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Unformatted text preview: 58 Propagation of Signals in Optical Fiber planned for the future. An important type of laser used for these applications is the vertical cavity surface-emitting laser (VCSEL; see Subsection 3.5.1). These lasers transmit at 850 nm and up to 10 Gb/s. VCSELs at 1300 nm have been demonstrated but are not in wide use commercially. A different launch condition, called the effective laser launch , in which only a small subset of modes are excited, better represents what happens when laser transmitters are used. This launch condition is used to specify OM3 and OM4 fibers, where the bandwidth is referred to as the effective modal bandwidth (EMB). The uses of lasers instead of LEDs results in a higher bandwidth–distance product, as can be seen from Table 2.1. In practice, variations in alignment between laser and fiber, and variations in the lasers themselves, cause different modes to be excited, complicating the assurance of effective bandwidth. The solution has been to improve the quality of OM3 and OM4 fiber, particularly near the center of core, such that the dispersion is kept sufficiently small regardless of which subset of modes are excited. Parallel Ribbon Fiber Although most fiber links are serial high-speed (and unidirectional) connections, there do exist parallel fiber connections as well. A typical parallel connection uses a standard 12-wide ribbon of fibers for a distance up to tens or hundreds of meters. The fibers are individually protected by a plastic jacket layer before being assembled side by side, where the spacing is 250 microns. The most common usage is multimode, where VCSEL arrays can be used as transmitters. Single-mode ribbon fiber exists as well. Connection specifications for parallel ribbon fiber are developed through multisource agreements (MSAs) among vendors. Example MSAs are SNAP12 and QSFP (Quad Small Form Factor Pluggable). 2.3 Optical Fiber as a Waveguide In order to completely overcome intermodal dispersion, you must use fibers whose core radius is appreciably smaller and of the order of the operating wavelength. Such fibers have only one mode, ray, or path in which light can propagate. These fibers are called single-mode fibers. A useful way to conceptualize propagation in a single-mode fiber is to treat the light as a single beam. The following physical explanation for the propagation of light in single-mode fiber is based on [Neu88]. In any medium with a constant refractive index, a narrow light beam tends to spread due to a phenomenon called diffraction. Thus, in such a medium, the beam width will increase as light propagates. Note that this diffraction phenomenon is what makes the geometric optical approach invalid for single-mode fibers, that is, an optical signal cannot be modeled as a ray. 2.3 Optical Fiber as a Waveguide 59 The diffraction effect can be counteracted by focusing the light with a lens. To illustrate this concept, imagine a chain of convex lenses that bring the beam back to size periodically. The beam center travels slightly slower than the beam periphery sosize periodically....
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- Spring '09