Lasers for optical broadband communication systems should have excellent frequency selectivity and modal stability. DFB lasers have low lasing frequency shift during high speed current modulation. In this paper, when a refractive index grating and a gain grating are simultaneously present in a DFB laser having a wavelength of 1.55 μm, the dielectric film is coated so that reflection does not occur on the right mirror surface, so that =0. For the first mode, which requires a minimum gain at the threshold, the beam distribution of the oscillation mode in the longitudinal direction and the radiated power ratio were analyzed and compared for the cases of the phase of = and . If the phase of = , in order to obtain a low threshold current and high frequency stability, should be greater than 8. In the case of the phase of =, for low threshold current, is necessary to be 1.0, where the oscillation frequency coincides with the lattice frequency. DFB lasers with an anti-reflection coated mirror have excellent mode selectivity than 1.55um DFB lasers with two mirror facets.