Piezoelectric Deformable Mirror For Adaptive Optics

Adaptive optics (AO) measures and corrects the wavefront distortion in real-time to overcome the dynamic wavefronts aberrations of the imaging system, such as atmospheric disturbances, achieving the diffraction limit resolution. With the development of the AO, it has been investigated in astronomical telescopes, retinal imaging, laser beam shaping, optical communication and so on. Deformable mirror (DM) worked as wavefront corrector is a core component and determines the performance of the AO system. Among various kinds of developed DMs, piezoelectric-driven DM is attracted by researchers due to its large deformation, high frequency response. In this dissertation, in order to develop DMs with low-cost, large deformation and low drive voltage, two types of DMs based on PZT thick films, PZT-thick-film MEMS DM and dual-drive mode unimorph DM, were studied.(1) For the PZT-thick-film MEMS DM, the correction capability of the DM was investigated using the influence function matrix method. The electrics-mechanics coupling model of the PZT-thick-film MEMS DM based on the theory of plates and shell and piezoelectric equations was developed to predict the DM deformation. Based on thses models, the parameters of the DM, such as actuator arrangement, actuator number, influnce function, actuator size, the PZT/Si layer thicknesses and the mirror thickness, were optimized. The fabrication processes of the PZT-thick-film MEMS DM, including PZT thick films grinding and polishing techniques, were intestigated. A61-element DM prototype was successfully developed, which meets the requirements of the wavefront aberration correction. The stroke was measured of approximately5μm at100V. The first order resonant frequency is18kHz. And peak to valley (PV) amplitude is2.1μm at16mm operture which controlled by19actuators. The generation of Zernike polynomials was performed using19actuators and37actuators, which demonstrates that the PZT-thick-film MEMS DM has strong correction capabilities even for higher-order aberrations.(2) A dual-drive mode unimorph DM driven by positive voltage is presented. The developed DM consists of both an inner actuator array and an outer ring actuator. The inner actuator array is used for aberration correction, while the outer ring actuator is used to generate an overall defocus bias. This dissertation describes the design, fabrication and characterization of the DM. The measurements show that the device has the maximum defocus amplitude of～15μm and the actuator deflection of～5um at100V. After correction, mirror surface better than λ/20RMS was achieved. Replication of Zernike polynomial up to5th order indicates it has excellent wavefront correction ability. Considering the advantages of low-cost, large deformation and low driving voltage, this DM is satisfactory for low-cost AO applications.(3) The control methods of DM are investigated. The control methods based wavefront feedback were studied, including influence function matrix method and the closed-loop control method based on steepest-descent algorithm are investigated. The correction of the initial surface and generation of Zernike polynomials shapes were characterized using influence function matrix method. The application of the DM for laser beam shaping was studied. Point Spread Function with wavefront aberrations was used to evaluate the spot quality. The control method based search algorithms, such as simulated annealing algorithm, were researched for wavefront sensor-less AO system. After the correction using the developed DM, the sysrem achieved the diffraction limit focal spot. This experiment lays the foundation for the applications of the DMs.There are some creative points in this dissertation as follows:(1) a piezoelectric MEMS DM which is suitable for the AO applications is develop, and the correction performances are characterized,(2) a new dual-drive mode unimorph DM is proposed, which can achieve bidirectional wavefront correction capability only using a positive drive voltage;(3) a test platform based on Twyman-Green interferometer for the DM performance measuement is developed. The experiments for both the wavefront feedback control algorithm and the search-based algorithm for wavefront sensor-less AO can be tested.