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12 pages, 8428 KiB

Open AccessArticle

Improving on Atmospheric Turbulence Profiles Derived from Dual Beacon Hartmann Turbulence Sensor Measurements

by Alexander Boeckenstedt, Jack McCrae, Santasri Bose-Pillai, Benjamin Wilson and Steven Fiorino

Appl. Sci. 2022, 12(12), 5822; https://doi.org/10.3390/app12125822 - 8 Jun 2022

Cited by 3 | Viewed by 1603

Abstract

Atmospheric turbulence is an inevitable source of wavefront distortion in all fields of long range laser propagation and sensing. However, the distorting effects of turbulence can be corrected using wavefront sensors contained in adaptive optics systems. Such systems also provide deeper insight into [...] Read more.

Atmospheric turbulence is an inevitable source of wavefront distortion in all fields of long range laser propagation and sensing. However, the distorting effects of turbulence can be corrected using wavefront sensors contained in adaptive optics systems. Such systems also provide deeper insight into surface layer turbulence, which is not well understood. A unique method of profile generation by a dual source Hartmann Turbulence Sensor (HTS) technique is introduced here. Measurements of optical turbulence along a horizontal path were taken to create

C_{n}^{2}

profiles. Two helium-neon laser beams were directed over an inhomogeneous horizontal path and captured by the HTS. The measured differential tilt variances imposed on the laser wavefronts were used in conjunction with a set of computed weighting functions to profile the turbulence over the sensing path. The weighting function matrix is inherently ill-conditioned, therefore, Tikhonov regularization was applied to produce accurate

C_{n}^{2}

profiles. A distribution of sonic anemometers and a co-located boundary layer scintillometer (BLS) collected independent

C_{n}^{2}

measurements to add confidence to the HTS profiles. The

C_{n}^{2}

profiles generated by this approach agree very well with the auxiliary anemometer and scintillometer measurements. This method of producing turbulence profiles may be useful in future multi-conjugate adaptive optics applications. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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11 pages, 2808 KiB

Open AccessArticle

Measurements and Modeling of Optical Turbulence in the Coastal Environment

by Sasha Barnett, Joseph Blau, Paul Frederickson and Keith Cohn

Appl. Sci. 2022, 12(10), 4892; https://doi.org/10.3390/app12104892 - 12 May 2022

Cited by 4 | Viewed by 2011

Abstract

The goal of this study was to characterize optical turbulence in the near-coastal environment. Measurements to obtain the refractive index structure parameter and other meteorological data were taken over the course of a month along the shore of Monterey Bay. The results were [...] Read more.

The goal of this study was to characterize optical turbulence in the near-coastal environment. Measurements to obtain the refractive index structure parameter and other meteorological data were taken over the course of a month along the shore of Monterey Bay. The results were compared to a new version of the Navy Vertical Surface Layer Model (NAVSLaM), a model of turbulence originally developed for maritime environments but now extended to terrestrial environments. The new version has not been previously validated by comparisons to experiments, particularly in a complex environment such as near the coastline. Our experimental results showed generally good agreement between measured and modeled levels of turbulence. Specifically, the differences between experimental and modeled values of the refractive index structure parameter were less than an order of magnitude in most conditions and followed the same diurnal trend. There were some greater differences during near-neutral conditions, but this is a known limitation of the model. Overall, this extended model appears to do a good job of predicting turbulence in this environment for the observed time period. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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10 pages, 2200 KiB

Open AccessArticle

First Observations of Cirrus Clouds Using the UZ Mie Lidar over uMhlathuze City, South Africa

by Nkanyiso Mbatha and Lerato Shikwambana

Appl. Sci. 2022, 12(9), 4631; https://doi.org/10.3390/app12094631 - 5 May 2022

Cited by 3 | Viewed by 1800

Abstract

Clouds cover more than two-thirds of the earth’s surface and play a dominant role in the energy and water cycle of our planet. Cirrus clouds are high-level clouds composed mostly of ice crystals and affect the earth’s radiation allocation mainly by absorbing outgoing [...] Read more.

Clouds cover more than two-thirds of the earth’s surface and play a dominant role in the energy and water cycle of our planet. Cirrus clouds are high-level clouds composed mostly of ice crystals and affect the earth’s radiation allocation mainly by absorbing outgoing longwave radiation and by reflecting solar radiation. This study presents the characterization of cirrus clouds observed on 10 and 11 April 2019 using the ground-based University of Zululand (UZ) light detection and ranging (lidar) for the first time. Dense cirrus clouds with an average thickness of ~1.5 km at a height range of 9.5–12 km on 10 and 11 April 2019 were observed by the UZ lidar. The UZ lidar observation on 10 April 2019 agreed with the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) observation. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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18 pages, 5211 KiB

Open AccessArticle

Re-Visiting Acoustic Sounding to Advance the Measurement of Optical Turbulence

by Steven Fiorino, Santasri Bose-Pillai and Kevin Keefer

Appl. Sci. 2021, 11(16), 7658; https://doi.org/10.3390/app11167658 - 20 Aug 2021

Cited by 7 | Viewed by 1981

Abstract

Optical turbulence, as determined by the widely accepted practice of profiling the temperature structure constant,

C_{T}^{2}

, via the measurement of ambient atmospheric temperature gradients, can be found to differ quite significantly when characterizing such gradients via thermal-couple differential temperature sensors [...] Read more.

