Fibre optic electronic speckle pattern interferometry: path length stabilised automatic phase stepping

Electronic speckle pattern interferometry (ESPI) is a well established technique for whole-field measurement of surface displacements and vibration. A fibre optic ESPI system has been demonstrated in which phase stepping was achieved at the frame rate of 25 Hz using a closed-loop control system. Satisfactory operation of the system was illustrated by producing correlation fringes by tilting a flat plate and using phase stepping to recover phase as a function of position on the plate. The expected linear dependence of phase on position was observed