Laser Beam Diameter Control for Atmospheric Turbulence Fading
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Solution Overview
Problem
Signal fading due to atmospheric turbulence significantly affects long-range ground-to-satellite optical communication systems, particularly caused by scintillation and beam wander, which existing methods fail to adequately address without introducing noise or increasing complexity.
Innovation Solution
A method and apparatus that dynamically adjust the diameter and pointing direction of the laser beam to match the averaged angular divergence and angle of arrival of the down-link optical beam, using a single-transverse-mode laser and adaptive control to minimize speckle formation and maintain signal power by optimizing the beam diameter and direction based on real-time measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing methods (automatic gain control, large receiving aperture, adaptive optics, phase conjugation) are used to minimize signal fading, then signal fading is reduced, but device complexity and cost increase significantly
Solution Approach 1:
The patent extracts and addresses only the specific problem of signal fading due to scintillation, rather than implementing comprehensive complex systems like adaptive optics or phase conjugation. The solution focuses on a simplified apparatus comprising a beacon receiver for measuring beam wander and a controller for adjusting the transmitter beam pointing direction, eliminating the need for complex receiving端 processing systems.
Solution Approach 2:
The system uses the down-link beacon beam itself to carry information about atmospheric turbulence, which is then used to control the up-link communication beam. The beacon receiver measures the beacon beam's characteristics, and this information feeds back to adjust the transmitter, creating a self-contained control loop that doesn't require external complex systems.
2Reliability
If existing methods are implemented to reduce signal fading, then communication reliability improves, but implementation cost increases
Solution Approach 1:
The beacon transmitter and receiver serve multiple functions: they enable satellite acquisition and tracking, provide turbulence measurement for beam pointing control, and facilitate communication. This multi-functionality eliminates the need for separate expensive systems, as the same beacon infrastructure is used for both navigation and signal fading compensation.
Solution Approach 2:
The patent changes the parameter being controlled from beam size or receiver aperture to beam pointing direction. By measuring the beacon beam's angular position and using this information to adjust the transmitter beam pointing, the system achieves fading reduction through a simpler parameter adjustment that requires less complex hardware.
3Reliability
If beam size is increased to reduce speckle formation, then signal fading decreases, but beam divergence increases and pointing precision deteriorates
Solution Approach 1:
Instead of changing the beam size to reduce speckles, the patent inverts the approach by maintaining a small, well-collimated beam and instead adjusting the beam pointing direction dynamically. The beacon receiver measures angular deviations caused by turbulence, and the controller adjusts the transmitter beam direction to compensate, achieving fading reduction without compromising beam quality or divergence.
4Reliability
If adaptive optics or phase conjugation are used to correct turbulence effects, then wavefront distortion is reduced, but system complexity and noise increase
Solution Approach 1:
The system performs preliminary measurement of atmospheric turbulence using the down-link beacon beam before the up-link communication occurs. By measuring the beacon beam's angular position and characteristics in advance, the controller can pre-adjust the transmitter beam pointing direction to compensate for anticipated turbulence effects, eliminating the need for real-time complex wavefront correction during communication.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively reduces signal fading by minimizing speckle formation and maintaining signal power at the satellite receiver, even under turbulent conditions, without introducing additional noise or complexity, thus enhancing the reliability of long-range optical communication systems.
Implementation Method 1
Time variation of the atmosphere parameters, such as local temperature and density of air, causes changes in the speckle distribution that appears as random 'boiling' of speckle pattern, in other words, random movement of points of intensity maximums and minimums across the beam. This represents the scintillation effect, which appears as temporal fluctuation of laser beam intensity spatial distribution.
Implementation Method 2
The random beam wander in turbulent atmosphere may also lead to significant reduction of the receiver signal power acquired when the laser beam shifts as a whole far enough outside the receiving aperture
Implementation Method 3
Laser beam propagation in turbulent atmosphere can be accompanied by random beam wander (changing central direction of laser beam propagation) and strong aberrations with formation of inhomogeneous, also called speckled, spatial structure of the laser beam. Cross section of intensity profile of the speckled beam consists of randomly distributed bright spots ('speckles') divided by regions with low or zero light intensity.
Data Source
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AI summary
A method and system for reducing optical signal fading in an optical communication system include: dynamically obtaining turbulence-induced divergence and deviations of pointing direction for the atmosphere where an optical signal to be transmitted through; generating a single-transverse-mode laser beam of a predetermined diameter as the optical signal; dynamically varying the diameter of the laser beam to match the obtained turbulence-induced divergence of the atmosphere with a backtrack pointing direction; and transmitting the laser beam with varying diameter to a remote transceiver, as the optical signal.