A broadband thermo-optic 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 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.
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