Phase diversity (PD) technique is an effective method for wavefront sensing and image restoration in adaptive optics (AO). Classical PD with Tikhonov regularization can achieve proper wavefront estimation but constantly results in overly smooth images. Nonlocal centralized sparse representation (NCSR) based on nonlocal self-similarity and the sparsity model is combined with PD to obtain high-resolution images. The proposed method contains two steps: the first step is obtaining wavefront from ordinary PD with Tikhonov regularization, and the second step is deblurring the image with NCSR other than Tikhonov regularization. Numerical simulations show that the peak signal-to-noise ratios and structural similarity index metrics of deblurred images by the proposed method are higher than those by the traditional method. This work also studies the influence of weak noise. Initially, the proposed method is applied to a liquid crystal AO system, where the highest spatial resolutions that can be clearly distinguished are 1.59 × diffraction limitation with AO on, 1.41 × diffraction limitation with traditional PD, and 1.26 × diffraction limitation with the proposed method. The proposed approach can be widely used for AO postprocessing in ground-based telescopes, fluorescence microscopes, and other applications.
The intrinsic hysteresis nonlinearity of the piezo-actuators can severely degrade the positioning accuracy of a tip-tilt mirror (TTM) in an adaptive optics system. This paper focuses on compensating this hysteresis nonlinearity by feed-forward linearization with an inverse hysteresis model. This inverse hysteresis model is based on the classical Presiach model, and the neural network (NN) is used to describe the hysteresis loop. In order to apply it in the real-time adaptive correction, an analytical nonlinear function derived from the NN is introduced to compute the inverse hysteresis model output instead of the time-consuming NN simulation process. Experimental results show that the proposed method effectively linearized the TTM behavior with the static hysteresis nonlinearity of TTM reducing from 15.6% to 1.4%. In addition, the tip-tilt tracking experiments using the integrator with and without hysteresis compensation are conducted. The wavefront tip-tilt aberration rejection ability of the TTM control system is significantly improved with the −3 dB error rejection bandwidth increasing from 46 to 62 Hz.
We demonstrate a low pulse repetition frequency (LPRF) Q-switched erbium-doped fiber (EDF) lasers based on acoustic optical modulator (AOM). The single wavelength fiber laser has a stable output at 1553 nm. In Q-switched operation, a pulse train with 3.3μs width and a repetition rate of 1kHz is obtained. The dual wavelength fiber laser is based on fiber Bragg gratings (FBGs) and a Faraday rotator mirror (FRM) as the laser cavity, which has a stable output at 1545 nm and 1557 nm with similar peak power and same repetition rate.