Sham N. Dixit

Kinoform phase plates for focal plane irradiance profile control

A versatile, rapidly convergent, iterative algorithm is presented for the construction of kinoform phase plates for tailoring the far-field intensity distribution of laser beams. The method consists of repeated Fourier transforming between the near-field and the far-field planes with constraints imposed in each plane. For application to inertial confinement fusion, the converged far-field pattern contains more than 95% of the incident energy inside a desired region and is relatively insensitive to beam aberrations.

Vibrational structure in the emission spectra of Yb3+-doped apatite crystals

Abstract We have determined the ground state energy levels of Yb 3+ in crystals with the apatite structure from the low temperature emission spectra of Yb-doped crystals of calcium, strontium and barium fluorophosphates, calcium chlorophosphate and strontium fluorovanadate. Analyses of the data reveal that the previously reported laser transition arises from an electronic level of Yb interacting with the local vibrational modes of the Yb-O bond.

Diffractive Alvarez lens

A diffractive Alvarez lens is demonstrated that consists of two separate phase plates, each having complementary 16-level surface-relief profiles that contain cubic phase delays. Translation of these two components in the plane of the phase plates is shown to produce a variable astigmatic focus. Both spherical and cylindrical phase profiles are demonstrated with good accuracy, and the discrete surface-relief features are shown to cause less than lambda/10 wave-front aberration in the transmitted wave front over a 40 mm x 80 mm region.

NIF final optics system: frequency conversion and beam conditioning

Installation and commissioning of the first of forty-eight Final Optics Assemblies on the National Ignition Facility was completed this past year. This activity culminated in the delivery of first light to a target. The final optics design is described and selected results from first-article commissioning and performance tests are presented.

Fused Silica Final Optics for Inertial Fusion Energy: Radiation Studies and System-Level Analysis

The survivability of the final optic, which must sit in the line of sight of high-energy neutrons and gamma rays, is a key issue for any laser-driven inertial fusion energy (IFE) concept. Previous work has concentrated on the use of reflective optics. Here, we introduce and analyze the use of a transmissive final optic for the IFE application. Our experimental work has been conducted at a range of doses and dose rates, including those comparable to the conditions at the IFE final optic. The experimental work, in conjunction with detailed analysis, suggests that a thin, fused silica Fresnel lens may be an attractive option when used at a wavelength of 351 nm. Our measurements and molecular dynamics simulations provide convincing evidence that the radiation damage, which leads to optical absorption, not only saturates but that a “radiation annealing” effect is observed. A system-level description is provided, including Fresnel lens and phase plate designs.

Magnetorheological finishing for imprinting continuous-phase plate structures onto optical surfaces

Magnetorheological finishing (MRF) techniques have been developed to manufacture continuous phase plates (CPPs) and custom phase corrective structures on polished fused silica surfaces. These phase structures are important for laser applications requiring precise manipulation and control of beam-shape, energy distribution, and wavefront profile. The MRF’s unique deterministic-sub-aperture polishing characteristics make it possible to imprint complex topographical information onto optical surfaces at spatial scale-lengths approaching 1 mm. In this study, we present the results of experiments and model calculations that explore imprinting two-dimensional sinusoidal structures. Results show how the MRF removal function impacts and limits imprint fidelity and what must be done to arrive at a high quality surface. We also present several examples of this imprinting technology for fabrication of phase correction plates and CPPs for use at high fluences.

High energy two-photon states in finite versus infinite polyenes

Optical nonlinearity in π‐conjugated polyenes depends crucially on a single two‐photon state that is located above the one‐photon allowed exciton. In the case of the infinite polyene this single state should be thought of as a collection of states with very narrow bandwidth. The location of this two‐photon state in the infinite polyene is of considerable interest. We present numerical data for finite chains and discuss in detail why extrapolation of finite chain data to obtain the energies of the high energy states in the infinite polyene requires considerable care. Based on upper and lower bounds that we put on the energy of the two‐photon state in question, we predict it to be extremely close to the one‐photon excition in the infinite chain limit. We present recent experimental data for a polydiacetylene that is consistent with our prediction.

Image processing and control of a programmable spatial light modulator for spatial beam shaping

Programmable spatial shapers using liquid-crystal-based spatial-light-modulators in the National Ignition Facility lasers enable spatial shaping of the beam profile so that power delivered to the target can be maximized while maintaining system longevity. Programmable spatial shapers achieve three objectives: Introduce obscurations shadowing isolated flaws on downstream optical elements that could otherwise be affected by high fluence laser illumination; Spatial shaping to reduce beam peak-to-mean fluence variations to allow the laser to operate at higher powers so that maximum power can be delivered to the target; And finally gradually exposing the optical regions that have never seen laser light because they have always had shadowing from a blocker that is no longer needed. In this paper, we describe the control and image processing algorithms that determine beam shaping and verification of the beam profile. Calibration and transmittance mapping essential elements of controlling the PSS are described along with spatially nonlinear response of the device such as scale and rotation.

Manufacture, optical performance, and laser damage characteristics of diffractive optics for the National Ignition Facility

We have fabricated demonstration diffractive optic plates at full scale for the NIF laser. These include an off-axis focusing beam sampling grating, a color separation grating, and a kinoform phase plate for spatial beam smoothing. Fabrication methods and optical performance of these DOPs are discussed. IT was discovered that the sol-gel antireflective coating normally applied to high-power transmissive optics partially planarizes the diffractive structures, particularly on the color separation grating used for color management at target, to the extent that optical performance and laser damage threshold are negatively impacted. The effect of sol-gel coatings on grating performance, the feasibility of placing all diffractive structures on a single surface, and future work in this area are discussed.

Effect of bandwidth on beam smoothing and frequency conversion at the third harmonic of the Nova laser

We present the results of experiments performed on the Nova laser system to determine the effect of bandwidth on third harmonic (3(omega) ) frequency conversion and beam smoothing by spectral dispersion (SSD). Our experiments utilized a wide bandwidth fiber optic cross- phase modulated (XPM) source and a narrower bandwidth microwave modulated (FM) source, each centered at 1053 nm (1 (omega) ). A significant fraction (> 50%) of the 1(omega) XPM bandwidth was transferred to the 3(omega) beam (22 cm-1 yields 36 cm-1), yielding 0.13% bandwidth at 3(omega) . The maximum intrinsic narrowband 3(omega) frequency conversion obtained using a type-II/type-II KDP crystal array was 62%. The intrinsic efficiency obtained at the Nova 10-beam chamber is typically > 65%. Frequency conversion was essentially unaffected by the 2 cm-1 bandwidth obtained from FM source. However, the 5 - 16 cm-1 of bandwidth from the XPM source reduced the conversion efficiency to approximately 24%. We have developed broadband frequency conversion codes and broadband pulse simulations to model our results, and have obtained good agreement with experiment.