Enhancing the Ultimate Sensitivity of Optical Sensors Based on Passive Ring Resonators Using a PT-Symmetric System of Optically Coupled Active and Closed-Loop Passive Waveguides

Abstract

This study investigates methods for enhancing the ultimate sensitivity of sensors based on passive ring resonators by utilizing the properties of PT-symmetry. A simple PT-symmetric system is considered, consisting of two parallel waveguides—one providing optical gain and the other introducing optical loss—where the passive waveguide forms part of a ring resonator coupled to the system. This configuration enables an increase in the ultimate sensitivity of sensors based on passive ring resonators. The system can operate in several regimes. The first regime is realized when the gain of the active waveguide does not exceed the coupling coefficient between the parallel waveguides. This regime makes it possible to increase the ultimate sensitivity by more than an order of magnitude—specifically, by a factor of 12 in the example analyzed. The second regime is obtained at an optimal ratio of system parameters. Theoretically, this regime enables an almost unlimited increase in the slope of the transmission coefficient of the system near frequencies lying between the resonator eigenfrequencies, thereby increasing the ultimate sensitivity. However, the amplification of the light passing through the system also increases almost without limit, which may hinder the practical implementation of this regime. Nevertheless, in practice, the use of the second regime can be appropriate when operating the sensor near the resonator eigenfrequencies, where the system does not amplify the transmitted light. In the example in question, this enabled a 76-fold increase in the ultimate sensitivity. The influence of the system parameters on the adjustment of these regimes is also analyzed.