Fiber lasers are nowadays one of the preferred coherent light sources when high average powers up to the multikW regime are targeted. Additionally, they are also widely employed for the generation of ultrashort pulses with moderate energy in the mJ-range. However, numerous physical limitations currently hinder a further performance scaling. One way to overcome these limitations is the coherent beam combination of parallel amplifiers. This approach has led to laser systems operating beyond the limits of a single amplifier, albeit at the cost of an increase in footprint and complexity. Amplifying multicore fibers hold the promise to combine the simplicity of fiber lasers with unprecedented performance scaling prospects and the additional benefit of enabling dynamic beam and pulse shaping. In this paper, basic considerations about design parameters of multicore fibers with non-coupling cores are given and the impact of physical effects on the output emission is discussed. State-of-the art results of multicore-fiber-based laser systems are reviewed with these fibers now being able to output kWlevel average powers and nanosecond pulses with over 100 mJ pulse energies. Finally, an outlook regarding future performance targets and possible applications is given.

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.

We present what we believe to be the first systematic experimental study investigating the impact of core size on transverse mode instability. This is a very complex measurement that requires a significant amount of attention to detail. For example, this study can only be done using fibers which modal characteristics do not change with the core size, a property offered by Ytterbium-doped large-pitch fibers. Additionally, it is mandatory to consider the influence of gain saturation and photodarkening to isolate the impact of the core size on the transverse mode instability threshold. The findings of the measurements reveal that the dependence of this threshold with the core size is weaker than predicted by some theoretical models. Additionally, during the course of these measurements, a mode instability threshold over 600 W was achieved in one of the fibers, which represents the highest, diffraction-limited, average output power reported from an Ytterbium-doped, rod-type fiber so far.

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.

This study experimentally compares the TMI thresholds of two few-mode large mode area fibers, one PM and one non-PM, drawn from the same preform. Both fibers are tested in the same setup under the same conditions. The results indicate that the TMI threshold of the PM fiber (operated along the slow axis) is approximately 300 W, while the non-PM fiber shows a threshold of about 330 W. This 10% difference in threshold is attributed to an increase in the refractive index and V-number resulting from the mechanical stresses in the PM fiber. However, rotating the polarization in the PM fiber can lead to higher TMI thresholds, demonstrating its potential advantages in terms of TMI suppression. Overall, while the PM fiber exhibits a slightly lower TMI threshold when aligned with the slow axis, it offers significant performance benefits when polarization is adjusted parallel to the fast axis or at an intermediate angle around 45 degrees between the slow and fast axis.

This contribution presents temperature-dependent and site-selective spectroscopy measurements of Yb, Al, F co-doped silica. White light absorption and fluorescence measurements using a multimode 915 nm diode for excitation were both made over the range 77K to 420 K. Low temperature measurements allow determination of the Stark levels. The high temperature measurements allow quantification of how the laser cross-sections vary with temperature over intervals applicable to high power laser operation. Between room temperature and 420 K, the cross-sections for some spectral regions change by more than 10%, whereas other regions are essentially unchanged over the same temperature range.

We investigate cladding pump light absorption in double-clad multicore amplifier fibers in dependence of the pump Numerical Aperture (NA). Results indicate that for the investigated case the assumptions for Beers law break down and need to be revised to include an NA dependent absorption. In this work we present a method for NA resolved absorption measurements and focus on the absorption behavior of a double clad multicore fibers, where experimental results show that the cladding absorption is changing drastically with the NA of the pump light. Additionally pump modes are investigated numerically to assist these findings. These results have not only implications for amplifier performance but impact characterization of cladding pump light absorption for such fibers as well. Lastly it is also expected that such behavior can be mitigated or exploited by fiber design.

Laser cooling of a 5 cm long, 1 mm diameter ytterbium doped (6.56×10 25 ions/m 3 ) silica rod by 67 K from room temperature was achieved. For the pump source, a 100 W level ytterbium fiber amplifier was constructed along with a 1032 nm fiber Bragg grating seed laser. Experiments were done in vacuum and monitored with the non-contact differential luminescence thermometry method. Direct measurements of the absorption spectrum as a function of temperature were made, to avoid any possible ambiguities from site-selectivity and deviations from McCumber theory at low temperature. This allowed direct computation of the cooling efficiency versus temperature at the pump wavelength, permitting an estimated heat lift of 1.42 W/m as the sample cooled from ambient temperature to an absolute temperature of 229 K.

