KEYWORDS: Atmospheric plasma, Plasma, Diffractive optical elements, High power lasers, Polishing, Chemical analysis, Chemical reactions, Surface finishing, Chemical elements
Continuous phase plate (CPP) is an important diffractive optical element, which is widely used in high power laser devices. The continuous phase plate with a small aperture period of 4 mm is processed by the atmospheric pressure plasma polishing (APPP). Through the study of the reaction mechanism, it is found that the removal volume has a non-linear relationship with the dwell time, which will lead to machining errors. Based on this, a dwell time compensation method is proposed, and the machining program is generated according to this relationship. A 70mm × 70mm × 20mm continuous phase plate was fabricated by using the processing program generated by this method. The processing time was 4.5h, and the surface residual converged to 57.188nm RMS. The experimental results show that the method can effectively calculate the removal function under different dwell time, and significantly improve the machining accuracy.
Reaction-sintered silicon carbide (RS-SiC) has been widely applied in space telescope mirrors, reflector, microelectronic mechanical systems due to its excellent properties. These applications require high surface accuracy and quality, but traditional mechanical contact polishing methods are hard to process this material due to the hardness and poor machinability. In this paper, atmospheric pressure plasma processing (APPP) which is a non-contact optical manufacturing technique with highly efficient and subsurface damage-free has been proposed to process this material, and optimal process parameter mainly about influence on removal function for machining processing RS-SiC have been found. The sample was observed and analyzed by scanning white-light interferometer (SWLI), which showed the calculated Peak- Maximum removal rate (MRR) and Volume-MRR were 12.526μm/min and 0.1298mm3 /min. Meanwhile, the surface chemical composition of RS-SiC was also investigated by XPS to reveal the plasma etching processes. The results illustrated that small number of radicals CxFy were introduced onto the RS-SiC surface during the plasma process which could be generated during the process of the reaction gas CF4 being excited. The process parameters and analysis of surface chemical composition in this paper will guide the further processing of RS-SiC.
The wavefront of coated optics is one of critical performances. Due to the interference between the coating layers, the measurement results will be totally different if the measurement wavelength is different from the working wavelength. However, all of the commercial interferometers have single measurement wavelength, which can’t treat the optical coatings working at various wavelengths. A wavelength-switchable interferometer (WSI) capable of detecting wavefront information in a wide wavelength range of 488-1064 nm is proposed in this paper. The principle of design and performance of the system are given in detail. Some typical measurement applications, such as reflection plate and optical filters will also be presented.
The axicon is a rotationally symmetrical optical element along the optical axis of the focal line, including concave shaft cone and convex shaft cone. It is widely used in laser beam shaping, laser drilling, optical detection, laser resonator and lithography illumination due to the property of long focal length and narrow transverse width. The axicon has only one optical axis, and the curvature radius of each point of the surface is different in the direction of the optical axis, that is, the line is a straight line, but the points of the adjacent bus are really circular torus, and the traditional equipment is difficult to polish. A new process suitable for axicon processing is proposed, including forming, traditional polishing, Atmospheric Pressure Plasma Processing for surface correction, flexible asphalt discs for smoothing. With it, the convergence of the polishing process is significantly improved, and the PV is decreases from 27λ to 3λ. There are two main factors that affect processing efficiency: temperature and gas flow rate. An analytical method is selected to estimate the complex interaction between the temperature field analysis and the polishing efficiency during the polishing process, and it is possible to achieve efficient and determined process.
In order to study the mist phenomenon on phosphate glass, the chemical composition of the hydrolysis layer on the glass surface of phosphate glass was investigated. The solid and liquid components of the used slurry and the surface composition of polished phosphate glass were tested. The experimental results show that the SIMS test results show that the content of K does not change with depth, and the content of Mg, Al, Nd firstly increases with depth and then tends to be gentle; K element does not participate in chemical reactions during polishing, just simply mechanical removed; but Mg, Al, and Nd elements are not only mechanically removed but also participate in chemical reactions and are precipitated as ions, which is consistent with the XRF test results. It is proposed a new substance generated on the surface of phosphate glass during polishing process, a hydrolysis layer formation model was proposed. The mist phenomenon is related to the hydrolysis layer, which is different from the substrate material and properties (density, light reflectance, etc.). So different scattered light is seen and the thicker the hydrolyzed layer, the stronger the scattered light. Moreover, the severity of mist is related to the thickness of the mist layer, and the thicker the thickness, the more severe the mist.
