Design And Fabrication Of A Water-cooled Piezoelectirc Deformable Mirror

In the adaptive optics system(AO),the deformation mirror(DM)is used as the wavefront corrector to correct the wavefront distortion by changing the shape of the mirror in real time.It is widely used in astronomy,retinal imaging,space remote sensing and reconnaissance satellite,biological microscope,high energy laser and other fields.Electrostatic film deformation mirror,electromagnetic deformation mirror,piezoelectric stack deformation mirror and unimorph/biomorph piezoelectric deformation mirror are common types in adaptive optics system.The AO system can be used to purify the beam on external cavity and intracavity and of high power laser system.However,thermal stress of DM is caused by thermal deposition of laser system in the field of high energy laser.The correction performance of the deformable mirror is decreased by the thermal deformation.The components of DM even are damaged by the excessive mirror temperature.As a crucial component of AO system,the sufficient thermal stability of the DM is required.In order to design a DM with large deformation,high precision and strong cooling capacity,a unimorph piezoelectric DM with special cooling structure was researched in this paper.The main research works are as follow:(1)The optimized simulation of water-cooled unimorph piezoelectric DM.According to the requirement of high power laser system,a water-cooled unimorph piezoelectric DM was designed.The central mirror of front side is driven by annular actuator,which arrange on the edge of clear aperture.This design can realize the physical separation of the coolant and electrical connection.The finite element method was used to establish the model of DM.The influence of structure parameters to correction capability is researched.The electrode size and the clear aperture of DM are optimized.Furthermore,the correction performance of optimized DM was analyzed.Then the structural parameters of the deformation mirror were determined for next research.In order to predict the stability of DM in high power laser field,finite element method also used to calculate the temperature change and thermal deformation of DM under the high power laser.The simulation results show that this DM can steadily work at 10 kW laser,with the temperature lower than 53 ℃,thermal deformation less than 0.6 μm.(2)The fabrication of DM.According to the optimization results,a prototype water cooled unimorph piezoelectric DM was fabricated.The fabrication process of DM,with some important steps which could influence the properties of DM,was described in this paper.The burn situation of laser cutting on PZT sheet was detected and a good quality on cutting area was obtained.Reasonable fixture configuration,room temperature curing scheme,quartz ring clamped method and some good packaging design are used during the fabrication process,then a water-cooled DM was successfully fabricated.(3)Performance characterization of a water-cooled unimorph piezoelectric DM.Firstly,the adaptive optics test platform was introduced,and the performance test was carried out by using the test platform.The influence function and initial shape of DM were tested.The initial surface was collected at the clear aperture 15 mm,with the residual error RMS only 26 nm.Then the first 14 Zernike modes are reconstructed using the prototype DM.The experiments results show that the DM reproduces the typical low-order aberrations accurately with relatively large amplitude under the voltage range from-50 V to + 50 V.The wavefront PV amplitudes of the reproduced tip/tilt,astigmatism,defocus,trefoil and coma are about 40 μm,24 μm,18.7 μm,10 μm and 6 μm,respectively.The normalized residual wavefront error of the first 9 Zernike modes are less than 3.2%.Finally,The effect of the lateral pressure of the water on the mirror surface was evaluated by filling the cooling cavity with water without flowing.The water pressure only causes some loworder aberration on the initial deformation mirror surface,such as astigmatism,defocus.the initial surface RMS increased to 1.33 μm,residual error is 42 nm after correction.The applied maximum voltage difference is about 20 V,the DM can still save enough capability for aberration correction,which meet the application demand of deformable mirror.