Several techniques for measuring high frequency vibrations are presented. The goal of the study is to develop a sensor for detecting acoustic emissions (AE) inside composite structures. The basics of wave propagation inside of materials has been presented along with an overview of typical acoustic emission testing. Surface acoustic waves (SAWs) were studied first and a novel, noncontact optical interferometric technique for measuring absolute amplitudes is presented. This technique has the added advantages in that it does not require that the interferometer be stabilized or phase biased. It is insensitive to laser fluctuations, random phase drifts, polarization changes and changes in mixing efficiency of the interferometer. SAW amplitudes between 7 and 2.5 angstroms were measured with the described technique. An intrinsic Fabry-Perot type interferometer was demonstrated for detecting SAW's and was then embedded into carbon fiber composite panels which were then put through tensile tests. AE's were captured, centered around 300 KHz, as is expected of a composite. These tests were repeatable and indicate that qualitative measurements of AE can be made. This sensor configuration was also used for detecting a variety of taps on the composite panel as well as pencil lead breaks, a standard calibration procedure for AE testing. / Master of Science

The detection and active monitoring of hydrogen gas levels is essential in many areas of industry including manufacturing, storage, and transport of this gas. Previously, methods used to detect hydrogen gas have relied upon monolithic technologies to create resistive-based sensors which are impractical for many applications requiring electrical isolation or operation in the presence of strong electromagnetic fields. Materials can be sputter deposited along cylindrical optical fiber-based sensors to create novel fiber-optic chemical sensors. Herein, the mechanisms allowing the detection of hydrogen gas using a modified Extrinsic Fabry-Perot Interferometric (EFPI) sensor are presented in this work. A new custom-designed, custom-built radio frequency (RF) magnetron sputtering system has been used to deposit thin films of palladium metal along with cylindrical substrates. The surface morphology of the deposited films are investigated using a variety of analytical tools, including Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (AES), and Auger Electron Spectroscopy (AES). The system is used to deposit thin films of palladium along a cylindrical EFPI fiber-optic sensor to produce a new fiber-optic hydrogen gas sensor. Experimental results obtained in a controlled hydrogen atmosphere are used to demonstrate the high resolution and fast response time associated with these new hybrid sensors. / Master of Science

The use of optical sensors to perform health monitoring in fault-tolerant multicomputers can allow the multicomputer to detect imminent failure in a particular section of the interconnection network due to damaging strain. This detection method allows the rerouting of critical data before data link failure occurs. This thesis investigates the implementation of the extrinsic Fabry-Perot interferometer into an optical hybrid communications/sensing network. A testbed of personal computers, acting as nodes of a multicomputer, are used to monitor the integrity of the network to a high degree of accuracy. When a node determines that an adjacent data link is no longer reliable due to physical damage, communications are rerouted and the node is shut down. Results of experiments with the testbed have shown that redundant nodes can be used to share computational loads, increasing the performance of the multicomputer, until network failure forces redundant nodes to assume full responsibility for computational tasks. Multicomputer performance suffers as a result of network damage, but full functionality is retained with no occurrence of errors or unknown conditions due to data link failure. / Master of Science

A liquid core fiber sensor can be used to sense both strain and temperature simultaneously. This liquid core fiber sensor is comprised of a hollow core optical fiber filled with a liquid of a known index of refraction which is slightly higher than that of the silica tube which acts as the cladding. The refractive index fluid is chosen such that the variation of its refractive index with strain and temperature is well defined and linear in the desired range of operation. The core of the sensing fiber contains a fluid which has a thermo-optic coefficient much larger in magnitude (-4.0x10⁻⁴/°C) than that of the silica cladding. This causes the fiber to be more sensitive to temperature changes than all-silica fibers. Both transmitted optical signal intensity and time-of-flight depend strongly on applied strain and temperature. Furthermore, the relative difference between the core and cladding refractive indices changes as a function of both parameters due to the inherently different material types used in the fiber construction. This results in critical strain and temperature regimes within which the refractive index difference is very small, and sensitivity is optimized. Testing of prototype sensors demonstrates these characteristics. A 0.47 m long liquid core fiber containing a liquid with a room temperature refractive index of 1.492 was analyzed. Both time- and intensity-domain behaviors around the device's critical temperature (95°C) confirm theoretical expectations. Simultaneous strain and temperature measurements were performed between 95 °C and 105 °C. Methods for multiplexing liquid core fibers for increasing the range of temperatures that can be monitored were also investigated as well as using those liquid core fibers for cooling purposes. / Master of Science

No description available.

