This year’s competition proposed to survey the state-of-the-art broadband, near-IR multilayer dielectric (MLD) mirrors designed for ultra-short, pulsed laser applications. The requirements for the coatings were a minimum reflection of 99.5% at 45-degree incidence angle for S-polarization from 830 nm to 1010 nm and group delay dispersion (GDD) < ± 50 fs2. The participants in this effort selected the coating materials, coating design, and deposition method. Samples were damage tested at a single testing facility to enable direct comparison among the participants using a 25 ± 5 fs OPCPA laser system operating at 5 Hz. A double blind test assured sample and submitter anonymity. The damage performance results, sample rankings, details of the deposition processes, coating materials and substrate cleaning methods are shared here. We found that multilayer coatings using tantala and/or hafnia as high index materials were top performers within several coating deposition groups. Specifically, dense coatings by ion-beam sputtering (IBS), magnetron sputtering (MS), and electron-beam ion assisted deposition (e-beam IAD) exhibited highest damage initiation onset (LIDT) while e-beam coatings were low performers. In addition, damage growth onset (LDGT) was also examined and the results are reported here for all samples as this performance metric plays an important role in establishing the safe operational conditions for larger aperture, ultrashort pulsed lasers. Lastly, not all coating samples in the survey met the GDD requirements stated above and associated measurements are discussed in the context of the present and past competitions focused on similar broadband, near-IR MLD coatings.
This work explores reactive ion etching parameters in order to identify and optimize key characteristics in gratings that govern their overall performance, including minimization of sidewall and trench structural defects and modification of fused silica via intrinsic molecular-level defects. This study is performed using grating-like samples with 5-µm-wide lines and trenches generated in fused silica by the photolithographic process and inductively coupled plasma-reactive ion etching. The analysis compiles metrology, simulation, and damage-testing results to obtain a better understanding of how to modify the fabrication process of gratings toward achieving better laser-induced–damage performance.
The Matter in Extreme Conditions Upgrade (MEC-U) project is a major upgrade to the MEC instrument of the LINAC Coherent Light Source (LCLS) X-ray free electron laser (XFEL) user facility at SLAC National Accelerator Laboratory. The envisioned MEC upgrade will significantly enhance the capabilities of the pump laser sources in current MEC experimental station, boosting the energy of the nanosecond shock driver from 100 J to the kJ level, and increasing the power and repetition rate of the short pulse laser from 25 TW at 5 Hz to 1 PW at 10 Hz rate. Building such high energy/power pump laser systems presents challenges to minimize and mitigate against laser-induced optical damage. As part of the system design, we have identified the optics at high-risk to damage and we have designed the laser systems to mitigate against these damage risks to ensure sustained facility operation.
Polarization smoothing of the 3w beam is highly desirable for direct-drive inertial confinement fusion experiments. However, fabrication methods of large-aperture optics that provide “randomized” beam polarization on the target are limited in-part to the challenging nature of the optical materials with suitable birefringence values. We report on the development of two new polarization-smoothing optics by separate and distinctly different approaches. In the first approach, a nematic liquid crystal with a high laser-induced–damage threshold is aligned between two fused silica substrates, with one substrate possessing a freeform, contoured imprint. In the second approach, freeform surface imprinting of potassium dihydrogen phosphate crystals is accomplished by fluid jet polishing.
Laser systems capable of delivering energetic nanosecond pulses with large incoherent bandwidth spanning more than 10 THz are currently under development for the study of laser–plasma instabilities. A consequence of such a spectrum is random intensity fluctuations, which may have a significant effect on laser damage thresholds. A special damage test station has been built for studying the effect this unique radiation has on common laser materials. We will share measurements of damage thresholds in coated optics, KDP, transmission gratings, and bare materials under various conditions aimed at understanding the role temporal fluctuations play in laser damage.
KEYWORDS: Contamination, Reactive ion etching, Etching, Laser damage threshold, Chemical analysis, Systems modeling, Silica, Scanning electron microscopy, Polymers, Optical fabrication
We investigate contamination induced in grating-like structures during the etching process as a possible precursor to laser-induced damage. Our experimental model utilizes 5-mm line structures fabricated in E-beam–deposited coatings of silica using reactive ion etching (RIE) and reactive ion beam etching (RIBE). This makes it possible to compare the behavior in the pillars and trench regions. The results suggest that surface contaminants are primarily fluorinated polymers, while embedded contaminants consist primarily of carbon with very low detection of fluorine. Samples fabricated by the RIBE method exhibit significantly reduced roughness in the trenches, yet still present similar embedded contamination.
