We have recently reported a method to improve the laser-induced damage threshold of UV laser optics by mitigating damage precursors (nano-sized absorbers) during the manufacturing of ion-beam sputtered, anti-reflective optical coatings. To further optimize parameters of the coating and particle mitigation schemes, we had to assess the damage resistance of laser optics with a low density of damage precursors. This was achieved using large area raster scans in which a highintensity laser beam was scanned over the optical surface. Via image processing of microscopic data, we then derived the density and size distribution of laser-induced damages for each raster scan. This advanced test method is described in this work and prepared the path to the manufacturing of improved laser optics that can be used to increase the performance and reliability of lasers in future space missions. Furthermore, we report on the comparison of standard and improved laser optics in tests of laser-induced contamination at 355 and 266 nm.
The ESA satellite Aeolus was successfully launched into space in August 2018 and measures global wind profiles using the Atmospheric Laser Doppler Instrument (ALADIN). ALADIN features a high-power UV laser source emitting nanosecond pulses at a wavelength of 355 nm. A crucial step in the development of ALADIN was the mitigation of laser-induced contamination (LIC). In this work we assess the opportunity of removing LIC deposits using UV/ozone cleaning with a mercury lamp. We find that UV/ozone cleaning is a very effective tool for removing laser-induced molecular contamination induced by the volatile components of a material mix representative of the ALADIN laser. Furthermore, we show that optical surfaces on which a contamination is removed via UV/ozone cleaning behave similar to pristine optical surfaces with respect to their susceptibility to subsequent LIC as well as laser-induced damage. These results demonstrate that UV/ozone cleaning is a useful and safe way of cleaning optical surfaces after ground-based thermal vacuum/lifetime testing.
As a consequence of the ongoing interest for deployment of laser systems into space, suitable optical components have to be developed and must be extensively space qualified to ensure reliable, continuous, and autonomous operation. The exposure to space environment can adversely affect the longevity of optics, mainly coatings, and lead to system degradation.
In this paper comprehensive investigations of laser induced deposit formation are reported. In a high vacuum chamber (p < 10-6 mbar) different space relevant materials containing epoxy, silicone and polyurethane based components were tested under space conditions. The experiments were performed with a pulsed Nd:YAG laser with peak fluences up to 2.5 J/cm2 at 355 nm wavelength and 3 ns pulse width. Additional tests were performed with an UV cw laser diode at 375 nm and 10 mW mean power. The onset and growth of the deposits was monitored in-situ and online by UV induced fluorescence imaging. The influence of roughness, temperature and chemical composition of the optical surface on the deposition process was investigated. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was used for chemical characterization of the deposits. Furthermore the influence of deposits on the UV-transmission of the optics was estimated.
We have investigated the formation of UV laser induced deposits on uncoated fused silica optics under simulated space conditions in presence of outgassing materials at 30°C and 100°C. We used a frequency tripled Nd:YAG laser with 355 nm wavelength, 3 ns pulse length and 100 Hz repetition rate. Optics were exposed to fluence values in the range of 0.5 – 1.0 J/cm2. As contamination samples epoxy, silicone and polyurethane containing materials were used. The depositions were monitored online and in-situ by measuring the fluorescence intensity distribution with CCD cameras, where the UV laser beam itself served as excitation source for fluorescence emission. This method allows for a very sensitive detection of the onset of deposit formation. Contaminant layers with a thickness down to 20 nm can be consistently detected. The influence of water on the formation of deposits was investigated. Time-of-flight secondary ion mass spectroscopy (ToFSIMS) was used for chemical characterization of the deposits.
L. Mondin, D. Wernham, F. Era, H. Kheyrandish, A. Bolkhovitinov, A. Baselga Mateo, A. Ciapponi, C. Heese, T. Ivanov, M. Hippler, D. Thibault, G. Van Papendrecht, W. Riede, Y. Butenko, H. Schröder
During the Aeolus laser and instrument transmitter development it was shown that atmosphere quality was one major limiting factor for high energy UV laser operation at ambient pressure. As already proven in literature operation can only be safely obtained in the presence of oxygen ([1] to [6]).
