Broadband optical nonreciprocity by emulation of nonlinear non-Hermitian time-asymmetric loop

Broadband and power-stable optical nonreciprocity is highly desirable for practical implementation of optical isolation of coherent sources and receivers in high-speed data-processing systems. Here, we experimentally demonstrate a 2 x 2 waveguide-coupler system that produces broadband optical nonreciprocity based on simple emulation of nonlinear non-Hermitian time-asymmetric loop. The coupler system consists of five lumped optical components including two linear attenuators and two saturable amplifiers. Under moderate operation conditions which are routinely obtainable with commercial erbium-doped fiber amplifiers, we obtain 20 dB isolation ratio persisting over the entire gain bandwidth of the amplifier and over a wide incident-intensity range from 0.1 mW to 10 kW. The isolation ratio can be enhanced up to 70 dB with the forward transmission efficiency adjustable at any desired stable point by tuning amplifier's internal dissipation in principle. Therefore, our result strongly encourages further experimental development of broadband on-chip optical isolators which have been elusive so far. In this work, the authors demonstrate a 2 x 2 waveguide-coupler system that produces broadband optical nonreciprocity based on the emulation of nonlinear non-Hermitian time-asymmetric loop. Under operation conditions functional for erbium-doped fiber amplifiers, they obtain 20 dB isolation ratio persisting over the entire gain bandwidth of the amplifier and a wide incident intensity range.