  We investigate the transmission spectrum of an optical cavity coupled with
four-level atoms. Multiple normal-mode splitting peaks of the strongly coupled
atom-cavity system are obtained as single cavity mode couples three separated
atomic transitions simultaneously. We employ a confocal optical cavity and an
ensemble of cold Rb-85 atoms for cavity quantum electrodynamics. We observe
four normal-mode splitting peaks of the cavity transmission in the strong
coupling regime due to collective enhancement of the atom-cavity coupling
strength by ensemble of cold atoms. The experimental observations are
consistent with the theoretical analysis. The multiple normal-mode excitation
in the strongly coupled multi-level atom-cavity system may lead to practical
application for realizing multi-channel all-optical devices.
