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Multicore fibers have the potential to combine the advantages of optical fibers (such as their high average power capability, high efficiency and well-defined beam quality) with those stemming from the large beam areas commonly used in other laser architectures. Coherent combination can then be employed to achieve one single, high-quality, output beam [1] , [2] . To match and even surpass the performance of state-of-the-art lasers systems comprising multiple separate fiber amplifiers, multicore fibers need to leverage the same technological advancements. One example is the use of a rod-type geometry with large core diameters to mitigate detrimental nonlinear effects. In this contribution, we present our high power laser results achieved with an in-house, all-glass, rod-type multicore fiber, whose basic structure is shown in figure 1 . The fiber contains 16 ytterbium-doped cores in a rectangular arrangement with a diameter of 22 µm each, operating at near single-mode output. The core-to-core pitch is 58 µm. An embedded octagonal fluoride ring is used as the guiding mechanism for the pump with a diameter of 310 µm and a NA of 0.22. A device length of 1.1 m was chosen to provide sufficient pump absorption.

In this work, we present a simplified method for manufacturing microstructured optical fibers, which was applied for an Yb-doped pentagonal large pitch fiber (LPF). Instead of using the classical stack & draw technique, which can become time-consuming and delicate, we used a one-step structuring of the cladding via deep-hole drilling in glass. The Ybdoped core was also realized in one-step by gas phase doping via MCVD. Additional nanostructuring steps of the core material were redundant due to the high radial homogeneity of the refractive index achieved, matched with high precision to the refractive index of the glass cladding. Numerical simulations were used to verify the method and analyze the scaling of the fiber towards larger mode field diameters, which is crucial for further scaling the power in fiber lasers.

Anti-Stokes fluorescence cooling of a Yb-doped silica glass optical fiber preform is achieved using a high-power laser in a double-pass configuration. The coherent laser beam illuminates the silica glass preform in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. The high-purity Yb-doped silica glass preform has low parasitic absorption and is codoped with modifiers to mitigate the quenching-induced non-radiative decay for sufficiently high concentrations of Yb ions in silica glass. Therefore, sufficiently large laser absorption could be achieved to observe cooling while maintaining a near-unity external quantum efficiency.

Laser cooling of a solid is achieved when a coherent laser illuminates the material, and the heat is extracted by annihilation of phonons resulting in anti-Stokes fluorescence. Over the past year, net solid-state laser cooling was successfully demonstrated for the first time in Yb-doped silica glass in both bulk samples and fibers. Here, we report more than 6 K of cooling below the ambient temperature, which is the lowest temperature achieved in solid-state laser cooling of silica glass to date to the best of our knowledge. We present details on the experiment performed using a 20 W laser operating at a 1035 nm wavelength and temperature measurements using both a thermal camera and the differential luminescence thermometry technique.

We report the observation of anti-Stokes fluorescence cooling of Yb-doped silica glass by 0.7 degrees Celsius. We conduct a detailed investigation of the cooling parameters of this glass, including the wavelength dependence of the cooling efficiency as a function of the wavelength and also the parasitic absorption of the pump laser. The measurements are performed on three different glass samples with different compositions and cooling is observed in all samples to varying degrees. The results highlight the possibility of using Yb-doped silica glass for radiation-balancing in fibers. Radiation-Balancing is a viable technique for heat mitigation in lasers and amplifiers.

Nonlinear effects and transverse mode instabilities (TMI) limit power scaling of single-mode fiber lasers. To overcome these limitations not only the fiber design but also laser relevant properties of the actively doped material itself need to be optimized. By being able to fabricate Yb-doped fibers for high power applications in-house, we have direct access to laser relevant material parameters.We fabricated fibers using three different co-doping systems, namely Yb:Al:P, Yb:Al:F, and Yb:Al:F:Ce. Afterwards we characterized and compared their laser relevant properties. All three co-doping systems showed nearly identical background losses and absorption cross-sections. In contrast, we found that the PD losses and the factor between PD losses @633nm and the laser wavelength range (1μm) to be significantly different. The retrieved characterization results were implemented into our simulations tool in order to improve the reliability of predictions. Finally, we characterized the fibers in kW-amplifier setups according to their power scaling limits, especially the TMI threshold. This cycle of fiber fabrication, characterization, and simulation enabled us to identify the impact of individual fiber parameters on the TMI threshold. We demonstrated that the impact of PD loss leads to a reductions of the TMI threshold for Yb:Al:F co-doping system of 13% to 23% (depending on the Yb-concentration). The PD loss for the two other systems was proved to be significantly lower and was found to have no impact on the TMI threshold. We experimentally proved that your in-house Yb:Al:P and Yb:Al:F:Ce fibers performed like PD-free fibers.

