A report discusses the difficulty of measuring scattering properties of coated mirrors extremely close to the specular reflection peak. A prototype Optical Heterodyne Near-angle Scatterometer (OHNS) was developed. Light from a long-coherence- length (>150 m) 532-nm laser is split into two arms. Acousto-optic modulators frequency shift the sample and reference beams, establishing a fixed beat frequency between the beams. The sample beam is directed at very high f/# onto a mirror sample, and the point spread function (PSF) formed after the mirror sample is scanned with a pinhole. This light is recombined by a non-polarizing beam splitter and measured through heterodyne detection with a spectrum analyzer. Polarizers control the illuminated and analyzed polarization states, allowing the polarization dependent scatter to be measured.
The bidirectional reflective or scattering distribution function is normally measured through use of a scattering goniometer instrument. The instrumental beam width (collection angle span) over which the scatterometer responds is typically many degrees. The OHNS enables measurement at angles as small as the first Airy disk diameter.
This work was done by Steven A. Macenka of Caltech and Russell A. Chipman, Brian J. Daugherty, and Stephen C. McClain of the University of Arizona for NASA’s Jet Propulsion Laboratory. This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel, NASA Management Office–JPL.
NPO-47310
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Coherent Detector for Near-Angle Scattering and Polarization Characterization of Telescope Mirror
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Overview
The document outlines the development and objectives of a coherent detector designed for near-angle scattering and polarization characterization of telescope mirror coatings, primarily for the Terrestrial Planet Finder (TPF) Mission. Led by Principal Investigator Steven A. Macenka and a team from the College of Optical Sciences at the University of Arizona, the project aims to detect extrasolar planets with a contrast ratio of 10^12, necessitating stringent optical system specifications.
The project addresses the challenges associated with near-angle scatter and the depolarizing effects of metallic coatings on image formation, which are not well understood. To tackle these issues, the team developed a small angle polarimetric scatterometer capable of high contrast measurements at small scattered light angles. Key objectives included designing a coherent imager to measure scattered light at 20 arcseconds from the point spread function (PSF) center, achieving a PSF contrast of 10^6, and characterizing the impulse response of the imager.
The results from FY09 indicate that the coherent detection imager successfully measures the polarization-dependent scattering properties of metallic mirror coatings at very small angles. The system employs heterodyne detection to analyze the PSF produced by reflecting a high f/# beam from sample mirrors. The first null of the PSF occurs at 20 arcseconds, with a contrast exceeding 10^6 at approximately 100 arcseconds. The imager's ability to measure electric field amplitude rather than irradiance allows for comprehensive scanning of the PSF in a single scan, enhancing the signal-to-noise ratio (SNR) by exploiting the high contrast in the nulls of the PSF.
The findings demonstrate the potential of coherent detection to meet the scattering metrology needs for mirror coatings in TPF missions. The scalable nature of the coherent imager allows for the measurement of larger mirrors within an arc second of the specular peak, facilitating the development of coating techniques for extremely low-scatter applications. Additionally, the project highlights the importance of studying and minimizing the effects of columnar microstructures found in metallic coatings, which can impact optical performance.
Overall, this research contributes significantly to advancing optical technologies for space exploration, particularly in the quest to identify and study distant planetary systems.
