In 2014, it was hypothesized that MCFs could act as shielded interferometers [1], where environmental perturbations would affect all cores in a highly correlated manner, leading to stable relative phases. However, this hypothesis has remained unverified. In this work, we present experimental evidence of relative phase stability in an Ytterbium-doped MCF at high power levels. Using two complementary measurement techniques, we demonstrate sub-wavelength relative phase stability at average output powers up to 55 W.

We report on our investigations on high-power, in-band pumped Thulium-doped fiber laser systems for high energy extraction in the 2 mu m wavelength region. The traditional 790 nm pumping scheme limits the output power scaling in Thulium-doped fiber lasers due to the quantum defect related heat load. Utilizing in-band pumping enables higher efficiencies, allowing to scale the average power further with high energy extraction. Therefore, a master-oscillator-power-amplifier configuration is presented that demonstrates significant average power scaling of up to 627 W with high slope efficiency (82 %) and allows for high pulse energy extraction of up to 5 mJ.

We present a femtosecond laser system based on a 49-core, Ytterbium-doped fiber. Phase control of the individual beams enables fast beam-shaping up to the kHz frequency range at 150 W total average power after compression.

Multi-core fibers have emerged as a promising solution for high-power fiber laser systems, which allow for the simultaneous mitigation of thermal and nonlinear effects through core-count scaling. This makes them highly attractive for high-average-power applications. However, there has been no demonstration of a multi-kilowatt, multi-core fiber laser system to date. In this work, we present a Yb-doped, multi-core fiber laser system, delivering up to 3.2 kW of average power (over all cores) with excellent short- and long-term stability. The system exhibited a slope efficiency with respect to launched power of 86.3%, and the output power was limited only by the available pump power.

A wide range of applications in material's processing, remote sensing and frequency conversion processes for mid-IR or EUV generation benefit from high-power laser sources in the 2 mu m wavelength region [1-3]. Thulium (Tm-) doped fiber lasers have shown already a tremendous scaling potential in terms of average power and high-energy extraction [4,5]. Usually, high-power, Tm-doped fiber amplifiers are pumped at 793 nm. Due to the large quantum defect of similar to 60 % such systems face considerable thermal challenges [5]. This large quantum defect can be overcome by applying in-band pump sources in the wavelength range between 1550 nm and 1900 nm [6-8]. A promising, power scalable concept is to use Raman fiber lasers as pump sources emitting at 1692 nm [9]. Due to their nonlinearity-based design, these pump sources contribute a certain amount of amplitude fluctuations. If the noise of the pump source is transferred to the signal noise within the amplification process, this could be detrimental and a disadvantage. Therefore, in this work we present our results in terms of efficiency and relative amplitude noise transfer to the amplified signal, while in-band pumping in the context of Thulium-doped fiber amplifier compared to the state-of the art pumping scheme at 793 nm.

This work demonstrates the potential of Q-switched 7x7 Yb-doped multi-core fiber lasers in achieving efficient frequency doubling at 515 nm, delivering up to 18 mJ pulse energy at 1.5 kHz with 57% conversion efficiency. Further investigations will focus on scaling the green light outputs beyond 100-W, 50-mJ, as well as exploring third harmonic generation at 343 nm.

Multicore fibers have redefined expectations on the performance level that can be reached by fiber lasers, with multi-kW, Joule-class systems already in development. This remarkable leap in performance is due to the fact that multicore fibers, due to their tight integration of the channels, allow solving some of the main issues of beam combining such as, for example, the linear dependence of cost, size and complexity on the channel count of such systems. In spite of this, multicore fiber lasers are still complex systems that require careful beam splitting and recombination stages and, in their current implementation, need to be actively stabilized. In this work we will show a design of multicore fibers that can help to reduce the overall complexity of the system by significantly simplifying the splitting and recombination stages as well as offering the possibility of passively stable operation.

