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In this book, the performance characteristics of distributed feedback semiconductor laser diodes and optical tunable filters based on DFB laser structures have been investigated. As discussed in Chapter 1, these lasers can be used as optical sources and local oscillators in coherent optical communication networks, in which a stable single mode (in both the transverse plane and the longitudinal direction) and narrow spectral linewidth become crucial. Based on the interaction of electromagnetic radiation with a two-energy-band system, the operating principles of semiconductor lasers were reviewed in Chapter 2. With partially reflecting mirrors located at the laser facets, a Fabry–Perot laser. | 12 Conclusion Summary and Suggestions 12.1 SUMMARY AND CONCLUSION In this book the performance characteristics of distributed feedback semiconductor laser diodes and optical tunable filters based on DFB laser structures have been investigated. As discussed in Chapter 1 these lasers can be used as optical sources and local oscillators in coherent optical communication networks in which a stable single mode in both the transverse plane and the longitudinal direction and narrow spectral linewidth become crucial. Based on the interaction of electromagnetic radiation with a two-energy-band system the operating principles of semiconductor lasers were reviewed in Chapter 2. With partially reflecting mirrors located at the laser facets a Fabry-Perot laser forms the simplest type of optical resonator. However due to the broad gain spectrum multi-mode oscillations and mode hopping are common for this type of laser. Nevertheless single longitudinal mode operation becomes feasible with the use of DFB LDs. The characteristics of the DFB laser were explained using the coupled wave equations. With a built-in periodic corrugation travelling waves are formed along the direction of propagation in which a perturbed refractive index and or gain are introduced. In fact DFB lasers act as optical bandpass filters so that only frequency components near the Bragg frequency are allowed to pass. The strength of optical feedback is measured by the strength of the coupling coefficient. Based on the nature of the coupling coefficient DFB semiconductor lasers can be classified into purely index-coupled mixed-coupled and purely gain- or loss-coupled structures. The discussion focused on the coupled wave equations in Chapter 3. In the analysis eigenvalue equations were derived for various structural configurations and consequently their threshold currents and lasing wavelengths were determined. From the lasing threshold characteristics impacts due to the coupling coefficient the laser cavity .
