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Abstract
The stacked array piezoelectric deformable mirror (DM) used in adaptive optics (AO) systems usually has actuator-corresponding high-frequency temperature-induced distortion (TID) on its mirror surface when the working temperature is different from the design temperature, which is harmful to beam quality. To effectively eliminate the actuator-corresponding high-frequency TID, we introduce a hybrid connection structure deformable mirror (H-DM), which adopts a magnetic connection structure besides the conventional adhesive connection structure. The TID characteristics of the H-DM are analyzed using the finite element method, and the wavefront compensation capability of the novel H-DM is also investigated in simulation. In the experiment, the initial surface shape and the TID characteristics of a lab-manufactured H-DM are measured. The experimental results show that the H-DM has a good initial surface shape, and no actuator-corresponding high-frequency distortion exists in the surface shape of the H-DM when the environment temperature changes. Thus it can be seen the TID could be well corrected by the H-DM itself, and thereby the environmental adaptability of the DM could be improved substantially.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Deformable mirror (DM) is widely used to compensate wavefront distortion in optics systems of many areas, such as laser systems [1,2], astronomical observation [3,4], biomedical microscopy [5,6], vision science research [7,8] and inertial confinement fusion facilities [9,10]. As reported, there are many types of DMs, including stacked array piezoelectric DM [11], membrane DM [12], thermal DM [13–16], MEMS DM [17], bimorph DM [18] and unimorph DM [19], etc. Among these DMs, the stacked array piezoelectric DM is widely adopted in adaptive optics (AO) systems for its advantages of high precision control ability, high laser damage threshold, quick response and large dynamic range. In practical application, the correction ability of the stacked array piezoelectric DM is greatly influenced by the local high-frequency aberration (HIFA) occurring on its surface shape [20–22]. With the development of the DM technology, various techniques have been studied to restrain the local HIFA [23–27]. The HIFA of the DM reportedly could be eliminated by re-grinding and re-polishing the assembled mirror [23], inserting annular pads between the mirror and the actuators [24], setting the actuators strictly perpendicular to the mechanical base [25], using the mirror with a post array [26] and optimizing the structural parameters of the DM [27].
However, actuator-corresponding high-frequency temperature-induced distortion (TID) still appears on the DM’s surface shape when the working temperature is different from the design temperature [28,29], although the local HIFA mentioned above could be depressed. Some research has been done to depress the actuator-corresponding high-frequency TID of the DM [30–32]. Structural parameters optimization was presented by increasing the thickness of the base and the mirror, increasing the actuator numbers and choosing the base and the mirror with similar thermal expansion coefficients [30]. A water-cooling channels structure embedded monolithically inside the DM’s faceplate was proposed to realize active cooling when the working temperature rises and thus maintain a good surface shape [31], while an auxiliary temperature compensation module was presented to depress the actuator-corresponding high-frequency TID of the DM based on the thermal stress characteristics [32]. Nevertheless, in practical cases, it’s difficult to choose materials with same thermal expansion coefficients for the base and mirror. The structure and the control method of the DM with water-cooling channels or the compensation module are very complicated, which makes the use of the DM more complex.
A hybrid connection structure deformable mirror (H-DM) to effectively eliminate the actuator-corresponding high-frequency TID is presented in this paper. Compared with the conventional adhesive connection structure DM (A-DM), a magnetic connection structure is added in the H-DM. This paper is organized as follows. In Section 2, the configuration of an A-DM is presented and the TID characteristics of the A-DM is investigated in simulation. In Section 3, a hybrid connection structure of the H-DM is introduced and the corresponding finite element model is built. The TID characteristics and the wavefront compensation capability of the H-DM are investigated and analyzed. In Section 4, the effective driving voltage range and TID characteristics of a lab-manufactured H-DM are measured in the experiment. The experimental results show no actuator-corresponding high-frequency distortion exists in the surface shape of the H-DM and the TID could be well depressed by the H-DM when the working temperature differs from the design temperature.
