Improved compensation of turbulence-induced amplitude and phase distortions by means of multiple near-field phase adjustments

An approach for compensation of turbulence-induced amplitude and phase dist ortions is described. Two deformable mirrors are placed optically conjugate to the collecting aperture and to a finite range from this aperture. Two c ontrol algorithms are presented. The first is a sequential generalized proj ection algorithm (SGPA) that is similar to the Gerchberg-Saxton phase retri eval algorithm. The second is a parallel generalized projection algorithm ( PGPA) that introduces constraints that minimize the number of branch points in the control commands for the deformable mirrors. These approaches are c ompared with the approach of placing the second deformable mirror conjugate to the far field of the collecting aperture and using the Gerchberg-Saxton algorithm to determine the optimal mirror commands. Simulation results sho w that placing the second deformable mirror at a finite range can achieve n ear-unity Strehl ratio regardless of the strength of the scintillation indu ced by propagation through extended paths, while the maximum Strehl ratio o f the far-field approach drops off with increasing scintillation. The feasi bility of the solutions is evaluated by counting the branch points containe d in the deformable mirror commands. There are large numbers of branch poin ts contained in the control commands that are generated by the Gerchberg-Sa xton SGPA-based algorithms, irrespective of where the second deformable mir ror is located. However, the control commands generated by the PGPA with br anch point constraints achieves excellent Strehl ratio and minimizes the nu mber of branch points. (C) 2001 Optical Society of America OCIS codes: 010. 1080, 010.1300, 010.1330.