Modulation Instability Of Fiber Couplers

Fiber coupler that is a kind of directional coupler is a necessary part of light-wave technique, it usually be used in many fields on fiber optics. Although we have only used it's linear property, but now more and more people are concerned about it's nonlinearity. It has been the intense research of fiber optics recent years.In this paper, the nonlinear coupled-mode equations are rewritten by even andodd modes, we study modulation instability of dispersion decreasing fiber couplerand dispersion-shifted fiber coupler when either even or odd mode is launched aloneand get it's gain spectra. The result shows that there are new types of modulationinstability in both the normal and the anomalous-dispersion regimes. In dispersiondecreasing fiber coupler, quasi-cw can be changed into pulses array with certainconditions and we can extract super short pulses from this. Furthermore, the shape ofgain spectra is invariable as function of propagation distance when input power isconstant. The bandwidth of gain spectra widens and it's strength heightens asalteration of input power when propagation distance is constant; In dispersion-shiftedfiber coupler, modulation instability is concerned with four phase dispersion and hasno relation with three phase dispersion. Quasi-cw can be changed into pulses arraywith certain conditions and we can extract super short pulses from this. Furthermore,the bandwidth of gain spectra widens and it's strength heightens as augment of input power.