2. Simulation investigation on the TID characteristics of the A-DM
Shown in Fig. 1 is the schematic diagram of the conventional DM with adhesive connection structure. As seen from Fig. 1(a), the surface shape of the A-DM is square with a size of 84 mm×84 mm while the 68 mm×68 mm red marked square is the effective aperture. The 49 posts of the mirror and the 49 actuators are in the same square array distribution with 12 mm spacing. In the conventional adhesive connection structure, the top of the flexure plate and the bottom of the mirror post are glued together. It should be noted that the post array structure of the DM’s mirror [Fig. 1(b)] could help to depress the local HIFA caused by uneven curing of the glue [23]. Because of the different thermal expansion coefficients of the base and the mirror, when the working temperature is inconsistent with the design temperature, the expansions of the base and the mirror are different in the horizontal direction, which results in the tilt of the flexure plates and the mirror posts. There are not only vertical displacement constraints but also horizontal displacement constraints between the connected mirror post and the flexure plate. Thus, when the base and the mirror expand or contract differently due to the temperature change, there will be a tilt for the flexure plate and the mirror post. The tilt of the flexure plates and the mirror posts leads to actuator-corresponding high-frequency TID in the surface shape of the A-DM [32].
Fig. 1. Schematic diagram of the conventional DM with adhesive connection structure. (a) is the actuator distribution of the A-DM, and (b) is the structure of the A-DM.
In order to investigate high-frequency TID characteristics of the A-DM, a finite element model is constructed, and simulation analysis is carried out in COMSOL Multiphysics software [33,34]. In the finite element model, the A-DM consists of a structure steel base, a BK7 mirror with 49 mirror posts, and 49 square-distributed piezoelectric ceramic transducer (PZT) actuators. Detailed structural and material parameters of the A-DM are listed in Table 1 and Table 2.
Table 1. Structural parameters of the A-DM/H-DM.
Table 2. Material parameters of the A-DM/H-DM.
In the simulation, the finite element model is based on the thermal stress interface in the structural mechanics module. The degrees of freedom for vertical deformation and horizontal rotation of the base undersurface is set to zero. The degrees of freedom of the center point of the base undersurface is set to zero. The initial surface shape of the A-DM under ${T_0} = 20^\circ{C}$ is set to an ideal plane surface. The heat transfer is carried out by natural convective heat flow, and the convective heat transfer coefficient of all outer surfaces is set to 10W/(m2K). The design temperature is set to ${T_0} = 20^\circ{C}$. The first natural frequency of the A-DM is calculated to be 2.6kHz using frequency analysis module in COMSOL Multiphysics software [35]. To investigate the TID characteristics, the temperature variations are set to $\Delta T ={-} 6^\circ{C}$ and $\Delta T ={+} 6^\circ{C}$, respectively. After each temperature variation, the surface distortion of the A-DM is measured when the steady state is achieved. Figure 2 shows the TID characteristics as well as self-compensation results of the A-DM under $\Delta T ={-} 6^\circ{C}$ and $\Delta T ={+} 6^\circ{C}$.
Fig. 2. TID characteristics of the A-DM. (a1) and (b1) are the surface shape distortions of the A-DM when $\Delta T ={-} 6^\circ{C}$ and $\Delta T ={+} 6^\circ{C}$, respectively. (a2) and (b2) are the 3D maps of (a1) and (b1), respectively. (a3) and (b3) are the distortions of the effective apertures of (a1) and (b1), respectively. (a4) and (b4) are the 3D maps of (a4) and (b4), respectively. (a5) and (b5) are the residues of (a4) and (b4) fitted by 3rd to 48th Zernike mode aberrations, respectively. (a6) and (b6) are the residues of (a4) and (b4) fitted by the A-DM itself, respectively.
