Optimization of Axially Strained Sapphire Microbend Fiber Optics Sensors For High Temperature Applications

The Johns Hopkins University is actively pursuing a single crystal sapphire fiber microbend sensor to
measure temperatures and surface strains at temperatures approaching 2000° C. Sapphire is the current material of choice due to its high strength, high melting temperature and low chemical reactivity. It is known that microbend sensors exhibit a sharp resonance at the point of maximum mode coupling. This resonance depends on the core/cladding index of refraction, fiber diameter and number of microbends.

The current state of the art in edge defined growth of single crystal sapphire produces core-only fibers that exhibit large optical attenuation. In the harsh high temperature environment a pliant mechanical protection layer is unrealistic. Stress analysis has shown that axial sensing places the highest stress regions of the deformed section into compression, thus extending operating limits. The core only means of production produces sensors with a huge index mismatch at the core/environment interface. This causes the resonance peak to occur at fiber distortion wavelengths that can not be fabricated. Through theoretical predictions and bench top models using plastic fibers, an optimal cladding material for sapphire is being sought in order to create a sensor with practical dimensions that can be fabricated with today’s technology.

High Temperature Quartz Oven Provides Testing Environment

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