Field conjugation adaptive arrays in atmospheric coherent optical links

Postprint (published version

Performance of Direct-Detection Mode-Group-Division Multiplexing using Fused Fiber Couplers

© 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works[EN] We present an end-to-end performance evaluation of a mode-group-division multiplexing system that uses direct detection instead of coherent detection, avoiding complex digital signal processing. The system transmits four data channels through a step-index fiber supporting six spatial modes comprising four mode groups, considering the two-fold degeneracy of the LPlm modes for l ≠ 0. Multiplexing and demultiplexing is performed using two- and three-core fused fiber couplers, each one phase-matched to a group of degenerate modes. These devices are analyzed through a field-based model that describes, for the first time to our knowledge, crosstalk between all the fiber modes. Propagation through the few-mode fiber is modeled considering differential modal attenuation, intermodal dispersion, chromatic dispersion, and both intergroup and intragroup modal coupling. The end-to-end link is described by a concatenation of matrix operators describing the optical field transfer functions for the multiplexer, fiber and demultiplexer. Error-free transmission of four 32-Gb/s OOK modulated data channels through a 1-km link proves the feasibility of the proposed direct-detection mode-group-division multiplexing approach.The work of I. Gasulla was supported by the Fulbright Commission and the Spanish Ministerio de Educacion through the Programa Nacional de Movilidad de Recursos Humanos del Plan Nacional de I-D + i2008-2011. The work of J. M. Kahn was supported by a Google Faculty Research Award.Gasulla Mestre, I.; Kahn, JM. (2015). Performance of Direct-Detection Mode-Group-Division Multiplexing using Fused Fiber Couplers. Journal of Lightwave Technology. 33(9):1748-1760. doi:10.1109/JLT.2015.2392255S1748176033

Understanding the performance of atmospheric free-space laser communications systems using coherent detection

Postprint (published version

Optimal modes for spatially multiplexed free-space communication in atmospheric turbulence

In near-field free-space optical (FSO) communication, spatial-mode multiplexing (SMM) increases transmission capacity by transmitting independent information streams in orthogonal modes. Propagation through atmospheric turbulence causes phase and amplitude distortions that can degrade SMM performance. In this paper, we show there exist optimal modes for transmission through turbulence with minimum degradation, under a realistic assumption that a transmitter knows the turbulence statistics but not the instantaneous state of the atmosphere. These modes are determined by performing a Karhunen-Loève expansion of the optical electric field in the receiver aperture. We show that these modes are Laguerre-Gauss (LG) modes whose beam waist is chosen depending on the field coherence length in the receiver plane. These adaptive-waist LG modes, when ordered by decreasing eigenvalue, can approximate a received signal field by a finite number of modes with lowest mean-square error among all orthonormal mode sets. Hence, they represent optimal transmit and receive bases for SMM FSO. Using numerical simulation, we study SMM FSO transmission at various turbulence strengths and signal-to-noise ratios. We compare the performance using the adaptive-waist LG modes to that using fixed-waist LG modes (which assume no knowledge of turbulence statistics) and instantaneous eigenmodes (which assume knowledge of the instantaneous state of the turbulence). We also study the performance using the orbital angular momentum subsets of the adaptive-waist LG mode and fixed-waist LG mode sets.FUNDING: Funding. Agencia Estatal de Investigación (PID2020-118410RB-C21).Peer ReviewedPostprint (published version

Frequency Diversity in Mode-Division Multiplexing Systems

In the regime of strong mode coupling, the modal gains and losses and the modal group delays of a multimode fiber are known to have well-defined statistical properties. In mode-division multiplexing, mode-dependent gains and losses are known to cause fluctuations in the channel capacity, so that the capacity at finite outage probability can be substantially lower than the average capacity. Mode-dependent gains and losses, when frequency-dependent, have a coherence bandwidth that is inversely proportional to the modal group delay spread. When mode-division-multiplexed signals occupy a bandwidth far larger than the coherence bandwidth, the mode-dependent gains and losses are averaged over frequency, causing the outage capacity to approach the average capacity. The difference between the average and outage capacities is found to be inversely proportional to the square-root of a diversity order that is given approximately by the ratio of the signal bandwidth to the coherence bandwidth.Comment: 8 pages, 6 figure