Compensation for Phase Anisotropy of a Metal Reflector

NASA’s Jet Propulsion Laboratory, Pasadena, California

Monday, January 01 2007

The magnitude of the compensatory phase anisotropy would be proportional to the thickness of the compensator. In choosing the thickness, one must take into account that incident light would pass through the dielectric layers, be reflected from the mirror surface, then pass through the dielectric layers again and, hence, the phase accrual through the compensation layer must therefore be doubled before being added to the reflection phase.

The free design parameters for a given application would be the choice of constituent dielectric layers (with their indices of refraction and dispersion characteristics), the thickness of the compensator (equivalently, the number of spatial periods), and the relative thickness of each constituent layer. In a typical design optimization, one would adapt the parameters to the reflector at hand and seek to keep the phase deviation below some maximum allowable value across the range of angles of incidence for the field of view of the instrument of which the reflector is a part. To obtain compensation over a spectral band, it would be desirable to perform a wider optimization involving the bandwidth of the light and the dispersion characteristics of each dielectric layer.

The lower part of Figure 2 illustrates an example of compensation for the anisotropy of Figure 1 for monochromatic light. In this case a combination of n_o = 1.5, n_e = 1.45, d₁ = d₂ = d/2, and an overall thickness of 0.5676 wavelengths was chosen to satisfy a requirement to keep the maximum phase anisotropy below 0.0075° at angles of incidence as large as 13°.

This work was done by John Hong of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-40728