Guipeng Tie

Novel magnetorheological f iguring of KDP crystal

A new process of magnetorheological figuring (MRF) based on the deliquescence theory is proposed to finish KDP crystals. A novel, non-aqueous, and abrasive-free magnetorheological (MR) fluid is explored, and polishing experiments are performed on a self-developed MRF machine. The removal mechanism is reckoned to be the result of a combination of dominant chemical etching and accessorial mechanical drag. The results indicate that the surface roughness of I plate KDP of 80×80 (mm) polished by MRF is 1.2 nm (root mean square (RMS)), and the tool marks are completely removed. The surface accuracy by MRF is 0.035\lambda (RMS), and the low/middle-frequency errors are significantly corrected after MRF.

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.

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 components 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.

Tool decentration effect in slow tool servo diamond turning off-axis conic aspheric surface

Off-axis conic aspheric mirrors are crucial components in some optical systems, such as three-mirror-anastigmatic telescopes (TMA). However, because of the swing limitation of lathe, off-axis aspheric mirrors are not easy to fabricate using a general-purpose diamond turning machine. This research demonstrates slow tool servo diamond turning process which allows fabricating off-axis conic aspheric mirrors on-axis. The figure error caused by tool centering error was studied on. An off-axis parabolic mirror was fabricated and actual machining data are discussed. The result proved that proposed approach is capable of fabricating copper off-axis parabolic mirror of 46mm diameter to a form accuracy of 0.736μm in PV error value.

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.

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.

Thermal analysis for ion beam processing of the unimorph deformable mirror

The unimorph deformable mirror (DM) is favored in the field of synchrotron radiation due to its simple structure, dynamic surface figure and adaptive adjustment. The request of mirror surface accuracy on the synchrotron radiation beam focus can be up to sub-nanometer RMS. Ion beam figuring is a high precision processing method with noncontacting and roughness damage. However, because it belongs to the type of thermal manufacturing, the adhesive layer characteristic is changed and the DM’s figuring accuracy is reduced by the thermal deformation. In this paper, thermal simulation and temperature test of the adhesive during ion beam processing are carried out; The variation law of temperature and thermal stress of the adhesive layer with different ion beam diaphragms and scanning times are obtained. Therefore, the selective guideline for the diaphragm is obtained. With the optimal process parameters, the temperature of the adhesive layer is decreased with the minimum temperature between the glass transition temperature Tg and 1/2 of the Curie temperature Tc.

Out-of-squareness measurement on ultra-precision machine based on the error separation

Traditional methods of measuring out-of-squareness of ultra-precision motion stage have many limitations, especially the errors caused by inaccuracy of standard specimens, such as bare L-square and optical pentaprism. And generally, the accurate of out-of-squareness measurement is lower than the accurate of interior angles of standard specimens. Based on the error separation, this paper presents a novel method of out-of-squareness measurement with a polygon artifact. The angles bounded with the guideways and the edges of polygon artifact are measured, and the out-of-squareness distraction is achieved by the principle that the sum of internal the angles of a convex polygon artifact is (n-2)π. A out-of-squareness metrical experiment is carried out on the profilometer by using an optical square brick with the out-of-squareness of interior angles at about 1140.2 arcsec. The results show that the measurement accuracy of three out-of-squareness of the profilometer is not affected by the internal angles. The measurementwith the method can be applied to measure the machine error more accurate and calibrate the out-of-squareness of machine.

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.

Study on surface roughness evolvement of Nd-doped phosphate glass after IBF

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.