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The adaptive optics system (AOS) has been used to eliminate the effect of atmospheric turbulence for free-space optical communications. The AOSs are designed based on the statistical average performance of the turbulence. In reality, the amplitude of turbulence changes drastically, and the AOS cannot always detect and correct the aberrations appropriately. To prevent the amplitude of turbulence from exceeding the designed range of the Shack–Hartmann wavefront sensor (SH WFS), the combination of the wavefront sensorless and SH WFS methods is proposed to extend the detection range. First, the control scheme is demonstrated, and the numerical simulation is performed. Then, the validation experiments are performed with static and dynamic turbulence. Results show that the proposed method is valid and that the effective communication time is greatly lengthened. • Turbulence amplitude often exceeds the detection range of adaptive optics systems. • Strong turbulence is detected with the combination of SPGD and wavefront sensor. • Wavefront sensor may detect the turbulence with the correlativity more than 0.81. • Communication BER is reduced from 10 − 1 to 10 − 6 with proposed correction method. [ABSTRACT FROM AUTHOR]

An eigenmode method is proposed to estimate the correction performance of atmospheric turbulence based on a deformable mirror (DM). On the basis of the eigenmode, a formula that describes the relationship among residual error, eigenmode N , and turbulence strength D/r 0 (D , telescope aperture, r 0 , atmospheric coherence length) is derived. The eigenmode and the Zernike polynomial methods are compared, too. An experiment is conducted, and results validate the theoretical equations. Hence, the correction performance of atmospheric turbulence may be estimated with the eigenmode method. This result is helpful for conveniently designing and evaluating the adaptive optics system used for turbulence correction. [ABSTRACT FROM AUTHOR]