“Performance analysis of Transient aspects for EDFA Amplifiers” A Thesis

Information revolution implies that multimedia networks need high bandwidth real-time communication services. At present, optical fibre is the only transmission medium offering such large bandwidth with low loss communication links. To amplify an optical signal with a conventional repeater, one performs photon to electron conversion, electrical amplification, retiming, pulse shaping, and then electron to photon conversion. Although this process works well for moderate speed single wavelength operation, it can be fairly complex and expensive for high speed multiple wavelength systems. Thus a great deal of effort has been expended to develop all-optical amplifiers. These devices operate completely in the optical domain to boost the power levels of multiple light wave signals over spectral bands of 30 nm .Optical amplifiers are in general bit rate transparent and can amplify signals at different wavelength simultaneously. Optical amplifiers are mainly of two types i.e. Semiconductor optical amplifiers and Fiber amplifiers and further classified into travelling wave’s semiconductor optical amplifier, fabry-perot semiconductor optical amplifier, Erbium doped fiber amplifier, Raman & Brillouin fiber amplifiers. First, the technique to control the transients by means of channel adding/dropping in cascades of erbium doped fiber amplifiers (EDFAs) in optical communication system is discussed. It is seen that as EDFAs are gradually added in the optical link, the transients are also reduced significantly. Also that when we use cascade of six EDFAs and note the transient reduction, it is not as much as in case of chain of ten edfa s in which the transients are greatly suppressed . Using the same model, the comparison of transient response of Compact EDFAs and Transient EDFAs is presented. It is observed that suppression of transients is much in Compact EDFAs than Transient EDFAs. Secondly the performance of the optical system consisting of chain of EDFA amplifiers for different data formats such as non return to zero (NRZ), return to zero (RZ) and Manchester are discussed. Their effect on the spectral loss variations produced in fiber output is analysed. It is seen that when the RZ raised cosine and Manchester raised cosine modulation formats are used, the non linear ties are produced in power spectrum plots which severely distort the signals obtained at the output of the chain of the EDFA amplifiers. On the other hand, the NRZ raised cosine modulation format best compensates the spectral loss variations in the power spectrum plots obtained at the output. Also NRZ raised cosine has good eye opening as compared to other modulation formats. Finally, we investigate the gain and noise figure characteristics of Physical EDFAs and Compact EDFAs in an optical system consisting of cascade of both the amplifiers. The Gain, Noise Figure variations of a forward pumped EDFA and Compact EDFA as functions of Er3+ fiber length, injected pump power and up-conversion coefficient is demonstrated. It is observed that the Gain becomes constant when the length of both the amplifiers reaches above 20m. The comparison shows that the higher gain with flatter output is obtained in case of Compact EDFAs than Physical EDFAs in a system consisting of chain of both the amplifiers. It is further investigated that the agreement between the Compact and Physical EDFA models is good up to 10 m with the no up-conversion co-efficient. Also, the noise figure obtained in case of Physical EDFA is higher than Compact EDFA when same amplifier length is more than 20 m and then becomes constant for both the amplifiers.