Streak Cameras are an essential diagnostic tool used in shock physics and high energy density physics experiments. Such experiments require well calibrated temporally resolved diagnostics for studying events that occur in the nanosecond to microsecond time scales. Although streak cameras are among the most common detectors used within the high energy density physics community, they require frequent calibration and typically lack reproducibility in the fine detail. A solid state device with similar temporal performance characteristics could provide several advantages to current streak camera systems by utilizing discrete spatial resolution set by the sensor diodes. National Security Technologies (NSTec) has built a multi-channel solid state streak camera (SSSC) prototype, in collaboration with Sandia National Laboratories, as part of an ongoing project to develop the technology to a level competitive with analog streak cameras. The device concept and results from electronic testing of our first prototypes will be discussed in this manuscript. These measurements will be used as a base for future SSSC development projects.
The Z-Backlighter Laser Facility at Sandia National Laboratories was developed to enable high energy density physics experiments in conjunction with the Z Pulsed Power Facility at Sandia National Laboratories, with an emphasis on backlighting. Since the first laser system there became operational in 2001, the facility has continually evolved to add new capability and new missions. The facility currently has several high energy laser systems including the nanosecond/multi-kilojoule Z-Beamlet Laser (ZBL), the sub-picosecond/kilojoule- class Z-Petawatt (ZPW) Laser, and the smaller nanosecond/100 J-class Chaco laser. In addition to these, the backlighting mission requires a regular stream of coated consumable optics such as debris shields and vacuum windows, which led to the development of the Sandia Optics Support Facility to support the unique high damage threshold optical coating needs described.
The Hippogriff camera developed at Sandia National Laboratories as part of the Ultra-Fast X-ray Imager (UXI) program is a high-speed, multi-frame, time-gated imager for use on a wide variety of High Energy Density (HED) physics experiments on both Sandia’s Z-Machine and the National Ignition Facility. The camera is a 1024 x 448 pixel array with 25 μm spatial resolution, containing 2 frames per pixel natively and has achieved 2 ns minimum integration time. It is sensitive to both optical photons as well as soft X-rays up to ~6 keV. The Hippogriff camera is the second generation UXI camera that contains circuitry to trade spatial resolution for additional frames of temporal coverage. The user can reduce the row-wise spatial resolution from the native 25 μm to increase the number of frames in a data set to 4 frames at 50 μm or 8 frames at 100 μm spatial resolution. This feature, along with both optical and X-ray sensitivity, facilitates additional experimental flexibility. Minimum signal is 1500 erms and full well is 1.5 million e-.
We designed an optical coating based on TiO2/SiO2 layer pairs for broad bandwidth high reflection (BBHR) at 45-deg angle of incidence (AOI), P polarization of femtosecond (fs) laser pulses of 900-nm center wavelength, and produced the coatings in Sandia’s large optics coater by reactive, ion-assisted e-beam evaporation. This paper reports on laser-induced damage threshold (LIDT) tests of these coatings. The broad HR bands of BBHR coatings pose challenges to LIDT tests. An ideal test would be in a vacuum environment appropriate to a high energy, fs-pulse, petawatt-class laser, with pulses identical to its fs pulses. Short of this would be tests over portions of the HR band using nanosecond or sub-picosecond pulses produced by tunable lasers. Such tests could, e.g., sample 10-nm-wide wavelength intervals with center wavelengths tunable over the broad HR band. Alternatively, the coating’s HR band could be adjusted by means of wavelength shifts due to changing the AOI of the LIDT tests or due to the coating absorbing moisture under ambient conditions. We had LIDT tests performed on the BBHR coatings at selected AOIs to gain insight into their laser damage properties and analyze how the results of the different LIDT tests compare.
Matthias Geissel, T. Awe, D. Bliss, M. Campbell, M. Gomez, E. Harding, A. Harvey-Thompson, S. Hansen, C. Jennings, M. Kimmel, P. Knapp, S. Lewis, R. McBride, K. Peterson, M. Schollmeier, D. Scoglietti, A. Sefkow, J. Shores, D. Sinars, S. Slutz, I. Smith, C. Speas, R. Vesey, J. Porter
Sandia National Laboratories is pursuing a variation of Magneto-Inertial Fusion called Magnetized Liner Inertial Fusion, or MagLIF. The MagLIF approach requires magnetization of the deuterium fuel, which is accomplished by an initial external B-Field and laser-driven pre-heat. While magnetization is crucial to the concept, it is challenging to couple sufficient energy to the fuel, since laser-plasma instabilities exist, and a compromise between laser spot size, laser entrance window thickness, and fuel density must be found. Nonlinear processes in laser plasma interaction, or laser-plasma instabilities (LPI), complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation and beam-spray. Key LPI processes are determined, and mitigation methods are discussed. Results with and without improvement measures are presented.
