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In Chapter 8 we introduced transmission-line laser modelling (TMLM). In this chapter, TLLM will be modified to allow the study of dynamic behaviour of distributed feedback laser diodes, in particular the effects of multiple phase shifts on the overall DFB LD performance. We can easily model any arbitrary phase-shift value by inserting some phaseshifter stubs into the scattering matrices of TLLM. This helps to make the electric field distribution and hence light intensity of DFB LDs more uniform along the laser cavity and hence minimise the hole burning effect. . | 9 Analysis of DFB Laser Diode Characteristics Based on Transmission-Line Laser Modelling TLLM 9.1 INTRODUCTION In Chapter 8 we introduced transmission-line laser modelling TMLM . In this chapter TLLM will be modified to allow the study of dynamic behaviour of distributed feedback laser diodes in particular the effects of multiple phase shifts on the overall DFB LD performance. We can easily model any arbitrary phase-shift value by inserting some phaseshifter stubs into the scattering matrices of TLLM. This helps to make the electric field distribution and hence light intensity of DFB LDs more uniform along the laser cavity and hence minimise the hole burning effect. 9.2 DFB LASER DIODES As explained in Chapter 2 the feedback necessary for the lasing action in a DFB laser diode is distributed throughout the cavity length. This is achieved through the use of a grating etched in such a way that the thickness of one layer varies periodically along the cavity length. The resulting periodic perturbation of the refractive index provides feedback by means of Bragg diffraction rather than the usual cleaved mirrors in Fabry-Perot laser diodes 1-3 . Bragg diffraction is a phenomenon which couples the waves propagating in the forward and backward directions. Mode selectivity of the DFB mechanism results from the Bragg condition. When the period of the grating A is equal to m B 2neff where AB is the Bragg wavelength neff is the effective refractive index of the waveguide and m is an integer representing the order of Bragg diffraction induced by the grating then only the mode near the Bragg wavelength is reflected constructively. Hence this particular mode will lase whilst the other modes exhibiting higher losses are suppressed from oscillation. The coupling between the forward and backward waves is strongest when m 1 i.e. first-order Distributed Feedback Laser Diodes and Optical Tunable Filters H. Ghafouri-Shiraz 2003 John Wiley Sons Ltd ISBN 0-470-85618-1 232 ANALYSIS OF DFB .