Abstract: An optical detector including an avalanche photodiode (APD) and protection circuitry for protecting the APD from excessive power dissipation therewithin, wherein the protection circuitry includes a switching element connected in parallel with the APD, which switching element is switched from a high resistance state to a low resistance state if the level of current through the APD exceeds a predetermined level, and wherein switching back of the switching element to the high resistance state is dependent on a reset signal from a controller. An optical detector including an avalanche photodiode (APD), and protection circuitry for protecting the APD from excessive power dissipation therewithin by switching the APD to a relatively low voltage power supply if the level of current through the APD exceeds a predetermined level, and wherein switching back of the APD to a higher voltage power supply is dependent on a reset signal from a controller.