Double drive modes unimorph deformable mirror for low-cost adaptive optics

Investigation of PZT Materials for Reliable Piezostack Deformable Mirror with Modular Design

This article presents a study of the electrophysical properties of a piezoceramic material for use in adaptive optics. The key characteristics that may be important for the manufacturing of piezoelectric deformable mirrors are the following: piezoelectric constants (d31, d33, d15), capacitance, elastic compliance values s for different crystal directions, and the dielectric loss tangent (tgδ). Based on PZT ceramics, the PKP-12 material was developed with high values of the dielectric constant, piezoelectric modulus, and electromechanical coupling coefficients. The deformable mirror control elements are made from the resulting material-piezoceramic combs with five individual actuators in a row. In this case, the stroke of the actuator is in the range of 4.1-4.3 microns and the capacitance of the actuator is about 12 nF.

Piezoelectric deformable mirror driven by unimorph actuator arrays on multi-spatial layers

In order to reduce the cost of the piezo actuator array deformable mirror (DM), a piezoelectric DM driven by unimorph actuator arrays on multi-spatial layers is proposed. The actuator density can be multiplied by increasing the spatial layers of the actuator arrays. A low-cost DM prototype with 19 unimorph actuators located on three spatial layers is developed. The unimorph actuator can generate a wavefront deformation up to 11 µm at an operating voltage of 50 V. The DM can reconstruct typical low-order Zernike polynomial shapes accurately. The mirror can be flattened to 0.058 µm in RMS. Furthermore, a focal spot close to Airy spot is obtained in the far field after the aberrations of the adaptive optics testing system being corrected.

Simulation and experimental investigation on the temperature-induced distortion characteristics of the hybrid connection structure deformable mirror

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.

Simulational and experimental investigation on the print-through high-frequency aberration of the shear-joint NIF deformable mirror

A shear-joint deformable mirror (DM) is important in the high-power laser systems such as the National Ignition Facility (NIF). In this paper, the print-through high-frequency aberration (PT-HIFA) that arises from adhesive curing and is corresponding to the actuator array is reported to exist in the initial surface shape of a lab-made shear-joint NIF DM. Meanwhile, experimental results show that the PT-HIFA has a great modulation on the laser intensity. This PT-HIFA could not be corrected effectively by the DM and may result in dangerous damage in the high power laser systems. In order to eliminate the PT-HIFA, we investigate the influence of the structural parameters on the PT-HIFA in a shear-joint DM through simulation and experiment. Simulational results show that the structural parameters have different influences on the PT-HIFA. In the experiment, by optimizing the structural parameters, the PT-HIFA could be eliminated in the shear-joint NIF DM and the modulation effect on the laser intensity could be removed. This investigation could help other researchers to determine appropriate structural parameters and to achieve a fine surface shape in their own shear-joint DMs.

Woofer-tweeter deformable mirror driven by combined actuators with a piezoelectric unimorph and stack for astronomical application

A woofer-tweeter deformable mirror (DM) driven by combined actuators with a piezoelectric unimorph and stack for astronomical applications is proposed. The piezoelectric unimorph "tweeter" part, made of a 200-μm-thick lead zirconate titanate film and 200-μm-thick silicon, has 234 separate elements for high-order correction. It is magnetically jointed with a seven-element "woofer" piezo-stack array, which is for low-order correction. The combined DM was fabricated and experimentally evaluated, showing a high resonant frequency near 1 kHz. The piezo-stack array together with the unimorph actuators enable the DM to produce wavefronts with RMS residue errors less than 20 nm. Experimental results indicate that the woofer-tweeter DM has the capability to compensate for the first 35 terms of Zernike aberrations with normalized RMS wavefront errors less than 20%. The woofer-tweeter DM has higher bandwidth than a conventional unimorph DM as well as simple structure, low cost, and good scalability, offering a potential alternative for large-aperture astronomical applications.

Theoretical and experimental research on temperature-induced surface distortion of deformable mirror

In a well-manufactured deformable mirror (DM), the temperature-induced distortion (TID) has been reported exist on the surface shape of the DM, when the working environment temperature is not equal to that of the manufacturing environment. The DM could not effectively correct this actuator-corresponding TID and the correction ability of the DM would be limited. In this paper, the the TID's essential mechanism is analyzed systematically based on the thermal stress characteristics. An efficient method based on an auxiliary temperature compensation module (TCM) and a hybrid closed-loop control algorithm are presented accordingly. A finite element model is built to evaluate the TID characteristics and the compensation capability of the TCM. In the simulation, by using the TCM, the the DM's improved surface shape does not contain the dynamic high-frequency distortion caused by the actuators tilt. In the experiment, which uses a designed TCM and a hybrid closed-loop control algorithm, a DM's TID is effectively depressed and a well-compensated DM surface shape is finally achieved.

Simulational and experimental investigation on the actuator-corresponding high-frequency aberration of the piezoelectric stacked array deformable mirror

The high-frequency aberrations (HIFAs) that are corresponding to the actuator array have been reported to exist on the initial surface shape of many deformable mirrors (DMs), such as the bimorph DM, the unimorph DM, the monomorph DM, and the membrane DM. This actuator-corresponding high-frequency aberration (AC-HIFA) could not be corrected effectively by the DM and would limit the correction ability of the DM. In this paper, we presented the AC-HIFA in a stacked array piezoelectric (PZT) DM, which may result in ghost damage that is dangerous in the high power laser system. More importantly, we investigated a solution through simulation and experiment that by using a mirror plate and a long thin post array, which were machined integrally from a piece of BK7 glass, the AC-HIFA could be eliminated completely. In addition, the structural parameters' influences on the AC-HIFA were investigated in the simulation, which could help other researchers to determine appropriate parameters of the mirror and the posts and to make a fine surface shape in their own DMs.

