Optical Noise in Interferometric Systems Containing Strongly Unbalanced Paths.
This dissertation addresses the topics of coherence multiplexing, noise in multimode laser diodes, optical noise associated with interferometers, speckle statistics in multimode optical fiber, and coupled-mode theory for dielectric waveguides. Coherence-multiplexing is a technique for transmitting signals from a number of interferometers through a single optical path in such a way that individual signals can be recovered. The principle of operation involves using a partially coherent optical source, so that well -balanced pairs of optical paths produce interference signals but unbalanced paths do not. Application to the multiplexing of fiber-optic interferometric sensors is emphasized, though the technique could also be applied to optical communications. Interferometrically processed noise from the optical source is found to limit the sensitivity of multiplexed sensors to much less than what is achievable with isolated sensors. This situation may be improved by the use of short-coherence-length sources or source modulation. Power-budget considerations may limit the number of sensors that can be multiplexed to on the order of ten. A model for the mode-partition and phase statistics of light from a multimode laser diode is presented. In the model spontaneous emission drives a random walk on a hypersphere representing the electric field in the various modes; carrier fluctuations associated with spontaneous emission produce additional phase noise common to all modes. Predictions of the model are confirmed experimentally using a bulk-optic interferometer, with discrepancies attributed to the model assumption that all modes have equal average optical power. Also presented is a theorem concerning the noise spectrum at the output of a strongly-unbalanced interferometer using an arbitrary source. Appendices treat multimode fiber speckle statistics using assumptions of hyperspherical symmetry, and develop a high-order coupled-mode theory for dielectric optical waveguides.
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- Physics: Optics