Exploring light coupling in short photonic integrated waveguides with Fourier space imaging

Fourier plane imaging microscopy offers an effective approach for identifying the optical modes excited in and scattered from photonic structures. In this work, we employ this technique to investigate the light coupling process between a focused free-space beam and a photonic integrated waveguide—just a few micrometers in length—vertically embedded within a multilayer stack on a silicon wafer. The reflected light is analyzed in the back-focal plane of the focusing lens under varying beam focal positions, incident angles, and numerical apertures of the excitation. This analysis provides insight into the coupling mechanisms in short waveguides, reveals the influence of the Gouy phase on the spatial frequency distribution of the reflected beam, and enables quantification of the coupling efficiency—without requiring access to transmitted light. The methodology and findings of this fundamental study will contribute to optimizing light coupling strategies for advanced image sensor engineering.