This paper reports a technique to enhance the magnitude and high-temperature stability of Rayleigh back-scattering signals in silica fibers for distributed sensing applications. With femtosecond laser radiation, more than 40-dB enhancement of Rayleigh backscattering signal was generated in silica fibers using 300-nJ laser pulses at 250 kHz repetition rate. The laser-induced Rayleigh scattering defects were found to be stable from the room temperature to 800 °C in hydrogen gas. The Rayleigh scatter at high temperatures was correlated to the formation and modification of nanogratings in the fiber core. Using optical fibers with enhanced Rayleigh backscattering profiles as distributed temperature sensors, we demonstrated real-time monitoring of solid oxide fuel cell (SOFC) operations with 5-mm spatial resolution at 800 °C. Information gathered by these fiber sensor tools can be used to verify simulation results or operated in a process-control system to improve the operational efficiency and longevity of SOFC-based energy generation systems.

This Letter presents an all-optical high-temperature flow sensor based on hot-wire anemometry. High-attenuation fibers (HAFs) were used as the heating elements. High-temperature-stable regenerated fiber Bragg gratings were inscribed in HAFs and in standard telecom fibers as temperature sensors. Using in-fiber light as both the heating power source and the interrogation light source, regenerative fiber Bragg grating sensors were used to gauge the heat transfer from an optically powered heating element induced by the gas flow. Reliable gas flow measurements were demonstrated between 0.066 m∕s and 0.66 m∕s from the room temperature to 800°C. This Letter presents a compact, low-cost, and multiflexible approach to measure gas flow for high-temperature harsh environments. Gas flow measurement plays important roles in various industrial sectors. It provides vital information for a large number of applications such as process controls, fossil fuel and nuclear electric power generation, transportation, and environment monitoring. To perform flow measurements, a large number of flow sensors based on various mechanical, electronic, and microelectromechanical system (MEMS) structures [1] have been developed. These sensors can perform effective flow measurements at room temperature or slightly elevated temperatures (e.g., <200°C). However, a number of industrial and aerospace applications demand flow sensors with much higher operational temperatures (>500°C). These are not attainable by current state-of-the-art technology, such as MEMS. To address this technical challenge, this Letter presents a low-cost and compact all-optical-fiber flow sensing technique that can provide rapid and accurate gas flow measurements from room temperature to 800°C. This is, to our best knowledge, the highest operational temperature for a flow sensor. Fiber-optic sensors are well-known for their resilience in many harsh conditions including in high-temperature, corrosive, and strong electromagnetic environments. Over the last decade, various optical-fiber-based flow sensors have been reported, largely based on two schemes: fiber optical interferometry [2,3] and optical hot-wire anemometry (optical HWA). Among different designs of optical flow sensors, the HWA is one of the most widely adopted flow measurement techniques, thanks to its simplicity and reliability. HWA-based flow sensors determine flow rate by measuring heat transfer between a heated element and adjacent temperature sensors [4].Compared with electrically heated HWA flow sensors, fiber-optic sensors can also be heated optically to perform flow measurements. Using various optical coupling schemes [5][6][7][8][9][10][11], in-fiber light has been used to heat up a section of fiber containing a sensing element (e.g., a fiber Bragg grating or FBG) to perform temperature measurements. The optical HWA can be more resilient than those using the electric heating for high-temperature applications. This is because the optical fiber, as a power delivery cable, can sustain much higher temperature...

This paper reports the development of a compact double-pulse laser system to enhance laser induced breakdown spectroscopy (LIBS) for field applications. Pumped by high-power vertical-surface emitting lasers, the laser system that produces 16 ns pulse at 12 mJ/pulse with total weight less than 10 kg is developed. The inter-pulse delay can be adjusted from 0μs with 0.5μs increment. Several LIBS experiments were carried out on NIST standard aluminum alloy samples. Comparing with the single-pulse LIBS, up to 9 times enhancement in atomic emission line was achieved with continuum background emission reduced by 70%. This has led to up to 10 times improvement in the limit of detection. Signal stability was also improved by 128% indicating that a more robust and accurate LIBS measurement can be achieved using a compact double-pulse laser system. This paper presents a viable and field deployable laser tool to dramatically improve the sensitivity and applicability of LIBS for a wide array of applications.

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