The desire to field space-based telescopes with apertures in excess of 10 meter diameter is forcing the development of extreme lightweighted large optomechanical structures. Sparse apertures, shell optics, and membrane optics are a few of the approaches that have been investigated and demonstrated. Membrane optics in particular have been investigated for many years. The MOIRE approach in which the membrane is used as a transmissive diffractive optical element (DOE) offers a significant relaxation in the control requirements on the membrane surface figure, supports extreme lightweighting of the primary collecting optic, and provides a path for rapid low cost production of the primary optical elements. Successful development of a powered meter-scale transmissive membrane DOE was reported in 2012. This paper presents initial imaging results from integrating meter-scale transmissive DOEs into the primary element of a 5- meter diameter telescope architecture. The brassboard telescope successfully demonstrates the ability to collect polychromatic high resolution imagery over a representative object using the transmissive DOE technology. The telescope includes multiple segments of a 5-meter diameter telescope primary with an overall length of 27 meters. The object scene used for the demonstration represents a 1.5 km square complex ground scene. Imaging is accomplished in a standard laboratory environment using a 40 nm spectral bandwidth centered on 650 nm. Theoretical imaging quality for the tested configuration is NIIRS 2.8, with the demonstration achieving NIIRS 2.3 under laboratory seeing conditions. Design characteristics, hardware implementation, laboratory environmental impacts on imagery, image quality metrics, and ongoing developments will be presented.
S. Burkhart, A. Awwal, M. Borden, T. Budge, J. Campbell, S. Dixit, M. Henesian , K. Jancaitis, D. Jedlovec, R. Leach, R. Lowe-Webb, B. MacGowan, S. Pratuch, J. Palma, T. Salmon, D. Smauley, L. Smith, S. Sommer, P. Wegner, K. Wilhelmsen, M. Witte, N. Wong
The National Ignition Facility (NIF) is the world’s most energetic laser, having demonstrated in excess of 1.9MJ @351nm with Inertial Confinement Fusion pulse-shapes in July, 2012. First commissioned with 192 operational beamlines in March, 2009, NIF has since transitioned to routine operation for stockpile stewardship, inertial confinement fusion research, and basic high energy density science.
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.
The desire to field space-based telescopes with apertures in excess of 10 meter diameter is forcing the development
of extreme lightweighted large optics. Sparse apertures, shell optics, and membrane optics are a few of the
approaches that have been investigated and demonstrated. Membrane optics in particular have been investigated for
many years. The majority of the effort in membrane telescopes has been devoted to using reflective membrane
optics with a fair level of success being realized for small laboratory level systems; however, extending this
approach to large aperture systems has been problematic. An alternative approach in which the membrane is used as
a diffractive transmission element has been previously proposed, offering a significant relaxation in the control
requirements on the membrane surface figure. The general imaging principle has been demonstrated in 50-cm-scale
laboratory systems using thin glass and replicated membranes at long f-number (f/50). In addition, a 5-meter
diameter f/50 transmissive diffractive optic has been demonstrated, using 50-cm scale segments arrayed in a
foldable origami pattern. In this paper we discuss Membrane Optical Imager Real-time Exploitation (MOIRE)
Phase 1 developments that culminated in the development and demonstration of an 80 cm diameter, off-axis, F/6.5
phase diffractive transmissive membrane optic. This is a precursor for an optic envisioned as one segment of a 10
meter diameter telescope. This paper presents the demonstrated imaging wavefront performance and collection
efficiency of an 80 cm membrane optic that would be used in an F/6.5 primary, discusses the anticipated areal
density in relation to existing space telescopes, and identifies how such a component would be used in previously
described optical system architectures.
A system of customized spatial light modulators has been installed onto the front end of the laser system at the National
Ignition Facility (NIF). The devices are capable of shaping the beam profile at a low-fluence relay plane upstream of the
amplifier chain. Their primary function is to introduce "blocker" obscurations at programmed locations within the beam
profile. These obscurations are positioned to shadow small, isolated flaws on downstream optical components that might
otherwise limit the system operating energy. The modulators were designed to enable a drop-in retrofit of each of the 48
existing Pre Amplifier Modules (PAMs) without compromising their original performance specifications. This was
accomplished by use of transmissive Optically Addressable Light Valves (OALV) based on a Bismuth Silicon Oxide
photoconductive layer in series with a twisted nematic liquid crystal (LC) layer. These Programmable Spatial Shaper
packages in combination with a flaw inspection system and optic registration strategy have provided a robust approach
for extending the operational lifetime of high fluence laser optics on NIF.
