A semiconductor optical amplifier with increased gain stability is described, comprising a signal waveguide that guides an optical signal along a signal path that intersects with one or more transverse laser cavities. A gain medium of the signal waveguide is integral with a gain medium of the transverse laser cavities at regions of intersection between the signal waveguide and the transverse laser cavities, resulting in gain-stabilized operation when the transverse laser cavities are biased above threshold. Successive transverse laser cavities are separated along the signal path by connecting zones having a higher electrical resistivity than the transverse laser cavities, providing a measure of electrical isolation between the transverse laser cavities and reducing parasitic lasing modes among them. The transverse laser cavities may be provided with separate bias currents for precise control of gain along the signal path. In another preferred embodiment, the transverse laser cavities are segmented in the direction of the lasing field into multiple segments with separate electrical contacts. For certain applications, one or more of the transverse laser cavities may be kept dark near its end mirrors to impede lasing and causing the gain medium to have a nonlinear gain profile for that transverse laser cavity, while other transverse laser cavities along the signal path maintain linear gain profiles. Preferred embodiments with reduced amplified spontaneous emission (ASE) noise are also described.