Cryogenic fuels are often considered as major energy alternatives to coal and petroleum based fuels. Safe and reliable
sensor networks are required for on-demand, real-time fuel management in cryogenic environments. In this paper, a new
sensor design is described that enhances the low-temperature performance of fiber sensors. FBGs inscribed in high
attenuation fiber (HAF) are used to absorb in-fiber power light to raise the local sensor temperature in the cryogenic
environment. When in-fiber power light is turned off, FBG sensors can serve as passive sensors to gauge temperature
and stress in the cryogenic system. When the in-fiber power light is turned on, the heated sensors can be used to rapidly
gauge fuel level and fuel leaks. In one example, a hydrogen gas sensor is demonstrated with a palladium-coated fiber
Bragg grating (FBG). The low-temperature performance of the sensor was improved by heating the gratings as much as
200 K above the ambient temperature, and hydrogen concentration well below the 4% explosion limit was measured at
123K. In a second example, an array of four aluminum coated fiber Bragg gratings was used to measure liquid level in a
We report an all-fiber hydrogen sensing system for low-temperature operation. The sensor consists of a fiber Bragg
grating written in high-attenuation fiber and coated in Palladium. Heating the sensor with in-fiber light power greatly
enhances sensitivity at low temperatures. A multi-functional infrared light source is used to provide both in-fiber heating
and sensor monitoring. This technology promises a single fiber feedthrough solution for low temperature multipoint
hydrogen leak detection.
Fiber optical components such as fiber gratings, fiber interferometers, and in-fiber Fabry-Perot filters are key
components for optical sensing. Fiber optical sensors offer a number of advantages over other optical and electronic
sensors including low manufacturing cost, immunity to electromagnetic fields, long lifetimes, multiplexing, and
environmental ruggedness. Despite the advantages of purely passive optical components described above, fiber sensor
performance and applications have been limited by their total passivity and solid-core/solid cladding structure
configurations. Passive sensors can only gather limited information. Once deployed; set point, sensitivity, trigging time,
responsivity, and dynamic range for each individual fiber sensor cannot be adjusted or reset to adapt to the changing
environment for active sensing. Further, the fiber sensor sensitivity is also limited by the traditional solid core/solid
In this paper, we present a concept of active fiber sensor that can directly powered by in-fiber light. In contrast to a
passive sensor, optical power delivered with sensing signal through the same fiber is used to power in-fiber fiber Bragg
grating sensors. The optical characteristics of grating sensors can then be adjusted using the optical energy. When optical
power is turned off, in-fiber components can serve as traditional passive sensor arrays for temperature and strain
measurements. When optical power is turned on, the fiber sensor networks are capable of measuring a wide array of
stimuli such as gas flow, wall shear stress, vacuum, chemical, and liquid levels in cryogenic, micro-gravity, and other
hostile environments. In this paper, we demonstrate in-fiber light powered dual-function active FBG sensor for
simultaneous vacuum, hydrogen fuel gas, and temperature measurement in a cryogenic environment.
A novel laser based corrosion sensor is being developed using embedded optical fibers, near- infrared (NIR) dyes and phase resolved fluorescence spectroscopy (PRFS) to detect corrosion by-products at the incipient stage. During the initial research effort, the practicality of using PRFS and NIR dye fluorescence lifetimes for characterization of metal ions was demonstrated. We have also demonstrated fiber optic strain measuring technology which will be integrated into the sensor design. The sensor will, thus, provide both early warning of corrosion as well as structural strain information.
The long-term mechanical reliability of polarization-maintaining and single-polarization single mode optical fibers drawn from ground preforms is of growing interest. This paper will report the results of the static fatigue tensile strength behavior under axial load at ambient conditions for PM and PZ types of single mode fiber produced from five ground preforms. The measured values of the fatigue resistance parameters N (22.6 +/- 1.3 to 27.9 +/- 1.5) are consistently higher than the N-parameter of 19.7 +/- 0.5 measured under identical conditions for standard commercially available single-mode fiber drawn from non-ground preforms. Large Weibull moduli of fiber from ground preforms suggest fiber of uniform strength distribution. Although fractographic analyses of failed fiber end faces do show extrinsic failure modes due to micro-cracks and zirconia particulates and intrinsic failures due to molybdenum inclusions, the analyses do not reveal failure modes peculiar to diamond grinding or high internal stresses.
Optical fibers are required to withstand high stresses associated with bending. These bends often occur during exposure to high-moisture environments in medical applications. Various polymeric coatings were developed to provide fiber protection. Three coatings on silica fibers were evaluated: acrylates, hard fluoropolymers, and polyimides. The evaluation included dynamic strength and static fatigue. In general, all three coatings produced strong fibers, but the acrylate and hard fluoropolymer provided superior protection. Also a strength dependence on fiber size was demonstrated. The polyimide fiber showed the highest static fatigue resistance with hard fluoropolymers outperforming acrylates. However, the poor abrasion resistance and toughness of the polyimide coatings can degrade both the dynamic and the static fatigue properties of the fiber.