This PDF file contains the front matter associated with SPIE Proceedings Volume 9614, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

A simple grid of 10×10 white-light LEDs allows for simultaneous measurement of several characteristics of atmospheric turbulence. With this device, an imaging sensor and the model of tilt anisoplanatism one can determine turbulence strength, anisotropy, outer scale and spectral slope of turbulence. We describe the theory and present preliminary results obtained over a 270-m path.

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 C_n² 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 C_n² 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 C_n² measurements collected in parallel.

The knowledge of the turbulence strength in the atmosphere is important for many applications. Imagery in the atmosphere experience significant blur when the turbulence is strong. This can be automatically improved (without user intervention) if the turbulence strength is known. The performance of a high-power laser emitting in the atmosphere can be predicted if the statistics of the turbulence strength is known. If not predicted correctly, the laser may unintentionally destroy a target or fail to be able to disable a target.

In this article, we review existing methods that estimate turbulence strength, provide a more in depth error analysis, and propose a new method for estimating and mitigating turbulence in the atmosphere. We focus on methods that are passive in design in order to prevent detection in surveillance scenarios and tactical situations. We also propose a new method, stereo image motion monitor (SIMM) which is a system containing two independent apertures. Our goal in this approach is threefold: 1) We can measure r0 using the DIMM method 2) We can simultaneously estimate r0 individually for each aperture and 3) We have multiple views of the same scene thus can increase the number of frames used in turbulence mitigation methods.

Atmospheric turbulence can significantly affect imaging through paths near the ground. Atmospheric turbulence is generally treated as a time varying inhomogeneity of the refractive index of the air, which disrupts the propagation of optical signals from the object to the viewer. Under circumstances of deep or strong turbulence, the object is hard to recognize through direct imaging. Conventional imaging methods can’t handle those problems efficiently. The required time for lucky imaging can be increased significantly and the image processing approaches require much more complex and iterative de-blurring algorithms. We propose an alternative approach using a plenoptic sensor to resample and analyze the image distortions. The plenoptic sensor uses a shared objective lens and a microlens array to form a mini Keplerian telescope array. Therefore, the image obtained by a conventional method will be separated into an array of images that contain multiple copies of the object’s image and less correlated turbulence disturbances. Then a highdimensional lucky imaging algorithm can be performed based on the collected video on the plenoptic sensor. The corresponding algorithm will select the most stable pixels from various image cells and reconstruct the object’s image as if there is only weak turbulence effect. Then, by comparing the reconstructed image with the recorded images in each MLA cell, the difference can be regarded as the turbulence effects. As a result, the retrieval of the object’s image and extraction of turbulence effect can be performed simultaneously.

Recently, a method referred to as aperture partitioning was suggested to improve imagery of space objects collected through strong turbulence. In these situations the ratio of aperture diameter to atmospheric coherence diameters is large but, otherwise, images are reconstructed over a single isoplanatic patch size. Here, aperture partitioning has been shown to improve reconstruction quality in speckle imaging by reducing redundancy among imaging baselines. In this work, we explore the possibility of using aperture partitioning in horizontal imaging scenarios where the ratio of aperture to coherence cell size is small but imagery is highly anisoplanatic.

We present a novel method for efficient and physically-accurate modeling & simulation of anisoplanatic imaging through the atmosphere; in particular we present a new space-variant volumetric image blur algorithm. The method is based on the use of physical atmospheric meteorology models, such as vertical turbulence profiles and aerosol/molecular profiles which can be in general fully spatially-varying in 3 dimensions and also evolving in time. The space-variant modeling method relies on the metadata provided by 3D computer graphics modeling and rendering systems to decompose the image into a set of slices which can be treated in an independent but physically consistent manner to achieve simulated image blur effects which are more accurate and realistic than the homogeneous and stationary blurring methods which are commonly used today. We also present a simple illustrative example of the application of our algorithm, and show its results and performance are in agreement with the expected relative trends and behavior of the prescribed turbulence profile physical model used to define the initial spatially-varying environmental scenario conditions. We present the details of an efficient Fourier-transform-domain formulation of the SV volumetric blur algorithm and detailed algorithm pseudocode description of the method implementation and clarification of some nonobvious technical details.

The usage of long-range optical systems for tracking applications encounters regions of deep turbulence throughout propagation. Such conditions lead to the inability to remain on target for a tracked object due to scintillation. To mitigate this issue, a double pass optical system is utilized as a means of tracking enhanced backscatter (EBS) and thus keeping alignment while characterizing turbulent conditions. EBS is detected through image processing algorithms that capture the returning constructive interference from the target. This paper evaluates EBS optical systems using a retro-reflector at a 1 kilometer distance in order to validate theoretical models that typify atmospheric turbulence regarding low-ground propagation. Meteorological conditions are also included in the empirical data obtained for the analysis of atmospheric conditions that contribute to non-homogenous turbulent conditions along the path.

