Demonstration of 2-Channel Mid-Infrared Mode-Division Multiplexing Using a Single Wavelength Converter for IM/DD and Coherent Free-Space Optical Communication

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Abstract
Free-space optical (FSO) communication in the mid-infrared (mid-IR) region has attracted increasing interest due to its lower atmospheric attenuation compared to near-infrared (near-IR) wavelengths. To further enhance link capacity, mode-division multiplexing (MDM) can be employed to transmit multiple orthogonal spatial channels simultaneously. Previous schemes for mid-IR MDM have converted Gaussian data beams from the near IR into the mid-IR before changing the beams into orthogonal modes. However, such a scheme employs a separate nonlinear wavelength converter for each mode (i.e., N converters for N channels), potentially causing high system complexity. In this paper, we demonstrate using a single wavelength converter from near-IR to achieve a mid-IR mode-division multiplexing (MDM) link. Two orbital angular momentum (OAM) channels with tunable mode orders are generated using spatial light modulators (SLMs) at 1550 nm. The channels are multiplexed and simultaneously converted to 3.4 μm via difference frequency generation (DFG) in a single PPLN crystal. We demonstrate mid-IR intensity modulation/direct detection (IM/DD) and coherent heterodyne communication. A total of 2-Gbit/s on-off keying (OOK) and 4-Gbit/s 16-quadrature amplitude modulation (QAM) MDM links are achieved, all with bit error ratios (BERs) below the 7% forward error correction (FEC) threshold.
Funding Information
  • Office of Naval Research (N00014-20-1-2558)
  • Airbus Institute for Engineering Research

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