Time-division multiplexing is a promising method for the interrogation of fiber-optic Bragg grating sensors arrays for measurement of strain and temperature. We examine the performance of these systems to determine the parameters for high-sensitivity, low-cross-talk operation. It is shown that the performance can be greatly improved by use of a short time resolution in the demultiplexing process. We propose a new method of demultiplexing with an electro-optic modulator to read out the sensor pulses by gating the signal with 400-ps resolution. The system is demonstrated experimentally to provide 0.15-microepsilon/square root(Hz) strain resolution in a 50-Hz bandwidth within a full-scale range of 8000 microepsilon. The system parameters are capable of handling at least 50 time-addressed sensors on a single fiber.
This paper presents an overview of the development and application of ISIS fibre optic sensor (FOS) technology by the University of Toronto Institute for Aerospace Studies and Department of Electrical and Computer Engineering. The primary focus of this technology has involved the use of fibre Bragg gratings (FBGs) to measure strain and temperature in concrete structures and fibre reinforced plastic (FRP) overwraps applied to concrete structures. A brief review of existing fibre optic sensor configurations and the advantages of using FOS compared to other strain sensors is first presented. Subsequently, the development of new sensor concepts such as a long gauge of arbitrary length, a distributed gauge for measuring local strain gradients, and multiple FBGs on a single fibre optic cable are discussed, with examples of their application to civil engineering structures. In addition, the specialized instruments under development that are essential for obtaining strain information from these sensors are also described. Finally, the issue of wireless remote monitoring of FOS systems is addressed.
To provide information to guide restoration of montane riparian willow communities, we investigated factors influencing the survival of prerooted and unrooted mountain willow (Salix monticola Bebb) cuttings in 2 degraded montane riparian areas in Rocky Mountain National Park, Colorado. We planted cuttings across a gradient of water table depths and soil textures and evaluated their survival using logistic regression analysis. Our results indicate that depth to groundwater was a critical factor influencing survival of both rooted and unrooted cuttings. We found that few cuttings (7.8% rooted, 3.9% unrooted) survived where summer water table depths exceeded approximately 90 cm. Soil texture was not a significant factor in our logistic models, potentially because of low silt and clay fractions in our plots. Rooted cuttings survived at a higher rate than unrooted cuttings after 1 (55.8% vs. 36.5 %, P < 0.001) and 2 (44.5% vs. 26.1 %, P < 0.001) years of growth. We conclude that, when combined with appropriate hydrologic data, the use of rooted cuttings represents an effective technique to restore and revegetate degraded montane riparian ecosystems. Resumen Para proveer información para guiar la restauración de las comunidades montanas ribereñas de "Willow" investigamos los factores que influyen en la sobrevivencia de esquejes pre-enraizados y sin enraizar de "Willow" (Salix monticola Bebb) en dos áreas ribereñas degradadas del Parque Nacional de las Montañas Rocallosas de Colorado. Plantamos los esquejes a lo largo de un gradiente de profundidad del manto freatico y texturas de suelo y evaluamos su sobrevivencia usando análisis de regresión logística. Nuestros resultados indican que la profundidad del manto freatico es un factor critico que influye en la sobrevivencia de los esquejes enraizados y sin enraizar. Encontramos que pocas estacas (7.8% enraizado, 3.9% sin enraizar) sobrevivieron donde la profundidad del mato freatico en verano excede aproximadamente los 90 cm. La textura de suelo no fue un factor significativo en nuestros modelos logísticos, probablemente debido a los bajos contenidos de limo y arcilla en nuestras parcelas. Los esquejes enraizados sobrevivieron a una tasa mayor que los esquejes sin enraizar, tanto 1 año (55.8 vs. 36.5 %, p < 0.001) como 2 años despues de plantados. Concluimos que si se combinan en forma apropiada con datos hidrológicos, el uso de esquejes enraizados representa una tecnica efectiva para restaurar y revegetar los ecosistemas ribereños montanos degradados.
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