Semiconductor laser device having strain buffer layer between compressive-strain quantum well layer and tensile-strain barrier layer

A semiconductor laser device comprising an active region which includes alternating layers of at least one quantum well layer and a plurality of barrier layers, where two of the plurality of barrier layers are outermost layers of said alternating layers, each of said at least one quantum well layer has a first thickness da and a compressive strain Δa, and each of said plurality of barrier layers has a second thickness db and a tensile strain Δb;

• in said active region, a strain buffer layer is formed between each of said at least one quantum well layer and each of two of said plurality of barrier layers adjacent to said each of said at least one quantum well layer, where each strain buffer layer has a third thickness dr and an intermediate strain Δr which is between said compressive strain Δa and said tensile strain Δb;
• and
• said first thickness da, said compressive strain Δa, said second thickness db, said tensile strain Δb, said third thickness dr, and said intermediate strain Δr satisfy a relationship,
• 0≦N·Δa·da+(N+1)·Δb·db+2N·Δr·dr≦0.08 nm,
• where N is the number of said at least one quantum well layer.