Yaguo Li

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.

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.

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.

Pressure and velocity dependence of the material removal rate in the fast polishing process

Based on the direct contact between the wafer and the pad, the pressure and velocity dependence of the material removal rate (MRR) in the fast polishing process (FPP) is investigated. There are three assumptions of the FPP material removal mechanism: the normal distribution of abrasive size, a periodic roughness of the pad surface, and the plastic contact between wafer-abrasive and pad-abrasive interfaces. Based on the particular FPP, a novel movement of the wafer is analyzed and a MRR equation is developed. The experiments with parameters of pressure and velocity are shown to verify the equation. Thus, a better understanding of the fundamental mechanism involved in FPP can be obtained.

Hybrid polishing of fused silica glass with bound-abrasive polishers in conjunction with vibration

We incorporated ultrasonic vibration into recently developed fixed-abrasive pellets in an attempt to enhance MRR and/or to improve manufactured surface quality. A prototype ultrasonic vibrator, the heart of the polishing head, was designed and the related experimental work was performed on an in-house built setup in conjunction with the constructed head. The vibrator is devised for the generation of 2-D tool path despite using only one actuator in lieu of two actuators in conventional 2-D ultrasonic machining systems. We then combined the ultrasonic vibration with fixed abrasive polishing pellets to machine fused silica glass. Machining experiments reveal that MRR is considerably increased up to <50% upon the introduction of ultrasonic vibration (UV) whilst surface roughness is not degraded appreciably.

Precision manufacturing of fused silica glass by combining bound-abrasive polishing with ultrasonic vibration

Ultrasonic vibration assisted machining with harder abrasives than the material to be machined can improve the quality of machined surface and manufacturing efficiency. Therefore, we integrated ultrasonic vibration (UV) into a recently developed technique chemo-mechanical bound-abrasive polishing in anticipation of further increasing the material removal rate (MRR) and/or surface roughness. The preliminary results indicate that ultrasonic vibration assisted chemo-mechanical bound-abrasive polishing can lead to increased material removal rate of manufactured optics while leaving the surface roughness comparable to conventional chemo-mechanical bound-abrasive polishing. The great MRR is attributed to the superiority of UV-assisted chemo-mechanical bound-abrasive polishing in discharging resultant swarf during machining.

The Surface Layer of Fused Silica Finished by Various Polishing Techniques

Our recent experiments show that the surface layer of fused silica manufactured by the magnetorheological finishing(MRF) is different from those finished by conventional polishing technique (e.g. pitch polishing or polyurethane polishing) in mechanical properties. We examined the hardness of the surface layer on fused silica polished by pitch/pad polishing and MRF respectively, and the results indicate that the hardness of traditionally polished layer is smaller than bulk material, decreasing with the distance from the sample surface, while the nanohardness of MRF-manufactured sample is greater than bulk, gradually increasing to the bulk value. We attribute the differences to the unique material removal mechanism of MRF. The distinctions among these polishing techniques may make contribution to varied laser-induced breakdown topography.