We present a coherently-combined Ytterbium-doped multicore-fiber-based ultrafast laser system emitting up to 261 W average power and 1.75 mJ energy pulses with 330 fs duration at 148 kHz repetition rate. Combination efficiencies of 75% are achieved at the highest average power and pulse energy.

The demand for high-power fiber lasers continues to grow, yet conventional single-mode fibers face physical limitations due to nonlinear effects and thermally induced transverse mode instabilities. Multicore fibers provide a promising approach to overcoming these constraints by enabling compact multi-channel amplification with the potential for coherent beam combination to enhance brightness. However, achieving high combination efficiency requires stringent control over the position and arrangement of individual cores, which becomes increasingly challenging as the number of cores increases. This study investigates two manufacturing techniques for high-core-count multicore fibers: deep-hole drilling and stacking. In the drilling approach, holes are machined into a glass cylinder and filled with doped glass rods. During fiber fabrication, the collapsing process introduces a cushion-shaped distortion, which depends on the gap size between rods and holes, the pitch, and the number of cores. As the core count increases, these distortions become a limiting factor. In contrast, the stacking method, employing an optimized arrangement of large and small rods, preserves the square core structure during fusion, avoiding geometric distortions. Our analysis shows that while drilled fibers allow for high core count integration up to approximately 100 cores, further scaling leads to unacceptable distortions. Stacked preforms, however, maintain geometric integrity, offering a viable alternative for even higher core counts. These findings provide critical insights into the design and fabrication of multicore fibers for coherent beam combination, highlighting the trade-offs between manufacturing feasibility and performance optimization for high-power fiber laser applications.

High power fiber lasers have become indispensable in industry and basic science due to their high efficiency, excellent beam quality, and compact design [1]. However, operating at high average powers and pulse energies introduces significant challenges, most notably thermal effects such as transverse mode instability (TMI) [2] and nonlinear phenomena [3], which degrade performance and limit a further power scaling. Coherent beam combining (CBC) offers a solution to these challenges by combining the outputs from multiple amplifier channels to form a single, high-quality beam [4]. Traditional CBC systems, however, rely on active phase control for all channels [5], leading to increased complexity and higher costs.

In this work we experimentally demonstrate an innovative approach to control the direction of the modal energy exchange during transverse mode instability (TMI) in high-power fiber amplifiers. Our technique involves seeding the amplifier with a traveling wave created by exciting two modes with slightly different frequencies. The resulting modal interference pattern moves along the amplifying fiber in either direction (depending on the sign of the frequency difference) at a speed determined by the absolute value of the frequency difference. By carefully selecting the traveling speed, it is possible to generate a constant phase shift between the modal interference pattern (travelling wave) and the thermally-induced index grating. This situation enables consistent energy transfer from higher-order modes to the fundamental mode (or vice versa). Additionally, the travelling wave can be forced to move so fast that it can even wash out the inversion profile, thus preventing any modal energy transfer and mitigating TMI. Both working regimes can effectively increase the TMI threshold of the high-power fiber amplifiers.

Numerous applications in material's processing, medicine, metrology and sensing as well as several frequency conversion techniques such as high- harmonic generation, mid-IR or EUV generation would strongly benefit from stable, high-power laser sources emitting in the 2 μm wavelength region [1]–[5]. Thulium (Tm-) doped fiber lasers have already shown a significant scaling potential in terms of average power and high-energy extraction while preserving a high beam quality [6], [7]. As the generation of inversion is not as straightforward as in Ytterbium-ions, obtaining a high efficiency is a significant challenge. Hence, slope efficiencies marginally exceeding 50% for pulsed systems, have been demonstrated employing matured ~790 nm diode-based cladding pumping. Consequently, there are high heat loads in the fiber, which cause thermal problems during high-power operation and, therefore, prevent scaling these systems beyond the kW-level so far [8]. The limitations imposed by the quantum defect can be largely overcome by pumping Tm gain media directly into the upper laser level, this technique is commonly known as in-band pumping. The most recent findings have identified the pump wavelength range of 1.6 μm to 1.7 μm as the optimal choice to achieve high efficiencies in combination with high energy extraction at highest average power levels from in-band pumped Thulium-doped fiber laser systems [9].