Understanding of the laser-induced damage threshold and impact of air–vacuum cycling of the optical components in short-pulse laser systems is of fundamental importance. We report the results of a damage-testing campaign that monitored representative pulse compression grating samples that were positioned inside the OMEGA EP grating compressor vacuum chamber during normal operation and routinely damage tested on every quarterly vent for a period of about 10 years. The evolution of their damage resistance under 10-ps and 100-ps pulse lengths are associated with a significant degradation of the laser-induced damage thresholds, which is comparatively larger at 100 ps, is described.
Multibeam lasers often require an output beam balance that specifies the degree of simultaneity of the laser output energy, instantaneous power, or instantaneous irradiance (power per unit area). This work describes the general problem of balancing a multibeam laser. Specific techniques used to balance the output power of the 60-beam pulsed OMEGA Laser System are discussed along with a measured reduction of beam-to-beam imbalance. In particular, the square-pulse distortion induced by a simple saturating amplifier operating with its output at some fraction of its saturation fluence is derived, and a method to exchange gain between saturated amplifiers in a single beam that have different saturation fluences to adjust balance is described.
Inertial confinement fusion (ICF) cryogenic experiments on the 60-beam OMEGA laser have strict requirements for the laser energy delivered on target to be power balanced in order to maximize target-irradiation uniformity. For OMEGA, this quantity (power balance) is inferred from measurements of the time-integrated energy and time-resolved, spatially integrated temporal profile of each of the 60 beams at the output of the laser. The work presented here proposes a general definition of power balance as measured at the laser output and discusses the conditions that are fundamental to achieving laser power balance. Power balance necessitates equal gain across all stages of amplification, equal net losses across each amplifier stage, equal frequency conversion (from 1053 nm to 351 nm) of all 60 beams, and equal beam path lengths (beam timing). Typical OMEGA ICF laser pulse shapes consist of one or more short (100-ps) "pickets" followed by a shaped "drive" pulse of 1 to 2 ns. For these experiments, power balance is assessed for the pickets and the drive independently, with the ultimate goal of achieving root-mean-square (rms) imbalance across all 60 beams of less than 2% rms on both. This work presents a comprehensive summary of laser shot campaigns conducted to significantly improve laser power balance from typical rms values of 4.7% and 5.2%, respectively, to the 3% level for both features along with a discussion of future work required to further reduce the rms power imbalance of the laser system.
The laser damage thresholds of various HfO2/SiO2-based thin film coatings, including multilayer dielectric (MLD) gratings and high reflectors of different designs, prepared by E-beam and Plasma Ion Assisted Deposition (PIAD) methods, were investigated in vacuum, dry nitrogen, and after air-vacuum cycling. Single and multiple-pulse damage thresholds and their pulse-length scaling in the range of 0.6 to 100 ps were measured using a vacuum damage test station operated at 1053nm. The E-beam deposited high reflectors showed higher damage thresholds with square-root pulse-length scaling, as compared to PIAD coatings, which typically show slower power scaling. The former coatings appeared to be not affected by air/vacuum cycling, contrary to PIAD mirrors and MLD gratings. The relation between 1-on-1 and N-on-1 damage thresholds was found dependent on coating design and deposition methods.
We report an investigation on the response to laser exposure of a protective capping layer of 1ω (1053 nm) high-reflector
mirror coatings, in the presence of differently shaped Ti particles. We consider two candidate capping layer materials,
namely SiO2 and Al2O3. They are coated over multiple silica-hafnia multilayer coatings. Each sample is exposed to a
single oblique (45°) shot of a 1053 nm laser beam (p polarization, fluence ~ 10 J/cm2, pulse length 14 ns), in the
presence of spherically or irregularly shaped Ti particles on the surface. We observe that the two capping layers show
markedly different responses. For spherically shaped particles, the Al2O3 cap layer exhibits severe damage, with the
capping layer becoming completely delaminated at the particle locations. In contrast, the SiO2 capping layer is only
mildly modified by a shallow depression, likely due to plasma erosion. For irregularly shaped Ti filings, the Al2O3
capping layer displays minimal to no damage while the SiO2 capping layer is significantly damaged. In the case of the
spherical particles, we attribute the different response of the capping layer to the large difference in thermal expansion
coefficient of the materials, with that of the Al2O3 about 15 times greater than that of the SiO2 layer. For the irregularly
shaped filings, we attribute the difference in damage response to the large difference in mechanical toughness between
the two materials, with that of the Al2O3 being about 10 times stronger than that of the SiO2.
