Proceedings Volume 4932

Laser-Induced Damage in Optical Materials: 2002 and 7th International Workshop on Laser Beam and Optics Characterization

Gregory J. Exarhos, Arthur H. Guenther, Norbert Kaiser, et al.
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Proceedings Volume 4932

Laser-Induced Damage in Optical Materials: 2002 and 7th International Workshop on Laser Beam and Optics Characterization

Gregory J. Exarhos, Arthur H. Guenther, Norbert Kaiser, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 30 May 2003
Contents: 6 Sessions, 80 Papers, 0 Presentations
Conference: XXXIV Annual Symposium on Optical Materials for High Power Lasers: Boulder Damage Symposium 2002
Volume Number: 4932

Table of Contents

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Table of Contents

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  • Fundamental Mechanisms
  • Surfaces and Mirrors
  • Fundamental Mechanisms
  • Surfaces and Mirrors
  • Materials and Measurements
  • Thin Films
  • Surfaces and Mirrors
  • Thin Films
  • Fundamental Mechanisms
  • Materials and Measurements
  • Surfaces and Mirrors
  • Fundamental Mechanisms
  • Surfaces and Mirrors
  • Fundamental Mechanisms
  • Thin Films
  • Surfaces and Mirrors
  • Thin Films
  • Fundamental Mechanisms
  • Materials and Measurements
  • Fundamental Mechanisms
  • Materials and Measurements
  • Thin Films
  • Surfaces and Mirrors
  • Thin Films
  • Fundamental Mechanisms
  • Thin Films
  • Materials and Measurements
  • Fundamental Mechanisms
  • Section
  • Materials and Measurements
  • Thin Films
  • Materials and Measurements
  • Thin Films
  • Surfaces and Mirrors
  • Section
  • Mini-Symposium on Optics Characterization
  • Section
  • Mini-Symposium on Optics Characterization
  • Section
  • Mini-Symposium on Optics Characterization
  • Section
  • Mini-Symposium on Optics Characterization
  • Section
  • Mini-Symposium on Optics Characterization
  • Section
  • Mini-Symposium on Optics Characterization
  • Section
  • Mini-Symposium on Optics Characterization
  • Section
  • Mini-Symposium on Optics Characterization
  • Fundamental Mechanisms
  • Section
Fundamental Mechanisms
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Calculation of electric field intensity at dielectric interfaces
This paper will develop the expression for the electric field in the vicinity of a dielectric interface. This work is an extension of previous calculations that specifically applied to total internal reflection (TIR) interfaces. This paper will show that some differences should be expected in damage thresholds for s and p polarization. Explicit derivation for a generalized interface will be given and previous work generalized to include sub and super critical angle reflection and reflection from non TIR components.
Accuracy and repeatability of laser damage threshold measurements made via order statistics
This paper develops expression for the outcome of a binary search measurement of laser damage threshold, using order statistics. Also developed is an expression for the repeatability of the measurement. Results of a recent experiment are used to evaluate the expression for the threshold to determine accuracy and repeatability of a measurement as function of the search length.
Trapping of electrical charges and laser damage
Janick Bigarre, Patrick Hourquebie, Ludovic Doucet
The polishing of optical components introduces defects in subsurface that can contribute to the damage initiation and the decrease of optical performance. These defects can trap electric charges or could be precursors to color center. We have proposed to study subsurface defects generated by the surface finishing by using trapping of charges and cathodoluminescence measurements. These two techniques are complementary: the cathodoluminescence gives information on defects having radiative decay and the grounded current can also be sensitive to other defects having non-radiative decay. It is shown that they are able to distinguish the surface finishing of natural silica. The defects are mainly concentrated in the first micron layer. We have also found that the repartition of defects on the surface in not homogeneous.
Surfaces and Mirrors
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Identification and elimination of fluorescent surface-damage precursors on DKDP optics
Mike C. Nostrand, Samuel L. Thompson, Wigbert J. Siekhaus, et al.
Fluorescing surface defects that led to damage upon 351-nm laser exposure below 7 J/cm2 (3-ns) in DKDP optics were reported in these proceedings by this group a year ago. Subsequent laser damage experiments have correlated the density of these damage precursors to single-pont diamond finishing conditions. Every diamond-finishing schedule contains brittle-mode cutting and ductile-mode cutting in a taper-down sequence. Finishing experiments have traced the occurrence of these defects to insufficient ductile-mode removal of subsurface damage incurred during piror brittle-mode cutting. Additionally, a correlation between defect fluorescence, laser-induced damage, and defect meorphology has been established. Laser-induced damage tests also suggest a correlation between growth method and damage probability. Current experiments indicate that damage-prone defects can be minimized with the proper choice of diamond finishing conditions.
Methods for mitigating growth of laser-initiated surface damage on DKDP optics at 351 nm
Lawrence W. Hrubesh, Raymond M. Brusasco, Walter Grundler, et al.
We report an experimental investigation of mitigating surface damage growth at 351 nm for machine-finished DKDP optics. The objective was to determine which methods could be applied to pre-initiated or retrieved-from-service optics, in order to stop further damage growth for large aperture DKDP optics used in high-peak-power laser applications. The test results, and the evaluation thereof, are presented for several mitigation methods applied to DKDP surface damage. The mitigation methods tested were CW-CO2 laser processing, aqueous wet-etching, short-pulse laser ablation, and micro-machining. We found that micro-machining, using a single crystal diamond tool to completely remove the damage pit, produces the most consistent results to halt the growth of surface damage on DKDP. We obtained the successful mitigation of laser-initiated surface damage sites as large as 0.14 mm diameter, for up to 1000 shots at 351 nm and fluences in the range of 2 to 13 J/cm2, ≈ 11 ns pulse length. Data obtained to-date indicates that micro-machining is the preferred method to process large-aperture optics.
Fundamental Mechanisms
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Analysis of raster scanning damage and conditioning experiments
The raster scan technique is used for large optics damage tests and laser conditioning. We show that the "effective area" concept enables the possibility to compare various scanning schemes and to use raster scan experiments for NIF optics damage prediction. It is shown that the hexagonal lattice of laser beam imprints yields optimal use of each shot for most of the typically used parameters. The effects of beam fluence fluctuations and pointing inaccuracies on experiments are evaluated. To analyze raster scan conditioning experiments, we introduce the concept of "effective dose," i.e. total dose averaged over a unit cell of the scan lattice. This allows various scanning schemes to be compared quantitatively.
Surfaces and Mirrors
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Mechanisms of CO2 laser mitigation of laser damage growth in fused silica
Theoretical models for heating, evaporation, material flow, and stress and strain generation accompanying CO2 laser damage mitigation and surface treatment of fused silica are developed to aid understanding of scaling with process parameters. We find that lateral nonlinear heat transport is an important cooling mechanism, more significant than evaporative cooling. Scaling laws relating experiments with different set of parameters are presented. Transverse conduction, together with the increased thermal conductivity at high temperatures, allows a gentle evaporation regime at low laser intensity in which the rate can be controlled via laser fluence. For higher laser intensity, recoil momentum imparted by rapid evaporation generates pressure, which can lead to transverse flow of the melted material. Only a very thin layer can flow because viscosity increases rapidly with depth. Evaporation and flow are subject to instabilities that can impact surace quality, especially surface flatness, if large areas are processed. Also material flow can heal cracks and improve material quality. Analysis of stress indicates that maximal tensile streses of order 0.1 GPa, comparable to the tensile strength, can be generated.
Overview of raster scanning for ICF-class laser optics
Recent work has shown that the damage resistance of both ICF-class (1600 cm2) DKDP tripler crystals and SiO2 components (lenses, gratings and debris shields) benefits from laser raster scanning using pulsed lasers in the 350 nm range. For laser raster scanning to be a viable optical improvement tool for these large optics, damage improvement must be optimized while maintaining scan times of less than 8 hours/optic. In this paper we examine raster scanning with small beams from tabletop laser systems. We show that 120 Watts of average power is required for a tabletop scanning system at one optic/day. Next, we develop equations for total scan time for square and round top heat beams and round and rectangular Gaussian beams. We also consider the effect of packing geometry (square vs. hexagonal), examine the deviations from uniform coverage with each scan geometry and show that hexagonal packing yields lower scan times but is less efficient in coverage than square geometry. We also show that multiple passes at low packing densities are temporally equivalent to a single pass with higher packing density, and discuss the advantages of each method. In addition, we show that the differences between hexagonal and square scan geometries are negated when pointing errors and fluence fluctuations from the laser are considered.
Materials and Measurements
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Effects of pulse duration on bulk laser damage in 350-nm raster-scanned DKDP
Michael J. Runkel, Justin Bruere, Walter D. Sell, et al.