In applications where a laser beam is propagated at high altitude or through layered zones of the atmosphere, there is a chance that atmospheric turbulence cells are not isotropic. In this paper we analytically describe how the spread of a laser beam is affected by anisotropic eddies of optical turbulence. We show that anisotropic turbulence is responsible for an elliptical beam spread and beam wander over the planes x-y, orthogonal to the direction of propagation z.

The ultimate limits on high accuracy laser ranging to satellites from the ground appear to be caused by the effects of the earth’s atmosphere. Other impediments in terms of lasers, timing equipment and calibration seem to be evolving to the point of providing very high accuracy. We shall address the role of the earth’s atmosphere for lunar laser ranging. In the near future, the robotic deployment of next generation lunar laser retroreflectors is planned. With proper robotic deployment, these retroreflectors may support single photo-electron ranging accuracy at the 100 micron level or better. In particular, there are questions of the random and systematic delays and broadening of a very narrow laser pulse. Theoretical and experimental results will be discussed that address estimates of the magnitudes of these effects and the issue of precision vs. accuracy. These effects may be roughly divided into three domains: High frequency effects due to atmospheric turbulence, low frequency effects due to atmospheric “slopes” and atmospheric waves and tides and spectral dispersion of the narrow pulse. In conclusion, the route to better ranging through the earth’s atmosphere appears to be more advance modeling of local meteorological effects, in a program that can be implemented at a reasonable cost.

The four year FSO link attenuation measurement concurrently with most important meteorological parameters was performed at our mountain observatory Milesovka. In this contribution we summarize and classify different atmospheric phenomena after the FSO link attenuation quantity. For all particular phenomena the CD curves, typical events and simple dependences on relevant atmospheric parameter(s) are presented. We consider the following phenomena (approximate specific attenuation in dB/km in brackets): 1. Fog and cloud (hundreds dB/km) 2. Rain and snow (tens dB/km) 3. Atmospheric turbulence (unit dB) 4. Clear air attenuation due to water vapour (unit dB or less)

A turbulent, atmospheric channel can be considered to be reciprocal at any one instance in time. Reciprocity is a powerful property that can be used to compensate for the distortions caused by turbulence such as beam scintillation, spreading, and wander. Here we investigate the use of reciprocity in instances where a beam is propagated to an uncooperative target. Theoretical work [V. P. Lukin and M. I. Charnotskii , Sov. J. Quantum Electron., 12(5), 602 (1982)] has shown that reciprocity principles indicate that properties of the beam incident on a target fluctuate synchronously with the intensity distribution scattered from the target. Here we extend this purely analytical treatment using phase screen simulations. We show that there exists a correlation between the intensity imaged by the receiver and the field incident on the target. Furthermore, we demonstrate that the intensity at a specific location could be used to drive an adaptive optics system that corrects for atmospheric phase distortions.

When a beam propagates through atmospheric turbulence over a known distance, the target beam profile deviates from the projected profile of the beam on the receiver. Intuitively, the unwanted distortion provides information about the atmospheric turbulence. This information is crucial for guiding adaptive optic systems and improving beam propagation results. In this paper, we propose an entropy study based on the image from a plenoptic sensor to provide a measure of information content of atmospheric turbulence. In general, lower levels of atmospheric turbulence will have a smaller information size while higher levels of atmospheric turbulence will cause significant expansion of the information size, which may exceed the maximum capacity of a sensing system and jeopardize the reliability of an AO system. Therefore, the entropy function can be used to analyze the turbulence distortion and evaluate performance of AO systems. In fact, it serves as a metric that can tell the improvement of beam correction in each iteration step. In addition, it points out the limitation of an AO system at optimized correction as well as the minimum information needed for wavefront sensing to achieve certain levels of correction. In this paper, we will demonstrate the definition of the entropy function and how it is related to evaluating information (randomness) carried by atmospheric turbulence.

The efficiency of laser communications systems is significantly limited by atmospheric effects. Notably challenging scenarios like a long horizontal path or strong scintillation lead to high failure rates of the electro-optical systems. Adaptive optics (AO) methods and components developed for astronomical applications cannot fulfill these higher requirements. The so-called Holographic Wavefront Sensor (HWFS) is a promising alternative for measuring the wavefront deformation of a laser beam. The basic elements of the HWFS are a diffraction pattern and a fast photodetector like a photodiode array. With these components the aberrations present in the beam are measured directly. In this paper we present an optical setup to verify two important features of the sensor type experimentally: high measurement speed and the sensor’s response on present scintillation. Measurements with the HWFS were performed at a bandwidth of 11 kHz for a single aberration and of 2.5 kHz for 6 aberrations simultaneously. Furthermore we operated the HWFS in a closed-loop adaptive optics system with a bandwidth of 600 Hz. We show that the wavefront correction based on the HWFS measurements is insensitive to dynamic partial intensity fluctuations of the laser beam.

