Free space optical communication utilizing modulating retro-reflectors (MRR) can greatly reduce the complexity of a system in both pointing requirements as well as the necessity for a laser transmitter at both ends of the link. Retroreflectors are susceptible to the same atmospheric turbulence effects of scintillation and beam wander of any laser communication system. An MRR link using an array (N>1) of retroreflectors is affected by self-interference of the return beams. This self-interference can create additional fluctuation that compound to increase the apparent scintillation of the received signal. Data were collected over a 1km outdoor path on the interference pattern returned from a pair of 7mm and 12.5mm retro-reflectors, with multiple spacing distances, in varying turbulence regimes with a 1550nm and 1070nm laser. The interference data of the retroreflectors were correlated with Cn2 data collected simultaneously over the same 1km horizontal path. Under weak turbulence, the self-interference fringes matched diffraction theory, under stronger turbulence regimes the self-interference fringes were either visibly reduced or completely destroyed. We also analyze the contrast of the interference fringes as a function of wavelength for varying turbulence regimes as well as the ability to measure Fried’s parameter from the retroreflector spacing and the returned self-interference pattern.
It is understood that atmospheric turbulence results in fluctuations in the received power of an electro-optical (EO) link, a phenomenon known as optical scintillation. The atmospheric variable relevant to optical scintillation is the structure function parameter (Cn2) which can be quantified through optical scintillation measurements or derived from measurements of high-rate sampled atmospheric turbulence, especially the temperature perturbations. In addition to this (Cn2) can be estimated using models, some of which are based on surface layer similarity theory. However, the near shore marine atmospheric surface layer (MASL) provides an optically heterogeneous and complex turbulent environment that can be difficult to model accurately. A better understanding of the characteristics of near shore surface layer scintillation will provide increased exploitation of the environment by current and future EO systems operating in littoral regions. In an effort to better determine the scintillation effects in the MASL, observations were taken during the 26-day Couple Air-Sea Processes and Electromagnetic ducting Research West coast (CASPER-West) field campaign in September - October 2017 off the coast of Pt Mugu, CA.
In this paper, we introduce the CASPER-West EO component to include a description of the operating area, major platforms and major instruments relevant to EO measurements, and sampling strategy. We show comparisons of the derived (Cn2) from scalar perturbation measurements, bulk model parameterization, and from concurrent scintillation measurements between the R/V Sally Ride and R/P FLIP. Slant path optical links between a remotely piloted hexa-copter and the R/P FLIP were also available. Both stable and unstable thermal stratifications of the MASL were encountered throughout the campaign and we will discuss the observed differences between the experiment and those from current similarity theories in these different stability conditions.
The growth of optical communication has created a need to correctly characterize the atmospheric channel. Atmospheric
turbulence along a given channel can drastically affect optical communication signal quality. One means of
characterizing atmospheric turbulence is through measurement of the refractive index structure parameter, Cn2. When
calculating Cn2 from the scintillation index, σΙ2,the point aperture scintillation index is required. Direct measurement of
the point aperture scintillation index is difficult at long ranges due to the light collecting abilities of small apertures.
When aperture size is increased past the atmospheric correlation width, aperture averaging decreases the scintillation
index below that of the point aperture scintillation index. While the aperture averaging factor can be calculated from
theory, it does not often agree with experimental results. Direct measurement of the aperture averaging factor via the
pupil plane irradiance covariance function allows conversion from the aperture averaged scintillation index to the point
aperture scintillation index. Using a finite aperture, camera, and detector, the aperture averaged scintillation index and
aperture averaging factor are measured in parallel and the point aperture scintillation index is calculated. A new
instrument built by SSC Pacific was used to collect scintillation data at the Townes Institute Science and Technology
Experimentation Facility (TISTEF). This new instrument’s data was then compared to BLS900 data. The results show
that direct measurement of the aperture averaging factor is achievable using a camera and matches well with groundtruth
Laser beam speckle resulting from atmospheric turbulence contains information about the propagation channel. The number and size of the speckle cells can be used to infer the spatial coherence and thus the Cn2 along a path. The challenge with this technique is the rapidly evolving speckle pattern and non-uniformity of the speckle cells. In this paper we investigate modern blob counting techniques used in biology, microscopy, and medical imaging. These methods are then applied to turbulent speckle images to estimate the number and size of the speckle cells. Speckle theory is reviewed for different beam types and different regimes of turbulence. Algorithms are generated to calculate path Cn2 from speckle information and path geometry. The algorithms are tested on speckle images from experimental data collected over a turbulent 1km path and compared to Cn2 measurements collected in parallel.
