The shape of the Brillouin gain spectrum (BGS) that is produced in an optical fiber undergoing strain varying linearly with respect to time, which is a typical example of temporally non-uniform strain, is theoretically derived through an analysis similar to that by which the BGS under spatially non-uniform strain would be derived. The BGS shape that is theoretically derived agrees well with the shape experimentally observed. The characteristics of the BGS deformation and strain measurement error under the temporally linear strain are discussed based on their similarity to the BGS shape derived under spatially linear strain.
A method is presented for measuring the displacement of a tunnel's ceiling and side walls using arc-shaped beams and
fiber Bragg grating (FBG) sensors. The beams are installed inside the tunnel so that the displacements of the ceiling and
side walls are transferred to the beams. The displacements are computed using mechanical analysis of the strains they
produce on the surface of the beams. Table-top experiments demonstrated that the method is valid. This method can be
used to construct a system for monitoring the displacements in tunnels with various cross-sections.
A number of distributed fiber optic strain sensing systems are proposed that are based on the frequency shift of the
Brillouin scattered light power spectrum in proportion to the strain produced in the fiber. Although the spectral shape
under a uniform strain distribution is generally given by a Lorentzian function, it is deformed under a non-uniform strain
distribution. It is important to investigate the relationship between the non-uniform strain distribution and the spectral
deformation, because it affects the strain measurement error. We have focused on a linear strain distribution where the
strain changes at a constant rate along the fiber as a typical non-uniform strain distribution. The power spectrum shape is
derived theoretically using the Brillouin frequency shift values at the both ends of the observation section. The power
spectrum of the Brillouin scattered light is then observed experimentally. The experimentally observed power spectrum
shape was in good agreement with that theoretically obtained and the power spectrum was widened according to the
slope of the linear strain distribution.
We have been researching a displacement measurement system for monitoring rock damage such as cracks in
tunnel walls and pillars in an underground mine. The system consists of a displacement sensor unit with a pair of
crossed optical fibers and a distributed fiber optic strain measuring device. The strain measuring device measures
displacement-induced strains in the optical fibers and, from these measured strains, displacements in two
directions are obtained. We analyzed the influence of the strain measurement error and sensor unit configuration
on the displacement measurement. Moreover, we confirmed the usefulness of displacement measurement by the
sensor unit through experiments using a prototype.