Coherent feedback induced transparency.

We propose a light transparency effect induced by coherent feedback. By studying a system comprising a linear optical cavity controlled by a linear coherent feedback loop, we show that the optical signal field passing through the system cavity exhibits novel transparency behaviors. Unidirectional coupling between the system and its feedback control loop enables the group velocity and transmission rate to be tuned separately, thus maintaining the unity transmission rate when the group velocity is significantly suppressed. Furthermore, we demonstrate that simply applying a certain phase shift to the output of the system cavity and feeding it back into the system can induce perfect transmission. Our proposal offers a simple and effective way to control light transmission and group velocity using only linear optics elements.