One third of the exchanged final turning mirrors on the National Ignition Facility have been laser damaged by 3ω (351nm) target backscattered light. The mirrors are spectrally complex hafnia and silica multilayer dichroic coatings with high 1ω reflection for the forward propagating beam and low 3ω reflection to suppress propagation of target backscatter. Alumina, a wider bandgap high index material than hafnia, was selected to improve the 3ω laser damage resistance despite a lower refractive index. A 2-4x improvement in 3ω LIDT was demonstrated with adequate 1ω laser damage resistance. Nodule ejection was the dominate laser damage morphology. An 18% decrease in the refractive index between alumina and hafnia results in coating designs that are 3x thicker. Similarly, a decrease in refractive index also decreases the angular bandwidth resulting in increasing the number of mirror types from two to six over an angular range of 16 degrees. The significant improvement in 3ω LIDT has justified beginning a scale up demonstration to full aperture (0.3m2).
The effect of oxygen partial pressure on the properties of Al2O3 films deposited by electron beam evaporation has been
investigated through a combination of spectrophotometric and interferometric characterization techniques. As oxygen partial
pressure increases, a decrease in the refractive index is observed, as well as a shift towards less tensile films once they are
exposed to ambient conditions. This decrease in tensile stress was observed to be correlated with water content in the films.
Increasing oxygen partial pressure during deposition improved film stoichiometry, absorption, and laser induced damage
threshold (LIDT) at 351 nm.
The use of high-power continuous wave (CW) lasers in the emerging directed energy (DE) market has put greater emphasis on the quality of optical coatings. These coatings (both high reflectance and anti-reflection) require high damage thresholds for use at irradiances up to and greater than 1 MW/cm2. The challenge in the coating process is to minimize the number of coating defects that can contribute to absorption and eventual coating failure. The industry lacks standardized testing to detect possible defects at the required irradiances for DE optics. To aid in the development of coating designs and production techniques for DE related optics, we have constructed a high-irradiance Ytterbium fiber laser-based scanning metrology system to detect absorbing defects in DE optical coatings. Defects are detected by their localized thermal heating creating a hot-spot. The goal of this work is to continually improve the coating, increase the laser damage threshold, and contribute to a standard for the testing and validation of directed energy optical coatings.
Successful commerce of science and technology requires standardization of measurement. Today more than ever, there is need for a new Optical Laser Damage standard to increase the efficiency of commerce in the optical industry. The current Laser Damage Standard, ISO 21254, is overly complicated for a general seller/buyer and can produce ambiguous results. To address the challenges to efficient commerce from ISO 21254, OEOSC Task Force 7 (TF7) is developing a new US laser damage standard. The new standard is based on a measurement theory approach and introduced and examined. The optical manufacturer (seller) directly benefits from an operationally effective means to determine acceptance criteria around the ability of categorizing optics that will meet customer’s requirements. OEOSC Task Group TF7 is seeking broad industry support to develop the most useful and robust standard possible. The new US Laser Damage Standard will benefit the optical manufacturing industry and customers alike that use optics susceptible to laser damage in their applications by providing a clear unambiguous pass or fail result. The paper will conclude with a discussion of the path forward in the development of the US laser damage standard.
In previous years, this committee reported on the need for a US National Laser damage standard, addressing the needs of domestic industry. In 2017, a process was reported that connected the measurement of the active defect density in a small area, a, with the likely density of such defects over a larger area, A. This was presented as the basis of a Type 1, go/no-go test. 2018’s achievement, is development of process starting from a user’s requirements and flowing into test parameters. This year’s report covers the resulting test procedure that implements the test process a useful workable standard.
Completing our suite of deposition equipment, we are developing a new Ion Beam Sputtering (IBS) System with different substrate configurations: the High Throughput version (HT) and the High Precision version (HP). The HT version enables the coating of 4 planets of up to 350mm diameter substrates, whereas the HP version allows coating of substrates up to 600mm diameter in a single planet configuration. The IBS system is configured with a Bühler proprietary Optical Monitoring System for layer termination, a large 22cm RF sputtering source, and a LION plasma source for assist. In this presentation the optical performance of this IBS coatings, including LIDT, absorption, total loss and residual coating stress, will be discussed and compared to the other available deposition techniques, such as Plasma Assisted Reactive Magnetron Sputtering, and Plasma Ion Assisted deposition (PIAD). Preliminary results of a 1064nm mirror show less than 5ppm absorption, reflectivity’s of 99.997%, and no visible damage in CW LIDT testing up to 10MW/cm2. Pulsed laser damage testing is in process and will be reported. These results will be compared to the coatings being done using PARMS and Evaporation.
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.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.