R. Edward English

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.

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.

Self-referencing Mach-Zehnder interferometer as a laser system diagnostic

We are incorporating a novel self-referencing Mach-Zehnder interferometer into a large scale laser system as a real time, interactive diagnostic tool for wavefront measurement. The instrument is capable of absolute wavefront measurements accurate to better than XIlOpv over a wavelength range > 300 nm without readjustment of the optical components. This performance is achieved through the design of both refractive optics and a catadioptric collimator to achromatize the Mach-Zehnder reference arm. Other features include polarization insensitivity through the use of low angles of incidence on all beamsplitters as well as an equal path length configuration that allows measurement of either broad-band or closely spaced laser-line sources. Instrument accuracy is periodically monitored in place by means of a thermally and mechanically stable wavefront reference source that is calibrated off-line with a phase conjugate interferometer. Video interferograms are analyzed using Fourier transform techniques on a computer that includes a dedicated array processor. Computer and video networks maintain distributed interferometers under the control of a single analysis computer with multiple user access.

Beam control and diagnostic functions in the NIF transport spatial filter

Beam control and diagnostic systems are required to align the National Ignition Facility laser prior to a shot as well as to provide diagnostics on 192 beam lines at shot time. A design that allows each beams large spatial filter lenses to also serve as objective lenses for beam control and diagnostic sensor packages helps to accomplish the task at a reasonable cost. However, this approach also causes a high concentration of small optics near the pinhole plane of the transport spatial filter (TSF) at the output of each beam. This paper describes the optomechanical design in and near the central vacuum vessel of the TSF.

Ghost reflection analysis for the main laser of the National Ignition Facility

Ghost reflections are a major consideration in the optical design of the National Ignition Facility. The first-order layout (e.g., spacing between components), the lens shape, and the dimensions of the building are strongly affected. In this paper we will describe the principal ghost reflections that drive the system configuration. Several specific examples will be shown to illustrate how dangerous ghost reflections are avoided and stray light concerns are managed.

Thermal analysis of transmissive elements in high-average-power laser beam delivery systems

We describe a computer code that aids in the thermal, structural, and optical analysis of laser optical systems. Inputs to the computer model include energy absorption by coatings and substrates, thermal diffusion, expansion, and heat removal parameters. The program performs steady-state thermal and structural analyses based on geometrical ray traces through the optical system. We show examples to illustrate how this capability can assist in the optical system design process.

Spatial filter lens design for the main laser of the National Ignition Facility

The National Ignition Facility (NIF), being designed and constructed at Lawrence Livermore National Laboratory, comprises 192 laser beams. The lasing medium is neodymium in phosphate glass with a fundamental frequency (1ω) of 1.053μm. Sum frequency generation in a pair of conversion crystals (KDP/KD*P) will produce 1.8 megajoules of the third harmonic light (3ω or λ= 0.351μm) at the target. The purpose of this paper is to provide the lens design community with the current lens design details of the large powered optics in the Main Laser. This paper describes the lens design configuration and design consideration of the Main Laser. The Main Laser is 123 meters long and includes two spatial filters: one 23.5 meters and one 60 meters. These spatial filters perform crucial beam filtering and relaying functions. We shall describe the significant lens design aspects of these spatial filter lenses which allow them to successfully deliver the appropriate beam characteristic onto the target.

Optical design of a high power fiber optic coupler

Specifications, design, and operation of an optical system that couples a high-power copper vapor laser beam into a large core, multimode fiber are described. The approach used and observations reported are applicable to fiberoptic delivery applications.

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.

Optical design of the National Ignition Facility main laser and switchyard/target area beam transport systems

The optical design of the main laser and transport mirror sections of the National Ignition Facility are described. For the main laser the configuration, layout constraints, multiple beam arrangement, pinhole layout and beam paths, clear aperture budget, ray trace models, alignment constraints, lens designs, wavefront performance, and pupil aberrations are discussed. For the transport mirror system the layout, alignment controls and clear aperture budget are described.