of LEDs is the optical pulse rise and fall times. Because light is not instantly turned on when the forward bias becomes operative, there is a delay in obtaining an applicable light pulse. Similarly, there is a second delay between toggling the forward bias off and the light pulse attenuating to black. As a result of these delays, the maximum achievable LED data transmission rate is approximately 300 Mbits/s. Figure 4.5 A light-emitting diode is a p-n semi- conductor that emits light when a forward bias is applied. Light Chapter Four OPERATIONAL RANGE The ratio of the number of emitted pho- tons to the number of electrons that cross the p-n junction represents the quantum efficiency of an LED. While the quantum efficiency of an LED is important, another important characteristic is the wavelength of emitted light. LED emissions occur in the visible part of the spectrum with wavelengths ranging from 400 to 700 nm or in the near-infrared band with wavelengths between 2.0 and 0.7 nm. Some commonly avail- able LEDs operate at wavelengths of 565 nm and generate green light, and some operate at 560 nm and generate red light. In fact, LEDs that operate at 660 nm are commonly used for panel readouts that generate information at sporting events and similar functions. Figure 4.6 is a cross-sectional view of a surface LED. Here the term sur- face indicates that light is emitted from the surface of the LED. Note that the doping material for this particular type of LED is indium gallium arsenide phosphorous (InGaAsP) and indium phosphorous (InP). Also note the use of silicon dioxide (SiO2) as an antireflection coating along the surface of the diode. Because an LED generates light over a wide angle, the ability to direct light into a fiber is difficult. In fact, LEDs have a coupling efficiency of approximately 2 percent, even when a coupler with a focusing lens is employed to direct light from the LED into an optical fiber. In one tech- nique for increasing the coupling efficiency of LEDs to optical fiber, the LED is bonded to 