DESIGN AND PERFORMANCE OF EXTERNALLY MODULATED 1.5-MU-M LASER TRANSMITTER IN THE PRESENCE OF CHROMATIC DISPERSION

The ultimate capacity and length of fiber-optic systems using optical amplifiers is primarily limited by noise and dispersion. Due to their nearly ideal electrooptic characteristics, externally modulated 1.5-mu m lasers are expected to be the technology of choice in these long unr epeatered systems. However, here too there are limitations. In this pa per we review key laser and modulator characteristics that impact the use of externally modulated lasers in the presence of chromatic disper sion excluding effects due to fiber nonlinearities. After a brief revi ew of transmission performance with directly modulated 1.5-pm DFB lase rs which have limited application of up to 80 km at 2.5 Gb/s, we discu ss the key design characteristics of externally modulated transmitters . We then present experimental results showing the effects of modulato r chirp and laser linewidth at a 2.5-Gb/s transmission rate. We find t hat lasers with CW linewidth under 100 MHz will have less than 2-dB di spersion penalty for 600 km of non-dispersion-shifted fiber. Furthermo re, lower dispersion penalties can be realized if the modulator chirp is tuned so as to narrow the transmitted pulses. In addition, excellen t modulator stability is demonstrated for 60 days of error- and degrad ation-free 2.5-Gb/s operation.