  We demonstrate the possibility of optical nonreciprocal response in a
three-mode optomechanical system where one mechanical mode is optomechanically
coupled to two linearly coupled optical modes simultaneously. The optical
nonreciprocal behavior is induced by the phase difference between the two
optomechanical coupling rates which breaks the time-reversal symmetry of the
three-mode optomechanical system. Moreover, the three-mode optomechanical
system can also be used as a three-port circulator for two optical and one
mechanical modes, which we refer to as optomechanical circulator.
