Fluoride Glass Fiber Optics

405 Pages·1991·5.58 MB·English

by  Ishwar D. Aggarwal and Grant Lu (Eds.)

Fluoride Glass
Fiber Optics
Edited by ISHWAR D. AGGARWAL
NAVAL RESEARCH LABORATORY
WASHINGTON, D.C.
GRANT LU
NORTON COMPANY
NORTHBOROUGH, MASSACHUSETTS
ACADEMIC PRESS, INC.
Harcourt Brace Jovanovich, Publishers
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Copyright © 1991 by Academic Press, Inc.
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ACADEMIC PRESS, INC.
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United Kingdom Edition published by
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Library of Congress Cataloging in Publication Data
Fluoride glass fiber optics/edited by Ishwar D. Aggarwal, Grant Lu.
p. cm.
Includes bibliographical references and index.
ISBN 0-12-044505-0 (alk. paper)
1. Fiber optics—Materials. 2. Fluoride glasses. I. Aggarwal,
Ishwar D., date. IL Lu, Grant, date.
TA1800.F568 1991
621.36'92—dc20 90-37415
CIP
Printed in the United States of America
91 92 93 94 9 8 7 6 5 4 3 2 1
Contributors
Numbers in parentheses refer to the pages on which the authors' contributions begin.
BERNARD BENDOW (85) TR W Space and Technology Group, One Space
Park, Redondo Beach, CA 90278
KENNETH J. Ε WING (141) United States Naval Research Laboratory,
Optical Sciences Division, Code 6505, Washington, DC 20375-5000
E. J. FRIEBELE (307) United States Naval Research Laboratory, Optical
Sciences Division, Code 6505, Washington, DC 20375-5000
D. L. GRISCOM (307) United States Naval Research Laboratory, Optical
Sciences Division, Code 6505, Washington, DC 20375-5000
HIROSHI IWASAKI (213) NTT Opto-Electronics Laboratories, Tokai, Ibaraki
319-11, Japan
YOSHINORI MIMURA (235) KDD Meguro R&D Labs., 2-1-15, Oohara,
Kamifukuoka, Saitama 356, Japan
TETSUYA NAKAI (235) KDD Meguro R&D Labs, 2-1-15, Oohara,
Kamifukuoka, Saitama 356, Japan
ROMULO OCHOA (37) Advanced Materials Research Center, Department of
Materials Science and Engineering, University of Florida, Gainesville,
Florida 32611
MARCEL POULAIN (1) Centre d'Etude des Matériaux Advances, Laboratoire
de Chimie Minerale, Universite de Rennes I, Campus de Beaulieu, Rennes
35042, France
R. S. QUIMBY (351) Department of Physics, Worcester Polytechnic
Institute, Worcester, Massachusetts 01609
CATHERINE J. SIMMONS (37, 275) Advanced Materials Research Center,
Department of Materials Science and Engineering, University of Florida,
Gainesville, Florida 32611
JOSEPH H. SIMMONS (37, 275) Advanced Materials Research Center,
Department of Materials Science and Engineering, University of Florida,
Gainesville, Florida 32611
ix
χ Contributors
JAMES Α. SOMMERS (141) Teldyne Wah Chang Albany, P.O. Box 460,
Albany, Oregon 97321-0136
SHIRO TAKAHASHI (213) NTT Opto-Electronics Laboratories, Tokai,
Ibaraki 319-11, Japan
ADRIAN C. WRIGHT (37) /. /. Thomson Physical Laboratory, Department
of Physics, University of Reading, Whiteknights, Reading RG6 2AF,
United Kingdom
Introduction
Heavy metal fluoride glasses were discovered by Michel Poulain (Poulain
et al., 1975) at the University of Rennes in France in 1974 during an
investigation of crystalline laser host materials. The discovery of an
amorphous fluoride material was quite unexpected since the only previously
known fluoride glasses were BeF and a complex multicomponent glass
2
based on A1F discovered by Sun (1949). Systematic investigations into the
3
glass-forming regions of heavy metal fluorides were undertaken by Michel
and Marcel Poulain, Jacques Lucas, and others at the University of Rennes.
