This Review summarizes the simultaneous transmission of several independent spatial channels of light along optical fibres to expand the data-carrying capacity of optical communications. Recent results achieved in both multicore and multimode optical fibres are documented.

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Space-division multiplexing in optical fibres

Internet data traffic capacity is rapidly reaching limits imposed by optical fiber nonlinear effects Having almost exhausted available degrees of freedom to orthogonally multiplex data, the possibility is now being explored of using spatial modes of fibers to enhance data capacity We demonstrate the viability of using the orbital angular momentum (OAM) of light to create orthogonal, spatially distinct streams of data-transmitting channels that are multiplexed in a single fiber Over 11 kilometers of a specially designed optical fiber that minimizes mode coupling, we achieved 400-gigabits-per-second data transmission using four angular momentum modes at a single wavelength, and 16 terabits per second using two OAM modes over 10 wavelengths These demonstrations suggest that OAM could provide an additional degree of freedom for data multiplexing in future fiber networks

http://science.sciencemag.org/content/sci/340/6140/1545.full.pdf

Terabit-Scale Orbital Angular Momentum Mode Division Multiplexing in Fibers

The rise in output power from rare-earth-doped fiber sources over the past decade, via the use of cladding-pumped fiber architectures, has been dramatic, leading to a range of fiber-based devices with outstanding performance in terms of output power, beam quality, overall efficiency, and flexibility with regard to operating wavelength and radiation format. This success in the high-power arena is largely due to the fiber’s geometry, which provides considerable resilience to the effects of heat generation in the core, and facilitates efficient conversion from relatively low-brightness diode pump radiation to high-brightness laser output. In this paper we review the current state of the art in terms of continuous-wave and pulsed performance of ytterbium-doped fiber lasers, the current fiber gain medium of choice, and by far the most developed in terms of high-power performance. We then review the current status and challenges of extending the technology to other rare-earth dopants and associated wavelengths of operation. Throughout we identify the key factors currently limiting fiber laser performance in different operating regimes—in particular thermal management, optical nonlinearity, and damage. Finally, we speculate as to the likely developments in pump laser technology, fiber design and fabrication, architectural approaches, and functionality that lie ahead in the coming decade and the implications they have on fiber laser performance and industrial/scientific adoption.

High power fiber lasers: current status and future perspectives [Invited]

We report simultaneous transmission of six spatial and polarization modes, each carrying 40 Gb/s quadrature-phase-shift-keyed channels over 96 km of a low-differential group delay few-mode fiber. The channels are successfully recovered by offline DSP based on coherent detection and multiple-input multiple-output processing. A penalty of ;28 dB.

Mode-Division Multiplexing Over 96 km of Few-Mode Fiber Using Coherent 6 $\,\times\,$ 6 MIMO Processing

High-power fibre lasers are in demand for industrial, defence and scientific applications. This review provides an overview of the present state of the art in the field and discusses present challenges and the future outlook.

High-power fibre lasers

The stimulated Brillouin scattering (SBS) gain efficiencies were measured in the LP08 and LP01 modes of a higher-order-mode optical fiber. Gain efficiencies CB of 0.0085 and 0.20 (m-W)-1 were measured for the LP08 and LP01 modes at 1083 nm, respectively. CB is inversely proportional to the optical effective area Aeff and the same core-localized acoustic phonon seeds the SBS process in each case. An acoustic modal analysis and a distributed phenomenological model are presented to facilitate the data analysis and interpretation. The LP08 mode exhibits a threshold power-length product of 2.5 kW-m.

SBS gain efficiency measurements and modeling in a 1714 mum(2) effective area LP(08) higher-order mode optical fiber.

We demonstrate a fiber device whose output resembles ideal, infinite-energy Bessel beams. We confirm the beam's two most-intriguing properties: depth-of-focus ~32x that of Gaussians of similar size, and its ability to completely reform past obstructions.

"Diffraction-Free," Self-Healing Bessel Beams from Fibers

Femtosecond fiber lasers are currently of great interest due to their small size, stable operation, long lifetime and low
cost compared to bulk lasers. However, for operation in the 1 µm wavelength range of Yb lasers, a major obstacle has
been the lack of suitable fibers with anomalous dispersion that can compensate for the normal dispersion of the
conventional active and passive fibers used. However, a new promising fiber device using a higher order mode (HOM)
with anomalous dispersion in the 1 μm range has recently been demonstrated. The device comprises integrated all fiber
mode converters based on long period gratings (LPG), and hence has the potential to be low loss and easy to splice,
while offering a large effective area, and the possibility of third order dispersion compensation.
In this paper, optimization of HOM fibers with anomalous dispersion in the 1 μm range has been investigated
theoretically and experimentally. Fibers with dispersion coefficients ranging from +50 to +300 ps/(nm·km) at 1060 nm
have been fabricated and devices including integrated LPG mode converters have been characterized. Modeled and
measured properties of the modules, such as dispersion, grating bandwidth etc., are found to correlate well. It is shown
that there is a tradeoff between a high dispersion coefficient and the bandwidth of LPG mode converters.
The characteristics of such HOM devices have been studied in a linear, passively mode-locked laser-cavity using
SESAM as saturable absorber.

Optimization of higher order mode fibers for dispersion management of femtosecond fiber lasers

Light propagation in higher-order modes of fibers yields a promising schematic for arbitrarily increasing modal areas. We demonstrate this with modes of record A eff ranging from 2100 to 3100 μm2, which experience negligible loss or mode coupling.

Scaling to ultra-large-A eff using higher-order-mode fibers

We compare various techniques to measure MPI in devices exhibiting distributed coherent crosstalk. Such devices are susceptible to deterministic noise as well as intermittent fading, and the measurement technique employed must capture both these manifestations of MPI. We evaluate several measurement techniques used to characterise MPI in few mode fibers. The coherent, distributed nature of MPI in few-mode fibers leads to fast fluctuations, manifesting in deterministic noise, as well as slow variations, reminiscent of fading in mobile communications systems. Slow temporal intensity variations of a CW DFB laser captures most of the relevant information.

Samir Ghalmi

Papers

SBS gain efficiency measurements and modeling in a 1714 mum(2) effective area LP(08) higher-order mode optical fiber.

"Diffraction-Free," Self-Healing Bessel Beams from Fibers

Optimization of higher order mode fibers for dispersion management of femtosecond fiber lasers

Scaling to ultra-large-A eff using higher-order-mode fibers

Measurement of multipath interference in the coherent cross-talk regime