Christopher T. Cotton

NIF optical specifications: the importance of the RMS gradient

The performance of the National Ignition Facility (NIF), especially in terms of laser focusability, will be determined by several key factors. One of these key factors is the optical specification of the thousands of large aperture optics that will comprise the 192 beamlines. We have previously reported on the importance of the specification of the power spectral density (PSD) on NIF performance. Recently, we have been studying the importance of long spatial wavelength phase errors on focusability. We have concluded that the preferred metric for determining the impact of these long spatial wavelength phase errors is the rms phase gradient. In this paper, we outline the overall approach to NIF optical specifications, detail the impact of the rms phase gradient on NIF focusability, discuss its trade-off with the PSD in determining the spot size, and review measurements of optics similar to those to be manufactured for NIF.

Compact Nd3+-based laser system with gain G ≤1013 and output energy of 20 J

We have developed a compact laser system capable of amplifying nanosecond-scale pulses form a few picojoules to 20J. The system has a 40-mm clear aperture and a 37-mm working aperture for high-energy output. We measured less than 1 wave phase distortion over full 37-mm aperture for a pulse with 18-J output energy at a shot repetition rate of one shot every 10 min. In experiments with a 30-mm diam beam, a flat-top spatial profile with 4 percent rms over the entire beam diameter was demonstrated for a 1-ns pulse with 20_j output energy. The amplifier has a net gain up to 1013 and fits easily on a 5-ft X 14-ft optical table.

Specification of large-aperture Nd:phosphate glass laser disks

The purchase of large (15- and 20-cm clear aperture) Brewster-angle laser disks involves the specification of a large number of often conflicting parameters, all of which bear on performance and cost. Furthermore, the laser requirements often approach the state-of-the-art in glass-melting technology and the parameter measurement. This paper enumerates the relevant parameters, the trade-offs made in their selection, and the test procedures and instrumentation required to ensure compliance with the specification.

The design of a multi-point probe for a scanning low-coherence distance-measuring interferometer

A highly efficient method for splitting the probe beam produced by a scanning low-coherence distance-measuring interferometer (SLCDI) is presented. The SLCDI is used to measure thicknesses of materials with thicknesses in the range of 12 microns-50 mm, with a repeatability of 0.1 microns. The measurements are made optically with a beam with a wavelength of 1.3 microns. The SLCDI is also capable of simultaneous measurement of a stack of multiple films. Splitting of the beam from the SLCDI probe to create a multi-point probe allows for multiple, simultaneous measurements to be made at a surface. An advantage of this capability is that it provides the ability of to measure the surface normal at each of the surfaces that are under test. The operation of the SLCDI will be described along with how the operation impacts the requirements for the multi-point probe system. The requirements are discussed from the standpoint of the coherence length of the SLCDI source and the operational usage of the probe. The splitting is achieved through the use of polarization components. The function and performance of the resulting probe is also discussed.

Low-coherence surface metrology using a multiple-beam optical probe

Despite increasing demand for freeform optical elements, at present there are no commercial systems to measure such components. In previously published research we demonstrated that a scanning low-coherence dual-wavelength interferometer can accurately measure the transmitted wavefront of hemispheric dome optics by mapping the optical thickness of the dome as a function of probe probe position. To address the issue of more generalized freeform surfaces, we have developed a new probe for the interferometer. This probe incorporates a reference surface and simultaneously projects four beams. This allows the instrument to measure both the position and orientation of the surface with respect to the probe as the probe is scanned over the object. Both the interior and exterior surfaces can be measured simultaneously. Furthermore, by overlapping the measurement regions, the redundant data can be used to minimize some forms of measurement error.

Surface and thickness metrology using scanning low-coherence dual interferometry (SLCDI)

We present experimental results for a low-coherence, dual-wavelength metrology system capable of measuring simultaneously both optical thickness and surface figure. The system measures optical thicknesses as thin as 12 microns to as wide as 12 mm with an accuracy of 0.1 microns. The current system scans at a resolution of 50 microns, which is limited by the spot-size of the measurement beam. We validate that SLCDI yields results in agreement with traditional interferometry and demonstrate its ability to measure aspheric “saddle” mirrors and complex three-dimensional surfaces.