Concentric Intensity-Based Adjacent OAM Mode Separation for High-Efficiency Free-Space Optical Spatial Multiplexing

The rapid growth of data traffic in modern communication networks has led to the development of advanced high-capacity multiplexing methods. Orbital angular momentum (OAM)–based mode division multiplexing (MDM) offers a promising scheme by utilizing the orthogonality of helical phase modes to transmit independent data streams simultaneously. In this work, we introduce a novel adjacent mode separation method exploiting OAM’s concentric intensity characteristics for free-space optical (FSO) spatial multiplexing. This method enables the detection of each OAM channel based on its distinctive ring-shaped intensity distribution, contrary to the conventional on-axis phase flattening approach. Two spatially multiplexed signals with different modes are separated by aligning its concentric intensity ring with the active area of an avalanche photodiode (APD), effectively suppressing crosstalk from adjacent modes. Experimental measurements demonstrate that our method achieves a bit-error-rate (BER) performance not exceeding the forward error correction (FEC) threshold, 3.8×10−3, at up to 160 Mbps of data rate, while the conventional detection scheme fails beyond 5 Mbps. The analysis of the eye diagram confirms that our concentric-ring demultiplexing system achieves a high signal-to-noise ratio (SNR) and mode selectivity. These results support the feasibility of the proposed concentric intensity-based mode separation methodology for constructing compact, high-throughput OAM-multiplexed FSO links.