 10 = ZDP or = Line width of source spectrum (such as for an LD transmitter), the product is over 100 GHz/km. For an equivalent 3-dB power penalty at a BER of 10-9, the bandwidth-length prod- uct in terms of GHzXkm is determined by the following:10 away from the ZDP (B) (L) at the ZDP (B) = Bandwidth (Gbps) (L) = Length (km) o = Root mean square half-width of the source spectrum (nm) T = Refractive index Multimode fibers cause source-pulse broadening that is due to the dif- ferent propagation speeds of the different modes. Because this usually has a larger effect than chromatic dispersion, multimode systems exhibit more pulse broadening. The different propagating modes of light carrying the information effectively zigzag down the fiber core, angling from side to side Optics Primer because of the total internal reflection between the core and its surround- ing cladding (see Figure 2-23). The zigzag path taken by light in the higher traverse modes means that those rays follow longer paths than the rays of the fundamental transverse mode that travel directly down the center of the fiber's core. Because of these different path lengths, pulses following the transverse modes are gradually spread. Those along the higher modes arrive at the receiver somewhat later than the rays of the fundamental mode. Thus, even though in each digital pulse all the rays have left the source at the same time, modal dispersion broadens the pulse as it travels down the fiber to the detector. The amount of broadening is a function of length and increases as distance increases. This phenomenon increases the BER at the receiving end. 2.3.3 Polarization Mode Dispersion (PMD) PMD is caused by slight fiber asymmetry. During the perform fabrication or during the drawing process, a nonperfectly circular core and/or cladding may result (see Figure 2-24). PMD is the result of the accumulation of weak birefringence along the various fiber spans, enabling a link of any length exceeding 2 or 4 km to be comprised of spliced fibers that originate from di