Results and Discussions The experimental research on speckle noise suppression is carried out by manually rotating single diffuser and dynamic-static double diffusers, respectively. In the experiments of the single diffuser, the speckle suppression research is performed based on diffusers with 600 grits and 1500 grits. Compared with that of the diffuser with 600 grits, the speckle contrast values of the diffuser with 1500 grits are reduced by 16.7%, 12.5%, 10%, and 6.3% in the condition of the superimposed numbers of 5, 15, 30, and 60, respectively (Fig. 5). In the experiments of double diffusers, the speckle contrast value of sixty phase superimposition is reduced by about 51.6% relative to five phase superimposition of diffusers with 600 grits. Similarly, the value of diffusers with 1500 grits is decreased by 33.3%. Compared with that of the double diffusers with 600 grits, the reduction range of the speckle contrast of the double diffusers with 1500 grits varies from 6% to 33% at different superimposed numbers (Fig. 6). It is shown that the speckle suppression effect becomes better as the grit number of the diffuser and superimposed number of phase increase. On this basis, the speckle suppression effects are compared using the single diffuser and double diffusers with 1500 grits, and the double diffuser system lowers speckle contrast values by 30%, 9.5%, 11.1%, and 6.7% in the condition of the superimposed numbers of 5, 15, 30, and 60, respectively (Fig. 7). The result is attributed to the more independent speckle fields for double diffusers. In addition, the phase grating etched in the glass base is selected as a sample. The grating height in the double diffusers with 1500 grits is reduced by 0.02 μm compared with the case without the diffuser (Fig. 9). The feasibility of the proposed method is verified, and the imaging effect with the higher quality can be acquired.

Digital holographic microscopy technique has been applied in biomedical imaging, particle tracking, microelectronic system detection, and other fields due to its advantages of non-contact, high precision, and three-dimensional imaging. As a light source with high coherence, the laser is widely used. However, coherent noise is inevitably introduced into the imaging, which thus degrades the imaging quality. In order to reduce the speckle noise in holographic imaging, a lot of approaches have been adopted. They are mainly divided into three categories. The first one is based on temporal integration by multiplexing holograms. The second category of digital processing methods is composed of the wavelet transform, neural network, and so on. The third one aims to reduce the coherence of the light source and adopt incoherent holography to suppress speckle. Among them, the rotating diffuser method has been studied due to its simple structure and implementation, and it can obtain multiple uncorrelated holograms by manually or electrically rotating diffusers. Although the electrically rotating diffuser method is suitable for dynamic measurement, it may be affected by vibration and thus brings additional noise to measurement results. Alternatively, the manually rotating diffuser can obtain more stable speckle fields and a better speckle suppression effect. In this paper, the speckle suppression method is proposed which performs by manually rotating double diffusers. Specifically, one diffuser is static, and another diffuser is rotated by manual operation. On this basis, multiple independent speckle fields are obtained. The superimposition of multiple reconstructed images realizes the speckle reduction. Compared with a rotating single diffuser, the proposed method can obtain lower speckle contrast and more accurate measurement results.

In theory, the spatio-temporal correlation functions of the dynamic-static and single diffusers are analyzed, respectively. From the theoretical simulation results, it can be concluded that the speckle suppression effect of dynamic-static diffusers is better than that of the single diffuser in decorrelated rate. Furthermore, the distance between the two diffusers is simulated, which provides a basis for the subsequent experiments. The experimental setup of speckle suppression by rotating diffusers is designed based on digital holographic microscopy. The double diffusers are manually rotated at different rotating angles. A series of corresponding holograms are captured and then processed by a numerical reconstruction algorithm. The multiple reconstructed phases are superimposed to produce a new phase with lower speckle noise. In order to make a better comparison, the speckle contrast is adopted as a parameter to evaluate the speckle effect.

In this study, a speckle suppression method is proposed based on manually rotating double diffusers, which are composed of dynamic and static diffusers. The speckle suppression effects of the single diffuser and dynamic-static double diffusers are analyzed from the view of the spatio-temporal correlation functions. The results show that the dynamic-static diffusers show a faster decorrelated rate compared with the single diffuser and have a better speckle suppression effect. The experiments obtain a series of independent speckle fields by manually rotating diffusers at different rotating angles. The experiments of the single diffuser and double diffusers for different and same grit numbers are compared and analyzed, respectively. It is shown that the maximal speckle contrast of the single diffuser with 1500 grits is reduced by 16.7% compared with that with 600 grits. Furthermore, the value of double diffusers with 1500 grits is decreased by 33% relative to that with 600 grits. Compared with that of the single diffuser, the reduction range of speckle contrast for double diffusers varies from 6.7% to 30% at the same grit number and different superimposed numbers. Therefore, more grits and superimposed numbers are accompanied by a better speckle suppression effect. Simultaneously, the speckle suppression effect of double diffusers is better than that of the single diffuser. The proposed method can be applied in many fields such as microfluidics and biomedicine.