Proceedings Volume 9632

Laser-Induced Damage in Optical Materials: 2015

Proceedings Volume 9632

Laser-Induced Damage in Optical Materials: 2015

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Volume Details

Date Published: 31 March 2016
Contents: 16 Sessions, 46 Papers, 0 Presentations
Conference: SPIE Laser Damage 2015
Volume Number: 9632

Table of Contents


Table of Contents

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  • Front Matter: Volume 9632
  • Thin Films I
  • Thin Films II
  • Thin Films III
  • Surfaces, Mirrors, and Contamination I
  • Surfaces, Mirrors, and Contamination II
  • Mini Symposium: Laser-Induced Damage to Multilayers in Femtosecond Regime
  • Fundamental Mechanisms I
  • Fundamental Mechanisms II
  • Fundamental Mechanisms III
  • Materials and Measurements I
  • Materials and Measurements II
  • Poster Session: Thin Films
  • Poster Session: Materials and Measurements
  • Poster Session: Fundamental Mechanisms
  • Poster Session: Surfaces, Mirrors, and Contamination
Front Matter: Volume 9632
Front Matter: Volume 9632
This PDF file contains the front matter associated with SPIE Proceedings Volume 9632, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.
Thin Films I
Comparative study of the laser damage threshold and optical characteristics of Ta2O5-SiO2 multilayers deposited using various methods
Roelene Botha, Silvia Schwyn Thöny, Martin Grössl, et al.
Manufacturing processes from the private and academic sectors were used to deposit anti-reflective and high-reflective coatings composed of Ta2O5 - SiO2 multilayers. Used deposition techniques included three Ion Assisted Deposition (IAD) systems and an Ion Beam Sputtering (IBS) system. Coatings were performed on fused silica (Corning 7980) substrates polished by two different suppliers. LIDT Measurements were performed using a Q-Switched Nd:YAG laser operating at 1064nm. The paper presents a comparison of the coatings in terms of laser damage threshold values, optical properties and surface quality.
Thin Films II
Investigating the relationship between material properties and laser-induced damage threshold of dielectric optical coatings at 1064 nm
Riccardo Bassiri, Caspar Clark, Iain W. Martin, et al.
The Laser Induced Damage Threshold (LIDT) and material properties of various multi-layer amorphous dielectric optical coatings, including Nb2O5, Ta2O5, SiO2, TiO2, ZrO2, AlN, SiN, LiF and ZnSe, have been studied. The coatings were produced by ion assisted electron beam and thermal evaporation; and RF and DC magnetron sputtering at Helia Photonics Ltd, Livingston, UK. The coatings were characterized by optical absorption measurements at 1064 nm by Photothermal Common-path Interferometry (PCI). Surface roughness and damage pits were analyzed using atomic force microscopy. LIDT measurements were carried out at 1064 nm, with a pulse duration of 9.6 ns and repetition rate of 100 Hz, in both 1000-on-1 and 1-on-1 regimes. The relationship between optical absorption, LIDT and post-deposition heat-treatment is discussed, along with analysis of the surface morphology of the LIDT damage sites showing both coating and substrate failure.
Characterization of laser-induced damage by picosecond pulses on multi-layer dielectric coatings for petawatt-class lasers
We investigate the optical damage performance of multi-layer dielectric (MLD) coatings suitable for use in high energy, large-aperture petawatt-class lasers. We employ small-area damage test methodologies to evaluate the damage resistance of various coatings as a function of deposition methods and coating materials under simulated use conditions. In addition, we demonstrate that damage initiation by raster scanning at lower fluences and growth threshold testing are required to estimate large-aperture optics’ performance.
Thin Films III
Impact of particle shape on the laser-contaminant interaction induced damage on the protective capping layer of 1high reflector mirror coatings
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.
The role of film interfaces in near-ultraviolet absorption and pulsed-laser damage in ion-beam-sputtered coatings based on HfO2/SiO2 thin-film pairs
The role of thin-film interfaces in the near-ultraviolet absorption and pulsed-laser–induced damage was studied for ion-beam–sputtered coatings comprised of HfO2 and SiO2 thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold were measured for a one-wave (355-nm)–thick HfO2 single-layer film and for a film containing seven narrow HfO2 layers separated by SiO2 layers. The seven-layer film was designed to have a total optical thickness of HfO2 layers equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO2 film. Absorption in both types of films was measured using laser calorimetry and photothermal heterodyne imaging. The results showed a small contribution to total absorption from thinfilm interfaces, as compared to HfO2 film material. The relevance of obtained absorption data to coating near-ultraviolet, nanosecond-pulse laser damage was verified by measuring the damage threshold and characterizing damage morphology. The results of this study revealed a higher damage resistance in the seven-layer coating as compared to the single-layer HfO2 film, in agreement with data recently reported for similarly designed electron-beam–deposited coatings. The results are explained through the similarity of interfacial film structure and structure formed during the co-deposition of HfO2 and SiO2 materials.
150-ps broadband low dispersion mirror thin film damage competition
Christopher J. Stolz, Raluca A. Negres, Kyle Kafka, et al.
