David M. Aikens

Power spectral density specifications for high-power laser systems

This paper describes the use of Fourier techniques to characterize the transmitted and reflected wavefront of optical components. Specifically, a power spectral density (PSD), approach is used. High power solid-state lasers exhibit non-linear amplification of specific spatial frequencies. Thus, specifications that limit the amplitude of these spatial frequencies are necessary in the design of these systems. Further, NIF optical components have square, rectangular or irregularly shaped apertures with major dimensions up to 800 nm. Components with non-circular apertures can not be analyzed correctly with Zernicke polynomials since these functions are an orthogonal set for circular apertures only. A more complete and powerful representation of the optical wavefront can be obtained by Fourier analysis in 1 or 2 dimensions. The PSD is obtained from the amplitude of frequency components present in the Fourier spectrum. The shape of a resultant wavefront or the focal spot of a complex multi-component laser system can be calculated and optimize using PSDs of the individual optical components which comprise the system. Surface roughness can be calculated over a range of spatial scale-lengths by integrating the PSD. FInally, since the optical transfer function of the instruments used to measure the wavefront degrades at high spatial frequencies, the PSD of an optical component is underestimated. We can correct for this error by modifying the PSD function to restore high spatial frequency information. The strengths of PSD analysis are leading us to develop optical specifications incorporating this function for the planned National Ignition Facility.

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

In the second half of the 1990s, LLNL and others will be designing and beginning construction of the National Ignition Facility. This new laser will be capable of producing the worlds first controlled fusion ignition and burn, completing a vital milestone on the path of 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) functions 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 (lambda) /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.

Origin and evolution of the optics specifications for the National Ignition Facility

In the second half of the 1990s, LLNL and others will be designing and beginning construction of the National Ignition Facility (NIF). At more than 10 times the power and size of the Nova laser system, 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. In order to optimize the performance of the laser system for a minimum cost, we have been conducting a campaign to properly specify the optical properties of the more than 7,500 large optical components to be deployed in the NIF. The draft optics specifications derived from this effort will be presented. The evolution of these specifications, both in language and in content, will be discussed, specifically transmitted wavefront (both P-V and PSD), scratch/dig, surface roughness, bubbles and inclusions specifications.

Implementing ISO standard-compliant freeform component drawings

Abstract. Successful fabrication of aspheres requires all parts of the process chain including design, production, and measurements. Aspheres now are well-established and accepted as an equal optical element, when done properly. Research and industry have now started to focus efforts to develop the next element that propels the field forward in capability, namely the optical freeform surface. An essential factor enabling wide use of freeforms is communicating requirements. This paper discusses form description and tolerancing additions to ISO 10110 to accommodate freeform surfaces. Information stating how ISO 10110 and related standards documents such as ISO 14999-4 are being continually developed to meet the requirements for specifying freeform surfaces is also provided. This paper further provides an example monolithic freeform element using the recently updated relevant parts of ISO 10110. The first manufacturing of this component has been successful, and this paper shows the role the ISO standard has played in success. Definitions for toleranced parameters, such as surface registration (centration) and form deviation (irregularity, slope, Zernike, PV, and PVr), are also indicated. The monolithic example also shows how to use the defined data and definitions for metrology and data handling. Metrology results for the freeform surface are given.

Overview of small optics for the National Ignition Facility

LLNLs project to construct the National Ignition Facility (NIF), a 192 beam laser system capable of generating enough light energy necessary to achieve fusion ignition, will require 26,641 small optics, many of which will be supplied in the form of cleaned, tested and aligned assemblies. These assemblies will be built to print, cleaned to specifications, and tested to performance specifications, ready to be installed in the laser system. A wide range of potential suppliers will participate in the manufacture of these sophisticated opto-mechanical systems. The injection laser system requires 7,440 precision optical components manufactured to state of the art performance specifications. In addition to 550 aspheric lenses, almost 2,000 precision spherical elements are required. Wave-fronts are specified in terms of P-V, RMS and RMS Gradient wave-front error, with strict requirements on the filtering and resolution which is required. Precision polarizers, high reflectors, leaking mirrors, high damage threshold coatings and cleanliness levels of 50 to 100 are also specified for this section of the NIF laser. The alignment and diagnostics systems for the NIF require 19,201 optics, many of which have requirements that exceed those of the injection laser system. All of these optics will be purchased using the ISO 10110 drawing notations. Other sections of the laser system will utilize commercial, off the shelf components to control cost. This paper will give an overview of the project and its objectives, with specific attention to the small optics required for the NIF.

Design of optical systems with both near-field and far-field system requirements

Abstract. The authors describe a simple technique for the generation of well-corrected optical designs with both far-field and near-field requirements. Several design examples illustrate the use of this technique and its range of applicability.

Database of wavefront measurements for laser system modeling, optical component development, and fabrication process qualification

This paper describes a database of wavefront measurements of large aperture optical components. This database is being compiled as part of our component development program in preparation for the National Ignition Facility (NIF). The data is stored electronically in the form of 3D phase maps of the reflected or transmitted wavefront, measured by phase shifting interferometry at 6328 angstrom. The database will serve both technical and administrative purposes for the NIF project. These purposes will be described, as well as the type of data and measurement techniques used.

Standards: a key element of optical design, engineering productivity, and time to market

Standards provide a conduit for understanding and communication in the global optics industry. Proper use and knowledge of standards is beneficial to global commerce and increases productivity. In this paper the design utility and efficiency afforded by standards is shown with examples that are congruent with current ANSI and ISO published documents.

Description and tolerancing of freeform surfaces in standards

In recent years, freeform surfaces have become increasingly important. This paper introduces new form description and tolerancing additions to ISO 10110 to accommodate freeform surfaces. Information stating how ISO 10110 and related standards documents such as ISO 14999-4 are being continually developed to meet the requirements for specifying freeform surfaces is also provided. The manuscript includes examples illustrating the increased features of the standard.