Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 129-133. / by A. Colbert Reisz. / Ph.D.

Distributed temperature and strain sensing is demanded for a wide range of applications including real-time monitoring of industrial processes, health monitoring of civil infrastructures, etc. Optical fiber distributed sensors have attracted tremendous research interests in the past decade to meet the requirements of such applications. This research presents a multiplexed sensor array for distributed temperature and strain sensing that can multiplex a large number of UV-induced sensors along a single fiber. The objective of this research is to develop a quasi-distributed sensing technology that will greatly increase the multiplexing capacity of a sensor network and can measure temperature and strain with a high accuracy and high resolution. UV-induced intrinsic Fabry-Perot interferometric (IFPI) optical fiber sensors, which have low reflectance and low power loss, are good candidates for multiplexed sensors networks. Partial reflectors are constructed by irradiating photosensitive fiber with a UV laser beam. A pair of reflectors will form a Fabry-Perot interferometer that can be used for temperature and strain sensing. A sensor fabrication system based on a pulsed excimer laser and a shadow mask is developed. A spectrum-based measurement system is presented to measure the interference fringes of IFPI sensors. A swept coherent light source is used as the light source. The spectral responses of the IFPI sensors at different wavelengths are measured. A frequency division multiplexing (FDM) scheme is proposed. Multiple sensors with different optical path differences (OPD) have different sub-carrier frequencies in the measured spectrum of the IFPI sensors. The multiplexing capacity of the sensor system and the crosstalk between sensors are analyzed. Frequency estimation based digital signal processing algorithms are developed to determine the absolute OPDs of the IFPI sensors. Digital filters are used to select individual frequency components and filter out the noise. The frequency and phase of the filtered signal are estimated by means of peak finding and phase linear regression methods. The performance of the signal processing algorithms is analyzed. Experimental results for temperature and strain measurement are demonstrated. The discrimination of the temperature and strain cross sensitivity is investigated. Experimental results show that UV-induced IFPI sensors in a FDM scheme have good measurement accuracy for temperature and strain sensing and potentially have a large multiplexing capacity. / Ph. D.

The conventional way of using a nonlinear Fabry-Perot cavity as an optical memory requires a negative pulse input to reset the state of the cavity. The possibility of using positive pulses to turn a system of nonlinear Fabry-Perot cavities on and off is studied and it was found that positive pulse switching is possible in a system of two coupled nonlinear cavities. First, Korpel and Lohmann's proposal of using polarization switching in a single nonlinear birefringent cavity was studied. After a detailed investigation of their proposal it was found that positive pulse switching in a single nonlinear Fabry-Perot cavity is not possible. One of the reasons is that the eigen-polarization states of the output of a nonlinear Fabry-Perot cavity cannot be switched independently. Although it is not possible to switch a single nonlinear Fabry-Perot cavity with positive pulses we were able to use the coupling of the eigen-polarization states to implement other kinds of optical switches which were demonstrated experimentally. The cross-talk effect in a metallic Fabry-Perot cavity was also studied. Next, a steady state model of a system of two coupled nonlinear Fabry-Perot cavities was developed and it was found that positive pulse switching is possible in such a system. The output can be turned on and off either by pulses sent into different cavities or by pulses of different magnitudes sent into one cavity. Finally, the dynamic behavior of the coupled cavities system was modeled by extending Goldstone and Garmire's model of a single cavity with one input to a system of two coupled cavities with two inputs. We verified by numerical calculations that positive pulse switching is also possible in the dynamic regime. / Ph. D.

