Study On Modes Distinction And Bistable Characteristics Of Two-Segment DFB Semiconductor Lasers

With the development of optical communications, the distributed feedback (DFB) lasers have become the necessary parts in optical communication systems with high speed and great capacity. Two-segment DFB semiconductor lasers, as some kind of tunable lasers, have become a hot point in present research work. They have many merits such as narrow linewidth, high mode-suppression ratio and high reliability, etc. Meanwhile, they also have the characteristics of bistability and wavelength conversion, so it can be expected that two-segment DFB lasers will have a very wide applied prospect in the domain of future optoelectronic technologies.In the former work of our research group, the carrier concentration in each segment is supposed to be uniform throughout the cavity length based on the fact of the carrier distribution in two-segment DFB lasers. Furthermore, the single-segment coupling-wave theory has been applied for two-segment DFB lasers, and the threshold expression of the carrier concentration in each segment and the lasing wavelength has been obtained. Subsequently in this paper, a new mathematical formula about the modes discrimination of two-segment DFB lasers has been derived from the obtained threshold expression. Compared with TMM model, this formula gives out a visible expression implying the physical meaning about the modes discrimination for the first time.In the aspect of steady-state characteristics, the strict coupling-wave theory is used to describe the photon density in the cavity to take place of the photon equation in current model of Kuznetsov's rate equations. After applying the boundary conditions and the continuous condition of two segments, the correlative dependence of average photon density in each segment has been obtained through mathematical calculus. Then, the bistable characteristics of two-segment DFB lasers have been studied by using the formula about the modes discrimination. The calculating results can well explain the characteristic of wavelength switching and explain the fact that the threshold point is not identical with the bistable turn-on point in special case.