Multimode interface between optical free-space- and waveguide modes

TL;DR

Problem: increasing data rates require bridging free-space spatial modes with on-chip multimode photonics. Approach: a four-plane MPLC-based interface converts a set of free-space $LG_{\ell,p}$ modes into the first few silicon waveguide TE modes across the telecom C-band in a passive, broadband manner. Key results: mode-conversion efficiencies of approximately $60\%$–$85\%$ with crosstalk visibility around $75\%$–$90\%$, and broadband operation over roughly 1528–1568 nm with ~40 nm span; demonstrated adaptability to different LG mode sets and orders. Significance: this provides a scalable, broadband, passive building block for multi-mode optical networks and on-chip processing, paving the way for higher capacity systems and potential polarization multiplexing.

Abstract

Free-space and on-chip photonic systems are key components in optical communication networks. While free-space beams allow for the flexible generation and manipulation of spatial modes, integrated waveguides provide compact and stable platforms for on-chip signal processing. Bridging these two domains is essential for scalable multi-mode communication networks. Here, we present an efficient, broadband interface capable of converting multiple higher-order free-space Laguerre-Gauss (LG) modes into corresponding waveguide modes using the multi-plane light conversion (MPLC) scheme. We experimentally demonstrate low-crosstalk mode conversion between various set of three LG modes, and the first three TE modes of a multimode silicon waveguide across the telecom C-band. The system operates passively without active switching and can be adapted to different spatial mode sets. This platform provides a pathway to increased data capacities and may enable more compact and efficient multi-mode optical communication and on-chip processing schemes.

Figures (4)

• Figure 1: Sketch of the experimental setup. ($LG_{00}$, $LG_{10}$, $LG_{20}$) modes are generated using the first hologram. Then, the multi-plane light conversion (MPLC) system is implemented with a spatial light modulator (SLM). The MPLC converts the input modes to the first waveguide modes, which are coupled into a multimode Si rib waveguide using an aspheric lens. After propagation through the waveguide, the output field is imaged onto a camera. Off-axis digital holography is performed by interfering the signal beam with a reference beam, which is separated before the MPLC and adjusted in intensity and polarization using waveplates. A similar interferogram is recorded before the waveguide to characterize the MPLC output.
• Figure 2: Characterization of the mode transformation at different stages. (a) Simulated MPLC input modes $LG_{00}, LG_{10}, LG_{20}$ and the corresponding simulated waveguide modes after the transformation. (b) Experimentally reconstructed waveguide modes at the MPLC output prior to coupling to the chip. (c) Modes coupled into the waveguide chip and imaged on a camera. All field reconstructions were obtained using off-axis holography. (d) and (e) Crosstalk matrices of the transformation before and after transmission through the waveguide chip, respectively.
• Figure 3: Spectral performance of the interface. Crosstalk matrices and visibility measured after coupling to the chip for wavelengths between 1528–1568 $nm$.
• Figure 4: Mode transformation with different Laguerre–Gaussian input sets.