| Time-to-wavelength mapping followed by spectral domain processing represents a unique and powerful method for processing of ultra-wideband waveforms. This dissertation presents the fundamentals of time-wavelength signal processing by a comprehensive analysis of one of its important applications: the ultra-wideband photonic time-stretched analog-to-digital conversion. By reducing the signal bandwidth prior to digitization, this technique offers revolutionary enhancements in the performance of electronic converters. The dissertation starts with a fundamental-physics analysis of the time-wavelength transformation including both continuous-time and discrete-time implementations, and the implication of time-dilation on the signal-to-noise ratio. A detailed mathematical description of the time stretch process is then constructed. It elucidates the influence of chromatic dispersion and optical nonlinearity on the fidelity of the stretched electrical signal. Two techniques for ultra-wideband operation, single sideband modulation and phase diversity with maximum ratio combining, are examined. Problems arising from the nonuniform optical power spectral density are explained, and two methods for overcoming them are described. Finally, Design issues and performance features of a time-stretched analog-to-digital converter array are discussed. The experimental results include a recent demonstration of a record 1 Tera-Samples/s digitizer. |
