This paper reports on the fundamental idea behind a US National Committee, The Optics and Electro-Optics Standards
Council (OEOSC) Task Force (TF) 7, proposal for a so-called Type 1 laser damage test procedure. A Type 1 test is
designed to give a simple binary, pass or fail, result. Such tests are intended for the transactional type of damage testing
typical of acceptance and quality control testing. As such is it intended for bulk of certification of optics for the ability
to survive a given fluence, useful for manufacturers of optics and their customers, the system builders. At the root of the proposed method is the probability that an optic of area A will have R or less damage occurrences with a user specified
probability P at test fluence Φ. This assessment is made by a survey of area and the observation of n events. The paper
presents the derivation of probability of N or less damage sites on A given n events observed in area a. The paper
concludes with the remaining steps to development of a useful test procedure based on the idea presented.
Sapphire presents many challenges to optical manufacturers due to its high hardness and anisotropic properties. Long lead times and high prices are the typical result of such challenges. The cost of even a simple 'grind and shine' process can be prohibitive. The high precision surfaces required by optical sensor applications further exacerbate the challenge of processing sapphire thereby increasing cost further. Optimax has demonstrated a production process for such windows that delivers over 50% time reduction as compared to traditional manufacturing processes for sapphire, while producing windows with less than 1/5 wave rms figure error.
Optimax's sapphire production process achieves significant improvement in cost by implementation of a controlled grinding process to present the best possible surface to the polishing equipment. Following the grinding process is a polishing process taking advantage of chemical interactions between slurry and substrate to deliver excellent removal rates and surface finish. Through experiments, the mechanics of the polishing process were also optimized to produce excellent optical figure. In addition to reducing the cost of producing large sapphire sensor windows, the grinding and polishing technology Optimax has developed aids in producing spherical sapphire components to better figure quality.
In addition to reducing the cost of producing large sapphire sensor windows, the grinding and polishing technology Optimax has developed aids in producing spherical sapphire components to better figure quality. Through specially developed polishing slurries, the peak-to-valley figure error of spherical sapphire parts is reduced by over 80%.
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 which challenge 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, finishing 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 specifically focusing on the use of fiducials in the manufacturing and measurement process and the adaptation of stitching interferometry to the measurement of a freeform conformal window.
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.
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.