Polarization-multiplexed and phase-stepped fibre optic shearography using laser wavelength modulation

A shearography system that measures the deformation gradient using two orth ogonal shear directions and performs phase stopping, using no moving mechan ical components and a single CCD camera, is described. The light exiting fr om a highly linearly birefringent optical fibre is switched between two ort hogonal linearly polarized states by tuning the optical wavelength of a las er diode via injection current modulation. A polarization sensitive Michels on interferometer is used to shear the image in orthogonal directions for p - and a-polarized light. The change in the optical wavelength is also used to provide a phase step in the pathlength imbalanced interferometer. In thi s way wavelength modulation of a laser diode source is used to accomplish s imultaneously polarization multiplexing and phase stepping in the shearing interferometer. By carefully matching the optical wavelength shift, the opt ical fibre, length and the pathlength imbalance in the interferometer, the pi/2 polarization shift can be matched to the required phase step.