The large and thin plane optical windows are used in large high power laser devices. Typically conventional methods such as stressed-lap polishing and small-tool pitch polishing are used to manufacture these optics. Nevertheless, the required wavefront accuracy cannot be achieved by the last smal-tool pitch polishing process which can lead to middle spatial frequency errors and high-slope errors because of the edge effect , unstable removal rate and pressure -loaded deformation. Ion Beam Figuring (IBF) technology is an optical fabrication method which can highly correct different spatial frequency errors due to the highly deterministic, highly stable, very small tools and noncontact. In this paper, IBF was employed to correct different spatial frequency errors of a large and thin plane optical windows. Before IBF, transmission wavefront error of the substrate was 0.51λ PV, 32.3nm/cm GRMS, 2.91nm PSD1, 0.27nm PSD2, 0.38nm Rq after being polished by double-sided polishing machine, and was improved to 0.07λ PV, 2.1nm/cm GRMS, 1.76nm PSD1, 0.13nmPSD2, 0.33nmRq after only two IBF(about thirty six hours processing time). All spatial frequency errors reached the required values.
The in-situ monitoring of subsurface defects and laser damages initiation using high resolution on-line microscope is performed on medium aperture fused silica optics manufactured by different procedures to investigate the specific damage precursors. The digital camera, Nomarski microscope and white light interferometer are used to characterize the subsurface defects. With shallow HF etching depth, the laser induced damages are mostly initiated on indents or invisible defects under the fluence of 8~10 J/cm2@355nm. The laser induced damages initiated on indents is gradually decreased with the increased etching depth and the laser induced damage density is also decreased. Besides, decrease of the indents by optimizing the polishing process could also make the laser induced damage density sharply decrease. These results prove that the indents are important damage precursors and the laser induced damage performance of fused silica optics could be substantially improved by decreasing the indents or deep HF etching.
Polishing pad becomes aging after finishing for a while, by reason of friction and wear, or polishing particles deposition, which will reduce polishing effects, increase polishing effects and make a low polishing quality. When the ultra-hardness of sapphire material needs high pressure and high rotate speed parameters, which will accelerate aging of polishing pad. When large scale sapphire optical window is finished by computer controlled optical surfacing (CCOS) method, the polishing pad aging problem is an important confinement factor for high quality of polishing. In this paper, we confirmed the stable polishing period and aging time node of pitch pad and polyurethane pad by polishing experiments. For pitch pad, time aging nodes are between 50min and 75min with polishing pressure 0.1Mpa and 0.2Mpa and between 25min and 50min with polishing pressure 0.4Mpa. For polyurethane pad, the time aging nodes are over 75min with polishing pressure 0.1Mpa and 0.2Mpa and between 50min and 75min with polishing pressure 0.4Mpa. The aging time of polyurethane pad is later than that of pitch pad. Increasing polishing pressure will help increase material removal rate, but will accelerate aging of polishing pad
HF-based etching has been an effective method to increase the laser induced damage thresholds (LIDTS) of fused silica optics. In this research, the effect of etching parameters on the surface quality and laser damage performance of fused silica with Megasonic-assisted HF acid etching has been investigated systematically. The fused silica samples (50mm in diameter and 5mm thick) were maufactured through the conventional grinding and chemical mechanical polishing process, these processed samples are etched with different etching parameters. Our results show that the frequency and distribution of megasonic field will bring great effect on the surface quality of optics. The LIDTS were measured by 1- on-1 mode, results showed that the 1.3MHz megasonic field and a certain amount of etching depth will benefit the laser damage performance of fused silica optics.
