Abstract: A superluminescent diode includes a substrate; a double heterojunction structure including a first conductivity type cladding layer, an undoped or first or second conductivity type active layer, and a second conductivity type cladding layer disposed on said substrate; a first conductivity type cap layer disposed on the second conductivity type cladding layer; and a second conductivity type stripe-shaped diffusion region penetrating the cap layer and reaching into the second conductivity type cladding layer. Current is injected into the active layer through the diffusion region. The stripe-shaped diffusion region extends from a front facet toward but not reaching a rear facet and is inclined an angle in a range from 3 to 20 degrees with respect to the front facet.