Interferometric imaging techniques can achieve non-invasion and high-resolution three-dimensional imaging for bio-tissue or industrial materials. Currently, these technologies have been widely applied in various fields including biomedicine, material science, and precision processing. By introducing polarization measurement techniques into interferometric imaging systems, the imaging system can display three-dimensional structures of samples, and quantify the structural anisotropy (birefringence) and absorption anisotropy (birefringence) of the sample, revealing physiological characteristics of biological tissues at the cellular and molecular levels, as well as damage and stress defects in materials. However, the existing polarization interferometric imaging technologies has two major limitations: to achieve multi-parameter measurement and improve measurement accuracy, it is necessary to increase the number of acquisition times, which leads to a decrease in imaging speed; to enable polarization detection, additional detection and acquisition devices need to be added to the original system, making the system structure complex and the cost significantly increased.

To address the above limitations, the team proposed a compact, rapid, and multi-parameters polarization interferometric method and combined it with optical coherence tomography (OCT) system, called spectral-polarization-modulation OCT (SPM-OCT), whose polarization state of the output light from light source was modulated in the spectral domain. By performing a single acquisition, it can extract the birefringence and dichroism parameters of the sample from the interference signal without reducing the acquisition speed. It was shown from the experimental results that SPM-OCT can avoid phase jump errors associated with optical axis parameters in common polarization-sensitive OCT (PS-OCT), enabling precise measurement of birefringence and dichroism parameters. Additionally, the dichroism parameter images provided by this system showed an imaging contrast of 20 to 25 dB between gold nanorods and biological tissues, far exceeding the contrast in intensity images, demonstrating the significant potential of this system for molecular imaging applications. Relevant research results were recently published in Photonics Research, Volume 13, Issue 4, 2025. [Di Yang, Weike Wang, Songwen Xu, Zhuoqun Yuan, Yanmei Liang, "Compact spectral-polarization-modulation method for rapid and versatile polarization measurements in interferometric imaging," Photonics Res. 13, 1049 (2025)]

By adjusting wavelength-sensitive multi-order wave plates (MPs) and an achromatic quarter wave plate (AP) appropriately, the spectral polarization modulation (SPM) method made the polarization modulation of the output light controllable in the spectral domain, as shown in Fig.1(a). The input light first passed through a linear polarizer with the transmission direction set horizontally to form horizontal linearly polarized light; then the light went through MPs with their fast axes at 45° to the horizontal direction, creating polarized light with retardation varying with wavelength; finally, the light passed through an AP with its fast axis set horizontally, adjusting the change in retardation to the change in polarization direction, thus forming linearly polarized light with modulated polarization direction in the spectral domain. Integrating the SPM module into the output of light source of the OCT system, as shown in Fig.1(b), constructed an SPM-OCT system that can obtain birefringence and dichroism parameters of the sample simultaneously with a single detection. In terms of data processing, the SPM method extracted polarization modulation information from the interference signal, separating different frequency components to calculate polarization parameters. Birefringence parameters were calculated from high-frequency components, and dichroism parameters were calculated from low-frequency components.

Fig.1 Schematic diagrams of SPM method (a) and SPM-OCT system (b)

In the experiments of the SPM-OCT system, standard optical components were firstly measured, including an AP and a linear polarizer. The results showed that the birefringence parameter error was only 0.2°, and the dichroism parameter is close to the ideal value, demonstrating the measurement accuracy of the SPM-OCT system. Next, the SPM-OCT system was used to image the leg of a mouse, and the result showed that SPM-OCT system overcame the phase jump problem commonly encountered in conventional PS-OCT, accurately distinguishing between muscle and fat, as shown in Fig.2. Finally, by comparing the SPM-OCT imaging results of gold nanorods and biological tissues, as shown in Fig.3, the team found that the dichroism of gold nanorods was much higher than that of biological tissue, with an imaging contrast reaching 25 dB.

Fig.2 (a) - (c) are the intensity, retardation, and optic axis en-face images of the skinned mouse leg, respectively. Labels 1 and 2 represent the images obtained by PS-OCT and SPM-OCT, respectively. The images of labels 3 and 4 are enlarged images of the areas selected by the dotted boxes in the images of labels 1 and 2, respectively. Scale bars of images of labels 1 and 2 are 1 mm. Scale bars of images of labels 3 and 4 are 400 μm.

Fig.3 (a)-(d) are the retardation, optic axis, diattenuation ratio, and transmission axis images of the sample, respectively. Labels 1 and 2 represent the skinned mouse leg and GNR phantom, respectively.

Utilizing the polarization characteristics of light for sample imaging or detection provides a unique perspective for precision detection of life sciences, clinical medicine, and industrial materials, which has received widespread attention in recent years. At present, polarization interferometric imaging is leading the development of optical detection toward the fusion of multidimensional information, and its industrial value has expanded to multiple fields such as medical diagnosis, precision manufacturing, and remote sensing. It can provide innovative solutions to critical challenges in microstructure analysis, weak signal detection, and complex environmental perception. The SPM method modulates the polarization state of output light in the spectral domain through a combination of multi-order waveplates and achromatic waveplates. This method overcomes the limitation of the acquisition times in existing time-sequenced polarization measurement methods and avoids the problems of slow acquisition speed, few parameters, and complex equipment in polarization interference imaging, enhancing the industrial value of polarization interferometric imaging. In this study, we combined the SPM method with OCT to achieve a function of polarization-sensitive OCT. Compared with the available PS-OCT, the SPM-OCT can detect more dimensional polarization information of samples and improve the accuracy and specificity of sample identification and performance evaluation, which is expected to identify the microscopic changes, defects, and stress distribution of samples simultaneously and accurately.

The team will further optimize the hardware structure and signal processing methods of the SPM-OCT system, reduce system complexity, improve imaging resolution and measurement accuracy, and expand the application range of SPM-OCT in the fields of biomedicine and precision manufacturing.