1. First read the 42-page FOURIER OPTICS summary above. It covers your junior-level EM study and the Fourier Optics study of Maxwell's Equations.
2. Then study the above syllabus IMAGE SCIENCE. It contains 39 separate thesis studies by individual superior doctoral scholars, each of whom obtained PhD degrees at The Institute of Optics. Look them over: they are separate roads to the PhD. Think of each paper as an independent scholarly study by the author listed.
3. Now pick one or two of these separate paths to a thesis proposal and find a faculty person to help you formulate a plan to extend this work. Do not be concerned with thinking about too many details. Also note that all Institute of Optics theses are available in the Institute offices. Good luck!