Customized spatial light modulators have been designed and fabricated for use as precision beam shaping devices in
fusion class laser systems. By inserting this device in a low-fluence relay plane upstream of the amplifier chain,
"blocker" obscurations can be programmed into the beam profile to shadow small isolated flaws on downstream optical
components that might otherwise limit the system operating energy. In this two stage system, 1920 × 1080 bitmap
images are first imprinted on incoherent, 470 nm address beams via pixelated liquid crystal on silicon (LCoS)
modulators. To realize defined masking functions with smooth apodized shapes and no pixelization artifacts, address
beam images are projected onto custom fabricated
optically-addressable light valves. Each valve consists of a large,
single pixel liquid cell in series with a photoconductive Bismuth silicon Oxide (BSO) crystal. The BSO crystal enables
bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in
turn modulates the amplitude of the coherent beams at 1053 nm. Valves as large as 24 mm × 36 mm have been
fabricated with low wavefront distortion (<0.5 waves) and antireflection coatings for high transmission (>90%) and
etalon suppression to avoid spectral and temporal ripple. This device in combination with a flaw inspection system and
optic registration strategy represents a new approach for extending the operational lifetime of high fluence laser optics.
Previously, we reported preliminary results for commercial thin borosilicate glass sheets evaluated for use as a frequently-replaced optic to separate the radiation and contamination produced by the inertial confinement fusion experiments in the National Ignition Facility target chamber from the expensive precision laser optics which focus and shape the 351-nm laser beam. The goal is identification of low cost substrates that can deliver acceptable beam energy and focal spots to the target. The two parameters that dominate the transmitted beam quality are the transmitted wave front error and 351-nm absorption. Commercial materials and fabrication processes have now been identified which meet the beam energy and focus requirements for all of the missions planned for the National Ignition Facility. We present the first data for use of such an optic on the National Ignition Facility laser.
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.
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.
Eyeglass is a very large aperture (25 - 100 meter) space telescope consisting of two distinct spacecraft, separated in space by several kilometers. A diffractive lens provides the telescope's large aperture, and a separate, much smaller, space telescope serves as its mobile eyepiece. Use of a transmissive diffractive lens solves two basic problems associated with very large aperture space telescopes; it is inherently fieldable (lightweight and flat, hence packagable and deployable) and virtually eliminates the traditional, very tight, surface shape tolerances faced by reflecting apertures. The potential drawback to use of a diffractive primary (very narrow spectral bandwidth) is eliminated by corrective optics in the telescope's eyepiece. The Eyeglass can provide diffraction-limited imaging with either single-band, multiband, or continuous spectral coverage.
Broadband diffractive telescopes have been built at LLNL and have demonstrated diffraction-limited performance over a 40% spectral bandwidth (0.48 - 0.72 μm). As one approach to package a large aperture for launch, a foldable lens has been built and demonstrated. A 75 cm aperture diffractive lens was constructed from 6 panels of 1 mm thick silica; it achieved diffraction-limited performance both before and after folding. This multiple panel, folding lens, approach is currently being scaled-up at LLNL. We are building a 5 meter aperture foldable lens, involving 72 panels of 700 μm thick glass sheets, diffractively patterned to operate as coherent f/50 lens.
This report discusses the design and installation of a static wavefront correction optic on the HELEN laser at AWE. The element is designed to compensate for static phase errors and prompt thermally induced aberrations on the backlighter beam of the laser. Partial compensation of cooling effects is also included in the design. A phase element has been fabricated using a recently developed novel wet etch figuring tool at LLNL. Performance evaluation through comparison of the focal spot pre- and post-installation is provided. The element has been tested on the laser to produce a 2x reduction in focal spot size.
