To solve the problem of machining accuracy of a convex blazed grating for an Offner imaging spectrometer by diamond turning limiting the imaging quality of the optical system, analyze the effect of Poisson burr on the diffraction efficiency of convex blazed grating, investigate the effect of the layout of blazed grating on the convex surface on the machining accuracy, and compare the performance of cutting convex gratings with microcrystalline aluminum RSA6061 and RSA6061 + chemically plated NiP for two workpiece materials. Turning with a 4-axis ultra-precision machining system yielded a convex blazed grating with a substrate radius of curvature R=41.104mm, substrate diameter of 14mm, the grating density of 53.97 line/mm, and blaze angle of about 3.8°. Cutting experiments show that RSA6061+ chemically plated NiP material is better for convex grating processing; the blaze angle error is better with the equal-along-projection layout than with the equal-along-arc layout; Poisson burr height is about 0.3μm; and the average roughness of the final grating blaze surface is less than 5nm to meet the processing quality requirements.
The improvement of surface shape accuracy and surface quality is an important aspect of ultra-precision grinding. In this paper, experiments were carried out by using a four-axis ultra-precision grinder to analyze the influence mechanism of grating machining and polar axis machining on surface shape accuracy and surface quality, and to clarify the advantages and disadvantages of the two machining processes. In the process of polar axis machining, the influence of polishing on the surface shape accuracy is analyzed, and the influence of different feed modes and x direction offset on the surface shape accuracy is clarified. By establishing the influence model of the error source on the surface shape accuracy, the error factors of the tool alignment were analyzed emphatically. Through numerical calculation and analysis of the specific form of tool alignment error, a reasonable grinding wheel center alignment scheme was put forward. The wear law of grinding wheel was analyzed through process test, and a reasonable dressing method of grinding wheel was put forward.
Metal mirrors, represented by alμminiμm alloys, have soft and easily damaged surfaces and can be coated with a nickel- phosphorus alloy by means of chemical nickel plating prior to polishing. In order to obtain better processing results, the characteristics of the nickel-phosphorus alloy must be studied. In this paper, the nickel-phosphorus alloy was characterised by means of a hardness tester and EDS, and the experimental results show that the nickel-phosphorus coating has a Vickers hardness of 566 Hv. Depending on the properties of the coating, magnetorheological polishing was carried out using different types of polishing abrasive grains. By comparing the surface roughness after processing, silicon dioxide was selected as the polishing abrasive particle and magnetorheological fluid parameters such as pH value were adjusted to obtain a removal function with a volμme removal efficiency of 5.22 x 107μm3 /min. At the same time, combining different processes such as CCOS polishing and analysing the processing principle, the obtained surface roughness Ra value is better than 0.5nm, exploring the process route of nickel-phosphorus coating and providing technical support for the subsequent processing.
Due to Metal mirror’s low hardness, we coat a layer of nickel-phosphorus (NiP) layer on the surface and then polish NiP layer simpler through various methods. However, when using CeO2 slurry in Magnetorheological Finishing(MRF), the NiP layer will appear "wise-tail" phenomenon. Based on the "double-edged circle" model, we analyze the shear force of the contact surface and optimize the polishing abrasive particles such as SiO2. In this way, we could obtain a surface without "wise-tail" and the surface roughness reaches 1.897 nm in Ra. By orthogonal experiments, we optimize the process parametersts and certify the stability and linearity of the removal function.The experiment results prove that the MRF fluid compounded with SiO2 could satisfy the requirements of NiP layer in magnetorheological machining.
In aerospace and high power laser devices, fused quartz material is in large demand because of its strong adaptability.The fused quartz optical mirror blank was processed by ultra-precision grinding, magnetorheological machining (MRF) and small smooth polishing (SP).Ultra-precision grinding machine was used for rough pre-polishing to obtain high precision surface shape, and then MRF was used for ultra-precision polishing to further improve the surface shape.The medium and high frequency errors generated by MRF processing were controlled by using smooth polishing.The experiments of ultra-precision machining of fused quartz mirror were carried out by using the combined process, and the rapid convergence of deterministic machining and full frequency error was realized.The combined process can not only improve the processing efficiency of optical mirror, but also reduce the processing cost and realize short process processing.
