Piezo Stage Research Articles

Lasing stability of quasi-CW driven cholesteric liquid crystal lasers

Lasing stability in dye-doped cholesteric liquid crystal (ChLC) lasers largely depends on repetition frequency of the pumping laser, and therefore stabilization of output power under quasi-continuous-wave operation with high repetition frequency greater than 1 kHz is a tough challenge. In this study, a significant improvement of lasing stability in dye-doped ChLC lasers is obtained by lowering dye concentration, increasing cell gap, and moving LC cells with a piezo stage. Considering the balance between lasing stability and slope efficiency, the optimal conditions regarding cell gap and dye concentration in a ChLC laser doped with Pyrromethene 597 are determined to be 50 μm and 0.3 wt%, respectively. Under severe conditions of high repetition frequencies of 1 kHz and 10 kHz, the optimized ChLC laser demonstrates durability retaining 100% and more than 72% against the initial output power for 30 min, respectively.

Multilevel phase supercritical lens fabricated by synergistic optical lithography

Abstract The advent of planar metalenses, including the super-oscillatory lens (SOL) and the supercritical lens (SCL) with distinctive interference properties, has profoundly impacted on the long-lasting perception of the far-field optical diffraction limit. In spite of its conspicuous success in achieving marvelously small focal spots, the planar metalens still faces tough design and fabrication challenges to realize high focusing efficiency. In this work, we demonstrated a dual-mode laser fabrication technique based on two-photon polymerization for realizing the multilevel phase SCL with focusing efficiency spiking. Synergistically controlling two types of movement trajectory, which is implemented with the piezo stage and the scanning galvo mirror, enables the fabrication of complicated structures with sub-diffraction-limit feature size. By utilizing such advantage, SCLs with discretized multilevel phase configurations are explicitly patterned. The experimental characterization results have shown that a four-level phase SCL can focus light into a sub-diffraction-limit spot with the lateral size of 0.41 λ/NA (NA is the numerical aperture), while achieve the focal spot intensity and the energy concentration ratio in the focal region 7.2 times and 3 times that of the traditional binary amplitude-type SCL with the same optimization conditions, respectively. Our results may release the application obstacles for the sub-diffraction-limit planar metalens and enable major advances in the fields from label-free optical super-resolution imaging to high precision laser fabrication.

Recurrent-Neural-Network-Based Predictive Control of Piezo Actuators for Trajectory Tracking

Precise trajectory tracking of piezo actuators (PEAs) in real time is essential to high-precision systems and applications. However, the real-time tracking accuracy is rather limited as the PEA cannot be accurately modeled over large bandwidth and displacement range due to its nonlinearities. In this article, we propose to use recurrent-neural-network (RNN) to model the PEA system and develop a nonlinear predictive controller for PEA trajectory tracking. Considering the computation efficiency, first, an RNN is trained to model the nonlinear dynamics of the PEA system at high-frequency range. Then, a second-order linear model is proposed to account for the PEA low-frequency dynamics. Therefore, the PEA dynamics is modeled by the nonlinear model consisting of the RNN and the linear model, which is further used for nonlinear predictive control of the displacement. To increase the prediction accuracy, an unscented Kalman filter is designed to estimate the states of the nonlinear model. The nonlinear predictive control problem is solved based on a gradient descent algorithm, in which a method for analytically calculating the gradient of the cost function is developed. The proposed technique was experimentally implemented on a nano piezo stage for demonstration and its performance was compared with that of a PID controller. The accuracy of an iterative learning control approach was used as a benchmark for comparison as well. The results showed that high precision trajectory tracking of PEAs in real time can be achieved using the proposed technique.

