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.
At the German Aerospace Center in Stuttgart a laser damage test bench is run to evaluate damage thresholds of various optical components. The system setup is based on the current ISO standards 11254 [1-3] for single shot and multiple pulse operation. The laser damage test bench contains two repetitively pulsed laser sources, a Ti:Sapphire and a Nd:YAG laser, operating at wavelengths of 775 nm and 1064 nm, respectively. Harmonic wavelength converters to the visible spectral range are available. Both lasers are supplying the same damage testing rig. Online damage assessment techniques like sensitive scatter probe monitoring and video microscopy monitoring are used. The system is suited and has been tested extensively in the past for dielectric coated optics like beam turning mirrors, reflectors and windows, nonlinear optical components, semiconductors, and laser crystals. The damage test bench is located in a class 10,000 cleanroom environment under a laminar flowbox providing an additional isolation factor of >10<sup>3</sup>. The tests can also be performed in sealed optical compartments in partial vacuum and under long term irradiation conditions. All experiments are supported by theoretical simulation of laser-material interactions, down to the sub-ps timescale .
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.
The Aladin instrument will fly on the European Space Agency’s ADM Aeolus satellite. The instrument is a Doppler wind
LIDAR, primarily designed to measure global wind profiles to improve the accuracy of numerical weather prediction models.
At the heart of the instrument is a frequency stabilized 355nm laser which will emit approximately 100mJ of energy in the
form of 20ns pulses with a fluence around 1Jcm<sup>-2</sup>. The pulse repetition frequency is 50Hz meaning that Aladin will eventually
have to accumulate 5Gshots over its 3 years planned lifetime in orbit. Due to anomalies that have occurred on previous spaceborne
lasers, as well as a number of failures that we have observed in previous tests, an extensive development and verification
campaign was undertaken in order to ensure that the Aladin instrument is robust enough to survive the mission. In this paper,
we shall report the logic and the results of this verification campaign.
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  and EarthCare
, 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.
In this paper, we report on a continuing multi-year empirical investigation into the nature of the laser
survivability curve. The laser survivability curve is the onset threshold as a function of shot number. This
empirical investigation is motivated by the desire to design a universal procedure for the measurement
of the so-called S on 1 damage threshold. In this year’s paper we investigate the usefulness of scaling
the fluence with shot number. First the scaling process is defined and applied to a result from our
experimental archives. The probability of damage curve for a single shot test is extrapolated to 10<sup>4</sup>
shots. The scaled result is shown to be very close the observed results providing a basis for extrapolation
to very large values of n.
This paper presents a first look at the application of maximum likelihood estimation methods to S on 1
testing by comparing results with an analysis that is typical of our previous reports and consistent with
ISO 21254. In traditional, ISO tests, the data collected from an S on 1 test is processed to give a set of
fluences representing the no-damage or safe operating fluence (SOF) as a function of the number of
shots. The (SOF,N) ordered pairs are then fitted to a model and the model is used to extrapolate the SOF
to large values of N. In the present report, the entire data set from an ISO S on 1 test is processed via
maximum likelihood methods to estimate the probability curve as a function of fluence, P(Φ). The
probability of survival to N shots is calculated, under the assumption that P is independent of N, to give
the final results. The maximum likelihood method shows promise for application to S on 1 testing.
In the determination of the laser-induced damage threshold (LIDT) of optical coatings former Round-Robin experiments
stand as the empirical foundation for the development of the International Standard as it is known today. In 1983 and
1997 such experiments were conducted at the fundamental wavelength of the Nd:YAG laser under atmospheric
conditions settling the International Standard as it is known today.
To cope with the growing demand of LIDT testing for satellite missions, existing test methods have to be extended to
deal with operation in space-like environments. This requires LIDT measurements performed under customized vacuum
conditions to validate the laser resistance capability and estimate the life time of optical components. To foster the
quality of measurements in such environments the need for an inter-laboratory comparison in vacuum conditions
To cover the preparatory test issues of upcoming ESA space laser missions, in joined effort amongst various
laboratories, an adaptation of existing laser damage test benches has been performed. Conventional S-on-1 tests were
extended with raster scanning procedures. Various aspects of characteristic damage curve issues are discussed.
Sensitive surface analysis like time-of-flight SIMS is used to identify potential low density low damage threshold
precursors. The inter-correlation of flight module testing and preceding single component testing is demonstrated.