In this paper we present the results of bulk damage experiments done on Type-II DKDP triple harmonic generator crystals that were raster conditioned with 351 - 355 nm wavelengths and pulse durations of 4 and 23.2 ns. In the first phase of experiments 20 different scan protocols were rastered into a sample of rapid growth DKDP. The sample was then rastered at damage-causing fluences to determine the three most effective protocols. These three protocols were scanned into a 15-cm sample of conventional-growth DKDP and then exposed to single shots of a 1-cm beam from LLNL's Optical Sciences Laser at fluences ranging from 0.5 - 1.5X of the 10% damage probability fluence and nominal pulse durations of 0.1, 0.3, 0.8, 3.2, 7.0 and 20 ns. The experiment showed that pulse durations in the 1 - 3 ns range were much more effective at conditioning than pulses in the 16.3 ns range and that the multiple pass "peak fluence" scan was more effective than the single pass "leading edge" scan for 23.2 ns XeF scans.
Thin Films
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Influence of BK7 substrate solarization on the performance on hafnia and silica multilayer mirrors
Christopher J. Stolz, Joseph A. Menapace, Francois Y. Genin, et al.
Transport mirrors within the National Ignition Facility, a 192-beam 4-MJ fusion laser at 1053 nm, will be epxosed to backscattered light from plasmas created from fusion targets and backlighters. This backscattered light covers the UV and visible spectrum from 351 - 600 nm. The transport mirror BK7 substrates will be intentionally solarized to absorb >95% of the backscattered light to prevent damage to the metallic mechanical support hardware. Solarization has minimal impact on the 351- 1053-nm laser-induced damage threshold or the reflected wavefront of the multilayer hafnia silica coating. Radiation sources of various energies were examined for BK7 darkening efficiency within the UV and visible region with 1.1 MeV gamma rays from a Cobalt 60 source ultimately being selected. Finally, bleaching rates were measured at elevated temperatures to generate a model for predicting the lifetime at ambient conditions (20°C), before solarized BK7 substrates exceed 5% transmission in the UV and visible region. Over a 30-mm thickness, BK7 glass will bleach in 10 years to 5% transmission at 600 nm, the most transmissive wavelengths over the 351 - 600 nm regions.
Surfaces and Mirrors
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Large-spot COIL irradiation of Ge samples
The large-spot out-of-band irradiation of Ge wafers and subsequent evaluation is discussed in this paper. The wafers were irradiated with a high-power, cw COIL system, operating at a wavelength of 1.315 μm. Damaging fluence values on the order of 1 kJ/cm2 were found for irradiation periods of several seconds. Thermal simulations were consistent with experimental findings. The damage morphology showed melt and microcrystallites. For sample evaluation, a compact modulation transfer function (MTF) test bench has been developed.
Environmental effects on optical component aging
The optical performances of components exposed to high power laser beams during long periods are generally decreasing with time. We analyze surface modifications that occur after some hundreds of hours at a fluence of 0.5 kW/cm2 to 10 kW/cm2. The results obtained with two dedicated test benches over thousands of hours are discussed. Exposition of tens of components has been achieved in the multi-component bench to simulate real optical systems and to acquire statistical confidence in the results. The single component bench allows continuous measurements of the surface temperature and of the scattered light. These measurements show how these beamprints can lead to the surface destruction. We investigate the effects of the beam fluence, the laser wavelength and the chamber pressure. Experiments at two wavelengths have shown that the deposition rate and the surface absorption increase with decreasing wavelength. The efficiency of oxygen in reducing the contamination speed has been investigated using absorption mappings. Laser cleaned surface absorptions are comparable to initial measurements, showing that damage did not occur. ESCA analysis of the beamprints showed that a few nanometers carbon layer has been implanted.
Influence of 527-nm laser light on debris- and shrapnel-contaminated optical surfaces
James E. Andrew, Karen R. Mann, Michael T. Tobin, et al.
The use of large aperture high power lasers to study plasmas formed from solid targets is well known. In this paper we consider the effects of second harmonic Neodymium laser radiation [527 nm] on optical surfaces that have been contaminated by solid shrapnel and liquid or gaseous debris arising from the use of laser irradiated solid targets. In typical operations large lasers use debris shields to protect expensive aspheric lenses from optical and mechanical damage. These debris shields have a finite lifetime before they need to be replaced. Criteria for replacement have often been derived empirically from operational experience of evacuated target chambers and may be based on transmission, reflectivity, scatter or obscuration measurements. Here we describe and characterize the debris produced from targets based on gas bags, spall packages and gold halfraums captured on fused silica borosilicate plates. The influence of contamination levels and the physical form of debris on laser beams of various fluences was measured. Laser cleaning, damage thresholds and growth rates were investigated and the techniques are described.
Thin Films
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Investigation of high-power laser beam-, plasma-, debris-, and shrapnel-induced damage of optical coatings on the HELEN laser facility
Joe C. Watson, James E. Andrew, Nicholas J. Bazin
This paper discusses laser beam, debris and shrapnel induced damage on optical components of a pulsed high power Neodymium laser system. Quantitative and qualitative investigations of the nature of laser damage on optical components and coatings that form the laser system are presented as well as investigations of debris and shrapnel induced damage on sol gel anti reflective coated optical components that surround the taget. In addition laser target debris and shrapnel induced damaged on sol gel thin film coatings have been studied where optical transmission was measured using an Ultra Violet Visible spectrophotometer before and after coated optics were used in the system and experienced damage. The problem with understanding causes of each type of damage is difficult because many processes occur simultaneously in an operational target chamber. Also investigated was the effect of low irradiance laser light at removing debris, and the rate of laser damage growth on contaminated HELEN optics, with number of laser shots.
Fundamental Mechanisms
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Self-focusing and rear surface damage in a fused silica window at 1064 nm and 355 nm
Herve Bercegol, Laurent Lamaignere, Bruno Le Garrec, et al.
Rear surface damage is known to occur preferentially to front surface damage on silica lenses and windows transmitting 1ω or 3ω light of a Nd:Yag or Nd:glass laser. Lab-scale Experiments were performed to assess the contribution of self-focusing to the measurement of damage statistics. The occurrence of filamentation in the bulk and the statistics of rear surface damage were studied at 1064 nm and 355 nm, for a pulse length around 3 ns. The measurements were performed on synthetic fused silica samples, 4 to 5 cm thick. The laser beam had dimensions around 0.6 mm at the sample. The results are interpreted with the help of the calculations made by J. H. Marburger et al. They are also compared with other experimental results.
Bulk damage and laser conditioning of KDP and DKDP crystals with Xe-F excimer light and the 3w of a Nd:Yag laser
Laurent Lamaignere, Marc Loiseau, Herve Piombini, et al.
Samples of deuterated and hydrogenated KDP were submitted to damage tests at 355 nm and 351 nm, with a Nd:Yag laser and the Xe-F line of a excimer laser. Bulk damage was observed; the statistical occurrence and the phenomenology of this type of damage was studied in various conditions. The crystals were raster-conditioned with the excimer source, which delivered a 16 ns pulselength. Laser-conditioning was performed with increasing fluences. The highest usable fluence was limited by the occurrence of surface damage on the front of the crystals. After this irradiation, a two-fold improvement of the damaging fluences was obtained when testing with the excimer beam. However, damage statistics were almost unchanged on the Nd:Yag installation, where the pulselength is about 3 ns. This result is discussed with respect to the large scale conditioning of crystals for high power lasers.
Materials and Measurements
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Growth of damage sites due to platinum inclusions in Nd-doped laser glass irradiated by the beam of a large-scale Nd:glass laser
Gerard Raze, Marc Loiseau, Daniel Taroux, et al.
Samples of Neodymium doped laser glass were irradiated by the 1ω beam of a Nd:glass laser that delivers up to 80 J during a 3-ns pulse duration. Prior to this experiment, platinum inclusions were revealed by a systematic scanning with a lab-scale Nd:Yag laser. The damage sites due to impurity inclusions were subjected to tens of shots of the centimeter-size beam. Several inclusions were irradiated by a series of shots, at a fixed fluence comprised between 10 and 20 J/cm2. The incidence on the optical component was taken at Brewster-angle. In each case, the damage zone began to grow, then the growth rate slowed down and finally stopped. Thus, a stabilization of the growth was obtained for this bulk damage as opposed to steady growth observed in the case of silica surfaces.
Surfaces and Mirrors
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Parametric study of the growth of damage sites on the rear surface of fused silica windows
Gerard Raze, Jean-Marie Morchain, Marc Loiseau, et al.
The growth of damage sites on the rear surface of fused silica plates was studied as a function of fluence and angle of incidence. At 1053 nm, a 70 J beam, 3 ns in pulselength, was directed to a 5 cm2 zone on a bare fused silica window. Initiation and growth was observed. The growth of previously initiated sites was also studied. Growth is exponential in nature. The experiments allow for the determination of the growth coefficient as a function of fluence. At 355 nm, damage sites were irradiated at various angles of incidence, with a tripled Nd:Yag laser, spatially Gaussian, 2.5 ns in pulselength. By fitting growth with an exponential law, it was determined that the relevant fluence for growth was that taken inside the material.
