Travelling-Wave Mach-Zehnder Modulators Work as Optical Isolators

Optical isolators, which allow light to pass in one direction but block light in the opposite direction, are one essential element in a laser or an optical amplifier system to protect the system from back reflection which can disturb the lasing/amplification operation. The direction-dependent light propagation in an isolator requires the breaking of Lorentz reciprocity, which holds for linear and time-independent devices with symmetric permittivity and permeability. In order to achieve optical isolation, current bulk and fiber optical systems utilize magneto-optical materials with asymmetric magnetic permeability to break Lorentz reciprocity. Although some waveguide-based isolators have been demonstrated by incorporating with such materials, it is difficult to integrate them with other silicon and III-V photonic integrated circuits (PICs). Isolators without the use of magneto-optical materials and with easy on-chip integration capability would be highly demanded. This has become a long-standing challenge in integrated optics. In recent years, there are increasing interests in this field to achieve isolation based on nonlinear optical effects and time-varying modulation methods. The use of nonlinear optics leads to unwanted power-dependent effects. Most of the previously demonstrated actively modulated isolators still suffer from either low isolation, high insertion loss, narrow operational bandwidth, or unwanted optical frequency shift. In this paper, we demonstrate that a traditional travelling-wave modulator can effectively function as an optical isolator, when driven under a prescribed modulation condition. By using an off-shelve lithium niobate modulator, we achieve more than 12.5 dB isolation over an 11.3-THz bandwidth at telecommunication wavelengths, using only a single radio-frequency drive signal. We also demonstrate that the proposed isolator can be functional in a laser system to effectively suppress strong back reflections. Our simple but efficient architecture provides a practical solution to non-reciprocal light routing in PICs.