In a non-polarization-maintaining fiber, the output polarization state is generally unpredictable and varies randomly due to environmental factors and fiber imperfections.

Polarization Behavior in Non-PM Fibers

Non-polarization-maintaining fibers do not have built-in birefringence to preserve a specific polarization state. As a result, light launched into such fibers experiences random coupling between polarization modes during propagation. This occurs because the fiber's circular symmetry allows multiple polarization modes to have nearly identical phase velocities, and small perturbations—such as bending, twisting, or stress—cause mode coupling, altering the net polarization state along the fiber length .

Factors Affecting Output Polarization

1. Fiber Stress and Bending: Mechanical stress or coiling introduces local birefringence, which changes the relative phase between polarization components, leading to elliptical or rotated polarization at the output .
2. Temperature Variations: Thermal fluctuations modify the refractive index and stress distribution in the fiber, causing time-dependent polarization changes .
3. Fiber Length: Longer fibers accumulate more random phase shifts between polarization modes, increasing the unpredictability of the output polarization .
4. Wavelength Dependence: The polarization evolution is also wavelength-dependent, as the phase difference between modes varies with wavelength .

Controlling or Stabilizing Polarization

Although non-PM fibers do not inherently preserve polarization, the output polarization can be partially controlled using external devices:

• Polarization Controllers: Adjustable fiber paddles or waveplates can rotate and compensate the polarization state to achieve a desired output .
• Feedback Loops: In high-power fiber amplifiers, a feedback system can monitor the output polarization and adjust the input or fiber paddles to stabilize the state .
• Coiling-Induced Birefringence: Deliberate coiling of the fiber can introduce a controlled birefringence to influence the output polarization, though this is less precise than using PM fibers .

Practical Implications

For applications requiring stable or known polarization, non-PM fibers are generally unsuitable without active control. Random polarization changes can affect:

• Interferometric measurements
• Nonlinear optical processes (e.g., stimulated Brillouin scattering)
• Quantum optics experiments, where polarization noise can degrade entanglement or squeezing In summary, the output polarization of a non-PM fiber is highly sensitive to environmental and mechanical factors, and without active control, it is unpredictable and fluctuates over time. For stable polarization, either PM fibers or active polarization control systems are recommended.