Laser configurations for high-order transverse mode selection and coherent beam combining

The paper briefly reviews two different approaches of obtaining high laser output powers with good beam quality. The first approach is operating a laser with a single high-order transverse mode, so that high output power is obtained, followed by efficient external mode conversion to obtain a nearly Gaussian beam. In this approach single high-order mode operation is achieved by inserting phase elements within the laser resonator, so as to introduce high losses to the undesired modes and low losses to the desired mode. Once a single high order mode beam is obtained, with well defined phase and amplitude distributions, external mode conversion by means of phase or interferometric elements can be performed. The second approach is efficient phase locking and coherent addition of several high-quality and low-power laser channels within a laser resonator, to obtain a single high-quality and high-power output beam. With this approach, self phase locking and coherent addition is achieved by use of intra-cavity interferometric combiners, so as to obtain a compact, stable and practical configuration.