The Ultra-Fast X-ray Imager (UXI) program is an ongoing effort at Sandia National Laboratories to create high speed, multi-frame, time gated Read Out Integrated Circuits (ROICs), and a corresponding suite of photodetectors to image a wide variety of High Energy Density (HED) physics experiments on both Sandia’s Z-Machine and the National Ignition Facility (NIF). The program is currently fielding a 1024 x 448 prototype camera with 25 μm pixel spatial resolution, 2 frames of in-pixel storage and the possibility of exchanging spatial resolution to achieve 4 or 8 frames of storage. The camera’s minimum integration time is 2 ns. Minimum signal target is 1500 e- rms and full well is 1.5 million e-. The design and initial characterization results will be presented as well as a description of future imagers.
We have designed and produced an optical coating suitable for broad bandwidth high reflection (BBHR) at 45° angle of
incidence (AOI), P polarization (Ppol) of petawatt (PW) class fs laser pulses of ~ 900 nm center wavelength. We have
produced such BBHR coatings consisting of TiO2/SiO2 layer pairs deposited by ion assisted e-beam evaporation using
the large optics coater at Sandia National Laboratories. This paper focuses on laser-induced damage threshold (LIDT)
tests of these coatings. LIDT is difficult to measure for such coatings due to the broad range of wavelengths over which
they can operate. An ideal test would be in the vacuum environment of the fs-pulse PW use laser using fs pulses
identical to of the PW laser. Short of this ideal testing would be tests over portions of the HR band of the BBHR coating
using ns or sub-ps pulses produced by tunable lasers. Such tests could be over ~ 10 nm wide wavelength intervals whose
center wavelengths could be tuned over the BBHR coating’s operational band. Alternatively, the HR band of the BBHR
coating could be adjusted by means of wavelength shifts due to changing the AOI of the LIDT tests or due to absorbed
moisture by the coating under ambient conditions. We conduct LIDT tests on the BBHR coatings at selected AOIs to
gain insight into the coatings’ laser damage properties, and analyze how the results of the different LIDT tests compare.
We studied theoretically the laser-plasma interaction, and performed experiments to investigate the mechanisms giving rise to optical damage in Borosilicate glass using nanosecond laser pulses at wavelength 1064 nm. Our experimental result shows that the optical damage process generated by nanosecond laser pulses is the result of an optically induced plasma. The plasma is initiated when the laser irradiance frees electrons from the glass. Although it may be debated, the electrons are likely freed by multi-photon absorption and the number density grows via impact ionization. Later when the electron gas density reaches the critical density, the electron gas resonantly absorbs the laser beam through collective excitation since the laser frequency is equal to the plasma frequency. The laser energy absorbed through the collective excitation is much larger than the energy absorbed by multi-photon ionization and impact ionization. Our experimental result also shows the plasma survives until the end of the laser pulse and the optical damage occurs after the laser pulse ceases. The plasma decay releases heat to the lattice. This heat causes the glass to be molten and soft. It is only as the glass cools and solidifies that stresses induced by this process cause the glass to fracture and damage. We also show the experimental evidence of the change of the refractive index of the focusing region as the density of the electron gas changes from sub-critical to overcritical, and the reflection of the over-critical plasma. This reflection limits the electron gas density to be not much larger than the critical density.
We measured the single-shot and multiple-shot optical breakdown thresholds leading
to optical damage of borosilicate glass, specifically BK7 glass, at 1.064 μm. We used 8-ns, single-longitudinal-mode, TEM00 laser pulses tightly focused inside a BK7 glass
window. The radius of the focal spot was measured using surface third harmonic
generation; it is equal to 7.5 μm. With this tight focus, the laser power at the breakdown
threshold of BK7 glass is below the SBS threshold, and the effect of self focusing is
small.
