Study Of Brillouin Gain Spectrum Characteristics In Conventional Single-Mode And Photonic Crystal Fibers

In fiber-optic communication,it is hoped to avoid the nonlinear effect of Brillouin scattering,so that higher power optical signals can obtain longer transmission distance and signal-to-noise ratio?SNR?.In fiber-optic sensing,the Brillouin scattering effect is used to obtain distributed temperature and strain information.Therefore,the characteristics of Brillouin Gain Spectrum?BGS?play a crucial role in the performance of the system in both fiber-optic communication and sensing.GeO₂ is commonly used as dopant to adjust the refractive index profile?RIP?and the acoustic velocity profile?AVP?in the fiber,thereby forming different BGS characteristics such as Brillouin gain,acoustic mode number and peak intensity difference.We have studied the BGS characteristics of conventional single-mode fiber and photonic crystal fiber?PCF?by finite element analysis method with wide parameter variation range and fine step size.The result can help design communication fibers with low Brillouin scattering effects and long-distance transmission,as well as designing sensing fibers with high-Brillouin gain and high sensing accuracy.On this basis,we have designed new fibers with multi-peak BGS with close intensity,which is expected to improve the SNR of fiber-optic sensing system based on Brillouin Beat Spectrum?BBS?detection.In the case of step-index and graded-index optical fiber,we take the RIP as the research object?the AVP changes accordingly?.We consider many possibilities for simultaneous changes in refractive index difference and core radius,and give the rule of the BGS characteristics as a function of them.In the case of complex-index optical fiber,we take large effective area fiber?LEAF?as an example to study and give the rule of BGS characteristics changing with several important parameters.In the design of new fiber with close intensity multi-peak BGS,for conventional single-mode fibers,we obtain a LEAF-type fiber with BGS peak relative intensities of 0,-2.7 and-9dB.It is expected to increase the SNR by3.1 times in BBS-based fiber optic sensing systems.In the case of PCF with uniform pitch,the rule of BGS characteristics changing with air hole pitch and diameter is given.In addition,we have studied the case of PCF with non-uniform air hole diameter.The results show that when the air hole diameter becomes larger from the inside to the outside,BGS with more and close intensity peaks can be obtained.According to this,a PCF with bimodal BGS in which the secondary peak is 8dB weaker than the main peak is designed.It is expected to increase the SNR by 2.5 times in BBS-based fiber-optic sensing systems.