Charles Nelson

Polarization-induced reduction in scintillation of optical beams propagating in simulated turbulent atmospheric channels

It is experimentally demonstrated that the class of partially coherent, partially polarized optical beams can be efficiently used for reduction in scintillations on propagation through turbulent air. The experiment involving the electromagnetic beam generation and its interaction with turbulent air simulator is discussed in details. The collected data is in solid agreement with the recently published theoretical predictions.

The dependence of the intensity PDF of a random beam propagating in the maritime atmosphere on source coherence

Measurements of intensity statistics of a spatially pre-randomized laser beam propagating in a turbulent atmosphere along two horizontal links in close proximity to grassy ground and above a creek have been carried out in the fall of 2012. The pre-randomization of the laser beam was made by using a nematic Spatial Light Modulator placed at the laser source exit. The beam’s phase is randomized with Gaussian statistics with zero mean and fully controllable degree of coherence. The receiver consisted of an amplified photodetector and data acquisition device which were used to capture the beam intensity fluctuations after propagation through turbulent media. The results for the intensity Probability Density Function (PDF), the Cumulative Distribution Function and, in particular, for the scintillation index are presented for a variety of the source’s states of coherence and environmental conditions. The analytical PDF is reconstructed from the statistical moments of intensity with the help of the Gamma-Laguerre model. The results clearly illustrate the dependence of the shapes of the density functions on the level of the source’s degree of coherence and on environmental parameters. It is shown that the optimal source coherence level may be experimentally found to obtain the narrowest profile for the intensity PDF.

Probability density function of the intensity of a laser beam propagating in the maritime environment

A number of field experiments measuring the fluctuating intensity of a laser beam propagating along horizontal paths in the maritime environment is performed over sub-kilometer distances at the United States Naval Academy. Both above the ground and over the water links are explored. Two different detection schemes, one photographing the beam on a white board, and the other capturing the beam directly using a ccd sensor, gave consistent results. The probability density function (pdf) of the fluctuating intensity is reconstructed with the help of two theoretical models: the Gamma-Gamma and the Gamma-Laguerre, and compared with the intensitys histograms. It is found that the on-ground experimental results are in good agreement with theoretical predictions. The results obtained above the water paths lead to appreciable discrepancies, especially in the case of the Gamma-Gamma model. These discrepancies are attributed to the presence of the various scatterers along the path of the beam, such as water droplets, aerosols and other airborne particles. Our papers main contribution is providing a methodology for computing the pdf function of the laser beam intensity in the maritime environment using field measurements.

Probability density function of partially coherent beams propagating in the atmospheric turbulence

Measurements of the intensity of a light beam produced by a laser source, reflected from a Spatial Light Modulator (SLM) and propagating in a weakly turbulent maritime atmosphere have been carried out in a recent campaign on the grounds of the US Naval Academy. The effect of the degree of spatial coherence in the beam at the source plane (right after the reflection from the SLM) on the Probability Density Function (PDF) of the intensity of the propagating beam at a single fixed position in space is studied in detail. The measured intensity histogram of the fluctuating intensity is compared with the Gamma-Laguerre analytical model for the intensity PDF.

Scintillation reduction in multi-Gaussian Schell-model beams propagating in atmospheric turbulence

The recently introduced class of Multi-Gaussian Schell-model [MGSM] beams is investigated via simulations and experiments with regards to its intensity fluctuations on propagation in atmospheric turbulence. The results indicate that the scintillation index of the MGSM beam is reduced for high values of the summation index, in agreement with previous theoretical results.

Measurements of partially spatially coherent laser beam intensity fluctuations propagating through a hot-air turbulence emulator and comparison with both terrestrial and maritime environments

Measurements of partially spatially coherent infra-red laser beam intensity fluctuations propagating through a hot-air turbulence emulator are compared with visible laser beam intensity fluctuations in the maritime and IR laser beam intensity fluctuations in the terrestrial environment at the United States Naval Academy. The emulator used in the laboratory for the comparison is capable of generating controlled optical clear air turbulence ranging from weak to strong scintillation. Control of the degree of spatial coherence of the propagating laser beam was accomplished using both infrared and visible spatial light modulators. Specific statistical analysis compares the probability density and temporal autocovariance functions, and fade statistics of the propagating laser beam between the in-laboratory emulation and the maritime field experiment. Additionally, the scintillation index across varying degrees of spatial coherence is compared for both the maritime and terrestrial field experiments as well as the in-laboratory emulation. The possibility of a scintillation index ‘sweet’ spot is explored.