Optical turbulence, as determined by the widely accepted practice of profiling the temperature structure constant,

C_{T}^{2}

, via the measurement of ambient atmospheric temperature gradients, can be found to differ quite significantly when characterizing such gradients via thermal-couple differential temperature sensors as compared to doing so with acoustic probes such as those commonly used in sonic anemometry. Similar inconsistencies are observed when comparing optical turbulence strength derived via

C_{T}^{2}

as compared to those through direct optical or imaging measurements of small fluctuations of the index of refraction of air (i.e., scintillation). These irregularities are especially apparent in stable atmospheric layers and during diurnal quiescent periods. Our research demonstrates that when care is taken to properly remove large-scale index of refraction gradients, the sonic anemometer-derived velocity structure constant,

C_{v}^{2}

, coupled with the similarly derived turbulence-driven index of refraction and vertical wind shear gradients, provides a refractive index structure constant,

C_{n}^{2}

, that can more closely match the optical turbulence strengths inferred by more direct means such as scintillometers or differential image motion techniques. The research also illustrates the utility and robustness of quantifying

C_{n}^{2}

from

C_{T}^{2}

at a point using a single sonic anemometer and establishes a clear set of equations to calculate volumetric

C_{n}^{2}

data using instrumentation that measures wind velocities with more spatial/temporal fidelity than temperature. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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13 pages, 3747 KiB

Open AccessArticle

Estimating Turbulence Distribution over a Heterogeneous Path Using Time-Lapse Imagery from Dual Cameras

by Benjamin Wilson, Santasri Bose-Pillai, Jack McCrae, Kevin Keefer and Steven Fiorino

Appl. Sci. 2021, 11(13), 6221; https://doi.org/10.3390/app11136221 - 5 Jul 2021

Cited by 5 | Viewed by 2011

Abstract

Knowledge of turbulence distribution along an experimental path can help in effective turbulence compensation and mitigation. Although scintillometers are traditionally used to measure the strength of turbulence, they provide a path-integrated measurement and have limited operational ranges. A technique to profile turbulence using [...] Read more.

Knowledge of turbulence distribution along an experimental path can help in effective turbulence compensation and mitigation. Although scintillometers are traditionally used to measure the strength of turbulence, they provide a path-integrated measurement and have limited operational ranges. A technique to profile turbulence using time-lapse imagery of a distant target from spatially separated cameras is presented here. The method uses the turbulence induced differential motion between pairs of point features on a target, sensed at a single camera and between cameras to extract turbulence distribution along the path. The method is successfully demonstrated on a 511 m almost horizontal path going over half concrete and half grass. An array of Light-Emitting Diodes (LEDs) of non-uniform separation is imaged by a pair of cameras, and the extracted turbulence profiles are validated against measurements from 3D sonic anemometers placed along the path. A short-range experiment with a heat source to create local turbulence spike gives good results as well. Because the method is phase-based, it does not suffer from saturation issues and can potentially be applied over long ranges. Although in the present work, a cooperative target has been used, the technique can be used with non-cooperative targets. Application of the technique to images collected over slant paths with elevated targets can aid in understanding the altitude dependence of turbulence in the surface layer. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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9 pages, 1905 KiB

Open AccessArticle

Method of Estimation of Turbulence Integral Parameters

by Hong Shen, Longkun Yu, Xu Jing and Fengfu Tan

Appl. Sci. 2021, 11(9), 4157; https://doi.org/10.3390/app11094157 - 1 May 2021

Viewed by 1846

Abstract

The optical effects of turbulence are directly related to turbulence integral parameters, which are integrals of the refractive index structure constant over a whole path with different path-weighting functions (PWFs). We describe a method that utilizes measurable turbulence integral parameters, such as angle-of-arrival [...] Read more.

