Active photonic systems with gain and loss are widely used for optical signal processing and amplification. Whilst generally such systems are described by non-Hermitian mathematical models, there is a particular type of pseudo-hermitian  systems attracting strong interest, with an important example of parity-time (PT) systems, due to a possibility of exact compensation between specially distributed gain and loss. The structure symmetry imposes specific mode geometry, which can facilitate non-reciprocal transmission, selective mode suppression or amplification, optical switching through phase transition, with applications in lasers and nonlinear optics.

In this talk, I will discuss different PT-symmetric and pseudo-Hermitian geometries such as dimers, trimers, chains of conservative waveguides with embedded non-Hermitian defects, and fiber ring resonators. I will outline their spectral properties and associated benefits for signal amplification, filtering, and lasing. I  will also consider a nonlinear bottle-type micro resonator in application to a frequency comb generation and signal transmission through reflectionless potentials.