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.
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.
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.
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 77 K 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.
Recent years have seen silica emerge as a viable material for optical refrigeration with potential applications in directed energy, integrated photonics, and precision metrology. Proper characterization of potential composition profiles is vital for optimization. Here, static photoluminescence spectroscopy of a Yb, Al co-doped silica sample over the temperature range 80 K to 300 K reveals the emission lineshape is dependent on the excitation wavelength. The impact this has on extracted laser cooling parameters is discussed.
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.
Record laser cooling of Yb-doped silica by 18.4 K and 6.3 K was observed in vacuum and atmospheric pressure conditions, respectively. We present a detailed investigation into the optical refrigeration of ytterbium doped silica glass for both in-air and in-vacuum conditions using various pump powers. Temperature measurements were made relative to the room temperature using thermal camera imaging and differential luminescence thermometry. Through analysis of the temporal behavior of the temperature differential at the start of the in-vacuum experiments, we find the cooling efficiency of the studied silicates to be 0.66 ± 0.07%.
Record laser cooling of Yb-doped silica by 18.4 K was recently observed in vacuum. There is a strong indication pointing to even stronger laser-cooling of Yb-doped silica. In light of these results, a radiation-balanced fiber laser in which cooling from spontaneous emission offsets waste heat generation appears more feasible than before. We will discuss the possibility of a radiation-balanced fiber laser, the similarities and differences with the conventional fiber lasers, and performance issues.
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.
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