Intracavity phase interferometry: frequency combs sensor inside a laser cavity

In traditional interferometric measurements, a physical quantity that changes the phase of a resonator is monitored through a change of its transmission. Interferometry inside a laser exploits the ultimate Q-factor of that resonator, and converts the phase to be measured into a frequency. A mode-locked laser with two intracavity pulses emits two frequency combs of the same repetition rate. The quantity to be measured (a sub-nano displacement, a nonlinear index, an acceleration or rotation, a magnetic or electric field) produces a minute phase change (10-10rad) in one of the two intracavity pulses, which is converted into a frequency, measured by beating the two pulse trains emitted by the laser. This paper presents methods of operating mode-locked lasers in which two independent pulses circulate, producing two frequency combs of the same repetition rate. Various examples of physical quantities that can be measured through this technique are presented.