Radial distribution of fictive temperatures in silica optical fibers

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Journal of Non-Crystalline Solids 217 (1997) 272-277

Radial distribution of fictive temperatures in silica optical fibers Yih-Lih Peng a,l, Anand Agarwal b, Minoru Tomozawa b,., Thierry A. Blanchet " " Department of Mechanical Engineering and Materials Science, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA b Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA Received 30 December 1996; revised 3 March 1997

Abstract The fictive temperature of glass samples influences many glass properties including mechanical strength and fatigue characteristics. A distribution in fictive temperature on a cross-section of a commercial silica glass communications fiber was determined by measuring the profile of the IR reflection band wavenumber of the silica fundamental structural band at ~ 1122 cm- t as a function of radial position using an FTIR with a microscope attachment. Results showed a lower peak wavenumber which corresponds to a higher fictive temperature near the surface of the as-received fiber. Upon annealing, this surface characteristic disappeared and a nearly constant peak wavenumber was obtained across the fiber cladding indicating that the high fictive temperature of the commercial fiber surface was produced during the fiber drawing process. The different mechanical fatigue behavior of silica fibers from those of bulk silica glasses may be due to the observed high fictive temperature of the fiber surface. © 1997 Elsevier Science B.V.

1. Introduction Many glass properties are known to vary with fictive temperature [1] of the glass. Silica glass is not an exception. For example, density [2-4], refractive index, electrical conductivity [5], viscosity [6], chemical durability [7], water diffusion [8] and mechanical strength and fatigue properties [9] of silica glass vary with the fictive temperature. Some of these properties, especially chemical and mechanical properties are sensitive to the surface conditions of the glass. Therefore, these properties are expected to vary with the fictive temperature of the glass surface. In general, a glass sample can have a fictive

* Corresponding author. Tel.: + 1-518 276 6659; fax: + 1-518 276 8554; e-mail: [email protected]. i Present address: SpecTran Specialty Optics Co., Avon, CT, USA.

temperature of its surface different from that of its bulk. This situation can occur, for example, when a glass is rapidly cooled from the liquid as is done in fiber glass production. In this case, a higher fictive temperature is expected on the fiber surface than in the bulk due to a faster cooling rate at the surface. The opposite situation, i.e., a lower fictive temperature on the glass surface, can be realized when a glass is heat-treated at a temperature lower than its annealing temperature. Due to the catalytic action of water vapor in the atmosphere [10], the surface relaxation can take place faster than the bulk relaxation producing a glass surface with a fictive temperature approaching the heat-treatment temperature while the interior retains the original, higher fictive temperature. One of the important concerns about silica optical fiber is its long time stability, especially its mechanical stability. Thus extensive studies on the static

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Y.-L. Peng et al./ Journal of Non-Crystalline Solids 217 (1997) 272-277

fatigue of silica glass fibers with and without plastic coating have been conducted [11]. In general, the plastic coating does influence the mechanical behavior, especially the fatigue properties of the glass fibers. Even when the plastic coating of the glass fiber is removed, its fatigue characteristics may not be the same as those of bulk silica glasses. One obvious difference between fiber and bulk silica glasses is the surface conditions. Fiber glass surface can be pristine [12], free of flaws, while the bulk silica glass usually has numerous flaws. The mechanical fatigue of the former, therefore, is expected to include crack initiation in addition to the general crack propagation [13,14] which is believed to be involved in the mechanical fatigue of bulk glasses. Even when silica glass fibers have surface flaws, their mechanical properties can still differ from those of bulk silica glasses due to their different fictive temperatures. It has been shown earlier that the same glasses having different fictive temperatures can also have different mechanical strengths and fatigue properties [9]. Silica glass communication fiber production involves a rapid cooling of a molten glass preform and the fiber produced is expected to have a higher fictive temperature than silica glass rods. Furthermore, it is possible that the surface of the fiber has a higher fictive temperature than the fiber interior. It is the objective of the present research to examine this possibility.

