Water diffusion into a silica glass optical fiber

Journal of Non-Crystalline Solids 324 (2003) 256–263

www.elsevier.com/locate/jnoncrysol

Water diffusion into a silica glass optical fiber

Stephanie Berger a, Minoru Tomozawa b,*

a Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USAb Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110, 8th Street, Troy, NY 12180-3590, USA

Received 30 July 2002

Abstract

The diffusion coefficient and solubility of water in silica glass optical fiber cladding were measured in the temperature

range of 600–800 �C and were compared with the corresponding values of bulk silica glasses. It was found that the

diffusion coefficient was slightly lower and the solubility was appreciably higher in optical fiber, especially at low

temperatures, compared with those in bulk silica glasses. The observed trend was consistent with the expected effect of

fictive temperature.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

Optical fibers are part of the important new

technology currently being used in the telecom-

munications industry. These fibers are composed

of pure silica glass cladding and a germanium-

doped silica glass core. Since the fibers are drawn

from high temperatures �2000 �C and are quen-

ched rapidly, the structure and properties of fibercladding glasses are expected to be different from

those of annealed silica glasses even though the

composition is same. Among various glass char-

acteristics, water diffusion is of particular concern

since water in glasses has a disproportionately

large influence on many glass properties. For ex-

ample, water in glass reduces both mechanical

* Corresponding author. Tel.: +1-518 276 6659; fax: +1-518

276 8554.

E-mail address: [email protected] (M. Tomozawa).

0022-3093/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/S0022-3093(03)00247-3

strength and chemical durability, and increases theoptical absorbance. Since the optical fibers can be

exposed to water vapor for an extended period

during their usage, the water diffusion into the

fibers may become an important concern.

Previous studies on water diffusion of silica

glasses [1] have found that the diffusion constant

of water in silica glass varies with fictive temper-

ature, or cooling rate from the melt. The fictivetemperature is the temperature at which the super-

cooled liquid changes to its glassy state [2]. In

general, glasses prepared using a faster cooling

rate acquire a higher fictive temperature. Due to

their fast cooling rate silica glass optical fibers

have a much higher fictive temperature, e.g. 1650

�C [3] compared to 1100–1200 �C of normal an-

nealed silica glasses. Silica glasses cooled at a fas-ter rate have a lower volume and therefore a higher

density and a higher index of refraction compared

with silica glasses cooled at a slower rate [4,5].

Roberts and Roberts [1] found that silica glass

ed.

mail to: [email protected]

S. Berger, M. Tomozawa / Journal of Non-Crystalline Solids 324 (2003) 256–263 257

with higher fictive temperatures had lower waterdiffusion constants and higher water solubility at

a constant temperature of 750 �C in the fictive

temperature range of 1100–1300 �C. However, no

water diffusion data are available for a silica glass

with fictive temperature as high as those of optical

fibers silica glasses. This study attempts to provide

the water diffusion constant and water solubility

data for silica glass optical fibers. The data areuseful for estimating the long-range stability of

optical fibers.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

3000 3200 3400 3600 3800 4000

Wavenumber (cm-1)

Abs

orba

nce

23hr

16hr

0hr

Fig. 1. Absorbance due to hydroxyl in silica glass fiber after

heat-treatment times indicated at 800 �C.

2. Experimental procedure

A commercial single mode silica glass optical

fiber with the diameter of 125 lm made byFurukawa Electric Co. was used. The fibers were

exposed to water vapor of constant water vapor

pressure, 335 Torr, for varied times at selected

temperatures. The water uptake was monitored

using Fourier transform infrared spectroscopy

(FTIR). From the water (hydroxyl) uptake data,

both the water diffusion coefficient and water sol-

ubility were obtained.First, the fibers were cut into pieces several

centimeters in length and the plastic coating was

removed from the fibers by placing them for 30 s in

a mixture of sulfuric acid and nitric acid (98%

H2SO4–2% HNO3), which was heated to 200 �C.This process was followed by ethanol and water

washing. The obtained bare fibers were then heat-

treated in an electric furnace for various lengths oftime up to 24 h at temperatures of 600, 700, and

800 �C. The constant water vapor pressure, 355

Torr, generated by a water bath kept at 80 �C, wasflown into the furnace where the fiber samples

were heat-treated. The fiber samples were placed in

a wire holder and then into a silica glass tube that

was placed in the electric furnace. The wire holder

allowed most of the surface area of the fiber to beexposed to water vapor. After heat-treatment for a

prescribed period of time, the fiber was quickly

taken out of the furnace and kept in a glass vial

until the FTIR measurement could be taken.

