AbstractSemiconductor lasers which oscillate at a single frequency have been developed. DFB lasers represent such lasers. Since coherent optical communication has many merits, e.g., longer distance between repeaters, many applications of such lasers have been investigated.In this study, the frequency characteristics of these lasers have been evaluated. In a conventional DFB laser, the frequency characteristic degrades when frequency modulation is carried out around several Megahertz.To solve this problem, a method to control the spatial hole‐burning phenomenon was investigated using a multielectrode DFB laser with a long cavity. As a result, a flat frequency modulation characteristic at frequencies from 100 kHz to 15 GHz was obtained and it was confirmed that this DFB laser was suitable for coherent optical communication system