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The above-threshold lasing characteristics of DFB semiconductor laser diodes were presented in the previous chapter using a modified transfer matrix. Instead of using an averaged carrier concentration, the inclusion of the actual carrier distribution allows phenomena such as the spatial hole burning effect and non-linear gain to be included. In the analysis, a parabolic gain model and high-order carrier recombination were assumed. Lasing mode characteristics such as the longitudinal distribution of carrier density, photon density, refractive index and the internal field intensity were shown for various laser structures | 7 Above-Threshold Analysis of Various DFB Laser Structures Using the TMM 7.1 INTRODUCTION The above-threshold lasing characteristics of DFB semiconductor laser diodes were presented in the previous chapter using a modified transfer matrix. Instead of using an averaged carrier concentration the inclusion of the actual carrier distribution allows phenomena such as the spatial hole burning effect and non-linear gain to be included. In the analysis a parabolic gain model and high-order carrier recombination were assumed. Lasing mode characteristics such as the longitudinal distribution of carrier density photon density refractive index and the internal field intensity were shown for various laser structures. In this chapter results obtained from the lasing mode characteristics will be used to determine the mode stability and noise characteristics of DFB LDs. For a coherent optical communication system it is essential that the LD used oscillates at a stable single mode and that a narrow spectral linewidth is achieved. Using the information obtained for the lasing mode characteristics a method derived from the above-threshold transfer matrix model will be introduced in section 7.2 which allows the gain margin to be evaluated. By introducing an imaginary wavelength into the transfer matrix equation characteristics of other non-lasing side modes can be evaluated and hence the single-mode stability can be obtained. Numerical results obtained using this method will be presented in section 7.3. In section 7.4 an alternative method which allows the theoretical prediction of the abovethreshold spontaneous emission will be presented. Based on the Green s function method one can use transfer matrices to help determine the single-mode stability of a DFB laser structure by inspecting the spectral components of oscillating modes. The TMM also allows the noise characteristics of the DFB LD to be evaluated. In section 7.5 it will be shown that various contributions to the spectral .