Applications such as directed energy, light detection and ranging (LIDAR) sensing, and free-space communications, require high average power, diffraction-limited lasers. Fiber lasers are already well suited for these tasks. However, the highest power fiber lasers operate at wavelengths at which the human eye is transparent (800–1100 nm), threatening to damage the retina of a battlefield participant or bystander.
We pursued two approaches to demonstrate high-power fiber lasers at safer wavelengths for the human eye: 15xx nm, Erbium / Ytterbium (Er/Yb) co-doped fiber, and 14xx nm, Neodymium (Nd) co-doped fiber. Lasers in these ranges combine high transmission in the atmosphere at wavelengths at which the cornea is effectively opaque, providing retina protection. Previously unknown material processing issues prevented success with Er/Yb. However, a 10 W Nd-doped fiber amplifier demonstrated greater than 500 times higher power than the previously-reported record with a path to even higher powers. The Nd fiber at 14xx nm can be used to resonantly pump Er in-band for emission at 1550 nm, although that feature was not demonstrated in this study. In addition to these results, a coupled mode theory model for multi-core fibers was developed.
The study advances Lawrence Livermore National Laboratory's core competency in lasers and optical science technology and supports the Laboratory's mission as a preeminent innovative science and technology contributor to the Department of Defense. Follow-on achievements from our work leveraged Livermore facilities and capabilities. For example, the first kW pump cladding at Livermore was demonstrated, and the National Ignition Facility-developed Advanced Mitigation Process was employed in the fiber stack-and-draw process that enabled Livermore's first kW fiber laser. The raised reservoir-type filtering fiber design and coupled mode theory model developed under this program positively impact fiber technology applications across the Laboratory. The study has generated interest from potential industry partners as well.
Drachenberg, D., 2018. "10W Single-Mode Nd+3 Fiber Laser at 1428nm." in Proc. SPIE LASE 2018. 10512. doi: 10.1117/12.2284876. LLNL-CONF-743918.
Drachenberg, D., et al. 2018. "Spectrally and Modally Selective Large Mode Area Fiber Coupler for High Power Applications." In Optics InfoBase Conference Papers. Vol. Part F111-SOF 2018. OSA - The Optical Society. doi:10.1364/SOF.2018.SoW2H.2. LLNL-PRES-754057.
——— . 2018. "Spectrally and Modally Selective Large Mode Area Fiber Coupler for High Power Applications." OSA Advanced Photonics Congress, Specialty Optical Fibers. LLNL-ABS-746808.
Khitrov, V., et al. 2018. "10W Single-Mode Nd3+ Fiber Laser at 1428nm." In Proc. SPIE, 105121P. doi:10.1117/12.2284876. LLNL-CONF-743918.
VanBlarcom, D., et al. 2018. "Novel AMP Surface Treatment for Improving Optical Fiber Strength and Laser Gain (Conference Presentation)." In Proc. of SPIE 10805, Laser Induced Damage in Optical Materials, 69. SPIE-Intl Soc Optical Eng. doi:10.1117/12.2501563. LLNL-PRES-758451.
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