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Modeling of Fiber Laser Refractive Index Profiles and Dopant Index Profiles for Predicting High Power Fiber Laser Performance

Description:

TECHNOLOGY AREA(S): Weapons 

OBJECTIVE: Develop innovative physics-based modeling methods for predicting high power fiber laser performance given refractive index and doping profile geometries of “as drawn” optical fibers. 

DESCRIPTION: This topic seeks mathematical and physics-based modeling and simulation source codes that import actual fiber refractive index and doping profile geometries to predict fiber performance and to estimate high power performance limitations. The need to model and verify, “as drawn,” fiber parameters to an intended design is critical to extending the laser reliability and power scaling designs beyond current state of the art. This modeling capability is critical to developing next generation high power fiber laser weapon systems as it will extend their efficiency, reliability, and multi-kW power scaling capabilities. Narrow-linewidth all-fiber laser sources are highly desired for directed energy applications, as they can be either spectrally or coherently beam combined for further power scaling to power levels. Develop and demonstrate concepts and hardware which enable high-brightness, high-power scaling of Ytterbium and Thulium fiber lasers/amplifiers to mature components and subsystems for robust system architectures. Fiber refractive index and rare earth doping profiles impact the ability of the fiber optic wave guide to maintain diffraction limited beam quality and produce good beam quality output at high power. Commercial measurement apparatus are available to provide refractive index and doping profile input parameters. Measurement of refractive index and tolerance of refractive index for typical large mode area rare earth doped fused silica fibers can be commercially manufactured with tolerances of 1x10-4, plus or minus 5x10-5. Therefore, refractive index differences between cores and cladding materials are critical parameters when propagating or combining multiple fiber lasers concurrently and is critical to the implementation of advanced beam control architectures. Similarly, non-destructive commercially available approaches can measure distribution and concentration of dopant materials in an optical fiber with measurement by % weight of rare earth dopants in an active fused silica cores to an accuracy of +/- 0.1 percent. 

PHASE I: Develop and mature innovative physics-based modeling and simulation software methods that import as measured refractive index and doping profile geometries of fibers designed and fabricated for high power fiber lasers/amplifiers. Modeling and simulations should incorporate physical configuration geometries and operational performance parameters from actual fiber laser implementations to provide prediction of deleterious effects. 

PHASE II: Expand the physics-based modeling and simulation software methods to include both commercially available and novel optical fibers fabricated for high power fiber lasers. Advanced modeling and simulations should incorporate realistic heating, fiber coiling, and changes to amplifier seeding methods and pumping. Software source codes should demonstrate that physics assumptions incorporate actual measurement parameters including minute changes to refractive index profiles and rare earth doping constituents distributions to verify fiber geometry of single mode, multimode and novel fibers including; conventional large mode area, photonic crystal, and photonic bandgap fibers. 

PHASE III: Commercialize the software and technologies for accurately modeling and predicting deleterious effects in novel fibers designed for high power operation. 

REFERENCES: 

1: Marcuse D. Principles of Optical Fiber Measurement, ch. 4, New York: Academic Press, 1981

2:  Yablon AD and Jasapara J. Hyperspectral optical fiber refractive index measurement spanning 2.5 octaves, SPIE Proceedings, vol. 8601, 86011V, 2013.

3:  Zhao Y, Fleming S, Lyytikainen K, and Poladian L. Nondestructive Measurement for Arbitrary RIP Distribution of Optical Fiber Preforms, Journal of Lightwave Technology, vol. 22, pp 478-486, 2004.

4:  Pace P, Huntington S, Lyytikäinen K, Roberts A, and Love J. Refractive index profiles of Ge-doped optical fibers with nanometer spatial resolution using atomic force microscopy, Optics Express, vol. 12, pp. 1452-1457, 2004.

5:  Dragomir NM, Goh X, and Roberts A. Three-Dimensional Quantitative Phase Imaging: Current and Future Perspectives, SPIE Proceedings, vol. 6861, pp. 686106, 2008.

KEYWORDS: Fiber Laser, Rare Earth Doped Fibers, Optical Fiber Refractive Index, Dopant Modeling 

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