Cover story: Huang HJ, Balčytis A, Dubey A, Boes A, Nguyen TG et al. Spatio-temporal isolator in lithium niobate on insulator. Opto-Electron Sci 2, 220022 (2023).

Integrated photonics is making strides towards hosting an increasing range of functionalities on a chip. Examples include information processing and computation as well as optical sensing and ranging applications. This has spurred advances in integrated laser light sources, needed for photonic chips to become truly autonomous devices. Thus, on-chip isolation likewise becomes important for suppressing feedback detrimental to their operation. Nonreciprocal optical devices can be realized using three methods: magnetic biasing, optical nonlinearity, and spatiotemporal modulation. Magnetic biasing is inherently broadband, but requires lossy magneto-optical materials. Nonlinear non-reciprocal devices are achievable monolithically in certain materials, yet their operation is complicated by dependence on input power. Conversely, isolators that leverage spatiotemporal modulation have no such power scaling issues, and can readily be integrated monolithically, particularly on platforms with excellent electro-optical characteristics, such as lithium niobate on insulator (LNOI). In this contribution, nonreciprocal operation is achieved by using spatiotemporal modulation of two cascaded travelling wave phase shifters. The microwave signal applied to the modulators and delay line ensures that their effect on forward-propagating light cancels out so that its spectral signature remains unchanged. However, reverse-propagating optical power is spectrally dispersed to the sidebands, which are then suppressed by a ring resonator filter, enabling an optical isolation of 27 dB.