Mechanical grinding and shaping of optical materials imparts damage that manifests itself as defects and cracks that can propagate well below the surface of the optic. Mitigation of damage is necessary to preserve the integrity of the optic and relieve residual stress that can be detrimental to its performance. Typically, a sequence of subsequent polishing steps with finer and finer grit sizes is used to remove damage, but the process can be painfully slow especially for hard materials such as silicon carbide and often fails to remove all the damage. Reactive Atom Plasma (RAP™) processing, a non-contact, atmospheric pressure plasma-based process, has been shown to reveal and mitigate sub-surface damage in optical materials. Twyman stress tests on thin glass and SiC substrates demonstrate RAP’s ability to relieve the stress while at the same time improving surface form.
Publications
Increasing demands are being made on optical surfaces, these being driven by shorter wavelengths, increased precision and the need for faster, smaller optics demanding complex aspherical surfaces to replace multiple spherical surfaces. Lately, there have been multiple innovations in final finishing (with sub-aperture, deterministic polishing steps) and metrology (stitching interferometry, computer generated holograms) of such optics. Issues such as sub-surface damage, mid-spatial frequencies, and cost and lead time of manufacturing still plague the development and deployment of precision optics.
A novel integrated manufacturing process comprising a split chemical mechanical polishing technique has been developed that addresses the issues with current modern optical manufacturing techniques.
Mechanical grinding and shaping of optical materials imparts damage that manifests itself as defects and cracks that can propagate well below the surface of the optic. Mitigation of damage is necessary to preserve the integrity of the optic and to relieve residual stress that can be detrimental to its performance. This damage can also cause instability in optical performance with changes in the ambient conditions. In certain high laser fluence applications or under transient stresses, it can also lead to catastrophic failure. This residual stress is usually relieved by grinding with progressively smaller grit sizes but often times this process will still leave behind a small amount of stress from the final polishing step.
Reactive Atom Plasma (RAP), a novel non-contact, atmospheric pressure plasma-based process, has been shown to reveal and mitigate sub-surface damage in optical materials. Twyman stress tests on thin glass and Silicon Carbide (SiC) substrates demonstrate RAP’s ability to relieve this stress. We will show how RAP processing can enhance and accelerate fabrication of precision SiC optics.
Polishing has traditionally been a process of mechanical abrasion with each iteration removing the damage from the previous iteration. Modern sub-aperture techniques such as CCOS, MRF polishing etc. have added a considerable amount of determinism to this iterative approach. However, such approaches suffer from one significant flaw, i.e., the algorithms are completely guided by figure error. This approach fails when there is a considerable amount of strain energy stored in the substrate and becomes very evident when the aspect ratio of the mirror increases significantly causing relaxation of strain energy to have deleterious and unpredictable effects on figure between iterations. This is particularly pronounced when the substrate is made of a hard ceramic such as silicon carbide requiring a considerable amount of pressure to obtain any appreciable material removal rate. This paper presents an alternate approach involving a stress-free figuring step and a buffing step intended to recover the surface roughness.
Modern day telescopes for astronomy have very complex requirements. Both ground and space based telescopes are getting much larger placing significant productivity requirements on the manufacturing processes employed. Conventional manufacturing paradigms involving mechanical abrasion have limitations related primarily to the material removal mechanisms employed. Reactive Atom Plasma (RAPTM) processing is a sub-aperture, noncontact, deterministic figuring technology performed at atmospheric pressures. The process has high material removal rates, and given the non-contact and atmospheric nature lends itself very well to scaling up for large aperture mirrors/segments. The process also benefits from its ability to simultaneously remove sub-surface damage (SSD) while imparting the desired figure to the surface. Developments are under way currently to scale the process up towards larger clear apertures while being able to figure in high spatial frequency features.
Reactive Atom Plasma (RAPTM) processing of substrates is a new technique developed to increase productivity of the manufacturing step by orders of magnitude without sacrificing precision. This is made possible by a paradigm shift in the approach to material removal and/or deposition. Instead of using iterative abrasive processes that are required to remove damage from prior iterations without adding significant amounts of damage/stress/strain in the current iteration, RAPTM relies on chemical etching of the underlying material. Further productivity enhancements are made possible by the use of a sub-aperture atmospheric-pressure plasma source. A large number of manufacturing problems can be solved by suitable modifications to the chemistry and/or processing parameters.