A CUBI, a simple geometric metal test-object, is placed in an outdoor environment to monitor the infrared signature. The (daily) temperature evolution of the individual facets is monitored as function of environmental parameters, such as solar irradiance and ambient temperature. This provides insight in the parameters that have the strongest effects on the thermal signature of the CUBI. The CUBI is also imaged by infrared cameras and these recordings are used to estimate the temperature of the CUBI. The recorded images are also used to provide insight in the amount of air turbulence generated by the radiance of the hot CUBI facets. The amount of turbulence is compared with the ambient turbulence as calculated by standard bulk theories.

Adaptive optics has been widely used in the field of astronomy to correct for atmospheric turbulence while viewing images of celestial bodies. The slightly distorted incoming wavefronts are typically sensed with a Shack-Hartmann sensor and then corrected with a deformable mirror. Although this approach has proven to be effective for astronomical purposes, a new approach must be developed when correcting for the deep turbulence experienced in ground to ground based optical systems. We propose the use of a modified plenoptic camera as a wavefront sensor capable of accurately representing an incoming wavefront that has been significantly distorted by strong turbulence conditions (C²_n <10^-13 m- ^2/3). An intelligent correction algorithm can then be developed to reconstruct the perturbed wavefront and use this information to drive a deformable mirror capable of correcting the major distortions. After the large distortions have been corrected, a secondary mode utilizing more traditional adaptive optics algorithms can take over to fine tune the wavefront correction. This two-stage algorithm can find use in free space optical communication systems, in directed energy applications, as well as for image correction purposes.

We propose a novel framework for the estimation of C²_n in the atmosphere by utilizing an inherent vertical scaling characteristics of the temperature fields. Observations from a field campaign over the Hawaii island are used for rigorous validation. Furthermore, the strength of the proposed approach is demonstrated by direct comparison against an alternative approach based on the so-called Thorpe scale.

In this paper, we propose a novel parameterization for optical turbulence (C²_n) simulations in the atmosphere. In this approach, C²_n is calculated from the output of atmospheric models using a high-order turbulence closure scheme. An important feature of this parameterization is that, in the free atmosphere (i.e., above the boundary layer), it is consistent with a well-established C²_n formulation by Tatarskii. Furthermore, it approaches a Monin-Obukhov similarity-based relationship in the surface layer. To test the performance of the proposed parameterization, we conduct mesoscale modeling and compare the simulated C²_n values with those measured during two field campaigns over the Hawaii island. A popular regression-based approach proposed by Trinquet and Vernin (2007) is also used for comparison. The predicted C²_n values, obtained from both the physically and statistically-based parameterizations, agree reasonably well with the observational data. However, in the presence of a large-scale atmospheric phenomenon (a breaking mountain wave), the physically-based parameterization outperforms the statistically-based one.

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) is a component of the Met Office NEON Tactical Decision Aid (TDA). Within NEON, the HT-FRTC has for a number of years been used to predict the IR apparent thermal contrasts between different surface types as observed by an airborne sensor. To achieve this, the HT-FRTC is supplied with the inherent temperatures and spectral properties of these surfaces (i.e. ground target(s) and background). A key strength of the HT-FRTC is its ability to take into account the detailed properties of the atmosphere, which in the context of NEON tend to be provided by a Numerical Weather Prediction (NWP) forecast model. While water vapour and ozone are generally the most important gases, additional trace gases are now being incorporated into the HT-FRTC. The HT-FRTC also includes an exact treatment of atmospheric scattering based on spherical harmonics. This allows the treatment of several different aerosol species and of liquid and ice clouds. Recent developments can even account for rain and falling snow. The HT-FRTC works in Principal Component (PC) space and is trained on a wide variety of atmospheric and surface conditions, which significantly reduces the computational requirements regarding memory and time. One clear-sky simulation takes approximately one millisecond. Recent developments allow the training to be completely general and sensor independent. This is significant as the user of the code can add new sensors and new surfaces/targets by simply supplying extra files which contain their (possibly classified) spectral properties. The HT-FRTC has been extended to cover the spectral range of Photopic and NVG sensors. One aim here is to give guidance on the expected, directionally resolved sky brightness, especially at night, again taking the actual or forecast atmospheric conditions into account. Recent developments include light level predictions during the period of twilight.

We use Mixture Density Networks (MDN) to estimate atmospheric particle size distributions based upon metrological parameters. Measurements of particle size spectra show that distributions are often multi-modal, composed of various underlying aerosol species that can grow from one mode to another. The flexibility of the MDN allows for the prediction of an arbitrary distribution. We show here that the MDN prediction engine can be useful in forecasting complicated, multi-modal particle size distributions. To inform and train the MDN we use meteorological, particle size and optical extinction measurements taken from a three week propagation measurement field campaign.

