An optical amplifier comprising a signal semiconductor optical amplifier (SOA) (1) and a control SOA (2) in an optical circuit comprising a signal path (4) passing through the signal SOA (1) and a laser cavity containing both the signal SOA (1) and the control SOA (2). The control SOA (2) may be arranged outside the laser cavity so that it does not amplify a signal in the signal path (4). Alternatively, the control SOA (2) is arranged in the signal path (4), but has insignificant gain in the predetermined signal band (3) so that it does not amplify a signal passing along the signal path (4). The laser cavity clamps the total gain of the signal SOA (1) and the control SOA (2). In use, the bias current supplied to the control SOA (2) is varied to control the clamped-gain of the signal SOA (1). Therefore the gain of a signal passing along the signal path (4) is clamped to a level which may be varied by control of the control SOA (2). In an alternative type of embodiment, a similar type of control is achieved in a structure integrated in a single semiconductor chip. In particular a waveguide (56) extends along a signal active layer (52) and a laser cavity arranged perpendicularly to the layered structure of the semiconductor chip contains both the signal active layer (52) and a control active layer (59) which corresponds to the control SOA (2).