Backscattering Interference Research Articles

Using Downstream DPSK and Upstream Wavelength-Shifted ASK for Rayleigh Backscattering Mitigation in TDM-PON to WDM-PON Migration Scheme

A migration scheme from time-division-multiplex passive optical network (TDM-PON) to wavelength-division-multiplex PON (WDM-PON) using differential phase-shift keying (DPSK) for the downstream signal and wavelength-shifted amplitude-shift keying (WS-ASK) for the upstream signal is demonstrated. The migration scheme does not change the existing fiber infrastructure. An optical filter is preinstalled at the optical networking unit (ONU) to select the desirable downstream wavelength for the WDM-PON and simultaneously demodulate the downstream DPSK signal. Signal remodulation is used to generate the upstream signal by reusing the downstream wavelength. In the ONU, by wavelength shifting the upstream optical spectrum with respect to the downstream optical spectrum, the Rayleigh backscattering (RB) interference beat noise affecting the upstream signal can be significantly mitigated. The optimum bandwidth for the downstream DPSK demodulation is analyzed. The downstream and upstream transmission performances at different split ratios are also discussed.

Slow Adaptive OFDMA-PON Through Channel Stabilization Employing Seed Carrier Clipping

We have proposed and experimentally demonstrated a novel slow adaptive orthogonal frequency division multiple access (OFDMA)-based passive optical network (PON) with a single-fiber loopback link. The proposed slow adaptive OFDMA-PON is realized by employing a clipping modulation of an optical seed carrier, which is fed to uplink transmitters at an optical network unit (ONU) in a carrier distribution scheme. Therefore, a fast changing characteristic of the single-fiber loopback channel, which is mainly attributed to Rayleigh backscattering interference, can be dramatically stabilized. Consequently, the uplink OFDMA-PON transmitters can reduce a large number of signal processing in updating channel state information and relevant subcarrier allocation for multiple access unless the required traffic condition changes for the given ONU.

Coherent electron backscattering interference in non-uniform disordered systems

We study the coherent electron backscattering interference in the presence of electron dephasing in 2D/3D non-uniform (NU) disordered systems, within our virtual electron trap scattering (VETS) model, where the dephasing rate 1/ τ φ is taken to be due to the inelastic electron–electron or electron–phonon scattering. A possible saturation mechanism of apparent electronic dephasing is examined. The system considered is composed of two kinds of subsystems, namely, L-islands and H-region with contrasting diffusion constants, of which the L-islands (of low diffusion constant) act as virtual electron traps randomly dispersed in the percolating H-background (of high diffusion constant). The physics of VETS model is characterized by the two important dwell times, τ f and τ b , with τ f / τ b being the mean duration for which an electron wanders in the H-region/L-island before it leaves the region, respectively. In order to make connection with experiments, we introduce the notion of an effective system of uniform ( U) disorder with a dephasing time τ φ ( e f f e c t i v e ) , which simulates in the aspect of backscattering the NU system being studied. The effective dephasing time τ φ ( e f f e c t i v e ) thus introduced is a function of τ φ , and the function, τ φ ( e f f e c t i v e ) ( τ φ ), is derived and examined. If τ b ⪢ τ f , an interesting phenomenon occurs in the range of temperature ( T) where the true dephasing time τ φ ( T) lies between τ f and τ b , i.e., τ f < τ φ ( T)< τ b . In this case, we obtain τ φ ( e f f e c t i v e ) ( τ φ )≈ τ f , which is insensitive to the variation in τ φ (or T) and shows the signature of τ φ ( e f f e c t i v e ) saturation. However, at the further lowered temperature where τ φ ( T)≥ τ b , τ φ ( e f f e c t i v e ) rises up sharply without being saturated.

Iterative Adaptive Kronecker MIMO Radar Beamformer: Description and Convergence Analysis

We introduce an iterative procedure for design of adaptive KL-variate linear beamformers that are structured as the Kronecker product of K-variate (transmit) and L-variate (receive) beamformers. We focus on MIMO radar applications for scenarios where only joint transmit and receive adaptive beamforming can efficiently mitigate multi-mode propagated backscatter interference. This is because the direction-of-departure (DoD) on one interference mode, and the direction-of-arrival (DoA) on the other, coincide with those of a target, respectively. We introduce a Markov model for the adaptive iterative routine, specify its convergence condition, and derive final (stable) signal-to-interference-plus-noise ratio (SINR) performance characteristics. Simulation results demonstrate high accuracy of the analytical derivations. In addition, we demonstrate, that for the considered class of multiple-input multiple-output (MIMO) radar interference scenarios, the diagonally loaded sample matrix inversion (SMI) algorithm provides additional performance improvement and convergence rate for this iterative adaptive Kronecker beamformer.