In this work tests for determination of ablation thresholds of various ceramic materials for pulsed laser irradiations at
wavelengths of 355 nm and 1064 nm in vacuum are presented. For comparison tests with copper and aluminium are also
reported. The ablation process was monitored insitu by long-distance microscopy. The morphology of ablation spots was
exsitu inspected by scanning electron microscopy. Furthermore, the redeposition of potentially released particles on
optics in the vicinity to the target was examined.
Laser-induced contamination (LIC) is still a major risk for space based laser systems. In this paper the mitigation of LIC by oxygen is investigated. Tests were performed with a pulsed laser at 355 nm. The partial pressure of the contamination material was in the range of 10-5 -10-4 mbar. The mitigation effect showed a threshold behavior concerning the ratio between contamination and oxygen pressure. Also a cleaning effect was successfully demonstrated: previously created depositions were completely removed by irradiation at several tens Pa oxygen pressure without any remaining degradation of the optical surface.
Contamination plays a major role in lifetime of vacuum optics. Several efforts have been made to derive suitable models
for lifetime prediction in laser-induced contamination related optical breakdown. But the broad spectrum of potential
contaminants present in the various applications with their very specific contamination mechanisms complicates the
derivation of universal optics degradation models.
As one possible contamination initiation process, the impact of optical absorption on the laser-induced contamination
and resulting optical breakdown is studied in this work. A set of specifically prepared samples using nanometer sized
gold particles embedded in dense IBS anti-reflecting coatings is exposed to radiation of 355nm in low pressure
naphthalene atmosphere. Even though the artificial defects are not in direct contact with the contaminant, their influence
on the long-term optics performance in dependence on the particle concentration in the coating is evident. In the
presence of naphthalene, the artificial nano-defects cause a significantly accelerated degradation compared to reference
samples without those defects or in absence of the contaminant. For this specific type of contaminant, a correlation of the
optical absorption and long-term durability is derived.
In this paper we will discuss the challenges of performing comparable laser damage testing as well as a detailed analysis
of the measurements conducted on the samples for this harmonisation activity. The goal of the activity is to enlarge the
test capacities within ESA’s EarthExplorer space program, especially for the missions ADM Aeolus [5] and EarthCare
[6], both having as main payload instruments containing high energy diode pumped nanosecond lasers. Four samples
have been compared with the S-on-1 method according to ISO21254-2, two AR1064/0° windows and two silicon wafers
leading to an agreement of better than 20% concerning the S-on-1 damage threshold.
Operating high power space-based laser systems in the visible and UV range is problematic due to laser-induced
contamination (LIC). In this paper LIC growth on high-reflective (HR) coated optics is investigated for UV irradiation of
355 nm with naphthalene as contamination material in the range of 10-5 mbar. The investigated HR optics were coated
by different processes: electron beam deposition (EBD), magnetron sputtering (MS) or ion beam sputtering (IBS). In-situ
observation of contamination induced damage was performed using a long distance microscope. Additionally the onset
and evolution of deposit formation and contamination induced damage of optical samples was observed by in-situ laserinduced
fluorescence and reflection monitoring. Ex-situ characterization of deposits and damage morphology was
performed by differential interference contrast and fluorescence microscopy.
It was found that contamination induced a drastic reduction of laser damage threshold compared to values obtained
without contamination. Contamination deposit and damage formation was strongest on IBS followed by MS and smallest
on EBD.
Within ESA’s ADM-Aeolus and EarthCARE missions Doppler-wind Lidar systems will be operated in the Earth’s orbit to measure global wind profiles. The active instrument will be based on a Nd:YAG laser, frequency tripled by nonlinear optical crystals. Different crystals are therefore to compare and qualify in regard of their space acceptability. A dedicated set-up to measure the maximum conversion efficiencies and its stability during longterm operation for KTP crystals (SHG) and BiBO crystals (SHG and THG) is presented in this work. In order to detect gray-tracking and its influence on thermal lensing in situ, measurements with a Shack-Hartmann sensor and a co-aligned HeNe laser were performed. Conversion efficiencies were 76±3 % at SHG for KTP and BiBO crystals and 48±2 % at THG with a combination of two BiBO crystals. During longterm experiments of 60 million laser pulses, conversion efficiencies were demonstrated to be stable over time (±1 % at SHG with KTP and ±2 % at THG with BiBO). The occurrence of gray-tracking was detected in the KTP crystal and the resulting thermal lensing with an exponential saturation over time was observed in situ.