Commercial thin borosilicate glass sheets have been evaluated for use as a single-shot optic debris shield to separate the radiation and contamination produced by the inertial confinement fusion (ICF) experiment from the expensive precision laser optics which focus and shape the 351-nm laser beam which irradiates the target. The goal of this work is identification of low cost materials that can deliver acceptable beam energy and focal spots to the target. The two parameters that dominate the transmitted beam quality are the transmitted wave front error and bulk absorption. This paper focuses on the latter. To date, the materials with the lowest linear 351-nm absorption have also generally demonstrated the lowest nonlinear absorption. Commercial materials have been identified which approach the beam energy and focus requirements for many ICF missions.
Wet-etch figuring (WEF), a computer-controlled method for generating arbitrarily shaped optical surfaces using wet chemical etching, has been developed. This method uses applicator geometry and surface tension gradients (the Marangoni Effect) to define and confine the footprint of a wetted etchant zone on the surface. Capillary forces attach the flowing etchant solution to the underside of the optic being figured. No mechanical or thermal stresses or residues are applied to the optic by this process. This enables interferometric measurement of the glass thickness while surfacing, which then controls the placement and dwell time of the wetted zone. The result is a truly deterministic, closed-loop figuring process with a high degree of optical precision. This process can figure submillimeter thickness, large-aperture plates or sheets that are very difficult to finish by conventional methods. Automated linear and circular spot etching tools were used to demonstrate surfacing on 380 micron-thick glass sheets, to Strehl better than 0.8, as specified by data array or Zernike polynomials.
Large aperture diffractive optics are needed in high power laser applications to protect against laser damage during operation and in space applications to increase the light gathering power and consequently the signal to noise. We describe the facilities we have built for fabricating meter scale diffractive optics and discuss several examples of these.
KEYWORDS: Wavefronts, Etching, Chemical elements, High power lasers, Optical amplifiers, Wet etching, Process control, Photoresist materials, Solid state lasers, Lithography
We describe development of passive phase correction elements to compensate for static phase errors and prompt thermally induced aberrations in the HELEN laser at AWE. Partial compensation of cooling effects is also included in the design. Phase elements have been fabricated through two processes, an indirect write lithographic process using amplitude masks generated from measured laser wavefronts and a direct write method using a novel wet etch figuring tool.
The NIF target chamber beam dumps must survive high x-ray, laser, ion, and shrapnel exposures without excessive generation of vapors or particulate that will contaminate the final optics debris shields, thereby making the debris shields susceptible to subsequent laser damage. The beam dumps also must be compatible with attaining and maintaining the required target chamber vacuum and must not activate significantly under high neutron fluxes. Finally, they must be developed, fabricated, and maintained for a reasonable cost. The primary challenge for the beam dump is to survive up to 20 J/cm2 of 1 micrometers light and 1-2 J/cm2 of nominally 200-350 eV blackbody temperature x-rays. Additional threats include target shrapnel, and other contamination issues. Designs which have been evaluated include louvered hot-pressed boron carbide or stainless steel panels, in some cases covered with transparent Teflon film, and various combinations of inexpensive low thermal expansion glasses backed by inexpensive absorbing glass.
Jerald Britten, Steve Herman, Leslie Summers, Michael Rushford, Lun Auyang, I. Barton, Bruce Shore, Sham Dixit, Thomas Parham, Curly Hoaglan, Charles Thompson, Colin Battersby, J. Yoshiyama, Ron Mouser
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.
Recent simulations and experiments on Nova indicate that some level of smoothing may be required to suppress filamentation in plasmas on the National Ignition Facility, resulting in the addition of 1D smoothing capability to the current baseline design. Control of stimulated Brillouin scattering and filamentation is considered essential to the success of laser fusion because they affect the amount and location of laser energy delivered to the x-ray conversion region (holhraum wall) for indirect drive and to the absorptive region for direct drive. Smoothing by spectral dispersion (SSD), reduces these instabilities by reducing nonuniformities in the focal irradiance when averaged over a finite time interval. We have installed SSD on Nova to produce beam smoothing on all 10 beam lines. A single dispersion grating is located in a position common to all 10 beam lines early in the preamplifier chain. This location limits the 1(omega) bandwidth to 2.2 angstroms with sufficient dispersion to displace the speckle field of each frequency component at the target plane by one half speckle diameter. Several beam lines were modified to allow orientation of the dispersion on each arm relative to the holhraum wall. After conversion to the third harmonic the beam passes through a kinoform phase plate (KPP) designed to produce an elliptical spot at best focus. The KPPs produce a focal spot having an elliptical flat-top envelope with a superimposed speckle pattern. Over 93% of the energy is contained in the central 400 micrometers . Calculations indicate a 16% rms intensity variance will be reached after 330 ps for a single beam.
