Qiao Xu

The Characteristics of Optics polished with a polyurethane pad

The effect of polishing an optical workpiece with a polyurethane pad was studied in this paper, including material removal rate, surface roughness and subsurface damage. Usually, optical polishing pitch is applied to polish optical workpieces, but the material removal rate (MRR) of pitch is quite low, and polyurethane foam is thus substituted for polishing pitch. With the polyurethane pad a much higher MRR was obtained. Surface roughness and subsurface damage of workpieces were also examined. We were gratified to find that there was almost no subsurface damage in the workpieces manufactured with pad polishing and surface roughness was comparable to the result of pitch polishing. Finally, a hypothesis was proposed in an attempt to explain the result that workpieces were defect-free.

A method for evaluating subsurface damage in optical glass

An alternative method for evaluating subsurface damage (SSD) in ground fused silica is presented. The method can acquire the knowledge of depth and morphology of subsurface damage at the same time. The fundamental support lent to the method is the fact that the depth of field reduces as the numerical aperture (NA)/magnification increases in optical microscopes. Large depth of field without undermining NA is preferred in most applications while the narrow range of focus depth is desired for our method. Using this method, we experimented on fused silica which was ground with bound-abrasive diamond wheels and the results show good agreement with the traditional method. The consistency indicates that the proposed method is practicable and effective in inspecting the subsurface damage in optical components.

Dwell-time algorithm for polishing large optics

The calculation of the dwell time plays a crucial role in polishing precision large optics. Although some studies have taken place, it remains a challenge to develop a calculation algorithm which is absolutely stable, together with a high convergence ratio and fast solution speed even for extremely large mirrors. For this aim, we introduced a self-adaptive iterative algorithm to calculate the dwell time in this paper. Simulations were conducted in bonnet polishing (BP) to test the performance of this method on a real 430  mm × 430  mm fused silica part with the initial surface error PV=1741.29  nm, RMS=433.204  nm. The final surface residual error in the clear aperture after two simulation steps turned out to be PV=11.7  nm, RMS=0.5  nm. The results confirm that this method is stable and has a high convergence ratio and fast solution speed even with an ordinary computer. It is notable that the solution time is usually just a few seconds even on a 1000  mm × 1000  mm part. Hence, we believe that this method is perfectly suitable for polishing large optics. And not only can it be applied to BP, but it can also be applied to other subaperture deterministic polishing processes.

Tentative investigation towards precision polishing of optical components with ultrasonically vibrating bound-abrasive pellets

Ultrasonic vibration has been employed to improve the quality of machined surface in the grinding of brittle materials. In this report, we transplant the philosophy of ultrasonic vibration assisted grinding to chemo-mechanical bound-abrasive-pellet polishing in anticipation of the improvement in either surface roughness or material removal rate. The preliminary experimental results show that the ultrasonic vibration assisted chemo-mechanical pellet polishing can yield desired results that material removal rate can be significantly raised while surface roughness is not degraded. The experimental results also indicate different mechanisms between ultrasonic-vibration-assisted chemo-mechanical pellet polishing and conventional chemo-mechanical bound-abrasive polishing.

Producing fused silica optics with high UV-damage resistance to nanosecond pulsed lasers

The laser induced damage to optics has been an issue of paramount importance in laser research community. The low damage threshold of fused silica surfaces predominantly restricts the development of high power and high energy systems. This paper is aimed at improving the surface damage threshold of fused silica substrates by researching the effect of mechanical and chemical defects on laser damage: cracks/scratches and metallic impurities. The cracks were found to close, at least in part, after thermal processing and the damage threshold of the indented region was little affected by the thermal processing. In contrast, the cracks were enlarged after chemical etching and the damage threshold was improved slightly. Concerning scratches, the damage threshold can be recovered significantly after different HF-based etching. The metallic contamination can be removed by HF-based etching and acid leaching. The etched surface shows that the damage threshold increased first to ~30J/cm2 and then decreased with etching time while the damage threshold stabilized at ~30J/cm2 for leaching >45min. The surface roughness may degrade after etching, from <1nm to 3~5nm RMS, but that is ~1nm after leaching. The leaching may be a potential method for dissolving metallic contaminants on the glass surface in order to get a smooth surface with high damage resistance.