The use of optical fiber is of great interest in developing extensive, high-speed networking infrastructures. Optical fiber provide many advantages over traditional copper cables and wireless links. Among them are high security, low electromagnetic interference, extremely low loss and high bandwidths, light weight and manageability. However, the very small wavelengths associated with optical radiation requires very small waveguide dimensions. Waveguide dimension of single mode fiber (SMF) are < 10µm, resulting in relatively poor yield in device manufacturing. For residential and other last-mile networks topologies, cost constraints limit the appeal of SMF. Multimode fiber (MMF) allow for less restrictive manufacturing tolerances; however, the distortion that results from the dispersion in propagation among the many modes can be prohibitively large for data rates approaching and exceeding 1 Gb/s. To improve the deployability of MMF, a method of dispersion compensation that maintains the ease-of-use characteristic of MMF is required This dissertation demonstrates an opto-electronic method of dispersion compensation by the use of a multisegment photodetector. It is shown the modes of the fiber can be seperated such that when the individual photodetector signals are combined, the resulting temporal response of the fiber link is improved from that of a conventional fiber link. This method is extremely robust to system variation and is independent of data rate and transmission format, allowing it to be employed in a wide variety of optical links. More importantly, the implementation demonstrated is comparable, in simplicity and alignment tolerance, to a conventional photodetector. System performance is shown using both temporal and frequency response as well as real bit error rate and eye diagram measurements.

Measuring acoustic pressure gradients is critical in many applications such as directional microphones for hearing aids and sound intensity probes. This measurement is especially challenging with decreasing microphone size, which reduces the sensitivity due to small spacing between the pressure ports. Novel, micromachined biomimetic microphone diaphragms are shown to provide high sensitivity to pressure gradients on one side of the diaphragm with low thermal mechanical noise. These structures have a dominant mode shape with see-saw like motion in the audio band, responding to pressure gradients as well as spurious higher order modes sensitive to pressure. In this dissertation, integration of a diffraction based optical detection method with these novel diaphragm structures to implement a low noise optical pressure gradient microphone is described and experimental characterization results are presented, showing 36 dBA noise level with 1mm port spacing, nearly an order of magnitude better than the current gradient microphones. The optical detection scheme also provides electrostatic actuation capability from both sides of the diaphragm separately which can be used for active force feedback. A 4-port electromechanical equivalent circuit model of this microphone with optical readout is developed to predict the overall response of the device to different acoustic and electrostatic excitations. The model includes the damping due to complex motion of air around the microphone diaphragm, and it calculates the detected optical signal on each side of the diaphragm as a combination of two separate dominant vibration modes. This equivalent circuit model is verified by experiments and used to predict the microphone response with different force feedback schemes. Single sided force feedback is used for active damping to improve the linearity and the frequency response of the microphone. Furthermore, it is shown that using two sided force feedback one can significantly suppress or enhance the desired vibration modes of the diaphragm. This approach provides an electronic means to tailor the directional response of the microphones, with significant implications in device performance for various applications. As an example, the use of this device as a particle velocity sensor for sound intensity and sound power measurements is investigated. Without force feedback, the gradient microphone provides accurate particle velocity measurement for frequencies below 2 kHz, after which the pressure response of the second order mode becomes significant. With two-sided force feedback, the calculations show that this upper frequency limit may be increased to 10 kHz. This improves the pressure residual intensity index by more than 15 dB in the 50 Hz-10 kHz range, matching the Class I requirements of IEC 1043 standards for intensity probes without any need for multiple spacers.

The work presented in this dissertation was aimed at understanding biology's application of soft materials to enhance sensing abilities and initiate innovative bio-inspired material-based approaches for flow (fluidic and air) sensors and photo-thermal sensors. A key aim is to help strengthen this niche of functional materials science referred to, here, as bio-inspired materials in sensing roles. The work aspires to traverse the boundaries of the subject in order to provide a strong foundation for future scientific explorations of the subject. The studies presented here, include studies of flow sensing in fish and implementing a bio-mimetic approach to microfabricated flow sensors. The work also includes studies of material based signal filtering in spiders, as well as, bio-inspired photo-thermal transduction mechanisms. The capabilities of the methodology are demonstrated with successful engineering studies.

Thesis (M.A.Sc.)--Naval Postgraduate School, 2000. / Includes bibliographical references (p. 157-159). Also available in electronic format via the Defense Technical Information Center website.

Thesis (Ph.D.)--Computing, Georgia Institute of Technology, 2009. / Committee Chair: Aaron Bobick; Committee Member: Charles Isbell; Committee Member: David Hogg; Committee Member: Irfan Essa; Committee Member: James Rehg