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Optical tunable filters are key components of the future dense wavelength division multiplexed (WDM) optical fibre networks. In such a network a number of information channels are simultaneously transmitted through a single fibre by putting each channel on a different optical carrier wavelength. The wavelength filter allows a single or multiple channel(s) to be isolated at the receiving or routing node. The tunability of the filter allows for dynamic network reconfiguration and increases versatility of the system | 11 Other Wavelength Tunable Optical Filters Based on the DFB Laser Structure INTRODUCTION Optical tunable filters are key components of the future dense wavelength division multiplexed WDM optical fibre networks. In such a network a number of information channels are simultaneously transmitted through a single fibre by putting each channel on a different optical carrier wavelength. The wavelength filter allows a single or multiple channel s to be isolated at the receiving or routing node. The tunability of the filter allows for dynamic network reconfiguration and increases versatility of the system. Ideally the wavelength filter should be tunable over the entire system bandwidth and should have no secondary pass bands or side lobes in its filter function. WDM systems require optical tunable filters not only as channel selectors but also as post-optical-amplifier filters that reduce amplified spontaneous emission ASE noise 1 . Following the recent rapid advances in lightwave technology wavelength tunable optical filters are now incorporated in wavelength-division-multiplexed transmission systems to increase the line capacity for lightwave telecommunication services. Optical filtering for selection of channels separated by 2 nm is currently achievable and narrower channel separations may be possible as filter technologies improve. This would give more than a hundred broadband channels in the low-loss fibre transmission region of pm and or pm wavelength bands with each wavelength channel having a transmission bandwidth of several gigahertz. Wavelength tunable optical filters have already been built into the receiver for each subscriber in distribution networks 2 . Basically a semiconductor wavelength tunable optical filter is a laser diode which is biased slightly below threshold. When an optical signal of a wavelength close to the oscillation wavelength of the device is incident upon the input the signal is amplified and emitted at the output. By .

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