Inverse Designed Photonics for Efficient Fiber-chip Interface

Description

Photonic integration plays a vital role in future optical communications, microwave photonics, sensing, and quantum technologies. Leveraging advanced microfabrication technologies, photonic devices can nowadays be densely patterned at high resolution on wafer scales, transforming traditional table-top optic systems into low-cost and mass-producible photonic integrated circuits (PICs).The much more compact optical devices in PICs on the one hand dramatically shrink the system sizes and enable unprecedented levels of light-matter interactions, but on the other hand also result in a large mismatch between the mode sizes of on-chip components (~ 1 micrometer) and optical fibers (~ 10 micrometers), leading to substantial optical losses at the fiber-chip interface.Conventional optical spot size converters typically rely on taper structures that adiabatically enlarge the optical mode. To match with optical fibers, the waveguides often need to be tapered to widths of ~ 100 nm with little tolerance, which can only be realized by expensive advanced lithography systems. This fiber-chip interfacing challenge is further exacerbated in emergingphotonic platforms, e.g. lithium niobate (LN) photonics to be studied in this work, where both the partially etched rib waveguides and the non-vertical sidewalls limit the largest mode size achievable in a single-layer taper. The lack of low-cost, robust and efficient fiber-chip couplers has become the “elephant in the room” in photonics.We propose to overcome this challenge by implementing a novel inverse design algorithm that works backward to find out the optimal solution without a pre-defined structural topology. Unlike conventional heuristic approaches that rely on certain physical analytic models and can only access limited parameter spaces, the inverse design algorithm could efficiently search the entire parameter space and often leads to highly counterintuitive yet effective designs. We will develop inverse design algorithms taking into consideration the practical design and fabrication constraints in LN photonics, experimentally demonstrate efficient and fabrication-tolerant fiber-chip couplers, and finally apply the couplers to a series of functional LN PIC demonstrations with unprecedented system performances. The ambitious goals proposed in this project are backed by the expertise in LN device fabrication in PI’s lab and preliminary results on functional inverse designed LN photonic devices.The successful accomplishment of this project will deliver not only an LN photonic fiber-chip interface that is readily compatible with existing LN PIC components and foundry processes, but also a brand-new concept in photonic fiber-chip interface applicable to a variety of material platforms, ultimately benefiting a wide range of future photonic applications including opticalcommunications and quantum photonics.