Research on the influence of the non-stationary effect of the magnetorheological finishing removal function on mid-frequency errors of optical component surfaces.

With the continuous development of modern optical systems, the demand for full spatial frequency errors of optical components in the system is increasing. Although computer-controlled sub-aperture polishing technology can quickly correct low-frequency errors, this technology significantly worsens the mid-frequency errors on the surface of the component, which greatly inhibits the improvement of optical system performance. Therefore, we conducted in-depth research on the non-stationary effect of the removal function caused by the fluctuation in magnetorheological polishing and their influence on the mid-frequency errors of the component surface. We established a non-stationary profile model of the removal function and applied this model to simulate the distribution of mid-frequency errors on the surface of the processed component, considering the non-stationary effect. The simulation results showed that the non-stationary effect of the removal function weaken the mid-frequency ripple errors but increase other mid-frequency errors. Therefore, we first proposed the optimal single-material removal thickness corresponding to the non-stationary effect and experimentally verified the effectiveness of the optimal material removal thickness in suppressing mid-frequency errors. The experimental results showed that when the magnetorheological finishing single-material removal thickness is set to the optimal value, both the mid-frequency ripple errors and the mid-frequency RMS on the surface significantly decrease. Therefore, this work provides a basis for improving the existing magnetorheological finishing process and effectively suppressing the mid-frequency errors on the surface of processed components. It also provides theoretical and technical support for the magnetorheological processing and manufacturing of high-precision optical components. At the same time, the non-stationary effect and the corresponding analytical models has the potential to be extended to other polishing tools.

Adaptive null interferometric test using spatial light modulator for free-form surfaces.

We report a method of using a liquid-crystal spatial light modulator (LC-SLM) as reconfigurable multi-level interferogram-type computer generated holograms (ICGHs) to perform dynamic null tests for aspheric and free-form surfaces. With the proposed multi-level ICGHs encoding method, amplitude and accuracy of the applicable aberration of LC-SLMs are both suitable for interferometric test. No other equipment is required to monitor the dynamic phase of LC-SLM for guaranteeing test accuracy. Moreover, complicated phase response calibration of the LC-SLM is not required. Besides being used in collimated beams, the LC-SLM is demonstrated for the first time to be used in divergent beams; hence, concave surfaces with apertures larger than that of the LC-SLMs can be tested. For realizing practical tests, the calibration of inherit wavefront distortion of the LC-SLM, diffraction orders isolation, and alignment are analyzed in detail. Two free-form surfaces with about 20 µm departure from flat and spherical surfaces are successfully measured in collimated beam and divergent beam, respectively. Cross tests are provided to verify the test accuracy.

Flexible interferometric null testing for concave free-form surfaces using a hybrid refractive and diffractive variable null.

Free-form surfaces have been applied in a wide range of modern optical systems. As a supporting technique for fabricating free-form surfaces, the interferometric null method for testing the surface figure error has very limited flexibility. In this Letter, we report a flexible interferometric null test method which can test free-form surfaces with a very broad departure varying range. In the presented flexible null method, a hybrid refractive and diffractive variable null (HRDVN) is utilized as the flexible null. The HRDVN has superb aberration types adaptability, amplitude adaptability, and moderate phase generating accuracy. A flexible interferometric null testing setup was established using the HRDVN. Its superb adaptive capacity and moderate test accuracy were successfully demonstrated by measuring a free-form surface with rotationally symmetric departure of 173.486λ (λ=632.8 nm) and non-rotationally symmetric departure of 23.786λ.

Near-null interferometry using an aspheric null lens generating a broad range of variable spherical aberration for flexible test of aspheres.