The temperature range of the heat-treatment

was chosen because of the relaxation kinetics. If a

high temperature such as 1200 �C had been used

the glass would quickly relax and lose memory ofits original structure. Even at lower temperature,

surface structural relaxation can take place [6]. In

the temperature range below 850 �C, however, therate of the surface relaxation is slower than the

rate of water diffusion [6] and water is expected

diffuse into the unrelaxed fiber.

The IR absorption spectrum of each fiber was

obtained using a Nicolet Magna 560 FT-IR withSpectra-Tech IR-Plan Advantage microscope at-

tachment. The beam size was fixed by an aperture

of 20 lm� 160 lm with the longer dimension

parallel to the fiber axis. An absorbance spectrum

across the fiber diameter was taken with 256 scans

for each reading. It is known that the absorption

band near 3670 cm�1 is due to the vibration of the

hydroxyl and that peaks at this wavenumber in-dicate the presence of hydroxyl water [6]. An ex-

ample of the absorbance data is shown in Fig. 1.

The sample before heat-treatment showed no ab-

sorbance at the hydroxyl band. The absorbance of

the heat-treated samples were obtained as the

heights of the peaks at 3570 cm�1 above the value

for the un-heat-treated sample. The obtained ab-

sorbance is proportional to the amount of theabsorbing species, hydroxyl in the present case.

The water uptake was measured as a function of

heat-treatment time and temperature. Water dif-

fusion into silica glass at the heat-treated temper-

ature is expected to exhibit the Fickian diffusion

0

0.02

0.04

0.06

0.08

0.1

0.12

0 2 4 6

Square Root Time (hr1/2)A

bsor

banc

e

800C

700C

600C

Fig. 2. Absorbance of the hydroxyl peak as a function of

square root of the heat-treatment time at three different tem-

peratures.

Table 1

Slopes of absorbance vs. square root time

Temperature (�C) Slope (absorbance/ph)

258 S. Berger, M. Tomozawa / Journal of Non-Crystalline Solids 324 (2003) 256–263

[4], following the relation Q ¼ Csð4=p

pÞpDt,where Q is the hydroxyl uptake, Cs is the surface

concentration, D is the diffusion constant, and t isthe heat-treatment time. This is the expression for

the uptake of a thick plate, water entering from

both surfaces [7]. The uptake was plotted against

the square root of the heat-treatment time and the

resulting slope was evaluated. In order to obtain

the diffusion coefficient from the slope, it is nec-essary to know the surface concentration or solu-

bility, Cs, of hydroxyl in the glass. The surface

concentration was determined by measuring the

hydroxyl content in the surface layer of the heat-

treated samples. For this purpose, the samples

were successively etched with a hydrofluoric acid,

25% HF–15% H2SO4 mixture and the corre-

sponding change in IR absorbance was monitored.After each acid etching, the fibers were washed

with ethanol, followed by water. The depth re-

moved was measured using a Unitron optical

microscope. The negative slope of the residual

absorbance vs. etch depth near the specimen sur-

face provides the surface concentration. The ob-

served surface concentration was also considered

to be the solubility of water under the water vaporpressure and temperature employed.

800 0.021

700 0.0183

600 0.0168
3. Results
The water uptake data expressed in terms of IR

absorbance values are plotted against the square

root of the heat-treatment time in Fig. 2 for allthree different heat-treatment temperatures. Error

bars are shown where they are larger than the

symbols. Data were least-square fitted with the

expected straight lines [7] at each heat-treatment

temperature. The slopes of the linear relationships

between the absorbance and the square root of the

heating time obtained at various temperatures are

shown in Table 1. The slopes show a slight increasewith increasing temperature indicating that at

higher temperatures water is absorbed at a faster

rate.

Fig. 3 shows the etched depth vs. etching time

for the glass fiber samples heat-treated at various

temperatures. Within the experimental error, all the

samples had the same etching rate of 1.6 lm/min.

