Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

Photonic crystal fibers are a kind of fiber optics that present a diversity of new and improved features beyond what conventional optical fibers can offer. Due to their unique geometric structure, photonic crystal fibers present special properties and capabilities that lead to an outstanding potential for sensing applications. A review of photonic crystal fiber sensors is presented. Two different groups of sensors are detailed separately: physical and biochemical sensors, based on the sensor measured parameter. Several sensors have been reported until the date, and more are expected to be developed due to the remarkable characteristics such fibers can offer.

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Photonic crystal fibers for sensing applications

A novel fiber-tip micro-cavity pressure sensor was fabricated by use of a fusion splicer and a pressurizing gas chamber. The fabrication process is simple and efficient without the need for careful cleaving, chemical etching, and bonding. Micro-cavities with wall thickness of a few micrometers demonstrated a pressure sensitivity of ~ 315 pm/MPa . The sensors have compact size, good mechanic strength, and high temperature stability up to 600°C, and may be potentially used for pressure sensing in a high-temperature environment.

A Compact Fiber-Tip Micro-Cavity Sensor for High-Pressure Measurement

Photonic crystal fiber sensors have potential application in environmental monitoring, industry, biomedicine, food preservation, and many more. These sensors work based on advanced and flexible phototonic crystal fiber (PCF) structures, controlled light propagation for the measurement of amplitude, phase, polarization and wavelength of spectrum, and PCF-incorporated interferometry techniques. In this article various PCF-based physical sensors are summarized with the advancement of time based on reported works. Some physical PCF sensors are discussed based on solid core as well as hollow core structures, dual core fibers, liquid infiltrated structures, metal coated fibers, grating incorporated fibers. With the advancement of sensing technology the possibilities of temperature, pressure, strain, twist, curvature, electromagnetic field, and refractive index sensing are discussed. Also, limitations as well as possible solutions and future hopes are outlined.

Prospects of Photonic Crystal Fiber as Physical Sensor: An Overview.

A micro structure folded cladding porous shaped with circular air hole photonic crystal fiber (FP-PCF) is proposed and numerically investigated in a broader wavelength range from 1.4 µm to 1.64 µm (E+S+C+L+U) for chemical sensing purposes. Employing finite element method (FEM) with anisotropic perfectly matched layer (PML) various properties of the proposed FP-PCF are numerically inquired. Filling the hole of core with aqueous analyte ethanol (n = 1.354) and tuning different geometric parameters of the fiber, the sensitivity order of 64.19% and the confinement loss of 2.07 × 10- 5 dB/m are attained at 1.48 µm wavelength in S band. The investigated numerical simulation result strongly focuses on sensing purposes; because this fiber attained higher sensitivity with lower confinement loss over the operating wavelength. Measuring time of sensitivity, simultaneously confinement loss also inquired. It reflects that confinement loss is highly dependable on PML depth but not for sensitivity. Beside above properties numerical aperture (NA), nonlinearity, and effective area are also computed. This FP-PCF also performed as sensor for other alcohol series (methanol, propanol, butanol, pentanol). Optimized FP-PCF shows higher sensitivity and low confinement loss carrying high impact in the area of chemical as well as gas sensing purposes. Surely it is clear that install such type of sensor will flourish technology massively.

https://cyberleninka.org/article/n/1462119.pdf

Folded cladding porous shaped photonic crystal fiber with high sensitivity in optical sensing applications: Design and analysis

We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interferometer for downhole high pressure sensing application. The PM PCF serves as a direct pressure sensing probe. The sensor is transducer free and thus fundamentally enhances its long-term sensing stability. In addition, the PM PCF can be coiled into a small diameter to fulfill the compact size requirement of downhole application. A theoretical study of its loss and birefringence changes with different coiling diameters has been carried out. This bend-insensitive property of the fiber provides ease for sensor design and benefits practical application. The pressure sensitivities of the proposed sensor are 4.21 and 3.24 nm/MPa at ∼1320  and ∼1550 nm, respectively. High pressure measurement up to 20 MPa was achieved with our experiment. It shows both good linearity in response to applied pressure and good repeatability within the entire measurement range. The proposed pressure sensor exhibits low temperature cross sensitivity and high temperature sustainability. It functions well without any measurable degradation effects on sensitivity or linearity at a temperature as high as 293 °C. These characteristics make it a potentially ideal candidate for downhole pressure sensing.

/pdf/high-pressure-sensor-based-on-photonic-crystal-fiber-for-1qxize3ygm.pdf

High pressure sensor based on photonic crystal fiber for downhole application

We propose a new hole-assisted polymer optical fiber design to eliminate the influence of dopant diffusion and to increase the ultra violet (UV) writing efficiency in fiber Bragg grating inscription. The optical waveguide is formed inside a solid core polymethyl methacrylate (PMMA) doped with photosensitive trans-4-stilbenemethanol, surrounded by a ring of three large air holes with double cladding. We determined a map of the single-mode and multi-mode phase transitions using a finite-element-based vectorial optical mode solver. A wide range of geometrical configurations for the single-transverse-mode (HE11) propagation in the visible was obtained. The design is optimized to operate at the low optical loss wavelengths of 580 and 770 nm.

Kei Chun Davis Cheng

Papers

High pressure sensor based on photonic crystal fiber for downhole application

Design of photosensitive microstructured polymer optical fibers