A null lens moving back and forth relative to a point source can generate variable spherical aberration for flexible test of aspheres. Different from the previous methods, variable spherical aberration null theory was developed by us to optimize the null lens. The optimized null was a plano-convex singlet containing a high order even asphere. Its attractive advantages are the simple structure and the broad range of testable surfaces. Most concave prolate conic and near conic surfaces with k∙R value varying between 0 and about 70000mm and with smaller relative aperture than that determined by each k∙R value can be tested. The testable asphericity range is between 0 and about 230λ. Relations among these testable surfaces were revealed as different groups of equidistant surfaces. To explicitly show the ability of the null, the measurable surfaces range map that contains all parameters defining a conic surface was offered. A practical near-null test system using this null was established. Alignment, near-null data processing, and error sources are analyzed in detail. To verify the broad testable surfaces range, three surfaces with widely varying amounts of asphericity were tested. Cross tests were provided to verify the test system accuracy.

Unimorph deformable mirror with an integrated strain feedback layer.

The correction accuracy of a pint-sized unimorph deformable mirror (DM) is significantly influenced by the nonlinear hysteresis error of piezoelectric ceramics, especially in an open-loop state. Moreover, the control bandwidth is also reduced by the nonlinearity. In this paper, we fabricated a three-unit pint-sized unimorph DM with strain gauges integrated on the actuators as a feedback layer for the first time. An experimental platform was built and utilized to test each electrode's strain signal. Testing results show that, under quasi-static condition, the hysteresis curve of the mirror's central displacement is corrected and the hysteresis rate could be reduced from 11% to less than 2% by adopting the strain feedback signal. More specifically, the DM's initial surface, Zernike defocus, together with spherical aberration can also be corrected by this method, and the correction accuracy is improved more than 20% compared to the open-loop state. By introducing a closed-loop control the gaps of the DMs under open loop are supplied. This demonstrates that adding a strain feedback layer is promising to enhance the performance of a unimorph DM.

Engineering a coaxial visible/infrared imaging system based on monolithic multisurface optics.

All-reflective coaxial visible/infrared imaging systems based on monolithic multisurface optics have been a hot point of research in recent years. Since multiple surfaces share a single substrate, their relative positions are fundamentally guaranteed as fabricated without any alignment. In this paper, the coaxial system is designed with multifolded ideas. Both the visible subsystem and the infrared subsystem are comprised of two monolithic optical modules, which are machined by single-point diamond turning (SPDT). A novel method based on a computer-generated hologram (CGH) is then proposed to simultaneously measure the shape and position of monolithic multisurface optics. The effects of surface shape and position error on the wavefront aberration of the system are also discussed with the help of Zernike annular polynomials. Then the wavefront aberration of the system is measured, from which we subtract the contribution of surface shape and position error. The aberration induced by misalignment of the two monolithic modules is then estimated. It indicates that the concentricity is about 3 µm. Finally, two similar systems with different clear apertures are assembled as a coaxial visible/infrared imaging system. Coaxial visible and infrared images are captured and fused to show clearer details.

Figuring process of potassium dihydrogen phosphate crystal using ion beam figuring technology.

Currently, ion beam figuring (IBF) technology has presented many excellent performances in figuring potassium dihydrogen phosphate (KDP) crystals, such as it is a noncontact figuring process and it does not require polishing fluid. So, it is a very clean figuring process and does not introduce any impurities. However, the ion beam energy deposited on KDP crystal will heat the KDP crystal and may generate cracks on it. So, it is difficult directly using IBF technology to figure KDP crystal, as oblique incident IBF (OI-IBF) has lower heat deposition, higher removal rate, and smoother surface roughness compared to normal incident IBF. This paper studied the process of using OI-IBF to figure KDP crystal. Removal rates and removal functions at different incident angles were first investigated. Then heat depositions on a test work piece were obtained through experiments. To validate the figuring process, a KDP crystal with a size of 200 mm×200 mm×12 mm was figured by OI-IBF. After three iterations using the OI-IBF process, the surface error decreases from the initial values with PV 1.986λ RMS 0.438λ to PV 0.215λ RMS 0.035λ. Experimental results indicate that OI-IBF is feasible and effective to figure KDP crystals.

