磁液变形镜的镜面动力学建模和实验验证

磁性液体是磁性纳米微粒分散在基液中形成的具有磁性又具有流动性的稳定胶体.磁性液体的流动性会随着周围磁场的变化而改变,基于磁性液体的变形镜的反射镜面通过液面下方驱动器阵列所产生的局部扰动磁场而变形.磁性液体变形镜与传统的变形镜相比,具有镜面连续平滑、变形行程大、制造成本低、易扩展等优点.本文以基于方形驱动器阵列的磁液变形镜为例,考虑磁性液体受重力场、电场、磁场多物理场耦合的作用,在笛卡尔坐标系中建立了磁液变形镜的动力学模型;然后基于推导出的理论模型,设计了磁液变形镜的结构和参数,并用MATLAB,COMSOL Multiphysics和Tracepro软件联合仿真了磁液变形镜镜面响应性能;最后搭建基于磁液变形镜原型样机的自适应光学系统,测试了磁液变形镜的镜面响应线性度和动力学特性,实验结果验证了所建模型的准确性和磁液变形镜面形控制性能.

Modeling and experimental verification of surface dynamics of magnetic fluid deformable mirror

As a key component of the adaptive optics (AO) system,wavefront corrector plays a crucial role in determining the performance of the AO system.At present,the typical wavefront correctors,including solid deformable mirrors and liquid crystal spatial light modulators,have the common drawbacks of high cost of per actuator channel,and the relatively low stroke deflection (normally less than 50 μm) due to the limitation of material and manufacturing technology.In the face of the growing demand for deformable mirrors with large stroke,low power dissipation and low cost,the magnetic fluid based deformable mirror (MFDM) is proposed in this paper.The magnetic fluid has the characteristic of the fluidity of liquid and can be magnetized by an external magnetic field.Therefore,the surface deflection of the MFDM can be controlled by the surrounding magnetic field generated by an array of electromagnetic coils located underneath the magnetic fluid layer.Compared with the conventional deformable mirrors,the MFDM has the advantages of a continuous and smooth mirror surface,large shape deformation,low manufacture cost,and easy extension.The surface dynamics model of MFDM with a circular geometry has been studied previously in the literature.In the present paper, considering the possible applications in the wavefront control of rectangular laser beams,we study the MFDM with a rectangular array of actuators. Firstly,based on the governing equations of the magnetic fluid,derived from the principles of conservation of fluid mass and magnetic field,the dynamics model of surface deflection of the rectangular MFDM is analyzed in Cartesian coordinates under the boundary condition of magnetic field and the kinematic conditions of magnetic fluid.The analytical solutions of the surface movement of the mirror subject to the applied currents in the electromagnetic coils are obtained by properly separating the variables with truncated model numbers.Secondly,based on the derived analytical model, the optimal design procedure for the structure and parameters of the MFDM to obtain the required performance,i.e. the largest stroke and inter-actuator stroke of the mirror,as well as the coupling coefficient of the influence function, is presented.The resulting surface response performance of the designed MFDM is validated by the co-simulation in MATLAB,COMSOL Multiphysics and Tracepro software.Finally,a prototype of square MFDM consisting of the square array of miniature electromagnetic coils,a Maxwell coil and the magnetic fluid filled in a rectangular container is fabricated for experimental evaluation.The experimental results of the surface response of the mirror subject to two adjacent active coils are first presented to validate the stroke performance and linear characteristics of the MFDM. A parabolic surface shape is then further produced in the AO setup system with the MFDM subject to the array of coils driven by the currents calculated from the analytical model.The experimental results verify the accuracy of the established dynamics model and show that the proposed MFDM can be used to effectively control the wavefront of laser beam.