carriers injected into the p type gaas active layer by the current flow recombine and emit light as the level of the injected current is increased stimulated emission begins and then the laser oscillation begins part of the laser light is guided into parts of the n type al 0 5 ga 0 5 as first cladding layer 2 the p type al 0 5 ga 0 5 as second cladding layer 4 and the p type al 0 5 ga 0 5 as third cladding layer 8 as shown in fig3 d the width of the groove 10 which is formed through the first second and third current blocking layers 5 6 and 7 and which is filled with the material of the third cladding layer 8 changes alternately between w 1 smaller width of fig3 b and w 2 larger width of fig3 c showing the cross section along y y in fig3 d the width changes with a period of u03bb because of this arrangement only the wavelength u03bb 0 expressed by the equation 1 is selected and the single longitudinal mode of oscillations results next referring to fig4 a through 4g the method of making the gain coupled type dfb semiconductor laser device according to the present invention shown in fig3 a through 3d is described 1 as shown in fig4 a by use of an epitaxial deposition technique there are successively deposited the n type al 0 5 ga 0 5 as first cladding layer 2 on the n type gaas substrate 1 the p type al 0 15 ga 0 85 as active layer 3 on the first cladding layer 2 the p type al 0 5 ga 0 5 as second cladding layer 4 on the active layer 3 the.