Public defence in Radio Science and Engineering, M.Sc.(Tech.) Samu-Ville Pälli

The title of the thesis: Frequency-diverse phase holograms for millimeter- and submillimeter-wave computational imaging

Thesis defender: Samu-Ville Pälli
Opponent: Prof. Thomas Fromenteze, University of Limoges, France
Custos: Prof. Zachary Taylor, Aalto University School of Electrical Engineering

This doctoral thesis introduces innovative methods for computational imaging at millimeter- and submillimeter-wave frequencies by utilizing frequency-diverse phase holograms. Addressing the inherent complexity and high cost of conventional imaging systems at these wavelengths, this research focuses on developing scalable and cost-effective imaging solutions. The core innovation is the design and application of dispersive phase holograms that leverage frequency diversity to encode spatial information, removing the necessity for complex mechanical or electronic beam steering. 

Frequency-diverse phase holograms presented in this work are carefully engineered dielectric structures that modulate the phase of electromagnetic waves. By precisely controlling their spatial phase profiles, these holograms generate distinct illumination patterns that vary with frequency. The thesis outlines a systematic approach to hologram design, including further optimization with spatial filtering techniques. Fabricated holograms demonstrated excellent phase accuracy and good diffraction efficiency. 

Two distinct imaging systems were developed and experimentally validated: a laboratory-grade system based on a vector network analyzer (VNA) and a compact, stand-alone frequency-modulated continuous-wave (FMCW) radar system. Experiments demonstrated good imaging accuracy, particularly at higher frequency bands, reaching resolutions close to theoretical diffraction limits. Neural network-based image reconstruction methods were effectively employed to reconstruct spatial images from reflected signals in real-time.

Thesis available for public display 10 days prior to the defence at Aaltodoc.