Coherent optical fiber transmission systems

The feasibility of coherent optical fiber transmission systems, in which laser coherence is utilized to carry information, are studied from the view point of device and system consideration and expected performance. Modulation scheme comparison among optical ASK, FSK, and PSK, transmitter and receiver configurations, single-polarization transmission through fibers, problems in providing stable local oscillator waves, and wide-band photodetectors are discussed together with alternative technologies to realize high performance systems. A study on digital system impairments caused by FM quantum noise of laser oscillators, interferometric FM-AM conversion noise, IF frequency fluctuation, and optical fiber transmission turbulence show the feasibility of the systems. Repeater spacing is estimated by considering the transmitting and receiving signal levels, optical fiber loss, and fiber transmission capacity. Repeater spacing of 240 km is feasible by 400 Mbit/s PSK homodyne-detection system and 220 km by 400 Mbit/s FSK heterodyne-discrimination detection system. The regenerative repeater spacing can be expanded further by employing intermediate repeaters with direct optical signal amplification. Based on the performance of semiconductor laser amplifiers, such as traveling wave type, Fabry-Perot cavity type, and injection locked devices, it is estimated that regenerative repeater spacing of more than 10⁴km is feasible with 50 km intermediate repeater spacing. These systems will find application in transoceanic optical fiber cable transmission as well as terrestrial long distance transmission systems.