The semiconductor optical amplifier device includes a plurality of active units. Each active unit includes an active stripe structure of an optical amplifying medium and a current circuit configured to inject current into the corresponding active stripe structure. Each active stripe structure extends from an input end to an output end. An optical splitter device is configured to split an incoming signal light and for distributing corresponding parts of the incoming signal light into the different input ends of the active stripe structures. The optical splitter device is configured to supply each active stripe structure with the same signals.