Improved model to discriminate adiabatic and transient chirps in directly modulated semiconductor lasers

An improved model to discriminate the adiabatic and transient frequency chirping in directly modulated lasers (DMLs) is proposed. Such a model is very useful for evaluating per se the effects of modulation-induced adiabatic and transient frequency chirping of DMLs on optical communication system performance. It is shown that the mostly used model, relating the frequency deviation to the optical power waveform at laser output, can lead to remarkable inaccuracy in performance evaluation of directly-modulated (DM) multi-span systems operating at very high bit rates, such as 40 Gbit s(-1), by under-estimating the DML transient chirp. Using numerical simulation, a good agreement of the frequency chirping predicted by the proposed model with the one estimated by the laser rate equations is shown. Its good accuracy on assessing the performance of a DM multi-span system operating at 40 Gbit s(-1) is also reported.