Leveraging Inverse Designed Silicon Metalens for Phase Initialization in Photonic Devices

Introduction

Silicon photonics has emerged as a leading platform for integrated photonic applications, including high-speed data transmission and sensing. Many of these applications, such as optical phased arrays (OPAs) and arrayed waveguide gratings (AWGs), rely on precise control of the phase of light propagating through waveguides. However, practical constraints like fabrication errors and sidewall roughness can introduce inherent phase errors between channels, hindering the intended performance. Conventional approaches to phase initialization often require multiple supplementary measurements, making the process complex and time-consuming.

This article introduces a simplified approach to phase initialization and error correction, utilizing an inverse-designed silicon-based on-chip metalens capable of operating across broad telecommunication bands.

Conceptual Scheme

The proposed phase initializer is based on a silicon on-chip metalens, as illustrated in Figure 1. Ideally, each waveguide channel should carry light with the same phase (φ0). However, due to fabrication errors, inherent phase errors (Δφi) are introduced in each channel.

Fig. 1. Conceptual scheme of phase initializer based on silicon on-chip metalens.

The on-chip metalens is designed to focus light only when all channels possess an identical phase. Prior to phase initialization, the output waveguide does not effectively confine the focused light due to varying phase errors. Photodetectors integrated at the end of the output waveguide monitor the confined power in real-time. By detecting this monitored power, a feedback loop can be established to adjust the phase shifters applied to each channel, compensating for the previously introduced phase errors. Through iterative adjustments, all channels can eventually be initialized to carry light with identical phases.

Inverse Design and Simulation Results

The silicon on-chip metalens was designed using the particle swarm optimization (PSO) technique, a widely used inverse design method in silicon photonics. Figure 2(a) and (b) depict the initial conditions and optimization parameters for the PSO method.

Fig. 2. (a) Overall schematic of analysis model of on-chip metalens for optimization. (b) Geometric parameters of on-chip metalens.

Simulations were conducted using 2.5-dimensional finite-difference time-domain (FDTD) analysis, optimizing a structure with four channels. The optimization objectives were to achieve high focusing efficiency and a broad operational bandwidth.

After more than 500 iterations, the geometric parameters of the on-chip metalens were optimized, as shown in Figure 3(a). Each meta-atom varies in length, influenced by mutual interactions to fulfill the role of the metalens effectively, as depicted in Figure 3(b).

Fig. 3. (a) Top-view and (b) E-field profile of optimized on-chip metalens structure at 1550 nm wavelength. The calculated focusing efficiency of optimized structure as a function of (c) wavelength and (d) phase difference between channels.

The designed metalens boasts a numerical verification of its performance, with a numerical aperture (NA) of 0.39 and a remarkable focusing efficiency of 61.3% at a wavelength of 1550 nm. Furthermore, as indicated in Figure 3(c), the designed metalens structure exhibits a broad bandwidth, with a calculated 1-dB bandwidth of approximately 380 nm, operating efficiently across the C-band, L-band, and O-band within the telecommunication spectrum.

To ascertain the feasibility of its role as a phase initializer, focusing efficiency calculations were performed based on the phase difference between channels, as shown in Figure 3(d). The metalens operates at its maximum focusing efficiency when all channels possess identical phases of light. Hence, by measuring the power of the focused light in the output waveguide in real-time, phase initialization can be facilitated by adjusting the channel phases to maximize the output power.

Conclusion

This article introduced a novel approach to phase initialization and error correction in silicon photonics using on-chip metalenses. Leveraging inverse design techniques, an on-chip metalens with exceptional performance characteristics was engineered, demonstrating high focusing efficiency, numerical aperture, and a broad operational bandwidth. Numerical simulations verified its potential for applications across the telecommunication spectrum. This approach simplifies the challenging task of managing phase errors, particularly in devices like OPAs, streamlining the phase initialization process and enabling improved performance in integrated photonic systems.

Reference

[1] J. Yoon, J.-Y. Kim, J. Kim, J. Park, H.-H. Park, H. Kurt, "Channel Phase-Error Initialization via Inverse Designed On-Chip Silicon Metalens," School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea; Dept. C4ISR Systems Center, Defense Agency for Technology and Quality, Republic of Korea, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.