In this paper some basic investigations about laser-induced contamination are reported. As contamination materials pure
aromatic hydrocarbons (naphthalene and anthracene) were used. A particular focus of the tests was to investigate the
impact of laser-induced contamination on damage threshold. Onset and evolution of deposit formation and damage were
observed in-situ by laser-induced fluorescence and transmission monitoring. As optical samples uncoated fused silica
substrates and AR and HR coated optics with different coating morphology, depending on coating process (e-beam,
magnetron sputtering) were investigated. Ex-situ characterization of deposits and damage morphology was performed by
differential interference contrast, fluorescence, and atomic force microscopy. The tests were run with pulsed UV light at
355 nm. Partial pressure of contamination material in the range of 10-4 mbar induced a drastic reduction of laser damage threshold compared to values obtained without contamination.
Operation of high fluence pulsed laser systems in space imposes various risks to optical components involved. Volatile
organic components are omnipresent in vacuum vessels housing space-borne laser systems and can be the source for
selective contamination of optics. Laser systems may respond very sensitively to absorption increases of their multiple
optical surfaces leading to inacceptable transmission losses and system degradation. In the recent past, thorough and
long term laser tests, performed at the optics qualification laboratories at DLR and at ESTEC using space relevant and
model substances, have revealed the onset, the built-up, and the later stages of the deposition process. It was found that
these deposits tend to accumulate preferably on the laser footprint area of the optic. Observed thicknesses are on the
order of several tens of nanometers, which can be sufficient to induce noticeable absorption. Sensitive techniques for insitu
and ex-situ monitoring of these molecular contaminative effects under vacuum conditions were developed and are
applied successfully. They are summarized in this paper, along with the phenomena, which are significant for the
appearance of deposits. In addition, adverse conditions, which are favorable for provoking deposits, are communicated.
Finally, mitigative and preventive methods are discussed.
Reliable, long term operation of high-power laser systems in the Earth orbit is not a straightforward task as the space
environment entails various risks for optical surfaces and bulk materials. The increased operational risk is, among others,
due to the presence of high energy radiation penetrating the metallic shielding of satellites and inducing absorption centers
in the bulk of optical components, and vacuum exposure which can deteriorate coating performance. Comprehensive testing
for analyzing high-energy radiation effects and mitigation procedures were performed on a set of frequency conversion
crystals and are discussed in this paper. In addition to a general resistance to space environmental effects, the frequency
conversion crystals were subjected to a comparative analysis on optimum third harmonic efficiency, starting from pulsed
1064 nm laser radiation, with the goal of exceeding a value of 30%. Concomitant modeling supported the selection of
crystal parameters and the definition of crystal dimensions.
As a consequence of the ongoing interest for deployment of laser systems into space, suitable optical components have to be
developed and must be extensively space qualified to ensure reliable, continuous, and autonomous operation. The exposure
to space environment can adversely affect the longevity of optics, mainly coatings, and lead to system degradation. An
increased operational risk is due to the air-vacuum effect, which can strongly reduce the laser damage resistance of optical
coatings. For this purpose, a vacuum laser damage test bench has been developed and is operated at DLR. In extensive test
campaigns, all damage-prone optics of the ALADIN laser system (being the laser source of the upcoming ESA ADM
Aeolus mission) were tested under operative conditions at the fundamental and at the harmonic wavelengths of Nd:YAG.
Further operational risks are due directly to operation under high vacuum. In the past, several space-based laser missions
have suffered from anomalous performance loss or even failure after short operation times. This degradation is due to
selective contamination of laser-exposed optical surfaces fed by outgassing constituents. These volatile components are
omnipresent in vacuum vessels. Various organic and inorganic species were tested at our facilities for their criticality on
deposit built-up. Finally, active optical components like Q-switch crystals or frequency converter crystals can also suffer
from bulk absorption induced by high-energy radiation (gray tracking) and dehydration. To analyze these effects, an ultrahigh
vacuum phase matching unit was set up to test various combinations of SHG and THG frequency converters.
