In general, in one aspect, an apparatus includes an optical bus arm and a ring resonator arm. The apparatus also includes first, second and third directional couplers between the optical bus arm and the ring resonator arm. A first tuner is included on one of the optical bus arm and the ring resonator arm between the first directional coupler and the second directional coupler. A second tuner is included on other of the optical bus arm and the ring resonator arm between the second directional coupler and the third directional coupler. A tuning mechanism is also included.