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| Home > Publications database > Thermal and optical characterization of cryogenically cooled Yb:YLF laser |
| Master Thesis | PUBDB-2024-01714 |
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2024
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Please use a persistent id in citations: doi:10.13140/RG.2.2.20360.43521
Abstract: This thesis focuses on two fundamental limitations that affect the power scaling of cryogenically cooled high-power solid-state lasers: (i) Fractional thermal load (FTL) and (ii) Thermal and population lensing. For FTL, a novel method is introduced for the direct measurement of fractional thermal load in cryogenically cooled laser crystals. The experimental methodology involves characterizing the liquid nitrogen evaporation rate in a dewar containing the laser crystal, which allows the accurate determination of FTL. The FTL is measured to be 1.7 × quantum defect (QD) for Yb:YLF and 1.5 × QD for Yb:YAG under continuous wave lasing conditions. The measured FTL values are then used to calculate the temperature distribution inside the crystals as a function of pump power, and the simulation results are found to be in a very good agreement with the in-situ temperature measurements using contactless optical luminescence thermometry. For thermal and population lensing, this thesis conducts the first detailed study of thermal and population lensing in cryogenically cooled Yb:YLF. Using a rod-shaped a-cut Yb:YLF crystal, the thermal lens strength is measured as a function of absorbed pump power for both E//a and E//c polarizations for pump power levels up to 600 W. Our experiments clarified that the thermal lensing behavior in Yb:YLF is quite asymmetric. For the E//a polarization, the thermal lens is rather strong and always converging for both horizontal and vertical axes. For the E//c configuration, thermal lensing is rather weak, and its sign could be different in different axes, and furthermore, the sign of thermal lensing is observed to be dependent on the absorbed pump power level. Simulations and detailed experimental measurements show that the observed peculiar behavior is a result of competition between the positive photo-elastic effect and negative thermo-optic effect: as their relative strength varies at different pump power levels. We have also measured the contribution of surface bulging into thermal lensing and found it to be relatively small in our crystals with un-doped end-caps. Lastly, the population lensing is measured in Yb:YLF for the first time, and it is found to be quite weak as well, and the polarizability difference parameter of Yb:YLF is determined as (3 ± 1.5) × 10−34 m3.
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