With the application of single point diamond ultra-precision turning technology in optical free-form surface processing, the development of high-frequency large amplitude fast tool servo system has become a research hotspot. In this paper, a double piezoelectric ceramic fast tool servo system is designed. Its performance index is 60μm stroke at 300Hz. In this paper, the structural design principle and processing technology of the system are systematically analyzed. The stiffness analysis of the system were carried out by using the finite element analysis software to verify the performance of the high-frequency response amplitude of the system. In this paper, the key technology research on the structure design of double piezoelectric ceramic fast tool servo system provides the basis for improving the machining accuracy and efficiency of the fast tool servo system.
The physical characteristics of silicon carbide materials, such as brittle texture and high hardness, make it very difficult to form complex structures. Additive manufacturing can realize the direct molding of complex structure silicon carbide mirrors, improve processing efficiency, and reduce processing costs. In this paper, an additive silicon carbide mirror substrate is processed by the combined process of grinding, lapping, and polishing with a small grinding head. After obtaining a certain surface shape accuracy, the surface was modified to obtain a silicon modified layer with a thickness of 10 μm. After two times of magnetorheological modification of the modified layer, the surface shape converged to 17.313nm RMS. After a conformal smoothing, the surface quality was 1.109nm Ra, which verified the machinability of the additively manufactured silicon carbide mirror.
In recent years, requirements such as over-the-horizon, high resolution, large field of view, wide spectrum and dexterity have pointed out new directions for the development of optical imaging systems. Common optical surfaces such as planes and spherical surfaces have been difficult to meet the requirements of optical indicators. It is often necessary to use multiple high-precision complex optical surfaces such as high-order aspherical surfaces, off-axis aspherical surfaces, non-rotational symmetric curved surfaces, microstructured optical array surfaces and freeform surfaces. This brings processing, testing and adjustment in terms of problems, the monolithic multisurface optics as a new type of optical element are increasingly attracting researchers' attention.
Computer-generated hologram (CGH) is an important component for high-precision aspherical surface testing. This paper puts forward a manufacturing method of amplitude-type CGH by femtosecond laser direct etching, and carries out detailed analysis and demonstration. Firstly, the demand of microstructure fabrication of amplitude-type CGH was analyzed by optical diffraction theory. Furthermore, we have established the theoretical model of femtosecond laser machining amplitude-type CGH fringe. Finally, we developed basic verification experiments, and analyzed the feasibility of fabricating CGH by direct etching process based on the testing results of processing fringe characteristics.
To further increase the laser-induced damage threshold (LIDT) of calcium fluoride (CaF2), combined technique of chemical mechanical polishing (CMP) and acid etching as a function of removal mechanical defect and impurity contamination was investigated. After CMP process, the surface roughness reduced to Ra 0.68nm, the scratch and subsurface defect were mitigated which rise the LIDT to 8.6 J/cm-2. Due to the acid etching, the surface roughness of CaF2 elements increased to Ra 0.8nm, while the LIDT reached 11 J/cm-2, benefit from the elimination of impurity contamination. The result laser-induced damage test shows that, comparing to mechanical abrasive polishing (MAP), the LIDT increased by 240% after combined technique, it demonstrated that combined technique is an effective way to improve the resistance of laser damage of CaF2.
With high resolution, low cost and short manufacturing cycle, the optical load of aluminum–based has attracted more and more attention from researchers. In this paper, the optical system is briefly introduced and the tolerance analysis is carried out. Secondly, the whole structure of aluminum space camera is designed. The main mirror adopts integrated design method to reduce two-stage assembly. In order to ensure the assembly is simple and reliable, the whole machine adopts cylinder support. Then, the structure of the integrated main mirror is optimized, the lightweight rate reaches 30%, and the machine's quality is 1.8kg. Finally, the mechanical characteristics and temperature adaptability of the whole machine are analyzed by means of finite element analysis. The analysis results show that the system's MTF value is more than 0.2 at 20±5°C temperature and static environment, which meets the imaging requirements; and each mode is larger than 100Hz, satisfying the dynamic requirements.
With the excellent optical performance especially some aspherical surfaces enhanced, aluminum alloy mirrors are widely used in many optical systems. The technique of Single-point diamond turning (SPDT) applied in the IR optical application directly. However, it is difficult to obtain higher precision for some complex mirrors and the remaining periodic diamond turning structure can cause scatter losses for shorter wavelengths. Hence, additional polishing steps are necessary. In this paper, a process chain contains SPDT and Magnetorheological finishing (MRF), is put forward for machining an aluminum alloy cylindrical mirror. A high accurate measurement setup, composed of an interferometer and an advanced Computer generated hologram (CGH) is built, and the data distortion is analyzed and corrected. Additionally, normal positioning error caused by tool setting is analyzed and simulated, and a workpiece self-localization system in MRF is used to control the error. The experimental result shows that the surface texture converges to PV 0.233λ (λ=632.8nm, 90% aperture), rms 0.040λ (90% aperture). It is concluded that the form accuracy of the aluminum alloy cylindrical mirror is improved by the process chain, moreover, the process chain is the reference for the fabrication of aluminum alloy mirrors with complex surfaces.
