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16 February 2018 Investigation of an ultra large mode area power amplifier stage for a pulsed 1550-nm laser system
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An Ultra Large Mode Area (ULMA) erbium doped optical fiber having a 52 micron core was investigated as the high power stage of a 1550 nm pulsed fiber laser system. The ULMA fiber was seeded with 1550 nm light from a 4 stage all fiber pulsed laser system with pulse energies of 11.35 and 6.25 μJ at pulse widths of 300 ns and repetition rates of 2 and 10 kHz, respectively. The ULMA fiber stage was counterpumped continuous wave in-band with a 1480 nm Raman laser. Maximum pulse energies (and amplifications) of 360 (15 dB) and 130 μJ (13.2 dB) at 2 and 10 kHz, respectively, were found when pumped with approximately 50 W of 1480 nm pump. More than 90% of the power at both repetition rates was found to reside in the pulse which indicated lower levels of amplified spontaneous emission. A lower conversion efficiency of approximately 53% -- which was significantly less than the theoretical maximum of 95% -- was estimated for the ULMA fiber amplifier. Possible causes are significant contamination on the order of 25% of the 1480 nm pump by lower Stokes orders, low coupling fractions of the 1480 nm pump into the core of the ULMA fiber, as well as pair induced quenching of Er due to clustering. Finally, creation of pump dumps at both the 1480 nm pump and 1550 nm signal ends by roughening the surface of the optical fiber enabled higher pump levels and pulse energies than would otherwise be the case.
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Michael Klopfer, Leanne J. Henry, and Ravinder Jain "Investigation of an ultra large mode area power amplifier stage for a pulsed 1550-nm laser system ", Proc. SPIE 10518, Laser Resonators, Microresonators, and Beam Control XX, 105181V (16 February 2018);

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