In recent years we have witnessed significant growth in the deployment of silicon photonic integrated circuits (PICs) for various applications spanning data communications to chem-bio sensing. The success of silicon photonics can be largely attributed to its compatibility with CMOS fabrication, enabling scalable manufacturing capitalizing on established silicon foundry infrastructures to drive cost reduction and performance improvement. Nonetheless, photonic packaging—the process of establishing optical, electrical and thermal interfaces of the PIC chip with the outside world—remains a standing bottleneck in terms of cost and throughput, which is mostly due to the difficulty of coupling light into and out of the photonic circuit. The conventional solution involves using diffractive gratings to direct light into on-chip waveguides, which however only works for a specific wavelength due to the inherent wavelength dependence of optical diffraction.

This paper experimentally demonstrated a novel route for efficient coupling into silicon waveguides using reflective freeform micro-optical couplers. Unlike diffraction, the dispersion-less nature of reflection underlies broadband low-loss operation of the couplers. The paper also proposes a design approach that circumvents computationally intensive optimization of freeform structures, and instead enables simple deterministic design of these couplers utilizing the Fermat's principle in optics. Couplers designed using this approach achieve a low coupling loss of 0.8 dB at 1550 nm wavelength with a remarkable 1-dB bandwidth exceeding 180 nm. Furthermore, the couplers are compatible with standard foundry processed photonic waveguides with no customization (for example removal of top waveguide claddings) needed. This is an important feature that facilitates seamless integration of the couplers with industry-standard PICs. Relevant research results were recently published in Photonics Research, Volume 12, No. 5, 2024. [ Luigi Ranno, Jia Xu Brian Sia, Cosmin Popescu, Drew Weninger, Samuel Serna, Shaoliang Yu, Lionel C. Kimerling, Anuradha Agarwal, Tian Gu, Juejun Hu. Highly efficient fiber to Si waveguide free-form coupler for foundry-scale silicon photonics[J]. Photonics Research, 2024, 12(5): 1055 ]

Luigi Ranno, the lead author of the paper commented: 'In addition to being superb fiber-to-chip couplers suitable for high throughput photonic packaging and universally applicable to any waveguide material or fiber type, these structures are also compatible with other applications requiring the emission of light out of the chip surface, e.g. chip-to-chip coupling or free-space beam generation. The versatility and excellent optical performance of this platform make it a compelling solution to address the photonics packaging bottleneck.'

Follow-on research will be focusing on developing a scalable fabrication route for the couplers. Currently the couplers are fabricated using two photon polymerization, a laser direct writing technique that has limited throughput. The team is working on a nanoimprint alternative which will enable high-throughput, cost-effective parallel fabrication of the micro-optical coupler arrays, thereby facilitating and expediting their deployment in the silicon photonics industry.

Figure 1: (a) Interference of waveguide input and fiber back-propagation used in the design process. (b) 3D schematic of the coupler. (c) Simulated electric field during coupling to an optical fiber. Relevant field profiles are shown on the side. (d) False-colored SEM of a fabricated device. (e) Optical spectrum of the reflector for TE and TM modes.