 -55.8 -49.9 -51.7 -51.9 -40.5 -45.2 -49.3 -38.7 -43.5 -41.5 -42.5 -39.5 -40.5 -37.5 -34.4 -33.2 -36.5 -31.3 -30.1 -36.6 -28.8 -32.6 aSAGM stands for separated absorption, grating, and multiplication regions. SAM stands for separated absorption and multiplication regions. Source: Ref. 8. Another type of detector is avalanche photodiode (APD), which is a gain device dis- playing gains on the order of 15-20 dB. The PIN diode is not a gain device. Table 9.7 summarizes various detector sensitivities with a "standard" BER of 1 X 10- for some common bit rates. 18 18 Some entities such as SPRINT (US) specify 1 X 10-12 9.4 FIBER-OPTIC COMMUNICATION LINKS Of the two types of photodiodes discussed here, the PIN detector is more economical and requires less complex circuitry that does its APD counterpart. The PIN diode has peak sensitivity from about 800 nm to 900 nm for silicon devices. The overall response time for the PIN diode is good for about 90% of the transient but sluggish for the remaining 10%, which is a "tail." The power response of the tail portion of a pulse may limit the net bit rate on digital systems. The PIN detector does not display gain, whereas the APD does. The response time of the APD is far better than that of the PIN diode, but the APD displays certain temperature instabilities where responsivity can change significantly with temperature. Compensation for temperature is usually required in APD detectors and is often accomplished by a feedback control of bias voltage. It should be noted that bias voltages for APDs are much higher than for PIN diodes, and some APDs require bias voltages as high as 200 V. Both the temperature problem and the high-voltage bias supply complicate repeater design. 9.4.7 Optical Fiber Amplifiers Optical amplifiers amplify incident light through stimulated emission, the same mechanism as used with lasers. These amplifiers are the same as lasers without feedback. Optical gain is achieved when the amplifier is pumped either electrically or optically to realize population inver