 to be as high as 1 ps/km ²/2, which could create severe problems at high datarates. More recently, manufacturers have invented methods of achieving values of less than 0.05 ps/km ²/2. The full details of their processes are not necessarily disclosed, but the main technique is called "spun fiber" (Damask, 2005). In the spun-fiber technique, after being pulled from the perform rod the fiber is still at a high temperature and therefore in a viscous state. At this point it is passed through a device called a spinner, the role of which is to create a fiber that consists of short alternating spans that act as concatenated birefringent fibers with their principal axes aligned at 90° with respect to each other. As the spinning is performed when the glass is still viscous, the result is different from that when the fiber is twisted at room temperature, because (ideally, at least) it does not impose stress in the glass. There are two mechanisms by which such a fiber reduces the PMD (Galtarossa et al., 2005). First, the correlation length, L, is significantly shortened with respect to the beat length, Lb, and this reduces the mean DGD. Second (and more important), any unintended accumulated birefringence in one of the spun half-periods tends to be canceled as the guided wave passes to the next half-period with its axis aligned perpendicularly. Polarization Mode Dispersion PMD Caused by Optical Components Optical components also contribute to the overall PMD of a system, and their influence is gaining in relative importance as operating bitrates increase. Sachiano (2005) argues that three potentially significant contributors are Dispersion-compensating fiber L-band erbium-doped fiber amplifiers Chirped fiber Bragg gratings DCF has waveguide designs and fabrication requirements that differ greatly from those of standardized transmission fibers, and manufacturers strive to ensure that these do not result in additional intrinsic contributions. In C-band EDFAs the rare-earth-doped fiber is typically 10 to 50 m in length and has