Large-aperture liquid crystal (LC) devices have been in continuous use since 1995 as polarization control devices in the
40-TW, 351-nm, 60-beam OMEGA Nd:glass laser system at the University of Rochester's Laboratory for Laser Energetics. The feasibility of using a noncontacting alignment method for high-peak-power LC laser optics by irradiation of a linearly photopolymerizable polymer with polarized UV light was recently investigated. These materials
were found to have surprisingly large laser-damage thresholds at 1054 nm, approaching that of bare fused silica (30 to 60 J/cm2). Their remarkable laser-damage resistance and ease in scalability to large apertures of these photoalignment materials, along with the ability to produce multiple alignment states by photolithographic patterning, opens new doorways for their application in LC devices for optics, photonics, and high-peak-power laser applications.
Previous ultraviolet-pulsed, laser-damage studies using model thin films with gold nanoparticles as artificial absorbing
defects revealed damage morphology in a form of submicrometer-scaled craters. It was also demonstrated that for
defects smaller than 20 nm, crater formation is preceded by plasma-ball formation around absorbing defects. In this work
an attempt is made to verify symmetry of the plasma ball by conducting film irradiation from the side of the air/film or
substrate/film interfaces. In each case, crater-formation thresholds are derived and crater morphology is analyzed by
means of atomic force microscopy.
Multilayer-dielectric (MLD) diffraction gratings are used in high-power laser systems to compress laser-energy pulses.
The peak power deliverable on target for these short-pulse petawatt class systems is limited by the laser-damage
resistance of the optical components in the system, especially the MLD gratings. Recent experiments in our laboratory
have shown that vapor treatment of MLD gratings at room temperature with organosilanes such as hexamethyldisilazane
(HMDS) produces an increase in their damage threshold at 1054 nm (10-ps, 370- μm spot size) as compared to uncoated
MLD grating control samples. The 1-on-1 laser-damage threshold of an HMDS-treated grating increased by 4.5% as
compared to the uncoated control sample, while the N-on-1 damage threshold of an MLD grating treated with
tetramethyldisilazane increased by 16.5%. For an MLD grating treated with bis-(trifluoropropyl)tetramethyldisilazane,
the N-on-1 and 1-on-1 damage thresholds increased by 4.8% and 5.3%, respectively. Such increases in laser-damage
threshold are unprecedented and counterintuitive because it is widely believed that the presence of organic materials or
coatings on the surfaces of optical substrates will inevitably lead to reduced laser-damage resistance.
Multilayer dielectric (MLD) diffraction gratings are an essential component for the OMEGA EP short-pulse, highenergy
laser system. The MLD gratings must have both high-optical-diffraction efficiency and high laser-damage
threshold to be suitable for use within the OMEGA EP Laser System. Considerable effort has been directed toward
optimizing the process parameters required to fabricate gratings that can withstand the 2.6-kJ output energy delivered by
each beam.
In this paper, we discuss a number of conventional semiconductor chemical cleaning processes that have been
investigated for grating cleaning, and present evidence of their effectiveness in the critical cleaning of MLD gratings
fabricated at LLE. Diffraction efficiency and damage-threshold data were correlated with both scanning electron
microscopy (SEM) and time-of-flight secondary ion-mass spectrometry (ToF-SIMS) to determine the best combination
of cleaning process and chemistry. We found that using these cleaning processes we were able to exceed both the LLE
diffraction efficiency (specification >97%) and laser-damage specifications (specification >2.7 J/cm2).
Thin-film polarizers are essential components of large laser systems such as OMEGA EP and the NIF because of the need to switch the beam out of the primary laser cavity (in conjunction with a plasma-electrode Pockels cell) as well as providing a well-defined linear polarization for frequency conversion and protecting the system from back-reflected light. The design and fabrication of polarizers for pulse-compressed laser systems is especially challenging because of the spectral bandwidth necessary for chirped-pulse amplification.
The design requirements for a polarizer on the OMEGA EP Laser System include a Tp greater than 98% over a spectral range of 1053±4 nm while maintaining a contrast ratio (Tp/Ts) of greater than 200:1 (500:1 goal) over the same range. An allowance must be made for the uniformity of the film deposition such that the specifications are met over the aperture of the component while allowing for some tolerance of angular misalignment. Production results for hafnia/silica designs will be shown, illustrating high transmission and contrast over an extended wavelength/angular range suitable for the 8 nm spectral bandwidth of OMEGA EP. Difficulties in production will also be illustrated, as well as the methods being implemented to overcome these challenges. A key challenge continues to be the fabrication of such a coating suitable for use on fused-silica substrates in a dry environment. Laser-damage thresholds for 1-ns and 10-ps pulse widths will be discussed.
