Numerical compensation in phase-shifting in-line lensless Fourier digital holographic microscopy

A numerical compensation technique is proposed to enhance the quality of reconstruction of phase-shifting lensless Fourier digital holographic microscopy. Introducing phase-shifting technique into the in-line lensless Fourier digital holographic microscopy is widely used, because this system possesses high resolution and is able to suppress the zero-order of diffraction and the twin image in reconstruction. Practically, it is likely to suffer from the lower resolution due to the blurred image in reconstruction plane in respect that the normal of object plane is not perpendicular to the CCD plane or the distance between the source of reference wave and the CCD plane is not equal to the distance between the object plane and the CCD plane accurately. Mechanical adjustment has difficulty in achieving high accuracy of adjustment and needs a great deal of time. Based on the theories of Fresnel diffraction and holography, it is found that numerical compensation can complete adjustment in high accuracy. It uses chirp function as compensation factor. The values of the three compensating parameters in three directions are obtained by repeatedly applying the numerical reconstruction procedure through gradual approach until the best in-focus image is obtained. The precisions of the three compensating parameters are decided by requirement. The main advantages are: (1) the accuracy of adjustment is high, (2) the quality of reconstruction can be greatly improved. In order to testify the feasibility of the numerical compensation technique, experiment is carried out. Its results show that fine numerical compensation is necessary to advance the quality and resolution of reconstruction.