Broadband low dispersion mirrors are fluence-limiting and pulse-shape-limiting components in short pulse lasers. To better understand the current technology state of broadband low dispersion mirrors, a laser damage competition was held at the 2015 Laser Damage Conference. Participants were asked to submit mirrors that met a minimum reflection of 99.5% at 45 degrees incidence angle at “P” polarization with a Group Dispersion Delay (GDD) of <± 100 fs2 over a spectral range of 773 nm ± 50 nm. The participants selected the coating materials, design, and deposition method. Laser damage testing was performed using the raster scan method with a 150 ps pulse length on a single testing facility to enable a direct comparison among the participants. GDD measurements were performed to validate specification compliance. Unfortunately nearly half of the submitted samples were found to not meet the GDD specifications. Details of the deposition processes, cleaning method, coating materials, and layer count are also shared.
Surfaces, Mirrors, and Contamination I
Dedicated contamination experiments in the Orion laser target chamber
J. Andrew, J.-M. Chevalier, D. Egan, et al.
The use of solid targets irradiated in a vacuum target chamber by focussed high energy, high power laser beams to study the properties of matter at high densities, pressures and temperatures are well known. An undesirable side effect of these interactions is the generation of plumes of solid, liquid and gaseous matter which move away from the target and coat or physically damage surfaces within the target chamber. The largest aperture surfaces in these chambers are usually the large, high specification optical components used to produce the extreme conditions being studied [e.g. large aperture off axis parabolas, aspheric lenses, X ray optics and planar debris shields]. In order to study these plumes and the effects that they produce a set of dedicated experiments were performed to evaluate target by product coating distributions and particle velocities by a combined diagnostic instrument that utilised metal witness plates, polymer witness plates, fibre velocimetry and low density foam particle catchers.
Surfaces, Mirrors, and Contamination II
Study of laser-induced damage at 1064nm in fused silica samples in vacuum environment
The influence of vacuum on nanosecond laser-induced damage at the exit surface of fused silica components is investigated at 1064 nm. In the present study, as previously observed in air, ring patterns surrounding laserinduced damage sites are systematically observed on a plane surface when initiated by multiple longitudinal modes laser pulses. Compared to air, the printed pattern is clearly more concentrated. The obtained correlation between the damage morphology and the temporal structure of the pulses suggests a laser-driven ablation mechanism resulting in a thorough imprint of energy deposit. The ablation process is assumed to be subsequent to an activation of the surface by hot electrons related to the diffusive expansion of a plasma formed from silica. This interpretation is strongly reinforced with additional experiments performed on an optical grating in vacuum on which damage sites do not show any ring pattern. Qualitatively, in vacuum, the intensity-dependent ring appearance speed VI1/2 is shown to be different than in air where VI1/3 . This demonstrates that the mechanisms of formation of ring patterns are different in vacuum than in air. Moreover, the mechanism responsible of the propagation of the activation front in vacuum is shown to be outdone when experiments are performed in air.
Laser-induced damage of fused silica on high-power laser: beam intensity modulation, optics defect, contamination
The wedged focus lens of fused silica, one of the final optics assembly’s optics, focuses the 351 nm beam onto target and separates the residual 1053 and 527 nm light with 351 nm light. After the experiment with beam energies at 3ω range from 3 to 5KJ, and pulse shapes about 3ns, the wedged focus lens has laser-induced damage at particular area. Analysis the damage result, there are three reasons to induce these damages. These reasons are beam intensity modulation, optics defect and contamination that cause different damage morphologies. The 3ω beam intensity modulation, one of three factors, is the mostly import factor to induce damage. Here, the n2 nonlinear coefficient of fused silica material can lead to small-scale self-focusing filament because of optics thickness and beam intensity. And some damage-filaments’ tails are bulk damage spots because there are subsurface scratches or metal contaminations.
Light scattering from laser-induced shallow pits on silica exit surfaces
E. Feigenbaum, R. N. Raman, N. Nielsen, et al.
We study the formation of laser-induced shallow pits (LSPs) on silica output surfaces and relate these features to optical performance as a function of incident laser fluence. Typical characteristics of the LSPs morphology are presented. Closed-form expressions for the scattered power and far-field angular distribution are derived and validated using numerical calculations of both Fourier optics and FDTD solutions to Maxwell’s equations. The model predictions agree well with the measurements for precise profile micro-machined shallow pits on glass, and for pitting caused by laser cleaning of bound metal micro-particles at different fluences.
Mini Symposium: Laser-Induced Damage to Multilayers in Femtosecond Regime
Optical coatings excited by femtosecond lasers near the damage threshold: challenges and opportunities
Luke A. Emmert, Cristina Rodriguez, Zhanliang Sun, et al.
The push for ever more powerful and shorter laser pulse sources demands optical coatings that can handle large bandwidths while operating near the threshold of permanent damage. We review some of the current challenges in the understanding of film behavior under load and opportunities that arise from tailored coating stacks for nonlinear optical harmonic generation.
Analysis of energy deposition and damage mechanisms in single layers of HfO2 and Nb2O5 submitted to 500fs pulses
Dam-Bé L. Douti, Mahmad Chrayteh, Serge Monneret, et al.
Laser Induced Damage Thresholds and morphologies of damage sites on thin films samples irradiated by sub-ps pulses are studied based on experimental and numerical studies. Experiments are conducted with 500fs pulses at 1030nm and 343nm and the irradiated sites are analyzed with phase imaging, AFM and SEM. The results are compared to simulations of energy deposition in the films based on the Single Rate Equation taking account transient optical properties of the films. Results suggest that a critical absorbed energy as a damage criterion give consistent results both with the measured LIDT and the observed damage morphologies.