One of the earliest and most important ternary systems to be investigated
was the ZrF —BaF —LaF system. In the 1980s, heavy metal fluoride
4 2 3
glasses without zirconium were also developed.
It was quickly realized that, compared with silicate glasses, these
glasses have extended transparency into the infrared wavelengths. The
multiphonon edge in fluoride glasses is shifted to longer wavelengths due to
the presence of heavier ions. Since Rayleigh scattering decreases rapidly at
long wavelengths, these glasses may potentially be used for ultra-low loss
optical fibers. One of the earliest predictions of the ultimate transmission
loss was made by Shibata et al. (1981), who predicted a theoretical loss of
10"3 dB/km at 4μιη. However, more recent estimates (Van Uitert et al.
9
1985, and France et al., 1987), based on better experimental data, show that
the ultimate attenuation is more likely to be 10" 2 dB/km at 2.5 μτη. .
The principal driving force behind fluoride glass research has been the
development of ultra-low loss optical fiber communication systems. The
ultimate goal is a transoceanic, repeaterless optical fiber. Such a develop-
ment would be of interest for commercial and defense needs. However,
fluoride glasses can be used in less demanding applications that require
infrared transmitting optical fibers or bulk components. Fluoride glasses are
being tested for use in the medical field, where fluoride optical fibers can
transmit a laser beam inside the body for surgery. Fluoride optical fibers
have also been tested for use in gas and liquid sensors. Absorption by the
gas or liquid is registered using a laser or LED at one end of the fiber and
xi
xii Introduction
a detector at the other end. Fluoride fibers are useful for infrared spectros-
copy because many important molecules have absorption bands in the
infrared. Remote monitoring is made possible by using the optical fiber as
the transmission medium for signal propagation.
Fluoride glasses are also advantageous in bulk components. High
energy HF and DF laser windows with ultra-low distortion are made
possible because of the low absorption coefficient in the mid-IR and the low
effective temperature induced optical path length change (Bendow, 1984).
The latter is a function of dn/dT, thermal expansion, and photoelastic
terms, and can be very close to zero for certain fluoride glass compositions.
Fluoride glass windows have much better performance than ZnS or ZnSe
and comparable performance to CaF . However, fluoride glass windows
2
can potentially be made in larger sizes then CaF windows.
2
By doping fluoride optical fibers with rare earth ions such as Tm, Ho,
Er, and Nd, fiber lasers can be fabricated. These fiber lasers can emit in the
wavelength region of 0.5 to 3.0μπι which is very useful for communication
applications and IR spectroscopy. Furthermore, the doped optical fibers
can also be used as amplifiers in fiber optic communication systems; this is
preferable to unreliable and expensive analog to digital signal converters
and repeaters. Fluoride glasses are especially interesting because rare earth
ions can be doped at a higher level in fluoride glasses than silica glasses,
thereby permitting a higher efficiency.
The optical loss of fluoride fibers can be divided into intrinsic and
extrinsic sources of loss. Each of these can be further subdivided into
absorption loss and scattering loss. There are three sources of intrinsic loss:
the UV absorption edge, Rayleigh scattering, and multiphonon absorption.
The dominant effect at very short wavelengths (UV-visible) is the UV
absorption edge caused by electronic transitions; however, this absorption
decreases very rapidly with increasing wavelength. Therefore, in the visible-
near IR range, Rayleigh scattering due to density and compositional
fluctuations is the main source of intrinsic loss. Rayleigh scattering
decreases inversely with (wavelength)4 so that at longer wavelengths, the
multiphonon absorption edge is the limiting factor.
Extrinsic losses can be attributed mainly to impurity absorption and
crystallite scattering. Most of the transition elements and some of the rare
earth elements absorb in the mid-IR; even 1 ppb level of contamination
cannot be tolerated for certain elements if extrinsic losses are to be reduced
below the intrinsic loss. Crystallite scattering is usually the dominant source
of extrinsic loss. Fluoride glasses devitrify much more readily than silicate
glasses, so it is difficult to avoid the formation of crystals.
