e Lasers With the advent of DWDM, the need arises to have tunable lasers that can be used in lieu of a large inventory of fixed-wavelength line cards. Today's DFB lasers are unique to a given wavelength; a DWDM system with, say, 100 channels presents a daunting challenge to the service provider in terms Optics Primer of sparing and maintenance. In case of an outage on a system, the carrier must be able to find the exact replacement laser. Tunable lasers also reduce system maintenance and troubleshooting activities, thereby reducing sys- tem operating and maintenance costs. A tunable laser is a single replace- ment for all static sources; sparing is done with a single, common, or configurable card. Tunable lasers are also at the basis of lambda bandwidth on demand and intelligent optical networking Hence, transport nodes can match available capacity to real-time demand. Routes can be set up across the network for an end-to-end lambda circuit; this also enables the dynamic setup and teardown of lambdas based on downstream requirements. Pro- ponents see these services entering the market on a wide scale in the near future. Also these devices support wavelength conversion, wavelength switching, and wavelength add/drop. The Intelligent Optical Network (ION), discussed in more detail in Chapter 7, "All-Optical Networks," and Chapter 8, "GMPLS in Optical Networks," is an architecture based on all- optical communication with a control mechanism for both the User- Network Interface (UNI) and the Network-Node Interface (NNI). Tunable laser families include Distributed Feedback (DFB) Distributed Bragg Reflector (DBR) Vertical Cavity Surface-Emitting Laser (VCSEL) External Cavity Diode Laser (ECDL) Some aspects of this technology can be seen in Figure 2-29. LEDs, the least expensive fiber-optic transmitters, are used for multi- mode transmission at 850 or 1,310 nm, but they are limited in speed. FP LDs are relatively inexpensive and are typically used for single-mode trans- mission at 1,310 nm. DFB LDs are the most expensive