We found the single-shot and multiple-shots optical breakdown thresholds to be
deterministic. At the single-shot damage threshold, the optical breakdown in BK7 glass
occurs on the trailing edge of the laser pulse, in contrast to fused silica in which the
breakdown always occurs at the peak of the laser pulse. However, the multiple-shot
damage threshold of BK7 glass occurs at the peak of the last laser pulse.
Our single shot damage threshold for BK7 glass is 4125
J/cm2, and our multiple shot
damage threshold ranges from 3974 J/cm2 for 2-shot damage to 3289 J/cm2 for 31-shot
damage. We also compare damage morphologies of BK7 glass with those of fused silica.
We discuss upgrades and development currently underway at the Z-Backlighter facility. Among them are a
new optical parametric chirped pulse amplier (OPCPA) front end, 94 cm 42 cm multi layer dielectric (MLD)
gratings, dichroic laser beam transport studies, 25 keV x-ray source development, and a major target area
expansion. These upgrades will pave the way for short/long pulse, multi-frame, multi-color x-ray backlighting
at the Z-Accelerator.
As high energy laser systems evolve towards higher energies, fundamental material properties such as the laserinduced
damage threshold (LIDT) of the optics limit the overall system performance. The Z-Backlighter Laser Facility
at Sandia National Laboratories uses a pair of such kiljoule-class Nd:Phosphate Glass lasers for x-ray radiography of
high energy density physics events on the Z-Accelerator. These two systems, the Z-Beamlet system operating at 527nm/
1ns and the Z-Petawatt system operating at 1054nm/ 0.5ps, can be combined for some experimental applications. In
these scenarios, dichroic beam combining optics and subsequent dual wavelength high reflectors will see a high fluence
from combined simultaneous laser exposure and may even see lingering effects when used for pump-probe
configurations. Only recently have researchers begun to explore such concerns, looking at individual and simultaneous
exposures of optics to 1064 and third harmonic 355nm light from Nd:YAG [1]. However, to our knowledge,
measurements of simultaneous and delayed dual wavelength damage thresholds on such optics have not been performed
for exposure to 1054nm and its second harmonic light, especially when the pulses are of disparate pulse duration.
The Z-Backlighter Facility has an instrumented damage tester setup to examine the issues of laser-induced
damage thresholds in a variety of such situations [2] . Using this damage tester, we have measured the LIDT of dual
wavelength high reflectors at 1054nm/0.5ps and 532nm/7ns, separately and spatially combined, both co-temporal and
delayed, with single and multiple exposures. We found that the LIDT of the sample at 1054nm/0.5ps can be
significantly lowered, from 1.32J/cm2 damage fluence with 1054/0.5ps only to 1.05 J/cm2 with the simultaneous
presence of 532nm/7ns laser light at a fluence of 8.1 J/cm2. This reduction of LIDT of the sample at 1054nm/0.5ps
continues as the fluence of 532nm/7ns laser light simultaneously present increases. The reduction of LIDT does not
occur when the 2 pulses are temporally separated. This paper will also present dual wavelength LIDT results of
commercial dichroic beam-combining optics simultaneously exposed with laser light at 1054nm/2.5ns and 532nm/7ns.
Sandia's Large Optics Coating Operation has extensive results of laser induced damage threshold (LIDT) testing of its
anti-reflection (AR) and high reflection coatings on substrates pitch polished using ceria and washed in a process that
includes an alumina wash step. The purpose of the alumina wash step is to remove residual polishing compound to
minimize its role in laser damage. These LIDT tests are for multi longitudinal mode, ns class pulses at 1064 nm and
532 nm (NIF-MEL protocol) and mode locked, sub-ps class pulses at 1054 nm (Sandia measurements), and show
reasonably high and adequate laser damage resistance for coatings in the beam trains of Sandia's Z-Backlighter terawatt
and petawatt lasers. An AR coating in addition to coatings of our previous reports confirms this with LIDTs of 33.0
J/cm2 for 3.5 ns pulses and 1.8 J/cm2 for 350 fs pulses. In this paper, we investigate both ceria and zirconia in doublesided
polishing (common for large flat Z-Backlighter laser optics) as they affect LIDTs of an AR coating on fused silica
substrates washed with or without the alumina wash step. For these AR coated, double-sided polished surfaces, ceria
polishing in general affords better resistance to laser damage than zirconia polishing and laser damage is less likely with
the alumina wash step than without it. This is supported by specific results of laser damage tests with 3.5 ns, multi
longitudinal mode, single shot pulses at 1064 nm and 532 nm, with 7.0 ns, single and multi longitudinal mode, single
and multi shot pulses at 532 nm, and with 350 fs, mode-locked, single shot pulses at 1054 nm.