The optical effects of turbulence are directly related to turbulence integral parameters, which are integrals of the refractive index structure constant over a whole path with different path-weighting functions (PWFs). We describe a method that utilizes measurable turbulence integral parameters, such as angle-of-arrival fluctuations and scintillation, to estimate turbulence integral parameters that cannot be measured directly. The estimates of the turbulence integral parameters are based on the linear combination of the PWFs of those measurable quantities. New measurable quantities and their PWFs under different propagation conditions were studied. Some interesting and meaningful results have been obtained. This method shows the prospect of characterizing anisoplanatism in adaptive optics and allows for the estimation of some optical turbulence parameters under non-ideal conditions, such as an isoplanatic angle in a finite distance. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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26 pages, 6653 KiB

Open AccessArticle

Atmospheric Turbulence Study with Deep Machine Learning of Intensity Scintillation Patterns

by Artem M. Vorontsov, Mikhail A. Vorontsov, Grigorii A. Filimonov and Ernst Polnau

Appl. Sci. 2020, 10(22), 8136; https://doi.org/10.3390/app10228136 - 17 Nov 2020

Cited by 29 | Viewed by 5007

Abstract

A new paradigm for machine learning-inspired atmospheric turbulence sensing is developed and applied to predict the atmospheric turbulence refractive index structure parameter using deep neural network (DNN)-based processing of short-exposure laser beam intensity scintillation patterns obtained with both: experimental measurement trials conducted over [...] Read more.

A new paradigm for machine learning-inspired atmospheric turbulence sensing is developed and applied to predict the atmospheric turbulence refractive index structure parameter using deep neural network (DNN)-based processing of short-exposure laser beam intensity scintillation patterns obtained with both: experimental measurement trials conducted over a 7 km propagation path, and imitation of these trials using wave-optics numerical simulations. The developed DNN model was optimized and evaluated in a set of machine learning experiments. The results obtained demonstrate both good accuracy and high temporal resolution in sensing. The machine learning approach was also employed to challenge the validity of several eminent atmospheric turbulence theoretical models and to evaluate them against the experimentally measured data. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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11 pages, 1943 KiB

Open AccessArticle

Characterization of Localized Atmospheric Turbulence Layer Using Laser Light Backscattered off Moving Target

by Victor A. Kulikov, Svetlana L. Lachinova, Mikhail A. Vorontsov and Venkata S. Rao Gudimetla

Appl. Sci. 2020, 10(19), 6887; https://doi.org/10.3390/app10196887 - 1 Oct 2020

Cited by 2 | Viewed by 2316

Abstract

A concept of atmospheric turbulence characterization using laser light backscattered off a moving unresolved target or a moving target with a glint is considered and analyzed through wave-optics numerical simulations. The technique is based on analysis of the autocorrelation function and variance of [...] Read more.

A concept of atmospheric turbulence characterization using laser light backscattered off a moving unresolved target or a moving target with a glint is considered and analyzed through wave-optics numerical simulations. The technique is based on analysis of the autocorrelation function and variance of the power signal measured by the target-in-the-loop atmospheric sensing (TILAS) system composed of a single-mode-fiber-based optical transceiver and the moving target. It is shown that the TILAS received power signal autocorrelation function strongly depends on the turbulence distribution and is weakly sensitive to the turbulence strength, while the signal variance equally depends on these parameters. Assuming the atmospheric turbulence model can be represented by a single spatially localized turbulence layer and the target position and speed are known independently, consecutive analysis of the autocorrelation function and variance of the TILAS signal allows evaluation of both the turbulence layer strength and position along the optical propagation path. It is also demonstrated that the autocorrelation function can potentially be used for the atmospheric turbulence outer scale estimation. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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Review

Jump to: Research

31 pages, 3904 KiB

Open AccessFeature PaperReview

Non-Classic Atmospheric Optical Turbulence: Review

by Olga Korotkova and Italo Toselli

Appl. Sci. 2021, 11(18), 8487; https://doi.org/10.3390/app11188487 - 13 Sep 2021

Cited by 20 | Viewed by 3187

Abstract

Theoretical models and results of experimental campaigns relating to non-classic regimes occurring in atmospheric optical turbulence are overviewed. Non-classic turbulence may manifest itself through such phenomena as a varying power law of the refractive-index power spectrum, anisotropy, the presence of constant-temperature gradients and [...] Read more.

Theoretical models and results of experimental campaigns relating to non-classic regimes occurring in atmospheric optical turbulence are overviewed. Non-classic turbulence may manifest itself through such phenomena as a varying power law of the refractive-index power spectrum, anisotropy, the presence of constant-temperature gradients and coherent structures. A brief historical introduction to the theories of optical turbulence, both classic and non-classic, is first presented. The effects of non-classic atmospheric turbulence on propagating light beams are then discussed, followed by the summary of results on measuring the non-classic turbulence, on its computer and in-lab simulations and its controlled synthesis. The general theory based on the extended Huygens–Fresnel method, capable of quantifying various effects of non-classic turbulence on propagating optical fields, including the increased light diffraction, beam profile deformations, etc., is then outlined. The review concludes by a summary of optical engineering applications that can be influenced by atmospheric non-classic turbulence, e.g., remote sensing, imaging and wireless optical communication systems. The review makes an accent on the results developed by the authors for the recent AFOSR MURI project on deep turbulence. Full article

(This article belongs to the Special Issue Atmospheric Optics Sensing, Mitigation and Exploitation)

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