Current digital holographic microscopy systems that can perform both transmission and reflection measurements are limited in availability. Typically, traditional digital holographic microscopes utilize Michelson interferometry for reflection measurements and Mach-Zehnder interferometry for transmission measurements, each confined to a single mode. To overcome these limitations, recent studies have aimed at improving traditional interferometric structures to support dual measurement modes. However, these systems often require complex adjustments to switching modes, such as installing cube beam splitters and repositioning complementary metal oxide semiconductor (CMOS), which adds to operational complexity and reduces efficiency. In addition, some systems using common-path off-axis interferometry reply on diffraction gratings for beam splitting, necessitating the replacement of gratings with different constants when changing objective lens magnification, along with adjustments to the filtering aperture. To address these challenges, we propose a low-cost dual-mode quantitative phase microscopic imaging method, using conventional optical components. The proposed method employs a Mach-Zehnder point diffraction interferometric structure, directly filtering the object light to produce a reference beam for interference with the object light. This design enhances flexibility in illumination for the microscopic imaging component and integrates transmission and reflection modes, broadening the range of measurable scenarios without requiring equipment changes or complex adjustments.

The dual-mode quantitative phase microscopic imaging method utilizes Mach-Zehnder point diffraction interferometry, with the imaging component functioning independently from the interferometric component. During transmission or reflection illumination, the object beam passes through the microscopic system into the Mach-Zehnder point diffraction optical path, where a beam splitter with a 9∶1 ratio divides it. One beam, accounting for 90% of the total intensity, is filtered through a pinhole to serve as the reference beam, ensuring similar intensity to the remaining 10% of the object beam to form interference. The phase retrieval algorithm based on the Fourier transform is then applied to reconstruct the phase from the interference pattern, revealing the true morphological details of the sample.

The constructed dual-mode quantitative phase imaging system is first tested on a transparent sample in transmission mode. Fig. 4 illustrates the interference pattern between the object wave containing the sample’s structural information and the reference wave. The true morphology is obtained via phase recovery from this pattern (Fig. 6). Repeated measurements across 30 data sets reveal an average depth of the etched letter “I” to be (101.0±1.6) nm, with a relative error of 0.50% compared to the nominal manufacturing value of (100.5±4.0) nm. This confirms the system’s accuracy and feasibility in transmission mode. To validate the system’s performance in reflection mode, the etching depth of deep grooves on a silicon substrate is measured. The three-dimensional morphology of the sample is shown in Fig. 8(a), with Fig. 8(b) displaying the statistical distribution of surface height values. Across 30 repeated measurements, the depth of the recessed structure is calculated as (210.7±1.5) nm. Due to an approximate deviation of 12.0 nm from the design value, a commercial white light interferometer (ER230, ATOMETRICS) is used for comparative analysis, yielding an average depth of 212.4 nm. The relative error of 0.80% between the two methods further validates the system's effectiveness in reflection mode.

In this paper, we propose a dual-mode quantitative phase microscopic imaging method based on the Mach-Zehnder point diffraction principle. Compared to existing methods, the proposed system enables rapid switching between transmission and reflection modes by simply inserting a reflective mirror, eliminating the need for complex optical path adjustments. In contrast to traditional common-path off-axis interferometric optical paths, which require the use of diffraction gratings for beam splitting and the replacement of gratings with different grating constants when changing the magnification of the objective lens, as well as adjustments to the position of the filtering aperture, this method reduces the dependence on specific optical components. Experiments conducted with the constructed dual-mode quantitative phase microscopic measurement system demonstrate that the etching depth of the quartz substrate sample surface measured in transmission mode is (101.0±1.6) nm, with a relative error of 0.50%, and the etching depth of the deep grooves on the silicon substrate surface measured in reflection mode is (210.7±1.5) nm, with a relative error of 0.80%. These results validate the effectiveness and reliability of the dual-mode quantitative phase microscopic measurement system proposed in this paper.