The paper reports results of the long distance (25 km range) distributed optical fiber sensing by means of <i>Tunable
Wavelength Coherent Optical Time Domain Reflectometry</i> (TW-COTDR) method. The tests were designed to verify the
accuracy and repeatability of the method in long distance measurements, as well as compatibility with various optical
fiber types. Results demonstrate the capability of the method to detect strain or temperature changes over long distances. This proposed method is compared to Brillouin sensing techniques, into the same fibers. Unlike the Brillouin-based methods, measurement uncertainty does not increase with increasing distance. We demonstrated 0.16°C uncertainty at 21km.
In this paper we present an engineering application of optical fiber distributed sensing for monitoring of the ground
settlement during a tunnel excavation. The design, installation methods, and implementation of 4D (space plus time)
warning system are outlined. The ground settlement was closely monitored for entire 6 months of tunnel construction
process, revealing a wide range of geomechanical phenomena. Obtained on-line vertical displacements allowed
engineers to control the excavation speed and monitor ground settlement, contributing substantially to the overall
reliability and safety.
The paper deals with the design and principles of the hybrid Brillouin-Rayleigh sensing system. The system is capable of
separating strain and temperature values in single SM fiber and offers spatial resolution as high as 2 cm. Furthermore,
the paper introduces a method to determine required separation coefficients for optical fiber cables. Finally, it presents
several validation examples and results of industrial applications.