Peak-to-valley Research Articles

Development and characterization of silicon dioxide clad silicon carbide optics for terrestrial and space applications

Silicon carbide (SiC), a non-oxide ceramic with superior thermo-mechanical stability, chemical and radiation resistive properties, finds extensive utilization in optical instruments for terrestrial and space applications. However, its inherent porous texture (α-HCP) becomes a deterrent for high-performance optical telescopes, although several techniques of surface alterations over sintered or reaction-bonded SiC are available. In the present work, the physical vapour deposition (PVD) technique is adopted to deposit a thick (∼5 μm) Silicon dioxide (SiO 2 ) clad layer on a sintered and optically polished SiC (SSiC) substrate. SiO 2 clad layer coated SSiC (SDO-SSiC) substrate reduces the surface porosity of SSiC which is found to be suitable for optical mirror application. Finally, an Al based reflective and oxides protective coatings are deposited on SiO 2 clad layer to achieve reflective behaviour. The surface figure of 75 nm PV (peak-to-valley) and less than 2 nm surface micro-roughness values are achieved which meets the stringent optical telescope specifications for terrestrial and space applications. The structural and nano-mechanical properties of presently developed SiO 2 clad layer-based SiC telescopic mirror have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-Ray Analysis (EDX), atomic force microscopy (AFM), and nanoindentation techniques. The optical properties are investigated by optical profilometry and wavelength based spectrometric (both in visible and infrared ranges) techniques. Finally, space worthiness studies viz., thermo-vacuum, thermal storage, thermal shock and relative humidity tests have been carried out successfully. The process of cleaning, grinding and polishing at each substrate preparation stage and coatings are also reported comprehensively.

Generation of Reference Softgauges for Minimum Zone Fitting Algorithms: Case of Aspherical and Freeform Surfaces.

Optical aspherical lenses with high surface quality are increasingly demanded in several applications in medicine, synchrotron, vision, etc. To reach the requested surface quality, most advanced manufacturing processes are used in closed chain with high precision measurement machines. The measured data are analysed with least squares (LS or L2-norm) or minimum zone (MZ) fitting (also Chebyshev fitting or L∞-norm) algorithms to extract the form error. Performing data fitting according to L∞-norm is more accurate and challenging than L2-norm, since it directly minimizes peak-to-valley (PV). In parallel, reference softgauges are used to assess the performance of the implemented MZ fitting algorithms, according to the F1 algorithm measurement standard, to guarantee their traceability, accuracy and robustness. Reference softgauges usually incorporate multiple parameters related to manufacturing processes, measurement errors, points distribution, etc., to be as close as possible to the real measured data. In this paper, a unique robust approach based on a non-vertex solution is mathematically formulated and implemented for generating reference softgauges for complex shapes. Afterwards, two implemented MZ fitting algorithms (HTR and EPF) were successfully tested on a number of generated reference pairs. The evaluation of their performance was carried out through two metrics: degree of difficulty and performance measure.

Modeling and experimenting of disc hydrodynamic polishing process for ultra-smooth surfaces

The machining of ultra-smooth surfaces has always been a significant research topic in the fields of optics fabrication. In this paper, disc hydrodynamic polishing (DHDP) process with a novel polishing tool is proposed to produce ultra-smooth surface with sub-nanometer scale surface roughness (Ra < 1 nm) and undamaged sub-surface. In order to explore the principle of the ultra-smooth surface generation in the DHDP process, the material removal mechanism and evolution mechanism of micro-morphology of polished surface were firstly studied by preliminary experiments. Then, theoretical material removal and surface simulation modeling are established on the basis of computational fluid dynamics (CFD), fractal theory for rough surfaces, particle contact mechanics, and erosion mechanisms in DHDP. Finally, a series of polishing experiments was performed on fused quartz glass to verify the theoretical model, which could be applied to predict surface topography and guide the ultra-smooth surface polishing process. The results indicated that the peak-to-valley (PV) value and waviness of the workpiece were reduced in the nanometer level under the erosion action of abrasive particles. Moreover, the surface roughness Ra and PV values decreased with the increase of rotation speed and polishing time. The analysis of power spectral density (PSD) results illustrates that high rotation speed of tool and polishing time (less than 60 min) can reduce the high-spatial-frequency errors distinctly, and the longer polishing time (above 60 min) can also influence on the mid-spatial-frequency errors.

