Driving an optical modulator is described. A control circuit generates first and second target voltages based on a target phase modulation between first and second optical waveguide arms of the optical modulator. An offset control circuit generates first and second offset signals. A linear modulator driver receives the first and second target voltages and the first and second offset signals, and generates a first output voltage for biasing the first optical waveguide arm and a second output voltage for biasing the second optical waveguide arm. Feedback circuitry can feed the first and second output voltages to the offset control circuit, which can generate the first and second offset signals using the first and second output voltages. The output voltages bias the waveguide arms so the optical modulator operates close to the target phase modulation, even in the presence of manufacturing errors.