Phase-locked fibre interferometer with intensity noise compensation

The authors present a phase-locked fibre interferometer in which the fluctuations caused by laser intensity noise or other spurious signals are eliminated by differential measurement. The interferometer makes use of a matched grating filter to transform the spatial movement of the interference fringes formed between the intersecting reference and signal beams into intensity modulation. With the grating filter split into two halves and shifted by 180°, the signals from the two halves contain antiphase variation related to the optical phase only. Other spurious intensity fluctuations are in common mode and can be eliminated. The use of such a filter also enables simple implementation of vibration isolation for maximum detection sensitivity by constantly moving the grating filter to track the null of the differential signal