Samuel Dolinar

100 Tbit/s free-space data link enabled by three-dimensional multiplexing of orbital angular momentum, polarization, and wavelength

We investigate the orthogonality of orbital angular momentum (OAM) with other multiplexing domains and present a free-space data link that uniquely combines OAM-, polarization-, and wavelength-division multiplexing. Specifically, we demonstrate the multiplexing/demultiplexing of 1008 data channels carried on 12 OAM beams, 2 polarizations, and 42 wavelengths. Each channel is encoded with 100 Gbit/s quadrature phase-shift keying data, providing an aggregate capacity of 100.8 Tbit/s (12×2×42×100 Gbit/s).

Capacity-approaching protograph codes

This paper discusses construction of protograph-based low-density parity-check (LDPC) codes. Emphasis is placed on protograph ensembles whose typical minimum distance grows linearly with block size. Asymptotic performance analysis for both weight enumeration and iterative decoding threshold determination is provided and applied to a series of code constructions. Construction techniques that yield both low thresholds and linear minimum distance growth are introduced by way of example throughout. The paper also examines implementation strategies for high throughput decoding derived from first principles of belief propagation on bipartite graphs.

Serial concatenated trellis coded modulation with rate-1 inner code

We develop new, low complexity turbo codes suitable for bandwidth and power limited systems, for very low bit and word error rate requirements. Motivated by the structure of previously discovered low complexity codes such as repeat-accumulate (RA) codes with low density parity check matrix, we extend the structure to high-level modulation such as 8PSK, and 16QAM. The structure consists of a simple 4-state convolutional or short block code as an outer code, and a rate-1, 2 or 4-state inner code. Two design criteria are proposed: the maximum likelihood design criterion, for short to moderate block sizes, and an iterative decoding design criterion for very long block sizes.

Low-rate LDPC codes with simple protograph structure

This paper provides a construction method for low-rate low density parity check codes. Inspired by recently proposed accumulate-repeat-accumulate (ARA) codes, and hybrid concatenated codes, in this paper we extend the construction to low rates. Such codes can be viewed as hybrid concatenations of simple modules such as accumulators, repetition codes, differentiators, and punctured single parity check codes. These codes constitute a subclass of LDPC codes with very fast encoder structure. They also have a projected graph or protograph representation that allows for high-speed decoder implementation. Based on density evolution, we show through some examples that low iterative decoding thresholds close to the channel capacity limits can be achieved, as the block size goes to infinity. Iterative decoding simulation results for short blocks are provided for a few examples that show near-capacity performance and very low error floor

A* decoding of block codes

The A* algorithm is applied to maximum-likelihood soft-decision decoding of binary linear block codes. This paper gives a tutorial on the A* algorithm, compares the decoding complexity with that of exhaustive search and Viterbi decoding algorithms, and presents performance curves obtained for several codes.

Fiber structure to convert a Gaussian beam to higher-order optical orbital angular momentum modes

We propose a fiber structure of a square core and ring refractive index profile that converts an input circular polarized Gaussian mode into optical orbital angular momentum (OAM) modes. By breaking the circular symmetry of the waveguide, the input circularly polarized fundamental mode in the square core can be coupled into the ring region to generate higher-order OAM modes, corresponding to the transference of spin angular momentum and orbital angular momentum. We show, by using simulation, the generation of OAM modes with a topological charge l up to 9 using <10 mm long fiber. The mode purity is above 96.4% and the extinction ratio can be 30 dB.

Constructing LDPC codes from simple loop-free encoding modules

Inspired by recently proposed accumulate-repeat-accumulate (ARA) codes, in this paper we propose a construction method for LDPC codes using simple loop-free encoding modules. Such codes can be viewed as serial/parallel concatenations of simple modules such as accumulators, repetition codes, differentiators, and punctured single parity check codes. Examples are accumulate-repeat-accumulate (ARA) codes, accumulate-repeat-accumulate-accumulate (ARAA) codes and accumulate-repeat-check-accumulate codes, and other variations. These codes constitute a subclass of LDPC codes with very fast encoder structure. They also have a projected graph or protograph representation that allows for high-speed decoder implementation. Based on density evolution, we show through some examples that low iterative decoding thresholds close to the channel capacity limits can be achieved with low maximum variable node degrees, as the block size goes to infinity. The decoding threshold in many examples outperforms that of the best known unstructured irregular LDPC codes constrained to have the same maximum node degree. Furthermore, by puncturing the accumulator modules, any desired higher rate codes can be obtained with thresholds that stay close to their respective channel capacity thresholds uniformly.

Efficient generation and multiplexing of optical orbital angular momentum modes in a ring fiber by using multiple coherent inputs

We propose an approach to efficiently generate and multiplex optical orbital angular momentum (OAM) modes in a fiber with a ring refractive index profile by using multiple coherent inputs from a Gaussian mode. By controlling the phase relationship of the multiple inputs, one can selectively generate OAM modes of different states l. By controlling both the amplitude and phase of the multiple inputs, multiple OAM modes can be generated simultaneously without additional loss coming from multiplexing. We show, by simulation, the generation of OAM modes (OAM state |l|<3) with mode purity greater than 99%. The power loss of generating and multiplexing seven modes is about 35%. A transmitter for an OAM-based mode-division multiplexing system is proposed based on the discrete Fourier transform between the data carried by the multiple inputs and the data carried by the OAM modes. The experimental implementation of the proposed approach could be achieved by integrating ring fiber, multicore fiber, and photonic integrated circuit technology.

Crosstalk mitigation in a free-space orbital angular momentum multiplexed communication link using 4×4 MIMO equalization.

We demonstrate crosstalk mitigation using 4×4 multiple-input-multiple-output (MIMO) equalization on an orbital angular momentum (OAM) multiplexed free-space data link with heterodyne detection. Four multiplexed OAM beams, each carrying a 20  Gbit/s quadrature phase-shift keying signal, propagate through weak turbulence. The turbulence induces inter-channel crosstalk among each beam and degrades the signal performance. Experimental results demonstrate that with the assistance of MIMO processing, the signal quality and the bit-error-rate (BER) performance can be improved. The power penalty can be reduced by >4  dB at a BER of 3.8×10-3.

Multicasting in a spatial division multiplexing system based on optical orbital angular momentum

We report multicasting data from a single orbital angular momentum (OAM) spatial channel onto multiple OAM channels of equally spaced OAM charge numbers. The designed sliced phase patterns for multicasting are loaded on the spatial light modulator. By optimizing the design of the phase pattern, the power of multicasted OAM channels can be equalized. We experimentally demonstrate multicasting five and seven OAM channels from a single-input OAM channel carrying a 100  Gbit/s quadrature phase-shift keying (QPSK) data stream.