Ultrafast imaging of soft materials during shear flow

The direct imaging of flow induced microstructural changes in complex fluids can have advantages over the use of scattering methods, since localized phenomena can be observed directly and more mechanistic insights can be obtained. This is useful in particular for materials with hierarchical or multiscale structures such as aggregated dispersions. Rheoconfocal instruments are ideally suited for this purpose but were, as yet, limited to relatively low imaging rates. In the present work, a stress-controlled rheometer was coupled to a fast scanning, instant structured illumination confocal microscope which uses a multi-array illumination and detection scheme. A second motor is integrated in a custom-made rheoconfocal instrument to achieve the counter-rotation of the lower glass plate. The resulting stagnation plane can be moved within the shearing gap in real time and allows the stable imaging of micro-structural features under steady shear. Velocity profiles were measured to validate the performance of the mechanical components, using particle image velocimetry on a sterically stabilized suspension. Structured illumination optics yielded an excellent inplane spatial resolution, while the multipoint scanning allows speeds as high as 1000 frames per second at full frame resolution. However, for rheological studies the 3D structure should ideally be resolved. The mechanical refocusing using a fast piezo stage at high speeds led to deformations of the lower thin glass plate. To circumvent this bottleneck, a focus-tunable lens was incorporated in the setup to acquire 3D image volumes at video rates. The excellent combination of temporal and spatial resolution under flow is demonstrated here using selected results from aggregated colloidal dispersions. The microstructure of a model depletion gel is studied over a broad range of shear rates under strong to moderate flow conditions. The ability to measure rheological properties while imaging the time-dependent microstructure is demonstrated with particles dispersed in a more viscous PDMS matrix. Transient rheology is reported simultaneously with high resolution imaging of the microscopic structural recovery. This novel tool enables the direct imaging of rheologically complex materials under conditions relevant to processing, to elucidate the physical phenomena underlying nonlinear rheology and thixotropy.

High-resolution interference microscopy of binary phase diffractive optical elements

In this work, we apply the high-resolution interference microscopy technique to investigate the intensity and phase of light transmitted through a binary phase element. Using a combination of simulation and experimental results, we identify specific features in the intensity and phase maps of the transmitted light that are associated with the positions of the edges of the ridges and grooves in the phase element. Specifically, we identify these features to be minima in the intensity recordings, as well as phase jumps and in some cases phase singularities in the phase maps. We focus on the former two features, as they can reliably be observed in a single z-plane intensity or phase image, respectively, focused at the top of the structure ridges. Using the edge locations extracted from the intensity and phase profiles, we estimate the dimensions of the structure ridges and grooves as well as measure the displacement of the sample on a piezo stage. With both methods, the absolute width of the ridges and grooves is measured with an accuracy of approximately 220 nm, and the sample displacement is detected to approximately 50 nm resolution.

In-situ measurement of SiC/Si interfacial tensile strength of reaction bonded SiC/Si composite

Reaction bonded SiC/Si (RBSC) ceramic composites have been extensively used in applications calling for outstanding mechanical and thermal properties such as high energy laser (HEL) mirrors, advanced armors and semiconductor fabrication platforms. The phase components of RBSC generally include preexisting α-SiC, reaction formed β-SiC, and residual crystalline silicon (c-Si). In addition, however, a minor phase, amorphous Si (a-Si), is often observed near the SiC/Si interface due to local temperature and structural fluctuations that can occur during the composite fabrication. An in-situ tensile testing system composed of piezo stages, force sensor and specimen holders is integrated and accommodated inside a field emission scanning electron microscope (FE-SEM) for the nanoscale tensile testing. Tensile specimens containing the SiC/Si interface are successfully prepared by focused ion beam (FIB). The interfacial tensile strength is directly measured for the first time and it reaches as high as 7.1 GPa. The a-Si phase on the fracture surfaces is confirmed by high resolution transmission electron microscopy (HRTEM).