tative of radial position, a small aperture size of 10 ~m was chosen for the infrared microscope. The sample position was adjusted by the manual stage controls. The reflection spectra were collected from 700 to 4000 cm-1 at 4000 scans and 2 cm-~ interval. The reflection spectra for the optical fiber had a well-defined band near 1120 cm-1 which represents the S i - O - S i stretching asymmetric vibration, as shown in Fig. 1. The S i - O - S i bending band may also be observed at just less than 800 cm -1. The exact peak positions were determined by fitting the data points in the vicinity of the peak to a 5th order polynomial. It is desired to determine the fictive temperature as a function of radial position in as-received fiber. Therefore, a calibration line for the S i - O - S i asymmetric stretching band position versus fictive temperature must be constructed. In order to achieve different uniform fictive temperatures in the calibration standards, optical fibers were held at various temperatures for long time periods. The temperatures and times selected were l l00°C for 50 h, 1200°C for 4.75 h and 1300°C for 0.5 h. Previous data on silica glass [2,8] were used to determine appropriate heattreatment times needed to attain equilibrium structure at each heat-treatment temperature. The structure reaches equilibrium faster at higher temperatures and hence the heat-treatment time is shorter. After heat treatment, these optical fibers were quenched in water to fix the uniform fictive temperature at the heat-treatment temperature. The IR reflection peak was profiled for the cross-sections of these heat-

2. Experimental procedure The optical fibers studied were made by Furukawa Electric Co., Japan. These fibers are single mode fibers consisting of a GeO2-SiO z core with a diameter of 10 Ixm surrounded by pure S i t 2 cladding with an outside diameter of 125 Ixm. Fibers were lightly scored using a blade, then broken. The resultant fracture surfaces were observed under an optical microscope and those samples with fracture surfaces perpendicular to the axis of the fiber were selected and used for the spectroscopic analysis. The infrared reflection spectra of these flat fracture surfaces were measured with a b'T-IR spectrometer (Mattson Cygnus 100) equipped with an infrared microscope (ATI Quantum). In order to collect spectra represen-

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E-L. Peng et al. / Journal of Non-Crystalline Solids 217 (1997) 272-277

274

treated fibers in a manner similar to the as-received fiber. Previously, silica glass preforms manufactured by Furukawa Electric Co., Japan, which have the same composition as the cladding of the fiber used in the present experiment were also studied [15,16]. 15 mm square plate samples were polished to a thickness of 1.40 _+ 0.05 mm and heated-treated for extended periods of time at various temperatures to achieve equilibrium, then water quenched. IR reflection peak positions of the silica asymmetric vibration band were determined using FTIR (Model 1800, PerkinElmer Corporation, Norwalk, CT). The details of the experimental procedure were reported previously [15,16]. IR measurements were also performed for the silica glass fiber surface after similar heat-treatment using a FI'IR (Nicolet Model 559, Madison, WI) with a microscope attachment. These results are included in the present paper for comparison.

3. Results Fig. 2 shows the change in the peak position of the S i - O - S i asymmetric stretching band in reflection as a function of the radial position for the as-received optical fibers. The material at the nearsurface of the fiber had the lowest wavenumber. The wavenumber of the peak increased toward the center of the fiber, until reaching a maximum at roughly 20 ~tm from the center. Thereafter the wavenumber

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decreased to the center of the fiber presumably due to the presence of the core containing germania. Spectra were taken along a line passing through the center of the fiber, with locations on either side of the center differentiated by positive and negative designations of radius. The wavenumbers were approximately symmetric about the center of the optical fiber. Fig. 3 shows the peak positions of the S i - O - S i asymmetric stretching band in IR reflection as a function of radial position for optical fibers heattreated at 1200°C. Compared to the as-received data in Fig. 2, the positions of the band peak shift to a higher wavenumber after heat treatment at 1200°C. Also, in contrast to Fig. 2, the heat-treated sample had a more uniform peak position. The wavenumbers remain relatively constant from radius 30 to 60 ~m in the pure silica region and within radius 20 Ixm near the core. Again, presumably because of the effect of the germania-containing core, the wavenumbers near the core of the fiber differ from those in the pure silica cladding region. The equilibrium peak positions for optical fibers as a function of the inverse of heat-treatment temperatures (fictive temperatures) are shown in Fig. 4, where the previous results by Agarwal [15] and Agarwal et al. [16] are included. The data points presented in Fig. 4 represent the average and +1 standard deviation of eight measurements on the cladding portion of the fiber cross-section. All the data in Fig. 4 have a similar trend with the peak positions shifting to lower wavenumbers when

