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Additive Manufacturing of Inorganic Transparent Materials for Advanced Optics



OBJECTIVE: Develop an Additive Manufacturing (AM) process for depositing inorganic glasses with sufficient quality and precision for free form and gradient index optics. 

DESCRIPTION: Naval Air Operations have a broad array of weapon and surveillance systems that utilize high performance optics. Many of these applications require greater wavelength transmission range, hardness, and temperature stability compared to polymers. The potential for utilizing AM technology to print glass lenses will provide the Navy the ability to (1) deposit net shape or near net-shape free-form optics, (2) locally adjust the index of refraction and other optical properties such as dispersion, (3) create high precision low thermal expansion meteorological frames that can form the basis for refractive optics, and (4) repair existing optical systems. The benefits of AM are widely realized for structural systems; however, work on printing optical systems is still in its comparative nascency. The majority of the work has been primarily focused on polymers. Processes that have been demonstrated for printing optically transparent polymers include ink-jet printing [Ref 1] with/without in-situ ultraviolet (UV) curing [Ref 2] and multiphoton stereolithography (SLA) to directly polymerize resin [Refs 3-4]. These techniques have been used for rapid prototyping of non-imaging optics using Poly(Methyl Methacrylate) (PMMA) like plastics [Ref 4]. They have also been used to create curved display surfaces, sensors, display devices, and interactive devices [Refs 1-2], and to print 3D Gradient Index (GRIN) devices by locally adjusting the index of refraction during the layer-by-layer fabrication [Ref 3]. This work has progressed to the point that it is beginning to be commercialized and while it is currently suitable for non-imaging purposes the technology is approaching viability for ophthalmic applications. Despite the benefits of inorganic glasses, there has been limited work applying AM processes to glasses. The high viscosity of glass when molten makes powder bed processes challenging because the bubbles fail to coalesce and escape due to buoyancy [Ref 5]. This can be overcome by using nanopowders to print a green part, followed by a slow high temperature burn-out/densification process [Refs 6-7]. Alternatively, fully dense material can be smoothly melted and deposited [Refs 8-9]. Both approaches have challenges and require significant development before optical devices can be successfully created. Currently no commercial AM system can be used to fabricate imaging quality optical glass with sufficient dimensional accuracy and surface finish. A robust AM process to fabricate optical materials with good optical properties and surface quality is needed. This AM process should be able to deposit optical materials within the desired transmission band and provide a smooth optical surface quality so that minimum post-processing is needed. This process will allow engineering of new optical systems with volumetrically varying properties such as the index of refraction (i.e., GRIN lenses). Even with homogeneous glasses, AM has the potential to rapidly realize free form optics or to repair existing systems with no or minimal post (such as least amount of time for a final polish to achieve a desired surface flatness, such as lambda / 4) processing. This will dramatically enhance the logistics and maintenance of Navy aircraft and other systems. To demonstrate a robust AM process, proposers are asked to develop an achromatic lens with a transmission window from 0.65 micrometer to 1.3 micrometer. • Clear aperture must be 3 inches in diameter • Effective Focal Length (EFL) must be 10 cm • Resolution must be 100 lp/mm • Athermalized design must work from -54 to 90 degrees Celsius • No adhesive should be used to combine the doublets Diameter Tolerance +0.00/-0.10 mm Focal Length Tolerance ±1% Surface Quality 40-20 Scratch-Dig Spherical Surface Power lambda /2 Spherical Surface Irregularity (Peak to Valley) lambda / 4 Centration =3 arcmin Clear Aperture >90% of Diameter Damage Threshold Pulse 5 J/cm2 (810 nm, 10 ns pulse, 10 Hz, 0.155 mm) CW 1000 W/cm (1070 nm, Ø0.971 mm) In addition, provide the following analysis and measured data: • Analyze Ray fan plots and spot diagrams • Demonstrate that the optical path length is equal to sigma n delta s for the two discontinuity between the doublets • Show the flatness of the wavefronts coming from a point source at the focus • Show what does n (lambda) curve looks like • Determine if power density at any image location is proportional to strength of corresponding object point? • Determine the birefringence of the material delta n • Determine the diffraction blur diameter • Determine the aberrations for the lens uinge the spot diagram to show these effects • Analyze spherical aberration; coma; astigmatism; distortion; transverse chromatic aberration • Determine the wavefront errors for Seidel aberration • Determine the Modulation Transfer Function (MTF) for this lens • Show what the optical plot of the optical transmission looks like 

PHASE I: Develop and demonstrate the feasibility of an AM process capable of the required optical properties, full densification, and smooth surface finish as provided in the description. The AM process should be able to realize a prescribed aspherical geometry with minimal post processing. Demonstration should include a fabrication plan of a representative achromatic lens with the specification provided in the description. Develop prototype plans to be developed under Phase II. 

PHASE II: Fully develop the AM process, demonstrated in Phase I, that can be applicable to an array of naval optical component geometries. Include, in the prototype demonstration, the effectiveness of fabricating fully densified optical components with precision control of the part geometry, and smooth surface quality. Fully characterize the resulting geometry, and mechanical and microstructural properties achieved through the process to validate the effectiveness of the AM process. 

PHASE III: Perform many experimental trials to define this additive manufacturing process since the development of an AM process for optical components is not a mature technology. Use simulation as a guide to help steer the direction of the experimentation; and to ensure the final product will meet the requirements of this topic as outlined in the specifications. Evaluate, by conventional metrology, the innovative achromatic doublet to ensure the AM process is on par with an achromatic produced by common practice. Transfer this process to platforms that have optical components. Perform testing and make improvements to the AM Process based upon the results. Begin producing optical components for testing and use in military systems. Laser manufacturers, camera manufacturers, and imaging technology manufacturers will benefit from this technology because they can now specify custom size optical components with unique transmission profiles that are not currently available with conventional optical processing. 


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KEYWORDS: Optical AM; Optical Additive Manufacturing; Aberration; Lens; Achromat; 3D Graded Index Lens; GRIN 

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