Research in glass compositions has therefore focussed on producing
compositions that are stable against devitrification and moisture attack but
Introduction xiii
that continue to have a low minimum loss in the infrared. More stable
fluoride glass compositions are continually being discovered. One of the
problems in this field is that it is difficult to assess accurately the stability
of any given composition. Therefore, major fluoride fiber laboratories
have selected different glass compositions for use in their fiber. More
compositional research, along with the stringent elimination of external
nuclei, should permit the fabrication of fibers with the level of scattering
needed for ultra-low optical loss. A scattering level of 0.025 dB/km
(Aggarwal et al., 1988) in a short length of fiber has been measured at the
Naval Research Laboratory. A virtually crystal-free window weighing 20 kg
has been fabricated by Owens-Corning Fiberglas. These recent develop-
ments show the tremendous progress that has been made in the last ten years.
Fluoride glasses are not as durable as silicate glasses when attacked by
atmospheric moisture or water. However, the static fatigue problem
appears to be less in the case of fluoride glasses. The use of hermetic
coatings appears to be essential for fluoride glasses and fibers. Coatings of
MgO and MgF have been deposited onto bulk components and fiber optics
2
by using rf plasma-assisted CVD. A large measure of protection has been
observed with these coatings.
In the first part of this book, the more fundamental aspects of fluoride
glasses are reviewed. In Chapter 1, Marcel Poulain discusses the wide range
of fluoride glasses that have been discovered with an emphasis on
fluorozirconate-based compositions. He also reviews the glass-melting
process. In Chapter 2, J. Simmons et al. discuss the work that has been
performed experimentally and theoretically to elucidate the structure
of simple fluoride systems—usually the ZrF—BaF binary glass. Little
2
information is available to help understand the complexities of present
experimental compositions that typically have four to six components. In
Chapter 3, Bendow discusses the intrinsic transparency of fluoride glasses
from the UV to the IR, with particular emphasis on the multiphonon edge
and on the electronic edge. The next three chapters are devoted to areas of
particular interest for ultra-low loss optical fibers. Ewing and Sommers
have extensively reviewed methods for purifying and analyzing the fluoride
glass raw materials. Promising results in the few ppb range have been
obtained. Takahashi and Iwasaki discuss preform fabrication and fiberiza-
tion techniques. A number of innovative techniques have been proposed in
the last few years, but none have supplanted the standard rotational casting
or built-in casting. Mimura and Nakai discuss sources of loss in Chapter 6.
They also propose that the absorption and scattering loss can be minimized
by using NF reactive atmosphere processing.
3
In the last part of the book, other aspects of fluoride glasses related to
applications are examined. C. Simmons and J. Simmons, in Chapter 7,
xiv Introduction
review work performed on the durability of fluoride glasses. Griscom and
Friebele discuss, in Chapter 8, the effects of radiation on fluoride glasses,
which appear to be more radiation resistant than silicate glasses. This could
be important for some applications. In Chapter 9, Quimby reviews the area
of active phenomena such as doping of fluoride glasses with rare-earth
elements for fluorescence and lasing, as well as frequency doubling.
Several excellent reviews on the subject of fluoride glasses are avail-
able. The reader is advised to consult Tran et al. (1984) or France et al.
(1987).
REFERENCES
Aggarwal, I., Lu, G., and Busse, L. Ε. (1988). Mat. Sei. Forum 32-33, 495.
Bendow, B. (1984). Proc. SPIE 505, 81.
France, P. W., Carter, S. F., Moore, M. W., and Day, C. R. (1987). Br. Telecom. Techn. J.
5, 28.
Poulain, Mi., Poulain, Ma., Lucas, J., and Brun, P. (1975). Mat. Res. Bull. 10, 243.
Tran, D. C, Sigel, G. H., and Bendow, B. (1984). J. Lightw. Tech. LT-2, 566.