Fundamental Mechanisms
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Modified Z-scan method for determination of nonlinear refraction of optical materials through the measurement of power-weighted time-averaged beam propagation factor
We demonstrate the possibility of extension of the open aperture Z-scan technique to measurement of nonlinear refractive indices for self-phase modulation and two-color cross-phase modulation of optical materials by measurement of power-weighted time-averaged propagation factor [M2(t)] of the beam after the sample. The influence of the temporal shape of the pulse is investigated for two cases: when the pulse peak intensity at the beam waist is known, or when the total energy of the pulse and the duration of the correlation function are known only. Numerical calculations show that the determined values of refraction indices might vary noticeably if the proper pulse shape is not taken into account.
Ultrashort pulse damage of Si and Ge semiconductors
An experimental and theoretical investigation of ultrashort pulse damage thresholds of Si and Ge semiconductors has been carried out. As the source of laser radiation, a commercial sub picosecond Ti:Sapphire laser system has been used. It produces laser pulses of 0.5 mJ pulse energy at 1 kHz repetition rate, providing a Gaussian-like beam profile. Compressor tuning allowed for varying the pulse duration from 150 fs to 5.5 ps. The laser damage thresholds were measured in air and for this pulse duration range. The damage morphologies were investigated with various microscopic inspection techniques like Nomarski DIC, atomic force and white light interference microscopy.
Synchrotron-radiation-induced damages in optical materials
Alexandre Gatto, Norbert Kaiser, Stefan Guenster, et al.
Synchrotron radiation sources and free electron lasers (FEL) represent practical approaches to produce high photon flux down to the VUV wavelength region or the X-ray spectral range. But operating brilliant photon sources requires to overcome severe and crucial damage problems. Optical material are exposed to high power broad band radiation extending from far IR to gamma rays. Typically, materials suffer series of picoseconds pulses at MHz repetition rate with multi photon energy ranging from few eV to several keV. Damages observed on some oxide and fluoride materials irradiated with synchrotron radiation and UV free electron lasers are reviewed. Surface and bulk damages, increased roughness, modification of spectral properties and change of crystalline structures are observed and described. Contamination elements like carbon are detected as major aging factor.
Surfaces and Mirrors
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Dependence of surface damage resistance on bulk quality in CsLiB6O10 (CLBO) crystal
Tomosumi Kamimura, Satoru Fukumoto, Munenori Nishioka, et al.
There has been great interest in high-repetition, high-power ultraviolet (UV) source for various applications in semiconductor processing, micro machining, and other fields. Discovery of CsLiB6O10(CLBO) crystals have enabled the production of such practical high-power all solid-state UV lasers. In 2001, UV output power up to 23.0 W by fourth harmonic generation of Nd:YAG laser was achieved. In general, one of the limiting factors for the development of high-power solid-state UV lasers is laser-induce damage of NLO crystal due to some kinds of defects inside the materials. Recently, we have succeeded to grow the high crystallinity CLBO with an enhanced bulk laser damage resistance. On these samples, an increase in the surface damage resistance could be expected. Measurement of the surface laser-induced damage threshold (LIDT) on CLBO crystals with various crystallinity was performed by using a 266 nm laser. For the crystal with high damage resistance (15-18 GW/cm2), LIDT of as-polished surface was 1.3 times higher than that of crystal with conventional damage resistance (9-12 GW/cm2). In addition, polishing compound embedded inside the crystal surface was removed by using an ion-beam etching process. We have observed 1.5 times improved surface LIDT by ion beam etching for both high damage resistance and conventional damage resistance samples. The relationships between vickers hardness and crystal quality will be presented.
Wet etching for the mitigation of laser damage growth in fused silica
Philippe Bouchut, Pierre Garrec, Catherine Pelle
On the 3ω part of the LIL laser many optical components will have to sustain fluences above 10J/cm2. Even if progress in silica substrate technology decreases the number of defects/cm2 which can induce a damage under such a laser flux, tens of damaged sites will appear on large surface optics. Knowing that these damaged sites grow exponentially with the number of laser shots, it is a necessity to stop the growth of these defects before the use of the optical component is impaired. In this paper we have used wet chemical etching as a way to circumvent the growth of laser-induced surface damages. SEM characterization of damages at all stages of the process has been carried out. We show that at a reduced damage creation fluence, the use of a highly concentrated HF acid leads to a 93% mitigation rate for those damaged sites that need mitigation. Due to the etching anisotropy, the HF acid concentration is more important than etch depth’s for the mitigation rate of laser induced surface damage in silica.
Fundamental Mechanisms
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Luminescence of UV thin films
Luminescence measurements have been set up in order to study the interaction of UV-laser radiation with dielectric thin films. The pulsed laser excitation was carried out at 193-nm (6.4eV), the coating materials comprised wide-band-gap oxides and fluorides. Experiments show the significant optical response of single- and multilayer coatings on the low fluence excitation at sub-band-gap energy. Time- and spectrally-resolved measurements indicate characteristic emission bands of color centers in the deep-UV and vacuum-UV coating materials. An assignment of these optical transitions can be derived from the comparison with known bulk-material studies.
Thin Films
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Calculated and thermally measured laser damage in metallic thin films as a function of pulse duration
Laser damage in metallic thin films is investigated in materials such as Gold and Nickel with different pulse durations from 1s to 10ns. In these materials, damage phenomena can be clearly investigated through thermal effects. Then, the heat equation is solved for monolayers which offers the prediction of transient temperature in each case. These results of calculations are compared to LIDT measurements at 1.O6μm and associated to morphology studies made by Atomic Force Microscope. Different mechanisms are highlighted, depending on the layer thickness. The experimental and theoretical results are found to agree well in the range of milliseconds to nanoseconds. This study offers a significant understanding for the design of metal-dielectric multilayers, such as broadband light absorbers, filters or mirrors.
Surfaces and Mirrors
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Long-term performances of very high-laser-damage resistant mirrors
Guillaume Ravel, Philippe Bouchut, Pierre Garrec, et al.
Achieving ultra-high laser resistance for high reflectivity coatings has become possible due to technological progress during the last years; keeping these performances up to their initial level over the storage or operation period seems a new challenge, as preliminary results reported last year in this conference showed a degradation of LIDT @ 1.06μm as a function of elapsed time and storage conditions. This work is clearly the continuation of last year's effort, as we report more observations on the same objects, with an emphasis on the use of surface-analysis techniques (TD-GCMS e.g.) to check potential correlation between degraded LIDT behavior and organic contamination.
Thin Films
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Optical characteristics of THV fluorothermoplastic (Abstract Only)
Kunio Yoshida, Takuo Nobori, K. Hatooka, et al.
Optical characteristics of a new commercially available perfluorinated organic polymer have been measured to prepare a quarterwave AR coating for Nd3+:YAG crystal and other optical materials. Coatings are carried out by dip method from solution at room temperature, they have a refractive index of 1.364. The laser-induced damage threshold at 1064nm, 532nm, 355nm, and 266nm is approximately same values with Teflon AR 2400.
Nodular defects in sputtered coatings
Keith L. Lewis, Gilbert W. Smith, Alan J. Pidduck
Significant advances have been made in the area of reactive sputtering to accommodate industry requirements for high performance coatings. A review is presented of progress in the field, highlighting the results of studies of specific films produced by various techniques. Ion beam sputtering, originally exploited for producing laser gyro coatings, has become one of the standard methods of producing narrow band filters for dense wavelength division multiplexing (DWDM) in the telecommunications industry. More recently, other techniques have also emerged exploiting advances made in the opto-electronics industry, notably high-density remote plasma techniques. It has been found that no process is able to generate films that are entirely free of defects, but significant differences occur in the geometrical form of the defects produced. Some are suggestive of the role that vapor-liquid-solid growth mechanisms may play in their formation.
Fundamental Mechanisms
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Comparison of numerical simulations with experiment on generation of craters in silica by a laser
The validation of numerical simulations of laser induced damage of fused silica requires detailed knowledge of the different parameters involved in the interaction. To approach the problem, we have performed simulations of laser energy deposition in spherical metallic defects and the surrounding fused silica. Our code DELPOR takes into account various laser/defect induced absorption mechanisms of SiO2, such as radiative ionization, avalanche and multiphotonic ionization. We have studied crater formation produced by the absorber explosion with a 2-D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. To validate our simulations, we have made and tested samples of ultra-pure silica thin film, containing gold nanoparticles of diameter 0.6 μm. The fused silica coating could have three different thickness. We compare experiment and simulations for two laser irradiations at wavelengths 0.351 and 1.053 μm.
Using a TOF mass spectrometer for studies of laser interaction with 3-nm diameter gold nanoparticles embedded in silica
Herve Bercegol, Florian Bonneau, Patrick Combis, et al.