Laser cooling of a solid is achieved when a coherent laser illuminates the material in the red tail of its absorption spectrum, and the heat is carried out by anti-Stokes fluorescence of the blue-shifted photons. Solid-state laser cooling has been successfully demonstrated in several materials, including rare-earth-doped crystals and glasses. Here we show the net cooling of high-purity Yb-doped silica glass samples that are fabricated with low impurities to reduce their parasitic background loss for fiber laser applications. The non-radiative decay rate of the excited state in Yb ions is very small in these glasses due to the low level of impurities, resulting in near-unity quantum efficiency. We report the measurement of the cooling efficiency as a function of the laser wavelength, from which the quantum efficiency of the Yb-doped silica is calculated. Laser cooling of rare-earth-doped silica may provide a route to vibration-free refrigeration of integrated photonic circuits and quantum sensors to reduce the thermal noise. Here, cooling of high-purity Yb-doped silica to 0.7 K below ambient temperature is demonstrated using sub-optimal laser parameters, and in spite of a substantial extra thermal load.

We report the successful laser cooling of a Ytterbium-doped silica optical fiber glass preform by 0.7K relative to the room temperature. The observed temporal behavior of cooling agrees well with the theoretical predictions.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

In this contribution the influence of fabrication technique (solution doping, gas-phase doping) and the choice of suitable material systems (Al, P, Yb:SiO2 and Al, F, Yb:SiO2) for high power fiber laser materials on their optical properties is analyzed. The materials under analysis contain low amounts of codopants (Yb < 0.15 mol%, other

In this contribution we investigate the transversal mode instability behavior of an ytterbium doped commercial 20/400 fiber and obtain 2.9 kW of output power after optimizing the influencing parameters. In this context, we evaluate the influence of the bend diameter and the pump wavelength within the scope of the absorption length and the length of the fiber. Furthermore with a newly developed fiber we report on 4.4 kW of single-mode output power at 40 cm bend diameter.

With their advantages like good beam quality, easy thermal management, high robustness and compact size, fiber lasers are one of the most promising solid state laser concepts for high power scaling with excellent beam quality. One issue of further power scaling is the reduction of nonlinear effects, especially Raman scattering, which consequently led to increased mode field areas. However, for large mode area fibers, new challenges, namely transversal mode instabilities (TMI) have to be taken into account. Beside our investigations in the power scaling of ytterbium doped fiber amplifiers up to 4.4kW output power, we present our investigations of the TMI threshold in dependence on bend diameter and absorption length of a well-known, commercial fiber. Within this scope, we used a 13m piece of the fiber and gradually reduced the bend diameter from 60cm slightly below 14cm within a pump wavelength of 976nm. Furthermore, we increased the fiber length to 30 m, presuming the bend diameter of 14 cm and all experimental conditions. However, in a next step, we detuned the pump wavelength up to 980 nm in order to increase the pump absorption length As a result, we achieved 2.9kW of single mode output at a bend diameter of 14cm. The 4.4kW result was obtained with a separately manufactured low-NA fiber, allowing for a slope efficiency of 90% with regards to the absorbed pump light and an extremely temporal stability.

We present the fabrication and properties of active fiber laser materials fabricated by a newly developed solution doping technique. The contribution focusses on Aluminum, Phosphorus, Ytterbium as well as Boron doped SiO2 for the use as fiber laser material. More specifically low doping concentration in the vicinity of the molar ratio of Al2O3:P2O5 = 1:1 will be elucidated. The effect of fabrication parameters on optical properties like refractive index, absorption and emission properties will be covered. Currently it is possible to achieve cw output powers greater than 4 kW using Al, P, Yb doped fibers fabricated with this method. Fibers additionally codoped with Boron are as well suitable for kW class applications as well.

