High-capacity optical wireless VCSEL array transmitter with uniform coverage

Abstract

Optical wireless communication (OWC) promises high-speed data transmission of multiple Gb/s per user and aggregate capacities beyond Tb/s. To achieve such data rates in an eye-safe environment, we envisage an array of arrays of optical emitters, with each emitter addressing a small atto-cell. To realise this vision, each array emitter must provide uniform illumination of the desired atto-cell while minimising interference to adjacent cells. Vertical Cavity Surface Emitting Laser (VCSEL) is an attractive optical emitter for such a design due to their high modulation bandwidth (BW), circular beam waist, low cost and commercial availability of low cost arrays in the near-infrared spectrum. However, available arrays are typically multi-mode devices developed for data communications, often exhibiting a doughnut-shaped beam profile. If used with a simple lens arrangement, the resulting illumination shows non-uniform SNR over the intended atto-cell area and interference into adjacent cells. In this work, a 5×5 VCSEL-array-based OWC multi-beam transmitter using microlens arrays is designed to homogenise each VCSEL output beam intensity at the receiver plane. The performance of the proposed transmitter is verified in simulation and experiments, demonstrating a beam intensity uniformity of up to 90% over a 1 𝑚 2 square area and 25.5 mW/m 2 uniform irradience distribution for each atto-cell area. We demonstrate the data transmission capability of a single array element achieving an 8 Gb/s data rate for a single channel using OOK modulation and Decision Feedback Equalization (DFE) with a Silicon Photomultiplier (SiPM) over a 3-metre free space link.