| Membrane mirror with flexible polymer film substrate is a new-concept ultra lightweight mirror for space applications. Compared with traditional mirrors, membrane mirror has the advantages of lightweight, folding and deployable, low cost and etc, and it has attracted widespread attention in space optics field. Due to the surface shape of flexible membrane mirror is easy to deviate from the design surface shape, it will bring wavefront aberration to the optical system. In order to solve this problem, a method of membrane mirror wavefront aberration correction based on the liquid crystal spatial light modulator(LCSLM) has been studied in this paper.The research background and status of membrane mirror in domestic and foreign are presented, and the research progress of LCSLM used as wavefront aberration corrector is introduced firstly. The electro-optic effects of the liquid crystal are elaborated in detail. The propagation process of incident light in the liquid crystal is analyzed by the way of Jones Matrix. Based on the liquid crystal continuum elastic deformation theory, the liquid crystal director distribution under different voltages is calculated by the method of difference iterative and is simulated by the MATLAB. And the relation between the LCSLM’s phase modulation and the drive voltage is derived in theory.It indicates that the phase of incident light can be modulated by controlling the drive voltage of LCSLM.Based on the principle of Tyman-Green interference,an experiment system is set up for measuring the phase modulation with the LCSLM LC2012 which is produced by the German Holoeye Company. The interferograms are processed by the fringe shift method, and then the phase modulation characteristics curve of LC2012 is obtained. In order to verify the correctness of the experiment system and the data processing method, the phase modulation characteristic of LC2012 is also analyzed by the ZYGO interferometer and the result is basically consistent with the above measurement. It lays a foundation for using the device to correct the membrane mirror wavefront aberration.Then, according to the optical properties of a membrane mirror and analyzing the principle of wavefront detection and wavefront aberration correction, the LCSLM and Hartmann-Shack sensor are used as a wavefront corrector and a wavefront detector, respectively. And the membrane mirror wavefront aberration correction system is designed and established. In order to verify the feasibility of the experiment system and the correctness of the program, the wavefront aberration of a glass plane mirror with a diameter of 70 mm is measured and corrected firstly. The PV value and RMS value of wavefront are reduced from 0.662λ and 0.135λ before correction to 0.315λ and 0.051λ after correction, respectively. The quality of the mirror wavefront is improved. Finally, the membrane mirror wavefront aberration correction is preliminary realized. The wavefront aberration of the aperture center Φ25 mm in a membrane mirror with a diameter of 200 mm is corrected. The PV value and RMS value of wavefront are reduced from 1.328λ and 0.261λ before correction to 1.037λ and 0.193λ after correction, respectively. And the experimental results are analyzed and discussed. The research of this paper would provide theoretical basis and technology support for the further research on the membrane mirror wavefront aberration correction. |
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