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In recent years, advances in wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) technology have enabled the deployment of systems that are capable of providing large amounts of bandwidth [1]. Wavelength tunable optical filters appear to be the key components in realising these WDM/DWDM lightwave systems. Optical filtering for the selection of channels separated by 2 nm is currently achievable, and narrower channel separations will be possible in the near future with improved technology [2–3] | 10 Wavelength TUnable Optical Filters Based on DFB Laser Structures 10.1 INTRODUCTION In recent years advances in wavelength division multiplexing WDM and dense wavelength division multiplexing DWDM technology have enabled the deployment of systems that are capable of providing large amounts of bandwidth 1 . Wavelength tunable optical filters appear to be the key components in realising these WDM DWDM lightwave systems. Optical filtering for the selection of channels separated by 2 nm is currently achievable and narrower channel separations will be possible in the near future with improved technology 2-3 . This would give more than 100 broadband channels in the low-loss fibre transmission region of 1.3 pm and or 1.55 pm wavelength bands with each wavelength channel having a transmission bandwidth of several gigahertz. In practice grating-embedded semiconductor wavelength tunable filters are among the most popular active optical filters since they are suitable for monolithic integration with other semiconductor optical devices such as laser diodes optical switches and photodetectors 4 . As a result A 4-shifted DFB LDs can be used as semiconductor optical filters when biased below threshold 5-6 . This is a grating-embedded semiconductor optical device which has the advantages of a high gain and a narrow bandwidth. However the drawbacks are that the bandwidth and transmissivity will change with the wavelength tuning 5 . Fortunately Magari et al. have solved these problems by using a multi-electrode DFB filter 7-8 in which a wavelength tuning range of 33.3 GHz 0.25 nm with constant gain and constant bandwidth has been obtained by controlling the injection current. Since then various DFB LD designs have been developed 9-11 . In this chapter the wavelength selection mechanism is discussed in detail. Subsequently the idea of the transfer matrix method TMM is again thoroughly explored and the derived solutions from coupled wave equations are also discussed in detail. By .