en لیزرها، تقویت کننده‌‏های اپتیکی و اپتیک لیزر Lasers, Optical Amplifiers, Laser Optics پژوهشي Research <span style="font-size:11pt"><span new="" roman="" style="font-family:" times=""><span style="font-weight:bold">In this paper, the thick thermal lens approximation is evaluated for a CTH:YAG laser with a standard telescopic resonator. An accurate relation between the thermal lensing parameter and lens-like media factor is proposed. Telescope adjustments were studied based on the practical values of the focal length of the thermal lens. The fundamental mode volume is determined regarding the Hermit-Gaussian theory, and the influence of the defocus parameter is examined. The study emphasizes the importance of considering both the fundamental mode volume and the distance to the stability boundary when selecting the optimal defocusing parameter. A merit factor was introduced that provides a comprehensive approach to selecting the most suitable defocus parameter for the system.</span></span></span><br> &nbsp; ABCD Matrix, CTH:YAG laser, Telescopic resonator, Thermal lens effect. 33 42 http://ijop.ir/browse.php?a_code=A-10-990-1&slc_lang=en&sid=1 Mahdi Shayganmanesh shaygan@iust.ac.ir 10031947532846005430 10031947532846005430 No School of Physics, Iran University of Science and Technology, Narmak, Tehran, Iran Davood Razzaghi davrazzaghi@yahoo.com 10031947532846005431 10031947532846005431 Yes Photonics and Quantum Technologies Research School, Nuclear Science and Technology Research Institute (NSTRI), Tehran, Iran Maryam Goli Hasanlo hasanlo_maryam@yahoo.com 10031947532846005432 10031947532846005432 No School of Physics, Iran University of Science and Technology, Narmak, Tehran, Iran Arezou Jalilzadeh Heydarlou a.jalilzadeh.heydarlou@gmail.com 10031947532846005433 10031947532846005433 No School of Physics, Iran University of Science and Technology, Narmak, Tehran, Iran Marzieh Akbari marziehakbari@iust.ac.ir 10031947532846005434 10031947532846005434 No Photonics and Quantum Technologies Research School, Nuclear Science and Technology Research Institute (NSTRI), Tehran, Iran Mohammad Reza Moghaddam mabdolhoseini@aeoi.org.ir 10031947532846005435 10031947532846005435 No Photonics and Quantum Technologies Research School, Nuclear Science and Technology Research Institute (NSTRI), Tehran, Iran