Design Of Highly Nonlinear Photonic Crystal Fiber Based On Double Cladding Structure And Analysis Of Their Brillouin Scattering Characteristics

In recent years,photonic crystal fiber(PCF)has attracted more attention because of its unique structure.The research and application of PCF are also making progress.PCF has many optical properties that single-mode fibers don't have,such as large mode area,tunable dispersion,high birefringence,low loss and high nonlinearity.Especially in properties of nonlinear,the nonlinear coefficient of optical fiber can be greatly improved through flexibly designing structure of PCF,which is several orders of magnitude higher than that of ordinary optical fiber.Stimulated Brillouin Scattering(SBS)is a classic nonlinear effect,which has potential value in optical fiber communication.At the beginning,the research on SBS was only carried out in ordinary single-mode fiber.With the reported of various special fibers,researchers began to study the SBS effect in special fibers,and the application of related research is more widely,such as slow light,distributed sensing,coherent optical storage,etc.In this paper,the structure design and performance analysis of highly nonlinear PCF are study based on double cladding structure.According to the process of Brillouin scattering in optical fiber,we analyzed the influence of PCF structure parameters on Brillouin scattering characteristics.The main research contents are as follows:Firstly,we introduced the characteristics and basic classification of PCF in detail,and analyzed the current research status of highly nonlinear PCF.At the same time,the research status and progress of Brillouin scattering effect in PCF are recommended.Then,the nonlinear effect of PCF is theoretically deduced,and several typical nonlinear effects are analyzed and introduced.The SBS effect is analyzed and deduced in detail,and the calculation formulas of Brillouin scattering spectrum,gain and threshold are derived.After theoretical analysis,the improvement of nonlinearity can be achieved mainly by reducing the mode field area of the fiber and using materials with higher nonlinear refractive index coefficients through theoretically analyzing.PCF with double cladding structure can design and change the refractive index distribution of fiber cladding more flexibly than single cladding structure,reducing the mode field area of the fiber.Moreover,the lower mode field area also helps to enhance acousto?optics coupling efficiency,which improves the intensity of the Brillouin scattering spectrum.Secondly,the finite element method is used to simulate the double cladding PCF with all elliptical hole in the inner cladding.We analyzed the effects of the number and spacing of hole in the outer cladding,the lattice constant,and the radius of major axis and minor axis of elliptical holes in the inner cladding on the nonlinear coefficient and loss of PCF.At the same time,the influence of the structure parameters of PCF on the Brillouin scattering characteristics is also analyzed.The simulation results show that the nonlinear coefficient of 39.24 w-1m-1 can be achieved at the wavelength of 1.55 ?m with the limiting loss is of 5.08 × 10-7 dB/m.And the Brillouin gain can reach 50dB around 10z.Then,a double cladding PCF with elliptical pores on both sides of the core and sulfide as background material was designed.The nonlinear coefficient and loss of PCF were analyzed by simulating calculation of the outer cladding hole spacing,lattice constant,inner cladding lattice constant and ellipticity of elliptical holes.The influence of structural parameters of sulfide PCF on Brillouin scattering characteristics was analyzed.The nonlinear coefficient of 8087.48 w-1-1an be obtained at wavelength of 2?m with the limiting loss of order of 10-8 dB/m,and the frequency shift of Brillouin scattering is about 7.8GHz.The Brillouin gain fluctuates around 48dB.Finally,the two PCF structures have both high nonlinearity and low loss characteristics,which can be applied in sensing systems designed based on SBS,which will help to improve the detection intensity of Brillouin scattering signals,and is useful for design of the distributed Brillouin fiber optic sensor.