| Popis: |
Single crystal (SC) fibers provide an interesting alternative to conventional glass fibers for high power laser applications. The limitations in power scaling in silica glass fibers arising from thermal lensing, onset of nonlinear phenomena, thermal shock and degradation of the fiber core, etc. hinder performance at high power densities. Since most of these are related to the amorphous structure of the silica glass in the active fiber gain medium, there has been a considerable push to move the fiber laser architecture to a crystalline material. High thermal conductivity and low nonlinear gain coefficients of the crystalline material, along with the high surface-area-to-volume ratio of the fiber geometry make yttrium aluminum garnet (YAG) SC fibers suitable for application involving high optical power density. The work presented in this dissertation discusses the growth and characterization of different YAG SC fibers grown by the laser heated pedestal growth technique. While SC fibers have theoretically been shown to have high power handling capabilities, in reality their application in practical devices has been severely limited by the high transmission losses involved in such waveguides. In this work, transmission losses in coil-able YAG SC fibers have been lowered to less than 0.8 dB/m at 1064 nm by optimizing growth conditions and fine-tuning the alignment of the optical components of the growth apparatus. One of the major obstacles in the development of SC fibers as effective laser medium is the difficulty in fabricating a core-clad structure like conventional glass fibers. Since SC fibers are grown from a melt, which is a true liquid, no cross-sectional geometry can be transferred from the preform to the final fiber. A few approaches to cladding have been discussed, including both post-cladding and in-situ cladding. Post-cladding techniques like sol-gel cladding, liquid phase deposition, and hydrothermal crystal growth have been explored. Among in-situ approaches, controlled radial distribution of dopant ions has been investigated as potential route to achieve a graded index profile. Er:YAG SC fibers from rod-in-tube preforms have been grown and the dopant distribution has been studied as a function of growth speed. Radial self-segregation of ions was also explored as a potential means to achieve an in-situ cladding. Such behavior of Nd ions in Nd:YAG SC fibers have been observed and reported. A sol-gel-based low-cost approach has been developed to vary the dopant concentration of YAG SC fibers. Using this technique Ho:YAG SC fibers of varying dopant concentration were grown and lasing was reported at 2090 nm. A maximum optical-to-optical slope efficiency of 58.5% was demonstrated in such fibers. Finally, the crystal quality of the YAG SC fibers was characterized by synchrotron white beam X-ray topography. This non-destructive characterization technique for large single crystal samples allowed the study of the strain and defect distribution of different YAG SC fibers. |
