Conformal windows pose new and unique challenges to manufacturing due to the shape, measurement of, and requested hard polycrystalline materials. Their non-rotationally symmetric shape and high departure surfaces do not lend themselves to traditional optical fabrication processes. The hard crystalline materials are another challenge due to increased processing time and possibility of grain decoration. We have developed and demonstrated a process for manufacturing various conformal windows out of fused silica, glass, zinc-sulfide multispectral, and spinel. The current process involves CNC generation/grinding, VIBE polishing, and sub-aperture figure correction. The CNC generation step incorporates an ultrasonic assisted grinding machine; the machine settings and tool are being continuously optimized for minimal sub-surface damage and surface form error. In VIBE, polishing to less than 5 nm rms surface roughness while maintaining overall form error is accomplished with a full aperture conformal polishing tool and with rapid removal rates. The final sub-aperture polishing step corrects the overall form error. Currently we utilize our CMM for surface form measurement and have shown that we can produce spinel conformal windows with form error within ±10 micrometers of the nominal shape, without grain decoration. This conformal window manufacturing process is continuously optimized for cost reduction and precision of the final optic.
Sapphire poses very difficult challenges to optical manufacturers due to its high hardness and anisotropic properties. These challenges can result in long lead times and high prices. Large optical sensor windows demand much higher precision surfaces compared to transparent armor (windshields) to achieve acceptable image quality. Optimax is developing a high speed, cost effective process to produce such windows. The Optimax high speed process is a two-step process that combines precision fixed abrasive grinding and high speed polishing. In-house studies have demonstrated cycle time reduction of up to 6X as compared to conventional processing.
For over 100 years, optical imaging systems were limited to rotationally symmetric lens elements, due to limitations in processing optics. However, the present rapid development and application of CNC machines has made fabrication of non-rotationally symmetric lenses, such as freeform surfaces, economical. The benefit of using freeform surfaces is that the lens designer has more flexibility to create innovative 3D imaging packages, while correcting for aberrations. This report details capabilities at Optimax for manufacturing freeform surfaces, with a specific example towards creation of freeform ZnS-multispectral optics for application as a corrector element. In addition to fabricating freeform optics, advances have been made in producing smooth surfaces on polycrystalline materials. In the past, achieving a smooth surface on polycrystalline materials during sub-aperture polishing has proven challenging, because of a phenomenon called grain highlighting. Significant progress has been made at Optimax in this field through utilization of proprietary pads, slurries, and processes.
Freeform optical shapes or optical surfaces that are designed with non-symmetric features are gaining popularity with lens designers and optical system integrators. This enabling technology allows for conformal sensor windows and domes that provide enhanced aerodynamic properties as well as environmental and ballistic protection. In order to provide ballistic and environmental protection, these conformal windows and domes are typically fabricated from hard ceramic materials. Hard ceramic conformal windows and domes provide two challenges to the optical fabricator. The material hardness, polycrystalline nature and non-traditional shape demand creative optical fabrication techniques to produce these types of optics cost-effectively. This paper will overview a complete freeform optical fabrication process that includes ultrasonic generation of hard ceramic surfaces, high speed VIBE polishing, sub-aperture figure correction of polycrystalline materials and final testing of freeform surfaces. This paper will highlight the progress made to each of the processes as well as the challenges associated with each of them.
The VIBE™ process is a full-aperture, conformal polishing process incorporating high frequency and random motion
designed to rapidly remove sub-surface damage in a VIBE pre-polish step and eliminate mid-spatial frequency (MSF)
errors in a VIBE finishing step. The VIBE process has potential to be introduced in two areas of today's modern optics
manufacturing process. The first instance is replacing the conventional pre-polishing step with the rapid VIBE pre-polish
step. Results will be discussed in this paper that show 10 - 50x higher removal rates compared to conventional
polishing for a variety of optical materials. High removal rates combined with the compliant lap results in damage-free
surfaces that have the same form that was generated by the CNC generation process for spherical and non-spherical
surfaces. The second potential area to incorporate VIBE into today's modern optics manufacturing process is as a
finishing step after deterministic sub-aperture polishing to remove mid-spatial frequency errors. By selectively altering
the compliant properties of the VIBE pad, and adjusting the frequency of the VIBE motion, VIBE finishing can reduce
the mid-spatial frequencies caused from sub-aperture polishing processes while maintaining the desired corrected surface
form. This paper will serve as an in-depth review of the VIBE process and how it complements other modern CNC
optics manufacturing technologies, as well as highlighting recent VIBE advances specifically in the area of conformal
Hard ceramic conformal windows and domes provide challenges to the optical
fabricator. The material hardness, polycrystalline nature and non-traditional shape demand
creative optical fabrication techniques to produce these types of optics cost-effectively.
VIBETM is a high-speed, high-pressure, conformal optical fabrication process that is capable
of rapidly polishing hard ceramic materials and non-traditional shapes such as toroids and
tangent ogives. This paper will overview the recent progress made to rapidly manufacture
hard ceramic conformal windows and domes as well as the challenges associated with it.
Results will show 10-50x increase in removal rates using the VIBE platform to polish hard
ceramic materials compared to conventional methods.