Large-mode-area Soliton Fiber Oscillator Mode-locked with Linear Self-stabilized Interferometer MARVIN EDELMANN, 1,2,3,* MALEK M. SEDIGHEH, 1,2 YI HUA, 4 ERWIN C. VARGAS, 1,2 MIKHAIL PERGAMENT, 1 AND FRANZ X. KÄRTNER 1,2 1 Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany 2 Department of Physics, Universität Hamburg, Jungiusstr. 9, 20355 Hamburg, Germany 3 Cycle GmbH, Notkestr. 85, 22607 Hamburg, Germany 4 Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany *Corresponding author: marvin.edelmann@desy.de Date of Submission: 10.08.2022 In this work, we investigate an approach to scale up the output pulse energy in an all polarization-maintaining 17 MHz Yb-doped fiber oscillator via implementation of 25 µm core-diameter large-mode-area fibers. The artificial saturable absorber in form of a Kerr-type self-stabilized fiber-interferometer enables highly stable mode-locked steady-states in the soliton-like operation regime with 170 mW average output power and a total output pulse energy of ~10 nJ distributed between two output ports. An experimental parameter comparison with a reference oscillator made of 5.5 µm core-sized standard fiber- components reveals an increase of pulse energy by a factor of 36 with simultaneously reduced intensity-noise in the high frequency range > 100 kHz. Mode-locked fiber oscillators for the generation of stable femtosecond pulse trains are key-elements for many state-of-the- art scientific and industrial applications including synchronization and timing [1], biological imaging and spectroscopy [2], seeding of high-power lasers [3] and photonic microwave generation [4]. The output characteristics of the oscillator in conjunction with its parameter stability and resistance against environmental perturbations determine the overall system performance and robustness in industrial applications. Especially the characteristics of the saturable absorber (SA) for initiation and stabilization of the mode-locked steady-states play a crucial role as it often limits the achievable parameters with respect to optical bandwidth, intracavity power, and output pulse energy [5,6]. For more than 3 decades, artificial SA mechanisms based on the optical Kerr-effect have revealed themselves as promising candidates for next-gen fiber oscillator technology. Nowadays, fiber oscillators mode-locked with nonlinear amplifying/optical loop mirrors (NALM/NOLM) [7- 9] or linear self-stabilized fiber-interferometers (LSI) [11-13] with all polarization-maintaining (PM) structures routinely achieve state-of-the-art environmental stability [10], timing jitter and intensity-noise [11]. In contrast to real SAs such as semiconductor saturable absorber mirrors (SESAM) or topological insulators, SAs based on the non-resonant optical Kerr-effect allow for large optical bandwidths and short pulse durations while being robust against optical damage and parameter degradation [12]. Besides these advantages, there are also limitations of fiber oscillators associated with the large roundtrip nonlinear phase shifts due to the strong confinement of the laser mode in the fiber segments. These nonlinear phase shifts limit the obtainable intracavity pulse energy as it initiates multi-pulse formation or breakthrough of continuous- wave lasing above a certain threshold, which ultimately deteriorates the stability of the mode-locked state [13,14]. The limitation of the pulse energy often necessitates an increased system complexity in form of additionally required amplification stages. In addition, it is further associated with additional limitations of the oscillators noise performance in terms of intensity and phase fluctuations [15,16]. Different approaches to overcome this limitation have been the subject of intensive research over the last decade. Besides techniques based on precise dispersion- management to reduce the average peak-power of the intracavity pulse in the dissipative soliton [17] or stretched-pulse regime [18], other works further revealed the possibility of periodic intracavity coherent pulse division and recombination [19,20]. In addition, the scaling of the core-size with large-mode-area (LMA) fibers in NALM/NOLM mode-locked lasers has enabled a significant increase of pulse energy due to the quadratic dependence on the core diameter [21,22]. In this work, we demonstrate the application of 25 µm core diameter LMA fibers and fiber-optic components in an LSI mode- locked fiber oscillator for the first time. The 17 MHz Yb-doped all- PM oscillator allows stable mode-locking in the soliton-like regime with a total pulse energy of 10 nJ and average power of 170 mW, distributed between two output ports. An experimental comparison with a reference oscillator operating at identical repetition rate and net-dispersion shows that the core-diameter enlargement from 5.5 µm to 25 µm results in an increase of the