High Efficiency Polarization-Diversity Two-dimensional Waveguide Grating Coupler for Perfectly Vertical Coupling

Introduction

In polarization division multiplexing (PDM) communications, two orthogonal polarizations are used to double the transmission capacity and increase spectral efficiency. This necessitates coupling of orthogonal polarizations from the optical fiber to the photonic integrated circuit. Two-dimensional waveguide grating couplers (WGCs) can couple the orthogonal modes in the fiber to the same single polarization in two silicon waveguides. However, commercially fabricated devices using deep UV photolithography still exhibit low performance, with the best reported coupling loss being around 6 dB for a standard 220 nm thick silicon wafer and minimum feature size above 180 nm.

Off-Vertical Coupling Challenges

To avoid second-order Bragg back reflection, off-vertical coupling is commonly employed in WGCs. While the best reported off-vertical polarization diversity two-dimensional WGC can achieve -1.8 dB coupling efficiency (CE), it requires additional bottom metal reflectors and benzocyclobutene bonding technology, and still exhibits a high polarization dependent loss (PDL) over 1 dB. Efforts have been made to reduce PDL by using special etched shapes or active phase control, but these approaches have limitations.

Perfectly Vertical Coupling

An alternative approach to reduce PDL is to use WGCs that couple to perfectly vertical optical fibers. The best previously reported polarization diversity two-dimensional WGC achieved 2.6 dB coupling loss using fine nanostructures as small as 40 nm, defined by electro-beam lithography (EBL), but introduced a high PDL of 3 dB. The best efficiency reported for perfectly vertical polarization-independent grating coupler is -2.5 dB with four-port coupling, using a thicker 340 nm silicon layer and feature sizes down to 123 nm.

Figure 1: Schematic view of the cross-section in the (a) lower silicon layer and (b) upper polysilicon layer. (c) Cross-section view and schematic of the proposed two-dimensional perfectly vertical coupled dual polarization grating coupler.

Novel Design Approach

The authors propose a novel design for a perfectly-vertical polarization diversity two-dimensional WGC with theoretically zero PDL and coupling loss of 2.61 dB. The proposed design can be fabricated by deep ultraviolet photolithography in an open access multi-project wafer (MPW) foundry, with a minimum feature size of 171 nm.

The basic approach combines the previous method to realize polarization diversity two-dimensional WGC with the optimized shift pattern polysilicon overlay method to realize high CE and perfectly vertical coupling. The genetic algorithm (GA) is used to optimize the performance of the dual polarization grating coupler, with the target set to realize high CE.

Design Specifications

The design uses an SOI wafer with a 220-nm-thick top silicon layer and a 2-μm-thick buried-oxide layer. The lower 220 nm thick silicon layer is 70 nm shallowly etched, and the upper 160 nm thick polysilicon layer is fully etched. The proposed dual polarization grating coupler can be fabricated using the standard fabrication process by deep UV photolithography developed by a commercial foundry (IMEC).

Figure 2: Parameters of the grating periods in the (a) lower silicon layer and (b) upper polysilicon layer.

The parameters of the grating periods in the lower silicon layer and upper polysilicon layer for the dual polarization grating coupler after GA optimization are shown in Figure 2. Three rectangular slabs are used as end reflectors with a width of 184 nm and spacing of 181 nm.

Simulation Results

3D FDTD simulation results of the proposed dual polarization grating coupler are shown in Figure 3. The simulation predicts a -2.61 dB CE for the orthogonal polarizations at 1552.6 nm with a 3 dB bandwidth of 30 nm from 1543 nm to 1573 nm. The symmetry and normal orientation of the fiber relative to the grating surface result in theoretically identical CEs for the X and Y polarizations in the fiber, as confirmed by the simulation results showing identical CEs for the X-port and Y-port inputs.

Figure 3: 3D simulation result of the proposed grating coupler.

Conclusion

The proposed design offers a novel method to realize polarization diversity two-dimensional WGCs for coupling orthogonal modes of a perfectly vertically orientated standard single-mode fiber to the TE polarization in two orthogonal silicon waveguides. The grating coupler can be fabricated in volume production using standard fabrication processes in a commercial foundry. It has the potential to be integrated with multicore fibers and used in polarization-division multiplexing or space division multiplexing communication networks, enabling high-capacity future communication systems.

Reference

[1] X. Zhou and H. K. Tsang, "High Efficiency Polarization-Diversity Two-dimensional Waveguide Grating Coupler for Perfectly Vertical Coupling," Dept. of Electronic Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, China, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.