Abstract: A semiconductor laser device with a resonator containing an active region for laser oscillating operation is disclosed which comprises a third-order diffraction grating with a periodic corrugation for producing feedback of laser light, the corrugation being of substantially rectangular shape, wherein the ratio of the width of each convex portion of the corrugation to the periodicity of the corrugation is in the range of 0.20 to 0.25, 0.40 to 0.60, or 0.70 to 0.95.

Abstract: A multi-layered dielectric film that is coated on the end surfaces or other surfaces of optical products, wherein said multi-layered dielectric film is composed of alternate layers consisting of two kinds of dielectric layer, one of which is a first dielectric layer of TiO.sub.2 or ZnS with a high refractive index n.sub.1 and the other of which is a second dielectric layer of Al.sub.2 O.sub.3 with a low refractive index n.sub.2.

Abstract: A semiconductor laser device comprising a double-heterostructure that is composed of an active layer and a pair of cladding layers sandwiching the active layer therebetween, and a stripe structure that is disposed on the double-heterostructure, the stripe structure being composed of a current blocking layer with a conductive type different from that of the adjacent cladding layer, and the current blocking layer having a striped groove constituting a current path, wherein the cladding layer that contacts the current blocking layer is composed of an In.sub.1-s Ga.sub.s P.sub.1-t As.sub.t crystal material (wherein 0.51.ltoreq.s.ltoreq.1, 0.ltoreq.t.ltoreq.1 and s=2.04t-1.04).