This contribution explores the average-power scaling potential of in-band pumped, Thulium (Tm-) doped fiber amplifiers for high peak-power operation. These results have been obtained by pumping a Tm-doped, photonic crystal fiber at a wavelength around 1700nm. In one of the experiments reported in this work an average output power of 242 W with a slope efficiency of 80% was obtained. Additionally, we also demonstrate operation up to an average output power of 80 W with no need for active cooling, which further highlights the benefits of this low-quantum defect pump approach. Furthermore, our simulation reveals the average power scaling potential of these systems to the multi-kilowatt level.

Multi-core fibers have been shown to scale the thresholds for nonlinear and thermal limitations by a factor approximately equal to the number of cores. A significant thermal limitation is transverse mode instability (TMI). During TMI, the beam experiences spatio-temporal fluctuations caused by power transfer between the fundamental mode and higher-order modes, rendering it unsuitable for many applications. However, by incoherently combining the multi-core fiber's unstable output, a more stable beam can be achieved, making it suitable for ultra-high average power applications with a significantly higher beam quality compared to commercial multimode fiber laser systems. In this manuscript, simulation results are presented, providing a quantitative analysis of the spatio-temporal stability improvement and the beam quality penalty associated of this approach.

High-power, ultrafast fiber lasers emitting in the wavelength region around 2 μm are in wide demand for applications such as high harmonic generation towards the soft-Xray, spectral conversion into the mid-infrared and semiconductor processing [1]. One of the main disadvantages of broadband laser sources in the range of 1.9 μm to 2.0 μm, however, is the overlap with atmospheric water-vapor absorption lines, which implies that high-power systems must be placed in a protective atmosphere or even vacuum [2]. Furthermore, crystal-based nonlinear frequency conversion towards the mid-IR would strongly benefit from a slightly longer wavelength as well. Thus, in this respect, it is advantageous to shift the central wavelength to a region above 2 μm where the impact of absorption is significantly reduced. Rare-earth elements offering gain above 2 μm wavelength include Thulium and Holmium, and both can be used as laser active ions in silica fibers [3]. Recent fiber-based demonstrations include cw-power levels of 100W and ns-pulses with energy of > 1mJ at wavelengths between 2.0 and 2.1 μm [4], [5]. Nevertheless, ultrafast fiber lasers appeared to be restricted to sub-μJ pulse energy, mainly due to the unavailability of low-nonlinearity Holmium- or Thulium/Holmium fibers.

A novel, to the best of our knowledge, approach for the modal decomposition of a fiber laser beam is demonstrated using a spatial mode multiplexer. Since the modal decomposition is carried out optically, this approach is able to obtain the modal content at speeds up to the GHz level. In order to demonstrate such performance, we have applied this approach to the modal analysis of a Q-switched pulse generated in a multimode fiber with alternating intra-pulse mode content. (c) 2025 Optica Publishing Group. All rights, includand similar technologies, are reserved.

Pump absorption is one of the most important and, at the same time, overlooked characteristics of double-clad fibers. Most people take it for granted, but it is actually a rather complex and delicate effect. In fact, the problem of pump absorption is only expected to become worse in the new breed of large core, rod-type, multicore fibers. Herein, the pump cladding diameter can reach several millimeters which, combined with the short length of the fiber, are expected to lead to a negligible contribution of mode mixing to pump absorption. Moreover, multicore fibers impose a new constraint to the pump process: amplification uniformity. In a multicore fiber it is important that all the cores emit the same power, but this is affected by the pump propagation and absorption. Therefore, there is an urgent need to study pump absorption and amplification uniformity in these fiber geometries to obtain an optimum pump configuration.