Optical angular momentum (OAM) based communication requires multiple OAM modes. Spiral phase plates for OAM generation are lithographically fabricated. Phase profile of the phase plate is evaluated by surface profiler as well as optically by using Mach Zehnder interferometer.

Optical beam propagation through the turbulent media such as the vehicle exhaust plumes need to be studied in order to estimate the communication performance degradations on vehicular visible light communication (V²LC) networks. When the optical transceiver is close to the exhaust plumes, the optical waves may be distorted in both phase and amplitude, by perturbations on the air's refractive index spatially and temporally. In this paper, the authors present the measurements performed on V²LC communication link using two different vehicles. In particular, several different transceiver configurations with respect to the position of exhaust nozzle have been tested and compared, and the turbulence strength is evaluated by scintillation index (SI). Since the V²LC applications are mainly based on intensity modulated, direct detection method, only the receiving optical power fluctuation (scintillation) will be discussed. Finally, the error rate performance under different log intensity variances is analyzed, demonstrating that scintillation effects introduced by the vehicle exhaust can be negligible in V²LC applications.

In this article the author’s team deals with problems of modulation formats for Free Space Optical (FSO) Communications. FSO communications have high bandwidth, low signal attenuation, quick installation, security, unlicensed band and low cost. In FSO communication occurs due to the influence of atmospheric effect (attenuation, and fluctuation the received power signal, influence turbulence). Here will be dealing modulation schemes OOK (On-Off keying) and Subcarrier Intensity Modulation (SIM) based on a BPSK (Binary Phase Shift Keying). In which will studied their characteristic and effect of atmospheric influence on the received signal. This results in decreased Eye-Diagram, Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER). To evaluate the modulation formats in atmospheric turbulence is used simulation box with heat and wind sources.

The technique of differential phase shift keying(DPSK) modulation is applied into demodulating phase information in the coherent optical receiver. The dual rate free-space receiving structure on the base of Mach-Zehnder delay interferometer with the lens is used suitably for differential delay which is equal to the one bit corresponding to a certain data rate. Delay distance at the interference receiver is varied with transmission rata from satellite to ground. Differential information is obtained by the subtraction of the two successive wave-front phases when made to interfere. The phase demodulation is extremely sensitive to phase fluctuation. Because of the incident light through atmospheric turbulence, the wave-front of optical signal became jittered in the temporal and spatial domain rapidly. In the paper, the dual rate free-space laser communication receiver for phase lock to stable signal light phase is proposed, increasing the homodyne efficiency and decreasing the bit error rate.

The atmosphere is unstable and unpredictable environment, where are continual changes of the air refractive index. These changes cause fluctuation of optical power at the receiver site. The prediction of behavior of the atmosphere and effect of this behavior on the FSO link is very complicated or even impossible. Aim of this article is focused on statistical analysis of measured level signal RSSI of the FSO link and atmospheric properties measured by hydro-meteorological station. For measured data the statistical analysis tools were used. Next part of article is focused on determination of the linear regression model to calculate level of RSSI depending on the atmospheric properties. Two empirical equations are result for day and night time. These equations describe behavior of signal RSSI in 30 days interval. Finally, comparison of the obtained mathematical model with real measured data of RSSI was introduced for one week before and one week after the analyzed time interval.

In practice, due to reasons related to the characteristics of the laser device and the inevitable error of the processing technique, a laser source emitted from a communication terminal is represented by the Gaussian Schell model (GSM). In a space to ground optical communication link, the incident optical intensity at the receiver aperture is affected by the source coherence parameter and the zenith angle. With full consideration of both the average optical intensity and scintillation, the statistical distribution of the optical intensity and the average bit error rate (BER) are obtained with different zenith angle and source coherence parameter. The effect of the source coherence parameter on the performance of an on-off-keying (OOK) receiver is systematically analyzed. The results indicate that the performance degrades seriously with increasing source coherence parameter or zenith angle. The hope of this work is to improve the redundancy design of the optical communication receiver system.

Signal laser propagation will pass through the random atmosphere turbulence channel in satellite-to-ground laser communication application. The turbulence will cause the wavefront distortion in the receiver telescope front. For direct detection laser communication system, atmospheric turbulence can affect the coupling efficiency from space laser to the detector. For coherent detection laser communication system, Atmosphere turbulence not only affects the coupling efficiency, but also can seriously reduce the heterodyne detection efficiency. Coherent detection communication receiver must use small aperture telescopes or large aperture telescope with adaptive wavefront compensation system.

To mitigate the influence of the atmosphere turbulence, we proposed a new method base orthogonal phase modulation with self homodyne detect. This method can not only mitigate the influence of the turbulence, but also adjust the communication date rate steplessly. The experiment results show that the method is very suitable to the satellite-to-ground link.