Experimental characterization of a 400 Gbit/s orbital angular momentum multiplexed free-space optical link over 120 m

Area of Science:

• Optical Communications
• Free-space Optics
• Wireless Communication

Background:

• Orbital Angular Momentum (OAM) offers a new degree of freedom for increasing optical communication capacity.
• Free-space optical (FSO) links are susceptible to atmospheric turbulence and pointing errors, impacting performance.

Purpose of the Study:

• To experimentally demonstrate and characterize a high-capacity 400-Gbit/s OAM-multiplexed FSO link.
• To investigate the impact of channel impairments on link performance, including received power, intermodal crosstalk, and power penalties.

Main Methods:

• Multiplexing four 100-Gbit/s quadrature-phase-shift-keyed (QPSK) channels onto OAM beams.
• Transmitting the multiplexed beams over a 120 m FSO link on a building rooftop.
• Analyzing received power fluctuations, intermodal crosstalk, and bit error rates under varying conditions.

Main Results:

• Fluctuations in received power (4.5 dB) and intermodal crosstalk (5 dB) were observed over 180 seconds without active compensation.
• Bit error rates remained below the forward error correction threshold for beam displacements up to 2 mm (pointing error < 16.7 μrad).
• Power penalties increased significantly with increasing beam displacement.

Conclusions:

• The 400-Gbit/s OAM-multiplexed FSO link shows promising performance despite environmental fluctuations.
• The system is robust to small pointing errors, crucial for practical FSO applications.
• Further research into compensation techniques could mitigate performance degradation due to larger displacements.