Growth of Single Crystal Fibers

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MRS BULLETIN/OCTOBER1988

one of the most important being the fiber growth method itself. While a wide variety of techniques can be used to produce Single crystal fibers, not all have equal simplicity and versatility. It is often difficult, for example, to control the diameter, length, and orientation of fibers grown by vapor and Solution methods. In general, melt growth techniques are more useful. Single-crystal fibers of various materials have been successfully grown by: (1) the EFG (edge-defined film-fed growth) method, (2) pulling through a die, (3) float-zone (pedestal) growth, (4) solidification in capillary tubes, (5) capillary drawing, and (6) pressurized capillary-fed growth. The choice of method will ultimately depend on the physical chemical properties of the material to be grown. Perhaps the most useful of the melt growth techniques is the float-zone method, sometimes known as the pedestal growth method if the fiber diameter is smaller than the source rod diameter. This technique, when used in conjunction with the small sample sizes characteristic of fibers, has advantages which make possible many of the research opportunities mentioned above. This fiber growth method evolved from the work of several investigators, beginning with Eickhoff and Gurs7 who first used laser-heating for float-zone crystal growth, Haggerty8,9 who used it for growing small diameter crystals, and finally Burrus and Stone10 who used a refined laser-heated pedestal growth (LHPG) technique for the growth of Nd:YAG fiber lasers. More detailed Information on the history of fiber growth is given in Reference 2. Fiber Devices The first scientists to extensively study the formation and properties of single-crystal fibers (or whiskers as they are sometimes known) 1 were excited that they were stronger than comparable bulk crystals (in fact, near the

theoretical maximum due to their very high crystalline perfection). Most fiber research activities then (circa 1950s) went into understanding the relationship between their growth behavior, crystalline perfection, and superior mechanical strength, but workers also explored the possibility of using these very strong materials in structural applications, in particular, fiber-matrix composites. Alternate strategies for strengthening materials were found more desirable, however, and work on these materials gradually subsided. During the early 1970s, Burrus and Stone10 successfully prepared the first single-crystal fibers for optical applications, and in 1978 Goodman11 discussed a number of potentially interesting optoelectronic devices which could benefit from single-crystal fiber components. Fiber crystals can be useful for a ränge of optical applications involving either passive, active, or nonlinear interactions. Examples of passive devices include light guides for thermometry, remote spectroscopy, laser delivery Systems, etc., polarizers, isolators, and filters. For these light-guiding applications, the practical lengths would be meters rather than the kilometer lengths needed for long-distan

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