To minimize the physical damage, phototoxicity, and photobleaching of the biological samples, microscopic imaging methods for the visualization of cells and tissues need to have the ability of noncontract and fast measuring of the three-dimensional (3D) sample information. Far-field optical microscopy, which has been widely applied for biomedical imaging, is one of the most direct and indispensable ways of capturing the dynamic 3D architecture of biological samples. In the optical imaging system, both the intensity and phase distribution of the illumination light field will be quantitatively modulated by the sample and finally transmitted to the detector plane. The demodulation of all the obtained information enables quantitative reconstructions of the samples’ 3D spatial structure, morphology profile, and refractive index distribution. However, the existing photon detectors are only sensitive to the intensity distribution of the input light signal. The phase of the light field, which cannot be directly measured by the detector, can be quantitatively coded and decoded from the two-dimensional intensity distribution of the interference pattern using the interference characteristic of light. These basic ideas enable, in principle, fast 3D imaging, tomography, and quantitative phase-contrast imaging and hence benefit the visualization of the dynamic structural and biophysical characteristics of the samples. Furthermore, the details of the fine structures inside the sample can be obtained with improved imaging performance through super-resolution imaging and nonscanning 3D imaging, which are enabled by fluorescent self-interference imaging techniques. All these potential advantages in biological imaging have promoted the rapid development of interference microscopic techniques in the past decades. The interference imaging methods are classified according to the coherence properties of the light source used. The interference microscopic techniques using different light sources enable quantitative phase-contrast imaging, nonscanning 3D imaging, and tomographic imaging to practically benefit the structural and functional visualization of 3D complex biological samples. The advantages, limitations, and potential applications of different interference imaging techniques are shown in Table 1.
Different optical systems and numerical methods have been designed to improve the spatial resolution, imaging speed, signal-to-noise ratio, and robustness to extend the application and modalities of the interference microscopic techniques. Among them, research is mainly focused on the applications of digital holography in quantitative phase-contrast imaging (Fig. 4) and nonscanning 3D imaging (Fig. 6) of the sample. Parallel phase shifting (Fig. 8) and compressive sensing (Fig. 9) methods have been combined with digital holography to improve temporal and spatial resolution. Digital holography’s large field-of-view, high speed, high resolution, and multidimensional imaging abilities have benefited both functional (Fig. 5) and structural (Fig. 7) imaging of biological samples. With optical sectioning imaging ability and less speckle noise, partially coherent digital holography has been applied for high accuracy phase-contrast imaging of cells (Fig. 10) and, more importantly, for the visualization of the structure behind the tissues (Fig. 11). Because of its 3D tomographic imaging ability, optical coherence tomography (OCT) has become one of the most important tools for ophthalmic imaging (Fig. 13). With extended imaging modalities, polarization sensitive OCT has provided proof-of-principle results in the diagnosis of bronchial disease (Fig. 14). Incoherent holography can considerably improve the temporal resolution of the existing 3D laser scanning fluorescence microscope. Nonscanning 3D imaging of the fluorescence sample has been demonstrated (Fig. 15) with inherent super resolution (Figs. 18 and 19). While some of the major limitations of incoherent holography, such as the low axial resolution, have been addressed and improved (Fig. 16), the potential of this technique for high-resolution, high-speed 3D fluorescence imaging is still being explored. Successes have been achieved, e.g., by optimizing the 3D imaging performance of fluorescence holography via computational adaptive optics (Fig. 17). In localization-based super-resolution microscopy, the basic idea of interference microscopy has also been used as a point spread function modulation method. Therefore, the system’s 3D resolution and imaging depth have been improved (Figs. 21 and 22).
In this paper, we have reviewed the basic principles, recent progresses, advantages, limitations, applications, and potential future directions of the techniques. The system’s 3D spatial resolution, imaging speed, and signal-to-noise ratio have been considerably improved during the past decades. Based on the multidimensional (3D spatial+ phase) imaging ability of interference microscopy, the applications of the methods for the structural and functional imaging of biological samples have been demonstrated. Further, the imaging modalities have been extended to provide even more data dimensions by combing the interference microscopic techniques such as OCT with polarization imaging method. In conclusion, in interference microscopy, the concurrently obtained structural and functional information of the sample is important for understanding the biological and biophysical mechanisms of the life processes. Interference microscopic techniques have benefited specific research in biological society by providing a powerful 3D imaging tool for both coherent and incoherent light sources. While several efforts have been made to improve system spatial resolution, another important direction in the future is to further develop functional imaging methods by exploring the potential of superior multidimensional data acquisition ability.
