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Proceedings Paper

Measurement and analysis of wavefront structure from large-aperture ICF optics
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Paper Abstract

This paper discusses the techniques, developed over the past year, for high spatial resolution measurement and analysis of the transmitted and/or reflected wavefront of large aperture ICF optical components. Parts up to 400 mm by 750 mm have been measured and include: laser slabs, windows, KDP crystals and lenses. The measurements were performed using state-of- the-art commercial phase shifting interferometers at a wavelength of 633 micrometer. Both 1 and 2-D Fourier analysis have been used to characterize the wavefront; specifically the power spectral density (PSD) function was calculated. The PSDs of several precision optical components are shown. The PSD(nu) is proportional to the (amplitude)2 of components of the Fourier frequency spectrum. The PSD describes the scattered intensity and direction as a function of scattering angle in the wavefront. The capability of commercial software is limited to 1-D Fourier analysis only. We are developing our own 2-D analysis capability in support of work to revise specifications for NIF optics. Two-dimensional analysis uses the entire wavefront phase map to construct 2-D PSD functions. We have been able to increase the signal-to-noise relative to 1-D and can observe very subtle wavefront structure. The physics of the NIF laser design dictate partitioning the wavefront into three regimes of spatial wavelength (or spatial frequency). We discuss the data in terms of the following three scale length regimes: (1) short scale, or 'micro roughness,' having scale lengths less than 120 micrometer; (2) mid-spatial scale, with scale lengths from 0.12 to 33 mm; and (3) long scale, or conventional 'optical figure/curvature,' having scale lengths greater than 33 mm. Regular repetitive wavefront structure has been observed in all three regimes, ranging from 10 micrometer to 100 mm in scale length. The magnitude of these structures are typically from lambda/100 to lambda/20. Structure has been detected in optical materials and on the surfaces of finished parts. We believe the sources of these structures are small fabrication errors. The Modeling Group at LLNL is using this data in beam propagation codes to assist in optimizing laser system design and to develop optics specifications for the NIF.

Paper Details

Date Published: 8 December 1995
PDF: 25 pages
Proc. SPIE 2633, Solid State Lasers for Application to Inertial Confinement Fusion (ICF), (8 December 1995); doi: 10.1117/12.228288
Show Author Affiliations
C. Robert Wolfe, Lawrence Livermore National Lab. (United States)
Janice K. Lawson, Lawrence Livermore National Lab. (United States)

Published in SPIE Proceedings Vol. 2633:
Solid State Lasers for Application to Inertial Confinement Fusion (ICF)
Michel Andre; Howard T. Powell, Editor(s)

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