The flexibility of the transfer matrix method allows one to evaluate the spectral behaviour of a corrugated optical filter/amplifier and the threshold characteristic of a laser source. To extend the analysis into the above-threshold biasing regime, the transfer matrix has to be modified so as to include the dominant stimulated emission. Based on a novel numerical technique, the above-threshold DFB laser model will be presented in this chapter. Using a modified transfer matrix, the lasing mode characteristics of DFB LDs will be determined | 6 Above-Threshold Characteristics of DFB Laser Diodes A TMM Approach INTRODUCTION The flexibility of the transfer matrix method allows one to evaluate the spectral behaviour of a corrugated optical filter amplifier and the threshold characteristic of a laser source. To extend the analysis into the above-threshold biasing regime the transfer matrix has to be modified so as to include the dominant stimulated emission. Based on a novel numerical technique the above-threshold DFB laser model will be presented in this chapter. Using a modified transfer matrix the lasing mode characteristics of DFB LDs will be determined. The new algorithm differs from many other numerical methods in that no first-order derivative of the transfer matrix equation is necessary. As a result the same algorithm can be applied easily to other DFB laser structures with only minor modification. In section the detail of the above-threshold laser model will be presented. Taking into account the carrier rate equation the dominant stimulated emission will be considered in building the transfer matrix. The numerical algorithm behind the lasing model will be discussed in section . Using the newly developed laser model numerical results obtained from various DFB lasers including QWS 3PS and DCC structures will be shown in section . Longitudinally varying parameters such as the carrier concentration photon density refractive index and the internal field intensity distributions will be presented with respect to biasing current changes. Impacts due to the structural variation in particular will be discussed. DETERMINATION OF THE ABOVE-THRESHOLD LASING MODE USING THE TMM In above-threshold analysis the lasing wavelength and the optical output power are important. For laser devices to be used in coherent communication systems the single-mode stability and the spectral linewidth should also be considered. Provided that the longitudinal distributions of the carrier photons and other .
