How does polarization mode dispersion affect the performance and reliability of optical systems?

1 What is polarization mode dispersion?

Polarization mode dispersion is a phenomenon that occurs when different polarization modes of light, which are the orientations of the electric field vector of the light wave, travel at different speeds in an optical fiber. This causes a pulse of light to spread or disperse as it propagates along the fiber, resulting in a loss of signal quality and integrity. Polarization mode dispersion is a form of chromatic dispersion, which is the general term for the dependence of the propagation speed of light on its wavelength or frequency.

2 How does polarization mode dispersion occur?

Polarization mode dispersion occurs because of the inherent birefringence of optical fibers, which is the difference in the refractive index of the fiber for different polarization modes. Birefringence can be caused by several factors, such as the asymmetry of the fiber cross-section, the stress or strain applied to the fiber during manufacturing or installation, the environmental conditions such as temperature or humidity, and the random fluctuations or perturbations of the fiber along its length. These factors create variations in the birefringence of the fiber, which in turn create variations in the propagation time of different polarization modes.

4 How can polarization mode dispersion be measured?

Polarization mode dispersion can be measured by using different methods and instruments, depending on the type and level of polarization mode dispersion. One of the most common methods is the interferometric method, which uses an interferometer to split a pulse of light into two orthogonal polarization modes, send them through the fiber under test, and recombine them at the output. The interference pattern at the output depends on the difference in the propagation time of the two polarization modes, which is called the differential group delay (DGD). The DGD is a measure of the polarization mode dispersion of the fiber. Another method is the polarization optical time domain reflectometry (POTDR) method, which uses a polarized light source and a polarizer to measure the backscattered light from the fiber under test. The backscattered light depends on the birefringence of the fiber, which is related to the polarization mode dispersion.

5 How can polarization mode dispersion be mitigated?

Polarization mode dispersion can be mitigated by using different techniques and devices, depending on the source and level of polarization mode dispersion. One of the most common techniques is the polarization mode dispersion compensation (PMDC) technique, which uses a device that introduces a negative or opposite polarization mode dispersion to the signal, canceling out the positive or original polarization mode dispersion. The PMDC device can be either fixed or adaptive, depending on whether it has a constant or variable polarization mode dispersion value. Another technique is the polarization diversity technique, which uses two receivers with orthogonal polarization orientations to detect the signal. The polarization diversity technique reduces the impact of polarization mode dispersion by combining the signals from both receivers.