In this paper, a test method based on oblique incidence is practically implemented in the interferometric measurement process. Three sets of wavefront data are achieved through cavity interference measurement with a Fizeau interferometer and one oblique incidence measurement. An iterative algorithm is applied to retrieve the matrix of transmission flatness and reference flatness. The new method can not only calibrate the reference flat error of large aperture interferometer, but also provide the absolute measurement method for large rectangular optical components applied in high power laser systems.
The surface figure control of the conventional annular polishing system is realized ordinarily by the interaction between the conditioner and the lap. The surface profile of the pitch lap corrected by the marble conditioner has been measured and analyzed as a function of kinematics, loading conditions, and polishing time. The surface profile measuring equipment of the large lap based on laser alignment was developed with the accuracy of about 1μm. The conditioning mechanism of the conditioner is simply determined by the kinematics and fully fitting principle, but the unexpected surface profile deviation of the lap emerged frequently due to numerous influencing factors including the geometrical relationship, the pressure distribution at the conditioner/lap interface. Both factors are quantitatively evaluated and described, and have been combined to develop a spatial and temporal model to simulate the surface profile evolution of pitch lap. The simulations are consistent with the experiments. This study is an important step toward deterministic full-aperture annular polishing, providing a beneficial guidance for the surface profile correction of the pitch lap.
The response characteristics of the pitch polishing pad under the pressure of conditioner is the key factor which affect the control efficacy of surface figure in annular polishing. The effect of the environmental temperature and humidity, rotation speed of the pad, uploading ratio and eccentricity of the conditioner on the response characteristics of pitch polishing pad were experimental researched. To this aim, a control system of the environmental temperature and humidity on the 2.8m-aperture annular polishing machine is established. The control precision of the local environmental temperature upon the pad is up to ±0.1℃, and the control precision of humidity is up to ±1% RH. The experimental results indicate that the matching degree of the pad and the conditioner decides the changing rule of the surface figure of workpiece with the eccentricity of the conditioner. The new adjustable parameters including the environmental temperature and humidity and the uploading ratio of the conditioner were introduced to change the matching degree and improve the controllability of the of the surface figure. Under the optimized process, the long-term stability of the surface figure of the meter-scale optical element is realized using the 2.8m annular polishing machine, which is an important step for the final realization of the deterministic processing in the large-aperture annular polishing.
The multi-object broadband imaging echellette (MOBIE) is the seeing-limited, visible-wavelength imaging multi-object spectrograph (MOS) planned for first-light use on the thirty meter telescope (TMT). The current MOBIE optical design provides two color channels, spanning the 310nm–550nm and 550nm-1000nm passbands. The involved large optics includes an atmospheric dispersion corrector (ADC) prism (1.4m in diameter), a collimator (1.7mx1.0m), a dichroic(680 mm x500 mm x 30 mm), a red folding mirror and two corrector lenses(570mm in diameter) for different channels. In the past two years, Shanghai Institute of Optics and Fine Mechanics (SIOM) has been included in the preliminary study of folding mirror sub-system in MOBIE, especially the study on the large optics manufacture techniques. The research progress of these large optics will be reviewed in this paper. The influence of optical quality of the large optics on the MOBIE is analyzed in order to define the specifications of the large optics. The manufacture methods are designed for different large optics. In order to testify the effectiveness of the manufacture methods, some samples have been processed and the final performance including wavefront error and spectral properties are tested. Finally, the future work including remaining problems and possible solutions are introduced.
