Optical clocks
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.
Cavity-stabilized lasers as frequency reference
Cavity-stabilized lasers can deliver the short-term stability required in new GNSS architectures such as Kepler, where the frequency references aboard the satellites are synchronized using optical links.
Optical cavities provide the frequency discriminator needed to stabilize a laser by means of a servo loop. The discriminator is generated using the Pound-Drever-Hall (PDH) method, which enables transferring the length stability of the cavity to the laser frequency. This implies that extremely stable materials are required.
An example of a viable design is a small (~5 cm) cavity realized with Ultra-Low Expansion (ULE) glass that reach thermal noise limits at or below the 10-15 level.
Efforts on portable cavity-stabilized systems are undergoing, and the very first cavity-stabilized laser in space is part of the laser ranging interferometer (LRI) on-board the GRACE-FO mission launched on 22 May 2018.