Exceptional Points in Fabry–Perot Cavities with Spatially Distributed Gain and Loss

We have studied the optical modes in Fabry–Perot (FP) cavities with spatially distributed gain and loss. Exceptional points (EPs) are observed not only in the parity-time (PT) symmetric FP cavity but also in these cavities with specially designed gain-loss distributions. Such phenomena result from the interference between the reflected and transmitted waves at the interface of the gain-loss regions, which cannot be explained by the dual-mode coupling theory that is widely used in the description of PT-symmetric coupled-cavity systems. The optical waves in the FP cavity traveling in the forward and backward directions have extremely different intensities at the gain-loss interface, and hence the interference will greatly affect the transmission of the weaker one and lead to unique mode properties such as EPs in the FP cavity. The mode properties are sensitive to the gain-loss distribution, but the split of modal loss is a universal phenomenon and can be used for lasing mode control.