1. The essence of "polarization-maintaining":
The goal of polarization-maintaining fiber is to maintain the polarization direction of linearly polarized light in the optical signal unchanged. It creates high birefringence by introducing strong and controllable asymmetry near the core (usually two symmetrical stress zones, such as the most common panda eyes). This high birefringence causes a significant difference in the effective refractive index of the optical fiber on the two perpendicular principal axes (slow axis and fast axis).
When linearly polarized light is incident precisely along one of the principal axes (slow axis or fast axis), due to the huge difference in propagation constants between the two orthogonal polarization components, almost no energy coupling occurs between them, thereby maintaining the incident polarization state.
2. Characteristics of multimode fiber:
Multiple transmission modes: The core diameter of multimode fiber is large (usually >50μm), allowing multiple spatial modes to be transmitted simultaneously.
Mode diversity: Each mode has a different electric field distribution in the cross section of the optical fiber, and its propagation path is also different.
3. Why multimode and "polarization maintenance" are incompatible:
It is impossible to unify the polarization axis of all modes: Even if you introduce stress areas or geometric asymmetries (such as elliptical cores) in multimode fiber similar to single-mode polarization-maintaining fiber, the impact of this asymmetry on different modes is completely different. One mode may experience strong birefringence, and its polarization axis is in a certain direction; while another mode may experience weaker or even different birefringence, and its polarization axis is in another direction. There is no unified "slow axis" or "fast axis" that allows all modes to be precisely aligned and maintain polarization.
Mode coupling destroys polarization characteristics: This is the most critical point. The inherent and unavoidable inter-mode coupling phenomenon in multimode fiber will completely destroy any attempt to maintain polarization. Even if a mode is initially well polarized, once it couples with another mode, the energy will be transferred to that mode.
So multimode fiber polarization maintenance is meaningless.