c for about 30 min to diffuse it to the depth to penetrate the undoped buffer layer 2 the doping concentration after the diffusion is about 5 u00d7 10 17 cm 3 then sio2 insulating film is formed on the buffer layer 2 on the substrate 1 followed by patterning it in the form that widths of a mask 4 and an interval portion 4 39 are as shown in fig4 a different in the longitudinal direction thus the widths of the mask 4 and interval portion 4 39 are varied at 50 u03bcm from a diffusion edge of the impurity diffusion region 3 i e the widths of the mask 4 at a tapered region 6 and ld region 5 are 10 u03bcm and 50 u03bcm respectively and the widths of the interval portion at the tapered region 6 and ld region 5 are 0 5 u03bcm and 0 8 u03bcm respectively then over the substrate 1 formed such mask 4 an undoped inp spacer layer 7 with a thickness of 50 nm a tapered waveguide layer 8 an active layer 9 and a n inp cladding layer 10 with a thickness of 0 1 u03bcm doping concentration of 2 u00d7 10 17 cm 3 are in turn grown herein the tapered waveguide layer 8 and active layer 9 may be continuously grown by the selective growth technique the multilayer structure in the active layer 9 as shown in fig5 consists of a unstrained ingaasp sch layer 21 with a thickness of 50 u03bcm with a pl photoluminescence wavelength composition of 1 1 u03bcm 1 0 compressively strained.