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A fiber optic fluid level sensor based on light transmission attenuation due to bending losses is designed, built and tested. Fibers formed with reverse curvatures of decreasing radii will induce and increasing amount of lower order mode light loss to the cladding as the light propagates along the step index multimode fiber. The sensor is arranged in the fluid in a vertical position such that the light travels along the fiber from the bottom or low fluid point to the top or full point. As the fluid covers increasing lengths of the exposed fiber, it strips even more power from the cladding (assuming the fluid refractive index is greater than the cladding refractive index). Data taken with a sensor of this configuration show a monotonic decrease of the output intensity as a function of increasing fluid level. As much as a 14 dB change occurs over a one-foot fluid level change. A mathematical model, based on both field theory and geometrical optics, is developed to evaluate and predict the performance of this fiber optic fluid level sensor. Comparisons of the theoretical predictions and the experimental results under laboratory conditions show very good agreement.

Multiplexing has evolved over the years from Emile Baudot's method of transmitting six simultaneous telegraph signals over one wire to the high-speed mixed-signal communications systems that are now commonplace. The evolution started with multiplexing identical information sources, such as plain old telephone service (POTS) devices. Recently, however, methods to combine signals from different information sources, such as telephone and video signals for example, have required new approaches to the development of software and hardware, and fundamental changes in the way we envision the basic block diagrams of communication systems. The importance of multiplexing cannot be overstated. To say that much of the current economic and technological progress worldwide is due in part to mixed-signal communications systems would not be incorrect. Along the vein of advancing the state-of-the-art, this dissertation research addresses a new area of multiplexing by taking a novel approach to network different-type sensors using software and signal processing. Two different sensor types were selected, fiber optics and MEMS, and were networked using code division multiplexing. The experimentation showed that the interconnection of these sensors using code division multiplexing was feasible and that the mixed signal demultiplexing software unique to this research allowed the disparate signals to be discerned. An analysis of an expanded system was performed with the results showing that the ultimate number of sensors that could be multiplexed with this technique ranges from the hundreds into the millions, depending on the specific design parameters used. Predictions about next-next generation systems using the techniques developed in the research are presented. / Ph. D.

This dissertation begins with a study of some of the linear and nonlinear optical properties of composite materials consisting of CdSₓSe₁₋ₓ microcrystallites embedded in a host glass matrix. These studies investigate changes in absorption, refractive index and nonlinear response time under a variety of experimental conditions. The data demonstrates that this class of materials exhibit: a strong saturation of absorption due to band filling; a large n₂ which also saturates; response times which range from <100ps to many nanoseconds; and a permanent darkening and change of n₂ induced by extended exposure to high energy pulses. These measurements were used to identify the optimum sample of the semiconductor doped glasses to demonstrate an efficient degenerate four-wave mixing process within a planar waveguide. High quality single mode waveguides were fabricated from the semiconductor doped glass by K⁺-ion exchange. Four wave mixing was performed in the waveguide that produced a peak reflectivity of ≅.003, which is 8 orders of magnitude larger than that achieved previously in a similar experiment where CS₂ was used as the nonlinear medium.

A study of the application of single-mode fiber optics to the multiple-beam interferometric recombination problem is presented. In the laboratory, the fibers have been used in wide bandwidth, two-arm, Mach-Zehnder test interferometers as well as a 5-telescope imaging interferometer connected to an all-fiber beam combiner. Based upon these experiments and some theoretical studies it is shown that fiber optics and fiber optic components such as directional couplers provide an excellent alternative to conventional optics such as mirrors, beamsplitters, and relay lenses. The equations describing the measurement of the complex degree of coherence in an interferometer with a single-mode fiber in each arm are derived. The equations reveal an important feature of the fibers: they filter phase fluctuations due to aberrations and turbulence at the input and convert them to intensity fluctuations at the output. This leads to a simplification of the calibration of measured visibilities. The coupling efficiency of light which has passed through a turbulent atmosphere is also studied as a function of fiber parameters and turbulence conditions for both image motion stabilized and non-stabilized cases. For the former case, coupling efficiency remains greater than 50% as long as telescope diameter is no larger than the turbulence coherence length. Beam combination architectures using arrays of directional couplers are fully discussed. Arrays accommodating up to 20 input beams are presented. The arrays require only N detector pixels for N input beams. A scheme of temporal multiplexing of the phase of each beam is used to identify individual fringe pairs. One possible scheme allows wide bandwidths even for large numbers of beams. A 5-telescope interferometer has been constructed and connected to an all-fiber beam combiner. Two extended objects were observed and reconstructed using standard radio astronomy VLBI software. The interferometer and beam combiner had good thermal and polarization stability and high throughput. Reconstructed images had dynamic ranges of about 50.