The Navy is actively developing diverse optical application areas, including high-energy laser weapons and free- space optical communications, which depend on an accurate and timely knowledge of the state of the atmospheric channel. The Optical Channel Characterization in Maritime Atmospheres (OCCIMA) project is a comprehensive program to coalesce and extend the current capability to characterize the maritime atmosphere for all optical and infrared wavelengths. The program goal is the development of a unified and validated analysis toolbox. The foundational design for this program coordinates the development of sensors, measurement protocols, analytical models, and basic physics necessary to fulfill this goal.
The growth of optical communication has created a need to correctly characterize the atmospheric channel. The measurement of turbulence, due to its ability to drastically effect signal quality, is an important part of this characterization and can be partially accomplished via calculation of the scintillation index. However, proper calculation of the scintillation index requires that the background (specifically the diffuse solar background) be accurately subtracted from the transmitted signal. While there are many methods to remove this background we introduce a hardware based method which seeks to overcome the weaknesses of traditional approaches while adding its own strengths. The corrected signal is allowed a greater dynamic range and atmospheric background variations are accounted for during transmission. We begin by discussing the scintillation index and traditional means of background subtraction followed with an introduction of our proposed optical design. We provide details of the experimental setup, data collection over a maritime location in San Diego, and analysis. Finally, we compare scintillation index calculations using our new method and a traditional method of background subtraction. Our results ranked our method favorably alongside common methods of background subtraction.
Current transmissometer designs can be physically bulky, electronically complex, and susceptible to background light; ultimately limiting performance. We describe a novel transmissometer design based upon a modulated LED source and an AC-coupled receiver to improve upon the aforementioned shortcomings. The design aims to reduce both complexity and SWAP through the use of a high frequency modulation technique, while ultimately improving SNR and measurement range over a variety of atmospheric conditions. The instrument is a dynamic atmosphere and range transmissometer (DART). First we discuss the theory associated with our technique; particularly addressing how the effects of atmospheric turbulence are handled. Next, we describe the radiometry and calibration procedures for the transmitter and the receiver. We describe the instrument hardware and how the DART was built and tested in the laboratory. Finally, we discuss the field experiment to test the DART against a commercial unit over a 700m coastal path in San Diego. The processed data are compared with concurrent measurements from the Optec LPV-3 commercial transmissometer. Transmission data from the DART tracks the commercial instrument very well over varying atmospheric conditions.
Obtaining accurate, precise and timely information about the local atmospheric turbulence and extinction conditions and aerosol/particulate content remains a difficult problem with incomplete solutions. It has important applications in areas such as optical and IR free-space communications, imaging systems performance, and the propagation of directed energy. The capability to utilize passive imaging data to extract parameters characterizing atmospheric turbulence and aerosol/particulate conditions would represent a valuable addition to the current piecemeal toolset for atmospheric sensing. Our research investigates an application of fundamental results from optical turbulence theory and aerosol extinction theory combined with recent advances in image-quality-metrics (IQM) and image-quality-assessment (IQA) methods. We have developed an algorithm which extracts important parameters used for characterizing atmospheric turbulence and extinction along the propagation channel, such as the refractive-index structure parameter C2n , the Fried atmospheric coherence width r0 , and the atmospheric extinction coefficient βext , from passive image data. We will analyze the algorithm performance using simulations based on modeling with turbulence modulation transfer functions. An experimental field campaign was organized and data were collected from passive imaging through turbulence of Siemens star resolution targets over several short littoral paths in Point Loma, San Diego, under conditions various turbulence intensities. We present initial results of the algorithm’s effectiveness using this field data and compare against measurements taken concurrently with other standard atmospheric characterization equipment. We also discuss some of the challenges encountered with the algorithm, tasks currently in progress, and approaches planned for improving the performance in the near future.
A model for predicting the radiometry for a resolved laser beam due to scattering in the maritime atmosphere, has been developed by the Electro-Optics group in the Atmospheric Propagation Branch at SPAWAR Systems Center, Pacific. The model predicts the power received at a sensor using local meteorological measurements of the current atmospheric conditions. Field experiments conducted in San Diego, CA were used to validate the predictions of the model. Radiometric calibrations of a CCD camera provided power measurements of a resolved laser to be taken from various senor-laser beam orientations. The performance of the laser scattering model is assessed from the field measurements.