Machine vision approach of process control during single crystal fiber growth via laser heated pedestal growth method

Laser heated pedestal growth (LHPG) method is a unique technique to grow single crystal fibers and fibrous materials of high temperature ceramics for various photonic and electronic applications. The stability of the solidification process and the floating molten zone created at the interface between the feed and seed materials is critical for ensuring high quality of the fiber including uniformity of diameter. To maintain the molten zone volume constant throughout the dynamic growth process, the typical LHPG system control scheme will modify the relative speed control of fiber-pull rate versus the source material feed rate as constrained by mass conservation. However, due to the dynamic nature of the growth process and the floating state of the molten material, it is prone to instability due to non-uniform heating, heat loss, melt convection, seed-fiber and pedestal material alignments, and other factors which impact growth processes. Sustainable growth process demands a combination of optimized optical components and real-time process controls. Here we present a detailed optical analysis of several candidate LHPG optical designs and compare details of the illumination at the molten zone region during the growth process. In addition, we explore the potential for enhancing the typical process control by utilizing (1) active laser power control and (2) machine vision methods for real-time characterization of the molten zone profile to be integrated into active control schemes. Impacts on the quality of the fiber grown in terms of uniformity in diameters upon active laser power feedback loop to mitigate the molten zone shape variation is also discussed.