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The result of a phase measurement is generally observed as an amplitude modulation of passively interfering sample and reference beams. Here, in contrast, the measuring interferometer and the laser source are combined as a single unit. A beat frequency (phase shift per round-trip) instead of a noisy amplitude modulation are recorded. This sensor circulates two pulses with the same group velocity in a laser or Optical parametric Oscillator (OPO) cavity; one pulse serves as reference while the phase of the other is affected at each round-trip by the parameter to be measured. We consider as an example a linear polarization maintaining (PM) fiber OPO in which two orthogonal pulses circulate. The cavity is terminated by a Michelson like interferometer consisting in a polarization splitter sending each pulse in a different arm. The classical noise in that configuration is very high, since a difference in length of the two arms produced a beat note in the tens of MHz. To analyze the quantum limit of noise, the Michelson termination is replaced by a single end mirror in a discrete components double comb OPO. The two combs to be interfered exist in exactly the same cavity, and thus have correlated noise. Fine control of the beat frequency is achieved with an intracavity phase modulator. Preliminary measurements show a bandwidth of 2 mHz at a beat note frequency of 1 Hz (phase shift of 1:5 · 10 -7). A theoretical simulation of the spectral evolution of the pulses at each round-trip shows how the beat note develops in presence or absence of coupling between the two beams. The simulation shows that the beat note frequency is enhanced, without noise penalty, by inserting resonant dispersion in the cavity. Preliminary results with a Gires Tounois as end cavity mirror agree with the simulation.
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