To enable laser-based radiography of high energy density physics events on the Z-Accelerator[4,5] at Sandia
National Laboratories, a facility known as the Z-Backlighter has been developed. Two Nd:Phosphate glass lasers are
used to create x-rays and/or proton beams capable of this radiographic diagnosis: Z-Beamlet (a multi-kilojoule laser
operating at 527nm in a few nanoseconds) and Z-Petawatt (a several hundred joule laser operating at 1054nm in the subpicosecond
regime) [1,2]. At the energy densities used in these systems, it is necessary to use high damage threshold
optical materials, some of which are poorly characterized (especially for the sub-picosecond pulse). For example,
Sandia has developed a meter-class dielectric coating capability for system optics. Damage testing can be performed by
external facilities for nanosecond 532nm pulses, measuring high reflector coating damage thresholds >80J/cm2 and antireflection
coating damage thresholds >20J/cm2[3]. However, available external testing capabilities do not use
femtosecond/picosecond scale laser pulses. To this end, we have constructed a sub-picoseond-laser-based optical
damage test system. The damage tester system also allows for testing in a vacuum vessel, which is relevant since many
optics in the Z-Backlighter system are used in vacuum. This paper will present the results of laser induced damage
testing performed in both atmosphere and in vacuum, with 1054nm sub-picosecond laser pulses. Optical
materials/coatings discussed are: bare fused silica and protected gold used for benchmarking; BK7; Zerodur; protected
silver; and dielectric optical coatings (halfnia/silica layer pairs) produced by Sandia's in-house meter-class coating
capability.
Sandia's Large Optics Coating Operation provides laser damage resistant optical coatings on meter-class optics required
for the ZBacklighter Terawatt and Petawatt lasers. Deposition is by electron beam evaporation in a 2.3 m × 2.3 m × 1.8
m temperature controlled vacuum chamber. Ion assisted deposition (IAD) is optional. Coating types range from antireflection
(AR) to high reflection (HR) at S and P polarizations for angle of incidence (AOI) from 0° to 47°.
This paper reports progress in meeting challenges in design and deposition of these high laser induced damage threshold
(LIDT) coatings. Numerous LIDT tests (NIF-MEL protocol, 3.5 ns laser pulses at 1064 nm and 532 nm) on the coatings
confirm that they are robust against laser damage. Typical LIDTs are: at 1064 nm, 45° AOI, Ppol, 79 J/cm2 (IAD 32
layer HR coating) and 73 J/cm2 (non-IAD 32 layer HR coating); at 1064 nm, 32° AOI, 82 J/cm2 (Ppol) and 55 J/cm2
(Spol ) (non-IAD 32 layer HR coating); and at 532 nm, Ppol, 16 J/cm2 (25° AOI) and 19 J/cm2 (45° AOI) (IAD 50 layer
HR coating). The demands of meeting challenging spectral, AOI and LIDT performances are highlighted by an HR
coating required to provide R > 99.6% reflectivity in Ppol and Spol over AOIs from 24° to 47° within ~ 1% bandwidth at
both 527 nm and 1054 nm.
Another issue is coating surface roughness. For IAD of HR coatings, elevating the chamber temperature to ~ 120 °C and
turning the ion beam off during the pause in deposition between layers reduce the coating surface roughness compared to
runs at lower temperatures with the ion beam on continuously. Atomic force microscopy and optical profilometry
confirm the reduced surface roughness for these IAD coatings, and tests show that their LIDTs remain high.
High intensity lasers require novel debris mitigation techniques in laser-target experiments. For a PW class
system (500 J in 500 fs at 1054 nm), the debris shield thickness is limited by the accumulated B-integral that the laser
acquires in transmission. In our case, this sets an upper limit of 500 micron for the debris shield thickness if the added Bintegral
is to stay below 1.5.
Therefore we have started to investigate the optical properties of various thin films such as Nitrocellulose,
Mylar, and Polyimide with respect to their application as laser debris shields. Those results were presented during the
last conference in 2007[1] and it was shown that Nitrocellulose and Polyimide are well suited. Damage testing was not
performed at this time. We now present short pulse (500 fs at 1054 nm) laser damage testing on these thin films in
vacuum. Energy, pulsewidth, beamsize and phase were closely monitored during the damage testing experiments.
