Optical mode expander

A semiconductor optical amplifier comprising an active gain region of the (In, Ga) (As, N) system is proposed, together with the use of (Ga, In) (As, N) as the base material for the fabrication of an SOA, and semiconductor optical amplifier comprising (Ga, In)(As, N) as the base material. The N content of the (In, Ga)(As, N) can be varied along a dimension of the active region in the direction of propagation of light signals therein, to create a varying bandgap such as for mode expanders. The active region can be supplied by a source of electrical bias which is applied in segments along the dimension of the active region, the segments being capable of independent variation. This should allow channel equalisation of WDM signals to be performed dynamically. This scheme could also be used to equalise device parameters such as differential gain, saturation output power and linewidth enhancement factor across the amplification bandwidth.