A magnetorheological finishing (MRF) method, which is named lap-MRF, is proposed in this paper. A lap, instead of a large polishing wheel, is used to expand the polishing area, which significantly improves the material removal rate. Lap-MRF uses the flexible magnetorheological fluid (MR fluid) as a polishing pad to match the surface well. Moreover, the MR fluid in the polishing zone can be renewed continuously to ensure the stability of the material removal rate. The principle of Lap-MRF and the theoretical model of the material removal rate are presented. The effects of the spindle speed and the working gap distance on the polishing normal pressure and the material removal rate are analyzed. Finally, a series of polishing experiments and simulations are carried out. The volume removal rate for K9 is up to 0.76 mm3 / min and the comprehensive material removal stability is 6.7%. Using MATLAB simulation process method, the convergence efficiency for a ϕ1000 mm mirror is 98.5%. The results verify the validity of the proposed method.
The aerostatic guide in the traditional three-coordinate measuring machine and profilometer generally use metal or ceramics material. Limited by the guide processing precision, the measurement accuracy of these traditional instruments is around micro-meter level. By selection of optical materials as guide material, optical processing method and laser interference measurement can be introduced to the traditional aerostatic bearings manufacturing field. By using the large aperture wave-front interference measuring equipment , the shape and position error of the glass guide can be obtained in high accuracy and then it can be processed to 0.1μm or even better with the aid of Magnetorheological Finishing(MRF) and Computer Controlled Optical Surfacing (CCOS) process and other modern optical processing method, so the accuracy of aerostatic bearings can be fundamentally improved and ultra high precision coordinate measuring can be achieved. This paper introduces the fabrication and measurement process of the glass guide by K9 with 300mm measuring range, and its working surface accuracy is up to 0.1μm PV, the verticality and parallelism error between the two guide rail face is better than 2μm, and the straightness of the aerostatic bearings by this K9 glass guide is up to 40nm after error compensation.
As the basis of fewer-axis grinding of complex surface, the grinding mathematical model is of great importance. A mathematical model of the grinding wheel was established, and then coordinate and normal vector of the wheel profile could be calculated. Through normal vector matching at the cutter contact point and the coordinate system transformation, the grinding mathematical model was established to work out the coordinate of the cutter location point. Based on the model, interference analysis was simulated to find out the right position and posture of workpiece for grinding. Then positioning errors of the workpiece including the translation positioning error and the rotation positioning error were analyzed respectively, and the main locating datum was obtained. According to the analysis results, the grinding tool path was planned and generated to grind the complex surface, and good form accuracy was obtained. The grinding mathematical model is simple, feasible and can be widely applied.
The aluminum mirrors are widely used as important optical components in some vital fields such as astronomical instruments or military installations due to the unique advantages of aluminum alloy. In order to simplify the structure of optical system and improve the performance at the same time, it’s a tendency that the optics will be designed to aspherical or other freeform shapes. However, the traditional techniques are falling to have adequate abilities to deal with the increasing demands of aluminum optics. For example, the tool marks leaved on the surface from single point diamond turning (SPDT) has obvious adverse effects to optical system. The deterministic and sub-aperture polishing process has showed the potential to fabricate complex shapes over the few years. But it’s still recognized as a problem to polish bare aluminum directly because of its soft surface and active chemical characteristics. Therefore, a combination of magnetorheological finishing (MRF) and small tool polishing (STP) is applied to obtain high performance aluminum optics in this paper. A paraboloid aluminum mirror was polished with this proposed method, and the results showed that the surface texture of the sample is restrained from rms 0.409λ (λ=632.8nm) to rms 0.025λ, and the surface roughness is improved from average Ra 6~7nm to Ra 3~4nm.
The subsurface damage and metal impurities have been the main laser damage precursors of fused
silica while subjected to high power laser irradiation. Light field enhancement and thermal
absorption were used to explain the appearance of damage pits while the laser energy is far
smaller than the energy that can reach the intrinsic threshold of fused silica. For fused silica optics
manufactured by magnetorheological finishing or advanced mitigation process, no scratch-related
damage site occurs can be found on the surface.