Microscale, scanning defocusing volumetric particle-tracking velocimetry

We present a novel defocusing particle tracking velocimetry (PTV) method for micro-fluidic systems. This method delivers 3-dimensional 3-component (3D3C) flow measurements, and does not require an additional calibration procedure to obtain the relationship between particle out-of-plane position and its diameter/intensity. A micro-fluidic device is mounted on a nano-positioning piezo stage that sweeps periodically in the out-of-plane direction. A high-speed camera is synchronized with the stage to capture oversampled two-dimensional microscopy images at different out-of-plane positions. 3D intensity volume is formed by stacking those 2D images. Flow tracers are identified from the intensity volume by a 3D Hessian filter, and segmented by erosion–dilation dynamic thresholding. Fitting of each identified-particle to a defocusing intensity model gives the parameters used in the hybrid algorithm of particle image velocimetry and a generalized multi-parametric PTV. Artificial image data, generated from direct numerical simulations of flow through porous media, are used for error analysis. When compared with classic nearest neighbor tracking our method shows improvements on tracking reliability by 2.5–12%, with seeding density as high as 1.6e−3 particles per voxel. Both mean and rms errors are improved by 80–95% and 49–74%, respectively. An application to micro-fluidic devices is presented by measuring the steady-state flow through a refractive-index-matched randomly-packed glass bead channel. The presented method will serve as a powerful tool for probing flow physics in micro-fluidics with complex geometries.

A MEMS-based measurement system for evaluating the force-length relationship of human induced pluripotent stem cell-derived cardiomyocytes adhered on a substrate

This paper reports on a method for evaluating the force-length relationship of adhering human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) on the substrate using a measurement system comprising of a micromachined movable plate and a piezoresistive force probe. The cells on the plate are stretched by pushing the movable plate with the piezoresistive cantilever, which is actuated by a piezo stage. The twitch forces and the applied stretch are measured quantitatively with the piezoresistive cantilever. The results demonstrated that the twitch forces of the hiPSC-CMs increased when a stretch was applied. This evaluation method improves the understanding of the intrinsic force-length relationship of hiPSC-CMs at the cellular scale.

Brownian Motion using a Piezo Actuated Microscope Stage

Single particle tracking (SPT) is an important collection of techniques and algorithms for the study of biomolecular transport and understanding the role of the dynamics of small molecules, proteins, and other organic material in health and disease on a cellular level. However successful these techniques are, it is difficult to compare them directly, especially as one is most interested in their performance on a specific experimental setup. Here, we demonstrate through Monte-Carlo simulation and experiment, the ability of a piezo actuated microscope stage to replicate faithfully a discrete-time sampled Brownian motion trajectory. Simulations consisted of an exact discrete model of the piezo stage including closed-loop control, the input Brownian motion trajectory, and feed forward model inverse control. A range of stage response times, discrete time step durations, and diffusion constants were modeled. The output of the system was compared to the input using mean squared displacement (MSD), Kullback-Leibler divergence comparing the single time step displacement distributions, and stage tracking error. We found that MSD was preserved over a large range of stage response times, discrete time step durations, and diffusion constant values. Physical experiments consisted of an epifluorescent microscope taking widefield images of quantum dots fixed to a microscope slide moved using the piezo actuated microscope stage following a Brownian motion trajectory control input. We envision this work will allow for the characterization and evaluation of SPT microscopes and localization algorithms by moving fixed fluorophores through a known Brownian motion trajectory to provide a known ground truth in an experimental setting. Future work includes using other biologically relevant motion models such as confined diffusion and elastic tethering as the motion model, and a comparison of SPT microscope configurations and estimation algorithms through simulated motion with a piezo stage.

Optimized alignment of X-ray mirrors with an automated speckle-based metrology tool.

X-ray mirrors are widely used in beamlines and laboratories as focusing or collimating optics. As well as the highly accurate processes used to fabricate them, optimized alignment of X-ray mirrors also plays an important role in achieving an ideal X-ray beam. Currently, knife-edge scans are the most often used method for aligning X-ray mirrors, which can characterize the focal size and tune the alignment iteratively. However, knife-edge scanning provides only one-dimensional information and this method suffers from being time-consuming and requiring a high-resolution piezo translation stage. Here we describe a straightforward and non-iterative method for mirror alignment by measuring the relationship between the tilt aberration and the misaligned pitch angle, which is retrieved by an at-wavelength metrology technique using a randomly shaped wavefront modulator. Software and a graphical user interface have been developed to automate the alignment process. Combining the user-friendly interface and the flexibility of the at-wavelength metrology technique, we believe the proposed method and software can benefit researchers working at synchrotron facilities and on laboratory sources.

Fast positioning for X-ray scanning microscopy by a combined motion of sample and beam-defining optics.