An ideal pitch button blocking process determines the level of workpiece deformation, especially the high-aspect-ratio optics, during the blocking process and process of polishing later. We have studied the pitch button blocking process by Finite Element Analysis (FEA) according to the thermoelastic equation. Meanwhile, the optimized pitch button blocking has been gotten by FEA which includes the thickness and material of blocking plate, as well as the radius, arrangement, elastic modulus and coefficient of thermal expansion of pitch buttons. The numerical simulation of Nd:glass (Ø100 mm×2 mm thickness) shows that the surface figure change (ΔPV) which is induced by the thermal stress during pitch button blocking process is influenced seriously by the thickness of blocking plate.
Dynamic Mechanical Analysis (DMA) and Bending Beam Rheometer (BBR) are applied to detect the property of the polishing pitch and get the creep compliance combining the craftwork of polishing. The simulation of workpieces’ surface in polishing is completed based on the Preston formula, Boltzmann superposition principle and Boltzmann superposition principle. A V-curve is gotten on studying the PV’s time history of the polished workpiece. Meanwhile, a confirmatory experiment whose parameter is agree with the simulation is completed, which the optimum result of PV, 0.3λ (λ=623.8nm) is similar with the optimum result of PV by simulation, 0.27 λ. A resemble simulated surface is gotten on the same PV with the experiment.
Different size polishing powder and different pH value ceria slurries were used to polish fused silica glass、K9 glass and Nd-doped glass on pitch plate. Material removal rates (MRR) of glass polished with different size powder and various pH value slurries, and textures of each sample were characterized. The results show that powder size has an effect on glass polishing performance: scratch densities increase with the increase of polishing powder size; surface textures become rougher with the increase of the size of polishing powder. The slurry pH value also affects glass polishing performance: MRR of fused silica glass are lowest under any pH value slurry while Nd-doped glass has the largest MRR; removal rates of all three kinds of glass will rise under both acidic and alkaline condition. Near neutral polishing environment and smaller size powder are useful for the surface polishing process. The results further reveal polishing mechanism and provide the guidance for glass surface process.
The effect of deep HF etching on the surface quality and figure of fused silica optics has been investigated systematically. Fused silica samples (100 mm in diameter x 10 mm thick) were manufactured using the conventional grinding and polishing process. These processed samples are etched with different removal depth. Initially, the surface quality of fused silica samples is characterized in terms of surface roughness and surface defects. Many digs not more than 1μm deep are emerged which originates from the micron grinding cracks and crack pits. These digs worsened the surface roughness and frosted the sample. While submillimeter subsurface damage exposed through etching appear as sparkling dots under the high power lamp. The average total length of millimeter scratches on single surfaces is over 200 mm. Not all millimeter scratches could be exposed until removal depth of up to 2 μm. Finally, the surface figure behavior during deep etching has also been figured out. Etching on the edge of the upper surface of samples placed horizontally went faster than on the inside parts. The surface of samples placed vertically assumed a more complicated removal distribution, which can be both explained in terms of "fringe tip effect". For the change of surface figure PV, the initial surface figure feature plays an important role as well as the etching removal distribution.
Smoothing is a convenient and efficient way to correct mid-spatial-frequency errors. Quantifying the smoothing effect allows improvements in efficiency for finishing precision optics. A series experiments in spin motion are performed to study the smoothing effects about correcting mid-spatial-frequency errors. Some of them use a same pitch tool at different spinning speed, and others at a same spinning speed with different tools. Introduced and improved Shu's model to describe and compare the smoothing efficiency with different spinning speed and different tools. From the experimental results, the mid-spatial-frequency errors on the initial surface were nearly smoothed out after the process in spin motion and the number of smoothing times can be estimated by the model before the process. Meanwhile this method was also applied to smooth the aspherical component, which has an obvious mid-spatial-frequency error after Magnetorheological Finishing processing. As a result, a high precision aspheric optical component was obtained with PV=0.1λ and RMS=0.01λ.
