The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to 351nm light or 3w. It was built to access the extreme high energy density conditions needed to support the nation’s nuclear stockpile in the absence of further underground nuclear tests, including studying Inertial Confinement Fusion (ICF) and ignition in the laboratory.
Over the last year, important results have been obtained demonstrated a fusion yield of 1.35MJ with 1.9MJ of laser energy (and 440 TW power) injected in the target, bringing the NIF to the threshold of ignition [2-3]. As the yield curve near ignition is steep, the laser performance team has focused on providing improved power accuracy and precision (better shot-to-shot reproducibility) with a high-fidelity pulse shaping system (HiFiPS), and also on extending the NIF operating power and energy space by 15% to 2.2MJ and 500TW.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a
192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to
351nm light or 3w. It has been designed to support the study of Inertial Confinement Fusion (ICF)
and High Energy Density Physics (HEDP). The NIF Precision Diagnostic System (PDS) was reactivated and new
diagnostic packages were designed and fielded that offer a more comprehensive suite
of high-resolution measurements. The current NIF laser performance will be presented as well as the preliminary results obtained with the various laser experimental campaigns using the new diagnostic tool suites.
KEYWORDS: Pulsed laser operation, National Ignition Facility, High power lasers, Diagnostics, Photodiodes, Picosecond phenomena, Laser systems engineering, Data modeling, Photodetectors, Signal attenuation, Amplifiers
Accurate characterization of pulse contrast for high peak power lasers is critical to the success of experiments exploring inertial confinement fusion. The Advanced Radiographic Capability (ARC) laser at the National Ignition Facility (NIF) is a petawatt class laser system that produces pulses in the picosecond regime for the creation of diagnostic x-rays. ARC leverages four of the NIF’s beamlines for final amplification while implementing a separate front-end and pre-amplification stage, known as the High-Contrast ARC Front End (HCAFE). To characterize pulse contrast at the output of HCAFE, a means of measurement at long times (>500 ps) has been developed using a photodiode that has achieved a dynamic range of over 100 dB and 125 dB after deconvolution. Within hundreds of picoseconds of the main pulse, a commercial third-order cross-correlator (Amplitude Technologies Sequoia) is used to characterize the pulse contrast. Together, these diagnostics provide the necessary data for ensuring pulse contrast requirements can be met on ARC. Efforts were made to mitigate existing pre-pulses and to increase the stability of the system as a long-term operational companion to the NIF.
We describe the development and testing of the photodiode diagnostic and the analysis of the data resulting from contrast measurements. Details are also provided regarding the identification and mitigation of pre-pulses within the HCAFE system.
The Advanced Radiographic Capability (ARC) at the National Ignition Facility (NIF) is a petawatt-class, short-pulse laser system designed to provide x-ray backlighting of NIF targets. ARC uses four NIF beamlines to produce eight beamlets to create a sequence of eight images of an imploding fuel capsule using backlighting targets and diagnostic instrumentation. ARC employs a front end that produces two pulses, chirps the pulses out to 2 ns, and then injects the pulses into the two halves of each of four NIF beamlines. These pulses are amplified by NIF pre- and main amplifiers and transported to compressor vessels located in the NIF target area. The pulses are then compressed and pointed into the NIF target chamber where they impinge upon an array of backlighters. The interaction of the ARC laser pulses and the backlighting material produces bursts of high-energy x-rays that illuminate an imploding fuel capsule. The transmitted x-rays are imaged by diagnostic instrumentation to produce a sequence of radiograph images. A key component of the success of ARC is the automatic alignment system that accomplishes the precise alignment of the beamlets to avoid damaging equipment and to ensure that the beamlets are directed onto the tens-of-microns scale backlighters. In this paper, we describe the ARC automatic alignment system, with emphasis on control loops used to align the beampaths. We also provide a detailed discussion of the alignment image processing, because it plays a critical role in providing beam centering and pointing information for the control loops.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is the first of a kind megajoule-class laser with 192 beams capable of delivering over 1.8 MJ and 500TW of 351nm light [1], [2]. It has been commissioned and operated since 2009 to support a wide range of missions including the study of inertial confinement fusion, high energy density physics, material science, and laboratory astrophysics. In order to advance our understanding, and enable short-pulse multi-frame radiographic experiments of dense cores of cold material, the generation of very hard x-rays above 50 keV is necessary. X-rays with such characteristics can be efficiently generated with high intensity laser pulses above 1017 W/cm² [3]. The Advanced Radiographic Capability (ARC) [4] which is currently being commissioned on the NIF will provide eight, 1 ps to 50 ps, adjustable pulses with up to 1.7 kJ each to create x-ray point sources enabling dynamic, multi-frame x-ray backlighting. This paper will provide an overview of the ARC system and report on the laser performance tests conducted with a stretched-pulse up to the main laser output and their comparison with the results of our laser propagation codes.
