Mokhtar Maklad

Active Fiber Bragg Grating Hydrogen Sensors for All-Temperature Operation

The use of liquid hydrogen as a fuel requires low-cost multipoint sensing of hydrogen gas for leak detection and location well below the 4% explosion limit of hydrogen. Herein is presented a multipoint in-fiber hydrogen sensor capable of hydrogen detection below 0.5% concentration with a response time of less than 10 s. Our solution entails use of a fiber Bragg grating (FBG) coated with a layer of hydrogen-absorbing palladium which, in turn, induces strain in the FBG in the presence of hydrogen. Infrared power laser light is used to induce heating in the palladium coating which dramatically decreases sensor response time and increases the sensors sensitivity at low temperatures. This technology promises an inexpensive fiber solution for a multipoint hydrogen detection array with only one fiber feed-through

Multiplexable Low-Temperature Fiber Bragg Grating Hydrogen Sensors

A palladium-coated fiber Bragg grating (FBG) inscribed in high attenuation fiber (HAF) capable of measuring hydrogen concentration at -150 degC, well below the 4% explosion limit, is presented. The low-temperature performance of the sensor was improved by heating the gratings as much as 300 K above the ambient temperature with highly attenuated infrared laser light. The sensitivity and response time of the FBG hydrogen sensor were studied as functions of fiber temperature. Through the selection of length and the loss per meter of the HAF, the self-heating scheme demonstrated here enables numerous FBG hydrogen sensors to be multiplexed in a single fiber for multipoint hydrogen sensing using a single fiber feedthrough.

Distributed high-temperature pressure sensing using air-hole microstructural fibers

We present spatially resolved Rayleigh scattering measurements in different polarization-maintaining (PM) fibers for high-temperature pressure sensing. The pressure-induced birefringence in the fiber cores is interrogated using polarization-resolved frequency-swept interferometry. The pressure responses of a PM photonic crystal fiber and a twin-air-hole PM fiber are investigated for a pressure range of 0 to 13.8 MPa (0-2000 psi) at room temperature and at temperatures as high as 800 °C. The proposed sensing system provides, for the first time to our knowledge, a truly distributed pressure-sensing solution for high-temperature applications.

Self-heated all-fiber sensing system for cryogenic environments

We present a new design of an all-fiber sensor system to enhance the low-temperature sensing performance of cryogenic fuel. Fiber Bragg gratings (FBGs) are inscribed in high attenuation fibers (HAFs), which absorb in-fiber light to raise the local sensor temperature in a cryogenic environment. The heated sensor displays improved sensitivity and response time to measure multiple parameters. Two sensing applications are described in this paper. First, a HAF–FBG sensor coated with palladium is used to detect hydrogen concentration well below the 4% explosion limit at 173 K. Second, an array of three aluminum-coated sensors is capable of measuring the liquid level at 77 K. In addition, through the selection of the length and attenuation coefficient of HAF, numerous FBG sensors can be multiplexed on a single fiber for multipoint sensing using a single fiber feedthrough.

Self-Heated Optical Fiber Sensor Array for Cryogenic Fluid Level Sensing

We present an array of aluminum-coated fiber Bragg gratings (FBGs) inscribed in high-attenuation fibers capable of measuring the liquid level in a cryogenic environment. The sensors are heated by the optical energy propagating in fiber core that is absorbed in high-attenuation fibers. Thermal responses of Bragg gratings to liquid nitrogen level at 77 K were observed. Precise control over the light absorption and heated FBG temperatures of individual level sensors using high-attenuation fibers enables multipoint liquid nitrogen level sensing using a single fiber and a single fiber feedthrough.

All-fiber low-temperature hydrogen sensing using a multi-functional light source

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.

Self-heated fiber Bragg grating sensors for cryogenic environments

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 cryogenic environment.

Self-powered multi-functional fiber sensors

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 cladding configuration. 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.