For the high demand of large aperture optical element, the regular trajectory errors in machining marks of double-side polishing need to be determinately controlled. The mechanism and control method of the regular trajectory errors in machining marks were deeply studied. The process was simulated and compared with the experiment. The method of active translation and pendulum motion and polishing plate correction were proposed, proved to be efficacious on eliminating the regular machining marks by the groove of the polishing pad and local surface figure errors of the polishing plate. The method of dynamic loading and motion combination was adopted, retaining the independence of the original fast convergence process on surface figure. For the optical element with 430mm×430mm×10mm, the surface figure was controlled below 1λ(PV, λ=632.8nm). Meanwhile, the regular machining marks repeatedly produced were eliminated, which provided the essential condition for the intermediate frequency index in the rear stage, small tool precision polishing, and the high efficiency and stable machining of the optical element in the index system was realized.
The influence of polishing parameters such as particle size, pad material and pressure on the surface roughness of glass optics were investigated and analysed. It reveals that the surface roughness will get worse with increase of the polishing particle size. The surface roughness would remain stable in a certain period of polishing pressure, but get worse with increase of the pressure beyond the period. The surface roughness is getting better when using smooth pitch polishing pad than polyurethane pad with lots of micropores. By optimizing the polishing parameters, the surface roughness of large aperture fused silica window is improved to 0.46nm before band-pass filtering and 0.084nm after band-pass filtering.
Advanced mitigation process (AMP) has been identified as a most effective method to improve the laser induced damage threshold of the fused silica optics used in the large laser facility, and as the most important sub-process HF etching with multi-frequency megasonic agitation plays a decisive role to improve the damage threshold of fused silica. But because of the physical characteristics of megasonic, the etched surface is apt to generate striated haze which not only modulates the optical field, but also reduces the damage threshold significantly. In this paper, the generating mechanism of the striated haze is discussed, and both of the uneven acoustic field distribution and the optics' redeposited substance are recognized as the primary factor resulted this phenomenon, ultimately based on these discussions a slight swing when etching is proposed to eliminate this phenomenon.
The effect of ion beam etching process on the surface quality, the surface roughness and the laser-induced damage threshold at 351nm was carried out. Research results reveal that the laser-induced damage threshold of fused silica was enhanced with the increase of etching depth when the etching depth was less than 800nm, and could be further enhanced about 30% at 800nm etching depth, however the laser-induced damage threshold began to decrease with the further increase of etching depth(more than 800nm). The test results of surface microtopography, laser damage morphology, and surface roughness reveal that the ion beam etching process can remove polishing re-deposition layer without degrading the surface condition at a smaller etching depth so as to enhance the laser-induced damage threshold of fused silica, however further ion etching which can produce impurity particle often results in a decrease rather than an increase of laser-induced damage threshold.
Subsurface damage (SSD) has been identified as a main initiator of laser-induced damage in fused silica, and the most of SSD is produced during grinding process. The distribution and morphology of SSD in fused silica samples ground with loose abrasive are investigated by magneto-rheological finishing (MRF) dimpling and buffered oxide etch (BOE) etching method. The results demonstrate that the SSD depth is most responsive to the loose abrasive size and the BOE etching is good for removing the SSD. Based on these results, an efficient grinding technique combined with BOE etching is proposed to reduce the SSD of fused silica, and the damage threshold is obviously improved by this routine as a result.
Conventional used ceria polishing would induce both of Ce contaminants and subsurface damages, which mainly restricts the laser induced damage resistance of fused silica optics. To control the near surface defects, nanometer sized colloidal silica are used to polish fused silica optics after the normal ceria polishing process. Then the contaminant elements and subsurface damages of the polished samples were analyzed by secondary ion mass spectrometry and Nomarski microscopy. It reveals that ceria polishing would introduce lots of subsurface damages whereas colloidal silica polishing induces much fewer subsurface damages especially no fracture induced severe subsurface damages. The laser damage tests reveal that subsequent colloidal silica polishing of the ceria pre-polished samples could gradually eliminate the ceria polishing induced subsurface damages and lower the laser induced damage density accordingly with the increased polishing time. But unlike the damage density, only the severe subsurface damages are totally eliminated could the damage threshold be substantially improved. These results incline to indicate that the subsurface damages have great influence on the laser induced damage density and the fracture related severe subsurface damages will greatly restrict the damage threshold in polished optics.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.