Design And Fabrication An Edge-Driven Piezoelectirc Deformable Mirror

Edge-driven deformable mirrors(DMs)are important technical directions in low-order aberration correction systems which have the advantages of simple structure,large wavefront correction stroke,and no interference from adhesive stress in the middle working area.Piezoelectric materials have excellent performance,and the unimorph DM is widely used in beam correction due to its large stroke,high bandwidth,and high mirror quality.In order to increase the amount of deformation,unimorph DMs usually use thinner piezoelectric ceramics.However,under the action of the traditional positive and negative voltage driving,the thin piezoelectric ceramic plate will experience repeated reversal of the electric field and mechanical load,which is prone to depolarization and even failure.At the same time,under the continuous irradiation of the laser(especially high-energy laser),the mirror surface will be thermally deformed,which will also affect its correction performance.Based on the design concept of edge-driven DMs and the structure of unimorph DMs,this paper aims to study a low-cost,large-deformation edge-driven DMs which correct low-order aberrations.We have conducted research to avoid depolarization of piezoelectric materials,reduce thermal effects,simplify optical path design,and reduce integration space and cost.First,an edge-driven actuator finite element model based on a single piezoelectric plate is established and optimized.Second,based on this model,a positive-voltage edge-driven reflective DM and an edge-driven transmissive DM are studied.The specific work is as follows:(1)Edge-driven actuators based on unimorph.Firstly,a finite element model of the edge-driven actuator was established,and the performance of the actuator was obtained by finite element analysis.With the influence function of the actuator and its ability to reconstruct low-order Zernike mode aberrations as optimization objectives,we carried out optimization work on the parameters such as the correction aperture and electrode number or size of the edge-driven actuator.According to the results,the optimal structural parameters are obtained,and the optimal correction performance of the edge-driven actuator is given under the parameters.(2)Reflective deformable mirror driven by positive-voltage actuators at edge.In order to improve the working stability of the electrode and reduce the influence of thermal load,a reflective DM driven by positive-voltage actuators at edge is proposed.The upper ring electrode of the DM is used to generate an overall defocus bias,while the underside electrodes which from the edge-driven actuator introduced in(1)are used to correct the wavefront aberrations.The bidirectional deformation of the mirror can be realized by the only positive voltage,and the thermal deformation of the DM is low due to the completely symmetrical structure.The DM prototype was fabricated and its actuator performance,mirror quality,and reconstruction ability were characterized.The results show that the wavefront peak and valley after initial mirror surface correction were reduced to 0.221 μm,and the corresponding root mean square was 0.036 μm(< λ/18),which reaches the diffraction limit.The typical low-order Zernike mode aberrations,such as astigmatism,defocus,trefoil and coma,are accurately reconstructed.The reconstructed amplitudes are 11.1 μm,9.7 μm,5.7μm and 4.2 μm,with the normalized residual errors of 1%,1%,3.3%,and 6%,respectively.In addition,non-diffraction Airy beam is generated experimentally,which shows the excellent reconstruction performance of the DM.(3)Edge-driven transmissive deformable mirror.Introducing a reflective DM into an existing optical system requires a folded optical path and relay optics,increasing of the bulk and cost.Based on the actuator model driven by the edge in(1),the traditional reflective mirror is replaced by transparent glass.This study proposes a low-cost transmissive DM that can be easily integrated into the existing system for correction of low-order aberrations.A transmissive DM prototype was fabricated and tested for performance.The results show that the wavefront peak and valley after initial aberration correction were reduced to 0.230 μm,and the corresponding root mean square was 0.030 μm(< λ/20),which reaches the diffraction limit.The fabricated prototype also accurately reproduced the first 9 Zernike mode shapes.The reconstruction amplitude is large,and the overall residual error is less than 5%.In order to further prove its correction ability,we corrected the system aberrations in the absence of wavefront and adopted an improved mountain climbing algorithm based on the Zernike mode.The corrected light spot is similar to the Airy spot,with a maximum light intensity increase of5 times.Furthermore,the transmissive DM was integrated into a customized microscopy imaging system to correct the aberrations,significantly improving the quality of biological cell microscopy images.