Nonlinear-optical loop mirrors (NOLMs) have historically been used primarily in applications utilizing pulsed signals, for which large peak powers are attainable, allowing for a useful nonlinear response with a relatively short fiber. However, NOLMs have not been intensively studied in applications using constant-intensity (CI) signals. When CI sources are used, the length of fiber needed to obtain modest nonlinear phase shifts can become very large. To reduce the NOLM's susceptibility to acoustic perturbations, we have folded the nonlinear loop, allowing the signals to pass through twice (once in each direction). When CI signals are used with this double-pass configuration, additional nonlinear interactions must be accounted for that, to the best of our knowledge, have never been studied.; In the research presented in this dissertation, we have studied the folded NOLM in an effort to improve its stability. The resulting improved understanding of the nonlinear effects occurring within the folded loop will have consequences for any folded-NOLM application using signals with peak and average powers of similar magnitude. To serve as a focus for our work, we chose to explore the notion of using folded NOLMs for gain equalization in optically amplified wavelength-division multiplexed (WDM) communication systems utilizing CI modulation schemes. Using this application as our motivation for developing a folded NOLM, we have observed two nonlinear effects for the first time: (1) Electrostriction-induced guided acoustic-wave Brillouin scattering (GAWBS); and (2) a phase-matched, degenerate four-wave mixing process that undermines the desired nonlinear action of the folded NOLM. Furthermore, we developed a model of the third-order nonlinear interactions in a folded NOLM that predicts the effects of (2). Using insight provided by this model, we were able to devise and demonstrate a folded NOLM that does not suffer from this effect by adding a phase modulator to one arm, thereby breaking the phase-matching condition in the folded loop. After making the aforementioned modification, we were able to demonstrate a folded NOLM that provides a power-dependent transfer function that selectively attenuates WDM channels when they become too strong, thereby providing automatic gain equalization. The proposed system does not require any feedback and can automatically adjust for system changes.
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