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.
The pyramid wavefront sensor (PWFS) is a novel sensor, equipped with superiorities in many aspects comparing with Shack-Hartmann wavefront sensor. The optical system of PWFS is more complicated, and relationships among different elements affect the static aberration of the system, which coupling with the turbulence would impact the sensing accuracy of the sensor. Aiming at improving the sensing accuracy, we analyzed the precise calibration of the system and verified the theory through experiments, which enhanced the closed-loop system’s ability to correct the static turbulence.
In this paper, the advances in liquid crystal adaptive optics system (LC AOS) are presented for Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences (CIOMP, CAS). The LC AOS has two bottlenecks of low energy utilization ratio and slow correction frequency. To solve these problems, a series of effective methods were utilized by the LC AOS working group. The problem of energy utilization ratio was solved and the energy utilization ratio was improved from 5% to 85%, which was similar to the deformable mirror based AOSs. Furthermore, the correction frequency of the LC AOS was also greatly improved from 5Hz to 140Hz, which is closed to the ability of correction the atmospheric turbulence. According to these research results, two LC AOSs, which correspond to a 2.16 meter Telescope (located at Xinglong Station of Beijing Astronomical Observatory) and a 1.2 meter telescope (located at CIOMP, CAS), were designed and fabricated. By using these LC AOSs, the star was observed with adaptive correction and the correction is effective. At last, the resolution ability of the star is up to 1.7 times of the diffraction limitation for the 1.2 meter telescope.
A new spot centroid detection algorithm for a Shack-Hartmann wavefront sensor (SHWFS) is experimentally investigated. The algorithm is a kind of dynamic tracking algorithm that tracks and calculates the corresponding spot centroid of the current spot map based on the spot centroid of the previous spot map, according to the strong correlation of the wavefront slope and the centroid of the corresponding spot between temporally adjacent SHWFS measurements. That is, for adjacent measurements, the spot centroid movement will usually fall within some range. Using the algorithm, the dynamic range of an SHWFS can be expanded by a factor of three in the measurement of tilt aberration compared with the conventional algorithm, more than 1.3 times in the measurement of defocus aberration, and more than 2 times in the measurement of the mixture of spherical aberration plus coma aberration. The algorithm is applied in our SHWFS to measure the distorted wavefront of the human eye. The experimental results of the adaptive optics (AO) system for retina imaging are presented to prove its feasibility for highly aberrated eyes.
We introduce the novel parallel-aligned liquid crystal wavefront corrector (LC WFC) with 1920×480 pixels designed to operate at phase-only mode. The optical characteristics of the LC WFC were measured. The theory of diffractive optics is used to correct the aberrated wavefront. The measured peak-valve (PV) value of the wavefront is 9.956 before correction and 0.837 after that. Moreover, the measured root mean square (rms) value of the wavefront is 2.202 before correction and 0.124 after that. It expands LC devices' application fields.