<title>Use of power spectral density (PSD) functions in specifying optics for the National Ignition Facility</title>

Abstract

In the second half of the 1 990ts, LLNL and others will be designing and beginning construction of the National Ignition Facility (NIF). This new laser will be capable of producing the worlds first controlled fusion ignition and burn, completing a vital milestone on the path to Fusion Energy. This facility will use more than 7,000 optical components, most of which have a rectangular aperture, which measure greater than 600 mm on the diagonal.In order to optimize the performance versus cost of the laser system, we have determined that specifications based on the Power Spectral Density (PSD) function are the most effective for controlling mid-spatial wavelength errors. The draft optics specifications based on a combination of PSD and conventional roughness and P-V requirements are presented, with a discussion of their origins. The emphasis is on the application of a PSD function for transmitted wavefront optical specifications, and the benefits thereof. The PSD function is the most appropriate way to characterize transmitted wavefront errors with spatial frequencies ranging from several centimeters to a few hundred nanometers, with amplitudes in the X/100 regime. Such errors are commonly generated by cost effective, deterministic finishing technologies, and can be damaging to the laser, as well as causing unnecessary energy loss and inability to focus, in a high energy laser application. In addition, periodic errors can occur as a result of errors at other steps in the fabrication process, such as machine vibration in a fixed abrasive step, or material homogeneity ripple. The control of such errors will be essential to the construction of future high energy lasers.

Study on the optical performance and characterization method of texture on KH2PO4 surface processed by single point diamond turning

“…It can reflect the impacts of frequency components with different wavelengths in different directions, and which is an important reference for the identification of processing texture direction. The 2-D PSD is defined as [24] PSD 2 (f x , f y ) = 1 L 2…”

Mid-frequency surface error test with a Foucault apparatus

“…With the development of Inertial Confinement Fusion (ICF), especially American National Ignition Facility (NIF) [2][3][4][5][6][7][8][9] in the second half of the 1990's, the importance of mid-frequency surface error, also called ripple or waviness, appears to clear up. Small-angle scattering caused by the error not only awfully decreases the performance of an imaging system because the Point Spread Function (PSF) of the system seems no longer sharp, but also may harm and even damages a high power solid-state laser system because of phase modulation resulting in non-linear gain.…”

“…Sub-aperture testing with Phase Shift Interferometry (PSI) is quite commonly used for NIF 4,5,6 and laser MégaJoule 10,11 . Meanwhile, some other approaches, such as High-Resolution Angle Resolved Scattering (HRARS) and Long Trace Profiler (LTP) for the X-ray Multi-Mirror Observatory (XMM) at the European Space Agency (ESA) 12,13 , are also described in corresponding papers.…”

Power spectral density measurement for large aspheric surfaces

“…Traditionally, simple statistical measurements such as Zernike polynomials, peak-to-valley wavefront error and root mean square (RMS) roughness are used to obtain an initial quantitative description of the surfaces [3][4][5][6]. These measurements are based on height information and, for certain types of surfaces, can serve as a good description of their roughness.…”