Nitrocellulose was measured to damage at 1.33 J/cm2. Polyimide showed signs of damage at 133 mJ/cm2 and began to
fully penetrate the film at 670 mJ/cm2. Surprisingly, these films do not rupture with tens of closely spaced damage sites
being present which makes them ideal candidates for short pulse laser debris shields. Damage testing procedure and
apparatus as well as the damage site morphology will also be discussed.
We show that GRENOUILLE, the experimentally simple version of frequency-resolved optical gating (FROG) can measure the two spatio-temporal distortions, namely spatial chirp and pulse-front tilt, in addition to the pulse intensity and phase. This is done without a single alteration in the experimental setup. Specifically, pulse-front tilt yields a displacement of the otherwise centered trace along the delay axis while spatial chirp causes a shear to the otherwise symmetrical GRENOUILLE trace. We develop a more general FROG pulse-retrieval algorithm based on the Levenberg-Marquardt algorithm, which can retrieve not only the pulse intensity and phase but also both the spatial chirp and pulse-front tilt from GRENOUILLE traces. Lastly, we also show that, by employing the exotic nonlinear crystal Proustite, GRENOUILLE can be extended to measure fiber-laser pulses with wavelengths near 1.5μm. The high nonlinearity of Proustite compensates for the lower output power of fiber lasers. Also Proustite has so high dispersion that it can spectrally resolve these relatively narrowband pulses. We experimentally test all of these innovations and obtain perfect agreement with the expected results.
Although previous direct measurements of the microstructure-fiber continuum have all showed a smooth and stable spectrum, our cross-correlation frequency-resolved optical gating (XFROG) full-intensity-and-phase characterization of the continuum pulse, utilizing sum-frequency-generation with a pre-characterized reference pulse and the angle-dithered-crystal technique, indicates that fine-scale spectral structure exists on a single-shot basis, contrary to previous observations. In particular, deep and fine oscillations are found in the retrieved spectrum, and the retrieved trace contains a "measles" pattern, whereas the measured trace and the independently-measured spectrum are rather smooth. The discrepancy is shown to be the result of unstable single-shot spectral structure. Although the XFROG measurement is not able to directly measure the single-shot fine structure in the trace, the redundancy of information in FROG traces enables the retrieval algorithm to correctly recognize the existence of the spectral fine structure, and restore the structure in the retrieved trace and spectrum. Numerical simulations have supported our hypothesis, and we directly observed the fine spectral structure in single-shot measurements of the continuum spectrum and the structure was seen to be highly unstable, the continuum spectrum appearing smooth only when many shots are averaged. Despite the structure and instability in the continuum spectrum, coherence experiments also reveal that the spectral phase is rather stable, being able to produce well-defined spectral fringes across the entire continuum bandwidth.
Implementation of optical imagery in a diffuse inhomogeneous medium such as biological tissue requires an understanding of photon migration and multiple scattering processes which act to randomize pathlength and degrade image quality. The nature of transmitted light from soft tissue ranges from the quasi-coherent properties of the minimally scattered component to the random incoherent light of the diffuse component. Recent experimental approaches have emphasized dynamic path-sensitive imaging measurements with either ultrashort laser pulses (ballistic photons) or amplitude modulated laser light launched into tissue (photon density waves) to increase image resolution and transmissive penetration depth. Ballistic imaging seeks to compensate for these 'fog-like' effects by temporally isolating the weak early-arriving image-bearing component from the diffusely scattered background using a subpicosecond optical gate superimposed on the transmitted photon time-of-flight distribution. The authors have developed a broadly wavelength tunable (470 nm - 2.4 micrometer), ultrashort amplifying optical gate for transillumination spectral imaging based on optical parametric amplification in a nonlinear crystal. The time-gated image amplification process exhibits low noise and high sensitivity, with gains greater than 104 achievable for low light levels. We report preliminary benchmark experiments in which this system was used to reconstruct, spectrally upcovert, and enhance near-infrared two-dimensional images with feature sizes of 65 micrometer/mm2 in background optical attenuations exceeding 1012. Phase images of test objects exhibiting both absorptive contrast and diffuse scatter were acquired using a self-referencing Shack-Hartmann wavefront sensor in combination with short-pulse quasi-ballistic gating. The sensor employed a lenslet array based on binary optics technology and was sensitive to optical path distortions approaching lambda/100.
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