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.
The European Space Agency is developing its first spaceborne LIDAR for global monitoring of wind velocities. ALADIN, to be launched on board ADMAeolus in 2008, is a pulsed Nd:YAG laser with about 120 mJ of pulse energy at 355 nm and a repetition rate of 100 Hz during bursts. Within the projected mission duration of three years, this gives a lifetime requirement of close to 5 billion pulses.
While laser-induced damage thresholds of optics in vacuum (possibly contaminated by small amounts of organic compounds) can differ from atmospheric conditions, their damage behaviour is generally poorly understood. The European Space Agency has therefore established a test campaign to measure the power handling of all the instrument optics with several European laboratories participating.
In the Optics and Opto-Electronics laboratory at ESTEC, a laser-induced damage threshold (LIDT) test facility has been set up with a 50 Hz Nd:YAG test laser. The pulse energy is 700 mJ at 1064 nm. This allows us to recreate the laser pulse conditions to which the ALADIN optics will be exposed. The flattop beam profile of the test laser irradiates the optics with uniform fluences and relatively large spots (up to 1mm across) at damaging intensities.
Damage tests are performed with up to 1 million pulses per test spot according to the S-on-1 test ISO-11254 standard, requiring typically 10 days to test one sample. With such extended tests, we can predict the laser-induced damage threshold over the ALADIN lifetime with improved accuracy.
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.
In the framework of the ADM-Aeolus satellite mission, successful test campaigns have been performed in ESTEC’s laser laboratory, and the efficiency of several mitigation techniques against Laser-Induced Contamination (LIC) have been demonstrated for the ALADIN laser. These techniques include the standard contamination control methods of materials identification with particular tendency to cause LIC, reduction of the outgassing of organic materials by vacuum bake-out and shielding of optical surfaces from the contamination sources. Also novel mitigation methods such as in-situ cleaning via partial pressures, or the usage of molecular absorbers were demonstrated. In this context, a number of highly sensitive optical measurement techniques have been developed and tested to detect and monitor LIC deposits at nanometre level.
This paper summarizes our results of S-on-1 testing carried out over the last few years. Our experimental data sets were
taken with nanosecond laser pulse durations. An attempt was made to use the same scaling laws with femtosecond pulse
widths but it was not successful. The conclusion was made: there is no single model than can universally applied to all
kinds of survivability curves. We present this summary with a particular goal of making recommendations to those
involved in the periodic review of ISO 21254. A preliminary review of models, describing damage threshold evolution
with respect to incident laser pulses, is made.
The European Space Agency is developing a direct detection Doppler Wind Lidar for measuring wind profiles from
space. The main objective of Aeolus is to provide tropospheric and lower stratospheric wind profiles globally for the
improvement of weather forecast on short and medium term. Aeolus data are expected to greatly contribute to weather
and air quality monitoring and to scientific advances in atmospheric dynamics. The UV Lidar instrument, ALADIN, will
deliver horizontally-projected single line-of-sight wind profiles from the Doppler shift of molecular and particle
backscatter. The development of the AEOLUS mission passed a major milestone with the integration of the full
instrument and its functional and performance tests in 2016 and a 6-month life test of the spare UV laser transmitter. The
satellite has been assembled and has successfully been subjected to a programme of functional and environmental
(vibration, acoustic, shock, EMC) tests. The preparation of thermal vacuum testing, including instrument performance in
vacuum, is close to completion.
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 ( to ).
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-2. 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 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 104
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.
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.
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.
The environment in space is a particularly harsh one for optical coatings. For porous coatings, space vacuum causes
a spectral shift and a resulting change in stress due to water release. Atomic oxygen present in Low Earth Orbits
causes erosion of coatings. Space also has a harsh radiation environment which can cause absorptive losses in
optics due to colour centre activation. Coatings exposed to solar radiation are subject to UV fixation of outgassing
contaminants. Similarly, high power laser irradiation of coatings in the presence of contaminant outgassing sources
results in laser-induced contamination, high absorption and potential laser damage. An important effect for high
power laser optics is the reduction of the laser-induced damage thresholds of porous coatings in vacuum. An
additional factor is the often high thermal excursion coatings can experience in space, typically ranging from -50°C
to +80°C, notwithstanding deep space missions which involve cryogenic temperatures where coatings which have
to withstand -270°C and coatings to the inner planets which may have to survive temperatures in excess of 300°C.
