The digital holographic microscopy technology can perform real-time quantitative imaging and visual measurement of phase objects and various microstructures under non-contact, non-staining experimental conditions. It plays an important role in life, medicine, environment, materials, manufacturing, and other fields. As the object and the reference beams pass through the same optical elements, the same phase noises are introduced into them. The structure makes the common path off-axis digital holographic microscopy (CO-DHM) more stable. However, the contrast of the hologram fringes recorded by the traditional CO-DHM system is often poor, and the utilization of the space bandwidth product of the recording device is insufficient. These defects seriously influence the filtering process and the quality of the reconstructed images. In this paper, a novel CO-DHM based on a beam splitter is proposed. The intensity ratio of the object and reference beams can be adjusted. The separation distance of three spectral terms in the hologram can also be flexibly controlled by this structure. The feasibility of adjusting stripe-contrast and spatial-frequency is analyzed theoretically and experimentally, and the imaging performance of the system is verified by the experimental analysis.

The setup of the proposed CO-DHM can be referred to Fig. 1. In order to analyze the system ability to separate and regulate the object and reference beams, the lateral offset distance range of the incident beam splitter is calculated (Fig. 2). In addition, the ability to adjust the fringe contrast is introduced. The polarization states of the object and reference beams when interference occurs are analyzed by the Jones matrix. The intensity of each light and the change in the contrast of the interference fringe are analyzed in different principal-axis azimuths of the polarizer (Fig. 3).

In the control performance of the system, we use (9.8±0.2) μm highly dispersed polystyrene microsphere (refractive index n=1.593) as the sample. The separation distance between the object and reference beams is modulated by laterally moving the beam splitter (Fig. 4). When the separation distance between the object and reference beams is 5 mm, the spectrum is aliased. If the spectral filter area including the zero-order center is too large, the fringe information is introduced in the phase diagram. Even if the filtering area is reduced, the low-frequency information of the 0 order is still introduced, which brings noise interference to the reconstructed phase diagram. When the separation distance between the object and reference beams is 9 mm, the spatial-frequency is separated fully, which avoids the above situation. It is helpful for the reconstruction of digital holography. In order to analyze the influence of interference fringe contrast on the experiment, we modulate the light intensity ratio of the object and reference beams to 1∶1, 6∶1, and 34∶1 by rotating the polarizer (Fig. 5). It can be seen that the fringe contrast of the hologram is significantly reduced as the light intensity ratio increases and the ±1 orders bandwidths are also reduced. The noise carried by the object beam is more obvious, and the quality of the reconstructed image becomes worse. Therefore, adjusting the contrast of interference fringes is an important part of digital holographic recording. In this system, the light intensity ratio of the object reference beams can be adjusted flexibly, which can effectively avoid the occurrence of the above situation. In the imaging performance of the system, the same polystyrene microsphere sample is used to perform a quantitative imaging test (Fig. 6). It can be seen that the reconstructed microsphere thickness is about 9.75 μm. In order to ensure the reliability of the experiment, the thickness of 100 microspheres is measured and analyzed, and the values are all within the range of (9.8±0.2) μm, which satisfies the nominal diameters of the microspheres and verifies that the quantitative phase measurement is accurate. We test the capability of the resolution by imaging the standard USAF1951 resolution plate (Fig. 7). It can be seen that the system can clearly image the sixth element of the 7 groups, which means that the resolution can reach 2.19 μm. It is suitable for the imaging of biological samples, which fully proves that the system has excellent imaging resolution. Onion epidermal cells are used as the experimental object (Fig. 8). The experimental results show that the thickness of onion cells is about 4.2 μm and the size of individual cells is about 190.0 μm, which fully proves that the system has excellent imaging performance for biological samples.

The experimental results show that the effect of arbitrarily adjusting the contrast of the hologram interference fringe is achieved by rotating the polarizer, the flexibility of the system is effectively improved. In addition, the degree of separation between the object and reference beams is controlled by rotating the beam splitter. The separation distance between the 0 order and ±1 orders in the Fourier transform is increased to extend a filterable region and avoid spectral aliasing. The method in this paper provides a technical support for the production of digital holographic microscopes.