PRECLUDING NONLINEAR ISI IN DIRECT DETECTION LONG-HAUL FIBER OPTIC SYSTEMS

Long-distance, high-rate fiber optic systems employing directly modulated 1.55-mu-m single-mode lasers and conventional single-mode fiber suffer severe intersymbol interference (ISI) with a large nonlinear component. Simple postdetection linear equalizers have been shown to reduce the linear portion of the ISI, but they are of limited effectiveness against the nonlinear component. In this letter we investigate a method to reduce the nonlinearity of the ISI, thereby making linear equalization more viable. We show that the degree of nonlinearity is highly dependent on the choice of laser bias current, and that in some cases the ISI nonlinearity can be significantly reduced by biasing the laser substantially above threshold. For the parameters considered here, simulation results predict that an increase in signal to nonlinear distortion ratio (SDR) as high as 25 dB can be achieved for synchronously spaced samples at an optimal sampling phase by increasing the bias current from 1.2 times threshold to 3.5 times threshold.