During continuous polishing, temperature is a significant source of processing uncertainty. Three work pieces of different kind material (K9, Nd:glass and ULE) were polished on 2.4m continuous polisher. It turns out that temperature difference has different influence on different material work pieces. It also indicates that temperature difference aggravates the processing uncertainy. The deformation caused by temperature difference is simulated using ANSYS. It shows that, with top-bottom temperature difference of 0.1°C, the deformation of Nd:glass, K9 and ULE are 0.444E-6 m (about 0.7025λ), 0.249E-6 m (about 0.3925λ ), and 0.105E-8 m (about 0.00166λ), respectively. With radial temperature difference of 0.1°C, the deformation of Nd:glass, K9 and ULE are 0.831E-7 m (about 0.1313λ), 0.465E-7 m (about 0.07348λ) and 0.196E-9 m (about 3.0973E-4λ), respectively. To explore the top-bottom temperature difference and radial temperature difference along the polishing surface, a small aperture Nd:glass and a large aperture Nd:glass in polishing have been measured using thermal infrared imager. The results showed that for Ø 260 mm × 26 mm Nd: glass, the radial temperature difference is about 0.1°C, while the top-bottom temperature difference is about 0.1°C ~ 0.21°C. Contrastively, for 810 mm×460 mm×40 mm Nd:glass, the radial temperature difference have reached 0.4°C, while top - bottom temperature difference ranges between 0.1°C ~ 0.27°C. When element gets larger, it will suffer greater temperature difference. These temperature differences are great enough to cause deformation far beyond the polishing accuracy required. Finally, methods are proposed to diminish the effect of temperature difference.
KEYWORDS: Polishing, Calibration, Temperature metrology, Laser glasses, High power lasers, Oxygen, Finite element methods, Abrasives, Optical simulations, Mechanics
The figures of the polishing pad are of great significance for the figures of optical workpieces in the continuous polishing process. Three main factors which affect the figures of the pad, including the polishing pad creep deformation, the calibration plate grinding and the ambient change in temperature, are analyzed in this work. Processing parameters including the eccentricity between the polishing pad and the calibration plate, the calibration plate thickness, angular velocities of the pad are introduced in this analytic process. With this method, the figures of the polishing pad in the continuous polishing can be obtained. This work provides theoretical guidance for the deterministic processing of the continuous polishing process.
The low surface laser damage threshold of fused silica components in high power laser systems such as NIF restricts the improvement of the output fluence of those systems. Once damage is initiated and grows under subsequent laser shots, the components will go unusable. Subsurface damage (SSD) introduced during manufacturing has been identified as a main damage initiator. A good knowledge of SSD and how manufacturing influences it is essential to optimize manufacturing processes for damage free optics. Using the magneto-rheological finishing (MRF) wedge technique of better accuracy attributed to a tip, we have characterized the subsurface damage on fused silica optical surfaces ground with loose Al2O3 abrasives of different sizes. Larger abrasives generates longer cracks and the number density of cracks decreases sharply with the depth for each size. Rogue particles account for the occurrence of trailing indent scratches. Addition of rogue abrasives into relatively small base abrasive extends SSD more deeply than that induced by rogue abrasives alone. The linear model, with the proportional coefficient 3.511, fits the relationship between SSD depth and surface roughness (SR) better than the quadratic polynomial one. We believe SSD depth relates to SR more statistically than following some specified physical law. The linear relationship between SSD depth and the abrasive size was also established. The abrasive size turned out not to be as a good indictor of SSD depth as SR.
Continuous ring polishing is the key process in large aperture optical elements. The surface figure of polishing pad is inferred by the offline testing surface figure of workpiece. The defects, low processing efficiency and uncertainty processing time in traditional continuous polishing, the real-time monitoring method of polishing is proposed. The realtime monitoring system is set up based on the computer, the dynamic interferometer, a beam expanding system and a beam reflecting system. There are a workpiece and a glass monitoring plate placing in same ring. The surface figure of workpiece, monitored by the monitoring plate, synchronize with the surface of glass monitoring plate in Peak-Valley (PV) and POWER. The new method with simple structure is fast measuring and judgmental directly to the changes of surface figures. The results of real-time monitoring and surface figure converging on the workpiece are valid for continuous polishing through experimental validation.