Stitching test of large flats by using two orthogonally arranged wavefront interferometers.

The most challenging problem in the stitching test of large flats with a small-aperture interferometer is the accumulation effect of the second-order error. As it is approximately enlarged by the square of the ratio of full aperture size to subaperture size, a very small amount of the second-order error in the reference surface of a transmission flat can be accumulated and gets far from negligible when the subaperture is far smaller than the full aperture. We present here a solution by using two orthogonally arranged wavefront interferometers. One is responsible for a subaperture test and the other for the simultaneous measurement of relative tilts. Because the accumulation effect originates from the lateral shift of the second-order error, only the tilt along the subaperture scanning direction needs to be measured accurately. It is no longer determined by stitching optimization instead to avoid the error accumulation. Piston and tilt perpendicular to the scanning direction are still determined by stitching optimization. The method is experimentally verified and compared to the stitching test with the reference surface error calibrated out, both referenced to the full aperture test result obtained with a 24-inch interferometer.

Research on temperature field of KDP crystal under ion beam cleaning.

KH2PO4 (KDP) crystal is a kind of excellent nonlinear optical component used as a laser frequency conversion unit in a high-power laser system. However, KDP crystal has raised a huge challenge in regards to its fabrication for high precision: KDP crystal has special physical and chemical characteristics. Abrasive-free water-dissolution magnetorheological finishing is used in KDP figuring in our lab. But the iron powders of MRF fluid are easily embedded into the soft surface of KDP crystal, which will greatly decrease the laser-induced damage resistance. This paper proposes to utilize ion beam figuring (IBF) technology to figure and clean the surface of a KDP component. Although IBF has many good performances, the thermal effect control is a headachy problem for the KDP process. To solve this problem, we have established its thermal effect models, which are used to calculate a component's surface temperature and thermal gradient in the whole process. By this way, we can understand how to control a temperature map and its gradient in the IBF process. Many experiments have been done to validate and optimize this method. Finally, a KDP component with the size of 200×200×12 mm is successfully processed by this method.

Effects of temperature on the removal efficiency of KDP crystal during the process of magnetorheological water-dissolution polishing.

As a kind of important nonlinear optical element, KDP crystal has great demand in the inertial confinement fusion system. Based on the dissolution mechanism of solid materials, the factors that affect the material removal rate of KDP crystal in magnetorheological (MR) water-dissolution polishing are investigated to improve the machining efficiency. It is found that the material removal rate is proportional to the product of the saturation concentration and diffusion coefficient, and the relationship between the removal efficiency and the temperature meets the unilateral Gaussian function. Polishing experiments are carried out on a magnetorheological finishing (MRF) machine with self-designed MRF fluid heating devices. The experimental results show that practical efficiency-temperature curve is consistent with the theoretical curve, and the maximum machining efficiency increases by about 50% with the rise of temperature from 294 to 302 K. Meanwhile, when the MR fluid temperature is lower than 308 K, the crystal surface quality and surface roughness in different processing temperatures have no remarkable difference with constant crystal temperature (294 K). This research indicates that it is feasible to drastically improve KDP crystal MRF efficiency by controlling the processing temperature.

Research of polishing process to control the iron contamination on the magnetorheological finished KDP crystal surface.

A new nonaqueous and abrasive-free magnetorheological finishing (MRF) method is adopted for processing a KDP crystal. MRF polishing is easy to result in the embedding of carbonyl iron (CI) powders; meanwhile, Fe contamination on the KDP crystal surface will affect the laser induced damage threshold seriously. This paper puts forward an appropriate MRF polishing process to avoid the embedding. Polishing results show that the embedding of CI powders can be avoided by controlling the polishing parameters. Furthermore, on the KDP crystal surface, magnetorheological fluids residua inevitably exist after polishing and in which the Fe contamination cannot be removed completely by initial ultrasonic cleaning. To solve this problem, a kind of ion beam figuring (IBF) polishing is introduced to remove the impurity layer. Then the content of Fe element contamination and the depth of impurity elements are measured by time of flight secondary ion mass spectrometry. The measurement results show that there are no CI powders embedding in the MRF polished surface and no Fe contamination after the IBF polishing process, respectively. That verifies the feasibility of MRF polishing-IBF polishing (cleaning) for processing a KDP crystal.

