Coherent Beam Combining via RF Delay and Linewidth Broadening
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Solution Overview
Problem
High energy fiber laser systems face challenges in coherent beam combining due to the need to broaden laser linewidths to avoid non-linear effects like Stimulated Brillouin Scattering, which reduces coherence lengths and makes phase-locking of beams difficult.
Innovation Solution
A system comprising a first sub-system with electro-optical modulators and output optics, and a second sub-system with a linewidth broadening RF generator, delay lines, and a system controller, which generates and adjusts radio frequency signals to phase-lock and synchronize the arrival times of optical signals across multiple channels, maximizing interference patterns and recovering coherence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the laser linewidth is broadened to avoid Stimulated Brillouin Scattering in high power applications, then the harmful non-linear effects are reduced, but the coherence length is reduced proportionately making phase-locking difficult
Solution Approach 1:
The patent applies preliminary action by pre-broadening the linewidth of each individual fiber laser channel before beam combining. This is achieved by modulating the phase of the laser signal in each channel using electro-optical modulators driven by pseudo-random binary sequence (PRBS) codes. By performing this linewidth broadening action beforehand, the system prevents Stimulated Brillouin Scattering from occurring during high-power amplification, while the patent subsequently compensates for the reduced coherence length through precise optical path length matching and phase-locking techniques.
2Power
If the coherence length is reduced to prevent non-linear effects, then the laser can propagate high optical power densities, but the coherence lengths become so small that mismatches between laser beam coherences arise
Solution Approach 1:
The patent employs feedback mechanisms to monitor and adjust the optical path lengths of individual channels. By using feedback signals from the interference pattern detection, the system dynamically adjusts the optical path length of each channel to ensure that the coherence lengths of all channels are properly aligned. This feedback control enables reliable coherent beam combining even when individual channel coherence lengths are short, thereby maintaining beam coherence matching while propagating high optical power densities.
Solution Approach 2:
The patent applies parameter changes by precisely controlling the optical path length parameter of each individual channel. By adjusting the optical path length to match within a fraction of the coherence length, the system compensates for the reduced coherence length caused by linewidth broadening. This parameter adjustment ensures that all channels maintain proper phase relationships, enabling reliable coherent beam combining while maintaining high optical power density propagation.
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
Enables effective coherent beam combining by synchronizing the arrival times of shortened coherence photons, overcoming the limitations of reduced coherence lengths and non-linear effects, thus achieving stable high-power laser combinations.
Implementation Method 1
each of the at least two channels of the first sub-system comprising a first electro-optical (EO) modulator
Implementation Method 2
phase locking optical signals in the at least two channels
Implementation Method 3
generating a reference interference pattern by propagating the master optical signal through the first sub-system
Implementation Method 4
adjusting a delay time of a respective delay line of one of the at least two optical channels
Data Source
AI summary
A method, apparatus and system for coherent beam combining (CBC) in high energy fiber laser (HEL) systems include generating a reference interference pattern of a signal source including at least two single-mode optical signals, capturing and evaluating the reference interference pattern, maximizing an intensity of the selected area of the captured, reference interference pattern, increasing a linewidth of the optical signals generating the reference interference pattern until the reference interference pattern is degraded, and adjusting a delay time of one of the at least two single-mode optical signals until the reference interference pattern is recovered, by adjusting a value of a delay of a delayed RF signal with a broaden linewidth to a respective EO linewidth broadening modulator in at least one channel of the at least two single-mode optical signals while evaluating the interference pattern on a display device.