103 research outputs found
The influrence of an optical receiving system on statistical characteristics of a lidar signal
The effects connected with correlation of direct and backward waves propagating through the same randomly inhomogeneous media can be observed along the path with refection in a turbulent atmosphere. In particular, the mean intensity of the reflected wave can increase in comparison with the wave propagating in the forward direction at a doubled distance; the intensity fluctuations can become stronger. These effects depend on the strength of optical turbulence, as well as on the diffraction sizes of the exit apertures of the source and the reflector. However, the focusing of radiation reflected with a receiving telescope leads, in some cases, to the fact that the dependence of amplification effects on the parameters becomes essentially different. This should be taken into account when alayzing the lidar signals. The effect of backscattering amplification and amplification of the intensity fluctuations is discussed
Influence of Phase Diffuser Dynamics on Scintillations of Laser Radiation in Earth Atmosphere: Long-Distance Propagation
The effect of a random phase diffuser on fluctuations of laser light
(scintillations) is studied. Not only spatial but also temporal phase
variations introduced by the phase diffuser are analyzed. The explicit
dependence of the scintillation index on finite-time phase variations is
obtained for long propagation paths. It is shown that for large amplitudes of
phase fluctuations, a finite-time effect decreases the ability of phase
diffuser to suppress the scintillations.Comment: 15 pages, 4 figure
Beam Wandering in the Atmosphere: The Effect of Partial Coherence
The effect of a random phase screen on laser beam wander in a turbulent
atmosphere is studied theoretically. The method of photon distribution function
is used to describe the photon kinetics of both weak and strong turbulence. By
bringing together analytical and numerical calculations, we have obtained the
variance of beam centroid deflections caused by scattering on turbulent eddies.
It is shown that an artificial distortion of the initial coherence of the
radiation can be used to decrease the wandering effect. The physical mechanism
responsible for this reduction and applicability of our approach are discussed.Comment: 16 pages, 5 figure
Photon Distribution Function for Long-Distance Propagation of Partially Coherent Beams through the Turbulent Atmosphere
The photon density operator function is used to calculate light beam
propagation through turbulent atmosphere. A kinetic equation for the photon
distribution function is derived and solved using the method of
characteristics. Optical wave correlations are described in terms of photon
trajectories that depend on fluctuations of the refractive index. It is shown
that both linear and quadratic disturbances produce sizable effects for
long-distance propagation. The quadratic terms are shown to suppress the
correlation of waves with different wave vectors. We examine the intensity
fluctuations of partially coherent beams (beams whose initial spatial coherence
is partially destroyed). Our calculations show that it is possible to
significantly reduce the intensity fluctuations by using a partially coherent
beam. The physical mechanism responsible for this pronounced reduction is
similar to that of the Hanbury-Braun, Twiss effect.Comment: 28 pages, 4 figure
Topological monoids of monotone injective partial selfmaps of with cofinite domain and image
In this paper we study the semigroup
of partial cofinal monotone
bijective transformations of the set of positive integers . We show
that the semigroup has algebraic
properties similar to the bicyclic semigroup: it is bisimple and all of its
non-trivial group homomorphisms are either isomorphisms or group homomorphisms.
We also prove that every locally compact topology on
such that
is a topological inverse
semigroup, is discrete. Finally, we describe the closure of
in a topological
semigroup
Continuous selections of multivalued mappings
This survey covers in our opinion the most important results in the theory of
continuous selections of multivalued mappings (approximately) from 2002 through
2012. It extends and continues our previous such survey which appeared in
Recent Progress in General Topology, II, which was published in 2002. In
comparison, our present survey considers more restricted and specific areas of
mathematics. Note that we do not consider the theory of selectors (i.e.
continuous choices of elements from subsets of topological spaces) since this
topics is covered by another survey in this volume
Consonance and Cantor set-selectors
It is shown that every metrizable consonant space is a Cantor set-selector. Some applications are derived from this fact, also the relationship is discussed in the framework of hyperspaces and Prohorov spaces.peer-reviewe
Antimicrobial de-escalation in the critically ill patient and assessment of clinical cure: the DIANA study
Purpose: The DIANA study aimed to evaluate how often antimicrobial de-escalation (ADE) of empirical treatment is performed in the intensive care unit (ICU) and to estimate the effect of ADE on clinical cure on day 7 following treatment initiation. Methods: Adult ICU patients receiving empirical antimicrobial therapy for bacterial infection were studied in a prospective observational study from October 2016 until May 2018. ADE was defined as (1) discontinuation of an antimicrobial in case of empirical combination therapy or (2) replacement of an antimicrobial with the intention to narrow the antimicrobial spectrum, within the first 3 days of therapy. Inverse probability (IP) weighting was used to account for time-varying confounding when estimating the effect of ADE on clinical cure. Results: Overall, 1495 patients from 152 ICUs in 28 countries were studied. Combination therapy was prescribed in 50%, and carbapenems were prescribed in 26% of patients. Empirical therapy underwent ADE, no change and change other than ADE within the first 3 days in 16%, 63% and 22%, respectively. Unadjusted mortality at day 28 was 15.8% in the ADE cohort and 19.4% in patients with no change [p = 0.27; RR 0.83 (95% CI 0.60\u20131.14)]. The IP-weighted relative risk estimate for clinical cure comparing ADE with no-ADE patients (no change or change other than ADE) was 1.37 (95% CI 1.14\u20131.64). Conclusion: ADE was infrequently applied in critically ill-infected patients. The observational effect estimate on clinical cure suggested no deleterious impact of ADE compared to no-ADE. However, residual confounding is likely
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