Frequency combs are revolutionizing metrology, providing a link between optical and RF fre-
quencies. The cavity of a mode-locked laser determines the wavelength of a particular comb
tooth, and the teeth spacing. It is demonstrated experimentally that the average tooth
spacing of a frequency comb can be tuned by tilting an etalon inserted in the cavity. This pro-
perty amounts to a control of the average group velocity of a pulse circulating in the resonator.
We have shown that a mode-locked laser in which two pulses circulate constitutes a
sensitive phase sensor. This is because any phase shift between the two pulses is converted
into a frequency shift, equal to the ratio of the phase shift to the cavity round-trip time at the phase
velocity. The two frequency combs issued from the laser are split in frequency, a split measured
as a beat note on a detector recording the interfering combs. A laser gyro is an example of such
an Intracavity Phase Interferometer. A structure with periodic discrete resonances matching
several teeth of the comb will, because of the dispersion associated with the resonances, decrease
or increase the beat note, since the split combs will see different cavity round-trip times. Such reso-
nant structures that applies to multiple teeth of the comb can be passive resonators (Fabry-Perot) in
transmission or reflection, narrow atomic two-photon resonances, phase matched frequency dou-
blers, etc. . . . Experimental demonstrations will be presented. No correlation is observed between
pulse velocity and beat note enhancement/reduction.
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