With the aim of observing and simulating laser initiated damage, ultra-pure silica "model" samples, seeded with gold nanoparticles (diameters 2 - 5 nm), were prepared and exposed to 3 ns laser pulses at wavelength 355 nm. These samples enable us to study the mechanism of damage initiation caused by inclusions of nanometric size, which is the typical size of defects occurring in optical quality glass. The samples were studied in a series of experiments using a time-of-flight mass spectrometer at Argonne National Laboratory. This installation is of great interest because it enables us to combine the laser irradiation of the sample with the chemical identification of material ejected from the exposed surface. An evaluation of the quantity of gold atoms emitted during irradiation can thus be obtained from the experimental results. These experimental data are completed with “Nomarski” and “atomic force” microscope observations, and then interpreted. In particular, a comparison is made to numerical simulations obtained with our DELPOR code. An encouraging result is the existence of a pre-damage phase at very low fluences that is not detectable by classical optical devices.
Numerical simulations of laser/defect-induced absorption in SiO2
Florian Bonneau, Patrick Combis, A. Pujols, et al.
We have performed simulations of laser energy deposit in sub-micrometric spherical defects and the surrounding fused silica. We have studied crater generation produced by the absorber explosion with a 2D/3D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. The comparison of the 2D simulations with experiment shows quite good agreement for shallow defects (depth < 2 μm). We have observed experimentally that the explosion of deeper absorbers results in a more complex crater morphology. Therefore we have begun performing 3D simulations in order to reproduce these features. Depending on the defect depth, the cracks may not reach the surface and a crater doesn't appear. Nevertheless, those cracks or pre-existing cracks can contribute to efficient electric field enhancement and breakdown on the surface. Different types of cracks (size, inclination, filled with a material or not) were investigated and the 2D or 3D electromagnetic field distributions were computed using a finite element code.
Materials and Measurements
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Experimental results of laser interaction with included gold particles in silica at 1w and 3w (Abstract Only)
Jean-Yves Natoli, Laurent Gallais, Michela Perra, et al.
In order to exhibit the role of laser damage precursor centers in silica, gold particles ranging from 0.2mm to 0.8mm have been included between two coatings of SiO2 deposited on silica substrates. UV and IR nano-second pulsed irradiations on samples with different SiO2 over-layer thicknesses (2, 5, 10 mm) has been performed. The damage morphologies observed with Nomarski and atomic force microscopes have shown to be dependent on fluence, wavelength and SiO2 over-layer thickness. In addition a localized irradiation study using 6mm spot size allows to aim accurately on an isolated particle. The measurement is compared to the experiments presented for a few hundred micrometers spot size. Indeed in this case we have to consider that several particles are simultaneously irradiated. A comparison between the respective morphologies could inform us on potential collective effects of the particles. The choice of experimental test conditions, and the whole associated results will be presented and discussed.
Fundamental Mechanisms
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Toward an absolute measurement of LIDT
The improvement of LIDT value of optical components had lead to develop a lot of experimental setups and procedures of test around the world. In this context it is often very difficult to make accurate comparisons of laser damage threshold values between the different apparatus. The differences are due to the procedure of test, the spatial and temporal beam variations, the laser damage criterion and others. A specific laser damage testing apparatus, with an accurate damage initiation detection and allowing a real time acquisition of the different shot parameters, lead to exhibit the influence of each parameter on the damage process. Laser beam profiling is performed in real time and give access to the fluence for one pixel (0.2μm2), therefore an effective fluence or a pixel fluence can be calculated in order to reach an “absolute” threshold value. The metrology developed will be detailed and some results obtained on silica and BK7 at 1064nm and 355nm with different procedures of test will be presented and discussed to illustrate the aim of this study.
Electromagnetic phenomena generated in laser-produced plasmas (Abstract Only)
When a laser plasma is produced on a target, various electromagnetic phenomena can occur. These can produce substantial currents and voltages in nearby structures. The effects depend on the target material and morphology, the pressure and species of the atmosphere, and the nature of the laser pulse. The following mechanisms are known to make a major contribution to electromagnetic signals detected near laser plasmas: (1) UV plume causing transient high conductivity in semiconductor targets, and ionisation in buffer gasses; (2) Laser plasma generating multi-GHz microwaves due to the generation of plasma waves; (3) Space charge and current charge travelling through vacuum due to differences in the electron and ion velocities; (4) Generation of transient magnetic fields that induce anomalous currents in conductors at the target point, and secondary induced current in nearby conductors. Many of which were first reported in the 1970s, and in this report we review their relative contributions and identify regimes where each dominate.
Self-guiding, supercontinuum generation and damage in bulk materials induced by femtosecond pulses
The dynamics of multiple pulse laser-induced damage in the form of cracking or nonlinear coloration in bulk materials (fused silica and borosilicate K8 glass) was studied under the irradiation by femtosecond pulses at 800 nm wavelength. A Ti:sapphire chirped pulse amplification system with ~130-fs pulse duration and ~1-mJ pulse energy at 1-kHz repetition rate was used in the experiment. Self-guided propagation of femtosecond pulses over greater than 1-cm lengths accompanied by intensive supercontinuum generation was observed and studied in an interaction geometry where the laser beam was focused in the middle of the thick (~4 cm) sample. The pulse energy value at which self-guided propagation and supercontinuum generation in fused silica was observed was ~60 times lower than the laser-induced damage threshold. The nonlinear coloration in K8 glass was present at pulse energy values which exceeded the threshold for self-guided propagation. Numerical simulations involving self-focusing, temporal dispersion and multiphoton absorption were found to be in good agreement with the experimental results.
Identification of point defects responsible for laser-induced ultraviolet absorption in LiB3O5 (LBO) crystals
W. Hong, Nelson Y. Garces, M. M. Chirila, et al.
Electron paramagnetic resonance (EPR) has been used to identify and characterize point defects in lithium triborate (LiB3O5) crystals grown for nonlinear optical applications. As-grown crystals contain oxygen vacancies and lithium vacancies (as well as trace amounts of transition-metal ions in a few samples). Exposing a crystal to ionizing radiation at 77 K produces “free” electrons and holes. These electrons are trapped at the pre-existing oxygen vacancies and give rise to an EPR signal with a large hyperfine from one 11B nucleus. The corresponding holes become self-trapped on oxygen ions as a result of the significant lattice relaxation of a nearest-neighbor fourfold-bonded boron ion. This gives rise to an EPR signal with a smaller 11B hyperfine pattern due to the oxygen’s threefold bonded boron neighbor. Warming the crystal to approximately 130 K destroys the self-trapped hole centers that were initially formed, and allows a second holelike signal to be observed (which in turn decays between 150 and 200 K). The structure of the second hole center is very similar to the self-trapped hole center and a neighboring lithium vacancy makes this latter center more thermally stable. The EPR spectra from Ni+ and Cu2+ ions are also reported.
Experimental study of wavelength-dependent damage threshold in DKDP
Using an OPO (optical parametric oscillator) laser we have measured the damage thresholds of deuterated potassium dihydrogen phosphate (DKDP) from 312 nm to 532 nm by the 1 on 1 method. Distinct steps are observed in the damage threshold at 318 nm and 487 nm. The wavelength dependence to the damage threshold is compared to various models from the literature to demonstrate the relevance of each to damage initiation in DKDP.
Materials and Measurements
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Evolution of bulk damage initiation in DKDP crystals
Christopher W. Carr, T. H. McMillian, Mike C Staggs, et al.
We investigate the evolution of laser-induced damage initiated in the bulk of DKDP crystals using in-situ microscopy. Experimental results indicate that at peek fluences greater than 10 J/cm2, damage sites are formed with increasing number as a function of the laser fluence. Following plasma formation, cracks are observed which grow in size for tens of seconds after the termination of the laser pulse. Subsequent irradiation leads to modest increase in size only during the initial 2-5 pulses. Experimental results suggest that there is also relaxation of the stresses adjacent to a damage site for several hours after initial damage.
Thin Films
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Femtosecond damage threshold of multilayer metal films
With the availability of terawatt laser systems with subpicosecond pulses, laser damage to optical components has become the limiting factor for further increases in the output peak power. Evaluation of different material structures in accordance to their suitability for high-power laser systems is essential. Multi-shot damage experiments, using 110 fs laser pulses at 800 nm, on polycrystalline single layer gold films and multi-layer (gold-vanadium, and gold-titanium) films were conducted. The laser incident fluence was varied, in both cases, from 0.1 to 0.6 J/cm2. No evidence of surface damage was apparent in the gold sample up to a fluence of 0.3 J/cm2. The multilayer sample experienced the onset of surface damage at the lowest fluence value used of 0.1 J/cm2. Damage results are in contrast with the time resolved ultrafast thermoreflectivity measurements that revealed a reduction of the thermoreflectivity signal for the multilayer films. This decrease in the thermoreflectivity signal signifies a reduction in the surface electron temperature that should translate in a lower lattice temperature at the later stage. Hence, one should expect a higher damage threshold for the multilayer samples. Comparison of the experimental results with the predictions of the Two-Temperature Model (TTM) is presented. The damage threshold of the single layer gold film corresponds to the melting threshold predicted by the model. In contrast to the single layer gold film, the multi-layer sample damaged at almost one third the damage threshold predicted by the TTM model. Possible damage mechanisms leading to the early onset of damage for the multilayer films are discussed.