As shown in Figs. 2(a1)–2(a2) and 2(b1)–2(b2), high-frequency distortion appears in the surface shape of the A-DM after temperature variation. For $\Delta T ={-} 6^\circ{C}$, a convexity with the peak-to-valley (PV) value of 2.0744 µm appears on the whole surface of the DM, while for $\Delta T ={+} 6^\circ{C}$, a concave with the PV value of 2.0744 µm appears on the whole surface of the A-DM. In addition, when the working environment temperature is increased and decreased for the same value, the corresponding distortions are opposite to each other. Distortions of the effective aperture of Figs. 2(a1) and 2(b1) are shown in Figs. 2(a3) and 2(b3), and the corresponding 3-dimensional (3D) maps are shown in Figs. 2(a4) and 2(b4). It could be seen that for the effective aperture of the A-DM, the TID represents a typical high-frequency distortion, of which the distribution is highly correlated with the position distribution of the actuators. The PV values of the distortions are both 0.4291 µm as shown in Figs. 2(a3) and 2(b3). The residues of the TID in the effective aperture fitted by 3rd to 48th Zernike mode aberrations are shown in Figs. 2(a5) and 2(b5) with PV values both to be 0.2978 µm. It could be seen that there still remain actuator-corresponding high-frequency distortions on the A-DM’s surface. Self-compensation for the TID is implemented on the A-DM, in which the influence function is measured and the TID in the effective aperture is set as the compensation target. According to the self-compensation residues shown in Figs. 2(a6) and 2(b6), the high-frequency distortions could not be well corrected and remain in the surface of the A-DM. The PV values of the fitting residues are both 0.2846 µm.
The simulation results show that for an A-DM, when the working temperature is inconsistent with the design temperature, the actuator-corresponding high-frequency TID appear on the A-DM’s surface shape, and the distortions could not be compensated by the A-DM itself. For a practical AO system using the A-DM, the actuator-corresponding high-frequency TID will be introduced into the beam wavefront and could not be corrected by the A-DM.
3. Simulation investigation on the H-DM
3.1 TID characteristics of the H-DM
The schematic diagram of the DM with hybrid connection structure is displayed in Fig. 3. As shown in Figs. 3(a) and 3(b), the mirror and the distribution of the actuators of the H-DM are the same as those of the A-DM. However, unlike the conventional adhesive connection structure of the A-DM, the H-DM adopts magnetic structure to connect the flexure plates and the mirror posts at 45 points. The flexure plates and the mirror posts at the remaining 4 corner points still adopt conventional adhesive connection structure to ensure the structural stability. Figure 3(c) shows the 3D schematic diagram of the H-DM.
Fig. 3. Schematic diagram of the DM with hybrid connection structure. (a) is the actuator distribution colored according to magnetic connection structure (orange) and adhesive connection structure (white), (b) is the structure of the H-DM, and (c) is the 3D schematic diagram of the H-DM.
Table 3. Magnetic parameters of the NdFeB magnet.
Compared with the conventional adhesive connection structure, an NdFeB magnet is added between the flexure plate and the mirror post in the magnetic connection structure. Structural, material and magnetic parameters of the NdFeB magnet are shown in Table 1, Table 2 and Table 3, respectively. In Table 3, Br refers to the remanence of the NdFeB magnet, Hcb the coercivity, Hcj the intrinsic coercivity, (BH)max the maximum magnetic energy product, and Tmax the maximum working temperature. The magnet is glued upwards with the mirror post, resulting in vertical and horizontal displacement constraints between them. The bottom of the magnet is connected downwards with the top of the flexure plate by magnetic force, mainly vertical force, thus chiefly resulting in vertical constraints between them. Thus, when the base and the mirror expand or contract differently due to the temperature changes, a horizontal relative displacement could occur between the bottom of the magnet and the top of the flexure plate. This could offset the tilt of the mirror post and prevent unwanted actuator-corresponding high-frequency distortion in the surface shape of the H-DM.
