High-power semiconductor laser device including resistance reduction layer which has intermediate energy gap

A semiconductor laser device comprising:

• a substrate made of a GaAs material having a first energy gap;
• a lower cladding layer formed on said substrate, and made of a material having a second energy gap;
• a lower optical waveguide layer formed on said lower cladding layer;
• an active layer formed on said lower optical waveguide layer;
• an upper optical waveguide layer formed on said active layer;
• an upper cladding layer formed on said upper optical waveguide layer, and made of a material having a third energy gap;
• a contact layer formed on said upper cladding layer, and made of a material having a fourth energy gap;
• and
• at least one of first and second resistance reduction layers, where said first resistance reduction layer is arranged between said substrate and said lower cladding layer, and made of an InGaAsP material having a fifth energy gap which is greater than said first energy gap, and smaller than said second energy gap, and said second resistance reduction layer is arranged between said upper cladding layer and said contact layer, and made of an InGaAsP material having a sixth energy gap which is greater than said fourth energy gap, and smaller than said third energy gap.