| Brillouin dynamic grating based on the stimulated Brillouin scattering has attracted much attention in the fields of optical fiber communication and optical fiber sensing.Brillouin dynamic grating has many advantages,such as all-optical generation,controllable position,controllable spectrum,adjustable parameters,and no damage to the fiber.Thus the application of Brillouin dynamic grating has be exploited in fiber distributed sensing,all-optical storage,tunable filter,high-precision spectral analysis etc.The theoretical analysis of Brillouin dynamic grating has significance for experimental research and applications.In this work,the Brillouin dynamic grating model is established based on the stimulated Brillouin scattering and elastic acoustic theory.The reflection spectrum of the Brillouin dynamic grating in single mode fibers is simulated.The effect of temperature and strain on velocity of acoustic is studied,the Brillouin frequency shift and central wavelength of Brillouin dynamic grating are investigated.The main research contents are as follows:1.The refractive index of Brillouin dynamic grating is derived according to the relationship between the pump wave,acoustic wave and Stokes wave in the process of stimulated Brillouin scattering and the elastic equation of matter in elastic mechanics.The reflectivity and linewidth are calculated according to the grating Bragg grating theory.2.Three different Brillouin dynamic gratings with double continuous waves,single continuous wave,single pulse wave and double pulse waves as pump waves are simulated.The result shows that Brillouin dynamic grating produced by single pump cannot be easily observed in the experiment.The reflection spectrum of the Brillouin dynamic grating generated by the double pulse pump is investigated.The results show that the Bragg wavelength which downshifts due to Brillouin frequency shift equals the Doppler frequency shift.The reflectivity of the Brillouin dynamic grating is positive correlation with the power and the pulse width of the pump wave.When the power increases to 30W,the pump pulse width reaches 10 ns,the peak reflectivity is 2.17×10-6,and 7.16×10-9,respectively.The reflectivity is negative correlation with the core diameter of optical fiber.When the fiber core diameter decreases from 10?m to 8?m,the peak reflectivity increases from 1.96×10-11 to 6.64×10-11.The spectral width of the reflection spectrum decreases with the increase of pulse width,and the minimum spectral width is 1.2×10-4nm.But the spectral width of the reflection spectrum is not subject to the pump wave power and core diameter.3.The refractive index,Young's modulus,elastic modulus and Poisson ratio in the fiber are affected by temperature or strain,which leads to the increment of acoustic velocity and Brillouin frequency shift.The calculation results show that temperature and strain induce central wavelength shift of Brillouin dynamic grating.In the Brillouin dynamic grating,the changes of acoustic velocity and Brillouin frequency shift affect the grating period.The changes of refractive index and period lead to the Brillouin dynamic grating central wavelength shift.We found that the central wavelength varies nonlinearly with temperature and stress.When the temperature changes 0-100?,the center wavelength drift is 5.32×10-4nm.When the strain is 0-300??,the center wavelength drift is 6.34×10-6nm. |
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