## Materials and Methods  
### Depolarization Fading System Model Based on Probe with Built-in Faraday Rotator  
The depolarization fading system model based on a novel probe with a built-in Faraday rotator, which is a Michelson interferometer, is shown in Figure 1. As the optical source of the fiber interferometer, a narrow linewidth tunable continuous wave laser (HP 8168F) with a linewidth of less than 100 kHz is used. The wavelength of the laser is tuned to 1550 nm, which is the low-loss working window of single mode fiber (SMF-28e) used in the system. The laser launches the light into a fiber circulator, and the second port of the circulator is connected to a \(3\times 3\) fiber coupler with equal splitting ratios. The \(3\times 3\) fiber coupler has a symmetrical structure with a \(2\pi/3\) phase difference between the outputs. Two output ends of the \(3\times 3\) fiber coupler are used to connect a single mode fiber at two of the interferometer arms, while the third arm is unused. At the distal end of one interferometer arm, a \(45^{\circ}\) FRM with an optic pigtail is connected and reflects the light. At the distal end of the other arm, a self-designed probe is fabricated. The light is transmitted through the probe and reflected on the surface of a reflector. The light in both interferometer arms transmits in reverse through the fiber and generates an interference in the \(3\times 3\) fiber coupler. All three outputs on the left-hand side of the \(3\times 3\) coupler, including the one coming back through the third port of the circulator, are detected by photodetectors (PDs). Three PDs convert the received optical power into voltage signals. The self-designed probe consists of a collimator, a Faraday rotator, and an achromatic lens, as shown in Figure 2a. Figure 2b is a corresponding picture of the self-designed probe. The collimator expands the light output from the fiber into a beam with a diameter of 7 mm. The Faraday rotator is a configuration of ferromagnetic crystals surrounded by strong permanent magnets, as shown in the  
Figure 1: Schematic of the depolarization fading system based on a probe with a built-in Faraday rotator. red square in Figure 2b. In this paper, the Faraday rotator is fabricated with a rotation angle of 45\({}^{\circ}\) by configuring the thickness of the ferromagnetic crystals and the magnetic flux density of the strong permanent magnets. In addition, the Faraday rotator can be installed and removed expediently. Its center wavelength is 1550 nm, and the transmittance is more than 90%. The size of the ferromagnetic crystals is 8 mm \(\times\) 8 mm, so that the entire beam from the collimator can pass through. Here, an achromatic lens with a focal distance of 20 mm is used to focus the beam on the surface of an object. In addition, with a numerical aperture of 0.172, it can receive more reflected light. If the two interference lights pass through two different single mode fibers, the polarization of light in the two interference arms must be considered, which affects the interference fringe visibility due to birefringence. For the arm with FRM, the working principle of FRM is detailed in [20], with the conclusion being that the input polarization corresponds to the output polarization when light passes through a single mode fiber, a 45\({}^{\circ}\) FRM, and transmits in reverse. Next, we analyze that the interference arm with the self-designed probe has the same effect as FRM. The net birefringence in a length of fiber can be represented by the Jones matrix formalism, as follows [18]:  
\[\overset{\rightarrow}{R}=\frac{\alpha}{d}\left[ \begin{array}{cc}a&-b^{*}\\ b&a^{*}\end{array} \right], \tag{1}\]  
where \(a\) and \(b\) are parameters related to the fiber birefringence; \(*\) denotes the conjugate; \(\alpha\) represents the fiber loss (including the loss of fiber itself and the fiber connection); and \(d=(a.a^{*}+b.b^{*})\). When light transmits through the fiber in the reverse direction, the Jones matrix can be expressed as:  
\[\overset{\leftarrow}{R}=\frac{\alpha}{d}\left[