Y.-L. Peng et al. / Journal of Non-Crystalline Solids 217 (1997) 272-277

heat-treated at higher temperatures. The slight difference between the fiber and silica plate is due to the different instrument conditions, especially the different angle of incidence of the IR beam. It is known that a change in the angle of incidence changes the IR reflection band position [17,18]. The FTIR spectrometer (Perkin-Elmer) used for silica plate samples employed an incident angle of 6.5 ° while the other instruments with microscope attachment used larger incident angle. The lines in the Fig. 4 are constructed using a linear regression of the data. The slope for the present data was similar to those for the previous data [15,16]. The equation obtained for the present data in Fig. 4 is v = 1114.4+ 12166/Tf

(1)

where v (cm l ) is the peak position of the Si-O-Si asymmetric stretching band and Tf (K) is the fictive temperature of the silica glass. The radial distribution of the fictive temperatures of the as-received optical fibers can be estimated, using Eq. (1), from the distribution of the IR peak position of the Si-O-Si asymmetric stretching band. Fig. 5 shows the estimated fictive temperature distribution on the cross-section of the as-received optical fiber together with that for the fiber heat-treated at 1200°C. Those data correspond to the IR peak positions along the cross-section of the optical fibers shown in Figs. 2 and 3. The fictive temperatures

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along the entire cross-section for the as-received optical fibers varied with radial position and were everywhere higher than those of the heat-treated fibers. The predicted fictive temperatures of the asreceived optical fibers at the near-surface region are higher than those in the interior of the fibers. Values near the surface of the fiber were only approximate, as they are obtained by extrapolating the wavenumber-fictive temperature relationship outside of the region of the calibrations provided in Fig. 4. For the optical fibers heat-treated at 1200°C, the fictive temperatures were relatively constant from radius 30 to 60 p~m. The fictive temperatures in the region near the core can not be estimated by Eq. (1) because the composition was not pure silica and the stress due to thermal expansion mismatch can influence the wavenumber of the IR peak position.

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Fig. 4. Equilibrium peak position of the S i - O - S i asymmetric stretching vibration band in IR reflection versus inverse of fictive temperature, for optical fibers in this study together with those previously reported by Agarwal for silica fiber surface [15] and Agarwal et at. for silica glass plate [16]. Error bars represent + 1 standard deviation.

The estimated distribution of fictive temperature across the fiber diameter of as-received optical fiber shown in Fig. 5, is based upon the empirical relationship between the fictive temperature of silica glass and the IR structural band reflection peak position, i.e., Eq. (1). This equation represents the experimental data, shown in Fig. 4, obtained in the fictive temperature range of 1100 to 1300°C. A similar relation was observed for silica plate samples in the temperature range of 950 to 1400°C. In these temper-

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Y.-L Peng et al. / Journal of Non-Crystalline Solids 217 (1997) 272-277