Most of the glass laser based inertial confinement fusion systems around the world
today employ non-linear frequency conversion for converting the 1.053 pm light at the
fundamental frequency (referred to as 1o light) to either its second harmonic (called 2o) at
527 nm or to its third harmonic (called 3w) at 351 nm. Shorter wavelengths are preferred
for laser fusion because of the improved coupling of the laser light to the fusion targets
due to reduced fast electron production at shorter wavelengths. The frequency conversion
process, however, is only about 60-70% efficient and the residual 30-40% of the energy
remains at la) and 2w frequencies.
KEYWORDS: Crystals, National Ignition Facility, Near field, Frequency converters, Prototyping, Diagnostics, Modulation, Spatial filters, Near field optics, Laser crystals
The Beamlet laser is a nearly full-scale, single-aperture prototype of the driver design for the National Ignition Facility (NIF). As part of a test and validation plan for the NIF design, Beamlet was recently equipped with final focusing optics and diagnostics for the purpose of evaluating integrated component performance and equivalent target-plane irradiance conditions at the 0.351-micrometers output wavelength specified for NIF targets. A 37-cm aperture two- crystal converter scheme generates the third harmonic of the Nd:glass 1.053-micrometers wavelength with high efficiency. The efficiency of the converter has been characterized and is reported, along with detailed measurements of the near-field and far-field UV irradiance distributions at operating conditions up to and exceeding red-line levels for the NIF. Dependences of observed beam quality on critical laser parameters including output power, B-integral, and spatial filtering are discussed and compared with numerical simulations.
We are presently adding the capability to irradiate indirectly-driven Nova targets with two rings of illumination inside each end of the hohlraum for studies of time-dependent second Legendre (P2) and time-integrated fourth Legendre (P4) flux asymmetry control. The rings will be formed with specially designed kinoform phase plates, which will direct each half of each beam into two separate rings that are nearly uniform azimuthally. The timing and temporal pulse shape of the outer rings will be controlled independently from those of the inner rings, allowing for phasing of the pulse shapes to control time dependent asymmetry. Modifications to the incident beam diagnostics will enable us to verify that acceptable levels of power balance among the contributing segments of each ring have been achieved on each shot. Current techniques for precision beam pointing and timing are expected to be sufficiently accurate for these experiments. We present a design for an affordable retrofit to achieve beam phasing on Nova, results of a simplified demonstration, and calculations highlighting the anticipated benefits.
It is now widely recognized that spatial beam smoothing (homogenization) is
essential in coupling the laser energy to the inertial confinement fusion (ICF) targets'. For
the indirect drive approach' to ICF, it is desirable to distribute the laser energy into a
uniformly speckled profile that has a flat-top super-Gaussian envelope (8th power or
higher) and contains greater than 95% of the energy inside the super-Gaussian profile.
Spatial smoothing is easily achieved by introducing a binary random phase plate (RPP) in
the beam. This produces a homogenized far-field pattern which consists of an overall
envelope function determined by the RPP element superimposed with a fine scale speckle
pattern arising due to the interference among the various RPP elements. Although easy to
fabricate and currently in routine use in many fusion laboratories2, the binary RPPs do not
meet the ICF requirements stated above since the far-field intensity profile is restricted to
essentially an Airy function containing only 84% (an upper limit) of the energy inside the
central spot. Approaches using lenslet arrays (refractive or diffractive)3'4 have limited use
since they operate in the quasi-far-field and have a short depth of focus.
We present fully continuous phase screens for producing super-Gaussian focal-plane irradiance profiles. Such phase screens are constructed with the assumption of either circular symmetric near-field and far-field profiles or a separable phase screen in Cartesian co-ordinates. In each case, the phase screen is only a few waves deep. Under illumination by coherent light, such phase screens produce high order super-Gaussian profiles in the focal plane with high energy content. Effects of beam aberrations on the focal plane profiles and their energy content are also discussed.