Nonlinear coherent backscattering

We theoretically examine diffusive transport and coherent backscattering of waves in dilute disordered media with weak nonlinearity. Depending on the type of nonlinearity, the coherent backscattering interference is either reduced or enhanced, as compared to the linear case.

Low-temperature diffusive transport of electrons in a disordered sandwich structure withquasi-two-dimensional charge transport characteristics

We present the theoretical study of weak localization in a disordered sandwich structure,which consists of a thin, dirty metallic film bounded by nearly insulating materials withelectrical current primarily located in the metallic film. The conductance correction,δG(B), due to backscattering interference is calculated within theweak-localization theory as a function of perpendicular magnetic fieldB. Itis found that, for a certain range of structural/material parameters, both the conductance correctionδG(B = 0) and magnetoconductance (MC) show a weak temperature dependence (i.e. saturation) overa range of low temperatures, as opposed to those of a freestanding metallic film, whichshow a strong temperature dependence. At further lowered temperatures, it shows thatδG(B = 0) and MC may or may not diverge, depending on the structure. If the structure is thin like a film,δG(B = 0) and MC both eventually diverge. On the other hand, if the structure is thick like a bulk, saturationof δG(B = 0) persists to 0 K but MC is likely to diverge. We also fit the calculated MC of sandwich structureswith the weak-localization theory suitable for a freestanding film, with the phase breaking timeτφ in the theory being the fitting parameter. This gives a nominal phase breaking timeτφ(eff), which is nearly constant over a range of low temperatures. The implication of the work isdiscussed in connection with the issue of dephasing time saturation. The limitation of thepresent theory is examined and directions for future extension are suggested.

COHERENT TRANSPORT OF LIGHT IN COLD ATOMS

We present an overview of the investigations on the transport of near-resonant light in laser-cooled atomic vapors, a field that emerged during the past decade. We first discuss time-resolved measurements yielding some informations on the nature of this transport, which is quasi-diffusive for low temperatures and limited optical thicknesses. We then focus on the coherence of the transport, which is probed using the coherent backscattering interference. We identify and analyze in detail several mechanisms limiting the coherence length in such media.

Particle backscatter and extinction profiling with the spaceborne high-spectral-resolution Doppler lidar ALADIN: methodology and simulations

The European Space Agency will launch the Atmospheric Laser Doppler Instrument (ALADIN) for global wind profile observations in the near future. The potential of ALADIN to measure the optical properties of aerosol and cirrus, as well, is investigated based on simulations. A comprehensive data analysis scheme is developed that includes (a) the correction of Doppler-shifted particle backscatter interference in the molecular backscatter channels (cross-talk effect), (b) a procedure that allows us to check the quality of the cross-talk correction, and (c) the procedures for the independent retrieval of profiles of the volume extinction and backscatter coefficients of particles considering the height-dependent ALADIN signal resolution. The error analysis shows that the particle backscatter and extinction coefficients, and the corresponding extinction-to-backscatter ratio (lidar ratio), can be obtained with an overall (systematic+statistical) error of 10%-15%, 15%-30%, and 20%-35%, respectively, in tropospheric aerosol and dust layers with extinction values from 50 to 200 Mm(-1); 700-shot averaging (50 km horizontal resolution) is required. Vertical signal resolution is 500 m in the lower troposphere and 1000 m in the free troposphere. In cirrus characterized by extinction coefficients of 200 Mm(-1) and an optical depth of >0.2, backscatter coefficients, optical depth, and column lidar ratios can be obtained with 25%-35% relative uncertainty and a horizontal resolution of 10 km (140 shots). In the stratosphere, only the backscatter coefficient of aerosol layers and polar stratospheric clouds can be retrieved with an acceptable uncertainty of 15%-30%. Vertical resolution is 2000 m.

Colorless Operation of WDM-PON Employing Uncooled Spectrum-Sliced Reflective Semiconductor Optical Amplifiers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We demonstrate a low-cost wavelength- division- multiplexed<?Pub _nolinebreak=""?> passive optical network based on colorless uncooled spectrum-sliced reflective semiconductor optical amplifiers. In this network, we achieve bit-error rates of better than <emphasis><formula formulatype="inline"><tex>$10^{-10}$</tex> </formula></emphasis> over 32 100-GHz-spaced channels from 0 <emphasis><formula formulatype="inline"><tex>$^{\circ}$</tex></formula></emphasis>C to 60 <emphasis><formula formulatype="inline"><tex>$^{\circ}$</tex></formula></emphasis>C in 155-Mb/s transmissions. The penalties due to dispersion effect and Rayleigh backscattering interference over 25 km are almost negligible. We also present colorless operations over 16-channel coverage in outdoor temperature range. </para>

Light Transport in Cold Atoms and Thermal Decoherence

We use the coherent backscattering interference effect to investigate experimentally and theoretically how coherent transport of light inside a cold atomic vapor is affected by the residual motion of atomic scatterers. As the temperature of the atomic cloud increases, the interference contrast decreases dramatically. This emphasizes the role of motion-induced decoherence for resonant scatterers even in the sub-Doppler regime of temperature. We derive analytical expressions for the corresponding coherence time.

