A method of compensating for or minimizing phase differences between orthogonal polarizations of light reflected from metallic mirrors at oblique incidence, as, for example, from weakly curved mirrors, is undergoing development. The method is intended to satisfy a need to maintain precise polarization phase relationships or minimum polarization differences needed for proper operation of telescopes and other scientific instruments that include single or multiple mirrors. The basic idea of the method is to optimally coat mirrors with thin engineered layers of materials that introduce phase differences that, as nearly precisely as possible, are opposite of the undesired phase differences arising in reflection with non-optimum coatings. Depending on the specific optical system, the method could involve any or all of the following elements:
- Optimization of a single coat on all the mirrors in the system.
- Optimization of a unique coat for each mirror such that the polarization phase effects of the coat on one mirror compensate, to an acceptably high degree over an acceptably wide wavelength range, for those of the coat on another mirror.
- Tapering the coat on each mirror.
Optimization could involve the choice of a single dielectric coating material and its thickness, or design of a more-complex coat consisting of multiple layers of different dielectric materials and possibly some metallic materials. Such designs and coatings are particularly significant and needed for obtaining very high quality of wavefront required in high-contrast imaging instruments such as the NASA Terrestrial Planet Finder Coronagraph.
This work was done by Kunjithapatham Balasubramanian 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-41396
This Brief includes a Technical Support Package (TSP).
Polarization Phase-Compensating Coats for Metallic Mirrors
(reference NPO-41396) is currently available for download from the TSP library.
Don't have an account?
Overview
The document titled "Technical Support Package for Polarization Phase-Compensating Coats for Metallic Mirrors" discusses advancements in mirror coatings aimed at controlling polarization and phase differences in light reflected from telescope mirrors, particularly for the Terrestrial Planet Finder Coronagraph (TPF-C) project. The work addresses the critical need for precise control of phase differences between orthogonal polarization fields, especially when light reflects off curved mirrors at non-normal incidence.
The motivation behind this research stems from the stringent requirements of the TPF-C, which necessitates minimal phase differences between x and y polarizations of light across a wide bandwidth. Traditional materials used for telescope mirrors, such as Aluminum and Silver, require protective overcoats to maintain performance over time. The document explores various dielectric materials, including MgF2, SiO2, and HfO2, that can be employed to achieve the desired optical performance while providing environmental protection.
A key innovation presented is the design of multilayer coatings that can compensate for polarization phase effects between different mirrors in a telescope system. By carefully selecting and designing the coatings on individual mirrors, it is possible to achieve a net compensation effect, thereby enhancing the overall performance of the optical system. The document outlines potential methods for reducing phase differences, including the use of very thin dielectric or metallic layers that do not significantly impact optical performance.
The findings are significant not only for the TPF-C but also for other applications requiring precise polarization control in optics. The document emphasizes the importance of overcoming manufacturing challenges associated with these complex coatings, particularly given the size of the TPF-C telescope mirrors.
In summary, this technical support package provides a comprehensive overview of innovative approaches to mirror coatings that enhance polarization control in telescope optics. It highlights the relevance of this work to current and future NASA projects, showcasing its potential impact on aeronautical and space activities. The research represents a significant step forward in the quest for improved optical performance in space exploration and related fields.
