The ability of chaotic carriers to synchronize in optical chaotic communication systems is studied experimentally under a spectral domain analysis. Synchronization of high-frequency components up to tens of gigahertz of the chaotic carriers can thus be evaluated. On the contrary, the traditional method of measuring the synchronization through chaotic carrier time-traces is constrained by the limited bandwidth of the real-time oscilloscopes used for such measurements, excluding the contribution of the high-frequency components of the chaotic carriers to the synchronization characterization of the system.