| Abstract [eng] |
Ever since the first demonstration of lasers, this new invention have attracted scientists with its exceptional properties: spatial and temporal coherence, low divergence, monochromaticity, and high energy density. Over time, lasers have improved, and their applications and parameter selections have expanded significantly. Nowadays, lasers are widely used in various scientific fields, military, medicine, industry, and even for entertainment purposes. However, as the desire for a wide range of parameters increased, the construction of lasers became more complex, involving more elements inside. Additional components introduced various unwanted defects and aberrations, such as deteriorating laser beam quality or astigmatism. In this work, an attempt was made to design a laser oscillator, correct the existing defects (astigmatism), study the properties of its fiber in both low and high power regimes, and apply theoretical models to understand and explain the principles of oscillator. The initial laser beam of the oscillator exhibited significant ellipticity and astigmatism. These defects were caused by spherical mirrors placed at an angle in the system, resulting in different distances that the beam had to travel in the sagittal and tangential planes. This defect was corrected by inserting a transparent plate into the oscillator, which introduced an opposite sign astigmatism and compensated for the astigmatism of the spherical mirrors. Furthermore, a configuration of the oscillator was found where the laser beam was least elliptical (stability parameter 0.25). During the measurements of the laser beam's quality with power, it was observed that the $M^2$ variation characteristic resembled that of a diode. At low powers, the $M^2$ value remained small and constant, but once the threshold value was reached, it began to increase sharply. Further research showed that such degradation of M^2 was partially related to the sizes of laser radiation and pumping diode modes on the crystal. By reducing the pumping diode mode on the crystal (using lenses with smaller focal lengths), the M^2 values decreased, indicating improved laser beam quality. The changing generation mode on the crystal showed no correlation with M^2, as these parameters changed independently of each other. The discrepancies between theoretical calculations (based on geometrical optics) and experimental data hinted that a more complex model was needed to fully explain the regularities of M^2 variation. For this purpose, the oscillator was described by solving the laser amplification problem and using the Furier description for light propagation. The obtained model is capable of generating a standing wave in the laser resonator, but obtained beam parameters do not match with experimentally measured ones, therefore the model still needs further improvements. |
