 are more suit- able than others for optical communications. Organic-dye lasers use a liquid medium, in which the dyes are used to produce a range of vibra- tion states that can generate a band of wavelengths from 300 to 1100 nm. Unfortunately, organic dyes degrade and result in the need for periodic replacement. A second more practical tunable laser is a vibronic laser. This type of tunable laser operates at a higher range of wavelengths than does a dye-based laser, and its solid-state composition makes it more practical for communications. The basis for the operation of vibronic lasers is the use of certain solid-state media that allow a link between the applied electronic energy level and the vibration rate of the crystalline lattice of the medium. The use of several materials now permits tuning between 660 and 1180 nm. The best type of tunable laser for communications is at present the tuning laser diode. Two different methods are used to tune this type of laser. In one method, which is referred to as a distributed-feedback laser, fre- quency is varied by changing either the temperature of the diode or the input current applied to the laser diode. This type of laser employs a grating structure fabricated along the active layer of the diode. The grat- ing structure is spaced to provide feedback only for the wavelength spec- ified by the grating spacing. When an external-cavity laser is used as the foundation for a tunable laser diode, the cavity is employed as the tuning mechanism. Here the cavity is adjusted by placing grating beyond the active laser instead of along the active layer used by a distributed-feedback laser diode. Now that we have an appreciation for some of the developments occurring in the field of laser technology, let's turn our attention to another key component of an optical fiber transmission system. That component is the photodetector. Photodetectors While a light source is an important element of a fiber-optic transmis- sion system, the photodetector is equally important. The photodetect