Surfaces and Mirrors
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Operation of the Jefferson Lab FEL: optics lessons learned
The IR Demo FEL User Facility at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) provided users with a unique source of laser radiation from 1999 to 2001. Utilizing superconducting RF linacs with electron recirculation and energy recovery, the machine lased with up to 2100 W of average power output at 3.1 microns with very high beam quality. It was capable of output wavelengths in the 1 to 6 micron range and produced ~0.7 ps (and shorter) pulses in a continuous train at ~75 MHz. This subjected the cavity optics to a unique combination of high average and peak irradiances. In addition, cavity optics were subjected to high energy X-rays in a high vacuum environment. In this talk I will summarize how the optics (cavity and transport) survived these conditions. Upgrades that are underway will extend operation beyond 10 kW average power in the near IR and kilowatt levels of power at wavelengths from 0.3 to 14 microns. Drawing from our experience and from research presented at these symposia, I will present the design of these new lasers.
Thin Films
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Ion-assisted versus nonion-assisted high-laser-damage resistant coatings on BBO (Abstract Only)
A comparison of ion assisted deposition (IAD) and non-IAD e-beam coatings on BBO crystals was performed. Samples of BBO were processed as a lot and prepared for coatings using standard solvent cleaning. Two types of coatings were tested; a dual peak anti-reflection, 1064nm and 532nm, and a triple peak anti-reflection, 1064nm, 532nm and 355nm. After depositing of these high laser damage resistant coatings the transmission, reflection and absorption of each coating was measured. Each of the coatings laser damage threshold was determined and an analysis of the two methods of deposition was performed. Additionally, after MIL spec humidity tests were performed on each of the samples, spectral shifts were analyzed for both types of coatings. Conclusions were drawn about the preferred method of deposition for the improvement for spectral characteristics, increased laser damage resistance and reduction of humidity induced spectral shifts.
Fundamental Mechanisms
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Photothermal microscopy for in-situ study of laser damage induced by gold inclusions
A photothermal microscope has been combined with an experimental set-up allowing damage threshold measurements at the same wavelength. The microscope is based on photothermal deflection of the transmitted probe beam: the CW pump beam (1.06 μm wavelength) and the probe beam are collinear and focused through the same objective. The diameter of the pump beam on the sample surface is 1 μm. Laser damage thresholds are measured thanks to a pulsed beam (1.06 μm wavelength and 6 nanosecond pulse) and the spatial position of the pulsed beam is controlled by a CCD camera. This experimental setup has been used to study the behavior of metallic inclusions in dielectric materials in laser damage processes. Results are presented with gold inclusions of about 600 nm in diameter in silica.
Thin Films
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Photochemical laminating of low-refractive-index transparent antireflective SiO2 film
Masataka M. Murahara, Yasuhiro Ogawa, Kunio Yoshida, et al.
A transparent low refractive index SiO2 film was laminated on a glass substrate with photochemical reaction by an Xe2* excimer lamp in the presence or NF3, 02 and silicon wafer at room temperature. The glass substrate and the silicon wafer were placed in the reaction chamber, which was filled with NF3 and O2 gases in the mixing ratio of 10:1 and under 330 Torr. The Xe2* excimer lamp was, then, irradiated for 20 minutes. The SiO2 film was spontaneously laminated on the glass substrate by repeating an adsorption of SiF4 and a photochemical oxidization with NO2, which was photo-dissociated from a mixed gas of NF3 and O2. The film thickness was 160 nm and the infrared spectrum was measured; the Si-O peaks were depicted at 600, 700, 1100[1/cm-1]. And Si-F peak was observed at 740[1/cm-1]. Then, the refractive index of the SiO2 film was 1.36. After annealing the film for one hour at 200 degree, the refractive index increased to 1.42.
Materials and Measurements
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Radiation resistance of optical materials against synchrotron radiation
Stefan Guenster, Holger Blaschke, Detlev Ristau, et al.
Radiation resistance of optical materials against synchrotron radiation is important, if optical components for the high energetic regime have to be produced. In the framework of the European project EUFELE, which deals with the development of optical coatings for the free electron laser at ELETTRA (Trieste), a set of CaF2 substrates was irradiated with synchrotron radiation. The synchrotron radiation was varied by wavelength, dose, and high energetic background illumination. Before and after irradiation, the CaF2 substrates were investigated spectrophotometrically in the VUV, VIS and IR range. The surface topology was characterized by Nomarski microscope methods. Structure investigations were carried out with X-ray diffraction measurements. CaF2 shows different types of degradation like color center formation, surface modification, and increased VUV absorption bands. Defect formation will be presented in dependence of synchrotron irradiation conditions.
Fundamental Mechanisms
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Optical limiting based on liquid-liquid immiscibility
Gregory J. Exarhos, Kim F. Ferris, William D. Samuels, et al.
Nonionic surfactants are used to stabilize a dispersed droplet phase in a continuous liquid phase when two immiscible liquids are mixed. As both liquid phases approach the index-matched condition, interfacial scattering is suppressed, and the mixture takes on the characteristics of a Christiansen-Shelyubskii filter. If, in addition, one of the liquids exhibits a substantial nonlinear optical response, then interfacial light scattering can be reversibly turned on when a laser beam incident upon the filter exceeds a critical fluence. To demonstrate this effect, an organic phase (dichloroethane) was dispersed in an aqueous phase containing sodium thiocyanate (NaSCN) using an alkyl end-capped polyethylene glycol ether. Optical limiting was observed through this transparent medium under conditions where the focused second-harmonic output of a Q-Switched Nd:YAG laser was on the order of about 50 mJ/cm2. An open-aperture z-scan technique was used to quantify the limiting behavior. Since the thiocyanate anion is both isostructural and isoelectronic with carbon disulfide which exhibits a large optical nonlinearity, the mechanism of optical limiting is proposed to be associated with a nonlinear shift in the aqueous fluid index of refraction, resulting in an index mismatch between the disparate phases at high laser fluence. Index mismatch between the two phases leads to multiple reflections, loss of coherence, and a significant transmission decrease due to Tyndall scattering. Fundamental studies of such systems are used to verify theoretical predictions of the limiting effect, and aid in the design and development of improved sub nanosecond limiters based upon this optical deflection approach.
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Calibration of laser power meters for high-power applications
Volker Brandl, Klaus Haensel, Mike Klos, et al.
The market offers a relatively wide range of laser power meters for high power applications, but when it comes to verifying measurements, a lot of know-how is required. Even the comparison of national standards between different countries in some cases has given evidence for discrepancies. For high power measurements, a major drawback has been that the primary standards of all the national calibration institutes are cryogenic radiometers, which are built for low power applications, while industrial applications often require lasers in the 0.1 - 12 kW range, thus creating the need for transfer standards from low to high power range. Primes GmbH currently is setting up a calibration laboratory for high power cw laser power meters in cooperation with the German institute for standards, the PTB, which will allow to trace high power laser measurements back to national standards and extend the measurement range substantially. Certified calibration services will be open to all users and manufacturers of laser power meters for high power applications.
Materials and Measurements
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Interferometric assessment of laser-induced damage to semiconductors
It is important to determine the onset of damage as well as the extent of the damage area when materials are illuminated by intense laser radiation. In this work, an optical interferometric technique was used to assess laser-induced damage in semiconductor materials based on the three-dimensional, topographic characteristics of the damage site. Both antireflection coated and uncoated materials were evaluated for variations of fluence level, focused spot size, and laser repetition rate. The interferometric technique was non-contact and nondestructive in nature, providing a high-resolution capability of assessing damage levels on the surfaces of the materials.
Thin Films
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Degradation of a multilayer dielectric filter as a result of simulated space environmental exposure
Peter D. Fuqua, Nathan Presser, James D. Barrie, et al.
The exterior optical surfaces of satellites are directly exposed to the harsh space environment. Here, a multilayer dielectric solar rejection filter was deposited on a silicon substrate and then subjected to electron and proton irradiation, simulating an orbital environment. Following the exposure, damage was observed that was attributed to dielectric breakdown. Optical and scanning electron microscopy revealed extensive pitting as a result of this exposure. The typical size of dischrage pits was 50 - 100 microns at the surface, extending to the substrate material, where a 10 micron diameter melt region was found. Pit damage occurred at pre-existing coating defects and was accelerated by pre-exposure to proton radiation. Pitting was not observed on similar samples that had also been overcoated with a conductive thin-film.
