The use of a modal domain fiber optic sensor for vibration control of a flexible cantilevered beam is experimentally demonstrated. The sensor utilizes mode-mode interference in a two mode elliptical core fiber. The sensor covers a major portion of the beam, and produces a measurement based on the strain distribution in the beam. A distributed-effect model is developed for the fiber optic sensor, and verified through both static and dynamic tests. This model is incorporated into a active control system employing dynamic output feedback. The control system is shown to add damping to the low order modes of the beam. Numerical simulations are presented which concur with the experimental responses, in both open and closed loop tests. / Master of Science

This paper investigates the application of single, low-moded, fiber optic sensors to the detection of structural vibration. Several laboratory vibration systems which demonstrate a range of component frequencies and dynamic range are analyzed in an effort to characterize the sensor's behavior. We compare frequency spectra of the sensor output with calculated frequency components of the structures to determine if the sensor is responding to the various structural harmonics. Specifically, we wish to demonstrate the dynamic range and frequency response of the sensing technique. Experimental results and observations are preceded by a brief review of sensor mechanisms in optical fibers and presentation of a theory describing the operation of the modal domain sensor. / M.S.

Wavelength Division Multiplexed (WDM) based broadband fiber optic networks offer an attractive approach to achieve extremely high throughputs while employing moderate speed electronics. Passive optical filters play a crucial role in such networks serving the function of combining and separating multiple wavelengths. Single and multiple cavity F-P filters are analyzed with particular emphasis on their bandwidth and tuning range. Fabry-Perot (F-P) filter tuning commonly employs piezoelectric techniques. A new tuning technique based on a F-P filter fabricated with a fiber having an electro-optic cladding is proposed. A novel F-P filter structure employing optical feedback is proposed and analyzed. The application of F-P filters in an incoherent multiwavelength star network and the associated concerns involving crosstalk and dispersion are evaluated. A comparative error performance analysis of one and two F-P filter receivers in a BFSK transmission system is performed. An overview of numerous multiwavelength network and switch proposals along with their associated merits and demerits is given. / Master of Science

Tunneling through a potential barrier is a well-known concept in quantum mechanics. It is shown that tunneling is not an inherently quantum-mechanical phenomenon but that it occurs in classical optics as well. Single-layer and double-layer dielectrics in which optical tunneling occurs are shown to serve as the basis of optical filters, the double-layer tunnel filter having a much higher resolution capability than the single-layer tunnel filter. Optical amplifiers used as a receiver preamplifier and as linear intermediate repeaters are evaluated. Performance parameters of particular interest are the receiver sensitivity and the total system gain. The effects of optical amplifier gain and bandwidth on those parameters are studied. Finally, the effects of gain variations are considered. It is shown that gain control gives very poor results, whereas power control does not severely limit the system performance, particularly when a small number of amplifiers are used. / Master of Science

A novel optical fiber sensor for the measurement of strain and temperature in graphite-epoxy composite materials using differential interferometry is described. The sensor uses two single-mode optical fiber waveguides imbedded within the composite during prepreg ply lay-up. Strain and temperature changes arc obtained as a motion of an optical interference pattern. Values arc calculated for the strain and temperature dependence of the fringe motion. The results of measurement which attempt to duplicate modeled loading conditions are reported and compared with analytical results. Analytical and experimental extensions of the technique to the measurement of the differential temperature in graphite-epoxy composite specimens during cure cycle processing are also considered. / M.S.

Refractive index discontinuities in fiber optic transmission systems are known to cause deleterious effects. Non-negligible return losses associated with connectors and splices in present day systems cause intermittent error bursts and bit-error-rate floors in gigabit per second systems. These are attributed to the interferometric conversion of laser phase noise into signal-dependent intensity noise. This relative intensity noise (RIN) is substantially higher than the intrinsic RIN of the laser. The power spectral density of the RIN and its impact on the performance of incoherent on-off keying digital systems are calculated. The combined effects of this noise and other degradations present in the system are studied using a simple model. It is shown that even though RIN is a bounded degradation, it, particularly in conjunction with other impairments, results in high and sometimes unacceptable power penalties. Previous analyses are extended to include the effects of multiple reflections from a single pair of reflectors, the effects of a multiplicity of reflection points and the combined effects of reflection-induced noise and other impairments. It is shown that the effect of multiple reflections, although having only a small influence on the reflection induced noise power, changes the distribution of the noise and has more serious system effects. In the case of a multiplicity of reflection points it is shown that for as few as four reflection points, the Gaussian approximation gives results in good agreement with results calculated from a Gram-Charlier series approximation to the actual distribution function. Power penalties as a function of reflection coefficient are calculated and compared using several different approximations for the distribution of the interferometric noise. The methodology presented, although applied specifically to reflection induced noise, is applicable to a broader class of problems in which there are other signal dependent noise phenomena. / Master of Science

In order to develop more compact optical fiber sensing systems, modal filtering can be performed in-line by using micro-optical devices. Two such devices are a laterally offset few-moded optical fiber mechanical splice and a modal conversion optical fiber coupler. A third device, the air-gap splice used with multimode fibers, can examine the modal content of an optical fiber. A basic theoretical understanding on how these devices operate is reviewed. A splice loss calculation for few-moded optical fibers is presented. Applications of the asymmetrical few-moded mechanical optical fiber splice, the modal conversion coupler, and the air-gap splice are discussed. / Master of Science

After a U. S. Navy ship is built but before it is placed into service, many performance tests of all shipboard systems are conducted. These tests vary greatly in who performs the test, the purpose of each test, which system or systems are being tested, and the duration of each test. As naval warfare ships become increasingly complicated, the performance tests that are conducted also become numerous and complex. The current test philosophy prescribes that for each test and test organization, telemetry cables for electrical sensors are strung throughout the ship immediately prior to the test being conducted. As the shipboard tests and trials become more numerous and complex this philosophy becomes expensive from a labor and materials point of view. This thesis proposes an economical solution to the current test and trials problem by offering a fiber optic network with optical sensors. The fiber optic network will be designed to accommodate as many different users as possible, and it will be installed once, during the new ship construction. Prior to the network design, optical fiber sensor schemes are discussed. One sensing scheme, using quartz crystal oscillators, looks promising for the test and trials application. This one sensing method can be applied to acceleration, velocity, displacement, temperature, current, and voltage. Thus economies can be realized by using one network and sensor type for the majority of tests and trials applications. / Master of Science