Abstract

The combination of the chirp of directly modulated semiconductor lasers and the chromatic dispersion of conventional single-mode fiber results in intersymbol interference which can cause significant performance degradations (e.g., see Refs. 1-3). Figure 1 shows a 1.55-μm fiber optic communication system using intensity modulation and direct detection. The laser is directly modulated at 8 Gbit/s. The current pulses (either NRZ or RZ with a 75% duty cycle) are square pulses distorted by laser parasitics and intentional filtering; we assume an equivalent overall response identical to that of an RC circuit with a 4-GHz bandwidth. The laser output is transmitted over an L-km length of single-mode fiber, which has a transfer function Hf(ω) = exp[j(Lλ²Dω²)/(4πc)], where D, the fiber dispersion parameter, is 17 ps/nm/km. The fiber output is passed through the photodetector and receiving filter (third-order Butterworth with a bandwidth of 6.24 GHz). In the equalized system, the fiber output is first operated by the equalizing H_eq(ω) before passing on to the photodetector. The control loop (Fig. 1) represents the means for adapting the frequency position of H_eq(ω).

© 1990 Optical Society of America