Simulation and measurement of systematic errors of stitching interferometry for high precision X-ray mirrors with large radius of curvature.

To accurately measure the surface figure of curved mirrors with large radius of curvature (RoC) using stitching interferometry methods, three types of measurement errors are systematically studied, including retrace error, defocusing error within a single subaperture, and stitching angle error among different subapertures. It was found that part of the retrace error caused by the mismatch between the reference wavefront and reflected wavefront has little effect, while the overall retrace error, including the influence of the imperfect optical elements, will cause an error of 1-2 nm RMS within a single subaperture. Defocusing error will enlarge the error due to amplification of optical path error caused by the deviation of the position of the CCD. Because the error is mainly in the edge area, a slope threshold, which controls the maximum surface slope of each subaperture, can be optimized to reduce the effect of the defocus on stitching measurement error. Constant angle error among neighboring subapertures has the biggest accumulation effect on the final stitched figure. For the spherical mirror with RoC of 100 m of 80mm×40mm, the error of the one-dimensional residual profile is 4.67 nm PV, assuming a constant angle error of 2×10-7rad. For the elliptical mirror with RoC of 60-140 m, it is more than 15 nm peak-to-valley (PV). It is because the profile difference caused by constant angle error is closer to a circle, which can be mostly removed after subtraction of a best-fit sphere. Based on the above error analysis, the developed algorithm-based stitching method was used to measure an elliptical cylindrical mirror of 74mm×40mm with RoC of 60-140 m, and the result was compared with a slope measurement instrument from the Beijing Synchrotron Radiation Facility. After removing the best fitting ellipse profile, the one-dimensional difference between the two results is only 0.77 nm RMS, which demonstrated good measurement accuracy.

High-Accuracy Surface Topography Manufacturing for Continuous Phase Plates Using an Atmospheric Pressure Plasma Jet.

The continuous phase plate (CPP) is the vital diffractive optical element involved in laser beam shaping and smoothing in high-power laser systems. The high gradients, small spatial periods, and complex features make it difficult to achieve high accuracy when manufacturing such systems. A high-accuracy and high-efficiency surface topography manufacturing method for CPP is presented in this paper. The atmospheric pressure plasma jet (APPJ) system is presented and the removal characteristics are studied to obtain the optimal processing parameters. An optimized iterative algorithm based on the dwell point matrix and a fast Fourier transform (FFT) is proposed to improve the accuracy and efficiency in the dwell time calculation process. A 120 mm × 120 mm CPP surface topography with a 1326.2 nm peak-to-valley (PV) value is fabricated with four iteration steps after approximately 1.6 h of plasma processing. The residual figure error between the prescribed surface topography and plasma-processed surface topography is 28.08 nm root mean square (RMS). The far-field distribution characteristic of the plasma-fabricated surface is analyzed, for which the energy radius deviation is 11 μm at 90% encircled energy. The experimental results demonstrates the potential of the APPJ approach for the manufacturing of complex surface topographies.

Characteristics of wavefront correction using stacked liquid lens based on electrowetting-on-dielectric

A stacked liquid lens based on electrowetting-on-dielectric (EWOD) is designed to analyze the ability of correcting the distorted wavefront caused by curvature, tilt, and piston. The model of the stacked liquid lens is constructed by COMSOL software which is used to simulate the change of liquid interface with different voltage combinations, and the change range of the interface. The correction ability of the stacked liquid lens at a certain point in the wavefront is assessed from wavefront image and point spread function (PSF) is got by ZEMAX software. The results show that different types of distorted wavefront can be compensated via the stacked liquid lens. The peak-to-valley (PV) value decreases from 19.7853 to 0.18, and the root mean square (RMS) value is down from 5.6638 to 0.0355. Concurrently, the Strehl ratio (SR) increased from near 0 to 0.962. The related research results have broad prospects in the field of wavefront correction.

