Fabrication of a diffractive optical element by the precision diamond micro-turning method
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-05-18 |
| Journal | Interexpo GEO-Siberia |
| Authors | Nikita A. Gurin |
| Institutions | Institute of Automation and Electrometry |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”The paper details the implementation of Precision Diamond Micro-Turning (PDMT) for the fabrication of complex Diffractive Optical Elements (DOEs), using a Fresnel diffractive lens as a specific example.
- Core Value Proposition: PDMT enables the simultaneous creation of flat, spherical, or high-order aspherical surfaces integrated with diffractive structures (e.g., sawtooth profile) in a single, continuous technological process.
- Surface Quality: The technology achieves superior surface roughness, reaching Ra 0.012 µm for general materials and less than 2 nm (Ra) for metal optics (Al, Cu), significantly surpassing conventional turning methods (Ra 0.8 µm).
- Tooling Requirement: Successful fabrication relies on selecting a specialized diamond cutter whose angle offset precisely matches the calculated microstructure profile of the diffractive element.
- Process Control: The manufacturing process is highly automated, requiring precise input of lens zone parameters (e.g., 52 zones, 425.64 µm max step height) and cutter geometry into the computer program.
- Material Versatility: PDMT supports a wide range of materials, including polymers (PMMA), brittle crystals (Crystalline Germanium, Silicon, Zinc Selenide/Sulfide), and soft metals (Gold, Platinum).
- Geometric Complexity: The method facilitates the production of optical parts up to 250 mm in diameter with complex geometries, including two-sided aspherical forms and kinoform elements.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Achievable Surface Roughness (PDMT) | 0.012 | µm | Precision class turning (Ra) |
| Achievable Surface Roughness (Metal Optics) | < 2 | nm | Aluminum and Copper mirrors [6, 7] |
| Conventional Lathe Roughness (DMG) | 0.8 | µm | Normal class turning (Ra) |
| Maximum Component Diameter | 250 | mm | Aspherical components capability |
| Example Diffractive Lens Zones | 52 | - | Fresnel design example |
| Example Max Step Height | 425.64 | µm | Calculated maximum step height for 52 zones |
| Minimum Surface Purity Class | II | - | Required purity class (GOST 11141-84) |
| Vacuum Chuck Material | D16 | - | Aluminum alloy used for fixture |
| Example Cutter Radius | 0.01 | mm | Input parameter (Figure 4) |
| Example Cutter Included Angle | 60 | degrees | Input parameter (Figure 4) |
| Example Zone Width | 0.5 | mm | Input parameter (Figure 1) |
Key Methodologies
Section titled “Key Methodologies”The fabrication process is divided into preparatory and manufacturing stages, ensuring high precision and minimal runout.
- Diamond Cutter Selection: A specialized diamond cutter is chosen based on its response angle offset relative to the calculated sawtooth microstructure of the diffractive element.
- Fixture Preparation: A vacuum chuck made of D16 material is manufactured.
- The chuck is initially turned using a classic cutter to match the mating radius of the PMMA optical blank.
- The classic cutter is then replaced with a diamond cutter, performing three passes to eliminate residual surface roughness on the vacuum holder.
- Blank Mounting and Alignment: The PMMA optical blank is secured to the vacuum chuck using vacuum clamping. A micron indicator head is used to precisely position and adjust the blank diameter to minimize runout.
- Software Configuration (Fresnel Zones): The specified parameters of the lens zones (e.g., number of zones, depth, half-diameter) are entered into the computer software, followed by a simulation of the zone profile (Figure 1).
- Software Configuration (Cutter Parameters): Parameters of the specific diamond cutter (radius, flank angles, clearance angles) are entered, referencing the manufacturer’s documentation (Figure 4).
- Surface Planing (“Kosiná”): The optical blank surface is leveled by performing a diamond turning pass across the plane until the required flatness specification is met.
- Diffractive Lens Manufacturing: The final program is launched to cut the diffractive lens structure using the “lapping” (выхаживания) method, which ensures the required surface quality and profile accuracy.
Commercial Applications
Section titled “Commercial Applications”The precision and versatility of the diamond micro-turning technology enable its use in several high-demand engineering fields:
- Infrared (IR) Optical Systems: Manufacturing kinoform and diffractive elements from IR materials (e.g., Crystalline Germanium, Silicon, Zinc Sulfide) to correct severe chromatic aberrations and reduce the complexity of optical assemblies.
- High-Energy Laser Systems: Production of ultra-smooth metallic mirrors (Aluminum, Copper) with surface roughness < 2 nm Ra, critical for minimizing scatter and absorption in high-power applications.
- Aerospace and Defense Optics: Fabrication of complex optical components, including two-sided aspheres and diffractive lenses, from materials like PMMA (aviation plastic) and specialized IR crystals.
- Precision Metrology and Imaging: Creation of optical details with complex aspherical working surfaces up to 250 mm in diameter, overcoming the limitations of conventional polishing tools regarding lens steepness.
- Space Astronomy: Manufacturing high-quality optical mirrors specifically designed for cosmic infrared observation, where stringent surface quality and dimensional accuracy are paramount [7].
View Original Abstract
The paper considers the manufacturing process of a diffractive optical element by precision diamond micro-turning using a diffractive lens as an example. The process consists of a preparatory stage and the stage of manufacturing a given diffractive element on specialized equipment with computer program control. The preparatory stage includes selection of a special diamond cutter with a response offset of the cutter angle relative to the calculated microstructure of the diffractive element, the preparation and manufacture of the necessary equipment, including fasteners, with the help of which the optical part is installed and adjusted, on the surface of which a diffractive lens is formed. After that, the specified values of the parameters of the lens zones, the values of the parameters of the diamond cutter are entered into the computer software, and the process of manufacturing a diffractive lens is started.