Optics and Photonics Society of Iran

Using unstable resonator is a promising idea for improving power and beam quality of a laser simultaneously. However due to the low gain active media in thin disk lasers, using standard unstable resonator is only applicable with low magnification which leads to poor beam quality. To solve this problem an additional flat retro-reflector mirror used as a part of the output coupler of unstable resonator. Based on the threshold condition the optimum coupling of the external reflector as well as the optimum magnification is calculated in this paper and an imaging confocal unstable resonator is designed. The results show that using an external reflector, designing a modified unstable resonator with appropriate magnification is possible for thin disk laser.