Free-space optical channel characterization in the maritime environment

The performance of a free-space optical (FSO) communications link in a maritime environment was evaluated during two field trials conducted off the mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Bidirectional, ship-to-shore data links operating at 2.5 Gbps and utilizing commercial, single-mode adaptive optics terminals were set up between a lookout tower located on Cedar Island, VA and a Johns Hopkins University Applied Physics Laboratory research vessel over a range of 2-22 km (optical horizon). This paper presents the test configuration, discusses the overall performance of the FSO channel, and compares it to the available turbulence and weather measurements. Additionally, modeling of the link configuration is presented, and comparisons are made to data measured throughout the experiment such as received power, received beam diameter, and local The performance of a free-space optical (FSO) communications link in a maritime environment was evaluated during two field trials conducted off the mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Bidirectional, ship-to-shore data links operating at 2.5 Gbps and utilizing commercial, single-mode adaptive optics terminals were set up between a lookout tower located on Cedar Island, VA and a Johns Hopkins University Applied Physics Laboratory research vessel over a range of 2-22 km (optical horizon). This paper presents the test configuration, discusses the overall performance of the FSO channel, and compares it to the available turbulence and weather measurements. Additionally, modeling of the link configuration is presented, and comparisons are made to data measured throughout the experiment such as received power, received beam diameter, and local Cn2.

Probability density function analysis for optical turbulence with applications to underwater communications systems

The Weibull and Exponentiated Weibull probability density functions have been examined for the free space regime using heuristically derived shape and scale parameters. This paper extends current literature to the underwater channel and explores use of experimentally derived parameters. Data gathered in a short range underwater channel emulator was analyzed using a nonlinear curve fitting methodology to optimize the scale and shape parameters of the PDFs. This method provides insight into the scaled effects of underwater optical turbulence on a long range link, and may yield a general set of equations for determining the PDF for an underwater optical link.

Measurements and comparison of the probability density and covariance functions of laser beam intensity fluctuations in a hot-air turbulence emulator with the maritime atmospheric environment

A hot-air turbulence emulator is employed for generating controlled optical clear air turbulence in the weak fluctuation regime in laboratory conditions. The analysis of the first and second-order statistical moments of the fluctuating intensity of a propagating infra-red (IR) laser beam through the turbulence emulator is made and the results are compared with bi-directional shore-to-ship maritime data collected during two 2009 mid-Atlantic Coast field tests utilizing single-mode adaptive optics terminals at a range of 10.7 km, as well as with a 633 nm Helium Neon laser propagating across land and water at the United States Naval Academy.

Characterization of data transmission through a maritime free-space optical channel with a custom bit error rate tester

Free Space Optical (FSO) Communications channels can exhibit high percentage availability, yet are subject to frequent intensity fades due to turbulence effects. For gigabit class links, tremendous amounts of data can still be transported through a fading channel, but an efficient network protocol is required to overcome the effects of fades. We describe a custom error detector that can process a digital signal from a channel which has frequent fades below system sensitivity and can provide data link statistics with bit-level timing accuracy. The statistics measured by the instrument include bit level counters that allow the device to be used as a traditional bit error rate tester (BERT), as well as block-based counters, which provide insight to the channel for packet based transmission formats. Synchronization parameters are adjustable to accommodate different link dynamics. Additionally, stretched error and sync pulse outputs provide useful live indicators of link performance when plotted against optical channel power. This paper will discuss the performance of the custom bit error rate tester (cBERT) in testing a 2.5 Gbps channel over a maritime FSO link trial conducted off the mid-Atlantic coast near Wallops Island, VA, in July and September 2009. Additionally, the overall design of the cBERT will be presented.