Effect of the polishing plane vibration on large-size optical workpieces in continuous polishing is studied. The vibration equation was deduced based on the existence of inclination between the polishing plane and z-axis direction. Influences of different parameters, such as the inclination, rotation speeds of the polishing plane and workpiece, the eccentricity and workpiece radius, on the polishing plane vibration were simulated. The simulations results show that rotation speeds of the polishing plane and workpiece is the most significant factor. The chaotic vibration of the polishing plane increases with increasing rotation speeds differences between the polishing plane and workpiece. When differences are small, periodic ups and downs of the polishing plane occur with the increase of polishing time. Experiments verified the influence of rotation speeds differences on the polishing plane vibration. The vibration affects PV of large-size optical workpieces in continuous polishing.
Large-aperture Nd-doped metaphosphate glasses are used in high-power/high-energy laser facilities. High transmission
wave front accuracy of Nd-doped metaphosphate glasses is required to decrease beam distortion, which affects the focus
ability and damages optical components. Elliptical shape astigmatism of the transmission wave front easily occurs when
Nd-doped metaphosphate glasses are detected and utilized at Brewster incident angle compared with zero angle. The
astigmatism is difficult to decrease for precision annular polishing because the surface profile adjustment of the polishing
pitch plane has low accuracy. Non-uniform removal at different velocity rates between the pitch plane and Nd-doped
metaphosphate glasses can be used to eliminate horizontal elliptical-shaped astigmatism. The improved process
contributes to the production of high-efficiency Nd-doped metaphosphate glasses.
We investigated the effects of various polishing additives added to cerium oxide slurry on the polishing properties of phosphate laser glass. The results showed that additives could change the polishing efficiency of the slurry and affect the surface quality of the laser glass. After polishing, the surface roughness of the glass increased and even micrometer-scale etch pits formed on the glass surface. The additive with 1.05% by weight LaF 3 improved the polishing efficiency significantly while controlling the surface roughness of the laser glass. From the chemical-reactions perspective, a model was proposed to analyze the polishing mechanism based on the chemical composition and structure of the phosphate laser glass material and on current and previous experimental phenomena.
Color filters are key components in an optical engine projection display system. In this paper, a new admittance-matching method for designing and fabricating the high performance filters is described, in which the optimized layers are limited to the interfaces between the stack (each combination of quarter-wave-optical-thickness film layers is called a stack) and stack, or between stack and substrate, or between stack and incident medium. This method works well in designing filters containing multiple stacks such as UV-IR cut and broadband filters. The tolerance and angle sensitivity for the designed film stacks are analyzed. The thermal stability of the sample color filters was measured. A good result in optical performance and thermal stability was obtained through the new design approach.
Color filters is one of the most important optical components in the projection display system. The design and fabrication of high performance filters were described in this paper. A new admittance-matching method was approached, which the optimized matching layers only exsited at the interfaces: stack|stack, stack|substrate and stack|air, and the satisfied design results were achieved. This method is especially used for wide cut-off and wide bandpass filters. The analysis of the tolerance and angle senstivity were carried for the designed film stacks . Thermal stability was also tested for the prepared samples. The results of the experiment and analysis indicate that film stack got from the new interface optimizing method presented good optical and thermal stability.
Adhesion between the interface of pure silver thin film and three kinds of low refractive index coatings MgF2,Al2O3,SiO2 were compared in this article. The results indicated that the adhesion of Al2O3 and Ag was evidently superior to that of MgF2 and Ag ,and the adhesion of MgF2 and Ag was evidently superior to that of SiO2 and Ag. Reasons were analyzed accordingly. On the other hand, we compared the effect on the optical characteristic of Ag film when these three kinds of films were used as protective coatings and protective coatings. Considering the difference of the adhesion between Ag and MgF2,Al2O3,SiO2,suited uses are given for each other.
Coatings with layer structures of Ag/glass, Ag/Cr/glass, and Ag/Cr-Ag/Cr/glass deposited with magnetron sputtering are investigated. The results indicate that the performance for reflectance, hardness, adherence, and humidity durability of the silver coatings with Ag/Cr/glass and Ag/Cr-Ag/Cr/glass structures are better than pure silver film. In addition, the silver coatings with an Ag/Cr-Ag/glass structure present more advantages than that of the Ag/Cr/glass. Reasons are analyzed accordingly.
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