Three-Mirror Off-Axis Systems for Laser Applications

Abstract

For the focussing of high power (>5kW) carbon dioxide laser radiation, mirror systems are preferable to lenses. While off-axis paraboloidal mirrors are often suggested for this application, their very small field causes severe alignment problems in practice. A series of all-spherical, off-axis 3-mirror systems has been designed as an alternative, which offers attractive features. The sequence of powers is + - +, and a fourth, flat, mirror can be placed conveniently behind the first, concave, mirror to set the exiting axis parallel or perpendicular to the incoming beam. This leads to four angular configurations (pnpn, nnpn, pppn and nppn) all of which have merits. At speeds of up to f/4, and with a back focus larger than the focal length, these systems are particularly attractive for applications in laser welding, but the excellent performance of the systems pnpn and pppn over field angles of more than 1 degree makes them suitable for other applications as well. Aberration characteristics and their dependence on configuration are discussed. A sample system has been built and tested. It verifies the expected performance and shows practical advantages over a comparable off-axis paraboloid.