Calculating strain from Brillouin frequency data

As described in section 1.1.6, the Brillouin scattering frequency, $vb$⁠, is a function of the refractive index of the fibre core, n, the acoustic wave velocity, $υa$⁠, in the fibre (~5800 m/s), and the wavelength of the incident light, $λ$ (Equation 1.4).

This Brillouin frequency shifts linearly with changes in longitudinal strain and temperature in the fibre core/cladding so that (Horiguchi et al., 1989):

or

which is the same as Equation 1.9 where $Δvb$ is the change in Brillouin frequency due to a simultaneous change in strain, $Δε$⁠, and in temperature, $ΔT$⁠. Here, C_ɛ and C_T are referred to as the strain coefficient and the temperature coefficient, respectively, of the Brillouin frequency shift.