Waveguide Lenses Using Multimode Interference for Low-Loss Slab Propagation

It is shown that a nearly ideal two-dimensional focusing Gaussian beam can be synthesized using a linear combination of the two lowest order even modes of an optical waveguide. This property can be used to couple laterally guided modes across slab waveguide regions with very low loss. The technique is illustrated using a conventional multimode interference (MMI) geometry, where the MMI coupler transforms the fundamental mode of an initial waveguide into a focusing Gaussian beam, which is then fed to a slab region. Two-dimensional beam propagation simulations show that the beam does not initially diverge in the slab region, but rather comes to a focus. A second MMI coupler then transforms the diverging beam back to the initial mode. A structure is designed which can couple the fundamental mode of a 9 micron wide waveguide across an 88-micron long slab region with only 0.036 dB loss. This technique can be applied to improving the performance of small-angle waveguide crossings and integrated turning mirrors.