129 research outputs found
Supersymmetric mode converters
Originally developed in the context of quantum field theory, the concept of
supersymmetry (SUSY) can be used to systematically design a new class of
optical structures. In this work, we demonstrate how key features arising from
optical supersymmetry can be exploited to control the flow of light for mode
division multiplexing applications. Superpartner configurations are
experimentally realized in coupled optical networks, and the corresponding
light dynamics in such systems are directly observed. We show that SUSY can be
judiciously utilized to remove the fundamental mode of a multimode optical
structure, while establishing global phase matching conditions for the
remaining set of modes. Along these lines, supersymmetry may serve as a
promising platform for a new generation of versatile optical components with
novel properties and functionalities.Comment: 14 pages, 4 figure
On Achievable Rates for Long-Haul Fiber-Optic Communications
Lower bounds on mutual information (MI) of long-haul optical fiber systems
for hard-decision and soft-decision decoding are studied. Ready-to-use
expressions to calculate the MI are presented. Extensive numerical simulations
are used to quantify how changes in the optical transmitter, receiver, and
channel affect the achievable transmission rates of the system. Special
emphasis is put to the use of different quadrature amplitude modulation
formats, channel spacings, digital back-propagation schemes and probabilistic
shaping. The advantages of using MI over the prevailing -factor as a figure
of merit of coded optical systems are also highlighted.Comment: Hard decision mutual information analysis added, two typos correcte
The Standard Quantum Limit of Coherent Beam Combining
Coherent beam combining refers to the process of generating a bright output
beam by merging independent input beams with locked relative phases. We report
the first quantum mechanical noise limit calculations for coherent beam
combining and compare our results to quantum-limited amplification. Our
coherent beam combining scheme is based on an optical Fourier transformation
which renders the scheme compatible with integrated optics. The scheme can be
layed out for an arbitrary number of input beams and approaches the shot noise
limit for a large number of inputs
Spectral Efficiency Optimization in Flexi-Grid Long-Haul Optical Systems
Flexible grid optical networks allow a better exploitation of fiber capacity,
by enabling a denser frequency allocation. A tighter channel spacing, however,
requires narrower filters, which increase linear intersymbol interference
(ISI), and may dramatically reduce system reach. Commercial coherent receivers
are based on symbol by symbol detectors, which are quite sensitive to ISI. In
this context, Nyquist spacing is considered as the ultimate limit to
wavelength-division multiplexing (WDM) packing.
In this paper, we show that by introducing a limited-complexity trellis
processing at the receiver, either the reach of Nyquist WDM flexi-grid networks
can be significantly extended, or a denser-than-Nyquist channel packing (i.e.,
a higher spectral efficiency (SE)) is possible at equal reach. By adopting
well-known information-theoretic techniques, we design a limited-complexity
trellis processing and quantify its SE gain in flexi-grid architectures where
wavelength selective switches over a frequency grid of 12.5GHz are employed.Comment: 7 pages, 9 figure
Nonlinearity compensation and information rates in fully-loaded C-band optical fibre transmission systems
Nonlinearity compensation and achievable information rates were investigated in fully-loaded C-band communication systems considering transceiver limitations. It is found that the efficacy of nonlinearity compensation in enhancing the achievable information rates depends on the modulation formats and transmission distances
Limit of achievable information rates in EDFA and Raman amplified transmission systems using nonlinearity compensation
Optical networks form an integral part of the world-wide communication infrastructure and nowadays over 95% of data traffic is carried over fibres. Erbium-doped fibre amplifiers (EDFAs) and Raman amplifiers have made it possible to extend the usable fibre bandwidth to increase the achievable capacity of optical communications in past decades to meet the ever-growing information rate demands. However, these amplification technologies are now viewed as limiting the accessible optical spectrum to ~5 THz and ~10 – 15 THz, respectively. Currently, the presence of Kerr effects in fibre channels has been largely regarded as the major bottleneck for enhancing achievable information rates of optical communications. Signal performance degradations due to fibre nonlinearities are more severe in the systems utilising larger transmission bandwidths, closer channel spacing and higher-order modulation formats. In this work, we will study the impact and compensation of Kerr effects to analyse the performance of long-haul optical fibre communication systems using EDFAs and Raman amplifiers. Achievable information rates of such ultra-wideband optical transmission systems will be discussed considering nonlinearity compensation and probabilistic shaping techniques
Group velocity equalisation in multimode waveguides using inverse scattering designs
In this paper, using an inverse scattering approach, we describe how the selection of mode effective indices and thus phase velocities can be used to control group velocity in a waveguide. As such it is shown that differential group delay can be equalised or minimised over a wavelength of choice. A particular feature of the new designs is the development of rings and a peaked core which may split depending upon the number of guided modes. These designs show characteristics comparable with commercially available fibres but with refractive index profiles that differ from typical graded-index designs
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