ce into an optical fiber vary, as do coupling efficiencies. To avoid ambiguous specifications on source output powers, such powers should be stated at the pigtail. A pigtail is a short piece of optical fiber coupled to the source at the factory and, as such, is an integral part of the source. Of course, the pigtail should be the same type of fiber as that specified for the link. LED lifetimes are in the order of 100,000 hr mean time between failures (MTBF) with up to a million hours reported in the literature. Many manufacturers guarantee a semicon- ductor laser for 20,000 hr or more. About 150,000 hr can be expected from semiconductor lasers after stressing and culling of unstable units. Such semiconductors are used in the latest TAT and PTAT series of undersea cables connecting North America and Europe. Current fiber-optic communication systems operate in the nominal wavelength regions of 820 nm, 1330 nm, and 1550 nm. Figure 9.30 is a plot of attenuation per unit length ver- sus wavelength. Based on this curve, we can expect about 3 dB/km at 820 nm, 0.50 dB/km at 1330 nm, and at 1550 nm some 0.25 dB/km of attenuation. Also take note that at about 1300 nm is a region of zero dispersion. For added expense, fiber is available with the dispersion minimum shifted to the 1550-nm band, where attenuation per unit length is minimum. There is mature technology at all three wavelengths. 9.4.6 Light Detectors The most commonly used detectors (receivers) for fiber-optic communication systems are photodiodes, either PIN or APD. The terminology PIN derives from the semiconductor construction of the device where an intrinsic (I) material is used between the p-n junction of the diode. CONCEPTS IN TRANSMISSION TRANSPORT dB/km Figure 9.30 Attenuation per unit length versus wavelength of glass fiber (Ref. 7). Table 9.7 Summary of Receiver Diode Sensitivities, Average Received Optical Power in dBm, BER = 1 X 10-9 InGaAs InGaAs InGaAs PIN Ge APD Photoconductor Bit Rate (Mbps) 1.3 um 1.55 um 1.3 um 1.3 um 1.55 um 1.3 um 1.55 um -52,5