Passive thermal management in high-power fiber laser systems

Parasitic and catastrophic thermal effects serve as the limiting factors for further power scaling in high-power fiber lasers. Among the different thermal energy generation processes, quantum defect (QD) heating ultimately serves as the dominant heat source in an optimized system. In a typical laser system, a positive QD gives net heating because the pump photon energy is higher than that of the signal. To reduce quantum defect heating, two approaches are investigated, both theoretically and experimentally. Since Yb-doped fiber lasers give the highest output power compared to any other fiber laser systems, the discussions herein mainly focus on Yb-doped fiber laser systems. The first approach is to reduce the QD heating by bringing the signal wavelength closer to the pumping wavelength. This requires exploring new host glasses for Yb3+ including fluorosilicates and phosphosilicates, so that efficient optical gain at shorter wavelengths can be achieved. Both low-power (< 1W pump power) linear cavity lasers and high-power (20 W pump power) amplifiers are demonstrated when pumping at ~975 nm. The former exhibited nearly 70% slope efficiency at 985.7 nm while the latter had an 87.4 % slope efficiency with respect to absorbed pump power when operating at 1005 nm. Theoretical models based on rate equations were built and the experimental results agree well with the simulations. Calculations are also performed to explore improved fiber designs and the possibility of a double-end pumping scheme. The second approach is to introduce a “cooling” mechanism so that the overall QD heating is reduced. To do so, a negative contribution to the QD is added via a second pumping wavelength positioned to the red side of the lasing wavelength (anti-Stokes pump). Since the balancing of the thermal energy is introduced through excitation of the system at two wavelengths, such a system is named excitation-balanced fiber laser or amplifier. Demonstrated first is a laser operating in a pulsed regime. The experimental results indicate that anti-Stokes pumping can effectively contribute to stimulated emission in a solid, resulting in a form of self-cooling. An FDTD model is built to further understand and optimize the system. Next, modeling of a high-energy excitation-balanced pulse amplifier is performed. Results indicate that a heat-free mJ-level pulse amplifier is possible. The influences from several input parameters, such as pump power, pump pulse width, and repetition rate are also discussed in detail, providing valuable guidance for future design and experiments. Last but not the least, since thermal energy can also be generated via nonradiative processes, a testing platform based on Brillouin scattering is proposed to quantify those sources of heating. The functionality of the platform is confirmed by testing a commercial Yb-doped fiber, and a few demonstrations of anti-Stokes fluorescence cooling are made in nanoparticle doped fiber. Such a flatform will be useful for optimizing glass quality. Finally, a brief conclusion is provided, and several future works are proposed.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Electrical & Computer Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Yu, Nanjie

Contributors dc:contributor

• Dragic, Peter D
• Choquette, Kent D
• Eden, J Gary
• Goddard, Lynford L
• Ballato, John

• optical fiber
• fiber laser

Statement dc:rights

• Copyright 2022 Nanjie Yu

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/115352

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F115352