A new sensing technique for the distributed measurement of temperature and strain, based on Brillouin optical frequency-domain analysis, is presented. Theoretical investigations and first experimental results of distributed measurements demonstrate the feasibility of this new concept.
One of the most dominant nonlinear effects in single-mode fibers is stimulated Brillouin scattering (SBS). The generation of SBS in a ring resonator can be considered as a lasing action, with the Stokes output downshifted in frequency by an amount equal to the Brillouin frequency. In the stationary case stimulated Brillouin scattering in a length of optical fiber can be described by the following partial differential equations for the slowly varying complex amplitudes of the pump wave E
p
and the backscattered first order Stokes wave E
B1
, with the wave amplitudes E
i
related to the intensities I
i
by I
i
= |E
i
|2 (i=p,B1) [1]: (1) In a ring resonator geometry as depicted in Fig. 1 the wave amplitudes at the ports of the directional coupler obey the relations [2] (2) where
V
k
2
is the coupler intensity radiation loss, K is the field coupling coefficient, and T is the field transmission coefficient.
Stimulated Brillouin scattering (SBS) is the dominant non-linearity in silica optical fibres. In highfinesse all-fibre ring resonators with fibre lengths of the order of 10 m stimulated Brillouin scattering can be observed at input power levels in the range of 10 to 100 μW [1]. Above pump threshold fibre optic ring resonators show intensive laser oscillation at the Stokes wavelength and therefore are called Brillouin ring lasers (BRL). An important application of Brillouin ring lasers is the all-fibre optic ring laser gyro [2].
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