A complete study of polarization mode dispertion (PMD) in optical fibers is presented. A new class of PMD emulators with random mode coupling lengths is proposed and compared experimentally and theoretically with other emulators exinting measurements, based in a optical filter transfer function is also presented and compared with the preceding method.With this new method the time domain spectral characteristics of some high frequency modulated DFB lasers were measured. First and second order PMD effects in high speed digital transmission were evaluated through both temporal distortion and power penalties measurement. The statistical treatment of the power penalties was discussed, showing that signal distortion by PMD may eventually improve the performance of the system. A theoretical model of simulate the signal propagation along the fiber was porposed, based on the emulator conception with random coupling lengths.