Distributed-feedback (DFB) lasers are semiconductor lasers with a single lasing mode thanks to a cavity design that suppress all other modes (called residual modes) by reducing their quality factors. Optical injection into a laser can lead to injection locking (i.e., synchronization), but also to unstable and ultra-fast chaotic dynamics. While optical injection into the main lasing mode of a DFB-laser has been extensively studied, injection into a residual mode remains unexplored. The objective is to study the dynamics caused by injection into a residual mode and to explore the possibility to create chaotic dynamics with increased bandwidth in view of applications like chaos cryptography. A systematic experimental investigation of the laser dynamics will be complemented by the development of a theoretical model and numerical simulations.