Fiber amplifiers are representing one of the most promising solid state laser concepts, due to the compact setup size, a simple thermal management and furthermore excellent beam quality. In this contribution, we report on the latest results from a low-NA, large mode area single mode fiber with a single mode output power beyond 4 kW without any indication of mode instabilities or nonlinear effects and high slope efficiency. Furthermore, we quantify the influence of the bending diameter of our manufactured low NA fiber on the average core loss by an OFDR measurement and determine the optimal bending diameter in comparison to a second fiber with a slightly changed NA. The fibers used in the experiments were fabricated by MCVD technology combined with the solution doping technique. The investigation indicates the limitation of the step index fiber design and its influence on the use in high power fiber amplifiers. We demonstrate, that even a slightly change in the core NA crucially influences the minimum bending diameter of the fiber and has to be taken into account in applications. The measured output power represents to the best of our knowledge the highest single mode output power of an amplifier fiber ever reported on.

We present our recent results of scaling low-NA fibers to high average power by overcoming the TMI-limitations. We obtained a record output power of 4.4kW with a higher stability and smaller bending compared to our previous result.

We report on a newly designed and fabricated ytterbium-doped large mode area fiber with an extremely low NA (~0.04) and related systematic investigations on fiber parameters that crucially influence the mode instability threshold. The fiber is used to demonstrate a narrow linewidth, continuous wave, single mode fiber laser amplifier emitting a maximum output power of 3 kW at a wavelength of 1070 nm without reaching the mode-instability threshold. A high slope efficiency of 90 %, excellent beam quality, high temporal stability, and an ASE suppression of 70 dB could be reached with a signal linewidth of only 170 pm.

We present a detailed investigation of different compositions of Yb3+-doped alumino-silicate glasses as promising materials for diode-pumped high-power laser applications at 1030 nm due to their beneficial thermo-mechanical properties. To generate comprehensive datasets for emission and absorption cross sections, the spectral properties of the materials were recorded at temperatures ranging from liquid nitrogen to room temperature. It was found that the newly developed materials offer higher emission cross sections at the center laser wavelength of 1030 nm than the so far used alternatives Yb:CaF2 and Yb:FP-glass. This results in a lower saturation fluence that offers the potential for higher laser extraction efficiency. Fluorescence lifetime quenching of first test samples was analyzed and attributed to the hydroxide (OH) concentration in the host material. Applying a sophisticated glass manufacturing process, OH concentrations could be lowered by up to two orders of magnitude, rising the lifetime and the quantum efficiency for samples doped with more than 6.1020 Yb3+ -ions per cm³. First laser experiments showed a broad tuning range of about 60 nm, which is superior to Yb:CaF2 and Yb:FP-glass in the same setup. Furthermore, measurements of the laser induced damage threshold (LIDT) for different coating techniques on doped substrates revealed the appropriateness of the materials for short pulse high-energy laser amplification.

In this contribution we demonstrate an approach to determine the longitudinal resolved thermal load in the core of an ytterbium-doped amplifier-fiber. The temperature distribution was measured by an OFDR in a single-mode amplifier in operation.

The fabrication of Yb doped fibers with low-NA (0.04) for high-power fiber laser applications in the multi-kW class is presented and discussed. Laser tests with ~3 kW output power and excellent beam quality are demonstrated.

The growing demand for high power fiber lasers and the need for higher and higher cw-output powers pose new challenges to fiber technologists. The fabrication of suitable low-NA fibers in the materials system of choice, Yb, Al, P-doped silica, is challenging. Hence, new, facilitating fabrication methods are required. In this contribution we report on fabrication principles of a promising alternative solution doping method — the so called all-solution doping method [1].

We report nearly an order of magnitude improvement over the lowest temperature achieved in laser-cooling of Yb-doped silica glass. The 6K cooling relative to the ambient temperature was achieved by pumping the sample at 1035nm.

We present our latest results of a newly developed low-NA Yb-doped fiber with single-mode 4.3 kW average output power. Additionally, a matched passive fiber is introduced to enable beam delivery and components towards monolithic integration.