Arun K. Majumdar

Effects of the atmospheric channel on free-space laser communications

In this paper we provide an overview of how the presence of atmospheric turbulence and scattering/absorbing media in the atmospheric channel degrade high-data-rate free-space laser communication performance. The impact of the atmospheric channel on overall link budget performance is discussed. Fog, rain, dust, snow, smoke, molecular absorption, and aerosol particulate matter all attenuate the signal-carrying laser beam, and to a certain extent can be compensated for by increasing the signal gain or by appropriate selection of the optical wavelength. In contrast, random fluctuations in the atmospheres refractive index severely degrade the wave-front quality of a signal-carrying laser beam, causing intensity fading and random signal losses at the receiver. This results in increased system bit error rates, especially along horizontal propagation paths. Atmospheric turbulence-induced signal losses increase as the distance between the transmitter and receiver is increased, and there is no wavelength window where these effects can be avoided, although longer wavelengths are less affected. With atmospheric turbulence, increasing the signal gain will not necessarily improve laser beam quality. For many cases of practical interest, the limiting factor in robust free-space optical communication link performance can be the presence of clear-air atmospheric turbulence in the optical channel. Various proposed probability density functions of laser intensity fluctuations through atmospheric turbulence will be discussed as they relate to laser communications performance and reliability under different weather conditions. Results from numerical simulations are presented for analyzing communications performance for various scenarios: Downlink, Uplink and Terrestrial (Horizontal) link.

Turbulence modeling for non-line-of-sight ultraviolet scattering channels

Recently ultraviolet (UV) scattering channels have received renewed interest for non-line-of-sight (NLOS) communication. Monte Carlo simulations and field experiments have yielded valuable results to predict channel path loss and impulse response at relatively short ranges, critical for communication link analysis. However, as communication range increases, the effect of turbulence becomes pronounced and inevitably induces additional impairments to system performance. This paper suggests a turbulence modeling method for NLOS UV channels incorporating the effects of scattering and absorption. The modeling results can be applied to study communication performance.

A parametric single scattering channel model for non-line-of-sight ultraviolet communications

Recent advances in ultraviolet (UV) semiconductor sources and detectors have inspired significant research activities in short-range UV communications, particularly in non-line-of-sight (NLOS) channel conditions due to atmospheric scattering. However, a scattering channel involves complex interactions of photons with atmospheric particles. This paper presents a parametric channel model that greatly simplifies channel characterization. For a short range link, single scattering may dominate in some scenarios. We model the channel impulse response with a gamma function as well as its variants to better fit the prediction by a widely adopted analytical single scattering model. Normalized mean square fitting error is adopted to validate our parametric model. Path losses and channel bandwidths are subsequently studied under different geometrical link configurations.

Reconstruction of probability density function of intensity fluctuations relevant to free-space laser communications through atmospheric turbulence

A new method of reconstructing and predicting an unknown probability density function (PDF) characterizing the statistics of intensity fluctuations of optical beams propagating through atmospheric turbulence is presented in this paper. The method is based on a series expansion of generalized Laguerre polynomials ; the expansion coefficients are expressed in terms of the higher-order intensity moments of intensity statistics. This method generates the PDF from the data moments without any prior knowledge of specific statistics and converges smoothly. The utility of reconstructed PDF relevant to free-space laser communication in terms of calculating the average bit error rate and probability of fading is pointed out. Simulated numerical results are compared with some known non-Gaussian test PDFs: Log-Normal, Rice-Nakagami and Gamma-Gamma distributions and show excellent agreement obtained by the method developed. The accuracy of the reconstructed PDF is also evaluated.

Laboratory simulation of atmospheric turbulence for laser propagation: design and characterization

A turbulence chamber has been designed and built for performing optical propagation experiments. The chamber can produce turbulence 1000 times stronger than that of atmospheric turbulence, and is isotropic and homogeneous. Scintillation measurements for optical propagation inside the chamber are presented and compared with real atmospheric tests. The chamber can be used to perform optical propagation experiments under well-controlled and reproducible conditions. The chamber can be used to develop experiments to test optical communications, lidar, directed energy weapons, electro-optical designers, imaging systems, and air-borne laser systems which have to operate under atmospheric turbulence conditions. The chamber can provide tests results which are much less costly compared to outdoor and field tests.