Finally, the successful execution of a flight module endurance test with more than 200 Mio. shots is detailed.
In this paper, we give the third installment of our ongoing investigation into the nature of the laser
survivability curve (LSC). In this year’s report, We examine a set of identically polished samples coated with
the same AR coating design. One set coated using IAD process and the other e-beam. In the samples
investigated show similar asymptotic behavior within manufacturing methods, but each technique behaves
In this paper, we report on the second installment of our ongoing investigation into the nature of the
laser survivability curve (LSC). The LSC has been traditionally viewed as a curve in the plane defined by
fluence,φ , and the number of shots, N, which defines the frontier of assured survival. In this year's
report we expand the concept of the survivability curve to a surface of survival, the laser survival surface
(LSS), which is in turn used to develop a conditional probability estimate for survival. This conditional
probability viewpoint is discussed as a possible basis for a
cost-efficient life time test. The LSS is
developed for test results at 1064 nm wavelength taken at atmospheric pressure and at vacuum.
In this paper, we report on the first steps in an empirical investigation into the nature of the laser survivability curve.
The laser survivability curve is the onset threshold as a function of shot number. This empirical investigation is
motivated by the desire to design a universal procedure for the measurement of the so-called S on 1 damage
threshold. Analysis is carried on the test results for first results from a large set of planned measurements from
identical samples produced for this investigation. The sample set and test conditions are discussed. A pair of
measurements, one taken at atmospheric pressure and one at vacuum are introduced and analyzed as an example.
Interim observations on the nature of the laser survivability curve, and its determination to be used in the remainder
of this investigation based on this initial look, are presented at the conclusion of this paper.
In this paper, we present test results and involved procedures of a comprehensive test campaign for S on 1 testing of laser
optics with large test areas allowing the generation of a profound test database for further analysis. This database will serve
as a starting point for an empirical study of the lifetime of laser optics, which will be discussed in companion paper
somewhere in these proceedings.
The optics are designed to operate as anti-reflective or high-reflective components at the respective test wavelengths for 0° angle-of-incidence. Both, coatings and substrates of 2.0 inch diameter are produced from the same batches to be as identical
as possible. There were two different coating technologies used, e-beam and IAD e-beam, to explore a possible effect of the
coating process on the long term laser irradiation behavior.
The laser damage test bench is operated with a laser source delivering laser pulses in a single longitudinal mode at a
repetition frequency of 100 Hz. The beam profile is of a
Gaussian-shape and of high spatial quality at the fundamental
Nd:YAG laser wavelength with a pulse duration of 3.5 ns at 1064 nm. Typical beam diameters on the samples were 400
μm, and usually more than 500 test sites are irradiated in one test to achieve statistical significance. The laser test procedure
itself is adapted from the ISO standard 11254-2 for multiple pulse irradiations, and the LIDT evaluation is done
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 report on a new experimental technique for monitoring laser-induced shock waves and thermal waves above the
sample surface called total internal reflection based photothermal or photoacoustic deflection (TIR based PTD/PAD
deflection). It is based on the changes in transmissivity of a prism which is operated near the condition of total internal
reflection for a HeNe laser beam propagating parallel to the sample surface at a small distance. The HeNe laser beam is
probing photoacoustic or photothermal waves originating from a sample surface due to interaction with a pulsed
Nd:YAG laser beam. The method is compared with standard online detection techniques like scatter probe monitoring
and plasma detection, and found to be a very sensitive and practical tool. It also showed its suitability for selectively
monitoring several surfaces (e. g. front and rear surface) of optical components, and attributing the damage starting
point. Therefore, the method might be used for monitoring of surface damage on laser crystals or valuable components.
Keywords: photothermal deflection, photoacoustic deflection, laser damage, total internal reflection.
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.
As a consequence of the ongoing interest for deployment of laser systems into space, optical coatings have to be
developed which allow for reliable long term operation under vacuum conditions. Extensive laser damage tests for
space qualification of laser optics have recently been performed at the DLR and LZH laser damage test facilities in the
IR, VIS, and UV spectral range within the ESA-ALADIN (Atmospheric Laser Doppler Instrument) test campaign.
These tests have consistently revealed the degradation of the LIDT values for e-beam evaporated dielectric coatings
under vacuum environment, which occurred independently of wavelength and type of coating (HR or AR) and other
parameters. Dense coatings like IAD-based coatings, on the other hand, did not show this effect. Water desorption and
diffusion processes seem to mediate the degradation under vacuum exposure.