One-dimensional inverse scattering problem for optical coherence tomography

Optical coherence tomography is a non-invasive imaging technique based on the use of light sources exhibiting a low degree of coherence. Low-coherence interferometric microscopes have been successful in producing internal images of thin pieces of biological tissue; typically samples of the order of 1 mm in depth have been imaged, with a resolution of the order of 10 µm in some portions of the sample. In this paper we deal with the imaging problem of determining the internal structure of a multi-layered sample from backscattered laser light and low-coherence interferometry. In detail, we formulate and solve an inverse problem which, using the interference fringes that result as the back scattering of low-coherence light is made to interfere with a reference beam, produces maps detailing the values of the refractive index within the imaged sample. Unlike previous approaches to the OCT imaging problem, the method we introduce does not require processing at data collection time, and it produces quantitatively accurate values of the refractive indexes within the sample from back-scattering interference fringes only.

Coherent backscattering of light by a layer of discrete random medium

We consider the problem of backscattering of light by a layer of discrete random medium illuminated by an obliquely incident plane electromagnetic wave. The multiply scattered reflected radiation is assumed to consist of incoherent and coherent parts, the coherent part being caused by the interference of multiply scattered waves. Formulas describing the characteristics of the reflected radiation are derived assuming that the scattering particles are spherical. The formula for the incoherent contribution reproduces the standard vector radiative transfer equation. The interference contribution is expressed in terms of a system of Fredholm integral equations with kernels containing Bessel functions. The special case of the backscattering direction is considered in detail. It is shown that the angular width of the backscattering interference peak depends on the polar angle of the incident wave and on the azimuth angle of the reflection direction.

The Opposition Effect and the Quasi-fractal Structure of Regolith: I. Theory

We present a new heuristic model combining the coherent-backscatter and shadow-hiding opposition effects. The model considers the aggregate structure of regolith in a realistic way and accounts for the fractal architecture of planetary surfaces both on microscopic particulate and macroscopic, texturally faceted size scales. It describes how the shadow-hiding opposition effect is manifested in a fractally arranged surface, the important contribution of incoherent, multiply scattered light among small scatterers within a surface composed of aggregate grains and its amplification by backscatter interference. We provide a general theoretical model as well as a preliminary, simple analytical approximation that is suitable for investigating the opposition behavior of planet, satellite, and asteroid surfaces. The analytical approximation utilizes only four model parameters; ω 0, the average, single-scattering albedo of elemental scatterers that are the building blocks of aggregates, h, Hapke's (1986, Icarus 67, 264–280) angular-width parameter for the shadow-hiding opposition effect, L, the diffusion scale length of light through the regolith, and q, a structural parameter that characterizes the relative size and complexity of aggregate particles. Our model explains the simultaneous occurrence of the coherent-backscatter enhancement and shadow-hiding in moderate to low albedo soils in addition to the dominance of coherent-backscatter in bright soils. It predicts that coherent backscatter and shadow-hiding can occur at a variety of size scales within the regolith. Individual regolith particles can exhibit their own opposition effects which are convolved with the contribution that arises from their packing within the planetarysurface. The size domains over which coherent-backscatter and shadow-hiding, respectively, each have their most pronounced effect is strongly controlled by the albedo of internal scatterers and the number of hierarchical generations of aggregates required to characterize the particulate-scale behavior of the regolith surface.

Shallow-water salmon and herring detection using 100-kHz sidescan sonars

Acoustic results of fish detection using fixed 100-kHz sidescan sonar installations in the Fraser River (British Columbia) and in the Oresund (near Copenhagen, Denmark) are reviewed. Looking transverse to the river flow, echo traces due to migratory salmon were observed at ranges up to 250 m in water 6 to 13 m deep. The salmon trajectories were readily identified and counted against the background of reverberation from the river surface and bottom sediments. Similarly in the Oresund, long-term monitoring detected migrating herring schools above background reverberation at ranges up to 500 m in water 10 to 13 m deep. In both cases, occasional strong backscatter interference from vessel wakes and wind-induced surface bubble layers was observed. In the Oresund, changes in acoustic propagation conditions due to bottom inflow of more saline oceanic water drastically reduced fish detectability. For salmon in the river, acoustic multipaths due to boundary reflections were distinguishable from direct echoes at ranges up to 150 m. The general problems of shallow-water boundary reverberation and multipath reflection focusing will be reviewed, with discussion on how to compensate for these waveguide effects.