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Remarkable progress in optical-fiber technology has now taken us beyond the first-generation optical fiber systems involving graded-index fibers and GaAlAs light emitting or laser diodes. When these first systems reached the stage of commercial use, noise related to laser diodes and fibers was hardly recognized as “unknown burst error phenomena.” This is mainly because those early systems were designed somewhat conservatively.

Mode partition noise may become one of the fundamental performance limitations1,2 for single-mode fiber systems. This noise is caused by the Instantaneous fluctuation of the power distribution among the laser longitudinal modes and the differential delay of the longitudinal modes due to chromatic dispersion. Since this noise is a generic function of the laser spectral distribution and the fiber chromatic dispersion, the SNR due to this noise is independent of the signal power. The error rate, therefore, approaches a floor value (asymptotic error rate) as the optical power is increased. We derived a simple formula describing mode partition noise based on two basic assumptions: (1) that the total power of each pulse is constant and (2) that the partition probability function is based on the time averaged spectral distribution. Figure 1 shows the analytic result for the bit rate-distance-mode partition coefficient product (asymptotic error rate of 10−9) for a silica single-mode fiber (minimum dispersion wavelength = 1.3 μm) due to the mode partition noise.

The development of single frequency 1.5 μm laser sources, e.g. distributed feedback (DFB) and coupled cavity (C3) lasers, has dramatically increased the capacity and performance of optical communication systems. These lasers, however, have several drawbacks. Firstly, DFB and lasers are more complex and hence are less reliable than regular Fabry-Perot lasers. Secondly, they suffer from chirp, i.e. a wavelength shift as the lasers are modulated. Thirdly, conventional single frequency lasers have rather broad linewidths, ~50–100 MHz which make them unsuitable as sources in coherent communication system. Further, DFB and C3 lasers are sensitive to optical reflections which may introduce mode partition noise, amplitude noise, and coherence collapse.

We present the newly defined parameter S to quantify the power penalty due to a laser’s mode partition noise (MPN) for a fiber-optic system by assuming the system has a raised cosine pulse shape, an exponential probability distribution for the power of each mode in the laser source, and a negligible sum of the power covariance between each mode for the laser. These assumptions are different from the previous assumptions that a laser has a Gaussian spectrum shape and a constant output power from which the k factor can be defined to characterize MPN and from which the link limited length due to MPN is obtained.1,2 Like the k factor, the parameter S can be used to estimate a laser’s MPN.2 Unlike the k factor, S can be easily measured because of the approximation made of the exponential distribution for the power of each mode.