Scanning X-ray microscopy such as X-ray ptychography requires accurate and fast positioning of samples in the X-ray beam. Sample stages often have a high mobile mass as they may carry additional mechanics or mirrors for position measurements. The high mobile mass of a piezo stage can introduce vibrations in the setup that will lead to imaging quality deterioration. Sample stages also require a large travel range which results in a slow positioning step response and thus high positioning overhead. Moving lightweight X-ray optics, such as focusing Fresnel zone plates, instead of the sample can improve the situation but it may lead to undesired variations in the illumination probe which may result in reconstruction artifacts. This paper presents a combined approach in which a slow sample stage mechanism covers the long distance range for a large field of view, and a light-weight optics scanner with a small travel range creates a superimposed motion to achieve a fast step response. The step response in the ptychographic tomography instrument used was thereby improved by an order of magnitude, allowing for efficient measurement without loss of imaging quality.

Spectral domain optical coherence tomography with sub-micrometer sensitivity for measurement of central corneal thickness

We demonstrated a method for measurement of central corneal thickness (CCT) with a sub-micrometer sensitivity using a spectral domain optical coherence tomography system without needing a super broad bandwidth light source. By combining the frequency and phase components of Fourier transform, the method is capable of measurement of a large dynamic range with a high sensitivity. Absolute phases are retrieved by comparing the correlations between the detected and simulated interference fringes. The phase unwrapping ability of the present method was quantitatively tested by measuring the displacement of a piezo linear stage. The human CCTs of six volunteers were measured to verify its clinical application. It provides a potential tool for clinical diagnosis and research applications in ophthalmology.

Quantitative phase imaging using spectral domain phase microscopy without phase wrapping ambiguity.

We demonstrate a method for quantitative phase (QP) imaging without 2π ambiguity using a spectral domain phase microscopy system. The method is capable of QP measurement of a large dynamic range with a high sensitivity. We determine an integer multiple of 2π to correct wrapped phases by calculating the phase shift difference between the detected interference fringe and simulated fringes. The presented method is quantitatively verified by measuring the vibration generated by a piezo linear stage and mapping the surface topography of a slanted mirror. QP imaging of red blood cells is also performed to demonstrate the method's capacity and application in biological imaging.

A novel inverse modeling control for piezo positioning stage

This paper focuses on the application of a novel inverse nonlinear autoregressive with exogenous input (NARX) structure and a fuzzy inference system on position control of a piezo positioning stage (PPS) system. The highly relationship between input voltage and output displacement is thoroughly modeled by using the inverse fuzzy NARX model-based identification process with the experiment training data. The unknown parameters of the proposed NARX fuzzy model was obtained base on a hybridization between backtracking search algorithm and gradient descent technique. A combination between fuzzy propositional–integral–derivative controller and inverse modeling feedforward controller was then applied to control PPS with several challenged working condition. The results show efficiency of the proposed method for the PPS system in terms of high tracking precision and excellent dynamic performance.

Focus on software, data acquisition, and instrumentation

Focus on software, data acquisition, and instrumentation

Visual Tracking of Six-Axis Motion Rendering Ultraprecise Visual Servoing of Microscopic Objects

This paper presents visual tracking of six-axis motion that renders ultraprecise visual servoing of microscopic objects. The principle of three-axis out-of-plane motion tracking is based on three-degree-of-freedom laterally sampled white light interferometry and real-time unbiased nonlinear parameter estimation of orientation-dependent interference fringes. A continuously shifting model of the warped shadow projection image of the object is derived and employed to achieve unbiased real-time image registration for three-axis in-plane motion tracking. Processing of images and parameter estimation are synchronized with real-time image acquisition to realize six-axis motion tracking. The ability of the proposed approach to eliminate measurement bias associated with previous approaches is validated through computer simulations. The visual motion tracking system is integrated with a six-axis compliant piezo stage to establish a six-axis visual servo control system. Experimental results are presented and used to examine positioning resolution and to demonstrate superior precision achieved for long-range motion tracking.

