mod-el 7000) operating in the wavelength band of 4.8-5.2gim. The emissivity of the sample was calculated fromthe transmission spectra on each sample (Fig. 1) fol-lowing the procedure described in Ref. 7. This value,which was typically near 0.5, was checked by observinga stable blackbody at 5000C through the thin silicasample. From the observed apparent change in tem-perature and suitable application of Planck's Law, theeffective emissivity of each sample at room tempera-ture could be determined. Values so obtained by thetwo methods agreed to within 0.05.

A 40-W cw CO2 laser beam was also directed at thesample to provide uniform irradiation over a 1-cmsquare area (Fig. 2). This laser beam was arranged sothat none of the radiation was incident on the detector-collecting optics of the probe laser beam. The radia-tion was also unchopped so that the detector wouldreject any stray scattered cw signal by means of itstuned amplifier.

Because of the thinness of the sample and the subse-quent minimum heat loss, it was possible to heat thesamples to 18001C. The pyrometer was arranged toview an area of 5-mm diameter at the center of theuniformly heated zone, coincident with the probe laserbeam.

111. Results and Discussion

The extinction coefficients of the samples were de-termined from the measured transmittance (eitherspectrophotometer or experimental measurement) by

1 May 1987 / Vol. 26, No. 9 / APPLIED OPTICS 1729

0.3

-z2

0

U.LI.LU00z

00

xw

0.2

0.1

UI.Ui.

00Z0

0zI-x

0 1 I I I10 11 12

WAVELENGTH (pm)Fig. 3. Values of the extinction coefficient calculated from the dataof Fig. 1: , pure fused silica; - - -, Vycor. The values werecalculated from spectrophotometer transmittance measurements at

250 C.

application of standard thin-film calculation proce-dures.89 These procedures take account of multiplereflections within the sample and require the refrac-tive index to be known to a reasonable accuracy.Within the wavelength region of interest (10-12 gim)sufficient literature values exist' 3 4 to reliably charac-terize the refractive index. Variations in these litera-ture values give rise to a maximum error of approxi-mately 0.003 in the extinction coefficient. Theresults of calculations applied to the spectrophotome-ter measurements of Fig. 1 are shown in Fig. 3 as well asthe results obtained with the Vycor sample. Thestructure in the 10-12-gm transmittance spectrum ofFig. 1 is due to interference fringes. Correspondingresidual minor fluctuations are still evident in the ex-tinction coefficient in Fig. 3 and these are due to minorinaccuracies in the determination of the thickness ofthe sample and/or the values of refractive index as-sumed for the fused silica.

Figure 3 shows that the extinction coefficient ofVycor exhibits a marked difference from that of fusedsilica by virtue of the presence of an absorption bandcentered at 10.9 gm. This band can be attributed to aSi-O-B bond as a result of the boron content in theVycor.10 Values of the extinction coefficient of theother fused silica samples (Table I) were found not todiffer significantly from the results shown in Fig. 3.These results were obtained from samples of differentthicknesses (as shown in Table I) and provided a usefulcross-check on the experimental procedure and calcu-lations.

A comparison between previously reported values ofthe extinction coefficient of fused silica and thepresent results at 250 C is shown in Fig. 4. As can beseen, good agreement with the recent results reportedby Philipp4 has been obtained. The results in Fig. 4are those for the Suprasil sample which, apart from theminor residual fluctuations, are not significantly dif-ferent from the other high-purity fused silica samples.

The experimental arrangement described in Fig. 2was used to determine the temperature dependence ofthe extinction coefficient by measuring the transmit-

0.3

0.2

0.1

o I I I10 11 12

WAVELENGTH (pm)Fig.4. Comparison of the present measurements ( ) with pre-viously reported values of the extinction coefficient of pure fused

silica.

w0z

I-l

CIco

0.3

0.2

0.1

00 500 1000 1500

TEMPERATURE (C)

Fig. 5. Example of the temperature dependence of the transmit-tance of fused silica (26.1-gm Suprasil) at a wavelength of 10.59 im.

tance of the samples at various sample temperaturesand over the range of wavelengths provided by theprobe laser. A typical result is shown in Fig. 5 whereeach point represents an experimental measurementon the Suprasil (26.1-gm thickness) sample. Figure 6shows the result when the extinction coefficient iscalculated from the data of Fig. 5 according to theprocedure described previously.8 9 The extinction co-efficient of fused silica at a wavelength of 10.26 gm isalso shown in the diagram as well as the result obtainedwith the Vycor sample at a wavelength of 10.59 gm. Aline of best fit has been fitted to the experimentalpoints and shows that the extinction coefficient varieswith temperature in a substantially linear manner.The temperature dependence of the extinction coeffi-cient of Vycor is significantly different from the fusedsilica, since it is determined by the Si-O-B absorptionband due to the 4% residual boron in Vycor.