Unimorph mirror for adaptive optics in space telescopes

This paper presents a unimorph deformable mirror intended to be used as secondary corrector in space telescopes. The deformable mirror consists of a single-crystal silicon wafer (76.2 mm diameter, 500 μm thickness) covered with an optical coating on the front side and an array of 25 independent piezoelectric transducer (PZT) actuators acting in d₃₁ mode on the back side. The mirror is mounted on an isostatic support with three position linear actuators controlling the rigid-body motion. The first part of the paper presents the experimental results obtained with the manufactured prototype. The mirror was tested in terms of root mean square (RMS) wavefront error, open-loop long-term stability, voltage budget for active control, rigid-body actuation, reflectivity, and dynamic response. The prototype is fully compliant with the requirements set by the European Space Agency (ESA). The second part of the paper, purely based on numerical simulations, presents a robust way to face thermal distortion, inherent to unimorph architecture.

Development of a unimorph deformable mirror with water cooling

Deformable mirror (DM) used for intracavity compensation in high-power lasers should be able to withstand very high laser intensity. This paper proposes a water-cooled unimorph DM which can withstand the laser power up to 10 kW in thermal simulation. The proposed DM consists of an annular PZT layer and a circular Si layer which are glued together with edge clamped. All the 32 piezoelectric actuators are distributed around the correction area and on the front side of the DM. The cooling water flows through the back side of the DM and cools the mirror directly. This design realizes the physical separation of the actuators and the coolant. The experimental results of a fabricated DM prototype show that the DM can reproduce typical low-order aberrations accurately with relatively large amplitude. The wavefront PV amplitudes of the reproduced tip/tilt, astigmatism, defocus, trefoil and coma shapes for 15 mm aperture are about 40 μm, 24 μm, 18.7 μm, 10 μm and 6 μm, respectively.

Folded monocentric imager with deformable mirror focus

We describe a catadioptric monocentric imager using an elastic central element focused by deformation of a fold mirror. We show the design of 6 mm and 12 mm focal length F/2.8 achromats and compare design performance when focusing by translating the sensor, translating the fold mirror, and by spherically deforming the fold mirror. We tested the 12 mm focal length design using a diamond turned polystyrene element as the outer meniscus lens (and mechanical mount), filled with an optical gel in contact with the central aperture and fold mirror, and demonstrate focusing by mechanical deformation of the fold mirror. The resolution at the spherical image surface was inspected by optical relay imaging, yielding a best focus MTF50 of 52.6  lp/mm.

Unimorph deformable mirror for space telescopes: design and manufacturing

Large space telescopes made of deployable and lightweight structures suffer from aberrations caused by thermal deformations, gravitational release, and alignment errors which occur during the deployment procedure. An active optics system would allow on-site correction of wave-front errors, and ease the requirements on thermal and mechanical stability of the optical train. In the course of a project funded by the European Space Agency we have developed and manufactured a unimorph deformable mirror based on piezoelectric actuation. The mirror is able to work in space environment and is designed to correct for large aberrations of low order with high surface fidelity. This paper discusses design, manufacturing and performance results of the deformable mirror.

Concept and modeling analysis of a high fidelity multimode deformable mirror

Conventional deformable mirrors (DM) cannot meet the requirement of aberration controlling for advanced lithography tools. This paper illustrates an approach using the property that deformation of a thin plate is similar to optical modes to realize a high fidelity multimode deformable mirror whose deformation has characteristics of optical aberration modes. The way to arrange actuators is also examined. In this paper, a 36-actuator deformable mirror is taken as an example to generate low-order Zernike modes. The result shows that this DM generates the fourth fringe Zernike mode (Z4) defocus, and primary aberration Z5-Z8 with an error less than 0.5%, generates the fifth-order aberration Z10-Z14, and generates the seventh-order aberration Z17-Z20 with an error less than 1.1%. The high fidelity replication of the Zernike mode indicates that the DM satisfies the demand of controlling aberrations corresponding to the first 20 Zernike modes in an advanced lithography tool.

Robustness properties of hill-climbing algorithm based on Zernike modes for laser beam correction

A modified hill-climbing algorithm based on Zernike modes is used for laser beam correction. The algorithm adopts the Zernike mode coefficients, instead of the deformable mirror actuators' voltages in a traditional hill-climbing algorithm, as the adjustable variables to optimize the object function. The effect of the mismatches between the laser beam and the deformable mirror both in the aperture size and the center position was analyzed numerically and experimentally to test the robustness of the algorithm. Both simulation and experimental results show that the mismatches have almost no influence on the laser beam correction, unless the laser beam exceeds the effective aperture of the deformable mirror, which indicates the good robustness of the algorithm.

Performance of a thermal-piezoelectric deformable mirror under 6.2 kW continuous-wave operation

The thermal-piezoelectric deformable mirror (TPDM) is a device employed to compensate for laser-induced mirror deformation and thermal lensing in high-power optical systems. The TPDM setup is a unimorph deformable mirror with thermal and piezoelectric actuation properties. Laser-induced thermal lensing is compensated for by heating of the TPDM. We show that this mirror can be applied to high-power laser systems of up to 6.2 kW laser power and high power densities of up to 2  kW/cm2. The piezoelectric stroke of the single actuators is between 1.5 and 4 μm and is not reduced by either the absorbed laser power or mirror heating.

Active diffraction gratings: development and tests

We present the realization and characterization of an active spherical diffraction grating with variable radius of curvature to be used in grazing-incidence monochromators. The device consists of a bimorph deformable mirror on the top of which a diffraction grating with laminar profile is realized by UV lithography. The experimental results show that the active grating can optimize the beam focalization of visible wavelengths through its rotation and focus accommodation.