To increase accuracy of the depth measurement,

the least-square fit line of the etching data was

taken as seen in Fig. 3, and was used to estimatethe thickness removed from the etching time em-

ployed.

As the fibers were etched, their remaining water

content decreased. This decrease in water content

was reflected in the decrease in height of the water

peak as shown in Fig. 4. Fibers heated at higher

temperatures had a slower rate of water decrease

than those heated at lower temperatures. The rateof water decrease in relation to thickness reduction

allows the surface concentration, Cs, to be calcu-

lated. For finding Cs the slope of the data in Fig. 4

near the surface was used for samples treated at

600 and 700 �C. However, for the sample treated

at 800 �C, the slope to the last (or deepest) point

was used because the change in water content was

0

1

2

3

4

5

6

7

8

9

10

4 5 6Time (min)

Tota

l Thi

ckne

ss E

tche

d (u

m)

800C

700C

600C

0 1 2 3

Fig. 3. Thickness of fibers etched vs. etching time for silica fiber

after heat-treatment at indicated temperatures.

0

0.02

0.04

0.06

0.08

0.1

0.12

0 2 4 6 8

Depth (um)

Abs

orba

nce

800C700C600C

Fig. 4. Decrease in absorbance due to hydroxyl with thickness

etched for fibers heat-treated at indicated temperature for 24 h.

Table 2

Surface hydroxyl concentrations

Temperature (�C) Cs (cm�1)

800 14.3

700 25.8

600 44.1

0

5

10

15

20

25

30

35

40

45

50

800 1000 1200 1400 1600

Temperature (K)

Cs

(cm-1

)

Davis andTomozawa [6]Present Data

Fig. 5. Surface concentration data for silica glass fibers com-

pared to previous data for bulk silica glass with fictive tem-

perature of 1075 �C as a function of heat-treatment temperature

in 355 Torr water vapor.


so small at early etching times that it was difficultto determine the slope accurately. The values for

the surface hydroxyl concentration, Cs, expressed

in terms of absorbance per unit sample thickness,

are shown in Table 2. The fibers heated at lower

temperatures had higher Cs values.

Fig. 5 shows the surface concentration data for

the fibers, compared to data previously taken by

Davis and Tomozawa for bulk silica glass [6]. Thesurface concentrations for the fibers are much

higher and decrease at faster rate than in bulk

silica glass with increasing temperature, especially

at lower temperature.

Using the slope of water uptake shown in Table1 and the surface concentration for each temper-

ature the diffusion constant was calculated. The

obtained diffusion coefficients are shown in Table

3. The diffusion data are also plotted as logðDÞ vs.the inverse of the absolute temperature in Fig. 6.

The diffusion coefficients obtained here are com-

pared with the diffusion coefficients for bulk silica

glasses obtained earlier by Davis and Tomozawa[6]. The water diffusion coefficients of the silica

optical glass fiber were slightly smaller and had a

steeper slope than those of the bulk silica glass.

The activation energy for the diffusion of water

Table 3

Effective water diffusion constants

Temperature (�C) D (cm2/s)

800 1.2� 10�10

700 2.8� 10�11

600 7.9� 10�12

-12

-11.5

-11

-10.5

-10

-9.5

-9

-8.5

-8

0.5 0.7 0.9 1.1 1.3 1.5

1000/T (K)

Davis andTomozawa [6]

Present Data

Fig. 6. LogD (cm2/s) plotted against 1000/temperature (Kelvin)

for both silica glass fibers and bulk silica glass samples by Davis

and Tomozawa [6].


into silica fibers was evaluated assuming the tem-

perature dependence of diffusion coefficient of

D ¼ D0 expð�E=RT Þ;where D0 and E are pre-exponential factor and

activation energy, respectively. The obtained value

of the activation energy for silica glass fiber, 104

kJ/mol, is larger than 83 kJ/mol obtained by Davis

and Tomozawa [6] for bulk silica glass.