Laser damage resistance of optical components in sub-picosecond regime in the infrared
Martin Sozet, Jérôme Néauport, Eric Lavastre, et al.
A rasterscan procedure is set to determine laser-induced damage densities in sub-picosecond regime at 1053nm on high-reflective coatings. Whereas laser-induced damage is usually considered deterministic in this regime, damage events occur on these structures for fluences lower than their intrinsic Laser-Induced Damage Threshold (LIDT). Damage densities are found to be high even for fluences as low as 20% of the LIDT. Scanning Electron Microscope observations of these “under threshold” damage sites evidence ejections of defects, embedded in the dielectric stack. It brings a new viewpoint for the qualification of optical components and for the optimization of manufacturing processes of coatings.
Fundamental Mechanisms I
What time-resolved measurements tell us about femtosecond laser damage?
Andrius Melninkaitis, Nerijus Šiaulys, Linas Smalakys, et al.
Time-resolved investigations of laser-matter interaction processes in dielectric coatings and bulk silica leading to laserinduced damage were performed with high temporal and spatial resolution. Distinct excitation geometries were used to study different aspects of laser matter interaction. Samples were irradiated at the pump fluence levels below and above their single shot laser-induced damage thresholds. The obtained results provide new insights about the sequence of interdependent processes. The fundamental differences between the so called 1-on-1 and S-on-1 damage morphologies are observed and discussed. New data of numerical simulations revealing the nonlinear properties of optical thin films are presented. Increased visibility in time resolved damage detection as well as observation of coherent oscillations in measured signals are introduced and discussed.
Laser damage threshold: useful idea or dangerous misconception?
The concept of a laser damage threshold as a safe operating level is both useful and confounding. This paper examines and analyzes the power of the concept of a laser damage threshold, identifying the good and the danger in its use. The history of the definition and concept of the laser damage threshold is traced from the earliest days of the laser through to the present day. Criteria for an accurate threshold measurement are presented and distribution of the weakest site on an optic is derived to provide a basis of analysis. Using a pedagogically selected defect distribution, the statistics of the weakest site are derived. The dependence of the weakest site on the area of the test is shown explicitly. It is argued that typical small area tests, characteristic of most main stream damage tests have insufficient area to include the true weakest site, and are therefore generally inaccurate. The paper concludes with some ideas on how to redefine the threshold measurement technique resulting in a more accurate test procedure.
Single-shot femtosecond laser ablation of copper: experiment vs. simulation
Enam Chowdhury, Kyle Kafka, Robert Mitchell, et al.
Single 5 and 40 femtosecond, near IR pulses with fluences varying from 0.4 – 80 J/cm2 from a Ti:Sapphire laser was focused onto a single crystal Cu sample surface with 2.0 μm focal spot at 15 and 45 degree angle of incidence. The surface profiles after interaction were studied with an interferometric depth profiler (Wyko NT9100), and benchmarked against crater size and morphology predicted by 2D Particle-In-Cell (PIC) laser damage simulation model.
Fundamental Mechanisms II
Morphology of ejected debris from laser super-heated fused silica following exit surface laser-induced damage
Laser induced damage (breakdown) initiated on the exit surface of transparent dielectric materials using nanosecond pulses creates a volume of superheated material reaching localized temperatures on the order of 1 eV and pressures on the order of 10 GPa or larger. This leads to material ejection and the formation of a crater. The volume of this superheated material depends largely on the laser parameters such as fluence and pulse duration. To elucidate the material behaviors involved, we examined the morphologies of the ejected superheated material particles and found distinctive morphologies. We hypothesize that these morphologies arise from the difference in the structure and physical properties (such as the dynamic viscosity and presence of instabilities) of the superheated material at the time of ejection of each individual particle. Some of the ejected particles are on the order of 1 µm in diameter and appear as “droplets”. Another subgroup appears to have stretched, foam-like structure that can be described as material globules interconnected via smaller in diameter columns. Such particles often contain nanometer size fibers attached on their surface. In other cases, only the globules have been preserved suggesting that they may be associated with a collapsed foam structure under the dynamic pressure as it traverses in air. These distinct features originate in the structure of the superheated material during volume boiling just prior to the ejection of the particles.
Multipulse degradation of fused silica surfaces at 351 nm
Sonny Ly, Ted A. Laurence, Nan Shen, et al.
We investigate the multipulse degradation of fused silica surfaces exposed at 351 nm for up to 109 pulses at pulse fluences greater than 10 J/cm2. In vacuum, the transmission loss increases as a function of the number of shots at low pulse intensity. However, as the pulse intensity increases, the transmission loss decreases and is not measureable above a certain intensity. Transmission loss is highest when measured at shorter wavelengths, and decreases towards the IR. Absorption is the primary mechanism that leads to transmission loss and is from photo-reduction of the silica surface.
