Experimental investigations of toric resonators

There is much work devoted to the design of new resonator configurations which could optimize efficiency of energy extraction and beam quality of high-power lasers with large- volume gain media. In general, operation with stable resonators at high beam quality limits filling of the gain volume and leads to a reduced efficiency. Unstable resonators exhibit large modal volume and excellent transverse mode discrimination. These are of primary concern in high-power laser systems. Output beams from unstable resonators with spheric mirrors have an annular intensity profile. Beam quality as well as the outcoupling increase as magnification increases. If the gain is not sufficiently high, the increase in the outcoupling reduces laser efficiency. From this it turns out, that in general the maximum beam quality and the maximum efficiency cannot be reached at the same time. This limits the use of these unstable resonators in lasers with low or medium gain. The problem can be overcome using the toric resonators. The beam quality of toric resonators is higher than in the case of unstable resonators with spheric mirrors and is not influenced by the magnification. Therefore, in the case of toric resonators the outcoupling can be matched to the gain in order to optimize the efficiency of energy extraction at high beam quality. Following theoretical analyses and design considerations, different configurations of toric unstable resonators were established for a fast axial flow CO2 laser with two rf-discharge tubes. The efficiency, the beam quality and the alignment sensitivity of toric resonators with different configurations were studied.