In this contribution, we study different types of mode interaction in high-average power, polarization maintaining (PM) ytterbium-doped fiber amplifiers. We analyze how they limit the performance of the fiber amplifier depending on the polarization of light used and introduce restrictions in the configuration of the fiber amplifier architecture. Avoided-crossings between core and cladding modes are studied in detail, with numerical simulations and systematic experiments, revealing that they are stronger when the input polarization is aligned parallel to the fast-axis of the fiber. We will show how the temporal dynamic characteristic of transverse mode instabilities depends on the polarization input angle of the seed laser. Moreover, a dramatic and unexpected deformation of the output beam was observed when operating one of our large-mode area PM fibers in the fast-axis, with a high mode content of the first high order mode in the slow-axis.

A thorough investigation of the spectroscopic properties of ytterbium-doped silica as a function of temperature (77-420 K) is carried out. Whitelight absorption and fluorescence collected under 915 nm excitation are used to calculate the temperature-dependent laser crosssections. These datasets are made publicly available in this work. Factors influencing the acquisition and interpretation of Yb-doped glass spectroscopic data are discussed, including spectrum fitting ambiguities, site-selective excitation, lifetime decay versus spectral integration, vibronic features, and the validity of the McCumber theory over the studied temperature range. Site-selectivity affects the measurement of the emission lineshape at standard pump wavelengths of 915, 940, and 976 nm at room temperature. Lifetime measurements under 915 nm excitation vary by up to 10%, depending on the choice of bandpass or long pass filter employed and hence the spectral region integrated over. The McCumber transform yields reasonable agreement with measured spectra over the range of similar to 200-420 K and then diverges rapidly at lower temperatures. Considering the measured cross-section data in the range applicable to contemporary fiber laser system operation, between 300-420 K, the cross-sections for some spectral regions change by more than 10%, including the absorption cross-section at 977 nm and the emission cross-section at 1030 nm. Some regions are essentially unchanged over the same temperature range, such as absorption at 940 nm and emission at 1045 nm. The provided data will be useful for future modeling and simulation efforts to consider the temperature-dependence of relevant quantities including, but not limited to, lifetime, cross-section, gain, and intensity saturation.

Multicore Fibers (MCF), the integrated alternative to spatially separated amplification in fibers, are a promising laser architecture thanks to their capability to deliver high pulse energy and average power in a compact format. The introduction of 7x7 MCF laser systems represents a significant advancement of this technology, bringing us closer to realizing multi-kilowatt and J-class fiber laser amplifiers. In this context, Ytterbium-doped MCFs have already demonstrated power scalability proportional to the number of amplifying cores. Using 4x4 MCFs already showcases high pulse energy and high average power and, if coherently combined, offers nearly diffraction-limited beam quality. This work complements the Coherent Beam Combination (CBC) testbed by Incoherent Beam Combination (IBC). IBC emerges as a straightforward and robust solution, providing the opportunity to achieve performance capabilities equivalent to Multimode Fibers (MMFs) while demonstrating a better beam quality. These IBC systems are appealing for various applications, including pumping solid-state lasers and incoherent frequency conversion towards shorter wavelengths, e.g., to the Extreme Ultraviolet (EUV) or soft-Xray through laser-produced plasma sources.

Fiber lasers are reliable and flexible sources of high laser power with excellent beam quality. However, limitations due to nonlinear and thermal effects, hamper further power scaling. We will give an overview over relevant influencing factors for these limitations, on the component side as well as regarding system design. Experimental examples in the 1µm and 2µm spectral region will be shown for the proposed techniques to tackle several of these obstructions, with a focus on ways to suppress transverse mode instabilities. Remaining limitations for single fiber systems can be overcome by parallelization of amplification, using multiple actively doped cores running below the critically power threshold each. Such fiber cores can be housed separately or in a single multi-core fiber. We will address coherent and spectral methods to (re-)combine multiple fiber laser output beams while maintaining beam quality and discuss scaling aspects and potential limitations to these architectures.

