Achieving Ultra-Broad Bandwidth with a Subwavelength-Assisted Power Splitter

Introduction

On-chip power splitters are crucial components in silicon photonic integrated circuits, enabling a wide range of applications such as quantum information processing, high-precision sensing, and biophotonics. These devices distribute optical signals efficiently on-chip and can be combined with other components to perform more complex functions, offering flexibility and scalability for modern telecommunications and data transmission systems.

However, traditional power splitters often face limitations in terms of bandwidth, footprint, polarization dependence, or sensitivity to fabrication processes. Fortunately, a novel approach using subwavelength grating (SWG) metamaterials has overcome many of these challenges, resulting in a remarkable achromatic subwavelength-assisted power splitter with ultra-low excess loss over an unprecedented bandwidth.

The proposed symmetric Y-junction splitter, illustrated in Fig. 1, was optimized for multimode operation in a silicon-on-insulator platform with a 200-nm-thick silicon core layer.

Fig. 1. Schematic of the proposed subwavelength-assisted symmetric Y-junction.

The splitter consists of a stem waveguide of width 1 μm that supports both fundamental and first-order transverse-electric modes (TE0 and TE1), and two single-mode S-bend output arms of width 0.5 μm. When the stem waveguide is excited with the TE0 mode, the power is evenly distributed between two in-phase TE0 modes at the output arms. Conversely, when excited with the TE1 mode, the Y-junction acts simultaneously as a power splitter and a mode converter, with the mode evolving into the TE0 mode of each output arm, but with a relative phase shift of π.

The key innovation lies in the inclusion of subwavelength metamaterials between the two branching arms. By leveraging this SWG section, a seamless and efficient modal transition from the stem waveguide to the arms is achieved, minimizing device losses.

Exceptional Performance

The excess loss (EL) performance of the proposed SWG-assisted symmetric Y-junction, obtained through three-dimensional finite-difference time-domain (3D FDTD) simulations, is truly remarkable, as shown in Fig. 2.

Fig. 2. Simulated excess loss for the SWG-assisted Y-junction (for TE0, TE1 and TM0 modes), using 3D FDTD.

The device EL is below 0.3 dB for the TE0 mode and under 0.07 dB for the TE1 mode, over a remarkably broad bandwidth of 700 nm, covering the wavelengths from 1300 nm to 2000 nm. The splitter also exhibits high performance for the fundamental transverse-magnetic (TM0) mode, with EL below 0.3 dB within the 1300–1800 nm wavelength range.

Experimental Validation

To validate the simulation results for the TE0 mode, a proof-of-concept device was fabricated using silicon-on-insulator (SOI) wafers with a 220-nm-thick silicon core and 2 μm buried oxide (BOX). Electron-beam lithography and a dry etching process were used to define and transfer the pattern to the silicon layer, with a silica upper cladding deposited for device protection.

Experimental characterization for the TE0 mode was performed using a Mach-Zehnder interferometer test structure. As shown in Fig. 3, measurements spanning the wavelength range from 1430 nm to 1680 nm demonstrated EL < 0.35 dB for the TE0 mode, in good agreement with simulations.

Fig. 3. Measured excess loss for TE0 mode of the SWG-assisted Y-junction. Nominal design (solid) and ± 10 nm biased splitters (dotted).

Notably, biased devices with deviations of ±10 nm were characterized, confirming the resilience of the device against fabrication errors.

Conclusions and Outlook

This novel approach using subwavelength metamaterials has demonstrated highly efficient multimode power splitting over a record bandwidth for silicon-on-insulator devices. Simulations predict low losses (<0.3 dB) over a 700 nm bandwidth (1300 nm - 2000 nm) for both TE0 and TE1 modes, and experimental results for the TE0 mode validate these findings within the measurement range of 1430–1680 nm.

The ultra-broadband behavior also holds for TM0 operation, with losses below 0.3 dB within a 500 nm wavelength range. This exceptional performance, combined with the device's robustness against fabrication biases, opens up unique possibilities for a wide range of applications, including telecommunications and datacom, light detection and ranging, multitarget spectroscopy, and optical sensing.

With its unprecedented bandwidth, low losses, and resilience to fabrication errors, this subwavelength-assisted symmetric Y-junction power splitter represents a significant advancement in silicon photonics, enabling more efficient and versatile on-chip signal distribution for next-generation integrated photonic circuits.

Reference

[2] R. Fernández de Cabo et al., "Achromatic Subwavelength-Assisted Power Splitter for Next-Generation Silicon Photonics," Instituto de Óptica, Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain; Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris-Saclay, Palaiseau, France; National Research Council Canada, Ottawa, Canada, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.