Page 689 - Introduction to Information Optics

12.2. Optical Network Elements 673

dispersion shifted fiber (NZ-DSF), such as LEAF (large effective area fiber)
fiber made by Corning and True Wave fiber by Lucent, with about ±2
ps/nm-km dispersion in the 1.55 /an region. NZ-DSF fibers are designed
mainly for high-speed long-distance transmission. Its nonzero local dispersion
helps to combat the fiber nonlinearities induced by the high signal power
required for long-haul transmission. The third generation of LEAF also aims
at bringing down PMD, which is quite crucial for 40 Gb/s systems.
The second type of fiber is multimode fiber, which has a large core about
50 /mi in diameter and thus can support the propagation of more than one
propagation mode at the wavelength of interest. In addition to chromatic
dispersion, this type of fiber leads to modal dispersion, which significantly
limits the bandwidth-distance product in system design. It is mainly used for
low speed and intraoffice applications.

The advent of semiconductor-laser-pumped, erbium-doped fiber amplifiers
(EDFAs) accelerated greatly the pace of deployment of high-capacity lightwave
systems, and, combined with DWDM technology, laid the foundation for
optical networking.
Figure 12.3 is a simplified schematic of an EDFA showing the erbium-doped
fiber pumped by a semiconductor purnp laser using a pump/signal multiplexer
[10]. In practice, additional components are usually added to EDFAs, includ-
ing optical isolators to prevent the amplifier from oscillating due to spurious
reflections, additional tap couplers to enable amplifier performance monitor-
ing, and optical filters to compensate the gain spectral profile for DWDM
applications. The critical component of the EDFA is the section of optical fiber

EDF

Pump
Input CouplerX / Isolator Output

Pump
Laser Diode

Fig. 12.3. Single-stage Erbium-doped fiber amplifier configuration.