Edge Couplers in Silicon Photonic Integrated Circuits: A Review

Abstract

Silicon photonics has drawn increasing attention in the past few decades and is a promising key technology for future daily applications due to its various merits including ultra-low cost, high integration density owing to the high refractive index of silicon, and compatibility with current semiconductor fabrication process. Optical interconnects is an important issue in silicon photonic integrated circuits for transmitting light, and fiber-to-chip optical interconnects is vital in application scenarios such as data centers and optical transmission systems. There are mainly two categories of fiber-to-chip optical coupling: off-plane coupling and in-plane coupling. Grating couplers work under the former category, while edge couplers function as in-plane coupling. In this paper, we mainly focus on edge couplers in silicon photonic integrated circuits. We deliver an introduction to the research background, operation mechanisms, and design principles of silicon photonic edge couplers. The state-of-the-art of edge couplers is reviewed according to the different structural configurations of the device, while identifying the performance, fabrication feasibility, and applications. In addition, a brief comparison between edge couplers and grating couplers is conducted. Packaging issues are also discussed, and several prospective techniques for further improvements of edge couplers are proposed.

Grating Couplers on Silicon Photonics: Design Principles, Emerging Trends and Practical Issues

“…Efficient coupling from SMF to waveguide with size of hundreds of nanometers is a challenge due to the large modal size mismatch. This problem is usually addressed using two solutions, in-plane (butt) edge coupling and off-plane (vertical) grating coupling [ 11 , 12 , 13 ].…”

“…It is reversed for fiber-to-chip propagation case, where Gaussian fiber mode distribution will be confined in the tapered waveguide as its size gradually increase. Edge couplers can achieve high coupling efficiency (CE), large bandwidth (BW) and low polarization dependent loss (PDL) [ 11 ]. However, a properly cleaved and polished facet with strict smoothness requirement is needed to reduce the loss, which adds extra fabrication cost considering mass manufacturing.…”

Mode-field Matching Down-Tapers on Single-Mode Optical Fibers for Edge Coupling Towards Generic Photonic Integrated Circuit Platforms

“…The two main coupling strategies being investigated are grating couplers [11] and edge couplers [12]- [19]. The latter approach (also called in-plane coupling) generally offers the best optical performance in terms of coupling efficiency, spectral transmission bandwidth and polarization independency [20], [21].…”

Mode-field matching design, 3D fabrication and characterization of down-tapers on single-mode optical fiber tips for coupling to photonic integrated circuits

“…This free-space approach of coupling will always be affected by Fresnel reflections at the optical interfaces and is limited in its design freedom on the shape and size of the focused spot, making it hard to exactly match the chip's modal fields. As many different types of on-chip coupling schemes are still being actively explored 6 , our design strategy is to optimize the fiber's side of the fiber-to-chip connection and tailor the design of the fiber taper to the on-chip waveguide schemes being employed by the PIC foundry. We propose to functionalize SMF end-facets with down-taper structures by means of two-photon polymerization (2PP)based direct laser writing (Figure 2(c)).…”