Materials and Measurements
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Application of spectroscopic ellipsometry to characterization of optical thin films
John A. Woollam, Corey L. Bungay, Li Yan, et al.
Spectroscopic ellipsometry is used to determine optical constants, layer thicknesses in multilayer stacks, and microstructure (voids, alloy fraction, or mixed phase composition), and is a well-developed technique for analysis of optical thin films. Ellipsometers now cover from 140 nm (≈ 9 eV) in the vacuum-ultraviolet to 200 microns (50cm-1) in the far infrared. Generalized anisotropy and depolarization are measurable using rotating compensator ellipsometers or controlled retarders for partial Mueller Matrix analysis. Rotating compensator ellipsometers allow accurate and rapid in situ diagnostics, including window birefringence calibration. Steady progress has been made in both ex situ and in situ ellipsometry hardware, software, and applications. These advances are reviewed and examples given.
Thin Films
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Damage behavior of SiO2 thin films containing gold nanoparticles lodged at predetermined distances from the film surface
Semyon Papernov, Ansgar W. Schmid
The impact of absorbing defect location inside thin film on measured laser-damage threshold and morphology is explored, using gold nanoparticles as artificial thin-film defects. In this work, 351-nm, 0.5-ns laser pulses were used to produce damage in SiO2 thin film containing gold nanoparticles located at predetermined but different distances from the film surface. As a result, laser-induced damage thresholds detected by means of dark-field optical microscopy and atomic force microscopy are measured and comparative trends are analyzed. Damage-crater geometry variations with particle-lodging depth are also obtained and compared with theoretical predictions.
Surfaces and Mirrors
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Comparison of antireflective coated and uncoated surfaces figured by pitch-polishing and magnetorheological processes
Robert Chow, Michael D. Thomas, Robert C. Bickel, et al.
When completed, the National Ignition Facility (NIF) will provide laser energies in the Mega-joule range. Successful pulse amplification to these extremely high levels requires that all small optics, found earlier in the beamline, have stringent surface and laser fluence requirements. In addition, they must operate reliably for 30 years constituting hundreds of thousands of shots. As part of the first four beamlines, spherical and aspherical lenses were required for the beam relaying telescopes. The magneto-rheological technique allows for faster and more accurate finishing of aspheres. The spherical and aspherical lenses were final figured using both coventional-pitch polishing processes for high quality laser optics and the magneto-rheological finishing process. The purpose of this paper is to compare the surface properties between these two finishing processes. Some lenses were set aside from production for evaluation. The surface roughness in the mid-frequency range was measured and the scatter was studied. Laser damage testing at 1064 nm (3-ns pulse width) was performed on surfaces in both the uncoated and coated condition.
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Laser beam characterization in uniaxial crystals
Gabriella Cincotti, Alessandro Ciattoni, Damiano Provenziani, et al.
We give a full characterization of laser modes propagating along the optical axis of an unbounded uniaxially anisotropic crystal, evaluating the analytical expressions of the beam parameters and putting the combined effects of diffraction and anisotropy into evidence. We consider a HG mode of any order Ex, linearly polarized along the x-axis at the entrance facet of the crystal, and we derive the spot-size evolution formula and the corresponding angular divergences: due to anisotropy, the beam parameters evaluated along the x- and y-axes have different expressions. The optical beam suffers a change of the polarization state, and a y-component of the electric field Ey arises from anisotropy: we evaluate the spot-sizes along the x- and y-axes, and the associated angular divergences. The mutual coupling between the Cartesian components of the field results in a reduction of the energy associated to Ex and a corresponding increase of the optical power associated to Ey: we calculate the analytical expression of the energy exchange between the Cartesian components of the optical beam and their asymptotic values. The analytical results are confirmed by a set of experimental measurements with a calcite crystal.
Mini-Symposium on Optics Characterization
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Characterization of particulate contamination of optics
The specification and characterization of particulate contamination is an area of active interest for ISO TC172/SC9/WG6. The starting point for the development of a new ISO standard is the US national standard, MIL-STD-1246. The paper will discuss the basic framework for MIL-1246 and introduce the issues that need to be addressed before a complete ISO standard for particulate contamination of optics can be issued. It is also hoped that this paper will stimulate discussion leading to a more complete and useful standard.
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Laser radiometry for UV lasers at 193 nm
Stefan Kueck, Klaus Liegmann, Klaus Moestl, et al.
A setup for the measurement and calibration of laser power at the excimer laser wavelength of 193 nm was realized. The investigations include the measurements of the damage threshold and the determination of the effective spectral reflectance of the material used for the standard detector, named LM4. The results of these studies allow the complete characterization of the LM4 standard detector including an uncertainty analysis according to the "Guide to the Expression of Uncertainty in Measurement." The LM4 standard detector is linked to the calibration chain established at the Physikalisch-Technische Bundesanstalt (PTB), the German National Metrology Institute. Furthermore, two transfer detectors were linked to the LM4 at the wavelength of 193 nm. Calibrations can be performed down to 193 nm with a relative expanded uncertainty (k = 2) below 1.6 %. Maximum average power, for which calibrations can be performed, is 2.8 W at a repetition rate of 200 Hz. An outlook for the calibration of the laser power at 157 nm is given.
Mini-Symposium on Optics Characterization
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Results of a round-robin experiment on reflectivity measurments at a wavelength of 1.06 micron
Detlef Nickel, Christoph Fleig, Andrea Erhard, et al.
Within the EUREKA-project EU 2359 (Instruments and Standard Test Procedures for Laser Beam and Optics Characterization) a "Round-Robin"-experiment on reflectivity measurements at the wavelength λ = 1.06 μm has been carried out. The topic of this "Round-Robin"-experiment was the evaluation of the Working Draft ISO/WD 13697 and Committee Draft ISO/CD 15368 and the comparison to simple transmission and reflectivity measurements. For the determination of transmittance and reflectance of laser components, commercially available spectrophotometers are used in industry to monitor the quality at a medium accuracy level. If a higher level of precision is required, especially to resolve the reflectivity of high-reflecting dielectric mirrors, the measurement set-up described in ISO/WD 13697 should be preferred. This standard procedure was elaborated in the framework of the CHOCLAB project. This paper analyzes the results of the "Round-Robin"-experiment; the accuracy and comparability of measurements according to different methods will be given.
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Laser beam and optics characterization with Z-scan method
Nelson V. Tabiryan, Vinay Jonnalagadda, Manuel J. Mora, et al.
We report about applications of nonlinear optical processes for laser beam and optics characterization. The basic mechanism of the measurements consists in scanning a thin film of a liquid crystal in the focal region of a laser beam and processing the self-phase modulation signal. This technique allows precise and quick determination of the focal waist position and radius, which, in conjunction with the parameters of the focusing system allows determination of the laser beam divergence. We have demonstrated the capabilities of the technique for measuring submicron waist sizes and characterizing astigmatic optical systems. The technique is applicable to short laser pulses. The measurements were performed using the device implementation of the technique, the Crystal Scan Optical Multimeter.
Characterization of capabilities of Z-scan technique for measuring of divergence and astigmatism of laser beams (Abstract Only)
We present the results of comprehensive characterization of capabilities of Crystal Scan device for measuring laser beams. (1) As small as 0.7 mm waist radius was measured for a laser beam of λ = 409 nm wavelength. The measurement lasts about 1 s with actual scan time being about 300 ms. (2) The focal waist position with accuracy better than 1% using focusing lenses of 6 mm to 50 mm focal length was determined. (3) Laser beam divergence in the range of 10-5rad to 10-2rad was measured and compared both to theoretically anticipated values as well as to the results obtained by conventional techniques. (4) We have identified the specific features of the signal received from astigmatic beams and measured the position and waist radii of both focuses. (5) The technique was successfully applied to measuring of laser beams at 1064 nm, 1341 nm and 10.6 mm wavelengths.
Estimation of beam parameters and coherence properties of laser radiation by use of an extended Hartmann-Shack wavefront sensor
Bernd Schaefer, Klaus R. Mann
A combination of a Hartmann-Shack sensor and a standard far-field measurement on one single detector is proposed. The technique is fast, manages without moveable parts, thus permitting a very compact design. It is not only suited for characterization of the wave-front distribution, but may also be considered for determination of the important parameters beam width, beam divergence and beam propagation ratio M2 of partially coherent laser beams. The experimental results in combination with findings from error analysis and numerical simulations indicate that a fairly thorough beam characterization including spatial coherence, propagation characteristics and beam quality can be achieved with this method, if a high resolution H.-S. sub-system with efficient crosstalk reduction is used.
General classification of partially polarized partially coherent beams
The behavior of the so-called generalized degree of polarization of partially coherent partially polarized beams upon free propagation is investigated. On the basis of this parameter a general classification scheme of partially polarized beams is proposed. The results are applied to certain classes of fields of special interest.