At the National Ignition Facility (NIF), home of the world’s largest laser, a critical pulse screening process is used to ensure safe operating conditions for amplifiers and target optics. To achieve this, high speed recording instrumentation up to 34 GHz measures pulse shape characteristics throughout a facility the size of three football fields—which can be a time consuming procedure. As NIF transitions to higher power handling and increased wavelength flexibility, this lengthy and extensive process will need to be performed far more frequently. We have developed an accelerated highthroughput pulse screener that can identify nonconforming pulses across 48 locations using a single, real-time 34-GHz oscilloscope. Energetic pulse shapes from anywhere in the facility are imprinted onto telecom wavelengths, multiplexed, and transported over fiber without distortion. The critical pulse-screening process at high-energy laser facilities can be reduced from several hours just seconds—allowing greater operational efficiency, agility to system modifications, higher power handling, and reduced costs. Typically, the sampling noise from the oscilloscope places a limit on the achievable signal-to-noise ratio of the measurement, particularly when highly shaped and/or short duration pulses are required by target physicists. We have developed a sophisticated signal processing algorithm for this application that is based on orthogonal matching pursuit (OMP). This algorithm, developed for recovering signals in a compressive sensing system, enables high fidelity single shot screening even for low signal-to-noise ratio measurements.
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.
A rectangular-core fiber that guides and amplifies a
higher-order-mode can potentially scale to much higher
average powers than what is possible in traditional circular core large-mode-area fibers. Such an amplifier
would require mode-conversion at the input and output to enable interfacing with TEM00 mode seed sources
and generate diffraction-limited radiation for various applications. We discuss the simulation and experimental
results of a mode conversion technique that uses two
diffractive-optic-elements in conjugate Fourier planes to
convert a diffraction limited TEM00 mode to the higher-order-mode of a ribbon core fiber. Our experiments
show that the mode-conversion-efficiency exceeds 84% and can theoretically approach 100%.
KEYWORDS: Calibration, Image processing, Liquid crystal on silicon, Control systems, National Ignition Facility, Spatial light modulators, Detection and tracking algorithms, Apodization, Process control, Optical components
The heart of the National Ignition Facility is a megajoule-class laser system consisting of 192 beams used to drive
inertial confinement fusion reactions. A recently installed system of programmable, liquid-crystal-based spatial light
modulators adds the capability of arbitrarily shaping the spatial beam profiles in order to enhance operational flexibility.
Its primary intended use is for introducing "blocker" obscurations shadowing isolated flaws on downstream optical
elements that would otherwise be damaged by high fluence laser illumination. Because an improperly shaped blocker
pattern can lead to equipment damage, both the position and shape of the obscurations must be carefully verified prior
to high-fluence operations. An automatic alignment algorithm is used to perform detection and estimation of the
imposed blocker centroid positions compared to their intended locations. Furthermore, in order to minimize the
spatially-varying nonlinear response of the device, a calibration of the local magnification is performed at multiple sub-image
locations. In this paper, we describe the control and associated image processing of this device that helps to
enhance the safety and longevity of the overall system.
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.
