Magnetless Microwave Circulators Based on Spatiotemporally Modulated Rings of Coupled Resonators

Nonreciprocal components are ubiquitous in electronic and optical systems. To date, the use of magneto-optical materials has been the prevailing method to achieve nonreciprocity. However, magnetic-based devices are accompanied by several drawbacks, such as the requirement of bulky biasing devices and their incompatibility with semiconductor technology, which make their integration challenging. Recently, strong magnetless nonreciprocity was demonstrated in spatiotemporally modulated coupled-resonator networks as a result of an effective spin imparted to the structure by an RF signal. These structures can be easily integrated, and they potentially exhibit good power and noise performance, as any parametric device. Here, we develop an analytical theory for such devices, which allows determining the conditions for designing them with optimum characteristics, and present two designs based on lumped- and distributed-element circuits for applications at the very high-frequency and wireless-communications bands, respectively. The circulators exhibit large isolation and low insertion loss within reasonable modulation requirements. Furthermore, they can be realized with a modulation frequency substantially lower than the input frequency, significantly simplifying the design. Measurements for the lumped-element design are provided and show good agreement with theory and full-wave simulations. The nonlinear characteristics of the presented designs are also studied, and possible ways to reduce nonlinear distortion by increasing the static bias of the varactors or using advanced varactor topologies are explored.