High Power Chirally-Couple-Core (CCC) Fiber Lasers for Coherent Combining Systems

Large core fibers with diffraction limited beam output are required by the rapidly developing high-power fiber lasers technology, with numerous current and future applications ranging from industrial to fundamental scientific. While PCF-based large-core solutions reached core sizes of more than 100µm, albeit at the cost of sacrificing their compatibility with compact integration, so-called chirally-coupled core (CCC) fibers demonstrate robust single-mode output with cores sizes reaching around 60µm in structures completely compatible with standard fiber fusion splicing and coiled packaging techniques, well suited for monolithically-integrated compact and robust high power fiber laser systems.

In this dissertation we present a detailed study of using this novel CCC fiber technology for high pulse energy and high average power systems, in particular for use in different types of coherently-combined fiber laser arrays, where compatibility of CCC fiber technology with monolithic integration becomes an enabling factor for constructing complex but practical high-power laser "circuitry".

We first present a detailed theoretical description of power handling and thermal characteristics in high power fiber amplifiers, and analyze impact of modal leakage from an effectively single-mode fiber core on the fiber amplifier and laser efficiency. Furthermore, unique polarization preservation characteristics of CCC fibers are explored, which provide the theoretical foundation for design guidelines to achieve stable polarization preservation in high power CCC fiber systems. In subsequent chapters we present experimental exploration of high average power scaling of CCC fiber amplifiers, reporting up to 576W of single frequency output from 37µm core CCC fiber amplifier, as well as laboratory study of large-core CCC fiber amplifier modal output performance at high average powers. Furthermore, we report demonstration of up to 9.1mJ at ~1MW peak power extraction from a 55µm core Yb-doped double-clad CCC fiber amplifier, the highest ever reported pulse energy from any effectively single-mode large-core fiber. Then we theoretically explore novel pulsed pumping approach, which could lead to an order of magnitude increase in extracted pulsed energies compared to what is currently possible with cw pumping.

Contributions of this work will be particularly important for the development of high intensity kHz-repetition rate ultrashort-pulse laser systems for driving laser plasma accelerators.