Fig. 2: Different PT-symmetric phases according to the theory.
From: Observation of parity-time symmetry in microwave photonics
a A normal single-loop OEO oscillates without a high-Q filter. All of the longitudinal modes with a positive net gain will oscillate. b Two coupled OEOs with same loop length oscillate without high-Q filters. All of the longitudinal modes with a positive net gain will oscillate. c Two coupled PT-symmetric OEOs oscillate in the PT-broken phase. The gain and loss of each loop are balanced. All of the losses overcompensate the gain for all of the longitudinal modes except for one mode by adjusting the gain and loss in the two loops. Then, a single-mode will emerge at the longitudinal mode with the highest gain, whereas other modes will be suppressed. d Two coupled OEOs degenerate to a single-loop OEO. The gain and loss are no longer balanced, and the loss exceeds the gain. Parity and time are no longer symmetric. The loss loop does not contribute to the oscillation
