The laser damage threshold of fused silica optics is affected by the surface/subsurface defects in optical fabrication and is related to surface contamination. In this paper, the evolution of surface roughness and photothermal weak absorption of fused silica treated by plasma cleaning and ion beam figuring (IBF) is studied. The results show that plasma cleaning has a certain change on the surface roughness of fused silica, while the change of photothermal weak absorption depends on the initial surface quality. The surface roughness of fused silica has a certain regularity after ion beam figuring, and the photothermal weak absorption is basically consistent with the initial.
Inductively coupled plasma processing (ICPP) is regarded as an unrivaled technique in the field of aspheric and freeform optics fabrication for its non-contact chemical etching and high efficiency. In this paper, to evaluate the obliquely incident machining behavior of removal function during inductively coupled plasma, we did some plasma beam line scanning test treatments on the fused silica surface with different obliquely incident angles in two vertical directions. Test results show that the full width at half maximum (FWHM) of the footprint of removal function increases from 13.1699 mm to 14.8368 mm with the increase of the obliquely incident angle from 0° to 30° along the X-direction line scanning processing, and that of another direction only increases from from 13.1699 mm to 14.0598 mm. Furthermore, the material removal rate in both directions reduces in a small range with the increase of the obliquely incident angle less than 10° under our processing condition. Therefore, test results demonstrate that the three-axis machining system can be effective supposing that the local slopes of the part to treat are less than about 10°. The presented conclusions can provide technical guidance for fused silica aspheric and free-form optical surface machining.
To improve the effective deformation of the unimorph deformable mirror (DM), ion beam figuring (IBF) is applied for manufacturing the initial surface figure. However, heat produced during IBF can cause comparable internal stress and thermal deformation on the adhesive layer, which causes the mirror’s figure to be unsatisfactory. Consequently, the whole manufacturing process, especially thermal distribution on the adhesive, should be monitored and controlled. A 19-unit thin-layer unimorph DM is adopted as the object to simulate and measure the distribution law of temperature field on the adhesive during the IBF process. IBF parameters are optimized according to the characteristics of the adhesive and the manufacturing simulation results. With a ϕ3-mm diaphragm integrated on IBF, the processing temperature is reduced below the adhesive’s glass transition temperature Tg. Actual figuring results report over 50% reduction on both peak-to-valley (PV) and root-mean-square (RMS) of DM with the value from 310 to 141 nm and from 38 to 17 nm, respectively. This precision has satisfied the self-corrected requirement of general unimorph DMs, which is usually <20 nm RMS. The voltage fitting coefficient of self-correcting for most electrodes is reduced to <0.05, which means the significant improvement in the effective deformation and correction ability.
Surface slope profile is commonly used to evaluate X-ray reflective optics, which is used in synchrotron radiation beam. Moreover, the measurement result of measuring instrument for X-ray reflective optics is usually the surface slope profile rather than the surface height profile. To avoid the conversion error, the slope-based figuring model is introduced introduced by processing the X-ray reflective optics based on surface height-based model. However, the pulse iteration method, which can quickly obtain the dell time solution of the traditional height-based figuring model, is not applied to the slope-based figuring model because property of the slope removal function have both positive and negative values and complex asymmetric structure. To overcome this problem, we established the optimal mathematical model for the dwell time solution, By introducing the upper and lower limits of the dwell time and the time gradient constraint. Then we used the constrained least squares algorithm to solve the dwell time in slope-based figuring model. To validate the proposed algorithm, simulations and experiments are conducted. A flat mirror with effective aperture of 80 mm is polished on the ion beam machine. After iterative polishing three times, the surface slope profile error of the workpiece is converged from RMS 5.65 μrad to RMS 1.12 μrad.
The ion beam figuring(IBF) process can produce high accuracy optical surfaces, but the material removal rate is usually lower than 5nm/s and it usually can’t reduce the surface roughness and the middle frequency. This paper study on material removal characteristic of reactive ion beam figuring(RIBF) for optics mirrors, the RIBF process combines physical and chemical effects to remove material, including physical sputtering, spontaneous chemical etching and simultaneous ion bombardment-enhanced desorption. The experiment results indicated that the RIBF process improved the surface quality for optics mirrors by decreased the surface roughness and the middle frequency, and it increased the removal efficiency compare with the IBF process. The research hopes to establish a combined process of RIBF and IBF, and aim to obtain high removal rate and high accuracy surfaces for complex optics mirrors.
