Abstract: An optical amplifier comprises a doped fiber and a pump source for providing pump light to the fiber. A power measurement circuit is provided for measuring the output power of the amplifier and a disable circuit is controlled to disable the pump source in response to the output power of an output pulse exceeding a predetermined limit. In this way, power surges can be avoided. Preferably, the predetermined limit takes into account the duration of the output pulse, so that the disable circuit can be tailored to pulse power versus pulse duration standardised limits.