Influence law of structural characteristics on the surface roughness of a magnetorheological-finished KDP crystal.

A new nonaqueous and abrasive-free magnetorheological finishing (MRF) method is adopted for processing potassium dihydrogen phosphate (KDP) crystal due to its low hardness, high brittleness, temperature sensitivity, and water solubility. This paper researches the influence of structural characteristics on the surface roughness of MRF-finished KDP crystal. The material removal by dissolution is uniform layer by layer when the polishing parameters are stable. The angle between the direction of the polishing wheel's linear velocity and the initial turning lines will affect the surface roughness. If the direction is perpendicular to the initial turning lines, the polishing can remove the lines. If the direction is parallel to the initial turning lines, the polishing can achieve better surface roughness. The structural characteristic of KDP crystal is related to its internal chemical bonds due to its anisotropy. During the MRF finishing process, surface roughness will be improved if the structural characteristics of the KDP crystal are the same on both sides of the wheel. The processing results of (001) plane crystal show we can get the best surface roughness (RMS of 0.809 nm) if the directions of cutting and MRF polishing are along the (110) direction.

Research on compensation of suction deformation error of potassium dihydrogen phosphate crystal.

Potassium dihydrogen phosphate (KDP) crystals used in high power laser systems require high figure accuracy. Deformation caused by vacuum suction is one of the most significant factors affecting the final surface figure accuracy. It is difficult to compensate the error caused by the suction deformation when using a single point diamond flycutting method. This paper presents a spiral turning method to machine KDP crystals on a common ultraprecision lathe, and the transfer rule and compensation theory of the suction deformation error are studied. The in situ measurement of deformation error is accomplished with a displacement test apparatus, which can acquire submicron precision, and then compensates for it by utilizing the three-axis servo control technology. Meanwhile, cutting parameters are strictly controlled to achieve full-aperture ductile material removal. The compensation experiments for both reflected and transmitted wavefront error are carried out on a Φ270 mm KDP crystal and the transmission wavefront error of 0.591λ (peak-to-valley, λ = 632.8 nm) and 25 nm/cm (gradient root-mean-square) are obtained after one compensation with full-aperture surface roughness of about 2 nm RMS.

Error analysis of compensation cutting technique for wavefront error of KH2PO4 crystal.

Considering the wavefront error of KH(2)PO(4) (KDP) crystal is difficult to control through face fly cutting process because of surface shape deformation during vacuum suction, an error compensation technique based on a spiral turning method is put forward. An in situ measurement device is applied to measure the deformed surface shape after vacuum suction, and the initial surface figure error, which is obtained off-line, is added to the in situ surface shape to obtain the final surface figure to be compensated. Then a three-axis servo technique is utilized to cut the final surface shape. In traditional cutting processes, in addition to common error sources such as the error in the straightness of guide ways, spindle rotation error, and error caused by ambient environment variance, three other errors, the in situ measurement error, position deviation error, and servo-following error, are the main sources affecting compensation accuracy. This paper discusses the effect of these three errors on compensation accuracy and provides strategies to improve the final surface quality. Experimental verification was carried out on one piece of KDP crystal with the size of Φ270 mm×11 mm. After one compensation process, the peak-to-valley value of the transmitted wavefront error dropped from 1.9λ (λ=632.8 nm) to approximately 1/3λ, and the mid-spatial-frequency error does not become worse when the frequency of the cutting tool trajectory is controlled by use of a low-pass filter.

Optimization of the Morphology of the Removal Function for Rotating Abrasive Water Jet Polishing.

