Results and Discussions The multifunctional Stokes-Mueller polarimeter based on embedded phase sensing is applied to the measurement of a lung cancer tissue section. 10 positions are selected from the normal and malignant areas for polarization and phase measurement, respectively. The results show that the morphology and arrangement of cells in different areas are not the same. In the malignant area, the arrangement of cells is disorderly, and the number of cells increases exceptionally due to excessive proliferation. Quantitative analysis shows that the depolarization and absolute-phase values of the malignant area are both large. That is, normal and malignant tissues have different refractive indexes, and this is crucial for optical diagnosis (Figs. 5 and 6). The malignant area has more cells than the normal area, and it also has a depolarization value higher than that of the normal area in both the whole image and a single cell (Table 1). The refractive index values of the cells in the malignant area measured at the wavelengths of 600, 633, and 700 nm are all larger than those of the cells in the normal area. To be precise, the refractive index values of the cells in the normal area measured at the three wavelengths are 1.383, 1.384, and 1.377, respectively, while those of the cells in the malignant area are 1.518, 1.516, and 1.511, respectively (Table 2). Therefore, depolarization parameters of polarization decomposition and the refractive index derived from the absolute phase can be used to determine whether a tissue is abnormal or early cancerous.

Since polarization imaging technology can extract richer structural and optical information of samples and is highly sensitive to changes in subwavelength microstructures, it has a bright application prospect in biomedicine. The Mueller matrix has been widely used in the pathological diagnosis of cancers because it can quantitatively provide complete polarization information of biomedical specimens. However, the existing Mueller imaging polarimeter loses the absolute phase information of the sample, and the relative phase information cannot reflect the change law of the phase to be measured. Absolute phase, as a basic property of light, quantifies the phase characteristics determined by the physical thickness and refractive index coefficient of the sample and reflects the changes to be measured. As one of the most important optical properties of biological samples, the refractive index has been proven to be useful for describing the optical properties of biological tissues and evaluating pathological tissues. As an absolute-phase detection method, the quadriwave lateral shearing interferometer is highly suitable for phase microscopic imaging and the measurement of absolute phase information of samples due to its advantages, such as no need for additional wavefront reference beam, no special requirements on the light source, and simple structure. However, the applications of the quadriwave lateral shearing interferometer in the pathological diagnosis of cancer tissues are rarely reported. Therefore, an embedded absolute-phase detection instrument needs to be developed to meet the detection requirements of different types of samples.

As the existing Mueller imaging polarimeter loses the absolute phase information of the sample, a multifunctional Stokes-Mueller polarimeter based on embedded phase sensing is built. Specifically, the analysis of the polar decomposition equation for the Mueller matrix shows that the Mueller matrix does not retain the absolute phase information of the polarized light, but only contains the phase delay information of polarized light. Then, the self-developed quadriwave lateral shearing interferometer is integrated into the polarimeter. A multifunctional Stokes-Mueller polarimeter based on embedded phase sensing is thereby obtained, and it solves the problem of missing absolute phase in the measurement results obtained by the Mueller matrix polarizer. Finally, the phase distribution of the sample is reconstructed by MATLAB according to the collected interferogram, and the refractive index is calculated. With a lung cancer tissue section as the research object, in addition to the extraction of polarization information of the biological sample, the refractive index is measured on the basis of the absolute-phase value. This instrument can serve as a new quantitative diagnostic index and enrich the measurement function of the polarizer.

The multifunctional Stokes-Mueller polarimeter based on embedded phase sensing solves the problem of missing absolute phase in the measurement results obtained by the Mueller matrix polarizer. The refractive index of the sample is measured on the basis of the absolute phase, and it can serve as a new quantitative diagnostic index for cancer diagnosis. An experiment is conducted with a lung cancer tissue section as the research object, and the results show that the depolarization parameter and refractive index of the malignant area are all larger than those of the normal area. The distinction between the normal and malignant areas can thus be achieved. The developed instrument can not only extract polarization parameter, but also quantify the refractive index information of the tissue. In this way, it further expands the functions of the traditional Mueller polarimetric imager. The combination of polarization information and phase information can provide a more comprehensive quantitative evaluation index for cancer diagnosis. In the future, this instrument can assist researchers in the preliminary screening of indicators, which reflects the application potential of the proposed instrument in pathological diagnosis research.