Theory Study Of Modulation Instability In Subwavelength-diameter Optical Fiber

Modulation instability is a common phenomenon in nonlinear systems. Opticalmodulation instability is originated from the interaction of dispersion and nonlineareffects. Its performance in the frequency domain is the appearance of sidelobes of thespectrum, and in the time domain is that the continuous or quasi-continuous lightbreak into a series of ultrashort pulse sequences. The subwavelength-diameter opticalfiber is a kind of micro/nano-scale optical waveguides. By choosing appropriatematerials and core diameter, the fiber with strong dispersion and nonlinearity providesa vector for the study of the modulation instability in micro/nano-scale opticalwaveguides. This paper will study the properties of propagating modes and nonlineartransmission properties of subwavelength-diameter optical fiber at first, then on thisbasis, study the modulation instability in it systematically. This paper includes:Based on the Maxwell equations, the field distribution of propagating modes,dispersion properties and nonlinear transmission properties of the subwavelength-diameter optical fiber and the subwavelength-diameter optical fiber withhigh-index-contrast film were discussed. Supposing the wavelength of laser source isat632.8nm, the dispersion curve of the subwavelength-diameter optical fiber withTiO₂-PMMA T80film just shift left compared with the subwavelength-diameteroptical fiber. The diameter of the subwavelength-diameter optical fiber withTiO₂-PMMA T80film at zero dispersion point and the maximum dispersion point bothsmaller than it of the subwavelength-diameter optical fiber. And the maximumnonlinear coefficient of the subwavelength-diameter optical fiber with TiO₂-PMMAT80film is nearly1,000times larger than that in the subwavelength-diameter opticalfiber in the normal dispersion fiber. When the fiber length is much longer than bothnonlinearity length and dispersion length, the ratio of nonlinearity length todispersion length is used to describe which one plays the leading role in thetransmission of pulse, the nonlinearity or the waveguide dispersion. When the coreradius of the subwavelength-diameter optical fiber is smaller than150nm, thenonlinearity and dispersion are both significant for nanofiber guiding properties. Ifthe core radius increases, the nonlinearity dominates over dispersion effect. But theratio of nonlinearity length to dispersion length of the subwavelength-diameter opticalfiber with TiO₂-PMMA T80film is very small, the nonlinearity almost dominates over dispersion effect in the pulse transmission all the time.Based on the nonlinear Schr dinger equation, we used the traditional linearstability analysis to study the modulation instability of the subwavelength-diameteroptical fiber and the subwavelength-diameter optical fiber with high-index-contrastfilm. The gain spectrum of modulation instability has been numerically calculated,and the generation of high repetition rate ultrashort pulse sequence has beennumerically simulated which determined by modulation instability of thesubwavelength-diameter optical fiber and the subwavelength-diameter optical fiberwith high-index-contrast film. It clearly shows that the subwavelength-diameteroptical fiber and the subwavelength-diameter optical fiber with high-index-contrastfilm are much suitable for this application.The numerical simulation shows that the subwavelength-diameter optical fiber andthe subwavelength-diameter optical fiber with high-index-contrast film both onlyneed very short transmission to generate high repetition rate ultrashort pulse sequenceby modulation instability. Modulation instability is the prelude of soliton, thereforethe adjustable dispersion properties and high nonlinear properties of thesubwavelength-diameter optical fiber and the subwavelength-diameter optical fiberwith high-index-contrast film make them have great potential application inmicro/nano optoelectronic devices.