4. Discussion

The present sample was glass fiber with a cir-

cular cross-section while a model for a thick plate

was used for the data analysis. Error associated

with this assumption will be evaluated first. Ac-

cording to Crank [7], the uptake of diffusing spe-

cies,Mt, at small time, t, by a fiber with radius, a, isgiven by

Mt=M1 ¼ ð4=ppÞpDt=a2 � Dt=a2 � ½1=ð3ppÞ

� ðDt=a2Þ3=2 þ � � � ;

where M1 is the uptake at time 1, and is given by

pa2Cs. When the second and higher order terms

can be ignored, the uptake per unit surface area is

given by Mt=2pa ¼ Csð2=p

pÞpDt, which is equiv-

alent to the quantity Q for a semi-infinite plate

employed in the present analysis. (The factor of 2

difference comes from the uptake from one side vs.

both sides.) Therefore, the error caused by ignor-ing the higher order terms in the above equation

will be evaluated. Taking the worst case scenario

of the highest diffusion coefficient of 1.17� 10�10

cm2/s at 800 �C and the longest heat-treatment

time of 24 h, the diffusion distance becomespDt ¼ 31:8 lm. Using the radius of the optical

fiber, 62.5 lm, the error in water uptake by ap-

proximating the fiber by a thick plate is at most25%. At lower temperatures and shorter times, the

error is smaller. Furthermore, the observed nearly

linear relationship between the uptake and square

root of time suggests that the higher order terms

are not playing an important role.

With the microscope attachment to the FT-IR

used in the present experiment, the IR beam dur-

ing the absorbance measurement is diverged andconverged instead of passing through the sample

vertically. The extent of the divergence is propor-

tional to the specimen thickness and the angle, a,of the diverging beam schematically shown in Fig.

7. In the Spectra-Tech IR-Plan Advantage micro-

scope employed, the average angle of incidence is

28�. If the refractive index of silica glass at �3500

cm�1 is assumed to be 1.42, the average value ofthe diverging angle, a, is estimated to be 19.3�.Under these conditions, the beam passes through

the fiber off-center across �118 lm distance as

shown in the figure. This distance is smaller than

the fiber diameter of 125 lm but is sufficiently

larger than the maximum water diffusion distance,

32 lm, and the water uptake equation employed

would still be valid.The general trends of the temperature depen-

dence of water diffusion into the silica glass fiber

were consistent with what was expected from pre-

vious research. For example, the observed higher

solubility ofwater at lower temperatures in themea-

sured temperature range is consistent with previous

research by Davis and Tomozawa [6]. The water

uptake data shown in Fig. 2 at three different

IR beam

20µ

α

m

Fig. 7. Schematic IR ray diagram of the absorbance measure-

ment of the fiber using FT-IR with a microscope attachment.


temperatures were very close. This is because,

while the water diffusion coefficient is greater at

higher temperature, the surface hydroxyl concen-

tration, or hydroxyl solubility, is lower at higher

temperature. The FT-IR absorbance measures the

total amount of water in the sample and the op-

posite temperature dependence of diffusion coeffi-cient and surface concentration makes the uptake

rates at different temperature very close.

The main cause of the different diffusion be-

havior between the fiber and the bulk silica glasses

is the different fictive temperature. Roberts and

Roberts [1] showed that a silica glass with higher

fictive temperature has a lower water diffusion

coefficient and higher hydroxyl solubility. Sincethe fibers have a higher fictive temperature they

should have lower diffusion constants and higher

surface concentration compared with the bulk sil-

ica glasses.

In Fig. 8, water diffusion data at 750 �C, ob-tained by interpolation in the present case, are

collected as a function of fictive temperature. Since

the solubility data by Roberts and Roberts [1] are

expressed in a different unit from that of the pre-

sent study, the value was normalized using the

data by Davis and Tomozawa [6] who used the

silica glass with the fictive temperature 1060 �C.The general trend is consistent but the diffusioncoefficients obtained by Roberts and Roberts [1]

are different from other data. The silica glass used

by Roberts and Roberts was type I [1,4] made by

fusion of crystalline quartz, while the glass used by

Davis and Tomozawa [6] as well as the glass fiber

used here were made by a CVD method. Different

types of silica glasses may be a cause of some

discrepancy. Another source of difference is thewater vapor pressure employed in the measure-

ment. While 335 Torr was used by the present

authors as well as by Davis and Tomozawa [6], 700

Torr was used by Robert and Roberts [1].

