Piston Error Control in Coherent Laser Arrays
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Solution Overview
Problem
High-power laser systems using arrays of fiber amplifiers face challenges in achieving high output power due to limitations such as Stimulated Brillouin Scattering, four-wave mixing, and optical damage, which restrict the energy concentration in a single lobe, and existing phase control systems become costly and complex as the number of elements increases.
Innovation Solution
A high-power laser system incorporating a laser master oscillator, fiber laser amplifier chains, a lenslet collimator array, and a piston error detection and processing system that uses a diffractive optical element (DOE) and phase modulators to correct piston error, allowing for improved energy concentration in a single lobe by distinguishing and correcting phase errors in each beamlet.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple fiber amplifiers are coherently combined to increase output power, then the output power is improved, but the beam quality deteriorates due to piston error and phase fluctuations
Solution Approach 1:
The patent implements a feedback control system where a photodetector array measures the far-field beam pattern, and this measurement is fed back to piezoelectric actuators that adjust the piston position of each fiber amplifier to minimize side lobes and maximize main lobe energy concentration
Solution Approach 2:
The patent replaces mechanical alignment adjustments with piezoelectric actuators that provide precise, electrically-controlled piston position adjustment, enabling dynamic phase control without mechanical intervention
2Productivity
If a lenslet array is used to combine beams from multiple fiber amplifiers, then the beam combination is improved, but the system complexity and cost increase due to precision manufacturing requirements
Solution Approach 1:
The patent employs a self-aligning fiber array configuration where individual fibers are independently mounted and adjustable, allowing each fiber to self-adjust its position and angle to achieve optimal alignment with the corresponding lenslet, eliminating the need for precision-manufactured rigid substrates
Solution Approach 2:
The patent divides the beam combining system into independent, modular units where each fiber-lenslet pair can be independently aligned and adjusted, replacing the monolithic precision-manufactured substrate approach with separable, adjustable components
3Power
If the number of fiber amplifiers in the array is increased to achieve higher power, then the output power is improved, but the phase control system becomes more costly and complex
Solution Approach 1:
The patent implements a universal control architecture where a single photodetector array and processing system serve all fiber amplifiers in the array, with each fiber controlled by a common feedback loop rather than requiring dedicated control systems for each element
Solution Approach 2:
The patent combines the control functions for multiple fiber amplifiers into a unified system where the photodetector array simultaneously monitors all beams and the processing system generates coordinated control signals for all piezoelectric actuators, reducing overall system complexity
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 effectively maximizes energy in the primary combined output beam, improving beam quality and reducing the impact of piston error, while being resistant to crosstalk and maintaining beam quality despite increased complexity.
Implementation Method 1
a diffractive optical element (DOE) having a grating profile or shape defined by a grating profile function... the grating profile or shape causing the plurality of intermediate beamlets to be combined into a single combined output beam
Implementation Method 2
phase modulators to correct piston error, allowing for improved energy concentration in a single lobe by distinguishing and correcting phase errors in each beamlet
Implementation Method 3
fiber laser amplifier chains producing intermediate output beamlets... optically-pumped preamplifier which receives and amplifies the signal from the master oscillator
Implementation Method 4
a lenslet collimator array to combine the intermediate beamlets into a combined output beam
Data Source
AI summary
A high-power laser system includes a laser master oscillator, a plurality of fiber laser amplifiers producing intermediate output beamlets, a combiner for combining the intermediate beamlets into a combined output beam, and a piston error controller for minimizing errors related to beam combination that may degrade the quality of the combined output beam. A piston error controller processes a sample of the combined output beam using a Diffractive Optical Element to isolate a signal representing the total piston error of the combined beam. The controller uses amplitude modulation based on Hadamard code words to tag each non-reference intermediate beamlet with a unique code sequence orthogonal to those used for the other beamlets. For each intermediate beamlet, the associated piston error contribution is recovered using a Hadamard decoder. A very small phase dither is also introduced to allow the sign or direction of the piston error to be recovered. The decoded piston error contribution is processed by a cascaded product detector unit to derive a piston error control signal, which is provided to a phase modulator to thereby adjust the phase of a beamlet and minimize the piston error contributed thereby.