Fundamental Mechanisms III
Energetic laser cleaning of metallic particles and surface damage on silica optics: investigation of the underlying mechanisms
Surface particulate contamination on optics can lead to laser-induced damage hence limit the performance of high power laser system. In this work we focus on understanding the fundamental mechanisms that lead to damage initiation by metal contaminants. Using time resolved microscopy and plasma spectroscopy, we studied the dynamic process of ejecting ~30 μm stainless steel particles from the exit surface of fused silica substrate irradiated with 1064 nm, 10 ns and 355 nm, 8 ns laser pulses. Time-resolved plasma emission spectroscopy was used to characterize the energy coupling and temperature rise associated with single, 10-ns pulsed laser ablation of metallic particles bound to transparent substrates. Plasma associated with Fe(I) emission lines originating from steel microspheres was observe to cool from <24,000 K to ~15,000 K over ~220 ns as τ-0.22, consistent with radiative losses and adiabatic gas expansion of a relatively free plasma. Simultaneous emission lines from Si(II) associated with the plasma etching of the SiO2 substrate were observed yielding higher plasma temperatures, ~35,000 K, relative to the Fe(I) plasma. The difference in species temperatures is consistent with plasma confinement at the microsphere-substrate interface as the particle is ejected, and is directly visualized using pump-probe shadowgraphy as a function of pulsed laser energy.
Delay dependency of two-pulse femtosecond laser damage
Mark Gyamfi, Peter Jürgens, Lars Jensen, et al.
In the femtosecond regime laser damage thresholds are determined by the electric field distribution within the optical components. Especially, for radiation sources with integrated frequency conversion the simultaneous presence of photons with different frequencies introduces additional ionization channels in optical materials by cross excitation and other effects. In this work we report on the pulse delay dependency of the LIDT of HR390/780nm mirrors under simultaneous exposure to fundamental and second harmonic femtosecond radiation. We perform Son1-tests according to ISO 21254 with the addition of a second harmonic pulse at different fixed pulse energies. To determine the influence of the cross excitation between fundamental and second harmonic radiation, these tests are repeated for different time delays between the two pulses. For the 1on1, single wavelength femtosecond LIDT testing, the Keldysh theory in combination with the Drude Model has been proven to reasonably describe the time dependent electron density in the conduction band, and hence the LIDT. We extend these approaches to the determination of the LIDT for the case of simultaneous interactions of photons of two separate wavelengths.
Self-consistent modeling of photoionization and the Kerr effect in bulk solids
Jeremy R. Gulley, Thomas E Lanier
In calculations of ultrafast laser-induced ionization the treatment of fundamental mechanisms such as photoionization and the Kerr effect are treated in isolation using monochromatic perturbative approaches. Such approaches are often questionable for pulses of ultrashort duration and multi-chromatic spectra. In this work we address this issue by solving the quantum optical Bloch equations in a 3D quasi-momentum space and show how to couple this model to ultrashort pulse propagation in dielectrics. This approach self-consistently couples a quantum calculation of the photoionization yield, the photoionization current, and the current from free-carriers with the traditional Kerr effect (self-focusing and self phase modulation) without resort to a perturbative treatment. The material band structure is taken in the tight binding limit and is periodic in the crystal momentum space. As this model makes no assumption about the pulse spectrum, we examine the laser-material interaction of strongly chirped pulses and multi-color multi-pulse schemes of laser-induced material modification. These results are compared to those predicted by standard treatments, such as the Keldysh model of photoionization, for pulses of ultrashort duration.
First principles simulation of laser-induced periodic surface structure using the particle-in-cell method
Robert A. Mitchell III, Douglass W. Schumacher, Enam A. Chowdhury
We present our results of a fundamental simulation of a periodic grating structure formation on a copper target during the femtosecond-pulse laser damage process, and compare our results to recent experiment. The particle-in-cell (PIC) method is used to model the initial laser heating of the electrons, a two-temperature model (TTM) is used to model the thermalization of the material, and a modified PIC method is employed to model the atomic transport leading to a damage crater morphology consistent with experimental grating structure formation. This laser-induced periodic surface structure (LIPSS) is shown to be directly related to the formation of surface plasmon polaritons (SPP) and their interference with the incident laser pulse.
Calculation of nonlinear optical damage from space-time-tailored pulses in dielectrics
Thomas E. Lanier, Jeremy R. Gulley
Control of the time duration of a laser pulse as it focuses spatially in a material provides a means for delaying the onset of nonlinear effects during propagation. We investigate simultaneous space-time focusing (SSTF) of femtosecond radially-chirped annular pulses in Kerr dielectrics. The energy and temporal chirp of pulses incident upon a grating-grating-lens system are varied in simulations that solve the unidirectional pulse propagation equation. This system is modeled by inserting transformations that act on the electric field obtained from propagation from one component to the next. The propagation is coupled to the time evolution of the free charge density as a function of space. The resulting “ionization tracks” are taken as a metric for predicting material modification and/or damage in bulk fused silica. As expected from linear-optical considerations, the temporal pre-chirp determines the overall pulse duration as the focusing annulus closes. We find in addition that, for a given pulse energy, the temporal pre-chirp also determines the on-axis intensity distribution as energy collapses onto the propagation axis. This effect determines how the local ionization-induced decrease in refractive index shifts energy in time relative to energy arriving on-axis from the spatially collapsing beam. The magnitude of the pre-chirp can thus control the spatial structure of ionization that may lead to material modification and/or damage.