Measurement of the Wigner distribution of a helium neon laser with a spherical aberration and a tapered semiconductor laser using moving slit technology
Bert J. Neubert, Wolf-Dieter Scharfe, Guenter Huber
We determine the Wigner distribution of a laser experimentally from intensity profiles obtained by moving slit technology. In order to find out whether the phase calculated from a measured Wigner distribution can be trusted, we add a known spherical aberration to a Helium Noen laser by a plan-convex collimating lens orientated "the wrong way." The magnitude of the aberration can be influenced by the beam diameter at the lens. The M2-values calculated from the Wigner distribution of the aberrated beam at different levels of aberration is in agreement with theory. The coefficient of spherical aberration obtained from the measurement agrees with the one predicted by aberration theory if the impact of the aberration on the beam profiles is large enough (M2 > 1.2). The Wigner distribution of a tapered semiconductor laser is also examined. The measured phase at the semiconductor facet is aberrated. The aberration however can not be identified to origin from exit of the beam from high index semiconductor to air through a planar interface.
Mini-Symposium on Optics Characterization
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Repetition rate dependence of two-photon absorption and self-trapped exciton luminescence in CaF2 at 193 nm
Christian Goerling, Uwe Leinhos, Klaus R. Mann
Repetition rate dependent measurements of nonlinear absorptance and self-trapped exciton luminescence (π-luminescence) of high-purity CaF2 crystals irradiated at 193 nm were performed in the range between 25 Hz and 300 Hz. The CaF2 samples showed different two-photon absorption coefficients ("effective" two-photon absorption coefficients βeff), which are attributed to different impurity or defect concentrations inside the crystals. For samples with βeff < 4 x 10-9 cm/W no dependence on repetition rate could be observed, while samples with higher βeff (⩾ 5 x 10-9 cm/W) reveal a strong nonlinear increase of βeff for increasing repetition rates. In contrast, the intensity of self-trapped exciton luminescence was nearly the same for all samples and repetition rates. A simple model for this behavior was developed, including the formation of long-living transient absorbing states, which contribute additionally to the two-photon absorption.
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Numerical phase retrieval from beam intensity measurements in three planes
A system and method have been developed at CEA to retrieve phase information from multiple intensity measurements along a laser beam. The device has been patented. Commonly used devices for beam measurement provide phase and intensity information separately or with a rather poor resolution whereas the MIROMA method provides both at the same time, allowing direct use of the results in numerical models. Usual phase retrieval algorithms use two intensity measurements, typically the image plane and the focal plane (Gerschberg-Saxton algorithm) related by a Fourier transform, or the image plane and a lightly defocus plane (D.L. Misell). The principal drawback of such iterative algorithms is their inability to provide unambiguous convergence in all situations. The algorithms can stagnate on bad solutions and the error between measured and calculated intensities remains unacceptable. If three planes rather than two are used, the data redundancy created confers to the method good convergence capability and noise immunity. It provides an excellent agreement between intensity determined from the retrieved phase data set in the image plane and intensity measurements in any diffraction plane. The method employed for MIROMA is inspired from GS algorithm, replacing Fourier transforms by a beam-propagating kernel with gradient search accelerating techniques and special care for phase branch cuts. A fast one dimensional algorithm provides an initial guess for the iterative algorithm. Applications of the algorithm on synthetic data find out the best reconstruction planes that have to be chosen. Robustness and sensibility are evaluated. Results on collimated and distorted laser beams are presented.
Mini-Symposium on Optics Characterization
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A coherent spectrophotometer for optical coating characterization
A laser spectrophotometer based on optical parametric oscillators for characterization of optical coatings in the spectral range 420 - 4500 nm is described. The characterization includes measurements of reflectance and transmittance, absorption losses, and laser-induced damage thresholds. The master oscillator of the pump-laser system is a diode-pumped, Q-switched Nd:YAG laser. Additional power, when needed for laser-induced damage threshold measurements, is obtained from flashlamp-pumped Nd:YAG amplifiers. Operation of the laser spectrometer is demonstrated by absorptance measurements and reflectance and transmission measurements of optical coatings over a wide range of angle of incidence. Damage thresholds are measured both with the OPO output and with the direct output and harmonics of the output of the amplified laser system.
S-on-1-induced damage thresholds of high-reflection metallic coatings at 1064 nm
Virginija Bingelyte, Valdas Sirutkaitis, Robert C. Eckardt
Measurements on several high-reflection (HR) metallic mirrors were performed, and multiple-pulse laser induced damage thresholds (LIDT) were estimate according to the draft International Standard ISO/DIS 11254 using S on 1 tests at 1064 nm. The experimental set-up was based on Q-switched, diode-pumped Nd:YAG laser. Transmission increase was used for an in situ measurement of damage. The characteristic damage curves were plotted and multiple-pulse damage thresholds were determined at 30-Hz pulse-repetition rate, while the number of pulses varied up to 300.
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Intrinsic and geometrical beam classification, and the beam identification after measurement
The goal of the paper is a better understanding of laser beams. The well-known method of second-order moments of Wigner function (to define beams) and of beam irradiance (to measure beams) is employed. After a short introduction and a summary of previous relevant results, the paper introduces new generalized beam propagation invariants and new beam classifications. As a result of a measurement, criteria to identify the beam are given. To achieve this task, certain approximations are suggested to assess whether some beam matrix elements are equal, or whether some beam matrix elements, or some beam propagation invariants are zero. The main application is in developing the ISO 11146 standard for characterizing and measuring the spatial properties of laser beams with any spatial symmetry.
Phase space analyzer with Gaussian slits
The phase space analyzer is an optical device that uses slits, lenses and an irradiance-calibrated image detector in order to characterize optical beams. With such a device it is possible to obtain the beam power distribution along the two-dimensional phase space coordinates corresponding to a given transverse direction. The usual setup includes hard edge slits, and it has been considered in previous studies to measure stigmatic and simple astigmatic beams. We analyze a phase space analyzer with Gaussian slits to measure Gauss Schell-model beams. Special attention is given to general astigmatic beams (such as twisted irradiance and/or twisted phase beams), where a characterization along two orthogonal transverse axes is not enough.
Mini-Symposium on Optics Characterization
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High-precision reflectivity measurements: improvements in the calibration procedure
Marco Jupe, Florian Grossmann, Kai Starke, et al.
The development of high quality optical components is heavily depending on precise characterization procedures. The reflectance and transmittance of laser components are the most important parameters for advanced laser applications. In the industrial fabrication of optical coatings, quality management is generally insured by spectral photometric methods according to ISO/DIS 15386 on a medium level of accuracy. Especially for high reflecting mirrors, a severe discrepancy in the determination of the absolute reflectivity can be found for spectral photometric procedures. In the first part of the CHOCLAB project, a method for measuring reflectance and transmittance with an enhanced precision was developed, which is described in ISO/WD 13697. In the second part of the CHOCLAB project, the evaluation and optimization for the presented method is scheduled. Within this framework international Round-Robin experiment is currently in progress. During this Round-Robin experiment, distinct deviations could be observed between the results of high precision measurement facilities of different partners. Based on the extended experiments, the inhomogeneity of the sample reflectivity was identified as one important origin for the deviation. Consequently, this inhomogeneity is also influencing the calibration procedure. Therefore, a method was developed that allows the calibration of the chopper blade using always the same position on the reference mirror. During the investigations, the homogeneity of several samples was characterized by a surface mapping procedure for 1064 nm. The measurement facility was extended to the additional wavelength 532 nm and a similar set-up was assembled at 10.6 μm. The high precision reflectivity procedure at the mentioned wavelengths is demonstrated for exemplary measurements.
Standard measurement procedures for the characterization of fs-laser optical components
Kai Starke, Detlev Ristau, Herbert Welling
Ultra-short pulse laser systems are considered as promising tools in the fields of precise micro-machining and medicine applications. In the course of the development of reliable table top laser systems, a rapid growth of ultra-short pulse applications could be observed during the recent years. The key for improving the performance of high power laser systems is the quality of the optical components concerning spectral characteristics, optical losses and the power handling capability. In the field of ultra-short pulses, standard measurement procedures in quality management have to be validated in respect to effects induced by the extremely high peak power densities. The present work, which is embedded in the EUREKA-project CHOCLAB II, is predominantly concentrated on measuring the multiple-pulse LIDT (ISO 11254-2) in the fs-regime. A measurement facility based on a Ti:Sapphire-CPA system was developed to investigate the damage behavior of optical components. The set-up was supplied with an improved pulse energy detector discriminating the influence of pulse-to-pulse energy fluctuations on the incidence of damage. Aditionally, a laser-calorimetric measurement facility determining the absorption (ISO 11551) utilizing a fs-Ti:Sapphire laser was accomplished. The investigation for different pulse durations between 130 fs and 1 ps revealed a drastic increase of absorption in titania coatings for ultra-short pulses.
