  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 one second, 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.