Materials and Measurements I
Laser damage of calcium fluoride by ArF excimer laser irradiation
M. Azumi, E. Nakahata
Artificially grown excimer grade calcium fluoride is one of key optical materials used in microlithography applications. Such calcium fluoride is required for optical components requiring high laser durability and laser induced bulk damage threshold (LIDT). The mechanical properties of calcium fluoride can vary depending on the crystal axis, <111>, <110> and <100>. For example, material hardness is highest in the {100} crystal orientation. Furthermore, it is also known to cleave in the {111} plane. Therefore there is a possibility of a property that originates in such a crystal structure that influences LIDT. In this study, we investigated the relationship between crystal structure, laser durability and LIDT. The influence in the relation between the polarization plane of the ArF excimer laser and the crystal orientation of calcium fluoride in regards to LIDT was examined. The samples were all prepared from the same CaF2 crystal with optical axis's of <111>, <110> and <100>. The azimuth of the samples was measured by the reflection Laue method. For the experiment, the samples were rotated to the polarization plane of the ArF excimer laser, and the change in the number of irradiation pulses that damage was observed and measured. As a result, we determined the position of the crystal orientation of the calcium fluoride relative to the polarization plane of the ArF excimer laser that produced the highest LIDT.
High-speed quantitative phase imaging of dynamic thermal deformation in laser irradiated films
Lucas N. Taylor, Andrew K. Brown, Kyle D. Olson, et al.
We present a technique for high-speed imaging of the dynamic thermal deformation of transparent substrates under high-power laser irradiation. Traditional thermal sensor arrays are not fast enough to capture thermal decay events. Our system adapts a Mach-Zender interferometer, along with a high-speed camera to capture phase images on sub-millisecond time-scales. These phase images are related to temperature by thermal expansion effects and by the change of refractive index with temperature. High power continuous-wave and long-pulse laser damage often hinges on thermal phenomena rather than the field-induced effects of ultra-short pulse lasers. Our system was able to measure such phenomena. We were able to record 2D videos of 1 ms thermal deformation waves, with 6 frames per wave, from a 100 ns, 10 mJ Q-switched Nd:YAG laser incident on a yttria-coated glass slide. We recorded thermal deformation waves with peak temperatures on the order of 100 degrees Celsius during non-destructive testing.
Materials and Measurements II
Comparative STEREO-LID (Spatio-TEmporally REsolved Optical Laser-Induced Damage) studies of critical defect distributions in IBS, ALD, and electron-beam coated dielectric films
Yejia Xu, Amir Khabbazi, Travis Day, et al.
The laser damage behavior of high quality coatings under nanosecond pulse illumination is controlled by statistically distributed defects, whose physical nature and defect mechanisms are still largely unknown. Defect densities are often retrieved by modeling the fluence dependence of the damage probability measured by traditional damage test (TDT) methods, based on ‘damage’ or ‘no damage’ observations. STEREO-LID (Spatio-TEmporally REsolved Optical LaserInduced Damage) allows the determination of the damage fluence (and intensity) in a single test by identifying the initiation of damage both temporally and spatially. The advantages of this test method over the TDT are discussed. In particular, its ability to retrieve detailed defect distribution functions is demonstrated by comparison of results from HfO2 films prepared by ion-assisted electron beam evaporation, ion-beam sputtering, and atomic layer deposition.
Heat treatment of fused silica optics repaired by CO2 laser
In the context of high power laser applications, we study the effect of a heat treatment on CO2 laser mitigation of laser damage sites on fused silica samples. The isothermal annealing in a furnace is investigated and then compared to the local annealing by CO2 laser irradiation that is applied to enhance laser damage resistance on mitigated sites. Before and after isothermal annealing, we study the sites morphology, the evolution of residual stress and the laser-induced damage threshold measured at 355nm, 3ns. The results show that the initial laser damage probabilities were significantly improved after annealing at 1050°C for 12 hours. These results are compared to simulations with a thermo-mechanical model based on finite-element method.
Analysis of optics damage growth at the National Ignition Facility
Z. M. Liao, M. Nostrand, P. Whitman, et al.
Optics damage growth modeling and analysis at the National Ignition Facility (NIF) has been performed on fused silica. We will show the results of single shot growth comparisons, damage site lifetime comparisons as well as growth metrics for each individual NIF beamline. These results help validate the consistency of the damage growth models and allow us to have confidence in our strategic planning in regards to projected optic usage.
Tunable laser source based on storage device using Bragg grating
Chinmayee V. Prabhu Dessai, I. V. Anudeep Kumar Reddy, Saidi Reddy Parne, et al.
Tunable LASER source is a device which emits a particular light wavelength based on the tuning done. The tuning depends on certain characteristic of the LASER source which makes it customised within a gamut of wavelengths. Most Conventional LASER sources in the market are bulky and complex. The Tunable LASER source designed is established on the simple idea that Optical Amplifier can act as a broadband source, and temperature and strain sensitive Fiber Bragg Grating can be used to filter the required wavelength. This makes the design very light and elementary.
Poster Session: Thin Films
How reduced vacuum pumping capability in a coating chamber affects the laser damage resistance of HfO[sub]2[/sub]/SiO[sub]2[/sub] antireflection and high-reflection coatings
Ella S. Field, John C. Bellum, Damon E. Kletecka
Optical coatings with the highest laser damage thresholds rely on clean conditions in the vacuum chamber during the coating deposition process. A low base pressure in the coating chamber, as well as the ability of the vacuum system to maintain the required pressure during deposition, are important aspects of limiting the amount of defects in an optical coating that could induce laser damage. Our large optics coating chamber at Sandia National Laboratories normally relies on three cryo pumps to maintain low pressures for e-beam coating processes. However, on occasion, one or more of the cryo pumps have been out of commission. In light of this circumstance, we decided to explore how deposition under compromised vacuum conditions resulting from the use of only one or two cryo pumps affects the laser-induced damage thresholds of optical coatings. The coatings of this study consist of HfO2 and SiO2 layer materials and include antireflection coatings for 527 nm at normal incidence, and high reflection coatings for 527 nm, 45° angle of incidence (AOI), in P-polarization (P-pol).