Abrasive water jet polishing has significant advantages in the manufacturing of complex optical components (such as high-slope optical component cavities) that require high-precision manufacturing. This is due to its processing process, in which the polishing tool does not make direct contact with the surface of the workpiece, and instead maintains a considerable distance. However, the removal functions of most existing abrasive water-jet polishing technologies do not possess strict symmetry, which significantly impacts the ability to correct surface figure errors. Therefore, this study implements rotating abrasive water-jet polishing based on traditional abrasive water jet processing to optimize the removal function, which turns it into a Gaussian form; thus, obtaining a type of removal function more suitable for CCOS polishing. This paper derives an empirical formula between the distance s' from the peak removal point of the removal function to the stagnation point and the nozzle tilt angle α, based on geometric relationships and experimental results, analyzes the relationship between material removal efficiency, nozzle tilt angle, and standoff distance. Finally, this paper verifies through experiments the validity of this empirical formula under different process parameters. Therefore, this study obtains the process conditions that allow rotating abrasive water-jet polishing technology to achieve a stable Gaussian form removal function, and the appropriate process parameters to be selected in conjunction with polishing efficiency; thereby, effectively improving the removal function's corrective ability and manufacturing efficiency. It provides theoretical support for the processing capability and process parameter selection of abrasive water-jet polishing technology, solves the problem of limited shaping capability of existing abrasive water jet tools, and significantly improves the manufacturing capability of high-end optical components.

Ultra-Smooth Polishing of Single-Crystal Silicon Carbide by Pulsed-Ion-Beam Sputtering of Quantum-Dot Sacrificial Layers.

Single-crystal silicon carbide has excellent electrical, mechanical, and chemical properties. However, due to its high hardness material properties, achieving high-precision manufacturing of single-crystal silicon carbide with an ultra-smooth surface is difficult. In this work, quantum dots were introduced as a sacrificial layer in polishing for pulsed-ion-beam sputtering of single-crystal SiC. The surface of single-crystal silicon carbide with a quantum-dot sacrificial layer was sputtered using a pulsed-ion beam and compared with the surface of single-crystal silicon carbide sputtered directly. The surface roughness evolution of single-crystal silicon carbide etched using a pulsed ion beam was studied, and the mechanism of sacrificial layer sputtering was analyzed theoretically. The results show that direct sputtering of single-crystal silicon carbide will deteriorate the surface quality. On the contrary, the surface roughness of single-crystal silicon carbide with a quantum-dot sacrificial layer added using pulsed-ion-beam sputtering was effectively suppressed, the surface shape accuracy of the Ø120 mm sample was converged to 7.63 nm RMS, and the roughness was reduced to 0.21 nm RMS. Therefore, the single-crystal silicon carbide with the quantum-dot sacrificial layer added via pulsed-ion-beam sputtering can effectively reduce the micro-morphology roughness phenomenon caused by ion-beam sputtering, and it is expected to realize the manufacture of a high-precision ultra-smooth surface of single-crystal silicon carbide.

Research on Laser-Induced Damage Post-Restoration Morphology of Fused Silica and Optimization of Patterned CO2 Laser Repair Strategy.

Fused silica has become the preferred optical material in the field of inertial confinement fusion (ICF) due to its excellent performance; however, these costly optical elements are vulnerable, and their manufacture is time-consuming. Therefore, the restoration of laser-induced damage for these optical elements is of great value. To restrain the post-restoration raised rim problem in the CO2 laser repair process to improve the restoration quality, the separate influences of key parameters of laser power, irradiation duration, and laser beam diameter on post-restoration pit morphology are compared in combined simulation and experimental studies. An optimized, patterned CO2 laser strategy is proposed and verified; the results indicate that, with the strategy, the rim height decreases from 2.6 µm to 1.52 µm, and maximal photo thermal absorption is decreased from 784.2 PPM to 209.43 PPM.

