Cassegrain reflector for hundredfold on-chip spot-size expansion

Emerging applications of integrated optics such as free-space beam projection, atomic spectroscopy, and interfacing with ultra-stable laser resonators often require beam sizes of the order of 100 μm or even higher, i.e., a hundred times larger than the mode field of common integration platforms such as silicon-on-insulator. Adiabatic tapers become prohibitively large for such extreme mode expansions, while other alternatives such as evanescent couplers, Bragg deflectors, and integrated lenses often suffer from narrow bandwidths or have not yet been proved beyond beam sizes of 40 μm. This study introduces an on-chip spot-size converter based on the Cassegrain dual-reflector geometry, adapted for the first time to the planar constraints of integrated optics platforms, alongside a novel, fully analytical design methodology. We experimentally demonstrate a device that expands the fundamental TE mode from a 0.5 μm waveguide to a 116 μm-wide beam, with an insertion loss of 0.6 dB at λ0 = 1.55 μm and a 1-dB bandwidth exceeding 200 nm (1.45–1.65μm. This design also offers a reduced area compared to its single parabolic analog. We, furthermore, use the device to feed a large-aperture optical antenna, producing a collimated free-space beam with a beam size of 116 × 235 μm2.