Monolithic multi-wavelength semiconductor laser unit

A semiconductor laser unit comprising:

• a substrate;
• a first laser emitting a light of a first wavelength, disposed on said substrate, having:
• a bulk active layer having thickness larger than the quantum-mechanical-maximum thickness, configured such that electrons in the bulk active layer can be treated by classical theories, but not larger than 0.1 μm, the quantum-mechanical-maximum thickness being so defined that only under the maximum thickness the electrons are allowed to be treated by quantum-mechanical analysis;
• a first band gap discontinuity relaxation layer directly disposed under said bulk active layer, constituted by a compound semiconductor in which a band gap is larger than said bulk active layer;
• a first cladding layer of a first conductivity type having a band gap larger than said first band gap discontinuity relaxation layer, directly contacting a lower portion of said first band gap discontinuity relaxation layer;
• a second band gap discontinuity relaxation layer directly disposed on said bulk active layer, constituted by a compound semiconductor in which a band gap is larger than said bulk active layer;
• a first cladding layer of a second conductivity type opposite to the first conductivity type, having a band gap larger than said second band gap discontinuity relaxation layer, directly contacting an upper portion of said second band gap discontinuity relaxation layer;
• and
• a second laser disposed on said substrate and isolated from said first laser, having a multi quantum well active layer in which the electrons are treated by the quantum-mechanical analysis, comprising a plurality of potential well structure composed of a quantum well layer and barrier layer adjacent to the quantum well layer, the second laser emitting a light of a second wavelength different from said first wavelength.