Research On Gain Flatness Characteristics Of Double-Clad Erbium-Ytterbium Co-Doped Fiber Amplifier

Fiber amplifier is an essential and important part of modern optical communications. With the generation of DWDM,CATV,FTTH and FSO technologies and the development of high-speed,large capacity,long-distance optical fiber communication systems, optical fiber communication networks put forward higher requirements for the performance,function and cost of the fiber amplifiers. Double-clad Erbium ytterbium co-doped fiber amplifiers have high output power, so the cost per unit power of optical amplifiers is greatly reduced, now become the preferred option of 1550nm band high-power fiber amplifiers for communication, and become a research hot spot at home and abroad.This paper analyses the theoretical model of erbium ytterbium co-doped system, based on the rate equations and the signal,pump and the ASE optical power transfer equations, using forward pumping, the above equation is numerically calculated with the use of fourth order Runge-Kutta iteration method. The double-clad erbium ytterbium co-doped fiber amplifier has been designed and optimized to improve its gain and reduce its noise figure with simulation.In DWDM systems, not only requires double-clad EYDFA having high gain, but also a flat gain spectrum. Long period fiber grating (LPFG) is a band-stop filter of transmission type, by cascading one or more LPFGs with different resonant wavelengths and transmission rates, we get a loss characteristics in contrast with the gain spectrum of double-clad EYDFA ,causing the high gain part of the double-clad EYDFA gain spectrum having large loss, so EYDFA gain spectrum becomes flatness. This paper uses the coupled mode theory of LPFG, simulates the mode coupling equation of LPFG to obtain the transmission spectrum of LPFG. And then according to the gain spectrum of double-clad EYDFA, three LPFGs are designed with different resonant wavelengths and the transmission rates. A loss characteristics is obtained in contrast with the gain spectrum of double-clad EYDFA through this three LPFGs cascaded, so the high gain part of the double-clad EYDFA gain spectrum is filtered, Finally making the double-clad EYDFA gain spectrum obtain high gain flatness which is less than 0.25dB in the range 1533 ~ 1560nm ,and the gain is maintained at more than 30dB, So that the double-clad EYDFA can absolutely meet the requirement of DWDM system.