Semiconductor Laser Amplifier for Single Mode Optical Fiber Communications

The recent advent of very low loss single mode optical fibers (< 0.2 dB/km at the 1.55μπι wavelength) opens the possibility for very long distance, high information bandwidth (^1 Gbit/s) communication systems, as transoceanic cable links or trunk networks. With such fibers, especially when chromatic dispersion is negligible, the range is only limited by attenuation, so there would be no need for complete signal regeneration (pulse shaping, timing, etc...). In these conditions, direct light amplifiers would be very attractive as "on-line" amplifiers as an alternative to sophisticated optoelectronic repeaters; they could also be used in a receiver as low noise preamplifiers, in combination with a pinphotodiode, especially at the 1.3 ... 1.55 μη wavelength, where avalanche photodiodes (APD) are more "noisy" than silicon APDs. It is our purpose to discuss the possibility for semiconductor laser amplifiers to be used in a single mode fiber communication system. We shall first derive the power budget improvement due to the insertion of a laser amplifier in a digital optical communication system. A review of semiconductor laser amplifiers (SCLA) characteristics of the traveling-wave type (TW), Fabry-Perot type (FP) and injection locked laser type (ILL) will then be presented, with emphasis on recent experimental results. Finally, possible applications of these devices in single mode fiber communication systems will be discussed. 2 Noise characteristics of a laser amplifier in an optical communication system