Improvement of beam quality in unstable resonators by a conical output coupler

A novel conical output coupler for positive-branch unstable resonators has been proposed to improve the output beam quality. With the application of an inner cone annular scraper and an outer cone reflector, a realization of the output beam from unstable resonator with solid core is introduced. The feasibility of this conical output coupler with solid beam is examined numerically and experimentally with the help of a commercial fast transverse flow (FTF) CO2 laser. The characteristics of the output coupling beam in near field and far field are discussed. Under the definition of 86.5 % power content, the Mpc2 factors of original annular beam and improved solid beam are 5.22 and 1.16 in simulation. In experiment, the Mpc2 factor of the original annular beam from the commercial FTF CO2 laser is more than 10, while the Mpc2 factor of the improved solid beam is 3.82. Both simulation and experimental results show that the beam quality can be improved a lot by introducing this output coupling configuration.