Abstract: A circuit arrangement including a controllable optical resonator having a filter characteristic with a transmission peak arranged to pass laser radiation at a wavelength in the region of the transmission peak, wherein the resonator is positioned to receive the laser radiation at an angle such that two modes of the radiation, having respectively different polarization states, and wavelengths, are capable of propagating in the resonator. The resonator has control electrodes connected for receiving a voltage which determines the wavelength of the transmission peak of the filter characteristic and a source connected for applying to the electrodes a square-wave modulating voltage which varies between a first value which causes the transmission peak of the resonator to be substantially at the wavelength of one of the modes and a second value which causes the transmission peak of the resonator to be substantially at the wavelength of the other one of the modes.