Study On Stimulated Brillouin Scattering Gain Spectrum Manipulation In Optical Fiber And Its Applications

StimulatedBrillouin scattering(SBS)is a nonlinear process that can occur in optical fibers,and is widely used in narrow-band optical filtering,microwave photonics,distributed sensing,laser manufacturing,slow light and other fields.The characteristics of SBS gain spectrum,such as the number of peaks,gain,bandwidth,polarization stability,have a great impact on the system performance in above applications,as a result,the SBS gain spectrum should be controlled.In this paper,the manipulation of SBS gain spectrum in optical fiber and its applications in lumped narrow-band optical filtering and distributed temperature strain sensing are studied.The main contents include:Firstly,the mechanism of SBS effect in optical fiber is studied.The main factors affecting the excitation and coupling of acoustic mode and optical mode are analyzed by the eigen value equation,and the interaction between pump light,Stokes light and acoustic wave are analyzed by SBS coupled amplitude equations at the same time.The simplified form of SBS coupled amplitude equations for quasi-continuous pumping and pulsed pumping are derived and the numerical simulation methods are also given respectively.Secondly,an SBS gain spectrum with the characteristics of single peak,narrow bandwidth and high gain is proposed,and the effects of doping concentration,geometry size,length of optical fiber and power of pump are analyzed.Then,a pump frequency depolarization structure based on a power-splitting Mach-Zehnder interferometer(MZI)is proposed,and the influence of parameters,such as the length of delay fiber and the power difference between two arms,on the the structure is simulated.On this basis,a polarization-independent narrow-band optical filter(NBOF)based on SBS effect is obtained.It is expected to maintain the narrowband filtering characteristics when the power and polarization state of input signal changed.Thirdly,the performance of the SBS-NBOF is experimentally studied.The experimental results show that the filter has the characteristics of ultranarrow band(?10MHz)and high isolation(?60dB),and the polarization dependent gain of SBS-NBOF can be reduced from more than 20 dB to less than 1dB with the pump frequency domain depolarization technique.Moreover,high resolution optical spectrum measurement utilizing this novel filter is investigated.The wavelength resolution of this system is about 0.1pm,which is two orders of magnitude better than that of the conventional grating-based optical spectrum analyzer,and the power stability is <0.5dB.Forthly,an SBS gain spectrum with the characteristics of multiple peaks is proposed,and the relationship between the temperature and strain measurement accuracy and the doping concentration and geometry size of fiber are analyzed.Moreover,a Brillouin gain fluctuation elimination scheme based on delay orthogonal pump is proposed and the influence of polarization fading on temperature and strain measurement accuracy is analyzed.On this basis,a distributed temperature strain simultaneous measurement technique based on multi-acoustic mode/single optical mode fiber is proposed,which is expected to keep high temperature/strain measurement accuracy when the polarization of probe light changed.Finally,the multi-parameter optical-fiber sensor is experimentally demonstrated.The experimental results show that multiple Brillouin gain peaks can be excited in the standard single-mode optical fiber,and the optimal temperature and strain measurement accuracy can reach 0.98°C and 19.6??,respectively.The experimental results also show that the standard deviation of the gain can be reduced from 0.668% to 0.055% with Brillouin gain fluctuation elimination scheme,which has reached a good level of the existing polarization fading suppression technology.Moreover,the cm spatial resolution can be achieved by combining this technique with the differential pulse-width pair Brillouin optical time domain analysis technique.Additionally,the multi-core fiber and the distributed strain measurement are applied to the three-dimensional curve reconstruction technology,and the verification experiment of isometric helical reconstruction is completed.