Monolithic magneto-optical oxide thin films for on-chip optical isolation
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The challenge of on-chip isolation Isolators, as the photonics counterpart of electrical diodes, play a critical role in photonic integrated circuits (PICs) by preventing harmful feedback between different parts of the circuit. For example, an isolator is often used to protect a laser source from destabilizing feedback or damage from back-reflected light. The need for on-chip isolation becomes imperative as the level of photonic integration continues to scale, since unwanted reflections between the many integrated devices in a PIC are common and can be highly disruptive in a complex optical network. Fundamentally, optical isolation requires breaking the time-reversal symmetry of light propagation. Such optical nonreciprocity can be realized by using magneto-optical (MO) effects, or nonmagnetic approaches such as indirect interband photonic transitions1,2 and nonlinear optics.3 Traditional optical isolators used in free-space and fiber-optic systems are almost exclusively based on Faraday rotation in MO crystals, in which a magnetic field is applied along the light propagation direction to induce circular birefringence and polarization rotation. These Faraday rotation isolators typically feature an isolation ratio (the transmittance contrast between forward
and backward propagating light) of the order of 30 to 40 dB, an insertion loss (loss of the forward propagating light passing through the isolator) below 1 dB, and an operation bandwidth exceeding 50 nm in the near-infrared (NIR) telecommunication window around 1.5-µm wavelength. Despite the high performance of these free-space isolators, they are bulky discrete devices not suitable for planar on-chip integration. In addition, at present, the price of a discrete isolator ranges anywhere from USD$30 up to more than USD$1,000, a prohibitive cost for large-scale PICs. On-chip magneto-optical isolation can be achieved by Faraday rotation,4–6 nonreciprocal phase shift (NRPS),7 nonreciprocal loss or gain,8 and nonreciprocal mode conversion.9–11 The Faraday rotator configuration used in free-space isolators is sensitive to waveguide birefringence and is not suitable for on-chip optical isolation,5,12 unless a quasi-phase match scheme is implemented (Figure 1b).6,13 Schemes based on nonreciprocal optical loss in an InGaAsP waveguide coated with iron14 and active devices, including an amplifier to compensate for the loss,15 have been demonstrated. Nonreciprocal mode conversion,16 cutoff,10,17 resonant delocalization,18 and coupling9,11 have also been theoretically explored, although
Qingyang Du, Massachusetts Institute of Technology, USA; [email protected] Takian Fakhrul, Massachusetts Institute of Technology, USA; [email protected] Yan Zhang, University of Electronic Science and Technology of China, China; [email protected] Juejun Hu, Department of Materials Science and Engineering, Massachusetts Institute of Technology, USA; [email protected] Caroline A. Ross, Department of Materials Science and Engineering, Massachusetts Institute of Technology, USA; [email protected] doi:
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