This article explores the use of distributed fiber optic sensing (DFOS) technology in monitoring civil infrastructure, with a concrete example of an elevated railway bridge in Taiwan. The field test utilized multiple strain-sensing fibers attached to a 1 km span of a bullet train railway bridge, which were combined to calculate the 3-dimensional bridge deformation. The installed sensing system and continuous measurements enabled quick safety confirmation after earthquakes of Richter scale 6.4 and 6.8 magnitudes occurred. Finally, the dynamic monitoring of a bullet train using Distributed Acoustic Sensing (DAS) demonstrated the merits of fiber optic sensing for both static and dynamic measurements. The empirical data gathered through this work aid in the evaluation of DFOS technology for structural-monitoring applications.

For the shape sensing of long-range cables, we study a shape sensing method that utilizes the strain data from optical fibers installed around the cable. We estimate the shape by solving the equation of a moving frame whose coefficient contains the curvature and twist rates obtained by the strain data. The estimation error process is formulated by a stochastic differential equation, and the error variance is obtained by solving the variance equation. The obtained theoretical error variance indicates that the shape estimation error becomes maximum when the curve is straight, and the general curve's error is smaller than that for the straight line. We validated these results by numerical simulation.

In a previous paper, we proposed synthetic Brillouin optical time-domain reflectometry (S-BOTDR) to overcome the resolution limit of a conventional BOTDR.We obtained this method's synthetic spectrum by combining the spectrums measured with different composite pump pulses and different composite low-pass filters. In this paper, we solve an optimization problem that minimizes the fluctuation of the synthetic spectrum and experimentally prove that 10-cm spatial resolution, which is much smaller than the conventional BOTDR limit, was attained by the optimal S-BOTDR.

Rayleigh scattering in an optical fiber has the property that its spectrum shifts in proportion to strain and/or temperature changes, which makes distributed sensing of strain/temperature possible. To estimate the spectral shift, cross-correlation between spectra before and after the shift can be used. However, when the shift is large, the overlap portion of the spectra becomes small and false detection of a correlation peak readily arises. In this paper, we analytically obtain the probability of false alarm for cross-correlation coefficients and determine the threshold that gives a specified constant false alarm rate (CFAR). It is shown that stable estimates of spectral shifts are obtained by using the CFAR threshold.

The spatial resolution of strain/temperature measurements using Brillouin scattering in an optical fiber is proportional to the pulse width of a pump laser light. However, because the phonon spectrum is broadened and the signal-to-noise ratio (SNR) decreases if the pulse width is narrowed to improve resolution, it was widely assumed that there existed a practical lower limit for pulse width about 10 ns, resulting in resolution of 1 m. In a previous paper, we proposed a spectral broadening suppression method called pulse-prepump Brillouin optical time-domain analysis (PPP-BOTDA), which was verified to attain a 2 cm resolution. In this paper, we apply the pulse compression techniques used in high-resolution radar to increase the SNR of Brillouin distributed sensors, which leads to far higher accuracy.

The equations of Brillouin distributed sensing using an optical fiber cannot be solved analytically due to nonlinear interactions between light waves and phonons. In a previous paper, an asymptotic solution was obtained using a perturbation method. However, since the solution does not depend on the length of the optical fiber nor the distance to the measurement positions, it is insufficient for performance analysis of long-range measurements. Moreover, the higher order terms in the perturbation expansion include secular terms. In this paper, a multiscale expansion method in the singular perturbation theory is used to solve the nonlinear equations for the performance analysis of long-range measurements. An asymptotic solution is obtained that contains the pump depletion effect.