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.
Spectral Beam Combining (SBC) is a useful tool for power scaling of optical systems as well as for multiplexing in communications. We will give an overview of different concepts for SBC and compare them regarding channel count, spectral properties and power handling. For this regard, we will present our examples of these concepts, ranging from a 20kW spectral combiner for high power application over systems in the 100W-range for free-space communications in different spectral regions down to miniaturized combiner concepts for multi-W applications to reduce the footprint.
We present the design of a high-power suitable processing-head used for laser material processing with compact piezo-driven kHz-range 3D beam steering in combination with a multiplexed output beam consisting of three different laser sources. The common beam consists of an 8-kW disc laser that is used to provide the bias-level of energy to preheat the work piece during welding or cutting close to the ablation threshold. The second laser is a MHz-switchable polarization-modulated kW-class Yb-doped fiber amplifier, which in combination with a high-power suitable polarizer transfers the polarization-modulation into a fast switchable amplitude-modulation that is finally used for the material processing. Furthermore, for the purpose of process control during cutting and welding a spectrally shifted 10 W-class fs-pulsed laser is incorporated and used for Laser-Induced Breakdown Spectroscopy (LIBS). Dynamic beam-steering and focusing in the range of several kHz at the output of the processing head is included utilizing a piezo-driven tip/tilt-mirror and a deformable mirror. Further details on the construction of the process head as well as on the different laser sources will be discussed in the presentation. Based on previous teaching of an AI-network with the obtained data from various parameter sweeps, such as camera images of the backscattered light, LIBS measurements and the inspection of the cutting edge or the welding seem, online process optimization shall be granted to improve process quality and cost-efficiency along with a decrease in yield during production.
Fiber lasers have evolved to be the most prominent laser systems for HEL applications due to their combination of ruggedness and excellent beam quality. Systems with multi-kW output power are becoming commercially available, sparking the question of further power scaling and its limits.
We will give an overview of current challenges of high power fiber and fiber laser development and point out options for further power scaling in different wavelength regions, also considering the required footprint.
We present the rapid output power modulation of a 200 W continuous-wave all-fiber ytterbium-doped polarization-maintaining fiber amplifier. Pulse durations of 3 ns and MHz modulation frequencies at full power could be achieved. A fiber-coupled electro-optical modulator was used to modulate the polarization of the seed light, transferring linearly to the polarization at the output of the fiber amplifier. Using a thin film polarizer, the perpendicular polarization components at the output were separated, yielding a polarization dependent amplitude modulation. The laser output power then follows the applied voltage signal to the EOM and could be rectangular, saw-tooth or any arbitrary signal.
Defense applications require intrinsically stable and resilient laser systems. Using single- or few-mode output fibers, fiber-based high-power lasers can address these challenges and also feature excellent beam quality, allowing to achieve high power density at long focusing distance. At high power levels, these diffraction limited output beams can be spoiled by thermally induced transverse mode instabilities (TMI), which cause beam profile fluctuations and thus increase the M²-factor.
As TMI are an interference-based effect, it is to be expected that there is a dependence on polarization. We have thus set up an analysis setup that allows to characterize the individual mode content of the fluctuating beam along with the full polarization (in terms of Stokes vector) of each individual contributing mode at kHz speed. We will present the setup and first results for high power systems.
In order to manipulate the TMI threshold, it has been shown to be beneficial to distribute the heat load evenly along the fiber. On the other hand, monolithic fiber component availability favors a co-propagating pump approach. We will present a dual-tone seeding setup that allows for variable modification of the heat load position and discuss the impact on the TMI threshold.
A module for spectral combination of five channels with an individual power of 20W each, designed for satellite-based communication, is presented. The channels are selected from the telecommunication ITU grid with a spectral spacing of 400 GHz. The combination is based on volume Bragg gratings, allowing for narrow spacing and combination without loss of near-diffraction-limited beam quality. Thermal effects due to high laser power are investigated. Due to the projected on-board satellite usage, volume and mass as well as vacuum conditions and radiation are considered in the module design.
Fiber-based high-power laser systems are advantageous for defense applications due to their intrinsic stability and resilience against challenging environmental conditions. Using single- or few-mode output fibers, an excellent beam quality can be achieved, allowing for tight focusing over long distance. However, thermally induced transverse mode instabilities (TMI) limit the obtainable diffraction-limited output power.
