Optimized Polarization Insensitive Grating Antenna in the SOI Platform

Introduction

Optical phased arrays (OPAs) are integrated photonic devices that enable the generation and dynamic steering of laser beams in free space. They exploit an array of individual emitters, such as integrated optical antennas, each precisely controlled in phase and amplitude. By adjusting the phase of the light emitted by each antenna, OPAs can steer and shape optical beams without bulky mechanical components, making them attractive for applications like light detection and ranging (LiDAR) and free-space optical communications.

The fundamental component defining an OPA's steering capabilities and overall efficiency is the optical antenna. Surface gratings are commonly used as antenna elements, but their strong polarization dependence for radiation angle and efficiency can be a major drawback, especially when the OPA is used for reception.

This tutorial covers an optimized design for a polarization-insensitive optical antenna grating in the silicon-on-insulator (SOI) platform, building on previous work that utilized L-shaped elements and subwavelength metamaterials. The updated design uses a thicker 500 nm silicon core for improved performance.

Grating Design and Structure

The grating is designed for an SOI platform with a 500 nm silicon layer, 3 μm buried oxide, and 2.2 μm oxide cladding. As shown in Fig. 1, the grating unit cell consists of two parts:

1. An L-shaped structure partially etched to 250 nm depth for a length Ls, with an un-etched section of length Lu.

2. A subwavelength metamaterial section with length Lswg, transverse period of 300 nm, and duty cycle ffy.

The effective refractive index nB of the grating's Bloch mode can be approximated as:

nB = (nswgLswg/Λx) + (nsLs/Λx) + (nuLu/Λx)

Where nswg, ns, and nu are the effective indices of the slab modes in the different grating sections. The value of nswg can be tuned by adjusting the metamaterial duty cycle ffy to ensure nBTE ~ nBTM, achieving the same diffraction angle for TE and TM modes.

The grating parameters were optimized using 3D FDTD simulations to maximize scattering efficiency and control the radiation angle. The final design uses Lswg = 244 nm, Ls = 180 nm, Lu = 219 nm, ffy = 0.6, ffx = 0.62 (duty cycle in x), and RL = 0.45 (L-shape duty cycle).

The antenna length of 6.43 μm (10 grating periods Nx) and width of 2.7 μm (9 metamaterial periods Ny) were chosen for a desirable far-field beamwidth. A 250 nm rib waveguide tapered to 50 nm at the grating input ensures fundamental TE/TM mode excitation.

Performance and Tolerance

The 3D FDTD simulations in Fig. 2 show the grating achieves a polarization-insensitive scattering efficiency of 53% for TE and 58% for TM at 1550 nm wavelength. The far-field patterns in Fig. 3 confirm the same 10° diffraction angle and 9°x104° beamwidth for both polarizations.

A fabrication tolerance analysis considered misalignment between the shallow and full etch sections. While efficiency varied more for TM, the diffraction angle remained relatively stable at ~10° in misaligned cases.

In summary, the key performance metrics from the optimized design are:

Table 1. Radiation angle, diffraction efficiency and beamwidth of the designed grating.

	TE	TM
Diffraction angle	10°	10.5°
Upward diffraction efficiency	53%	58%
Beamwidth (ϑxφ)	9°x104°	9.5°x104°

This grating demonstrates similar upward diffraction efficiency and radiation characteristics for TE and TM polarizations, enabling polarization-insensitive operation suitable for OPA applications.

Fig. 1. (a) 3D and (b) 2D side view of the polarization insensitive grating antenna SWG: subwavelength metamaterial.

Fig. 2. (a) TE and (b) TM scattering efficiency of the designed grating for the nominal design (black solid lines) and when a positive (red lines) or negative (blue lines) misalignment of the shallow etched section is considered in the x direction.

Fig. 3. (a) TE and (b) TM farfield radiation patterns of the optimized grating at a wavelength of 1550 nm.

Conclusion

This tutorial covered the design of a polarization-insensitive grating antenna optimized for the 500 nm silicon-on-insulator platform. By combining L-shaped antenna elements with subwavelength metamaterials in the grating structure, the design achieves a diffraction angle of 10° with comparable 53-58% efficiency for both TE and TM input polarizations at 1550 nm wavelength. The optimized parameters, fabrication tolerance, and performance metrics were presented, demonstrating the feasibility of this approach for realizing high-efficiency polarization-independent optical antennas required in integrated OPA systems.

Reference

[2] S. Salhi, X. Xin, D. Benedikovič, C. Alonso-Ramos, L. Vivien, D. Marris-Morini, E. Cassan, W. N. Ye, and D. Melati, "Optimized Polarization Insensitive Grating Antenna in the SOI Platform," Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, CNRS, Palaiseau, France, and Department of Electronics, Carleton University, Ottawa, Canada, 2024.