Reactive atom plasma (RAP) processing of mirrors for astronomy

Subrahmanyan, Pradeep; Gardopee, George J.

doi:10.1117/12.786991

Cited by 5 publications

(5 citation statements)

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“…Like other sub-aperture CNC polishing technologies, one can map the optical surface before polishing and then compute the tool path required to achieve the desired final figure. [4] …”

Section: Reactive Atom Plasma (Rap™) Processmentioning

confidence: 97%

Simultaneous figuring and damage mitigation of optical surfaces

Subrahmanyan¹,

Pang²,

Yu³

et al. 2008

Laser-Induced Damage in Optical Materials: 2008

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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.

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“…Like other sub-aperture CNC polishing technologies, one can map the optical surface before polishing and then compute the tool path required to achieve the desired final figure. [4] …”

Section: Reactive Atom Plasma (Rap™) Processmentioning

confidence: 97%

Simultaneous figuring and damage mitigation of optical surfaces

Subrahmanyan¹,

Pang²,

Yu³

et al. 2008

Laser-Induced Damage in Optical Materials: 2008

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“…The FWHM of the removal profile can be calculated as, FWHM = 2 √ ln 2c , (10) and the removal volume of the Gaussian-shape removal profile can be calculated as,…”

Section: Effects Of Substrate Thickness On Removal Ratementioning

confidence: 99%

“…Therefore, it can avoid the subsurface damage caused by mechanical stress and allow, in principle, the high-speed removal equivalent to chemical polishing [9] . For the optical figuring by an atmospheric plasma torch, the dwell time at a certain point is obtained by deconvolving the torch footprint from the figure error at that point [10] . Therefore, the footprint of the torch determines the figuring capability and efficiency.…”

Section: Introductionmentioning

confidence: 99%

“…Arnold et al presented high power and compact microwave-driven plasma jet sources and the miniature radio frequency-driven source [7] . Subrahmanyan et al obtained different footprints by adjusting the aperture size of the inductively-coupled plasma torch [10] . Yamamura et al combined a rotary electrode and a pipe electrode to get a mutual balance between efficiency and spatial resolution [11] .…”

Section: Introductionmentioning

confidence: 99%

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Atmospheric Pressure Plasma Processing of Fused Silica in Different Discharge Modes

Xin

Zhang

et al. 2015

Plasma Sci. Technol.

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One of the major advantages of utilizing atmospheric pressure plasma processing (APPP) technology to fabricate ultra-precision optics is that there is no subsurface damage during the process. In APPP, the removal footprint and removal rate are critical to the capability and efficiency of the figuring of the optical surface. In this paper, an atmospheric plasma torch, which can work in both remote mode and contact mode, is presented. The footprints and the removal rates of both modes are compared by profilometer measurements. The influences of process recipes and substrate thickness for both modes are investigated through a series of experiments. When the substrate is thinner than 12 mm, the removal rate in contact mode is higher. However, the removal rate and width of the footprint decrease dramatically as the substrate thickness increases in contact mode.

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“…Subsequently, the surface was handpolished to improve the roughness and to obtain specular surface suitable for interferometric measurement purposes. In particular, such polishing wasn't intended for subsurface damage removal which is known to influence the post-plasmaetching surface texture [15].…”

Section: Surface Preparationmentioning

confidence: 99%

Fast figuring of large optics by reactive atom plasma

et al. 2012

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The next generation of ground-based astronomical observatories will require fabrication and maintenance of extremely large segmented mirrors tens of meters in diameter. At present, the large production of segments required by projects like E-ELT and TMT poses time frames and costs feasibility questions. This is principally due to a bottleneck stage in the optical fabrication chain: the final figuring step. State-of-the-art figure correction techniques, so far, have failed to meet the needs of the astronomical community for mass production of large, ultra-precise optical surfaces. In this context, Reactive Atom Plasma (RAP) is proposed as a candidate figuring process that combines nanometer level accuracy with high material removal rates. RAP is a form of plasma enhanced chemical etching at atmospheric pressure based on Inductively Coupled Plasma technology. The rapid figuring capability of the RAP process has already been proven on medium sized optical surfaces made of silicon based materials. In this paper, the figure correction of a 3 meters radius of curvature, 400 mm diameter spherical ULE® mirror is presented. This work demonstrates the large scale figuring capability of the Reactive Atom Plasma process. The figuring is carried out by applying an in-house developed procedure that promotes rapid convergence. A 2.3 μm p-v initial figure error is removed within three iterations, for a total processing time of 2.5 hours. The same surface is then re-polished and the residual error corrected again down to λ/20 nm rms. These results highlight the possibility of figuring a metre-class mirror in about ten hours.

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Reactive atom plasma (RAP) processing of mirrors for astronomy

Cited by 5 publications

References 0 publications

Simultaneous figuring and damage mitigation of optical surfaces

Simultaneous figuring and damage mitigation of optical surfaces

Atmospheric Pressure Plasma Processing of Fused Silica in Different Discharge Modes

Fast figuring of large optics by reactive atom plasma

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