High-Precision Machining Method of Weak-Stiffness Mirror Based on Fast Tool Servo Error Compensation Strategy.

Weak-stiffness mirrors are widely used in various fields such as aerospace and optoelectronic information. However, it is difficult to achieve micron-level precision machining because weak-stiffness mirrors are hard to clamp and are prone to deformation. The machining errors of these mirrors are randomly distributed and non-rotationally symmetric, which is difficult to overcome by common machining methods. Based on the fast tool servo system, this paper proposes a high-precision machining method for weak-stiffness mirrors. Firstly, the clamping error and cutting error compensation strategy is obtained by analyzing the changing process of the mirror surface morphology. Then, by combining real-time monitoring and theoretical simulation, the elastic deformation of the weak-stiffness mirror is accurately extracted to achieve the compensation of the clamping error, and the compensation of the cutting error is achieved by iterative machining. Finally, a weak-stiffness mirror with a thickness of 2.5 mm was machined twice, and the experimental process produced a clamping error with a peak to valley (PV) value of 5.2 µm and a cutting error with a PV value of 1.6 µm. The final machined surface after compensation had a PV value of 0.7 µm. The experimental results showed that the compensation strategy proposed in this paper overcomes the clamping error of the weak-stiffness mirror and significantly reduces cutting errors during the machining process, achieving the high precision machining of a weak-stiffness mirror.

Effects of taping on grinding quality of silicon wafers in backgrinding

Taping is often used to protect patterned wafers and reduce fragmentation during backgrinding of silicon wafers. Grinding experiments using coarse and fine resinbond diamond grinding wheels were performed on silicon wafers with tapes of different thicknesses to investigate the effects of taping on peak-to-valley (PV), surface roughness, and subsurface damage of silicon wafers after grinding. Results showed that taping in backgrinding could provide effective protection for ground wafers from breakage. However, the PV value, surface roughness, and subsurface damage of silicon wafers with taping deteriorated compared with those without taping although the deterioration extents were very limited. The PV value of silicon wafers with taping decreased with increasing mesh size of the grinding wheel and the final thickness. The surface roughness and subsurface damage of silicon wafers with taping decreased with increasing mesh size of grinding wheel but was not affected by removal thickness. We hope the experimental finding could help fully understand the role of taping in backgrinding.

Numerical Simulation of Thermo-Optic Effects in an Nd: Glass Slab with Low Thermally Induced Wavefront Distortion

A gain slab configuration with a low thermally induced wavefront distortion, which is based on heating the edge by the cladding layer, is proposed. The gain slab will be applied to a helium-cooled Nd: glass multislab laser amplifier with an output of 100 J at a repetition rate of 10 Hz. Additionally, a 3D numerical simulation model is developed to analyze the thermo-optic effects in the gain slab. Some parameters, including the absorption coefficient (α) of the cladding layer, the shape of the pump beam, and the gap between the pump area and absorbing cladding layer, are optimized to eliminate the thermo-optic effects. The results indicate that the peak-to-valley (P-V) of the thermally induced wavefront distortion of the specific gain slab can be reduced by 61% if other parameters remain constant.

Impact of microvibration on the optical performance of an airborne camera.

Aiming to investigate the connection between camera structure and optical systems, a comprehensive analysis needs to be performed for the airborne camera. An integrated analysis method was proposed to design and analyze optical and mechanical structures. Based on the designed small airborne camera, the impact of microvibration on the optical performance of the airborne camera was studied by integrated optomechanical analysis. In addition, the change of optical surface accuracy was analyzed. First, static and dynamic analysis of the designed airborne camera was performed to verify the stability of the camera structure and obtain the data for integrated optomechanical analysis. Then, a calculation method for rigid body displacement was proposed, and the impact of rigid body displacements on the optical system was analyzed. To evaluate the change of surface accuracy, the parameters root mean square (RMS) and peak to valley (PV) were calculated by fitting the surface distortion data. Based on the Zernike polynomial coefficients, the response of the optical system was calculated and analyzed utilizing ZEMAX to analyze the impact of microvibration on the optical performance of the airborne camera. The analysis results show that microvibration has no significant impact on optical performance of the designed small airborne camera. Finally, the analysis results were verified through experiments.

