Photonic Techniques for Real-Time Analog-to-Digital Conversion

Doktorsavhandling, 2006

All-electronic analog-to-digital conversion (ADC) technology is limited by the timing accuracy and inherent speed of semiconductor materials. Photonic techniques, however, can potentially enable real-time ADC at a considerably wider bandwidth, higher sample rate and/or resolution, thanks to the availability of extremely accurate optical clocks and the immense bandwidth of optical systems. Together with the availability of very wide bandwidth components developed for fiber-optical communication, optical techniques for high speed ADC now have a unique opportunity for a break-through in demanding applications. In this thesis, the ADC process is separated into three distinct processes: sampling, binary encoding and thresholding. Photonic techniques for realization of these processes are reviewed in the first part of this thesis, as an introduction to the appended papers. In the first five appended papers, two schemes for photonic binary encoding are studied. The first scheme is based on a spectrometer-like setup using a tunable laser, and experiments are presented in Paper A with 100 MHz signals and 3.2 effective number of bits (ENOB). In the following paper, a design method for the diffractive optical element array in such an ADC is proposed, avoiding destructive inter-element interference while retaining diffraction efficiency. A more accurate model of the ADC is presented in Paper C, where it is shown that the spectrometer resolution can be relaxed if more accurate thresholders are used, giving an increase in signal bandwidth. A novel interferometric scheme for photonic binary encoding is proposed in Paper D, and demonstrated with optical sampling at 40 gigasamples/s with 3.6 ENOB in Paper E. This scheme uses only one standard phase modulator for sampling, and is thus suitable for very high frequency operation. An analytical model of the ADC performance is presented, showing good agreement with the experimental results. Paper F deals with the so called "photonic time-stretch"(PTS) system, and proposes a novel signal post-processing method. A very efficient numerical propagation method is also presented, and Monte-Carlo simulations are carried out to show that the post-processing successfully compensates for linear as well as nonlinear distortion. In the last paper, we demonstrate a method for tunable GHz-THz signal generation with a very stable phase. The setup can easily be added to a PTS system for characterizing the chromatic dispersion of the stretch element.