Performance of the ultra-violet laser transmitter during ESA's Doppler wind lidar mission Aeolus

Abstract

The European Space Agency's Aeolus mission was a groundbreaking achievement in Earth observation and space laser technology. Over its nearly five-year lifetime, the space-borne Doppler wind lidar instrument onboard Aeolus utilized two redundant ultraviolet (UV) lasers to measure atmospheric wind profiles globally, significantly enhancing the accuracy of numerical weather predictions. The laser transmitters were frequency-tripled, injection-seeded Nd:YAG systems, configured in a master oscillator power amplifier arrangement, generating single-longitudinal-mode pulses at 354.8 nm with a pulse duration of around 20 ns. Across the mission, both transmitters together generated more than 7 billion UV laser pulses. The performance of both the nominal and redundant laser was optimized and stabilized by carefully regulating their thermal environment, which influenced the laser energy depending on the emission frequency. At the optimum laser bench temperature, both lasers delivered stable UV output with pulse energies exceeding 60 mJ. The energy of the second laser, operational from June 2019 to October 2022, was further enhanced to over 100 mJ through step-wise increases in master oscillator pump power and adjustments of the amplifier pump phase. Following the mission's operational phase, a series of tests were conducted close before the mission end-of-life (EOL) to address instrument-related questions. During these EOL activities, the laser power was boosted to more than 150 mJ for 20 days and even over 180 mJ for 33 hours, setting a new record for a UV space laser. Additionally, the frequency stability of the two lasers was evaluated, revealing detrimental impact from micro-vibrations caused by the satellite's reaction wheels. EOL tests showed that adjustments to the master oscillator cavity control sequence significantly mitigated these effects, improving the laser frequency stability by a factor of two to better than 7 MHz (standard deviation over the period of one wind observation of 12 seconds). This paper provides a comprehensive overview of the ALADIN laser transmitters' architecture, operation, and performance during the Aeolus mission from 2018 to 2023, with a focus on energy and frequency stability improvements relevant to current and future space lidar missions such as EarthCARE and Aeolus-2.