Shibata, S., Horiguchi, M., Jinguji, K., Mitachi, S., Kanamori, T., and Manabe, T. (1981).
Electron. Lett. 17, 775.
Sun, Κ. H. US Patent 2,466,509 (April 5, 1949).
Van Uitert, L. G., Bruce, A. J., Grodkiewicz, W. H., and Wood, D. L. (1985). Mat. Sei.
Forum 6, 591.
— 1—
Fluoride Glass Composition
and Processing
MARCEL POULAIN
University of Rennes, Rennes, France
1. Introduction 1
2. General Aspects of Glass Formation 2
2.1. Prediction of Glass Formation 2
2.2. Evaluation of Glass Forming Ability 3
2.3. The Concept of Glass Progenitor 5
3. Glass Synthesis and Processing 6
3.1. The Various Steps of Fluoride Glass Preparation 6
3.2. The Chemistry of Glass Melts 9
3.3. Ammonium Bifluoride Processing (ABP) 10
3.4. Reactive Atmosphere Processing (RAP) 11
3.5. Dry Processing 12
3.6. Future Directions 13
4. Glass Forming Systems 14
4.1. Introduction 14
4.2. Zirconium Fluoride Glasses 14
4.3. Fluoroaluminate Glasses 22
4.4. Trifluoride Glasses 23
4.5. Divalent Fluoride Glasses 24
4.6. Other Fluoride Glasses 25
4.7. Poly anionic Fluoride Glasses 26
5. Physical Properties 28
5.1. Thermal Properties 28
5.2. Thermal Expansion 31
5.3. Other Physical Properties 31
6. Conclusion 32
References 33
1. Introduction
Exotic glasses form a group of increasing importance among advanced
materials as they offer a set of attractive features relating to their com-
position and the properties of the vitreous state. For some time, optical
applications have exemplified the advantages of glasses over crystals.
1
Copyright © 1991 by Academic Press, Inc.
All rights of reproduction in any form reserved.
ISBN 0-12-044505-0
2 Marcel Poulain
The development of fiber optics demonstrates that technological and
economical problems can be more easily solved using vitreous materials.
The development of fluoride glasses as high-tech materials arose from
need and serendipidity. Transoceanic communication links required that
fibers be transparent over thousands of kilometers, and therefore scientists
searched for glasses more transparent than silica. In the mid-1970s,
chemists discovered a new family of glasses while trying to synthesize low-
symmetry crystals in the complex fluoride system.
This discovery has been beneficial in many respect. Infrared fibers are
now available for various technical purposes. Prospects for long-haul
repeaterless telecommunications are promising. Glass formation, which was
considered an exceptional event, is now common in many fluoride systems.
These developments have expanded the horizons of glass science.
2. General Aspects of Glass Formation
2.1. PREDICTION OF GLASS FORMATION
Although their occurrence was highly probable in the chemical systems being
studied at the time, the first heavy metal fluoride glasses were discovered by
accident (Poulain, Mi. etal., 1975). Since then, numerous investigations have
been carried out in order to find new fluoride glasses or more stable glass
compositions. Guidelines for the prediction of glass formation form the basis
of current investigation methods even though these methods are rarely
explicit.
There are a number of observed correlations between composition and
ability to vitrify. These may be summarized as follows:
— in multicomponent systems, glass formation usually occurs near a
eutectic composition (Scholze, 1977)
— some fluorides enhance vitrification at medium or high concentra-
tion. They are BeF , ZrF , HfF , A1F , and some others. In a
2 4 4 3
general way, these fluorides display a rather high binding energy
(Baldwin and Mackenzie, 1979) and field strength (Poulain, Ma.,
1981; Portier et al., 1988a,b)
— the more stable glass compositions usually lie near the center of the
vitreous area
— the mixing of two fluoride—or halide—glasses usually results in a
vitreous composition
— large-size divalent cations (e.g., Ba 2+, Pb2+, Sr2+) and medium-size
monovalent cations (Na+, Li+) are found in most fluoride glasses