A molecular iodine-referenced absolute readout unit for kilohertz-accurate laser frequency determination

This paper presents a new iodine-referenced absolute frequency readout unit (ARU), developed to complement a high-finesse optical cavity-stabilized laser. The system enables precise absolute frequency determination and cavity drift tracking with accuracy at the kHz-level, while maintaining the short-term stability of a simultaneous cavity stabilization with a fractional frequency stability of σ = 3 · 10-15 at τ = 1 s. The system is developed with emphasis on deployment in future satellite missions, and it operates as a complementary module that does not interfere with the cavity stabilization. Using an iodine cell and a third-derivative lock technique, the required optical power is only 35 mW and thus reduced by one order of magnitude compared to conventional iodine-stabilized systems. Experimental results confirm that the ARU provides reliable long-term stability with a fractional frequency stability of σ < 1 · 10-11 for τ < 104 s, and it accurately follows large frequency drifts with MHz-amplitude. The presented concept marks a promising step toward compact, low optical power frequency references for laser interferometry, particularly suited to space missions.