Proceedings Article | 11 November 1999
KEYWORDS: Wavefronts, National Ignition Facility, Laser systems engineering, Spatial filters, Optical testing, Optical components, Linear filtering, Optics manufacturing, Tolerancing, Spatial frequencies
In a high energy laser system such as the National Ignition Facility (NIF), the ability to focus light into as small a spot as possible at the highest possible fluence is highly dependent on the quality of the optics used in the system. Typically, surface form errors and transmitted and reflected wavefront errors are specified in terms of a peak-to-valley wavefront error (P-V), or occasionally in terms of an RMS wavefront error (RMS). It has been shown, however, that the parameter that most closely correlates with beam focusability is neither of these, but the RMS of the gradients of the wavefront error (RMS Gradient). Further, the spatial frequency of the wavefront error plays a significant role in the way that a given error effects the performance of the laser system, so careful attention must be paid to how the spatial filtering is both specified an accomplished. Since ISO 10110 has no specific provisions for a gradient specification, LLNL has developed its own notation and procedures for these critical specifications. In evaluating surface figure errors as specified by the NIF drawings, modern phase modulating interferometers (PMI) will be used. In addition to performing QA testing of the optics, LLNL intends to utilize the software capabilities of the instruments to obtain the information to model the wavefront of the 131 passes through various optical elements comprising the NIF 'front end.' A typical transmitted wavefront error call-out for an optic in the front end is: (lambda) /8 P-V, (lambda) /40 RMS, (lambda) /30/ cm RMS gradient, with these values evaluated for spatial periods greater than 2 mm only. Test will be performed and documented after coating and as installed in the specified mechanical mounts. This paper describes the evaluation of the wavefront error for NIF small optics including specifications over a given spatial period call-out, the proper low pass filtering of the data and the allowable filtering and settings that can be applied to obtain proper wavefront data. This paper also describes the origin and evolution of other NIF wavefront and roughness specifications, and gives examples. Since the wavefront requirements and hence the specifications vary for the different systems in the NIF, we will focus on one system, the injection laser system (ILS) or 'front end.' Also discussed will be the metrology and data manipulation requirements for the large aperture optics. Finally, clarification will be given to the differences between various versions of the RMS wavefront and roughness specifications allowed in ISO 10110, and how they contrast to the RMS roughness specifications used in ANSI-Y14.5.
