a fiber using epoxy resin. By eliminating a connector, Metal Indium Phosphorous Metal Figure 4.6 Contact Contact A cross-sectional view of a surface LED. Light is emitted over a broad spectrum, and even the use of a focusing lens results in a coupling Multimode fiber efficiency of approxi- mately 2 percent. Silicon Silicon Dioxide Antireflection coating Dioxide Indium Gallium Arsenide Phosphorous Team-Fly® Light Sources and Detectors it becomes possible to eliminate a small portion of attenuation associat- ed with coupling of an LED to fiber. For short-range fiber-optic transmission at distances of up to 100 m, you can use visible light generating LEDs with either glass or plastic fiber. For longer-range transmission it is important to use an LED that generates light that has the lowest level of attenuation in dB/km for the fiber to be used. This results in infrared LEDs becoming more suitable than LEDs that generate visible light. Thus, LEDs with a wavelength between 800 and 900 nm or above 1000 nm are commonly used for transmission beyond distances of 100 m. The selection of a particular LED wavelength is commonly related to the optical windows associated with the properties of optical cables. As we noted in Chapter 3, the rela- tionship between attenuation and wavelength of optical signals is such that there are three wavelength bands or windows where attenuation is minimal. By using an optical transmitter functioning within a window, it becomes possible to minimize optical attenuation, which, in turn, enables longer cable runs before optical amplification becomes necessary. The most popular optical window used by LEDs is currently the first one that occurs at wavelengths between 800 and 850 nm. While LEDs operating at lower wavelengths are still used for relatively short distances, at distances beyond 100 m the use of an 850-nm LED is most popular. COUPLING LOSS One important limitation associated with all LEDs regardless of the type of light emitted is the coupling loss associated with mating the LED to an o