Chemical mechanical polishing for sapphire wafers using a developed slurry

At present, corrosive ingredients are widely employed in chemical mechanical polishing (CMP) of sapphire, bringing a potential threat to both people and environment. This results in high cost and long processing time on the treatment of CMP slurry of sapphire. It is a challenge to develop a novel green CMP for sapphire to meet the stringent requirements of high performance products. In this study, a novel green CMP is proposed for sapphire, consisting of silica nanoparticles of 50 nm, triethanolamine (TEA), sodium metasilicate nonahydrate, and deionized water. After green CMP, surface roughness Ra, root mean square (rms), and peak-to-valley (PV) are 0.11, 0.139, and 1.65 nm, respectively. Material removal rate is 3.31 μm/h during green CMP. To the best of our knowledge, surface roughness Ra and rms after green CMP developed are the lowest on a sapphire wafer, compared with those reported previously. The CMP mechanism is elucidated by X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectra. Sapphire formed Al(OH) 4 − ions in an alkaline environment, which was chelated by TEA, and removed from the surface of sapphire. These findings provide new insights to fabricate high performance devices of sapphire for the use in semiconductor and microelectronic industries.

Dwell Time Algorithm Based on Bounded Constrained Least Squares Under Dynamic Performance Constraints of Machine Tool in Deterministic Optical Finishing

The dwell time algorithm is one of the most important techniques within the deterministic optical surfacing technologies. The existing dwell time algorithms are generally based on non-negative least squares (NNLS) without considering the dynamic performance constraints of machine tools. This is a circumstance that leads to poor convergence accuracy. In this paper, a dwell time algorithm, based on bounded constrained least-squares (BCLS) under dynamic performance constraints of the machine tool, has been developed. The upper and lower constraints of the dwell time model could be derived through the acceleration and deceleration mechanism of the CNC (Computer Numerical Control) machine tools. A two-metric projection Newton iteration algorithm was used to solve the large-scale dwell time model, which greatly improved the computation efficiency. The results of the experiments and simulations showed that the proposed algorithm will give a very high convergence accuracy for optical finishing with machine tools with different dynamic performances. When the machine acceleration was set to a value as low as 0.1 g, the accuracies of the surface figures PV (Peak-to-Valley) and RMS (Root Mean Square) till improved by 40.8% and 55.2%, respectively, when using the BCLS algorithm. The influences of different dynamic performances of the machine tools on the dwell time solutions have also been investigated, which will provide a good guidance in the design of deterministic polishing machine tools.

Numerical Simulation of Static Seal Contact Mechanics Including Hydrostatic Load at the Contacting Interface

A finite element model of a static seal assembled in its housing has been built and is utilized to study how the seal deforms under varying loading conditions. The total contact load on the sealing surface is balanced by the sealed fluid pressure and the friction between the seal and the housing sidewall perpendicular to the sealing surface. The effect of the sealed fluid pressure between the sealing surfaces was investigated and the simulation showed that the surface profile is distorted due to the hydrostatic pressure. We study the distorted contact profile with varying sealed fluid pressure and propose five parameters to describe the corresponding contact pressure profile. One of these parameters, overshoot pressure, a measure of the difference between maximum contact pressure and the sealed fluid pressure, is an indicator of sealing performance. The simulations performed show different behaviors of the overshoot pressure with sealed fluid pressure for cosinusoidal and parabolic surfaces with the same peak to valley (PV) value.

