Performance Test And Application Study Of A Bimorph Deformable Mirror

At present, most deformable mirrors(DMs) adopted in domestic adaptive optics(AO) systems, which have continuous face plate with discrete stacked PZT actuators(stacked PZT DM), are self-designed and constructed. This kind of DM is very expensive for its long and complex fabrication process. Besides, the size of stacked PZT DM surface is generally lager than 30mm which makes the whole adaptive optics system hard to be compact. Thus, high cost and large volume are the major factors that obstruct the adaptive optics technology from extending its application. In the field which limits the cost or volume of AO system, it's an effective approach to search for a substitutive DM with a simpler structure and a lower cost. Bimorph DM is a newtype wave-front corrector with more and more widespread application in the AO systems of astronomy, military and civil use. Simpler and flexible structure, the advantage at compensating low-order aberrations makes bimorph DM a pop subject in recent years. The fabrication material and process of bimorph DM are similar to those of the stacked PZT DM. So, it's feasible and of great significance to fabricate bimorph DM and use it as corrector to make existing adaptive optics system lower cost and more compact.This paper is devoted to the performace test and application study of a 20-element bimorph DM, which includes response function, displacement dependence on voltage, surface thermal stability and response frequency test.First, the deformation of bimorph DM is analyzed and the expression of response function with circle clamped edge is deduced. Results show that, the displacement of bimorph mirror is directly proportional to the loading voltage, inversely proportional to the square of the thickness of the PZT layer, and it increases about 1.4 times when the area of the electrode doubles. According to the multilayer development tendency of bimorph DM, the advange and disadvantage of multilayer is presented by comparison between a 2-layer, 3-layer and 5-layer structures. Besides, a finite element model is established to verify the conclusions obtained by analysis.Performances of bimorph DM, such as displacement dependence on voltage, linear superposition of response functions, creep, thermal stability of the surface flatness and frequency response, are tested and analyzed. Results show that, the displacement is determined by the loading voltage and the previous state of surface. The surface P-V of bimorph DM increases nearly linearly with ambient temperature. The ratio is 0.11μm /℃and the major component of surface displacement is defocus, compared with which, astigmatism, coma and spherical aberration contribute very small. Thermal stability of bimorph DM is determined by thermal expansion coefficient and also affected by Young's modulus and thickness of material. The first response frequency is about 4kHz.A modified control algorithm, based on controlling gradients directly, is presented to solve the coupling between actuators of a 20-element bimorph DM, which is used to correct the aberrations detected by a Shack-Hartmann wave-front sensor. The decoupling operation is suitable for the adaptive optics system, in which two or more DMs are used to correct aberrations simultaneously.An experimental setup is established to test the aberration compensation ability of bimorph DM with the modified control algorithm. Results show that, the bimorph DM compensates its initial surface shape very well. Its good repeatability of response to voltage is observed and intensity distribution at the image plane of distorted wave-front is effectively improved when closed-loop control is on. Besides, fitting ability to 3-20 Zernike aberrations of the bimorph DM is measured. Almost all the fitting errors of low-order distorted wave-fronts are smaller than 0.5 and that verifies its advantage beyond the discrete PZT DM. The effect of any mismatch between the bimorph mirror actuator structure and the wave-front sensor sampling grids is studied.A scheme of 36-element bimorph DM is proposed to serve as corrector in the existing human retina imaging adaptive optics system. Calculation results show that, the displacement of defocus electrode with 400v voltage would be 2.89μm and the total displacement exceeds 8.49μm , which satisfies the demand of retina imaging system. Theoretical comparison between the 20-element bimorph DM, 36-element bimorph DM and the 37-element discrete PZT DM used at present is carried out to verify the rationality of the new scheme. The 36-element bimorph DM shows a better correction for low-order aberrations than the other two. The fitting errors for 3-25 Zernikes are less than 0.5.