Phase transfer between three visible lasers for coherent population trapping

Stringent conditions on the phase relation of multiple photons are a prerequisite for novel protocols of high-resolution coherent spectroscopy. In a recent experiment, we have implemented an interrogation process of a Ca⁺-ion cloud based on three-photon coherent population trapping, with the potential to serve as a frequency reference in the THz range. This high-resolution interrogation has been made possible by phase-locking both laser sources for cooling and repumping of the trapped ions to a clock laser at 729 nm by means of an optical frequency comb. The clock laser, a titanium-sapphire laser built in our lab locked onto two high-finesse cavities reaches a linewidth of a few Hertz and a frequency stability below 10^-14 at 1 s, performances which can be copied onto the two other sources. In this paper, we discuss the performances of the phase transfer between the three involved lasers via the optical frequency comb.