Fabrication and on-line evaluation of single crystal fiber via laser-heated pedestal growth

Although single crystal sapphire fiber has been fabricated extensively for decades, many details surrounding the impacts of growth conditions on fiber quality are still unreported. Traditional fiber quality measurements require stopping the fiber growing process, cutting the fiber into short pieces, and measuring the transmission which is time consuming and highly variable. We developed a very simple method to monitor the fiber quality in real-time. During the fiber growth process, the melting pool shape becomes stable and incandescence from the molten zone can be used as an active light source. By connecting the other end of the growing fiber to a spectrometer, we can monitor the light intensity as fiber length increases continuously. Not only we are able to deduce the current fiber quality being grown, but also to identify the optimum growing conditions include the growth rate, rod-to-fiber ratio, and position of the crystallization interface, as well as monitoring impacts from laser power instability. These measurements can be further compared to cutback style measurement, or loss measurements using Raman interrogation. Different single crystal fibers, including sapphire and YAG are grown while measuring throughput during growth.