Impact of Nonlinearities on Fiber Optic Communications

Volume 7 of the series Optical and Fiber Communications Reports pp 219-252


Power-Efficient Modulation Schemes

  • Magnus KarlssonAffiliated withPhotonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology Email author 
  • , Erik AgrellAffiliated withCommunication Systems Group, Department of Signals and Systems, Chalmers University of Technology

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Coherent optical fiber communications had a brief period of popularity in the early 1990s, mainly because the optical links of that day were significantly power limited. Coherent detection provided a possibility of optically amplifying the signal to a power level that, after photodetection, made the thermal noise negligible. Two things, however, caused those coherent systems to be abandoned. The first was the sheer technical difficulties: a coherent receiver requires a local oscillator laser that is to be phase- and polarization-locked to the received signal. This gave rise to significant technical obstacles, and only a few limited and expensive coherent receiver solutions were demonstrated [17, 27]. The second was the development of the Erbium-doper fiber amplifier (EDFA) that provided an elegant and practical solution to the problem of the thermal noise. By 1995, the EDFA was a commodity in fiber communication systems, simple on-off keying modulation worked well enough, and coherent communication was forgotten.