Phase retrieval for multiple-wavelength in-line digital holography

In-line digital holography, because of its most simplified holographic recording architecture without extra reference beam, has a larger field of view compared with off-axis digital holography. Thus, it has been applied in many fields. However, both the defocused conjugate images and zero-order autocorrelation are superimposed on the real image of the object in the reconstruction process, which blurs the real image and degrades the measurement accuracy. A phase retrieval method is proposed to eliminate the unwanted conjugate image based on four diffraction intensity patterns recorded at different illumination wavelengths for in-line digital holography, in which the appropriate constraints are imposed both on the hologram plane and object plane. A synthetic wavelength is used to retrieve wrap-free phase distributions with a much extended vertical measurement range, thus the reconstructed phase distribution directly provides the height distribution of the surface of the sample after eliminating the twin image. In comparison with previous research, the proposed method requires a fewer number of illumination wavelengths and has a faster rate of convergence, as well as a better elimination effect, which is demonstrated by the simulation and experimental results.