Two-wavelength contouring by iterative phase retrieval using volume speckle field

Determining the shape of an object can prove to be very handy in various sectors such as manufacturing industries. Out of many optical technologies, interferometric methods are one of the most effective and widely used optical testing methods. However, interferometric methods may encounter problems pertaining to phase ambiguity while dealing with largely deformed aspheric structures. To obtain true phase information, phase unwrapping algorithms are employed which may encounter problems in the case of complex shape measurement. This issue can be addressed by employing the two-wavelength contouring approach for shape measurements. Moreover, the conventional interferometric setups involve two-beam geometries leading to low temporal stability and making the system bulky. Thus, the proposed technique is a single beam two-wavelength contouring technique that is developed based on the iterative phase retrieval technique where multiple intensity patterns of the object at various axial planes are recorded for extracting complex amplitude by iterative use of angular spectrum propagation approach of the scalar diffraction theory. The technique is successfully employed to determine the shape of 3D objects as well as their artifacts in transmission as well reflection mode where an aspheric lens and a parabolic mirror are used as an object respectively.