The variation of the extinction coefficient k withtemperature T for all wavelengths measured is sum-marized in Table II which also gives the coefficients ofthe line of best fit k = a + bT, where T is expressed in'C. It should be noted that these coefficients are

1730 APPLIED OPTICS / Vol. 26, No. 9 / 1 May 1987

Table II. Temperature Dependence of the Extinction CoefficientExpressed for the Least-Squares Line of Best Fit k = a + bT(Tin C) for

the Temperature Range and Wavelengths Indicated

Wavelength a b TemperatureMaterial (Am) (X 10-2) (X 10-5) range (C)

Fused silica 10.61 2.25 7.71 25-400Fused silica 10.59 1.82 10.10 25-1800Fused silica 10.57 2.15 8.53 25-400Fused silica 10.55 1.82 11.04 25-1600Fused silica 10.26 3.41 16.67 25-1200Fused silica 10.21 4.70 14.36 25-300Fused silica 9.6 3.71 8.27 25-300

Vycor 10.61 9.74 2.79 25-400Vycor 10.59 8.87 4.59 25-600Vycor 10.57 8.50 3.71 25-400Vycor 10.55 7.89 4.61 25-400Vycor 10.26 4.04 16.63 25-600

applicable for the temperature ranges indicated sincemeasurements were not taken outside these statedranges.

The significance of the results shown in Fig. 6 and inTable II is that the absorption depth : of the radiation,given by A = X/47rk, varies significantly with tempera-ture, from 34 gim at 250C to 4 gim at 1800'C. Theinteraction of high power CO2 laser radiation withfused silica thus appears to be dominated by surfaceheating without any substantial absorption of the radi-ation in the bulk of the material.

IV. Conclusion

The extinction coefficient of high purity fused silicafrom several suppliers has been determined in the 10-12-gm wavelength region by transmission measure-ments on thin (25-30-gm) samples. The values ob-tained were found to agree with previously reportedresults. In addition, the temperature dependence ofthe extinction coefficient was determined up to18000C and found to vary substantially in a linearmanner. The absorption depth of CO2 laser radiationin fused silica was found to vary from 34 gm at 250C to4 gim at 18000C indicating that laser interaction pro-cesses are dominated by surface heating without anysignificant absorption of the radiation in the bulk ofthe material.

0.3I-z

U.-L

0

0z

I-zI-

x

0.2

0.1orI , , , II0 500 1000 1500

TEMPERATURE (OC)

Fig. 6. Measured temperature dependence of fused silica at wave-lengths of 10.59 and 10.26 jum. The temperature dependence of

Vycor at 10.59 um is also shown.

The authors would like to acknowledge the skillfulwork of G. N. Sprott in fabricating the samples and thetechnical assistance of P. Dixon in painstakingly con-ducting the measurements.

References1. G. W. Cleek, "The Optical Constants of Some Oxide Glasses in

the Strong Absorption Region," Appl. Opt. 5, 771 (1966).2. R. Hanna, "Infrared Absorption Spectrum of Silicon Dioxide,"

J. Am. Ceram. Soc. 48, 595 (1965).3. I. Simon and H. 0. McMahon, "Study of the Structure of Quartz

Cristobalite, and Vitreous Silica by Reflection in Infrared," JChem. Phys. 21, 23 (1953).

4. H. R. Philipp, "Silicon Dioxide SiO2 (Glass)," in Handbook of

Optical Constants of Solids, E. D. Palik, Ed. (Academic, Lon-don, 1985), pp. 749-763.

5. G. N. Sprott, "A Method of Fabricating Optically Thin, Parallel-Sided Glass Samples," MRL Technical Note, to be published.

6. I. H. Malitson, "Interspecimen Comparison of the RefractiveIndex of Fused Silica," J. Opt. Soc. Am. 55, 1205 (1965).

7. H. 0. McMahon, "Thermal Radiation from Partially Transpar-ent Reflecting Bodies," J. Opt. Soc. Am. 40, 376 (1950).

8. 0. S. Heavens, Optical Properties of Thin Solid Films (Butter-worths, London, 1955), p. 55.

9. H. A. Macleod, Thin Film Optical Filters (Adam Hilger, Ltd.,London, 1969), p. 37.

10. A. S. Tenney and J. Wong, "Vibrational Spectra of Vapor-Deposited Binary Borosilicate Glasses," J. Chem. Phys. 56,5516(1972).

0

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