High power fiber lasers have revolutionized many areas of both basic research and industry with their high power, high brightness output and capability for a compact, monolithic design. These systems are often run in multimode operation with few constraints on output. However, for many directed energy applications, such as spectral beam combining, the necessary laser parameters are far more stringent, requiring systems with state-of-the-art characteristics, such as single transverse mode operation, narrow linewidth and multi-kW output powers. Currently, further growth in output power of fiber laser systems at 1 and 2 µm is hindered by various nonlinear and thermal limitations within the fiber gain medium. Specifically, 1 µm systems are primarily hindered by transverse mode instability, while 2 µm lasers at high average powers often encounter an effect known as modulation instability. This dissertation explores the power scaling potential of both Yb-doped and Tm-doped fiber lasers along with related applications. Power scaling of two different Yb:fiber amplifiers is examined, where first, a novel confined-doping fiber geometry is used to generate 450 W single mode output, and second, through a demonstration of a Yb:fiber amplifier with > 2200 W output power and quasi-single mode beam quality. At 2 um, a new pumping architecture known as in-band pumping is utilized in an effort to take Tm-doped fiber amplifiers first to the multi-100 W level and then into the multi-kW regime. An in-band pumped laser is developed and characterized with over 75 W output power and > 80% slope efficiency. The last laser system constructed is a tunable, narrow linewidth Tm-doped fiber laser used to systematically induce and study the effects of thermal blooming. Results obtained from scanning the wavelength across individual atmospheric absorption resonances are presented, along with time-resolved data obtained at a fixed wavelength. Finally, two candidate fiber designs are presented for power scaling of in-band pumping at 2 µm to the multi-kW power level. Numerical studies detail the performance of these two fiber designs at high average powers and their ability to suppress the onset of modulation instability. Taken together, the results discussed in this dissertation further the science of fiber lasers for directed energy applications.


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Graduation Date





Richardson, Martin


Doctor of Philosophy (Ph.D.)


College of Optics and Photonics


Optics and Photonics

Degree Program

Optics and Photonics




CFE0008812; DP0026091





Release Date

December 2026

Length of Campus-only Access

5 years

Access Status

Doctoral Dissertation (Campus-only Access)

Restricted to the UCF community until December 2026; it will then be open access.