We will show how thermally induced mode instabilities can be conveniently detected in-line in an all-fiber system. Towards this aim, we compare different detection locations and methods and show that characteristic signals can be detected in backwards direction.
Expanding the system and applying these detection methods, we will investigate Raman-based high-power amplification for spectral shift in a passive fiber. We present the observation and identification of TMI due to Raman amplification and the tracing of their origin, locating it in the passive fiber section. By comparing different fiber and seed laser parameters, we deduce respective limiting parameters.
The analysis of TMI has advanced over the last decade, with added observation parameters depending on the complexity of the experimental system. Increasing levels of information have been extracted, from camera images in the beginning over modal decomposition, time trace and frequency analysis, on towards bi directional measurements at multiple system positions and separation of spectral components. We will give an overview of the evolution of TMI analysis for different model systems and discuss the applicability and the additional insight that can be gained from advanced observation methods.
We report on the observation and experimental characterization of backward power fluctuations with the temporal characteristics of transverse mode instability (TMI). A quasi-monolithic, counter-pumped amplifier system in 20/400 μm geometry was developed to investigate forward and backward propagating core- and cladding power as well as their temporal evolution. By experimentally observing the backward propagating core power on a photodiode, we can correlate the temporal traces to those in forward direction. The degree of correlation is found to be highly increased above the TMI threshold. Simultaneous investigations on the modal content in forward and backward direction were enabled by a free-space optical coupling between the first and second amplification stage and performed utilizing a high-speed camera (HSC). In the case of TMI mode content fluctuations are found to occur only in forward direction. Additionally, the evaluations reveal a varying core power content in both directions. The forward core power fluctuations are shown to be induced by the partial coupling of higher-order mode (HOM) content to the cladding. Meanwhile the backward core power fluctuations appear to be a consequence of the ones in forward direction induced by backreflections. Our measurements demonstrate the detection of TMI at various amplifier positions and could be helpful for scientific as well as industrial applications.
We investigated the limitations in output power generated by a high power narrow-linewidth Raman fiber amplifier. The pump was produced by a kW-level all-fiber Yb-doped amplifier emitting at 1060 nm, whose seed linewidth could be changed. The Raman seed was a narrow-linewidth signal at 1110 nm co-propagating with the laser at 1060 nm. The main Raman conversion occurred in the passive fiber at the amplifier output. We identified cross-phase modulation (XPM) as a main reason for broadening of the Raman light by using different pump sources, which is a first limitation. An improved setup was limited at approximately 600 W of Stokes output power by a threshold-like onset of a transverse mode instability. Since the instability was not observed without a Stokes seed and the temperatures of the active fiber with and without Stokes seed are equal, this constitutes the first direct observation of transverse mode instabilities (TMI) induced by SRS in a passive fiber.
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.
We present and compare the performance of bidirectionally pumped Yb-doped monolithic amplifier and oscillator setups in 20/400 μm geometry tested up to signal powers of 3.5 kW and 5 kW without the occurrence of transverse mode instabilities and maintaining a single mode beam quality of M2 ~ 1.3. The scaling was primarily limited by the nonlinear effect of Stimulated Raman Scattering. This contribution contains detailed analysis of the temporal and spectral behavior of both configurations. The results show the excellent feasibility of monolithic oscillators and FBG for high power operation, even outperforming the amplifier pendant in terms of output power.
We present highly efficient three-channel dual-grating spectral beam combining with a combined signal output power of 5.5 kW at an excellent beam quality of M2 = 1.5. Three 2-kW all-fiber narrow-linewidth continuous-wave Ytterbium-doped fiber amplifiers at 1050 nm, 1070 nm and 1090 nm were combined using in-house fabricated polarizationindependent dielectric reflection gratings. The total combining efficiency was 94% at full power level, which is close to the expected value referred to the incorporated grating’s efficiency in a dual-grating setup.
We demonstrate a quasi-continuous wave laser amplifier with a peak output power of 6.8 kW pumped by an industrial thin-disk laser. A high slope efficiency of 84 % has been obtained within a duty cycle of 10 % at a signal wavelength of 1071 nm. For cw-pumping we measured a maximum output power of 985W. The amplifier fiber had a step index profile with a core diameter of 45μm and a pump core diameter of 120μm. A signal to ASE peak ratio of 48 dB could be determined.
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