Hafnia is one of the most utilized high-index materials in thin-film multilayer coatings for high-power lasers. It is well established that in the HfO2/SiO2 multilayers for 351 nm, nanosecond-pulse applications the damage is driven by the nanoscale absorbers localized in the hafnia layers. In this work, damage-crater formation thresholds and morphology are investigated for the undoped HfO2 monolayer films and films containing isolated gold nanoparticles of 2- and 5-nm average diameter. Atomic force microscopy is used for characterization of the damage craters produced by 351-nm, 0.5 ns laser pulses. This comparative study of crater-geometry variation with laser fluence for doped and undoped films allowed the estimation of properties of the intrinsic absorbing defects in the hafnia films.
Multilayer dielectric (MLD) diffraction gratings are a key component for the construction of high-peak-power, pulse-compressed laser systems. While a great deal of effort has been devoted to the design of optimal grating structures and the etching of these structures into the MLD coating, there has not been the same effort put into the optimization of the MLD coating itself. The primary characteristics of the multilayer that must be considered during design include minimization of the standing wave created in the photoresist because of the reflectivity of the coated optical surface, creation of a sufficiently high reflectivity at the use wavelength and incidence angle in a dry environment, proper balance of the individual layer materials to yield a coating with an overall neutral or slightly compressive stress, and a high laser-damage threshold for the wavelength and pulse duration of use. This work focuses on the modification of a standard MLD mirror, while considering these characteristics, to allow the fabrication of a diffraction grating with higher efficiency and laser-damage threshold than is typically achieved. Scanning electron microscopy (SEM) images of the grating structures demonstrate smoother shapes with lower roughness due to the holographic exposure. Damage testing performed at 1053 nm with a pulse width of 10 ps demonstrates the MLD coating has a sufficiently high laser-damage threshold to form the basis of reflection gratings that survive in high-fluence applications.
The OMEGA EP Facility includes two high-energy, short-pulse laser beams that will be focused to high intensity in the OMEGA target chamber, providing backlighting of compressed fusion targets and investigating the fast-ignition concept. To produce 2.6-kJ output energy per beam, developments in grating compressor technology are required. Gold-coated diffraction gratings limit on-target energy because of their low damage fluence. Multilayer dielectric (MLD) gratings have shown promise as high-damage-threshold, high-efficiency diffraction gratings suitable for use in high-energy chirped-pulse amplification [ B. W. Shore et al., J. Opt. Soc. Am. A14, 1124 (1997).] Binary 100-mm-diam MLD gratings have been produced at the Laboratory for Laser Energetics (LLE) using large-aperture, holographic exposure and reactive ion-beam etching systems. A diffraction efficiency of greater than 99.5% at 1053 nm has been achieved for gratings with 1740 grooves/mm, with a 1:1 damage threshold of 5.49 J/cm2 diffracted beam fluence at 10 ps. To demonstrate the ability to scale up to larger substrates, several 100-mm substrates have been distributed over an aperture of 47 × 43 cm and successfully etched, resulting in high efficiency over the full aperture. This paper details the manufacture and development of these gratings, including the specifics of the MLD coating, holographic lithography, reactive ion etching, reactive ion-beam cleaning, and wet chemical cleaning.
Several tests were completed to measure the laser-damage threshold of coated optics processed through different cleaning methods. Initial results indicate that the mechanical-scrub cleaning step is critical to high-laser-damage performance.
Large-aperture laser-resistant mirrors are required for the construction of the National Ignition Facility, a 1.8 MJ laser. In order to fabricate the 1408 mirrors, a development program was started in 1994 to improve coating quality, manufacturing rate, and lower unit cost. New technologies and metrology tools were scaled to meter size for facilitization in 1999 at Spectra-Physics and the Laboratory of Laser Energetics at the University of Rochester. Pilot
production, to fabricate 5-10% of each component, commenced in 2001 and full production rates were achieved in 2002. Coating production will be completed in 2008 with the coating of 460 m2 of high-damage-threshold precision coatings on 100 tons of BK7 glass with yields exceeding 90%.
A model SiO2 thin film system with nanoscale absorbing defects (gold nanoparticles) is employed with the goal of unraveling the connection between the pulsed-laser-energy absorption process inside a single nanoscale defect and the resulting film damage morphology. For this purpose, gold nanoparticles are lodged at a well-defined depth inside a SiO2 monolayer film. Particle sites, as well as damage craters generated at these locations after 351-nm pulsed- laser irradiation, are mapped by means of atomic force microscopy. The results of this mapping confirm mechanism of damage that involves initiation in the nanoscale defect followed by absorption spreading out to the surrounding matrix. At low laser fluences (below optically detected damage onset), the probability of damage crater formation and the amount of the material vaporized is, to within +/- 25% of the average value, almost independent of the particle size. Inhomogeneities in the particle environment are held responsible for variances in the laser-energy absorption process and, consequently, for the observed particle/damage crater correlation behavior. The nanoscale damage threshold is introduced as a laser fluence causing localized melting without significant vaporization.
