A method of operating an optical modulator. The optical modulator has: a rib waveguide which includes a junction which is either a PIN or PN junction, the junction having a breakdown voltage. The method comprises: applying a reverse bias to the junction, so as to operate the optical modulator around the breakdown voltage of the junction; operating the modulator in an avalanche multiplication and/or band-to-band tunnelling mode by increasing the reverse bias past the breakdown voltage.