Sol-Gel Processing for Fiber Optic Applications
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SOL-GEL PROCESSING FOR FIBER OPTIC APPLICATIONS* IAN M. THOMAS* University of California, Lawrence Livermore National Box 5508, L-490, Livermore, CA 94550
Laboratory, P.O.
ABSTRACT The sol-gel process is a versatile process for the preparation of many types of glass and ceramics. This includes optical fibers. In this paper, the general principles of sol-gel processing are described first. This is followed by details of the specific methods that can be used for the fabrication of optical fibers. Finally, some conclusions are drawn as to the commercial potential of the various methods. 1.
INTRODUCTION
The sol-gel process for the preparation of glass and ceramics has been known for many years. It has received increasing attention in the last decade because many of the advantages inherent in the method have been recognized. Bound volumes of papers given in Symposia dedicated exclusively to sol-gel chemistry are now available.[l-5] Three of these have been published by the MRS under the titles "Better Ceramics Through Chemistry". In addition, a book giving a general review of the more practical aspects of the process has recently been publlshed.[6] The first part of this paper is devoted to a general description of the sol-gel process and the types of product that can be prepared. There then follows a description of how the process can be adapted to the preparation of optical fibers and a brief review of the research that has been carried out in the area. 2.
THE SOL-GEL PROCESS 2.1
General
The sol-gel process can be used for the preparation of single or multicomponent products. Most of the applications are for oxide systems, specifically for those containing silica. As these are the only sol-gel systems that have been applied to fiber optic fabrication details that follow will be directed exclusively to silica-containing materials. Three basic steps are involved in the process: 1. Preparation of a solution or suspension of all components (the SOL). 2.
Gellatlon of the SOL by polymerization or coagulation (the GEL).
3.
Conversion of the GEL to a useful product.
We shall consider each of these three steps in turn. In the first step, a solution of all components In the form of precursor species is prepared. More will be said about precursors later but they are chosen so that they can readily be converted to oxides. The prime objective at this stage is to obtain a mixture of all components at the molecular level. In addition there are also advantages in having a solution for certain processing steps. *Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-4B. Mat. Res. Soc. Symp. Proc. Vol. 172. ©1990 Materials Research Society
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The second step is gellatlon of the solution. This is normally accomplished by chemical polymerization or destabilization of the precursor species. This leads to crosslinking and a rigid three dimensional structure is formed. All components are therefore "frozen" in this structure and homogeneity is t
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