This dissertation describes the techniques involved in using a fiber optic coupled spectrometer to do multiple object spectroscopy of astronomical objects. The Medusa spectrograph, with optical fibers fixed in a focal plane aperture plate, was used to study clusters of galaxies via velocity distributions. Some relevant problems in the study of the structure and dynamics of clusters of galaxies are outlined as motivation for building a multiple fiber spectrometer. The history of fiber optic spectroscopy in astronomy is presented along with an outlook for the future. The results and experience gained from the Medusa spectrograph are used to design a second generation instrument. The MX Spectrometer uses optical fibers which are positioned remotely under computer control. These fibers are optically matched to the telescope and spectrograph optics to achieve optimum performance. The transmission, flexibility, and image scrambling properties of step-index silica fibers allow efficient reformatting of multiple objects into the spectrometer entrance aperature. By allowing spectra of 32 objects to be obtained simultaneously, the MX Spectrometer will make an order of magnitude increase in the quantity of spectroscopic data that can be recorded with a large telescope. Mechanical, control, and optical elements of the MX design are discussed. Telescope and fiber parameters influencing the design of the fishermen-around-the-pond mobile fiber head are detailed. Results of testing the stepper motor driven fiber positioner probes are described. The algorithm for controlling the motion of 32 positioners in the telescope focal plane without collisions is outlined. Detector performance and spectrometer efficiency are considered for both the Medusa and MX systems. The use of a Charge Coupled Device (CCD) array detector provides increased quantum efficiency, dynamic range, and stability, as well as allowing digital sky subtraction. Microlenses and their use in correctly coupling optical fibers to the telescope and spectrograph are discussed. In particular, the pupil-imaging technique for microlens matching to fibers is introduced.

Approved for public release; distribution is unlimited / This thesis presents the design test and evaluation of a fiber optic remote monitoring system. Practical aspects of loss measurement, link analysis, receiver design, and controller implementation are examined. The fundamental operation of the system relies on conversion of the voltage data to be a variable frequency TTL pulse train. The pulse train modulates a 1300 nm laser, which transmits the telemetry data via single mode fiber to the shore station. One of the two test voltages can be selected by the shore-based controller, via the bidirectional link. Laboratory test results are included / This thesis presents the design, test and evaluation of a fiber optic remote monitoring system. Practical aspects of loss measurement, link analysis, receiver design, and controller implementation are examined. The fundamental operation of the system relies on conversion of the voltage data to a variable frequency TTL pulse train. The pulse train modulates a 1300 nm laser, which transmits the telemetry data via single mode fiber to the shore station. One of two test voltages can be selected by the shore-based controller, via the bidirectional link. Laboratory test results are included. / http://archive.org/details/longhaulunderwat00dena / United States Navy / Lieutenant Commander, United States Navy

M.Phil. (Electrical and Electronic Engineering) / A numerical model of an Erbium-Ytterbium co-doped distributed feedback (DFB) fiber laser is developed. The DFB fiber laser is a short length fiber laser whose feedback is distributed throughout the cavity. Its main advantage is its single longitudinal mode operation. The amplifying medium of a DFB fiber laser is a few centimetres long rare earth doped fiber. The feedback is obtained by a fibre Bragg grating printed in the core of the rare earth doped fiber. This type of laser emits naturally in two longitudinal modes. To obtain the single longitudinal mode operation, a π phase shift is introduced in the middle of the grating. Erbium doped DFB fiber lasers present the advantage of emitting single frequency light in the 1550 nm region where telecommunication fibers present the minimum loss. However due to the relatively short length of the gain medium, the number of available Erbium ions is small; as a result pump power absorption is low and the efficiency of the fiber laser is strongly reduced. The straightforward solution to this problem could be increasing the concentration of Erbium ions. This solution however has the disadvantage of increasing the Erbium ions interactions, thus leading to detrimental effect like cooperative upconversion and excited state absorption, which in term reduce considerably the laser efficiency. The best solution is to use Ytterbium ions as sensitizers along with Erbium ions to enhance the pump absorption, hence the efficiency of the laser. A model of the DFB fiber laser is an indispensable tool for its design, because it allows one to predict characteristic behaviour that would be both difficult and costly to deduce in laboratory conditions. The model developed in this project is based on rate equations of the Er3+-Yb3+ gain medium and coupled mode equations describing the laser field propagation in the fibre Bragg grating structure. The equations are solved using a quasi-analytical iterative method along with transfer matrix method with appropriate boundary conditions. The quasianalytical method used in this thesis is more robust than numerical solutions because it does not require providing an initial guess on the solution. Furthermore this method is hundreds time faster than the exact numerical solution while giving almost similar results.

The objective of this thesis was to evaluate the feasibility of three approaches to thermal imaging that used fiber-optics to decouple the imaging optic from a detector in order to form images without a direct line of sight. All images were formed using 2 m-long fibers, at a working distance of 19 cm, and the ability to detect temperatures between 100 °C to 200 °C. The three approaches were designated active, hybrid, and passive. The active approach involved scanning a single fiber in the image plane to formulate thermal images. Although images had sufficient field of view and spatial resolution, they were not formulated at a frequency of at least 1 Hz using either linear stages or a piezoelectric actuator for scanning. The hybrid approach involved scanning a bundle of fibers in the image plane to formulate a thermal image. Although it was possible to form individual images from fibers within a simple bundle and a seven-fiber bundle with sufficient field of view and spatial resolution, an image formulation frequency of at least 1 Hz was not achieved. While considered feasible, future work is required to identify a suitable motion platform, design a new bundle, and develop a method for stitching images from individual fibers into a single spatially-coherent image in order to achieve an image formulation frequency of at least 1 Hz. Lastly, the passive approach was evaluated in which images were formed using a fixed seven-fiber bundle for direct thermal imaging. The seven-fiber bundle produced images with sufficient field of view and an image formulation frequency on the order of kHz, but without sufficient spatial resolution. Additional work is required to determine the feasibility of designing a passive bundle for a specific spatial resolution, as well as, if overall costs are prohibitive. While no approach satisfied all functional requirements specified herein, evaluation of each approach and a potential solution are provided.