Coherent Beam Combining with Electronic Path Length Matching
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Solution Overview
Problem
High-energy laser systems face challenges in scaling power due to coherence length limitations and path length mismatches in fiber amplifiers, leading to inefficiencies in beam combination and SBS suppression.
Innovation Solution
Electronically tuning the delay of phase modulation in coherent beam combining systems allows for dynamic path length matching, reducing the importance of physical fiber lengths and enabling broader bandwidth compatibility, SBS suppression, and high-speed beam steering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If spectral width is increased to scale to higher single-amplifier powers, then SBS suppression is improved, but coherence length becomes shorter than differential path length changes
Solution Approach 1:
The patent implements dynamic path length adjustment through electronic control of phase modulators in each amplifier arm. This allows the system to adaptively compensate for path length differences in real-time, maintaining coherence despite the shortened coherence length from broadband operation. The dynamic adjustment converts a static limitation into a controllable parameter.
Solution Approach 2:
The system changes the operational parameters by using electronic phase modulation to alter the effective optical path length dynamically. By adjusting the phase shift parameter in response to measured path length differences, the system maintains constructive interference and coherent combining efficiency even with broadband sources having short coherence lengths.
2Manufacturing precision
If precise path length matching is performed manually, then combining efficiency is improved, but system setup time becomes very time consuming
Solution Approach 1:
The patent replaces manual mechanical adjustment of optical path lengths with electronic control of phase modulators. Instead of physically adjusting optical components to match path lengths, the system uses electronic phase shifting to achieve the same effect, dramatically reducing setup time while maintaining precision.
Solution Approach 2:
The system implements feedback control by measuring the actual path length differences between amplifier arms and using this information to dynamically adjust the phase modulation parameters. This closed-loop approach automatically achieves precise path length matching without manual intervention, eliminating the time-consuming setup process.
3Manufacturing precision
If physical fiber lengths are precisely controlled, then path length mismatch is reduced, but fabrication tolerances become more stringent
Solution Approach 1:
The patent introduces phase modulators as intermediary devices between the fixed physical fiber paths and the beam combination process. These modulators act as mediators that can electronically adjust the effective optical path length, compensating for physical length variations without requiring precise fabrication control of the fiber lengths themselves.
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 simplifies system fabrication, enhances combining efficiency, and allows for sub-second turn-on times and high-speed beam steering, while maintaining high power output and SBS suppression across multiple amplifiers.
Implementation Method 1
phase modulation between 0 and π phase
Implementation Method 2
suppression of stimulated Brillouin scattering (SBS) in the fiber amplifiers
Implementation Method 3
A photodetector measures a cross-correlation of an electric field the first laser beam with an electric field of the second laser beam
Implementation Method 4
The beams from several of these fiber amplifiers can be combined coherently such that the fiber amplifiers form a High Energy Laser (HEL) system
Data Source
AI summary
In coherent beam combining, the beams can be phase-modulated with a pseudo-random bit sequence (PRBS) to prevent stimulated Brillouin scattering (SBS) downstream. To coherently combine the phase-modulated beams, however, the PRBS waveforms should be true-time-delayed to within a small fraction of the bit duration. Traditionally, this true time delay is achieved by cutting optical fibers to length or with optical trombones. But trimming fibers is hard to do precisely, and optical trombones have large insertion loss. In addition, the path length mismatch varies as the fibers heat up and/or vibrate. Here, the beams are generated from a kilohertz linewidth seed split among N>1 (e.g., N=100) arms. Each arm is phase-modulated with a separate copy of the unique PRBS pattern. The relative phase of the PRBS patterns is stabilized by phase-locking the master oscillators used to read out the PRBS patterns. The PRBS patterns can be phase shifted with respect to one another to compensate for physical path length mismatches of the optical fibers. Scanning the relative phase of the PRBS pattern used to modulate different arms yields a cross-correlation peak in combined power when the phases are matched at the combination plane.