A manufacturing chain that revolutionizes the production of fibers with highly precise refractive index matching is presented. By integrating iterative refractive index adjustments using reference fibers, a remarkable improvement in homogeneity, reaching 1E-5, was achieved. Conventional methods, such as the MCVD process, are limited to similar to 1E-4. This enhanced precision was demonstrated through meticulous measurements of core materials and validated in the final fiber. Importantly, the presented manufacturing method considers that the refractive index of fused silica is not a fixed property of the material. It depends on the thermal treatment history and the exact conditions during fiber drawing. This breakthrough manufacturing process ensures the reproducible manufacturing of fibers with fundamental mode guiding properties and mode field diameters exceeding 100 mu m. These specialized fibers play a pivotal role in scaling the power of fiber lasers for ultrashort pulse applications.

We investigate transverse mode instability (TMI) in an in-house large-mode-area polarization maintaining (PM) fiber amplifier. The TMI threshold was systematically measured at different linear polarization input angles with respect to the slow axis of the fiber. At a polarization input angle of 50 degrees, the TMI threshold increased by more than 100% with respect to the threshold of the slow axis and 60% with respect to one of the fast axis. Furthermore, the temporal characteristic of TMI were studied in detail at different polarization input angles but fixed power of 290W, which was above the TMI threshold of the slow and fast axis. This analysis revealed the three different temporal regimes associated to TMI: chaotic fluctuations in the slow-axis, stable at 50 degrees, and periodic fluctuations in the fast axis. These new results provide with valuable insights into the effect of TMI, especially concerning PM fibers, as well as with a relatively simple way of mitigating TMI in these fibers.

In this work we present a comprehensive parameter study on core-to-core power coupling in multicore fibers (MCFs). In order to do this, a simulation tool has been developed. We chose MCFs with 3×3 cores in a squared pattern with core sizes ranging between 15 and 50 μm and core-to-core distances of 1.5 to 5 times the core diameter. The central core is seeded by a perfectly matched Gaussian beam and the power evolution in each core along the fiber is calculated up to lengths of 2 m. We will show that coupling effects not only depend on the core distance and the core NA, but also on the core diameter and the wavelength. Our simulations predict that a simplified 3×3 core arrangement can be even used to quantify coupling effects in MCFs with more cores when the core-to-core power coupling is kept low. This comprehensive study is crucial for designing laser-active rod-type MCFs.

Fiber laser systems are known for being able to provide emission at high average powers. However, achieving pulsed operation with high peak-powers has been a challenge due to the small confinement of the light in the fiber causing limitations, e.g. due to nonlinear effects. As a strategy to overcome these limitations, parallelization using coherent combination of pulses emitted by multiple amplifier [1] has been worked on and has resulted in the achievement of record performance values [e.g. 2] so far. Recently, our focus has been multicore fibers which have the potential to allow for laser systems with a massive number of amplification channels while maintaining a compact footprint and manageable component count. In this contribution, we will show the latest results of performance values from Ytterbium-doped multicore fibers with $4\times 4$ and $7\times 7$ cores, respectively.

Multicore fibers (MCF) offer a groundbreaking method that allows compact setups with an unprecedented number of channels [1]. In fact, MCFs offer an opportunity to integrate the parallel amplifiers in one fiber with a shared pump cladding and sharing amplification load across multiple cores. Therefore, MCFs are potential option in many applications, such as generation of extreme UV, incoherent beam combination, and pump source for other lasers. MCFs have scalable performance by the number of amplifying cores. While $4\times 4$ MCFs already provided high pulse energy [2] and high average power [3] with a diffraction limit beam quality, $7\times 7$ MCFs are the new generation of these systems and have the potential for J-class fiber laser amplifiers. In this work, a rod-type Ytterbium doped $7\times 7$ MCF is used both as an oscillator and amplifier in a Master Oscillator Power Amplifier (MOPA) system. The employed fibers are based on a step-index design with shared pump cladding and a rectangular core arrangement. This optical system containing 49 cores generates up to 10-mJ pulse energy as a seed for the amplifier (see Fig. 1).