Fig. 4. (a1) and (b1) are the surface shape distortions of the H-DM when $\varDelta T ={-} 6^\circ{C}$ and $\varDelta T ={+} 6^\circ{C}$, respectively. (a2) and (b2) are the 3D maps of (a1) and (b1), respectively. (a3) and (b3) are the distortions of the effective apertures of (a1) and (b1), respectively. (a4) and (b4) are the 3D maps of (a4) and (b4), respectively. (a5) and (b5) are the residues of (a4) and (b4) fitted by 3rd to 48th Zernike mode aberrations, respectively. (a6) and (b6) are the residues of (a4) and (b4) fitted by the A-DM itself, respectively.
To investigate the TID characteristics of the H-DM, a finite element model is built in COMSOL Multiphysics software, and simulation analyses are carried out. In the H-DM’s model, the constraint setup is the same as that of the A-DM’s model. The initial surface shape of the H-DM under ${T_0} = 20^\circ{C}$ is set to an ideal plane surface. Natural convective heat flow is employed to complete the heat transfer, and the convective heat transfer coefficient of all outer surfaces is set to 10W/(m2K). In the simulation, the constraint between the bottom of the magnet and the top of the flexure plate is set in the vertical direction. The first natural frequency of the H-DM is calculated to be 1.7kHz. According to the A-DM’s measurement results mentioned in Section 2, it could be concluded that compared with the A-DM, the H-DM has a lower first natural frequency, which makes the achievable dynamical correction performance of the H-DM lower than that of the A-DM.
The difference between the H-DM’s and the A-DM’s model is that the magnetic connection structure is adopted in 45 points in the H-DM’s model. In the simulation, the design environment temperature is set to ${T_0} = 20^\circ{C}$, and the temperature variation are set to $\Delta T ={-} 6^\circ{C}$ and $\Delta T ={+} 6^\circ{C}$, respectively. When the steady state is achieved after each temperature variation, the surface shape distortion of the H-DM is measured. Shown in Fig. 4 are the TID characteristics and self-compensation ability of the H-DM under $\Delta T ={-} 6^\circ{C}$ and $\Delta T ={+} 6^\circ{C}$.
Figures 4(a1) and 4(b1) display the 2-dimensional (2D) TID of the H-DM under $\varDelta T ={-} 6^\circ{C}$ and $\varDelta T ={+} 6^\circ{C}$ respectively, of which the PV values are 2.6733 µm and 2.6567 µm, respectively. Figures 4(a2) and 4(b2) demonstrate the 3D maps of Figs. 4(a1) and 4(b1), respectively. It could be seen that the distortions under symmetric environment temperatures are opposite to each other, and no actuator-corresponding high-frequency distortion exists in the surface shape of the H-DM. Distortions of the effective apertures of Figs. 4(a1) and 4(b1) are shown in Figs. 4(a3) and 4(b3), and corresponding 3D maps are shown in Figs. 4(a4) and 4(b4), with the PV values to be 0.5519 µm and 0.5472 µm, respectively. The residues of the TID in the effective aperture fitted by 3rd to 48th Zernike mode aberrations are shown in Figs. 4(a5) and 4(b5) with PV values to be 0.0894 µm and 0.0886 µm, respectively. It could be seen that there is no actuator-corresponding high-frequency distortion on the H-DM’s surface. Figures. 4(a6) and 4(b6) are the self-compensation residues of Figs. 4(a3) and 4(b3) with PV values to be 0.1170 µm and 0.1165 µm, respectively. Before and after self-compensation, there is no actuator-corresponding high-frequency distortion in the surface shape of the H-DM, which is different from the A-DM.
In conclusion, due to the magnetic connection structure adopted in the H-DM, no actuator-corresponding high-frequency distortion exists in the surface shape of the H-DM and the TID could be well corrected by the H-DM itself when the working temperature differs from the design temperature. The result indicates that the H-DM would not introduce actuator-corresponding high-frequency distortion into the beam wavefront in a practical AO system.
