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An Exploration in Fiber Optic SensorsSeng, Frederick Alexander 01 September 2016 (has links)
With the rise of modern infrastructure and systems, testing and evaluation of specific components such as structural health monitoring is becoming increasingly important. Fiber optic sensors are ideal for testing and evaluating these systems due many advantages such as their lightweight, compact, and dielectric nature. This thesis presents a novel method for detecting electric fields in harsh environments with slab coupled optical sensors (SCOS) as well as a novel method for detecting strain gradients on a Hopkinson bar specimen using fiber Bragg gratings (FBG). Fiber optic electric field sensors are ideal for characterizing the electric field in many different systems. Unfortunately many of these systems such as railguns or plasma discharge systems produce one or more noise types such as vibrational noise that contribute to a harsh environment on the fiber optic sensor. When fiber optic sensors are placed in a harsh environment, multiple noise types can overwhelm the measurement from the fiber optic sensor. To make the fiber optic sensor suitable for a harsh environment it must be able to overcome all these noise types simultaneously to operate in a harsh environment rather than just overcome a single noise type. This work shows how to eliminate three different noise types in a fiber optic sensor induced by a harsh environment simultaneously. Specifically, non-localized vibration induced interferometric noise is up converted to higher frequency bands by single tone phase modulation. Then localized vibrational noise, and radio frequency (RF) noise are all eliminated using a push-pull SCOS configuration to allow for an optical measurement of an electric field in a harsh environment. The development and validation of a high-speed, full-spectrum measurement technique is described for fiber Bragg grating sensors in this work. A fiber Bragg grating is surface mounted to a split Hopkinson tensile bar specimen to induce high strain rates. The high strain gradients and large strains which indicate material failure are analyzed under high strain rates up to 500 s-1. The fiber Bragg grating is interrogated using a high-speed full-spectrum solid state interrogator with a repetition rate of 100 kHz. The captured deformed spectra are analyzed for strain gradients using a default interior point algorithm in combination with the modified transfer matrix approach. This work shows that by using high-speed full-spectrum interrogation of a fiber Bragg grating and the modified transfer matrix method, highly localized strain gradients and discontinuities can be measured without a direct line of sight.
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Optical fibers with periodic structuresHaakestad, Magnus W. January 2006 (has links)
<p>This thesis concerns some experimental and theoretical issues in fiber optics. In particular, properties and devices based on photonic crystal fibers (PCFs) are investigated.</p><p>The work can be grouped into three parts. In the first part we use sound to control light in PCFs. The lowest order flexural acoustic mode of various PCFs is excited using an acoustic horn. The acoustic wave acts as a traveling long-period grating. This is utilized to couple light from the lowest order to the first higher order optical modes of the PCFs. Factors affecting the acoustooptic coupling bandwidth are also investigated. In particular, the effect of axial variations in acoustooptic phase-mismatch coefficient are studied.</p><p>In the second part of the thesis we use an electric field to control transmission properties of PCFs. Tunable photonic bandgap guidance is obtained by filling the holes of an initially index-guiding PCF with a nematic liquid crystal and applying an electric field. The electric field introduces a polarization-dependent change of transmission properties above a certain threshold field. By turning the applied field on/off, an electrically tunable optical switch is demonstrated.</p><p>The third part consists of two theoretical works. In the first work, we use relativistic causality, i.e. that signals cannot propagate faster than the vacuum velocity of light, to show that Kramers-Kronig relations exist for waveguides, even when material absorption is negligible in the frequency range of interest. It turns out that evanescent modes enter into the Kramers-Kronig relations as an effective loss term. The Kramers-Kronig relations are particularly simple in weakly guiding waveguides as the evanescent modes of these waveguides can be approximated by the evanescent modes of free space. In the second work we investigate dispersion properties of planar Bragg waveguides with advanced cladding structures. It is pointed out that Bragg waveguides with chirped claddings do not give dispersion characteristics significantly different from Bragg waveguides with periodic claddings.</p>
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Optical fibers with periodic structuresHaakestad, Magnus W. January 2006 (has links)
This thesis concerns some experimental and theoretical issues in fiber optics. In particular, properties and devices based on photonic crystal fibers (PCFs) are investigated. The work can be grouped into three parts. In the first part we use sound to control light in PCFs. The lowest order flexural acoustic mode of various PCFs is excited using an acoustic horn. The acoustic wave acts as a traveling long-period grating. This is utilized to couple light from the lowest order to the first higher order optical modes of the PCFs. Factors affecting the acoustooptic coupling bandwidth are also investigated. In particular, the effect of axial variations in acoustooptic phase-mismatch coefficient are studied. In the second part of the thesis we use an electric field to control transmission properties of PCFs. Tunable photonic bandgap guidance is obtained by filling the holes of an initially index-guiding PCF with a nematic liquid crystal and applying an electric field. The electric field introduces a polarization-dependent change of transmission properties above a certain threshold field. By turning the applied field on/off, an electrically tunable optical switch is demonstrated. The third part consists of two theoretical works. In the first work, we use relativistic causality, i.e. that signals cannot propagate faster than the vacuum velocity of light, to show that Kramers-Kronig relations exist for waveguides, even when material absorption is negligible in the frequency range of interest. It turns out that evanescent modes enter into the Kramers-Kronig relations as an effective loss term. The Kramers-Kronig relations are particularly simple in weakly guiding waveguides as the evanescent modes of these waveguides can be approximated by the evanescent modes of free space. In the second work we investigate dispersion properties of planar Bragg waveguides with advanced cladding structures. It is pointed out that Bragg waveguides with chirped claddings do not give dispersion characteristics significantly different from Bragg waveguides with periodic claddings.
