Optical semiconductor device for changing a beam diameter

摘要

An optical semiconductor device has a greatly reduced transmission constant even when a reduction rate in the thickness of a core layer is small so that a relatively high beam-diameter increasing efficiency is obtained. The optical semiconductor device has a multilayered structure with a first end surface perpendicular to an optical axis of the optical semiconductor device and a second end surface opposite to the first end surface. A core layer has a first refractive index. A lower cladding layer is provided on a lower side of the core layer, the lower cladding layer having a second refractive index. An upper cladding layer is provided on an upper side of the core layer, the upper cladding layer having the second refractive index. A transmission constant reduction enhancing layer is provided between the lower cladding layer and the upper cladding layer, the transmission constant reduction enhancing layer having a third refractive index less than the first refractive index and the second refractive index. The optical semiconductor device includes a first area and a second area, a portion of the core layer in the first area having a uniform thickness, a portion of the core layer in the second area having a thickness gradually decreasing toward the first end surface of the optical semiconductor device.