Ultrafast beam dump materials and mirror coatings tested with the ELI beamlines LIDT test station
Michal Ďurák, Daniel Kramer, Praveen K. Velpula, et al.
The ELI Beamlines project will deliver ultrafast laser pulses with peak powers up to 10PW available every minute and PW class beams at 10Hz complemented by a 10TW 1kHz beamline. To properly determine damage thresholds of involved optical components in conditions similar to the operational environment and with expected laser parameters, a high vacuum LIDT test station was constructed at PALS facility. Our study presents results of ISO based S-on-1 and R-on-1 tests in femtosecond regime (50fs, 800nm, 10Hz/1kHz) performed on two different types of coatings: a) highabsorption black coatings with low outgassing rates, intended for use as a beam dump surface; and b) high-reflectivity, low-dispersion 45° AOI ultrafast mirror coatings. Testing of absorptive coatings was accompanied with QMS residual gas analysis to verify, that high intensity laser radiation approaching the damage threshold does not increase concentration of volatile organic compounds in the vacuum chamber. In case of HR mirror coatings, we also investigate the effect of cleaning on LIDT value, comparing characteristic S-on-1 curves of given sample with values obtained after 12h immersion in ethanol-water solution.
Test station development for laser-induced optical damage performance of broadband multilayer dielectric coatings
K. R. P. Kafka, E. A. Chowdhury, R. A. Negres, et al.
Laser-induced damage threshold (LIDT) testing was performed on commercially-available multilayer dielectric coatings to qualify for use in the High Repetition-Rate Advanced Petawatt Laser System (HAPLS) for Extreme Light Infrastructure Beamlines. Various tests were performed with uncompressed pulses (150 ps) from a 780 nm-centered Ti:Sapphire regenerative ampliflier, and the raster scan method was used to determine the best-performing coatings. Performance varied from 2–8 J/cm2 across samples from 6 different manufacturers.
Measurement and compensation of wavefront deformations and focal shifts in high-power laser optics
K. Mann, B. Schäfer, M. Stubenvoll, et al.
We demonstrate the feasibility of passive compensation of the thermal lens effect in fused silica optics, placing suitable optical materials with negative dn/dT in the beam path of a high power near IR fiber laser. Following a brief overview of the involved mechanisms, photo-thermal absorption measurements with a Hartmann-Shack sensor are described, from which coefficients for surface/coating and bulk absorption in various materials are determined. Based on comprehensive knowledge of the 2D wavefront deformations resulting from absorption, passive compensation of thermally induced aberrations in complex optical systems is possible, as illustrated for an F-Theta objective. By means of caustic measurements during high-power operation we are able to demonstrate a 60% reduction of the focal shift in F-Theta lenses through passive compensation.
Design and laser damage properties of a dichroic beam combiner coating for 22.5 degree incidence and S polarization with high-transmission at 527nm and high-reflection at 1054nm
John C. Bellum, Ella S. Field, Damon E. Kletecka, et al.
We have designed a dichroic beam combiner coating consisting of 11 HfO2/SiO2 layer pairs deposited on a large fused silica substrate. The coating provides high transmission (HT) at 527 nm and high reflection (HR) at 1054 nm for light at 22.5° angle of incidence (AOI) in air in S polarization (Spol). The coating's design is based on layers of near half-wave optical thickness in the design space for stable HT at 527 nm, with layer modifications that provide HR at 1054 nm while preserving HT at 527 nm. Its implementation in the 527 nm/1054 nm dual wavelength beam combiner arrangement has two options, with each option requiring one or the other of the high intensity beams to be incident on the dichroic coating from within the substrate (from glass). We show that there are differences between the two options with respect to the laser-induced damage threshold (LIDT) properties of the coating, and analyze the differences in terms of the 527 nm and 1054 nm E-field intensity behaviors for air → coating and glass → coating incidence. Our E-field analysis indicates that LIDTs for air → coating incidence should be higher than for glass → coating incidence. LIDT measurements for Spol at the use AOI with ns pulses at 532 nm and 1064 nm confirm this analysis with the LIDTs for glass → coating incidence being about half those for air → coating incidence at both wavelengths. These LIDT results and the E-field analysis clearly indicate that the best beam combiner option is the one for which the high intensity 527 nm beam is incident on the coating from air and the 1054 nm high intensity beam is incident on the coating from glass.
Poster Session: Materials and Measurements
Improved parametric spectroscopic performance of an optical fiber doped with erbium
M. Ghoumazi, N. Demagh, A. Adouane, et al.
In recent years, the rare earth ions and primarily Er played a crucial role in the development of the technology of optical telecommunications. The Emission of erbium ions at 1.53 microns is important for optical telecommunications because this emission corresponds to minimum mitigation of silica fibers which used as purpose to transport information. At first, we study the evolution of the signal powers and the pump powers along the propagation in the optical fiber amplifier Erbium doped. In addition, we study the variation of Erbium ions concentration for different spectroscopic parameters such as signal strength with (0, 1μW, 1mW) and the power of the pump going up 200 mW.
