Shaping a focused laser beam by continuous wavefront control using a deformable mirror

Uniform irradiation of laser beam is needed for the directly driven inertial confinement fusion, laser-chemistry and industrial applications such as laser welding, semiconductor processing and laser microfabrication. For improving the irradiation uniformity on a fusion target, several beam smoothing techniques such as a random phase plate (RPP)1, induced spatial incoherence (ISI)2, smoothing by spectral dispersion (SSD)3 and kinoform phase plate (KPP)4 have been proposed. The RPP has energy loss of ~ 15 % by a large diffraction angle, and the ISI and the SSD have imprint problems by instantaneous speckle pattern. The precise KPP with a continuous phase distribution is difficult to fabricate, even if the energy loss can be reduced to 5%. We are developing an active optical system to compensate (or control) a wavefront aberration included in a large-scale laser system for fusion. Shack-Hartmann wavefront sensor using a large F-number plastic micro-lens array5 and continuous faceplate type deformable mirror6 have been developed and their good performance has been demonstrated.