Abstract: There is some axial extension in the applying scale of linearly chirped fiber Bragg(LCFBG) with transversal strain. The effect of the axial one should be considered on condition that the length of the applying scale is sharp to the same order of magnitude compared with the diameter of fiber. Based on the analysis method of spatial elasticity, phase shift in the modulated refractive index distribution of LCFBG is obtained because of the axial extension. And a transmission spectrum peak is created which is related to the value and position of phase shift. Furthermore, the radial refractive index of LCFBG is also changed for the birefringence with transversal strain. Considered with the conditions above, the theoretical sensing model of transversal strain in small applying scale that can be treated as a point is introduced, in which the Stokes parameter characteristics of LCFBG are very important and useful. Simulation and analysis results show that the transmission coefficient grows exponentially with transversal strain but the peak value of transmission Stokes parameter spectrum is linearly dependent to the value of transversal strain applying in LCFBG. There is a one-to-one correspondence between the peak wavelength of transmission Stokes-s1 parameter spectrum and the applying position of transversal strain. And the peak wavelength of transmission Stokes-s1 parameter spectrum grows linearly with the value of transversal strain. So the position and value of transversal strain in small applying scale can be converted through the analysis method of measuring the transmission Stokes-s1 parameter spectrum of LCFBG.

Key words: fiber optics, Stokes parameter, fiber Bragg grating, transversal strain, sensing