Study On The Target-In-the-Loop Adaptive Optics Systems

Due to the advantages such as optic feedback and no need of auxiliary beacons, the target-in-the-loop adaptive optics (TIL AO) is one of the promising and efficient ways to make the high-energy laser systems miniaturization and practicality, which has been under intensive research in recent years. The TIL AO is studied theoretically and experimentally in an all-round way in the paper. Three essential issues, i.e., the feasibility of TIL AO systems, the selection of proper experimental scheme and its realization, propagation of high-power fiber laser in real environments, are studied and analyzed.The paper begins with reviewing the history of traditional AO technology and TIL AO technology. The basic principles are briefly introduced, and the advantages of TIL AO over the traditional ones are investigated. The dynamic development and trends of TIL AO technology are reviewed and analyzed in detail. Some recently realized remarkable experiments in TIL AO technology are introduced to demonstrate the fast development of TIL technology.The theoretical model of TIL fiber laser array AO systems is built up using a combination of the parabolic equation describing coherent wave propagation, and the equation describing the scattering off the target surface. The feasibility of its application in adaptive phase locking of fiber laser array in atmospheric turbulence is validated by theoretic derivation.Experimental system of TIL high power adaptive phase-locked fiber laser array is set up in the lab and fiber laser array of two/nine fiber amplifiers are studied. Adaptive phase-locking of two/nine 10-W-level fiber amplifiers is accomplished successfully, which is the highest record for adaptive phase-locked fiber laser array based on TIL technology in both laser numbers and power level. The experimental results demonstrate that the optical phase noise for each beam channel can be effectively compensated by the TIL adaptive optics system under high power applications, and the long-time exposure fringe contrast of spot on the remotely located extended target is advanced from less than 15% to higher than 70%. The effecting factors of close-loop performance of the system, i.e., rotation of the target, intensive turbulent disturbance, are studied experimentally. It is shown that the rotation of the target has a negative influence on the system close-loop performance and the influence of turbulent disturbance is negligible.An analytical expression for the beam quality parameter of high power laser beams propagating through a multilens optical system is derived as a function of the system parameters. It is shown that parameters of lenses, such as materials, structures, have significant influence on real performance of high-power fiber laser collimating systems and a best input power can be found. A best optical beam quality can be achieved through reasonably designing the collimating systems. Considering the effects of the practical properties of beam projection system and real atmospheric environment, the high power fiber laser propagation model, including the influence of collimating system, mechanic jitter of operating platform, beam correlation, atmospheric turbulence, thermal blooming, is built up. The influence of collimating system and thermal blooming is considered together with the other factors for the first time. The laser energy concentrating of laser beam in the far-field is calculated and analyzed based on the theoretical model, which presents good reference for evaluating the performance of high-power laser system.