Broadband optical switch for multiple spatial modes based on a silicon densely packed waveguide array.

A broadband thermo-optic 2×2 Mach-Zehnder switch that can control two spatial modes simultaneously on a silicon chip is demonstrated in this Letter. A broadband multimode 3 dB coupler is first realized based on symmetrically coupled waveguides with a sub-wavelength structure. The length of the coupler is only 24.2 μm. By employing such a multimode 3 dB coupler together with symmetrical delay arms, an optical switching functionality is realized experimentally with excess losses less than 1.3 dB and crosstalks less than -15  dB over a 60 nm bandwidth for the two spatial modes. An arbitrary splitting ratio at two output ports for the two modes simultaneously is also demonstrated when applying different currents on the heater.