The use of aspheric surfaces in the optical systems design allows the designer to achieve better spot size performance, or alternatively achieve similar performance while using fewer elements in the system. These aspheric surfaces are extremely difficult to fabricate using conventional polishing processes. Our diamond-turning facility at II-VI includes two-axis machines which can produce precision optical finishes with aspheric geometry. Infrared materials suitable for this machining process are germanium, zinc selenide, zinc sulfide, and silicon.The sketch below shows a plano-convex aspheric lens element with the aspheric curve parameters definition.Asperic Lens Parameterswhere,
R is radius of curvature at vertex
K is conic constant
A4...A20 is Aspheric coefficient
For single element lens designs, the designer may use an aspheric surface to correct for spherical aberration, thus the theoretical spot size is limited only by diffraction. The table below shows the theoretical spot size for 2.50” focal length lenses and a 21 mm diameter Gaussian beam at 1/e2 points and an M2 value of 1.