Measuring optical turbulence using cell counting algorithms
Inventors
Cauble, Galen D. • Wayne, David T.
Assignees
Publication Number
US-9959612-B2
Publication Date
2018-05-01
Expiration Date
2036-08-31
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Abstract
Methods for characterizing atmospheric turbulence along an optical path from a laser transmitter to a laser receiver can include the steps of counting the number of laser speckles at the receiver imaging plane, and then finding Fried's parameter r0 using the counting result to characterize the turbulence along the path. Before counting speckles, images at the receiver image plane can be preprocessed by capturing the images. The captured images at the image plane can then be blurred and a threshold can be chosen so that only certain pixels in the image are further processed. The thresholding can be via Otsu's methods or via variants of a Gaussian fit. Kostelec's method can then be used to count speckles in the portions of the image that have made it through the thresholding step. Other counting methods could be used. Fried's can then be found using the speckle count.
Core Innovation
The invention provides methods for characterizing atmospheric turbulence along an optical path from a laser transmitter to a laser receiver by counting the number of laser speckles at the receiver imaging plane, and then finding Fried's parameter r0 using the counting result to characterize the turbulence along the path. The method includes preprocessing the captured images at the receiver image plane by blurring the images and applying thresholding methods such as Otsu's or Gaussian fit variants to select pixels for further processing, followed by applying counting algorithms such as Kostelec's method to count the speckles.
The problem addressed by the invention stems from the effect of atmospheric turbulence, which causes refractive-index fluctuations leading to intensity fluctuations and characteristic beam breakup into speckles at the pupil plane of a receiving system. These intensity fluctuations limit the performance of free-space optical communication. Existing methods to measure atmospheric turbulence include estimating Fried's coherence length r0 by analyzing the number and size of speckles, but accurately and efficiently counting these speckles in turbulent conditions is challenging due to artifacts and poor contrast. The invention leverages interdisciplinary cell counting algorithms from life sciences, adapted and enhanced by image preprocessing, to accurately count laser speckles and hence characterize atmospheric turbulence cost-effectively without relying on expensive scintillometers.
Claims Coverage
The patent includes three independent claims covering methods for characterizing atmospheric turbulence using laser speckle counting with image preprocessing, and a system for measuring atmospheric turbulence employing these techniques.
Method for characterizing atmospheric turbulence by speckle counting
Capturing an image of the laser beam at the receiver image plane; preprocessing the image to limit pixels and enhance it by blurring and thresholding (using methods such as Otsu's or Gaussian fits); counting the number of laser speckles in the enhanced image; finding Fried's parameter r0 using the speckle count as an indicator of atmospheric turbulence.
Method for measuring refractive index fluctuations using speckle counting
Capturing and recording an image of the laser beam at the receiver image plane; preprocessing via blurring and thresholding (similar to the first method); counting speckles; calculating Fried's parameter r0 to measure atmospheric turbulence along the path based on the speckle count.
System for measuring atmospheric turbulence with components and processing
A system comprising a laser transmitter, a receiver with image plane, a detector to capture the image, a recorder to record the image, and a processor programmed to preprocess the image by reducing pixels through blurring and thresholding, count the number of speckles in the preprocessed image, and find Fried's parameter r0 from the speckle count to indicate atmospheric turbulence along the path.
The claims cover methods that integrate image capture, preprocessing by blurring and thresholding, speckle counting using interdisciplinary algorithms, and determining Fried's parameter r0 to characterize atmospheric turbulence, as well as a system incorporating these elements for real-time turbulence measurement.
Stated Advantages
Provides a quick and effective measurement of Fried's coherence length r0 for atmospheric turbulence characterization without the need for costly and sensitive scintillometers.
Compatible with any pupil plane imaging system, camera, and wavelength, offering flexibility over traditional scintillometers which often operate within specific wavelength ranges.
Allows adaptability to new and future speckle counting methods and can benefit from hardware improvements such as better cameras, lenses, or faster processors to optimize and speed up measurement.
Documented Applications
Characterizing atmospheric turbulence along an optical path for assessing the performance and effective range of free-space optical communication systems.
Using laser speckle counting methods adapted from life sciences to estimate the refractive index structure constant Cn2 and Fried's coherence length r0 in atmospheric laser propagation paths.
Real-time or near real-time measurement of turbulence-induced intensity fluctuations to aid in laser beam coherency evaluation over turbulent atmospheric distances.
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