Characteristics of a recirculation fibre interferometer at a high-frequency radiation-phase modulation

The signal sensitivity and detectability of a recirculation fibre-ring interferometer (RFRI) are studied experimentally at various parameters of the radiation-phase modulation and gains of an intracavity fibreoptic amplifier (FOA). It is established that, at an optimal phase-modulation amplitude, the signal sensitivity of the RFRI with a FOA based on an Yb – Er fibre is independent of the modulation frequency in a wide FOA-gain range up to the prethreshold operating mode. In this case, the modulation frequency may be much higher than the width of the resonance lines of the optical resonator, so that the quasi-static modulation condition imposes no limitations on the sensitivity. It is also shown that the RFRI output noise is minimum at a phase-modulation frequency close to the middle of the intermode interval, which is explained by the filtering of an excess noise of a superfluorescent fibre source (SFS) in the resonator. Comparing the RFRI to an ideal Sagnac fibre-ring interferometer shows that, under identical conditions, the signal sensitivity and detectability of this RFRI prototype are higher by factors 16 and 4, respectively. The latter is determined by the excess noise of the SFS.