Absorptance measurements for the DUV spectral range by laser calorimetry
The determination of absorptance in optical components is of crucial interest in respect to their power-handling capability. As a fundamental loss mechanism in the DUV/VUV spectral range, the absorptance is a significant parameter to decide the applicability of an optic in the industrial facilities. The established measuring technique of laser-calorimetry in accordance to the standard ISO 11551 was adapted to the wavelength of the ArF-laser (λ = 193 nm). The present paper describes the developed set-up considering the problems of calibration in the DUV spectral range. In detail, the electrical and optical calibration method were performed and different effects are discussed with regard to the practicability of both procedures. Furthermore, the reduction of scattered light in combination with a remarkable increase in sensitivity is demonstrated. As typical substrate materials for optical components at this laser wavelength, fused silica and CaF2 were investigated determining the absorptance. Simultaneously, the characteristic luminescence spectrum has been recorded during the investigations.
DUV/VUV spectrophotometry for high-precision spectral characterization
Holger Blaschke, Jürgen Kohlhaas, Puja Kadkhoda, et al.
The development of characterization tools for the deep-ultraviolet (DUV)/vacuum-ultraviolet (VUV) spectral range gains of increasing importance considering the applicability of optics in adequate facilities. At the Laser Zentrum Hannover, procedures for the investigation of optical parameters, i.e. transmittance, reflectance, absorption and scattering, are developed. In the last two years, a spectrophotometric unit was redesigned allowing a comprehensive characterization of optical components in the wavelength range between 115nm and 310nm. The paper describes the developed device in detail and discusses the sources of error with regard to their influence on measured data. Different investigations were performed and are presented in dependence on the wavelength, the adjusted angle of incidence (AOI) and the polarization of the incident beam. Furthermore, numerous measuring methods are explained which are supported by the in-house compiled software package.
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Characterization device for diode-laser-stack beam propagation
Markus Roehner, Holger Muentz, Olaf Schroeder, et al.
High beam quality can be achieved by accurate adjustment of the mechanical and micro-optical components in the manufacturing process of high power diode laser stacks. A charaterization device which can determine these parameters by automatically measuring the radiation properties of high-power diode-laser stacks has been developed. The result is a mechanically robust, easy to use characterization device of high reliability suited for applications in quality control and product optimization.
Mini-Symposium on Optics Characterization
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Characterizing CaF2 for VUV optical components: roughness, surface scatter, and bulk scatter
Methods for evaluating the quality of CaF2 substrates for vacuum ultraviolet (VUV) low loss optical components are presented. Today superpolished CaF2 is available. However, major differences might still occur between batches and careful control is therefore necessary. By using roughness data from AFM measurements combined with total scattering measurements at 193 nm and 157 nm surface roughness as well as inhomogeneities in the bulk of the material can be studied. Results are also presented of anti-reflective (AR) and highly reflective (HR) multilayer coatings on CaF2 where reduced total backscatter scattering was found for the AR-coating as compared to the substrate.
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Characterization of the near-field profile of semiconductor lasers and the spot size of tightly focused laser beams from far-field measurements
Characterization of the near field of typical semiconductor lasers and the spot size of tightly focused laser beams poses significant challenges to direct near-field profile measurement techniques. Far-field measurements are considerably easier to perform and offer an attractive alternative for this characterization. To assess this alternative, profiles of edge-emitting laser diodes and VCSELs, and the spot size of focused laser beams were determined from far-field and near-field measurements. In the far field, measurements were made using a 3D-scanning goniometric radiometer that provides irradiance profiles with angular extent to approximately ±70°. Indirect measures derived from these data using different methods are reported, including the spot size using the M2 times-diffraction-limited approximation, the Hankel transform Petermann II mode-field diameter used for optical fiber characterization, and a measure obtained from 2D Fourier transform inversion of the far field using phase retrieval. In the near field, direct profile measurements were made using a scanning-slit profiler and a CCD camera with magnifying lenses.
Mini-Symposium on Optics Characterization
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System for angle-resolved and total light scattering, transmittance, and reflectance measurements of optical components at 157 nm and 193 nm
Stefan Gliech, Henning Gessner, Angela Duparre
A system is presented that measures total and angle resolved light scattering, reflectance and transmittance at 193 nm and 157 nm. Substrates and coatings for VUV lithography components can be investigated with high sensitivity, down to scattering levels of 1 ppm.
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Single-shot laser beam profiling using multilevel imaging (Abstract Only)
Andrew M. Scott, Simon C. Woods, Paul Harrison
We describe a compact laser beam profiler that records multiple images of a laser beam at different positions along the beam waist in a single shot. This is used to determine the laser beam parameter M2. It is known that if light is incident on a curved grating such as a section of a Fresnel zone plate, then the +1 and -1 diffracted orders are focused or defocused. By combining two curved gratings and a conventional lens, it is possible to build a system in which a mosaic of nine images are created, corresponding to nine separate object planes. These nine images can be processed and fitted to an appropriate beam profile. This type of system measures beam quality in real time without any moving parts, and has the potential to provide real time monitoring of pulsed lasers, lasers with dynamic spatial behaviour, and in systems where beam handling optics may produce transient aberrations. Unlike other 'single shot' techniques, this technique makes no assumptions about beam propagation, and produces a value of M2 based only on measurements of beam waists.
Practical absolute wavelength meter using iodine-stabilized diode laser
Yoshiaki Akimoto, Lee Yong-Chol, Satoshi Hatano, et al.
A practical absolute wavelength meter was assembled separately a reference wavelength source and wavelength measurement system. The frequency (wavelength) stability was obtained to be 2×10-12 for a reference wavelength source. The other hand accuracy of wavelength measurement was estimated to be ** Michelson interferometer with vacuum chamber. The wavelength was estimated to be 1523.48813 nm for a 1523nm He-Ne laser as test laser. Moreover, the pressure dependence for wavelength was measured to be 0.0044 [pm/hPa]. Measurement reproducibility of the wavelength meter was guaranteed to be ±0.035pm.
Mini-Symposium on Optics Characterization
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Spectrophotometry in the vacuum UV
We report on our investigations of spectrophotometric measurements in the vacuum UV. A commercially available VUV setup at Fraunhofer IOF is used to illustrate spectrally and angle-resolved reflection and transmission measurements together with an in-situ sample irradiation by a F2-excimer laser.
Calcium fluoride for ArF laser lithography: characterization by in-situ transmission and LIF measurements
Christian Muehlig, Wolfgang Triebel, Gabriela Toepfer, et al.
An experimental setup was established for in situ transmission and laser induced fluorescence (LIF) measurements of CaF2 at 193 nm laser irradiation. The known rapid damage process in CaF2 upon ArF laser irradiation is shown to terminate for all tested samples within 3×104 laser pulses for the applied fluences. Furthermore, it is demonstrated that for typical application values the fluence dependent transmission (FDT) at the end of the rapid damage process is independent of the irradiation history and determined by the specific crystal quality. From the lifetimes and signal strengths of different present fluorescence bands the excitation and recording conditions for LIF investigations are derived. The results of laser induced fluorescence measurements at 193 nm excitation make evident that certain impurities or defects are responsible for the different transmission properties even of high purity CaF2 crystals. Comparing transmission and LIF data a quantitative correlation was found between selected emission bands and ArF laser stability of CaF2 material.
Fundamental Mechanisms
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Femtosecond pulse damage and predamage behavior of dielectric thin films
Mark Mero, Jianhua Liu, Ali Sabbah, et al.
The damage behavior of five different oxide dielectric thin films (Ta2O5, TiO2, Al2O3, HfO2, and SiO2) has been investigated with ultrashort laser pulses with durations from 25 fs to 1 ps. At all pulse durations the damage threshold is well defined and scales with the bandgap energy of the material. The damage behavior can be described with a phenomenological model taking into account multi-photon excitation, impact ionization, and electron relaxation. The temporal evolution of the dielectric constant of the film following the excitation with pulses below the damage threshold has been measured with time-resolved pump-probe spectroscopy. The complex dielectric constant was retrieved from transient reflection and transmission data.
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On-line control of laser beam quality by means of diffractive optical components
At LBOC6 meeting we presented an alternative approach for laser beam characterization, based on the decomposition of the electrical field distribution at certain cross section of the laser beam into a system of orthogonal functions. As such orthogonal function systems we selected "natural" laser eigenmodes of either GL or GH type. The looked for strength of the individual modal components then can be achieved by measuring the output signal of multi-channel correlation filters placed in a Fourier set-up, whereas the correlation filters themselves have been realized as DOEs by laser lithography. In between different systems of such GL and GH correlation filters have been designed, manufactured and experimentally tested with miscellaneous laser beams. Achieved results demonstrate a very good conformity between optical experiment and computer simulation. First attempts to compare results of our method with results of “standard” beam characterization methods (ISO11146) indicated principal conformity, but illustrated the continuing demand for a sophisticated adjustment procedure for the filter during application.