Multilayer coatings with high damage resistance at 1054 nm can be produced with a metallic hafnium starting material and silicon dioxide (the damage characteristics of these coatings are discussed in detail in a companion paper submitted to this conference). These films have the potential of exhibiting good performance for a high fluence laser if all other optical specifications can be met by the coating. This paper discusses the methods used to prepare a large substrate and coat it with a multilayer meeting damage, spectral, surface flatness, and uniformity specifications. The highly stable processes developed to e-beam coat hafnia and silica are also conducive to producing well-calibrated, highly deterministic uniform films, essential to meeting some of the NIF specifications. The residual stress in the films is controlled mainly through control of residual gas pressures during deposition of the silica layer and is specific to the substrate material and relative humidity in the use environment. Process details for production of these coatings will be discussed.
Degradation of sol-gel coated KDP surfaces has been observed in crystals used on the OMEGA laser system in 40-50% relative humidity environments. The defect characteristic is an etch pit which develops under the sol-gel coating and produces a significant loss in the crystal due to scatter. Both diamond-turned and polished KDP surfaces show evidence of defect growth after sol-gel coating and exposure to ambient conditions; however, there is no relationship between defect growth and exposure to laser radiation. The defect size, orientation, and density is uniform across a diamond-turned surface, and the growth rate is accelerated with exposure to higher relative humidity. Experiments with a thermosetting polysiloxane and polymethylmethacrylate (PMMA) demonstrate these materials are successful barriers to prevent the transport of water vapor via the sol-gel coating to the KDP surface. Both materials meet the OMEGA laser damage threshold requirement and have been successfully applied to 300mm diameter KDP crystals.
Vacuum surface damage to fused-silica, spatial-filter lenses is the most prevalent laser-damage problem occurring on the OMEGA laser system. Approximately one-half of the stage C- input and output, D-input, E-input, and F-input spatial- filter lenses are currently damaged with millimeter-scale fracture sites. With the establishment of safe operational damage criteria, laser operation has not been impeded. These sol-gel-coated lenses see an average fluence of 2 to 4 J/cm2 at 1053 nm/1 ns. Sol-gel coatings on fused-silica glass have small-spot damage thresholds at least a factor of 2 higher than this peak operational fluence. It is now known that the vacuum surface of OMEGA's spatial-filter lenses are contaminated with vacuum pump oils and machine oils used in the manufacture of the tubes; however, development-phase damage tests were conducted on uncontaminated witness samples. Possible explanations for the damage include absorbing defects originating form ablated pinhole materials, contamination nucleated at surface defects on the coating, or subsurface defects from the polishing process. The damage does not correlate with hot spots in the beam, and the possibility of damage from ghost reflections has been eliminated. Experiments have been initiated to investigate the long-term benefits of ion etching to remove subsurface damage and to replace sol-gel layers by dielectric oxide coatings, which do not degrade with oil contamination.
Terrance Kessler, Ying Lin, Lawrence Iwan, Bill Castle, C. Kellogg, J. Barone, E. Kowaluk, Ansgar Schmid, Kenneth Marshall, Douglas Smith, Amy Rigatti, Joy Warner, Arthur Staley
Energy-efficient laser phase conversion, using fully continuous distributed phase plates, has been achieved for solid-state laser drivers in ICF. Optical lithography has been demonstrated to be an excellent means of generating deep, continuous surface-relief structures in photosensitive materials that subsequently are replicated with embossing or etching techniques. In addition, the method of simulated annealing has been shown to be a superior technique for designing continuous phase plates to control the focal-plane profile.
The 60-beam OMEGA laser has sustained approximately 1000 target shots without significant damage to the optics. Approximately 3000 optics on the OMEGA laser system were closely monitored during their installation, and inspections continue throughout the operation of the system. A review of the condition of these optics at each stage of the laser and a summary of the peak incident fluences are presented. The most severe damage on OMEGA is seen on the input, fused- silica, spatial filter lenses. Since these optics are under vacuum, inspection of damaged lenses occurs on a more frequent cycle to track the growth of the defect and to maintain the system's safety. An optic is replaced well before massive failure is expected to occur. Other optics on the system that exhibit different types of damage are BK7 spatial filter lenses, focus conversion crystals, primary pickoff lenses, calorimeters, and liquid-crystal optics. Laser glass and development optics such as distributed phase plates are not covered in this review.
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