Abstract: Nowadays laser have found great applications in various industries. Because of coherency, monochromaticity, directionality, brightness, high optical quality and high power, the laser beam have an enormous application in medical, transportation, military and communications industry. Naturally, the laser has no difference from the light and it only differs in physical properties. Lasers based on the active substance are categorized to solid, liquid and gas. The solid state lasers are formed from host mediums that are doped by an active ions. The host environment is built in the form of slab, rod, disk and fiber according to the geometric shape. In most cases, the solid lasers can be stimulated by diode pumping. Pumping process is applied in different ways that side and end pumpings are common types of pumping. During the pumping, some of the pumping energy at the crystal is converted to heat and this heat causes the temperature gradient in the crystal. the temperature gradient can produce the thermal stresses and even lead to fracture in the crystals. In the fiber lasers, these phenomona disturb the laser operation and decrease the maximum input power of the laser. In this research, the thermo-mechanical behaviour of single clad Yb:YAG fiber laser is discussed under continuous end pumping. The single clad fibers have been modelled with single and four cores and they are simulated in finite element software (Ansys). Since the thermal loading and geometry of the fiber have symmetry planes, in the modelling, one-quarter model is used to determine the temperature and stress distributions. In the Ansys software, the coupled thermal-structural analysis is utilized. First, the temperature distribution is calculated in the thermal environment of the software and then thermal stress fields for single core and four core fiber lasers are obtained by applying the temperature distributions calculated through the thermal analysis. In the analysis, the mesh dependency is eva‎luated and for achevinig convergency, the reuquired number of element is determined. The temperature distribtions in single and four core fibers, with two positions for cores, are calcluated. For any case, the calculated temperatures are compared with each other and the position of core related to smaller temperature is selected. The results show that, in comparison to single-core fiber, a four-core fiber has smaller maximum stress at the same power and higher pumping fracture power. Finally, the other thermal effects of the pumping, including thermal lensing, for single and four-core fiber are compared with each other and it is shown that the four core fiber laser has better performance in terms of thermomechanical output.

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