3.2 Wavefront compensation capability of the H-DM
Fig. 5. Influence function of the 25th actuator of (a) Gaussian function and (b) H-DM. (c) is the difference between them.
To investigate the wavefront correction capability of the H-DM, the 3rd to 14th Zernike mode aberrations are used as the compensate targets in the simulation. The influence function of the H-DM is measured first by using the finite element model on condition that the initial ideal plane mirror surface of the H-DM is set as the reference. Then the top of each actuator is set to shift 1 µm in the vertical direction. Finally, the mirror surface of the H-DM differs from reference and the difference is the influence function of each corresponding actuator. The influence function of the 25th actuator of the H-DM is displayed in Fig. 5(b) as an example. Figure 5(a) shows the Gaussian influence function of the 25th actuator. The Gaussian influence function is an influence function representation method, using a 2D Gaussian radial basis function rotationally symmetric around the axial center of the actuator as the influence function of the actuator [36]. The Gaussian influence function is used in simulations and experiments of many studies as a standard influence function [36–38]. The difference between the Gaussian and the H-DM influence function is shown in Fig. 5(c). It could be seen that compared to the concentric circle distribution of the Gaussian influence function, the H-DM influence function follows square distribution, which is consistent with the actuators’ square distribution.
Fig. 7. (a) PV and (b) RMS value comparison between Zernike mode aberrations before and after compensation by the H-DM.
In the wavefront compensation process of the simulation, the 3rd to 14th Zernike mode aberrations are used as the compensation targets. The PV value of each Zernike mode aberration is set to 1µm. For each Zernike term, the driving voltage applied to each actuator is calculated using the influence function and the compensation target. Figure 6 displays the 3rd to 14th Zernike mode aberrations and the corresponding compensation residues. Shown in Fig. 7 is the comparison of PV and root-mean-square (RMS) values of Fig. 6.
It could be seen from Figs. 6 and 7 that after compensation, the 3rd to 14th Zernike mode aberrations are well corrected, as their PV and RMS values are significantly reduced. Taking the 8th Zernike mode aberration as an example, its PV value is reduced by 93% from 1 µm to 0.0684 µm, while the RMS value is reduced by 98% from 0.2890 µm to 0.0054 µm. The compensation results for other Zernike mode aberrations are better than that of the 8th Zernike term. It could be concluded that the H-DM model has good capability in low order aberration compensation.
4. Experimental investigation of the H-DM
4.1 Effective driving voltage range of the H-DM
In the experiment, an H-DM was manufactured in our lab [Fig. 8] at the design temperature ${T_0} = 23.7^\circ{C}$, of which the structure is the same as that of the simulation model described in Section 3.1 [Fig. 3, Tabs. 1–3]. The measured influence functions of the 11th, the 17th and the 25th actuators at the driving voltage of 1 V are shown in Fig. 9 as an example.
Fig. 9. Measurement results of the influence functions of (a) the 11th, (b) the 17th and (c) the 25th actuator in the H-DM.
Different from the conventional adhesive connection structure, the magnetic connection structure of the H-DM is based on the magnetic force connecting magnets and flexure plates. When the DM is working, the surface shape deformation will bring the force on the end of the magnet connection structure. When the tensile force on both ends of the magnet connection structure is greater than the magnetic force, the magnet will detach from the flexure plate. We take an actuator as example. When the actuator is driven by negative voltage to produce shrinkage deformation, a tensile force is generated in the magnetic connection structure. The negative voltage at which the magnet detaches from the flexure plate is defined as ΔUmin. When the actuator is driven by positive voltage to produce tensile deformation, tensile forces are generated in the magnetic connection structure of adjacent actuators. The positive voltage at which the adjacent magnets detach from the flexure plate is defined as ΔUmax. ΔUmin to ΔUmax is defined as the effective driving voltage range of the H-DM.
