We present a tunable experimental setup to obtain the three-dimensional refractive index distribution of microscopic biological structures. We introduce an adjustable system to change the position of the focal plane and perform stitched reconstruction. There are two main approaches for obtaining the projections of sample in optical diffraction tomography: beam scanning and rotating sample. Compared to beam scanning, the method of rotating sample allows the sample to be rotated 180° to capture uniformly distributed data, which improves the accuracy of the phase measurement and the resolution of the reconstruction result. The depth-of-field in the optical diffraction tomography setups is very small and the method of rotating sample inevitably causes the sample to deviate from the depth-of-field during the rotation, making it difficult to obtain ideal data. We divided the sample position deviation area into several ideal data acquisition areas and collected the ideal data in each area by shifting the position of the focal plane. By the combination of 180° projection method and stitched reconstruction method, we have obtained high measurement accuracy results with uniform resolution.
Optical diffraction tomography is an important method to obtain the microstructure of biological samples. We present a reconstruction process for the 3D refractive index distribution of samples in optical diffraction tomography. First, obtaining an accurate phase image by preprocessing the interference image is especially important for the reconstruction. The quality oriented method is used to perform phase unwrapping to avoid the influence of residual error. Then, the background phase is eliminated by curve fitting method, which reduces the experimental complexity and requirements. The positional deviation caused by the rotation of the sample is solved by autocorrelation algorithm. We apply a filtered back propagation algorithm based on Fourier diffraction theory to improve the accuracy of reconstruction. Finally, we have carried out experiments on samples such as photonic crystal fiber and pollen, and obtained the detailed internal structure of the samples with satisfactory results.