Unstable imaging resonator

The invention relates to an unstable imaging resonator. In one aspect a power amplifier comprises a first plurality of laser disks disposed in a first vertical plane and a second plurality of laser disks disposed in a second vertical plane, opposite the first vertical plane, wherein the plurality of laser disks are disposed in a central horizontal plane, a first plurality of reflecting mirrors are disposed in the first vertical plane and a second plurality of reflecting mirrors are disposed in the second vertical plane. A first set of reflecting are disposed in a lower horizontal plane and a second set of reflecting mirrors are disposed in an upper horizontal plane, wherein respective laser disks and reflecting mirrors adjacent along an optical axis are positioned to provide a 1:1 imaging system therebetween. Other aspects may be described.