Hybrid Coherent Incoherent Diffractive Beam Combining
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Solution Overview
Problem
Current high power laser technologies face limitations in achieving a diffraction-limited focus due to physical constraints and imperfections in optics, leading to reduced power delivery per unit area, and existing beam combining methods either suffer from fill factor issues or increased complexity and energy loss when dealing with multiple wavelengths.
Innovation Solution
A hybrid beam combining system that combines both coherent and incoherent beams by using N oscillators to split each wavelength into M beams, phase-locking them, and then using a diffractive optical element to coherently combine them, followed by spectral beam combining to achieve a composite beam with a fill factor of nearly 100%, reducing thermal distortions and bandwidth requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If coherent beam combining using a two-dimensional array of optical fiber emitters is used to increase power, then the power delivered by the laser beam is increased, but the fill factor is reduced to about 70% due to voids between fibers and Gaussian beam shape, resulting in reduced brightness and poor focusability
Solution Approach 1:
The patent segments the beam combining process into two distinct stages: first, coherent combining of beams at the same wavelength to achieve high power; second, incoherent spectral combining of the coherent beams at different wavelengths to achieve the final composite beam. This segmentation allows each stage to optimize for its specific function, with the coherent stage achieving high fill factor and the spectral stage adding wavelength diversity without compromising the fill factor.
Solution Approach 2:
The patent transitions from two-dimensional spatial array combining to three-dimensional combining by adding the wavelength dimension. Spectral beam combining combines beams at different wavelengths along the spatial path, effectively utilizing the spectral dimension to increase power while maintaining the spatial fill factor achieved in the coherent combining stage.
2Shape
If spectral beam combining is used to combine incoherent beams of different wavelengths, then the composite beam shape is substantially identical to constituent beams, but the system complexity increases and each wavelength requires different oscillators with precise angular adjustment
Solution Approach 1:
The patent performs preliminary coherent combining of beams at each wavelength before spectral combining. By first achieving high-fill-factor coherent beams at individual wavelengths, then combining these pre-optimized beams spectrally, the system avoids the complexity of directly combining many individual incoherent beams while maintaining the beneficial beam shape properties.
Solution Approach 2:
The patent merges the advantages of coherent combining (high fill factor) and spectral combining (beam shape preservation) into a hybrid approach. The coherent combining stage consolidates beams at each wavelength into high-quality beams, and the spectral combining stage merges these consolidated beams while preserving their shapes, achieving both goals simultaneously.
3Power
If coherent beam combining is used to achieve high power, then the focused composite beam has central peak intensity equal to fill factor ratio times maximum intensity, but the composite beam shape is dramatically different from constituent beams resulting in poor focusability
Solution Approach 1:
The patent segments the combining process to preserve beam shape. By performing coherent combining first (which preserves the Gaussian shape of individual beams) and then spectral combining (which maintains the shape while adding wavelengths), the final composite beam retains the desirable Gaussian shape characteristics despite high power consolidation.
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 high efficiency in combining multiple beams into a single diffraction-limited beam with improved focusability and reduced energy loss, overcoming the limitations of existing methods by maintaining the shape of the constituent beams and minimizing thermal distortions.
Implementation Method 1
The M beams in each of N groups are coherently combined by a diffractive optical element (DOE) into a single beam at each of N wavelengths
Implementation Method 2
The N beams are incident and spectrally combined on spectral beam combining (SBC) grating, which outputs a composite beam
Implementation Method 3
the M beams in each of N groups are phase locked by phase modulators according to phase correction signals provided to the phase modulators in a feedback loop
Data Source
AI summary
A hybrid beam combining system or method combines a plurality of coherent and incoherent light beams into a composite high power diffraction limited beam. N oscillators each transmit light at one of N different wavelengths and each wavelength is split into M constituent beams. M beams in each of N groups are phase locked by a phase modulator using phase correction signals. The phase locked beams are amplified and coupled into an M×N fiber array. Beams emerging from the array are collimated and incident on a diffractive optical element operating as a beam combiner combining the M outputs at each N wavelength into a single beam. The N single beams are incident and spectrally combined on a grating which outputs a composite beam at a nominal 100% fill factor. A low power sample beam, taken from the N beams emerging from the diffractive optical element, is measured for phase deviations from which the phase correction signals are derived and fed back to the phase modulators. The diffractive optical element may include a weak periodic grating for diffracting the low power sample. The diffractive optical element may also be combined with the spectral combining grating into a single optical element.