Edfa Gain Flattening. Optical Communication Research And Theoretical Modeling

Erbium doped fiber amplifier (EDFA) is a key component of optical communication. It can amplify all the signal in WDM system taking the places of an array of electronic regenerators, furthermore, it has advantages such as high gain, low noise and so on. But for the final energy structure of Er3+, property of EDFA's gain has its inherent disadvantage: EDFA's gain is different VS wavelength, in other words, its gain spectrum is not ideally flat, so the output signal will be different if the input signal have the same power. This effect will accumulate as several EDFAs are used in the long-distance backbones system, hi the end, the signal will have low Signal-to-Noise. In this thesis, scheme is present to solve this problem, simulation is given as well. The work of this thesis mainly includes three sections as follows:一. Equalizer based on coupler. Model of gain spectrum was made based on the coupled theory. Parameters of coupler were adjusted so that the coupler can have the inverse gain spectrum comparing with EDFA's. After coupling the coupler, flattened gain can be attained.二. Ameliorate the algorithms. In order that our program can work as a software for design of gain equalizer, adaptability and less amount of computing are necessary. Algorithm of solve nonlinear equation was ameliorate to decrease the amount of computing. In the aspect of adaptability, several algorithms of optimization were investigated and the genetic algorithm was selected in the end. Using this optimize algorithm, less interventions were needed and the compute result was more precious as well.三. Dynamic gain equalizer (DGE) based on volume phase grating (VPG ). EDFA's gain spectrum has dependence on the EDFA's operate point. Thus, the equalizer should alter its gain spectrum as the EDFA's operate point changes. Based on the theory of thick grating, Characters of VPG's transmissive gain spectrum were studied. Using the genetic algorithm, novel DGE (including several VPGs) was designed. Besides the simulations, some VPGs were also made using technology of H-PDLC and some of their important characters have been measured. Further research is now going on.