<title>Compensation of object beam perturbations in interferometric measurements with a phase conjugation mirror</title>

Abstract

Optical measurement methods and especially interferometric methods suffer from a great drawback. The beam that illuminates the investigated object travels through the air. When the air is moving due to thermal or other perturbation, the object beam propagates through a turbulent flow of air. It deviates from its expected optical path due to air refractive index changes produced by the turbulent flow. Those perturbations become very critical when interferometric measurements are achieved in industrial environment. We have developed an original solution to address that drawback of interferometers by introducing a phase conjugation mirror into the interferometer. First we record the shape of the perturbed object beam by means of interference with a reference (pump one) beam in a non- linear photosensitive polymer allowing phase conjugation. The object beam travels back and forth between the visualization part of the interferometric set-up and the object (perturbed zone). Then the interference pattern is read by another reference beam (pump two) antiparallel to the first pump beam to produce the phase conjugate of the object beam. The conjugate beam travels also back and forth the perturbed zone but in opposite direction in regard with the object beam. At the end of the perturbed zone the conjugate beam is unaffected by the optical path modifications due to refractive index changes. Finally the conjugate beam is used as object beam (in classical sense) and interferes with a reference beam (classical part of the interferometer). The complete interferometric set-up is only sensitive to optical path changes due to object displacements and not to optical path changes due to perturbed air.