We propose a circuit design for a broadband tunable 2x2 waveguide coupler, consisting of a two-stage Mach-Zehnder interferometer with electro-optic phase shifters in each stage. We demonstrate that such design can be configured as a tunable coupler with arbitrary coupling ratio and with a uniform response over 50-nm spectral range around 1550~nm. The design is also tolerant to fabrication variations that affect the coupling ratios of the directional couplers.
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