Achieving Wide Far-Field Beams with Ultra-Compact Silicon Optical Antennas

Introduction

Optical beam steering plays a crucial role in silicon photonics, enabling dynamic control over light propagation direction. Traditional beam steering systems rely on mechanical assemblies and bulk optic components, making them slow, power-hungry, and expensive. Optical phased arrays (OPAs) have emerged as a compelling alternative, offering non-mechanical beam steering capabilities. At the heart of on-chip OPA systems are optical antennas, which significantly influence the OPA's performance.

Key factors impacting optical antenna performance include efficiency, size, and far-field radiation beamwidth. To extend the beam steering range of OPAs, wide-beam antennas are essential. However, achieving broader far-field beams necessitates the use of smaller antennas, which poses a challenge in maintaining antenna performance at reduced sizes due to the significant impact of aperture size on beam width and efficiency.

Theory and Design

The geometry of an optical antenna directly influences its far-field distribution, as the far-field distribution is related to the antenna's near-field properties through Fraunhofer's transformation. In this study, a novel strategy was introduced to effectively separate the minimum antenna length required for achieving a given scattering efficiency from the far-field beam width. This separation was achieved through meticulous control of the phase of the complex near-field by implementing a transverse chirping method.

The antenna design consists of two single-etch, periodic gratings on a standard silicon-on-insulator (SOI) platform. Each grating comprises four longitudinal sections: two fully etched sections (W1 and W3) and two un-etched sections (W2 and W4), interleaved transversally at a subwavelength pitch of 105 nm. The design was optimized for transverse electric (TE) polarization using a genetic algorithm and 2D Finite-Difference Time-Domain (FDTD) simulations, with rigorous 3D FDTD simulations for validation.

The optimized longitudinal period for the first grating (Λg1) is 753 nm, and the second grating pitch (Λg2) is 620 nm. Incorporating a Bragg reflector at the grating's termination enhances the grating's efficiency by redirecting any remaining power back into the antenna.

Fabrication and Characterization

The antenna was fabricated on a 300-nm SOI platform with a 1 μm-thick buried oxide (BOX) layer. The fabricated device is shown in Fig. 1.

Fig. 1. Scanning electron microscope (SEM) image of the fabricated antenna.

For experimental characterization, a tunable laser was employed to assess the antenna's performance. An input lensed optical fiber coupled with a polarization controller was used. The angular distribution of the upward diffracted light was captured using a photodetector situated far away from the chip, as shown in Fig. 2.

Fig. 2. Far-field radiation profile along the (a) x and (b) y axis for measured and simulated (49°×47°) results. (The measurement is in an asterisk green line, while the simulation is depicted with a dashed gray line.)

At a wavelength of 1.55 μm, the measured full-width at half-maximum (FWHM) of the output power in the longitudinal (x) and transverse (y) directions is 44°×52°. The experimental observations closely align with the simulation results, demonstrating strong agreement.

Conclusion

In this study, the successful use of transverse chirping to significantly widen the antenna's far-field pattern within a specific aperture was demonstrated. The experimental antenna design achieved a broad far-field beam width of 44°×52° while maintaining an ultra-compact ~5 μm² footprint. These results open new possibilities for advancing integrated photonic antennas for optical phased arrays (OPAs).

The ability to achieve wide far-field beams from ultra-compact optical antennas addresses a significant challenge in extending the beam steering range of OPAs. By overcoming the trade-off between antenna size and beam width, this research paves the way for more efficient and versatile on-chip OPA systems with enhanced capabilities in various applications, such as optical communications, LiDAR, and free-space data transmission.

Reference

[2] S. Khajavi, M. Vachon, J. Zhang, D. Melati, P. Cheben, J. H. Schmid, C. A. Alonso Ramos, A. Atieh, and W. N. Ye, "Experimental Investigation of Silicon Optical Antennas," Department of Electronics, Carleton University, Ottawa, Canada; Advanced Electronics and Photonics Research Center, National Research Council Canada, Ottawa, Canada; Centre for Nanoscience and Nanotechnologies, CNRS, Université Paris-Saclay, Palaiseau, France; Optiwave Systems Inc., Nepean, Canada, 2024.