ature ranges, the volume of silica glass decreases with increasing fictive temperature [3] and the corresponding shift of the IR structural band to lower wavenumber is attributed to the decreasing S i - O - S i bond angle with increasing temperature [16]. In order to obtain high fictive temperature of the as-received silica fiber surface, this equation was extrapolated to higher temperatures. Since above ~ 1550°C, the volume of glass increases with increasing fictive temperature [3] the validity of this extrapolation may be questioned. However, Devine and Marchand [19] reported that silica glass made from the vapor phase, which is expected to have an extremely high fictive temperature, had a low wavenumber, 1064 cm- 1, for the IR absorption peak of the silica structural band as well as a large Raman peak due to defects. The absorption peak and reflection peak are related by the Kramers-Krt~nig relationship and this IR absorption peak wavenumber observed by Devine and Marchand [19] corresponds to a much smaller IR reflection wavenumber ( ~ 1086.5 cm -1 estimated using the Kramers-KriSnig relationship [15]) than observed here for the as-received silica fiber surface. In fact, Devine and Marchand estimated the fictive temperature of the vapor deposited silica film to be in excess of 2400°C. Thus, it appears safe to attribute the low wavenumber of the IR peak to a high fictive temperature even above 1550°C, although the linear relationship employed to estimate the fictive temperature of the as-received silica fiber surface may not hold quantitatively. Apparently, the correlation between the IR peak wavenumber and the volume of the glass observed at lower temperature does not extend to the higher temperature beyond 1550°C. Devine and Marchand [19] attribute this apparent discrepancy to the increasing 'free volume' at high temperature. If there is a residual stress in the glass fiber it can influence the IR peak position; compressive stress shifts the IR peak to lower wavenumbers [7]. Therefore, the observed lower wavenumber of the IR structural band near the surface of the as-received fiber may be due to the residual compressive stress in the fiber cladding. Kurkjian and Paek [20,21] showed, both theoretically and experimentally, that the residual stress in the axial direction of optical fibers has both thermal and mechanical origins. The thermal stress is caused by the thermal expansion

mismatch between core and cladding while the mechanical stress is caused by the difference in viscosities. The thermal stress is independent of fiber drawing stress while the mechanical stress increases with the fiber drawing stress. Consistent with the prediction by Kurkjian and Paek, GeO2-doped core/SiO 2 cladding fiber was found to have an axial compressive stress, on silica cladding, of ~ 1 k g / m m 2 when the drawing force is small. The residual stress becomes a tensile stress with increasing fiber drawing force [22,23]. Thus the maximum compressive stress on the cladding of the present optical fiber is estimated to be in the range of ~ 1 k g / m m 2 in the axial direction. A larger magnitude of tensile stress can exist on the fiber cladding but that would shift the IR structural band to the higher wavenumber, a direction opposite to the observed trend. From the comparison of the IR peak position of the silica structural (absorption) band near 1100 cm-~ and the density of silica glass, Devine [24] found a peak shift of 40 cm -1 for a 23% volume change. The same magnitude of shift by the volume change is expected for the corresponding reflection peak [15]. Assuming that the stress effect on the peak shift is due primarily to the volume change, a 1 k g / m m 2 uniaxial compressive stress is estimated to produce a peak shift < 0.02 cm -1, which is much smaller than the observed peak shift attributed to the fictive temperature change. The higher tensile stress would shift the peak to higher frequency, which is opposite to the observed trend. Therefore, the effect of the residual stress on the observed IR structural band peak wavenumber of the as-received glass fiber surface is negligible and the observed surface features are almost exclusively due to the fictive temperature effect. Although the present paper refers to only one type of optical fiber made by one manufacturing method, our preliminary measurement indicated a similar high surface fictive temperature for an optical fiber made by other method, e.g., MCVD method as well as for a pure SiO 2 glass fiber. The IR peak wavenumber near the core of the fiber, i.e., +20 ~zm from the center, had unusual features both in the as-received sample and the annealed sample, as shown in Figs. 2 and 3. These features are probably due to the different composition of the core which contains germania and the stress due to the thermal expansion coefficient mis-

Y.-L. Peng et al. / Journal of Non-Crystalline Solids 217 (1997) 272-277

match between core and cladding. The thermal residual stress in the core is expected to be much larger than that in the cladding due to its smaller cross-sectional area [20,21].

5. Conclusions As-received silica glass communication fiber was found to have a higher fictive temperature than annealed fiber. Also, the surface of the as-received fiber had a higher fictive temperature than its interior while the annealed fiber had a constant fictive temperature in the cladding silica region. This observed feature should be taken into account when static fatigue of the glass fiber is considered.

Acknowledgements

A part of this research was supported by the National Science Foundation under Grant No. DMR9616313. Careful reading of the manuscript by Dr Steven Crichton of Rensselaer Polytechnic Institute is greatly appreciated. Discussion with Dr Charles R. Kurkjian of Bellcore Corp. is greatly appreciated. References [1] A.Q. Tool, J. Am. Ceram. Soc. 29 (1946) 240. [2] R.W. Douglas, J.O. Isard, J. Soc. Glass Technol. 35 (1951) 206.

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