Phase Sensitive Beam Tracking for Free Space Optical Links
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Solution Overview
Problem
Existing communication networks face challenges in maintaining accurate alignment and correction of free space optical communication beams due to scintillation and pointing errors caused by atmospheric turbulence and mechanical vibrations, which require high precision and speed in beam tracking.
Innovation Solution
A beam tracking system that converts amplitude-driven differential analog signal processing into a phase/timing problem measurable using digital electronics, employing a multi-axis position sensitive detector, signal processing hardware for phase shifting and gain compensation, and a mirror control system to adjust the beam alignment, eliminating errors associated with analog division.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If analog division is used to solve amplitude-driven differential signal processing, then the system can process signals, but numerous errors are generated and manufacturing precision deteriorates
Solution Approach 1:
The patent replaces the mechanical/analog division operation with a digital signal processing approach using phase-locked loops and frequency counting. Instead of using analog dividers that generate errors, the system uses digital electronics to measure phase differences and calculate position, thereby eliminating division errors and improving measurement precision.
Solution Approach 2:
The patent transforms the signal processing from amplitude-driven to phase-driven. By changing the measurement parameter from amplitude ratio to phase difference, the system avoids the errors inherent in analog division while maintaining the ability to process differential signals. This parameter transformation enables digital processing with higher precision.
2Measurement precision
If high precision beam tracking is implemented to correct pointing errors, then alignment accuracy improves, but system complexity increases
Solution Approach 1:
The patent uses digital signal processing techniques (phase-locked loops, frequency counters) to replace complex analog tracking circuits. This substitution simplifies the overall system architecture while achieving high precision beam alignment through software-based algorithms running on digital signal processors.
Solution Approach 2:
The patent introduces a digital signal processor as an intermediary between the photodetector and the control system. This intermediary handles the complex signal processing and position calculation, isolating the simplicity of the optical front-end from the complexity of the control algorithms, thereby managing system complexity while maintaining high precision.
3Speed
If fast beam tracking is implemented to compensate for dynamic pointing errors, then response speed improves, but measurement precision may deteriorate due to dynamic effects
Solution Approach 1:
The patent implements continuous phase measurement and feedback through the phase-locked loop, ensuring that the system continuously tracks beam position without interruption. This continuous action allows the system to respond to dynamic pointing errors in real-time while maintaining measurement precision through uninterrupted phase detection.
Solution Approach 2:
The patent uses feedback mechanisms where the phase-locked loop continuously adjusts its reference signal based on the detected phase differences. This feedback enables the system to correct for dynamic effects in real-time, maintaining both fast response and high precision by continuously adapting to changing beam conditions.
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
The system achieves precise alignment and correction of optical communication beams with position measurement accuracy down to 0.1% despite large amplitude changes, effectively mitigating scintillation and pointing errors, ensuring stable communication links.
Implementation Method 1
a multi-axis position sensitive detector configured to sense a position of a communication beam incident thereupon and output axis signals corresponding to the sensed position of the communication beam
Implementation Method 2
shifting a phase of the axis signals by approximately 90 degrees
Data Source
Figure 1A
Figure 1B
Figure 1C
AI summary
A method includes receiving axis signals (520, 530, 540, 550) from a multi-axis position sensing detector (510), generating a reference signal (660) by summing the axis signals, determining a mirror position of a mirror (880) directing the optical beam based on the beam position error (780, 790) of each axis of the multi-axis position sensing detector, and actuating the mirror to move to the mirror position. Each axis signal is indicative of a beam position (122) of an optical beam incident on the multi-axis position sensing detector, each axis signal corresponding to an axis of the multi-axis position sensing detector. For each axis, the method includes converting a phase of an axis to have a 90 degree phase difference from a signal of the axis, generating an axis-phasor signal (640, 650) by summing the axis signals, and comparing the axis-phasor signal and the reference signal to determine a phase difference.