Jaume Recolons

Aperture averaging effects on the probability density of irradiance fluctuations in moderate-to-strong turbulence

The lognormal (LN) and gamma-gamma (GG) distributions are compared to simulated and experimental data of the irradiance fluctuations of a Gaussian beam wave propagating through the atmosphere along a horizontal path, near the ground, in the moderate-to-strong turbulence regime. Irradiance data were collected simultaneously at three receiving apertures of different sizes. Atmospheric parameters were inferred from the measurements and scintillation theory and were used to develop the parameters for the theoretical probability density functions. Numerical simulations were produced with the same C(n)(2) value as the experimental data. Aperture-averaging effects were investigated by comparing the irradiance distributions for the three apertures at two different values of the structure parameter C(n)(2), and, hence, different values of the coherence radius rho(0). For the moderate-to-strong fluctuation regime, the GG distribution provides a good fit to the irradiance fluctuations collected by finite-sized apertures that are significantly smaller than rho(0). For apertures larger than or equal to rho(0), the irradiance fluctuations appear to be LN distributed.

The curious arithmetic of optical vortices

The superposition of noncoaxial light beams containing screw wave-front dislocations is shown to create light patterns with a richer vortex content than that given by the arithmetic of the topological charges of the individual beams. We report the experimental observation of this phenomenon.

Analysis of beam wander effects for a horizontal-path propagating Gaussian-beam wave: focused beam case

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.

Propagation of flattened Gaussian beams in the atmosphere: a comparison of theory with a computer simulation model

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.

Guided-to-leaky mode transition in uniaxial optical slab waveguides

The guided-to-leaky hybrid mode transition in slab optical waveguides made on uniaxial crystals such as LiNbO/sub 3/, or LiTaO/sub 3/, is analyzed. Two different guided-to-leaky transitions have been identified, namely the ordinary cutoff and the extraordinary cutoff, which occur when considering negative and positive birefringent materials, respectively. Analytical but transcendental expressions have been obtained, yielding the critical optical axis orientation, relative to the waveguide axis, above which the totally guided hybrid modes become leaky. The results indicate that the value of the critical orientation strongly depends on the waveguide parameters. The possibility of tuning this critical orientation to a desired value through the waveguide parameters is discussed. >

Temporal analysis of laser beam propagation in the atmosphere using computer-generated long phase screens.

Temporal analysis of the irradiance at the detector plane is intended as the first step in the study of the mean fade time in a free optical communication system. In the present work this analysis has been performed for a Gaussian laser beam propagating in the atmospheric turbulence by means of computer simulation. To this end, we have adapted a previously known numerical method to the generation of long phase screens. The screens are displaced in a transverse direction as the wave is propagated, in order to simulate the wind effect. The amplitude of the temporal covariance and its power spectrum have been obtained at the optical axis, at the beam centroid and at a certain distance from these two points. Results have been worked out for weak, moderate and strong turbulence regimes and when possible they have been compared with theoretical models. These results show a significant contribution of beam wander to the temporal behaviour of the irradiance, even in the case of weak turbulence. We have also found that the spectral bandwidth of the covariance is hardly dependent on the Rytov variance.

Accurate calculation of phase screens for the modelling of laser beam propagation through atmospheric turbulence

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.

One-wavelength two-component laser Doppler velocimeter system for surface displacement monitoring

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.

Observation of optical vortex streets in walking-second harmonic generation

We report what is believed to be the first experimental observation of spontaneous nucleation of arrays of optical vortices in the process of second-harmonic generation in a crystal with moderate Poynting vector walk-off, pumped with focused beams containing screw phase dislocations. Our experiments were conducted in lithium triborate under conditions of small-efficiency second-harmonic generation pumped by nanosecond pulses at 1064 nm.

Aperture averaging in a laser Gaussian beam: simulations and experiments

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.