Diffusion of water in silica glass is believed to

involve the motion of molecular water and the

following reaction between water and the glass

network [8]

H2OþBSiAOASiB $ BSiAOHþHOASiB;

where B represents three chemical bonds to the

neighboring oxygen. It is believed that the reaction

is fast at high temperature and the local equilib-

rium is maintained during the water diffusion, with

the equilibrium constant

K ¼ ½OH2=½H2O;where [OH] and [H2O] represent the activity or

concentration of hydroxyl and molecular water,

respectively and the activity of silica network is

considered to be unity. When the water concen-

tration is low, the major part of water in silicaglass exists as hydroxyl. The measured water dif-

fusion coefficient under such condition is an ef-

fective diffusion coefficient, Deff , given by

Deff ¼ 4D½OH=K;where D is the diffusion coefficient of molecular

water. Silica glasses with higher fictive temperatureare expected to contain greater concentration of

defects such as vacancy and strained bonds. These

0

5

10

15

20

25

800 1000 1200 1400 1600 1800

Fictive Temperature (oC)

Cs

(cm

-1)

Roberts andRoberts [1]

Davis andTomozawa[6]

Present Data

0.00E+00

5.00E-11

1.00E-10

1.50E-10

2.00E-10

2.50E-10

1000 1200 1400 1600 1800

Fictive Temperature (oC)

Diff

usio

n C

oeffi

cien

t (cm

2 /s)

Roberts andRoberts [1]Davis andTomozawa [6]Present Data

(a)

(b)

Fig. 8. Fictive temperature dependence of (a) water solubility

and (b) diffusion coefficient at 750 �C, under 355 Torr water

vapor pressure.N: present data,r: Roberts and Roberts [1],j:

Davis and Tomozawa [6]. (Roberts and Roberts� data [1] were

obtained under 700 Torr water vapor pressure. Their solubility

data were shifted to match that by Davis and Tomozawa data

[6].)


defects are expected to increase the reactivity of

silica glass with water. Furthermore, silica glasses

with higher fictive temperatures have a structure in

which Si–O–Si bond angles are smaller. It was

reported [9] that silica structures with smaller Si–

O–Si bond angles have a greater reactivity with

water. This structural feature would also increase

the hydroxyl concentration. Thus, the solubility ofwater, which exists predominantly in the form of

hydroxyl, is expected to increase with increasing

fictive temperature. On the other hand, the con-

centration of molecular water in a silica glass,

[H2O], would change only slightly with fictive

temperature. This would make the resulting value

of K greater for a glass with higher fictive tem-

perature at a constant temperature. The effectivewater diffusion coefficient, Deff , is expected, then,

to decrease with increasing fictive temperature due

to a larger value of K. Thus, the observed trend of

water diffusion in silica glasses, both bulk and

fibers, appears to be consistent with the trend ex-

pected for glasses with different fictive tempera-

tures.

5. Conclusion

Silica glass optical fibers were found to have

higher surface hydroxyl concentrations (or solu-

bility) and lower effective water diffusion constants

than bulk silica glass. The observed features are

attributed to the higher fictive temperature of thesilica glass optical fibers.

Acknowledgements

This research was performed as a part of the

NSF sponsored Research Experience for Under-

graduates (REU) program at Rensselaer Poly-technic Institute under grant number DMR-97589.

Careful reading of the manuscript by Mr Michael

Magyar of Rensselaer is greatly appreciated.

References

[1] G.J. Roberts, J.P. Roberts, Phys. Chem. Glasses 5 (1964)

26.

[2] A.Q. Tool, J. Am. Ceram. Soc. 29 (1946) 240.


[3] D.-L. Kim, M. Tomozawa, J. Non-Cryst. Solids 286 (2001)

132.

[4] R. Bruckner, J. Non-Cryst. Solids 5 (1970) 123.

[5] J. Murach, R. Bruckner, J. Non-Cryst. Solids 211 (1997)

250.

[6] K.M. Davis, M. Tomozawa, J. Non-Cryst. Solids 185 (1997)

203.

[7] J. Crank, Mathematics of Diffusion, 2nd Ed., Clarendon,

Oxford, 1975, p. 32 and p. 74.

[8] R.H. Doremus, in: J.W. Mitchell, R.C. DeVries, R.W.

Roberts, P. Cannon (Eds.), Reactivity of Solids, Wiley, NY,

1969, p. 667.

[9] T.A. Michalske, B.C. Banker, J. Am. Ceram. Soc. 76 (1993)

2613.

Water diffusion into a silica glass optical fiber

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