单压电执行器阵列驱动的变形镜的制备与表征

Fabrication and Characterization of Deformable Mirror Driven by Piezoelectric Unimorph Actuator Array

为降低压电执行器阵列式变形镜的成本,提出了一种由单压电执行器阵列驱动的低成本变形镜。制备了19单元50 mm口径的变形镜样机,建立了基于Shack-Hartmann传感器的自适应光学测试系统,并对单个执行器性能、校正性能、镜面校平性能、变形镜频率响应特性进行了表征。测试结果显示,制备的变形镜单个执行器在50 V电压下波前变形量可达10μm,可精确重构前14项Zernike多项式像差,校平后远场光斑接近艾里斑,变形镜一阶固有频率约为1.8 kHz。提出的单压电执行器阵列驱动的变形镜具有比较好的性能,在低成本自适应光学系统中具有应用前景。

Adaptive optics system can detect the wavefront distortion of the optical system in real time and then control the mirror deformation of a deformable mirror to compensate the wavefront aberrations,so that the optical system can reach or approach the diffraction limit. It plays an important role in the fields of astronomical imaging,visual science and high-energy laser. Deformable mirrors driven by piezoelectric materials are widely used in the adaptive optics systems due to their high driving force and fast response.Piezoelectric deformable mirrors can be divided into two categories. One is the piezoelectric actuator array deformable mirror which is driven by the discrete piezoelectric actuators through the connecting pillars to deform the thin mirror. It is mainly used for the applications which require high number of actuators and high correction bandwidth. The other one is the bimorph deformable mirror which is composed of one or more piezoelectric layers bonded with the mirror layer. The transverse piezoelectric effect of piezoelectric materials is used to drive the mirror to produce bending deformation. Bimorph deformable mirror has a high interactuator coupling value and a strong correction capability for low-order aberrations. The traditional piezoelectric actuator array deformable mirror is driven by stack piezoelectric actuators,which has the advantages of large deformation and high resonant frequency. However, the high price limits its applications. Chengdu Institute of Optoelectronics Technology of Chinese Academy of Sciences and Xinetics Company improved the actuator structure with transverse piezoelectric effect to improve the actuator density of the deformable mirror and promote the compactness of the overall system design.However,the manufacturing process is complicated and is mainly applied in the astronomical field. In order to reduce the cost,OKO Technologies has proposed a deformable mirror driven by piezoelectric ceramic tubes which are cheaper than traditional stacked actuators. The price is still expensive. In addition,YANG E H and Ma Jianqiang proposed a deformation mirror driven by piezoelectric thin film and piezoelectric thick film array of micro-piezoelectric actuators, respectively. These deformable mirrors have the advantages of low drive voltage and large stroke. However,the manufacturing process of such deformation mirrors using micro-machining technology is complicated. This kind of deformation mirror is still in the stage of principle prototype. In order to further reduce the cost of deformable mirror with piezoelectric actuator array,a low-cost deformable mirror driven by piezoelectric unimorph actuator array is proposed in this paper. The deformable mirror comprises a mirror,a piezoelectric unimorph actuator array,connecting pillars and a substrate. The piezoelectric unimorph actuators are hexagonally arranged on the substrate with corresponding through holes. The piezoelectric unimorph actuator consists of a copper layer and a piezoelectric layer with both sides covered by electrodes. When a voltage is applied to the piezoelectric actuator,the actuator bents due to the transverse piezoelectric effect. The out of plane displacement of the actuator is transmitted to the mirror through the connecting pillar fixed at the center of the piezoelectric unimorph actuator,resulting in the deformation of the mirror. When all the piezoelectric actuators work together,the mirror can produce a conjugate shape with the distorted wavefront to compensate the wavefront aberrations. A deformable mirror prototype with 19 actuators and an effective pupil of 50 mm was fabricated. The piezoelectric unimorph actuator is commercially available,which is made of a brass disc and a piezoelectric ceramic disc bonded together with the diameters of 15 mm and 11 mm,and the thickness of 80 μm and 70 μm,respectively. The substrate material is optical quartz glass with 19 through holes. The mirror is a 3-inch single crystal silicon wafer with a thickness of 350 μm. Since the materials used are all commercially available and the manufacturing process is relatively simple,the cost of the proposed deformable mirror is much lower than other types of piezoelectric actuator array deformable mirrors(especially the stacked piezoelectric deformable mirrors). In order to verify the performances of deformable mirror driven by piezoelectric unimorph actuator array,an adaptive optics test system based on Shack-Hartmann sensor is built. A microlens array with 33×33 lenses is used for the measurement. Sixtyfive Zernike polynomials are used for wavefront aberration fitting. The characteristics of actuator stroke,correction performance,mirror performance and frequency response are performed experimentally. The experimental results show that the wavefront deformation of a single actuator of the fabricated deformable mirror can reach 10 μm under a voltage of 50 V. The deformable mirror can accurately reconstruct the first14-term Zernike polynomial aberrations. The far-field spot after correction is close to the Airy disc. The first-order natural frequency of the deformable mirror can reach 1.8 kHz. Compared to the traditional piezoelectric actuator array deformation mirror,this deformation mirror has the advantages of large stroke,low working voltage and low cost. The actuator number can be further increased for the implementation of the deformation mirror with large diameter and high number of actuators. The proposed deformable mirror has a promising application in the low-cost adaptive optics systems and even in astronomical imaging applications.