Amorphous silica optical fibers which guide light via total internal reflection have become a cornerstone of modern communication, structural monitoring, remote sensing, and fiber laser technologies. However, glass fibers possess several drawbacks including low thermal conductivity, opaqueness to mid-infrared light, and a lack a uniform crystal field which inherently limits their usefulness as dopant host materials. To address these problems, solid-core all-crystalline fibers have been proposed and fabricated for the first time using high pressure chemical vapor deposition (HPCVD).<br/>Similar to hollow-core reflective Bragg fibers, HPCVD fibers typical rely on thin layers of silicon and germanium to guide light through constructive interference of reflected waves due to their large refractive index difference. Layers are deposited in a bottom-up approach by successive thermal decompositions of silane and germane, while growth kinetics are used to control layer thicknesses.<br/>The ideal layer thickness of Bragg fibers is determined using finite element analysis, followed by precisely controlled growth of smooth amorphous Si and Ge layers. These layers are crystallized by rapid thermal annealing.<br/>Active crystalline materials such as ZnSe, ZnS, or ZnO may then be deposited into the hollow-core fiber using additional HPCVD reactions. ZnSe was most closely studied because of its applications in mid-IR lasers and its potential uses in passive and active devices owing to its high nonlinear susceptibility and noncentrosymmetric crystal structure. Furthermore, ZnSe suffers from a low thermal conductivity which significantly impacts its performance at high optical powers, meaning that it benefits highly from being bonded to thermally conductive materials that are IR transparent.<br/>In addition to basic step-index Bragg fibers, progress has been made to model and synthesize structures with desirable optical and electronic properties. For example, chirped films of silicon and germanium can be used to tune the spectral reflectivity of such fibers, enabling the design of transmission or reflection windows. Utilizing the solubility of germanium and silicon, it is also possible to synthesize graded-index Bragg layers through thermal annealing. In-situ doping of the ZnSe core is concurrently being explored to access specific materials properties such as lasing, opening the future for fully integrated fiber devices.

Synthesis and design of semiconductor-core crystalline Bragg fibers

2019 Fall National Meeting

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