A semiconductor laser comprises two optical cavities, each comprising an optical waveguide bounded by two partially reflecting elements. The two optical waveguides are disposed on a substrate to form a substantially V-shaped geometry with substantially no cross-coupling at the open end and a predetermined cross-coupling at the closed end for achieving an optimal single-mode selectivity of the laser. The first cavity has a length such that its resonant wavelengths correspond to a set of discrete operating channels. The second cavity has a slightly different length so that only one resonant wavelength coincides with one of the resonant wavelengths of the first cavity over the operating spectral window. The lasing action occurs at the common resonant wavelength. In operation, at least a portion of the optical waveguide in each of the first and the second cavities are forward biased to provide substantially equal round-trip optical gains. The second cavity is tuned by varying the effective refractive index of a portion of the waveguide through an electrical means, resulting in wavelength switching among the set of discrete operating wavelengths as determined by the first cavity.