Adaptive-gain fast nonsingular terminal sliding mode for position control of a piezo positioning stage

This article proposed a variable gain fast terminal sliding mode controller with an estimator of the uncertainty model for a piezo positioning stage system. The designed terminal sliding mode control has some advantages over the linear sliding mode control such as fast convergence and chattering reduction while maintains its robustness to the uncertainties. Next, an indirect technique is developed to enable the elimination of the singularity problem corresponding to initial terminal sliding mode control. In addition, a cerebellar model articulation controller is carried out to estimate the nonlinear dynamics of the piezo positioning stage. To deal with unknown bounds of uncertainties and disturbances, the proposed scheme consists of using online tuning control gains that ensure the establishment of a real terminal sliding mode in a finite time. Moreover, a fuzzy logic scheme is presented to smooth out the discontinuity part of the control signal, hence improve the control performance. Stability analysis of closed loop system is provided using the Lyapunov function method. Experiment results are presented to evaluate the effectiveness of the designed control approach.

High accuracy step gauge interferometer

Step gauges are convenient transfer standards for the calibration of coordinate measuring machines. A novel interferometer for step gauge calibrations implemented at VTT MIKES is described. The four-pass interferometer follows Abbe’s principle and measures the position of the inductive probe attached to a measuring head. The measuring head of the instrument is connected to a balanced boom above the carriage by a piezo translation stage. A key part of the measuring head is an invar structure on which the inductive probe and the corner cubes of the measuring arm of the interferometer are attached. The invar structure can be elevated so that the probe is raised without breaking the laser beam. During probing, the bending of the probe and the interferometer readings are recorded and the measurement face position is extrapolated to zero force. The measurement process is fully automated and the face positions of the steps can be measured up to a length of 2 m. Ambient conditions are measured continuously and the refractive index of air is compensated for. Before measurements the step gauge is aligned with an integrated 2D coordinate measuring system. The expanded uncertainty of step gauge calibration is .

An efficient fluorescent single-particle position tracking system for long-term pulsed measurements of nitrogen-vacancy centers in diamond

A simple and convenient design enables real-time three-dimensional position tracking of nitrogen-vacancy (NV) centers in diamond. The system consists entirely of commercially available components (a single-photon counter, a high-speed digital-to-analog converter, a phase-sensitive detector-based feedback device, and a piezo stage), eliminating the need for custom programming or rigorous optimization processes. With a large input range of counters and trackers combined with high sensitivity of single-photon counting, high-speed position tracking (upper bound recovery time of 0.9 s upon 250 nm of step-like positional shift) not only of bright ensembles, but also of low-photon-collection-efficiency single to few NV centers (down to 103 s-1) is possible. The tracking requires position modulation of only 10 nm, which allows simultaneous position tracking and pulsed measurements in the long term. Therefore, this tracking system enables measuring a single-spin magnetic resonance and Rabi oscillations at a very high resolution even without photon collection optimization. The system is widely applicable to various fields related to NV center quantum manipulation research such as NV optical trapping, NV tracking in fluid dynamics, and biological sensing using NV centers inside a biological cell.

Optimization of the GPU-based data evaluation for the low coherence interferometry

Abstract Optical interferometers as non-contact measurement devices are very desirable for the measurement of surface roughness and topography. Compared to phase shifting interferometers (PSIs) with a limited measurement range and a scan step of maximum λ/4, the optical interferometers like low coherence interferometers (LCIs) evaluating the degree of fringe coherence allow a larger vertical measurement range. Their vertical measurement range is only limited by the scan length allowed by the linear piezo stage and the coherence length of the light source. To evaluate the obtained data for a large range, the common LCIs require much computation time. To overcome this drawback, we present an evaluation algorithm based on the Hilbert-Transform and curve fitting (Levenberg–Marquardt algorithm) using Compute Unified Device Architecture (CUDA) technology, which allows parallel and independent data evaluation on General Purpose Graphics Processing Unit (GPGPU). Firstly, the evaluation algorithm is implemented and tested on an in-house developed LCI, which is based on Michelson configurations. Furthermore, we focus on the performance optimization of the GPU-based program using the different approaches to further achieve efficient and accurate massive parallel computing. Finally, the performance comparison for evaluating measurement data using different approaches is discussed in this paper.