Baoqing Li

Design, fabrication and characterization of a bulk-PZT-actuated MEMS deformable mirror

This paper describes the design, fabrication and characterization of a bulk-PZT-actuated MEMS deformable mirror (DM). An analytical model was employed to optimize the DMs structure. The fabrication techniques for PZT thick film actuators were also experimentally explored, including the bonding of bulk PZT ceramics and a silicon wafer by epoxy resin, and the thinning of the bulk PZT ceramics using a wet-etching method. A 10 × 10 array of 1.75 mm × 1.75 mm PZT thick film actuators was successfully fabricated. The PZT actuators showed a stroke of about 4.5 µm at 100 V. When a 36 µm thick silicon membrane mirror was assembled, the measured mirror deflection at 100 V was approximately 3.8 µm. The assembled DM showed an operating frequency bandwidth of 21 kHz and an influence function of approximately 30%. The displacement hysteresis was greatly eliminated by using the method of staying on the same segment.

Hill-climbing algorithm based on Zernike modes for wavefront sensorless adaptive optics

Abstract. To solve the local optimum problem, a modified hill-climbing algorithm based on Zernike modes is presented for wavefront sensorless adaptive optics. This algorithm adopts the Zernike mode coefficients, instead of the actuators’ voltages in a traditional hill-climbing algorithm, as the adjustable variables to optimize the object function. First, the principle of the algorithm is described. Then the efficiency of the modified and traditional hill-climbing algorithms are analyzed numerically and experimentally using a He-Ne laser beam shaping system with a 37-element unimorph deformable mirror. The results of both simulations and experiments demonstrate that the modified hill-climbing algorithm can eliminate the local optimum problem with a fast speed of about 100 iterations.

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.

Vortex-induced Vibration Control by Micro Actuator

The control of vortex-induced vibration of two side-by-side circular cylinders in a cross flow is carried out experimentally. One cylinder is elastically supported and the other is fix-supported at both ends. The two cylinders vibrate under the action of the unsteady flow-induced forces. A micro actuator is embedded on the surface of each cylinder to perturb the boundary layer. The spacing ratio is set at 1.2. The measurement shows that the structural vibration can be suppressed significantly when the reduced excitation frequency is around 2.655.

Characterization of close-loop performance of double drive modes unimorph deformable mirror

Unimorph deformable mirrors are attractive in adaptive optics system due to their advantages of simplicity, compact, low cost and large stroke. In this paper, a double drive modes unimorph deformable mirror is discussed, which comprises a 100 μm thick PZT layer and a 200 μm thick silicon layer. This deformable mirror (DM) can achieve two different directions deformation of concave and convex driven by only positive voltage. The dual direction maximum defocus deformations are -14.3 μm and 14.9 μm. The close-loop performance of this DM is also tested in an experimental adaptive optics system based on Hartman-Shack wavefront sensor. In experiments, the DM is controlled by the steepest descent algorithm (SD) to corrected the aberrations in a close-loop manner. The ability of this DM of correction for the system aberration and reconstruction for the low order Zernike mode aberration is tested. The root mean square (rms) value of the system aberration after close-loop correction is about 28 nm. The reconstruction results for most low order Zernike mode aberrations have a relative error less than 10%.

Double drive modes unimorph deformable mirror for femtosecond laser beam wavefront correction

Once a laser beam suffers from wavefront aberrations, the intensity of the focal spot degrades and the shape of the focus spot distorts. The same problem also exists in femtosecond laser fabrication system. The aberrations in the femtosecond laser fabrication system contain two main components: system aberrations and aberrations from the refractive index mismatch problem. Recently, adaptive optics (AO) has been used for laser beam aberrations correction to improve the light beam quality. In this paper, we introduce an adaptive optics system with double drive modes unimorph deformable mirror (DM) into the femtosecond laser fabrication system. In the experiments, the hill-climbing algorithm based on Zernike modes is used to control the DM to correct the aberrations in the close-loop manner. After correction for system aberrations and the refractive index mismatch aberrations, the shape and maximum intensity of the focal laser spot is much improved.

Closed-loop performance of double drive modes unimorph deformable mirror

Abstract: Unimorph deformable mirrors are attractive in adaptive optics system due to their advantages of simplicity, compact, low cost and large stroke. This paper reports the close-loop performance of a double drive modes unimorph deformable mirror.

Characterization of a 61-element bulk-PZT thick film deformable mirror and generation of Zernike polynomials

This paper describes the characteristics of a 61 element piezoelectric deformable mirror (DM) based on bulk-PZT thick film and the generation of Zernike polynomials. This device consists of a continue silicon mirror supported by 61 element piezoelectric unimorph actuators which are arranged in a hexagonal grid with spacing of 5mm. Measurements of the displacement using a laser Doppler vibrometer demonstrated that the stroke of DM was 3.8μm at 100 volt with a displacement hysteresis of approximately 9% and the operating bandwidth was greater than 10KHz. A custom phasing-shifting interferometer based on Twyman-Green interferometer was developed to measure the mirror surface shape in response to the applied voltage. The influence function of the mirror measured accorded with Gaussian function with inter-actuator coupling of approximately 5%, which was similar to the traditional piezoelectric DM with stacked actuators. To examine the ability of the mirror to replicate optical aberrations described by the Zernike polynomials, low-order Zernike modes were reproduced by calculating the voltage on each actuator using an influence function matrix. The measurement demonstrated that the deformable mirror could produce the Zernike modes up to the ninth term. Considering the low-voltage actuation as well as the capability for miniaturization of the actuator size, deformable mirror actuated by bulk-PZT thick film has a potential application for low-cost adaptive optics.

Experimental evaluation of a positive-voltage-driven unimorph deformable mirror for astronomical applications

Abstract. A modified low-cost unimorph deformable mirror (DM) driven only by positive voltages for atmospheric turbulence compensation is presented. The 214 patterned inner actuators generate convex deformations for aberration correction, while one outer ring actuator generates an overall concave bias. To evaluate the aberration correction capability of the proposed DM, the iterative reconstruction of Zernike aberrations and correction were performed in an adaptive optics test system. The experimental results indicate that the fabricated DM has an excellent aberration correction capability, particularly matching the first 20 term Zernike aberrations with the normalized residual root-mean-square (RMS) error <5%. Furthermore, the random atmospheric turbulence aberrations were simulated based on Karhunen–Loève coefficients and reconstructed using the fabricated DM. The simulative and experimental results show that the atmospheric turbulence aberrations can be steadily compensated with λ/40 (λ=2.2u2009u2009μm) RMS residual error, indicating the prospect for atmospheric applications.

Double Drive Modes Unimorph Deformable Mirror with high actuator count for astronomical application

Unimorph deformable mirrors are attractive in adaptive optics system due to their advantages of simplicity, compact, low cost and large stroke. In this paper, a double drive modes unimorph deformable mirror is presented, which comprises a 200 μm thick PZT layer and a 400 μm thick silicon layer. This deformable has 214 inner actuators in the 50-mm active aperture, which are for the aberration correction and a outer ring actuator for generating an overall defocus bias. An analytical model based on the theory of plates and shells is built to predict the behavior of the deformable mirror. The stroke of the deformable mirror is tested in the experiments. In order to test the performance for aberration correction, the deformable mirror is used to correct the aberration from its imperfect initial mirror surface in the close-loop manner. The root-mean-square value of the mirror surface after the close-loop correction for ten iterations is about λ/40, which indicates this deformable mirror has a good aberration correction performance. This DM has the potential to be used for astronomical adaptive optics.