Eun Oh

Passive optical monitor for atmospheric turbulence and windspeed

Measurement of the atmospheric index of refraction structure constant (Cn^2) is critical for predicting the performance of a free-space optical laser communication (FSO lasercomm) link. A Cn^2 monitor based on angle-of-arrival (AOA) fluctuations has been built for characterization of atmospheric conditions at the NRL FSO Lasercomm Test Facility across the Chesapeake Bay. The monitor used existing lights in various locations as point sources for determining AOA fluctuations. Real time analysis of the AOA fluctuations was performed to determine the power spectrum of the fluctuations every few seconds. This additional power spectrum information allows much greater understanding of atmospheric conditions including estimation of average wind speed based on frequency shifts in the power spectrum distribution. The performance of the monitor was tested over short paths by comparison to a commercial scintillometer. In addition, the monitor was used at other sites to determine atmospheric conditions at a variety of locations. Results of these experiments are presented.

A comparison of optical turbulence models

The U.S. Navy has an interest in the use of laser systems for surface ships. Such systems must operate within a thin near-surface environment called the marine atmospheric surface layer. There exist substantial gradients in temperature and momentum within this layer which make turbulence a strong function of height. We are interested in robust and simple optical turbulence models that can be used to predict turbulence along near-horizontal paths. We discuss several different models that are based upon similarity theory, and we compare the models with field transmission data taken from both over-water and over-land propagation paths.

Estimating optical turbulence effects on free-space laser communication: modeling and measurements at ARL's A_LOT facility

We evaluate a similarity-based optical turbulence model that estimates diurnal values for Cn2 from easily obtained local terrain and environmental information by comparing it with scintillometer data taken at the Army Research Laboratory’s A_LOT facility in Adelphi, Maryland. The A_LOT facility is characteristic of many planned urban sites for free-space laser communication. One end of the test site is on top of a two-story building, and the other end is a water tower about 70 meters high. This comparison examines the effects of the asymmetric location, such as the non-uniform height above ground and surface roughness length. We found that by emphasizing the terrain type directly in front of the receiver and assuming the height above ground to be the height of the receiver, model results compare favorably with experimental data.

Spatial intensity correlation and aperture averaging measurements in a 20-mile retroreflected lasercom link

The Naval Research Laboratory has established a lasercom test bed across the Chesapeake Bay. The test bed uses a bi-static transmitter/receiver arrangement on the western shore of the Chesapeake Bay and various configurations of 5 cm retro-reflectors on the eastern shore to produce a 32 km retro-reflected lasercom test range. Experiments measuring the laser’s transverse spatial profile after propagation over the test range have been performed. These experiments use an InGaAs CCD to image the pupil plane of the 40 cm receiver telescope and a frame grabber to store contiguous images for analysis. Analysis of these image sequences allows measurement of transverse spatial correlations across the received beam after 32 km retro-reflected propagation of the beam. Various configurations and numbers of retro-reflectors were studied to investigate the impact of number and arrangement of retro-reflectors on the received beam’s spatial profile and spatial correlations. Additionally, since the CCD output is stored as a contiguous stream of images, analysis of these images’ intensity variance in time allows measurement of aperture averaging effects as a function of number of retro-reflectors and their geometry. Results from these experiments are presented.

Tripartite entanglement generation via four-wave mixings: narrowband triphoton W state

We propose a method to generate a narrowband triphoton W state entangled in time (or energy) via two four-wave mixing processes in cold atomic gas media. The calculation of such a triphoton W state is performed with second-order perturbation theory. To characterize the optical properties of the state, we analyze the two-photon and three-photon temporal correlations in the photon coincidence counting measurement. Considering the role of determining the time coherence of triphotons between the nonlinear susceptibilities and phase matchings, we concentrate on two regimes, damped Rabi oscillation and group delay, to look at the temporal correlations. To further enhance the nonlinear interactions, it may be promising to consider cold atoms confined within hollow fibers or loaded into a high-Q cavity.

Estimating Optical Turbulence Using the PAMELA Model

We present an optical turbulence model that has evolved from the PAMELA model. After a preliminary report in SPIE 2003 it became apparent that more data was needed to refine this adaptive model. This led us to take twelve months of over-land data (~100 meters pathlength) at the Chesapeake Bay Detachment of the Naval Research Lab. We present data throughout the year with varying environments with comparison with the model prediction. Our recent modification includes segmenting the windspeed to 3 sections, morning, afternoon, and night for better fitting. This is an attempt to incorporate variable wind speed into the model which is known to contribute significantly to the turbulence in the atmosphere. In addition, we present preliminary results from the over-the-bay data (10 km pathlength).

Humidity's influence on visible region refractive index structure parameter C n 2

In the near-infrared and visible bandpasses optical propagation theory conventionally assumes that humidity does not contribute to the effects of atmospheric turbulence on optical beams. While this assumption may be reasonable for dry locations, we demonstrate that there is an unequivocal effect owing to the presence of humidity upon the strength of turbulence parameter, C(n)(2), from data collected in the Chesapeake Bay area over 100 m length horizontal propagation paths. We describe and apply a novel technique, Hilbert phase analysis, to the relative humidity, temperature, and C(n)(2) data to show the contribution of the relevant climate variable to C(n)(2) as a function of time.

Humidity contribution to the refractive index structure function C2n.

Humidity and C2n data collected from the Chesapeake Bay area during the 2003/2004 period have been analyzed. We demonstrate that there is an unequivocal correlation between the data during the same time periods, in the absence of solar insolation. This correlation manifests itself as an inverse relationship. We suggest that C2n in the infrared region is also function of humidity, in addition to temperature and pressure.

Applying the Hilbert-Huang decomposition to horizontal light propagation C n 2 data

The Hilbert Huang Transform is a new technique for the analysis of non-stationary signals. It comprises two distinct parts: Empirical Mode Decomposition (EMD) and the Hilbert Transform of each of the modes found from the first step to produce a Hilbert Spectrum. The EMD is an adaptive decomposition of the data, which results in the extraction of Intrinsic Mode Functions (IMFs). We discuss the application of the EMD to the calibration of two optical scintillometers that have been used to measure Cn2 over horizontal paths on a building rooftop, and discuss the advantage of using the Marginal Hilbert Spectrum over the traditional Fourier Power Spectrum.

Optical turbulence model for laser propagation and imaging applications

We evaluate a simple model for predicting and understanding the structural behavior of Cn2 for a specific location, date, time, and given environmental parameters. This model is compared with Cn2 data taken at the Chesapeake Bay Detachment of the Naval Research Laboratory in Chesapeake Beach, Maryland. This simplified model predicts and explains the fluctuation in Cn2 reasonably well, and also shows that Cn2 is a strong function of solar irradiation.