This thesis demonstrates the use of fiber optic methods for nondestructive testing of composite materials and aluminum specimens using the acousto-ultrasound approach. A noncontact method using a hybrid interferometer is devised for measuring absolute surface acoustic wave (SAW) amplitudes. The J1..J4 spectrum analysis technique is used for calibrating the piezoelectric transducer cylinder (PZT) and JO/J2 spectrum analysis technique is used for demodulating the SAW signal from the interferometer. An extrinsic Fabry-Perot interferometric (EFPI) sensor is utilized for sensing acoustic emission, measuring speed and attenuation in aluminum and composite specimens. A broadband preamplifier is designed for amplifying signals from the EFPI sensor. Theoretical and practical minimum detectable air gap change of an EFPI sensor are calculated for the system. The directional sensitivity of the EFPI sensor to SAW is studied. / Master of Science

The developing field of smart structures and skins provides an application to which fiber optic sensors bring unique capabilities and benefits. The primary cost in a network of fiber sensors is in the sources, receivers and associated hardware and can be prohibitive for a large number of sensors. Multiplexing of sensors based on spread spectrum techniques offers an efficient and cost effective solution to this limitation. The system hardware developed in this research work is capable of the real time monitoring of a four sensor network. Experimental results with embedded and attached Extrinsic Fabry Perot Interferometers (EFPI) are presented. The system can be used to multiplex any type of sensor which translates the measurand into intensity variations of the light. A measure of the system efficiency is obtained using crosstalk measurements. A suppression of 40 dB has been obtained between the desired sensor signal and the interference. The effect on system performance has been observed by varying system parameters such as code length and separation between codes. Highly sensitive embedded interferometric sensors have been used in a multimeasurand environment to measure temperature and strain. A solution to the inherent 'near-far' problem in an optical COMA system has been proposed and results for the same are presented. A novel WDM/CDM hybrid (Wavelength Division Multiplexing/Code Dhtision Multiplexing) scheme has been proposed to increase the light intensity at the detector thereby increasing the number of sensors in the system. Methods to optimize and upgrade the system are discussed. / Master of Science

This thesis presents the design and simulation of a digital frequency demodulator applied to a Michelson interferometer-based scanning laser vibrometer. The laser vibrometer is a velocity transducer that produces frequency-modulated signals that have traditionally been optically or electronically frequency shifted from baseband to an intermediate frequency. This shifting produces a narrow band modulation that may be demodulated with standard analog frequency demodulators. This thesis proposes replacing these traditional optics or electronic circuits with a digital frequency demodulator. The optics of a laser vibrometer can be constructed such that a near-perfect baseband quadrature representation is available for the frequency modulation. It will be seen that this representation is well suited for use by a digital frequency demodulator. This thesis applies the arctangent-type digital frequency demodulator to a laser vibrometer and demonstrates its superiority over other digital demodulation techniques. In addition some specialized signal processing to handle the special case of sine-dwell vibration tests is presented. / 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

Conventional fiber optic interferometric sensors employing a monochromatic source prove to be inadequate for applications requiring absolute, real-time value of magnitude and direction of the applied perturbation. This limitation can be overcome by using a broadband light source to extract unambiguous information from the sensor in the wavelength domain. Several variations in the signal processing techniques for white light interferometry are discussed and compared in terms of resolution, bandwidth and cost. A detailed analysis is made of the principle of operation and basic features of the commercially available absolute sensing system. This compact system is self calibrating, has a 100 micro-strain Ole) strain and 2°C temperature resolution and is ideal for applications in environments where the parameter to be measured is static or quasi-static. High finesse Fabry-Perot cavities are employed to obtain almost an order of magnitude sensitivity improvement over conventional low finesse cavities. The principle of white light interferometry is extended to absolute axial stain and temperature sensing in two-mode, elliptical-core fibers. Other novel applications, such as to operation of photo induced refractive index gratings and fiber characterization are proposed. / 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

The FP interferometer was found to be feasible for detecting changes in machining forces. The fiber optic sensor was able to detect increases in strain corresponding to force increases detected by a dynamometer. The FP interferometer system must progress in several ways. A better data acquisition and data analysis system must be developed. A robust sensor must be made to withstand the harsh environment of machining. Also a method for eliminating the affects of thermal strain must be created. Finally, the placement of the FP sensor must also be determined. The FP has the potential to effectively monitor machining forces without affecting the rigidity of a turning operation setup. / Master of Science