This paper attempts to give a general overview of the effects of the space environment on optical coatings giving
some examples from tests carried out by the European Space Agency.
Laser-induced contamination (LIC) is a phenomenon that can lead to the degradation of the properties of optical
components in vacuum due to the formation of deposits in the area irradiated by a laser beam. The deposit growth is
proposed to be the result of photochemical and photothermal mechanisms triggered by the interaction of UV laser
radiation and outgassing species from polymeric materials on the surface of the optics. In the framework of ESA's ADM-Aeolus
satellite mission, a successful test campaign has been performed, which has demonstrated the efficiency of
several mitigation techniques against LIC for the ALADIN laser. These include the standard contamination control
methods of identification of materials with particular propensity to cause LIC, reduction of the outgassing of organic
materials by vacuum bakeout and shielding of optical surfaces from contamination sources as well as novel methods
such as in-situ cleaning. These methods are now being applied at satellite level in order to guarantee the success of the
mission. The subject of this paper is to summarise the various mitigation techniques from the large number of studies
that have been performed and is applicable to any use of high power pulsed lasers in vacuum in the presence of organic
Photon-induced contamination of optical surfaces is a major obstacle for space-bound laser applications. At
Laser-Laboratorium Göttingen, a setup was developed that allows monitoring transmission, reflection and fluorescence of
optical components under well-controlled vacuum conditions, in order to assess their possible optical
degradation due to radiation-induced contaminant deposition in orbit. In cooperation with the European Space Agency
ESA optical elements for the ADM-Aelolus mission were investigated. In order to perform global wind-profile
observation based on Doppler-LIDAR, the satellite ADM-Aelolus will be launched in 2011 and injected into an orbit 400
km above Earth's surface. ADM-Aeolus will be the first satellite ever that is equipped with a UV-laser (emitting at a
wavelength of 355 nm) and a reflector telescope.
For both high-reflecting mirrors and an anti-reflective coated windows long-term irradiation tests (up to 500 million laser
pulses per test run) were performed at a base pressure < 10-9 mbar, using a XeF excimer laser (λ=351 nm, repetition rate
1kHz). At this, samples of polymers used inside the satellite (insulators for cabling, adhesives, etc.) were installed into
the chamber, and the interaction of their degassing with the sample surfaces under laser irradiation was investigated.
Optical degradation associated with contaminant adsorption was detected on the irradiated sample sites as a function of
various parameters, including pulse repetition rate, view factor and coating material
The European Space Agency ESA is running a series of earth observation missions. In order to perform global windprofile
observation based on Doppler-LIDAR, the satellite ADM-Aelolus will be launched in April 2011 and injected
into an orbit 400 km above Earth's surface. ADM-Aeolus will be the first satellite ever that is equipped with a UV-laser
(emitting at 355 nm) and a reflector telescope.
At LLG, a setup was developed that allows monitoring transmission, reflection and fluorescence of laser-irradiated
optical components, in order to assess their possible optical degradation due to radiation-induced contaminant deposition
in orbit. For both a high-reflecting mirror and an anti-reflective coated window long-term irradiation tests (up to 500
million laser pulses) were performed at a base pressure < 10-9 mbar, using a XeF excimer laser (wavelength 351 nm,
repetition rate 1kHz). At this, samples of polymers used inside the satellite (insulators for cabling, adhesives, etc.) were
installed into the chamber, and the interaction of their degassing with the sample surfaces under laser irradiation was
investigated. Various paramters were varied including pulse repetition rate, view factor and coatings. Optical degradation
associated with contaminant adsorption was detected on the irradiated sample sites.
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.
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.
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.
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.
In the context of lifetime of optics in laser systems in space, spot size dependencies of laser induced damage thresholds have been investigated. The measurements were performed with two different Nd:YAG laser systems at 1064 nm with pulse durations of 8 and 50 ns and repetitition rates of 10Hz and 3kHz, respectively. The effective beam diameter was varied in the range of a few microns up to some tens of millimetres using several focussing optics. In preparation of the experimental analysis, simulations have been performed to determine the difference of the linear evaluation algorithm and a more sophisticated theoretical description of the damage threshold. Correspondence of simulated and experimental results should reveal information concerning the applicability of small spot sizes to standardized damage tests.
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/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.