A zone plate is an optical element used to separate and focus individual wavelengths of light. The zone plate consists of small circular rings that are opaque to the light separated by gaps, which are transparent. The incident light diffracts from the zone plate at an angle that depends on the wavelength. Different wavelengths are focused to different distances behind the zone plate. By placing a small aperture at the appropriate distance from the zone plate, a single wavelength can be selected and transmitted to a detector or other optical instrument. The figure to the right illustrates this.
This project is developing zone plates to be used in a solar radiometer to observe the intensity of the emission from our sun at a wavelength of 30.4 nm, which is emitted from ionized helium in the solar transition region. The zone plates are designed with gold rings to focus the 30.4 nm light to a distance 25 mm behind the zone plate. The outer diameter of the zone plates is 4 mm, and there is a 2-mm diameter central occulter to block the undiffracted light. The remaining space is filled by several thousand zones.
Two zone plates have been fabricated at the Stanford Linear Accelerator Facility (SLAC), and they were characterized at the EUV Radiometry Beamline (BL-9) at NIST's Synchrotron Ultraviolet Radiation Facility (SURF III). The zone plate's absolute efficiency was measured and found to be in reasonable agreement with the values determined by modeling the structure.
At its conclusion, this project will develop a compact radiometer for 30.4 nm radiation that has been qualified by test to technology readiness level 6 (TRL-6) by characterization at NIST. The instrument will be made available to flight teams to advance to the technology to TRL-7 by flight demonstration of a working prototype. A compact radiometer based on zone plates has several advantages over grating based technologies:
- small focal spot enables the used of small-area, low-noise detectors;
- nearly constant efficiency over 1° field-of-view;
- circular symmetry means no polarization sensitivity; and
- out-of-band wavelengths are not well-focused at the aperture and are blocked.