This paper reports a novel optoelectronic nonlinear oscillator, which produces a chaotically phase-modulated light beam over a bandwidth greater than 5 GHz. The setup is intended for high-speed chaos-based secure communications. The nonlinear process is performed by the transient response of a fiber interferometer. The flexibility in the definition of such an all-optical nonlinearity features a potentially high-dimensional key space of the encryption process. Modeling, simulations, and experimental results of the dynamical behaviors are reported.