  We study entanglement of the cavity modes in a double-cavity optomechanical
system in strong-coupling regime. The system consists of two optomechanical
systems coupled by a single photon hopping between them. With the radiation
pressure of the photon, entanglement of the cavity modes can be generated. The
concurrence between the cavity modes is at least twice larger than that between
the mechanical modes. Moreover, when we change the ratio between coupling
strength and resonant frequency of mechanical modes, the entanglement in cavity
and mechanical modes are influenced differently.