The ISO 11254 standard for LIDT tests suggests two possible spatial beam profiles for damage testing. Accordingly, an equal set of samples was tested with a Gaussian TEM<sub>00</sub> as well as with a top-hat beam profile at different beam diameters. It was found that for the investigated HfO<sub>2</sub>/SiO<sub>2</sub> high reflectors there was no threshold dependence on the beam diameter at 355nm. The damage threshold values measured with the Gaussian and the top-hat beam were in good correlation.
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.
Laser optics being used in space laser systems are usually exposed to high vacuum conditions under the absence of air or
oxygen. In the past, several space-based laser missions have suffered from anomalous performance loss or even failure
after short operation times. To mitigate the risks involved with long-term operational conditions, a laser damage test
bench has been developed and is operated at the German Aerospace Center (DLR) to test laser optics in the IR, VIS, and
in the UV spectral range.
The testing is performed under application oriented conditions, i.e. under high-vacuum using dry pump systems. The
main goal of the test campaign is to identify the critical components in terms of their laser damage threshold for very
high pulse numbers applied per site. Characteristic damage curves according to ISO 11254 are evaluated for each
component under investigation for up to 10 000 shots per site. The characteristic damage curves are used for the
estimation of the performance at very high pulse numbers.
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.
This work summarizes the results from an extensive test campaign in which space-based laser optics were qualified for the upcoming ESA ADM-Aeolus mission. 14 different types of optical components from different suppliers were tested at the Nd:YAG laser wavelength according to the ISO standard 11 254 - 2 for multiple pulse testing. A new technique based on transient pressure sensing was developed to monitor the occurrence of damage on a sample surface exposed to a vacuum environment. Parallel testing of reference samples showed a distinct degradation under vacuum compared to atmospheric or pressurized environment. For all samples tested we found a typical behavior in the characteristic damage curves attained: A sharp drop in LIDT for small pulse numbers followed by a smooth decrease for larger pulse numbers (laser fatigue effect).
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/cm<sup>2</sup> at 0.1 mbar, 0.8 J/cm<sup>2</sup> at 0.01 mbar toluene). The deposits strictly followed the spherical geometry of the laser spot. No deposit accumulated on MgF<sub>2</sub> 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.
For the development of standard measurement procedures in optics characterization, comparative measurement campaigns (Round-robin experiments) are indispensable. Within the framework of the CHOCLAB project in the mid-90s, several international Round-robins were
successfully performed qualifying procedures for e. g. 1 on 1-LIDT, laser-calorimetry and total scattering. During the recent years, the demand for single pulse damage investigations has been overtaken by the more practically relevant S on 1-LIDT. In contrast to the
industrial needs, the comparability of the multiple-pulse LIDT has not been proven by Round-robin experiments up to now. As a consequence of the current research activities on the interaction of ultra-short pulses with matter as well as industrial applications, numerous fs-laser systems become available in universities and research institutes. Furthermore, special problems for damage testing may be expected because of the intrinsic effects connected with the interaction of ultrashort pulses with optical materials. Therefore, a Round-robin experiment on S on 1-damage testing
utilizing fs-pulses was conducted within the framework of the EUREKA-project CHOCLAB II. For this experiment, seven parties investigated different types of mirrors and windows. Most of the partners were guided by the International Standard ISO 11254-2, but one partner employed his own damage testing technique. In this presentation, the results of this comparative experiment are compiled demonstrating the problems induced by special effects of damage testing in the ultra-short pulse regime.
An investigation of ultra short pulse damage threshold of AR coated beta-barium borate (BBO) used for SHG or as Q-switching material has been carried out. As a laser source, a CPA Ti:Sapphire laser system operating at a wavelength of 775 nm tuned to various pulse durations (150 fs, 1 ps, 3 ps) has been used. The online damage detection system is based on a scatter probe unit. It is capable of counting the number of pulses prior to damage and logging of the damage level, and allows for the interruption of irradiation immediately after damage. For S-on-1 irradiation and pulse numbers of up to 10<sup>4</sup> per site, LIDTs of 0.3, 0.8 and 1.9 J/cm<sup>2</sup> were detected for the given pulse durations. Characteristic damage curves were measured according to the ISO standard 11254 2.0. Coating delamination was observed as the damage morphology.
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.