  We propose a scheme in which an arbitrary incidence can be made perfectly
reflected/transmitted if a phase setup is adjusted under a specific condition.
We analyze the intracavity field variation as well as the output field with
changing closed-loop phase of atomic system and relative phase of input probe
beams. And we obtain the condition for perfect transmitter or reflector. By
adjusting two phase setups, the medium absorption and light interference can be
controlled so that photon escape from cavity can be modulated, thus the
intensity switching based on phase control can be realized. Then based on the
transmission/reflection analysis, total absorption of this system can be
investigated. Therefore our scheme can be used as an absorption interferometer
to explore the optical absorption in some complicated system. The phase delay
dependent on phi_1 or phi_2 in output light intensity can be applied in the
realization of quantum phase gate and subtle wave filter. And based on this
scheme, we implement the state transfer between perfect transmitter/reflector
and non-perfect coherent photon absorber via relative-phase modulation.
