This work focuses on the results and analyses performed on long-period optical-fiber gratings (LPFG) designed for use as refractometric sensors. The study was conducted with non-uniform LPFG specially designed for refractometric applications. First, LPFG transmission spectra were calculated for nominal period, propagation constant of the cladding mode and phase shift between the adjacent LPFG sections. Then, a random variation of the above parameters was introduced and the effect of this variation on the transmission spectrum of the LPFG, with the external refractive index as parameter, was evaluated. Finally, a change in the resonant wavelength against the external refractive index was investigated under random variation of parameters. We have found that in some long-period fiber grating refractometric sensors fabrication tolerances should as stringent as 0.2 % of nominal parameter. The methodology of tolerance analysis and quantitative data obtained in this work might be useful in establishing tolerances for fabrication parameters of long-period fiber grating refractometric sensors.
An optical-fiber sensor system for structural health monitoring of concrete elements such as beams and columns is
presented. The system employs arrays of conventional optical fibers embedded in the concrete elements as crack sensors.
Twelve types of optical fibers as well as several embedding techniques have been tested for this role. The survival rate of
optical fibers embedded in concrete could be as high as 80%. The loss of fibers during the embedding process was
acceptable provided that the number of fibers in the array had redundancy. The optical transmission of all fibers in the
array was monitored in a time-division multiplexed mode at a high repetition rate, in the kHz range. The monitoring
scheme allowed a quasi-continuous data acquisitions of large optical fiber arrays. A sharp decrease in the optical
transmission of one or more optical fibers was a clear indicator of the development of cracking in the element subjected
to flexural loads. The system was successful in detecting not only the initiation but also the propagation of cracks in
concrete elements subjected to incremental flexural loading. In this work, the relation between the mechanical properties
of the optical fibers and their behavior for the described application is discussed. Also, considerations towards a rational
design of the system are proposed. The damage detection system may be used for the mapping and monitoring of cracks
in concrete elements. The simplicity of the operation and relatively low cost of the proposed system make it a great
candidate for applications in structural health monitoring of critical elements in civil infrastructure.
We present an optical fiber liquid-level sensor which employs an array of plastic optical fibers coupled to a single semicylindrical
refractometric detection element of plastic. The sensor measures the level of liquids in a discrete way and
also is capable of discriminating between different liquids (such as gasoline and water) in a tank or reservoir.