Diffractive Optical Element Beam Combiner Error Control
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Solution Overview
Problem
Existing laser systems combining output beams from multiple fiber amplifiers face challenges with beam quality due to piston and tilt errors, leading to power loss and secondary beamlets, which degrade the primary output beam.
Innovation Solution
A high-power laser system incorporating a diffractive optical element (DOE) for beam combination, along with piston and tilt error controllers using phase and amplitude modulation, respectively, to minimize errors and maximize energy in the primary output beam.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a lenslet array is used to combine beams from multiple fiber amplifiers, then beam combination is achieved, but beam quality deteriorates due to the Fourier transform of the lenslet array being imprinted on the far-field pattern
Solution Approach 1:
The patent extracts the harmful Fourier transform pattern imprinting by removing the lenslet array and replacing it with a diffractive optical element (DOE) that combines beams without imprinting its own pattern, thus eliminating the source of beam quality degradation while maintaining combination capability
Solution Approach 2:
The patent introduces a diffractive optical element (DOE) as an intermediary component that performs beam combination through diffraction rather than refraction, avoiding the fill-factor losses and pattern imprinting associated with lenslet arrays while achieving the desired beam combination
2Productivity
If a lenslet array is used to combine beams, then beam combination is achieved, but power is lost due to overfilling of lenslet aperture and interstitial space
Solution Approach 1:
The patent introduces a diffractive optical element (DOE) as an intermediary that combines beams through diffraction, allowing full utilization of the aperture area without the fill-factor losses inherent in lenslet arrays, thereby reducing energy loss while maintaining combination functionality
Solution Approach 2:
The patent replaces the mechanical lenslet array system with a diffractive optical element that uses wave optics (diffraction) instead of geometric optics (refraction), eliminating the interstitial space losses and aperture overfilling issues that cause energy loss in lenslet-based systems
3Device complexity
If beamlets are incident on DOE at incorrect angles or with incorrect phase, then beam combination is simplified, but secondary beamlets are generated reducing power and beam quality
Solution Approach 1:
The patent implements feedback control by monitoring the output beam for satellite beamlets and adjusting the incident beam angles and phases accordingly, using the presence of secondary beamlets as an error signal to fine-tune the system and eliminate harmful diffraction orders while maintaining simple operation
Solution Approach 2:
The patent controls the parameters of incident beams (angles and phases) to match the DOE design specifications, ensuring that beams are incident at the correct eigenangles with appropriate phase relationships, thereby maximizing energy concentration in the primary beam and minimizing secondary beamlet generation
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 reduces secondary beamlets and enhances beam quality by correcting piston and tilt errors, thereby maximizing energy in the primary combined output beam.
Implementation Method 1
a diffractive optical element (DOE) may also be used to combine the beamlets from several fiber amplifiers into one output beam
Implementation Method 2
A piston error controller provides for each intermediate beamlet a unique dithering signal used to phase modulate the beamlet before combination
Implementation Method 3
A tilt error controller provides for each non-reference beamlet a tagging signal with which the beamlet is amplitude-modulated prior to combining
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 diffractive optical element for combining the intermediate beamlets into a combined output beam, and one or more error controllers for minimizing errors related to beam combination that may degrade the quality of the combined output beam. A piston error controller uses phase modulation to tag each non-reference intermediate beamlet with a unique dither signal harmonically unrelated to those used for the other beamlets. For each intermediate beamlet, the associated piston error is recovered using a synchronous detector, and an error control signal proportional to the piston error is supplied to a phase modulator to control the piston error for that beamlet. A tilt error 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 tilt error is recovered using a Hadamard decoder, and an error control signal proportional to the tilt error is supplied to a beam steerer to control the tilt error for that beamlet.