Enhanced modulation bandwidth in complex cavity lasers can be obtained by exploiting the coupling between two adjacent cavity modes having proper spectral separation. We investigate the conditions that a complex cavity injection laser device must satisfy in order to take advantage of this effect to significantly increase its modulation bandwidth and we present a general design rule. According to the proposed guidelines, a structure is designed and then dynamically analyzed using a finite-difference time-domain program to show the validity of the proposed method. As an example the proposed guidelines have been adapted also to the design of a distributed Bragg reflector laser and extended modulation was found.