Innovative Pure-Erbium-Doped VLMA Fibers Delivering High Peak Power at $\boldsymbol{1.5 \mu} \mathbf{m}$ for Eye-Safe Solutions

Applications such as TOF (time-of-flight) LIDAR, free-space optical communications, and remote sensing require laser systems capable of delivering high-energy, high peak-power pulses at wavelengths of high atmospheric transmission. Erbium-doped fiber lasers, which operate in the eye-safe region of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.5 \mu \mathrm{m}$</tex> provides excellent beam quality, compact architecture, and ease of fabrication thanks to the technological maturity of adapted components. All these attributes make them highly suited to these demanding applications. However, scaling these laser sources to higher peak powers and pulse energies remains challenging. Limitations include the low absorption cross-section of erbium ions, parasitic <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1 \mu \mathrm{m}$</tex> emission from the common erbium-ytterbium codoping scheme, and the difficulties of achieving single-mode propagation with large mode areas (required to mitigate nonlinear effects) using conventional MCVD preforms. In this work, we demonstrate significant progress in performance using a novel Very Large Mode Area (VLMA) triple-clad fiber with a 55 μm core diameter, doped exclusively with erbium ions as active medium. As shown in Fig. 1(a), the fiber was fabricated via stack and draw technology of differently doped rod elements. The individual rod elements were drawn from preforms that were synthesized using the REPUSIL powder sintering technology, ensuring high doping uniformity and precise refractive index control. This approach enables efficient amplification while maintaining excellent beam quality. The low numerical aperture (NA = 0.048) of the active core ensures single-mode operation despite a large mode area, critical for peak power scaling. As a matter of fact, preliminary studies showed an efficiency of 43% in CW operation. As shown in Fig. 1(b), in a Q-switched oscillator configuration, the fiber delivered a pulse energy of 1.8 mJ at 1560 nm, with a pulse duration of 37 ns and a repetition rate of 1 kHz. The beam quality was measured at M = 1.46. Furthermore, in a second set-up, the fiber was set in a MOPA configuration using 3 ns, 1550 nm (± 0.2 nm), 25 kHz pulses as a seed. Fig. 1(c) depicts the fiber's ability to amplify up to 150 kW of peak power, corresponding to a pulse energy of 0.4 mJ, with a beam quality of M = 1.3. To the best of our knowledge, these results represent the state-of-the-art for conventional singlemode step-index double-clad erbium-doped fibers pumped at 976 nm [1]–[4]. These results establish a new benchmark for erbium-doped fiber amplifiers, demonstrating their capability to deliver high-energy pulses for demanding applications. The absence of significant nonlinear effects or amplified spontaneous emission (ASE) during these experiments emphasizes the robustness and scalability of this novel fiber design. Future work will focus on optimizing the fiber design to further increase energy and peak power levels while exploring its implementation in compact, polarization-maintaining architectures tailored to real-world applications. The ability to generate laser pulses with exceptional beam quality and record-setting performance establishes this technology as a key enabler for the next generation of high-power, eye-safe laser systems.