Two-stage cross-talk mitigation in an orbital-angular-momentum-based free-space optical communication system

Area of Science:

• Optical Communications
• Signal Processing

Background:

• Orbital angular momentum (OAM) multiplexing is a promising technology for free-space optical communication.
• Mode crosstalk is a significant challenge that degrades system performance.

Purpose of the Study:

• To propose and demonstrate a two-stage crosstalk mitigation method for OAM-based free-space optical communication systems.
• To enhance the robustness of OAM communication against crosstalk and atmospheric turbulence.

Main Methods:

• Implementing spatial offset for adjacent OAM modes (±2 and ±6) to reduce mode crosstalk.
• Utilizing low-density parity-check (LDPC) coded nonuniform signaling, specifically 5-quadrature amplitude modulation (5-QAM) and 9-QAM, based on Huffman coding.
• Combining spatial offset with LDPC-coded nonuniform modulation formats.

Main Results:

• Spatial offset reduced crosstalk by 1.6 dB for OAM states ±2 and 1 dB for OAM states ±6.
• LDPC-coded 5-QAM and 9-QAM demonstrated significant performance improvements (1.1 dB and 5.4 dB, respectively) compared to traditional formats under atmospheric turbulence.
• The proposed method effectively mitigates crosstalk-induced performance degradation.

Conclusions:

• The two-stage crosstalk mitigation method combining spatial offset and LDPC-coded nonuniform signaling is effective in OAM free-space optical communication.
• This approach enhances system performance and tolerance to atmospheric turbulence.
• The study presents a viable strategy for improving the reliability of OAM communication systems.