Spatially resolved fibre cavity ring down spectroscopy.

This paper presents a fibre cavity ring down spectroscopy probed by Rayleigh scattering optical frequency domain reflectometry (OFDR), which provides spatial location of stimuli and improved signal to noise ratio for distributed sensing measurements. A section of optical fibre was integrated into an active fibre ring cavity with optical gain and interrogated by the OFDR system for 11 cycles with a single laser scan. Through the cavity ring down configuration, root-mean-squared (RMS) noise of distributed temperature and strain measurements was reduced to 6.9 mK and less than 0.1 µÎµ, respectively for 1-cm spatially resolved measurements. Our work shows that the active fibre cavity configuration can be combined with distributed fibre sensing schemes to achieve both high spatial resolution and high sensitivity measurements.

Distributed Optical Fiber Sensors with Ultrafast Laser Enhanced Rayleigh Backscattering Profiles for Real-Time Monitoring of Solid Oxide Fuel Cell Operations.

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.

Potential to Detect Hydrogen Concentration Gradients with Palladium Infused Mesoporous-Titania on D-Shaped Optical Fiber.

A distributed sensing capable high temperature D-shaped optical fiber modified with a palladium nanoparticle sensitized mesoporous (∼5 nm) TiO2 film, is demonstrated. The refractive index of the TiO2 film was reduced using block copolymer templating in order to realize a mesoporous matrix, accommodating integration with optical fiber. The constructed sensor was analyzed by performing direct transmission loss measurements, and by analyzing the behavior of an integrated fiber Bragg grating. The inscribed grating should reveal whether the refractive index of the composite film experiences changes upon exposure to hydrogen. In addition, with frequency domain reflectometry the distributed sensing potential of the developed sensor for hydrogen concentrations of up to 10% is examined. The results show the possibility of detecting chemical gradients with sub-cm resolution at temperatures greater than 500 °C.

A compact field-portable double-pulse laser system to enhance laser induced breakdown spectroscopy.

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.

High resolution monitoring of strain fields in concrete during hydraulic fracturing processes.

We present a distributed fiber optic sensing scheme to image 3D strain fields inside concrete blocks during laboratory-scale hydraulic fracturing. Strain fields were measured by optical fibers embedded during casting of the concrete blocks. The axial strain profile along the optical fiber was interrogated by the in-fiber Rayleigh backscattering with 1-cm spatial resolution using optical frequency domain reflectometry (OFDR). The 3D strain fields inside the cubes under various driving pressures and pumping schedules were measured and used to characterize the location, shape, and growth rate of the hydraulic fractures. The fiber optic sensor detection method presented in this paper provides scientists and engineers an unique laboratory tool to understand the hydraulic fracturing processes via internal, 3D strain measurements with the potential to ascertain mechanisms related to crack growth and its associated damage of the surrounding material as well as poromechanically-coupled mechanisms driven by fluid diffusion from the crack into the permeable matrix of concrete specimens.

Fiber-optic flow sensors for high-temperature environment operation up to 800°C.

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.

Regenerated distributed Bragg reflector fiber lasers for high-temperature operation.

This Letter presents distributed Bragg reflector (DBR) fiber lasers for high-temperature operation at 750°C. Thermally regenerated fiber gratings were used as the feedback elements to construct an erbium-doped DBR fiber laser. The output power of the fiber laser can reach 1 mW at all operating temperatures. The output power fluctuation tested at 750°C was 1.06% over a period of 7 hours. The thermal regeneration grating fabrication process opens new possibilities to design and to implement fiber laser sensors for extreme environments.