A method of processing optical information is described in which the use of absorptive spatial filters is replaced by suitable mirrors with spatially varying reflectances. When these mirrors are made the terminal planes of a laser resonator, the light which is lost in conventional spatial filtering systems, is stored in the laser cavity. It is shown that shallowly modulated phase objects may be imaged outside the resonator without any appreciable effect on the modes of the resonator.
R. V. Pole, H. Wieder, and E. S. Barrekette, "Reactive Optical Information Processing. I: Theory of Information Recovery and Resonator Mode Structure," Appl. Opt. 6, 1571-1575 (1967)