An optical modulator is arranged to compensate for the thermo-optical modulating effects induced by charge-injection based phase modulators. It comprises first modulator means (3), that receive optical radiation (11, 21), direct it along an optical path, apply a first predetermined optical phase modulation by the injection of free charges into the optical path, and output optical radiation (13, 22) so modulated. Compensator means (4) apply, to optical radiation output from (13, 22), or to be received by, the first modulator means (3), a second predetermined optical modulation chosen to substantially compensate for stray thermo-optical phase modulations imposed upon received optical radiation by the first modulator means (3) as a result of thermal dissipation within the first modulator means.