Improvement on contamination resistance to volatile organics and moisture of sol–gel silica antireflective coating for 351 nm laser system by structural modulation with fluorinated compounds

A new silica antireflective (AR) coating with excellent spectral stability and laser-induced damage threshold (LIDT), which are crucial characteristics in high-powered laser systems, was prepared using the sol–gel method. First, aided by 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoro-1,8-octanediol (simplified as DFDO), cross-linking among silica particles was facilitated. Thus high and adjustable inter-particle porosity was obtained and hence the refractive indices of the coatings were modulated. Next, poly(2,2,3,4,4,4-hexafluorobutyl) acrylate (simplified as PHFBA) was utilized to occupy micro-pores inside the coating in order to protect them from contamination. Consequently, the volatile organic and water vapor resistance of resultant DFDO/PHFBA/silica coatings was improved dramatically. By characterizing aggregating manner and stacking condition between particles, the modification process of the proposed AR coating was clarified in details.

Highly efficient deterministic polishing using a semirigid bonnet

Abstract. This paper presents a semirigid (SR) bonnet tool which has the advantages of high efficiency and determinacy for material removal on optical elements and also has the potential to be used on aspheric optics. It consists of three layers: a metal sheet, a rubber membrane, and a polishing pad, from inside to outside. It inherits the flexibility of a normal bonnet but has a higher stiffness. Finite element analysis was performed to determine that the stainless steel is the best-suited material for use as the metal sheet. An SR bonnet with a stainless-steel metal sheet was fabricated and tested. Its tool influence function (TIF) is Gaussian-like, and the TIF stability is more than 90%. The peak-to-valley of its uniform removal area is less than 0.1λ. Tool ripples are highly depressed and the surface profile is well preserved in the prepolishing test. In 12 min, ∼36  mm3 of material is removed.

Improving UV laser damage threshold of fused silica optics by wet chemical etching technique

Fused silica is widely used in high-power laser systems because of its good optical performance and mechanical properties. However, laser damage initiation and growth induced by 355 nm laser illumination in optical elements have become a bottleneck in the development of high energy laser system. In order to improve the laser-induced damage threshold (LIDT), the fused silica optics were treated by two types of HF-based etchants: 1.7%wt. HF acid and buffer oxide etchant (BOE: the mixture of 0.4%wt. HF and 12%wt. NH4F), respectively, for varied etching time. Damage testing shows that both the etchants increase the damage threshold at a certain depth of material removal, but further removal of material lowers the LIDT markedly. The etching rates of both etchants keep steady in our processing procedure, ~58 μg/min and ~85 μg/min, respectively. The micro-surface roughness (RMS and PV) increases as etching time extends. The hardness (H) and Young’s modulus (E) of the fused silica etched for diverse time, measured by nano-indenter, show no solid evidence that LIDT can be related to hardness or Young’s modulus.

Generation of Scratches and Their Effects on Laser Damage Performance of Silica Glass

Scratches are deleterious to precision optics because they can obscure and modulate incident laser light, which will increase the probability of damage to optical components. We here imitated the generation of brittle and ductile scratches during polishing process and endeavored to find out the possible influence of scratches on laser induced damage. Brittle scratches can be induced by spiking large sized abrasives and small abrasives may only generate ductile scratches. Both surface roughness and transmittivity are degraded due to the appearance of brittle scratches while ductile scratches make little difference to surface roughness and transmittance. However, ductile and brittle scratches greatly increase the density of damage about one order of magnitude relative to unscratched surface. In particular, ductile scratches also play an unignorable role in laser induced damage, which is different from previous knowledge. Furthermore, ZrO2 and Al2O3 polished surfaces appear to perform best in terms of damage density.

Unicursal random maze tool path for computer-controlled optical surfacing

A novel unicursal random maze tool path is proposed in this paper, which can not only implement uniform coverage of the polishing surfaces, but also possesses randomness and multidirectionality. The simulation experiments along with the practical polishing experiments are conducted to make the comparison of three kinds of paths, including maze path, raster path, and Hilbert path. The experimental results validate that the maze path can warrant uniform polishing and avoid the appearance of the periodical structures in the polished surface. It is also more effective than the Hilbert path in restraining the mid-spatial frequency error in computer-controlled optical surfacing process.