I show that in a composite cavity and two-level absorber system coupled by two coherent input fields, the output light fields from the cavity are asymmetrical and can be controlled by the relative phase between the two input fields. Under appropriate conditions, when the two input fields have a particular phase difference, one of the cavity output field increases linearly with the input field while the other output field vanishes. Thus, the CQED system functions like an optical transistor and the two output fields can be switched on or off alternatively by varying the relative phase between the input fields.
