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.
Elements of spectroscopic continuous-wave, frequency-modulated ladar (CW-FM-ladar) concept based on principles of both CW-FM-range-finding and modulation spectroscopy, and also on modern techniques of optical signal transmission, reception and processing are presented. Features of heterodyning methods for ladar echo-signal reception are considered. The comparison of CW-FM-ladar with CW-FM-range-finder and incoherent pulse lidar is carried out. Estimations of the achievable signal-to-noise ratio, the operation range and the range resolution are performed using frequency-dependent parameters of the transmitting and receiving subsystems. Preliminary experimental results on the range-finding subsystem characteristics of the CW-FM-laser diode (LD)-ladar are discussed.
The European Space Agency (ESA) has launched the geostationary data-relay satellite ARTEMIS with one of its payloads being a laser communication terminal (LCT). The LCT is used within the semiconductor-laser intersatellite link experiment (SILEX) to receive Earth observation data transmitted from a similar LCT onboard the SPOT-4 satellite. To support SILEX, ESA has also reached an agreement with the Instituto de Astrofisica de Canarias (JAC) to build the Optical Ground Station (OGS), in the Teide Observatory ofthe IAC. ARTEMIS and the OGS are an ideal and unique test-bed to study and characterise laser beam propagation through atmospheric turbulence. Theoretical models of laser beam propagation through atmospheric turbulence have been reviewed and developed, to predict the performance of the optical links from the propagation and communication point of view. Special effort has been invested in modelling the uplink effects. Optical link experiments have been planned in detail, to gather the necessary data required to be statistically representative, to compare the results with theoretical predictions and to validate and adjust the theoretical models. This comparison will pave the way towards the implementation of deep-space laser communication links. The first results ofthese experiments, presenting the theoretical models, analysing separately downlink and uplink measurements, and comparing the data with the theoretical predictions, are presented.
The nonlinear directional coupler is analyzed in the presence of asymmetrically distributed and saturable nonlinear refractive index. We make use of the linear supermodes of the structure and phase-space techniques, which allows to obtain the main features of its behavior, without the need of solving the coupled mode equations.
In the present work we analyse the nonlinear, nonlocal response tensor describing optical second-harmonic processes
in centrosymmetric free-electron-like bulk metals with a flat surface. On the basis of the classical infinite barrier
(CIB)-model and the Boltzmann equation in the relaxation time approximation we present new analytic results for
the fully nonlocal nonlinear response tensor. Via numerical calculations the nonlinear, fully nonlocal response tensor's
dependence on the fundamental frequency is discussed and compared with that of the near-local (hydrodynamic)
response tensor. Finally, the significance of the contribution to the nonlinear, nonlocal optical response stemming
for single-particle excitations, i.e. Landau interactions, is considered.