Thin disk laser with unstable resonator and reduced output coupler

•Unstable resonator with reduced output coupling is studied theoretically in a thin disk laser.•Mode of laser beam in unstable resonator calculated by Collins integral and iterative method.•Amplification of laser beam in the active medium is calculated based on the Rigrod method.•Width, divergence, M2 factor, output power, near and far-field beam profiles are calculated.•Feasibility of using unstable resonator for a single thin disk laser is shown. In this paper, feasibility of using unstable resonator with reduced output coupling in a thin disk laser is studied theoretically. Unstable resonator is modeled by wave-optics using Collins integral and iterative method. An Yb:YAG crystal with 250 micron thickness is considered as a quasi-three level active medium and modeled by solving rate equations of energy levels populations. The amplification of laser beam in the active medium is calculated based on the Beer-Lambert law and Rigrod method. Using generalized beam parameters method, laser beam parameters like, width, divergence, M2 factor, output power as well as near and far-field beam profiles are calculated for unstable resonator. It is demonstrated that for thin disk laser (with single disk) in spite of the low thickness of the disk which leads to low gain factor, it is possible to use unstable resonator (with reduced output coupling) and achieve good output power with appropriate beam quality. Also, the behavior of output power and beam quality versus equivalent Fresnel number is investigated and optimized value of output coupling for maximum output power is achieved.