Cavity optomechanics becomes a promising field in quantum and nano technologies. Motivated by the op-tomechancial experiment with the membrane located in a high-finesse optical cavity and theoretical treatment on two membranes cavity optomechanics, we here study the optomechanical interaction of the system consisting of triple mem-branes within an optical cavity. The increase of membranes will increase the normal modes of the cavity and mechanical fields, and thus enrich the forms of optomechanical interaction. Firstly, we use the transfer matrix and resonance trans-mission methods to obtain the dispersion relation between the eigen-frequencies of the optical modes and the mechanical motions. Owing to the existence of triple mechanical membranes, the system possesses different forms of collective mechanical motion, and here we focus on the center-of-mass (COM) motion and relative motion of the equally placed membranes. The numerical solutions of the dispersion relation show that the optical eigenmodes are comprised of a group of closely spaced avoided-crossing quaternion of wave numbers, which arise from the transmission and reflection of the optical field at the membranes and the tunneling couplings between subcavity modes. Moreover, the change of each eigen wave number along each form of the mechanical motion is different, which implies the different forms of optomechanical coupling between eigenmodes and mechanical motions. Then, to achieve the explicit expressions of the optomechanical coupling, it is sufficient to use the perturbation method under the equilibrium condition of the system, where the amplitude of mechanical motion is much smaller than the optical wavelength. With using the implicit function differentiation theorem, the optomechanical coupling strengths between the four optical modes and the COM and relative mechanical motions are obtained respectively. We find that the strong quadratic optomechanical coupling between the optical modes and COM motion can be achieved, and the linear and quadratic couplings between the optical modes and relative motion can both be realized. By tuning the laser to pump different optical modes, we can choose either the linear or the quadratic coupling to the relative motion. Our method is universal to multi-membrane system, and the results may provide some references to theoretical and experimental investigations on the multi-membrane cavity optomechanics.
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