Horizontal Propagation Deep Turbulence Testbed

Abstract

The Navy is interested in horizontal laser propagation studies in a maritime environment, near the ocean surface, for applications including imaging and high-energy laser propagation. The Naval Postgraduate School (NPS) in Monterey, California, and the Naval Research Laboratory (NRL) Wavefront Sensing and Control Division in Albuquerque, New Mexico, are collaborating in the development of a horizontal propagation testbed with adaptive optics for correction and simulation of atmospheric deep turbulence conditions. Atmospheric turbulence near the ocean surface is mostly dominated by scintillation, or intensity fluctuations, which degrade beam quality as propagation distance increases. While statistical data has been collected and analyzed for decades on the vertical turbulence profile, horizontal, deep turbulence data collection has begun only relatively recently. No theoretical model currently exists to describe horizontal turbulence that parallels the familiar Kolmogorov statistical model used in vertical AO applications, and investigations are underway to develop such models. The main purpose of the NPS testbed is to develop an adaptive optics system which is capable of simulating scintillation effects. Since it is known that branch points and scintillation are characteristic of the deep turbulence problem, the testbed developed for this research is used to simulate the effects of intensity fluctuations and intensity dropouts on the Shack-Hartmann WFS. This is accomplished by applying atmosphere on two separate Spatial Light Modulators (SLMs), both individually and simultaneously, and extending the beam path between them to observe the atmospheric disturbances produced. These SLMs allow the implementation of various atmospheric turbulence realizations, while the use of two in combination allows the simulation of a thick aberrator to more closely approximate horizontal turbulence behavior. The short path length allows a propagation distance of approximately 2 meters between the SLMs, while the long path allows approximately 22 meters. Images of Shack Hartmann wavefront sensor spots show intensity fluctuations similar to those observed in actual maritime experiments. These fluctuations are used to quantify intensity dropouts in both the short and long paths. Intensity dropouts due to the atmospheric distortions in the short path represent 0.5–1% of total sensor subapertures, while the long path distortions introduce 5–10% intensity dropouts. Further analysis is performed on the images from the long path to estimate the atmospheric structure constant, 2 Cn, from scintillation calculations, resulting 2 in a 2 Cn value of approximately 9.99x10-11m -2/3 for the short path and 4.26x10 -11 m-2/3 for the long path. This paper presents a description of the optical testbed setup, the details of horizontal turbulence simulation, and the results of the scintillation calculations resulting from data collected in the laboratory. The success of this experiment lays an important foundation for simulating maritime-like horizontal atmosphere in the laboratory for beam control in HEL ship systems.The Navy is interested in horizontal laser propagation studies in a maritime environment, near the ocean surface, for applications including imaging and high-energy laser propagation. The Naval Postgraduate School (NPS) in Monterey, California, and the Naval Research Laboratory (NRL) Wavefront Sensing and Control Division in Albuquerque, New Mexico, are collaborating in the development of a horizontal propagation testbed with adaptive optics for correction and simulation of atmospheric deep turbulence conditions. Atmospheric turbulence near the ocean surface is mostly dominated by scintillation, or intensity fluctuations, which degrade beam quality as propagation distance increases. While statistical data has been collected and analyzed for decades on the vertical turbulence profile, horizontal, deep turbulence data collection has begun only relatively recently. No theoretical model currently exists to describe horizontal turbulence that parallels the familiar Kolmogorov statistical model used in vertical AO applications, and investigations are underway to develop such models. The main purpose of the NPS testbed is to develop an adaptive optics system which is capable of simulating scintillation effects. Since it is known that branch points and scintillation are characteristic of the deep turbulence problem, the testbed developed for this research is used to simulate the effects of intensity fluctuations and intensity dropouts on the Shack-Hartmann WFS. This is accomplished by applying atmosphere on two separate Spatial Light Modulators (SLMs), both individually and simultaneously, and extending the beam path between them to observe the atmospheric disturbances produced. These SLMs allow the implementation of various atmospheric turbulence realizations, while the use of two in combination allows the simulation of a thick aberrator to more closely approximate horizontal turbulence behavior. The short path length allows a propagation distance of approximately 2 meters between the SLMs, while the long path allows approximately 22 meters. Images of Shack Hartmann wavefront sensor spots show intensity fluctuations similar to those observed in actual maritime experiments. These fluctuations are used to quantify intensity dropouts in both the short and long paths. Intensity dropouts due to the atmospheric distortions in the short path represent 0.5–1% of total sensor subapertures, while the long path distortions introduce 5–10% intensity dropouts. Further analysis is performed on the images from the long path to estimate the atmospheric structure constant, 2 Cn, from scintillation calculations, resulting 2 in a 2 Cn value of approximately 9.99x10-11m -2/3 for the short path and 4.26x10 -11 m-2/3 for the long path. This paper presents a description of the optical testbed setup, the details of horizontal turbulence simulation, and the results of the scintillation calculations resulting from data collected in the laboratory. The success of this experiment lays an important foundation for simulating maritime-like horizontal atmosphere in the laboratory for beam control in HEL ship systems.The Navy is interested in horizontal laser propagation studies in a maritime environment, near the ocean surface, for applications including imaging and high-energy laser propagation. The Naval Postgraduate School (NPS) in Monterey, California, and the Naval Research Laboratory (NRL) Wavefront Sensing and Control Division in Albuquerque, New Mexico, are collaborating in the development of a horizontal propagation testbed with adaptive optics for correction and simulation of atmospheric deep turbulence conditions. Atmospheric turbulence near the ocean surface is mostly dominated by scintillation, or intensity fluctuations, which degrade beam quality as propagation distance increases. While statistical data has been collected and analyzed for decades on the vertical turbulence profile, horizontal, deep turbulence data collection has begun only relatively recently. No theoretical model currently exists to describe horizontal turbulence that parallels the familiar Kolmogorov statistical model used in vertical AO applications, and investigations are underway to develop such models. The main purpose of the NPS testbed is to develop an adaptive optics system which is capable of simulating scintillation effects. Since it is known that branch points and scintillation are characteristic of the deep turbulence problem, the testbed developed for this research is used to simulate the effects of intensity fluctuations and intensity dropouts on the Shack-Hartmann WFS. This is accomplished by applying atmosphere on two separate Spatial Light Modulators (SLMs), both individually and simultaneously, and extending the beam path between them to observe the atmospheric disturbances produced. These SLMs allow the implementation of various atmospheric turbulence realizations, while the use of two in combination allows the simulation of a thick aberrator to more closely approximate horizontal turbulence behavior. The short path length allows a propagation distance of approximately 2 meters between the SLMs, while the long path allows approximately 22 meters. Images of Shack Hartmann wavefront sensor spots show intensity fluctuations similar to those observed in actual maritime experiments. These fluctuations are used to quantify intensity dropouts in both the short and long paths. Intensity dropouts due to the atmospheric distortions in the short path represent 0.5–1% of total sensor subapertures, while the long path distortions introduce 5–10% intensity dropouts. Further analysis is performed on the images from the long path to estimate the atmospheric structure constant, 2 Cn, from scintillation calculations, resulting 2 in a 2 Cn value of approximately 9.99x10-11m -2/3 for the short path and 4.26x10 -11 m-2/3 for the long path. This paper presents a description of the optical testbed setup, the details of horizontal turbulence simulation, and the results of the scintillation calculations resulting from data collected in the laboratory. The success of this experiment lays an important foundation for simulating maritime-like horizontal atmosphere in the laboratory for beam control in HEL ship systems.