1,721,096 research outputs found
Thermal lensing in high-power end-pumped Nd:YLF lasers
No full textOne of the major limitations of scaling diode-end-pumped solid-state lasers to high powers is introduced by thermal effects. An attractive feature of Nd:YLF has been its superior thermo-optical properties compared to other laser crystals. This is due to a decrease of refractive index with increasing temperature, creating a negative thermal lens, which partially compensates for the positive lens due to bulging of the rod end faces. Other advantages of Nd:YLF include its natural birefringence and its long fluorescence lifetime. The latter feature is of interest for high-power Q-switched operation. Problems in realising the true potential of the laser, however, have often been encountered, for underlying spectroscopic reasons as indicated, e.g., in [1]. We investigated the thermal lensing under lasing and non-lasing conditions within a diode-bar-pumped system. Under non-lasing conditions the thermal lens was measured using a Nd:YAG probe laser which double-passed the Nd:YLF rod. The resulting change in beam divergence was measured. Under lasing conditions the laser-beam waist size on the output coupler was measured. Hence, using the ABCD-matrix formalism, focal-length values for the thermal lenses were determined. The results showed a significant difference in the thermal lens under lasing and non-lasing conditions. In the former case a weak thermal lens was observed which varied linearly with pump power. Under non-lasing conditions a much stronger thermal lens was measured, whose power increased non-linearly with pump power. With 11 W of pump power incident on the crystal, a factor of 6 difference between lasing and non-lasing values of focal length was determined (Pi-polarisation, plane perpendicular to c-axis). These measurements demonstrate that significant additional heat is generated in the non-lasing case. A finite-element calculation, which considered the relevant processes including interionic upconversion, their contribution to thermal loading, as well as the temperature distribution in the Nd:YLF crystal, was performed. An experimentally observed fluorescence saturation at 1.05µm of more than 50 % under Ti:sapphire pumping was numerically reproduced, and the value of the published upconversion parameter [2] was thereby confirmed. With this information, the heat generation, spatial temperature distribution, and thermal lens under diode pumping were determined. The calculated thermal lens powers were in reasonable agreement with experimental results. Upconversion processes as well as the temperature dependencies of heat conductivity and thermo-optical parameters were found responsible for strong thermal lensing under non-lasing conditions and its non-linear behaviour with respect to absorbed pump power. Design improvement by a significant decrease of thermal lens power and spherical aberrations under Q-switched conditions can be achieved by increasing the pump-spot size, decreasing the dopant concentration and using a longer crystal, or detuning the pump wavelength from the absorption peak
Upconversion lifetime quenching and ground-state bleaching in Nd<sup>3+</sup>:LiYF<sub>4</sub>
Full text linkSince the Nd3+:LiYF4 system has some advantage over Nd3+:YAG and Nd3+:YVO4 for high-power scaling of diode-end-pumping, this system has been investigated under strong excitation. in this case using a Ti:sapphire pump. The interionic processes responsible for fluorescence saturation have been determined, due allowance being taken for the significant ground-state bleaching under these conditions. Their temperature dependence, which is relevant to scaling consideration, has been investigated theoretically, and found to be rather small over a wide temperature range. By comparing the experimental data with finite-element rate-equation calculations, the influence of interionic upconversion is determined quantitatively, and a published value of the upconversion parameter is confirmed. The spatial dependence of ground-state bleaching and quenching of the fluorescence lifetime is calculated. Analytical expressions are derived, including the influence of interionic upconversion, for the dependence of ground-state bleaching, excitation density, and storage time on pump parameters and dopant concentration
The influence of energy-transfer upconversion on thermal lensing in end-pumped Nd:YLF and Nd:YAG lasers
Full text linkA large increase i n thermal lens power in Nd:YLF (factor ~6) and Nd:YAG (factor ~2), caused by upconversion, has been experimentally measured and theoretically calculated under conditions of high excitation density (typical of Q-switched lasers) when compared to CW lasing conditions
Microstructured KY(WO4)2:Gd3+, Lu3+, Yb3+ channel waveguide laser
Full text linkEpitaxially grown, 2.4-μm-thin layers of KY(WO4)2:Gd3+, Lu3+, Yb3+, which exhibit a high refractive index contrast with respect to the undoped KY(WO4)2 substrate, have been microstructured by Ar beam milling, providing 1.4-μm-deep ridge channel waveguides of 2 to 7 μm width, and overgrown by an undoped KY(WO4)2 layer. Channel waveguide laser operation was achieved with a launched pump power threshold of only 5 mW, a slope efficiency of 62% versus launched pump power, and 76 mW output power
Thermal lensing in end-pumped Nd:YAG under lasing and non-lasing conditions
Full text linkInterferometric measurements and finite-element calculations of thermal lensing in end-pumped Nd:YAG demonstrate that interionic upconversion significantly increases lens power and, hence, aberrations under non-lasing conditions. This has strong implications for Q-switched and amplifier operation
Energy-transfer upconversion and thermal lensing in high-power end-pumped Nd:YLF laser crystals
Full text linkThermal lensing in an end-pumped Nd:YLF rod, under lasing and non-lasing conditions, has been investigated. Under lasing conditions a weak thermal lens, with dioptric power varying linearly with pump power, was observed. Under non-lasing conditions where higher inversion densities are involved, hence relevant to Q-switched operation or operation as an amplifier, a much stronger thermal lens was measured, whose power increased non-linearly with pump power. This difference has been attributed to the increased heat deposition due to the subsequent multiphonon decay following various interionic upconversion processes, which increase strongly under non-lasing conditions, and is further exacerbated by the unfavourable temperature dependencies of heat conductivity and rate of change of refractive index with temperature. A strategy for reducing upconversion and its associated thermal loading, without degrading laser performance, is discussed
Upconversion-induced heat generation and thermal lensing in Nd:YLF and Nd:YAG
Full text linkWe investigate the influence of interionic upconversion between neighboring ions in the upper laser level of Nd:YLF and Nd:YAG on population dynamics, heat generation, and thermal lensing under lasing and non-lasing conditions. It is shown that cascaded multiphonon relaxations following each upconversion process generate significant extra heat dissipation in the crystal under non-lasing compared to lasing conditions. Owing to the unfavorable temperature dependence of thermal and thermo-optical parameters, this leads, firstly, to a significant temperature increase in the rod, secondly, to strong thermal lensing with pronounced spherical aberrations and, ultimately, to rod fracture in a high-power end-pumped system. In a three-dimensional finite-element calculation, excitation densities, upconversion rates, heat generation temperature profiles, and thermal lensing are calculated. Differences in thermal lens power between non-lasing and lasing conditions up to a factor of six in Nd:YLF and up to a factor of two in Nd:YAG are experimentally observed and explained by the calculation. This results in a strong deterioration in performance when operating these systems in a Q-switched regime, as an amplifier, or on a low-gain transition. Methods to decrease the influence of interionic upconversion are discussed. It is shown that tuning of the pump wavelength can significantly alter the rod temperature
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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