Analysis of cumulative versus ISO-recommended calculation of damage probability using a database of real S-on-1 tests
Alexandru Zorila, Aurel Stratan, Ioana Dumitrache, et al.
Data collected in real S-on-1 LIDT experiments performed with a nanosecond, 1064 nm automated station are used to calculate the damage probability with the ISO-recommended (conventional) method and the recently-suggested cumulative method. The damage probability points versus fluence for each type of calculation are fitted using both, linear and nonlinear curves. The resultant four data sets corresponding to each real experiment are used to compare important parameters as: statistical uncertainty of damage probability points, fitting errors, damage threshold fluences for actual number of pulses, and the extrapolated threshold fluences for very large number of pulses. We suggest and analyze also a limit case of the cumulative method, when the damage probability points are calculated for each interrogated site. Both, the recently-suggested cumulative method, and our limit case, look very promising.
Lowering evaluation uncertainties in laser-induced damage testing
Lars O. Jensen, Marius Mrohs, Mark Gyamfi, et al.
As a consequence of the statistical nature of laser-induced damage threshold measurements in the nanosecond regime, the evaluation method plays a vital role. Within the test procedure outlined in the corresponding ISO standard, several steps of data reduction are required, and the resulting damage probability distribution as a function of laser fluence needs to be fitted either based on an empirical regression function or described by models for the respective damage mechanism.
Direct comparison of statistical damage frequency method and raster scan procedure
G. Batavičiūtė, M. Ščiuka, V. Plerpaitė, et al.
Presented study addresses the nano-size defects acting as damage precursors in nanosecond laser pulse irradiation regime. Defects embedded within the surface of glass are investigated in terms of defect ensembles. Damage frequency method and raster scan procedure are directly compared on the set of two samples: uncoated fused silica substrates and SiO2 monolayer films. The extracted defect ensembles appear to be different from each other. The limitations of compared methods such as pulse-to-pulse variation of laser intensity and sample contamination induced by laser ablation were identified as the main causes of observed differences.
Characterization of damage precursor density from laser damage probability measurements with non-Gaussian beams
Damage induced by nanosecond laser in optical materials can often be attributed to the presence of laser damage precursor in the material. The presence of these precursors within dielectric optics can be successfully described by so called distributed defect ensembles. The physical parameters of these precursor presence models can be deduced by fitting experimental laser damage probability data. For a degenerate defect ensemble these parameters are the precursor threshold and the precursor density in the sample. To deduce precursor densities correctly it is essential to consider the real shape of laser beam that often deviates from Gaussian or hat-top models. To address these issues we discuss a new fitting procedure that minimizes significant errors in the deduced model parameters using experimental beam profile images. We suggest two methods: Defining a Gaussian replacement beam or using a numerical approximation of the surface over threshold (SOT) of the real beam. Both methods are discussed at the example of a degenerate damage precursor population but apply to any type of damage precursor population.
Transmittance measurements of laser components using a combination of cavity ring-down and photometry
H. Cui, Y. Han, C. Gao, et al.
A combined cavity ring-down (CRD) and photometry technique is employed to measure the transmittance of optical laser components in a range extending from below 0.01% to over 99.99%. In this combined technique, the conventional photometric configuration is used to measure, by ratioing the transmitted light power to the input power, the transmittance ranging from below 0.01% to over 99% with a typical relative uncertainty below 0.3%, and the CRD configuration is used to measure the transmittance higher than 99% with an uncertainty below 0.01%. Eight test samples with transmittance in the range of nearly 99.99% to approximately 0.013% are experimentally measured. Uncertainties of approximately 0.0001% for the transmittance of 99.9877% and of 0.003% for the transmittance of 0.013% are achieved with respectively the CRD and photometric schemes of a simple experimental apparatus. The experimental results showed that the combined technique is capable of measuring the transmittance of any practically fabricated optical laser components.
Poster Session: Fundamental Mechanisms
Direct absorption measurements in thin rods and optical fibers
Christian Mühlig, Simon Bublitz, Martin Lorenz
We report on the first realization of direct absorption measurements in thin rods and optical fibers using the laser induced deflection (LID) technique. Typically, along the fiber processing chain more or less technology steps are able to introduce additional losses to the starting material. After the final processing, the fibers are commonly characterized regarding losses using the so-called cut-back technique in combination with spectrometers. This, however, only serves for a total loss determination. For optimization of the fiber processing, it would be of great interest to not only distinguish between different loss mechanisms but also have a better understanding of possible causes.

For measuring the absorption losses along the fiber processing, a particular concept for the LID technique is introduced and requirements, calibration procedure as well as first results are presented. It allows to measure thin rods, e.g. during preform manufacturing, as well as optical fibers. In addition, the results show the prospects to also apply the new concept to topics like characterizing unwanted absorption after fiber splicing or Bragg grating inscription.
Bulk damage and absorption in fused silica due to high-power laser applications
F. Nürnberg, B. Kühn, A. Langner, et al.
Laser fusion projects are heading for IR optics with high broadband transmission, high shock and temperature resistance, long laser durability, and best purity. For this application, fused silica is an excellent choice. The energy density threshold on IR laser optics is mainly influenced by the purity and homogeneity of the fused silica. The absorption behavior regarding the hydroxyl content was studied for various synthetic fused silica grades. The main absorption influenced by OH vibrational excitation leads to different IR attenuations for OH-rich and low-OH fused silica.

