a hetero- junction (also referred to as a heterostructure) resulted in two general types of laser diodes: single heterojunction and double heterojunction. The single-heterojunction laser diode has a high output power, however, it requires a high driving current and has a limited duty cycle. In com- parison, the double-heterojunction laser diode is not limited by its duty cycle. While the ratings of most double-heterojunction laser diodes are between 3 and 10 mW, their optical power can range up to 20 mW. The Light Sources and Detectors wavelengths of both types of laser diodes range from 750 mn at the highest near-infrared energy level down to 390 nm at the lower ultravio- let energy level. Many laser diodes have a wavelength near 900 nm and a rise time to achieve full output between 0.1 and 2 ns. In the evolution of fiber-based networks, the use of lasers at 800 nm represents a first generation of optical networks. Laser systems now com- monly operate at approximately 1300 and 1500 nm, which represents two low-loss windows in fiber where the attenuation level is minimal. As we noted earlier in this chapter, a design goal of a long-distance fiber-optic system is to minimize the number of amplifiers required. To accomplish this goal, communications carriers use laser diodes that operate at wave- lengths where attenuation is minimized, allowing the optical signal to be transmitted further before requiring the signal to be amplified. Fig- ure 4.10 is a cross-sectional view of a typical laser diode. The laser diode is manufactured using a process very similar to the production of large-scale integrated (LSI) and very-large-scale integrated (VLSI) semiconductor circuits. For this reason, the laser diode is also Positive Contact Figure 4.10 A cross-sectional view of a laser diode. Light Emitting Layer Negative Contact Coherent light with narrow emission spectrum Chapter Four commonly referred to as a semiconductor laser and results in the fabrica- tion of a laser diode that is relatively inexpensive to manufacture in q