Optical frequency references
The atomic clocks employed in modern GNSSs lock to the atomic or molecular electronic transitions in the microwave regime providing stabilities between 10-9 to 10-13 at 1s, and between 10-14 to 10-15 at 105 s [s/s] (Allan deviations for a reference averaging time interval). Optical frequency references evolved over the last decades, recently demonstrating frequency stabilities at the 10-18 level for integration times of a few thousand seconds, surpassing microwave clocks’ performance by several orders of magnitude.
Optical references are composed by three elements: a component capable of providing a reference frequency characterized by a very narrow optical absorption line, a feedback system that locks a laser to the reference frequency, and a frequency comb. The latter is a device capable of accurately measuring the stabilized frequency of the laser.
While ultimate frequency stability is shown using optical ion clock and lattice clock technologies in complex laboratory setups, optical frequency references based on Doppler-free spectroscopy can be realized in space compatible compact and ruggedized setups in a relatively short time. Up to integration times of 10.000 s, these setups have demonstrated frequency instabilities comparable to the active hydrogen maser as currently integrated for the ACES (Atomic Clock Ensemble in Space) mission. Furthermore, cavity-based optical frequency references can provide high short-term frequency stability at and below the 10-15 level for integration times up to a few seconds, also demonstrated in compact setups developed for space applications.
Iodine-stabilized lasers: a stable space clock
While different methods for laser frequency stabilization are conceivable, stabilization to an atomic or molecular transition offers high long-term stability and the provision of an absolute frequency reference. Setups based on Doppler-free spectroscopy of molecular iodine near 532 nm demonstrate frequency stabilities at the 10-15 level at long integration times (up to several hours).
In a collaboration between the German Aerospace Center (DLR Institute of Space Systems, Bremen), the University Bremen (Center of Applied Space Technology and Microgravity, ZARM), the Humboldt-University Berlin and the space company Airbus Defence & Space (Friedrichshafen), two compact setups on elegant breadboard (EBB) and engineering model (EM) level, respectively, were realized. The spectroscopy units use a baseplate made of glass material in combination with a dedicated easy-to-handle assembly-integration technology for the optical components ensuring high pointing stability of the two counter-propagating laser beams in the iodine cell and therefore high long-term stability. For realizing a compact and ruggedized setup, a special designed compact multi-pass gas cell was produced. A frequency stability of 6 · 10-15 at 1 s integration time and a noise floor below 3 · 10-15 for integration times between 100 and 1000 s was measured.
The EM spectroscopy unit was subjected to thermal cycling from -20°C to +60°C and vibrational loads with sine vibration up to 30 g and random vibration up to 25.1 grms. The frequency stability was measured before and after the tests where no degradation was observed. The light source is a 1064 nm solid-state Nd:YAG laser, which is also available in a space-qualified version. For iodine spectroscopy, the laser output is frequency doubled using second harmonic generation based on PPLN waveguide technology. The 1064 nm laser wavelength is the baseline for space missions employing high-sensitivity laser metrology such as Gravity Recovery and Climate Experiment (GRACE) Follow-On – launched in May 2018, LISA (Laser Interferometer Space Antenna) and LISA Pathfinder (launched 2015, successfully operated in space) and is also the operating wavelength of the Laser Communication Terminals (LCTs).
In a recent activity, a very compact spectroscopy setup for use on a sounding rocket mission has been integrated and was successfully launched in May 2018, using micro-integrated external cavity diode lasers (ECDLs) as light source. An iodine-based reference has been compared to a microwave reference via an optical frequency comb, resulting in a test of local position invariance (LPI).
Within the current project ADVANTAGE (Advanced Technologies for Navigation and Geodesy), optical clock technologies based on Doppler-free spectroscopy of molecular iodine and on optical cavities are investigated experimentally with respect to applications in GNSS, also – in collaboration with the DLR Institute of Communication and Navigation – in combination with optical link technology.