Adrian Tighe

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.

Preliminary Flight Data From the Materials Exposure and Degradation experiment (MEDET)

The Materials Exposure and Degradation Experiment (MEDET) was recently launched to the ISS on Space Shuttle Flight IE, as part of the EuTEF payload on the external payload facility of ESA’s Columbus module. The experiment will operate in‐orbit for at least 1.5 years, and has the overall objectives of evaluating the effects of the complex low Earth orbit space environment on material properties, investigating material degradation due to contamination, characterising the local ISS environment and measuring the local micro‐particle flux. This paper gives a brief overview of the experiment function and the material samples which are being exposed, before presenting some of the early flight data. In this phase of the mission, all of the instruments are operating successfully, and continuously acquiring data. The preliminary results mainly concern the environmental sensors, which are operating at relatively high acquisition rates (e.g. one reading every few seconds). It has been shown that the docking of the Sp...

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.

A new ECSS standard for environmental durability testing of optical coatings for space application

Many different environmental factors can have an effect on optical coating durability for space applications. This includes in-orbit effects such as vacuum exposure, UV radiation, particle radiation, atomic oxygen, thermal cycling, contamination and orbital debris, as well as ground based effects such as cleaning, contamination and humidity [1].

Characterization of Novel Lightweight Radiation Shielding Materials for Space Applications

Novel materials or multilayers can help to reduce the mass requirement for radiation shielding of electronic components significantly. In this paper, potential alternatives to the standard aluminum shielding approach are assessed by the Monte-Carlo simulations and promising candidates are manufactured and characterized by radiation tests including proton and electron tests. The transmission of energetic protons of up to 39 MeV through the shielding solution was assessed as well as the dose deposited by energetic electrons up to 12 MeV in Radiation sensing field-effect transistor (RADFETs) and Alanine dosimeters behind the shield.

Characterization of novel lightweight radiation shielding materials for space applications

Novel materials or multilayers can help to reduce the mass requirement for radiation shielding of electronic components significantly. In this study, potential alternatives to the standard aluminum shielding approach are assessed by Monte Carlo simulations and promising candidates are manufactured and characterized by radiation tests including proton and electron tests. The transmission of energetic protons of up to 39 MeV through the shielding solution was assessed as well as the dose deposited by energetic electrons up to 12 MeV in RADFETs and Alanine dosimeters behind the shield.

Post-flight Analysis of Materials Exposed on the Spectrometer Sub-unit of MEDET (18 Months On-Board ISS)

A variety of materials were exposed to the Low Earth Orbit (LEO) environment, mainly Ultraviolet Radiation and Atomic Oxygen, through The Material Exposure and Degradation Experiment (MEDET). The degradations of polymeric films with and without atomic oxygen protection coating, quartz with and without thermal control coating, and solar cell cover glass adhesives, were studied in flight and post-flight. Yellowing for AO protected polymeric films and silicone resins due to UV exposure and the decrease of thicknesses for other samples like polymeric films due to AO erosion have been highlighted. Thermo-optical properties have been determined and the surface morphology classified as cone-like due to surface erosion or containing microcracks in silicone layers.

Long-term laser induced contamination tests of optical elements under vacuum at 351nm

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 laser-irradiated 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 Earths 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

Design and Testing of a Mini-spectrometer System for On-Orbit Degradation Studies of Optical Materials

The degradation of optical materials exposed to the near earth orbit space environment is of primary interest for spacecraft designers and must be accurately predicted. Optical materials generally have highly polished and accurately formed surfaces so that even low levels of degradation may have a significant effect on the long-term performance of the component. This paper describes the design and testing of a miniature spectrometer system which will be used to actively monitor the on-orbit degradation of optical materials placed on the exterior of the International Space Station.