Modulation Formats for High-Speed, Long-Haul Fiber Optic Communication Systems
The goal of an optical fiber communication system is to transmit the maximum number of bits per second over the maximum possible distance with the fewest errors. Electrical data signals are converted to optical signals via a modulator. A "1" is transmitted as a pulse of light while a "0" has no light output. The number of "1s" and "0s" transmitted per second determines the speed of the link (bit rate). Glass optical fibers have a wide transmission window over which a number of optical signal channels may be transmitted simultaneously by wavelength division multiplexing (WDM). The power of all the channels combined is boosted by an optical amplifier before being launched into an optical fiber. The launched power generally compensates for the fiber transmission loss of a given fiber stage (span). After each span, the signals are amplified by an optical line amplifier (e.g., Erbium doped fiber amplifier), or repeater. Since transmission fiber is a dispersive medium, implying that pulses spread as they travel through the fiber, some form of dispersion compensation is applied at each repeater stage. At the receiving end of the link, the WDM optical signal is de-multiplexed. Each channel is optically pre-amplified and then detected by an optical-to-electrical (O/E) converter (e.g., a photodiode). A decision circuit identifies the "1s" and "0s" in the signal. An optical filter can be inserted before the O/E converter to filter out amplifier noise.
In this paper, we will focus on the modulation scheme for long-haul systems, i.e., the format used to create the optical pulses. The simplest modulation scheme is a non-return-to-zero (NRZ) format, where the pulse is on for the entire bit period.