Silicon Photonics Mode-Selective Phase Shifter

TL;DR

This paper introduces a mode-selective thermo-optic phase shifter using subwavelength grating structures on silicon photonics chips, enabling independent phase control of TE modes for advanced optical processing.

Contribution

It presents the first experimental demonstration of a mode-selective phase shifter with high selectivity and low loss, utilizing SWG structures for mode-specific phase manipulation.

Findings

Mode selectivity of 1.44 demonstrated experimentally.

TE0 thermo-optic coefficient is 44% larger than TE1.

Insertion loss is at most 2.5 dB with -13.1 dB crosstalk.

Abstract

A novel mode-selective thermo-optic phase shifter (MS-TOPS) enabled by subwavelength grating (SWG) structures is proposed and experimentally demonstrated on a 220 nm waveguide thick silicon photonics chip for the first two quasi-transverse electric modes (TE0, TE1). Mode-selective relative phase manipulation of modes unlocks several processing tasks in mode division multiplexing systems. This integrated solution provides a direct phase manipulation of modes without converting them to their fundamental modes. A Mach-Zehnder interferometer is deployed as a test structure incorporating the proposed MS-TOPS in one arm and a mode-insensitive thermo-optic phase shifter (MI-TOPS) in another. The effect of the SWG duty cycle ratio is investigated by both numerical simulations and experimental measurements. A mode-selectivity of 1.44 is experimentally demonstrated. In other words, the thermo-optic coefficient of TE0 is 44% larger than the one for TE1. The phase shifter's insertion loss is at most 2.5 dB and a worst-case crosstalk of -13.1 dB over a 40 nm wavelength range from 1520 to 1560 nm. A cascaded configuration of the proposed MS-TOPS and an MI-TOPS provides sufficient degrees of freedom to manipulate the relative phase of each mode independently. Potential numerous applications of such devices include optical switching, multimode quantum optical processors, and scaling-up conventional optical processors with a mode selective building block.