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Deterministic Precision Machining of Miniature Optics in Hard Ceramics

Description:

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics; Nuclear; Space Technology

 

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

 

OBJECTIVE: Develop the manufacturing capability to fabricate miniature, high-precision, high-numerical aperture optical components in hard ceramic materials.

 

DESCRIPTION: The Navy requires enhanced capabilities for precision machining of hard ceramic optical mirror surfaces with high numerical aperture (NA). In spite of fabrication challenges, hard ceramics such as silicon carbide are seeing increasing adoption as substrate materials for optical mirrors due to their high thermo-mechanical stability and high stiffness. These products find use in applications such as space-based telescope primary mirrors, for instance. Grinding and polishing are traditional methods of achieving final surface form and are well-suited to a broad range of materials, including hard ceramics. Traditional grinding methods, however, are limited to relatively low-NA optical surfaces. For soft metals, single point diamond turning (SPDT) can produce a wide range of surface forms, including high-NA and aspheric surfaces, while achieving extraordinarily precise surface form tolerances [Ref 1]. SPDT has also recently shown promise as a method for machining certain hard ceramics, although further research may be required to optimize cutting parameters [Ref 2]. Techniques such as ion etching and magnetorheological finishing (MRF) may also be used to produce arbitrarily precise surface forms in a variety of materials [Ref 3], although they are slow and labor-intensive. The Navy has an interest in developing a deterministic and cost-effective fabrication capability for high-NA optical surfaces in hard ceramics (such as silicon carbide, silicon nitride, or similar) with optically precise surface form tolerances. In addition to the machining challenge, high-NA surfaces add challenges to the process of validating the achieved surface form (interferometry and profilometry are common methods) which must also be overcome. The capability will be demonstrated by the production of small (sub-centimeter-scale) convex and concave hemispherical mirror substrate test articles to be delivered at the conclusion of Phase II.

 

PHASE I: Develop methods for producing hemispherical mirror substrates in hard ceramic materials. Perform a feasibility study for achieving the target surface form and radius of curvature thresholds listed below for two types of mirror substrates: a concave hemispherical mirror (substrate A) and a convex hemispherical mirror (substrate B). Assess the scalability of the proposed approach in terms of per unit labor hours and throughput. Material and threshold mirror surface specifications for test articles to be produced in Phase II include:

• Material: Hard, non-porous ceramic (such as silicon carbide, silicon nitride, or material of similar hardness)

• Radius of curvature (both convex and concave): 3 mm +/- 500 nm

• Nominal outer diameter (OD) of mirror surface: 3mm

• Spherical surface irregularity may not exceed 500 nm

• Surface roughness may not exceed 30 nm RMS • Mirror substrates shall be uncoated

• Localized surface imperfections (scratch and dig): Best effort (Goal is 10-5 per mil spec MIL-PRF-13830)

 

Methods that provide a path toward deterministic production of aspheric surfaces are of interest, but not strictly required. Methods of metrology for validating the final surface form must also be proposed and assessed for scalability for arbitrary surface forms and numerical aperture. A detailed risk assessment of the proposed fabrication and metrology methods should be provided.

 

PHASE II: Implement the methods proposed in Phase I for the production of a set of test articles for delivery to the Navy by the conclusion of Phase II. These deliverables consist of five (5) prototypes of substrate A and five (5) prototypes of substrate B with key specifications listed in Phase I above. Parts will be evaluated based on the form of the primary hemispherical surface; other dimensions and surfaces are not critical. Each prototype must be delivered with metrology data indicating the achieved surface form.

 

PHASE III DUAL USE APPLICATIONS: The machining capabilities demonstrated in Phase II advance the state of the art for optical component fabrication in durable materials. Support the Navy in transitioning the technology to Navy use. The prototypes will be evaluated for compatibility with existing and planned strategic system component designs. The technology will be used in Phase III to develop components according to specific design requirements for strategic sensors. The end product technology is applicable to a range of dual use applications that benefit from the stiffness and thermal stability properties of hard ceramics. These include space-based and airborne optical systems and high power laser applications.

 

REFERENCES:

  1. 1. Higginbottom, D. B.; Campbell, G. T.; Araneda, G.; Fang, F.; Colombe, Y.; Buchler, B. C. and Lam, P. K. “Fabrication of ultrahigh-precision hemispherical mirrors for quantum-optics applications.” Scientific Reports, 2018, 8, 221. https://doi.org/10.1038/s41598-017-18637-8
  2. Goel, S. “The current understanding on the diamond machining of silicon carbide.” Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47, 243001. https://dx.doi.org/10.1088/0022-3727/47/24/243001
  3. Golini, D.; Kordonski, W. I.; Dumas, P. and Hogan, S. J. “Magnetorheological finishing (MRF) in commercial precision optics manufacturing.” Optical Manufacturing and Testing III, SPIE, 1999, 3782, 80 – 91. https://doi.org/10.1117/12.369174

 

KEYWORDS: Non-porous Ceramic; Silicon Carbide; Silicon Nitride; High-precision Machining; Precision Optics, High numerical aperture

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