Spatial Mode Multiplexing for Fiber-Coupled IM/DD Optical Wireless Links with Misalignment

TL;DR

This work addresses misalignment-induced crosstalk in spatial mode multiplexing for fiber-coupled IM/DD OWC using a mutually coherent, non-linear channel model. It develops a LG-mode-based coupling framework that captures pointing errors $d$ and AOA fluctuations $\\varepsilon$ and derives the coupling matrix $\\mathbf{H}$ with entries $h_{ik}$, enabling a non-linear IM/DD capacity analysis. By applying zero-forcing beamforming at the transmitter, the authors derive closed-form expressions for the aggregated data rate $C_{ZF}$ under a total power constraint and show substantial capacity gains (over $200\%$) in simulations when optimizing aperture size and the transmitting mode set. The results provide practical guidance for designing SMM-OFC systems with FMF under realistic misalignment, highlighting the trade-offs between aperture geometry, mode order, and power budgets for indoor data-center-like links.

Abstract

Optical wireless communication (OWC) emerges as a pivotal solution for achieving terabit-level aggregate throughput in next-generation wireless networks. With the mature high-speed transceivers and advanced (de)multiplexing techniques designed for fiber optics, fiber-coupled OWC can be seamlessly integrated into existing ultra-high-speed networks such as data centres. In particular, OWC leveraging spatial mode multiplexing (SMM) and few-mode fiber (FMF) coupling can significantly increase capacity, though misalignment may reduce performance. This paper presents a thorough investigation into the SMM-enabled FMF coupling OWC systems affected by link misalignment, specifically focusing on systems with intensity modulation with direct detection (IM/DD) receivers. A theoretical analysis is conducted to assess the fiber coupling efficiency of the considered system in the presence of both pointing error and angle of arrival (AOA) fluctuations caused by random device vibrations. Our model elucidates the dependence of coupling efficiency to the order of the incident modes, highlighting the critical role of beam properties in system performance. To mitigate the intermodal crosstalk arising from link misalignment, we employ zero-forcing beamforming (ZFBF) to enhance the overall aggregated data rate. Through extensive numerical results, we identify optimal system configurations encompassing aperture design and mode selection, leading to a capacity boost exceeding 200%.

Figures (9)

• Figure 1: The considered FMF-coupled spatial mode multiplexing OWC system with mutually coherent channels: EOM: electro-optic modulators; SMF: single-mode fiber; SPP: spiral phase plate; (DE)MUX: modal (de)multiplexing; PD: photodetector.
• Figure 2: The lens with diameter $D$ couples $i$th incident beam ${E^{(i)}_{\mathrm{I}}}$ to the $k$th fiber mode field ${E^{(k)}_{\mathrm{F}}}$. Plane A: aperture; plane B: fiber end face.
• Figure 3: Illustration of the transmitter vibration-induced pointing error, showing the displacement $\Vec{d}$ between the beam center and the aperture center.
• Figure 4: Illustration of the receiver vibration-induced AOA fluctuation, showing the cross-section of the wavefront intersecting the aperture at an angle $\varepsilon$ at distance $z$.
• Figure 5: The coupling efficiency for a six-mode FMF as the function of coupling parameter $\beta$ in the absence of misalignment for an incident (a): $\mathrm{LG}_{00}$ mode, (b): $\mathrm{LG}_{01}$ mode, (c): $\mathrm{LG}_{02}$ mode; and in the presence of misalignment for an incident (d): $\mathrm{LG}_{00}$ mode, (e): $\mathrm{LG}_{01}$ mode, (f): $\mathrm{LG}_{02}$ mode.