Key Engineering Materials Vol. 552

Abstract: Ultra-precision diamond turning can deliver very accurate form, often less than 100nm P-V. A possible manufacturing method for thin Wolter type-1 mirrors in hard X-ray space telescopes thus involves generating electroless nickel plated mandrels by diamond turning, before coating them with a reflective film and substrate. However, the surface texture after turning falls far short from the requirements of X-ray and EUV applications. The machining marks need to be removed, with hand polishing still widely employed. There is thus a compelling need for automated finishing of turned dies. A two step finishing method is presented that combines fluid jet and precessed bonnet polishing on a common 7-axis CNC platform. This method is capable of finishing diamond turned electroless nickel plated dies down to 0.28nm rms roughness, while deterministically improving form error down to 30nm P-V. The fluid jet polishing process, which consists of pressurizing water and abrasive particles for delivery through a nozzle, has been specially optimized with a newly designed slurry delivery unit and computer simulations, to remove diamond turning marks without introducing another waviness signature. The precessed bonnet polishing method, which consists of an inflated membrane rotated at an angle from the local normal to the surface and controlled by geometrical position relative to the work-piece, is subsequently employed with a novel control algorithm to deliver scratch-free surface roughness down to 0.28 nm rms. The combination of these two deterministic processes to finish aspheric and freeform dies promises to unlock new frontiers in X-ray and EUV optics fabrication.

Abstract: Quality of the groove shap made by original diffraction grating is the main factor to affect the diffraction efficiency .The process of diffraction grating scratch mainly depends on the experience of the operator. However, its shortage is time-consuming, difficult and waste. In order to overcome the issues mentioned above, the new method has been put forward in this paper: simulate the process of diffraction grating scratch by limited element analysis and cross experiment, find out the process factor that affects the quality of the groove by rang analysis and sort. This method provides the reference to the process design of mechanical scratch grating and instruction to the adjustment of the grating trial scratch .It has practical significance on improving the quality of mechanical grating.

Abstract: Free-form lens is a special element with non-rotating body and aspheric structure. The surface can’t be expressed by stationary analytic formula, and generally it expressed by discrete list point. Free-form lens can distribute light intensity freely,and control light angle、optical path difference etc. Specific light spot is formed in illumination surface, meanwhile the light energy utilization ratio is improved greatly. There are two kinds of methods about Free-form fabrication, and the first kind of method is that free-form is grinded by a small grinding head ,which is controlled by computer; its disadvantage is low precision、high waster rate，especially for optical material; The second kind fabricating method is that multi-degree-of-freedom single-point diamond turning equipment; its disadvantage is that plenty of sub-micron crack formed when brittle material fabricated. In addition, micro-lens array element can’t be fabricated by the two kinds of methods. In allusion to the feature of free-form without stationary analytic formula ,which varying dose exposure lithography method for free-form micro-optical element is presented for free-form micro-lens and array. The method is from the theory that the relation between different exposure energy and different developing depth, adopting laser direct writing equipment free-form photoresist structure is formed in material surface, then the free-form photoresist structure is transfered on the substrate by ion etching method. Any surface figure can be fabricated by controlling every point laser energy for this method, and surface roughness can’t be effected because ion etching belongs to nanofabrication. In this paper, that low Distribution of light amplitude and relation of between exposure energy and exposure time was analysised on base of optical propagation theory and mathematical model of light distribution law was built and simulated by computer program.The results indicate that light energy distributes in accordance with specific law; the exposure depth increases with exposure time. The experiment of exposure and developing finished, used the BOL-500 laser direct writing system in Changchun University of Science and technology,american Futurrex62A photoresist, 412nn He-Ne laser device, 5‰NaOH developing solution. The experimental data coincide with simulation result via comparative analysis. Meanwhile，a set of free-form micro-lens array testing system was established and focal spot shape,energy distribution,focal distance,F parameter were tested about the micro-lens array.

Abstract: More and more optical elements are needed in modern industry, which is reflected by both the bigger amount of requirement, and more complicated designed surface such as asphere and even free form surface. As a result, optical polishing using small tools on CNC machines is applied widely in the whole world, which has a lot of advantages such as fast speed, high accuracy, high flexibility and so on. But the small tool polishing can also cause in some problems. For example, the middle spatial frequency errors (MSFE) may be increased during the polishing. The reasons may consist of the following aspects: 1) the positional error of the workpiece; 2) the mistake of error data input; 3) the unexpected stop of the polishing process; 4) the stabilization of the slurry density during the process; 5) the temperature change of the contact area between the tool and the workpiece and so on. As is known, pitch is usually used in traditional optical manufacture, and it can reduce the middle and high spatial frequency errors in a large extent because of the different removal rate between the high points and low points on the optical surface. Therefore, the application of the combination of pitch and small tool is described in this paper in order to solve the MSFE problem, and we have got good polishing results of reducing the error from 2.155nm to 0.267nm by using the new pitch tool on the CNC machine after two runs.

