摘要： An optical amplifier is a widely used device in optical fiber communications for the purpose of amplification of the optical signal generated by an optical transmitter. The optical signal is directly amplified to yield optical signal without any conversion to electrical signal first and then restoring it to amplified optical signal. This implies that optical amplifiers operate on photons in all-optical domain. In essence, optical amplifiers do require source and photodetector as optoelectronic devices and additional electronic circuits for other operations such as shaping of optical pulses including retiming. But why do we need optical amplifiers? We know that typical fiber loss around 1500-nm wavelength is approximately 0.2 dB/km. Let us examine what happens after the optical signals travel about 100 km down the fiber. The signals are attenuated by 20 dB. So they need to be amplified, otherwise the signal-to-noise ratio of detected signals is too low and bit-error-rate becomes too high (typically desired value of BER is <10-9). One way to resolve this issue is to detect the weak signals, followed by modulating a new laser (optical-to-electrical-to-optical conversions) which requires high-speed (>10 GHz) electronic circuitry. Therefore, the best way to amplify the signal is only optically and the preferred method is fiber amplifier which is the most efficient, the most stable and the one with the lowest loss.