To mitigate risks due to laser-induced contamination (LIC) for the LISA space mission, we have carried out an extensive LIC test campaign, including a series of short duration tests with different test parameters, as well as a long-duration test. Those previous experimental results as well as theoretical considerations indicate that LIC might be less of a concern for the LISA mission. A remaining concern is whether LIC could occur in the presence of metallic particles on optical surfaces and whether a higher pressure does have an impact. Our ongoing research thus aims at testing for a possible deposit formation in a combined LIC and metallic particulate contamination test. Therefore, a HR optics is contaminated with metallic (aluminum) particles, mounted in the sample holder and tested within a similar test setup used for previous tests. The test is performed at a pressure similar to the actual expected pressure of 10-5 mbar at the optical bench during the mission (previous tests at 10-8 mbar). These tests do not indicate that LIC is a concern and metallic particulate contaminants seem not to accelerate or trigger LIC in this laser regime.
We present the European development of an engineering model Laser Head for LISA. This single box includes a seed laser, an electro-optical phase modulator, a fiber amplifier and all PCBs to operate the Laser Head.
Within the DLR project COMPASSO, optical clock and link technologies will be evaluated in space on the Bartolomeo platform attached to the Columbus module of the ISS. The system utilizes two iodine-based frequency references, a frequency comb, an optical laser communication and ranging terminal and a GNSS disciplined microwave reference. While COMPASSO is specifically dedicated to test optical technologies relevant for future satellite navigation (i.e. Galileo), the technologies are also crucial for future missions related to Earth observation and science. The optical frequency reference is based on modulation transfer spectroscopy (MTS) of molecular iodine near a wavelength of 532 nm. An extended cavity diode laser (ECDL) at a wavelength of 1064 nm is used as light source, together with fiber-optical components for beam preparation and manipulation. The laser light is frequency-doubled and sent to a mechanically and thermally highly stable free-beam spectroscopy board which includes a 20 cm long iodine cell in four-pass configuration. The iodine reference development is lead by the DLR-Institute of Quantum Technologies and includes further DLR institutes, space industry and research institutions. Phase B of the project will be finalized soon and an Engineering Model of the iodine reference, which represents the flight models in form, fit and function, will be realized by mid 2023. The launch of the COMPASSO payload is planned for 2025. Additional presentation content can be accessed on the supplemental content page.
We investigate effects of laser-induced contamination (LIC) with contaminant materials and laser parameters relevant for the LISA space mission. To accelerate outgassing and a possible deposit formation, the contaminant materials have been heated to a temperature of up to 100 °C (nominal operating temperature of 20° C), and LIC tests were performed with a laser power density of up to 300 W/cm2 (to be compared with expected 125 W/cm2). Neither in-situ measurements (laser transmission, polarization and wavefront), nor a careful microscopic inspection of the optical surfaces after the LIC tests showed indications of a laser-induced deposit formation. Condensation on optical surfaces could be observed. This is cautiously encouraging for LISA and indicates that the LIC concern may be reduced compared to what observed at shorter wavelength or with pulsed laser radiation.
Additional presentation content can be accessed on the supplemental content page.
This paper presents the design and breadboarding of the proof of concept demonstrator for the so called retro-reflector interferometer scheme in off-axis configuration for the ‘Next Generation Gravity Mission’ (NGGM) studied at the European Space Agency (ESA). This configuration can offer benefits in terms of overall satellite configuration compared to the transponder scheme, which is currently flying on board of GRACE-FO. However, it relies on very low received laser signal levels due to the fact that the laser light is travelling about 100 km from the master satellite to the remote satellite and is reflected back to the master satellite by a retro-reflector. In comparison to the transponder scheme, where the signal is amplified on the remote satellite using a laser, which is optically phase locked to the laser signal of the master spacecraft, this reflection does not amplify the signal. Thus, even with higher emitted laser power, instead of some nanowatt, only a few picowatt are available on the according science detector. Therefore, less than a femtowatt of straylight within the detectable heterodyne frequency and angular range is allowed on the detector to fulfil the ranging noise requirement. The paper gives insights into the main opto-mechanical design topics of the Optical Bench Assembly (OBA). It includes the optical analysis results as well as mechanical design to suppress straylight below the required limit. The optomechanical design of the OBA is complemented by the opto-mechanical design of the test setup and by the electro-optical design of the phase read-out chain. Finally, preliminary results from the test campaign are presented.
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