A multi-wavelength semiconductor laser is formed by monolithically integrating a plurality of laser diodes (1, 2) with at least one isolator section (3) and a coupler (4), which couples the different emission wavelengths ₁, ₂ into one output port (5). The isolator section can be either a light absorptive type or wavelength selective type, including a Bragg grating type isolator or a photonic bandgap crystal type isolator. The coupler is preferably a Y-junction coupler, but can also be a multi-branch waveguide coupler or a waveguide directional coupler.