 no exposed active parts and do not require hermetic packaging. In addi- tion, this also means that VCSELs can be packaged in plastics instead of ceramics, which further reduces the manufacturing cost. Because VCSELs have no exposed active parts, they have a higher level of reliability than do edge-emitting devices, where the active material is exposed. Because the VCSEL is considerably smaller than an edge emit- ter, it can support less costly light-coupling methods. In fact, connector coupling tolerances for VCSELs can be as liberal as 5 to 10 um, compared to the much tighter 1-um tolerance required by edge emitters. Although VCSELs have many advantages, they are not problem-free. One key problem associated with the VCSEL is the fact that p-type material absorbs light much more strongly at 1300 nm than at 850 nm. As a result, it is difficult to design a VCSEL structure that has a suffi- ciently low level of absorption but is still laser-capable. Until the year 2000, this was the key problem preventing the development of long- wavelength VCSELs. However, during that year several manufacturers developed techniques that enable VCSELs to be fabricated with nonab- sorbing mirrors. In addition, the use of different materials combined with nonabsorbing mirrors resulted in the creation of very efficient 1.3- nm VCSEL devices whose use in WAN communication systems is expected to represent a viable emerging technology. TUNABLE LASERS A second generation of lasers that warrant discussion are tunable lasers. Although tunable lasers based on the use of a liquid medi- um have been available for many years, far more compact and less costly solid-state lasers were not available until quite recently. The development of tunable lasers is a necessity for implementing a lower-cost method of wave- length division multiplexing (WDM) that enables an array of wavelengths to be generated, each transporting a separate signal on a common fiber. Team-Fly® Light Sources and Detectors There are several types of tunable lasers, some of which