LOSS AND PERIODIC COUPLING EFFECTS IN DIELECTRIC DIRECTIONAL COUPLERS

This dissertation is concerned with understanding the causes and effects of a new loss mechanism in dielectric directional couplers, namely dissimilar normal mode loss, as well as introducing a new class of all-fiber devices based on the periodic coupling of fiber modes.;Theoretical arguments and experimental evidence are presented to demonstrate that the coupler modes usually have different losses. Dissimilar mode loss causes the relative phase between the light in the guides to be modified and prevents complete power transfer from occurring. Interferometers using such couplers will exhibit phase errors in their outputs and all-fiber resonators will display an asymmetry in their resonance peaks. In integrated optics lower limits are set on switching extinction ratios.;It is shown that much of the analysis presented in the literature on three waveguide couplers is based on approximations that may not be valid in the regimes where the couplers are to be used. A three-waveguide coupler interferometer with dissimilar mode loss is studied and shown to have two independent outputs whose phases are environmentally insensitive to changes in coupler loss and power transfer.;Uniform and periodic coupling functions are analyzed and it is shown that complete power transfer can occur when the period of the sinusoidal coupling matches the beat length between the coupled propagating waves. A birefringent fiber polarization coupler and a two-mode fiber modal coupler are demonstrated and evaluated. These compact and simple devices are used to fabricate all-fiber amplitude modulators, notch filters, in-line Mach Zehnder interferometers, and polarizers. Further applications include polarization controllers, signal processing operations such as fast word generation, and in-line artificial dispersion elements.;A formal introduction to coupled mode theory is developed from which directional couplers can be described by using linear propagation operators. Comparisons to the standard coupling matrix approach are made and examples are given.;Allowing a periodic coupling function to propagate along the coupler can result in single sideband frequency shifting. This is demonstrated experimentally by pressing a block of aluminum with a surface acoustic wave on it against a birefringent fiber. Alternative configurations and approaches to all-fiber frequency shifting are presented.