Off-axis phase modulation for SU(2) mode sorting.

Area of Science:

• Optical communications
• Quantum information science

Background:

• Multi-singularity structured beams with SU(2) modes offer enhanced communication capacity through orthogonal degrees of freedom.
• Current SU(2) mode recognition faces challenges in signal demultiplexing, specifically in mode sorting and separation.

Purpose of the Study:

• To propose and demonstrate a novel SU(2) mode sorting strategy for efficient signal demultiplexing.
• To overcome limitations in current SU(2) mode sorting techniques, including multi-mode conversion and separation.

Main Methods:

• Developed an off-axis phase modulation strategy combining conjugate and gradient phase modulation.
• Converted SU(2) modes into quasi-Gaussian modes with tailored diffraction angles for spatial separation.
• Implemented a proof-of-concept SU(2) mode multiplexing communication link.

Main Results:

• Successfully sorted 18 SU(2) modes (central OAM, sub-beam OAM, coherent-state phase) with crosstalk below -13.8 dB.
• Achieved a 100 Gbit/s quadrature phase-shift keying (QPSK) transmission with bit error rates below 10^-5.
• Demonstrated flexible spatial reallocation of SU(2) modes.

Conclusions:

• The proposed off-axis phase modulation strategy offers an effective solution for SU(2) mode sorting.
• This method advances multidimensional multiplexing and high-efficiency shift-keying (SK) communication.
• The technique shows promise for future high-capacity optical communication systems.