Overcoming pulse energy and peak power limitations in narrow-linewidth Tm3+-doped fiber lasers

Scaling the pulse energy and peak power in high-power, narrow-linewidth Thulium-doped fiber lasers (TDFLs) is fundamentally limited by parasitic lasing, amplified spontaneous emission (ASE), and nonlinear effects. We present a nanosecond-pulsed Tm-doped photonic crystal fiber MOPA that successfully pushes these performance boundaries, delivering 2.07 mJ nearly rectangular-shaped pulses, with 52 kW peak power at 207 W average power, linear polarization, a narrow 390 pm linewidth at 2048 nm and excellent beam quality (M²<1.3). Based on these results, we provide a detailed analysis of these limiting mechanisms, supported by theory and experiment. We examine the impact of fiber geometry (core diameter, fiber length) and operational parameters of the TDFL (pump and signal wavelength, temporal pulse shape, pulse repetition frequency) on the spatial, temporal and spectral output characteristics. Thus, this work provides a comprehensive framework for understanding and optimizing high-energy pulsed fiber lasers and aims to clarify common misconceptions in the field.