Dispersion compensation-An Introduction

Fiber dispersion is one of the most critical parameters that need to be considered for high speed transmission design. Dispersion typically varies with wavelength and accumulates along a fiber length. Therefore it is difficult to precisely compensate for all the propagating channels with fixed optical compensation modules at the same time. When preferred, dispersion at each channel can be fully compensated electronically in the DSP of the receiver.

Dispersion compensation basically means eliminating the compounded dispersion originating from length of the fiber. There might be a misconception that having a fiber with zero dispersion would avoid such a drama. However this is not true as verified from older experience with dispersion-shifted fiber (DSF). A DSF is designed with zero dispersion between 1525 nm and 1575 nm. This would work perfectly for a single channel transmission inside this window. However in DWDM transmission, it gives rise to undesired levels Four Wave Mixing which renders DWDM transmission practically impossible. Therefore, the goal is not to reduce the dispersion to zero but to avoid excessive temporal broadening of the pulses so that the residual dispersion is still within the tolerable limits of the system.

Several technologies exist for chromatic dispersion compensation, such as dispersion compensating fiber/ unit (DCF/U), dispersion managed cables, higher-order mode DCF, fiber Bragg gratings and optical phase conjugation. The widely used compensation is via the dispersion compensating fiber. In general, chromatic dispersion can be compensated in lump or according to dispersion map/management. In lump compensation, the accumulated dispersion is compensated in bulk using in-line dispersion compensation while with dispersion management the local dispersion evolution along the link is compensated utilizing the DCFs as shown in the figure.