## 2 Experimental Setup  
### System Setup of Fiber-Based Polarization Sensitive Optical Coherence Tomography  
A scheme of the PSOCT system is shown in Figure 1. The laser (SL131160, Thorlabs, Bergkirchen, Germany) employed was a MEMS-based swept source with a central wavelength of 1310 nm, a spectral bandwidth of 90 nm, a repetition frequency of 100 kHz, and an output power of 25 mW. The light from the laser is split into the reference arm and sample arm by a 20:80 optical coupler (OC\({}_{1}\)). In the reference arm, the light from port2 of the circulator (C\({}_{1}\)) sequentially passes through a collimator (CL\({}_{1}\)), an achromatic lens, and a mirror. The achromatic lens (AC254-050-C, Thorlabs, Germany) uses glasses of two different refractive indices to cancel chromatic aberrations, which can achieve a tighter focus and superior off-axis performance. The reflected reference light back from the mirror finally delivers to a 50:50 optical coupler (OC2) with port3 of the C1. In the sample arm, the light from port2 of the circulator (C2) sequentially passes through a polarization controller, a collimator (CL2), a quarter-wave plate (QWP), a pair of galvo scanners (GVSM002-EC, Thorlabs, Germany), and an achromatic lens before finally arriving to the sample. The sample light backscattered from the tissue also follows its original path back and delivers to a 50:50 optical coupler (OC\({}_{2}\)) with port3 of the C\({}_{2}\). In the detection part, both the s-polarized and y-polarized interference lights from the sample are obtained using two fiber-based polarizing beam separators (PBSs) and two balanced detectors (PDB480C-AC, Thorlabs, Germany). These two interference signals were collected using Channel A and Channel B of a high-speed acquisition card (ATS9371--12 bit, Alazar, Pointe-Claire, QC, Canada). The system utilizes a swept-source light with a central wavelength of 1310 nm and a full optical bandwidth of 100 nm. The axial resolution of the system can be expressed as \(\triangle\)z = \(\frac{2\Omega\ln 2}{\pi}\cdot\frac{\lambda_{0}^{2}}{\Delta\lambda}\), where \(\lambda_{0}\) represents the central wavelength of the light source, and \(\Delta\lambda\) represents the spectral bandwidth of the light source. Therefore, the axial resolution of this system can reach 7.55 \(\upmu\)m. The lateral resolution is also influenced by the beam diameter at the sample arm and the achromatic lens focal length, which can be expressed as \(\triangle\)x = \(\frac{4\lambda_{0}\cdot f}{\pi\cdot d}\). There is an inverse relationship between lateral resolution and axial resolution. In this system, the beam diameter is 2.04 mm, and after passing through a chromatic aberration lens with a focal length of 50 mm, the lateral resolution of the system can be calculated as 40 \(\upmu\)m. The maximum imaging depth of the system is approximately 8 mm. The key point of PSOCT is obtaining a circularly polarized light incident to the sample. PSOCT achieves distinct contrast within samples by observing alterations in the polarization state of backscattered light, resulting from interactions between light and tissues within the sample. A circularly polarized beam exhibits particular sensitivity to birefringent properties. The imaging process of the sample arm on the in vitro biological sample is shown in Figure 2. The two-dimensional galvo scanners consist of two mirrors in different directions, x and y. The driving voltage of the galvanometer is controlled by a motion control card (PCI6221, National Instruments, Austin, TX, USA), which can be rotated at any angle on the x- and y-axes to change the position of the focus to achieve a lateral and longitudinal scanning of the sample. A set of A-lines is collected at each focus position, and a lateral scan can be completed by rotating only the x-axis. Finally, calculated using the Matlab 2019 complete algorithm, reflectivity, and phase retardation, a picture of a sample PSOCT depth profile is obtained.