Transient Characteristics Of High-speed Modulation Of Semiconductor Lasers

High-speed modulated semiconductor lasers and tunable semiconductor lasers play an important role in fiber-optic communication systems. They are the essential components in high-capacity WDM systems and coherent transmission systems.Starting from the rate equations of semiconductor lasers, using the small-signal approximation, the analytical expressions for the evolutions of the carrier and photon densities have been derived when the laser is subject to small gain and loss modulations. Comparing the difference between the analytical and numerical solutions, analyzing the variation of first- and second-order small quantities, we discuss the feasibility of the small-signal analysis. The solutions show that it may not be correct to use the relative magnitude of initial perturbation to judge whether the small-signal analysis is applicable. The validity of small-signal approximation depends on the nature of the perturbation. More specifically, if the perturbation comes from a loss (or the threshold carrier density) variation, it is likely that the small-signal analysis becomes invalid.Considering the gain compression factor, we analyze the transient process of gain and loss modulation again in order to check the influence of the gain compression factor on the feasibility of perturbation analysis. It is found that this factor does not change the validity of small-signal approximation in nature, although it may change the decay rate, reduce the degree of oscillation and lessen the difference between the analytical and numerical solutions to certain extent.Solving the multimode rate equations analytically, the dynamic process has been described when the laser is switched from one oscillation mode (or frequency) to another. Meanwhile, we discuss in detail the decay rate of the deviations of the carrier and photon densities. And the decay rate of these deviations depends on not only the carrier lifetime but also the photon lifetime, which is different from that of an ordinary diode laser experiencing turn-on process.The dynamic characteristics of grating tuned bistable external cavity semiconductor lasers (ECLDs) have been described numerically. The effects ofthe tuning angle of the grating on the transient process have been discussed. It is found that a narrow and strong pulse may be observed before the switched-off mode dies out when the output of the bistable ECLD jumps up. This phenomena does not show up in the jump down process. In addition, the ECLD settles down more quickly for the jump-up process. The transient characteristics have also been described numerically when the oscillation frequencies are approximately symmetric about the peak frequency of the gain line. And an account has also been given to the distinct difference of the dynamic processes under these two tuning schemes.