Evaluating the coupling efficiency of OAM beams into ring-core optical fibers

Transmission of optical communication signals through ring core fiber using perfect vortex beams

Orbital angular momentum can be used to implement high capacity data transmission systems that can be applied for classical and quantum communications. Here we experimentally study the generation and transmission properties of the so-called perfect vortex beams and the Laguerre-Gaussian beams in ring-core optical fibers. Our results show that when using a single preparation stage, the perfect vortex beams present less ring-radius variation that allows coupling of higher optical power into a ring core fiber. These results lead to lower power requirements to establish fiber-based communications links using orbital angular momentum and set the stage for future implementations of high-dimensional quantum communication over space division multiplexing fibers.

Silicon metaoptics for the compact generation of perfect vector beams in the telecom infrared

Perfect vortices have attracted considerable attention as orbital angular momentum (OAM) beams with customizable ring-like intensity distribution. More recently, the non-separable combination of perfect vortices with opposite OAMs and spins, yielding so-called perfect vector beams, has further expanded their applications in the fields of optical manipulation and imaging, high-resolution lithography, and telecommunications. Exploiting the combined manipulation of dynamic and geometric phases using silicon anisotropic metaunits, here we present the design, fabrication, and characterization of novel, to the best of our knowledge, dielectric metaoptics for the compact generation of perfect vector beams in the telecom infrared using a single metasurface. These devices pave the way to integrated optical architectures with applications in information and communication technologies in both the classical and quantum regimes.

Enhancing fiber-coupling efficiency of beam-to-fiber links in turbulence by spatial non-uniform coherence engineering

We present a general formula for the fiber-coupling efficiency of various types of non-uniformly correlated beams propagating in a turbulent atmosphere. With it, we calculate the fiber-coupling efficiency of a specific type of non-uniformly correlated beams, Laguerre non-uniformly correlated (LNUC) beams, to investigate how the non-uniform correlation structure plays a role in enhancing the fiber-coupling efficiency. Compared with conventional Gaussian Schell-model beams, the LNUC beams possess better coupling behavior, and the initial coherence length and beam order of such beams can be adjusted to further improve the fiber-coupling efficiency in turbulence. Our results demonstrate how non-uniformly correlated beams can be used for fiber-coupling applications, and demonstrate their intriguing potential for free-space optical communications.

Few-mode optical fiber with a simple double-layer core supporting the O + C + L band weakly coupled mode- division multiplexing transmission

A weakly-coupled few mode fiber (FMF) with a simple double-layer core is designed and fabricated that support five (seven) weakly coupled mode groups with average attenuation of 0.21 dB/km (0.39 dB/km) at the C + L (O) optical wavelength bands. Two data transmission experiments are demonstrated utilizing the fiber. A 2-km orbital angular momentum (OAM) mode-group multiplexing (MGM) experiment in the O band achieves error-free transmission for all five multiplexed mode groups without multiple multiple-input multiple-output (MIMO) processing. A 40-km OAM mode division multiplexing (MDM) experiment supporting 14 mode channels in the C + L bands achieves bit error rates (BER) of below 2.4 × 10^-2 (20% soft-decision forward-error-correction threshold) for all channels, based on low-complexity 4 × 4 or 2 × 2 MIMO equalization. These demonstrations prove the capability of the fiber to support weakly coupled MDM/MGM transmission across O + C + L optical wavelength bands.

The Performance of Orbital Angular Momentum Mode (|l| = 1~3) Amplification Based on Ring-Core Erbium-Doped Fibers

We demonstrated that a ring-core erbium-doped fiber amplifier (RC-EDFA) can support orbital angular momentum (OAM) modes with topological charges (|l| = 1~3). The dependence of the characteristics on the length of the RC-EDF was investigated experimentally, including an investigation of gain and 3 dB gain bandwidth over the whole C band (i.e., 1530~1565 nm). The 3 dB gain bandwidth was improved to 21 nm. At a signal wavelength of 1550 nm, the maximum gain of all signal modes was up to 30.1 dB. Differential modal gain was maintained below approximately 1.3 dB.

1-Pbps orbital angular momentum fibre-optic transmission

Space-division multiplexing (SDM), as a main candidate for future ultra-high capacity fibre-optic communications, needs to address limitations to its scalability imposed by computation-intensive multi-input multi-output (MIMO) digital signal processing (DSP) required to eliminate the crosstalk caused by optical coupling between multiplexed spatial channels. By exploiting the unique propagation characteristics of orbital angular momentum (OAM) modes in ring core fibres (RCFs), a system that combines SDM and C + L band dense wavelength-division multiplexing (DWDM) in a 34 km 7-core RCF is demonstrated to transport a total of 24960 channels with a raw (net) capacity of 1.223 (1.02) Peta-bit s−1 (Pbps) and a spectral efficiency of 156.8 (130.7) bit s−1 Hz−1. Remarkably for such a high channel count, the system only uses fixed-size 4 × 4 MIMO DSP modules with no more than 25 time-domain taps. Such ultra-low MIMO complexity is enabled by the simultaneous weak coupling among fibre cores and amongst non-degenerate OAM mode groups within each core that have a fixed number of 4 modes. These results take the capacity of OAM-based fibre-optic communications links over the 1 Pbps milestone for the first time. They also simultaneously represent the lowest MIMO complexity and the 2nd smallest fibre cladding diameter amongst reported few-mode multicore-fibre (FM-MCF) SDM systems of >1 Pbps capacity. We believe these results represent a major step forward in SDM transmission, as they manifest the significant potentials for further up-scaling the capacity per optical fibre whilst keeping MIMO processing to an ultra-low complexity level and in a modularly expandable fashion.

Enhanced fiber-coupling efficiency via high-order partially coherent flat-topped beams for free-space optical communications

The fiber-coupling efficiency of signal beams is crucial in free space optical (FSO) communications. Herein, we derived an analytical expression for the fiber-coupling efficiency of partially coherent flat-topped beams propagating through atmospheric turbulence based on the cross-spectral density function. Our numerical calculation results showed that the fiber-coupling efficiency of partially coherent flat-topped beams in a turbulent atmosphere could be enhanced by increasing the beam order. Under the same conditions, the fiber-coupling efficiency of the high-order partially coherent flat-topped beams was larger than those connected to the Gaussian and Gaussian Schell-model (GSM) beams. Our results will improve the quality of partially coherent beams used in FSO communications.