Nonreciprocal optical devices, allowing transmission of light with different efficiencies in opposite directions, are key elements for modern optical communication and even quantum information technologies, but elusive to be integrated on a chip to date. Such devices exploring nonlinearity can. Optical nonreciprocity is of fundamental importance for signal processing in modern optical communication systems. An all-fiber device, containing two mutually coupled Fabry-Perot (FP) resonators to realize broken parity-time (PT) symmetry, is demonstrated to achieve nonreciprocal light. A reflective all-fiber optical current transformer based on a spatial non-reciprocal phase modulation technique is investigated by theoretical analysis and experimental measurement. They are technically related to Faraday isolators, and on a broader scale similar to electronic circulators. Typically, a circulator has three or four optical ports (inputs / outputs). Lightwave systems, including fiber optic and integrated optic, are becoming more and more complex, new function blocks ( or components) and networking strategies are very important for future highly integrated lightwave circuits.
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