Equalization Techniques for mitigating transmission impairments

Most current multi-gigabit per second digital fiber optic systems use simple modulation and detection techniques such as on-off keying with matched-filter receiver techniques. However, more complex techniques such as equalization, error control coding, and/or multilevel signaling, can be used in lightwave systems to significantly increase the data rate and/or reduce the effect of transmission impairments and improve performance, and, in many cases can be easily implemented. There are numerous sources of transmission impairments in lightwave systems. These include chromatic dispersion and polarization mode dispersion (PMD) in the fiber, laser and fiber nonlinearities, nonideal receiver response, echo, and distortion caused by semiconductor optical amplifiers. Fortunately, there are also numerous techniques to reduce these impairments, including error control coding, equalization, and modulation techniques with multilevel signaling. These techniques can be used for upgrading existing systems (e.g., reducing chromatic dispersion and PMD in systems with previously installed fiber) or as an alternative to the use of more costly transmitters and receivers (e.g., using coding to permit less stringent laser specifications). The purpose of this chapter is to provide an overview of equalization techniques, which become increasingly important as device technology matures, in which case substantial increases in performance (especially for already installed systems) may only be achieved through these techniques. We will examine the prospects for the application of digital equalization techniques to overcome the above impairments, where the improvement is large and the implementation is relatively simple.