The effective driving voltage range of the H-DM was tested in the experiment. In the experiment, a voltage of 20 V was applied to all the actuators. Then, a voltage difference (ΔU, from −20 V to 16 V at intervals of 2 V) was applied to the center actuator, and the PV value of the surface shape deformation was measured. The upwards $\mathrm{\varDelta }U$ was set up to 16V to protect the mirror of the H-DM from fracture. According to the measurement result shown in Fig. 10, the PV value varies with the ΔU ranging from −14V to +16V, indicating that the H-DM is responsive within this voltage range. When the downward ΔU reaches over −14V, the change of PV value is almost static, which indicates that H-DM is no longer responsive due to the failure of the magnetic connection structure. This could lead to the conclusion that the effective range of driving voltage is from ΔUmin = −14 V to ΔUmax = +16 V (the corresponding stroke range of the actuator is measured to be from −1.20 µm to 3.52 µm) for this lab-manufactured H-DM. Practically, magnets with stronger magnetic force and mirror with thinner thickness could be considered if a larger range of effective driving voltage is needed.
Fig. 10. PV values variation of the H-DM’s surface shape deformation with driving voltage (ΔU) of the center actuator.
4.2 TID characteristics of the H-DM
Figure 11 shows the initial surface shape of the H-DM under the design temperature ${T_0} = 23.7^\circ{C}$. A 632.8 nm ZYGO interferometer (VerifireTM XPZ) was used to measure the surface shape of the H-DM. Shown in Figs. 11(a), 11(b) and 11(c) are the 3D map, the 2D map and the fringe pattern of the H-DM’s surface shape, respectively. Note that the wavelength of light employed by the interferometer is 632.8 nm, therefore the PV and RMS values of the surface shape of the H-DM are 0.4413 µm and 0.0861 µm, respectively. The Zernike decomposition coefficients of Fig. 11(b) are displayed in Fig. 11(d). Figures 11(e) and 11(f) demonstrate the 2D map and fringe patterns of the self-compensation residues of the H-DM. As seen from Fig. 11, the lab-manufactured H-DM has a good initial surface shape without actuator-corresponding high-frequency distortions.
Fig. 11. Initial surface shape of the H-DM at the design temperature ${T_0} = 23.7^\circ{C}$. (a) and (b) are the 3D and 2D maps of the surface shape measured by the interferometer, respectively. (c) is the fringe pattern of (b). (d) shows the Zernike decomposition coefficients of (b). (e) is the self-compensated residue of the H-DM’s surface shape, and (f) is the fringe pattern of (e).
Based on the well-manufactured H-DM, the TID characteristics were investigated in the temperature response experiment. First, the surface shape of the H-DM was measured as the reference at the design environment temperature ${T_0} = 23.7^\circ{C}$. Then the environment temperature was changed to $T = 17.6^\circ{C}$ ($\varDelta T ={-} 6.1^\circ{C}$). When the H-DM reached a steady state, the surface shape remained stable and then was measured [Fig. 12(a1)]. After that, the environment temperature was changed to $T = 29.7^\circ{C}$ ($\varDelta T ={+} 6.0^\circ{C}$). The surface shape was measured [Fig. 12(b1)] when the H-DM reached a steady state again. As shown in Fig. 12(a1), when $\varDelta T ={-} 6.1^\circ{C}$, the TID of the H-DM is mainly a convex surface shape with a PV value of 2.1746 µm and an RMS value of 0.3318 µm. As shown in Fig. 12(b1), when $\varDelta T ={+} 6.0^\circ{C}$, the TID of the H-DM is mainly a concave surface shape with a PV value of 2.1413 µm and an RMS value of 0.3151 µm. It can be seen that when the temperature increases or decreases by the same amount, the surface shape distortions of the H-DM are almost conjugate with each other, which is consistent with the simulation results. Besides, under both temperature variations, there is no visible actuator-corresponding high-frequency distortion in the surface shape of the H-DM. It should be noted that the conditions in the experiment are not as simple and ideal as those in the simulation experiment, therefore the TID in the experiment are not strictly the same as that in the simulation. Figures 12(a2) and 12(b2) exhibit the Zernike decomposition coefficients of Figs. 12(a1) and 12(b1), respectively.
