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Collimating a Free-Space Gaussian Beam by Means of a Chip-Scale Photonic Extreme Mode Converter

Integration of nanophotonics with micromechanical, biological, and chemical systems can miniaturize the reference samples for SI traceable calibration. However, an efficient conversion between photonic modes and free-space beams remains a main hurdle due to a gigantic mode mismatch (106 times in modal area). Here, we report an extreme mode converter realized on a photonic chip, which enables projecting a 160-micrometer wide Gaussian beam in free space by a sequential mode expansion from a waveguide to a slab mode and then to free space using an apodised grating. The design can be easily adapted for visible, telecom, and UV wavelengths.