Passive phase demodulation and laser-noise compensation scheme for fibre interferometers

A scheme for passive phase demodulation and laser intensity noise suppression for fibre interferometers is presented. The scheme uses a matched Ronchi grating filter to transform the interference fringe pattern formed between the reference and the signal fibres into intensity variation that can be related to the optical differential phase in the interferometer. By dividing the grating filter into four sectors with their fringe positions slightly shifted to introduce an equivalent of π/2 phase difference between the signals detected by individual sectors, one can apply cross-subtraction on diagonal sectors to remove intensity noise. In addition, the phase information due to the detected sample vibration can be recovered by taking root-mean-square on the differential signals. The new scheme greatly reduces the complexity of practical fibre interferometers and an experimental setup has been implemented to demonstrate its expected merits.