Power scaling of fiber lasers has always been pursued, being limited by nonlinear effects and heat generation in the active fiber and various components. Among the most critical components are cladding light strippers (CLS) between amplifier chains, removing light from leaked higher order modes, the unabsorbed pump or losses from splices and components. Polymer-based CLS work sufficiently well for the near-IR including the pump wavelength at 793 nm but suffer from high absorption at the signal wavelength near 2 μm and have not been evaluated in detail in this regime. Therefore, it is necessary to examine different acrylates and siloxanes at both the pump and signal wavelengths individually concerning their performance as CLS and test their limits. We present a CLS with an improved design which can withstand 7.5 W at 2039 nm while stripping >46 dB. For higher powers to >800 W, we examine CO2-laser inscribed CLS at the pump wavelength, reaching 21 dB stripping efficiency within only 15 mm of length.
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.
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.
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.
Thulium-doped fiber amplifiers have been limited to average output powers of around 1 kW for over a decade. To achieve multi-kW powers around 2 μm wavelength, we propose using dual-grating Spectral Beam Combining (SBC). Three customized kW-class Tm-doped fiber amplifiers operating in the 2030-2050 nm range were developed. The amplifiers consist of three stages and are pumped with non-stabilized, fiber-coupled diode lasers at 790 - 795 nm. Singlemode, TMI-free output powers exceeding 800 W, with narrow linewidths of FWHM ⪅115 pm, were achieved and subsequently combined using highly efficient in-house fabricated reflection gratings. With an overall combining efficiency of 90 % and a thermal slope of the combining grating measured as 6.8 K/kW, scalability to kW-level powers is enabled. The combined output power achieved a record-breaking 1.91 kW with good beam quality (M2 ⪅2) and potential for further optimization. Finally, the potential power scalability of this non-coherent combining approach to power levels exceeding 20 kW is discussed.
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°, 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°, 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.
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 novel high-speed and mode-resolved polarization measurements with PM and non-PM Ytterbium-doped fiber amplifiers up to their TMI thresholds. The implemented full-Stokes polarimetry technique is based on the simultaneous detection of four parallel channels on a high-speed camera. It enables spatially and mode-resolved polarization analysis with sub-ms temporal resolution, allowing to monitor the polarization of individual modes during TMI. We believe that this high-speed mode-resolved polarization measurement technique is highly interesting for the characterization of PM fiber lasers and could help to explore and analyze new TMI mitigation strategies.
In this work we present experimental results of transverse mode instabilities in dependence of the polarization input angle in a large-mode area polarization maintaining fiber amplifier. The transverse mode instabilities threshold was found at 300 W for an input polarization angle aligned parallel to the slow-axis. We demonstrate that at a constant output power of 300 W the temporal stability can significantly be improved by rotating the input polarization angle with respect the slow-axis, indicating an increased threshold. This allowed for further scaling of the fiber amplifier for linear polarization input angles detuned from the slow-axis of the fiber. For operation in the fast-axis (90° to the slow-axis), the power was scaled to up to 475 W without the onset of transverse mode instabilities. However, a static energy transfer from the fast-axis into the slow-axis was observed at powers above 400 W.
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.
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.
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.
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.
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.
Supported by both experimental and simulated results, this contribution demonstrates the heat load distribution in a co-pumped, ytterbium (Yb)-doped fiber amplifier seeded with two different wavelengths can be significantly changed depending on the seed power ratio. Longitudinal temperature measurements in a Yb-doped 10.5 m 20/400 μm fiber confirm a significant shift of the heat load maximum by 3.5 m towards the fiber output when decreasing the seed power ratio from P1030nm/P1080nm = 1.7 to 20. In single-tone operation with a seed power of P1080nm = 3.5 W, the amplifier is limited by the onset of transverse mode instabilities at a power-level of 1950 W. However, dual-tone seeding with a seed power ratio up to P1030nm/P1080nm = 10 reduces the TMI-threshold dramatically down to 1050 W. Additionally we show, that the modal instability threshold is very susceptible to 1030 nm seed noise in the frequency regime up to 10 kHz.
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.
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.
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.
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