The linewidth enhancement factor is the crucial design parameter with others, such as gain and refractive index changes for semiconductor laser diodes (LD). The changes of characteristics are measured according to the variation of the thickness of SCH (Separate Confinement Heterostructure, 1.24 micrometer p-InGaAsP). The gain spectra were obtained from the spontaneous emission for three LD's. The threshold current; Ith were about 15 mA. The optical field profiles for various SCH thickness was calculated from the effective index and transfer matrix method and the corresponding optical confinement factors are compared with the variation of measured (alpha) . It is meaningful to find the SCH thickness, SCH(Gamma max), of maximum confinement (Gamma) for given LD structure. Although the improvement of the linewidth enhancement factor ((alpha) ) in thicker SCH (greater than SCH(Gamma max)) has been known, it has not been measured for thin SCH (less than SCH(Gamma max)). Here, we compare the measured linewidth enhancement factors of three DFB-LD's with different SCH thickness of 500, 750, and 1000 angstrom, which are all smaller than the SCH(Gamma max) for given structure. It is shown that (alpha) would be improved as SCH thickness increases up to 1500 angstrom (approximately SCH(Gamma max)) for given structure if other design parameters permit.