The frequency reference tests were carried out on 13 May on board the sounding rocket TEXUS54. The centerpiece of the payload, a compact laser system, which was primarily developed by HU Berlin and the Ferdinand-Braun-Institut, demonstrated its suitability for space. In the JOKARUS experiment (German acronym for iodine comb resonator under weightlessness), an active optical frequency reference based on molecular iodine was qualified for the first time in space. The results are an important milestone towards using optical clocks in space. Such clocks are required, inter alia, for satellite-based navigation systems that provide data for accurate positioning. They are equally important for fundamental physics research, such as the detection of gravitational waves and measurements of the gravitational field of the Earth.
The experiment demonstrated the fully automated frequency stabilization of a frequency-doubled 1064 nm extended cavity diode laser (ECDL) on a molecular transition in iodine. Thanks to integrated software and algorithms, the laser system worked completely independently. For the sake of comparison, a frequency measurement with an optical frequency comb in the separate FOKUS II experiment was carried out during the same space flight.
The JOKARUS payload was developed and implemented under the direction of the Humboldt-Universität zu Berlin (HU Berlin) as part of the Joint Lab Laser Metrology. The lab, which is collectively operated by Ferdinand-Braun-Institut (FBH) and HU Berlin, combines the know-how of both institutions in the field of diode laser systems for space applications. A quasi-monolithic spectroscopy module was provided by the University of Bremen, the operating electronics came from Menlo Systems.