Patent ID: 8811447

Claim:
A semiconductor laser for outputting laser light from an output facet of a waveguide having an index waveguide structure, via a lens system, wherein the waveguide is formed by an active layer, the active layer of the waveguide includes, in order from a rear facet opposite to the output facet, a first narrow portion configured to filter a higher-order waveguide mode, a wide portion that is wider than the first narrow portion, a second narrow portion narrower than the wide portion, a first tapered portion formed between the first narrow portion and the wide portion, the first tapered portion expanding toward the wide portion, and a second tapered portion formed between the wide portion and the second narrow portion, the second tapered portion narrowing toward the second narrow portion, and each of the first narrow portion, the wide portion, and the second narrow potion has a uniform width, the width of the second narrow portion of the waveguide is in a range of 2.0 micrometers to 5.0 micrometers, an inclination angle θ represented by θ=arctan[(ΔW/2)/Lt 2 ] is 0.6 degrees or less, where ΔW is a difference between the width of the wide portion and the width of the second narrow portion, and Lt 2 is a length of the second tapered portion, the inclination angle θ and Ln 2 , which is a length of the second narrow portion in micrometers, satisfy Ln 2≧106θ−0.00681 when 0.47<θ<0.60, Ln 2≧317θ−100 when 0.32<θ<0.47, and Ln 2≧0 when θ≦0.32, an upper limit of Ln 2 is 200 micrometers in all ranges of the inclination angle θ, and a length of the first narrow portion is equal to or longer than 30% of a cavity length between the output facet and the rear facet and equal to or shorter than 65% of the cavity length.