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Characterization of Brillouin Scattering Spectrum in LEAF FiberLiu, Xuan 06 December 2011 (has links)
Fiber optic sensors are designed to measure various parameters. The distributed fiber optics sensor has been a very promising candidate for the structural health monitoring. In this thesis, we characterized LEAF (Large Effective Area Fiber) fiber’s Brillouin scattering spectrum and investigated its potentiality for the distributed Brillouin temperature and strain sensor.
Optical fibers with complex refractive index profiles are applied to improve the Brillouin threshold by varying the Brillouin linewidth. As LEAF fiber has a modified refractive index profile, we investigated its Brillouin linewidth’s dependence on the square of the pump light’s frequency. We verified the Brillouin frequency’s variation with input SOP experimentally for LEAF fiber in the spontaneous regime. This sets a limitation for the frequency resolution of distributed Brillouin sensors. We also realized a simultaneous temperature and strain sensor with LEAF fiber applying the Brillouin optical time domain analysis. Based on the direct detection of LEAF beat frequencies, a simultaneous strain and temperature sensor was demonstrated.
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Characterization of Brillouin Scattering Spectrum in LEAF FiberLiu, Xuan 06 December 2011 (has links)
Fiber optic sensors are designed to measure various parameters. The distributed fiber optics sensor has been a very promising candidate for the structural health monitoring. In this thesis, we characterized LEAF (Large Effective Area Fiber) fiber’s Brillouin scattering spectrum and investigated its potentiality for the distributed Brillouin temperature and strain sensor.
Optical fibers with complex refractive index profiles are applied to improve the Brillouin threshold by varying the Brillouin linewidth. As LEAF fiber has a modified refractive index profile, we investigated its Brillouin linewidth’s dependence on the square of the pump light’s frequency. We verified the Brillouin frequency’s variation with input SOP experimentally for LEAF fiber in the spontaneous regime. This sets a limitation for the frequency resolution of distributed Brillouin sensors. We also realized a simultaneous temperature and strain sensor with LEAF fiber applying the Brillouin optical time domain analysis. Based on the direct detection of LEAF beat frequencies, a simultaneous strain and temperature sensor was demonstrated.
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An experimental investigation of the sensitivity of a buried fiber optic intrusion sensorKuppuswamy, Harini 12 April 2006 (has links)
A 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.
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Continuous phase modulation for high speed fiber-optic linksDetwiler, Thomas Frederick 10 November 2011 (has links)
Fiber-optic networks are continually evolving to accommodate the ever increasing data rates demanded by modern applications and devices. The current state-of-the art systems are being deployed with 100 Gb/s rates per wavelength while maintaining the 50 GHz channel spacing established for 10 Gb/s dense wavelength division multiplexed (DWDM) systems. Phase modulation formats (in particular quadrature phase shift keying - QPSK) are necessary to meet the spectral efficiency (SE) requirements of the application. The main challenge for phase modulated optical systems is fiber nonlinearities, where changes in intensity of the combined optical signal result in changes to the fiber's refractive index. Limiting launch power is the primary means to avoid dramatic intensity fluctuations, a strategy which in turn limits the available signal-to-noise ratio (SNR) within the channel.
Continuous phase modulation (CPM) is a format in which data is encoded in the phase, while the amplitude is constant throughout all transmission (even during transitions between symbols). With the goal of reducing the impact of nonlinearities, the purpose of this research was to identify a set of CPM signals best suited for high speed fiber-optic transmission, and quantify their performance against other formats. The secondary goal was to identify techniques appropriate for demodulation of high speed fiber-optic systems and implement them for simulation and experimental research.
CPM encompasses a number of variable parameters that combine to form an infinite number of unique schemes, each of which is characterized by its own SE, minimum distance, and implementation complexity. A method for computing minimum distance of DWDM-filtered CPM formats is presented and utilized to narrow down to a range of candidate schemes. A novel transmitter design is presented for CPM signal generation, as well as a number of novel reception techniques to achieve proper demodulation of the CPM signal from the coherent optical receiver. Using these methods, the identified range of candidate schemes was compared in simulation to the conventional QPSK format, showing that some modest gain can be expected from CPM.
Through these and other simulations, it is revealed that fiber nonlinearities depend on the aggregate sum of all wavelengths rather than the imposition of each separate carrier on its neighbors. Therefore the constant envelope of CPM does not directly impact the nonlinearities since multiple carriers will photonically interfere and result in intensity fluctuations regardless of modulation format. Additionally, dispersive effects in fiber decompose the underlying channels so that the intensity throughout propagation is nearly Gaussian distributed, regardless of format. The benefits gained from CPM are thus limited to schemes that attain a higher minimum distance than alternative formats (in the given channel passband), and for optically compensated links in which low dispersion is maintained throughout the fiber link.
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Versatile photonic processor based on fiber optical parametric amplifiersLiang, Yu, January 2009 (has links)
Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.
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Gain, noise and bandwidth of avalanche photodiodes with thin multiplication regions /Yuan, Ping, January 2000 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 148-159). Available also in a digital version from Dissertation Abstracts.
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Experimental methodology for embedding fiber optic strain sensors in fiber reinforced composites fabricated by the VARTM/SCRIMP process /Fifield, Samantha D., January 2002 (has links)
Thesis (M.S.) in Civil Engineering--University of Maine, 2002. / Includes vita. Bibliography: leaves 134-141.
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