Phase plates are required to remove aberrations from laser beams caused by inhomogeneities in the optical components of the laser. The first type of plate that we prepared consisted of a bilevel optical component that caused spatial smoothing of the beam by breaking it up into a fine scale spatial structure. This was made by etching a pattern directly into the substrate using HF/NH4F. Components up to 80 cm in diameter were prepared but these are only 85% efficient because of beam losses in secondary maxima. Multilevel designs are more efficient and we have prepared 5 inch diameter samples with 16 levels. These require four separate etch steps but have efficiencies greater than 90%.
We report on the design and fabrication of continuous contour (kinoform) phase plates for homogenizing the focal plane irradiance of high-power, inertial confinement fusion laser systems. These kinoform phase plates are designed using an iterative algorithm. They offer the flexibility of controlling the overall shape of the far-field irradiance profile and the ability to concentrate the energy within a central region of the focal profile. These properties make kinoforms superior to the conventional, binary random phase plates for many applications. Potential methods for fabrication of such kinoform phase plates are discussed.
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.
Spectral broadening of single-frequency laser pulses by optical cross-phase modulation (XPM) with chaotic laser pulses in birefringent single-mode optical fibers is investigated numerically and results are compared with experiments. By this process we have generated laser pulses of variable bandwidth (1 - 25 angstrom) at the fundamental wavelength (1053 nm) for amplification in high power solid-state Nd:glass lasers used for inertial confinement fusion research. Simulations indicate that a temporally smooth XPM pulse can be generated with intensity fluctuations of less than 10% and spectral width greater than 50 angstrom using a short length (approximately 5 m) of special low dispersion and low birefringence fiber, e.g., D equals 10 ps/nm-km (normal dispersion) and (Delta) n equals 2 X 10-5. Readily available fibers of similar length, with parameters of D equals 40 ps/nm-km and (Delta) n equals 6 X 10-5, can give spectral widths exceeding 25 angstroms, but the noise will range from 25 to 60%. Broadband laser pulses generated by XPM are now routinely used at Lawrence Livermore National Laboratory for active smoothing of the laser irradiance on targets by the technique of smoothing-by-spectral dispersion.
We present numerical simulations of transverse stimulated Brillouin scattering (SBS) in large aperture optical elements and investigate its suppression due to finite laser bandwidth. A specific model for the laser bandwidth, namely, a sinusoidal phase modulation of the electric field, is considered. Our calculations indicate that bandwidths larger than the SBS linewidth lead to significant reduction in the SBS gain. We also investigate the dependence of the SBS gain on the modulation frequency and find that for a fixed overall bandwidth, a modulation frequency of the order of the SBS linewidth leads to an optimal reduction of the SBS gain.
The authors have outfitted one beamline of Nova with random phase plates (RPPs) to spatially smooth the beam. The fabrication and characterization of large aperture (80-cm diameter) RPPs for use at 526.5 nm along with the performance of these optics on Nova are discussed. The present RPPs were fabricated by a conventional, although large scale vacuum deposition process. Fabrication of RPPs using a unique sol-gel coating process, which has been demonstrated at 15-cm diameter and can be scaled to the 80-cm diameter required for Nova, is also discussed.
We present a microscopic mechanism of optical nonlinearity in quasi-one-dimensional semiconductors within the context of rigid band Peierls-extended Hubbard models. A detailed configuration space analysis is done to predict the dominant excitation paths. We show that only two channels contribute to the bulk of the optical nonlinearity, even though an infinite number of channels are possible in principle. Most importantly, these channels involve a virtual two photon excited state whose relative energy should be nearly parameter independent in the infinite chain limit. This would imply that the mechanism of optical nonlinearity, as well as the frequency dependence of the third order optical susceptibility, are also largely parameter independent. This universality is a consequence of the one dimensionality alone and remains valid for arbitrary convex Coulomb interactions. These conjectures are confirmed by exact numerical calculations on finite chains that do very careful analysis of finite size effects.
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