4 X 20 Gbit/s mode division multiplexing over free space using vector modes and a q-plate mode (de)multiplexer

Abstract: Vector modes are spatial modes that have spatially inhomogeneous states of polarization, such as, radial and azimuthal polarization. They can produce smaller spot sizes and stronger longitudinal polarization components upon focusing. As a result, they are used for many applications, including optical trapping and nanoscale imaging. In this work, vector modes are used to increase the information capacity of free space optical communication via the method of optical communication referred to as mode division multiplexing. A mode (de)multiplexer for vector modes based on a liquid crystal technology referred to as a q-plate is introduced. As a proof of principle, using the mode (de)multiplexer four vector modes each carrying a 20 Gbit/s quadrature phase shift keying signal on a single wavelength channel (~1550nm), comprising an aggregate 80 Gbit/s, were transmitted ~1m over the lab table with <-16.4 dB (<2%) mode crosstalk. Bit error rates for all vector modes were measured at the forward error correction threshold with power penalties < 3.41dB.

• The paper introduces a q-plate based mode (de)multiplexer that enables 4×20 Gbit/s transmission using vector modes with crosstalk below -16.4 dB.
• It reports BERs at the forward error correction threshold with power penalties under 3.41 dB, confirming robust signal integrity.
• The approach paves the way for future research on atmospheric turbulence compensation and fiber integration for mode division multiplexing.

Optimization and Evaluation of Mode Division Multiplexing Using Vector Modes and $q$-Plate Technology

The paper "4 $\times$ 20 Gbit/s mode division multiplexing over free space using vector modes and a $q$-plate mode (de)multiplexer" presents a notable advancement in the utilization of vector modes to enhance the information capacity of free space optical communication systems through mode division multiplexing (MDM). Vector modes, characterized by their spatially inhomogeneous states of polarization such as radial and azimuthal polarization, offer significant applications due to their ability to produce smaller spot sizes and stronger longitudinal polarization components.

Key Contributions and Results

This study introduces a mode (de)multiplexer for vector modes based on $q$-plate technology, which is leveraged to transmit data via free space optical communication. The $q$-plate serves as a crucial component for matching the spatially inhomogeneous state of polarization, effectively enabling the separation and combination of spatial modes without significant crosstalk, which remains below -16.4 dB or approximately 2%. Notably, the research demonstrates the transmission of four distinct vector modes, each carrying a 20 Gbit/s quadrature phase shift-keyed signal, resulting in an aggregate data rate of 80 Gbit/s over a short distance.

The paper reports bit error rates (BERs) being measured at the threshold for forward error correction with power penalties less than 3.41 dB for all vector modes. The mode crosstalk was assessed across the channels, highlighting minimal interference and validating the system’s efficacy.

Theoretical and Practical Implications

The research addresses both the potential and limitations of employing vector modes in mode division multiplexing. While offering an increase in capacity, the propagation of such modes through varying media, particularly turbulent atmospheric conditions, remains unexplored within the scope of this paper. Future work could explore adaptive optics to mitigate turbulence effects akin to those utilized in OAM-based MDM systems.

Moreover, the alignment of vector modes with optical fibers, particularly in the context of multi-path interference and mode coupling, is an intriguing aspect for further exploration, as indicated by potential applications in optical fiber technology. The compatibility of $q$-plates with ring-core optical fibers may open new possibilities for extending this research beyond free space environments.

Future Directions

The study opens several avenues for further research:

1. Atmospheric Turbulence: Investigations into how vector modes behave under atmospheric disturbances could enhance understanding and improve the robustness of free space optical communication in varying environmental conditions.
2. Scattering Media: Analyzing the effects of light scattering on vector modes compared to other multiplexing methods may reveal additional insights into their relative advantages and limitations.
3. Integration with Optical Fibers: Exploring mode coupling and multiplexing in fiber optics, especially for long-distance communication, could expand the applicability of vector modes beyond laboratory settings.
4. Higher-Order Poincaré Beams: Research on the interactions between vector modes and beams with complex polarization states could advance optical data transmission techniques.

In conclusion, the paper provides substantial contributions to the field of optical communication, particularly in demonstrating vector modes as a viable method for increasing data capacity via mode division multiplexing. The innovative application of $q$-plate technology further highlights the potential for continuing advancements in both theoretical and practical realms of optical data transmission.