This thesis mainly focuses on the experimental investigations of electromagnetically induced transparency (EIT) related phenomena in various systems involving multilevel atoms inside an optical ring cavity. Semiclassical methods, e.g. density-matrix equations, are used through out this thesis to simulate the experimental results. First, the cavity transmission spectrum can be significantly modified when multilevel atoms are placed inside an optical ring cavity. Such coupled atom-cavity systems are well explained by the intracavity dispersion/absorption properties. Specifically, three-level lambda-type, four-level N-type and double-lambda-type atoms inside an optical ring cavity are investigated by examining their cavity transmission spectra. Second, optical multistability (OM) has been demonstrated with EIT atoms inside an optical ring cavity. Such OM has been utilized to realize a controllable optical multistate switch, which can be modeled as a triple-well system. Third, self-Kerr nonlinearities of multilevel atoms are measured in an optical ring cavity by scanning the cavity length. Fourth, bright Stokes and anti-Stokes fields are generated simultaneously in a doubly-resonant atomic optical parametric oscillator (AOPO). In addition, vacuum-induced absorption and noise correlations are studied in the AOPO system. Last, a theoretical model is proposed to realize parity-time (PT) symmetry in a four-level N-type atomic system by spatially modifying the complex refractive index in free space. Moreover, the experimental progress is made to observe discrete diffraction pattern in an optically induced lattice by interfering with plane waves in a coherent atomic medium