We have investigated the growth mechanisms for laser induced contamination of space optics in vacuum, particularly
during the early stages of the deposit formation. Experiments have been performed in vacuum to study the influence of
the environmental conditions and the condition of the optical surface, using a variety of physical and chemical
techniques. In particular, different methods of conditioning the surface prior to irradiation and cleaning the surface after
irradiation have been tested.
In this paper, we present the continued joint effort of ESA/ESTEC and DLR laser laboratories of improving the fluorescence monitoring technique towards a quantitative means for analysis of UV laser-induced deposit formation on optical samples in vacuum. In addition, a separate low power UV fluorescence excitation light source was implemented into the system allowing the investigation of laser-induced deposition occurring during irradiation of optics with IR and VIS light beams.
We investigated the formation of UV laser induced deposits on uncoated and coated fused silica optics under vacuum
conditions in presence of outgassing materials. As contamination samples epoxy, silicone and polyurethane containing
materials were used. To realize low partial pressures of the contaminants in the gas phase they were slightly heated
(40°C). The formation of the depositions was monitored in situ and online by detecting the fluorescence emission of
the deposits, excited by the UV laser beam. The influence of different optical coatings on the deposit formation was
studied. By analysing the surface profiles of the deposits, growth rates were estimated. Time-of-flight secondary ion
mass spectroscopy was used for chemical characterization of the deposits.
It is well known that optical dielectric coatings show a change in performance when altering the environmental
condition from air to vacuum. Evacuating or venting a set-up will shift the spectral characteristic and also the damage
behavior of the specimen. With respect to the spectral shift it has been observed that dense dielectric coatings
manufactured by Ion Assisted Deposition and Ion Beam Sputtering do not show this modification.
This work was performed to investigate AR coatings of different deposition processes to determine whether the LIDT of
dense layers can also be kept stable in vacuum. It was found that the damage threshold of these dense coatings is also
stable in an evacuated environment.
Spaceborne lidars carry much promise for Earth observation and interplanetary missions to measure atmospheric parameters (wind velocity, optical extinction or species concentrations) and planet topologies. As the first European lidar mission, the European Space Agency is developing a Doppler wind lidar, ALADIN, to be launched on board ADM-Aeolus in 2008. ALADIN is a pulsed laser, emitting about 120 mJ of pulse energy in the UV. The mission duration is envisaged to be three years, which corresponds to several billion emitted pulses, thus imposing very stringent criteria on the longevity of the system. Laser-induced damage is one of the most significant issues here, in particular since laser-induced damage in space vacuum is still poorly understood. The European Space Agency has therefore established a test campaign to measure the power handling of all the instrument optics with laboratories in Germany, Italy, the Netherlands, the United Kingdom and France participating. Measurements are conducted at three wavelengths (1064nm, 532nm and 355nm) and with the introduction of several contaminants. The presentation covers laser-induced damage risk mitigation, the ESA test campaign and some test results.
We investigated laser-induced deposition processes on BK7 substrates under the influence of pulsed Q-switched Nd:YAG laser radiation, starting from small toluene partial pressures in a background vacuum environment. The composition and structure of the deposit was analyzed using microscopic methods like Nomarski DIC, dark-field and white-light interference microscopy, TEM, EDX and XPS. We found a distinct threshold for deposition built-up dependant on the partial pressure of toluene (0.2 J/cm2 at 0.1 mbar, 0.8 J/cm2 at 0.01 mbar toluene). The deposits strictly followed the spherical geometry of the laser spot. No deposit accumulated on MgF2 AR coated BK7 samples even at high toluene partial pressures. The onset of deposit was accompanied by periodic surface ripples formation. EDX and XPS analysis showed a carbon-like layer which strongly absorbed the 1 μm laser radiation. The typical number of shots applied was 50 000. In addition, long term lifetime tests of more than 5 Mio. shots per site were run.
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