In terrestrial free-space laser communication, aside from pointing issues, the major problem that have to be dealt
with is the turbulent atmosphere that produces irradiance fluctuations in the received signal, greatly reducing
the link performance. Aperture averaging is the standard method used to mitigate these irradiance fluctuations
consisting in increasing the area of the detector, or effectively increasing it by using a collecting lens with a
diameter as large as possible. Prediction of the aperture averaging factor for Gaussian beam with currently
available theory is compared with data collected experimentally and simulations based in the beam propagation
method, where the atmospheric turbulence is represented by linearly spaced random phase screens. Experiments
were carried out using a collecting lens with two simultaneous detectors, one of them with a small aperture to
emulate an effective point detector, while the other one was mounted with interchangeable diaphragms, hence
measurements for different aperture diameters could be made. The testbed for the experiments consists of a
nearly horizontal path of 1.2 km with the transmitter and receiver on either side of the optical link. The analysis
of the experimental data is used to characterize the aperture averaging factor when different values of laser
divergence are selected.
A two-component laser Doppler velocimeter (2D-LDV) prototype for surface-displacement measurement is presented. The system proposed is based on the Doppler differential technique with a backwardscattering configuration. A reference and two frequency-shifted probe beams are generated departing from a single laser beam by means of two beamsplitters and acousto-optic modulators. This configuration has the advantage of using a single-wavelength laser source and a single detector system, while permitting sign detection of the two vector components of the velocity in surface-displacement measurements. It also enables the implementation of a low-cost multitrigger data acquisition system, which is useful in low signal-to-noise ratio (SNR) situations, such as the velocity measurement of low-dispersion surfaces or in hydrodynamic applications. The prototype has been designed to measure velocities up to 3 m per second, both in industrial (solid target) and hydrodynamic applications, with an error well below 1% in the tests performed. The system works in real time with the use of an acquisition card and C++ based software on a PC.
Failure of the first-order Rytov approximation to properly predict the scintillation index of a large-aperture focused beam, or an uplink collimated (or focused) beam, has been discussed in several recent publications, which cite beam wander effects as the main reason for this failure. We use computer simulations to examine several aspects of beam wander phenomena on a propagating convergent beam in the weak-fluctuation regime over a horizontal path at high altitude for which the refractive index structure parameter is on the order of C=1.39×10−16 m−2/3. Simulation results are presented at various ranges up to 10 km for (1) the beam wander centroid displacement, (2) the kurtosis excess of the irradiance profile, (3) the irradiance profile, (4) the mean-square hot spot displacement from the boresight and from the centroid, and (5) the scintillation index at the optical axis of the beam. In addition, simulation results are compared with theoretical models.
In an attempt to mitigate the effects of the atmosphere on the coherence of a laser beam, interest has recently been shown in changing the beam shape to determine if a different power distribution at the transmitter will reduce the effects of the random fluctuations in the refractive index. We develop a model for the scintillation index of a flattened Gaussian beam and compare this with that of the standard TEM00 Gaussian beam. We verify our results by comparison with a computer simulated model for the flattened beam.
The lognormal and gamma-gamma distributions are compared to simulated and experimental data of the irradiance
fluctuations of a Gaussian beam wave propagating through the atmosphere on a horizontal path, near ground, in the
moderate-to-strong turbulence regime. Irradiance data was collected simultaneously at three receiving apertures of
different size. Atmospheric parameters were inferred from the measurements and used to reproduce the experimental
data with numerical simulations and calculate the parameters for the theoretical probability density functions (PDFs).
The simulation values agree well with the experimental data for all three aperture sizes, while the support for the
theoretical PDFs depends on the size of the receiving aperture.
In this work, simulation of beam propagation through atmospheric turbulence is made by means of the split-step method, including spatially separated two-dimensional phase screens, which represent the existing turbulence. These phase screens may be generated mainly by two techniques, namely fractal interpolation or in the spatial frequency domain. We report some important considerations to take into account in order to generate phase screens with enough accuracy to properly reproduce the structure function of the turbulence. It is shown that slight deviations from the theoretical structure function in the set of screens used along the propagation may increase in an appreciable way the statistical error inherent to any particular realization. Some comparisons are made with analytical results based on the second order Rytov approximation. One of the conclusions that may be clearly drawn from these comparisons is that beam wander effects are not included in these theories.
In this paper we introduce the use of normalized parameters to study an electrooptically active
Y-Junction, by means of a five-layer model and the step approximation method. In a similar way
as it is done for three-layer and four-layer waveguides, we find a set of dimensionless parameters for
five-layer waveguides that allows the description of their waveguiding features.
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