Abstract: Ion Beam Figure (IBF) is believed to be one of the most effective technics that can fabricate lens with nano or even sub-nano accuracy. For different sizes of IBF removal functions, the correct effects in different spatial frequency range are different. Power Spectral Density (PSD) curve can describe surface errors in full spatial frequency range, so it is a very convenient way to evaluate the quality of lens’ surface. In this paper, firstly, the principles of IBF and PSD are introduced briefly; Secondly, IBF removal functions with sizes from 2 mm to 15 mm are generated. A lens with surface error more than PV value 400nm is simulated with different sizes of IBF removal functions by Lucy-Richardson algorithm. Finally, experiments are done by IBF plant. A lens is fabricated by different sizes of removal functions and the fabricate results are tested by interferometer precisely and calculated to PSD curves. By the comparison of these curves, the IBF fabricate effects with different removal sizes are analyzed, which show that the smaller the removal size, the better the removal effect in higher spatial frequency range, but in the meantime, it will take a much longer time. Also the reasons of the difference between theory simulation and actual fabrication result are taken into account, and several influence factors are analyzed.

Abstract: In order to satisfy the special requirements of IR optical instrument, multi-band filter film on the substrate of ZnS is deposited by adopting electron beam vacuum depositing method with the ion assistant deposition technology. At incident angle of 0°～25°, 660nm high reflection and 1064nm and 3～5μm high transmission are realized. ZnS and YbF3 are selected as the high and low refractive index materials, and the film structure prepared easily is getting by optimizing the film system design curve ceaselessly. The precision of thickness control has been improved by adjusting film process parameters and improving film thickness control methods. The adhesion and firmness of film and substrate has been increased by pre-coating and mixed evaporation technology, and the laser induced damage threshold of film has been improved with vacuum annealing. The deposited film can endure the environmental tests such as rain pour, salt fog, and high and low temperature etc, and meet its practical requirement.

Abstract: According to the requirement of the filter for laser tracking system, we chose Ta as the target material of high refractive index and SiO2 as the target material of low refractive index, the film system was designed and optimized with the TFCalc software, adopted the double ion sputter coaters by VEECO company for plating system, using two sides of the substrate to broaden the long and short wavelength cut-off band respectively, we improved the peak transmittance and simplified the difficulty of experimental analysis, the filter to meet the demand has been deposited. The average transmittance of the filter at 532nm, 808nm, 1064nm and1550nm wavelength was less than 0.2% and the peak transmittance at 905nm wavelength was 96.3%. The filter can withstand the test of bad environment and satisfy the requirements of the laser tracking system.

Abstract: A method of symmetric distortion correction is described and successfully applied to correct the distortion mapping errors caused by CGH as well as internal imaging optics. The aspheric surface is 160 mm in diameter and has a maximum departure of 145 μm. According to the model established by the fiducial spots on the aspheric surface, we show experimental results after direct measurement and distortion-corrected error map respectively. The results show that the method of symmetric distortion correction is useful and effective.

Abstract: The samples of Ag/ITO multilayer films with different Ag insert layer thickness (0.5, 2, 4 nm) were prepared on sapphire and p-GaN substrates. The effects of the Ag layer thickness, annealing temperature and annealing time on the transmittance, sheet resistance and specific contact resistance of Ag/ITO films were investigated. The experiment results show that the transmittance is obviously affected by Ag insert layer thickness. The Current–voltage (I–V) measurements indicate that the sheet resistance and specific contact resistance of Ag/ITO film on p-GaN are lower than those of single ITO film. The samples with Ag(0.5nm)/ITO film on p-GaN produce the low specific contact resistance of ~1.386×10-4Ω•cm2 , low sheet resistance of ~11Ω/sq and high transmittance of ~ 90% at 455nm when the samples are annealed at 600°C for 10 minutes.

Abstract: This paper makes an researches on high precision polishing process of SiC mirror by using mechanical and chemical polishing combined methods. The chemical compositions, mechanical characteristics of SiC and the factors which affect the polishing precision and quality are analyzed. Also, the components of polishing liquid, polishing pad and the pressure of the millstone are also optimized. Then the author polishes a 220mm diameter SiC mirror which the roughness is less than 1 nm. The results show that the combination of chemical and mechanical polishing can overcome the traditional shortcomings of mechanical polishing and chemical etching method, improve the removal rate, further improve the polishing surface roughness, get the small surface damage layer and high integrity of smooth surfaces.