During the conventional optical shaping process of fused silica, lapping is generally used to remove grinding damage layer. But this process is of low efficiency, it cannot meet the demand of large aperture optical components. Therefore, Inductively Coupled Plasma Processing (ICPP) was proposed to remove grinding damage layer instead of lapping. ICPP is a non-contact, deterministic figuring technology performed at atmospheric pressure. The process benefits from its ability to simultaneously remove sub-surface damage (SSD) while imparting the desired figure to the surface with high material remove rate. The removing damage capability of ICPP has preliminarily been confirmed on medium size optical surfaces made of fused silica, meanwhile serious edge warping was found. This paper focused on edge effect and a technique has been designed to compensate for these difficulties. Then it was demonstrated on a large aperture fused silica mirror (Long320mm×Wide370mm×High50mm), the removal depth was 30.2μm and removal rate got 6.6mm3/min. The results indicate that ICPP can rapidly remove damage layer on the fused silica induced by the previous grinding process and edge effect is effective controlled.
Ion beam figuring (IBF) provides a highly deterministic method for the final precision figuring of optical components. According to the Sigmund sputtering theory, the mass of incident ions is an important factor to the sputtering rate and the optical surface quality. Both Ar+ and Kr+ are alternative ions in IBF, but the mass of Kr+ equals two times that of Ar+. In order to achieve the nanometer and sub-nanometer precision fabrication with IBF, the optical material removal property of Ar+ and Kr+ ions was researched. The bombardment process had been simulated with the software TRIM, and the sputtering yield of Ar+ and Kr+ ions for different incident angles was calculated. Then the removal function experiments on Si were conducted. The simulations and experiments result indicated that Ar+ ion beam achieves higher removal rate at 0° incident angle, but Kr+ ion beam performs more efficiently when the incident angle gets across a critical point.
Ion beam figuring (IBF) is a deterministic and powerful optical figuring process to high-end optics. To perform an IBF process, some computations are inevitably involved, such as to extract beam removal functions, to calculate dwell times, to determine scan velocities and to recognize the removal rate. These computations influence the process time and the process result of the IBF. In this paper, the computations involved in an IBF process are introduced and the software IBFCAM, which is developed by us to implement all the computations, is also introduced. IBFCAM is well designed and it consists of 6 modules, including IBFCAM. Read, IBFCAM. Removal, IBFCAM.BRF, IBFCAM. Time, IBFCAM. Code and IBFCAM. PostAnalysis. Each module deal with a relatively independent function. Particularly, The IBFCAM. Post Analysis is newly added to IBFCAM in version 5.0. It is used to recognize the actual the removal rate. especially useful to a new material or a new optics. This is very useful to a new optics, especially to a new material.
Nd doped phosphate glass is widely used as gain media in high power laser system. It is traditionally polished with the annular polishing technology. The edge effect is inevitable in annular polishing process and it results in the low manufacturing efficiency. Ion Beam Figuring (IBF) is a highly deterministic, non-contact method for the ultra-precision optics fabrication. So the edge effect is avoided. Nanometer and sub-nanometer precision is realizable in IBF. In this paper, Nd doped phosphate glass was polished with IBF, and the evolvement of surface roughness was emphasized. The roughness of surface polished with ion beam at normal and oblique incidence was researched. The oblique incident angle was 45°. The surface roughness was measured with the white light interferometer. No evident change was observed. This means that the pre-finish roughness can be preserved in IBF. The results denote that IBF is a feasible method to correct the contour errors of Nd doped phosphate glass, and the roughness will not be coarsened.
Material removal accuracy has a direct impact on the machining precision and efficiency of ion beam figuring. By analyzing the factors suppressing the improvement of material removal accuracy, we conclude that correcting the removal function deviation and reducing the removal material amount during each iterative process could help to improve material removal accuracy. Removal function correcting principle can effectively compensate removal function deviation between actual figuring and simulated processes, while experiments indicate that material removal accuracy decreases with a long machining time, so a small amount of removal material in each iterative process is suggested. However, more clamping and measuring steps will be introduced in this way, which will also generate machining errors and suppress the improvement of material removal accuracy. On this account, a free-measurement iterative process method is put forward to improve material removal accuracy and figuring efficiency by using less measuring and clamping steps. Finally, an experiment on a φ 100-mm Zerodur planar is preformed, which shows that, in similar figuring time, three free-measurement iterative processes could improve the material removal accuracy and the surface error convergence rate by 62.5% and 17.6%, respectively, compared with a single iterative process.
An ion beam figuring system (KDIBF2000) for final figuring of large size optics has been designed and built by National University of Defense Technology in China. It can figure optics up to the maximum dimensions of 2.0m×2.0m×0.4m with five axes of servo-motion used to control ion source movement. For operational facility, there are two vacuum chambers with main work chamber and a small supplementary chamber isolated by a flapper valve. The main chamber has two work zones, which can meantime hold a large optics with Φ1.5m and a small optics with 0.4m. The small optics can be transferred through supplementary chamber with a moving vehicle. By this way, it is very convenient and economical to gain the material removal function and check the system’s process performance. Now, this system has been gone into running to figure large SiC off-axis aspheric optics. Next step, a 1.2m SiC aspheric primary mirror will be figure by this system.
