Multipass Optical Amplifier Using a Double Confocal Resonator Geometry.

We examine multipass optical amplifier configurations designed for amplification of short optical pulses. The multipass configurations are based on a double confocal resonator which is formed by introducing small angular rotations of the four mirrors forming two adjacent confocal resonators. Further small rotations and displacements of these mirrors convert the double confocal resonator into a multipass configuration which causes an incident beam to pass through two beam waists on each round trip. These beam waists occur at one spatial location, or at two different spatial locations, depending on the orientation and location of the mirrors and the imaging of the incident beam. Configurations based on symmetric confocal mirror pairs maintain a constant diameter at the beam waists.