Two holograms are used. A first hologram is recorded by the light that passes through the phase object with an ordinary off-axis holographic system. A second hologram is recorded by the light that passes through the deformed object and is diffracted by the first hologram and processed nonlinearly. Phase difference amplification is performed by the superposition of two higher harmonic waves from the second hologram. In a resultant interferogram of eight-fold phase difference amplification, phase change, due to only the deformation of the object, is amplified; and wavefront aberration, caused by poor quality of an optical system and by inhomogeneity of the object, is completely eliminated.
Satoru Toyooka, "Elimination of Wavefront Aberration of Optical Elements Used in Phase Difference Amplification," Appl. Opt. 13, 2014-2018 (1974)