Zone Plate
This surface is used to model a Fresnel Zone Plate (FZP). A FZP consists of a refractive or reflective surface upon which annular zones of varying depth are cut or etched. Typically, the spacing between grooves is large compared to the wavelength, and therefore FZP's are entirely refractive, and not diffractive, devices. The varying depth of the annular zones imparts a slight phase change to different zones of the beam, due to the change in path length through the material.
The FZP is modeled by altering the local depth of the surface. Edge effects, such as rays striking the side of the zone, and diffraction from the edges, are ignored. The FZP is defined by a normalized optical thickness of each zone. Each zone is spaced a fixed radius apart, and up to 240 zones may be defined. Three parameter values are required: the mode, the Δr , and the reference wavelength, λ0.
If the mode is zero, the steps are not interpolated, and each zone has a constant optical thickness. This results in discontinuous phase steps, like a staircase. If the mode is unity, then the optical thickness is interpolated between adjacent zones. Although the phase will be continuous, the first derivative of the phase will generally be discontinuous at the zone boundaries. The " Δr " is the step in radius between adjacent zones in lens units. The "reference wavelength" is the wavelength in micrometers used to normalize the optical thickness. For example, if the FZP defines the boundary between a material with index 1.55 and air, with index 1.00, and the reference wavelength is 0.45 micrometers, then an optical thickness of 1 wave corresponds to a groove depth of
This thickness is the actual groove depth if the optical thickness is given to be 1.0. OpticStudio uses this normalized optical thickness for convenience. To specify a zone that introduces one-half wave of phase at the reference wavelength, enter an optical thickness of 0.5. At other wavelengths, the phase will be greater or less depending upon the dispersion of the two adjacent medi The optical thickness is defined at n points, separated by Δr : r1, r2, r3, ...rn .specified in the first column of data in the extra data file to import (see the Import section of the Surface Properties). The remaining terms are defined in columns 2 through n+1 in the extra data file.
PARAMETER DEFINITIONS FOR ZONE PLATE SURFACES
| Parameter # | Definition |
| 0 | Mode |
| 1 | Δr |
| 3 | Ref. wave |
| 13 | Number of points (up to 240) |
| 14 | Optical thickness at Δr |
| 15 | Optical thickness at 2 Δr |
| 16 | Optical thickness at 3 Δr |
| n | Optical thickness at (n-13) Δr |
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