117,354 research outputs found
Dual Polarization Full-Field Signal Waveform Reconstruction Using Intensity Only Measurements for Coherent Communications
Conventional optical coherent receivers capture the full electrical field,
including amplitude and phase, of a signal waveform by measuring its
interference against a stable continuous-wave local oscillator (LO). In optical
coherent communications, powerful digital signal processing (DSP) techniques
operating on the full electrical field can effectively undo transmission
impairments such as chromatic dispersion (CD), and polarization mode dispersion
(PMD). Simpler direct detection techniques do not have access to the full
electrical field and therefore lack the ability to compensate for these
impairments. We present a full-field measurement technique using only direct
detection that does not require any beating with a strong carrier LO. Rather,
phase retrieval algorithms based on alternating projections that makes use of
dispersive elements are discussed, allowing to recover the optical phase from
intensity-only measurements. In this demonstration, the phase retrieval
algorithm is a modified Gerchberg Saxton (GS) algorithm that achieves a
simulated optical signal-to-noise ratio (OSNR) penalty of less than 4dB
compared to theory at a bit-error rate of 2 times 10-2. Based on the proposed
phase retrieval scheme, we experimentally demonstrate signal detection and
subsequent standard 2x2 multiple-input-multiple-output (MIMO) equalization of a
polarization-multiplexed 30-Gbaud QPSK transmitted over a 520-km standard
single-mode fiber (SMF) span
Analog hardware for learning neural networks
This is a recurrent or feedforward analog neural network processor having a multi-level neuron array and a synaptic matrix for storing weighted analog values of synaptic connection strengths which is characterized by temporarily changing one connection strength at a time to determine its effect on system output relative to the desired target. That connection strength is then adjusted based on the effect, whereby the processor is taught the correct response to training examples connection by connection
Intelligent search for distributed information sources using heterogeneous neural networks
As the number and diversity of distributed information sources on the Internet exponentially increase, various search services are developed to help the users to locate relevant information. But they still exist some drawbacks such as the difficulty of mathematically modeling retrieval process, the lack of adaptivity and the indiscrimination of search. This paper shows how heteroge-neous neural networks can be used in the design of an intelligent distributed in-formation retrieval (DIR) system. In particular, three typical neural network models - Kohoren's SOFM Network, Hopfield Network, and Feed Forward Network with Back Propagation algorithm are introduced to overcome the above drawbacks in current research of DIR by using their unique properties. This preliminary investigation suggests that Neural Networks are useful tools for intelligent search for distributed information sources
Reference-less measurement of the transmission matrix of a highly scattering material using a DMD and phase retrieval techniques
This paper investigates experimental means of measuring the transmission
matrix (TM) of a highly scattering medium, with the simplest optical setup.
Spatial light modulation is performed by a digital micromirror device (DMD),
allowing high rates and high pixel counts but only binary amplitude modulation.
We used intensity measurement only, thus avoiding the need for a reference
beam. Therefore, the phase of the TM has to be estimated through signal
processing techniques of phase retrieval. Here, we compare four different phase
retrieval principles on noisy experimental data. We validate our estimations of
the TM on three criteria : quality of prediction, distribution of singular
values, and quality of focusing. Results indicate that Bayesian phase retrieval
algorithms with variational approaches provide a good tradeoff between the
computational complexity and the precision of the estimates
Magnon dark modes and gradient memory
Extensive efforts have been expended in developing hybrid quantum systems to
overcome the short coherence time of superconducting circuits by introducing
the naturally long-lived spin degree of freedom. Among all the possible
materials, single-crystal yttrium iron garnet has shown up very recently as a
promising candidate for hybrid systems, and various highly coherent
interactions, including strong and even ultra-strong coupling, have been
demonstrated. One distinct advantage of these systems is that the spins are in
the form of well-defined magnon modes, which allows flexible and precise
tuning. Here we demonstrate that by dissipation engineering, a non-Markovian
interaction dynamics between the magnon and the microwave cavity photon can be
achieved. Such a process enables us to build a magnon gradient memory to store
information in the magnon dark modes, which decouple from the microwave cavity
and thus preserve a long life-time. Our findings provide a promising approach
for developing long-lifetime, multimode quantum memories.Comment: 18 pages, 12 figure
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