In this work, we implemented a photothermal absorption technique based on thermal lens method
to characterize the subsurface defects of fused silica optics. The pump beam is CW 532 nm
wavelength laser. The probe beam is a He-Ne laser. They are collinear and focused through the
same objective. When pump beam pass through the sample, optical absorption induces the local
temperature rise. The lowest absorptance that we can detect is about the order of magnitude of
0.01 ppm. When pump beam pass through the sample, optical absorption induces the local
temperature rise.
The photothermal absorption value of fused silica samples range from 0.5 to 10 ppm. The damage
densities of the samples were plotted. The damage threshold of samples at 8J/cm2 were gived to
show laser damage performance of fused silica.The results show that there is a strong correlation
between the thermal absorption and laser damage density. The photothermal absorption technique
can be used to predict and evaluate the laser damage performance of fused silica optics.
The precision of traditional squareness measurement on ultra-precision motion stage is mostly determined by the standard component, such as bare L-square and optical pentaprism, and the measurement precision is lower than the precision of the standard component. Based on the error separation, this paper presents a novel method of squareness measurement with an optical square brick. The angles between the guide and the four lines of the section of the brick are measured, and the squareness error distraction is achieved by the truth that the internal angle sum of a tetragon is 360°. A squareness measurement test is carried out on the coordinate measuring machine (CMM) by using an optical square brick with the squareness error of 0.3°. The results show that the local squareness and uncertainties between the X axis and Y axis of the CMM are 2.5arcsec and 0.5arcsec respectively. The measurement precision is higher than the precision of the brick and the feasibility and accuracy of the method are validated.
Based on the elastic–plastic deformation theory, status between abrasives and workpiece in magnetorheological finishing (MRF) process and the feasibility of elastic polishing are analyzed. The relationship among material removal mechanism and particle force, removal efficiency, and surface topography are revealed through a set of experiments. The chemical dominant elastic super-smooth polishing can be fulfilled by changing the components of magnetorheological (MR) fluid and optimizing polishing parameters. The MR elastic super-smooth finishing technology can be applied in polishing high-power laser–irradiated components with high efficiency, high accuracy, low damage, and high laser-induced damage threshold (LIDT). A 430×430×10 mm fused silica (FS) optic window is polished and surface error is improved from 538.241 nm [peak to valley (PV)], 96.376 nm (rms) to 76.372 nm (PV), 8.295 nm (rms) after 51.6 h rough polishing, 42.6 h fine polishing, and 54.6 h super-smooth polishing. A 50×50×10 mm sample is polished with exactly the same parameters. The roughness is improved from 1.793 nm [roughness average (Ra)] to 0.167 nm (Ra) and LIDT is improved from 9.77 to 19.2 J/cm 2 after MRF elastic polishing.
Combination technology of MRF and sub-aperture smoothing in off-axis asphere manufacture was researched. The
asphere polishing with computer control polishing is not very deterministic , as removal rate of computer control
polishing is variation over time in asphere polishing due to the tool misfit, pad wear, or slurry variation.
Magnetorheological Finishing is deterministic, subaperture finishing technology in asphere manufacture, but it is limited
to smooth mid-spatial-frequencies and high-spatial-frequencies. An example was given in a 290mm circle aperture offaxis
aspherical optic polishing process with combination technology of MRF and sub-aperture smoothing. The final
figure error was λ/50 rms from the initial 0.8λ rms. The result shows that the combination technology is practical and
have high convergence efficiency.
We demonstrate a method to solve the main problem involved in lattice design for subaperture stitching interferometry. That is calculation of the aspheric departure of a subaperture whose location is given arbitrarily. The simulated measurement data with unknown phase values are transformed into the global model frame, which satisfy the nominal surface equation. Then the phase values can be solved and used to calculate the aspheric departure with piston, tilts, and power removed. Accordingly, the subaperture lattice is determined with preferred interferometer parameters.
The removal function is the fundamental function for the computer controlled optical surfacing (CCOS) process. The removal function of magnetorheological jet polishing is investigated to obtain an optimum one by the method of CCOS. The initial annular footprint is revolved around an eccentric axis resulting in a time-averaged footprint with the largest removal at the center, monotonically decrease to zero at the edge. Similarly if the rotational footprint is traversed in a straight line, the convolved shape of stripe shows a Gaussian-like character. We also calculate the cycloid pattern from parallel scanning by rotational footprint and predict the residual errors on the surface. Calculation results show that the eccentric rotation motion of the jet can lead to improvements in footprint shapes and removal profiles.
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