Browsing by Subject "fiber optics"
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Item An experimental investigation of the sensitivity of a buried fiber optic intrusion sensor(Texas A&M University, 2006-04-12) Kuppuswamy, HariniA distributed fiber optic sensor with the ability of detecting and locating intruders on foot and vehicles over long perimeters (>10 km) was studied. The response of the sensor to people walking over or near it and to vehicles driving nearby was observed and analyzed. The sensor works on the principle of phase sensitive optical time domain re ectometry, making use of interferometric effects of Rayleigh backscattered light along a single mode fiber. Light pulses from a highly stable Er:doped fiber laser emitting single longitudinal mode light and exhibiting low frequency drift are passed through one end of the buried fiber. The backscattered light emerging from the same fiber end was monitored using a photodetector. The phase changes produced in the light pulse due to the pressure of the intruder walking directly above or near the sensor or from the seismic disturbances created by vehicles moving in the vicinity of the sensor are detected using the phase sensitive Optical Time Domain Re ectometer (OTDR). Field tests were conducted with the sensing element as a single mode fiber in a 3-mm diameter cable buried at depths ranging from 8 to 18 inches in clay soil. It was observed that the sensor could detect intruders walking transverse to the cable line at a distance of 40 ft from it. A car moving at a speed of 30 mph on a rough road could be consistently detected up to a distance of 480 ft from the sensor, while a car driven on a smooth road 200 ft from the sensor could be detected only when passing through rough patches on the road. Tests were also performed with an intruder walking near the sensor while a car was driven at a speed of 30 mph on a rough road. The effect on the signal due to the intruder on foot could be distinguished clearly only when the car was at least 200 ft away from the sensor. The results in this thesis represent the first quantitative study of the sensitivity of the sensor under varied test conditions. It is expected that these findings will be helpful in the practical implementation of the long perimeter intrusion sensor along high security domains like national borders, military bases and government buildings.Item The monitoring and multiplexing of fiber optic sensors using chirped laser sources(Texas A&M University, 2004-09-30) Wan, XiaokeA wide band linearly chirped erbium-doped fiber laser has been developed. The erbium-doped fiber laser using a rotating mirror/grating combination as one of the reflectors in a Fabry-Perot laser cavity has been tuned over a 46 nm spectral range. Linearization of the chirp rate has been achieved using feedback from a fiber Fabry-Perot interferometer (FFPI) to adjust the voltage ramp which drives the rotating mirror. In a demonstration of monitoring an array of two fiber Bragg grating (FBG) sensors, a wavelength resolution of 1.7 pm has been achieved. The linearly chirped fiber laser has been used in measuring the optical path difference (OPD) of interferometric fiber optic sensors by performing a Fourier transform of the optical signal. Multiplexing of an array of three FFPI sensors of different lengths has been demonstrated, with an OPD resolution ranging from 3.6 nm to 6.3 nm. Temperature was measured with one of the sensors over the range from 20?C to 610?C with a resolution of 0.02?C. Short FBGs are used to form the two mirrors of a fiber Bragg grating pair interferometer (FBGPI) sensor, so that the mirror reflectances change gradually as a function of temperature. Modulating the drive current of a DFB laser produces chirping of the laser frequency to scan over ~2.5 fringes of the FBGPI reflectance spectrum. Because the fringes are distinguished due to the FBG reflectance change, the ambient temperature can be determined over the range from 24 oC to 367 oC with a resolution of 0.004 oC. Multiplexing of FBGPI sensors of different lengths with a linearly chirped fiber laser has demonstrated improved sensitivity and multiplexing capacity over a conventional FBG WDM system. The FBG spectral peak position and the phase shift of an FBGPI are determined through the convolution of the sensor reflected signal with an appropriately matched reference waveform, even though the reflectance spectra for the FBGs from different sensors overlap over a wide temperature range. A spectral resolution for the FBG reflectance peak of 0.045 GHz (0.36 pm), corresponding to a temperature resolution of 0.035 oC, has been achieved.