Preparation of Flat and Smooth Copper Surface by Jet Electrochemical Machining and Electrochemical Polishing

Ultra-flat and ultra-smooth copper (Cu) surfaces are widely used as optical mirrors, heat sinks, and substrates for functional material growth. Traditional polishing methods that rely on abrasive particles are easy to induce mechanical defects such as abrasives embedding and scratches on surfaces. A new stress-free machining process is proposed in this research to fabricate an ultra-flat and ultra-smooth Cu surface by combining jet electrochemical machining (Jet-ECM) and electrochemical polishing (ECP). With the accurate manipulating of material removal rate (MRR) and planning of nozzle trajectory, an ultra-flat surface can be obtained efficiently in the Jet-ECM process. The surface roughness of the workpiece can be further improved by ECP with the same electrolyte used in the Jet-ECM process. The results show that the surface peak-to-valley (PV) value which indicated the surface form error of the Cu surface was reduced from 4.4 μm to 1.7 μm and the surface roughness Sa was reduced from 70.3 nm to 13.5 nm. The combination of Jet-ECM and ECP which share the same electrolyte and apparatus can improve the surface flatness and roughness significantly. This study improves the machining accuracy of stress-free machining methods and has great implications for the further understanding of the electrochemical removal mechanism.

Phase diversity method based on an improved particle swarm algorithm used in co-phasing error detection.

The phase diversity (PD) algorithm is an image-based co-phasing error detection method for segmented mirror synthetic aperture optical systems. Particle swarm optimization (PSO) is suitable for PD for its fast convergence speed and simple structure. However, with the increase of the numbers of subapertures, optimization of cost function formed by the PD algorithm becomes a high-dimension non-linear optimization problem, which would lead PSO to result in a premature solution. Regarding the problem above, this paper presented a modified PSO in the PD algorithm to co-phase the segmented primary mirror, which overcame drawbacks of the traditional PSO, such as premature convergence and deactivation of particles, and enhanced the dig ability of the algorithm. Vast simulation results show that the modified PSO in the PD algorithm can effectively restore the segmented primary mirror, reducing the peak-to-valley (PV) value to 0.0012λ and the root mean square error to 0.0007λ, and raising the Strehl ratio to over 0.999. A comparison was also conducted to show that the modified PSO has advantages over two existing algorithms in higher accuracy and faster convergence speed.

Freeform machining of ophthalmic toric lens mould using fast tool servo-assisted ultra-precision diamond turning process

This research aims to establish a methodology for machining of toric lenses, using fast tool servo-assisted single point diamond turning and to assess the generated surface for its characteristics. Using the established mathematical model, toric surface is explained to understand the geometry and to generate the parameters required for fast tool servo machining. A toric surface with a major diameter of 18.93 mm and a minor diameter of 15.12 mm has been cut on the intelligent ultra-precision turning machine (iUPTM). The surface profile and surface roughness were measured. After analysing the measurement data of the machined surface, on two perpendicular axes of the toric lens, form accuracy of 0.49 µm peak-to-valley (PV), and surface roughness of 12 nm in Ra, 4–8 nm in Sa are obtained. From the experimental results obtained, it can be concluded that the proposed method is a reasonable alternative for manufacturing toric lens mould.

An imaging performance analysis method correlated with geometrical deviation for the injection molded high-precision aspheric negative plastic lens

High-precision aspheric negative plastic lenses are widely used in optical systems owing to their excellent performance and ease of high-efficiency manufacturing. The imaging performance of the lens is difficult to control, because it is unable to perform optical measurements directly and is sensitive to manufacturing processing meanwhile. Generally, the imaging performance is guaranteed by a strict control of geometrical deviation, such as Peak-to-Valley (PV) and Root-Mean-Square (RMS). In this study, an optical ray-tracing algorithm with the measured geometrical deviation data is proposed to perform an imaging performance analysis correlated with geometrical deviation for the injection molded high-precision aspheric negative plastic lens. Taguchi experiments are applied to investigating the effect of processing parameters. The geometrical deviation of the convex surface is found to be an order of magnitude greater than that of the concave surface. The geometrical dimension of the concave surface is mainly determined by the machining precision of the mold cavity surface, whereas the convex surface dimension is mainly affected by the lens shrinkage. However, the imaging performance has a nonlinear correlation with the geometrical deviation. Modulation Transfer Function (MTF) and Spot Diagram are equivalently affected by the concave and convex geometrical deviations, and depend on the object field height. The effect of processing parameters on geometrical deviation and imaging performance is uncorrelated. Thus, the imaging performance should be simultaneously considered as a criterion as well as the geometrical deviation in the optimization of aspheric negative lens injection molding processing. The imaging performance prediction using an optical ray-tracing algorithm with the measured geometrical deviation data is instrumental to optimize the manufacturing processing.