Analysis of optical communications through the random air-water interface: feasibility for under-water communications

New results for investigating optical propagation through the random wavy air-water interface relevant to underwater optical communications are presented. A laser beam propagating through the air-water interface reaching a receiver below the water surface, as well as propagated through the water towards an airborne receiver, is significantly distorted due to the high geometric phase aberrations introduced by the random motion of the water surface waves. This causes a significant reduction in the received communications signal resulting in limiting the data transfer capability and the transmitting and receiving data rates. This research develops probabilistic models for optical propagation at the random air-water interface for both reflection and transmission cases under various wind speed conditions. Preliminary results from a laboratory water tank experiment provide information about histograms or the probability density functions of intensity fluctuations measured by a CCD camera for both reflection and transmission cases. Angular displacements of the centroid of the fluctuating laser beam generates the beam wander effects. Finally preliminary results for BER estimates for an on-off keying (OOK) for air-water interface only are presented for a communication system where random air-water interface is a part of communication channel.

Atmospheric turbulence measurements over desert site using ground-based instruments, kite/tethered-blimp platform, and aircraft relevant to optical communications and imaging systems: preliminary results

New results of the (temperature) refractive index structure parameter (CT2), Cn2 are presented from fast response sensor observations near the ground and also using a kite/tethered blimp platform and an aircraft, at the Edward Air Force Base in Mojave Desert, California. Additional optical measurements include near-ground scintillation observations over horizontal paths. Atmospheric turbidity were also calculated from direct beam solar radiation measurements using pyrheliometer. Comparisons were made of the observed profiles of refractive index structure parameters (Cn2) with theoretical modeled profiles, and two derived quantities such as transverse coherence length (r0) and isoplanatic angle (θ0) for a slant path are discussed. All of these parameters are the major indicators of turbulence and are important to design an aircraft or space-craft-based free-space laser communication and high resolution optical synthetic-aperture imaging systems. Non-isotropic turbulence observations from some of the data will be pointed out. Probability density functions (PDF) of the distribution of Cn2 will be described using histograms. Fundamental limits imposed by atmospheric effects in high data rate communication and optical synthetic-aperture imaging systems will be discussed.

Non-line-of-sight ultraviolet communication channel characterization: modeling and validation

Non-line-of-sight (NLOS) ultraviolet (UV) scattering channel impulse response in the deep UV solar-blind spectrum band is investigated. Taking into account a light source power angular distribution and applying a photon tracing technique, Monte Carlo simulation is performed to obtain the channel impulse response and associated path loss. Some comparisons are carried out with an existing single scattering channel model, as well as field measurements in both the impulse response model and path loss, to demonstrate modeling accuracy. The importance of considering multiple versus single-scattering in the analysis is shown. The results provide guidelines for study of limitations to data rate and communication range in NLOS UV communications.

Demonstration of adaptive optics for mitigating laser propagation through a random air-water interface

This paper describes a new concept of mitigating signal distortions caused by random air-water interface using an adaptive optics (AO) system. This is the first time the concept of using an AO for mitigating the effects of distortions caused mainly by a random air-water interface is presented. We have demonstrated the feasibility of correcting the distortions using AO in a laboratory water tank for investigating the propagation effects of a laser beam through an airwater interface. The AO system consisting of a fast steering mirror, deformable mirror, and a Shack-Hartmann Wavefront Sensor for mitigating surface water distortions has a unique way of stabilizing and aiming a laser onto an object underneath the water. Essentially the AO system mathematically takes the complex conjugate of the random phase caused by air-water interface allowing the laser beam to penetrate through the water by cancelling with the complex conjugates. The results show the improvement of a number of metrics including Strehl ratio, a measure of the quality of optical image formation for diffraction limited optical system. These are the first results demonstrating the feasibility of developing a new sensor system such as Laser Doppler Vibrometer (LDV) utilizing AO for mitigating surface water distortions.

Measurement and characterization of laboratory-simulated turbulence parameters of interest to adaptive optics imaging and laser communications

Further results from the previously reported atmospheric turbulence chamber are discussed in this paper. The generated turbulence has been characterized by evaluating isotropy and homogeneity within the chamber by measuring temperature structure functions for various nozzle pressures and temperature differences. Measurements of higher-order intensity moments were utilized to reconstruct the probability density functions and were compared with Log- Normally Modulated Exponential, Log-Normal and Gamma distributions. The power spectrum of intensity fluctuations of a laser beam propagated through the generated turbulence provided additional characterization of the chamber regarding scale sizes and frequency cut-offs. Frieds parameter (r0) and the Greenwood frequency of the artificially generated turbulence were computed and were shown that it is possible to simulate the real atmospheric parameters with a modified design of the present chamber.