Industrial laser systems aim for the maximum energy extraction possible. Heraeus Quarzglas developed an Yb-doped fused silica fiber to support this growing market. But the performance of laser welding and cutting systems is fundamentally limited by beam quality and stability of focus. Since absorption in the optical components of optical systems has a detrimental effect on the laser focus shift, the beam energy loss and the resulting heating has to be minimized both in the bulk materials and at the coated surfaces. In collaboration with a laser research institute, an optical finisher and end users, photo thermal absorption measurements on coated samples of different fused silica grades were performed to investigate the influence of basic material properties on the absorption level.

High purity, synthetic fused silica is as well the material of choice for optical components designed for DUV applications (wavelength range 160 nm - 260 nm). For higher light intensities, e.g. provided by Excimer lasers, UV photons may generate defect centers that effect the optical properties during usage, resulting in an aging of the optical components (UV radiation damage). Powerful Excimer lasers require optical materials that can withstand photon energy close to the band gap and the high intensity of the short pulse length. The UV transmission loss is restricted to the DUV wavelength range below 300 nm and consists of three different absorption bands centered at 165 nm (peroxy radicals), 215 nm (E’-center), and 265 nm (non-bridging oxygen hole center (NBOH)), which change the transmission behavior of material.
Refined metrology of spatio-temporal dynamics of nanosecond laser pulses
Seeded nanosecond Q-switched Nd:YAG lasers working with an unstable resonator and a variable-reflectivity-mirror are widely used for they represent useful sources for stable and repeatable light-matter-interaction experiments. Moreover, in most setups, the fundamental wavelength is converted to higher harmonics. When the injection seeder is turned off, random longitudinal mode beating occurs in the cavity, resulting in strong variations of the temporal profile of the pulses. The generated spikes can then be ten times higher than the maximum of equivalent seeded pulses. This strong temporal incoherence is shown to engender spatial incoherence in the focal plane of such unseeded pulses leading to an instantaneous angular displacement of tens of µrad. This effect is even more pronounced after frequency conversion.
Poster Session: Surfaces, Mirrors, and Contamination
Improved laser damage threshold performance of calcium fluoride optical surfaces via Accelerated Neutral Atom Beam (ANAB) processing
S. Kirkpatrick, M. Walsh Jr., R. Svrluga, et al.
Optics are not keeping up with the pace of laser advancements. The laser industry is rapidly increasing its power capabilities and reducing wavelengths which have exposed the optics as a weak link in lifetime failures for these advanced systems. Nanometer sized surface defects (scratches, pits, bumps and residual particles) on the surface of optics are a significant limiting factor to high end performance. Angstrom level smoothing of materials such as calcium fluoride, spinel, magnesium fluoride, zinc sulfide, LBO and others presents a unique challenge for traditional polishing techniques. Exogenesis Corporation, using its new and proprietary Accelerated Neutral Atom Beam (ANAB) technology, is able to remove nano-scale surface damage and particle contamination leaving many material surfaces with roughness typically around one Angstrom. This surface defect mitigation via ANAB processing can be shown to increase performance properties of high intensity optical materials. This paper describes the ANAB technology and summarizes smoothing results for calcium fluoride laser windows. It further correlates laser damage threshold improvements with the smoothing produced by ANAB surface treatment. All ANAB processing was performed at Exogenesis Corporation using an nAccel100TM Accelerated Particle Beam processing tool. All surface measurement data for the paper was produced via AFM analysis on a Park Model XE70 AFM, and all laser damage testing was performed at Spica Technologies, Inc. Exogenesis Corporation’s ANAB processing technology is a new and unique surface modification technique that has demonstrated to be highly effective at correcting nano-scale surface defects. ANAB is a non-contact vacuum process comprised of an intense beam of accelerated, electrically neutral gas atoms with average energies of a few tens of electron volts. The ANAB process does not apply mechanical forces associated with traditional polishing techniques. ANAB efficiently removes surface contaminants, nano-scale scratches, bumps, particles and other asperities under low energy physical sputtering conditions. ANAB may be used to remove a precisely controlled, uniform thickness of material without any increase of surface roughness, regardless of the total amount of material removed. The ANAB process does not involve the use of slurries or other abrasive polishing compounds and therefore does not require any post process cleaning. ANAB can be integrated as an in-situ surface preparation method for other process steps in the uninterrupted fabrication of optical devices.
Scaling of laser-induced contamination growth at 266nm and 355nm
M. Ließmann, L. Jensen, I. Balasa, et al.
The growth of laser-induced contamination (LIC) on optical components in extraterrestrial missions is a known issue especially for the UV spectral region. The Laser Zentrum Hannover e.V. is responsible for the development of a pulsed laser-system operating at a wavelength of 266 nm for the ExoMars mission and for the qualification of used optics and materials regarding LIC. In this context, toluene was utilized which is an often used model contaminant in LIC studies. Test cycles based on the application of the two UV wavelengths 355 nm and 266 nm on fused silica substrates and ARcoated optics are conducted and the observed contamination effects are compared. This scaling allows for a rough estimate of the destructive influence of LIC on space optics degradation at 266 nm. Further tests will be performed with materials integrated into the ExoMars-laser-head under near-operation environmental conditions.