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Investigation On Processing Of Fused Silica Substrate Used In High Power Laser System

Posted on:2016-11-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:X L JiangFull Text:PDF
GTID:1108330473961545Subject:Synchrotron radiation and its application
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Laser induced damage of fused silica has become a big bottleneck which limits the output energy of high power laser facilities designed for inertial confinement fusion (ICF). It is of great importance to improve the laser damage resistance to prolong the lifetime of fused silica optics. It is generally believed that there are two factors serving as damage precursors. The first is metal contamination such as ceria originating from polishing slurry. The other one is subsurface damage (SSD) created by the repeated indentation of mechanically loaded hard abrasives sliding on the surface of an optic during various cutting, grinding and polishing processes.In this work, we investigate on improving laser damage resistance of fused silica using HF acid etching and a combined machining process. The main contributions are as follows.1. We systematically optimize the HF etching process to reduce the precipitates of reaction byproducts. HF solution instead of BOE (HF:NH4F) is chose as etching solution to avoid the formation of hardly soluble hexafluorosilicate compound. High-frequency ultrasonic agitation and excessive spray rinsing are also employed to improve the mass transport of reaction product away from the etched surface. Metal contaminations and SSD are effectively removed after the optimized HF etching. After 20 μm removal, laser induced damage threshold (LIDT) increases from 5.7 J/cm2 to 13.1 J/cm2. In addition, HNO3:H2O2 and HC1:H2O2 leaching are found to be very effective in removing Ceria and other metal contaminations (like Fe, Al) respectively. LIDT increases from 5.7 J/cm2 to 12.3 J/cm2.2. A combined machining process is proposed in this work. Its process flow is: grinded substrate-300μm HF acid etching-super polishing-dry etching-HF acid cleaning. After 300μm HF acid etching, the SSD layer of grinded substrate gets completely removed and subsurface defects (cracks and scratch) involve into surface cusps. Moreover, during the extended etching, the individual cusps coalesce with one another, providing a means of reducing the depth of subsurface damage and the peak-to-valley roughness. After super polishing, surface roughness drops to sub-nanometer and a shallow SSD layer (sub-micron) is created, which will get removed in the following dry etching. At last, a shallow HF acid etching is conducted to remove the possible metal contamination introduced during the dry etching. Thus, a SSD-free substrate without metal contamination is obtained. According to LIDT tests, the proposed combined machining process greatly improves the laser damage resistance. LIDT increases from 20.0 J/cm2 to as much as 42.0 J/cm2 with surface roughness of ~1nm.3. We systematically studied the plasma induced surface damage (PISD) of ICP etching. First, the morphology features and evolution behaviors are investigated. PISD is identified by comparing the surface morphology of different etched substrates with different SSD densities. After identification, the evolution behaviors in both reactive and physical etching are studies. At last, PISD is successfully eliminated by employing an isolation device and optimizing etching parameters.
Keywords/Search Tags:fused silica, laser induced damage threshold (LIDT), HF etching, inductively coupled plasma (ICP) etching, RIBE etching, plasma induced surface damage (PISD)
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