Improving displayed resolution in convolution reconstruction of digital holograms

In digital holographic microscopy, when the object is placed near the COD, the Fresnel approximation is no longer valid and the convolution approach has to be applied. With this approach, the sampling spacing of the reconstructed image plane is equal to the pixel size of the CCD. If the lateral resolution of the reconstructed image is higher than that of the CCD, Nyquist sampling criterion is violated and aliasing errors will be introduced. In this Letter, a new method is proposed to solve this problem by investigating convolution reconstruction of holograms. By appending enough zeros to the angular spectrum between the two FFT's in convolution reconstruction of digital holograms, the displayed resolution of the reconstructed image can be improved. Experimental results show a good agreement with theoretical analysis.