Modified J₁....J₄ method for linear readout of dynamic phase changes in a fibre-optic interferometer

Fibre optic interferometers have been extensively studied for sensing applications because of the very high sensitivities with which many physical parameters can be measured using these devices. A limiting feature of fibre interferometers is the random phase changes which occur at the interferometer output because of general environmental fluctuations, for example, due to changes of ambient temperature. These fluctuations can be compensated by active phase tracking techniques which keep the interferometer biased at quadrature. Passive demodulation techniques are of greater practical interest and the J₁....J₄ technique has been recently proposed as a method for the linear measurement of dynamic phase changes in a fibre interferometer. In this technique the interferometer requires no phase bias. The measurement is unaffected by random phase fluctuations in the interferometer. However, the J₁....J₄ technique is limited by the fact that only the magnitude of the J₁....J₄ Bessel components can be measured on a spectrum analyser so that no information about the sign of the Bessel function is available thereby generating some ambiguity in the read-out. In this paper, a modified technique which overcomes this limitation is demonstrated.