Fig. 12. Experimental results of the TID characteristics of the H-DM. The measured TID at (a1) $\varDelta T ={-} 6.1^\circ{C}$ and (b1) $\varDelta T ={+} 6.0^\circ{C}$. (a2) and (b2) are the Zernike decomposition coefficients of (a1) and (b1), respectively. (a3) and (b3) are the self-compensation residues of (a1) and (b1), respectively. (a4) and (b4) are the fringe patterns of (a3) and (b3), respectively.
In the self-compensation process, the influence function of the H-DM was measured and the TID was set as the compensation target. Figures 12(a3) and 12(b3) are the self-compensated residues of the TIDs shown in Figs. 12(a1) and 12(b1), respectively. It could be seen that only random aberration occurs and apparently there is no actuator-corresponding high-frequency distortions in the surface shape of the H-DM. As shown in Fig. 12(a3), the compensation residue under $\varDelta T ={-} 6.1^\circ{C}$ has a PV value of 0.2669 µm with an 88% reduction and an RMS value of 0.0493 µm with an 84% reduction. As shown in Fig. 12(b3), the residue under $\varDelta T ={+} 6.0^\circ{C}$ has a PV value of 0.2624 µm with an 88% reduction and an RMS value of 0.0380 µm with an 88% reduction. After self-compensation, the convex and concave TID that appeared in the surface shape of the H-DM under temperature variations are well corrected by the H-DM, which is consistent with simulation results. Thus, the TID was well compensated without any actuator-corresponding high-frequency distortion occurring, and the PV and RMS values of the TID were well reduced. Figures 12(a4) and 12(b4) exhibit the fringe patterns of the surface shape under $\varDelta T ={-} 6.1^\circ{C}$ and $\varDelta T ={+} 6.0^\circ{C}$, respectively. It could be concluded that both in the TID and in the self-compensated residues, there is no actuator-corresponding high-frequency distortion left and the TID could be well corrected by the H-DM itself.
5. Conclusion
This paper presents a hybrid connection structure DM (H-DM) used to effectively eliminate the actuator-corresponding high-frequency TID in the surface shape. Different from the conventional adhesive connection structure DM (A-DM), the H-DM adopts magnetic structure to connect the flexure plates and the mirror posts at 45 points, and adhesive connection structure at the rest 4 corner points. In the magnetic connection structure, the NdFeB magnet is employed to connect the mirror post upwards via adhesion, and connect the flexure plate downwards via magnetic force. Finite element models are built to investigate the TID characteristics of the A-DM and the H-DM. Simulation results show that for the A-DM, the change of environment temperature results in actuator-corresponding high-frequency distortion on the mirror surface shape that could not be self-compensated, while for the H-DM, no actuator-corresponding high-frequency distortion occurs in the surface shape after the temperature variation, and the TID could be well self-compensated. Furthermore, the wavefront compensation capability of the H-DM is investigated in simulation. The results reveal that the H-DM has good compensation capability for the 3rd to 14th Zernike mode aberrations. In the experiment, the effective driving voltage range and TID characteristics of a lab-manufactured H-DM are investigated. According to the experiment results, the lab-manufactured H-DM not only has a good initial surface shape without actuator-corresponding high-frequency distortions but also possesses an excellent stroke response within the driving voltage of −14V to +16V. The experiment results of temperature response indicate the TID of the H-DM caused by the environment temperature variation does not contain actuator-corresponding high-frequency distortion and could be well corrected by the H-DM itself.
Funding
National Natural Science Foundation of China (61775112); Laser Fusion Research Center, China Academy of Engineering Physics (6142A04180304).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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