Denny Wernham

Laser-induced contamination on space optics

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.

Growth mechanisms for laser induced contamination on space optics in vacuum

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.

Investigation of laser induced deposit formation under space conditions

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.

Risk mitigation in spaceborne lasers

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 situ observation of UV-laser-induced deposit formation by fluorescence measurement

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.

Investigation of UV laser induced depositions on optics under space conditions in presence of outgassing materials

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.

Fluorescence monitoring of organic deposits

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-induced contamination control for high-power lasers in space-based LIDAR missions

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.

ESA space wind lidar mission: Approaching launch

ESA is launching a Doppler Wind lidar mission within its Earth Explorer Program. The objective of ADM-Aeolus is to provide tropospheric and lower stratospheric wind profiles globally for the improvement of weather forecasts on short and medium term. Spin-off products are profiles of atmospheric backscatter and extinctions coefficients and lidar ratio. The observations will also be used as input to air quality models and to verify climate model parameterization and predictability. In this way, ADM-Aeolus data is expected to greatly contribute to weather and air quality monitoring and to scientific advances in atmospheric dynamics. The Aeolus Mission is also a technology demonstrator, demonstrating the potential of an operational space-based Doppler Wind Lidar. After a long development phase the ADM-Aeolus mission is now gearing up for the last satellite level tests, preparing the mission for launch at the end of 2017.

An empirical investigation of the laser survivability curve: VIII-summary

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.