In the previous chapter, the transfer matrix method (TMM) was introduced to solve the coupled wave equations in DFB laser structures. Its efficiency and flexibility in aiding the analysis of DFB semiconductor LDs has been explored theoretically. A general N-sectioned DFB laser model was built which comprised active/passive and corrugated/planar sections. In this chapter, the N-sectioned laser model will be used in the practical design of the DFB laser. The spatial hole burning effect (SHB) [1] has been known to limit the performance of DFB LDs. As the biasing current of a single quarterly-wavelength-shifted (QWS) DFB LD increases, the gain. | 5 Threshold Analysis and Optimisation of Various DFB LDs Using the Transfer Matrix Method INTRODUCTION In the previous chapter the transfer matrix method TMM was introduced to solve the coupled wave equations in DFB laser structures. Its efficiency and flexibility in aiding the analysis of DFB semiconductor LDs has been explored theoretically. A general N-sectioned DFB laser model was built which comprised active passive and corrugated planar sections. In this chapter the N-sectioned laser model will be used in the practical design of the DFB laser. The spatial hole burning effect SHB 1 has been known to limit the performance of DFB LDs. As the biasing current of a single quarterly-wavelength-shifted QWS DFB LD increases the gain margin reduces. Therefore the maximum single-mode output power of the QWS DFB LD is restricted to a relatively low power operation. The SHB phenomenon caused by the intense electric field leads to a local carrier depletion at the centre of the cavity. Such a change in carrier distribution alters the refractive index along the laser cavity and ultimately affects the lasing characteristics. By changing the structural parameters inside the DFB LD an attempt will be made to reduce the effect of SHB. As a result a larger single-mode power and consequently a narrower spectral linewidth may be achieved. A full structural optimisation will often involve the examination of all possible structural combinations in the above-threshold regime. On the other hand the analysis of the structural design may be simplified in terms of time and effort by optimising the threshold gain margin and the field uniformity. The structural changes and their impacts on the characteristics of DFB LDs will now be presented. By introducing more phase shifts along the laser cavity a three-phase-shift 3PS DFB LD will be investigated in section . In particular impacts due to the variation of both phase shifts and their positions on the lasing characteristics of the 3PS
