Simultaneous figuring and damage mitigation of optical surfaces

Abstract

Sub-surface damage is a serious issue in the manufacturing of precision optical elements. For very lightweight mirrors, changes in surface stresses through various process steps that sequentially relieve stored up strain energy lead to poor convergence to eventually desired figures. For high fluence laser applications damage sites can prove to be deleterious to the functioning of the optic. For precision refractive optics, birefringence resulting from damage and stress can be an issue as well.Conventional methods of optical finishing rely mostly on mechanical abrasion, requiring an iterative process of subsurface damage mitigation from earlier process steps while minimizing damage from the current process step. This manufacturing paradigm leads to very long lead times and costs in producing high precision optics.Reactive Atom Plasma (RAP) based figuring is introduced as a technique to simultaneously remove damage from prior steps while imparting no further damage and figuring the surface of the optic. RAP based figuring demonstrates a new approach to the figuring of precision optics using a non-contact sub-aperture atmospheric plasma footprint to shape the surface. RAP figuring has been illustrated to remove Twyman stresses caused by conventional optical processing technologies. Twyman stresses on coupons of various glass materials and ceramics have been characterized and RAP removals of the damage layer have led to removal of the strains and thence the associated stress. The process is deterministic, enabling the figuring of high-precision surfaces with little to no sub-surface damage.

Analysis on principle and key techniques of deterministic finishing

“…7 Atmospheric pressure plasma deterministic machining system developed by HIT PV 3nm [12] 1nm [13] LLNL SiC [14,25] Fig.8 Optics surface with sin structure machined by APPM [15] 1nm [16] 7 2mm 600 25 8 IV. 50535020 50775055…”

Proposal for Machining Optical Free Form Surfaces Using Atmospheric Pressure Plasma Jet

“…Atmospheric pressure plasma machining is a pure chemical etching process, there is no damage layer on the optical surface and it is lower cost, more time saved and more material removal rate than vacuum plasma etching. Since introduced in 1990s, several researches on atmospheric pressure plasma chemical machining optical elements had been studied [3][4][5]. But these studies were focused on improving surface quality, such as correcting form errors, decreasing surface roughness or removing damage layer, there were no reports on machining surface topography using atmospheric plasma technology.…”