A developed formalism1 for analyzing the power scaling of diffraction limited fiber lasers and amplifiers is applied to a
wider range of materials. Limits considered include thermal rupture, thermal lensing, melting of the core, stimulated
Raman scattering, stimulated Brillouin scattering, optical damage, bend induced limits on core diameter and limits to
coupling of pump diode light into the fiber. For conventional fiber lasers based upon silica, the single aperture,
diffraction limited power limit was found to be 36.6kW. This is a hard upper limit that results from an interaction of the
stimulated Raman scattering with thermal lensing. This result is dependent only upon physical constants of the material
and is independent of the core diameter or fiber length. Other materials will have different results both in terms of
ultimate power out and which of the many limits is the determining factor in the results. Materials considered include
silica doped with Tm and Er, YAG and YAG based ceramics and Yb doped phosphate glass. Pros and cons of the
various materials and their current state of development will be assessed. In particular the impact of excess background
loss on laser efficiency is discussed.
The National Ignition Facility (NIF) is currently the largest and most energetic laser system in the world. The main
amplifiers are driven by the Injection Laser System comprised of the master oscillators, optical preamplifiers, temporal
pulse shaping and spatial beam formatting elements and injection diagnostics. Starting with two fiber oscillators
separated by up to a few angstroms, the pulse is phase modulated to suppress SBS and enhance spatial smoothing,
amplified, split into 48 individual fibers, and then temporally shaped by an arbitrary waveform generator. Residual
amplitude modulation induced in the preamplifiers from the phase modulation is also pre-compensated in the fiber
portion of the system before it is injected into the 48 pre-amplifier modules (PAMs). Each of the PAMs amplifies the
light from the 1 nJ fiber injection up to the multi-joule level in two stages. Between the two stages the pre-pulse is
suppressed by 60 dB and the beam is spatially formatted to a square aperture with pre-compensation for the nonuniform
gain profile of the main laser. The input sensor package is used to align the output of each PAM to the main laser and
acquire energy, power, and spatial profiles for all shots. The beam transport sections split the beam from each PAM into
four main laser beams (with optical isolation) forming the 192 beams of the NIF. Optical, electrical, and mechanical
design considerations for long term reliability and availability will be discussed. Work performed under the auspices of
the U. S. Department of Energy under contract W-7405-Eng-48.
Microring resonators can serve as key elements in the realization
of engineerable photonic media. A sequence of resonators coupled
to an optical waveguide can be viewed as an optical transmission
line with highly controllable dispersive and nonlinear properties,
similar to those of photonic crystals or gratings. We have
constructed and characterized several optical micro-ring
resonators with scale sizes of the order of 10 microns. These
devices are intended to serve as building blocks for engineerable
linear and nonlinear photonic media. Light is guided vertically by
an epitaxially grown structure and transversely by deeply etched
air-clad sidewalls. In this work, we chose to construct ring
resonators in AlGaAs and probe them at a photon energy below the
half-gap of the material. Our motivation for this choice was to
maximize the ultrafast bound (Kerr) nonlinearities resulting from
virtual transitions while minimizing the two-photon contribution
to carrier generation. We report on the spectral phase transfer
characteristics of such resonators. We also report the observation
of a pi-radian Kerr nonlinear phase shift accumulated in a single
compact ring resonator evidenced by all-optical switching between
output ports of a resonator-enhanced Mach-Zehnder interferometer.
We describe the dispersive and nonlinear optical properties of
microresonator-modified waveguides. While many applications of
microresonators demand ultra-high quality factors and as a result
impose strict fabrication tolerances, we examine a variety of
useful devices that may be may be constructed using small numbers
of only moderately-high Q resonators using the current state of
the art in fabrication technology.
All known polarizers operate via a separation of orthogonal electric field components, one of which is subsequently discarded. As a result, 50% of the unpolarized incident light is wasted in the process of conversion to polarized light. We demonstrate a new method by which the optical power in the ordinarily discarded component is used as the pump to amplify the retained component in a nonlinear amplifier. We achieve greater than 50% throughput. These new polarizers are self-organizing in that they form internal gratings which, based only on the phase relationship between the two components, best optimize the transfer of power to the desired component.
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