An optical amplifier for amplifying an optical signal is disclosed wherein a four port coupler provides multi-functionality. A pump light source is coupled to a first port, a transmission signal to be amplified is coupled to a second port, a multiplexed signal is received on the third port and a monitor signal is provided on the fourth port for monitoring one of the pump or transmission signal. Conveniently a detector is optically coupled with the fourth port to provide a feedback signal.