The Cause of Ribbon Fluctuation in Magnetorheological Finishing and Its Influence on Surface Mid-Spatial Frequency Error.

In the high-power laser system, the mid-spatial frequency error of the surface of the high-power laser component will affect the normal operation of the high-power laser system. In order to improve the mid-spatial frequency error of the high-power laser component after magnetorheological finishing, the causes and influencing factors of the ribbon fluctuation in magnetorheological finishing are studied, and the influence of different ribbon fluctuation on the mid-spatial frequency error of the surface is studied. Firstly, the influence of different ribbon fluctuations on the mid-spatial frequency error of the machined surface is simulated by a computer. Secondly, the magnetic field in the circumferential direction of the polishing wheel, the fluctuation amount and frequency of the magnetorheological polishing ribbon are measured, and then the causes of the fluctuation of the magnetorheological polishing ribbon are analyzed. Moreover, through the principle of a single variable, the influence of process parameters on the fluctuation of magnetorheological polishing ribbon is explored. Finally, the fused silica component is scanned uniformly under the process parameters of magnetorheological polishing ribbon fluctuation of 40 µm, 80 µm, 150 µm, and 200 µm. The experimental results show that the greater the ribbon fluctuation, the greater the surface mid-spatial frequency error of the component, and the ribbon fluctuation is approximately linear with the RMS of the PSD2 in the mid-spatial frequency band on the surface of the component. Therefore, the fluctuation of the ribbon can be controlled by controlling the magnetorheological processing parameters, and the mid-spatial frequency band error on the surface of the high-power laser component can be significantly reduced by optimizing process parameters after magnetorheological finishing.

Nonlinear Effects of Pulsed Ion Beam in Ultra-High Resolution Material Removal.

Ion beam sputtering is widely utilized in the area of ultra-high precision fabrication, coating, and discovering the microworld. A pulsed ion beam (PIB) can achieve higher material removal resolution while maintaining traditional ion beam removal performance and macro removal efficiency. In this paper, a 0.01 s pulse width beam is used to sputter atom layer deposition (ALD) coated samples. The nano-scale phenomenon is observed by high-resolution TEM. The results show that when the cumulative sputtering time is less than 1.7 s, the sputtering removal of solid by ion beam is accompanied by a nonlinear effect. Furthermore, the shortest time (0.05 s) and lowest thickness (0.35 nm) necessary to remove a uniform layer of material were established. The definition of its nonlinear effect under a very small removal amount guides industrial ultra-high precision machining. It reveals that PIB not only has high removal resolution on nanoscale, but can also realize high volume removal efficiency and large processing diameter at the same time. These features make PIB promising in the manufacturing of high power/energy laser optics, lithography objective lens, MEMS, and other ultra-high precision elements.

Figuring Method of High Convergence Ratio for Pulsed Ion Beams Based on Frequency-Domain Parameter Control.

The continuous phase plate (CPP) provides excellent beam smoothing and shaping impacts in the inertial confinement fusion application. However, due to the features of its dispersion, its surface gradient is frequently too large (>2 µm/cm) to process. When machining a large gradient surface with continuous ion beam figuring (IBF), the acceleration of the machine motion axis cannot fulfill the appropriate requirements, and the machining efficiency is further influenced by the unavoidable extra removal layer. The pulsed ion beam (PIB) discretizes the ion beam by incorporating frequency-domain parameters, resulting in a pulsed beam with a controlled pulse width and frequency and avoiding the extra removal layer. This research evaluates the processing convergence ability of IBF and PIB for the large gradient surface using simulation and experiment. The findings reveal that PIB offers obvious advantages under the same beam diameter. Compared with the convergence ratio (γ = 2.02) and residuals (RMS = 184.36 nm) of IBF, the residuals (RMS = 27.48 nm) of PIB are smaller, and the convergence ratio (γ = 8.47) is higher. This work demonstrates that PIB has better residual convergence in large gradient surface processing. It is expected to realize ion beam machining with a higher convergence ratio.