| Recently,photonic crystal fiber(PCF)has attracted extensive attention from researchers for its flexible adjustability in structural and material.This flexibility gives it unparalleled advantages in many fields,and great progress has been made in relevant application research.Compared with traditional optical fibers,PCF has many unique optical characteristics,such as high nonlinearity,high birefringence,large mode field area,low loss and adjustable dispersion.Due to the flexibility of structures and materials,the nonlinear coefficients of PCF are usually much higher than that of traditional optical fibers.Among many nonlinear effects,stimulated Brillouin scattering(SBS)has great application prospects in the field of optical fiber sensing and communication.With the deepening of research on PCF,the research of SBS has also expanded from traditional optical fibers to PCFs.PCF with three-layer structure has incomparable advantages over single-cladding structure and doublecladding structure in various scenarios,including better ability to constrain optical energy,more flexible design structure and more convenient access to multi-peak Brillouin scattering spectrum.The work of this paper is mainly to explore the structure design scheme of PCF with high nonlinear,and to study and analyze the Brillouin scattering spectrum of the designed PCF.The main work is as follows:(1)The current research status and progress of PCF with high nonlinear and Brillouin scattering in PCF are introduced in detail.Then,several typical nonlinear effects are introduced,and the generation principles of these nonlinear effects are analyzed,especially the stimulated Brillouin scattering process.Through the above introduction of PCF characteristics and SBS effect,the simulation software and simulation module are determined.The simulation analysis method of PCF,the simulation calculation of optical and acoustic modes and the mathematical model of Brillouin scattering spectrum are derived.After the introduction of basic theory and calculation formula,it will pave the way for the subsequent PCF simulation.(2)An asymmetrical PCF with three claddings,high nonlinear,high birefringence and large negative dispersion is designed.Energy is limited to the fiber core area through the interaction of the air hole of the inner cladding and the intermediate cladding.The inner cladding consists of two elliptical holes and four round holes,while the middle cladding is composed of round air holes with two different sizes.By changing the structural parameters of the air holes in the three claddings,the nonlinearity,birefringence,loss and dispersion of the fiber are deeply investigated.Then the optimal structural parameters are determined to obtain a PCF with high nonlinear and high birefringence large negative dispersion.At the same time,the influence of the change of PCF structural parameters on the Brillouin scattering is also explored and analyzed.Simulation results show that at a wavelength of 1.55 μm,the PCF can obtain a high nonlinearity of 46.76 W-1km-1,a high birefringence of 3.85×10-2,a low loss of 4.19×10-3 dB/km and a large negative dispersion of-881 ps/km/nm,which ensures the application prospect of the PCF in polarization maintaining and dispersion compensation.Besides,the peak ratio of the two polarization modes of PCF is 51.2:48.8,and the difference of Brillouin frequency shift is 273.1 MHz.It can be seen that the size of the two peaks is close and the Brillouin frequency shift difference is relatively large,which means that the PCF has great potential in the application of distributed Brillouin sensing system(3)A symmetrical PCF with three claddings and high nonlinear is designed,and it can realize dual-parameter sensing.By changing the structural parameters of the air hole in the three claddings of the PCF,it has the characteristics of the refractive index distribution of M-type optical fiber.Then the optical characteristics and the multi-peak Brillouin scattering spectrum of the PCF are studied.In order to find better sensing performance,on the one hand,it is necessary to reduce the peak difference of the double peaks of the Brillouin scattering spectrum and improve the Brillouin frequency shift;on the other hand,it is important to obtain the best temperature and strain dual-parameter sensing performance by adjusting structural parameters and material doping,which can be achieved by improving the sensitivity of temperature and strain,and reducing the differentiation error of measurement.The results show that at 1.55 μm,the PCF can achieve a nonlinearity of 6.42 W-1km-1,a low loss of 1.14×10-1 dB/km,a negative dispersion of-252 ps/km/nm,a peak ratio of 49.0:51.0 and a Brillouin frequency shift difference of 172.0 MHz.Higher nonlinearity means higher Brillouin gain.In addition,the temperature and strain sensitivity of the two peaks are CTL01=4.4005 MHz/℃,CSL01=0.1920 MHz/μs,CTL02=4.2208 MHz/℃ and CSL02=0.1896 MHz/e.The temperature and strain sensing sensitivity are 0.01℃ and 0.75με,respectively.This scheme has high sensing sensitivity,and the measurement discrimination error is far less than the existing research results.The two different structures of PCF designed in this paper provide a certain reference value for the research of SBS-based optical sensors.In addition,this study has certain guiding significance for exploring the stimulated Brillouin scattering of PCF,and shows new ideas for the application of PCF in nonlinear optics. |
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