Bradley A. Clare

Characterizing the propagation path in moderate to strong optical turbulence

In February 2005 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation and the University of Central Florida. A Gaussian beam was propagated along a horizontal 1500 m path near the ground. Scintillation was measured simultaneously at three receivers of diameters 1, 5, and 13 mm. Scintillation theory combined with a numerical scheme was used to infer the structure constant C2n, the inner scale l0, and the outer scale L0 from the optical measurements. At the same time, C2n measurements were taken by a commercial scintillometer, set up parallel to the optical path. The C2n values from the inferred scheme and the commercial scintillometer predict the same behavior, but the inferred scheme consistently gives slightly smaller C2n values.

Scintillation : theory vs. experiment

In May 2004 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation, the Office of Naval Research and the University of Central Florida. A 45 mm divergent Gaussian beam was propagated along a horizontal 1500 meter path approximately 2 meters above the ground. At the receiver were 3 apertures of diameter 1mm, 5mm, and 13mm. The scintillation was measured at each aperture and compared to scintillation theory, recently developed for all regimes of optical turbulence. Three atmospheric parameters, Cn2, lo and Lo, were inferred from these optical measurements. Simultaneously, a commercial scintillometer, which recorded values for Cn2, was set up parallel to the optical path. In this paper, a numerical scheme is used to infer the three atmospheric parameters and comparisons are made with the Cn2 readings from the scintillometer.

Scintillation index of the free space optical channel: Phase screen modelling and experimental results

Scintillation index (SI) is a key metric for free space optical communications (FSOC), and measures the normalised intensity variance caused by atmospheric turbulence. It is a function of the refractive index structure parameter Cn2, range, and receiver aperture. There is a need for an atmospheric simulation model of the effects of scintillation because testing of FSOC performance in the environment is difficult and time consuming. In this paper we compare experimental results with numerical simulations using phase screens for channels involving three receivers of different size apertures. There is good agreement in the results of experiment and model.

Comparison of probability density functions for analyzing irradiance statistics due to atmospheric turbulence

A large number of model probability density functions (PDFs) are used to analyze atmospheric scintillation statistics. We have analyzed scintillation data from two different experimental setups covering a range of scintillation strengths to determine which candidate model PDFs best describe the experimental data. The PDFs were fitted to the experimental data using the method of least squares. The root-mean-squared fitting error was used to monitor the goodness of fit. The results of the fitting were found to depend strongly on the scintillation strength. We find that the log normally modulated Rician and the log normal PDFs are the best fit to the experimental data over the range of scintillation strengths encountered.

Development and implementation of a robust angle of arrival turbulence measurement system

Abstract. Free space optical communications (FSOC) systems are a promising complement to existing wireless communications technologies. FSOC systems have many significant advantages over traditional radio frequency links, including high bandwidth, no spectrum licensing requirements, low-power consumption, small payloads, low probability of intercept, and greater immunity from interference or jamming. However, atmospheric turbulence (scintillation) imparts significant phase noise onto the laser beam, resulting in intensity fluctuations at the receiver. In order to develop scintillation mitigation strategies, it is necessary to monitor scintillation in parallel to the communications channel. We report on the development and implementation of a robust angle of arrival (AoA) turbulence measurement instrument that is suitable for this task. Several key data acquisition and processing techniques were designed to enhance the reliability and robustness of the scintillation measurement.

Differential oversampling data converters in SEED technology

Abstract Differential architectures for both first order error diffusion and first order sigma–delta modulators are presented in this paper. Techniques required to transform single-ended architectures to differential architectures are discussed which are suitable for implementation in both p–i–n and n–i–n SEED technologies. Descriptions of common SEED circuit modules, together with SPICE behavioural simulations are also presented. A feature of the architectures presented is that they can be fully integrated into a single substrate using MEMS technology. This can be done by incorporating integrated optical waveguides together with MMIC technology. The goal of this work is a fully integrated differential optical oversampling modulator with extremely high resolution and linearity.

Long range, analog RF free space optical communication link in a maritime environment

The Naval Research Laboratory (NRL) in collaboration with the Defence Science and Technology Organisation (DSTO) of Australia has performed long distance experiments with analog modulated free space optical communication links across the Chesapeake Bay. Results will be presented on estimating the probability density functions of the RF parameters of gain, noise factor, and linearity after propagating an RF modulated, 1550nm laser beam over a 32km distance (folded round-trip across Chesapeake Bay). In addition, results from the transmission of video using analog FM modulation of a 1550nm laser beam over the link will be presented.

Initial Measurements of Atmospheric Parameters in a Marine Environment

April 2005, a laser propagation experiment was conducted over a 470m horizontal maritime path. Scintillation measurements of a divergent Gaussian beam wave were taken simultaneously for different receiver aperture sizes. Terrestrial scintillation theory combined with a numerical algorithm was used to infer the atmospheric parameters Cn2 and lo from the optical maritime scintillation measurements. This paper presents the initial results.

Numerical experiments in atmospheric scintillation correlation for applications in dual channel optical communications

A method for reducing noise in near-IR laser communications has been proposed that relies upon the dual wavelength output of the He-Xe laser having a high level of noise coherence. However, in transmissions through the atmospheric boundary layer, an additional and significant noise component is added by atmospheric scintillation. These scintillations are mainly limited to frequencies of less than 1 kHz and are correlated in the two laser channels to a degree determined by the channel wavelength separation, the transmission range and the severity of the turbulence regime. To analyze the propagation of waves in random media one normally considers the statistics of the field. In the case of small angle forward scattering, which is the case of interest in laser propagation, field moments higher than the fourth are so difficult to solve that no solutions are known outside of the asymptotic weak and strong approximations. An alternative approach is to conduct numerical experiments in which one generates a realization of the random medium (with the desired statistics) and then calculates the wave field. We have numerically modeled the spatial irradiance intensity as a function of range from a point source under turbulence regimes typical of daytime conditions near the Earth’s surface. Simulations were performed for two closely separated channels in the near-IR (1556.5 and 1558.1 nm). We present the results of these simulations together with the implications for the mitigation of atmospheric scintillation noise by common mode rejection.

Real-time scintillation noise mitigation for free space optical transmission of analogue and digital signals

It is well-known that free space optical communications through a turbulent atmosphere are adversely affected by scintillation noise. This paper reports on the experimental demonstration of a two-colour common mode rejection technique to mitigate atmospheric scintillation noise. Real-time equalisation was achieved for both analogue (amplitude modulated PAL composite video) and digital (quadrature amplitude modulated) signals.