Lissajous scanning structured illumination microscopy.

High-resolution fluorescent microscopic imaging techniques are in high demand to observe detailed structures or dynamic mechanisms of biological samples. Structured illumination microscopy (SIM) has grabbed much attention in super-resolution imaging due to simple configuration, high compatibility with common fluorescent molecules, and fast image acquisition. Here, we report Lissajous scanning SIM (LS-SIM) by using a high fill-factor Lissajous scanning micromirror and laser beam modulation. The LS-SIM was realized by a Lissajous scanned structured illumination module, relay optics, and a conventional fluorescent microscope. The micromirror comprises an inner mirror and an outer frame, which are scanned at pseudo-resonance with electrostatic actuation. The biaxial scanning frequencies are selected by the frequency selection rule for high fill-factor (> 80%) Lissajous scanning. Structured illumination (SI) was then realized by modulating the intensity of a laser beam at the least common multiple (LCM) of the scanning frequencies. A compact Lissajous scanned SI module containing a fiber-optic collimator and Lissajous micromirror has been fully packaged and coupled with relay optics and a fiber-based diode pumped solid state (DPSS) laser including acousto-optic-modulator (AOM). Various structured images were obtained by shifting the phase and orientation of the illumination patterns and finally mounted with a conventional fluorescent microscope. The LS-SIM has experimentally demonstrated high-resolution fluorescent microscopic imaging of reference targets and human lung cancer cell PC-9 cells. The LS-SIM exhibits the observable region in spatial frequency space over 2x, the line-edge sharpness over 1.5x, and the peak-to-valley (P-V) ratio over 2x, compared to widefield fluorescent microscopy. This method can provide a new route for advanced high-resolution fluorescent microscopic imaging.

Characterization of thermal sprayed Si on sintered SiC for space optical applications

ABSTRACT Silicon carbide (SiC) ceramic exhibits several outstanding properties, such as superior chemical and thermal stabilities, high radiation resistance and low thermal expansion, that qualify it as a prime candidate for space applications. Optical quality with surface figure accuracy of the order of 60 nm peak-to-valley (PV) and micro-roughness (root-mean-square, RMS) less than 2 nm is necessary for space optical applications. Sintered SiC (α-HCP) is inherently porous and challenging to make high-grade optics. Hence, it calls for a surface modification of the sintered SiC by means of a clad layer that has superior adhesion to the substrate and facilitates optical polishing. This paper describes a typical method of synthesis and characterization of a thick silicon coating on sintered SiC substrate, directly machinable to aspheric for optical telescope applications. Present approach describes the quantitative opto-mechanical characterization of silicon film over SiC by thermal spray coatings, eventually leading to its space flight-worthiness.

Maximum a posteriori-based digital holographic microscopy for high-resolution phase reconstruction of a micro-lens array

A novel approach is presented for improving phase reconstruction in digital holographic microscopy. In this process, a maximum a posteriori (MAP) image restoration technique is combined with a digital holographic phase reconstruction algorithm and used to extract the three-dimensional phase of a measured sample. High-resolution restoration of a digital hologram was first performed using the MAP method, a digital holographic microscopy device, and a USAF1951 resolution plate. The low- and high-resolution holograms were then used, in combination with a micro-lens array, to reconstruct corresponding phase information. Experimental results indicated the peak-to-valley (PV) and root-mean-square (RMS) values for the phase data, reconstructed using a super-resolution hologram, were respectively 31% and 59% lower than that of the low-resolution hologram. This suggests the proposed technique could enhance hologram resolution and reduce noise in reconstructed phase information.