Laser-induced damage threshold (LIDT) tests were performed at 1064 nm and 20 ns. Nodule defects were identified as the LIDT-limiting factor. The results suggest that the scale of the nodules is associated with the size of defects residing on the aluminum substrate surface. 3D finite-difference time-domain (FDTD) simulation was employed to calculate the electric field intensity (EFI) enhancement at the nodular defects with a seed diameter ranging from 0.35 μm to 2.5 μm. A direct linkage between the EFI enhancement and laser-induced damage morphology was established. Additional LIDT tests were conducted on surface modified aluminum substrate by using Corning aluminum process (CAP). The surface modification led to a 10x increase of the LIDT. Finally, LIDT of the multiband mirrors was predicted based on the absorption-driven damage and defect-driven damage. The results suggested that a combination of the CAP-modified Al6061 and low defect deposition process of the dielectric enhanced layers lead to high laser durability.
Protective and enhanced Silver coatings enable broadband high reflectance and durability. Energetic deposition provides densified layers to protect Silver mirrors and enhance optical reflectance in desired spectral bands. We employed variable angle spectroscopic ellipsometry to analysis ion beam assisted SiO2 and Nb2O5 films for optimizing enhanced silver performance in the near UV. We generated a visualized field guide for coating process development based on spectral reflectance measurement. Adjusted ion beam assisted deposition parameters included ion energies, ion currents, gas flow rates and deposition rates. Appropriate ion interaction with the deposited materials led to an increase of both film homogeneity and densification. Accelerated UV ozone exposure tests confirmed that the optimized ion beam assisted deposition yielded stable optical performance.
Thick HfO2 single layers derived from a reactive plasma ion assisted deposition were investigated with a designed film thickness of 800 nm. The film structure was modeled by fitting the corresponding variable angle spectroscopic ellipsometric data and correlated to the ratio of plasma ion momentum transfer during the film deposition. Scatter loss was calculated according to a multilayer model as well as a single surface model. Water absorption in the MWIR was used to confirm the revealed film structure. The results indicate that the scatter loss of the HfO2 based high reflective optics can be estimated by using a single surface model in a first-order approximation from the DUV to the MWIR. A linear relationship between the refractive index inhomogeneity and the amount of plasma ion momentum transfer during the deposition was established. The total loss at 2.95 μm is dominated by the absorptance loss, whereas both the absorptance and the scatter losses are reduced as the ratio of plasma ion momentum transfer increases. Appropriately optimizing and selecting deposition parameters enable low loss and environmentally stable HfO2 coatings, leading to numerous defense applications from the DUV to the MWIR.
HfO2/SiO2 multilayer based reflective optics enable threat detection in the short-wave/middle-wave
infrared and high power laser targeting capability in the near infrared. On the other hand, HfO2/SiO2
multilayer based transmissive optics empower early missile warning by taking advantage of the extremely
low noise light detection in the deep-ultraviolet region where solar irradiation is strongly absorbed by the
ozone layer of the earth’s atmosphere. The former requires high laser damage resistance, whereas the
latter needs a solar-blind property, i.e., high transmission of the radiation below 290 nm and strong
suppression of the solar background from 300 nm above. The technical challenges in both cases are
revealed. The spectral limits associated with the HfO2 and SiO2 films are discussed and design concepts
are schematically illustrated. Spectral performances are realized for potential A and D and commercial
applications.
Due to advances in manufacturing processes, the substrate options for high performance diamond machined mirrors are
expanding. Fewer compromises have to be made to achieve the needed weight, stiffness and finish while maintaining
reasonable costs. In addition to the traditional mirror materials like aluminum and beryllium, there are some less
common materials that can now be included in the trade space that fill the cost and performance continuum between
wrought aluminum and beryllium mirrors. Aluminum and beryllium, respectively, had been the low cost/fair
performance and very high cost/very high performance bounds for substrate selection. These additional substrates
provide multiple near net shape blank options and processes, mostly within these bounds, that can be considered in a
mirror cost versus performance trade analysis.
This paper will include a summary of some advances in manufacturing processes that provide more substrate options for
diamond machined mirrors with some sample performance analysis and data. This is merged with the traditional
substrate options to illustrate the now larger mirror substrate trade space. Some benchmark structural analysis is
provided to back up a generic mirror design trade study.
The demand for high performance, lightweight mirrors was historically driven by aerospace and defense (A&D) but now we are also seeing similar requirements for commercial applications. These applications range from aerospace-like platforms such as small unmanned aircraft for agricultural, mineral and pollutant aerial mapping to an eye tracking gimbaled mirror for optometry offices. While aerospace and defense businesses can often justify the high cost of exotic, low density materials, commercial products rarely can. Also, to obtain high performance with low overall optical system weight, aspheric surfaces are often prescribed. This may drive the manufacturing process to diamond machining thus requiring the reflective side of the mirror to be a diamond machinable material.
This paper summarizes the diamond machined finishing and coating of some high performance, lightweight designs using non-exotic substrates to achieve cost effective mirrors. The results indicate that these processes can meet typical aerospace and defense requirements but may also be competitive in some commercial applications.
An ultra-low surface finishing process for 6061 T6 type aluminum has been developed by Corning Incorporated, Specialty Materials Division, and has been successfully applied to mirrors up to 13 inches in diameter. This paper presents finish and figure data achieved from the mirror finishing process. Mirror stability is demonstrated through Pre and post thermal cycle surface figure measurements; temperature range of cycle -55°C to +70°C. As an added benefit, the process enables the use of deterministic finishing and enhances the reflective optics resistance to corrosion. Survivability of the reflective optic is evaluated through extended humidity testing.
Extreme light-weighting is important in many aerospace and defense applications but the cost associated with beryllium or other exotic materials can be prohibitive. The current standard for producing cost effective, high performance mirrors is to diamond machine mirror blanks from aluminum alloy stock. About 80% material removal is the limit for geometrical lightweighting while still retaining the structural integrity required for optical fabrication. To reduce weight further requires alternative materials. This paper summarizes the status of diamond machined finishing and coating of magnesium alloys to produce cost effective, lightweight mirrors with high, broadband reflectivity and low scatter finish.
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