| In this paper the phase distortion compensation existed within numerically reconstructed images in digital holography is investigated, and the causes of digital holographic phase distortion are analyzed, when the reference light is were plane wave and spherical wave, respectively. The accurate mathematical expression about phase distortion is deduced. The digital phase mask has been gotten, which has been a general applicable method to compensate the phase distortion. By improving the fitting method, a good compensation result is achieved and an experimental validation is performed. Main works in the paper are as follows:1. The conception of phase distortion in digital holography is recommended. The mathematical formula to express the phase distortion has been derived for the cases of off-axis Fresnel and pre-magnification digital holography with the reference light of plane wave. Meanwhile, the phase distortion has been classified. For the lensless Fourier transform digital holography, three reconstruction algorithms have been analyzed, which are single Fourier inverse transform algorithm, Fresnel algorithm and angular spectrum algorithm, respectively. Moreover, the mathematical expression of the phase distortion has been discussed toward different reconstruction algorithms. The exclusive nature of the angular spectrum algorithm is proved.2. On the basis of three reconstruction algorithms, a method of spatial frequency filtering with respect to lensless Fourier transform digital holography is proposed. By tracking the centers of+1and-1order diffraction, the pixel number of their reconstruction images is increased effectively. Thus, the magnified images facilitate the next phase unwrapping process and phase compensation. Finally, a comparison is made with the zero-padding approach.3. The digital phase mask is researched to compensate the phase distortion. A novel multiple-profile fitting method is put forward, which is based on the least square method for curve fitting. According the method, the phase distortion is corrected better. At last, using well-applied lensless Fourier transform digital holography as an example, an experimental validation is conducted by combination with spatial spectral filtering method. The experiment proves that it is better to correct phase distortion compared to frequently-used two-profile fitting method. In addition, in the case of a weak phase, the direct use of the built-in interpolation function V4in Matlab to fit DPM for phase compensation is also another simple and valid method. From the view of calculating time on a computer, the time-consuming operation limits its applicable utilization. |
