f- ferent manufacturing events. An input light pulse of an initial polarization will decompose into two pulses with two different polarizations separated Lost light Figure 2-23 Angle of acceptance Zigzag rays in a multimode fiber. Cladding Low-order mode High-order mode Light rays Cladding (Three modes are shown in the figure; in actuality hundreds of discrete modes typically propagate in a single fiber.) Low-order modes, those launched into the fiber at small angles with respect to the axis, travel a shorter ray distance to reach a given point along the fiber than do the high-order modes, those launched at large angles. Chapter 2 Cross Section Figure 2-24 Longitudinal view Elliptical fiber core PMD = Time delay Circular Linear Elliptical polarization polarization polarization Fiber segments (typically spliced together) in time. The time separation leads to decreased system margin or even out- ages. This issue becomes more pronounced and problematic at higher speeds (such as OC-192 and OC-768). In addition to noncircular cores and cladding from the manufacturing process, bends and/or twist-induced stresses also introduce PMD. Further- more, PMD also depends on the temperature of the fiber, sometimes mak- ing transmission problematic on a seasonal basis. System components such as isolators and circulators also have a bearing on the issue. The problem is more pronounced on older fibers and systems. At this time, fiber can be drawn in a manner that minimizes PMD; also, components can be manu- factured that keep PMD low. Older fibers have PMD in the range of 0.5 ps/km ²²² newer fibers range around 0.1 ps/km ¹/². The expectation is that this figure will decrease to 0.05 for future fibers. Forward error correction (FEC) methods may also be used as an effective tool against nonlinear effects. Tunable PMD compensators are also being developed at this time. 2.3.4 Other Impairments Noise is measured in terms of the optical signal-to-noise ratio (OSNR). Each (optical) amplifier along a fiber path adds noise, giving rise to 