Phase is one of the important attributes of light waves, and its distribution directly affects the spatial resolution of optical imaging and is related to the three-dimensional topography of objects or the refractive index distribution of transparent objects. However, the phase distribution of light waves cannot be directly detected. How to accurately obtain the phase distribution of light waves has become a hotspot in the field of optics. The invention of phase-contrast microscopy has opened the curtain of phase imaging, which has epoch-making significance. It successfully converts the phase distribution of light waves into intensity changes, solving the problem of difficult direct microscopic observation of transparent samples such as cells.

Nevertheless, the conversion between phase distribution and intensity change is not a linear relationship in phase contrast microscopy, resulting in phase information that cannot be observed quantitatively. By measuring the phase of light waves, the three-dimensional topography or refractive index distribution of transparent objects can be quantitatively obtained. The refractive index is one of the essential characteristic physical quantities that reflect the internal structure and state of the sample. Therefore, conducting quantitative phase microscopy methods has scientific significance. Quantitative phase imaging has important application value in industrial detection, biomedicine, special beam generation, adaptive optics imaging, and synthetic aperture telescopes.

The current quantitative phase microscopy imaging technology mainly obtains the quantitative distribution of phase through interference. Therefore, factors such as the stability of interference devices, limitations on optical diffraction, phase wrapping, coherent noise generated by laser illumination, and sample refocusing during dynamic observation affect the imaging resolution and accuracy of quantitative phase microscopy. Thus, systematic and in-depth research on improving measurement accuracy and stability, spatial resolution, expanding the longitudinal measurement range, suppressing coherent noise, and autofocusing of quantitative phase microscopy imaging has been carried out. A theoretical and technical system centered on high-precision quantitative phase microscopy imaging has been formed.

A simultaneous phase shift digital holographic microscopy (DHM) with a common-path configuration has been proposed, which allows the object light and reference light to share the same optical path and components, solving the impact of environmental disturbances on phase imaging fundamentally (Fig. 3), simultaneously recording multiple phase-shift interferograms within one exposure and achieving real-time high-precision quantitative phase imaging. The optical path fluctuation of the system is only 3 nm within 35 min, and the real-time phase microscopy imaging accuracy reaches 4.2 nm, which is 2.2 times the accuracy of conventional off-axis interference quantitative phase microscopy imaging (Fig. 5). A super-resolution quantitative phase imaging method based on structural illumination has been proposed. Using the structured light illumination, the spatial resolution of quantitative phase microscopy can be doubled when the spatial frequency of the structural illumination stripe is the same as the highest spatial frequency of the microscopic objective, and super-resolution phase imaging is realized (Fig. 7). A slightly off-axis interference dual-wavelength illuminated digital holographic microscopy has been proposed to expand the longitudinal unwrapped phase measurement range from the wavelength to the micrometer level (Fig. 8), meeting the high-precision phase imaging requirements of thicker samples. Using a low-coherence LED as an illumination light source, the coherent noise in the common laser-illuminated DHM can be reduced by 68% (Fig. 10), and the signal-to-noise ratio (SNR) of images can be improved. The phase measurement accuracy is 2.9 nm, providing a high-precision solution for the measurement of micro/nano structures and micro electro mechanical system (MEMS) surfaces. Two autofocusing methods based on dual-wavelength illumination and dual beam off-axis illumination have been proposed to meet the autofocusing requirements of high-resolution quantitative phase microscopy imaging for long-term tracking and observation of samples under different conditions (Fig. 11). The former does not rely on the characteristics of the tested sample or other prior knowledge, making it suitable for both amplitude and phase objects. The latter has a simple criterion and can easily determine the optimal imaging surface by reproducing the differences and changes between images, without the need for tedious iterative calculation and with relatively fast processing speed.

Digital holographic microscopy is one of the representative achievements with significant influence and widespread application in the field of quantitative phase imaging, playing an increasingly important role in biomedical, material science, industrial testing, flow field display research, and other fields. We focused on the theoretical and technical issues of high-precision quantitative phase imaging and conducted systematic research on improving measurement accuracy and stability, improving lateral spatial resolution, expanding longitudinal unwrapped measurement range, suppressing coherent noise, and achieving automatic image focusing. With the promotion and application of quantitative phase microscopy imaging technology in other fields such as biological research, high-precision quantitative phase topography microscopy imaging methods will be our future research direction. It is expected that quantitative phase microscopy imaging technology can play a greater role in industrial testing, materials science, and biomedical fields, becoming an indispensable tool for studying the micro world.