Equalization of Multimode Optical Fiber Systems

It has been recognized for some time1 that conventional linear equalization is of limited benefit in improving the performance of multimode optical fiber channels. The major reason is the rapid falloff in the equivalent baseband frequency response of the optical channel, which results in considerable noise enhancement when linear equalization is attempted using a filter which emphasizes the high frequencies. However, for data rates above 100 Mb/s, multimode systems are generally modal dispersion rather than loss-limited2 and some form of equalization is highly desirable. This is particularly true considering that index-grading imperfections in presently manufactured fibers produce a wide spread in fiber bandwidths and that equalization could improve the yield of usable fibers. Decision feedback equalization (DFE) is known to be superior to linear equalization for channels which exhibit amplitude distortion.-1,4 Previous theoretical investigations0,6 of DFE for fiber transmission systems have shown that power penalties are significantly lower than 1367