The effects of pre-chirping on the repeaterless transmission distance of high-speed optical communication systems are investigated. For an externally modulated system, the pre-chirping increases the transmission distance by decreasing the spectral broadening of optical signal due to self-phase modulation. The optimum pre-chirping coefficient decreases as the optical power coupled to the transmission fiber increases. For 10 Gbit/s signals, the transmission distance for 1 dB dispersion penalty can be extended up to 140 km by optimizing the pre-chirping and the transmission power, while the maximum transmission distance of the linear transmission system with pre-chirping is about 100 km. It is possible to decrease the dispersion penalty by reducing the extinction ratio of the modulated signal, because a higher 'space' power level can delay the dispersion-induced signal distortion more effectively. However, there is a trade-off between the dispersion penalty and the extinction ratio penalty. For 200 km transmission distance, the optimum extinction ratio is about 10 dB with the input optical power of 16 dBm and the pre-chirping coefficient of -0.6. The maximum transmission distance is limited to about 200 km for the power penalty less than 2 dB.
In case of a directly modulated system the effects of chirping are more complex than those in an externally modulated system. For the directly modulated signal with a small chirping parameter, a long transmission distance can be achieved when the frequency modulation index is comparable to the amplitude modulation index. In this region, one side band of the amplitude modulation is suppressed and the spectrum of the modulated signal is similar to a single side-band large-carrier spectrum. By considering the harmonic components and the phase mismatch between AM and FM of the directly modulated signal, the maximum transmission distance is obtained when $f_r \approx 4 f_m$ and ξ$＼lapprox 0.4$. Here, $f_m$ is the modulation frequency, $f_r$ and ξ are the relaxation oscillation frequency and the damping ratio of the small signal modulation response of the laser, respectively. For the semiconductor laser with large chirping parameter, there exist windows for long-haul transmission in $f_r$ and ξ domain. The windows result from narrowing of -20 dB spectral width due to the interaction of AM and FM. For $f_ > 2 f_m$, the optimum ξ's are about 0.4 and 1.1. Effects of the laser diode driver bandwidth, the extinction ratio, and the fiber nonlinearity on the transmission distance are also discussed.
In the dispersion-managed 10 Gbit/s transmission system using a dispersion compensating fiber the interaction of pre-chirping and SPM is numerically investigated. The pre-chirping suppresses the chirping induced by self-phase modulation in the post-compensation scheme, while enhancing the signal distortion caused by self-phase modulation in the pre-compensation scheme. The physical mechanism of signal distortion in a high power dispersion compensated transmission system is also explained. In the post-compensation scheme, the maximum power margin is obtained for the pre-chirping coefficient of 1 ~ 2 and the dispersion compensation ratio of 70%~80%
