Correlation method for processing speckles of signals from single-fibre multimode interferometers by using charge-coupled devices

The correlation method for processing signals from a single-fibre multimode interferometer by using a digital charge-coupled device is studied experimentally and theoretically. Optimal conditions are determined for recording multimode interference patterns with charge-coupled devices. It is found that the nonlinearity of characteristics of such devices affects the results of correlation measurements. The method for eliminating this influence is proposed. The correlation method considered in the paper allows one to measure a linear deformation of the interferometer within 0–80 μm with an accuracy of ~±3 μm for typical multi-mode fibres with the core diameter 50 μm.