A new ion beam figuring (IBF) technique, obliquely incident IBF (OI-IBF), is proposed. In OI-IBF, the ion beam bombards the optical surface obliquely with an invariable incident angle instead of perpendicularly as in the normal IBF. Due to the higher removal rate in oblique incidence, the process time in OI-IBF can be significantly shortened. The removal rates at different incident angles were first tested, and then a test mirror was processed by OI-IBF. Comparison shows that in the OI-IBF technique with a 30 deg incident angle, the process time was reduced by 56.8%, while keeping the same figure correcting ability. The experimental results indicate that the OI-IBF technique is feasible and effective to improve the surface correction process efficiency.
This paper proposes a new chemical mechanical polishing (CMP) process method for CaF2 single crystal to get ultraprecision surface. The CMP processes are improving polishing pad and using alkaline SiO2 polishing slurry with PH=8, PH=11 two phases to polish, respectively, and the roughness can be 0.181nm Rq (10μm×10μm). The CMP process can’t get high surface figure, so we use ion beam figuring (IBF) technology to obtain high surface figure. However, IBF is difficult to improve the CaF2 surface roughness. We optimize IBF process to improve surface figure and keep good surface roughness too. Different IBF incident ion energy from 400ev to 800ev does not affect on the surface roughness obviously but the depth of material removal is reverse. CaF2 single crystal can get high precision surface figure (RMS=2.251nm) and still keep ultra-smooth surface (Rq=0.207nm) by IBF when removal depth is less than 200nm. The researches above provide important information for CaF2 single crystal to realize ultra-precision manufacture.
We propose a deterministic figuring method for fabricating high-precision optical surfaces by combining ion beam material adding (IBA) and material removal [ion beam figuring (IBF)] technology. The IBA is first utilized to improve the accuracy and quality of the optical surface by deterministically adding material to the local pits, and then the IBF is applied to further improve the surface accuracy through removing the local protuberances. Compared with current IBF technology, this combined method can realize the uniform convergence of the surface errors and simultaneously increase the convergence rate of the figuring process.
In deterministic ion beam figuring (IBF) technology, the application of small ion beam enhances the corrective capability for mid-to-high spatial frequency errors on the optical surface, which directly determines the surface accuracy of the figuring process. But when the diameter of the ion beam becomes smaller, the machining errors will have a stronger influence on the final figuring result, so these errors must be controlled through corresponding methods. We investigate the corrective principle in IBF for surface errors of different spatial frequencies and establish the selection criterion for removal function in different figuring stages to realize the rapid convergence of surface accuracy. Then, through analyzing and controlling the machining errors in the figuring process, high-precision mirrors can be rapidly obtained. Finally, experiments on fused silica planar and spherical samples are conducted on our self-developed IBF system, and their final surface accuracy are both smaller than 1.1 nm RMS (root-mean-square) and 12.0 nm PV (Peak-to-Valley) after several iterations within 20 min.
KEYWORDS: Ion beams, Ion beam finishing, Temperature metrology, Optical components, Ions, Thermal modeling, Copper, Optical engineering, Thermal effects, Monte Carlo methods
Ion beam figuring (IBF) Technology to fabricate optical materials with a high thermal expansion coefficient, such as BK7, we were aware that the thermal effect is a troublesome problem that generates high thermal stress. If the thermal stress is over the component mechanical stress limit, the component may crack or break. We discuss this problem in detail and set up a thermal model. Using this model, we gain the machined component temperature field and its corresponding thermal stress field. A filtered IBF method is created to correct the thermal stress field which that gain a more even thermal stress distribution and decrease the thermal stress about 20%.
KEYWORDS: Ion beams, Ion beam finishing, Surface finishing, Silicon carbide, Ions, Process control, Polishing, Chemical vapor deposition, Defense technologies, Plano
Ion beam figuring (IBF) is an optical fabrication technique that provides highly deterministic process to correct surface figure error of previously polished surfaces by using a directed, inert and neutralized ion beam to physically sputter material from the optic surface. Recently, an ion beam figuring system KDIFS-500 has been designed and built in National University of Defense Technology (NUDT) of the P.R. China. KDIFS-500 is capable of processing workpiece up to Φ500mm. Line scanning process was discussed in detail for estimating the parameters of the beam removal function (BRF) in process. Experiments were conducted to demonstrate that the BRF increases gradually in process and by employing a stability control, the BRF can be kept stable in process. Finally, a Φ95 mm plano optical sample of CVD coated SiC substrate has been figured in two process iterations for demonstrating the correction capability of the KDIFS-500. Their figure convergence ratios reached 5.8 and 2.1 respectively. The actual figure residual errors were basically consistent with the predicted error. These consistencies indicated that the IBF processes on KDIFS-500 are predictable deterministic processes.
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